Long-term oxygen plasma etching of UDMA:TEGDMA methacrylate resin: Etching rate and physical property changes

Abstract

The effect of low-pressure oxygen plasma etching on the physical properties of a dental photopolymer, namely UDMA:TEGDMA in 3:1 wt ratio, was investigated. Samples of the photopolymer were exposed to oxygen plasma from 10 to 120 min time intervals at different power (60, 100 and 140 W) and pressure (0.2, 0.4, and 0.6 mbar) conditions. The etching rate and changes in the surface roughness of the sample were measured with white light interferometry, while the optical properties and chemical changes were monitored by optical and Raman spectroscopy, respectively. It was found that by controlling the power and pressure parameters, steady etching rates can be set up in the 0.11–0.48 μm/min range. Longer etching times (e.g. above 1 h) were found to increase the surface roughness of the polymer significantly, while reducing its optical transparency. This loss in transparency was successfully restored by depositing a second polymer layer on top of the plasma-etched one. Raman spectroscopy investigations revealed that the degree of conversion, the amount of aromatic molecules and the introduced stress are all depend on the applied etching pressure and power conditions. Actinometry with Ar was performed to measure the internal plasma parameters, and the measured etching rates and observed physical-chemical property changes were correlated with the electron temperature and atomic O densities.