Syndiotactic Polystyrene: A New Polymer for High-Performance Medical Applications

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published July 1998

Syndiotactic polystyrene (SPS) is a semicrystalline polymer synthesized from styrene monomer using a single-site catalyst, such as metallocene. First synthesized in 1985 by Idemitsu Kosan Co. Ltd. (Tokyo, Japan), the material has been under joint product and process development by Idemitsu and Dow Plastics (Midland, MI) since 1988.

Because of its semicrystalline nature, SPS products exhibit performance attributes that are significantly different from those of amorphous styrenic materials. These properties include a high melting point, good chemical and moisture resistance, and a high degree of dimensional stability.

Resin, tensile bars, and impact discs of glass-filled, impact-modified syndiotactic polystyrene (top) and impact-modified syndiotactic polystyrene (bottom). Photo: Dow Plastics

Syndiotactic polystyrene can be differentiated from conventional styrenic polymers on the basis of molecular structure.Atactic, or general-purpose, polystyrene is produced with random stereochemistry, resulting in nonspecific placement of the cyclic aromatic portion of the molecule. In contrast, isotactic and syndiotactic polystyrene are made using stereo-specific catalysis techniques that result in highly ordered molecular structures.

Figure 1 shows the molecular structure of syndiotactic polystyrene. This high degree of molecular order gives rise to the ability of syndiotactic and isotactic polystyrene to crystallize from the melt, forming discrete crystalline domains and hence a semicrystalline microstructure. The kinetics of crystallization for syndiotactic materials are several orders of magnitude faster than for isotactic resins. For these reasons, the syndiotactic configuration provides rapid crystallization during processing and fabrication. On a microscopic scale, SPS crystalline polymer has a narrow molecular-weight distribution, so that the polymer, upon cooling from the melt, forms a crystalline structure that appears as tightly packed spherulites. The density of both the amorphous and the crystalline regions is 1.05, a value that favors isotropic part shrinkage and enhanced dimensional stability.

Figure 1. Molecular structure of syndiotactic polystyrene, showing side chains arranged in a symmetrical, regularly alternating pattern on either side of the polymer backbone.

As SPS cools from the melt, molecules organize and form distinct regions of crystallinity. The rate at which crystallinity develops in SPS can be controlled through the use of comonomers or the addition of nucleating agents. In injection molding, the temperature of the mold surface has also shown a major effect on crystallization. Each of these factors can be utilized to vary both the rate and level of crystallization in molded parts. Typical injection-molded SPS components have a level of crystallinity of approximately 50%.

Impact modifiers have been added to SPS to improve the impact resistance and tensile elongation of the neat resin. Glass reinforcement has also been used to produce SPS resins with high-heat resistance, good dimensional stability, and rigidity. Table I lists the typical physical properties of an impact-modified resin, XU72108.01L (hereafter designated SPS-IM), and a glass-filled, impact-modified resin, XU72107.02L (hereafter designated SPS-GF).

An important feature of resins that are appropriate for use in the medical industry is their compatibility with key sterilization methods. Testing has been completed on SPS for autoclave, ethylene oxide (EtO), and gamma sterilization. Following processing by each of these sterilization methods, tensile properties and instrumented dart impact were examined, along with any resin color changes.

Tensile elongation and color changes in a plastic are generally the first properties affected by sterilization. Figures 2 and 3 provide tensile data for SPS-IM and SPS-GF after 750 autoclave cycles. The autoclave cycle was for 13 minutes at 275°F, with pressure at 35 psi. Cycles were repeated in rapid succession, with all 750 occurring within two weeks. Results show that tensile strength remains relatively constant, whereas approximately 50% of the tensile elongation is retained. After the 750 cycles, both materials give evidence of a slight color change. This is observed as a yellowing of the natural samples, a discoloration that can be masked with colorants.

Figure 2. Tensile strength of impact-modified (SPS-IM) and 30%-glass-filled, impact-modified (SPS-GF) syndiotactic polystyrene following autoclave cycles.

Figure 3. Elongation of impact-modified (SPS-IM) and 30%-glass-filled, impact-modified (SPS-GF) syndiotactic polystyrene following autoclave cycles.

