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Even though gelcoats were not used in the early days of the GRP industry the need for resin-rich surfaces to protect structural laminates was an established practice:
In order to reduce drainage, introduce colour and improve air-release, fabricators used additives in the resin to ensure the resin-rich surface layer was of appropriate thickness with sufficient integrity to provide the desired finish. Of course, manufacturing a coating in the moulding shop resulted in variability from batch to batch and gradually the resin manufacturers introduced a range of formulated gelcoats offering the following advantages:
Hence, the fabricator now had access to quality surface coatings for laminates of consistent quality and whilst there was a price to pay for such systems, it was more than off-set by the savings made in labour and wastage, by attempting to manufacture complex coatings in the workshop.
Gelcoats are available in brush and spray versions and are best put down at a thickness of 0.5mm (approximately 500 g/m²). If they are too thin, poor cure occurs and fibre pattern will result. If they are too thick, crazing and cracking can occur and the laminate will be more susceptible to reverse impact, star cracking damage.
A variety of base resins can be used to manufacture gelcoats with the most common based on isophthalic acid type resins. The choice of isophthalic acid based resins rather than orthophthalic acid based resins results from the superior water resistance of isophthalic acid based resins, their superior blister resistance and their superior toughness as indicated by their excellent tensile elongation to break compared to orthophthalic acid based resins.
Over the years the need for improved gloss and colour retention in gelcoats have been recognised and the development of improved base resins with improved UV resistant additives has resulted in gelcoats that can be weathered under severe conditions without loss of gloss or colour, discernible to the naked eye.
As well as the demand for improved durability, there is a demand for reduced styrene emission gelcoats and now low styrene content, low styrene emission gelcoats are available in both brush and spray forms. Such systems have been well tested and proven. They are blister resistant and are approved by the major marine approval authorities, such as Lloyds Register of Shipping and det Norske Veritas.
The importance of the matched performance system which requires matching performance characteristics of the gelcoat and the ‘skin coat’ immediately behind it to ensure optimum water absorption characteristics and hence, reduce the possibility of blister formation, has been proven over the past 15 – 20 years by boat builders in the UK.
The use of isophthalic acid – neopentyl glycol (NPG) based gelcoats and skin coats offer even better resistance to blister formation that isophthalic acid-propylene glycol based resins but at increased cost. The contribution of the polyvinyl acetate (PVA) binders, used on glass fibre as a size and binder, must not be ignored as a major contributor to blistering in GRP. Hence, skin coat construction should be with powder bound (PB) chopped strand mat (CSM) and not emulsion bound (EB) CSM.
Published courtesy of Dr L S Norwood, Scott Bader Company Ltd
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