Novel Transparent Plastics Offer High Stress Crack Resistance

ESC is the premature embrittlement of a plastic when it contacts specific fluids while under stress, which can cause surface defects that lead to localized fluid absorption. Consequently, this can result in craze initiation, crack growth, and eventual fracture.^1–4 In the medical device industry, it is critical to decrease the failure rate of plastic parts to protect patients and minimize device replacement. As a result, there is a growing demand for plastics that offer improved resistance to the stress cracking that can result from chemical attack.

Additionally, some plastics currently used in fluid-contact applications are under regulatory scrutiny for having leachable components. FDA is investigating bisphenol A (BPA) in medical devices as a possible health risk because of its estrogenic activity. The outcome of this investigation could limit the use of BPA-containing polymers in medical devices.⁵ Although alternative materials have been developed to fill the gap left by anticipated regulation, these new resins tend to be either more expensive or difficult to process than BPA-containing polymers. The challenge for plastics suppliers is to provide materials that are safe, affordable, easy to process, and resistant to ESC.

Improving Acrylic’s ESC Resistance

Acrylic resins have a number of attractive properties. They provide medical devices with optical clarity, toughness, biocompatibility, resistance to gamma radiation sterilization, bondability to flexible PVC, dimensional stability, and UV transmittance. They also do not contain BPA or other potentially harmful leachables. Up to now, however, acrylic resin development has concentrated on optimizing the properties that are mentioned above but it has fallen short of providing the exemplary ESC resistance that other polymers offer.

This logically leads to the following question: How can acrylic polymers be made more resistant to ESC without diminishing their other positive attributes? To answer that question, the authors assembled a proprietary toolbox based on mechanistic insights into ESC. As illustrated in Figure 1 (p. 80), this toolbox uses a variety of compositional features to address the required balance of properties for fluid contact medical devices.

Plexiglas CR50 and Plexiglas CR30 medical resins are highly chemical resistant. Both resins were engineered with a focus on achieving outstanding ESC resistance through high polymer-chain entanglement and polymer modification. Using industry-accepted protocols, their performance was assessed against other polymers currently used in fluid-delivery devices.⁶ In brief, annealed injection-molded parts were placed under a predetermined constant strain and exposed to a chemical for a finite period of time. After exposure, retention of tensile elongation at break was measured.⁷ Figure 2 demonstrates the retention of mechanical strength of the new resins versus comparable materials, when exposed to 70% aqueous isopropanol (IPA) and Cremophor RH40. The difference is most remarkable in the case of the Cremophor drug excipient, a lipid known to be aggressive towards plastic materials in medical applications. These data strongly suggest that the engineered acrylics show enhanced ESC resistance over traditional acryclic resins. As such, these plastics are suitable for fluid-delivery medical devices that require low failure rates and long service time.

In addition to chemical resistance, the ease of melt processing is an important property of polymer resins when molding medical devices. This is especially true with regard to those devices that are used in thin-walled and multicavity molds. To minimize the melt viscosity of the resin, select processing aids were empoyed that offered optimized moldability while retaining ESC resistance. Based on spiral flow test data presented in Figure 3, the melt processability of the engineered acrylics is similar to that of styrene-acrylic multi-polymers.⁸ In comparison, lipid-resistant polycarbonates require much higher processing temperatures, while copolyesters typically need to be rigorously dried prior to melt processing because they are prone to degradation when exposed to heat and moisture. The melt processability of the new acrylic medical grades makes them suitable for injection molding at moderate temperatures. They can be readily adapted to tooling designed for other thermoplastic polymers, such as polystyrene, ABS, and polycarbonate.

Another challenge of working with transparent medical resins is retaining their optical properties after they are sterilized by gamma radiation. This sterilization process is common in the medical industry and can be described as controlled exposure to ionizing radiation to reduce the bioburden to a safe level. Plastic devices subjected to irradiation sterilization often discolor or yellow as a result of ionization processes, although this phenomenon may fade over time in a process known as color recovery.

The new acrylic resins were formulated to inhibit the effects of such ionization processes. After exposure to 40 kGy of gamma radiation, these materials exhibited minimal yellowing and fast recovery of their initial optical properties when compared to a number of other transparent medical-grade polymers. These acrylic resins retained optical transparency and mechanical strength even when exposed to gamma radiation.

Conclusion

Plexiglas CR50 and Plexiglas CR30 are high-impact acrylics engineered to exhibit ESC resistance. Medical device companies looking to address ESC should learn about these plastics and consider whether they can be applied to a device project.

References

1. DC Wright, Environmental Stress Cracking of Plastics (Shawbury, UK: Smithers Rapra Technology, 1996).

2. JC Arnold, “The effects of physical aging on the brittle fracture behavior of polymers,” Polymer Engineering & Science 35, No. 2, (1995): 165–169.

3. M Kamal et al., “Residual Thermal Stresses in Injection Molding of Thermoplastics: A Theoretical and Experimental Study,” Polymer Engineering & Science 42, (2002): 1098–1114.

4. K Jansen et al., “Residual Stresses in Quenched and Injection Moulded Products,” International Polymer Processing 9, (1994): 82–89.

5. Federal Register, 73:200, October 15, 2008.

6. ASTM D543. 2006, “Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents,” (West Conshohocken, PA: ASTM International, 2006).

7. ASTM D638. 2010, “Standard Test Method for Tensile Properties of Plastics,” (West Conshohocken, PA: ASTM International, 2010).

8. ASTM D2123 (withdrawn), 1987, “Specification for Rigid Poly (Vinyl Chloride-Vinyl Acetate) Plastic Sheet,” (West Conshohocken, PA: ASTM International, 1987).