Stabilizers for polymers containing aliphatically-bound bromine

Abstract

Aliphatic bromine-containing polymers are stabilized using a mixture of an alkyl phosphite and an epoxy compound. This stabilizer package is very effective at preventing cross-linking reactions from occurring when the aliphatic bromine-containing polymer is subjected to high temperatures as are seen in melt processing operations. The stabilized aliphatic bromine-containing polymer is useful as a flame retardant for other polymers, notably polystyrene foam.

Claims

1. An extruded polymer foam composition comprising (a) a styrene homopolymer or copolymer, (b) a brominated styrene/butadiene block copolymer in which fewer than 1% of the carbon-bromine bonds are at allylic or tertiary carbons in an amount sufficient to provide the extruded polymer foam composition with 0.5 to 5 weight-percent bromine, and (c) a mixture of at least one alkyl phosphite selected from the group consisting of bis (2,4-dicumylphenyl)pentaerythritol diphosphite, distearylpentaerythritol diphosphite and di(2,4-di-(t-butyl)phenyl)pentaerythritol diphosphite and at least one epoxy cresol novolac resin, wherein from 1 to 20 parts by weight of the alkyl phosphite are present per 100 parts by weight of the brominated styrene/butadiene block copolymer and further wherein from 1 to 20 parts by weight of the epoxy cresol novolac resin are present per 100 parts by weight of the brominated styrene/butadiene block copolymer.

2. A method for producing an extruded polymer foam composition of claim 1, comprising forming a pressurized melt of a mixture containing molten polystyrene and a brominated styrene/butadiene block copolymer in an amount sufficient to provide the extruded polymer foam composition with 0.5 to 5 weight-percent bromine in the presence of a blowing agent and a mixture of at least one alkyl phosphite selected from the group consisting of bis (2,4-dicumylphenyl)pentaerythritol diphosphite, distearylpentaerythritol diphosphite and di(2,4-di-(t-butyl)phenyl)pentaerythritol diphosphite and at least one epoxy cresol novolac resin wherein from 1 to 40 parts by weight of the alkyl phosphite are present per 100 parts by weight of the brominated styrene/butadiene block copolymer and further wherein from 1 to 40 parts by weight of the epoxy cresol novolac resin are present per 100 parts by weight of the brominated styrene/butadiene block copolymer and forcing the melt through an opening into a zone where the blowing agent expands and the polystyrene solidifies to form a foam.

3. A method for reducing gel formation in a brominated styrene-butadiene copolymer during melt processing, comprising performing the melt processing of the brominated styrene-butadiene copolymer in the presence of a mixture of at least one alkyl phosphite and at least one epoxy cresol novolac resin compound, wherein from 1 to 40 parts by weight of the alkyl phosphite are present per 100 parts by weight of the brominated styrene/butadiene block copolymer and further wherein from 1 to 40 parts by weight of the epoxy cresol novolac resin are present per 100 parts by weight of the brominated styrene/butadiene block copolymer.

Description

EXAMPLES 1-4

(1) Screen experiments are done to evaluate the ability of various stabilizers to prevent thermally-induced gelling in a brominated butadiene polymer. The brominated butadiene polymer in the screening experiments is a styrene/butadiene/styrene triblock polymer containing 60% by weight butadiene prior to bromination. This starting polymer is brominated using elemental bromine as the brominating agent as described in WO2008/021418, and the resulting brominated material has a bromine content of 62% by weight. Three percent of the aliphatic carbon-carbon double bonds in the starting polymer remain after the bromination. 3.5% of the carbon-bromine CBr bonds are to allylic or tertiary carbon atoms, which are less thermally stable than the other CBr bonds in the structure.

(2) In each screening experiment, the brominated butadiene is melt-blended with a stabilizer in the amount shown in Table 1 below. The blended material is ground in a mortar and pestle and then is immersed in methylene chloride at a proportion of 1 g of the blend per 10 mL of methylene chloride. A film of this blend is cast and dried in a vacuum oven at 30 C. The film sample in each case is equilibrated at 30 C. under nitrogen for 5 minutes, and then heated to 180 C. under nitrogen at the rate of 20 C./minute on a thermogravimetric analyzer (TGA). The samples are maintained at 180 C. for 20 minutes and then cooled to 30 C. at the rate of 50 C./minute, all under nitrogen. The sample is then placed in 2 mL of methylene chloride and inspected visually to determine whether the brominated butadiene polymer dissolves. The presence of undissolved and/or gelled material indicates that cross-linking has occurred under the conditions of the heating regimen, and so indicates the effectiveness of the various stabilizers tested to prevent thermally-induced crosslinking.

