Note: Descriptions are shown in the official language in which they were submitted.
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Description
EXPANDABLE GRAPHITE AS A FLAME RETARDANT
IN UNSATURATED POLYESTER RESINS
Technical Field
The present invention relates to flame retardant compositions, useful for
providing
protection from fire to a substrate, such as fibreboard or strandboard. More
specifically, the
present invention relates to unsaturated polyester resin intumescent
compositions containing
particles of expandable graphite. The flammability of the crosslinked
polyester resin is
reduced by the addition of particles of expandable graphite, even at levels as
low as 7 pph.
The expandable graphite is particularly effective when used in conjunction
with ammonium
polyphosphate or a halogen compound as a synergist.
Background Art
Chemical intumescent systems have been used as flame retardants for over 50
years.
Typically based on phosphates, melamine, and pentaerythritol, these
intumescents rely on
heat-induced decomposition to generate a char layer that insulates the
substrate from the heat
source. As such, they are most useful as coatings for flammable materials.
When added into
many materials, however, the expansive force of chemical intumescents is often
insufficient
to generate an effective char layer. For example, it has been found that many
thermoset
phenolic or unsaturated polyester resins cannot be protected by the
incorporation of chemical
intumescents.
Expandable graphite flake is now being used in a growing number of
applications as
an intumescent fire retardant additive, as an expansive agent, and as a smoke
suppressant.
Ford et al. describe the use of expandable graphite in oriented strandboard
panels to reduce
the flame spread in U.S. Patent 5,443,894. Hutchings et al. developed an
especially effective
intumescent coating containing expandable graphite that reduced the flame
spread and smoke
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of highly combustible fibreboard panels, as described in a paper titled
"Expandable Graphite
Flake As An Additive For A New Flame Retardant Resin," presented at the Fire
Retardant
Chemicals Association Fall Meeting, Naples, Florida, pp. 137-146 (October
1996). Many
intumescent putties, caulks and firestop systems rely on expandable graphite
to provide the
expansive force necessary to close off gaps and holes during the course of a
fire. Expandable
graphite is also used in various polyurethane foams to pass the rigorous tests
required of
aircraft seating and construction insulation, as described in U.K. Patent
Application
2,168,706.
The fire retardant properties of a number of thermoplastic polymers with
expandable
graphite and synergists was described by Okisaki in "Flamecut GREP Series New
Non-
Halogenated Flame Retardant Systems," presented at the Fire Retardant
Chemicals
Association Spring Meeting, San Francisco, California, pp. 11-24 (March 1997).
For many applications, there is a need for unsaturated polyester resins (UPRs)
to be
fire retardant or self extinguishing. Fibre-reinforced plastic (FRP) materials
containing
UPRs are of particular concern, since this material is often used in critical
applications such
as aircraft, shipbuilding, and building construction.
The fire retardancy of UPRs has been recognized in the past. Chlorine and
bromine
compounds (e.g. HET acid, tetrachlorophthalic anhydride, tetrabromophthalic
anhydride,
dibromoneopentyl glycol) can be built into the UPR molecule, or added to the
UPR directly
as pentabromoethylbenzene or various chloroparaffins. The mechanism of fire
retardancy
involves the formation of Cl or Br radicals that terminate the radical chain
reaction of flame
propagation. Antimony trioxide is often used to enhance the fire retardant
efficiency of
halogen compounds. However, the use of halogenated compounds and antimony
oxide has
come under some criticism due to the possible formation of toxic or corrosive
combustion
products.
Aluminum trihydroxide Al(OH)3 and magnesium hydroxide Mg(OH)2 exhibit a
different mechanism of fire retardancy. Both materials evolve water at high
temperatures,
thus decreasing the temperature of the substrate and diluting the combustible
pyrolysis gases.
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The primary disadvantage of these fire retardants is that they require
relatively high loadings
to be effective. This makes impregnation of the fiber reinforced composite
more difficult
while reducing its final physical properties. The metal hydroxides do have an
important
advantage in that they can significantly decrease the evolution of smoke.
A still different mechanism of fire retardancy is observed when red phosphorus
and
phosphorus compounds (e.g. triethyl phosphate or ammonium polyphosphate) are
used. The
oxidation and pyrolysis of these compounds result in the formation of
polyphosphoric acid.
