Why syndiotactic PS is hot.

Syndiotactic polystyrene is a new class of semicrystalline thermoplastics that are first cousins in chemistry, but not in performance, to familiar amorphous PS. "SPS" is said to be in extensive precommercial customer development by Idemitsu Petrochemical Co. in Tokyo and its U.S. licensee and research partner, Dow Plastics, Midland, Mich.

SPS is unusual among polystyrenes for its high crystalline melting point (518 F), and consequently high practical heat resistance. Compared with conventional PS, SPS also boasts improved chemical resistance. These characteristics, along with very good electrical properties and lack of moisture sensitivity, could make SPS potentially competitive--in both price and performance--with engineering resins like thermoplastic polyesters, nylon, and perhaps even PPS, Dow and Idemitsu say.

In the syndiotactic form of PS, the pendant benzene rings alternate regularly on opposite sides of the polymer TABULAR DATA OMITTED backbone. In commercial atactic PS, they attach randomly along the chain. (A third form, isotactic PS, was made in laboratories back in the 1950s. It theoretically could have even better crystalline properties than SPS, but tends to crystallize too slowly to develop these properties fully before the melt solidifies.)

SPS is a product of the new metallocene or "single-site" catalyst technologies that are making waves in polyolefins these days because of their ability to make novel resins (such as syndiotactic PP) with very narrow and molecular-weight distribution. Dow and Idemitsu both have composition-of-matter and applications patents, but the overwhelming preponderance were filed by Idemitsu, which has several hundred patents covering catalyst, material, process and applications, substantially more than any other company. Other companies did SPS research in the late '80s--including Montedison in Italy and Sumitomo Corp. and Asahi Denka Kogyo KK in Japan--but all are thought to have backed off in view of the firm Idemitsu/Dow patent position.

Dow and Idemitsu each say they have sampled 50 to 100 companies and are doing extensive premarket development work with a small number of them. The two companies have cooperated on SPS process development since 1988. Each has been developing SPS grades at a pilot plant capable of 40,000-100,000 lb/yr. Idemitsu's pilot plant in Chiba started up three years ago, and Dow's in Midland two years ago. Dow v.p. of R&D Dennis McKeever says Dow plans to "more than double" its pilot plant production this year.

THE NEW SPS GRADES

The initial injection molding and extrusion grades that Idemitsu and Dow are sampling include a high-impact SPS blended with 20% rubber, and the same filled with 15% or 30% glass fiber in standard and flame-retardant (UL 94V-0) versions (see table for preliminary properties on some developmental grades). Neat grades are for extrusion and thermoforming; glassfilled grades are for injection molding. Both companies are developing copolymer versions; alloys are another likely possibility.

Dow says it is initially targeting automotive and electronic applications, including injection molded underhood electrical parts (the kinds now made primarily of PPS and nylon). Dow has also done extensive internal research on a semipermeable membrane made of syndiotactic PS for use in the chemical processing industry to filter corrosive materials from solutions.

Idemitsu's initial target applications appear to be broader in scope, but Idemitsu says they will eventually converge, and that there is no division of markets between the two companies. Idemitsu is courting a variety of automotive applications, as well as appliances. Another big potential market is printed circuit boards, says Idemitsu's manager of SPS development Narumi Hata, because "the dielectric constant of these resins is very low, close to PTFE." SPS has a dielectric constant of 2.6, vs 2.4 for PTFE.

In packaging, Dow sees an obvious market for high-heat thermoformed trays. Idemitsu has intriguing patents on melt-to-mold forming of five-layer sheet of SPS, EVOH and adhesive tie layers. This Idemitsu process is rumored to involve holding the material at temperatures 140|degrees~ F higher than the melt temperature (518 F) of the material before molding. Both companies also seem keen on microwavable packaging. Neither has yet applied for FDA approval for the new resins, but Dow says unfilled grades would likely already comply.

SPS does have one drawback: it tends to be brittle. This probably makes it unsuitable for snap-fit parts.

HOW SPS PROCESSES

Idemitsu and Dow say their new grades process "extremely well" in standard injection molding and extrusion machinery. Because of its crystalline nature, cycle times are slightly faster for syndiotactic than for conventional PS, says Dow development associate Thomas Wessel. SPS has a sharp melting point, while amorphous PS melts gradually over a temperature range. (Wessel plans to deliver a technical paper on SPS at this year's Society of Automotive Engineers convention in Detroit at the beginning of March.)

"The only |processing~ problem is mold temperature," Hata notes. Normally PS molds are controlled with hot water to a maximum of 212 F. But processing temperatures for SPS are considerably higher--530 F for extrusion, 570 F for injection, vs. 350-450 F for standard PS. Thus, some SPS applications will require molds heated to a minimum of 300 F for proper crystallization and good surface quality.

THE PACE QUICKENS

When Idemitsu and Dow first announced their R&D partnership last September, the timetable for commercialization was for Idemitsu to build a commercial plant of greater than 20-million-lb/yr capacity in Japan by 1996 and Dow to build one in the U.S. somewhat later.

Idemitsu's Hata says, "The decision to build the commercial plant will be made soon. It could take 18 months for construction." Dow people also say that they expect material could become available from Japan as early as late next year. Dow is expected to build its plant some time later. But whether Dow builds a plant, and how big it might be, both depend on precommercial applications work being done this year and next.

As noted above, the new resins will ultimately be priced to compete with engineering resins, partly due to the high manufacturing cost. Dow says they could be available in the U.S. by 1995 at a developmental price of about $2/lb. The company cautions processors not to imagine that SPS will ever achieve commodity PS pricing levels.