Polycarbonate

Polycarbonate is one of the very few polymers that have glass-transition (T_g) and heat-deflection temperatures (HDT) high enough to withstand normal steam sterilization conditions in medical device applications.^1­4 Other steam-sterilizable polymers include polysulfones and poly(etherimide).^2­4 Among these materials, polycarbonate remains the most attractive and economically viable because of its excellent combination of performance, processibility, and cost. However, it has been widely reported that polycarbonate quickly loses its ductility or impact properties following repeated steam sterilization cycles.^2­4

Numerous studies in the past have shown that polycarbonate is subject to hydrolysis by hot-water aging.^2­9 This conclusion was derived from evidence that showed a loss in molecular weight⁶ or viscosity,^2­4 an increase in melt-flow rate,⁶ or the presence of monomers (bisphenol A) and oligomers on the sample surface.⁵ Condensation polymers such as polyesters and copolyesters are also known to undergo similar hydrolysis and can lose mechanical properties as a consequence of hot-water aging.^10­15 Most of the studies involved long-term aging on the order of days or months. Other investigations focused on the effects of repeated heat-aging cycles and reported that cyclic exposure conditions are more detrimental to the mechanical properties of polycarbonate than are conditions of continuous exposure.^2­4,16

These previous studies have led to a general belief that the ductility loss of polycarbonate following steam sterilization is immediately attributable to hydrolysis. Some authors ascribe the embrittlement to a combination of hydrolysis and the formation of "microcavitation" or "microvoids" as a result of water supersaturating at room temperature.^2­4,7,9

The purpose of the present study is to reexamine the root causes of ductility loss and occasional brittle failure of polycarbonate that typically arise following short-term steam-sterilization processes.

Experimental

The materials used in this study were blue-pigmented polycarbonates containing less than 2% of titanium dioxide, less than 0.3% of ultramarine blue, and traces of carbon black. The molecular weight, polydispersity, and melt-flow rate (MFR) for the six polycarbonates used are summarized in Table I. MFR was measured at 300°C/1.2 kg. (Tables and figures not yet available on-line.)

Prior to MFR measurement, pellets were dried in a vacuum oven at 125°C for 72 hours under 29 mmHg. Following rigorous drying, the resins were injection molded into standard 0.125-in.-thick tensile bars according to ASTM procedures. The bars were then placed in a steam sterilization vessel and sterilized at 250°F under 30-psig pressure for a specific amount of time up to 120 minutes.

After the specimens were cooled down to room temperature and conditioned overnight, tensile testing was performed at a speed of 10 in./min, with elongation at break recorded as a measure of ductility. Ten specimens were tested for each sample variable.

The fracture surfaces of the tensile bars were examined with both a light microscope and a scanning electronic microscope (SEM) to search for clues about the failure mechanism. Any foreign matter on the fracture surfaces was identified with microscope infrared (Digilab FTS-60). Gel-permeation chromatography was employed to determine weight average and number average of molecular weight for the raw resins and the molded tensile bars.

The aging effect of steam sterilization was determined by measuring the area under the endothermic peak (overshoot) near T_g on a differential scanning calorimeter (DSC) thermogram. The energy measured is referred to as recoverable enthalpy.

Results And

Discussion

Figure 1 illustrates the effects of steam sterilization on the ductility of polycarbonate resin samples MW-1 (MFR = 32) and MW-3 (MFR = 21), with each tensile-elongation-at-break data point representing an average of 10 test specimens. The ductility of the lower-molecular-weight sample MW-1 falls off more rapidly with increasing steam exposure time than does that of the higher-molecular-weight sample MW-3.

The sterilized specimens were tested for molecular weight, as summarized in Figure 2. The molecular weight of each sample remains constant after various sterilization cycles, despite the substantial decay in ductility. There was a slight decrease in molecular weight from the control sample (in pellet form) to the sterilized samples (molded tensile bars), which can be attributed to minor thermal degradation from the injection molding process.

In order to discover the underlying cause of the ductility loss, tensile data for each test specimen were examined. The scattering of the ductility data for the lowest-molecular-weight sample MW-1 is enormous, as shown in Figure 3. Six out of 10 samples (specimen numbers 1, 4, 5, 6, 7, and 8) exposed to 45 minutes of steam sterilization exhibited a brittle failure mode. It is interesting to note that the four other samples ruptured in a ductile mode. The heterogeneous nature of the failure mode suggests that the brittleness cannot be caused by an inherent homogeneous parameter such as molecular weight.

