Flame retardant polyphenylene oxide thermoplastics

Thermoplastic flame retardant compositions of polyphenylene oxide and styrene are provided exhibiting superior resistance to stress cracking.

CROSS REFERENCE TO RELATED APPLICATIONS 
Reference is made to my copending application Ser. No. 651,542, filed Sept. 
17, 1984, now U.S. Pat. No. 4,593,058, for Flame Retardant Polyphenylene 
Oxide Thermoplastics, assigned to the same assignee as the present 
invention and incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
Prior to the present invention, as shown by Cizek U.S. Pat. No. 3,383,435, 
assigned to the same assignee as the present invention and incorporated 
herein by reference, thermoplastic resin compositions were provided 
comprising a polyphenylene ether and a styrene resin. Other thermoplastic 
compositions are shown by Izawa et al., U.S. Pat. Nos. 3,929,930 and 
3,929,931, incorporated herein by reference, which are polyphenylene ether 
having polystyrene grafted onto the backbone. Although these materials in 
the form of shaped articles have high impact strength, stiffness, good 
surface appearance, heat resistance and other desirable properties during 
or after molding, a major shortcoming of these materials as thermoplastics 
resins is their normally flammable nature. As a result, flame retardant 
and/or drip retardant agents are commonly incorporated into such blends of 
polyphenylene ethers and styrene resin prior to molding. 
The flammability of normally flammable thermoplastic polymers have been 
reduced by using antimony-, halogen-, phosphorous- or nitrogen-containing 
additives commonly referred to as flame retardant agents. For example, 
aromatic phosphates such as triphenylphosphate or a combination of such 
compounds with other compounds such as halogenated aromatics have been 
added as flame retardant agents as shown by Haas, U.S. Pat. No. 3,639,506. 
Experience has shown, however, that although these phosphate compounds 
have been found to impart good flame resistant properties to such 
polyphenylene ether thermoplastic blends, physical properties of the 
molded blends such as the heat distortion temperature (HDT) are often 
adversely affected. 
In my copending application Ser. No. 651,542, there is taught that the 
addition of a small amount of an organothiophosphate, for example, a 
triarylthiophosphate, to polyphenylene oxide-polystyrene resin blends, or 
grafted copolymers thereof, significantly reduces the flammability of the 
resulting molded thermoplastic materials. However, it has been found that 
even though improved flame retardant properties can be achieved by using 
an effective amount of an organothiophosphate in polyphenylene oxide 
blends, the resulting flame retardant compositions exhibit a significant 
degree of stress cracking after heating aging the molded composition. As 
defined hereinafter, the term "stress cracking" means the formation of 
small hairline cracks along the edges of a molded thermoplastic part. It 
results from stresses created due to variations in the cooling rate of 
different areas of the molded part after the part is released from a mold. 
In measuring stress cracking, the thermoplastic is molded to a 
22".times.73/4".times.2" tub shaped container having 1/8" thick walls. The 
presence of small cracks in centimeters near the edges is determined by 
optical microscopy. 
As shown by Lohmeijer, U.S. Pat. No. 4,529,761, polyphenylene oxide 
compositions, having both improved flame retardant and stress cracking 
resistance, can be obtained by using aromatic phosphates, such as 
triphenylphosphate, with a mixture of certain alkali metal salts of alkyl 
sulfonates. Although improved stress cracking can be achieved utilizing 
the aforementioned combination, the resulting flame retardant 
polyphenylene oxide composition nevertheless shows some degree of stress 
cracking. It would be desirable to obtain flame retardant polyphenylene 
oxide compositions substantially free of stress cracking. 
The present invention is based on my discovery that the addition of a small 
amount of an alkali metal salt of a C.sub.12 to C.sub.18 alkyl sulfonate 
having the formula, 
EQU (RSO.sub.3).sub.n M, (1) 
where R is an aliphatic radical, and preferably a polymethylene radical 
having from 12 to 18 carbon atoms, and n is an integer equal to 1 or 2, 
and when n is 1, M is sodium or potassium, and when n is 2, M is calcium 
or magnesium. These sulfonate salts, referred to hereinafter as "metal 
alkyl sulfonate" or "metal alkyl sulfonates" when used in combination with 
an effective amount of triarylthiophosphate in polyphenylene oxide-styrene 
blends or graft copolymers thereof can provide high performance 
thermoplastic molding compositions. The resulting compositions exhibit 
improved flame retardance and are substantially free of stress cracking as 
compared to moldable polyphenylene oxide styrene resin compositions of the 
prior art. 
