Lesser-Known Silicates: It's All Relative

The properties and uses of the functional silicate fillers in coatings have been the subject of a series of articles in PCI.1-6 This, the concluding article of the series, describes four additional fillers, pyrophyllite, chlorite, perlite and vermiculite, which serve limited but specific functions in coatings. Also discussed are two silicate minerals, smectite clay and hormite clay, that have structural similarities to silicate fillers, but that serve a distinctly different function in coatings.

Special Silicates

Pyrophyllite, chlorite and vermiculite share certain structural features and physical properties with the more commonly used kaolin, talc and mica, but their use in coatings is restricted due to limitations in purity, color or availability. Their basic properties are compared in Table 1. Perlite is described as an amorphous aluminosilicate7, but its lack of defined crystallinity makes it structurally unrelated to the other silicates.

Pyrophyllite

Pyrophyllite, Al2Si4O10(OH)2, is the aluminum analogue of talc, and a pure, platy specimen of it would share many of talc's physical characteristics. In North America, however, pyrophyllite is rarely found in deposits as the pure mineral, and it is usually intimately associated with one or more of the accessory minerals such as quartz, mica, kaolinite, andalusite and diaspore. Rather than being a detriment, these associated minerals, to a large degree, determine the appropriate commercial application for a given pyrophyllite ore. Little emphasis is placed on producing high-purity pyrophyllite in the United States. In fact, the mineral pyrophyllite is typically not the major component of products sold under its name.

Worldwide, paint is the largest filler market for pyrophyllite products. In the U.S., however, this is a minor market, after refractories and ceramics.8 Relatively coarse-ground grades are used to impart mud-crack resistance to high-build coatings such as textured paints and block fillers, and checking, cracking and frosting resistance to exterior latex paints. The platy nature of the pyrophyllite, mica and kaolin components promotes good dispersion by inhibiting the settling of pigments, helps film dry, and increases resistance to film cracking. Figure 1 shows the platy character of a filler-grade pyrophyllite product. The considerable quartz content of these products contributes hardness and wear resistance. Fine-ground grades have also been considered for use in powder coatings and traffic paints. Although the combination of soft, platy and hard, nodular particles in pyrophyllite products provides unique performance possibilities in coatings, their generally off-white color, and the safety concern relating to their high quartz content (up to 60%) have inhibited widespread use.

Chlorite

Chlorite, (Mg,Fe)3(Al,Si)4O10(OH)2·(Mg, Fe)3(OH)6, is a platy mineral composed of alternating talc-like and brucite, Mg(OH)2, sheets. Up to half of the tetrahedral Si4+ and up to one third of the octahedral Mg2+ in the talc-like layer may be replaced by Al3+. Both Fe2+ and Fe3+ commonly substitute for part of the Mg2+ as well, usually to the detriment of mineral color. The charge imbalance from tetrahedral substitution is generally balanced by octahedral substitution either in the talc structure or the brucite structure. Hydroxyl-bearing brucite sheets between the talc sheets allow for hydrogen bonding and corresponding resistance to delamination.

Chlorite is often associated with talc deposits, and offers many of the same benefits as talc in flatting ability, barrier properties and exterior durability. Chlorite is more hydrophilic, but generally lower in brightness than coatings grade talcs. Its use in coatings is limited.

Perlite

Perlite is a volcanic glass containing 6-9% aluminum. Perlite rock contains 2-6% combined water and is distinguished by its concentric fractures, the result of rapid cooling of the molten volcanic glass, which gives rise to an onion-skin appearance. When heated above 870 oC the water expands the perlite grains, forming minute bubbles within the glass matrix. The result is low-density particles with cellular interiors. These particles are used for their acoustical and thermal insulating properties, chemical inertness, physical resilience, fire resistance, water retention ability and low bulk density.

Perlite expansion is carried out in regionally located plants in order to reduce the cost of transportation per unit weight of product. The perlite is expanded by quick exposure in furnaces to temperatures of 870 oC to 1100 oC. This allows simultaneous softening of the glass matrix and volatilization of the contained water. The perlite expands in volume up to 2000% and decreases in bulk density by up to 90%. After expansion, the perlite is air classified to remove any unexpanded particles and fines. Depending upon the intended end use, the expanded product may be further size-classified, surface-treated or milled.

