Nepheline Syenite

Nepheline syenite is a micronized functional filler and extender used as a performance enhancer in a broad range of paints, coatings, adhesives, sealants, and inks. Excellent brightness, tint retention, and weatherability can be achieved in exterior paints. Improved color, sheen uniformity, chemical and stain resistance, and durability result when used in interior paint formulations. It is easily dispersed in all conventional vehicle systems, and low oil absorption permits high loadings in adhesives, sealants, and aqueous and alkyd-based paints. Two grades are available to provide optimal performance in a variety of applications. The fine grade offers cost-effective gloss and sheen control and can add texture and application versatility for a wide range of uses. The extra-fine grade is preferred for higher gloss coatings and is also the preferred choice for powder and transparent paint systems.

Source

Nepheline syenite is a naturally occurring, silica-deficient, anhydrous sodium-potassium alumina silicate. Scanning electron microscopy confirms this filler contains less than one-tenth of one percent crystalline silica. No free crystalline silica is detectable in the mineral complex. Although feldspar-like in its chemistry, mineralogically, it is an igneous rock combination of nepheline, microcline, albite, and minor minerals like mica, hornblende, and magnetite. The rocks are mostly pale colored, grey or pink, and in general appearance, they are not unlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusive equivalent.

Nepheline is a feldspathoid, a solid-solution mineral that does not coexist with quartz; rather, nepheline would react with quartz to produce alkali feldspar. They are distinguished from ordinary syenites not only by the presence of nepheline but also by the occurrence of many other minerals rich in alkalies and rare earths.

Use

Nepheline syenite is composed of angular nodular particles. With its attendant low surface area, this shape accounts for easy dispersibility, low vehicle demand—lower, in fact, than would be suggested by oil absorption values—and the ability to achieve high loadings. The shape and size distribution of these filler particles can significantly improve the integrity of dried film through their ability to interlock and pack more tightly and rigidly than smooth-surfaced spherical or nodular fillers.

Dry film durability confers crack resistance and, in combination with relatively high filler hardness, provides high abrasion, scrub, and burnish resistance. Shape and particle size distribution also promote flatting and uniform low-angle sheen. Coating grades of nepheline syenite are each differentiated primarily by particle size distribution, which in turn determines surface area, oil absorption, and Hegman fineness.

Nepheline syenite has very low tint strength, contributing minimal color at high loadings, thus allowing the maximum development of deep tone and pastel colors with minimal colorant. Film reinforcement, chemical stability, and resistance to chalking promote high tint and gloss retention on prolonged exterior exposure. A relatively coarse Hegman 4 grind of nepheline syenite is typically used to achieve the optimal balance of properties. The low refractive index of nepheline syenite allows for higher loadings in transparent and semi-transparent coatings and stains.

Nepheline syenite is often favored over feldspar in coatings because it is silica-free and is an effective alkaline in-can buffer. If an exterior coating is sufficiently permeable, however, the nepheline component is subject to attack by acid rain and acidic atmospheric moisture. This can eventually result in the frosting of deep tones. Feldspar is preferred in this case. An application of emerging interest for nepheline syenite is powder coatings. Low resin demand allows for low melt viscosity and high loadings. Improved abrasion resistance and melt flow have been noted.

Particle Size Grades

For a complete explanation of the available particle size grades and their use in artist's paints, please read Oil Paint Extenders: Enhance Your Oil Painting with Texture and Transparency.