Wollastonites

Abstract

Wollastonite is a metasilicate having an inosilicate (chain silicate) structure. It belongs to the pyroxenoid group of minerals. It is the only white acicular mineral and became famous and widely used in the industry in the 1970s 1980s as a nonhazardous replacement for asbestos fibers. During crushing and grinding, it forms acicular particles having an aspect ratio (length vs. diameter ratio) in the range of 1:3 up to 1:15. The structure of the wollastonite particles depends partly on geological conditions of genesis but also to a large extent on the grinding technology. The applications are widely spread reaching from additive in metallurgy (protective slag, welding electrodes) over ceramics and ceramic boards to reinforcement in thermoplastics and thermosets (car interiors, fenders, trims) and corrosion inhibitor as well as reinforcement in paints and coatings.

Especially in cost-efficient PP systems, high-aspect-ratio (HAR) wollastonite can confer the compound properties necessary to replace other high-price polymeric systems, e.g., in automotive industry. Properties asked for by automotive applications are low thermal expansion (CLTE), zero-gap bumpers, painting ability, high flow, mold in color, high stiffness, reduced shrinkage, and improved resistance to UV and petrol. Besides the fibrous structure of the mineral and its size distribution, the surface treatment plays a major role when looking at dispersing ability, scratch resistance, and mechanical properties.

LAR, aminosilane-coated wollastonite flours have been successfully used for many years as functional fillers for polyamides. With HAR wollastonite the rigidity of the compound is even higher than with LAR wollastonites. Polyamides reinforced in this way offer a range of different possibilities for the design of low warpage, rigid moldings/parts. They are used in, e.g., wheel covers, air filter housings, and electrical appliance parts.

Polyurethane for reaction injection molding (PUR RIM) is a microcellular (integral) hard foam for thin-walled moldings exposed to compressive and flexural stress. The PUR R-RIM-reinforced variants with HAR wollastonite are used when a higher elastic memory is required.

In comparison to other fillers in mid-voltage insulation applications based on epoxy resin molding materials, wollastonite is used due to its reinforcing properties when wall thickness in technical parts shall be reduced or resistance toward cracking – i.e., better flexural modulus – should be improved. The coefficient of linear thermal expansion is comparably low.

In recent years the development of brake pads/linings based on phenolic resins has led to the use of environmentally friendly raw materials like wollastonite, which is critical to health.

LAR, silane-coated wollastonite flours are successfully used for many years as functional fillers in shaft seals based on fluoroelastomers in order to adjust the hardness. Apart from the good reinforcement in fluoroelastomers, LAR wollastonite (due to its bright and neutral color) provides also the advantage of colored bright mixtures. These optimized fluoroelastomers are ideally suited for the applications under special conditions, e.g., when high mechanical load and high temperatures apply and whenever chemical resistance is crucial.