Coatings Introduction

Many applications where reinforced plastics are used are structural but, generally, there is a need for an aesthetic, protective surface finish with low maintenance requirements. The most common finish used on glass fibre reinforced polyester resin (GRP) is gelcoat. Apart from affording protection to the structural laminate gelcoats improve durability, reduce fibre pattern on the surface and provide a finished surface from the mould, thereby eliminating the need to paint.

In the early days of the GRP industry it was not uncommon to mix a thixotrope with the laminating resin for use as a resin-rich surface but demands from the fabricator for reduced air entrapment and coloured finishes gradually lead to the availability of formulated gelcoat systems, generally, compounded by resin producers. It is not surprising that the early use of gelcoats was mainly in the marine market, since this was the first major industry to use GRP on a large scale for structural component construction.

Not any resin system can be used successfully in environments where resistance to water is essential. Hence, the development of isophthalic acid based resin systems with improved water resistance and low water absorption, compared to orthopthalic acid based systems, resulted in a range of gelcoats becoming available for the GRP market. Water absorption of resins and its effect on the durability of GRP has been the subject of many publications and a summary for orthophthalic acid based versus isophthalic acid based resins was presented by Clarke and Norwood in 1979.

Over the years a range of gelcoats has been developed to meet demands other that simply water resistance, for example, for mould making, for brush application, for spray application and to provide fire resistance. In addition, gelcoats can be supplied coloured to a standard colour range or matched to meet specific customer needs.

It is often important to protect a structural laminate with more than just a gelcoat alone; in the 1970’s blister formation in GRP boats and swimming pools resulted in investigations to find the means to reduce or even eliminate the problem. The mechanism of blister formation was well established over 25 years ago but solutions were not then, readily available. At first an important lesson to be learnt was that the problem was not simply concerned with the resin alone but was influenced by glass binder type, workshop conditions, workshop practices, the level of cure and the surface condition after delays in laminating. From this understanding, research resulted in the matched performance skin coat for boats where the gelcoat and laminate system in the first one or two layers of reinforcement were matched to give similar water resistant and mechanical performance characteristics. In fact, accelerated testing showed that GRP systems could be produced, cost effectively, that were unlikely to blister during the life-time use of many components and certainly not within the first 20 years or so.

The storage of chemicals is demanding for any material but GRP is resistant to many environments and is often chosen as the material of construction in preference to more traditional materials. However, some chemicals are particularly corrosive to glassfibre reinforcement, via a strain corrosion mechanism and, hence, the structural laminate requires protection by a barrier layer. Such layers are considered non-structural and are often manufactured from a different resin to that used for constructing the structural laminate. The barrier layer is often constructed using surface tissue, which can be based on glass or polymer fibres depending upon the environment to be continued, supported by several layers of laminate to a thickness of a least 3mm.

Solvents are very corrosive to many polyester resins and careful consideration must be given to the choice of resin for the barrier layer for the storage of solvents and fuels. With the correct choice of resin and barrier layer construction, underground fuel storage tanks have been shown to survive, in perfect condition, for in excess of 25 years.

Published courtesy of Dr L S Norwood, Scott Bader Company Ltd