(217b) Heat Resistance Effect of CaSO4 Whiskers to PVC Composites

Abstract: Calcium sulfate(CaSO₄) whiskers which have been widely used in many  polymers just like PP, EPDM and epoxy resin are a kind of low cost and environmental fibrous monocrystal with good heat resistance and heat-insulating properties. In this paper, the heat resistance effect of PVC composites with CaSO₄ whiskers as fillers was investigated and compared with fumed silica/PVC and CaCO₃/PVC composites. PVC composites(1#, 2# and 3# samples) were prepared from PVC (100phr), DOP(40phr), stearic acid(0.5phr), co-stabilizers(0.5phr), external lubricant(0.3phr), heat stabilizer(5phr) and the fillers(5phr) which is CaSO₄ whiskers, fumed silica and CaCO₃, respectively, by mixing on a double roller plastic refining machine at 170°C for 10 minutes. The CaSO₄whiskers used in the present experimental study have been proved to be good crystallinity and titanate-modified by characterizing through FT-IR, XRD and optical microscope.

The mass loss during heating of the three composites were measured by a TA Q50 thermogravimetric analyser (TGA) with heating at a rate of 10°C min^-1 from 30°C to 150°C, 2°C min^-1 from 150°C to 250°C and 10°C min^-1 from 250°C to 650°C under 40 cm³ min^-1 nitrogen gas flow. At the 1% mass loss from CaSO₄ whisker/PVC, fumed silica/ PVC and CaCO₃/PVC, the temperature was 179.6°C, 179.8°C and 177.8°C, respectively. At the 5% mass loss from three composites, the temperature was 216.4°C, 214.2°C and 214.1°C,respectively. This shows that the CaSO₄whisker/PVC had minimum mass loss compared to the others in this temperature range( PVC processing temperature scope is about 170-210°C).

DSC curves of the composites were obtained at a rate of 10°C min^-1 from 0°Cto 250°C under 40 cm³ min^-1 nitrogen gas flow using a TA Q200 differential scanning calorimeter(DSC). the glass transition temperature of CaSO₄ whisker/PVC, fumed silica/ PVC and CaCO₃/PVC was 88.3°C, 82.9°C and 83.6°C. The result indicates that CaSO₄whiskers can improve the glass transition temperature of PVC composites.

The three composites were tested for air oven heat aging properties at 100±2°C for 168h under forced convection air circulating inside the oven. The tensile strength and elongation at break before and after aging were carried out by means of Taiwan GOTECH electronic testing machine at a tensile rate of 250 mm/min under room temperature. The tensile strength before aging from CaSO₄ whisker/PVC, fumed silica/ PVC and CaCO₃/PVC was 21.11MPa, 20.90MPa and 20.64MPa, and the elongation at break was 297.5%, 241.0% and 252.8%. The change rate before and after aging in tensile strength was 5.7%, 3.3% and 5.2%, and those in elongation at break was -16.3%, -0.9% and -7.8%, respectively. The results showed that CaSO₄ whisker exhibited excellent mechanical properties, the tensile strength of CaSO₄whisker/PVC became a little better after aging, but the flexibility decreased because PVC resin and the whiskers formed crosslinked structures and became harder. However, the change rate of elongation at break was within ±20% and could meet the need of GB/T 8815-2008.

Thermal deformation experiments were taken according to the test method specified by item 6.4 in GB/T8815-2008 at 120±2°C under forced convection air circulating inside the oven. The thermal deformation from CaSO₄ whisker/PVC, fumed silica/ PVC and CaCO₃/PVC was 25.03%, 17.7% and 28.20%, respectively. The results obtained from the present study showed that the thermal deformation of CaSO₄ whisker, which was better than CaCO₃, but worse than that of fumed silica, was much lower than 40% which is specified in GB/T8815-2008.

The dispersing states of the fillers in three composites before and after aging were observed by means of scanning-electron microscopy. Electron microscopy revealed that CaSO₄ whiskers whose lenght×diameter are about 35.5μm×3.92μm and length-to-diameter ratio considerably is 9.1 dispersed uniformly and had a good consistency and interfacial configuration with PVC resin. After aging, the interfacial configuration isn’t so clear as that before aging. fumed silica and CaCO₃ dispersed in the form of particles, and most of the fumed silica existed on the surface of PVC resin and some of them integrated into the resin after aging. CaCO₃particles which dispersed in PVC resin uniformly didn’t give a apparent change before and after aging.

In conclusion, CaSO₄ whiskers can improve the thermal stability and glass transition temperature of PVC composites because of its good compatibility with PVC resin. The PVC composites with CaSO₄ whiskers as fillers exhibit a better properties to thermal aging resistance and the protection to thermal deformation.