Polyphenylene ethers are a class of polymers which are widely used in industry, especially as engineering plastics in applications which require such properties as toughness and heat resistance. For many such applications polyphenylene ether resins are blended with various types of polystyrene resins to improve their processability. Recently it has become necessary to produce such compositions which are both substantially odorless and tasteless for food contact applications. It is therefore necessary that the polyphenylene ether or polyphenylene ether/polystyrene composition be substantially free from any volatile materials which have undesirable odors or would otherwise harm the food. Materials of this kind which are known to be present in polyphenylene ether or polyphenylene ether/polystyrene resins include toluene, dialkylamines such as di-n-butylamine, which are components of the catalyst used in preparing polyphenylene ether resins; styrene monomers, from degradation of any styrene resin and other by-products resulting from polyphenylene ether resin synthesis. In the case of poly(2,6-dimethyl-1,4-phenylene ether); 2,4, 6-trimethylanisole, 7-methyldihydrobenzofuran, 2,3 -dihydrobenzofuran, 2,6-dimethylcyclohexanone and 2-ethylhex 2-enal are commonly present. Removal of sources of volatile odoriferous amines and oxygenated species is especially important because they are detectable in human organoleptic tests even in very low quantities.
Methods are known, to those skilled in the art, for removing volatile substances from polyphenylene ether or polyphenylene ether/polystyrene resins during vented extrusion, with or without the injection of water into the melt. For example, Kasahara et al., U.S. Pat. No. 4,369,278 extrudes polyphenylene ether and rubber reinforced polystyrene in a single pass, single stage vacuum vented extruder with optional water injection; Newmark, U.S. Pat. No. 3,633,880, extrudes plastic materials, which could include polyphenylene ether, in a single pass through an extruder with multiple vents and employs a specially designed screw to compress and decompress the melt aiding devolatilization without employing water. U.S. Pat. No. 4,831,115 describes the extrusion of polyphenylene ether in a single pass single stage system using up to 15 percent water and vacuum venting. Although all three of the above-mentioned methods reduce the amount of volatile substances in the resin, none of them reduce the amount of volatile odoriferous species down to a level where such materials are hardly detectable by analytical methods thus providing that articles made from them are substantially odorless, especially in food packaging applications.
U.S. Pat. No. 4,906,700, issued to J. Banevicius, discloses a method for producing polyphenylene ether resin with a reduced content of phenolic by-products of the polymer synthesis, such as 2,4,6-trimethylanisole (2,4,6-TMA) and 7-methyldihydrobenzofuran (7-MDBF) in the initial production stage, but not the reduction of sources of residual amine components during post-production extrusion stages.
U.S. Pat. No. 4,994,217, issued to Banevicius et al, discloses method of producing low odor polyphenylene ether/polystyrene resins by solution blending and employing a sequence of devolatilizers to reduce the content odoriferous impurities present in the resin.
It has now been discovered that if the prior art processes are modified by using a single pass through an extruder and dividing the water injection/vacuum venting stage into at least two stages, each consisting of at least one water injection step and at least one vacuum venting step, remarkable and highly unexpected reductions in volatile content and in sources of odoriferous amine content are achieved. By way of illustration, in comparison with a single pass, single stage extrusion, a polyphenylene ether composition containing 8.5 ppm of 2,4,6-trimethylanisole (TMA) is reduced to 2.6 ppm, but in a single pass, two stages, the TMA content is reduced to levels barely detectable by gas chromatography (0.27 ppm). If a 50:50 blend of polyphenylene ether and polystyrene is sent through an extruder in a single pass, single stage with 3 percent water injection and vacuum venting, the styrene content is reduced to 1130 ppm and the toluene content is reduced to 271 parts per million; a back-to-back repitition using a single pass but two water injection/vacuum venting stages, using 1.5% by weight of water in each such stage, reduced the styrene content to 694 ppm and toluene content to 84 parts per million.
Furthermore, it has been surprisingly discovered that the use of a polyphenylene ether resin essentially free of phenolic by-products in the improved process of the present invention produces a resin with a very low content of any odoriferous components. This novel low odor resin can then be shaped into pellets for further processing or directly into solid sheets, molded or extruded articles, and foams which are highly desirable for use in food contact applications.