Paint & Coatings Industry - March 2009 - (Page 23) Use of Glass Flake Principally, there are two manufacturing techniques employed to produce glass flake. The first is the bubble method, which has been in use for over 40 years, albeit in various incarnations. The second is the spun method. The bubble method is much more cost effective, allowing for high throughput production and normally produced from pre-melted glass marbles. The drawback to this method is the limitation on control of product parameters. In the earlier years, flake produced using this technique typically displayed a product thickness variation of 1-18 µm and a high degree of fines, which are small particles/splinters of glass resulting from fracture of the glass during production and milling. In recent times the process has been developed to produce product typically with a variance of 2-9 µm. It is generally accepted that it is difficult to improve further on this process economically. Additionally, the technique can result in curved flake, the degree of which depends on the bubble size. The spun glass method has the advantage of tighter product control, typically with thickness deviation at +/-1 micron for a given product, and no curved flake is generated. The drawback of this method is cost; it is typically 30% higher. The spun process is able to produce glass flake of a magnitude thinner than the bubble method. Thicknesses as low as 100 nm with a typical deviation of +/-25 nm are achievable at commercial level, with the lower limit in trials being 30-50 nm. Consideration should also be given to glass composition. There are many different types of glass available; for the purpose of anti-corrosion coatings a chemically resistant glass (‘C’ glass) should be employed. Most glass flakes are produced from either Electric glass or ‘C’ type glass. Within the category of ‘C’ type glass there are many variations, with differing degrees of hydrolytic stability. Of the various types, ‘ECR’ glass, which is a modified C glass, is considered one of the more stable. As with thickness control improvements, the cost of more chemically resistant glass compositions is higher. This is because these types of glasses have been produced from raw materials rather than pre-melted marbles, and also because those compounds impart extra chemical resistance. acrylics. Although flake addition will generally improve the MVT resistance of almost any organic coating film or membrane there may be other benefits with new properties being imparted or existing ones improved. However, the level at which the glass flake should be added, the particle size distribution and ensuring adhesion to the carrier are of paramount importance. Although glass flakes with aspect ratios as low as 10:1 will give benefit, generally the higher the aspect ratio the better the barrier presented. This premise has however to be tempered to some extent, as out of alignment largeaspect-ratio flakes can afford a direct path through the film where the film is less in thickness than the nominal diameter of the flake or cause stress raisers for crack propagation. In addition there are some properties that may be adversely affected when using large flakes such as flexibility and elongation to break. Also for consideration is the practicality of using large flakes e.g., when a coating has to be sprayed, the gun tip size is limited by several factors and the flake will have to be small enough to pass through the spray tip. It is therefore common that flakes with a nominal screen dimension across the flake (sdaf) of around 500 µm and below are used for spray application and flakes above this size i.e., as large as (sdaf) 1500 µm are rarely used except for hand applied materials. A further consideration with large flake is surface disruption and in consequence large flakes tend to produce rough surface finishes. There is also the possibility of blending flakes of differing size (sdaf) and not thickness, for instance incorporating a quantity of micronized (average diameter 30 µm) flakes with large flakes. Flake size and thickness are only two of the issues involved in performance; the quantity of glass flake added and particle distribution are also critical. It is obvious that if thin glass flakes are used there are many more flakes than if thick ones are used for the same weight added Basic Parameters It is important to understand that although the glass flake is impervious to moisture vapour and gas diffusion it does not present a continuous barrier in a resin matrix. The resin carrier therefore plays a very important role too, i.e., glass flake cannot make a poor resin film into an excellent coating, although it may substantially improve it. On the other hand, even excellent resins can benefit from flake addition. Additionally, glass flake offers differing aspects of mechanical reinforcement and fire resistance than those attained by adding fibre. Today many different types of coating resins are used with glass flakes, including, but not limited to, polyesters, epoxies, chlor-rubbers, alkyds, coal tars, vinyls and water-based 2- to 3-micron flake with consistent thickness. High-quality glass flake. Low-quality flake thickness Av 5 μm. Poor-quality GF with curvature. 23 PA I N T & C O A T I N G S I N D U S T R Y
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