CA2092444A1 - Use of polyphenylene ether resins to improve the flame retardancy of high molecular weight silicones - Google Patents

Abstract

ABSTRACT OF THE INVENTION

A novel silicone composition having significantly improved flame retardance comprising a blend of polyorganosiloxane gum, a polyphenylene ether and a reinforcing filler is disclosed.

Description

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USE OF POLYPHENYLENE E~HER RESINS TO IMP~OV~ ~H~ F~AME
RETA~D~NCY OF HIGH MO~ECU~AR WEI~HT SILICON~S

The present invention relakes to blending polyphenylene ether resins with high molecular weight silicones to improve the flame retardancy of the silicones.

BACXGROUND OF THE PRESENT INVEN~ION

A common drawback of polymeric resins is their flammability. Accordingly there have been numerous attempts in the prior art to provide flame retardant thermoplastics. Typically it has been necessary to heavily fill the polymeric material with additives until the desired degree of flame retardancy has been achieved.
Silicone flame retardants for thermoplastics such as polyphenylene ethers are disclosed in Frye, United States Patent No. 4,387,176. The patentee teaches adding a combination of metal soaps, polydiorganosiloxanes and silicone resins to the thermoplastic resin.
Also, Abolins et al., United State~ Paten~ No.
4,497,925 disclose improving the flame ~e~is~ance of pol~meric resins by adding a blend of hydrated alumina, polydiorgano~iloxane and polyphenylene ether.
There are also di~closures in the patent literature to the preparation of polyphenylene ether-siloxane polymer block copolymers. Clark et al., United States Patent No. 3,696,137 disclose preparing block copolymers of polyphenylene ether-polydimethylsiloxane~
wherein the block unit lengths are of critical importance, which are useful as surface tension depressants for dielectric fluids.

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2 60~I-1427 Shea et al., United States Patenk No 4,814,392 disclose silicone-polyarylene ether block copolymers formed by reacting an amino terminated polydiorganosiloxane and and anhydride functionaliæed polyphenylene ether.
Mention is also made of p~tents to Lovgren et al., United States Patent Nos. 4,487,858 and 4,446,090 which discloses a method for blending temperature sensitive components into a silcone modified thermoplastic by injecting the silicone fluid into a molten thermoplastic re~in in an extruder. Further, Cooper et al., United States Patent No. 4,365,038 disclose preparing blends of polyphenylene ether resins and ~PDM-polystyrenes modified with a silicone oil as having improved physical properties.
However, none o~ the prior art disclosures teach or suggest a high molecular weight silicone composition which has improved flame retardancy.

SUMMARY OF TH~ PRESE~T INVENTION

According to the present invention there i8 provided a novel ~ilicone compositlon exhibiting excellent ~lame re~ardancy comprising ~a) a organosiloxane gum; (b) a polyphen~lene ether resin and tc) a reinforcing ~iller. In pre~erred embodiments, the composition~ of the present invention can further comprise one or more of the following components (d) a plasticizer; (e) an extending filler and (f) a compatibilizing agentO
According to the present invention there is also provided a method for preparing a novel silicone composition having excellent flame retardancy comprising adding a polyphenylene ether resin to an organosiloxane gum and a reinforcing filler.

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3 60S~ 27 Also according to the present invention, there are provided novel articles exhibiting excellent flame xetardancy prepared from a composition comprising (a) an organosiloxane gum, tb) a polyphenylene ether ~e~in and (c) a reinforcing filler.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention provides novel silicone compositions having improved flame retardancy.
~ he organosiloxane ~ums (a) of the present invention are high molecular weight organosiloxane polymers, generally having a viscosity of from about 3,000,000 to about lOO,OOO,OOO centipoi~e. The high molecular weight polyorganosiloxane gums useful in the practice of the present invention are genreally random polymers containing alkyl, vinyl and/or phenyl substituted siloxane units.
The alkyl substituted polyorganosiloxanes are of the general ~ormula:

20R3SiO ~ Si-O ~ 5iR3 wherein R i~ an alkyl group, mo~t preferably me~hyl; and X i9 a number such that the viscosity o~ the polymer ranges from about 3,000,000 to about 100,000,000 centipoise.
The vinyl substitutad polyorganosiloxanes are of the general ~ormula:
R' R3Si-O - - Si-O ~ Si~3 R
wherein R is an alkyl group, most preferably methyl; R' is an alkyl group, preferably methyl, or a vinyl group with a sufficient number of units having R' as a vinyl ;,~, .

