CA2029533A1 - Thermally-stabilized thermoplastic compositions which contain polyphenylene ether and conjugated diene polymers - Google Patents

Abstract

ABSTRACT

An improved thermoplastic composition is described herein, comprising polyphenylene ether resin, at least one conjugated diene polymer or blend containing such a polymer, and an effective amount of an organic or inorganic nitrite material, Such compositions exhibit excellent thermal stability during blending or other processing operations. Also described herein is an improved method for preparing thermally stable compositions based on polyphenylene ethers and diene materials.

Description

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THERMALLY-STABILIZED THERMOPLASTIC COMPOSITIONS WHICH
CONTAIN POLYPHENYLENE ETHER AND CONJUGATED DIENE POLYMERS
This invention relates generally to thermoplastic polymers, and more particularly to blends of polyphenylene ethers and diene polymers which are processed under high-temperature conditions.

Polyphenylene ethers (often alterna~ively referred to as "PPE" or "polyphenylene oxides") are a well-known class of engineering resins which exhibit a highly desirable combination of various properties. Examples of these properties are toughness, impact strength, heat resistance, hydrolytic stability, and dielectric characteristics. Some of these properties are enhanced by the presence of elastomeric materials, such as natural or synthetic rubbers, or mixtures which contain such rubbers.

Methods for processing PPE resin blends are familiar to those with some skill in the art. Regardless of the particular method utilized, processing temperatures often rise above about 200 C. These temperatures sometimes adversely affect the final proper~ies of the PPE composi-tion, especially when the composition includes some of the elastomeric materials mentioned above. The major property so affected is impact strength, thereby making these types of compositions somewhat less attractive for use in applications where high impact resistance is a requirement, such as machine housings and various automotive parts.

It is thus clear that a need exists for a PPE-based composition which exhibits the excel~ent properties described above, such as impact strength, and which retains those properties after being processed at high temperatures.

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-~ - 2 - 8CN-8427 2~2~5~,7 SUMMARY OF THE INVENTION

The requirements discussed above have been satisfied by the present invention~ a thermoplastic composition comprising:
a) polyphenylene ether resin;

b) at least one conjugated diene polymer or blend containing such polymer; and c) about 0.005% by weight to about 1% by weight of a nitrite material selected from the group consisting of metal nitrites, mixtures of metal nitrites, organic nitrites, and mixtures of organic nitrites. The amount of nitrite material is based on the combined weight of components (a) and (b).

The me~al nitrite or mixtures thereof which are suitable for this invention have a melting point below about 325 C.

Included within the scope of this invention i 5 an improved method for preparing thermally-stable compositions comprising polyphenylene ethers and conjuga~ed diene polymers. In one embodiment, the PPE is dissolved in an organic solvent solution and combined with a diene polymer 25 solution in the presence of a nitrite material of the type described above. This mixture is then devolatilized and extruded to yield the desired compositions.

DETAILED DESCRIPTIQN OF THE INVENTION

PPE resins suitable for use in ~he compositions of this invention are gener~lly well-known in the art. Many of them are described in U.S. patents 3,306,874; 3,306,875; and ~ , 2~Z~

3,432,469 of Allan Hay; U.S. patents 3,257,357 and 3,257,358 of Gelu Stamatoff; and U.S. patent 4,806,602 of Dwain M.
White et al., all incorporated herein by reference. Many PPE resins are also described in two applications for B.
Brown et al., serial numbers 210,547 and 210,266, both filed on June 23, 1988 by the assignee of the present invention and incorporated herein by reference. Both homopolymer and copolymer polyphenylene ethers are within the scope of this invention.

The preferred PPE resins are homo- and copolymers which comprise a plurality of structural units of the formula wherein each Q1 is independently halogen, primary or secondary lower alkyl, phenyl, haloalkyl, aminoalkyl, hydrocarbonoxy, or halohydro-carbonoxy, wherein at least two carbon atoms separate the halogen and oxygen atoms; and each Q2 jS independently hydrogen, halogen, primary or secondary lower alkyl, phenyl, haloalkyl, hydrocarbonoxy or halohydro-20 carbonoxy as defined for Q1.