The acids tend to promote charring of the plastic during heating, which acts
to reduce
burning. Moreover, the char formation decreases the concentration of gaseous
hydrocarbons
due to pyrolysis. The concentration of phosphorus is usually much less than
that required for
the Al or Mg hydroxides. In fact, if the amount of the phosphorous-containing
fire retardants
is too large some properties of the UPRs may be adversely affected and
processing problems
may arise.
There have been many acidic metal compounds suggested for use as flame
retardants.
The fire retardant mechanism of these compounds may be similar to that of
phosphorus-
containing compounds. In addition to the well-known zinc borates, tin
compounds (zinc
stannate, ZnSn03 and zinc hydroxystannate, ZnSn(OH)6) have also been suggested
as
possible fire retardants and smoke suppressants.
As mentioned above, a flame retardant system must reduce the generation of
smoke
as well as suppress the flame propagation. In addition to A1 and Mg hydroxides
and the
borates and stannates, molybdenum trioxide (Mo03) is an efficient smoke
suppressant. The
effect of the various smoke suppressants depends on the temperature of
pyrolysis.
Combinations of various fire retardants can exhibit an elevated efficiency
when the
mechanism of fire retardancy is different. For example, halogen compounds form
synergistic
systems with Al(OH)3 or phosphorus compounds. Halogen-free systems containing
Al(OH)3
with red phosphorus or ammonium polyphosphate can be somewhat effective.
Phosphorus-
phosphorus synergism was found when triethyl phosphate was added together with
melamine
polyphosphate.
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What is desired, therefore, is an additive effective to increase the fire
retardancy of
compositions based on crosslinked polymers (e.g. unsaturated polyester
resins). The additive
should be capable of improving fire retardancy and related properties at
relatively low levels,
and act synergistically with other fire retardant compositions to improve the
fire retardancy
achieved.
Summary of the Invention
It is an object of the present invention to provide an unsaturated polyester
resin
having improved fire retardancy.
It is another object of the invention to provide an unsaturated resin having
particles of
expandable graphite as an intumescent additive.
It is yet another object of the present invention to provide a fire retardant
composition
comprising an unsaturated polyester resin having particles of expandable
graphite and a
synergistic fire retardancy-enhancing compound mixed therein.
These objects and others which will become apparent to the artisan upon review
of
the following description can be accomplished by providing a fire retardant
composition
which comprises an unsaturated polyester resin which includes as an additive
particles of
expandable graphite. The particles of expandable graphite are present in the
polyester resin
at levels of at least about 7.5 parts by weight of expandable graphite
particles per 100 parts
resin (phr) when the particles of expandable graphite are the only additive,
and at least 5 phr
when the expandable graphite particles are used in combination with a
synergistic additional
fire retardant compound.
Graphites are made up of layer planes of hexagonal arrays or networks of
carbon
atoms. These layer planes of hexagonally arranged carbon atoms are
substantially flat and
are oriented or ordered so as to be substantially parallel and equidistant to
one another. The
substantially flat, parallel equidistant sheets or layers of carbon atoms,
usually referred to as
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basal planes, are linked or bonded together and groups thereof are arranged in
crystallites.
Highly ordered graphites consist of crystallites of considerable size, the
crystallites being
highly aligned or oriented with respect to each other and having well ordered
carbon layers.
In other words, highly ordered graphites have a high degree of preferred
crystallite
orientation. Briefly, graphites may be characterized as laminated structures
of carbon, that is,
structures consisting of superposed layers or laminae of carbon atoms joined
together by
weak van der Waals forces. In considering the graphite structure, two axes or
directions are
usually noted, to wit, the "c" axis or direction and the "a" axes or
directions. For simplicity,
the "c" axis or direction may be considered as the direction perpendicular to
the carbon
layers. The "a" axes or directions may be considered as the directions
parallel to the carbon
layers (parallel to the planar direction of the crystal structure of the
graphite) or the directions
perpendicular to the "c" direction.
As noted above, the bonding forces holding the parallel layers of carbon atoms
together are only weak van der Waals forces. Graphites can be treated so that
upon the
application of heat, the spacing between the superposed carbon layers or
laminae can be
appreciably opened up so as to provide a marked expansion in the direction
perpendicular to
the layers, that is, in the "c" direction and thus form an expanded graphite
structure (also
referred to as exfoliated or intumesced graphite) in which the laminar
character of the carbon
layers is substantially retained. Upon exposure to a flame, the graphite
particles expand (or
exfoliate) to form a mechanical and insulative barrier to fire.