Compared with sample MW-1, the medium-molecular-weight sample MW-3 produced more uniform tensile elongation data after the same 45 minutes of steam sterilization. Only three specimens fall off the bulk average region (as shown in Figure 4), and only two brittle failures were observed (specimens 2 and 10). Specimen 10 underwent a catastrophic failure even though most of the other samples were very ductile, which suggests that the brittle failure is sporadic and random. In view of the fact that not all the finished devices in the field are subject to the same tensile stress level as the samples in the current designed experiments--and that not all the parts are brittle even under severe stress conditions--the observed results can be translated into a realistic defect rate in the parts-per-million level often found in the field. Comparing the results of MW-3 with those of MW-1 suggests a greater resistance of the higher-molecular-weight polycarbonate to steam-induced brittle failure.

To confirm the molecular-weight effect, the data from the highest-molecular-weight sample MW-6 were examined. As shown in Figure 5, all specimens were broken in ductile mode and the elongation-at-break data have higher values and are very consistent and uniform. It is evident that molecular weight does play a very crucial role in ductility retention during or after steam sterilization.

In an attempt to further define the root cause of the random brittle failure of the polycarbonate, the fracture surfaces of the tested tensile bars were examined with an SEM. It was discovered that for every brittle fracture surface there was a speck that initiated radial cracks that critically failed the sample before it was fully elongated. The size of the specks ranges from a few to several hundred microns (see Figure 6). Infrared microscopic analysis identified these specks as foreign matter. To minimize the possibility of contamination, the injection molding experiments were repeated with thorough cleaning and purging, and were performed at more than three independent locations including the cleanroom facilities of the material suppliers. Results indicated that the natural inclusion of minor foreign specks appears to be inevitable, although it can be reduced.

The foreign specks or contaminants identified in a wide range of samples include unpigmented polycarbonate, natural polyethylene, degraded PVC, degraded nylon, cardboard chips, aluminum fragments, chromium, titanium, silicone, and sulfur. These specks could have been introduced during any of a number of processes, including polymerization, compounding, pigment formulating, material transfer and handling, or injection molding.

Although the high-molecular-weight sample MW-6 demonstrated 100% ductile failure, contaminants were also found on the fracture surface. The only difference is that no microcracks radiating from the specks were found in these samples. The higher-molecular-weight polycarbonate appears to tolerate the presence of contaminants better than the lower-molecular-weight grade.

In addition to the sporadic brittle failure caused by contaminants, there is a clear reduction in overall elon-gation after steam sterilization. It is well known that amorphous polymers lose some of their impact strength upon aging^17­19 and that this aging effect can be accelerated by heat. To find out whether steam sterilization imposes any aging effect on a material, compression-molded polycarbonate film was subjected to autoclave at 121°C for different periods up to 32 hours. The aging effect was measured by the level of recoverable enthalpy in the DSC scan. The endothermic energy absorbed during heating at T_g is the energy required to relax the molecular segments that have been compacted as a result of the effects of aging. As shown in Figure 7, steam sterilization does impose aging effects on polycarbonate, because the degree of aging increases with increasing steam sterilization time. This aging effect can be confirmed by the similar results from samples subjected to a dry-oven heating experiment (see Figure 7). The aging effect thus accounts for the general decay of elongation for those samples failed in a ductile mode.

Conclusion

Steam sterilization causes ductility loss of polycarbonate in two different ways: through a general reduction in ductility and through sporadic brittle failure of the material. Based on molecular-weight data, it can be concluded that hydrolysis did not occur during steam sterilization cycles lasting up to 120 minutes.

Drastic brittle failure of the parts can be induced during the first 30 minutes of steam exposure without molecular-weight breakdown. The rate of brittle failure increases with steam exposure time or with the number of sterilization cycles, and is caused mainly by the presence of foreign specks or contaminants. The adhesion between these particles and the surrounding polycarbonate matrix may be destroyed during exposure to heat and moisture. As a result, the polycarbonate relaxes and separates from the particle interface, enabling the specks to act as seeds for the radial propagation of cracks that become the weakest physical link when the part is subjected to mechanical stress or impact.

