Dynamic mechanical properties of polypropylene composites filled with ultrafine particles

Abstract

SynopsisThe dynamic moduli of isotactic polypropylene (PP) filled with ultrafine SiOn and micron sized glass particles are measured in the temperature range 30-130°C at frequency 10 Hz. The storage moduli of PP composites, E;, increase with filler content and decreasing filler size in the whole range of temperature. The loss moduli of PP composites, Ef, increase with filler content and decreasing filler size above 4OOC. The intensity of the broad dispersion which appears at ca. 60°C increases with filler content and decreasing filler size. By assuming that the energy is not dissipated in the effective volume, namely, filler volume plus that of immobilized interfacial region, the effective volume fraction is evaluated from the relative loss modulus, E:/EU at 60°C. The effective volume fraction increases with filler content and decreasing filler size. The effect of addition of ultrafine particles on the broad dispersion at ca. 6OoC resembles the effect of increasing crystallinity of pure PP. It is concluded that the broad dispersion which appeared at ca. 60°C seemed to be assigned to the grain boundary of PP composites or crystalline boundary of pure PP. INTRODUCTIONIn a previous paper we have studied the reducible properties of drawing temperature, rate of strain, and filler content with respect to the tensile yield stress of PP composites filled with ultrafine particles. The yield stress is reducible by three factors, i.e., drawing temperature, strain rate, and filler content. The conversion law is relatively simple':(1) The first shift factor, a T , is the Arrhenius type for temperature and for strain rate. The activation energy is not dependent on the filler volume fraction and size.(2) The logarithm of the second shift factors, log a,, for strain rate and for filler content is proportional to the square root of the filler volume fraction.The Arrhenius plots of shift factor, log aT, against the reciprocal temperature have two breaks at 60°C and llO°C for filled and nonfilled PP. These breaks are related to some kinds of transition of matrix PP. Many authors have reported about the nature of the broad dispersion which appears at ca. 60°C of isotactic PP. However, the assignment of this transition is not known clearly.24 In the present study, we discuss the effect of filler content and size on this dispersion from the measurement of dynamic modulus of PP filled with ultrafine particles. Table I shows the average diameters of these particles. Mixing and Molding. Polymer and filler were mixed in a two-roll mill for 15 min a t 180°C. The content of the fillers was 5,10,15 and 20 wt %. Films about 0.5 mm thick were molded from the mixtures a t 200°C under a pressure of 100 kg/cm2. Since thermal degradation of matrix polymer took place more or less during the mixing and molding, films without filler were prepared by the same procedures. It was assumed that the degradation of polymer took place in the composites to the same extent as in the pure polymer sample. All the samples were annealed a t 130°C for 1 h. EXPERIM...

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