Wollastonite, acicular

Fuchs (1971) identified acicular wollastonite from the Allende meteorite. Miyamoto et al. (1979) called these extraterrestrial crystals whiskers. Massive deposits of fibrous wollastonite in minable quantities occur in the Adirondack Mountain region of New York State. 

Particle Shape and Structure. Some materials exhibit particular properties owing to their particle shape or form, eg, the plate-like minerals talcum and mica or acicular wollastonite. It is often desired to maintain particle shape in such cases, an impact-type mill is usually chosen rather than a ball mill, as the latter tends to alter the original particle shape. 

Figure 14.1 Microphotograph of acicular wollastonite (VANSIL WG, R.T. Vanderbilt Co., Inc.), 170x.

PJ Wright. Acicular wollastonite as filler for polyamides and polypropylene. In A Whelan, JL Craft, eds. Development in Plastics Technology-3. New York Elsevier, 1986, p 119. 

Acicular wollastonite - This is produced from ore containing a suitably high percentage of long needles. The ore is milled and air separated in such a way that very fine, needle-like particles are preserved and recovered. Acicular grades typically have aspect ratios of 15 1 to 20 1. 

Table 10. Acicular Wollastonite vs. Talc in Ethylene-Propylene Copolymer ...

The main problems with mica are poor knit line strength and impact resistance. When mica-reinforced plastic flows around inserts and rejoins on the other side, the knit line may only have 60% of the strength of the rest of the part. Impact resistance of mica-reinforced plastics is poor, as it is with talc and kaolin. These problems are often overcome by combinations of mica with glass fibers, acicular wollastonite, calcium carbonate or glass microspheres, and by special molding machinery. 

Table 18. Treated Acicular Wollastonite in Polyurethane/ Polyurea ...

Table 19. Treated Acicular Wollastonite vs. Glass Fiber in Polyurethane RRIM Automobile Fascia...

Solubility of fillers is also important in dental cements and biomaterials. Unlike in construction cements, the cost of the end product is not a major issue in dental cements, but purity, biocompatibility, and performance of the final product is very important. Each filler component has its own role in modifying the behavior of the final product. As we shall see later in Chapter 18, wollastonite improves the flexural strength and toughness of the dental cements because the grains of wollastonite have elongated needle shape (acicular). Hydroxyapatite is added to provide biocompatibiUty to the product. Attributes of these fillers govern the final properties of the products and hence it is necessary that, not only one understands detailed properties of the binder components, but also that of the additives and filler components. For this reason, this chapter is devoted to describe the genesis and properties of the binder and important filler components. 

In Section 13.2, we mentioned that whisker reinforcement aids in increasing the flexural strength and fracture toughness of CBPCs. Acicular crystals of wollastonite act like whiskers. In this section, the effect of hber reinforcement is discussed. 

Is the tradename for wollastonite. It is available in three grades varying in particle size. An acicular calcium silicate, it is used as a filler and reinforcing agent in plastics, cement, wallboard, soil conditioner and in casting plasters. 

Wollastonite is a natural calcium metasilicate mineral with an acicular structure. Silanized and uncoated versions with maximum fiber lengths up to 25 pm are used as inexpensive secondary reinforcements in friction materials. Short fiber attapulgite clays have also been developed as secondary reinforcements. 

Wollastonite. This is a calcium silicate mineral, acicular, with aspect ratios of 3 to 20/1. It is of interest for reinforcement of strength and as a safe replacement for asbestos. 

The structure of wollastonite is characterized by chains formed from silica tetrahedra connected side-by-side through calcium in octahedral coordination [2]. These chain arrangements account for the formation of acicular crystals and preservation of their shape upon cleavage (Figure 14.1). The moderate value of wollastonite Mohs hardness (4.5-5) is attributed to the high density of the silica chains. In addition to the most important property of acicular shape, the plastics industry places high value on properties such as... 

Calcium silicate, natural n. CaSiOs. It has white color and acicular particles. It is characterized by high flatting action, combined with low oil absorption, and is used as an extender in paints, ceramics and plastics. Density, 2.9g/cm (24.2lb/gal) O.A., 25—30 particle size, 7 pm (fine particle grade) hardness (moh), 4.5. Syn wollastonite and calcium metasilicate. 

Wollastonite China (320), USA (150), Mexico, India 640,000 Acicular grade ... 

The polymer applications of wollastonite are mainly based on its combination of acicularity, chemical inertness, whiteness and low water adsorption. Primary consuming polymers include thermoplastics, such as polypropylenes and polyamides, and thermosets, snch as polyester dough and sheet-moulding compounds, epoxies and phenolics. Manufacturers are predicting considerable growth in reinforced reaction injection moulding (RRIM) systems. 

In the one-dimensional form, filler may be available in the thicker variety as a fiber or thinner (acicular-needle-shaped) variety as a whisker. Fillers available as fibers are glass, nylon, polyester, carbon and so on. Wollastonite stands out as a good example of an acicular... 

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. 

Wollastonite is a naturally occurring calcium metasilicate. It is the only commercially available pure white mineral that is wholly acicular typical length to diameter ratios range from 3 1 to 20 1. The world s largest known and purest deposit is owned and operated by Nyco. 

By special milling techniques, Nyco is able to retain the acicular integrity of the Wollastonite and produce many high aspect ratio grades. 

---