STATEMENT OF THE INVENTION 
There is provided by the present invention, flame retardant thermoplastic 
molding compositions of polyphenylene oxide and styrene resin, or graft 
copolymers thereof, comprising by weight 
(A) 100 parts of polyphenylene oxide 
(B) 20 to 300 parts of styrene resin, 
(C) an amount of organothiophosphate having the formula, 
##STR1## 
which is sufficient to provide from 0.5% to 5% by weight of phosphorous 
based on the weight of the flame retardant thermoplastic composition and 
(D) an effective amount of metal alkyl sulfonate, 
where R.sup.1 is a C.sub.(1-13) monovalent hydrocarbon radical or 
substituted C.sub.(1-13) monovalent hydrocarbon radical, Q is a monovalent 
group selected from --OR.sup.1 and 
##STR2## 
and R.sup.2 is selected from divalent C.sub.(2-20) hydrocarbon radicals 
and substituted divalent C.sub.(2-20) hydrocarbon radicals. 
Radicals included within R.sup.1 of formula (1) are, for example, 
C.sub.(1-8) alkyl radicals such as methyl, ethyl, propyl, butyl, pentyl, 
hexyl, etc.; C.sub.(6-13) aryl radicals such as phenyl, tolyl, xylyl, 
naphthyl, and C.sub.(1-8) alkoxy and halogenated derivatives of such aryl 
radicals. Radicals included within R.sup.2 are, for example, alkylene 
radicals such as dimethylene, trimethylene, tetramethylene, hexamethylene 
and branched alkylene radicals; arylene radicals such as phenylene, 
toluene, xylylene, and divalent radicals having the formula 
EQU --R.sup.3 (X).sub.a X.sup.1 -- 
where R.sup.3 is a divalent C.sub.(6-13) arylene radical, X is a divalent 
radical selected from O, S and C.sub.y H.sub.2y, X.sup.1 is selected from 
R.sup.3 radicals, y is a whole number from 1 to 5 inclusive, a is 0 or 1, 
and when a is 0, the sum of the carbon atoms in R.sup.3 and X.sup.1 is the 
same as R.sup.3. 
Some of the organothiophosphates which are included within formula (1) are, 
for example, triphenyl thiophosphate, tri o-cresyl thiophosphate, tri 
m-cresyl thiophosphate, tri p-cresyl thiophosphate, trixylyl 
thiophosphates, tris-trimethylphenyl thiophosphates, trimethyl 
thiophosphate, triporpyl thiophosphate, phenyldimethyl thiophosphate, 
tri-p-chlorophenyl thiophosphate and the like. 
The polyphenylene ether or polyphenylene oxide resin which can be used in 
the practice of the present invention is shown by the following formula 
##STR3## 
where R.sup.4 is a monovalent radical selected from the class consisting 
of hydrogen, halogen, hydrocarbon radicals free of a tertiary 
.alpha.-carbon atom, halogenated hydrocarbon radicals having at least two 
carbon atoms between the halogen atom and the phenyl nucleus, 
hydrocarbonoxy radicals and halogenated hydrocarbonoxy radicals having at 
least two carbon atoms between the halogen atom and the phenyl and b is an 
integer having a value of at least 50. 
A more preferred class of polyphenylene ether resins for the compositions 
of the present invention include those of formula (3) where R.sup.4 is 
alkyl and, most preferably, having from 1 to 4 carbon atoms. 
Illustratively, members of this class include 
poly(2,6-dimethyl-1,4-phenylene)ether; 
poly(2,6-diethyl-1,4-phenylene)ether; 
poly(2methyl-6-ethyl-4-phenylene)ether; 
poly(2-methyl-6-propyl-1-4-phenylene)ether; 
poly(2,6-dipropyl-1,4-phenylene)ether; 
poly(2-ethyl-6-propyl-1,4-phenylene)ether; and the like. 
Especially preferred is poly(2,6-dimethyl-1,4-phenylene)ether, preferably, 
having an intrinsic viscosity of about 0.45 deciliters per gram (dl./g.) 
as measured in chloroform at 30.degree. C. 
