Thermal Properties of Nonmetallic Restorative Materials

Abstract

Thermal capacity, diflusivity, and conductivity were determined for nonmetallic restorative materials. Thermal characteristics were affected by composition, powder-liquid ratio, and water sorption. Diflusivity and conductivity followed the same order of values. The least conductive material investigated was an unfilled acrylic resin. Highest diffusivities and conductivities were exhibited by a resin composite. Values obtained with the composite were about the same as those obtained for a thick mix of unmodified zinc oxide-eugenol cement.Thermal properties of tooth structure and dental restorative materials have been investigated by several authors.'-" Most of the studies involve direct thermal conductivity measurements. Difficulties in obtaining steady-state conditions and in estimating and controlling heat losses have contributed toward large variations in reported values even for the same investigators.7,8 Additional difficulties in correlating results arise from an incomplete definition of compositions and powder-liquid ratios or a lack of identification of commercial brands used. The omissions are particularly obvious with liners and zinc oxide-eugenol cement preparations. To circumvent some of the technological difficulties, Braden' has introduced a simple method of determining diffusivity and has suggested that, under transient thermal conditions of the oral cavity, diffusivity, rather than conductivity, is the more important parameter.1030 conductivity determinations, to measure thermal properties of typical conventional nonmetallic restoratives, and to obtain information on some of the newer materials, such as a polymer-reinforced zinc oxide-eugenol cement and a direct filling resin composite. Materials and MethodsMaterials used in this study are listed in Table 1. Diffusivity and conductivity were determined for zinc phosphate, silicate, unmodified zinc oxide-eugenol cements; a resinreinforced zinc oxide-eugenol material; an unfilled direct filling acrylic resin; and a direct filling resin composite. Diffusivity was determined by the method described by Braden.' The specimen was a cube with the hot junction of a copper-constantan thermocouple* embedded in its center. Specimen molds were made of silicone rubber held in approximately 5 cm (-2 inch) sections of a 3.18 cm (1 1/4 inch) diameter copper pipe (Fig 1). The materials were mixed for 30 seconds and packed into the molds holding the thermocouples. The open ends of the molds were covered with glass plates. Except for zinc oxide-eugenol cement, the specimens remained in the molds 24 hours at 37 C and a relative humidity of 100%; they were then defiasked and placed in a desiccator at 37 C for seven hours for dry specimen determinations, or in 50 ml distilled water for seven days at 37 C for wet specimen measurements. To ensure complete set, unmodified zinc oxide-eugenol cement specimens were defiasked 48 hours after starting the mix.The surface moisture of the wet specimens was blotted off with tissue paper before test-* Type T insulated 30 gauge (Brown...

Thermal Diffusivity of Glass-ionomer Cements

“…The rise in thermal diffusivity of the polycarboxylate cement was particularly steep when compared with those of the glass-ionomer cements. The same effect was observed by Watts and Smith (1984) and by Braden (1964) ain Civjan et al (1972) with zinc phosphate and zinc oxide/eugenol cements. Watts and Smith attributed this effect to the fact that most zinc-oxide-based cements contain a small amount of magnesium oxide, 5% in the case of zinc polycarboxylate (Bertenshaw and Combe, 1972).…”

“…Civjan et al (1972) employed a similar technique to determine the thermal diffusivity of non-metallic restorative materials. Pearson et al (1980) also adopted Braden's method for direct measurement of the thermal diffusivity of composite filling materials, using equation (2), and compared the values obtained with those calculated with equation (1) from separate measurements of thermal conductivity, specific heat, and density.…”

Thermal diffusion in some polyelectrolyte dental cements: the effect of powder/liquid ratio

“…Consequently the magnitude of the thermal diffusivity is a factor of clinical importance. Thermal diffusion through liners has been compared without the determination of diffusivity values by Phillips and co-workers (Voth, Phillips & Schwartz, 1966;Tibbetts et al, 1976), while absolute values of thermal diffusivity have been obtained for cements at the standard (P/L) ratio by Civjan et al, (1972), Carter (1978 and by Watts & Smith (1981). In clinical mixing of cements the powder/liquid ratio is sometimes varied, either deliberately or inadvertently, from that recommended by the manufacturer.…”

Thermal conductivity studies of composite dental restorative materials

“…Because of experimental errors they cross somewhat to the left, to give the values for k of 14.1 X and 13. 8 x If the conductivity is considerably greater than that of water, the simple procedure outlined above runs into practical difficulties because of the long extrapolation of a gently curving line, whose curvature is established only far from the intersection with the 45" line. Under these circumstances a hyperbolic extrapolation of the thermal resistance has some advantages.…”

“…8 A t the present time a number of composite restoratives are on thc market, some developed for specific applications. The largest group is still those designed for filling cavities (Adaptic, Addcnt, Concise, HL-72, I'restigc, DFR, Exact, and Smile are examples), but others arc.…”