(3) In addition, the 5% weight loss temperature of the heat-treated product is evaluated using thermogravimetric analysis. 10 milligrams of the polymer blend is analyzed using a TA Instruments model Hi-Res TGA 2950 or equivalent device, with a 60 milliliters per minute (mL/min) flow of gaseous nitrogen and a heating rate of 10 C./min over a range of from room temperature (nominally 25 C.) to 600 C. The mass lost by the sample is monitored during the heating step, and the temperature at which the sample has lost 5% of its weight at 100 C. (after i.e., after volatiles have been driven off) is designated the 5% weight loss temperature (5% WLT).

(4) The various stabilizers that are evaluated, the amount of stabilizer used in each case, the solubility after thermal aging and the 5% WLT are as reported in Table 1.

(5) TABLE-US-00001 TABLE 1 Amount, Soluble parts/100 parts after Stabilizer type resin 5% WLT Aging? None 0 195 No di-(2,4-di-(t-butyl)phenyl) 4 243 Yes pentaerythritol diphosphite.sup.1 distearylpentaerythritol 8 241 Yes diphosphite.sup.2 (2,4-dicumylphenyl) 8 246 Yes pentaerythritol diphosphite.sup.3 Epoxy cresol novolac resin 14 237 No Epoxidized soybean oil 14 218 No Brominated epoxy resin 14 225 No Ester-modified sulfide.sup.4 8 210 No Commercial antioxidant A.sup.5 8 ND No Commerial antioxidant B.sup.6 8 195 No Commercial antioxidant C.sup.7 8 207 No Commercial Antioxidant D.sup.8 8 200 No Commercial organotin stabilizer 8 226 No A.sup.9 Dioctyl tin maleate 8 233 No BHT 8 ND No Commercial organotin stabilizer 8 ND No B.sup.10 Tris (2,4-di-tert- 8 202 No butylphenyl)phosphite .sup.1Irganox 126, from Ciba, CAS No. 26741-53-7. .sup.2Doverphos S682, from Dover Chemical Corporation. .sup.3Doverphos S9228 from Dover Chemical Corporation. .sup.4Irganox PS800FL8, from Ciba, CAS No. 123, 28-4. .sup.5Irganox 38, from Ciba, CAS No. 145650-60-8. .sup.6Irganox 565, from Ciba, CAS No. 991-84-4. .sup.7Irganox 1076, from Ciba, CAS No. 2082-79-3. .sup.8Irganox B215, from Ciba. .sup.9Baerostab 0M36, from Baerolocher GmbH. .sup.10Thermchek 835 from Ferro Corporation.

(6) On the basis of the screening experiments, di-(2,4-di-(t-butyl)phenyl) pentaerythritol diphosphite, distearylpentaerythritol diphosphite and (2,4-dicumylphenyl) pentaerythritol diphosphite are identified as materials which provide both good suppression of crosslinking in the brominated butadiene polymer as well as a significant increase in 5% WLT.

(7) A blend of 50 grams of a commercial foam-grade polystyrene resin, 1.5 grams of the same brominated styrene/butadiene/styrene block copolymer and 0.25 grams of an epoxy cresol novolac resin is made as follows. The polystyrene resin is charged to a Haake Rheocord 90 with controller and mixing bowl containing roller blade mixers. The bowl is preheated to 180 C. The polystyrene is blended for 2 minutes at 40 rpm, and then the brominated copolymer and epoxy resin are added as a dry blend. Blending is continued for another 8 minutes at the same temperature and speed. The resulting blend is designated Example 1.

(8) Examples 2-7 and Comparative Sample A are made in the same manner by varying the stabilizer package in each case. The stabilizer package in each case is as indicated in Table 2 below. Comparative Sample A contains no stabilizer package.

(9) The amount of the brominated block copolymer that remains soluble (and thus ungelled) in each of Examples 1-7 and Comparative Sample A is estimated in the following manner. The sample is in each case dissolved in toluene and filtered, and bromine content in the both the original unfiltered and filtered solutions is determined by x-ray fluorescence, using a bench-top energy dispersive x-ray spectrometer. Calibration standards are prepared from pure samples of the brominated butadiene polymer, using the Compton peak correction method. The ratio of these measured bromine contents correlates to the percentage of cross-linked brominated butadiene polymer. The estimates in each case are as reported in Table 2. In each case, the margin of error is believed to be 5 percentage points.

(10) A portion of each blend is separately heated to 230 C. on a thermogravimetric analyzer, and held at that temperature. The time at which the sample exhibits a measurable weight loss is determined as an indication of the thermal stability of the blend. Results are as reported in Table 2.

