According to William Shaker, market development manager of Polymer Additives at Potters Industries, "No one conventional additive can match the multiple performance benefits of SPHERICEL hollow glass microspheres. Because they are made of colorless glass they do not discolor light or pastel formulations. Their hollow structure, low density (0.60 and 0.34 g/cc) and small particle size make them ideal for use as extenders for paint formulations."
Paint that is extended with SPHERICEL microspheres has a lower viscosity than a paint filled to an equivalent volume with a non-spherical extender. Spherical particles have a low-energy surface that minimizes friction and drag. As a result, an equal volume substitution of these microspheres for irregularly shaped extenders will decrease the coating's viscosity. "Lower viscosity is a significant benefit in offsetting VOC levels in solventborne paint," says Shaker. "Adding microspheres to a high-VOC paint formulation allows formulators to remove some of the solvent and still maintain a viscosity that facilitates application and spreading properties."
With particle sizes considerably finer than previously available, SPHERICEL hollow microspheres can be used in thin-film coatings to improve integrity. "Because glass spheres do not absorb resin, more resin is available to create the film," explains Shaker. "The result is a tighter and more uniform film with improved durability, even under adverse conditions."
SPHERICEL microspheres may also be added to improve hiding properties or to replace some of the titanium dioxide (TiO2). The hollow glass spheres redirect the angle of light, imparting opacity. "Depending on the formulation," says Shaker, "equivalent tint strength can be achieved with 5-10% replacement of TiO2."
Study ResultsAccording to Chris Smith, technical service manager at Potters, a recent study conducted for Potters Industries on flat latex wall paints found that 60P18 glass microspheres enhance important performance properties. In ASTM tests for scrub resistance, burnish resistance, gloss, hiding and stain properties, the glass microspheres replaced clay and calcium carbonate and "appear to outperform an established ceramic additive."
Scrub ResistanceIn this test, says Smith, several versions of acrylic latex flat interior wall paint were formulated for a test program. The basic standard formula using both calcium carbonate and clay extenders is shown in Table 1, along with a modified formula that replaces clay with an equal volume of hollow microspheres. Tests included volume substitutions for both extenders with various glass microspheres.
Formulas with equal volume substitution of a minus 24 micron ceramic microsphere additive were also included in the test series. Sample preparation and scrub testing were undertaken by DCA Coatings, a paint test facility in Ohio. Scrub testing was done according to ASTM D 2487 and was reported as scrubs to failure. The data is shown in Table 2.
The test design was set up to see the effect of substituting the glass microspheres for either the clay or the calcium carbonate at equal volume % loadings. Since the glass microspheres differ substantially in density, all formulating was done by volume %. The standard formula with clay and calcium carbonate extenders failed at 178 scrubs. The solid ceramic product performed better, giving 200 scrubs when substituted for calcium carbonate, and 285 scrubs when substituted for clay.
As a replacement for calcium carbonate, the glass microspheres performed in a similar fashion with 200 to 212 scrubs. However, as a replacement for clay, the performance was improved substantially compared to the ceramic product. Scrubs ranged from 250 to as high as 500 depending on the grade of glass microsphere tested.
Burnish ResistanceIn this test, explains Smith, the formulas used for the scrub tests were also subjected to burnish testing according to ASTM 6736-01. "This method employs a BYK-Gardner test unit and 20 passes with 4-ply grade-20B cheesecloth," he says. Testing was done using an 85-degree gloss measurement. Results are shown in Table 3.
The standard showed a 104% increase in gloss while the microsphere-modified formulas performed substantially better. As calcium carbonate substitutes, the glass spheres showed gloss increases of 57 to 79%. This compared to 56% for the ceramic sphere formula. The results were quite different with glass microspheres as a clay substitute. The glass sphere substitutions exhibited gloss increases in the 32 to 50% range, while the ceramic product showed a 69% increase. Glass microspheres, properly formulated into a flat paint, appear to have significant potential for burnish resistance.
GlossAccording to Smith, the test's gloss figures show mixed results for the substitutions depending on the extender that was replaced. For calcium carbonate substitution there was a substantial range of results at 85 degrees, but the 20- and 60-degree tests show little variation. Compared to the standard with a gloss of 2.3 at 85 degrees, the calcium carbonate substituted with ceramic product increased gloss to 2.7. The glass spheres exhibited gloss reduction with readings ranging from 1.3 to 2.1 depending on grade. For the clay substitutions, the ceramic additive and the glass spheres were a lot closer in gloss with a range of 1.3 to 1.6. "It appears that some consideration must be given to gloss reduction," says Smith, "and that there may be some benefit to incorporation of glass microspheres in that regard."
Stain ResistanceTests for stain resistance were run according to methods specified in ASTM D 3258, except coffee was used instead of ink. Black coffee was allowed to stand on the chart for five minutes before a water rinse and drying. The glass microspheres outperformed the standard and ceramic product in all cases. "This may be accounted for by the tighter film that results when a smooth-surfaced microsphere is replacing extenders that have a more irregular or porous surface," explains Smith. "Or, it may be from the stain accumulating on the irregular and perhaps porous surfaces of some particles while the smooth glass surface is more resistant."
