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Determination of the activation energies of and aggregate rates for exothermic physico-chemical changes in UHMWPE by isothermal heat-conduction microcalorimetry (IHCMC)

Paper ID Volume ID Publish Year Pages File Format Full-Text
12603 803 2003 7 PDF Available
Title
Determination of the activation energies of and aggregate rates for exothermic physico-chemical changes in UHMWPE by isothermal heat-conduction microcalorimetry (IHCMC)
Abstract

Exothermic heat flow rates (Q=μW=μJ/s), as a function of elapsed time, were measured by isothermal heat-conduction microcalorimetry (IHCMC) in order to study the aggregate rate of physico-chemical change in specimens of unsterilized and sterilized ultra-high-molecular-weight polyethylene (UHMWPE). Standard protocols for performing the IHCMC tests were developed and are described. Use of the standard protocols yielded the desired results—data that were not significantly different among either replicate sets of unsterilized specimens or as a function of which calorimeter test well was used. Heat flow rates measured in air at 20°C, 25°C, 35°C, and 45°C yielded estimates of activation energies of 47, 11, and 41 kJ/mol for unsterilized, γ-radiation sterilized, and ethylene oxide gas (EtO) sterilized polymer, respectively. These results support the ideas that (a) initial exothermic degradation takes place much more easily in the radiation-sterilized material, due to direct oxidation of readily available free radicals, and (b) the much slower degradation process in EtO-sterilized UHMWPE is not appreciably different than in unsterilized polymer. Comparison with other activation energy data suggests that the rate-limiting process in EtO- or un-sterilized polymer is oxygen diffusion into the polymer. For shelf storage in air, for periods up to 8 months, the mean exothermic heat flow in air, at 25°C (Qm) [determined from the Q values averaged over the time period between 15 and 20 h after test start], from UHMWPE γ-radiation sterilized in air was significantly higher than for unsterilized material (2.91±0.11 vs. 0.73±0.11 μW). The higher rate can be attributed to oxidation of radiation-induced free radicals in the polymer near its surface. For the γ-irradiated polymer, the decline in Qm with shelf storage time suggests that, eventually, degradation might become oxygen diffusion limited in this case also. However, in vivo, surface wear of an UHMWPE articular component may continue to expose unoxidized free radicals, keeping the exothermic reaction rate high and, possibly, continuing to produce an oxidized UHMWPE surface prone to wear.

Keywords
Ultra-high-molecular-weight polyethylene; Isothermal microcalorimetry; Activation energy; Sterilization
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Determination of the activation energies of and aggregate rates for exothermic physico-chemical changes in UHMWPE by isothermal heat-conduction microcalorimetry (IHCMC)
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Publisher
Database: Elsevier - ScienceDirect
Journal: Biomaterials - Volume 24, Issue 28, December 2003, Pages 5145–5151
Authors
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Subjects
Physical Sciences and Engineering Chemical Engineering Bioengineering
Get Full-Text Now
Don't Miss Today's Special Offer
Price was $35.95
You save - $31
Price after discount Only $4.95
100% Money Back Guarantee
Full-text PDF Download
Online Support
Any Questions? feel free to contact us