Textile auxiliaries are defined as chemicals of formulated chemical products which enables a processing operation in preparation, dyeing, printing of finishing to be carried out more effectively or which is essential if a given effect is to be obtained. Certain Textile Auxiliaries are also required in order to produce special finishing effects such as wash & wear, water repellence, flame retardancy, aroma finish, anti odour, colour deepening etc. The prime consideration in the choice of Textile materials is the purpose for which they are intended, but colour has been termed the best salesman in the present scenario. The modern tendency is towards an insistence on colour which is fast to light, washing, rubbing, and bleaching; this movement makes a great demand on the science of dyeing. Auxiliaries, dyes and dye intermediates play a vital role in textile processing industries. The manufacture and use of dyes is an important part of modern technology. Because of the variety of materials that must be dyed in a complete spectrum of hues, manufacturer now offer many hundreds of distinctly different dyes. The major uses of dyes are in coloration of textile fibers and paper. The substrates can be grouped into two major classes-hydrophobic and hydrophilic. Hydrophilic substances such as cotton, wool, silk, and paper are readily swollen by water making access of the day to substrate relatively easy. On other hand hydrophobic fibers, synthetic polyesters, acrylics, polyamides and polyolefin fibers are not readily swollen by water hence, higher application temperatures and smaller molecules are generally required. Dye, are classified according to the application method. Some of the examples of dyes are acid dyes, basic or cationic dyes, direct dyes, sulfur dyes, vat dyes, reactive dyes, mordant dyes etc. Colorants and auxiliaries will remain the biggest product segment, while faster gains will be seen in finishing chemicals. World demand for dyes and organic pigments is forecast to increase 3.9 percent per year through 2013, in line with real gains in manufacturing activity. Volume demand will grow 3.5 percent annually. While the textile industry will remain the largest consumer of dyes and organic pigments, faster growth is expected in other markets such as printing inks, paint and coatings, and plastics. Market value will benefit from consumer preferences for environmentally friendly products, which will support consumption of high performance dyes and organic pigments.
Some of the fundamentals of the book are antimony and other inorganic compounds, halogenated flame retardants, phosphorous compounds, dyes and dye intermediates, textile fibers, pigment dyeing and printing, dry cleaning agents, dry cleaning detergents, acrylic ester resins, alginic acid, polyvinyl chloride, sodium carboxy methyl cellulose, guar gum, industries using guar gum, gum tragacanth, hydroxyethyl cellulose, polyethylene glycol, industries using polyethylene glycols, etc.
The book covers details of antimony and other inorganic compounds, halogenated flame retardants, silicone oils, solvents, dyes and dye intermediates, dry cleaning agents, different types of gums used in textile industries, starch, flame retardants for textile and many more. This is very resourceful book for new entrepreneurs, technologists, research scholars and technical institutions related to textile.
Antimony and Other Inorganic Compounds
In many polymers
the high concentration of halogenated
organic compounds needed to impart flame retardancy adversely affects
their
physical properties. In practice halogen containing flame retardants
are
formulated with inorganic compounds that behave synergistically with
the
halogen. This enables formulators to use less additives without
diminishing
flame retardance. Indeed in many instances flame retardancy is improved
when
inorganic halogen synergists are used.
Antimony
Compounds
Antimony
Trioxide. In 1979
approximately 15 900 metric
tons of antimony trioxide (commonly referred to as antimony oxide) was
used to
impart flame retardance to a variety of plastics. Antimony trioxide is
manufactured by oxidizing molten antimony sulfide ore and/or antimony
metal in
air at 600 800°C. Typical properties for antimony trioxide are listed
in Table
1.
Antimony
trioxide is a white pigment (qv). Its pigment
strength is a function of the average particle size and the particle
size distribution.
Particle size can be controlled during its manufacture to produce
either a high
tint or a low tint product. The difference in the particle size and
particle
size distribution between high tint and low tint antimony trioxide is
illustrated in Figure 1. Both grades have the same flame inhibiting
efficacy
but have different effects on pigmentation and physical properties.
Domestic
products and trade names of antimony trioxide are
summarized in Table 2.
Special grades
are available at higher costs. They include
White Star S15 from the Harshaw Chemical Company and ultra fine
antimony oxide
from PPG Industries.
Antimony
Trioxide in Cellulosics. Antimony
trioxide can be used as a condensed phase flame retardant in cellulosic
materials. In these substrates it reacts endothermically with the
hydroxyl
groups and forms a variety of products. The endothermic reaction
absorbs heat
needed to propagate the flame. The products formed are difficult to
ignite and
shield the underlying cellulose from the flame minimizing pyrolytic and
oxidative degradation.
Antimony
Pentoxide. Antimony
pentoxide is
manufactured by the oxidation of antimony trioxide with nitrates or
peroxides
(5 9). For Sb205 the
wt % of antimony is 72.8 and the specific
gravity is 3.8.
When the
pentoxide is heated above 380°C it disproportionates
into antimony tetroxide with the evolution of oxygen.
Commercially
antimony pentoxide is primarily available as a
stable colloid (Nyacol Inc.) or as a redispersible powder (Nyacol Inc.
PPG
Industries Inc.). It is significantly more expensive than antimony
trioxide and
is designed primarily for highly specialized applications. Antimony
pentoxide
manufacturers suggest fiber and fabric treatment applications as a
potential
area for its use. The redispersible powder form of antimony pentoxide
which is
also recommended for plastics contains 88% antimony pentoxide and 12%
dispersing agents. Care must be exercised when this product is
incorporated
into plastic since the dispersing agents can adversely affect the
thermal
stability and physical properties.
Sodium
Antimonate. Sodium
antimonate Na2OSb2O5.½H2O is a free
flowing white powder
made by the oxidation of antimony trioxide in a basic medium. A few of
its
properties are shown in Table 3.
The pigmenting
strength of sodium antimonate is less than
antimony trioxide. It is recommended for formulations in which deep
tone colors
are required. Because it contains 62 wt % antimony somewhat higher
concentrations are needed to make it as effective as antimony trioxide
which has
83 wt % antimony.
Mixed Metal
Antimony Compounds. Recent
developments in inorganic flame retardant synergists have centered on
mixed
products that contain antimony and other metals which reportedly give
excellent
performance at reduced cost.
Thermoguard CPA
(M & T Chemicals
Inc.) appears to be as effective as antimony trioxide in most flame
retardant
applications and has a significantly lower price. Although it contains
a lower
level of antimony compared to antimony trioxide other metals contained
in the
product significantly boost its flame retarding properties.
NL Industries
has developed a series
of antimony silico complexes under the trade name Oncor. These products
contain
up to 50% antimony trioxide. They are less opacifying than either high
or low tint
antimony oxide. Generally antimony silico complexes are less effective
flame retardants
than antimony trioxide. Therefore although the cost per kilogram is
less than
antimony trioxide the cost effectiveness of the antimony silico
complexes can
be higher.
Antimony Halogen
Mechanisms. Antimony
trioxide is used almost exclusively with heat labile
halogen compounds. Most of the mechanisms proposed indicate that
antimony
trioxide is activated by reaction with halogens forming antimony
trihalides or
antimony oxyhalides.
Antimony
trichloride and antimony oxychloride work primarily
as flame phase flame retarders. The type of antimony halide formed
depends on
the concentration of the hydrogen halide and the temperature of the
reaction.
In this study a
typical aliphatic
chlorinated paraffin containing 70 wt % chlorine (Chlorowax Diamond
Shamrock)
was heated alone at a rate of 20°C/min. A 67% weight loss was noted at
250 360°C
(see Fig. 2). The loss is equivalent to 93 wt % of the theoretical
stoichiometric quantity of hydrogen chloride.
When an equal
weight of antimony
trioxide was added to the chlorinated paraffin and the mixture was
heated at
the same rate a 76% weight loss at 310 400°C was noted (see Fig. 3). If
there
were no reaction the loss would have been only 37.5% since only half of
the
mixture was the chlorinated paraffin and antimony trioxide does not
volatilize
below 656°C. The higher weight loss indicates that some reaction
between either
the decomposition products of the chlorinated paraffin or the
chlorinated
paraffin itself and antimony trioxide have taken place. The gas
generated by
the reaction has been analyzed and identified as antimony trichloride.
The
weight loss is equivalent to 90% of the theoretical quantity of
antimony
trichloride that can be formed from the mixture. From this thermal
analysis it
is apparent that antimony trichloride is the predominate antimony
species
formed from combinations of antimony trioxide and aliphatic chlorine
compounds
that generate high concentrations of hydrogen chloride upon thermal
degradation.
When a cyclic
halogenated organic compound Dechlorane 5 10
(Hooker) that contains 77% chlorine was heated at a rate of 20°C/min
90% of its
weight was lost between 280 and 400°C. It does not generate hydrogen
chloride directly
upon decomposition (see Fig. 4). When equal weights of Dechlorane 5 10
and
antimony trioxide were heated at the same rate a different weight loss
pattern
was noted (see Fig. 5).
Instead of the
smooth continuous decomposition pattern
observed for either the chlorinated paraffin antimony trioxide mixture
or
Dechlorane 5 10 itself a two stage decomposition pattern was observed.
There
was a 45% weight loss between 305 and 410°C and another between 490 and
680°C.
It appears that
antimony oxyhalides are the primary antimony
compounds formed when organic halogen compounds which do not generate
hydrogen
chloride directly upon thermal exposure and antimony trioxide are
heated
together.
Antimony
trihalides are the flame retarding species whether
they are generated directly from the starting antimony halogen mixture
or from
antimony oxyhalide. They inhibit combustion by altering the manner and
type of
decomposition products formed by the plastic and by modifying the
reactions in
the flame to make them less exothermic. In the condensed phase or
molten
polymer just beneath the flame antimony trihalide promotes reactions
that form
carbonaceous chars instead of highly volatile reactive gases. The chars
act as
heat shields which deflect the heat of the flame and slow down the
thermal and
oxidative decomposition of the polymer. The chars also form a seal
around the
polymer preventing potentially flammable gas from escaping and entering
the
flame.
Once in the
flame the antimony trihalides decompose into
various antimony oxides and halogen compounds. The decomposition
mechanism has
not been completely determined.
The antimony
oxides formed also participate directly in
reactions with the hydrocarbons to give water and molecular hydrogen
instead of
flame propagating radicals. Since the formation of the nonpropagating
molecules
is less exothermic than the formation of flame propagating radicals
less heat
is generated.
Boron
Compounds
Approximately
2700 metric tons of borates was used as flame
retarders for poly(vinyl chloride) cellulosics and unsaturated
halogenated
polyesters in 1979. Zinc borate is by far the most widely used of this
class of
compounds (see Boron compounds). There are a variety of zinc borates
available
that vary in zinc boron and water content.
Manufacturers
and trade names of commercially available
borate flame retardants are shown in Table 4.
Zinc borate is
rarely used alone. It acts synergistically
with antimony oxide enabling compounders to extend antimony trioxide in
some
formulations.
Zinc borate is
also used with high levels of alumina
trihydrate in some halogenated unsaturated polyester resins.
Boric Acid
Sodium Borate. Boric acid and
sodium borate
(borax) are two of the oldest known flame retardants. They are used
primarily
to flame retard cellulosics such as cotton (qv) and paper (qv). Both
products
are inexpensive and fairly effective in these applications. Their use
is
limited to products for which nondurable flame retardancy is acceptable
since
both are very water soluble.
Boron Mechanism. Boron compounds
function as flame retardants
in both the flame and condensed phases. Flame phase active boron
compounds are
generated from combination of borates and halogenated organic
compounds. These
compounds usually generate boron trihalides which have been used to
reduce the
flame volatility of air hexane mixtures.
Boric acid and
borax are effective condensed phase flame retardants
in polyhydroxyl compounds especially in cellulosic fibers. When these
compounds
are exposed to a flame they melt and form a glasslike coating around
the
fibers. Prolonged exposure causes the coating to dehydrate generating
water which
cools the flame and cause it to extinguish. The boron residue also
reacts with
the hydroxyl groups of the cellulose to generate additional quantities
of water
and form an inorganic char that is difficult to ignite and burn. The
char is an
insulator that slows down the rate of polymer degradation and fuel
formation.
Boron compounds
that also contain other metals are active in
both phases. Although zinc borate is not used alone to flame retard PVC
it does
inhibit flammability in the condensed and flame phases. Upon exposure
to the
flame the PVC generates hydrogen chloride which can react with the zinc
borate
to form nonvolatile zinc compounds as well as volatile and nonvolatile
boron
compounds.
The nonvolatile
zinc compounds and boric acid promote char reducing
fuel formation and the boron trichloride and water cool and extinguish
the
flame.
Ammonium
Fluoroborate. Ammonium
fluoroborate NH4BF4 is
another boron containing compound that has
some utility as a flame retardant. It can decompose to yield both
halogen and
boron functionalities to the flame retarding process. Flame retardant
plastic
formulations recently published suggest that ammonium fluoroborate
should be
used primarily in combination with antimony trioxide. Manufacturers
propose
that the following reaction describes functionally what takes place
when the
two products are exposed to flaming conditions.
The products
formed contribute to extinguishing the flame by
the mechanisms proposed in proceeding paragraphs.
Alumina
Hydrates
Approximately
159 000 metric tons of alumina trihydrate
(ALTH) was used to flame retard unsaturated polyesters and foam carpet
backing
in 1979. ALTH is made either from bauxite by the Bayer process from
recovered
aluminum by the sinter process. Physical properties listed in Table 5
and
principal suppliers in Table 6.
Alumina
trihydrate is the only aluminum compound of
commercial significance as a flame retardant. It functions as a flame
retardant
in both the condensed and flame phases.
When alumina
trihydrate is exposed to temperatures above 250°C it forms water and
alumina.
The evolution of
water absorbs heat. The water cools the
flame and dilutes the flammable gases and oxidant in the flame. The
alumina
residue an excellent heat conductor increases removal of heat from the
flame
zone.
Although ALTH is
an inexpensive compound it is a
comparatively inefficient flame retardant. High add on levels up to
four times
as much as the plastic itself are needed to impart acceptable flame
retardance.
It is used alone only in polymers in which large amounts of filler can
be
tolerated and increased weight (or density) is desired. The major
application
areas for ALTH are filled thermoset polyesters and styrene butadiene
rubber
latex rug backing.
Alumina
trihydrate is also used as a secondary synergist to
improve the flame retardance of polymer systems that already contain
antimony
trioxide zinc borate or some phosphorus flame retardants.
Molybdenum Oxides
Molybdenum
compounds have been used
as flame retardants of cellulosics for many years. Recently they have
found
some use in other polymers. Molybdenum compounds appear to function as
condensed phase flame retarders (32). After ignition of PVC
formulations
containing molybdenum oxide (MoO3) and antimony
oxide 90% of the molybdenum remained in
the ash and only 10% of the antimony was found.
Since most of
the molybdenum
remained in the ash and the formulation did have flame retardant
properties molybdenum
is probably a condensed phase flame retardant that promotes char. The
precise
mechanism of action has not been sufficiently defined to warrant
further
speculations.
Halogenated Flame Retardants
The development
and extensive use of
synthetic polymers in both old and new types of applications has
intensified
the concern for combustibility. Although these new polymers are not
necessarily
more flammable than natural polymers they are more readily used in
forms eg foams
electrical applications etc that can result in an increased fire
control
problem.
Along with the
development of many
synthetic polymer systems during the 1930s and 1940s a significant
advance in
the science of imparting flame resistance occurred ie the use of
halogenated
organic materials to impart ignition resistance to these new polymer
systems.
In early
plastics applications the
small size of fabricated articles and the relative scarcity of these
articles
made fire retardancy a secondary consideration. Advances in plastics
technology
have led to increasingly large scale applications especially in the
construction industry. Since many polymers have fuel values (heats of
combustion) comparable to common fuels eg wood oil alcohol etc. it is
readily
understandable that they contribute to the burning process in a typical
fire.
Commercial
halogenated products used
as flame retardants for plastics currently in use are mainly compounds
containing high (50 85 wt %) levels of either chlorine or bromine ie
decabromodiphenyl
oxide chlorendic acid tetrabromophthalic anhydride etc. These materials
fall
into two distinct types additives and reactives. The additives have the
advantage of being readily added to a polymer by mechanical means with
a
minimum of reformulation being required. The reactives on the other
hand require
the development of essentially new polymer systems.
Only massive
polymer forms are considered though the
materials and concepts discussed are almost similarly applicable to
fibers fabrics
coatings and elastomers. Halogenated phosphorus compounds are included
under
Flame retardants phosphorus compounds.
Principles
of Developing Flame Retardant Polymers
Any discussion
of the principles of developing flame retardant
polymer systems must acknowledge the chaotic situation that exists at
present.
This situation has arisen for a variety of reasons technical economic
legal and
semantic.
The semantic
problem is the worst in that it is at the root
of most of the other problems and is caused by the fact that the term
fire or
flame retardant may be perceived in a variety of ways depending upon
the user s
viewpoint. The term as defined above means simply that some change has
been
made in a polymer system so that it will pass one or more of at least a
hundred
different flammability tests. These tests are normally designed to
minimize but
not eliminate the fire risk associated with the use of a polymer in
some
specific use or product. As a consequence a modification of a polymer
that
makes it suitable for one use does not necessarily make it suitable for
others.
