Feature 2 |MARINE COMPOSITES & CONSTRUCTION MATERIALS
drop of around 25% from its dry performance after four weeks. Longer term tests will determine whether moisture uptake will continue and further degradation occurs or whether the flax composites are just more rapidly affected but remain at these performance levels. Currently, the research into the water
ageing of natural resin composites shows that they’re performing better3
. But
herein lies another challenge for natural composites.
One set of accelerated Figure 2. Strain rate dependency of Castor oil compared to a standard epoxy composite.
natural resins: glass fibres combined with an epoxidised linseed oil (ELO), which is actually 97% bio-derived with a photoinitiator hardener making up the balance; and glass fibre combined with a polyurethane derived from castor oil. Comparing the specific properties, the glass/linseed composite compares favourably with glass/polyester and the glass/castor composite competes well against the glass/epoxy benchmark. Whilst it is evident that the specific
properties of natural resin composites are competitive, it’s worth noting that, like the natural fibre composites, they also can show dramatic strain rate behaviour. Figure 2 shows laboratory measurements of the response to very high strain rate of glass/epoxy and glass/ castor composites. Whilst the absolute Young’s modulus of the castor oil resin composite is significantly lower than the glass/epoxy benchmark, it rapidly stiffens at higher strain rate. Natural composites clearly offer an
opportunity to reconsider structural design and ‘box clever’ with their inherent material properties, balancing dynamic and static properties against design for structural strength or stiffness.
However, for natural fibre
composites, it is crucial to increase the 46
fibre volume fraction to dramatically gain the true weight advantages they can afford. One obvious question is in the
longevity and durability of the natural composites. Natural fibres are well known for moisture uptake and significant displacement increases can undo the lightweight advantages initially seen. Preliminary measurements on flax fibre composites, combined with a natural resin, Greenpoxy, show that after four weeks of non-accelerated ageing in distilled water (distilled water is more aggressive in moisture penetration than saltwater), the natural composite has absorbed up to 10% of its original mass in water whilst the conventional glass/ epoxy has gained only 1%. Furthermore, the impact of this
moisture absorption isn’t only in weight gain but in significant performance reduction. The flexural properties are considerably affected by the high moisture content of the immersed specimens: tests at the University of Southampton have shown flexural breaking strengths and flexural modulus of flax/Greenpoxy to be reduced to around 50% of its dry performance after just four weeks. This compares to a conventional glass/epoxy performance
ageing experiments are being conducted on glass/linseed specimens which are manufactured using UV light. After 36 weeks in a water bath of distilled water at 40°C, many of these specimens have developed osmotic blisters and consequently the moisture uptake is close to 6%, with performance degradation in the region of 25% of its original flexural strength. However, those specimens that haven’t blistered show a moisture uptake of about 2% which is directly comparable with glass/epoxy after 36 weeks in the same conditions. For interest, ‘durable’ glass/epoxy
flexural strength after 36 weeks of accelerated ageing in distilled water is about 45% of its virgin properties. The blisters are obviously key to this particular natural composite’s performance and durability, and highlights how manufacturing is a challenge. UV curing is a highly attractive
method for boatbuilding. With only a photoinitiator acting as the hardener, boat layups will not cure until they’re exposed to UV light, giving as much work time as is required by the builder. Furthermore, any excess resin can be poured back into the resin container and used again. But the issue lies with the penetration
of UV light into the composite to be cured and currently the manufacturing method only allows two layers of cloth to be laid up, cured and keyed before the next two layers are applied and results in each pair of co-cured layers having a weak interface, which is attacked rapidly by moisture, undermining the laminate properties. More conventional
Ship & Boat International May/June 2013
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