Mechanisms for Flame
Retardancy and Smoke
suppression -A Review
JOSEPH GREEN
FMC Corporation
P.O. Box 8
Princeton, NJ 08543
(Received May 7, 1996)
(Reemsed June 13, 1996)
ABSTRACT: The prevailing mechanisms for halogen and phosphorus flame
retardancy are reviewed. Halogens act in the vapor phase and phosphorus can
act in either the vapor or condensed phase depending on the specific phosphorus compound and the chemical composition of the polymer. Halogenantimony synergy is discussed. Convincing evidence is presented for brominephosphorus synergy in specific polymers. The mode of decomposition of
polycarbonate is shown and the effect of salts of organic acids in changing the
mode of decomposition hence producing a more flame resistant polymer is
shown. Intumescence in polyolefins is discussed. Inorganic metal hydrates used
in large concentration cool by endothermically releasing a large concentration
of water. The effect of boron compounds is discussed. Methods of smoke suppression are presented as is the role of zinc borate, molybdenum and tin compounds
acting as Lewis acids in PVC.
INTRODUCTION
THE OBJECTIVE OF flame retarding polymers is to increase the resistance of the material to ignition and to reduce the flame spread with
minimal degradation of its properties. The resultant products are not
non-combustible, and the use of the flame retardant is to minimize, not
eliminate, the fire risk associated with the use of a polymer in a specific
© 1996, FMC Corporation.
426
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application. The use of flame retardants may prevent a small fire from
becoming a major catastrophe. Approximately 80% of fire deaths can be
attributed to smoke inhalation. The objective of this paper is to attempt
to explain, qualitatively, the prevailing thoughts on the mechanisms
involved in flame retardancy and smoke suppression of polymers.
The combustion of polymers is a process comprising endothermic
pyrolysis to flammable gases which mix with air and ignite, leading to
exothermic processes of flame propagation and heat release. Thermal
feedback reinforces pyrolysis, fueling the flame at an increasing level.
Flame retardants can act chemically and/or physically in the condensed or vapor phase. They interfere with the combustion process during heating, pyrolysis, ignition, or flame spread. The most significant
chemical process interfering with the combustion can take place in
either the vapor or condensed phase.
Halogen, phosphorus, and antimony can function in the vapor phase
by a radical mechanism. The exothermic processes are thus interrupted and combustion is suppressed.
Phosphorus can also function in the condensed phase, promoting char
formation on the surface which insulates the substrate from heat and
air and also interferes with the loss of decomposition products to the
flame zone.
The combustion process can be inhibited by physical means. If the
concentration of the gases from the flame retardant, e.g., hydrogen bromide, is sufficiently high, the mechanism may be in part physical such
as the suppressant effect of an inert gas like carbon dioxide or nitrogen.
Phosphorus is known to promote char formation, forming a protective
coating. Phosphoric acids which form coat and protect the substrate
similar to borate glasses which form when boric acid and borax are
used. The incorporation of large quantities of a filler will dilute the
polymer and thereby reduce the concentration of decomposition gases.
Hydrated fillers additionally will cool the substrate as they decompose,
lowering the temperature below that needed for sustained pyrolysis.
Decomposition of the polymer can be accelerated by an additive such as
a peroxide causing pronounced flow or dripping of the polymer. This
dripping of flaming polymer removes heat from the flame zone and contributes significantly to flame extinguishment.
Commercial flame retardant additives include organohalogen and organophosphorus compounds, inorganic synergists for halogen, red
phosphorus, inorganic phosphorus compounds, and hydrated minerals
such as aluminum trihydrate and magnesium hydroxide. Commercial
smoke suppressants include zinc borate and molybdenum and tin com-
pounds.
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HALOGEN FLAME RETARDANTS
It is generally agreed that the combustion of gaseous fuel proceeds
via a free radical mechanism [1,2]. A number of propagating and chain
branching reactions critical for maintaining the combustion process
are illustrated below. Methane is used in the examples as the fuel or
the decomposition gases coming from a polymer.
Here H, OH, and 0 radicals are chain carriers, and the reaction of the
H radical with O2 is an example of chain branching in which the
number of carriers is increased. The reaction of CO with the OH radical
converting CO to CO, is a particularly exothermic reaction.
In the radical trap theory of flame inhibition, it is believed that HBr
competes in the reactions above for the radical species that are critical
for flame propagation.
