Study On A Novel Oxidation Resistant And Highly Cross-linked Ultra-High-Molecular-Weight Polyethylene

Ultra high molecular weight polyethylene (UHMWPE) has been thechoice for bearing material used in total joint arthroplasty due to itsexcellent properties, such as high mechanical properties, low friction,high abrasive and chemical resistance. However, the long time wear invivo of total joint implants may cause osteolysis. γ-ray or electron beamirradiation was used to improve its wear resistance, while the mechanicalproperties were significantly decreased as a result of irradiation andpost-irradiation thermal treatment.Vitamin E (-tocopherol) was widely added to UHMWPE before orafter irradiation to stabilize the residual free radicals produced byirradiation. Trace amount of-tocopherol addition (such as0.1wt%) wasefficient to increase the oxidation resistance of cross-linked UHMWPE asthe phenolic hydroxyl of the-tocopherol molecule can capture the freeradicals generated during irradiation process. However, the addition of-tocopherol may inhibit the cross-linking of UHMWPE. On the otherhand, irradiation may reduce the activity of-tocopherol, especiallywhen the content of-tocopherol is low.Considering that polyphenols may remain anti-oxidation potencyeven after high dose irradiation, we used polyphenols to capture themacro-radicals generated during high energy e-beam irradiation in thispaper. Polyphenols including gallic acid, dodecyl gallate, and caffeic acidetc. were respectively blended with UHMWPEs, followed byconsolidation and subsequent e-beam irradiation.Accelerated aging experimental results indicated that UHMWPEsadded with0.3wt%polyphenol remained excellent oxidation resistancesafter e-beam irradiation and accelerated aging at70°C in O2for one week.The results of oxidation process of UHMWPEs added with lowconcentrations (0.05%&0.1%) of polyphenols at high temperatures (100,110,120°C) in air showed that UHMWPEs didn’t take oxidation untilthey reach the point of oxidation induction time and polyphenols couldsignificantly improve their oxidation resistances at high temperatures inair. We also studied the oxidation products of UHMWPEs generatedduring oxidation process at high temperatures and proposed the chemical reaction pathway of polyphenols within the UHMWPEs.Gallic acid and caffeic acid hardly inhibited the cross-linking ofUHMWPE while vitamin E and dodecyl gallate did. The mechanicalproperties of UHMWPEs including tensile properties and impactresistance dropped after e-beam irradiation and accelerated aging in O2,but phenolic anti-oxidants could maintain the mechanical properties ofUHMWPEs compared to the virgin samples.The isothermal crystallization of e-beam irradiated cross-linkedUHMWPEs was also investigated as a function of radiation dose,crosslink density, and crystallization temperature in this paper. Theresults indicated that the cross-linked network within the UHMWPEaccelerated the crystallization and hindered the crystal growth afternucleation. We further hypothesized that the polymer network washeterogeneous since very few cross-links could form in the crystallamellae and thus the crosslink density was higher in the amorphousphase than in the crystalline phase. UHMWPEs and cross-linkedUHMWPEs were sequentially cooled from melt down to differenttemperatures for isothermal crystallization at each temperature. The studyindicated that a distribution of lamellae size of cross-linked UHMWPEwas formed during stepped isothermal crystal process, which was causedby the not uniform network of the polymer.