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4 60SI-14~7 group such that the mole percent of vinylalkylsiloxy units is from about 0.025 to about 25 mole percent, and is preferably from about 0.025 to about 3.0 mole percent and x is a number such that the viscosity of the vinyl substituted polyorganosiloxane range~ from about 3,000,000 to about 100,000,000.
The phenyl substituted polyorganosiloxanes are of the general formula:

R,Si-O ~ Si-O ~ Si-O ~ S~R3 wherein R is an alkyl group, preferably methyl; R' is an alkyl group such as methyl or is a vinyl group and R" is phenyl; wherein y is an integer such that the phenyl siloxane units are present in an amount ranging from about 1 to about 30 mole percent, preferably from about 5 to about 6 mole percent; and wherein x+y equals a number such that the viscosity of the phenyl substituted polyorganosiloxane ranges from about 3,000,000 to about lOO,OGO,OOO centipoise.
~ he polyoxganosiloxanes of the present invention are prepared by methods known to tho~e ~killed in the art or are available commerclally. They c:an typically bo prepared by the hydrolysls of the correspondlng chlorosilanes ~ollowed by acid or based catalyzed equillbration.
The polyphenylene ether resins (b) useful in the present invention are well known in the ar~ and are described in the patent literature, e.g. United States Patent Nos. 3,306,874 and 3,306,875. Numerous modifications and variations have since been developed but, in general, they are characterized as a class by the presence of aryleneoxy structural units. The present invention includes all such variations and modifications, including but not limited to those described herelnafter.

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~0~-1427 The polyphenylene ethers ~b) ~avored for use in the practice of the present invention generally contain structural units of the following formula:
Q2 Ql ~~; - ' Q2 ~1 in which in each of these units independently each Ql is hydrogen, halogen, primary or secondary lower alkyl (i.e., alkyl containing up to 7 carbon atomq), phenyl, lQ haloalkyl ox aminoalkyl wherein at least two carbon atoms separate the halogen or nitrogen atom from the benzene ring, 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 Ql.
Examples o~ suitable primary lower alkyl groups are methyl, ethyl, n-propyl, n~butyl, isobutyl, n-amyl, isoamyl, 2-me~hylbutyl, n-hexyl, 2,3-dimethylbutyl, 2-, 3- or ~-methylpentyl and the corresponding heptyl group~.
Examples o~ secondary lower alkyl group~ are isopropyl, ~ec-butyl and 3-penkyl. Preferably, any alkyl radicals are straight chain rather than branched. Most often, each ~1 is alkyl or phenyl, especially Cl 4 alkyl, and each Q2 is hydrogen.
Both homopolymers and copolymers are included. ;~
Suitable homopolymers are those containing, or example, 2,6-dimethyl-1,4-phenylene ether units. Suitable copolymers include random copolymers containlng such units in combina~ion with, for example, 2,3,6-trimethyl-, -, . . .: . . : ~ : . , ~ . ;

" ~3~2~4 6 60~I-1427 1,4-phenylene ether units. Many suitable random copolymers, as well as homopolymers, are disclosed in the patent literature, including the various Hay patents.
Also contemplated are graft copolymers, including tho~e prepared by grafting onto the polyphenylene ether chain such vinyl monomers as acrylonitrile and vinyl aromatic compounds (for example, styrene), and such polymers as polystyrenes and elastomers. Still other suitable polyphenylene ethers are the coupled polyphenylene ethers in which the coupling agent is reacted with the hydroxy groups of the two polyphenylene ether chains to increase the molecular weight of the polymer. Illustrative of the coupling agents are low molecular weight polycarbonates, quinones, heterocycles and formals.
The polyphenylene ether ge~erally has a molecular weight (number average, as determined by gel permeation chromatography, whenev~r used herein) within the range of about 5,000 to 40,000. The intrinsic viscosity of the polymer is typically in the range of about 0.45 to 0.7 deciliters per gram (dl/g), as measured in solution Ln chlorofoxm at 25C.
The polyphenylene ethers may be prepared by known method~, and typlcally by the oxidative coupllng of at least one correspondlng monohydroxyaromatic (e.g., phenolic) compound. A particularly useful and readily available monohydroxyaromatic compound is 2,6-xylenol ~in which for the above formula each Ql is methyl and each Q2 is hydrogen, the corresponding polymer of which may be characterized as a poly(2,6-dimethyl-1,4-phenylene ether). Preferably the oxidative couplln~ reaction is carried out in the presence of a suitable catalyst as known to those skilled in the art, such as those containing a heavy metal compound, including copper, manganese or cobalt, usually in combination with various other materials.

