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...
Output of three commercially available electric pulp testers were compared. Clinical threshold settings of the instruments could not be correlated to peak voltage, root mean square voltage, or power outputs determined by use of a simulated load impedance. A mathematical expression to establish a relation between threshold settings and output information is suggested.
The effect of varying burnout and casting temperatures on the surface roughness of dental gold castings has been studied. Data relating the degree of surface roughness of various alloys to nominal tempê ratures of the mold as removed from the oven after burnout and that of the molten gold alloy when cast have been recorded for 273 dental gold alloy castings cast with burnout temperatures varying from 800°F to 1600 °F and alloy casting temperatures varying from 1725 °F to 2400 °Fo Extremely high mold and metal temperatures appear to increase surface roughness irrespective of alloy used. The effect of high temperatures seems to be an additive one. At burnout temperatures to 1300 °F and metal casting temperatures to 2000 °F and in some instances to 2100°F surface roughness appears to be constant at approximately 30 microinches. In repeat castings of one alloy^seventeen times^surface roughness was constant when burnout and alloy casting temperatures were held constant. Reusing the alloy did not affect surface roughness.
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