There is no single fire retardant chemical or method that is applicable
to all
polymer systems or even to all uses of a single polymer.
It is therefore
necessary that early in the development of a
flame retardant polymer system the question Why? is answered before
much effort
is put into answering the question How? .
It is not
unusual to see many compounds proposed as flame retardant
chemicals that are clearly unusable in any practical sense but that
allow a
polymer system to pass a specific flammability test. A polymer system
can be
easily modified so that it can be called flame retardant by some test.
It is
difficult however to do so and keep a polymer system that is low cost
environmentally
and physiologically acceptable and also mechanically and esthetically
not too
dissimilar from nonfire retardant counterparts.
One of the
most common approaches used to modify the burning properties of
polymers at the
present time is by incorporation of halogen into the polymer matrix
either
directly or through the use of halogenated additives. The usual
rationale for
the use of the halogens as flame retardants is based on the theory that
they
function in the gas phase as radical traps. It is generally agreed that
the
combustion of gaseous fuels is a high temperature process which
proceeds via a
free radical mechanism.
In the radical
trap theory of flame inhibition it is thought
that equations 6 10 effectively compete with equations 2 5 for those
radical
species that are critical for flame propagation ie .OH and .0. thereby
slowing
the rate of energy production and resulting in the extinction of the
flame.
Hydrogen fluoride does not significantly enter into the flame chemistry
thus
fully fluorinated compounds are generally considered to be ineffective
as flame
retardant agents. The radical trap theory of flame inhibition although
attractive in that it can be adapted to any situation tends to lead to
the
belief that the simple inclusion of small amounts of halogen into a
polymer
system will render the system flame retardant.
A recent
physical theory of flame suppression by the halogens
although conceding that the halogens enter into flame chemistry
suggests that
this participation per se cannot be the primary
mechanism by which the
halogens function. Rather it is postulated that the halogens act by
altering
the physical properties ie the density and mass heat capacity of the
gaseous
fuel oxidant mixture so that flame propagation is effectively
prevented. The
physical theory is primarily based on the observations that any gaseous
mixture
of fuel and halogenated agent generally propagates flame when mixed
with air as
long as the mass fraction of halogen in the mixture is less than ca 0.7
and the
relative effectiveness of the halogens is directly proportional to
their atomic
weights ie F CI Br I = 1.0 1.9 4.2 6.7. The halogenated agents probably
act by
the same basic mechanisms as the inert gases ie CO2 N2 etc
and their
suppressant effects are additive to those of the inert gases.
Mo is
the mass fraction of oxygen in the
combustion zone Hc is
the net heat of combustion of the sample
(J/g) r is the stoichiometric mass oxygen/fuel ratio Cp is
the specific heat of the gases in the
combustion zone Ts
is
the
surface temperature of the sample (ºC) Ta is
the ambient
temperature (ºC) and HG is
the apparent heat of gasification (J/g).
The B number contains the fundamental properties of the polymeric
materials.
Thus the mass burning rate or burning intensity can be related to the
fundamental properties of the material.
Where Mi is
the mass fraction of the inert components
of the mixture and ma mN mf
and
mo are
the weights
of agent nitrogen fuel and oxygen respectively. If all of the terms in
the B
number remain constant an increase in the mass of inert gas in the
combustion
zone (ma O)
results in a lower oxygen mass fraction mo a
lower B number and a corresponding reduction
in the polymer burning rate.
When applied to
liquid fuels the Spalding B number in its
simplest form can be visualized as the ratio of the heat of combustion
and the
heat of vaporization (Hc/Hv). Table 1 shows
the significance of
this ratio applied to several halogen containing fuels. In Table 1 the
flash
and fire points are expressed both in °C as normally reported and as
the weight
of compound present in the gas phase over the surface of the liquid at
this
temperature (mg/L). The introduction of halogen has a lesser effect
upon Hv per
milligram of compound evaporated. The ratio Hc/ Hv decreases
with added halogen indicating that
less energy is available from the flame for gasification and in order
to keep
the flame burning additional heat from some outside source is required.
Hv is
the amount of
heat required to vaporize the weight of fuel (latent heat of
vaporization)
present in the gas phase at the appropriate flash and fire points after
the
fuels have been raised to these temperatures by the outside source.
Note the
large increase in mass that must be vaporized in order to obtain
sustained
burning in the case of bromobenzene at least 100 times the mass that
must be
vaporized in the case of benzene itself.
The physical
theory apparently accounts for the effects seen
when halogenated agents are used as flame retardants. In view of the
fact that
the halogen content of a typical plastic is generally ca 1 30 wt % it
is
obvious that if the typical polymer were totally vaporized the gases
given off
would be quite capable of flame propagation.
In order to
visualize the role of halogen it is necessary to
examine the heat balance that occurs at the surface of the polymer.
Figure 1
shows a schematic of this balance (10). Heat received by the polymer
surface
may arise either as a heat flux from the flame (T) or as an
externally applied heat flux (E) derived from
another source. Heat
is lost either as the heat required for gasification (G) of the polymer
or as heat lost L through
radiation conduction convection dripping
etc. T and
G are
agent
dependent whereas E is
obviously agent independent except in char forming
systems. L may
be agent dependent if the agent acts by
increasing the drip rate of the burning polymer. Halogenated agents
affect the
heat balance through T G and
L. Although
phosphorus may act in the
gas phase it appears to be the most important element affecting G and
E through
char
formation.
Qualitatively
the burning process involves heating of the
substrate to a temperature high enough to drive off flammable vapors.
When the
rate of vapor evolution becomes high enough to generate a flammable
mixture the
mixture ignites. If the rate of vapor or gas evolution becomes
sufficiently
high the heat produced by the combustion process may return enough heat
to the
substrate so that the evolution offuel becomes self sustaining.
When a flame
retardant that acts in the vapor phase is added
to the system part of the vapor that distills from the polymer does not
contribute to the heat of combustion but results only in a reduction in
the
mass fractions of the oxygen and fuel in the combustion zone. Hence
there is an
increase in the total mass of material that must be vaporized per unit
time in
order to keep the fire burning. A corresponding increase in the amount
of
energy must be added to the system from an external heat source (E Figure
1) in order to vaporize the extra
material.
Both dripping
and char formation interfere with the energy
feedback cycle (T and
E) and
consequently cause an increase in the intensity
of the external heat flux required to balance the energy fuel cycle.
Where the flame
is actively spreading over the surface of a
material the elemental composition of the vapor being evolved ahead of
the
moving flame is not necessarily the same as the elemental composition
of the
polymer. The composition of the vapors may vary considerably between
the
temperature at which the material first begins to evolve vapors and the
temperature at which the rate of evolution supports the flame. With
this type
of dynamic burning condition changes in the substrate and the structure
of the
agent are more important than they are under steady state conditions.
There are five
fundamental methods used to fire retard both
natural and synthetic polymer systems. They are
Raise the
decomposition temperature of the polymer. This is
generally accomplished by increasing the cross linking density of the
polymer as
with ladder polymers (increase G).
Reduce the fuel
content of the system. This
approach generally involves halogenating the polymer backbone adding
halogenated additives adding inert fillers or by resorting to inorganic
systems
(increase G decrease
T).
Induce polymer
flow by selective chain scission. This
approach is generally applicable to thermoplastic polymer systems where
interrupting the polymer backbone results in reduction of the viscosity
of the
polymer and promotes dripping (increase L).
Induce selective
decomposition pathways. This method
is most applicable to cellulosics where the introduction of phosphorus
compounds generates phosphorus acids which catalyze the loss of water
and the
retention of the carbon as char (increase G decrease
T).
Mechanical means
include (1) bonding a
nonflammable
skin on the polymer (2) covering the polymer with an intumescent
coating (3)
design of the system and (4) the use of sprinklers (decrease E).
Antimony Halogen
Synergism. Antimony oxide
a commonly
employed fire retardant adjunct for halogen containing polymer systems
is
usually employed as a means of reducing the halogen levels required to
obtain a
given degree of flame retardancy with the polymer system. This
reduction is
often desirable since the required halogen content for the system may
be so
high that
it affects the physical properties of the system. In other cases the
antimony
oxide is used simply to give a more cost effective system.
Antimony halogen
systems have been widely studied in attempts
to explain the apparent synergistic effects obtained with this
combination of
elements. No completely satisfactory theory is available as yet but it
is
generally agreed that the active agents antimony trihalides or antimony
oxyhalides act principally in the gas phase (12 13). As with the
halogens it is
generally postulated that the antimony halides act as radical traps.
Small scale
tests show that the optimum halogen (CI
Br)/antimony atom ratio in most systems is 3/1 (14) corresponding to
the atom
ratio found in the antimony trihalides ie SbCI3 SbBr3. On the usual
weight basis this
corresponds to a ratio of ca 0.9/1 for the chlorine antimony system and
ca 2/1
for the bromine antimony systems.
Although the
antimony halides appear
to act principally in the gas phase some effect on the condensed phase
chemistry
cannot be ruled out. Antimony halogenflameretardant compositions
usually
produce a carbonaceous residue even in polymers such as polypropylene
which
produces none in the absence of fire retardants. The production of the
carbonaceous residue probably results from the antimony trihalides
strong Lewis
acid catalysts which are capable of promoting the dehydrohalogenation
of
organic halides and coupling and rearrangement reactions in organic
systems.
Phosphorus
Halogen Systems. A large
number of phosphorus containing compounds have been used in halogen
containing
polymer systems as a means of improving their ignition resistance. In
many of
these cases both the phosphorus and the halogen reside in the same
molecule although
there is little if any evidence to indicate that having both elements
in the
same molecule has any particular advantage. Because there is no fixed
optimum
phosphorus halogen ratio in contrast to the antimonyhalogen system it
is
frequently easier to optimize the ignition resistance when the
phosphorus and
halogen are adjusted separately.
Generally
phosphorus appears to act
as an acid precursor in the solid phase to induce selective
decomposition
pathways that result in a reduction in the rate of fuel formation and
an
increase in charring. This mode of action is most applicable to
cellulosics but
may also be important in other oxygen or nitrogen containing polymers
such as
polyesters (qv) polyamides (qv) and polyethers (qv).
In polymers such
as polyolefins and
polystyrene the formation of acids has little affect on the mode of
polymer
decomposition and much of the phosphorus may be volatilized in some
cases as
much as 50 99%. Even in these cases some of the phosphorus may end up
as
polyphosphoric acids which serve to protect the substrate from the heat
produced by the burning gases. The phosphorus that volatilizes will
show some
beneficial flame retarding effects in that it has gas phase flame
suppressing
activity similar to the halogens.
Phosphorous Compounds
The main fire
retardants currently used in plastics and
textiles fall into several distinct classes (1) alumina trihydrate (2)
halogenated compounds usually used in combination with antimony oxide
(3) borax
and boric acid and (4) the phosphorus phosphorus nitrogen and
phosphorus halogen
compounds.
Mechanism
of Action of Phosphorus Flame Retardants
The overview
article presents a broad discussion of flame retardant
mechanisms. The following discussion deals specifically with phosphorus
flame retardants.
Condensed Phase
Mechanisms. The mode of
action of phosphorus based flame retardants in cellulose has been more
extensively studied and is better understood than in most other polymer
systems. Two alternative routes of cellulose (qv) pyrolysis are known
to occur one
route proceeds first to a tarry depolymerization product called
levoglucosan
(1) which decomposes to volatile combustible fragments the other route
(catalyzed by acids) leads primarily to water and difficultly
combustible char.
Although mineral
acids in
general catalyze the desired water and char forming pyrolysis route
phosphoric
acid is particularly advantageous because of its low volatility. Also
when
strongly heated phosphoric acid yields polyphosphoric acid which is
even more
effective in catalyzing the desired dehydration reaction. The flame
retardant
action of phosphorus compounds in cellulose is believed to proceed by
way of
initial phosphorylation of the cellulose. The phosphorylated cellulose
then
breaks down to water phosphoric acid and an unsaturated cellulose
analogue eventually
char by repetition of these steps. Certain nitrogenous compounds such
as
melamines guanidines ureas and other amides appear to catalyze the
cellulose
phosphate forming steps and are found to enhance or synergize the flame
retardant
action of phosphorus on cellulose.
In poly
(ethylene
terephthalate) and poly (methyl methacrylate) the mechanism of action
of
phosphorus based flame retardants has been shown to involve both a
similar
decrease in the amount of combustible volatiles and a similar increase
in the
amount of residue (aromatic residues and char). The char thus formed
also acts
as a physical barrier to heat and gases.
In rigid
polyurethane foams the
action of phosphorus flame retardants also appears to involve char
enhancement.
The physical
character of the
char from rigid urethane foams was found to be affected by the
retardant. The
presence of a phosphorus containing flame retardant caused rigid
urethane foam
to produce a more coherent char possibly serving as a physical barrier
to the
combustion process. There is evidence that a substantial fraction of
the
phosphorus may be retained in the char.
In polymers such
as
polystyrene that do not readily undergo charring phosphorus based flame
retardants tend to be less effective and such polymers are usually
flame
retarded by antimony halogen combinations. However even in noncharring
polymers
phosphorus additives exhibit some activity that suggests at least one
other
mechanism of action. It has been proposed and some evidence adduced
that
phosphorus compounds may produce a barrier layer of polyphosphoric acid
on the
burning polymer.
There is
evidence that
phosphorus containing additives can act in some cases by catalyzing
thermal
breakdown of the polymer melt reducing its viscosity and favoring the
flow or
drip of molten polymer from the combustion zone. In polystyrene tris(2
3 dibromopropyl)
phosphate acts at least in part by this mechanism.
Several
commercial polyester fabrics are flame retarded with
low levels of phosphorus additives or reactives which cause them to
melt and
drip more readily than fabrics without the flame retardant. This
mechanism can
be counteracted or completely defeated by the presence of
nonthermoplastic
fibers such as cotton which can serve as wicks or by silicone oils
which can
form pyrolysis products capable of impeding melt flow.
Vapor Phase
Mechanisms. In addition
to the condensed phase mechanisms discussed above phosphorus flame
retardants
can exert vapor phase flame retardant action. It has been demonstrated
that
trimethyl phosphate retards the velocity of a methane oxygen flame with
about
the same molar efficiency as SbCl3. Both physical
and chemical vapor phase mechanisms
have been proposed for the flame retardant action of certain phosphorus
compounds. Since tris(dibromopropyl) phosphate was found not to change
the
activation energy of thermo oxidative degradation of polypropylene
although it
raised the oxygen index a vaporphase physical shielding action was
postulated.
Possibly this action may be produced by bromine containing pyrolysis
products
rather than by the phosphate itself.
Triphenylphosphine
oxide and
triphenyl phosphate as model phosphorus flame retardants were shown by
mass
spectroscopy to break down in a flame to give small molecular species
such as
PO HPO2 PO2 and
P2. The rate
controlling hydrogen atom
concentration in the flame was shown spectroscopically to be reduced
when these
phosphorus species were present. These data indicate the existence of a
vapor phase
mechanism however the stable volatile compounds used in this study are
not
typical of many of the phosphorus based flame retardants used
commercially.
Physical or chemical vapor phase mechanisms may be reasonably
hypothesized in
cases where a phosphorus flame retardant is found to be effective in a
noncharring
polymer and especially where the flame retardant or phosphorus
containing
breakdown products are capable of being vaporized at the temperature of
the
pyrolyzing surface. In General Electric s engineering thermoplastic
Noryl which
consists of a blend of a charrable poly (phenylene oxide) and a
noncharrable
polystyrene experimental evidence indicates that effective flame
retardants
such as triphenyl phosphate act in the vapor phase to suppress the
flammability
of the polystyrene pyrolysis products.
A comparison of
a variety of
phosphorus additives at equivalent phosphorus loadings was made in poly
(methyl
methacrylate) which can be retarded by condensed phase action but
should also
be subject to vapor phase inhibition because it depolymerizes to
monomer. Poor
flame retardancy was found with trimethylphosphine oxide a volatile
stable
species whereas a much larger oxygen index elevation was observed with
phosphoric acid this result suggests that the condensed phase mechanism
is the
more efficient one in poly (methyl methacrylate).
Poly (ethylene
terephthalate) exhibits a higher oxygen index
with 5 wt % phosphorus incorporated in the backbone of the polymer as
phenylphosphinyl groups as contrasted to 5 wt % phosphorus incorporated
as a
relatively volatile additive triphenylphosphine oxide. This result
suggests
that a condensed phase mechanism is more effective than a vapor phase
mechanism
in this polymer.
The question as
to whether a flame retardant operates mainly
by a condensedphase mechanism or mainly by a vapor phase mechanism is
especially complicated in the case of the haloalkyl phosphorus esters.
A number
of these compounds upon thermal degradation release volatile
halogenated
hydrocarbons which are plausible flame inhibitors. At the same time
their phosphorus
content remains as relatively nonvolatile phosphorus acids which are
plausible
condensed phase flame retardants. There is no evidence for the
formation of
phosphorus halides.