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7 60~-1427 The reinforcing ~illers (c) of the present invention are generally employed in amounts ranging from about 5 to about 200 parts by weight, preferably from about 10 to about 100 parts by weight, and most preferably from about 30 to about 75 parts by weighk, based upon the weight of the siloxane gum (a). The reinforcing filler is added to provide compositions having high tensile and tear s~rength. The reinforcing fillers o~ the present invention are available commercially and typically include, but are not limited to, fumed silica, silica aerogel and precipitated silica, with fumed silica being preferred. The filler may also be treated with various agents so as to prevent the cured composition from structuring. Exemplary treatments include cyclopolysiloxanes, such as those disclosed in Lucas, United States Patent No. 2,938,009; and silazanes, such as those disclosed in Smith, United States Patent No. 3,635,743, or both. The cyclopolysiloxane present may be, for example, a cyclotetramethylsiloxane present ln an amount of from about 15 to about 20 weight percent of the reinforcing filler (c).
The preferred fumed silica ~iller may have a surface area of ~rom about 50 m2/gm to about 300 mZ/gm and praferably rom about 100 m2/gm to about 240 m2/gm. ~he ~umed ~illca may be treatod in situ wl~h a linear vinyl containing silazane.
Component (d) of the present invention is a plasticizer or processing aid. Any conventional plasticizer or processing aid known to be useful for facilitating incorporation of fillers into organosiloxane gums may be us~d in the practice of the present invention. Preferred are low viscosity silanol-stopped siloxane fluids having a viscosity of about 3 to 500 cps : ~ , : . .
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and preferably 3 to 50 cps at 25C. The siloxane fluid is an equilibrium mix of low molecular weight oligomers of from about 4 to about 10 repeating units, and preferably between 4 and 6 repeating units in length with a minimum amount of cyclics in equilibrium with the oligomersO Component (d) is generally of the formula:

R~o(R25io)xR

where R' is h~drogen or Cl-Ca monovalent hydrocarbon radical free of aliphatic unsaturation; R is lower alkyl, preferably methyl; and x is between 4 and 10 and preferably between 4 and 6, with resulting cyclics in the same number of units in equilibrium.
In the present invention the processing aid or plasticizer is typically present in amounts up to about 20 parts by weight, based upon the amount of siloxane gum, preferably from about 0.5 to about 6 parts by weight, and more preferably about 3 parts by weight. It should however be understood that as the amount of reinforcing filler increases, the greater the amount o~
processing aid or pla~tlcizer i9 generally added~
Extending illers (e) can be used in the compo~ition of the pre~enk invention in combination with ~he reinforcing ~illexs, treated or untreated, ~o obtain proper balance in the ~inal physical properties. A wide vari0ty of extending fillers known ~o those skilled in the art may be employed. Typically they include, for example, titanium dioxide, zinc oxide, zirconium silicate, iron oxide, diatomaceous earth, calcium carbonate, glass fibers, magnesium oxide, chromic oxide, zirconium oxide, aluminum oxide, alpha quartz, calcined clay, carbon, graphite, cotton, synthetic fibers, aluminum silicate, calcium silicate and mixtures of any of the foregoing.

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9 ~O~II~1427 The compositions of the present invention may also comprlses a compatibilizing agent (f). In instances where the polyphenylene ether resin and polyorganosiloxane gum blends do not exhibit su~ficient compatibility, a compatibilizing agent may be added to the blend. The compatibilizing agents o~ the present invention are preferably those which do not react with either the polyphenylene ether resin or the polyorganosiloxane gum to form covalently bonded compounds, but may react to form ionic pairs. These are well known to those of skill in the art~
A compatibilizing agent particularly suitable for use i~ the present lnvention is a bis-aminopropylpolydimethylsiloxane.
~he compos~tions of the present invention are generally prepared by blending the polyorganosiloxane gum, polyphenylene ether and reinforcing filler together in conv~ntional mannner, i.e. such as in a Banbury mixer.
Other additives may also be incorporated into the blends.
The blends are physical mixtures with no, or substantially no, graft or block polymexization occurring.
The polyphenylene ether and polyorgano~iloxane gum can be combined in any proportlon~, as long as su~flcien~ polyphenylene ether i~ added ko provide improved ~lame retardancy on the compositlon.
The blended compositions can then be formed into a wide varlety of flame retardant silicone articles, such as wire and cable insulation. Typically, these forming operations include but are not limited to ln~ection molding~ ex~rusion and compression molding.

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60SI~1427 DESCRIPTIO~ OF THE PREF~RR~D EMBODIMEN~S

The following examples illustrate the present invention. They are not to be construed to limit the scope of the appended claims in any manner whatsoever.