Hiding PowerCalculation of contrast ratio was done by measuring the Y reflectance over black and over white. Substituting hollow spheres for calcium carbonate showed contrast ratios similar to the standard, while the ceramic product gave a significant reduction in hiding. In the tests employing a clay substitution, the hollow glass microspheres were very close but had less than the contrast ratio for the standard. However, glass outperformed the ceramic additive. The light-scattering ability of a hollow sphere may be able to add hiding power to various paint formulas, and some reduction in titanium dioxide was possible in other experiments. The ability of voids to scatter light is well known, and the controlled size of the voids inside hollow glass spheres can be counted on to do this in a highly controlled way.
Formulating and EconomicsNormal-density microspheres have been substituted for mineral fillers in many systems with direct weight substitution and few technical concerns. Although they have densities somewhat lower than typical extenders, the lower viscosity contribution tends to still allow easy processing and application even though volume percentages are higher. For hollow glass microspheres however, it is definitely necessary to think in terms of volume substitution. For instance, a 0.6 g/cc microsphere such as 60P18 is less than 1/4 the density of most mineral extenders or solid ceramic additives. When substituting into existing formulas, it is appropriate to take out 4 Kg of the heavy material for each 1 Kg of 60P18 added, as a starting point. Substantial economies can be had when formulating a batch of scrub-resistant paint. In the test formulas used in this study, based on ingredient list prices, the substitution of 60P18 for clay increased the cost per gallon of paint from $6.39 to $6.44, which is only 0.8 %. Compared to the ceramic-modified paint at $6.70, the 60P18 formula costs were down 3.9 %.
To achieve the best economics with hollow microspheres, attention to formulating and mixing technique is necessary. Although higher-density glass microspheres are extremely strong and resist virtually any shear applied, some consideration may be appropriate when hollow glass microspheres are used in paint manufacture. With a smooth glass surface, controlled particle size distribution and low surface area, as well as freedom from agglomerates, the hollow spheres wet out easily in virtually all systems. Adding spheres at the final mixing stage is usually best, and only relatively low shear needs to be applied to get spheres dispersed. Long periods of mixing employing high shear could result in some breakage of hollow spheres and should be avoided.
ConclusionsThe study found that glass microspheres are performance-enhancing additives for flat latex wall paints. In ASTM tests for scrub resistance, burnish resistance, gloss, hiding and stain properties, the glass microspheres replaced clay and calcium carbonate and appear to outperform an established ceramic additive. Formulators will find that variations in the levels of other extenders will play an important role in getting optimum performance. Substituting lightweight additives must be done on a volume basis to get the performance advantages and to understand the economies of using hollow glass microspheres vs. heavy extenders. Cost calculations prove out the benefits by comparing volume economics for the systems. SPHERICEL 60P18 is designed to give all the advantages of hollow spheres at a particle size, strength and volume cost acceptable to latex paint formulators.
Glass Microspheres - A Performance AdditiveGlass microspheres, both solid and hollow, have been used for various applications within the paint and coatings industry for many years. Improved performance in epoxy primers, powder coatings, floor applications, aircraft paints and industrial high-build coatings has been well established. VOC reduction has been a driving force for inclusion of glass microspheres in some applications. Hollow spheres are finding a place in thermal insulating coatings for various construction and transportation applications, and acoustic insulation effects are claimed for some coatings. Both solid and hollow glass microspheres are specified, depending on the critical performance needs.
More recently, an additive based on ceramic technology has been used in scrub-resistant latex applications. Glass microspheres, because of size control, superior surface smoothness and lack of irregularly shaped particles, take performance to a new level. Under the microscope, the glass microspheres look like tiny ball bearings, and in fact, promote good flow during all stages of paint manufacture and application. The unique flow properties have allowed higher PVC while retaining or enhancing important performance properties.
Tests with hollow glass microspheres in anti-corrosive primers have demonstrated that enhancement of film integrity permits reduction of the rust-inhibiting additive while getting far superior salt spray performance. Recent innovations in glass technology have allowed the production of hollow glass microspheres at 0.6 g/cc true density with a particle size distribution suited to flat wall paint. This addition to the Potters SPHERICEL line is called 60P18.
Potters Industries, a wholly owned subsidiary of PQ Corporation, is the world's leading manufacturer of engineered glass materials for highway safety, industrial and specialty applications. For more information on the properties and performance advantages of SPHERICEL hollow glass microspheres, call Bill Shaker at 610/651.4715 or visit the Potters Industries website at www.pottersbeads.com.
The technical information and suggestions for use and applications presented herein represent the best information available to us and are believed to be reliable. They should not, however, be construed as controlling suggestions. Potters Industries Inc. makes no warranties, either expressed or implied, with respect to our materials, including warranties of merchantability or fitness for any particular purpose. We urge users of our materials to conduct tests to determine final suitability for their specific end uses.