Interactions
With Other Flame Retardants. Some claims
have been made for a phosphorus halogen synergism but unlike the firmly
established antimony halogen synergism phosphorus halogen interactions
are
often merely additive and in some instances slightly less than
additive. Cases
of phosphorus halogen synergism (ie activity greater than that
predicted by
some additivity model) usually do not hold up to careful analysis and
some
supposed cases are artifacts of nonlinear response concentration
relationships.
Nevertheless combinations of phosphorus and halogen in separate
compounds or in
a single compound are often quite useful even if not truly synergistic.
Antagonism
between antimony oxide and phosphorus flame retardants
has been reported in several polymer systems and has been explained on
the
basis of phosphorus interfering with the formation or vaporization of
antimony
halide. This phenomenon is also not universal and some useful
commercial PVC
formulations have been described for antimony oxide and triaryl
phosphates.
An interesting
case of synergism has been described involving
a bisphosphine oxide American Cyanamid s RF 699 and ammonium
polyphosphate.
Phosphorus
Based Flame Retardants in Commercial Use
Since the
original report of
ammonium phosphate as a flame retardant by Gay Lussac in 1821 and the
commercial introduction of tricresyl phosphate as a flame retardant
plasticizer
for cellulosics early in the present century many thousands of
phosphorus
compounds have been described as having flame retardant utility. A
broad
sampling of these is covered in ref. 33. The more specialized topics of
phosphorus monomers and polymers containing built in phosphorus have
been
reviewed. This article is confined to the much more limited groups of
compounds
that found commercial or semi commercial use.
Inorganic
Phosphorus
Compounds. Red Phosphorus. This allotropic
form of phosphorus
is relatively nontoxic and unlike white phosphorus is not spontaneously
flammable (although easily ignited). It is a polymeric form of
phosphorus with
thermal stability up to ca 450°C. In finely divided form it has been
found to
be outstandingly effective as a flame retardant additive. In Europe it
has
found commercial use in molded nylon electrical parts. Handling hazards
such as
flammability odor partial reversion to toxic white phosphorus and the
imparting
of color have deterred broader usage. A product Exolit 505 available
from
Hoechst (FRG) consists of red phosphorus treated with caprolactam and
is
reported to be safer than the untreated material (38). Related products
are
marketed in Japan.
Ammonium
Phosphates. These salts
were recommended for treating theater
curtains in 1821. Their use in forest fire control is well established.
Monoammonium phosphate and diammonium phosphate or mixtures of the two
which
are more water soluble and nearly neutral are still used in large
amounts for
nondurable flame retarding of paper textiles disposable nonwoven
cellulosic
fabrics and wood products. Their advantage is high efficacy and low
cost.
Ammonium phosphate finishes are not resistant to laundering or even to
leaching
by water but they are resistant to organic solvents such as dry
cleaning
solvents. One important advantage of ammonium phosphates as flame
retardants and
phosphorus flame retardants in general over borax (also used for
nondurable
cellulosic flame retardants) is their effectiveness in preventing
afterglow.
The crystalline
nature of
ammonium phosphates may produce a gritty texture on the surface of some
substrates. This characteristic is lessened by commercial ammonium
phosphate
formulations containing softening and penetrating agents.
Self cross
linking acrylic
latexes have been formulated with diammonium phosphate and organic
phosphates
to obtain flame retardant textile backcoatings and nonwoven binders
with a
small but useful degree of durability to laundering and dry cleaning.
Insoluble
ammonium polyphosphate. When
ammonium phosphates are heated with urea or by themselves under ammonia
pressure relatively water insoluble ammonium polyphosphate (Phoschek
P/30 Monsanto)
is produced. These products are long chains having repeating units of
the
structure OP(O)(ONH4) . This
product a finely divided solid is a principal ingredient of intumescent
paints
and mastics. In such formulations ammonium polyphosphate is considered
to
function as a catalyst. Thus when the intumescent coating is exposed to
high
temperature the ammonium polyphosphate yields a phosphorus acid which
then
interacts with an organic component such as dipentaerythritol to form a
carbonaceous char. A blowing (gas generating) agent such as melamine or
chlorowax
is also present to impart a foamed characteristic to the char thus
forming a
fire resistant insulating barrier to protect the substrate. In addition
the
intumescent formulations typically contain resinous binders pigments
and other
fillers. Mastics are related but generally more viscous formulations
intended
to be applied in thick layers to girders trusses and decking they
generally
contain mineral fibers to increase their coherence.
Ammonia/P2O5 products. The reaction
of ammonia gas
with phosphorus pentoxide at high temperature yields an amorphous
colorless
solid slowly soluble in water to form a nearly neutral solution. The
product
consists of a mixture of ammonium salts of metaphosphorimidic acid.
Analysis
shows about two ammonium nitrogen atoms and one imide nitrogen atom for
every
two phosphorus atoms. Stauffer s Victamide is known to be a complex
mixture but
a typical component is believed to have structure (2).
Victamide as an
aqueous solution can
be applied to paper cotton cloth cotton batting and nonwovens. When dry
it
produces a smoother surface texture than that produced by the
crystalline
ammonium phosphates. Proprietary formulations have been developed
affording
some degree of water resistance presumably the Victamide acts therein
as a phosphorylating
agent. Ammoniation of Victamide by concentrated ammonia produces a
product
which when applied to a cellulosic substrate and heated yields a
semidurable
flame retardant finish that withstands several aqueous washes.
Phosphoric
Acid Based Systems for Cellulosics. Semidurable
flame retardant
treatments for cotton can be attained by phosphorylation of cellulose.
This was
originally accomplished by heating cotton or paper with phosphoric acid
in the
presence of basic compounds such as urea at ca 145 180°C. Commercial
formulations have been developed utilizing as coreactants either
phosphoric
acid and cyanamide or phosphoric acid and a dicyandiamide formaldehyde
resin.
The nitrogenous component catalyzes the phosphorylation of cellulose
retards
the acid degradation of the cellulose and synergizes the flame
retardant action
of the phosphorus. A fair degree of durability to laundering is
achieved by
such treatments. Typically several launderings can be tolerated and dry
cleaning
resistance is good. A substantial part of the decline in flame
retardancy
during laundering is caused by ion exchange of the protons of the
phosphoric
acid groups by sodium calcium and magnesium cations that suppress the
flame retardant
effectiveness of the phosphorus groups. Such finishes also have limited
utility
because of fabric damage during cure although applications have been
made on
draperies nonwoven fabrics and paper products.
Commercial
formulations have
been developed based on phosphoric acid ureaformaldehyde resins and
dicyandiamide as leach resistant clear flame retardant coatings for
wood.
Organic
Phosphorus Flame
Retardants Additive Types. Alkyl
Acid Phosphates. The lower alkyl
acid phosphates have found some limited use as additive flame
retardants in
cast thermoplastics and polyester resins. In cast poly(methyl
methacrylate) methyl
and haloalkyl acid phosphates are effective in combination with halogen
containing additives.
Trialkyl
Phosphates. Triethyl
phosphate is a colorless liquid boiling at
209 218°C and containing 17 wt % phosphorus. It is manufactured from
diethyl
ether and phosphorus pentoxide via a metaphosphate intermediate.
Triethyl
phosphate has been used commercially as an additive for polyester
laminates and
in cellulosics. In polyester resins it functions as a viscosity
depressant and
as a flame retardant. The viscosity depressant effect of triethyl
phosphate in
polyester resin permits high loadings of alumina trihydrate a fire
retardant
smoke suppressant filler. Triethyl phosphate has also been employed as
a flame resistant
plasticizer in cellulose acetate. Because of its water solubility the
use of
triethyl phosphate is limited to situations where weathering resistance
is
unimportant. The halogenated alkyl phosphates are generally used for
applications where lower volatility and greater resistance to leaching
are
required.
Trioctyl
phosphate has been employed
as a specialty flame retardant plasticizer for vinyl compositions where
low
temperature flexibility is critical eg in military tarpaulins. It can
be
included in blends with general purpose plasticizers such as phthalate
esters
to improve low temperature flexibility.
Dimethyl
Methylphosphonate. Dimethyl
methylphosphonate (DMMP) is made by molecular rearrangement of
trimethyl
phosphite. It contains 25 wt % phosphorus (near the maximum possible
for a
phosphorus ester) and it is therefore highly efficient on a weight
basis as a
flame retardant. DMMP is a low viscosity colorless liquid bp 185°C.
Because of
its volatility it has been useful mainly in thermoset systems. DMMP is
an
efficient viscosity depressant in polyester resins and epoxy resins. As
a flame
retardant it has somewhat greater efficiency than triethyl phosphate
which is
used in similar systems. DMMP is used commercially in mineral filled
and glass reinforced
polyester compounds where its viscosity depressant effect permits use
of higher
filler loadings. The use of alumina trihydrate as filler and DMMP in
the dual
role of viscosity depressant and flame retardant affords reinforced
polyester
resin formulations with low flame spread suitable for bathtubs and
shower
stalls. DMMP has been used commercially for boosting the phosphorus
content of
flame retardants used in rigid foams. DMMP is also used as a chemical
intermediate for the manufacture of several other flame retardants.
Urea Formaldehyde Resins
Although the
urea and melamine resin reactions have certain
similarities they also have definite differences therefore it will be
better to
describe them separately. One mole of urea may be reacted with 1 or 2
moles of
formaldehyde to produce different products. The reactions may be
carried out
under either acidic or basic conditions and again different products
will be
obtained. In addition the dimethylol urea may be produced from urea and
formaldehyde and etherified with butanol separately or the
etherification may
be carried out simultaneously with the condensation by reacting the
urea formaldehyde
and butanol together. Figure 1 illustrates the condensation reactions
between 1
mole of urea and both 1 and 2 moles of formaldehyde under acidic and
basic
conditions.
Under acidic
conditions insoluble compounds are formed which
cannot be used for coating resins. Under basic conditions the monoor
dimethylol
ureas are produced which can be used as intermediate products for
coating
resins and other purposes. The dimethylol urea is known commercially as
DMU and
is available as a white solid containing 88 90% DMU and 10 12% water.
It is
soluble in water and alcohol but it polymerizes slowly at room
temperature to
the insoluble stage.
The usual source
of formaldehyde is formalin which is a 37%
solution of formaldehyde in water. The urea is a white crystalline
solid having
a melting point of 133°C and soluble in water to the extent of 80
gm/100 ml it
has a molecular weight of 60. For the production of DMU the correct
amounts of
urea and formalin are adjusted to a pH of 7 8.5 and reacted at about
50°0.
Sodium hydroxide is the usual basic catalyst but others may be used
including
various amines. When the reaction is complete the mass is concentrated
under
vacuum to the desired solid content and may be tray dried spray dried
or
crystallized. The DMU is used in some of the non coating applications
referred
to earlier since it may be polymerized to the insoluble stage by
heating. The
structure of the insoluble polymer has not been proved but there can be
no
doubt that it is highly complex. It is probable that the structure
contains
cross linked linear polymers and six membered rings as indicated in
Fig. 2 A
and B respectively.
In order to form
the six membered ring structure (B Fig. 2)
the two –NH2 of
urea may react differently with
formaldehyde. One may react as a primary amine to form the Schiff s
base followed
by trimerization to the ring structure. The other may then react as an
amide followed
by elimination of water and formation of methylene linkages connecting
the ring
structures as indicated in Fig. 2 B. Until more positive evidence is
obtained
regarding the structure of these insoluble polymers the above theories
afford
some idea of their possibilities and their complexity.
Alkylation or
Etherification. In order to
change the DMU from a water soluble material to an organic solvent
soluble
material it must be made less polar. This is accomplished by alkylation
with various
alcohols. Obviously the lower alcohols with very short carbon chains or
small
non polar groups are less effective than the higher alcohols with
longer carbon
chains. For example the methoxy methylol ureas are water soluble the
ethoxy
products are soluble in ethanol but good solubility in organic solvents
is not
obtained until butyl alcohol is used. It will be shown later that
better
solubility and compatibility with other resins are obtained if the
higher
alcohols are used such as capryl or octyl. However these are more
expensive and
they retard the curing rate of the resin.
Butylated Urea
Resin. A typical urea
formaldehyde resin
suitable for use in baking coatings may be prepared by dispersing the
DMU in
butanol which has been slightly acidified. The dispersion is heated and
both
etherification and polymerization reactions occur. It is essential that
sufficient etherification take place before excessive polymerization
occurs so
that the product will have good solubility and stability. Conversely if
a high
degree of etherification occurs and relatively low polymerization the
resin
will have low viscosity and will be slower curing. These factors are
controlled
by the amount and type of acidic catalyst the temperature and the ratio
of the
components. A variety of acids may be used including phosphoric formic
oxalic and
phthalic. In general the ratio of combined butanol in the final resin
is from
0.5 to 1.0 moles per mole of DMU but of course a considerable excess of
butanol
is used during the resin manufacture.
The water
eliminated in the etherification and polymerization
reactions together with any water with the original DMU is removed
either by
straight azeotropic distillation or by a continuous decantation
procedure. When
the desired degree of etherification and polymerization is reached as
indicated
by solubility and compatibility tests the resin is neutralized and
concentrated. For a resin solution which is marketed as 50% resin 30%
butanol and
20% xylol the original butanol solution would need to be concentrated
to 62.5%
resin and 37.5% butanol. When 100 parts of this solution are thinned
with 25
parts of xylol the resulting product would meet the requirements
indicated
above. Every effort is made to remove as much water and free
formaldehyde as
possible because these detract from stability curing speed and gloss in
the
finished enamel. In general the final resin solution does not contain
more than
0.5% water and somewhat less of free formaldehyde. Resins of this type
may be
prepared from the original ingredients without first preparing the DMU
as an
intermediate. In such cases the ingredients are reacted first under
alkaline
conditions to permit the necessary amount of condensation then finished
under
slightly acidic conditions as indicated above.
The simplified
formulas for a butylated urea formaldehyde
resin are shown in Fig. 3. This resin is based on a mole ratio of 1
mole urea 2
moles formaldehyde and 1 mole butanol.
The partially
polymerized product in Fig. 3 is a highly
simplified and idealized representation. The actual product would be
much more
complex and cross linked and would probably contain ring structures as
indicated in Fig. 2. However the diagram will serve to illustrate the
effect of
the type of alcohol and the degree of etherification on the solubility
and rate
of cure of the resin.
Solubility and
Compatibility. It should be
apparent that the 4carbon chain alkyl group in the butoxymethylol urea
serves
three purposes (1) it decreases the amount of cross linking (2) it
confers
hydrocarbon solubility on the resin (3) it increases compatibility with
alkyds
and other resins. . Decreasing the possible cross linkages retards the
curing
rate and hydrocarbon solubility permits the use of xylol to replace
part of the
more expensive butanol. In the manufacture of the resin isobutanol may
be used but
the secondary and tertiary butanols react too slowly. If the ratio of
butanol
were reduced from the 1 mole shown in Fig. 3 to 2/3 mole there would be
fewer
butoxyl groups in the resin. This would reduce the hydrocarbon
solubility and
would provide another point for cross linking in the trimer
illustrated. Higher
ratios than 1 mole of butanol would increase the solubility but these
are
seldom used since they would retard the curing rate and amount of
polymerization
excessively.
It should be
apparent that increasing the carbon chain from a
4 to a 10 or 12 carbon chain would provide another method for
increasing
hydrocarbon solubility without reducing the number of possible cross
linkages.
This may be done by using capryl octyl or other alcohols instead of
butanol.
However these longer chain alcohols are not good solvents for the
intermediate
DMU. Therefore the DMU polymerizes excessively before any appreciable
amount of
etherification takes place and a heterogeneous product is obtained
which is not
suitable for coating resins. However the higher alcohols may be
incorporated
into the resin by the transetherification procedure. The resin is
prepared
first with one of the lower alcohols such as methanol and the
methylated
methylol urea reacted with capryl or octyl alcohol. The
transetherification
takes place because the liberated methanol may be removed by
distillation at a
temperature low enough not to affect the higher boiling alcohol.
Mineral Spirits
Tolerance. Amino resins are
used
frequently with the medium oil length alkyd resins which are thinned
with
mineral spirits instead of xylol. Since mineral spirits is not as
strong a
solvent as xylol it will be necessary for the amino resins to have
better
hydrocarbon solubility. One method for accomplishing this is the use of
the
higher alcohols referred to above. The long carbon chain on the resin
makes it
much more non polar and therefore more soluble in aliphatic
hydrocarbons. The
degree of solubility of the resin is referred to as its mineral spirits
tolerance. This is measured by adding mineral spirits to the resin
solution
slowly until turbidity develops.
The mineral
spirits tolerance is usually expressed as the
pounds of mineral spirits tolerated by 100 lb of resin solution before
turbidity develops. The mineral spirits tolerance may also be increased
by (a)
increasing the ratio of formaldehyde to urea (b) reducing the degree of
polymerization of the resin (c) increasing the amount of alkylation. It
will be
evident that all these methods tend to reduce the curing rate of the
resin which
means a longer baking time for the finish in order to obtain the same
hardness.