Examples of suitable primary lower alkyl groups aremethyl, ethyl, n-propyl, n-butyl, isobutyl, n-amyl, isoamyl, 2-methylbutyl, n-hexyl, 2,3-dimethylbutyl, 2-, 3- or ~5 4-methylpentyl and the corresponding heptyl groups. Examples of secondary lower alkyl groups are isopropyl, sec-butyl and 3-pentyl. Preferably, any alkyl radicals are straight chain, rather than branched. Most oftenl each Ql is alkyl or phenyl, especially C._4 alkyl, and each Q2 is hydrogen.

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- 4 - 8CN-g427 2~2~

Especially preferred polyphenylene ethers will be comprised of units derived from 2,6-dimethyl phenol, wherein the resulting polymer is commonly known as poly(2,6-dimethyl -1,4-phenylene ether). Also contemplated are PPE copolymers 5 comprised of units derived from 2,5-dimethyl phenol and 2,3,6-trimethyl phenol.

The conjugated diene polymers suitable for the present invention are well-known in the art, and are usually derived from a monomer containing about 4 to about 15 carbon atoms.
Examples of such monomers are 1,3-butadiene; isoprene;
piperylene; 2,3-dimethyl-1,3-butadiene; 2-methyl-1,3-hexadiene;
1,3-octadiene; 1,3-dodecadiene; 2,5-dimethyl-1,3-decadiene;
4,5-diethyl-1,3-octadiene. A mixture of any of the foregoing may also be used. Some of the resulting materials are often alternatively referred to as "rubbers": for example, cis-1,4-polybutadiene. Methods for preparing these materials are also known in the art.

Furthermore, the conjugated diene polymer may be a component in a blend, for example, a blend or combination with a vinyl aromatic polymer such as polystyrene, polychlorostyrene, poly-alpha-methylstyrene, or polyvinyltoluene. One particularly preferred material of 25 this type is known as "rubber-modified polystyrene" or "high impact polystyrene", and can be prepared by dissolving rubber in a styrene-solvent solution, and then polymerizing the styrene within the solution. The resulting product usually contains a graft polymer of polystyrene and rubber, 30 alon~ with some ungrafted polystyrene and rubber.

The conjugated diene may also be a copolymer derived in part from a comonomer such as a vinyl aromatic compound, an acrylic ester, or an alkylacrylic ester. Examples of 35 suitable vinyl aromatic compounds are styrene, ' .

2~2953 chlorostyrene, alpha-methylstyrene, and vinyl toluene.
Examples of suitable acrylic esters are ethyl acrylate, butyl acrylate, and methyl acrylate. Examples of suitable alkylacrylic esters are methyl methacrylate, ethyl 5 methacrylate, and butyl methacrylate.

The conjugated diene could also be in the form of a rubbery copolymer of styrene with either isoprene, butadiene, or mixtures of these dienes. These particular materials are described more completely below, since they can also be used as an additional component for the composition, i.e., an impact modifier.

As mentioned above, component (c) of the present 5 invention is a nitrite material selected from the group consisting of metal nitrites(i.e., inorganic nitrites), mixtures of metal nitrites, organic nitrites, and mixtures of organic nitrites. The nitrites or mixtures thereof should have a melting point below about 325 C to be 20 effectively incorporated into the resin compositions.

When component (c) is inorganic-based, the metal is usually one from the group consisting of Groups ~A, IIA, IIIA, and IIB of the Periodic Table, with the proviso that 25 the resulting nitrite has a melting point as described above. Examples of inorganic nitrites which are thought to be suitable are sodium nitrite, calcium nitrite, magnesium nitrite, zinc nitrite, aluminum nitrite, and lithium nitrite, as well as mixtures of the foregoing, such as a 30 mixture of sodium nitrite and potassium nitrite.

Examples of organic nitrites are as follows: n-amyl nitrite, n-hexyl nitrite, n-heptyl nitrite, n-dodecyl nitrite, benzyl nitrite, and mixtures of any of these 35 nitrites.

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2~29~3?.1 The preferred materials for component (c) are either sodium nitrite or the mixture of sodium nitrite and potassium nitrite. When such a mixture is used, the weight ratio of sodium nitrite to potassium nitrite usually ranges 5 from about 99:1 to about 30:70.

The nitrite material of component (c) is usually present in the composition at a level of about 0.005% by weight to about 1% by weight, based on the combined ~eight c of components (a) and (b). A more preferred level is about 0.01% by weight to about 0.5% by weight, with a most preferred level being about 0.05% by weight to about 0.3% by weight.