The active chain carriers are replaced with the much less active Br
radical, slowing the rate of energy production resulting in flame extinguishment.
It also has been suggested that halogens simply alter the density and
mass heat capacity of the gaseous fuel-oxidant mixture so that flame
propagation is effectively prevented [3]. This physical theory is equivalent to the way inert gases such as carbon dioxide and nitrogen may influence combustion.
Suggestions have been made that some bromine compounds act
mainly in the condensed phase and depend on the polymer [4]. Reaction
of the flame retardant or its decomposition products with the polymer
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inhibit the decomposition of the polymer, thereby influencing the
flame retardancy.
The performance of halogens as flame retardants is rated as follows:
can
Iodine compounds, apparently the most effective, are not used in
polymers because they do not have adequate thermal stability.
Fluorocarbons are inherently non-burning, but they generally do not
impart flame retardancy to other plastics because either the C-F bond
is too thermally stable or the highly reactive hydrogen fluoride or
fluoride radicals that may form react rapidly in the condensed phase.
An exception is that small amounts of Teflon will significantly increase
the oxygen index of polycarbonate resins.
Commercial organohalogen flame retardants include aliphatic, alicyclic, and aromatic chlorine and bromine compounds. Aliphatic compounds are the most effective and the aromatic compounds are the least
effective with the alicyclic compounds in-between.
aliphatic >
alicyclic >
aromatic
This is in the same direction as the thermal stability, indicating that
the more easily available the halogen, the more effective. The actual
type of compound used in an application will depend on the processing
temperature of the plastic. Bromine compounds are about two times
more effective than chlorine, which is proportional to their atomic
weights. If the relative effectiveness of the halogens is directly proportional to their atomic weights, then their relative effectiveness would
be expected to be F : C1: Br :I = 1.0 :1.9 : 4.2 : 6.7. This would be consistent with the physical theory of inert gases mentioned above.
It takes only 3% of a brominated flame retardant plus 1.5% antimony
oxide to obtain a polypropylene composition with a V-2 rating in the
UL-94 test. The product drips flaming polymer removing heat from the
flame zone. Addition of inert fillers to inhibit drip leads to a burning
product demonstrating the effect of dripping as a valid method of passing a small scale laboratory test [5] (Figure 1). Also in large scale tests,
dripping allows heat to be removed from the flame zone, significantly
reducing the total heat release [6]). The flaming dripping polymer will
extinguish and the actual amount of polymer burned could be significantly less, reducing the heat released.
Antimony oxide itself imparts no flame inhibition to polymers, but it
is known as a synergist for halogen compounds. It is not volatile, but
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Figure
1.
Polypropylene containing 4% bromine flame retardant and 2% antimony
oxide without and with talc
antimony oxyhalide (SbOX) and antimony trihalide (SbX3) formed in
the condensed phase by reaction with the halogenated flame retardant
are volatile and facilitate the transfer of halogen and antimony into the
gas phase where they can function [ 7 ]. Laboratory flammability tests
indicate that the optimum halogen/antimony atom ratio in many polymers is 3/1. It has been suggested that antimony is also a highly active
radical trap [8].
Interference with the antimony-halogen reaction will affect the flame
retardancy of the polymer. For example, metal cations from color pigments or seemingly inert filler such as calcium carbonate may lead to
the formation of stable metal halides, rendering the halogen unavailable for reaction with antimony oxide. The result is that neither the
halogen or the antimony is transported into the vapor zone [9]. Silicones have also been shown to interfere with the flame retardant
mechanism. As a result, the total plastic composition must be considered in developing a flame retarded product.
Other members of Group V of the periodic chart such as arsenic and
bismuth function as synergists for halogen. Little work has been done
with these compounds for obvious reasons. A Diels-Alder diadduct of
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hexachlorocyclopentadiene and 1,5-cyclooctadiene (Dechlorane Plus)
can be used to flame retard nylons, epoxies, and polybutylene terephthalate (PBT) using synergists other than antimony oxide. These
compounds include zinc compounds such as the borate, oxide, stannate,
phosphate, and sulfide and iron oxides such as Fe203, Fe204, and Fe203
H20 [10]. The use of mixed synergists is also reported to lower the level
of the total flame retardant package.
