Originally published January 1997
TECHNICAL PAPER SERIES
An ongoing trend in the medical manufacturing industry is the replacement of metal components and devices with advanced engineering thermoplastics. The wide variety of available thermoplastics offers designers opportunities to decrease costs and increase both production rates and profits. Most medical devices require sterilization after they are packaged, and another trend has been the move to sterilization by gamma radiation as opposed to other methods such as ethylene oxide (EtO) gas. Advantages of gamma irradiation include speed, cost-effectiveness, and the elimination of the need for special packaging.
A potential drawback of gamma processing, however, is that this form of high-energy irradiation can affect both the color and the physical properties of many thermoplastics. Although there is ample information available pertaining to unmodified grades of thermoplastics, data are limited regarding the specialty lubricated and reinforced compounds that are becoming more prevalent in the medical industry. Additives such as glass reinforcement, pigment, and lubricants have a significant effect on the properties of unsterilized thermoplastics. These fillers will also influence the compounds after processing, given their potential to mask or intensify the effects of sterilization. This paper investigates the effect of 3.5-Mrd gamma radiation on nylon, polycarbonate, and acetal compounded with different internal lubricants, pigments, and levels of glass-fiber reinforcement. Specifically, the tensile strength, impact strength, and color characteristics were evaluated both before and after sterilization. The results should prove useful in the development of thermoplastic composite parts designed for single sterilization during their life cycle.
A total of 10 different materials were molded into ASTM test specimens for evaluation. Properties examined in this study were tensile strength (ASTM D 638), with specimens at 3.18 mm (0.125 in.); Izod impact (ASTM D 256), with specimens at 3.18 mm (0.125 in.); and Yellowness Index (ASTM D 1925). Specimens were irradiated with gamma radiation at levels of 3.5 Mrd. In general, the greater the radiation level, the greater the effect on plastic materials, and the dosage was chosen because it represents a worst-case scenario compared with the industry standard of 2 Mrd.
Tensile testing was conducted on both the sterilized samples and control samples so as to obtain a direct comparison and to determine the effect of sterilization on tensile strength. Additionally, some of the sterilized samples were tested 6 months after sterilization to ascertain whether the property changes are time-dependent. Izod impact data were generated in the same fashion, with some of the sterilized samples retested 6 months after the irradiation procedure.
In order to evaluate the color shift due to sterilization, test samples remained concealed from light. Control samples were measured for the initial Yellowness Index to obtain a direct comparison. The time between sterilization and color measurement was about 5 days.
Because the materials evaluated in this study vary significantly in actual tensile strength, the most effective means to evaluate the effect of sterilization on the composites is to examine the percentage of tensile-strength retention after exposure. This method allows for a more realistic comparison among materials, regardless of the overall strength of any specific material. The actual tensile-strength data are listed in Table I. The tensile strengths of the three glass-fiber polycarbonates (GF PCs) and carbon-fiber polycarbonate (CF PC) were largely unaffected by the gamma radiation, as illustrated in Figure 1. These materials maintained nearly 100% of their original tensile strength after sterilization, with the slight variances evident in Table I most likely the result of testing variation.
Figure 1. Tensile-strength retention (per ASTM D 638 test method) after exposure to 3.5-Mrd gamma radiation.
Table I. Tensile strength data per ASTM D 638 test method, with specimens at 3.18 mm (0.125 in.). Gamma irradiation was at 3.5 Mrd.
In the nylon family, the glass-fiber nylon 6/6 (GF PA 6/6), nylon 6/6 alloy (PA 6/6 alloy), and glass-fiber polyphthalamide (GF PPA) also were unaffected by the gamma radiation at this dosage level. However, both the PTFE-lubricated acetal and PTFE-lubricated/glass-fiber acetal compounds experienced significant losses in tensile strength--35 and 45%, respectively. This was expected, in that acetal is extremely sensitive to gamma radiation and acetal compounds are not suitable for applications requiring gamma sterilization. It is interesting to note, however, that the addition of glass fiber further decreased the percentage of acetal's tensile-strength retention.
In order to evaluate the stability of the measured tensile strength, additional sterilized specimens were tested 6 months after sterilization and compared with the same controls (see Figure 2). The polycarbonate and PPA (low-moisture, high-temperature nylon) compounds remained very stable, with minimal variation, and the acetal materials were as equally degraded as in the initial testing. Both the GF PA 6/6 and PA 6/6 alloy showed some reduction in tensile strength; however, this is most likely due to moisture absorption as opposed to the effects of sterilization. Because the PA 6/6 alloy is less moisture-sensitive than standard PA 6/6, it consequently demonstrated a higher percentage of tensile-strength retention. In summation, tensile strength does not appear to be a time-dependent variable unless combined with other factors such as environment.
