Suitability of serpentinized rock material as mineral filler in polymer matrices

Mineral fillers can significantly affect the application properties of plastic materials. The structural and chemical properties of phyllosilicates provide the conditions to change the properties of polymeric material, e.g. flexural and tensile strength or thermal properties, according to the required application. Mineral fillers frequently used are clay minerals with a two-layer structure (serpentine-kaolin group) or three-layer structure (talc-pyrophyllite group, mica group, smectite group). The mineral fillers can be directly used or after surface modification, depending on the polar nature of the polymer. Polymers containing polar groups (hydrophilic polymers) are water-soluble, like polyvinyl alcohols and polysaccharides or can form hydrogen bonds, like polyamides, polyesters and polyvinyl fluorides. Hydrophobic (non-polar) polymers show an absence of polar groups (e.g. polyethylene, polypropylene) or mutual cancelling electrical dipole moments (e.g. polytetrafluorethylene). Minerals with a hydrophilic surface are directly applicable with polar polymers. For the application with non-polar polymers their surface require hydrophobization, whereas non-polar two-layer silicates are directly applicable with these polymers.

Serpentinized rock material is investigated with regard to its suitability as a polymer filler and its influence on the performance characteristics of various polymers. The samples origin from the Kraubath Ultramafic Massif, which represents part of an Early Paleozoic ophiolite, at the basement of the Austro-Alpine. The Kraubath complex is dominated by metamorphosed dunites and harzburgites, which origin from fractionation processes of the primary peridotite magma. Hydrothermal alteration led to a partly or entirely serpentinization of the ultramafic rocks. The serpentinization process of dunite, ortho-pyroxenite and harzburgite transformed Mg-containing silicates, like olivine and pyroxene to serpentine group minerals. Rock material with a high grade of serpentinization offers favourable conditions for the application as mineral filler.

The qualitative and quantitative XRD-analyses reveal a predominant occurrence of the antigorite. Further serpentine group minerals, like lizardite, occur in small amounts. Talc represents the second largest mineral phase. The rock material contains a few percentage of amphibole, chlorite, olivine (forsterite) and less than two percent of chromite and bronzite. In the two-layer structure of the main component antigorite, the charge of the tetrahedral layer is compensated by the charge of the octahedral layer. The three-layer structure of talc is electrostatically neutral, with no interlayer material. Therefore, serpentine minerals and talc are suitable for the application as mineral fillers in non-polar polymers, like polypropylene. Both influence the mechanical and tribological properties of polymers. Serpentine improves elasticity, tensile strength, stress at break, elongation at break, the mass wear rate and the coefficient of friction of the polymer but reduces the impact strength. Talc positively influences rigidity, shrinkage, creep properties, heat distortion under load and the coefficient of linear thermal expansion, however reduces toughness, long thermal ageing, impact strength and tensile strength. The further mineral phases are not considered to affect the application properties negatively. Regarding tensile strength and elasticity the ratio of serpentine to talc can influence the increase and decrease of these properties in non-polar polymers. The applicability of the practical implementation is investigated with nanoparticles of the serpentinized rock material in combination with polypropylene in varying proportions.