Abstract
Composites have been used to build boats since the 1950s and now dominate the boatbuilding world. George Marsh reports on the materials used today and the challenges they face.
Experimenters first tried moulding boats in the then-revolutionary glass reinforced plastic (GRP) prior to World War 2, but the take-off in GRP construction from the 1950s led to this material's domination in the boatbuilding world.
The strength and low weight of glass (a single filament, a quarter of the thickness of a human hair, has an ultimate tensile strength of some 250 tons per square inch) along with the watertight integrity of the plastic matrix in which it is embedded, created a revolution in boat structures. Also key were the composite's ability to be moulded into the monocoque forms that were smooth and hydro/aerodynamic as well as aesthetically pleasing, and the entry level accessibility of the technology.
Yet today GRP is being challenged by other materials — light, corrosion-resistant steels and marine-grade aluminium as well as more advanced fibre reinforced plastics (FRP), and by environmentally-driven legislative pressures. Additionally, its maritime reputation has suffered with the awareness that, like any other boatbuilding material, it is subject to degenerative attack by water (see article on blistering on page 39).
A beauty of polymer-based materials is that polymers can be modified chemically to meet different property requirements. Unfortunately, these requirements are not always compatible. For example, modifications used to cut emissions of hazardous air pollutants (HAPs) can reduce resistance to hydrolysis, the mechanism behind that insidious enemy of marine GRP, osmotic blistering. At a time when failure to meet Maximum Achievable Control Technology (MACT) limits in the USA and similar limits elsewhere could put companies out of business, the drive for low styrene emissions (LSE) and other pollutants may take precedence over properties like blister resistance, weatherability and processability. Nevertheless, a number of companies say they are able to walk the tightrope and deliver a satisfactory blend of all the desired properties.
One such is DSM Composite Resins, which offers resin products differently optimised for different structural applications. Thus below the waterline, where hydrolytic stability is key, Synolite isophthalic resin can be used, possibly with a tie coat of Atlac vinyl ester, vinyl ester resins being particularly resistant to hydrolysis. Neogel Eco, launched at the JEC Composites Show 2003 as the latest member of its Neogel gel-coat family, has 25% less styrene than previous-generation coatings yet is said to cover well and resist weathering. For decks and superstructures DSM recommends a standard dichloropentadiene (DCPD) resin for rapid cure and superior cosmetics. Where bonding and filling is the requirement, Freefix is a group of low-weight polyester pastes. To ensure best compatibility with its own products, DSM will also source other materials, from resins and reinforcements to stiffening and ancillaries.
New gel-coats launched at JEC by GRP pioneer Scott Bader (an early Crystic resin was used in dinghies produced by W&J Tod in 1951) included two (one filled, the other unfilled) isophthalic products intended for spray-up and offering improved weathering and osmosis-resistance. Another new Crystic Envirotec product, however, was a spray gel-coat having 15% less styrene content than its immediate predecessor.
US company AOC says that little has been sacrificed in terms of blister or weather resistance in developing new gel-coats having styrene contents of 21–32%, depending on colour. Its new Hydropel products include a vinyl ester barrier coat for increasing resistance to hydrolysis and blistering. Dow Chemical offers variants of its Derakane epoxy vinyl ester laminating resin series in reduced styrene formulations.