To a doughmixer is added a siloxane gum and processing aid. These components are thoroughly mixed.
Next, a filler i~ gradually added and mixed, followed by the addition of an extending filler and other components.
A solution o~ polyphenylene ether in toluene is then added and mixed. The mixture is then heated with steam heating to remove toluene.
Specimens are prepared from the mixture by curing with 1.2 parts per hundred of Lupersol~-101 on calcium carbonate.
For comparative purposes, blends with no polyphenylene ether and with polycarbonate instead of polyphenylene ether were also prepared. rrhe results, along with compositional data are ~e~ forth in Table 1 below.

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From ~able 1 above, it can be seen that flama retardant silicone compositions can be prepared in accordance with the present invention while essentially maintaining physical properties. The addition of the polyphenylene ether resin to the polyorganosiloxane gums provides significantly improved flame retardancy on the yums, converting a non-self-extinguishing material into a self extinguishing material.

The above-mentioned patents are hereby incorporated by reference.
Many vaxiations of the present invention will suggest themselves to those skilled in the art in light of the above-detailed description. A wide variety of reinforcing fillers and extending fillers may be employed in the in~ention. Further, other conventional additives, such as pigments, anti-oxidants, W stabilizers and the like can be added for their specific pxoperties. All such obvious modifications are within the full intended scope of the appended claims.

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Claims (21)

1. A silicone composition having improved flame retardancy comprising a physical blend of:
(a) a high molecular weight polyorganosiloxane gum;
(b) a polyphenylene ether resin; and (c) a reinforcing filler.

2. A silicone composition as defined in Claim 1 wherein said polyorganosiloxane gum (a) has a viscosity of from about 3,000,000 to about 100,000,000 centipoise.

3. A silicone composition as defined in Claim 1 wherein said polyorganosiloxane gum (a) comprises polymer units selected from dimethylsiloxane, methylvinylsiloxane, hydrovinylsiloxane, diphenylsiloxane and mixtures of any of the foregoing.

4. A silicone composition as defined in Claim 3 wherein said polyorganosiloxane gum (a) has from about 0.025 to about 25 mole percent vinyl alkyl siloxy units.

5. A silicone composition as defined in Claim 3 wherein said polyorganosiloxane gum (a) has from about 1 to about 30 mole percent phenyl siloxane units.

6. A silicone composition as defined in Claim 1 wherein said polyphenylene ether resin (b) is selected from the group consisting of poly(2,6-dimethyl-1,4-phenylene ether), poly(2,3,6-trimethyl-co-2,6-dimethyl-1,4-phenylene ether, and mixtures thereof.

7. A silicone composition as defined in Claim 6 wherein said polyphenylene ether resin comprises poly(2,6-dimethyl-1,4-phenylene ether).

8. A silicone composition as defined in Claim 1 wherein said reinforcing filler (c) is selected from the group consisting of treated and untreated fumed silica and precipitated silica.

9. A silicone composition as defined in Claim 8 wherein said reinforcing filler comprises silazane treated fumed silica.

10. A silicone composition as defined in Claim 1 further comprising (d) a plasticizer.

11. A silicone composition as defined in Claim 10 wherein said plasticizer comprises a silanol stopped siloxane fluid having a viscosity of from about 3 to about 500 centipoise.

12. A silicone composition as defined in Claim 1 further comprising (e) an extending filler.

13. A silicone composition as defined in Claim 12 wherein said extending filler is selected from the group consisting of titanium dioxide, zinc oxide, zirconium silicate, silica aerogel, iron oxide, diatomaceous earth, calcium carbonate, glass fibers, magnesium oxide, chromic oxide, zirconium oxide, aluminum oxide, alpha quartz, calcined clay, carbon, graphite, cotton, synthetic fibers, aluminum silicate, calcium silicate and mixtures of any of the foregoing.

14. A silicone composition as defined in Claim 1 further comprising (f) a compatibilizing agent.

15. A silicone composition as defined in Claim 1 wherein said compatibilizing agent (f) does not react with said polyorganosiloxane gum (a) or said polyphenylene ether resin (b) to form a covalently bonded compound, but may react to form ion pairs.

16. A method for perparing a novel silicone composition having excellent flame retardancy comprising physically blending a polyphenylene ether resin with an organosiloxane gum and a reinforcing filler.

17. A method as defined in Claim 16 wherein said polyphenylene ether comprises poly(2,6-dimethyl-1,4-phenylene ether).

18. A method as defined in Claim 16 further comprising blending an extending filler, a plasticizer, a compatibilizing agent, and mixtures of any of the foregoing.

19. An article of manufacture prepared from the silicone composition as defined in claim 1.

20. An article of manufacture as defined in Claim 19 which is wire or cable insulation.

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