Synergistic action between organochlorine and organobromine compounds has been reported in ABS [11], polyolefins [12], and HIPS [13].
The chlorinated flame retardant used
oxygen index and UL-94 data show this
was
Dechlorane Plus. Both
synergism.
PHOSPHOItUS CONTAINING COMPOUNDS
Phosphorus containing flame retardants include phosphate esters,
phosphonates, phosphine oxides, chlorophosphates, chlorophosphonates, red phosphorus, and inorganic phosphates. The mechanism
whereby phosphorus flame retardants function varies depending on
both the type of phosphorus compound and the specific polymer. They
appear to function both in the condensed phase where they can promote char or coat the char surface with viscous phosphoric acids, in the
vapor phase where they can function by the free radical trap theory, or
physically by promoting dripping of the burning polymer.
Phosphorus compounds are effective flame retardants for oxygencontaining polymers and show little efficacy in polyolefins and
styrenics. This is demonstrated by the activity of red phosphorus (Table
1). It takes only 1 and 3% of red phosphorus in polycarbonate and
polyethylene terephthalate, respectively, for a UL-94 V 0 rating, but 10
and 15% in polyethylene and polystyrene, respectively.
The flame retarding mechanism for red phosphorus and oxygen containing polymers is believed to be through the formation of
Table 1. Red phosphorus concentration for
UL-94 V-0 Rating.
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phosphorus-oxygen bonds. Ester cleavage of polyethylene terephthalate or polymethyl methacrylate takes place 75-100 ° C lower
than normal. The resulting products are less volatile and crosslinking
takes place giving polyaromatic structures. The formation of flammable pyrolysis products is therefore suppressed and the formation of a
heat shield on the polymer surface is promoted. Red phosphorus has
been shown to retard the nonoxidative pyrolysis of polyethylene. Therefore, the scavenging of radicals in the condensed phase also has been
proposed for red phosphorus [14].
The mode of action of phosphorus containing flame retardants in cellulose is perhaps best understood. Cellulose decomposes to tarry depolymerization products, but when catalyzed by acids, the decomposition is an endothermic dehydration to char. Phosphoric acids formed by
decomposition of phosphorus containing compounds is highly effective
in this dehydration. The non-volatile phosphoric and polyphosphoric
acid also may coat the char protecting it from oxidation.
The major application for phosphate ester flame retardants is in
plasticized polyvinyl chloride (PVC). PVC is a rigid polymer containing
57% chlorine and does not burn at ambient conditions. Flexible PVC
contains large quantities of plasticizer, 20 to 80 parts per hundred of
resin (phr). The chlorine content of the resulting resin is significantly
reduced and organic plasticizers are highly flammable. Phosphate ester plasticizers are non-flammable and the resulting flexible PVC is
flame resistant. In this application, the flame retarding mechanism is
in large part physical, i.e., the flammable plasticizer is replaced with a
non-flammable plasticizer.
Two types of phosphate ester are used commercially, triaryl and alkyl
diaryl phosphates. The former are thermally stable to about 400 ° C and
mainly volatilize where they function in the vapor phase. The alkyl diaryl phosphates are not as stable and decompose in the condensed
phase to yield a volatile flammable hydrocarbon and a phosphoric acid.
The result is that the triaryl phosphates are more efficient flame retardants, but the alkyl diaryl phosphates yield significantly more char (intumescent char) and as a result less smoke. This is very obvious when
the samples are burned in the cone calorimeter at heat fluxes of 30-60
kw/ml [15]. The hydrogen chloride functions as the blowing agent.
Modified polyphenylene oxide (PPO) is a blend or alloy of PPO and
HIPS with as much as 65% HIPS in commercial grades. The flame
retardants used commercially are phosphate esters. These esters are
stable up to temperatures of 400°C and presumably volatize into the
vapor phase without significant decomposition. Evidence has been presented that the mechanism of flame retardancy is in the vapor phase
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[16] similar to the halogen radical trap theory. Triphenyl phosphine oxide
was
shown to volatilize and to decompose in the vapor phase to
acids and subsequently to HPO2, HPO, and PO radicals
phosphoric
[17].
Triphenyl phosphate and resorcinol diphosphate are used to flame
retard polycarbonates and polycarbonate blends such as polycarbonate/ABS. They may be used in combination with a bromine flame
retardant. The mode of action is presumably in the vapor phase as
these materials are volatile. In the preparation of the diphosphate
higher molecular weight, oligomers may form which are not volatile.