Brief Description of the Drawings
The present invention will be better understood and its advantages more
apparent in
view of the following detailed description, especially when read with
reference to the
appended Figure, which is a photomicrograph of a particle of expanded
graphite.
Detailed Description of the Invention
Expandable graphite is manufactured using natural crystalline graphite flake.
Deposits of crystalline graphite are numerous and found around the world,
usually as
inclusions in metamorphic rock, or in the silts and clays that result from
their erosion.
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Graphite is recovered from the ore by crushing and flotation, and is usually
beneficiated to
give graphite flake that is 95-98% carbon.
As discussed above, graphite is a crystalline form of carbon comprising atoms
covalently bonded in flat layered planes with weaker bonds between the planes.
By treating
particles of graphite, such as natural graphite flake, with an intercalant of,
e.g. a solution of
sulfuric and nitric acid, the crystal structure of the graphite reacts to form
a compound of
graphite and the intercalant. The treated particles of ~aphite are referred to
as "particles of
intercalated graphite." Upon exposure to high temperature, the particles of
intercalated
graphite expand in dimension as much as 80 or more times their original volume
in an
accordion-like fashion in the "c" direction, i. e. in the direction
perpendicular to the
crystalline planes of the graphite, thus the particles of intercalated
graphite can also be
referred to as particles of expandable graphite." Exfoliated graphite
particles are vermiform
in appearance, and are therefore commonly referred to as worms. The worms act
as a barrier
to fire, both mechanically and because of their insulative value.
A common method for manufacturing particles of expandable graphite is
described by
Shane et al. in U.S. Pat. No. 3,404,061, the disclosure of which is
incorporated herein by
reference. In the typical practice of the Shane et al. method, natural
graphite flakes are
intercalated by dispersing the flakes in a solution containing e.g., a mixture
of nitric and
sulfuric acid. The intercalation solution contains oxidizing and other
intercalating agents
known in the art. Examples include those containing oxidizing agents and
oxidizing
mixtures, such as solutions containing nitric acid, potassium chlorate,
chromic acid,
potassium permanganate, potassium chromate, potassium dichromate, perchloric
acid, and
the like, or mixtures, such as for example, concentrated nitric acid and
chlorate, chromic acid
and phosphoric acid, sulfuric acid and nitric acid, or mixtures of a strong
organic acid, e.g.
trifluoroacetic acid, and a strong oxidizing agent soluble in the organic
acid.
In a preferred embodiment, the intercalating agent is a solution of a mixture
of
sulfuric acid, or sulfuric acid and phosphoric acid, and an oxidizing agent
such as nitric acid,
perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide,
iodic or periodic
acids, or the like. Although less preferred, the intercalation solution may
contain metal
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halides such as ferric chloride, and ferric chloride mixed with sulfuric acid,
or a halide, such
as bromine as a solution of bromine and sulfuric acid or bromine in an organic
solvent.
After the flakes are intercalated, any excess solution is drained from the
flakes and the
flakes are water-washed. The quantity of intercalation solution retained on
the flakes after
draining may range from 20 to 150 parts of solution by weight per 100 parts by
weight of
graphite flakes (pph) and more typically about 50 to 120 pph. Alternatively,
the quantity of
the intercalation solution may be limited to between 10 to 50 parts of
solution per hundred
parts of graphite by weight (pph) which permits the washing step to be
eliminated as taught
and described in U.S. Pat. No. 4,895,713, the disclosure of which is also
herein incorporated
by reference.
When the intercalated graphite is exposed to heat or flame, such as in a fire,
the
inserted molecules decompose to generate gas. The gas forces apart the carbon
layers and
the graphite expands. The expanded graphite is a low density, non-burnable,
thermal
insulation, often referred to as a "worm" because of its curved shape
(Figure). For use in the
inventive composition, the particles of expandable graphite should preferably
expand at
temperatures no higher than about 300°C, preferably no higher than
about 250°C. Suitable
expandable graphites are commercially available from UCAR Graph-Tech, Inc. of
Lakewood, Ohio and include UCAR Graph-Tech's GRAFGUARD~ Grade 220 expandable
graphite flake and GRAFGUARD~ Grade 160 expandable graphite flake. The
principal
functional difference between the two grades is onset temperature, that is,
the temperature at
which expansion begins. For the GRAFGUARD~ Grade 220 expandable graphite
flake,
expansion begins at 220°C, whereas for GRAFGUARD~ Grade I60 expandable
graphite
flake, expansion begins at 160°C.
