Mechanical properties of polypropylene/talc composites: Role of the mineral morphology and particle surface modification

Abstract

Talc filled Polypropylene (PP) is a very used versatile composite as it combines a relative good performance with easy processability and low cost. Talc addition strongly affects PP properties mainly by their effect on crystallization. It is an efficient nucleating agent, changing the morphology, chain orientation, crystal type and crystallinity degree of PP. Such properties can indeed be tailored by changing the volume fraction, shape, and size of the talc particles. According to the genesis, talc has different morphologies and different associated minerals. These aspects conferred it specific whiteness, particle shape, oil absorption, and amphiphilic behavior. On the other hand, the geomorphologic and geochemical characteristics, as well as the mineral processing (milling, treatments, among others), also affect thermal, mechanical and optical properties of PP/talc composites. In order to obtain a higher mechanical performance of PP/talc composites it is necessary to assure a good dispersion and distribution of particles, having very large aspect ratio, and a good particle/matrix interfacial adhesion. Generally, to improve the filler/polymer interaction it is a common practice to modify the filler surface. In this work, a systematic analysis of the influence on talc geomorphology and different surface treatments on the PP/talc composites structure and their mechanical properties is performed. Two well characterized talc samples, micro and microcrystalline were used. Thus, a low costs surface treatment is proposed in order to improve the mechanical performance, by the way of adhesion enhancement and particle size strong reduction, but without increase the material price. An accurate particle characterization, between and after the treatment, was carried out in order to determined their morphological and surface aspects. A methodology to improve dispersive and distributive talc mixing in PP using a twin screw extruder is also proposed. Composite structure were characterized by differential scanning calorimetry, electronic microscopy, Fourier transform infrared and X-ray spectroscopy. Tensile properties were assessed and their relationship with the structure carefully analyzed. Very strong adhesion improvements were found either when macrocrystalline talc or treated one is used. The results confirm that composites processed with the higher aspect ratio particle, show a better mechanical behavior than the other. In the case of PP/treated talc composites, an improvement on dispersion, distribution and interfacial adhesion are observed, as a consequence of talc functionalization. However, the talc surface treatment has more influence on mechanical properties in only one of the composites, due to its efficiency depends on the intrinsic talc characteristic.