In some instances, the appropriate amount of nitrite material may be expressed as a value relative to the amount of conjugated diene present. Usually, about 0.05/c by weight to about 15% by weight nitrite material is appropriate, based on the total weight of conjugated diene (such as 20 polybutadiene), with the most preferred range being about 0.5% to about 3% by weight.

A particular level of nitrite most suitable for an application of this invention can be determined without 25 undue experimentation, based on various fac~ors, such as the particular resins present, as well as the physical property performance desired, e.g., in terms of impact strength.

Component ~c) can be incorporated into the other 30 components by various techniques. For example, a metal nitrite in nea~, finely-divided form can be compounded with one or more of the resins, either as a dry premix or within an extruder.

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In some preferred embodiments of this invention, the nitrite material may be used in the form of an aqueous solution or an aqueous/alcohol solution. For example, a metal nitrite could be dissolved in water or ethanol to 5 form a solution which is then added to the resins.

Likewise, the organic nitrites could be used in the form of a solvent solution, for example, that prepared by dissolving the nitrite in an organic solvent such as acetone, ethanol, or n-hexane.

Compositions of this invention sometimes include effective amounts of impact modifier materials, such as various core-shell polymers or block copslymers. Examples 5 of suitable block copolymers (as a component which would be used in addition to component (b)) are those characterized by an A-B, A-B-A', or (A-B)m-X structure, or mixtures of these structures, wherein A and A' are each polymerized vinyl aromatic hydrocarbon blocks, each B is derived from 20 at least one polymerized conjusated diene, X is ~he radical of a multifunctional coupling agent, and m is an integer of at least 2. For example, the block copolymer may comprise blocks of polystyrene and polybutadiene, with the polybutadiene being completely-, partially-, or 25 non-hydrogenated.

As mentioned above, many of these block copolymers (i.e., those containing the conjugated diene~ can ~hemselves constitute component ~b). Suitable block copolymers are also 30 described in U.S. Patents 4,166,055; 3,670,054; and 3,431,323, as well as in the patent application of B. Haaf et. al., U.S. S.N. 407, 6?0, file~ September 15, 1989, the contents of all of these references being i ncorporated herein by reference.

- 8 - sCN-8427 ZlZ3 295~?, Examples of core-shell polymers which would function here as suitable impact modifiers are those which comprise a cross-linked polyacrylate rubber core which is surrounded by a cross-linked vinyl aromatic resin. An illustrative rubber 5 core is polybutyl acrylate; an illustrative vinyl aromatic resin for the surrounding "shell" is polystyrene.

The level of impact modifier used is usually between about 1% by weight and about 30% by weight, based on the weight of the entire composition, and more preferably, between about 2% by weight and 10% by weight.

Furthermore, compositions of this invention may include homopolystyrene, sometimes referred to as "crystal" poly-15 . styrene.

Compositions of this invention may also includeeffective amounts of various additives, such as flame retardants, plasticizers, stabilizers, antistatic agents, 20 fillers, reinforcing agents, lubricants, colorants, dyes, pigmer,ts, processing aids, and drip retardants. The proper amounts for each additive can be determined without undue experimentation. The levels each usually range from about 0.1% by weight to about 50% by weight, based on the weight 25 of the entire composition.

Included within the scope of this invention is an improved method for preparing thermally-stable compositions which comprise PPE resin and conjugated diene polymers.
30 The method comprises:
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a) combining a PPE/organic solvent solution with an organic solvent solution which contains the diene polymer, in the presence of a nitrite material of the type 35 described previously, i.e., metal nitrites, mixtures of .

~ ~ Z9 metal nitrites, organic nitrites, or mixtures of organic nitrites; and b) passing the mixture prepared in step (a) through a devolatization system to remove substantially all of the 5 organic solvents;

Particular examples of suitable nitrites or nitrite combinations are provided above, as are suitable addition levels. Furthermore, examples of particular PPE resins and conjugated diene polymers or blends thereof are described above in both the general and preferred embodiments for the -compositions of this invention.

Step (a) may be carried out by various techniques, such lS as by mixture in an agitated vessel, a static mixer, or in a single- or twin-screw extruder. Mixing temperatures usually are in the range of about 120C to about 300C, and more preferably, in the range of about 160C to about 200C.