The unsaturated polyester resin (UPR) into which the particles of expandable
graphite
can be incorporated to form the inventive intumescent composition comprise
solutions of
unsaturated polyesters (UPs) and similar products in unsaturated monomers that
can be cured
by radical copolymerization. UPs are composed of glycol and dicarboxylic acid
units which
include unsaturated copolymerizable acid units and saturated or unsaturated
non-
copolymerizable acid units. The unsaturated copolymerizable acid units are
primarily
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derived from malefic acid and fumaric acid residues. The saturated or
unsaturated acid units
are primarily derived from orthophthalic acid, isophthalic acid, terephthalic
acid,
tetrahydrophthalic acid, endomethylenetetrahydrophthalic acid, hexachloro-
endomethylenetetrahydrophthalic acid, adipic acid, sebacic acid and
tetrabromophthalic acid.
A variety of glycol units can also form a part of the structure of UPs, namely
ethylene
glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol,
dipropylene glycol, 1,3-
propylene glycol, 1,4-butylene glycol, neopentyl glycol, dibromoneopentyl
glycol, 2-methyl-
1,3-propanediol, chloromethylethylene glycol, 2,2,4-trimethyl-1,3-pentanediol,
ethoxylated
bisphenol A, propoxylated bisphenol A, and 1,4-cyclohexanedimethanol.
The UP molecules contain terminal hydroxyl and carboxylic groups. They can
also
be terminated with dicyclopentadiene, i.e. with dihydrodicyclopentadienyl
ester groups.
The UPs that are chain terminated with brominated monofunctional units, e.g.
2,3-
dibromopropyl residues, are included also. Additionally, oligo (ethylene
terephthalate)
segments can be incorporated.
UPRs commonly contain mostly styrene as the monomer, although the styrene can
be
partially or entirely replaced by vinyl monomers, e.g. vinyltoluene or p-t-
butylstyrene,
acrylic monomers, e.g. methyl methacrylate, ethyl acrylate, 2-hydroxyethyl
methacrylate, 2-
hydroxypropyl methacrylate, ethylene glycol dimethacrylate, diethylene glycol
dimethacrylate or triethylene glycol dimethacrylate, and allyl esters, e.g.
diallyl phthalate,
isophthalate or terephthalate. Allyl ethers, e.g. trimethylolpropane monoallyl
ether or
trimethylolpropane diallyl ether, can also be used as one of the monomers or
built into the
molecule of an UP.
So-called vinyl ester resins are also UPRs and are cured and processed in the
same
way. Vinyl ester resins are solutions of unsaturated esters in the above
listed unsaturated
monomers, mostly styrene. The unsaturated esters are addition products of
methacrylic acid
to epoxy groups in epoxy resins, mostly bisphenol A diglycidyl ether type,
tetrabromobisphenol A diglycidyl ether and epoxynovolak resin. UPRs,
comprising vinyl
ester resins, are also used in urethanized form. The urethanization consists
of the reaction of
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a diisocyanate with a part of hydroxyl groups contained in an UP with the
maleate/fumarate
unsaturation or in the vinyl esters with the methacrylate unsaturation.
The above described UPRs are cured at ambient temperature using an initiator-
accelerator system, such as a hydroperoxide or a ketone peroxide with a cobalt
salt or a
vanadium compound, or benzoyl peroxide with a tetiary aromatic amine (e.g. N,N-
dimethylaniline or N,N-dimethyl-p-toluidine); or at elevated temperatures
using peroxy
compounds with an elevated decomposition temperature, e.g. benzoyl peroxide or
dicumyl
peroxide.
Fillers containing UPRs can also be used in the compositions according to this
invention, together with expandable graphite and optionally other fire
retardants. Powdered
fillers, e.g. chalk and talc, and fibrous reinforcing fillers, e.g. glass
fiber, juts and sizal can
also be included.
In forming the inventive flame retardant compositions, particles of expandable
graphite are incorporated into the UPR prior to curing. The graphite particles
are included in
an amount sufficient to exhibit flame retardancy as compared with the UPR
without graphite.