Composition
Variables
The
variation in type of alcohol was the only composition variable
considered in
the preceding discussion of urea resins. However both the amino and the
aldehyde components may also be changed. The only other amino resin
which has
achieved commercial importance to date is the melamine formaldehyde
type
described in the following section. . However mention should be made of
thiourea since it was one of the early amino compounds investigated for
use in
resins.
The
formula shows that the oxygen of urea has been replaced with sulfur.
Resins
made with thiourea have slightly better water and alkali resistance
than
comparable urea resins but they are not as pale in color are somewhat
odorous and
are inferior in exterior durability.
Formaldehyde is
the most useful aldehyde for amino resins
utilized in surface coatings. An aldehyde with more carbons such as
acetaldehyde may be expected to increase the solubility of the resin
but the
curing rate color retention and film properties have been reported to
be
inferior (8). Parker (8) also points out that mixtures of formaldehyde
and
acetaldehyde are impractical because very little of the higher aldehyde
is
combined under such conditions. It can be removed quite readily by
distillation
of the resinous material. Furfural has not been used extensively in
coating
resins to date but it is employed in amino resins for adhesives and
molding
compounds.
Melamine
In 1834 Justus
von Liebig (9) produced a new chemical which
he believed was the amine of melam and which he called melamine.
Subsequent
investigations have shown that his analysis was not entirely correct
but the
chemical has retained its original name. It is a member of the class of
compounds known as triazines and may be designated 2 4 6 triamino 1 3 5
triazine.
It may also be considered a trimer of cyanamide. It may be prepared
from
dicyandiamide by heating under pressure in the presence of a diluent
such as
alcohol or ammonia. The relation of melamine to cyanamide and
dicyandiamide is
shown in Fig. 4.
The
chemistry historical
development and methods of production of melamine are given in
considerable
detail by McClellan (11). Hughes (12) studied crystalline melamine and
found it
to be a resonance hybrid with the position of the atoms as shown in
Fig. 5 (a).
Ostrogovich (13) suggested both amino and imino structures in view of
the
possibility of tautomerism Fig. 5 (b). The amino form is generally used
in the
discussion of melamine in coating resins because its highmelting point
and heat
stability suggest this benzenoid structure. Melamine is a white
crystalline
powder with a melting point of 354°C. It has a molecular weight of 126
and
specific gravity of 1.57 at 25°C. Its solubility in water has been
reported by
Chapman (14) to be 0.5% at 25°C 1.0% at 50°C 2.5% at 75°C and. 4.0% at
90ºC.
Solvents
Terpene
Solvents. The terpene
solvents are the oldest in use by the paint industry and are obtained
from pine
trees. They have been replaced by lower cost aliphatic hydrocarbon
solvents in
many coatings. Their chemical properties make them quite valuable as
raw
materials for synthetic resins and other compounds. The chemical
structures of
important constituents of terpene solvents are shown in Fig. 1.
Turpentine is
the most widely used terpene solvent its principal use being in house
paints
and in some varnishes. The production of gum turpentine from the
exudation of
the pine tree is described. The production of wood turpentine dipentene
and
pine oil from solvent extraction of pine stumps followed by steam
distillation
is described.
Gum turpentine
contains 60 65% a pinene and 30 35% pinene.
Wood turpentine contains about 80% a pinene the remainder being
dipentene terpinene
and terpene alcohols. Dipentene has a higher boiling point than
turpentine and
excellent solvent properties. It is also used to retard the skinning of
varnishes synthetic resins and enamels. The heavy fractions obtained
from the
production of wood turpentine are known as pine oil. These fractions
consist of
terpene tertiary and secondary alcohols plus varying percentages of
highboiling
terpene hydrocarbons. Small percentages of phenol ethers and ketones
are also
present. The polar non polar structure of pine oil makes it suitable
for a wide
range of uses. It has excellent solvent properties improves the flow of
enamels
retards skinning is an antifoaming agent and has some bactericidal
action. A typical
group of terpene alcohol solvents is given in Table 1.
p Cymene and p
menthane are obtained from the
catalytic disproportionation of dipentene. As a result they have a
higher
degree of purity and are used as chemical raw materials as well as
solvents.
p Menthane is a
saturated terpene and therefore not
susceptible to oxidation like the unsaturated terpenes.
The specific
uses for terpene solvents in varnishes resins and
coatings are given in other sections of this book and in Volume II. The
physical characteristics of a typical group of commercial terpene
solvents are
given in Table 2.
Hydrocarbon
Solvents. The petroleum
and coal tar
hydrocarbon solvents are used extensively because of their low cost
good
solvent power for oils and resins and effectiveness as diluents for
nitrocellulose lacquers. The petroleum solvents are the lighter
fractions
obtained by the distillation and fractionation of the crude oil. They
include
the original turpentine substitutes Varnish
Makers and Painters Naphtha (VM &
P Naphtha) and mineral spirits. Vast improvements have been made in
large scale
fractionation apparatus with the result that today many grades of
hydrocarbon
solvents are available as shown in Tables 3 and 4.
The coal tar
hydrocarbons are obtained by distillation of the
material from the coke oven by product recovery process. They include
benzene
(benzol) toluene (toluol) xylene (xylol) and other aromatic
hydrocarbons. The
tremendous wartime demand for toluene to make explosives stimulated
research to
produce it from other sources. Today more aromatics are produced from
petroleum
than from coal tar.
The hydrocarbon
solvents may be classified chemically in
three groups
Aliphatics straight or open chain
saturated
hydrocarbons.
Naphthenics
cyclic saturated hydrocarbons with or without alkyl side chains.
Aromatics
cyclic hydrocarbons carbons containing the benzene ring structure.
The commercial
hydrocarbon solvents are usually mixtures of
closely related compounds and isomers hence the range in distillation
temperatures
for single solvents shown in Tables 3 and 4. The structures of typical
hydrocarbons are shown in Fig. 2 with the boiling points of the pure
chemicals.
The boiling
points increase with increase in molecular weight
in a given series but the effect of molecular shape is shown by the
first three
compounds in Fig. 2. They are isomers of hexane and therefore have the
same
composition and molecular weight but the boiling point decreases with
the
shortening of the main carbon chain. The increase in boiling point of
the
normal straight chain saturated hydrocarbons or alkanes is shown in
Fig. 3. The
normal alkanes containing from 5 to 16 carbon atoms in the chain are
liquids at
room temperature. The solid paraffin wax contains from 18 to 25 carbons
per
chain and polyethylene contains several hundred carbons. This topic was
discussed in Chapter 1 with respect to the secondary valence forces and
chain
length and their effect on the physical properties.
The wide range
of properties in hydrocarbon solvents which
are available commercially is illustrated in Table 3 with the Amsco
solvents of
the American Mineral Spirits Co. ASTM Designation D86 46 gives the
procedure
for distillation of petroleum hydrocarbons. The report usually contains
the
temperatures of the initial boiling point (when the first drop falls
from the
end of the condenser) the points at which 50% and 90% by volume have
been
distilled the dry point (at which the bottom of the flask becomes dry)
and the
end point or temperature at which the last drop is obtained. Additional
heat
must be applied after the bottom of the flask is dry to obtain the last
drop.
It is advantageous in many cases to have the spread in distillation
temperatures kept as small as possible but this requires closer
fractionation
with an increase in cost.
In Table 3 the
solvent power is indicated by the range of KB
values from 34 to 37 for the regular mineral spirits type of solvent to
105 for
toluol. The straight aniline point is used for the aliphatic or mineral
spirits
ype solvents and the mixed aniline point for the aromatic type as
explained
previously. The results from the nitrocellulose dilution ratio test are
given
for the solvents having fast enough evaporation rates and sufficient
solvent
power to be used as diluents. The test was run with butyl acetate as
the true
solvent portion. The values would be different if another solvent were
used as
explained previously. It will be noted that the aromatics are higher in
weight
per gallon than the aliphatics.
A typical set of
characteristics of aromatic solvents as
produced by the coal tar industry is given in Table 4.
Fire and
Explosion Hazard. A comparison of
the flash
point autoignition temperature and explosive limits in air for a
variety of
hydrocarbon solvents may be obtained from Table 5. The fire hazard is
related
to the volume of vapor in the air the volumes of solvent vapor per
gallon of
solvent evaporated at 80°F and 212°F are given. Also given is the
solvent vapor
density in comparison with air.
The volume of
solvent vapor at a given temperature may be
calculated from the relationship between molecular weight and volume.
For
example when the molecular weight is expressed in pounds 1 lb mole of
vapor
occupies 400 cu ft at 80°F and 500 cu ft at 212°F. These factors are
expressed
in the following formula
Using this
formula to calculate the concentration of toluol
at 80°F one obtains about 31 cu ft of vapor per gallon evaporated. When
using a
factor of safety of 4 this means that 4 × 100 × 31 = 12 400 cu ft of
air must
be supplied for each gallon of toluol evaporated to keep the
concentration
safely below the explosive point.
Dyes and Dye Intermediates
Dyes are
intensely colored substances that can be used to
produce a significant degree of coloration when dispersed in or reacted
with
other materials by a process which at least temporarily destroys the
crystal
structure of the substances. This latter point distinguishes dyes from
pigments
which are almost always applied in an aggregated or crystalline
insoluble from.
Modern dyes are products of synthetic organic chemistry. To be of
commercial
interest dyes must have high color intensity and produce dyeings of
some
permanence. The degree of permanence required varies with the end use
of the
dyed material.
All molecules
absorb energy over various parts of the
electromagnetic spectrum. The characteristic of dye molecules is that
they
absorb radiation strongly in the visible region which extends from 4000
7000
angstroms. Only organic molecules of considerable complexity which
contain
extensive conjugation systems linked to electron withdrawing and
attracting
groups give sufficient absorption (tinctorial value) in the visible
region to
be useful as dyes. The shade and fastness of a given dye may vary
depending on
the substrate due to different interactions of the molecular orbitals
of the
dye with the substrate and the ease with which the dye may dissipate
its
absorbed energy to its environment without itself decomposing.
The primary use
for dyes is textile coloration although
substantial quantities are consumed for coloring such diverse materials
as
leather paper plastices petroleum products and food.
The manufacture
and use of dyes is an important part of
modern technology. Because of the variety of materials that must be
dyed in a
complete spectrum of hues manufacturers now offer many hundreds of
distinctly
different dyes. An understanding of the chemistry of these dyes
requires that
they be classified in some way. From the viewpoint of the dyer they are
best
classified according to application method. The dye manufacturer on the
other
hand prefers to classify dyes according to chemical type.
Both the dyer
and the dye manufacturer must consider the
properties of dyes with relation to the properties of the materials to
be dyed.
In general dyes must be selected and applied so that color excepted a
minimum
of change is produced in the properties of the substrate. It is
necessary therefore
to consider the chemistry of textile fibers as a background for an
understanding the chemistry of dyes.
The major uses
of dyes are in coloration of textile fibers
and paper. The substrates can be grouped into two major classes
hydrophobic and
hydrophilic. Hydrophilic substances such as cotton wool silk and paper
are
readily swollen by water making access of the day to the substrate
relatively
easy. On the other hand the ease of penetration also allows easy
removal in
aqueous systems and special techniques must be used where a high degree
of wet fastness
is required.
On the other
hand hydrophobic fibers such as the synthetic
polyesters acrylics polyamides and polyolefin fibers are not readily
swollen by
water hence higher application temperatures and smaller molecules are
generally
required.
The polymer
chemist has increased the versatility of the
newer fibers by incorporating dye sites of a varying nature as needed
to
achieve dyeability with a predetermined class of dyes. It is now
possible to
have polyesters acrylics and polyamide fibers which can be dyed with
positive
(basic cationic) negative (acid anionic) or neutral (disperse) dyes.
These
recent developments have allowed the fabric designer to produce
materials
(textiles carpets) fabricated in patterns which can be dyed three
different
colors from one dyebath containing three types of dyes. This concept is
called cross
dyeing and is becoming increasingly popular as a low cost method of
coloration.
Textile
Fibers
Cotton
and Rayon
Cotton and rayon
(regenerated cellulose) fibers are composed
of cellulose in quite pure from. Cellulose lacks significant acidic or
basic properties
but has a large number of alcoholic hydroxyl groups. It is hydrolyzed
by hot
acid and swollen by concentrated alkali. When cotton is swollen by
concentrated
alkali under tension so that the fibers cannot shrink lengthwise it
develops a
silk like luster. This process is called mercerization. The affinity of
mercerized cotton for dyes is greater than that of untreated cotton.
Cotton and rayon
fibers are easily wetted by water and afford
ready access to dye molecules. Dyeing may takes places by adsorption
occlusion or
reaction with the hydroxyl groups. It is also possible to make cotton
and rayon
receptive to a variety of dyes by pretreatment or mordanting with a
material
capable of binding the dyes.
Wool
and Silk
Wool and silk
fibers are protein substances with both acidic
and basic properties. They are destroyed by strong alkali. Strong acid
causes
hydrolysis but the process may be controlled to permit dyeing from
acidic
solutions.
Wool and silk
are wetted by water and are dyed with either
acid or basic dyes through formation of salt linkages. They may also be
dyed
with reactive dyes that from covalent bonds with available amino
groups.
Mordanting is sometimes used to alter the dyeability of wool and slik.
Cellulose
Acetates
Acetylated
cellulose fibers differ from cellulose fibers in
that they are more hydrophobic and lack large numbers of free hydroxyl
groups.
The higher the degree of acetylation the more unlike cotton and rayon
the
acetates become. Strong acid and strong alkali degrade cellulose
acetates although
the initial attack is slow under moderate conditions because of the
difficulty
of wetting the fiber. The triacetate is the most hydrophobic and the
most
stable.
Dyeing of
cellulose actetates is effected with dyes of low
water solubility which become dissolved in the fiber or by occlusion of
dyes
formed in situ. Acid basic and reactive dyes cannot
be used because of
the lack of sites for attachment.
Polyamides
Polyamide fibers
(nylon) are synthetic fibers possessing
properties somewhat like those of wool and slik. They are more
hydrophobic however
with only a limited numbers of basic or acidic groups. Polyamides are
degraded
by strong acid but may be dyed from acidic dye baths under controlled
conditions.
Polyamide fibers
are dyeable near the boiling point of water
with acid dyes that from salt linkages with basic sites. Dyeing by this
means
is limited by the availability of these sities. Dyes like those used on
cellulose acetates (i.e. that dissolve in the fiber) or reactive dyes
that bond
to available amino groups may also be used.
Polyester
Polyester fibers
are
synthetic fibers unlike any produced in nature. They are hydrophobic
and posses
good stability to acid and alkali as a result of this hydrophobicity.
They are
hydrolyzed under sufficiently drastic conditions however. Some
polyester fibers
lack functional groups others are provided with acidic groups or
otherwise
modified to make them more hydrophilic.
Unmodified
polyester fibers
are dyed by solution of dyes in the fiber or to a limited extent by
occlusion
of dyes formed in situ. Modified polyester fibers may be dyed in these
ways or
with dyes selected according to the nature of the sites introduced by
the
modification. Both unmodified and modified polyester fibers must be
dyed under
vigorous conditions often with the assistance of a swelling agent to
open up
the fiber.
Acrylics
Acrylics fibers
are
hydrophobic synethetic fibers with excellent chemical stability. They
do not
resemble any natural product. The only funcational groups pressent are
those
introduced for the purpose of providing sites for dyeing.
Acrylic fibers
are dyed by
solution of dyes in the fiber by occlusion of dyes formed in
situ and by
formation of salt linkages with dyes capable of attachement to sites
provided
for that purpose. Basic dyes are used on acrylic fibers bearing
sulfonic acid
groups for examples.
Vinyls
Vinyl polymers
and copolymers
make up a class of fiber forming materials that varies greatly in
properties depending
on constitution. Some vinyl fibers are very resistant to degradation by
acids.
Dyes are selected accoding to the nature of the specific polymer to be
dyed.
Polyolefins
Polyolefin
fibers are formed
from the products of polymerization of unsaturated compounds of carbon
and
hydrogen for example propylene. They do not absorb water and are
chemically
quite inert. They can be dyed with special disperse dyes but are
colored best
by introducing a colorant into the polymer before the fibers are spun.
Some
types of polypropylene incorporate metal ions such as Ni++ to
act as dye sites for chelatable dyes.
Glass
Fibers
Glass fibers are
used for special purpose for example where
flammable materials cannot be tolerated. They are often colored during
manufacture but can be dyed by special techniques which involve the use
of
surface coatings that have affinity for dyes.
Paper
Paper is a
nonwoven material made up primarily from cellulose
of varying degress of refining (see chapter 15). Paper may be colored
in the
pulp as a watery fibrous slurry by either continuous or batch methods.
The
dyeing process takes place at ambient temperature and the dyes are
adsorbed on
the pulp by their affinity for the cellulose. Direct dyes are most
commonly
used. In continuous coloration the dye solutions are metered directly
into a moving
stream of pulp. In batch operations dye is added to a pulper beater or
blending
chest containing a given quantity of slurry.