Increasing concentration of these oligomers in the diphosphate results
in decreasing flame retardant efficiency. This is presumptive evidence
that the major flame retardant mechanism is vapor phase.
Triphenyl phosphate is used to flame retard HIPS to a UL-94 V-2
rating by plasticizing and promoting drip, allowing the molten polymer
to drip away from the flame zone thus removing heat from the combustion
zone.
Phosphorus compound may act
in
some
polymers by catalyz-
ing thermal decomposition of the polymer resulting in drip [18].
Ammonium polyphosphate is used as the acid source in intumescent
plastics and coatings. Commercial intumescent coatings are composed
of a binder, a carbonific (char former) such as a polyol, frequently dipentaerythritol, a catalyst (acid source) such as ammonium polyphosphate,
and a spumific (blowing agent) such as melamine. The mechanism involves decomposition of the phosphate to phosphoric acid, esterification
of the polyol followed by decomposition and regeneration of the
phosphoric acid. Decomposition of the melamine helps blow the forming char that finally insulates the substrate from heat, flame, and air.
Intumescent systems for thermoplastics are commercially available.
They perform best in polypropylene, presumably because the softening/decomposition temperature of the polypropylene closely matches
the decomposition of the intumescent compounds. A number of single
compounds which intumesce are known. Here all three functions of an
intumescent system are combined into a single organic compound, for
example, a melamine salt of a polyol and phosphoric acid reaction product. These products are shown in Figure 2. An ethylenediaminephosphoric acid product is also used as an intumescing additive.
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Figure
2. Intumescent system and
compounds
Monoammonium and diammonium phosphates can be used alone or
together to impart fire retardant properties to a wide variety of cellulosic materials such as paper, cotton, wood, etc. They are highly effective
in preventing afterglow. They are non-durable additives since they are
water soluble and are easily leached out.
PHOSPHORUS-HALOGEN SYNERGY
The flame retardant mechanism for haloalkyl phosphorus esters in
polyurethanes is not understood. A number of these compounds are
sufficiently volatile to enter the flame zone intact while others could
decompose in the condensed phase to liberate halogen hydrocarbons. It
has been suggested that the endothermic vaporization and heat capacity of the intact chloroalkyl phosphates may be a major part of their
action [19,20].
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Phosphorus-bromine combinations are perhaps the most effective
flame retardant combination [21] and claims have been made for
synergy. The formation of phosphorus tribromide or oxybromide has
been postulated by analogy to that of antimony tribromide and oxybromide, but there is no evidence for this formation. Some reports of
synergy appear to be a result of a nonlinear response to concentration.
A comparison of bromine and phosphorus compounds on the flammability of PET fiber shows phosphorus (as phosphine oxide) to be 3.7
times more effective than aromatic bromine. But, combinations of the
two show no synergy [22].
Bromine-phosphorus synergy was convincing demonstrated in ABS,
HIPS, and polymethyl methacrylate [23-25]. The phosphorus compound used was the highly volatile triphenyl phosphite. The
mechanism may therefore be a staging of free radical traps into the
flame zone, the highly volatile phosphite followed by decomposition of
the bromine compound. When the bromine and phosphorus are in the
same compound, the synergy is further enhanced. If the brominated
phosphates are not volatile but decompose at the combustion temperature, then the mode of reaction may be a combination of vapor phase
with the bromine and condensed phase with the phosphoric acid that
would form.
Bromine-phosphorus flame retardant synergy was also convincingly
demonstrated in polycarbonate blends with PET and ABS [26,27]. Enhanced synergy was also reported when both elements were in the
same
compound.
3 compares a bromine additive (brominated polycarbonate
with a phosphorus additive (triphenyl phosphate) as flame
retardants for a 2/1 polycarbonate/polyethylene terephthalate (PET)
blend. The bromine concentration increases from left to right and the
phosphorus concentration from right to left. The two curves intersect at
5% bromine and 0.5% phosphorus, indicating that phosphorus is ten
times more effective than aromatic bromine in this polymer blend.