An example of a suitable technique for carrying out step (b) of this embodiment is provided by the disclosure of U.S. Patent 4,808,262 of V. Aneja and J. Skilbeck, the entire contents of which are incorporated herein by reference. Thus, the combined PPE/conjugated diene solution 2s can be passed through a zone of indirect heat exchange which is made up of a number of channels having specific dimensions which permit very even heat treatment o~ the polymers. These channels terminate in a vaporization chamber, wherein most of the solvents evaporate, and the 30 polymer mixture is usually recovered from the bottom of the chamber.

The recovered polymer may then be utilized in any conventional manner. Fo, example, it can be extruded and ~ ~2 ~3 then cooled by various techniques, e.g., use of a water bath, fol1owed by pelletization.

A key aspect of this method is that the PPE/conjugated 5 diene polymer system remains thermally stable because of the use of the nitrite material described above, even when high devolatilization temperatures are applied, thereby resulting in the maintenance of important physical properties such as impact strength.

The examples which follow are provided to illustrate some embodiments of this invention.

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- 11 - 8cN-s427 2 ~9 53 EXAMPLES

In the following examples, the PPE employed was a poly(2,6-dimethyl-1,4-phenylene) ether resin having an intrinsic viscosity (as measured in chloroform at 25C) of 0.46.

The conjugated diene was in the form of high impact polystyrene, prepared by dissolviny polybutadiene in styrene monomer, followed by polymerization to a conversion level of at least about 85% of the styrene.
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The nitrites used here are described below.

KRATON~ D1101, a product of Shell Chemical Company, is a styrene-butadiene-styrene block copolymer.

IRGANOX~ 565, a product of Ciba-Geigy Corporation, is a hindered phenolic compound commonly used as a thermal-oxidative stabilizer for various organic polymers.

23 One technique for measuring thermal stability is the swell index test. Since much of the instability in the polymer compositions discussed above is thought to result from excessive cross-linking of the conjugated diene during heat exposure, a measurement of the degree of swell of the 2~ diene within a solvent, which is proportional to the degree of its cross-linking, is a good measure of stability. In these examples, a predetermined amount of the polymer sample was pressed in a heated press at about 282C, and then dissolved in a measured volume of toluene, followed by 3~ centrifuging for 70 minutes at 18,000 rpm. The toluene was decanted from the gel which resulted. The gel was then mixed with more toluene, and was centrifuged a~ain under the .. . . ~ ~ .
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2 ~ ~ 5 same conditions. The remaining free toluene was then removed, and the weight of the "wet" polymer (gel) was determined. The gel was dried in a vacuum oven to remove all toluene, and the weight of the dried polymer was measured.
s The swell index value was calculated as the ratio of the wet polymer gel to the weight of the dry polymer gel. Higher swell index values indicate greater thermal stability for the composition.

lQ The gel percent is also a useful measurement, and is calculated from the ratio of the dry polymer gel to the initial weight of polymer sample.

In most of the examples, several residence times in the 15 processing apparatus (i.e., an extruder or molding machine, unless otherwise indicated) were utilized in order to evaluate the compositions throughout a range of heat exposure times.

20 The sample number designations correspond to the various residence times.

Heat Distortion Temperature (HDT) was measured by ASTM
D648.
Izod Impact Strength was measured by ASTM D256, notched, at room temperature.

Dynatup Impact Strength was measured by ASTM D3763, at 30 Poi nt of fracture.
All values given below are in parts-by-weight (pbw), unless otherwise indicated.

-~ - 13 - 8CN-8427 Z ~ 2 95 33 The materials set forth in Table 1 were compounded at 271C on a 28mm Werner & Pfleiderer twin screw extruder, cooled, pelletized, and then pressed into films at 282. The swell index values were determined as described above.
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~ ~2 9 5 3~.1 MATERIAL ¦ AMOUNT (pbw) _ _ SamPle 1 SamPle 2 SamPle 3 s PPE ¦ 35 35 35 HIPS (a) I 65 65 65 SODIUM NITRITE ~ 0.02 I

SWELL INDEX VALUES

TIME(Min ! ¦ SamPle 1 SamPle 2 SamPle 3 I

2 1 9.4 10.4 11.8 1 9.0 10.2 11.4 1 8.1 8.4 11.4 (a) Rubber-Modified Polystyrene * Comparative Samples 2~2~5~?~
The results set forth on page 14 demonstrate that compositions of this invention exhibit significantly improved heat stability as compared to those compositions which omit stabilizers, or which contain a conventional s stabilizer like the IRGANOX compound.