This study also indicated that the general reduction in ductility of polycarbonate is caused by the aging effects imposed by thermal exposure during steam sterilization. Resistance to both the aging effects and contaminant-induced brittle failure increases with increasing molecular weight. The retention of polycarbonate ductility can be achieved by using higher-molecular-weight resins or by minimizing contamination during material processing.

Acknowledgments

The authors wish to thank Bayer Corp. and GE Plastics for providing the materials, samples, and related technical information needed for this study.

References

01.Hong KZ, "Clear Plastics for Medical Applications," Med Plast Biomat, 1(1):48, 1994.

02.Rosato DV, "Polymer Resistance to Hot-Water and Steam Sterilization," Med Dev Diag Indust, 7(7):48, 1985.

03.Robeson LM, Dickinson BL, and Crisafulli ST, "Engineering Resins Hold Up in Heat Sterilization," Mod Plast, September, p 108, 1985.

04.Robeson LM, Dickinson BL, and Crisafulli ST, "Hydrolytic Stability of High T_g Engineering Polymers: Relevance to Steam Sterilization," Polym News, 11: 359, 1986.

05.Bair HE, Falcone DR, Hellman MY, et al., "Formation of BPA on the Surface of Hydrolyzed Polycarbonate," Polym Prepr, 20(2):614, 1979.

06.Pryde CA, Kelleher PG, Hellman MY, et al., "Hydrolytic Stability of Some Commercially Available Polycarbonates," Polym Eng Sci, 22:370, 1982.

07.Narkis M, and Bell JP, "An Unusual Microcracking/Healing Phenomenon in Polycarbonate at Room Temperature," J Appl Polym Sci, 27:2809, 1982.

08.Joseph EA, Paul DR, and Barlow JW, "Boiling Water Aging of a Miscible Blend of Polycarbonate and a Copolyester," J Appl Polym Sci, 27:4807, 1982.

09.Narkis M, Nicholais L, Apicella A, et al., "Hot Water Aging of Polycarbonate," Polym Eng Sci, 24:211, 1984.

10.Gordon RJ, and Martin JR, "Effect of Relative Humidity on the Mechanical Properties of Poly (1, 4-Butylene Terephthalate)," J Appl Polym Sci, 25:2353, 1980.

11.Borman WFH, "The Effect of Temperature and Humidity on the Long-Term Performance of Poly(butylene tere- phthalate)," Polym Eng Sci, 22:883, 1982.

12.Kelleher PG, Wentz RP, and Falcone DR, "Hydrolysis of Poly(butylene terephthalate)," Polym Eng Sci, 22:248, 1982.

13.Bastioli C, Guanella I, and Romano G, "Effects of Water Sorption on the Physical Properties of PET, PBT, and Their Long Fibers Composites," Polym Compos, 11(1):1, 1990.

14.Sawada S, Kamiyama K, Ohgushi S, et al., "Degradation Mechanisms of PET Tire Yarn," J Appl Polym Sci, 42:1041, 1991.

15.Gallucci RR, Dellacoletta BA, and Hamilton DG, "Hydrolysis-Resistant Thermoplastic Polyesters," Plast Eng, November, p 51, 1994.

16.Maslyar KD, and Thomas JR, "Sterilization of Polycarbonate," in Proceedings of the Society of Plastics Engineers Regional Technical Conference (RETEC), Brookfield, CT, Society of Plastics Engineers, p 619, 1980.

17.LeGrand DG, "Crazing, Yielding, and Fracture of Polymers. I. Ductile Brittle Transition in Polycarbonate," J Appl Polym Sci, 13:2129, 1969.

18.Struik LCE, Physical Aging in Amorphous Polymers and Other Materials, New York, Elsevier, 1978.

19.Woo L, and Cheung YW, "Physical Aging Studies in Amorphous Polyethylene Terephthalate Blends," Thermochim ACTA, 192:209, 1991.

K. Z. Hong, PhD, is manager of materials technology and engi- neering at the Medical Materials Technology Center of Baxter Healthcare Corp. (Round Lake, IL). He specializes in polymeric materials and related processing and is responsible for medical material development, selection, qualification, and approval. The coauthors are also at Baxter: Chuan Qin, PhD, is an engineering specialist concentrating on polymer structure and physical properties; and Lecon Woo, PhD, is the Baxter Distinguished Scientist specializing in biomedical polymer development and polymer rheology and processing.