Figure 2. Tensile-strength retention (per ASTM D 638 test method) 6 months after exposure to 3.5-Mrd gamma radiation.
IZOD IMPACT PROPERTIES
Impact testing was carried out on the sterilized and control samples and again presented in the form of percentage of property retention (see Figure 3; actual data are listed in Table II). Although there was virtually no change in the 30% GF PPA, all other materials did experience a decrease in impact strength after sterilization. Whereas the acetal compounds suffered major losses in impact strength, the PA 6/6 alloy and PC compounds were minimally affected.
Table II. Izod impact strength data per ASTM D 256 test method, with specimens at 3.18 mm (0.125 in.). Gamma irradiation was at 3.5 Mrd.
Figure 3. Impact-strength retention (per ASTM D 256 test method) after exposure to 3.5-Mrd gamma radiation.
As with the tensile specimens, some of these materials were tested again 6 months after sterilization (see Figure 4). These test results were extremely consistent with results of the impact testing performed immediately after gamma sterilization. This was unexpected, especially given the variability of impact testing. However, the general trend in the data indicated that impact strength is slightly more susceptible to gamma sterilization than is tensile strength.
Figure 4. Impact-strength retention (per ASTM D 256 test method) 6 months after exposure to 3.5-Mrd gamma radiation.
Most thermoplastics experience a color shift after exposure to gamma radiation. This shift is most often characterized by a yellowing effect, and the degree of shift can be measured by evaluating the Yellowness Index of the specimens both before and after exposure to the radiation dose.
Table III. Yellowness Index data per ASTM D 1925. Gamma irradiation was at 3.5 Mrd.
Table III shows that the presence of pigment systems or glass fiber has a significant impact on the color stability of thermoplastic compounds following sterilization. For example, the unfilled grade of polycarbonate has a color shift (E) of 41, whereas a similar material with 10% glass fiber has a E of 21. Increasing the glass-fiber content to 40% and adding a white pigment (WT) further reduces the E to 5.82, which represents an 85% reduction in color shift in comparison with the unfilled grade. Therefore, even white materials will exhibit improved color stability after sterilization when compared with unmodified base resins.
The gray (GY) PPA and blue (BL) acetal specimens were not measurable on the Yellowness Index. However, any color shift in these materials was virtually indistinguishable. Additionally, the PA 6/6 alloy did appear to have improved color stability compared with the 30% GF PA 6/6.
The effect of gamma sterilization on the tensile and impact properties of thermoplastic composites appears to be primarily dependent on the sensitivity of the base resin. For example, PC and nylon resins are not significantly affected by low dosages of gamma radiation, and neither are their composites. Conversely, acetal compounds are severely degraded by low levels of gamma radiation, and the same is true of their composites. In addition, the changes in tensile and impact characteristics appear to be both permanent and constant, at least through a period of 6 months following sterilization.
The effect of gamma sterilization on the color of thermoplastics is extremely sensitive to both the type and level of additives in the composites. The addition of glass fiber has a significant effect on reducing the color shift in PC. Combining glass with white pigment results in a composite that undergoes only a minor color shift. The initial color of a material also plays a significant role. For instance, there was no discernible change in the dark gray PPA or the black CF PC. The blue acetal experienced massive physical degradation, but the color change was highly masked.
An understanding of the potential effects of gamma sterilization on both the physical and color properties of thermoplastic materials increases a designer's ability to develop products with the necessary characteristics. Selecting a base resin that is compatible with the sterilization process is critical to successful part design. Equally important is selecting the proper filler or pigment, which, among other benefits, can help minimize color shift. Information regarding the commercially available grades of the materials tested can be found in Table IV.
Table IV. Generic descriptions and commercial grades of materials tested.
The author wishes to thank Greg Jarrell and Joe Combs for their assistance with physical and color evaluations.
Joshua McIlvaine works as an application development engineer in the Customer Application Center of LNP Engineering Plastics (Exton, PA), where his current responsibilities include developing new plastic applications for various industries. At LNP since 1992, he previously worked in the company's technical service group focusing on injection molding optimization and customer-support activities.