Paper may also
be colored on its surface after the inital
sheet is formed pressed and partially dried. This can be done at the
size press
of the paper machine or color can be carried by a calender roll for
heavier
sheets. A wide vareiety of low cost dyes can be used for surface
coloration.
The
Properties of Dyes
The properties
of dyes may be classified as application
properties and end use properties. Application properties include
solubility affinity
and dyeing rate. End use properties include hue and fastness to
degrading
influences such as light washing heat (sublimation) and bleaching. Dyes
are
selected for acceptable end use properties at minimum expense. Involved
application procedures are used only when necessary to acheive
unusually good
results.
It has become
common practice to treat dyed textiles with
agents designed to improve resistance to shrinking wrinkling and the
like.
These agents frequently alter the appearnce and fastness of dyes.
Stability to
after treatments must therefore be considered as an important end use
property
of dyes.
The amount of
dye required to obtain a light shade is usually
about 1 per cent of the weight of the fiber heavier shades may require
as much
as 8 per cent. These values are very approximate since dyes differ in
colour
strength and are usually sold in diluted form. These amounts of dye are
not
sufficient in most cases to markedly affect the properties other than
color of
the fiber. Care must be exercised however to apply the dye under
conditions
that do not cause fiber degradation.
It is obvious
from the list
above that many basic dyes have about 10 20 times the color value per
molecule
as the anthraquinone types. Unfortunately light fastness is in the
reverse
order the anthraquinones being used where maximum durability to light
is
needed. The challenge to be dye chemist or engineer is to increase the
strength
of the light fast dyes or to increase the fastness of the strongest
dyes.
Classification
of Dyes
Dyes are
classified according
to application method for the convenience of the dyer. The best
classification
method available is that used in the color index a publication
sponsored by the
society of dyers and colourists (England) and the American Association
of
Textile Chemists and Colorists.
Acid dyes
Acid
dyes depened on the presence of one or more acidic groups for their
attachment
to textile fibers. These are usually sulfonic acid groups which serve
to make
the dye soluble in water. An example of this class is Acid yellow 36
(Metanil
Yellow).
Acid dyes are
used to dye fibers containing basic groups such
as wool slik and polyamides. Application is usually made under acidic
conditions which cause protonation of the basic cause protonation of
the basic
groups.
It should be
noted that this process is reversible. Generally
acid dyes can be removed from fibers by washing. The rate of removal
depends on
the rate at which the dye can diffuse through the fiber under the
conditions of
washing. For a given fiber the diffusion rate is determined by
temperature size
and shape of the dye molecules and the number and kind of linkages
formed with
the fiber.
Chrome dyes. A special kind
of acid dye used mainly
on wool they posses improved fastness when converted to chromium
complexes. A
suitable chromium salt is applied to the fiber (1) before the dye (2)
at the
same time as the dye or (3) after the dye. All these methods are
staisfactory but
more complicated than is desired. In recent years manufactures have
made
available dyes in which chromium is already a part of the molecule.
These dyes
are simpler to apply than the older types and as a consequence are
increasing
in importance.
Basic
or Cationic Dyes
Cationic dyes
become attached to fibers by formation of salt
linkages with anionic or acidic groups in the fibers. Basic dyes are
those which
have a basic amino group which is protonated under the acid conditions
of the
dyebath. Cationic dyes can be divided into the three classes which are
illustrated.
Basic
brown 1 (Bismark brown) is an amino containing dye which is redily
protonated
under the pH 2 5 conditions of dyeing.
Crystal
violet (Basic violet3) is an example of a cationic
dye in which the cationic charge is delocalized by resonance and may be
present
at any one of the basic centers at any time. These resonance forms of
almost
equivalent energy are one of the reasons that crystal violet is among
the
strongest dyes known. This high color value (tinctorial strength) has
important
commercial interest in the hectograph copying system. In this system
crystal
violet in a wax base is transferred to the back of a typewritten copy
sheet. By
using paper moistened with alcohol more than 200 good copies may be
made from
the master.
Dry Cleaning Agents
Drycleanings of
garments is done in much the same manner as
laundering except that organic solvents are used in place of water. As
in
laundering detergents are added to the solvent to enhance its cleaning
quality.
Other solvent additives are used to give the textile the desired
finish. This
may be done merely to improve the hand or drape of the textile or
chemical
additives may be used to acheive water repellency insect repelleney or
flame
resistance.
Drycleaning
washes are similar in construction to commercial
laundry washers but in drycleaning provision is made for clarifying the
solvent
for reuse. In laundering the used wash water is discarded this cannot
be done
with the more expensive drycleaning solvents.
In the
drycleaning system the solvent is continuously pumped
through the washes and then through some type of after designed to
remove all
suspended soil. Provision is also made for distillation of the solvent
to free
it form the solvent soluble soil.
The filters also
contain activated carbon to absorb dissolved
dye which would otherwise build up in the solvent.
Other chemical
products used in small quantities by
drycleaners are formulted to remove stains by local appplicaton to the
affected
area of the garment.
Only two classes
of solvents have proved suitable for
drycleaning petroleum fractions and a few halogenated hydrocarbons. All
other
classes of solvents fail to meet the following eight major requirements
of a
drycleaning solvent.
It
must not weaken dissolve or shrink the ordinary textile fibers.
It
must not remove the common dyes from fibers.
It
must be an acceptable solvent for fat and oils.
It
must not impart an objectionable order to drycleaned textiles.
It
must be sufficiently volatile to permit reclamation by distillation and
to
permit garments to be tried without prolonged heating at excessive
temperatures.
It
must be noncorrosive to metals either when dry or in the presence of
water.
It
must be relatively nontoxic.
It
must have a flash point of 1000F or above.
The major
drycleaning solvents used in the U.S. are the
petroleum fraction called Stoddard solvent of which there are four
types
perchlorethylene and to a limited extent trichlorethylene and
trichlorotrifluoroethane.
With the
exceptions of the solvents the chemicals used in
drycleaning are sold as brand name formulations and the only tests
performed on
them are the determination of the amount of detergent in the solvent
and the
amount of water in a solution of detergent in solvent. However these
chemicals
are tested to determine how well they perform the function they are
designed
for according to a number of procedurs developed by the National
Insitiute of
Drycleaning (NID).
Stoddard
Solvent
Much of the
drycleaning done in the United States employs a solvent
corresponding to a petroleum fraction with a minimum flash point of 1000C.This solvent
has been named
Stoddard solvent for W.J. Stoddard. The first commercial standard for a
drycleaning solvent CS3 28 was issued in 1928 by the National Bureau of
Standards.
The latest revision of this specification is CS3 41 it also became an
ASTM.
Table 1 summarizes the current specifications of regular Stoddard
solvent.
Today three
other petroleum fractions are also broadly termed
as Stoddard solvents. These are the 1400F solvent the
low end point solvent and the ordoless
solvent.
1400F Solvent. This
solvent is safer than the regular stoddard solvent. Therefore it may be
used in
locations where stoddard solvent is prohibited. Also building codes for
plants
using 1400F solvent
are not so rigorous. For example explosion proof motors and other
electrical
fixtures are not required. Specifications of 1400F solvent which
differ from the specificantions of the
regular stoddard solvent are listed in Table 2.
Low End Point
Solvent. This type of
stoddard solvent
has a dry point in the range of 330 3620F compared with
368 4090F for the
regular stoddard solvent. The result is a
rapid drying solvent. There is no specification covering this solvent.
It is
regaeded as a premium grade because of the fast drying feature.
Odorless
Solvent. Whereas regular
stoddard solvetn is
specified to be free of objectionable odor this new class of stoddard
solvent
is free of all odors. This is acheived by removing or hydrogenating all
aromatic compounds. The solvent also meets all requitements for a
nonsmog producing
solvent since smog production is related to the aromatic content.
Odorless solvent
is also regarded as a premium grade of
stoddard and is not conered by a separate specification.
Specification
Tests
Some comments to
the specification tests for stoddard solvent
are given here.
Odor. The term sweet
as used in the specification means the
opposite of rancid or sour. Although the usual methods of clarifying
solvent in
a drycleaning plant remove odors that accumulate during continued
drycleaning these
processes do not always remove dodrs caused by improper refining.
Therefore the
solvent when received from the refinery should be free from undesirable
odor.
There is nothing to show whether or not a solvent meets the requirement
except
the opinion of the examining chemist. Many samples of stoddard solvent
have a
rather strong odor but it is easily removed from the fabric by
conventional
drying methods.
Flash point. The flash point
is governed by those
portions of the solvent that have the lowest boiling points and are
therefore
the most volatile. Since these portions evaporate more rapidly than the
rest of
the solvent the flash point of Stoddard solvent in a drycleaning system
gradualy rises with use. Soaps prespotters or other added materials
sometimes
contain low flash solvents (such as some alcohols) that lower the flash
point
of the solvent and increase the fire hazard. The introduction of even
small
amounts of methyl ethyl or isopropyl alcohol into a washer lowers the
flash
point of the solvent below normal room temperatures.
A lighted match
held over Stoddard solvent at ordinary room
temperatures does not ignite the solvent because the solvent is not
giving off
enough vapor to form a combustible mixture with the air. If the
tempetature of
the solvent is raised vapor pressure is increased and the air above the
solvent
becomes richer in solvent vapors. Finally a temperature is reached
where enough
solvent has vaporized to form a combustible mixture with the air If a
flame is
then introduced above the solvent the vapors will flash. The lowest
temperature
at which this occurs is called the flash point. Since the flash point
of
stoddard may not be under 1000F there is
no danger of fire from solvent vapors until the temperature of the
solvent
rises to 1000F or above.
The flash point
specification is frequently violated. In some
cases the refinery may have set the lower limit of the distillation
range too
low. Such violations usually result in a solvent with a flash point of
98 or 990F. A more
dangerous type of
violation however results from careless handling of the solvent. For
example stoddard
is sometimes transported in tank trucks that were previously used for
carrying
gasoline and still contain small quanities of gasoline. As little as 1%
of
gasoline in stoddard solvent lowers its flash point considerably.
The flash point
is determined
by ASTM method.
Corrosive
properties. Improperly
refined solvent may contain traces of dissolved free sulfur which can
corrode
the metals of storage tanks and equipment. The corrosion test is
carried out
according to ASTMD 1616 60 at an elevated temperature 121ºF. Under
these
conditions corrosion that would be apparent after considerable use of
the
solvent at room temperature can be seen after only 3 hr.
Distillation
Range. From the
standpoint of a drycleaning solvent there are disadvantages in products
containing very low or very high boiling hydrocarbons. Low boiling
hydrocarbons
petroleum ether and gasoline cause fires and high evaporation loss high
boiling
hydrocarbons such as kerosene cause excessive drying time. The
distillation
range for Stoddard solvent is between 300 and 4100F a range not
low enough to cause undue fire hazard and
evaporation loss or high enough to prolong the drying time.
The distillation
range of
stoddard solvent is determined according to ASTM D 86 66 and D 1078 67.
Residue. Excessive
nonvolatile matter in the solvent often contributers to odors and
lengthens
drying time. Because of the high temperatures used a small amount of
odorous
residue is usually formed during the distillation test. A sample of the
same
solvent evaporated on a steam bath where temperature is not raised
above 2120F yields a
smaller and less odorous
residue.
Acidity. If the
solvent is given a sulfuric acid treatment in the refinery and not
followed by
a neutralizing treatment (such as with caustic soda) it will contain
small
amounts of sulfuric acid or other acidic materials. Even small amounts
of
sulfuric acid are undesirable in a drycleaning solvent as they corrode
equipment and damage garments. Fortunately almost without exception
drycleaning
solvents pass the acidity test.
Sulfuric acid
has a very high
boiling point. The presence of this substances in the solvent will be
as
residue in the flask after the distillation. If a residue of 1 ml
remains from
distilling 100 ml of solvent any sulfuric acid present is concentarted
there
100 times. Thus it is logical to test the residue from distillation for
sulfuric acid.
According to
ASTM D 1093 65 the solvent sample is shaken with
water and one drop of a 0.1% methyl orange indicator solution is added
to the
aqueous layer there should be no change in the color of the indicator.
Doctor Test. Mercaptans
impart to the solvent
unpleasant odors which may be absorbed onto the garments during
drycleaning.
The doctor test is a qualitative method to determine whether the
treatment for
mercaptans was properly done in the refinary. Sulfur and sodium
plumbite are
added to the solvent in a test tube. If mercaptans are present in the
solvent the
reaction proceeds and the black lead sulfide formed is indicative of a
positive
test
Sulfuric Acid
Absorption Test. This test
determines if the solvent contains appreciable amounts of unsaturated
hydrocarbons. These would be in the solvent if it was inadequately
treated with
sulfuric acid during refining. Since unsaturated hydrocarbons turn
ranoid and
cause undesirable odors in drycleaned garments it is imperative that
they be
removed before the solvent leaves the refinery.
In the test
concentrated sulfuric acid is added in a
graduated cylinder to the solvent and shaken. Sulfuric acid reacts with
any
unsaturated hydrocarbons present and most of the products of the
reaction
settle into the acid layer thus the volume of the solvent is decreased.
Since
some of the products formed from the reaction remain in the solvent
layer the
test does not give a quantitative measure of unsaturated hydrocarbons
that is a
5% absorption of the solvent by sulfuric acid actually represents a
greater
percentage of unsaturated hydrocarbons in the solvent.
Variations in
the strength of commerically available
concentrated sulfuric acid cause variations in the sulfuric acid
absorption
test. Therefore the acid strength must be standardized if
reproducibility is
desired.
Perchloroethylene
Perchloroethylene
(tetrachloroethylene) became an important drycleaning
solvent because of its nonflammability which permits its use in places
where
all types of flammable solvent are either forbidden by codes or
inhibited by
high insurance rates. Its general properties are given in Table 3 and
the
specifications proposed by the NID are listed in Table 4.
Residual Odor. Any residual
odor left in a fabric
after treatment in the solvent is objectionable. Detection of such
odors by
smelling is more sensitive if the fabric is steamed immediately prior
to the
test. A swatch of bleached but unfinished cotton poplin Style A 400W
Testfabrics
Inc. is used and subjected to the following test.
Procedure
Condition the
cotton at 60% relative humidity for at least 8
hr prior to use. Soak the swatch in perchlororthylene for 5 min then
remove it
and hang it to drain dry for about 4 hr. Tumble the swatch in a tumble
dryer
for 30 min at 1400F.
To test for odor
grasp the swatch in the center with a
forceps hold it in live steam for 5 sec and smell it immediately. Test
an
untreated swatch smiultaneously. There should be no discernible
difference in
odor between the two swatches.
Nonvolatile
Residue. This test
detects the presence of
nonvolatile impurities in the solvent. It is determined gravimetrically
by
evaporating a measured quantity of solvent and weighing the residue as
follows.
Procedure
Dry a 4 in.
diameter evaporating dish and weigh it to the
nearest 0.1 mg. Place it on a steam bath in a hood and add the
perchloroethylene to be tested by pipet in two 50 ml portions.
Perchloroethylene
has a high specific gravity. 1.62 and is difficult to handle in a 100
ml pipet.
Add the second portion afted the first is partially evaporated.
After the
solvent has completely evaporated on the steam bath
heat the dish further in an oven at 1050C for I hr then
cool it in a desiccator and weigh. The
increase in weight of the dish in grams for a 100 ml sample is %
nonvolatile
residue.
Stability Test.
Perchloroethylene is stabilized by
adding traces of chemicals known to inhibit its decomposition. Loss of
stabilizer or the pressure of certain impurities can lower the
stability of the
solvent.
Procedure
Wash two strips
foil 2.0×7.5×0.005 cm in concentrated
hydrochloric acid. Rinse dry and weigh to the nearest 0.1 mg. Add 75 ml
of the
test solvent and 3 ml of water to a 300ml Soxhlet extractor. Place one
copper
strip in the flask and the other into the condenser of the soxhlet.
Heat the
Soxhlet at a rate that will cause it to empty 8 10 min.
After 24 hr
remove the strips wash them again in concentrated
hydrochloric acid and weigh. The combined weight loss of the two strips
should
not exceed 30 mg.
Note Do not fail to
add the water with the solvent. The
test is worthless in the absence of water.
Fluorocarbon
Solvent
Around 1960 du
pont introduced trichlorotrifluoroethane as a
drycleaning solvent under the trade name valclene. This solvent has
aroused
much interest because of its ideal properties but it is too volatile to
be used
in machines designed for perchloroethylene. Therefore its full
utilization must
a wait machine development. A number of companies have introduced small
machines for the solvent but it will be several years before use of the
solvent
is widespread.
No special
specifications or test methods have been developed
for this solvent.
Used
Drycleaning Solvents
In addition to
the tests given under the specifications there
are several analytical methods designed for quality control purposes in
drycleaning operations. These methods are normally performed on used
solvent
taken from plant washers. The following tests are made routinely on
used
solvent.
Detergent
Concentration. The method of
fessler for
anionic detergents is used. There is no satisfactroy method for
drycleaning
detergents that are all nonionic however manufacturers of niononic
detergent formulations
normally include some anionic surfactant in the mixture to serve as a
tracer.
