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

Andy Hardison, Gladius Lewis, A. U. Daniels, Richard Smith

Research output: Contribution to journalArticle

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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 41kJ/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 20h after test start], from UHMWPE γ-radiation sterilized in air was significantly higher than for unsterilized material (2.91±0.11 vs. 0.73±0.11μW). 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.

Original languageEnglish (US)
Pages (from-to)5145-5151
Number of pages7
JournalBiomaterials
Volume24
Issue number28
DOIs
StatePublished - Jan 1 2003

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Ultrahigh molecular weight polyethylenes
Heat conduction
Polymers
Activation energy
Hot Temperature
Ethylene Oxide
Free radicals
Air
Radiation
Free Radicals
Ethylene
Gases
Heat transfer
Degradation
Oxides
Flow rate
Wear of materials
Oxygen
Oxidation
Exothermic reactions

All Science Journal Classification (ASJC) codes

  • Bioengineering
  • Ceramics and Composites
  • Biophysics
  • Biomaterials
  • Mechanics of Materials

Cite this

Determination of the activation energies of and aggregate rates for exothermic physico-chemical changes in UHMWPE by isothermal heat-conduction microcalorimetry (IHCMC). / Hardison, Andy; Lewis, Gladius; Daniels, A. U.; Smith, Richard.

In: Biomaterials, Vol. 24, No. 28, 01.01.2003, p. 5145-5151.

Research output: Contribution to journalArticle

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