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 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 useful 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 (DHc/DHv). 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 DHv per milligram
of compound evaporated. The ratio DHc/DHv 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. DHv 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.<![endif]>
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%, b-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:
1. Aliphatics: straight- or open chain,
saturated hydrocarbons.
2. Naphthenics: cyclic, saturated
hydrocarbons with or without alkyl side chains.
3. 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.
a. It must not weaken,
dissolve, or shrink the ordinary textile fibers.
b. It must not remove the
common dyes from fibers.
c. It must be an acceptable
solvent for fat and oils.
d. It must not impart an
objectionable order to drycleaned textiles.
e. It must be sufficiently
volatile to permit reclamation by distillation and to permit garments
to be tried without prolonged heating at excessive temperatures.
f. It must be noncorrosive to
metals, either when dry or in the presence of water.
g. It must be relatively
nontoxic.
h. 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:
2 RSH + S + Na2PbO2 ® R2S2 + PbS + 2 NaOH
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 b-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 b-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 b-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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