When incorporated without a synergistic agent, the particles of expandable
graphite should
be incorporated into the resin at a level of at least about 7 parts by weight
graphite per 100
parts by weight of UPR (pph), more preferably at least about 7.5 pph. Most
preferably, the
particles of expandable graphite are included in the resin composition at a
level of at least
about 10 pph. Although there is no strict upper limit of the amount of
graphite particles to be
included, generally, no more than about 25 pph should be included, in order to
maintain the
mechanical strength of the cured resin composition.
In addition to the particles of expandable graphite and UPRs, the inventive
flame
retardant compositions can also include other components that act
synergistically with the
graphite particles to create further flame retardancy for the inventive
composition. Such
synergistic compositions include ammonium phosphate type compositions like
ammonium
polyphosphate, which tends to form polyphosphoric acid on oxidation and
pyrolysis, and
promote charring which reduces burning, and halogen compounds such as chlorine
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compounds and bromine compounds like pentabromoethylbenzene, optionally with
an
antimony compound like antimony trioxide or antimony pentoxide. These
synergistic
compounds can be included at levels of at least about 2 pph, and generally not
more than
about 15 pph. When the synergistic compounds are included the minimum level of
graphite
particles can be reduced to about 4 pph in the inventive composition.
In use, the compositions of the present invention can be incorporated in or
coated on
the surface of the substrate to be protected, and then cured. For instance,
the inventive
compositions can be coated on strandboard or fibreboard, and provide enhanced
fire
retardancy to the surfaces; or, the particles of expandable graphite can be
incorporated in
glass fiber reinforced polyester resin (GRP) products, such as GRP products
made by the
hand-lay-up method, by spraying etc.
In another embodiment of the invention, particles of expandable graphite can
also be
added to unsaturated polyester-based resin binders with fibrous and powdered
fillers. The
best example of these are sheet molding compounds (SMCs) which consist of
UPR, chalk, cut glass fibers, a thickener, peroxide initiators, pigments and,
optionally, a low-
profile additive. Expandable graphite (optionally with other fire retardant
additives) may be
used to make the compression molded parts fire resistant. In such
applications, the
expandable graphite can be included at lower levels, as low as 5 parts by
weight per 100 parts
by weight of UPR or even as low as 3 parts by weight if used in conjunction
with a
synergistic agent like an ammonium phosphate type compound. An exemplary
formulation,
in parts by weight: general purpose UPR 100, chalk 120, tert-butyl perbenzoate
1.5, Mg0
(thickener) 4.5, zinc stearate 4.5, cut glass fiber 9.5, expandable graphite
particles S or, when
ammonium polyphosphate is present at 5 parts by weight, then expandable
graphite particles
can be present at 3 parts by weight.
The following examples are presented to further illustrate and explain the
present
invention and should not be viewed as limiting in any regard. Unless otherwise
indicated, all
parts and percentages are by weight, and are based on the weight of the
product at the
particular stage in processing indicated.
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Example I
The resin employed was Polimal 109 unsaturated polyester resin obtained from
Chem. Works "Organika-Sarzyna", Nowa Sarzyna, Poland. The resin is a general-
purpose,
halogen-free system made of malefic anhydride, phthalic anhydride, 1,2-
propylene glycol and
styrene. The cold curing system consisted of methylethylketone peroxide and
cobalt octoate.
The expandable graphite employed was GRAFGUARD~ 220-80N Expandable
Graphite Flake, obtained from UCAR Graph-Tech, Inc. of Lakewood, Ohio. As
noted
above, in the grade nomenclature, 220 refers to the temperature (in °C)
at which the graphite
begins to expand; in addition 80 refers to the mesh size of the particles (
177 micron or
larger), and N refers to a neutral flake.
The liquid UPR was mixed with the expandable graphite particles (denoted G),
at
differing levels, and 1 pph Aerosil 200 (to avoid sedimentation). The
components of the
curing system consisting of 1.5 pph methylethylketone peroxide (40%) and 0.4
pph cobalt
octoate solution (1% Co) were added, and the compositions were cast into 4 mm
x 10 mm x
100 mm steel molds. The curing was carried out by holding the cast pieces at
20°C for 24
hours, followed by a postcuring treatment at 80°C for 2 hours.
