Paint & Coatings Industry - March 2009 - (Page 22) I Understanding and moisture vapour and, being a synthetic material, they are consistent in composition and colour, and unlike mica they are not stepped. Other barrier pigments commonly used are opaque and often strongly coloured. Micaceous iron oxide in particular makes coatings and paints difficult to tint in light shades, while glass flake is clear. Not only that, glass flake manufactured from ‘C’ or ECR glass offers high levels of chemical resistance and is inert in most mixtures and environments. It has good mechanical properties and is generally considered non-hazardous or a simple mechanical dust hazard, particularly when compared with thin fibres and some other fillers. Early glass flakes and the coatings they were used in were somewhat crude. The flakes were quite variable in thickness and plate size; the coatings were simple trowel or brush-applied materials basically designed as a glass fibre composite layer but with glass flake substituting for fibres. It was the mid seventies before good spray-applied glass flake coatings were available and these were generally thought to be exotic, unstable, difficult to apply and expensive. They were produced predominantly with the polyester resins used previously for GRP hand lay-up or manufactured as a vinyl ester variant for improved chemical resistance. Epoxy formulations containing glass flake were not common, and until more recent times were few and far between. From the early 80s glass flake coatings started to become more accepted, as their performance and the benefits of long life and hence low whole life-cycle cost became apparent. At the same time prices, compared with other coatings, dropped due to the larger scale production of raw materials. This led to greater acceptability of these coatings within the market place. It was during this period, that much research was carried out regarding the use of glass flake as a barrier pigment, and the types of coatings using it increased significantly. This work eventually spilled into other fields. Unfortunately the effects of using different concentrations of flake, flake aspect ratios, particle size distribution, critical pigment volume concentration and the unusual effects on viscosity were rarely understood and little work was done in this area. There was also relatively poor understanding of how the glass bonded within the various resin matrices. t is said that glass flake was developed in the United States around 1959 and was used initially for the reinforcement of what was, at that time, new-technology roof-light panels made from a polyester resin. However, it was found that the panels distorted in strong sunlight and a means of improving modulus and dimensional stability was sought. Glass fibre, although it provided improvement when used at the required volume for stiffness, severely reduced light transmission. Apparently, when glass flake was used not only was the modulus substantially improved but also light transmission. Although this seems to have been one of the first commercial uses of the product, glass flake quickly found its way into the coatings industry and it is here that the bulk of flake was used for many years and where most investigative work was carried out, only later progressing into other materials. As much of the research and development work with glass flake, and hence understanding, has been carried out with coatings, it is through these materials that we can introduce most of the facets of incorporating glass flakes into product. Apart from coatings for small-component parts, most coatings are based on organic resins. However, all organic coatings will, to some extent or another, convey moisture vapour and allow gas diffusion; preventing or resisting this is desirable and it is in this area that glass flakes initially found their niche. Particles of a high aspect ratio e.g., low thickness to surface area, as for example with platelets or flakes, can overlap each other and extend the path length for diffusion in a film, presenting a barrier to the passage of moisture and gas diffusion by creating a tortuous path through it. Particles of a granular or spherical nature do not overlap and offer only limited resistance to diffusion through the film. High aspect ratio fillers are, therefore, termed barrier fillers and are desirable not just as fillers or extenders but for performance improvement in other areas also. These improvements can extend not only into the areas of diffusion resistance and mechanical reinforcement but also other areas such as fire retardancy. The benefits of using plate-like barrier fillers, such as mica and micaceous iron oxide in anti-corrosive coatings in order to reduce moisture vapour transmission (MVT) have been known for a number of years. Other barrier fillers with varying attributes such as aluminium and zinc flakes have also been used as combination anodic and barrier fillers with varying degrees of success but may be problematical due to oxidation of the metal. Glass flakes, introduced into coatings around 1960, gradually gained popularity for several reasons. Glass flakes have a large aspect ratio and they do not oxidize or corrode in the normal sense. They are totally impervious to Glass Flake Qualities With the advent of glass flake markets developing, particularly in the anti-corrosion coatings sector, many different glass flake qualities were developed to suit the differing application requirements. Qualities are available with varying thicknesses, particle size diameters and glass compositions giving flake of varying performance. It is important to understand the effect of flake thickness and glass composition on overall coating performance. By Simon J. Brigham and Charles Watkinson | Glassflake Ltd., Glassflake Australia PTY LTD 22 MARCH 2009 | W W W . P C I M A G . C O M http://WWW.PCIMAG.COM
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