This serves the purpose of quality control with a known product but not
for
analysis of an unknown mixture.
Nonvolatile
Residure. Except that
10ml samples are used
instead of 100 ml.
Moisture
Content. The moisture
content of the used
solvent can be determined by the Karl Fischer method.
Acid Number. This test was
originally designed to
measure the buildup of fatty acids in the solvent. Its value has
diminished in
recent years because of the widespread use of amine sulfonate
detergents. These
detergents react quantitatively with the titrant giving a high value
for the
fatty acid content of the solvent. However the test is still useful for
control
purposes where proper correction can be applied for interfercence by
the
detergent.
In other fields
acid number is defined as the mg of potassium
hydroxide neccessary to neutralize l g of sample. In drycleaning. The
NID has
defined acid number as the mg of potassium hydroxide necessary yo
neutralize
1.28 ml of solvent.
The titration is
made in the usual manner using a 0.06 N
alcoholic solution of potassium hydroxide and phenolphthalein
indicator. It was
found that 2 methyl 2 4 pentanediol is a better solvent than ethanol
because of
its solubility in petroleum solvents.
Residual Odor. The test is
carried out according to the procedure given on p 608.
Color. The color
of used drycleaning solvents may be due chiefly to dyes dissolved from
the
textiles. The balance is caused by colored soils or colloidally
suspended
pigments. The latter are removed by microfiltration prior to
determining color.
At NID color is determined on a Coleman universal spectrophotometer
using a 40 mm
cuvet at 500nm. The instrument is standardized against water.
Greying of
Cotton. The cotton
fabric used for the residual odor test is read on the reflectometer to
determine the decrease in % green reflectance. Although this is called
greying it
is actually a measure of the amount of dye and colored impurities
dissolved in
the solvent because the insoluble material has been removed by
microfiltration
through 0.2 µm membranes.
Sizing. Many
drycleaners use certain resins in the solvent as sizes or bodying
agents for
fabrics to replace the finishing materials removed during wear or
cleaning.
Natural terpene resins are widely used and the amount of resin in the
solvent
is determined at NID by extracting the nonvolatic residue with boiling
ethanol.
This reagent dissolves everything except the terpene resins. The
procedure has
not been validated however for all types of sizes.
Suspended Soilds. After
microfiltration of a measured volume of the solvent the membrane witch
has been
previously weighed is oven dried at 1050C and weighed to
determine the quantity of insoluble
material suspemded in the solvent. The NID standard for this is
50mg\liter.
Larger quantities can cause excessive greying of white fabrics and is
an
indication of poor solvent filtration.
Drycleaning Detergents
Detergency in
nonaqueous
solvents follows much the same principles as in water particularly in
the
removal of insoluble soil. The major differences come in the attack on
water soluble
and solvent soluble soils. In aqueous detergency the major attack is on
the
oily soils because the water soluble soils are removed by simple
solution. In
drycleaning on the other hand the major attack by detergents is on the
water soluble
soil because the soil is removed by simple solution.
In both
lanundering and drycleaning the
process of emulsification and solubilization effects the removal of
soil from
the fiber surface. In both types of eleaning the detergents used are
based on
surface active agents.
Laundry
detergents generally contain
not more than 20% surface active agents (surfactants) the balance being
various
types of builders. Drycleaning detergents may consist of a single
surfactant.
The product may also contain a consolvent or coupling agent to enhance
the
capacity for dissolving or emulsifying water and a fluorescent
whitening agent.
Frequently two or more surfactants are mixed.
A drycleaning
detergent performs
three functions in the cleaning process. It acts as a dispersant or
peptizing
agent for insoluble soils. It not only disperses this kind of soil but
also
keeps it in suspension while it is being flushed out of the fabric and
pumped
to the filter. Insoluble soils may be dispersed to particle size in the
submicron range by good detergents and while so dispersed the particles
of soil
are small enough to escape between the tightly packed fibers in textile
yarns. In
the absence of a good detergent this kind of soil is difficult to
remove and
readily redeposits on other fiber surfaces causing what is generally
called
greying a phenomenon also common in laundering particularly with
polyester
fibers. Thus the first two roles of a drycleaning detergent are to
assist in
the removal of insoluble soil and to prevent it from redepositing on
other
fabrics in the bath.
The third
function of a drycleaning
detergent is to emulsify water in the solvent and promote the removal
of water soluble
soil by the emulsified or solubilized water. Although the water plays
the major
role in detaching water soluble soil from the fiber surface the
detergent
itself can dissolve some of these soils within its micelles.
Progress in the
formulation of
drycleaning detergents is slow compared to the formulation of laundry
detergents. One reason for the lack of progress has been the absence of
reliable test methods for drycleaning detergents. The literature on
drycleaning
detergent test methods is scanty and the few methods that have been
described
have received little attention or use. The methods described here have
been in
use at the National Institute of Drycleaning and are designed to test
the
ability of a detergent to perform its three functions.
Methods
of Analysis
The tests to be
carried out on drycleaning detergents can be
divided into two groups specification tests resulting in information on
the
properties of the detergnets and performance tests.
Specification
tests
Physical
Composition. Drycleaning
detergents almost
without exception are liquids so it is describle to know how much of
the
material consists of an active ingredient and how much is solvent or
water. The
determination is made on a perchloroethylene solution of known
concentration of
the detergent. An aliquot is evaporated to dryness as described on p.
608 for
the determination of the nonvolatile residue of a solvent. The amount
of water
is determined on a separate sample by the karl fischer titration.
Some drycleaning
detergents are diluted with mineral oil so
that the nonvolatile residue is not all surfactant but it still
established the
upper limits of surfactant concentration.
Specific Gravity. The main
purpose of this test is to
establish what types of solvents are used as diluents. Most surfactants
have
specific gravities close to unity whereas drycleaning solvents have a
specific
gravity of about 0.8 (Stoddard solvent) or 1.62 (perchloroethylene).
The
determination can be carried out by any of the conventional methods.
pH. A drycleaning
detergent should be essentially neutral
because of the adverse effect of acids and alkalis on some types of
dyes. The
test is made by thoroughly shaking the detergent with water and
determining the
ph of the water phase.
Distillation
Test. Since used
dryeleaning solvent is
reclaimed by distillation it is important that the detergent cause no
problems
in the still. This is checked qualitatively by distilling a 1% solution
of the
detergent in perchloroethylene in an all glass laboratory still. The
process is
observed for any signs of foaming flooding over or decomposition. The
distillate should be pure perchloroethylene presence of other volatile
solvents
is undesirable.
Detergents
intended for use in Stoddard solvent must be
tested by vacuum distilling a 1% solution in this solvent.
Solubility in
Dryeleaning Solvents. The purpose
of this test is to ascertain that the detergent is soluble in both
solvents. A
simple qualitative test is sufficient.
Chemical Type. It is desirable
to know whether the
sufactant in the detergent is anionic cationic nonionic or a mixture of
ionic
and nonionic surfactants. This can be determined by studying the
infraed
spectrum of the sample as well as the methylene blue titration method
given
blow.
Detergent
Concentration by Methylene Blue Titration. This method
is widely used as a control test to determine the amount of a prticular
detergent in a drycleaning solution. It was originally described by
Fessler in
1951. The following procedure is from an NID publication.
Procedure
Anionic
Surfactants. Place 25ml of
chloroform into a 100
ml glass stoppered graduated cylinder. Take at least a 5 ml sample of
solution
to be tested dilute to 100 ml and then add a proper aliquot to the
chloroform.
Add 25 ml of water containing 1 drop of a 0.5% methylene blue solution
and
shake. The methylene blue enters the chloroform layer as a result of
solubilization by the surfactants. Start to add a 0.02% aqueous
cetylpyridinium
chloride solution in 0.5 ml increments and shake the mixture vigorously
after
each addition. As long as any free anionic surfactant remains in the
chloroform
layer its blue color will presist. Near the end point the methylene
blue begins
to pass into the aqueous layer. Eventually this is complete and the
lower layer
is colorless. A sharp and reproducible end point is the point of equal
color
distribution between the two phases. Prepare a calibration curve for
each
detergent by titrating a number of samples of known volume volume
concentration
over the expected range and plotting ml of titrant againt deteregent
concentration.
Cationic
Surfactants. Carry out the
determination in a
similar way but by using a standard anionic surfactant such as Aerosol
OT as
the titrant.
Nonionic
surfactants cannot be titrated in this manner. However
detergents consisting of nonionic surfactants generally contain a small
amount
of anionic surfactant as a tracer so the solution can be titrated to
control
concentration.
Alginic Acid
General
Information
As early as 600
B.C. seaweed was used as a food for man but
algin a component of seaweed was first discovered by British chemist E.
C.C.
Stanford in 1880. In 1896 A Krefting prepared a pure alginic acid. In
1929 Kelco
Company began commercial production of alginates and introduced milk
soluble
again as an ice cream stabilizer in 1934. In 1944 propylene glycol
alginate was
developed.
Algin is a
polysaccharide found in all brown seaweeds phaeophycea
which grow on rocky shores or in ocean areas that have clean rocky
bottoms.
Although some species can be found at the high tide line other exist
along the
shore where depths are less than about 40 m (125 ft) the maximum depth
to which
sunlight will penetrate. (Since algae do not have true roots stems or
leaves nourishment
comes directly from sunlight and mineral nutrients in ocean water.)
Only a few
species of brown seaweeds are used for commercial
production of algin. The principal source of the world s supply of
algin is the
giant kelp Macrocystis pyrifera found along the
coasts of North and
South America New Zealnd Austrila and Africa. Other seaweeds used for
algin
manufacture are Ascophyllum nodosum and species of
Laminaria and
Ecklonia.
Algin exists in
the kelp cell wall as the insoluble mixed
salt (calcium magnesium sodium potassium) of alginic acid. Alginic acid
is a
high molecular weight linear glycuronan comprising solely D mannuronic
acid and
L guluronic acid.
Algin is used in
foods and general industrial applications
because of its unique colloidal behaviour and its ability to thicken
stabilize emulsify
suspend form films and produce gels. These properties are discussed in
greater
detail in later sections of this chapter such as solution Properties
and
commercial Uses.
Chemical
Structure
It has been in
recent years only that the composition of
alginic acid has become understood. Table 1 shows the composition of
alginic
acid whereas Table 2 shows the proportions of polymannuronic acid
segments ployguluronic
acid segments and alternating segments of these two uronic acids in
three
commercial samples of alginic acid. Figures 1 to 3 illustrate the
structures of
mannuronic and guluronic acids the apparent discrepancies between the
date of
Tables 2 and 3 are accounted for by variations between alginates
derived from
different species of brown algae.
Chemical
derivatives The propylene
glycol ester of
alginic acid is the only orgainc derivative of alginic acid currently
on the
market. Propylene glycol alginate has improved acid stability and
resists
precipitation by calcium and other polyvalent metal ions.
Amine alginates
can be made by reacting alginic acid with
orgainc amines. Suitable amines are triethanolamine triisopropanolamine
butylamine
dibutylamine and dimylamine. Algin acetate and algin sulfate esters
have been
prepared but have no known applications. Carboxymethyl alginate can be
made by
treating sodium alginate with chloroacetic acid and alkali. A number of
alkylene glycol esters of alginic acid have been prepared and evaluated.
Ethylene oxide
can be reacted with alginic acid to form 2 hydroxyethyl
alginate. Alginamides can be prepared by reacting propylene glycol
alginate
with primary amines such as ammonia ethanolamine ethylenediamine
ethylamine propylamine
isopropyl amine and butylamine. Very little reaction occurs with
secondary
amines.
Manufacture
Macrocystis
pyrifera the brown
seaweed that is the main
source of algin grows in relatively calm waters and in large dense
beds. The
plant is a perennial and can be harvested on a continuing basis. Its
rapid
growth permits up to four cuttings per year.
Only mature beds
are cut. At the time harvesting a dense mat
of fronds floats on the ocean surfac. Cutting the dense mat on the
surface
allows light to penetrate the water and reach the immature fronds this
stimulates their growth. Harvesting is actually a massive pruning of
the kelp
bed. Underwater blades mow the kelp approximately 3 ft below the water
surface then
the cut kelp is automatically conveyed into the hold of the barge by a
moving
belt.
Although
commercial methods of producing sodium alginate from
seaweed are proprietary the fundamental steps in a typical process
essentially
one of ion exchange are shown in Fig. 4. In the seaweed the algin is
apparently
present as a mixed salt of sodium and /or potassium calcium and
magnesium and
is a high molecular weight polymer. The exact composition varies
considerably
with the type of seaweed but does not affect processing.
It is possible
to extact sodium alginate from seaweed with a
strong solution of a sodium salt however for the production of purfied
alginates the commercial processes are much more efficient. Alginic
acid may
also be neutralized with bases to give salts and reacted with propylene
oxide
to make propylene glycol alginate.
Physical
Properties
Commercially
available water soluble alginates include the sodium
potassium ammonium calcium and mixed ammonium calcium salts of alginic
acid propylene
glycol alginate and alginic acid itself. The physical properties of
several of
these alginates are given in Table 3.
Powdere
Alginates Alginate as a
hydrophilic polysaccharide
absorbs moisture from the atmosphere therefore equilibrium moisture
content is
related to relative humidity as shown in Fig. 5. The dry storage
stability of
alginates is excellent at moderate temperatures 25oC (77oF) or less
however they should be stored in a cool dry
place. Table 4 gives datea showing the effects of storage for 1 year at
24.9oC (75oF) on typical
alginates. Table 5 shows the effects of
variious storage temperatures on the stablilities of alginates.
Solution
Properties
Pure alginates
dissloved indistilled water from smooth
solutions with long flow characteristics. The physical variables that
affect
the flow properties of alginate solutions are temperature shear rate
polymer
size concentration and the presence of solvents miscible with the
distilled
water. The chemical variables that affect algin solutions are pH and
the
presence of sequestrants monovalent salts polyvalent cations and
quaternary
ammonium compounds.
Rheological
Properties The flow
properties of sodium
alginate solutions are concerntration dependent. A 25% medium viscosity
sodium
alginate solution is pseudoplastic over a wide range of shear rates (10
to 10
000s 1) wheras a
0.5% solution is Newtonian at low shear rates (1 to 100s 1) and
pseudoplastic only at high
shear rates (1000 to 10 000s 1) as shown
in fig. 6.
Because of high
molecular weight and molecular rigidity sodium
alginate forms solutions of unusally high apperant viscosity even at
low
concentraions. Propylene glycol alginate solutions are shear thining
over a
wide range of shear rates at 3% concentrations. However at 1% or lower
concentrations solutions have almost constant viscosity below shear
rates of
100 s 1(fig. 7).
Figure 8 shows
that viscosity shear curves of medium viscosity
sodium and potassium alginates are virtually the same over the entire
shear
range. On the other hand in comparing low viscosity propylene glycol
and sodium
alginates the curves are indentical at shear rates greater than 10 000s
1 but
diverge at low shear rates.
The effects of
solution soilds on shear thinking are
illustrated in fig. 9. The viscosity shear curves of a 2% solution of
medium viscosity
sodium alginate were the same as those of a 9% solution of a low
viscosity
sodium alginate. Measurements were taken using shear rates in the
brookfield viscometer
range (1 to10 000 s 1). At high
shear rates such as those experienced at 100 000 s 1 measured
with a capillary viscometer the
curves diverge.
Figure 10
illustrates the effect of temperature on the flow
of a high viscosity propylene glycol alginate. Addition of a
sequestrant sodium
hexametaphosphate to a medium viscosity sodium alginate (fig. 11) gives
a
viscosity shear curve comparable to that of a low calcium sodium
alginate.
Xanthan gum can
be used to modify the rhelogical behavior of
sodium alginate solutions (fig. 12) As shown the curves for the 0.5%
solutions
of sodium alginate and xanthan differ greatly. A combination of the two
gums
produces flow properties intermediate between the two materials.
Latex paints
illustrate the importance of rheological
properties to the design of product performance. If the paint is highly
pseudoplastic application will be easy and sagging will be prevented
but flow
and leveling will be minimal and brush marks will be left on the dried
paint film.
Elimination of the yield value will result in setting out of the
pigments in
the can. If dilatancy occurs stirring will be difficult and brush drag
will be
excessive.
Effect of
temperature The viscosities
of algin solutions
decrease as temperatures increas approximately 12% for each 5.6ºC (100F) increase in
temperature. The
decrease is reversible if the high temperatures are not held for long
periods.
Table 6 shows the effect of time and temperature on solution viscosity.
It is
apparent that the heating of sodium alginate results in some thermal
depolymerization the amount being related to both temperature and time.
Although a
reducation in temperature of an alignate solution
in an increase in viscosity it does not produce a gel. A sodium
alginate solution
can be frozen and thawed without any change in its appearance or
viscosity
after remelting. It is possible to form a freeze dried sodium calcium
alginate
gel with an absorptive capacity of more than 5000%.
Effects of
Solvents Addition of
increasing amounts of
nonaqueous water miscible solvents such as alcohols glycols or acetone
or an
aqueous alginate solution increases solution viscosity and eventually
causes
precipitation of the aligante. Tolerance of the alignate solution to
such
solvents is influenced by the source of the alginate the degree of
polymerization the cation type present and the solution concentration.