Blends of the present invention and control blends - lO were prepared by extruding as described above and then molding in a molding machine at 282C. Table 2 sets forth the sample compositions and the results of physical tests performed on molded test pieces.

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MATERIAL
Samole 4 Sample 5 SamDle 6 Sample 7 SamPle 8 Sample S
s PPE (pbw) 35 35 35 35 35 35 HIPS (pbw) 65 65 65 65 65 65 SODIUM NITQITE -- ~~ ~~ 0.2 0.2 0.2 RESIDENCE TIME Z.7 6.3 8.52.7 6.3 8.5 (mins. j TEMP. (-F) IZOD IHPACT STRENGTH4.0 3.43.3 4.5 4.5 4.5 (ft-lbs/in. ) 2~
DYNATUP (in-lbs) 300 2:D 216432 ;36 384 ~ Comparative Samples 2 ~ ~ ~5 ~ 3 The data in Table 2 demonstrate that impact values in general are initially higher for compositions of this invention (samples 7-9) as compared to compositions without the nitrite material (samples 4-6). Furthermore, although s the Dynatup values drop somewhat over time for samples 7-9, the values are still superior to those for samples 4-6.
Moreover, the Izod values for samples 7-9 remain very consistent over the residence time range, in contrast to the results for samples 4-6.

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The compositions set forth in Table 3 were prepared and processed as described in Example 2. As shown, two different levels of sodium nitrite were employed (except for the control materials), and various material residence times were 5 utilized. The results for physical tests performed on molded test parts are also described in the table; the tests were identical to those used in Example 2.

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, ~ ~29 53 It is clear from Table 3 that both the Izod and the Dynatup values for compositions of this invention (samples 13-18) are much higher than for the control compositions (comparative samples 10-12), throughout the range of residence times.

The compositions set forth in Table 4 were prepared and processed as in Example 2. As shown, an amalgam of nitrite - materials was used: 52pbw sodium nitrite to 48pbw potassium nitrite. The results for physical tests performed on molded test parts, using tests like those in Example 2, are set forth below.

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2 ~ 95~-~9 Table 4 demonstrates that the Izod impact strength values for samples of this invention were initially higher than those of the comparative samples. Furthermore, these values were substantially retained over the course of s molding residence times.

In this example, Dynatup values for the comparative samples 19-21 were equal to or substantially higher than those for samples 22-24.

However, the Izod test is often thought to be a better indicator of impact strength for many of the particular parts formed with compositions of this invention.

Tensile elongation and flexural values were slightly lower for samples 22-24 as compared to samples 19-21, but were still very acceptable.

The compositions described in Table 5 were prepared by compounding on a 28mm Werner & Pfleiderer twin screw extruder at 271C. The Percent Gel and Swell Index values for test pieces were then determined as described above.

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- 22 - aCN-a427 2~95~t~

MATERIAL
SamDle 25Samole 26SamDle 27Sam~le 28 PPE (pbw) SO 50 SO SO

(pb~) SODIUII NITRITE -- 0.2 -- 0.2 TRIBLOCK COPOLYIlER**
~EL (%) 3.0 0.97 7.1 ~.3 . .
S~ELL INDEX VALUE 14.0 29.9 11.0 17.6 Comoarative Examples KRATON''- D1101 - 23 - 8~N 8427 2~2~i33 Table 5 demonstrates that the compositions o~ this invention contained less gel content and higher swell indexes, indicating that less cross-linking of the polybutadiene phase in the triblock copolymer had occurred.
s This example demonstrates some of the advantages of the present invention as applied to a process for producing blends - of PPE and high impact polystyrene (HIPS) in solution. This type of blending process often involves subjecting the polymers to a more rigorous temperature regime, which can result in a decrease in physical properties (such as impact strength) for the final product.