Also, the oxygen indices are equivalent at 10% bromine and 1%
phosphorus. Since both curves are linear, a straight line drawn between the two extremes should represent the theoretical oxygen index
for a polymer containing physical blends of the bromine and phosphorus additives. The experimental data, however, give products with
much higher oxygen indices, as shown. The author concludes that this
conclusively demonstrates bromine-phosphorus synergy. Two brominated phosphates were evaluated with bromine/phosphorus ratios of
60/4 and 70/3. The latter compound gave a much higher oxygen index,
indicating that when both elements are in the same molecule the
Figure
oligomer)
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Figure
3.
Bromme-phosphorus synergy m
2/1
polycarbonatelPET blend
synergy could be enhanced. The latter
compound is both higher in
molecular weight and less thermally stable than the former compound.
This may indicate that the phosphorus acts in the condensed phase and
the bromine acts in the vapor phase accounting for this enhanced
synergy, while the former compound may volatize and decompose in
the vapor state [26].
The brominated phosphate also gives much more char when pyrolyzed in thermogravimetric analysis (TGA) up to 500 ° C than a physical
blend of the bromine- and phosphorus-containing compounds [28]
(Table 2). The concentration of the blend was specifically chosen
Table 2. Residue yields formed in TGA up to
500°C (NZ) 211 polycarbonatelPET.
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Table 3. Residue yields in TGA up to
500°C (N2) 3/1 polycarbonatelABS.
because it gave an Underwriters Lab rating (UL-94) of V 0 at 1.6 mm.
In a similar study, bromine-phosphorus synergy was also conclusively demonstrated in polycarbonate/ABS blends of 5/1 to 8/1 [27]. In
this polymer blend, however, the phosphorus appears to function in the
condensed phase as measured by TGA (Table 3).
SALTS OF ORGANIC ACIDS
It is well known that salts of aromatic sulfonates in very low concentration (0.1%) are very effective flame retardant additives for polycarbonate polymers. The thermal degradation of polycarbonates proceeds
by two mechanisms. Isomerization leads to linear oligomeric ethers
with phenol end groups. Loss of carbon dioxide and water leads to cross-
linking.
Intramolecular exchange leads to formation of cyclic oligomers which
will react with water vapor and liberate carbon dioxide to form aromatic phenols.
The addition of catalytic levels (0.1%) of aromatic sulfonate salts,
sodium and potassium, changes the mechanism of thermal degradation ; oxygen transfers to the ortho position forming ketones (Fries rearrangement). The end result is crosslinking.
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TGA data show that the sulfonate salt accelerates the decomposition
and lowers the temperature at which carbon monoxide evolves. Aromatic sulfonates are not effective in polycarbonate/ABS blends; the
ABS lowers the decomposition temperature of the polycarbonate.
INORGANIC METAL HYDRATES
metal
hydrates decompose endothermically, giving up
off-gases. Major applications include unsaturated
and
polyester
polyethylene. They are used at concentrations of about
65%. They contain a high concentration of bound water and as a result
of the endothermic decomposition, the polymer is cooled, delaying ignition. The water vapor liberated also has a diluting effect in the gas
phase and forms an oxygen displacing protective layer over the condensed phase. Furthermore, at the high concentrations used, the
amount of fuel is decreased significantly and the less fuel the lower the
heat release and smoke. The oxide that forms can also provide for a protective layer especially if it combines with other additives.
Alumina trihydrate has 34.6% bound water and decomposes at about
205°C. Because of its low temperature thermal stability, it is used
mainly in thermoset polymers and in those thermoplastics which can
be processed at low temperatures (Table 4). Magnesium hydroxide has
Inorganic
water to dilute the
31% bound water and is stable to about 320 ° C. As a result, it can be
used in thermoplastics which are processed at a high temperature.
Magnesium carbonate is another metal hydrate and it is compared
with the other two. Other hydrates include boric acid, zinc borate, and
zinc hydrostannate.
Table 4.
Properties of inorganic
metal
hydrates.
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BORON COMPOUNDS
Mixtures of borax and boric acid are used as flame retardants for cellulose. Boric acid decomposes endothermically, releasing water in two
stages first at 130-200°C to form HBO, and at about 265 ° C to B103The mixture dissolves on heating in its own water of hydration, froths,
and finally fuses to a surface coating. Similar to the phosphoric acids
resulting from phosphate esters, boric acid dehydrates oxygencontaining polymers yielding char. The glassy coating and the char
protect the substrate from oxygen and heat. Borates are also known to
be glow inhibitors.
