8th June 2020
As part of our continued dive into the company archives, we are sharing a technical paper titled ‘Understanding and Use of Glass Flake’. The paper, co-authored by Charles Watkinson and Simon Brigham was originally published by PCI in 2009. Paint & Coatings Industry shared this paper on their website, a full copy of which can be found here. The paper focusses on the applications in protective coatings, common for glassflake at the time. Since publication, there are now a far broader set of applications for glassflake materials.
Excerpts from the paper can be found below. If you are working in any of the areas mentioned, we would love to hear from you.
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.
Glass flake coatings are often criticized in their flexibility. The addition of glass flake more often, although not always, increases rigidity and reduces elongation; however, careful choice of coating thickness may in many instances remove any associated problems. We used a flexural testing rig for testing a coated steel plate at 1 mm and 3 mm applied coating through tensile and compressive flexing modes. The rig was stopped and the plate spark tested on a fixed cycle basis. The coating applied at one millimeter, although showing surface stress cracks after 90,000 cycles, was spark free when tested at 15 kv right until the steel plate fractured at just under 140,000 cycles. The coating applied at 3 mm only completed one cycle before failure. The glass flake applied at 1 mm out performed five non-glass flake epoxies applied at between 350 μm and 1 mm one of which was stated to be a ‘High Build Flexible Epoxy’.
There are many instances of polyester glass flake coatings in aggressive seawater duty being in service for more than 20 years and still showing no signs of degradation. More recently glass flake coatings have been developed with both polyester and epoxy resin bases that can be applied as single coats to structural steel work giving excellent edge coverage and obviating the necessity for stripe coating in many instances. This has been achieved by the use of glass flakes with differing particle size distributions changing the surface tension characteristics whilst allowing normal spray application.
There are several properties that can be achieved and substantial benefits gained from using glass flake barrier fillers in coatings. First, however, the choice of flake to be used must be followed by ensuring that the formulation is specifically tailored for optimum performance. It is of no use to add arbitrary amounts of glass flake; the type and amount of bonding agent will vary not only with resin type but the surface area of the glass flake pigment and other components within the formulation.
There are some negative aspects, and these relate primarily to impact resistance and stiffness, although not all materials are affected in this way. Glass flake coatings properly formulated not only give outstanding performance but also long life cycles. Many of the advantages in using glass flake, such as fire retardancy, mechanical properties and speed back to service have been ignored and yet there is still work to be done before all the benefits of glass flake fillers are understood.