The fire performance of the bars was tested by heating the samples in a
horizontal
position using a gas flame for 60 seconds, then monitoring the self
extinguishing (afterflame)
time (and time to extinguish after application of flame) and the remaining non-
burnt length of
the bar (up to 80 mm). This test corresponds to the Polish Standard PN-82/C-
89023 [see ISO
1210:1992 (E) "Plastics-determination of the burning behaviour of horizontal
and vertical
specimens in contact with a small-flame ignition source"]. In addition, the
Limiting Oxygen
Index (LOI) was also determined.
The results are shown in Table I.
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Table I
Behavior in fire
Sample G, pph
No. AfterflameNon-burnt
Self extinguishing
time, length, LOI,
sec mm
1 - I 0 No 280 0 19.5
2 - I 5 No 365 0 21.6
3 - I 7.5 Yes 7 78
4 - I 10 Yes 0 80 23.3
- I 15 Yes 0 80 23.3
6 - I 25 Yes 0 80
7 - I 25 Yes 0 80
8 - I 30 Yes 0 79
At some of the lower loading levels of G there was observed some "sparking" of
the
graphite during expansion. Presumably, this was the result of the graphite
expanding through
the rigid polyester, causing some small particles of the resin to be ejected
from the sample.
In a few cases, the sparks were hot enough to ignite a sheet of filter paper
placed 20 cm
below the burning bar.
Example II
Phosphorus in the form of ammonium polyphosphate (APP) was also included in
the
UPR composition, along with expandable graphite, in the amounts indicated.
The results are shown in Table II.
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Table II
Fire Behavior
retardant, in fire
pph
Sample
No. G APP Self AfterflameNon-
extinguishintime, burnt
sec
g length,
mm
2 - I 5 0 No 365 0
1 - IV 5 5 No 13 73
2 - IV 5 10 Yes 10 75
3 - N 5 15 Yes 0 80
4 - IV 10 0 Yes 0 80
4 - IV 10 5 , Yes 0 80
- IV 10 10 Yes 0 80
6 - IV 10 15 Yes 0 80
7 - IV 0 15 No 40 55
Whereas APP by itself at 15 phr was not self extinguishing and had an
afterflame
time of 40 seconds, the addition of 5 phr G gave immediate self extinguishing.
No sparks
were observed at this concentration of additives. In addition, at a loading
level of 10 phr G +
phr APP, the amount of smoke produced was reduced significantly. Thus, the
combination of G and APP is distinctly synergistic in this unsaturated
polyester resin system.
Example III
Pentabromoethylbenzene [PBEB] with antimony trioxide (Sb203, ATO) were also
included in the UPR/expandable graphite composition in the amounts shown
below.
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The results are shown in Table III.
Table III
Fire Behavior
retardant, in Fire
pph
Sample G PBEB ATO Self AfterflameNon-burnt
I No. extinguishingtime, sec length,
mm
1 - I 5 0 0 No 36~ 0
1 - V 5 3 2 Yes 0 74
2 - V 5 5 , 2.5 Yes 0 78
3 - V 5 10 3 Yes 7 80
It is apparent that the self extinguishing properties appear at a relatively
low
concentration of the fire retardants. Zero burning time was found at loadings
as low as 5 phr
G and 3 phr PBEB with 2 phr ATO.
Expandable graphite flake added to unsaturated polyester resin in the amount
of 7 pph
or more makes the resin self extinguishing. The use of expandable graphite
with ammonium
polyphosphate further improves the fire retardant behaviour of UPR: G added at
5 phr with
15 phr APP inhibits burning of the sample while eliminating any afterflame
(immediate self
extinguishing). Moreover, G and APP together suppress the formation of smoke.
Good fire
retardancy can also be obtained by adding expandable graphite to a brominated
fire retardant
and antimony trioxide. Sparks generated by pieces of the rigid resin ejected
by the expansion
of the graphite are eliminated with APP or (PBEB + ATO) when applied in a
proper ratio.
The above description is intended to enable the person skilled in the art to
practice the
invention. It is not intended to detail all of the possible variations and
modifications which
will become apparent to the skilled worker upon reading the description. It is
intended,
however, that all such modifications and variations be included within the
scope of the
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invention which is defined by the following claims. The claims are intended to
cover the
indicated elements and steps in any arrangement or sequence which is effective
to meet the
objectives intended for the invention, unless the context specifically
indicates the contrary.
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