Perlite is a chemically inert flatting filler with high oil absorption and good dry hide. Expanded perlite is used in lightweight, fire-resistant coatings for structural steel and concrete, as a filler in textured paints, and in auto underbody plastisol coatings to reduce weight and improve sound insulation. Expanded perlite is milled and sized to fine particles with jagged interlocking shapes that promote good film integrity and substrate adhesion. The United States is the leading producer of perlite with 670,000 tons/year. Of this, less than 5% is used in coatings applications.9

Vermiculite

Vermiculite, Mg0.3(Mg,Fe)3(Al,Si)4O10 (OH)4·8H2O, is a platy sheet silicate with a talc-like structure, but similar in appearance to the mica from which it was geologically altered. Like talc, the vermiculite structure is composed of an octahedral magnesia layer sandwiched between two tetrahedral silica layers. There can be some substitution of Fe3+ and Al3+ for Mg2+ in the octahedral layer, but there is always some substitution of Al3+ for Si4+ in the tetrahedral layer, creating a charge imbalance. Two oriented water layers and charge-balancing magnesium cations separate vermiculite plates. Vermiculite is nearly as soft as talc, but delamination is prevented by the attraction of opposing plates to the interlayer Mg2+, as well as the simultaneous hydrogen bonding of oriented water to plate faces. Most commercial vermiculite is heat-treated to volatilize its interlayer water and produce an expanded (exfoliated) product. Upon heating, the flat, macroscopic "books" of vermiculite plates expand into elongated, concertina-like particles. Expanded vermiculite possesses absorptive, insulating and ion-exchange properties that are the basis for its industrial uses.

Expanded vermiculite has a very low bulk density, so, as with perlite, exfoliation is carried out in regionally located plants. Vermiculite concentrates are expanded in furnaces by brief exposure to 900 oC or more. The heat rapidly turns the interlayer water to steam, which pushes the individual vermiculite plates apart. The vermiculite increases in volume by 1500-2000% and decreases in bulk density by 85-90%. The expanded particles are 90% entrapped air by volume and possess excellent insulation properties. The expanded material is separated from nonexpanding impurities by air classification, and is either bagged directly or ground to sizes suitable for various applications.

The size and color (light to dark brown) of generally available expanded vermiculite products precludes their use in most coatings. Nevertheless, about one quarter of the U.S. use of expanded vermiculite is in ready-mix plaster and cement coatings to impart low density, high acoustical insulation, high thermal insulation and low thermal conductivity.10 These mixes are used as acoustical coatings, or are sprayed on structural steel and concrete as lightweight, fire-resistant coatings. The platy nature of the mineral also protects these coatings against cracking and spalling when subjected to temperature extremes and mechanical shock.

Functional, But Not Fillers

There are two silicate clays used by the coatings industry that serve as rheology modifiers or thixotropes. They are described here because of their structural similarity to silicate fillers, and to indicate how relatively subtle differences in structure can result in fundamental differences in performance properties. The structural similarities among the platy silicate fillers and one of the smectite clays (montmorillonite) are depicted in Figure 2.

Smectite Clays

Smectites are platy, water-swellable colloidal clays that are used to control the rheology of waterborne coatings. Because they are readily delaminated in water, they are also fundamentally nanoclays, with the potential to contribute to the physical and optical properties of a coatings film.

Like mica, smectite clay, more commonly known as bentonite, can have either a pyrophyllite- or talc-like structure. Montmorillonite can be described as the pyrophyllite structure with a minor octahedral substitution of Mg2+ for Al3+. Saponite is similar in structure to talc, but with limited tetrahedral substitution of Al3+ for Si4+, while hectorite has the talc structure but with limited substitution of Li+ for octahedral Mg2+. The charge imbalance resulting from the substitutions in these clays is compensated for by exchangeable cations, usually Na+ or Ca2+, between the platelets. In addition to these counterions, oriented water, similar to that in vermiculite, occupies the interlayer space. When Ca2+ is the main exchangeable cation, there are two water layers, as in vermiculite; when Na+ is the counterion, there is usually just one water layer.

Unlike vermiculite, the smectite structure can accommodate additional interlayer water, due at least in part to its lower counterion density. This allows for hydraulic delamination. When sheared in water, sodium smectites incorporate enough additional water layers through diffusion and osmosis to overcome weak platelet-platelet attractions. The result is separation into individual high aspect ratio platelets one nanometer thick by several hundred nanometers across.