SPS after EtO sterilization shows no significant loss in tensile properties or instrumented dart impact. There is also no resin color change associated with EtO sterilization.

Following 100 kGy (10 Mrd) of gamma sterilization, the tested properties are not significantly different, and there is once again no change in resin color. Therefore, the data suggest that SPS is suitable for autoclave, EtO, and gamma sterilization.

Another important material attribute for many medical applications is chemical resistance. Semicrystalline thermoplastics generally have good resistance to chemicals, and SPS offers resistance to a wide range of solvents, including acids, bases, and most organic solvents. The exceptions are organic solvents with solubility parameters close to styrenic polymer, such as benzene and toluene.

Figure 4 shows the tensile strength that is retained after SPS samples were immersed in various solvents for 30 days. The tested samples were compared to controls that had no exposure to solvents to determine the effects of the chemicals.

Figure 4. Chemical resistance of syndiotactic polystyrene.

SPS is typically molded using standard injection molding machines. Barrel temperatures normally range from 520° to 620°F (270°–326°C) with the nozzle and melt temperatures set at 600°F (315°C). The mold temperature varies from 160° to 300°F (71°–149°C), depending on part geometry. As mentioned earlier, the mold temperature does have an effect on crystallinity; for example, thinner-walled parts require a hotter mold to attain the needed crystallinity.

The unique combination of properties of SPS makes it well suited for a number of potential medical device applications. SPS is a nonpolar polymer that does not attract appreciable moisture: in most cases, predrying before fabrication is not required. The polymer's nonpolar nature is also of benefit when parts made from SPS are exposed to autoclave cycles, where moisture could possibly accelerate mechanical failure.

In addition to being nonhygroscopic, SPS also features a low melt viscosity, which means that it is easy to process compared with competitive high-heat materials. Capillary melt-viscosity measurements of SPS versus atactic polystyrene at their processing temperatures are presented in Figures 5 and 6, which show that the capillary melt-viscosity curves for the two materials are similar. These results support the experience of fabricators who report that SPS is able to fill thin-wall sections and flow long lengths without excessive injection pressures. SPS also has a lower specific gravity than comparable resins, enabling more parts to be made from a given volume of polymer.

Figure 5. Capillary melt viscosity for SPS polymers (tested at 320° and 340°C) versus HIPS (tested at 220° and 240°C).

Figure 6. Capillary melt viscosity for SPS polymers (tested at 280° and 300°C) versus HIPS (tested at 180° and 200°C).

Among specific medical uses are sterilization trays, surgical instruments, and dental equipment. Other key application areas include thin-walled parts or parts with long, difficult flow paths. SPS has the potential to satisfy a range of product designs for which sterilization compatibility, resistance to moisture, and dimensional stability are required.

This article reviewed the results of tests conducted on standard tensile bars and disks and concluded that SPS can be sterilized by repeated autoclave cycles, gamma radiation, or ethylene oxide gas. SPS has good dimensional stability and is resistant to many chemicals, including alcohols. SPS resins are also able to fill thin-walled parts without excessive injection pressures. These results are a good indication of the material's suitability for various applications in the medical industry. Of course, this study is intended to serve as a useful tool for the preliminary stages of material selection, and in no way diminishes the need for testing final devices in real-use situations.

Pusko Jezic Z, Polimeri, 18(3–4): 150–157, 1997.

QUESTRA Crystalline Polymers Suggested Injection Molding Conditions, Dow Plastics form number 301-02684-197, Midland, MI, Dow Plastics, 1997.

Schut JH, "Why Syndiotactic Polystyrene Is Hot," Plast Tech, February, p 26, 1993.

Yamato H, Styrenics 1993 Conference, Session 1-1, December 1993.

Nancy J. Hermanson, medical market technical leader, and Tom E. Wessel, development leader, work in technical service and development for Dow Plastics (Midland, MI). Nancy has 14 years of plastics experience and specializes in the medical device industry. Tom has been with Dow for 33 years, with his most recent project involving the development and commercialization of Questra crystalline polymers.

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