Wollastonite, surface treated

Figure 2.73. SEM micrographs of polypropylene fracture area. Top - fracture at room temperature, bottom -fracture at liquid nitrogen, left - surface treated wollastonite, Tremlin 939, right - untreated wollastonite, Tremlin939. Courtesy of D. Skudelny, Quarzwerke GmbH, Frechen, Germany.

B Industrial Minerals to mine and process wollastonite in China this has since been discontinued. Most suppliers provide surface-treated grades and high aspect ratio forms that are the grades most commonly found in engineering plastics. 

Surface treated Nyglos 4 and 5 grades provide good wet out, with much improved impact strength at ambient and low temperatmes. There is a market for wollastonite in phenolic commutators, RRIM products, and PVC pastes. In Russia wollastonite has been investigated as a replacement for asbestos in linoleum compositions. 

Fibertsc Wollastonites are acicular (fibrous or needle-lika) mineral products milled to various bulk densities and particle sise distributions. These products can be surface treated to enhance performance in selected resin systems. 

Surface-treated wollastonite - Both powder and acicular forms of wollastonite are readily available with silane, organosilicone, and titanate treatments. 

Micronized and surface treated mieas and wollastonites are used in automobile and van fascias and fenders made by Reinforced Reaction Injection Molding (RRIM) of polyurethanes and polyureas. These minerals add strength, stiffness and heat resistanee, and lower the coefficient of thermal expansion so that the plastie part mates better with the steel frame of the vehiele. Combinations of wollastonite with mica are used, since the mica gives a smooth surface (improved Distinetness of Image), an essential charaeteristie for aesthetics after painting. Properties of miea- and wollastonite-reinforced RRIM are shown in Tables 17 to 20. 

The moisture resistance properties of filled molding compounds are enhanced by the treatment of the fillers with silane adhesion promoters prior to compounding. Silane promoters on wollastonite fillers in thermoplastic polyester molding compounds (50 percent filled) will improve the flexural strength after 16 h in 50°C water by as much as 40 percent. Silane-treated silica fillers have been found to significantly increase the moisture resistance of epoxy adhesives used in the electronics industry for chip, surface-mounted, and printed-circuit processes.8... 

When glass fibers are compounded in polyamides in high content, warping of the molded product can become a problem. Wollastonite exhibits better properties in this aspect." Wollastonite is a white mineral that consists essentially of calcium metasilicate. It is commonly used as an inorganic filler material of thermoplastic polymers for molding. The wollastonite fibers are treated with silane surface treatments by using y-aminopropyltri-ethoxysilane or y-glycidylpropylmethoxysilane. 

The effect of fillers on creep phenomena (essentially the inverse of stress relaxation) is also of interest a detailed study by Nielsen (1969b) of creep in filled polyethylene is illuminating. Kaolin and wollastonite were used as fillers, both treated with a silane coupling agent and untreated. A major aim was to discover whether the major effect of a filler is due to its effect on elastic modulus, or whether a filler also changes the viscoelastic nature of the system. As reported in previous work by others, the presence of a filler did in fact reduce the creep, the relative effect being nearly independent of the applied stress. The nature of the filler and the surface treatment were also found to be important. In experiments at a constant volume fraction (0.2), kaolin was more effective than wollastonite. Silane treatment of the filler surface tended to decrease creep, especially if the specimens had been soaked in water. 

Most fillers - calcium carbonate, kaolin, mica and wollastonite - have polar surfaces. Conversely, many polymers, such as the polyolefins, are hydrophobic. These will not readily wet hydrophilic fillers. It is therefore necessary to treat the filler surface to facilitate intimate polymer-mineral contact. Surface treatments also act as internal lubricants and improve the dispersion of the filler in the plastic matrix and the flow characteristics of the filled polymer. A further effect of some surface treatments is to improve the mechanical properties when exposed to water in vapor or liquid form, especially at high temperatures. These topics are discussed in detail in the chapter Mi era/ Surface Modification. 

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