Wollastonite

Wollastonite is a calcium metasilicate mineral, available in two powder grades of fineness. Each grade exhibits lower viscosity in paint than is indicated by its oil absorption value. The low oil absorption characteristic of wollastonite enables you to reduce raw material cost by replacement of the vehicle. The relatively high pH (10–11) of wollastonite is particularly useful in maintaining the desired alkaline pH level of water-based paints and preventing early rusting of water-based coatings. Wollastonite is an excellent extender pigment for powder coatings as well as solvent-based and water-based paints.

The fine grade of wollastonite disperses to 4 Hegman fineness at three pounds per gallon. This grade provides smooth paint films and is a general purpose extender. The extra-fine grade disperses to 6 to 7 Hegman fineness at three pounds per gallon. This grade is suitable for liquid industrial and corrosion resistant coatings, powder coatings and semi-gloss architectural coatings. In powder coatings, this grade is desirable for smoother films and improved gloss.

Source

Wollastonite is a calcium metasilicate mineral (CaSiO₃) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids as in metamorphic rocks. Associated minerals include garnets, vesuvianite, diopside, tremolite, epidote, plagioclase feldspar, pyroxene and calcite. It is named after the English chemist and mineralogist William Hyde Wollaston (1766–1828).

Some of the properties that make wollastonite so useful are its high brightness and whiteness, low moisture and oil absorption, and low volatile content. Wollastonite is used primarily in ceramics, friction products (brakes and clutches), metal-making, paint filler, and plastics.

Use

Wollastonite has industrial importance worldwide. It is used in many industries, mostly by tile factories which have incorporated it into the manufacturing of ceramic to improve many aspects, and this is due to its fluxing properties, freedom from volatile constituents, whiteness, and acicular particle shape. In ceramics, wollastonite decreases shrinkage and gas evolution during firing, increases green and fired strength, maintains brightness during firing, permits fast firing, and reduces crazing, cracking, and glaze defects. In metallurgical applications, wollastonite serves as a flux for welding, a source for calcium oxide, a slag conditioner, and to protect the surface of molten metal during the continuous casting of steel. As an additive in paint, it improves the durability of the paint film, acts as a pH buffer, improves its resistance to weathering, reduces gloss, reduces pigment consumption, and acts as a flatting and suspending agent. In plastics, wollastonite improves tensile and flexural strength, reduces resin consumption, and improves thermal and dimensional stability at elevated temperatures. Surface treatments are used to improve the adhesion between the wollastonite and the polymers to which it is added. As a substitute for asbestos in floor tiles, friction products, insulating board and panels, paint, plastics, and roofing products, wollastonite is resistant to chemical attack, inert, stable at high temperatures, and improves flexibility and tensile strength. In some industries, it is used in different percentages of impurities, such as its use as a fabricator of mineral wool insulation, or as an ornamental building material.

Wollastonite was investigated as a functional filler for coatings as far back as the mid-1940s. In 1950, wollastonite was cited as a filler in coatings. Attractive properties of the mineral then included white color, moderate hardness, and a high natural brightness.

In 1979, a review of wollastonite and its suitability in coatings systems noted that this mineral had been important for over twenty-five years as an "extender pigment which contributes to rust resistance, exterior durability, and color retention." The article described a study comparing wollastonite to a variety of extenders in both interior and exterior latex paints. Also reported was the use of wollastonite in an acrylic alkyd modified latex paint containing zinc oxide. Using a 400 mesh wollastonite, the zinc oxide was stabilized without a high dispersant demand. Two benefits noted were efficient raw material cost management and better package stability. The author concluded, after comparing extenders in many types of latex paints, that wollastonite offered the following.

• Versatility based on its acicular shape for low sheen, film strength, and overall durability
• Moderate oil absorption for higher PVC contribution
• High dry brightness for cleaner tints
• Moderate hardness for mar and scrub resistance
• Alkaline pH, likely to become one of wollastonite's most important properties, requiring less ammonia and improving rust and mildew resistance

Five years later, a number of functional extender pigments were compared for their contribution to the performance of anti-corrosive epoxy metal primers. Because legislation restricted the use of lead- and hexavalent chromium-based corrosion inhibitors, new types of molybdate, borate, and phosphate inhibitors were emerging. These had their own performance issues due to high oil absorption and lower efficiency. Examining the PVC/CPVC ratio in order to regain efficient corrosion control led to the reassessment of the choice of extender. Among the nine extender pigments examined, wollastonite was preferred for improved corrosion and blistering resistance, with additional improvement derived from epoxysilane treatment of this mineral.

More recently, work was reported on the use of wollastonite surface-treated with reactive silanes, such as amino and epoxy functional types, in corrosion resistant coatings. In this article the role of wollastonite as a "synergist" with inhibitive pigments was described. While the mechanism is not fully known, numerous authors have described the synergistic behavior of wollastonite with the majority of primary inhibitive pigments. What is known is that when used in combination with inhibitors, wollastonite, especially the surface-treated grades, enables the inhibitive pigment to be more effective for overall corrosion protection than when the pigment is used by itself. One author, in a study focusing on waterborne epoxy primers and anti-corrosive pigments, reached the following conclusions regarding treated wollastonite and its performance:

Pigment volume concentrations (PVCs) in the range of 38 to 45%, and a PVC/CPVC ratio of 0.6 to 0.9, showed the best overall coating performance when the extender package included 150 pounds per 100 gallons of treated wollastonite and the anti-corrosive pigment level was 100 pounds per 100 gallons.

The past several years have seen the incorporation of wollastonite in a wide variety of corrosion resistant coatings. While the waterborne epoxies seem to prevail, especially since the formulating trend is toward reduced VOCs for environmental compliance, acrylic copolymer systems are using wollastonite, as are water-reducible polymer systems.

Chromate and phosphate pigments are being developed to improve the effectiveness of inhibitive metal primers, especially in wash primers, aircraft primers, and automotive primers. To completely replace the performance of hexavalent chromium remains a challenge, particularly at low loadings. Because strontium chromate remains, for many systems, the inhibitor of choice, the formulator looks to wollastonite to reduce the amount of strontium chromate used, without sacrificing corrosion protection. Further, zinc phosphate has been used to help offset dependence upon strontium chromate, and over the last 30 years has become the leading nontoxic inhibitive pigment. For instance, a surface-treated wollastonite has been successfully used to improve the performance of zinc phosphate in alkyds.

In barrier-type formulations where corrosion resistance is achieved through means other than inhibitive pigments, surface-treated wollastonite has been shown to be effective on its own. Because the protection afforded by these types of systems is more related to controlling ionic permeability and chemical reactivity of the binder with the filler surface, a working hypothesis is that surface-treated wollastonite contributes to performance with its acicular shape and via the chemical bond occurring between the surface treatment and the binder.

Further, wollastonite's acid solubility can be controlled by proper selection of the surface treatment chemistry. By carefully matching the chemistry to the inhibitive pigment and binder system, the contribution of wollastonite to passivation can be maintained, and the synergistic relationship can be maximized.