The preparation of the polyphenylene ether resins is described in Hay, U.S. 
Pat. Nos. 3,306,874 and 3,306,875 and in Stamatoff, U.S. Pat. Nos. 
3,257,357 and 3,257,358, which are incorporated herein by reference. 
The preferred styrene resins are those having at least 25% by weight of 
repeat units derived from a vinyl aromatic compound of the formula: 
##STR4## 
where R.sup.5 is selected from hydrogen, C.sub.(1-5) alkyl and halogen, Z 
is selected from vinyl, hydrogen, halogen and C.sub.(1-8) alkyl, and p is 
a whole number equal to 0 to 5 inclusive. 
The term "styrene resins" is used broadly to define components fully 
described in Cizek, U.S. Pat. No. 3,383,435, the disclosure of which is 
incorporated herein by reference Such resins include homopolymers, such as 
polystyrene, polychlorostyrene and polyvinyl toluene, the modified 
polystyrenes such as rubber modified polystyrene blended or grafted high 
impact products, e.g., the rubber being a polybutadiene or an elastomeric 
copolymer of styrene and a diene monomer. Also included are styrene 
containing copolymers, such as styrene-acrylonitrile copolymers (SAN), 
styrene-butadiene copolymers, styrene-acrylonitrilebutadiene terpolymers 
(ABS), styrene-maleic anhydride copolymers, polyalpha-methylstyrene, 
copolymers of ethyl vinyl benzene and divinylbenzene, and the like. In 
instances where the method of Izawa et al. is used to make thermoplastic 
materials, U.S. Pat. Nos. 3,929,930 and 3,929,931, polyphenylene oxide is 
heated with a styrene monomer shown by formula 4 in the presence of a free 
radical initiator resulting in a graft polyphenylene ether polystyrene 
copolymer. 
Special mention is made of a preferred class of styrene containing resins. 
These are known as "HIPS" resins, for high impact polystyrenes, in which 
the impact modifier comprises one or more of an ethylene/propylene/diene 
terpolymer, or a hydrogenated derivative, a vinylaromatic/diene block 
copolymer resin, or a hydrogenated derivative, a hydrogen saturated 
vinylaromatic/diene random copolymer, a radial teleblock copolymer of a 
vinyl aromatic compound and a diene, a vinyl aromatic/methacrylic or 
acrylic acid or ester/diene terpolymer, and the like. These specialty HIPS 
resins are commercially available or can be prepared by those skilled in 
this art. 
The compositions of the invention can also further include reinforcing 
agents, preferably fibrous glass reinforcements, alone or in combination 
with non-glass reinforcing fillers. The fibrous glass is especially 
preferably fibrous glass filaments comprised of lime-aluminum borosilicate 
glass which is relatively soda free, known as "E" glass. However, other 
glasses can be used such as low soda glass or "C" glass. The filaments are 
made by standard processes, e.g., by steam or air blowing, flame blowing 
and mechanical pulling. The preferred filaments for plastics reinforcement 
can be made by mechanical pulling having filament diameters range from 
about 0.000112 to 0.00075 inch. 
It is preferred to use sized filamentous glass reinforcement at from about 
1 to about 80% by weight, based on the combined weight of glass and 
polymer and preferably, from about 10 to about 50% by weight. Especially 
preferable, is where glass comprises from about 10 to about 40% by weight, 
based on the combined weight of glass and resin. Generally, for direct 
molding use, up to about 50% of glass can be present without causing flow 
problems. However, it is useful also to prepare the compositions 
containing substantially greater quantities, e.g., up to 70 to 80% by 
weight of glass. 
Other ingredients, such as stabilizers, pigments, plasticizers, 
antioxidants, and the like, can be added for their conventionally employed 
purposes. 
An effective amount of the alkali metal sulfonate as used in the practice 
of the invention is that amount sufficient to provide from 0.2 to 10 parts 
of metal alkyl sulfonate, and preferably 0.2 to 3 parts, per 100 parts of 
polyphenylene oxide, while maintaining sufficient organothiophosphate to 
provide from 0.5% to 5% by weight of phosphorus based on the weight of 
thermoplastic molding composition. 