In each of these experiments, polybutadiene dissolved in 5 styrene was fed into a series of reactors (along with small amounts of various additives), and polymerized at a temperature of about 120C to about 180C, resulting in about 92% conversion of the styrene monomer. The HIPS
product contained, by weight, about 70%-75% polystyrene, and about 5%-10~/o polybutadiene.

A stream of this product, flowing at 15.9 kg/hr, was directed to a mixing vessel, while a stream of a 30% solution of PPE resin (of the type used in the previous examples) in 25 toluene was fed into the HIPS stream, at a ra~e of 23 kg/hr.
The two streams were homogenized in the vessel at a temperature of about 180C, and then devolatilized in multiple flash tanks.

In a series of trial runs, various nitrite materials or combinations thereof were pumped into the PPE/HIPS stream just prior to the first flash tank stage. For example, a 25~io .

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sodium nitrite water solution was used in one run, while a 50/50 weight percent mixture of sodium nitrite and potassium nitrite as a 25% by weight solution in water was used in another run. The results of these trial runs, for test 5 pi eces molded from the devolatilized materials, are described in Table 6.

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Z ~ 95 Amount of Nitrite ** **
SamPle No.(weiq~ht % of Salt) Swell_Index Izod ImPact 29 0.0 8.0 2.1 0.1 11.1 3.2 31 0.2 13.2 3.5 - 32 0.4 16.0 3.7 * Comparative Sample `
** Measured as described previously - 26 - 8CN-8~27 ~ ~ g~ 3 The results set forth in Table 6 clearly demonstrate that compositions of this invention containing the nitrite material exhibit much better Izod impact strength than those without the nitrite, even when such compositions are 5 prepared by a solution blending process at relatively high temperatures.

All of the above-mentioned patents and/or publications are incorporated herein by reference. It should also be noted that 15 other modifications and variations of the present invention are ~ possible, in light of the above teachings. Such changes should fall within the scope of this invention, as set forth in the claims which follow.

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Claims (34)

1) A thermoplastic composition comprising:
a) polyphenylene ether resin;

b) at least one conjugated diene polymer or blend containing such polymer; and c) about 0.005% by weight to about 1% by weight of a nitrite material selected from the group consisting of metal nitrites, mixtures of metal nitrites, organic nitrites, and mixtures of organic nitrites, said metal nitrite or mixture thereof having a melting point below about 325°C, and the weight percentage of said nitrite material being based on the total weight of components (a) and (b).

2) The composition of claim 1 wherein the polyphenylene ether resin comprises a plurality of structural units of the formula:

wherein each Q1 is independently halogen, primary or secondary lower alkyl, phenyl, haloalkyl, aminoalkyl, hydrocarbonoxy, or halohydrocarbonoxy wherein at least two carbon atoms separate the halogen and oxygen atoms; and each Q2 is independently hydrogen, halogen, primary or secondary lower alkyl, phenyl, haloalkyl, hydrocarbonoxy or halohydrocarbonoxy as defined for Q1.

3) The composition of claim 2 wherein each Q1 is an alkyl group having from 1 to 4 carbon atoms, and each Q2 is hydrogen.

4) The composition of claim 3 wherein the polyphenylene ether resin is poly(2,6-dimethyl-1,4-phenylene ether).

5) The composition of claim 2 wherein the polyphenylene ether is derived from the group consisting of 2,6-xylenol; 2,3,6-trimethylphenol, and mixtures of these monomers.

6) The composition of claim 1 wherein the conjugated diene polymer of component (b) is derived from a monomer containing about 4 to 15 carbon atoms.

7) The composition of claim 6 wherein the monomer is selected from the group consisting of 1,3-butadiene;
isoprene; piperylene; 2,3-dimethyl-1,3-butadiene;
2-methyl-1,3-hexadiene; 1,3-octadiene; 1,3-dodecadiene;
2,5-dimethyl-1,3-decadiene; and diethyl-1,3-octadiene.

8) The composition of claim 1 wherein the conjugated diene is a copolymer derived in part from a comonomer selected from the group consisting of vinyl aromatics, acrylic esters, and alkylacrylic esters.

9) The composition of claim 8 wherein the vinyl aromatic material is selected from the group consisting of polystyrene, polychlorostyrene, poly-alpha-methylstyrene, and polyvinyl toluene.