Zinc borate is used in polyvinyl chloride to replace in part antimony
oxide. The hydrogen chloride generated from the PVC reacts with the
zinc borate.
SMOKE SUPPRESSANTS
Polymers which unzip to monomer on heating, such as polyacrylates
and polyformaldehyde, burn cleanly and give very little smoke. Polyolefins decompose to hydrocarbons and burn with some smoke. Aromatic
polymers such as styrenics or polymers which decompose and rearrange
to aromatic decomposition products, such as PVC, are very smoky.
A number of methods to reduce smoke are listed in Table 5. If the
polymer is loaded with fillers which do not burn, then the smoke will
be accordingly reduced. However, for a substantial reduction, the
Table 5. Methods of smoke
suppression.
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polymer must be filled with a high concentration of filler which will
degrade the polymer properties. Polyvinylidine chloride smokes to a
much lesser extent than PVC presumably because it degrades by a different mechanism. PVC pyrolyzes to form volatile aromatics such as
benzene which burn with considerable smoke. The literature is full of
references to catalytically changing the mode of decomposition. These
methods are mainly effective with PVC and there has been little success in reducing the smoke liberation of other polymers.
Polyvinyl chloride (PVC) which contains 57% chlorine is inherently
flame resistant. When it is forced to burn, it behaves like most organic
materials and evolves smoke. Burning PVC forms volatile aromatics
such as benzene which burn with considerable smoke.
Certain metals and metal salts acting as Lewis catalysts (form
chloride salts) can alter the mode of decomposition and promote crosslinking. On continued heating, the crosslinked polymer forms char.
The most effective smoke suppressants for polyvinyl chloride are compounds of transition metals. They function because they either are or
are converted to Lewis acids. Essentially all of the effective metal-based
smoke suppressants appear to work in the solid state, not in the vapor
state. These compounds have the ability to alter the mode of decomposition of PVC, promoting crosslinking, and greatly reduce the yield of
aromatic hydrocarbons during pyrolysis in either an inert atmosphere
or in air. On continued heating, the crosslinked polymer forms char.
There is a direct relationship between char formation and smoke reduction and the reduced yield of aromatic hydrocarbons [29].
Zinc borate is claimed as a smoke suppressant for rigid and plasticized polyvinyl chloride. The hydrogen chloride generated from the
PVC reacts with the zinc borate to form non-volatile zinc chloride and
oxychloride as well as volatile boron trichloride and boric acid. The zinc
chloride is a Lewis acid which has been shown to promote crosslinking
and char formation. The boron trichloride is also an effective Lewis
acid.
Many
metal
compounds
available oxidation state
in which the metal is found in its
highest
effective smoke suppressants for PVC.
They include molybdenum and tin compounds. Such metal compounds
or the metal chlorides formed from them in situ are known to be potent
Lewis acids. Though highly effective in small fires, Lewis acids tends to
lose their utility when enthalpy inputs are high, because they also promote cracking chemistry with the formation of volatile flammable hyare
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drocarbons that burn readily. Hence, those Lewis acids that are the
most effective in small fires may prove to be of minimal value in fires
where enthalpy inputs are excessive.
Copper compounds are very effective smoke suppressants for PVC. It
has been proposed that crosslinking by reductive coupling takes place
with resultant crosslinking [30]. Where RCI is PVC and Me is a metal,
one can show the coupling as follows:
The patent literature reports a large number of smoke retarding
systems which consist of a combination of two or more chemical comcombination include cuprous oxide with molybdenum trioxide and zinc borate with alumina trihydrate. A combination recommended for low smoke and reduced flammability wire and
cable compound consists of 11 phr alumina trihydrate, 2 phr antimony
oxide, 4 phr zinc borate, and 3 phr ammonium molybdate.
Molybdenum oxide (MoO3) when combined with some compounds of
copper, iron, and tin can form synergistic systems for reducing smoke
from rigid PVC. The nature of the synergism is unknown. Magnesium
oxide, hydroxide, and carbonate show smoke retarding ability in PVC.
The activity of the basic magnesium compounds may be related to the
ease with which they can react with HCI and water at low temperature
to form oxychlorides [31]. Tin compounds have been proposed as smoke
suppressants. Zinc stannate and hydroxystannate are suggested smoke
pounds. Synergistic
suppressants.
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