Materials: The Elastosil LR3003/50 LSR used in this study was obtained from Wacker Chemie AG (www.wacker.com). The Elastosil R4000/50 platinum-cured HCR used in this study also came from Wacker. HV3 622 base-rubber peroxide-cured HCR was obtained from GE Bayer Silicones GmbH & Co. KG (www.gesilicones.com). It was cured by Di-2,4-dichlorobenzoylperoxide with 1% loading.

Sterilization By Gamma Exposure: Three levels of gamma irradiation experiments were performed in air on the three silicone rubbers by STERIS Isomedix Services using 60 Co as a radiation source. The dosing levels of gamma radiation were 25, 50, and 75 kGy.

Sterilization By E-beam Exposure: Three levels of e-beam irradiation experiments were performed in air on the three silicone rubbers by STERIS Isomedix Services using an 80-kW 5-MeV system electron beam. Dosing levels were 10, 40, and 80 kGy.

Sterilization By EtO Treatment: The three silicone rubbers were exposed to 100% ethylene oxide at room temperature in a fully automated conveyor system at the STERIS Isomedix Services facility in Spartanburg, SC.

Mechanical Testing: We evaluated tensile, modulus, and elongation properties on an Instron material testing machine using the ASTM D-412 standard. Tear tests were performed on the same machine according to the ASTM D-624 standard. Hardness measurements were carried out on a Shore A durometer from Rex Gauge Co. (www.rexgauge.com), following the ASTM D-2240 procedure, and measured in “shA” units. Each data point reported on the plots in Figures Figures 318 is an average of five specimens from the same sample.

Results and Discussion

Effect of Gamma Irradiation: As the gamma radiation dose increases, the hardness and modulus of silicone rubber also increase because they are directly proportional to the crosslink density of the rubber. In our study, hardness (increase by >10 shA after 75-kGy radiation) and modulus (>200% increment after 75-kGy radiation) of the peroxide-cured HCR were most affected by gamma radiation (Figures 3 and 4). This can be explained by the presence of radiation-sensitive oxygen-containing active species in peroxide cured silicone rubbers that promote the formation of free radicals, resulting in increased chain branching. The hardness and modulus of platinum cured LSR and HCRs increase slightly upon gamma radiation exposure, possibly due to unreacted SiH functionalities inducing further crosslinking. The slight initial drop (after 25 kGy gamma radiation) in the durometer hardness and the tensile modulus of the platinum cured HCR may be attributed to a disruption of the hydrogen bonds on the filler surface (10). This behavior has also been observed by Patel et al. (11).

Unsurprisingly, gamma radiation reduced the tensile elongation of each of the three silicone rubbers tested, as illustrated in Figure 5. Property degradation of the peroxide cured rubber was more extensive, with up to 75% reduction in tensile elongation occurring after exposure to a 75 kGy dose of gamma radiation. Only 45% and 35% reductions in tensile elongation were observed at the same radiation dose for platinum cured LSR and HCR, respectively.

The effect of gamma radiation on tensile strength is mostly due to the silicone rubber filler content and treatment (proprietary information for the commercially available materials evaluated during this study). So it is difficult to predict or rationally explain the data trend plotted in Figure 6 We observed that gamma radiation had a minimal effect on the tensile strength of LSR and platinum-cured HCR. By contrast, the tensile strength of the peroxide-cured HCR was reduced by 50% after the sample was exposed to a 75-kGy dose of radiation.

Tear strength is primarily affected by the arrangement of crosslinking, bimodality (ratio of short polymer chains to long chains), and the amount of crosslinker used for curing rather than by the crosslink density itself. It is also well known that the tear properties of silicone rubber are dictated by the amount and type of filler used (11). Again, information on the content and nature of fillers is unavailable for the materials evaluated here. However, it is known that the tear strength of platinum-cured silicone rubbers is generally higher than that of peroxide-cured HCR due to the nature of the crosslinks (Figure 7).

As Figure 8 shows, gamma radiation reduced the tear strength of all three products. The platinum-cured HCR was most resistant to gamma radiation: A 30% decrease of tear strength occurred when it was exposed to a 75-kGy dose of radiation, whereas the tear strength of the peroxide-cured HCR exhibited a 45% reduction after receiving the same dosage.