The manner in which the present compositions are prepared is not critical 
and conventional methods can be employed. Preferably, however, each of the 
ingredients can be as part of a blend premix, and the latter passed 
through an extruder, e.g., a 28 mm. WP twin screw extruder, at an 
extrusion temperature of from about 500.degree. to about 600.degree. F., 
dependent on the needs of the particular composition. The strands emerging 
from the extruder can be cooled, chopped into pellets and molded to any 
desired shape. 
In order to allow those skilled in the art to better practice the present 
invention, the following example is given by way of illustration and not 
by way of limitation. All parts are by weight.

EXAMPLE 
Several moldable blends were prepared consisting of 50 parts polyphenylene 
oxide having an intrinsic viscosity of about 0.45 deciliters per gram 
(dl/g) as measured in chloroform about 30.degree. C., 50 parts of 
Foster-Grant high impact polystyrene and 15 parts of one of several flame 
retardants. Each blend was then divided into two equal parts. Into one of 
the blend series was added 2% by of weight Hostastat antistatic additive 
(a sodium C.sub.12 -C.sub.18 alkyl sulfonate salt). The blends were 
separately added to a Henchel mixer and thoroughly mixed for 5 minutes. 
The respective mixtures were then extruded at 300.degree. C. using a 
Werner and Pfleiderer extruder. The resin strands which were obtained were 
chopped into 1/8" by 1/8" pellets which were molded in a 5".times.1/2" 
bars in accordance with Underwriters Laboratories Bulletin No. 94 
requiring a flame out time (FOT) of less than five seconds. The various 
blends were then molded at 250.degree. C. and evaluated for flame 
retardancy. 
The flame retardants used included K-50 of the FMC Corporation, or 
Kronitex-50 which is the product obtained by distilling the reaction 
product of POCl.sub.3 and a mixture of isopropylphenol and phenol. 
Stress cracking was determined by the following procedure: 
There were molded 1/8" by 1/8" pellets into a 22" by 73/4" by 2" tub with 
1/8" thick walls on an Engel 700 Ton molding machine at 
525.degree.-540.degree. F. A total of five tubs were evaluated for each 
formulation. The tubs were placed in an oven at 170.degree. F. for 24 
hours to heat age the parts. After heat aging, the parts were examined for 
stress cracking. The total length of cracks in centimeters for each of the 
five parts were measured. 
The stress cracking results were shown in the following Table, where the 
figures are in parts by weight: 
______________________________________ 
Poly- Extent of.sup.c 
phenylene 
Poly- Flame Hostastat.sup.b 
Crack 
Oxide styrene Retardant Antistat 
(cm) 
______________________________________ 
50 50 15 None 32.3 .+-. 8.8 
(Kronitex-50).sup.a 
50 50 15 2 24.7 .+-. 2.5 
(Kronitex-50).sup.a 
50 50 15 None 38.8 .+-. 4.8 
(triphenyl- 
phosphate) 
50 50 15 2 0.1 .+-. 0.1 
(triphenyl- 
phosphate) 
50 50 15 None 8.4 .+-. 2.4 
(triphenyl- 
thiophosphate) 
50 50 15 2 0.0 .+-. 0.0 
(triphenyl- 
thiophosphate) 
55 45 15 2 0.0 .+-. 0.0 
(triphenyl- 
thiophosphate) 
______________________________________ 
.sup.a TM of FMC Corporation 
.sup.b Hostastat is a .TM. of American Hoechst Company for C.sub.12 to 
C.sub.18 Sodium Sulfonate. 
.sup.c Extent of crack is the average total edge crack measured for five 
test samples. 
The above results show that the molding compositions having the 
triphenylthiophosphate and alkali alkyl sulfonate exhibit the best stress 
cracking resistance as compared to the compositions of the prior art. It 
was further found that the polyphenylene oxide compositions containing the 
triphenylthiophosphate flame retardant exhibited superior UL-94 flame 
retardance and F.O.T. (sec) as compared to the prior art compositions. 
Although the above example is directed to only a few of the very many 
variables which can be utilized in the practice of the present invention, 
it should be understood that the present invention is directed to a much 
broader variety of molding compositions, polyphenylene oxide and styrene 
resin utilizing triphenylthiophosphate and metal alkyl sulfonate salts to 
impart improved flame retardance and stress cracking resistance thereto.