In Part 1 of this series, ceramic microspheres and glass bubbles were evaluated in a high-solids, two-part epoxy coating for industrial applications and other end uses such as concrete floor coatings. Conventional 100% solids UV-curable coatings offer excellent performance properties, but can be difficult to flatten based on the low shrinkage that occurs during the film curing stage, making it difficult for the matting agents to function effectively at the surface. Dry Film Thickness and Gloss: Dry film thickness readings were measured on aluminum with a Positector 6000 per ASTM D7091, and gloss readings were taken on aluminum and glass with a BYK micro-TRI-gloss instrument per ASTM D523. Stain Resistance:A modification of ASTM D1308 was used with a 4 h open spot test followed by a water rinse and visual rating system. Scratch Resistance:A standard abrasion tester (Elcometer Model #1720) was used for the scratch studies.
Abrasion Resistance: A standard Taber Abraser Model 5150 was utilized with CS-10 wheels and 1,000 gram loading. Clarity and Haze:The clarity and haze were measured with a BYK-Hazeguard instrument on glass.
The effects of loading levels on viscosity and package stability were the next properties to be studied. The ceramic microspheres (A and E) did not have a significant impact on viscosity as the weight loading increased to >20%. Sedimentation studies showed that the silicas stayed in suspension very well, while the ceramic microspheres and nepheline syenite showed slight settling over time (30+ days at room temperature) at the higher loading levels. Haze measurements using the BYK-Hazeguard meter show an increase in haze as the filler load increases, as shown in Figure 5.
Fillers D, C and F exhibit higher haze values at lower weight percentages because they exhibit lower gloss values at those levels (as shown in Figure 1). Figure 6 shows the results of a 4 h spot stain test using five household chemicals for three different weight loadings of filler E.
Three loading levels were evaluated for each filler pigment except D and F where only two levels were evaluated. In general, the conventional silica matting agents have a slightly higher cost structure in this formulation, even at the low weight loadings needed for matting efficiency. Many different functional fillers may be utilized as matting agents in waterborne UV-curable coatings, such as the polyurethane dispersion used in this study. Ceramic microspheres may offer formulators alternatives in waterborne UV-curable coatings to serve as both a matting agent and functional performance filler. Our September issue focuses on sustainable solutions, specialty chemicals, architectural coatings and the latest pigment technology. Benefits included higher volume loadings and lower VOCs at lower viscosities, in addition to better film integrity, as manifested by improved abrasion resistance. Many manufacturers still desire UV-curable coatings for their fast-cure footprint, minimal VOCs, and high-performance capabilities and aesthetic attributes on many substrates including wood. The aluminum panels were used for gloss, scratch, stain and friction testing, and the glass panels were used for gloss, clarity and haze testing. A rating of 0 signifies no change, while a rating of 5 signifies severe softening, gloss loss or change of color.
The ASTM D2486 head and brush were used with an additional 3M Scotch-Brite®non-scratch blue pad attached to the bottom of the brush.

The silica matting agents were very effective and yielded a sharp decrease in gloss at relatively low loading levels (1-5% weight), while the ceramic microspheres and nepheline syenite behaved comparably. Both of them exhibited viscosity increases of less than 1,000 cps (baseline with no additive is approximately 1,550 cps).
This settling was very soft, and the coating was able to be remixed without any adverse effects on application.
It should be noted that even though the clarity readings were low in the low-gloss region, the corresponding visual appearances on cherry wood panels looks similar among the matting agents. At comparable gloss levels, the clarity and haze values were similar for each of the fillers studied. Mustard was the most severe and showed only very slight staining at all loading levels with all fillers.
The 60° gloss readings were taken before and after testing, and the % change was plotted versus the filler loading.
Fillers B and D could actually offer cost benefits at higher loadings depending on the costs of the other raw materials in the formulation.
The key is to finding one that can serve as a matting agent while offering excellent performance and application properties.
Depending on other formulation parameters, higher ceramic loadings could equate to higher solids and lower binder demands.
Technical information, recommendations and other statements contained in this document or provided by 3M personnel are based on tests or experience that 3M believes are reliable, but the accuracy or completeness of such information is not guaranteed. Part 2 of this series evaluates the same ceramic microspheres, silica matting agents and nepheline syenite in a waterborne, UV-curable polyurethane dispersion. Manufacturers want to use the same coating chemistry at varying gloss levels without sacrificing application or performance properties.
After 5 min mixing, the matting agent was added and the mixing speed was increased to 4,000 rpm for 10 min. Five household stains were evaluated (ketchup, mustard, vegetable oil, red wine and hot coffee). This test was performed on the aluminum panels, and the gloss was read before and after testing. Two layers of white cheesecloth (Bleached Grade 40) were wrapped around the sled to simulate a sock. To obtain a flat gloss (<20 on a 60 meter) would require 4% or less of the silica matting agents (C, D and F) versus about 20-25% for the ceramic microspheres (A and E) or nepheline syenite (B) materials.
Nepheline syenite (B) and two of the silicas (C and F), however, did yield increases of two to three times the base viscosity with the increased loading levels. Settling is dependent on many factors such as particle size, density, shape, concentration and the overall viscosity of the coating solution.
Fillers E and C did give slightly higher COF values, but all of the fillers studied had low values compared to coatings that would be considered slip resistant. Filler A offers comparable costs at higher loadings and does not impart any surface roughness at lower dry film thicknesses. In addition, many manufacturers desire a matting agent that does not impart cloudiness or haze to the coating and yields a film that exhibits warmth, thus being decorative and functional on substrates such as wood cabinetry.

Long-term viscosity stability can equate to consistent application without the need for formula adjustments. Waterborne radiation-curable coatings are seeing increased use for their performance properties and relative ease of matting.
The speed was then reduced to about 1,500 rpm, and the photoinitiator and deionized water were added under agitation and allowed to mix for an additional 10 min.
The percentage change in the 60° gloss was measured at various loadings for each of the matting agents studied.
Testing was conducted in triplicate, and the average kinetic COF was plotted for each filler type.
These could require additional dilution for application, which could result in lower application solids. In many instances, one may be able to minimize settling by controlling rheology through the use of thickeners or anti-settling agents. The effective top size for filler E is approximately 40 microns, and the dry film thickness for this study was approximately 20 microns maximum. Values greater than 100 signify a rub up or higher gloss after testing, and values less than 100 indicate a dulling or gloss loss.
Filler D varied from 6 to 25%, filler B from 10-42%, and fillers A and E (ceramic microspheres) from 1-10%.
It may be interesting to see if formulations involving both silica and ceramic microspheres offer the benefits of cost and performance.
In addition, ceramic microspheres offer excellent scratch, chemical and abrasion resistance, which are highly desirable properties for many industrial applications on wood and other substrates. This application study evaluates two different particle size ceramic microspheres, three silicas and nepheline syenite for effects on application viscosity, gloss, film clarity, scratch and friction resistance. The precipitated silica gel (D) performed very well and did not exhibit an increase in viscosity with a loading of up to 5% by weight.
The appearance will be different at higher film thicknesses, but the resulting gloss and performance properties would need to be evaluated. Properties such as matting efficiency, scratch resistance and viscosity control were studied in addition to the effect each material had on clarity and haze.
The nepheline syenite was chosen for its similar particle size to the smaller ceramic microsphere (A). Figure 4 shows select examples of the general appearance of fillers A, B, E and F on oak boards. All of these properties are important in obtaining a final finish that is both decorative and functional.

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