Surface Oxidative Degradation Research Articles

Unilateral Blow-Off and Periodic Smoldering Holes in Upward Flame Spread Over Thin Charring Material

ABSTRACT In this study, upward flame spread over thin charring material under varied ambient pressures and oxygen concentrations diluted with different inert gases was investigated. After symmetric ignition, a small systematic disturbance at the flame base region triggers asymmetric two-sided flames, while the flame tips remain aligned. Afterward, one branch of flame dwells and speeds up in a cycle, associated with periodic smoldering holes, mainly ascribed to the secondary pyrolysis and surface oxidative degradation (smoldering) of the remaining char, while the other branch of flame extinguishes eventually, primarily attributed to the self-induced buoyant blow-off at a low critical Damköhler number associated with finite-rate kinetics. The temperature distribution over the sample surface is obtained by infrared thermal imagers, and six stages of the whole pyrolysis process are roughly divided, which gives a better perspective on the underlying mechanism of these unique combustion behaviors. With the burnout (end) of the primary or secondary pyrolysis processes, the local fuel/oxidant mixture ratio at the flame base region approaches that of the inflammability limit (neutral stable point) where the near-limit flames are extremely sensitive to the environmental disturbances, and thus the flame base instability appears. With the increase of the oxygen concentration, the pressure range of the existence of the unique combustion behaviors narrows down due to intensified chemical reactions and thus the stability of the flame base. Furthermore, compared with the environments diluted in N2, the unique combustion behaviors are more conspicuous in that of CO2 (radiative intervening medium) due to larger molecular weight and lower Lewis number. This investigation into the upward flame propagation will enrich the combustion science at transition regimes such as the fire scenarios after the application of the carbon dioxide fire extinguisher.

Comparative analysis of in vitro oxidative degradation of poly(carbonate urethanes) for biostability screening

The resistance to oxidation and environmental stress cracking of poly(carbonate urethanes) (PCUs) has generated significant interest as potential replacements of poly(ether urethanes) in medical devices. Several in vitro models have been developed to screen segmented polyurethanes for oxidative stability. High concentrations of reactive oxygen intermediates produced by combining hydrogen peroxide and dissolved cobalt ions has frequently been used to predict long-term oxidative degradation with short-term testing. Alternatively, a 3% H₂O₂ concentration without metal ions is suggested within the ISO 10993-13 standard to simulate physiological degradation rates. A comparative analysis which evaluates the predictive capabilities of each test method has yet to be completed. To this end, we have utilized both systems to test three commercially available PCUs with low and high soft segment content: Bionate PCU and Bionate II PCUs, two materials with different soft segment chemistries, and CarboSil TSPCU, a thermoplastic silicone PCU. Bulk properties of all PCUs were retained with minor changes in molecular weight and tensile properties indicating surface oxidative degradation in the accelerated system after 36 days. Soft segment loss and surface damage were comparable to previous in vivo data. The 3% H₂O₂ method exhibited virtually no changes on the surface or in bulk properties after 12 months of treatment despite previous in vivo results. These results indicate the accelerated test method more effectively characterized the oxidative degradation profiles than the 3% H₂O₂ treatment system. The lack of bulk degradation in the 12-month study also supports the hydrolytic stability of these PCUs.

Wear of '100 Mrad' cross-linked polyethylene: effects of packaging after 30 years real-time shelf-aging

Studies have shown that gamma-irradiation of polyethylene (PE) generally results in degradation by surface oxidation. However, from 1970 to 1978 Oonishi et al. used ultra-high-molecular-weight polyethylene (UHMWPE) cross-linked and sterilized by 100 Mrad of gammairradiation in air (100 Mrad PE) for total hip prostheses, and obtained excellent clinical results extending for 30 years. In the present study, we used a hip joint simulator to investigate the wear characteristics of 100 Mrad PE cups which had been shelf-aged for an extremely long period (30 years). The PE cups, aged in an air-containing triple polyethylene package for 30 years (packaged 100 Mrad PE), showed low wear with 3.4 mg of weight loss, even after 5 × 106 cycles. In contrast, non-packaged 100 Mrad PE showed considerable wear: 47.0 mg at run-in ((0–0.25) × 106 cycles) and 114.1 mg at the end of 5 × 106 cycles. The substantially, lower wear even in the presence of an oxidized surface layer for the packaged 100 Mrad PE, was comparable to the low wear seen on retrieved 100 Mrad PE after 30 years of clinical use. The long-term shelf-storage conditions, which affect the surface oxidative degradation of PE, are assumed to be the key factor in the wear-resistance of gamma-irradiated UHMWPE.

Abrasive erosion of polyurethane

The surface physical and chemical effects of abrasive erosion of polyurethane in three different fluids (H2O, polyacrylamide and NaOH) containing quartz particles have been investigated respectively. The surface profiles, elements constitutions, binding energies and functional groups of polyurethane surfaces before and after abrasive eroding have been analyzed with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectrometer (FT-IR). The surface physical effects of the abrasive erosion process were found to be microcutting, high-cycle fatigue and plastic fracture. The surface chemical effects turned out to be mechanical rupture of macromolecular chains, thermal decomposition of the allophanate groups and the biuret groups, hydrolysis of carbamate groups, and surface oxidative degradation.