Table 7
gives data on solvent tolerances of various types of alginates in
solution.
Effect of
Concentration Figure 13 shows
the effect of
solution concentration on selected grades of sodium ammonium postassium
and
propylene glycol alginates.
Effect of pH Sodium alginates
with some residual
calcium content increase in viscosity at a ph of 5.0 and are unstable
at pH
levels of about 11.0. Sodium alginates with minimal calcuim content do
not show
the viscosity increase until the pH reaches 3.0 to 4.0 Lower molecular
weight
sodium alginates are stable at a pH as low as 3.0 if calcuim is
completely
sequestered.
Propylene glycol
alginates do not gel until the pH is below
3.0 but they do saponify at pH levels above 7.0 The long term stability
of
sodium alignate solutions is poor when the pH reaches 10.0. At even
higher pH
values there is depolymerization with an accompanying viscosity loss.
Figure 14
illustrates the effect of pH on viscosity for several types of
alignates in
solutions.
Gelation Algin polymers
will react with most polyvalent cations
(magnesium excepted) to form crosslinkages. As the content of
polyvalent ion
increases the algin solutions thicknes then gels and finally there is
precipition. The proposed
structure of an alignate gel in which the calcuim ions are bound
between the
associated segments of the polymer chain is shown in fig. 15.
All alginate
gets are the result of interactions between the
alginate molecules which produce a three dimensional structure
controlling the
mobility of the water molecules. They are not thermally reversible. By
the
proper selection of gelling agent gel structure and rigidity are
controlled.
Loss of water to the atomsphere and resulatant shrinkage is very slow
in algin
gels.
Metallic
polyvalent ions e.g. zinc aluminum copper and silver
from complexes with aliginates in the presence of excess ammonium
hydroxide.
When the ammonia is driven from the system the insoluble metal
alignatic is
formed. Calcium is the polyvalent cation most often used to change the
rheological properties and get characteristics of algin solutions.
Calcium is
also used to form insoluble aliginate filaments and films.
The method of
calicum addition to an alginate system greatly
influcences the properties of the final gel. If calcium is added too
rapidly the
result is spot gelation and a discontinuous gel structure. The rate of
calcium
addition can be controlled by use of a slow dissloving calcium salt or
by the
addition of a sequestrant such as tetrasodium pyrophosphate or sodium
hexametaphosphate.
Effect of
sequestrants The purpose of
squestrants in
alginate solutions can be either to prevent the alginatic from reacting
with
polyvalent ions present in the solution or to sequester the calcium
inherent in
the alginate. Polyvalent ion contaminants can come from water chemicals
pigments
or various natural origin materials. Figure 16 and 17 show viscosity
concentration
relationships for two types of alginates which and without sodium
hexametaphosphate as the sequestrant.
In fig. 16 a low
calcium sodium alginate shows a very small
viscosity change up addition of the ployphosphate sequestrant to the
solution.
In contrast fig. 17 should that a sodium calcium alginate solution has
a major
change in viscosity when the sequestrant is added. Sequestered alginate
solutions are more Newtonian in behavior than are those with some
available
calcium.
Effect of
Monovalent Salts Monovalent salts
depress the viscosities
of dilute sodium alginate solutions. The maximum effect on viscosity is
attained at a salt level of 0.1N in the solution. Except for alginates
high in
calcium an increase in alginates concentration decreases the effect of
the
monovalent electrolyte.
Figure 18 shows
the effect of sodium chloride on the
viscosity of several kinds of alginate solutions whereas Table 8 shows
the
effects of 1% and 5% sodium chloride concentrations over a 210 day
period at
temperatures of 4.4 23.9 and 48.90C (40 75 and 1200F). The effect
of a salt on an alginate solution will
vary with the source of the alginate as well as with its degree of
polymerization the concentration of alignate in the solution and the
type of
salt.
Insolubilization
Normally
insoluble adducts result
when sodium alginate reacts with cationic organic ammonium compounds.
This
insolubilization can be prevented by adding an electroylte e.g. NaCl to
suppress the activity of the cation. Salt concentrations needed to
solubilze
insoluble alginate adducts are listed in Table 9.
Compatibilities
Alginates in
solution have compatibility with a wide variety
of materials including other thickeners synthetic resins latices sugar
oils fats
waxes pigments various surfactants and alkali metal solutions.
Incompatibilities
are generally the result of a reaction with divalent cations (except
magnesium)
or other heavy metal ions cationic quaternary amines or chemicals that
cause
alkaline degradation or acid precipitation. In many cases the
incompatibility
can be avoided by sequestration of the metal ion or by careful control
of the
solution pH.
Table 10 lists
materials that were tested for compatibility
with solutions of a medium viscosity purified sodium alginate.
Preservaties Alginates have
compatibility with
most commonly used preservatives except quaternary ammonium compounds.
The
polysaccharide is quite resistant to the common enzyme systems produced
by
bacteria however since the solutions will support microbiological
growth a
preservative should be used if alginate solutions are to be stored for
any
considerable period of time. The preservatives listed in Table 10
exhibit good
compatibility.
Sodium benzoate
can be used to protect against bacterial
action in acid systems. For additional protection against yeast and
mold potassium
sorbate or calcium or sodium propionate can be effective.
Thickeners The alginates
show compatibility
with most commercially available thickeners both synthetic and natural.
With
some thickeners a synergistic viscosity increase may be noticed. If the
residual polyvalent ion content of a natural gum causes gelation of an
algin
solution the gelation can be controlled by the proper use of a
sequestrant.
Water Soluble
Resins The
compatibility of the alginates
with most water soluble resins is excellent. Polyvinyl alcohol exhibits
definite synergism with sodium alginate in the formulation of grease
resistant
films.
Latices Those latices
normally used in the formulation of
paints paper coatings and adhesives have compatibility with the
alginates. However
latex emulsions with pH of 4.0 or less will cause gelation of the
alginate. The
apparent incompatibility may be overcome by proper buffering. High
viscosity
ammonium alginate may be used as a creaming agent for natural rubber
latex and
for several types of synthetic latex.
Organic Solvents
As shown in
Table 10 alginate
solutions will tolerate up to 30% water miscible solutions. However
viscosity
increases may occur with long term storage. To prevent localized
gelations it
is necessary that there is good agitation of the solution at the time
the
organic solvent is added.
Enzymes Enzymes commonly
encountered as by products or as
commercially available products e.g. protease cellulase amylase
galactomannanase
have no effect on the alginate molecule. Storage test data is given in
Table 11
for representative enzymes.
Surfactants Although
alginate solutions have
compatibility with anionic non ionic and amphoteric surfactants high
concentrations of surfactants will result in a loss of viscosity and
eventually
the aliginate will out salt of solutions.
Non ionic
surfactants can be used at concentrations higher
than those allowable for the anionics or amphoterics. Some cationic
surfactants
may be used if approximately 2.5% of a soluble salt such as sodium
chloride is
added to the system. The exact salt level required depends upon the
particular
cationic material in the system.
Plasticizers Plasticizers
such as glycols or
glycerol may be used to improve the flexibility of alignate films. Data
for a
number of plasticizers and their effects on alginate solutions are
given in
Table 10.
Inorganic Salts The
compatibility of alginate
solutions with inorganic salts is limited to ammonium magnestium or the
alkali
metal salts. Divalent or higher valence cationic salts will unless
sequestered cause
gelation or precipitation of the alginate. The alginates will also be
precipitated by molar solutions of monovalent salts.
Salts which are
sightly acidic may produce large viscosity
increase after prolonged storage. Sequenstrants in many cases will
improve the
salt compatibility and stability of the sodium alginate. Mixed alginate
salts
(sodium/calcium alginate) are much more salt sensitive than are the
alkali
metal alginates.
Cellulose Ethers
Cellulose a
large volume renewable agricultural raw material is
transformed into hundreds of products affecting every phase of daily
life. Its
use and versatility are exploited by the chemical industry much as the
meat
industry exploits its raw materials using everything but the squeal.
The production
of water soluble cellulose derivatives in
contrast to that of polymers based on petrochemical resources starts
with a
preformed polymer backbone of either wood or cotton cellulose instead
of a
monomer. Cellulose is a linear polymer of anydroglucose with the O
glucopyranosyl
structure shown below
The properties
of a specific cellulose ether depend on the
type distribution and uniformity of the substituent groups. For each
O glucopyranosyl
ring there are three hydroxyl groups available for the nucleophilic
substitution reaction. Reactions at these sites can occur either on a
one to one
basis or with formation of side chains depending on choice of reagent
employed
to modify the cellulose. In the former case the term degree
of
substitution (DS) is used to identify the average number of
sites reacted
per ring. The maximum value is 3 corresponding to the number of
hydroxyls
available for reaction. When side chain formation is possible the term molar
substitution (MS) is used and the value can exceed 3.
The water
soluble cellulose ethers
possess a range of multifunctional properties resulting in a broad
spectrum of
end uses.
General Information
This family of
commercial water soluble
cellulose ethers comprises methylcellulose (MC) and the methylcellulose
derivatives
hydroxypropylmethylcellulose (HPMC) hydroxyethylmethyl cellulose (HEMC)
also
possesses many properties and end uses in common with the
methylcellulose
products and is included in this section.
Methylcellulose
and
hydroxypropylmethylcellulose are two examples of this versatile class
of water soluble
hydrocolloids derived from the etherification of cellulose. MC and HPMC
are
polymers having the useful properties of thickening thermal gelation
surfactancy
film formation and adhesion. Those characteristics earn them
application in
areas such as foods cosmetics paints construction pharmaceuticals
tobacco
products agriculture adhesives textiles and paper. Additionally to
tailor a
product for a specific end use the properties of MC and HPMC may be
modified by
changing the molecular weight or the relative amounts of etherifying
ragents.
Commercial MC
products have an
average degree of substitution (DS) ranging from 1.5 to 2.0 hence one
half to
two thirds of the available hydroxyl units are substituted with methyl
groups
(Table 1). In commercial HPMC products the DS for methyl groups ranges
from 0.9
to 1.8 and the substitution (MS) of hydroxypropyl groups range from 0.1
to 1.0.
MC and HPMC
possess the rather
unusual property of solubility in cold water and insolubility in hot
water so
that when a solution is heated a three dimensional gel structure is
formed. By
modifying production techniques and by altering the ratios of methyl
and
hydroxypropyl substitutions it is possible to produce products whose
thermal gelation
temperature ranges from 50 to 900C (122 to 1940F) and whose gel
texture ranges from firm to rather
mushy.
Altering the
amounts of methyl and hydroxypropyl substitution
also affects the solubility properties of the cellulose ether.
Decreasing the substituent
groups below a DS of 1.4 gives products whose solubility in water
decreases.
Concentrations of 2 to 8% sodium hydroxide are required for solubility
as the
level of substitution decreases. Increasing the substitution above an
MS of 2.0
improves solubilty in polar organic solvents.
Kalle &
Co. A. G. of west Germany also produces
hydroxyethylmethylcellulose. The small amount of hydroxyethyl
substitution
increases the solubility of the polymer and raises the thermal gel
point from
about 55oC to about
70oC. The more
polar nature of the hydroxyethyl group versus the hydroxypropyl group
allows
for the formation of a slightly stiffer gel than is possible with an
HPMC
material of comparable gelation temperature.
A third product
ethylhydroxyethylcellulose is similar in many
properties to MC and HPMC. The small amout of hydroxyethyl substitution
raises
the thermal gel point from about 55ºC (131ºF) to about 70ºC (158ºF).
The more
polar nature of the hydroxyethyl group allows for the formation of a
slightly stiffer
gel than is possible with an HPMC material of comparable gelation
temperature.
In many respects
the properties and uses of EHEC are also
very similar to those of methylcellulose and
hydroxypropylmethylcellulose. The
product has the characteristic properties of thickening surfactancy
film
forming binding solubility in cold water and insolubility in hot water
plus a
broad range of solubility in many organic solvents.
The properties
of EHEC are quite dependent upon the relative
amounts of ethyl and hydroxyethyl substitution. By varying the ratio of
substituents the gelation temperature the gel characteristics the
solubility
properties in different solvents and the surfactancy can be modified.
Increasing the amount of ethyl substitution increases the solubility in
organic
media and the tendency to form a firm gel while increasing the
hydroxyethyl
substitution improves the water solubility reduces the tendency to from
a gel
on heating and improves the brine tolerance of the polymer in various
salt
solutions.
Methylcellulose
was first produced commercially in the United
States in 1938 by The Dow Chemical Co. under the registered trademark
of
Methocel. Hydroxypropylmethylcellulose achieved commerical significance
in the
early fifties. In additon to The Dow Chemical Co. other suppliers of
these
products are Shin Etsu Chemical Products Ltd. (Metolose) of Japan
British
Celanese Ltd. (Celacol) of Great Britain and Kalle & Co. A G.
(Tylose) Henkel
and Cie GmbH (Culminal) and Wolff A. G. of Germany.
The worldwide
capacity of MC and HPMC in 1979 is estimated to
be about 159 million pounds per year and is growing. Water soluble
ethylhydroxyethylcellulose is produced by Berol Kemi AB (formerly
Modokemi AB)
of Sweden.
Main Reaction
Side Reaction
The relative
amounts of methyl and hydroxypropyl substitution
are controlled by the weight ratio and concentration of sodium
hydroxide and
the weight ratios of methyl chloride and propylene oxide per unit
weight of
cellulose.
EHEC is prepared
by reacting dissolving grade wood pulp with
aqueous sodium hydroxide and then with ethyl chloride and ethylene
oxide as
schematically illustrated below
The amount of
ethylation is controlled by the amount of
caustic used in the formation of alkali cellulose and the amount of
hydroxyethylation is controlled mainly by the amount of ethylene oxide
added to
the reactor. The addition is stepwise since ethylene oxide is far more
reactive
than ethylene chloride and hence reacts with the cellulose first.
Manufacture
There are three
main steps used in the manufacture of MC and
HPMC.
Preparation of
Alkali Cellulose Alkali
cellulose is prepared by contacting cellulose and 35 to 60% aqueous
caustic
according to several procedures that include dipping a cellulose sheet
in a
caustic solution spraying the caustic onto agitated cellulose flock
slurrying
the cellulose in aqueous caustic and removing the excess or mixing the
cellulose and aqueous caustic in an inert diluent.
Viscosity
control of the
final product is obtained by choice of pulp by aging the alkali
cellulose in
warm air and by controlling the amount of oxygen left in the reactor
during
methylation. For high viscosity products the higher molecular weight
cotton
linters are used with minimum aging. Since alkali cellulose is
susceptible to
oxidative degradation exposing it to air for varying time peroids is an
effective method for viscosity reduction.
Reaction The alkali
cellulose methyl chloride and (if required propylene oxide are loaded
into a
jacketed nickel clad agitated vessel and heated under controlled
conditions to
a maximum pressure of 1.38 MPa (200 psig.) The heat of reaction is
removed by
condensation of the solvents. In additon to controlling the
substitution levels
variations in the amounts of methyl chloride and changes in the
reaction profile
will affect the properties of the final product.
Purification Since MC and
HPMC are insoluble in hot water the reaction by products are removed by
slurrying the crude product in water heated to above 900C (1940F) and then
filtering. The purified wet product in then
dried ground to > 95% through 40 mesh screen and commonly
packaged in 22.68
kg (50 lb) bags.
Toxicity and Handling
Commercial MC
and HPMC
products have been used by the food pharmaceutical and cosmetic
industries for
many years. They are odorless. Tasteless powders and are considered to
be
physiologically insert.
MC products are
listed in the
United States Pharmacopeia XIX and Food Chemicals Codex and
are listed
by the FDA as Generally Recognized as Safe (GRAS).
HPMC compounds
whose methoxyl
substitution ranges from 19 to 30% and hydroxypropyl substitution
ranges from 4
to 12% are also listed in the United States Pharmacopeia XIX
and Food
Chemicals Codex. Both products can meet the requirements of
Food Additive
Regulations 182.1480 and 172.874 as a miscellaneous and/or general
purpose food
additive for nonstandardized foods.
While a gross
exposure to MC
or HPMC can conceivably cause temporary mechanical irritation to skin
and eyes exposure
to normal amounts presents no significant health hazards from either
contact or
inhalation.
In storage good
housekeeping is suggested to prevent dusts
from building up to possibly explosive levels. All the cellulose ethers
are
organic materials that will burn under the right conditions of heat and
oxygen
supply. Fires can be extinguished by conventional means. Gross powder
spills
should be swept up to avoid accidents caused by slippery floors or
equipment and
the trace residual product can be flushed to a sewer. These products
showed no
biochemical oxygen demand (BOD) with the standard 5 day test. However
radioassay
tests with activated sludge showed breakdown over a 15 to 20 day
period. These
products should provide no ecological hazard. The products may be
disposed of
by either landfill or incineration.
When the
viscosity is known at one concentration the
viscosity can be calculated for any other concentration by using Eq.