10) The composition of claim 8 wherein component (b) is a copolymer characterized by an A-B, A-B-A', or (A-B)m-X

structure, or mixtures of these structures, wherein A and A' are each polymerized vinyl aromatic hydrocarbon blocks, each B is derived from at least one polymerized conjugated diene, X is the radical of a multifunctional coupling agent, and m is an integer of at least 2.

11) The composition of claim 10 wherein component (b) comprises blocks of polystyrene and polybutadiene.

12) The composition of claim 1 wherein the conjugated diene polymer of component (b) is a constituent of a rubber-modified vinyl aromatic material.

13) The composition of claim 12 wherein the vinyl aromatic material is selected from the group consisting of polystyrene, polychlorostyrene, poly-alpha-methylstyrene, and polyvinyl toluene.

14) The composition of claim 12 wherein the conjugated diene itself is either isoprene, polybutadiene, or a rubbery copolymer of styrene and butadiene.

15) The composition of claim 1 wherein the level of nitrite material present is about 0.01% by weight to about 0.5% by weight.

16) The composition of claim 1 wherein component (c) is derived from metals from the group consisting of Groups IA, IIA, IIIA, and IIB of the Periodic Table.

17) The composition of claim 16 wherein the metal nitrite is selected from the group consisting of sodium nitrite, calcium nitrite, magnesium nitrite, zinc nitrite, aluminum nitrite, lithium nitrite, and mixtures of any of such nitrites.

18) The composition of claim 16 wherein component (c) comprises a mixture of sodium nitrite and potassium nitrite.

19) The composition of claim 18 wherein the weight ratio of sodium nitrite to potassium nitrite ranges from about 99:1 to about 30:70.

20) The composition of claim 17 wherein the metal nitrite or mixture thereof is present at a level of about 0.05% by weight to about 0.3% by weight.

21) The composition of claim 1 wherein the organic nitrite is selected from the group consisting of n-amyl nitrite, n-hexyl nitrite, n-heptyl nitrite, n-dodecyl nitrite, benzyl nitrite, and mixtures of any of these nitrites.

22) The composition of claim 1, further comprising an impact modifier.

23) The composition of claim 22 wherein the impact modifier is selected from the group consisting of block copolymers and core-shell polymers.

24) The composition of claim 23 wherein the block copolymer comprises blocks of polystyrene and polybutadiene.

25) The composition of claim 1, further comprising homopolystyrene.

26) The composition of claim 1, further comprising an effective amount of at least one additive selected from the group consisting of flame retardants, plasticizers, stabilizers, fillers, reinforcing agents, lubricants, antistatic agents, colorants, dyes, pigments, drip retardants, and processing aids.

27) An improved method for preparing thermally-stable compositions comprising polyphenylene ethers and conjugated diene polymers, or blends containing said conjugated diene polymers, said method comprising:

a) combining a polyphenylene ether/organic solvent solution with an organic solvent solution which contains the diene polymer, in the presence of a nitrite material;
aid nitrite material selected from the group consisting of netal nitrites, mixtures of metal nitrites, organic nitrites, and mixtures of organic nitrites; and b) passing the mixture prepared in step (a) through a devolatization system to remove the organic solvents;

28) The method of claim 27 wherein the composition is extruded after step (b).

29) The method of claim 27 wherein the polyphenyiene ether is derived from the group consisting of 2,6-xylenol;
2,3,6-trimethylphenol, and mixtures of these monomers; and wherein the diene polymer is either a polybutadiene or a rubbery copolymer of styrene and butadiene.

30) The method of claim 27 wherein the nitrite naterial is a metal nitrite selected from the group consisting of sodium nitrite, calcium nitrite, magnesium nitrite, zinc nitrite, aluminum nitrite, lithium nitrite, and mixtures of any of such nitrites.

31) The method of claim 27 wherein the nitrite material comprises sodium nitrite and potassium nitrite.

32) The method of claim 27 wherein the nitrite material is present at a level of about 0.005% by weight to about 1%
by weight, based on the total weight of the polyphenylene ether and the diene polymer or blend containing said diene polymer.

33) The method of claim 27 wherein the diene polymer is a constituent of a polystyrene-based material formed by dissolving the diene in a solution of organic solvent and styrene monomer, and then polymerizing the styrene in said solution.

34) The invention as defined in any of the preceding claims including any further features of novelty disclosed.