Coloration of polypropylene gel and temperature conditions for gel formation

Abstract

We investigated, on the basis of light-scattering theory, the relationship between the color of polypropylene gel and the refractive index in the vicinity of the interface of polypropylene networks in the gel when prepared at different cooling temperatures after dissolving the polypropylene at 160°C. The color of the gels was found to differ depending on the cooling temperature upon gel formation. SEM images of these gels showed polypropylene networks with smaller diameters with decreases in the cooling temperature. Based on measurements using the maximum transmittance method, it was found that the origin of color changes in the gels is a shift in the wavelength dispersion of the refractive index for polypropylene to a higher refractive index. Changes in the solvent content of polypropylene in the gels, which were evaluated using experimental values of the refractive indices and densities for the solvent and polypropylene, revealed that the solvent causes swelling of the polypropylene interface which is a source of light-scattering. The degree of swelling appears to depend upon the cooling temperature when gels are prepared by cooling beyond the glass transition temperature of polypropylene.