JEC COMPOSITES MAGAZINE - Issue #75 - August/September 2012 - 26

sustainability

Fig. 1: Flax fibres tested in mat, woven and unidirectional reinforcement forms

polymers, composites and sandwich cores, performing ten test campaigns since then. They decided to focus on thermoset resins, even though most of these are still only available as partially bio-sourced resins and cannot be recycled, but are easily processed by infusion. Other groups in Brittany

are concentrating on fully bio-sourced thermoplastics such as PLA, and developments in this area were described in a recent JEC Composites Magazine article [2]. The reinforcements tested include flax, hemp, jute and cellulose from various suppliers, in unidirectional, woven and mat forms. Cork was also studied in detail as a bio-sandwich core. In all these tests, particular attention was paid to the comparison with traditional boat-building materials, so glass-reinforced polyester and epoxy specimens produced using similar manufacturing methods were tested systematically along with the bio-composites. All the samples were produced in the Kaïros workshops. Early tests revealed considerable variability and poor properties but more recent campaigns have shown that the quality of reinforcements has improved as suppliers adapt their manufacturing methods to these natural fibres. New resins have also become available and have been integrated in the test programme. The first results with bio-composites show that more material is usually needed to achieve equivalent stiffness

and strength to traditional marine laminates, although this can be compensated by their lower density. When a cork core is added to produce a biosandwich, the resin tends to infiltrate the core, making resin selection a critical step. For example, two core sandwich materials with the same flax fibre reinforcements and cork cores but different matrix resins, produced using the same infusion technique, showed a 15% difference in overall weight.

Durability
The behaviour of bio-composites in marine environments also needs to be characterized, and already published studies [ 3 ] have highlighted the sensitivity of natural fibres to water. Water diffusion is controlled by the matrix resin, and the use of partially bio-sourced epoxy resins offers a number of benefits as these resins are not very sensitive to water and can protect the fibres. The figure below shows the results of tensile tests performed on traditional glass/epoxy and cellulose-reinforced epoxy composites after various ageing periods of up to 18 months fully-immersed in natural sea water at 40°C. This is a very severe test since the higher temperature accelerates degradation while the immersion of tensile test specimens with unprotected edges and no gel-coat results in much faster water ingress than in a real boat structure. The two composites had the same weight content of reinforcement fibres and were infused with the same resin. After ageing, break load decreases in a similar way for both materials but their stiffness, although initially similar, drops more rapidly for the cellulose composites, with an increase in failure strain. A second important aspect of the long-term behaviour of these materials is their damage tolerance. Few data are available to make compari-

Fig. 2: Evaluation of bio-sandwich materials by extensive testing – here, flax fibre-reinforced facings on cork cores, showing different failure mechanisms (Photos: R. Gladu)

26 jec composites magazine / No75 August - September 2012

JEC COMPOSITES MAGAZINE - Issue #75 - August/September 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #75 - August/September 2012