Effect of Antioxidants on Ultra High Molecular Weight Polyethylene (UHMWPE)

This thesis presents a study on the effects of antioxidants on the oxidative induction time (OIT), thermal behavior, and hardness property of ultra high molecular weight polyethylene (UHMWPE). The antioxidants were dissolved into methanol alcohol before the solution was mixed with UHMWPE powder. The mixture was degassed under vacuum, and then compression-molded into disks 1.25 in. in diameter. Different antioxidants were evaluated at concentrations up to 0.6 percent. A 0.6 percent concentration was used to screen the effectiveness of various antioxidants. Effect of antioxidant concentration was investigated in the range between 0 and 0.6 percent. OIT was evaluated on UHMWPE with various antioxidants to understand the effects of antioxidants on oxidation resistance of the polymer. The experiments were performed using high pressure differential scanning calorimetry (HP DSC) with an isothermal temperature of 165 °C, and pure dry oxygen under a pressure of 500 psi. Regular differential scanning calorimetry (DSC) was used to evaluate the thermal behavior and changes in molecular structure of the polymer with antioxidants, in which a scanning rate of 10 °C/min. was utilized from room temperature to 200 °C. With the regular DSC, thermal stability of UHMWPE with various antioxidants was assessed from room temperature up to 300 °C. Durometer hardness (Type D) was tested on the polymer in order to understand changes in mechanical property and polymeric structure of UHMWPE due to antioxidants. The OIT of UHMWPE was increased with the addition of antioxidants except β-carotene. Quercetin, at a concentration of 0.6 percent, was the most effective antioxidant, which increased the OIT 1133 percent. The effectiveness of antioxidant increased as their concentrations increased in the tested range. The effectiveness can be arranged in descending order as: quercetin \u3e BNX 1010 \u3e GSPE \u3e BHT \u3evitamin E \u3e TBHQ \u3e vitamin C \u3e β-carotene. There was a weak synergy between BHT and vitamin E. A strong synergy was demonstrated between GSPE and quercetin. Results indicated that the addition of antioxidants had very little or no effect on the thermal behavior of UHMWPE. This fact suggests antioxidants at the concentrations studied had little effect on the molecular structure of UHMWPE. Thermal stability of UHMWPE was increased with the addition of antioxidants. The effectiveness of the antioxidants on thermal stability in descending order was: quercetin \u3e BNX 1010 \u3e vitamin E \u3e GSPE \u3e BHT \u3e TBHQ \u3e vitamin C. Hardness test results determined that the addition of antioxidants did not significantly change or increase the hardness of UHMWPE, which indicates no significant cross-linking of the polymer