Here's more on the newest SMA resins.

A new line of styrene maleic-anhydride (SMA) engineering thermoplastics known as Stapron S, which will target automotive injection molding applications, will be introduced later this month at the Society of Automotive Engineers (SAE) show in Detroit. This introduction will make DSM Engineering Plastics North America Inc., Reading, Pa., the third U.S. supplier of SMA resins, alongside Arco Chemical Co., Newtown Square, Pa., and Monsanto Chemical Co., St. Louis. When contacted, neither Arco nor Monsanto said they had plans to introduce new SMA resins in the near term, though Arco hinted that it may have some new Dylark grades in the works.

As reported here in December (p. 10), Stapron S is an amorphous, rubber-modified SMA copolymer to be domestically compounded in six neat and glass-filled grades. Details on the properties of the new grades are now available (see table). [Tabular Data Omitted]

Stephen M. Hartig, automotive manager for DSM, explains that the company's polymerization technology is able to control accurately the amount of maleic anhydride that's built into the polymer chain. This capability enables DSM to engineer a wider range of both toughness and heat resistance, and to tailor a specific balance of properties, says Hartig. Maleic anhydride content can be accurately varied up to 50%, while also providing variations in the resin's molecular weight.

Stapron S will compete for applications now using PC/ABS alloys, PPO alloys and existing SMA resins. Hartig says Stapron S offers better chemical resistance than PPO or PC/ABS, and better uv stability than PPO.

The amorphous properties of Stapron S give it low moisture absorption, low shrinkage, a high degree of dimensional stability, and good part-to-part repeatability. As a polar polymer, Stapron S also offers good adhesion to glass fillers. Hartig points out, however, that Stapron S can't match the chemical and fuel resistance of a crystalline polyester or nylon.

The first generation of Stapron S will concentrate on semi-structural automotive applications such as dashboard carriers, front-end fascias, windscreen plenums, underbody shields and exterior trim. Injection molding is the main process technology for the resin, but DSM is conducting research in blow molding and thermoforming. Second-generation grades of Stapron S, now under development, will focus on vertical body panels.

DSM is building a new Stapron S production facility in the Netherlands, which is slated to go on line in the first quarter of 1993. The DSM process involves the pure SMA resin being removed from the reactor and then modified with a proprietary elastomer by extrusion compounding. The resin will be compounded in the U.S. to better conform to local market specifications, according to Hartig.