Effect of saline exposure on the surface and bulk properties of medical grade silicone elastomers

Medical-grade silicone elastomers were subjected to accelerated aging in saline to verify the hydrolytic stability of the elastomer. Tensile strength, elongation at break, and the elastomer stress measured at 100% or 200% elongation did not change significantly for peroxide-cured sheeting aged in 37 degrees C or 100 degrees C saline for 45 h. Under similar condition, hydrosilylation cured sheeting behaved similarly; however, increases in stresses measured at 100% and 200% elongation were observed after the first hour of treatment. After the first hour, the physical properties remained relatively constant. On either elastomer, initial liquid drop advancing contact angles for water ranged from 110 degrees to 115 degrees, and in no case was a change of > 6 degrees observed as a result of aging in saline for 45 h at temperatures up to 97 degrees C. The high advancing angles indicated that the surface remained largely hydrophobic. Initial liquid drop receding contact angles ranged from 48 degrees to 64 degrees, with receding contact angles being more sensitive to accelerated aging, in one case decreasing to 14 degrees. Similar decreases in receding contact angle were observed on polyethylene subjected to the same accelerated aging conditions. Decreases in receding contact angle were not considered to be indicative of extensive hydrolysis. The observed contact angle phenomena are consistent with current views of contact angle hysteresis being caused by surface heterogeneity. There was no evidence of significant surface or bulk siloxane hydrolysis under these accelerated aging conditions.