Feature 2 |MARINE COMPOSITES & CONSTRUCTION MATERIALS Natural resourcefulness


Demand for natural or bio-derived composites is growing in tandem with tighter environmental legislation. Dr James Blake & Adam Sobey update Ship & Boat International on the latest composite research at the University of Southampton, including a boatbuilding project


design, build and operate products that are more sustainable. In the marine industry, much of this original focus has been on the emissions during vessel operation but this has been broadened to include the sustainability of a vessel through-life and end-of-life disposal. Composite materials have been a


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success story for many boatbuilders, offering excellent strength to weight ratios (providing vessels wi th lightweight efficiency) and minimal maintenance through life. However, these materials are highly dependent on petroleum derivatives, especially resin systems, and it will be vital for companies to consider other ways to use these materials more sustainably, both to keep ‘green’ credentials for sales and to pre-emptively reduce the impact of legislation and keep material supply in balance with product demand. An emerging interest is the


opportunity of using naturally sourced materials for the constituents for composite construction. ‘Natural’ or ‘bio-derived’ composites are where at least one of the constituents, either the fibre and/or the resin, is derived from non-petroleum derivatives (or in the case of sandwich composites, the core may be bio-derived). By taking advantage of their inherent characterist ics, it is anticipated that through-life financial and environmental costs can be lower due to further vessel lightweighting (by providing a lower density substitute for glass and polyester), reduced emissions in manufacture (no styrene with many of the natural resin systems) and end-of-life opportunities to reduce disposal costs (suitable composite combinations can be biodegradable).


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he current global climate is creating a socio-economic and political push for companies to


Figure 1. (a) Computed Tomography image of a glass/linseed oil specimen subjected to bending showing cracks (in red) through the laminate; (b) SEM image showing ideal cohesive failure before glass/epoxy is aged, and (c) adhesive failure after ageing


Range of choices There is a large choice of different fibres and resins. For example, for fibres there are plant fibres such as flax, hemp and jute; animal fibres such as wool, hair or silk; fibres from seeds such as coir, kapok and cotton; leaf fibres like banana or sisal; and grasses such as bamboo, miscanthus and wheat. The engineered resins can be mouldable thermoplastics, rigid thermosets, and biodegradable variants of the same. Flax (linseed oil), castor, soybean,


cashew nut and sugarcane are a few examples of feedstock. The resulting composite resins can vary in natural content from a few percent to almost 100% but, typically, the higher fraction of natural content reduces the mechanical performance of the resulting composite resin. Currently most research is centred


on replacing the glass fibre in a typical composite with natural fibres but evidence suggests that this constituent,


from an environmental perspective, does not deserve the most attention. However the constituents are derived or combined, a challenge exists surrounding the quality of the fibre to resin bond, the synergy of which defines a ‘composite’. Typical glass, carbon or aramid fibres, combined with conventional polyester, vinylester, phenolic or epoxy resins, are highly engineered to complement each other. Te immaturity of natural fibre and resin interaction research is a fundamental block to the uptake of these new composite materials for structural applications in boatbuilding. This is reinforced by the importance of the bond, and the degradation of that bond, in a wet and diverse loading environment. Figure 1(a) shows an X-ray Computed


Tomography image of a glass/linseed oil specimen, where the areas in red show the crack propagation through a laminate, subjected to bending, following the interface of the fibre with the resin. Two scanning electron


Ship & Boat International May/June 2013


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