(2) to
first calculate K for the sample at the concentration for which the
viscosity
is known and then using Eq. (2) again to calculate the viscosity at the
new
concentration knowing the value of k.
EHEC may be
dissolved in cold water to yield clear smooth
solutions. Commercial products range in viscosity from 0.050 to
12.000Pa.s (50
to 12 000 cP) at 2% concentration. The solutions are pseudoplastic in
that the
apparent viscosity decreases with increasing rate of shear the
solutions are
not thixotropic unless they are gelled. (see fig. 1)
Rheology Solutions of MC
and HPMC generally show pseudoplastic
nonthixotropic flow properties at 200C (680F) that is
not a function of substitution within the range of available commercial
products and whose deviation from Newtonian character increases with
increasing
molecular weight (figs. 2 and 3). Dilute solutions of low viscosity
products
(fig. 4) do closely approach Newtonian flow but increasing the
concentration of
the gum to over 5% may give a solution showing same thixotropy due to
weak
chain to chain interactions. Since flow properties are dependent on the
molecular weight and the molecular weight distribution of the polymer a
blend
of high and low molecular weight polymers can have different flow
properties
than a polymer having the same solution viscosity as the blend but
having a
narrow molecular weight distribution (fig. 5). This effect is generally
not
important for dilute solutions of higher viscosity materials (fig. 6)
but can
be significant when applied to solutions of over 5% of the low
viscosity
derivatives.
Heating a
solution of MC or HPMC shows the normal effect of
lower viscosity until the gelation temperature is reached at that point
the
viscosity of the solution increases rapidly and highly thixotropic flow
is
observed.
The normal
effect of temperature in the range of 0 to 450C (32 to 1130F) is roughly a
3% reduction in
viscosity for every degree celsius increase in the temperature of the
solution
(when applied to aqueous solutions containing no added solutes and
showing no
evidence of gelation).
Thermal Gelation
MC and HPMC
solutions show the
unusual property of forming a structured gel when heated.
In solution
these polymers exist as aggregates of long
colloidal molecules. These molecules are highly hydrated with the
solvent water
in layers that are held through hydrogen bonding thereby giving the
chains some
lubricity and smooth flow. As the temperature is raised the hydrogen
bonding
between the water molecules weakens and the interactions between chains
become
significant eventually leading to the formation of a structured gel.
Unlike
many chemical gels those made from MC and HPMC are primarily a result
of phase
separation and are susceptible to shear thinning (a mechanical breaking
up of
the gel without affecting the molecular weight). With cooling this
process is
reversible and the gel reverts back to a solution whose flow properties
are not
changed.
The gelation
temperature is dependent on the relative amounts
of methyl and hydroxypropyl substitution and may be used as an
indication of
the relative hydrophilicity of the derivative. In general the more
highly
substituted derivatives have lower gelation temperatures and will be
less
compatible with added solutes or electrolytes. The gelation temperature
of
products currently produced varies from about 50 to 850C (122 to 1850F) with the
resultant gels ranging
from firm to rather mushy in consistency (when determined by heating a
2%
solution of the gum in pure water). The gelation temperature of a
product is
affected by the concentration of the gum and more importantly by other
dissolved solutes. Presence of salts (Table 3) will lower the gel point
addition
of ethanol or propylene glycol can raise the gel point as much as 200C (36OF).
Upon reaching
the thermal gelation temperature EHEC will
separate out of solution as either a floc or a gel depending upon
molecular
weight and the concentration (see Table 4).
Surface Activity MC and HPMC
reduce the surface
tension and interfacial tension of aqueous sustems to values of 41 to
55 dyn/cm
and 18 to 28 dyn/cm respectively (depending on chemical structure)
thereby
functioning as moderate emulsifiers for two phase mixtures. Since they
are
polymeric materials they are active surfactants at very low use levels
ranging
from 0.001 to 1.0%. Their status as approved additives in foods makes
them
useful as edible surfactants. EHEC also behaves as a moderate
surfactant lowering
the surface tension of water to 47 to 52 dyn/cm.
Moderate foaming
is usually encountered. This can be
controlled if desired by use of commercially available defoamers
Polyglycol P 1200
(The Dow Chemical Co.) Antifoam A AF B or FG (Dow corning corp.) Nopco
KFS
(Nopco Chemical Co.) or tril n butylphosphates.
Sodium Carboxy Methyl Cellulose
Sodium carboxy
methyl cellulose is a water soluble anionic
linear polymer. It is universally known as CMC and will sometimes be so
designated here. In the food pharmaceutical and cosmetic industries the
highly
purified types required are referred to as cellulose gum. The
United
States Food and Drug Administration (FDA) has defined cellulose gum
(see
section on toxicological properties) also the Food Chemicals
Codex and
the Food and Agriculture Organization (FAO) of the United Nations have
established specifications for identity and purity of sodium
carboxymethylcellulose for food uses worldwide.
Purified sodium
carboxy methyl cellulose is a white to buff colored
tasteless odorless free flowing powder. Less purified grades contain
the
reaction salts (sodium chloride and sodium glycolate) and can be off
white to a
light brown for the low assay types (50% purity).
Sodium carboxy
methyl cellulose is probably used in more
varied applications worldwide than any other water soluble polymer
known
today. Applications vary from the large worldwide detergent use to the
specialized barium sulfate suspension for medical diagnosis.
Worldwide
applications of CMC in order of size of estimated
end use are given in Table 1. These estimated usages demonstrate the
versatility throughout the world for this modified natural long chain
water soluble
polymer.
It is estimated
that over 250 types of sodium
carboxymethylcellulose are manufactured throughout the world by over 50
producers with outputs ranging from as little as 200 metric tons to
over 35 000
metric tons per year.
The growth of
CMC was accelerated by the world conflict in
the early 1940s when fatty acids usage was drastically shifted from
civilian
soap manufacture to wartime manufacture of explosives. Even though CMC
was
developed shortly after World War I as a possible replacement for some
gelatin
uses the major growth in the use of CMC began after it was discovered
that it
improved the efficiency of synthetic detergents. Usage during the early
1940s
was primarily for detergent systems although many new applications were
developed on a laboratory scale where control of water movement was
important.
With the end of
the world conflict in 1945 and with the huge
demand for consumer products CMC backed with several years of
laboratory
studies began finding uses in all types of areas requiring water
control in
systems with various levels of soluble and insoluble solids.
In the United
States a landmark in the growth of purified
sodium carboxymethylcellulose (cellulose gum) was the approval by the
FDA for
its use as an intentional food additive (see section on toxicological
properties for details). Following this was the definition in the United
States Pharmacopoeia for subsequent use in pharmaceutical
applications.
Chemical Nature
Cellulose is a
linear polymer of
anhydroglucose units. Each anhydroglucose unit contains three hydroxyl
groups.
The extent of
the reaction of
cellulose hydroxyls to form a derivative is called the degree
of
substitution (DS) and is defined as the average number of
the three
hydroxyl groups in the anhydroglucose unit which have reacted. Thus if
only one
of the three hydroxyl groups has been carboxymethylated the DS is 1.0.
Commercial products have DS values ranging from 0.4 to about 1.4. The
most
common grade has a DS of 0.7 to 0.8 and if the DS is not specifically
mentioned
it can be assumed to be in this range. CMC is commercially available in
several
different viscosity grades ranging from 4.5 Pa.s (4500 cP) in 1%
solution to
0.010 Pa.s (10 cP) in 2% solution. The various viscosity grades
correspond to
products having molecular weights from about 1 000 000 to 40 000. Table
2 shows
that 19 different DS viscosity combinations are available from one
producer.
CMC is a salt of
a carboxylic acid
having approximately the same acid strength as acetic acid. The pK
varies
somewhat with degree of substitution. The pure commercial product of DS
0.8 has
a pK value of 4.4 the corresponding value of K
the ionization
constant is 4 × 10–5. A dilute
solution of such a product has a pH of about 7 and has over 99% of its
carboxylic acid groups in the sodium salt form and very few in the free
acid
form.
CMC forms
soluble salts with alkali
metal and ammonium ions. Calcium ion present in concentrations normally
found
in hard water prevents CMC from developing its full viscosity and thus
its
dispersions are hazy. At much higher concentrations calcium ions
precipitate
CMC from solution.
Magnesium and
ferrous ion have a
similar effect on CMC dispersions. Heavy metal ions like silver barium
chromium
lead and zirconium precipitate CMC from solution. Quaternary salts
attached to
a long hydrocarbon chain such as dimethylbenzylcetylammonium chloride
also
precipitate CMC from solution.
CMC is
precipitated from solution by the polyvalent cations
Al3+ Cr3+ or
Fe3+. If the ion
concentration is
carefully controlled for example by the presence of a chelating agent
such as
citric acid it is possible to form more viscous solutions soft gels or
very
rigid gels. In these instances the polyvalent ion functions as a
crosslinking
agent.
CMC reacts with
certain proteins. For example soy protein which
is insoluble in its isoelectric range can be solubilized by CMC. Thus
the
solubility can be extended over a wider pH range. CMC has a similar
solubilizing effect on casein but in the case of gelatin which is a
more
soluble protein the reaction with CMC manifests itself as a rise in
solution
viscosity.
Like all
polymers CMC may be salted out of solution. However CMC
being a very hydrophilic polymer is more tolerant of alkali metal salts
than
many other water soluble polymers. Its salts compatibility is much
greater if
the salt is dissolved in the CMC solution than if the CMC is dissolved
in the
salt solution (see Figure 1). Such behavior relates to the fact that
CMC of DS
0.7 is aggregated in solution. This is discussed later in connection
with its
rheological properties.
Commercial
grades of CMC have most of their carboxyl groups
in the sodium salt form. Such products may be converted into the free
acid form
e.g. by passing a solution through a suitable ion exchange resin. If
the
resulting free acid is freed from water by drying a film or
precipitating with
alcohol and drying the product is no longer water soluble. It may be
dissolved however
in aqueous NaOH that is by re forming a soluble salt.
Physical
Properties
The general
physical properties of CMC are summarized in
Table 3. Other physical properties follow.
Equilibrium
Moisture Content CMC is a
very hydrophilic polymer whose equilibrium moisture increases with DS.
Figure 2
gives the equilibrium moisture content for products of DS 0.4 0.7 and
1.2 at
different humidities. These data were obtained on dry commercial
samples by
conditioning the powder to constant weight at 25°C.
Molecular
Weights The molecular
weights shown in Table
4 were calculated from intrinsic viscosity measurements in 0.1% NaCl at
25°C
using the relationship [M] = 2.9 × 10–4 M0.78 where
M is the weight average molecular
weight.
Solubility The only good
common solvent for CMC
is water. The degree of dispersion in water varies with the DS and the
molecular weight. CMC with a DS of 0.7 may be dissolved in glycerin
particularly
in the presence of a slight amount of water by heating with good
agitation.
Aqueous solutions of CMC will tolerate considerable quantities of water
miscible
organic solvents such as methanol ethanol and acetone. For example a 1%
solution of the high viscosity grade will tolerate 1.6 volumes of
ethanol per
volume of CMC solution before it becomes hazy and precipitates. Low
viscosity
grades will tolerate as much as 3.5 volumes. Aqueous solutions of CMC
will
tolerate large amounts of alkali metal salts and small amounts of
calcium and
magnesium salts. Heavy metals and multivalent salts precipitate CMC as
discussed earlier under Chemical Nature.
Film Properties Table 5 gives
mechanical properties
of 0.508 mm (2 mil) films containing about 18% moisture for three
different
viscosity grades of CMC with a DS of 0.7. It is evident that the
strength and
flexibility are greater for the types which have high viscosities or
molecular
weights. Films may be insolubilized by crosslinking at the hydroxyl
groups
using suitable water soluble resins such as Hercules Kymene 917 and
Kymene 754
Resin or Aerotex M 3. The crosslinks are formed by reaction of
cellulosic
hydroxyls with the aldehyde functionality of the resins. A film is cast
from an
aqueous solution of the resin and CMC. Upon drying and further curing
the film
becomes insoluble. The degree of insolubilization depends on the extent
of the
curing treatment. Dry CMC films may be insolubilized by treatment with
aqueous
solutions of aluminum salts.
Manufacture
The manufacture
of CMC involves treatment of cellulose with
aqueous sodium hydroxide followed by reaction with sodium chloroacetate
Cellulose is a
fibrous solid. Chemical cellulose which is
used for the manufacture of CMC is derived from cotton linters or wood
pulp. To
obtain uniform reaction it is essential that all the fibers be wetted
out with
the aqueous NaOH. One process for accomplishing this is to steep
sheeted
cellulose in aqueous NaOH and then press out the excess. The sheets are
then
shredded and the sodium chloroacetate is added. Reactions are generally
conducted at 50 to 70°C. In some cases a greater amount of NaOH is
added and
the monochloroacetic acid is added as such the sodium salt being formed
in the
presence of the cellulose.
In an alternate
process the steeping and pressing steps are
eliminated by conducting the reaction in the presence of an inert water
miscible
diluent such as tertiary butyl alcohol or isopropanol. At the end of
the
reaction the excess alkali is neutralized and the crude product which
contains
sodium chloride and sodium glycolate is purified or partially purified
(see
Table 6). There are many variations of these processes depending on the
DS
level and the quality of the product desired.
Biological
Properties
Water soluble
cellulose derivatives are all subject to
microbiological attack under certain conditions. The magnitude of the
microbiological degradation is influenced by a number of factors which
include
contaminants present temperature pH of system oxygen available and
concentration. The first sign of biological degradation is usually loss
of
viscosity (i.e. chain length). This loss can be rapid under extreme
conditions or
very slow under much less severe conditions. Biological attack is
greater in
solution systems than on the dry form of sodium carboxymethylcellulose.
Usually
the moisture content of sodium carboxymethylcellulose is from 5 to 10%
and
biological degradation is generally not severe under normal dry storage
conditions at this moisture level.
Manufacturing
conditions and subsequent packaging systems can
greatly influence the microbiological stability of the final product.
In some
commercial manufacturing systems using solvents the final packaged
sodium
carboxymethylcellulose is essentially aseptic. In the dry process
production of
CMC there is a greater possibility of residual biological
contamination.
Generally however the caustic present in the system necessary for the
alkali
cellulose stage is detrimental to any microorganisms present.
Packaging
environment is also extremely important. Clean containers
and air free of microbiological organisms are necessary in the
packaging of
CMC. Generally speaking the product as produced and packaged is
relatively free
of microbiological organisms which would promote degradation of the
cellulose
chain. It has been found in actual application that most biological
organisms
causing CMC degradation have been introduced from outside sources
(other than
in the CMC). Each manufacturer of sodium CMC usually runs periodic
bacteriological examinations. An example of these analyses is as
follows
Other specific
bacteriological testing has been done on the
example purified sodium carboxymethylcellulose testing for the presence
of
coliforms thermophilic anaerobic spores pathogenic staphylococci beta
hemolytic
streptococci Salmonella species and Pseudomonas
aeruginosa.
It is stressed
that most individual manufacturers of CMC
throughout the world have had bacteriological examination of their
product and
will have data comparable to the preceding. Varying manufacturing
processes
(i.e. different solvents and dry process techniques) will yield
different
biological analyses but generally speaking most known processing does
not
promote or support bacterial growth.
The major
bacteriological problems are generally caused when
the sodium carboxymethylcellulose is used in solution. Contaminating
spores can
be introduced in the system from influent water used for solution
preparation as
well as from the air surrounding the mixing and makeup vessels. This is
especially true in warm humid climates which are supportive of
bacteriological
spores. Care should always be taken in handling containers of sodium
carboxymethylcellulose that are stored in locations exposed to warm
humid air.
Introduction of
bacteriological contamination has been observed
in plant operations where the vessels piping pumps etc. have not been
cleaned
after each use. In many fluid handling systems the possibility exists
for areas
which could retain residual product. In these areas under the proper
conditions
microbiological growth can take place at a rapid rate thus
contaminating
subsequent batches pumped or handled in the same system. Industrial
experience
has shown that this is a major source of microbiological contamination.
Simple
clean out measures in any solution handling system where CMC is used
are all
that are necessary to prevent subsequent contamination of other batches
of
product.
Toxicological
Properties
In the United
States toxicological information on sodium
carboxymethylcellulose has primarily been developed on the food
additive grade or
cellulose gum which is of 99.5% purity. Extensive testing has been
performed on
this purified grade of sodium carboxymethylcellulose to assess its
safety in
foods as an additive. Details on testing and results follow. However a
definition of food grade sodium carboxymethylcellulose is necessary to
assure
safe use. The United States Food and Drug Administration defines
cellulose gum
as the sodium salt of carboxymethylcellulose not less than 99.5% on a
dry
weight basis with a maximum substitution of 0.95 carboxymethyl groups
per
anhydroglucose unit and with a minimum viscosity of 0.025 Pa.s (25 cP)
in a 2%
(dry weight) aqueous solution at 25°C.
Sodium
carboxymethylcellulose (cellulose gum) is classified
under Substances That Are Generally Recognized As Safe (GRAS) by Title
21 Section
182.1745 (formerly 121.101) of the Code of Federal
Regulations (U.S.A.).
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