Sodium smectites are used as rheology-control agents because of the colloidal structure their delaminated particles form in water. The platelets have negatively charged faces, from the lattice substitutions, but carry a slight positive charge along their edges. After delamination, platelet edges and faces are mutually attracted. They form a cohesive, three-dimensional structure commonly described as a "house of cards." This structure will thicken water, but its primary benefit is the ability to segregate and trap any dispersed phase, whether solid, liquid or gas.11 Sodium smectites are therefore used as suspension and emulsion stabilizers. Calcium smectites also swell through interlayer water absorption, but will not proceed to complete delamination because of the greater bonding effect of their divalent cations.

Most smectite clays are processed by drying, crushing and milling to a 200-mesh or 325-mesh powder, with mineralogical purity determined by ore selection. The impurities most commonly associated with commercial smectites are silica, feldspar, zeolites and carbonate minerals. Some white or light-colored smectite clays are water-washed to produce products of high mineralogical and chemical purity for optimum performance efficiency. Smectites can also accommodate certain organic cations in exchange for their native counterions. This makes them hydrophobic and enables them to be used as organoclay rheological agents in nonaqueous systems. Organoclays of this type are widely used as thixotropes in solventborne coatings.

Smectite clays are typically used at low loadings, about 0.3% by weight, as thixotropes in waterborne coatings because their colloidal structure effectively controls settling, syneresis and sagging. They are often used synergistically with cellulosic thickeners and associative thickeners. Because of the small size of the individual clay platelets, recent attention has focused on their use for TiO2 extension. In this work, loadings of up to 1.2 weight percent of sodium smectite and calcium smectite were evaluated and shown to improve whiteness and contrast ratio. Significant savings were suggested by using the smectite clay to decrease the loading of TiO2.12

Hormite Clays

Hormite clays are chain silicates having certain structural features in common with both tremolite and antigorite, although their properties are quite unlike either. As in the case of antigorite, silica sheets are continuous but periodically inverted. Since hormites have a silica layer on both sides of the octahedral layer, silica sheet inversions limit the width of the octahedral sheet, allowing it to grow in only one direction. The result is talc-like strips resembling those in tremolite. However, these strips are joined by shared tetrahedral oxygens at the lines of inversion. This creates channels that are filled with water. Removal of this water confers highly absorptive properties. The lattice structure accounts for the high surface area and acicular particle shape of the commercial hormites - attapulgite and sepiolite. The hormite clay used in North American coatings mainly derives from the extensive deposits of attapulgite in Georgia and Florida. Minor amounts of sepiolite from Spain are used as well.

The processing of hormite clays involves crushing, extruding, drying, milling, screening and air classifying, as required, to produce a range of products from partially calcined, coarse absorbent granules to fine colloidal powders for thickening applications. Products for rheology control are typically dried to 12-16% free moisture and micronized. The needles of gelling-grade Spanish sepiolite are about 1 micrometer long with a high, 100:1, aspect ratio.13 The needles of domestic gelling-grade attapulgite are typically shorter, with a lower aspect ratio. Gelling grades may be extruded before drying to facilitate the separation of their acicular particles. Because the preparation of aqueous hormite dispersions is energy intensive, concentrated, predispersed suspensions are sold as a convenience to coatings manufacturers and other major consuming industries.

When hormite clays are dispersed in water, they do not swell like smectites. The needles deagglomerate in proportion to the amount of shear applied, forming a random colloidal network. A hormite dispersion is alkaline in pH, with a less cohesive colloidal structure than that produced by a smectite, since it is not based on ionic bonds. This structure offers rheological properties similar to those of smectite clays, with somewhat less physical stability, although with considerably greater compatibility with electrolytes and other water-solubles.10 Attapulgite is widely used as a thixotrope in waterborne paints in North America, although this use accounts for only about 2% of attapulgite sales.

Conclusion

Although they are functional fillers, pyrophyllite, chlorite, perlite and vermiculite find relatively minor use in coatings. Except for perlite and vermiculite in the insulating coatings noted, their use tends to be limited to applications where price or local availability, rather than performance, provides an advantage over the more commonly used silicates. The smectite and hormite thixotropes, on the other hand, are commonly used in waterborne coatings. In North America, attapulgite is the most widely used of these because of its favorable price/performance ratio and availability in predispersed form. In solventborne coatings, however, chemically modified smectite organoclays are the preferred thixotropes. The applications in which these filler and thixotrope silicates are used are summarized in Table 2.

For further information, contact RT Vanderbilt Co., phone 203/853.1400; fax 203/853.1452; visit www.rtvanderbilt.com; or e-mail pciullo@rtvanderbilt.com or srobinson@rtvander bilt.com.