(11) TABLE-US-00002 TABLE 2 di-(2,4-di-(t- butyl)phenyl) Example or Dioctyltin pentaerythritol Epoxy cresol % Soluble Comparative maleate, diphosphite, novolac Brominated Block 230 C. Onset Sample No. pphr pphr resin, pphr Copolymer.sup.1 Time, min..sup.2 A* 0 0 0 30 5.7 1 0 0 0.5 48 9.7 2 0 0 1.0 59 16.8 3 0 0.2 0.5 61 16.1 4 0 0.4 0.5 63 17.3 5 0.2 0 0.5 54 15.9 6 0.2 0.2 0.5 43 19.9 7 0.4 0.2 0.5 42 19.2 .sup.1The weight percent of the brominated butadiene block copolymer that remains ungelled after treatment at 180 C. in the Haake blender. .sup.2The amount of time at 230 C. before the blend shows evidence of degradation (as weight loss). *Not an example of this invention.

(12) The brominated butadiene copolymer used in this set of experiments contains a somewhat high level of bromine weakly bonded to allylic or tertiary carbons. With no stabilizer package present (Comparative Sample A), the copolymer gels very significantly and begins to show thermal degradation after less than 6 minutes at 230 C. Adding an epoxy resin alone, as in Examples 1 and 2, reduces gelling and provides greater thermal stability. However, one weight percent of the epoxy resin (as in Example 2) is a somewhat high level, as the epoxy can plasticize the polystyrene when present at such a level. Accordingly, it is desired to reduce the epoxy resin loading and maintain equivalent or better results.

(13) Example 3 shows the effect of replacing half of the epoxy resin used in Example 2 with 0.2% of the alkyl phosphite. Gelling is comparable in these two cases, and only a small loss of thermal stability is seen on the 230 C. thermal aging test.

(14) Example 4 shows that by increasing the alkyl phosphite level to 0.4%, gelling is reduced significantly and the blend is slightly more thermally stable. Total additive level remains below that of Example 2.

(15) Examples 5, 6 and 7 show the effect of adding a small amount of an organotin stabilizer to the blends of Examples 1 and 3. Thermal stability is improved significantly in each case. Less of the brominated butadiene remains soluble than in Examples 1 or 3, but this may be due to a change in solubility parameter caused by the presence of the organotin stabilizer, rather than an actual reduction in effectiveness of the stabilizer package. At the 0.4% level, the organotin stabilizer can begin to interfere with the cell structure of a polystyrene foam.

EXAMPLES 8-17 AND COMPARATIVE SAMPLE B

(16) Examples 8-17 and Comparative Sample B are made in the same manner as the previous examples. The brominated butadiene polymer in this case is a styrene/butadiene/styrene triblock polymer containing 60% by weight butadiene prior to bromination. This starting polymer is brominated using a quaternary ammonium bromide as the brominating agent as described in WO2008021417. The resulting brominated material has a bromine content of 63%. The brominated butadiene polymer contains 7% residual aliphatic carbon-carbon double bonds. Fewer than 1% of the carbon-bromine bonds in this brominated polymer are at allylic or tertiary carbon atoms. The antioxidant packages used in this set of experiments are as indicated in Table 3. The amount of soluble brominated butadiene polymer in each blend and the 230 C. onset time for each blend are determined as described in the previous examples. Results are as indicated in Table 3.

(17) TABLE-US-00003 TABLE 3 di-(2,4-di-(t- (2,4- Epoxy Ex. or butyl)phenyl) dicumylphenyl) Distearyl cresol % Soluble 230 C. Comp. pentaerythritol pentaerythritol pentaerythritol novolac Brominated Onset Samp. diphosphite, diphosphite, diphosphite, resin, Block Time, No. pphr pphr pphr pphr Copolymer.sup.1 min..sup.2 B* 0 0 0 0 58 7 8 0 0 0 0.5 87 10 9 0.4 0 0 0 83 10 10 0.8 0 0 0 90 11 11 0.4 0 0 0.5 89 22 12 0 0.4 0 0 84 11 13 0 0.8 0 0 88 11 14 0 0.4 0 0.5 91 27 15 0 0 0.4 0 88 12 16 0 0 0.8 0 89 11 17 0 0 0.4 0.5 88 22 *Not an example of the invention.

(18) The data in Table 3 shows that each of di-(2,4-di-(t-butyl)phenyl) pentaerythritol diphosphite, (2,4-dicumylphenyl) pentaerythritol diphosphite, distearyl pentaerythritol diphosphite and the epoxy cresol novolac resin are effective in reducing gellation of the brominated butadiene polymer and in retarding the degradation of the brominated butadiene polymer. However, increasing the levels of the phosphites from 0.4 to 0.8 pphr has little additional beneficial effect. When the alkyl phosphite and epoxy cresol novolac resin are used together (as in Examples 11, 14 and 17), a very significant lengthening of the 230 C. onset time is seen.