Influence of curing protocol on selected properties of light-curing polymers: degree of conversion, volume contraction, elastic modulus, and glass transition temperature.

OBJECTIVES: The purpose of this study was to investigate the effect of light-curing protocol on degree of conversion (DC), volume contraction (C), elastic modulus (E), and glass transition temperature (T(g)) as measured on a model polymer. It was a further aim to correlate the measured values with each other. METHODS: Different light-curing protocols were used in order to investigate the influence of energy density (ED), power density (PD), and mode of cure on the properties. The modes of cure were continuous, pulse-delay, and stepped irradiation. DC was measured by Raman micro-spectroscopy. C was determined by pycnometry and a density column. E was measured by a dynamic mechanical analyzer (DMA), and T(g) was measured by differential scanning calorimetry (DSC). Data were submitted to two- and three-way ANOVA, and linear regression analyses. RESULTS: ED, PD, and mode of cure influenced DC, C, E, and T(g) of the polymer. A significant positive correlation was found between ED and DC (r=0.58), ED and E (r=0.51), and ED and T(g) (r=0.44). Taken together, ED and PD were significantly related to DC and E. The regression coefficient was positive for ED and negative for PD. Significant positive correlations were detected between DC and C (r=0.54), DC and E (r=0.61), and DC and T(g) (r=0.53). Comparisons between continuous and pulse-delay modes of cure showed significant influence of mode of cure: pulse-delay curing resulted in decreased DC, decreased C, and decreased T(g). Influence of mode of cure, when comparing continuous and step modes of cure, was more ambiguous. SIGNIFICANCE: A complex relationship exists between curing protocol, microstructure of the resin and the investigated properties. The overall performance of a composite is thus indirectly affected by the curing protocol adopted, and the desired reduction of C may be in fact a consequence of the decrease in DC.

Softening and elution of monomers in ethanol.

OBJECTIVES: The purpose of this study was to investigate the effect of light-curing protocol on softening and elution of monomers in ethanol as measured on a model polymer. It was a further aim to correlate the measured values with previously reported data on degree of conversion and glass transition temperature for the same polymer and curing protocols. METHODS: Different light-curing protocols were used in order to investigate the influence of energy density, power density, and mode of cure on the properties of a model polymer. The modes of cure were continuous, pulse-delay, and stepped irradiation of the specimens. Wallace hardness was used to determine the softening of the polymer after storage in ethanol for 24h. Elution of monomers from the polymer was assessed after 7 days in ethanol by means of high-pressure liquid chromatography (HPLC). Data were submitted to two- and three-way analysis of variance (ANOVA), Newman-Keuls' multiple comparison test, and linear regression analysis. RESULTS: Energy density, power density, and mode of cure of the polymer influenced the softening and elution of monomers in ethanol. As energy density increased, softening and elution in ethanol decreased. At same energy density, the influence of power density varied with the mode of cure. When compared to the continuous mode of cure, and at same energy density, pulse-delay irradiation resulted in polymers that in general were more susceptible to softening, but eluted monomers to a lower extent. Less elution was also found with step-cured polymers. Significant, negative correlations were detected between softening and elution in ethanol, respectively, and degree of conversion and between softening and elution in ethanol, respectively, and glass transition temperature. SIGNIFICANCE: A complex relationship exists between curing protocol and the properties selected for investigation. The effect of different combinations of exposure periods and power densities are important to understanding how the curing protocol affects the properties of polymer-based materials.

Plasticizers in denture soft-lining materials: leaching and biodegradation.

The leaching from soft lining materials into an aqueous buffer with the same esterase activity as that of saliva was compared to leaching into buffer without esterase activity. The buffer contained 0.1% non-ionic detergent, giving the medium a capacity to dissolve phthalates to the same degree as saliva. The hypothesis that esterase in the immersion medium will increase the rate of diffusion of plasticizers from denture soft-lining material was confirmed by the results. The average leaching of phthalates from 5.5 g of one of the materials was 4.5 mg kg(-1) d(-1) within the first 2 d and 1.1 mg kg(-1) d(-1) within the first 28 d. These levels may be compared to a LOAEL (lowest observed adverse effect level) of 52 mg kg(-1). It is concluded that an esterase activity, equivalent to that in saliva, in the immersion medium for soft lining materials increased the rate of diffusion of plasticizer from the materials. The measured levels of phthalates leaching from these materials might in vivo only be slightly less than 1/10 of the LOAEL given above.