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Two main interests – strength and low weight – have led to continual growth in the vast field of composite materials. Now, minimising the environmental impact has become an additional concern, opening up possibilities for materials reinforced with vegetable fibres such as flax, which is a fibre that offers stiffness close to that of common glass, with less than 2/3 its density. Manufacturers are also interested in price. Here is a new way to valorize agricultural by-products such as flax tow fibres.
(Published on May-June 2006 – JEC Magazine #25)
BY ANTOINE BONNESCOEUR, CONSERVATOIRE NATIONAL DES ARTS ET MÉTIERS DE HAUTE-NORMANDIE, MONT SAINT AIGNAN, FRANCE STÉPHANE MARAIS, FABRICE GOUANVÉ, JEAN GRENET, POLYMERS, BIOPOLYMERS & MEMBRANES LABORATORY, UMR6522 CNRS / UNIVERSITY OF ROUEN – UFR DES SCIENCES, MONT SAINT AIGNAN, FRANCE
Amajor factor in the quality of a composite is a good cohesion at the reinforcement-matrix interface. With respect to this, the work we report here has been guided by a concern for reducing the fibres’ sensitivity to water and for adopting a treatment solution that is environmentally friendly, i.e. free from all chemical deposit. Plasma and autoclave were the preferred treatments.
We used Cray Valley’s Norsodyne G 703 thermosetting resin. Cross-linking was obtained by adding an accelerator and a catalyst comprising respectively 0.2wt.% of the resin and 1.5wt.% of the total unit. The resin was reinforced with flax tow (short fibres) from the industrial scutching of flax fibres from the stem of linum usitatissimum, used in the textile industry. By weight, 60% of the fibres were of the “Hermes” variety. To get the benefit of a reproducible industrial-type fibrous reinforcement and for easier fibre handling, we chose the form of a non-woven material, borrowing a technique from the textile industry whereby a strip of fibrous webbing is folded on itself using a “spreader-topper” mechanism, and then pressed to obtain a type of felt.
Steam and cold plasma treatments
The above-mentioned manufacturing process involves a steam treatment in autoclave and a cold plasma helium treatment. The steam treatment serves to modify the fibre envelopes in order to decrease the sorption of water. The fibres are heated to 130°C for 30 minutes, dried at 23°C, and then heated again at 130°C for 2 hours.
The plasma helium treatment involves placing the non-woven material in contact with ionized helium for five minutes under a power of 50 watts. The plasma treatment creates radicals at the surface of the fibres that react with the oxygen in the air to produce peroxides. These sites react in turn with the crosslinking resin. The plasma treatment is also accompanied by a light stripping of the fibres and leads to their densification.
The laminate samples were obtained through a wet compression process (24 hours at ambient temperature). After stripping, they underwent two post-curing steps, the first at 80°C for 6 hours and the second, at 130°C for 2 hours.
The physicomechanical properties measured were the variations in water sorption of the fibres, in particular, and the variations in laminate permeability and modulus of elasticity. The tests were completed by scanning electron microscopy (SEM) for preliminary comparative observations between untreated fibres and plasmatreated fibres, and then the examination of fracture topography of laminates at cryogenic temperatures (fig.1).
The quality of untreated and treated fibres and the conditioning of the laminates in both dry and wet state were studied to determine the influence of the treatments on laminate stiffness.
In the dry state, the evolution of the interface with the contribution of the plasma treatment was examined, while for wet samples, we wanted to know if autoclave treatment is effective at preserving the properties observed in dry samples.
The sorption tests on fibres showed that the autoclave treatment decreased the amount of moisture held, but the plasma treatment did not (fig.2, top). Concerning permeability, a delay in the diffusion of water in the material is noted for both treatments, and the autoclave treatment particularly reduced the solubility to water of fibres (fig.2, bottom).
Mechanical tests on the dry materials showed an increase in stiffness (+17%) after the plasma treatment (fig.3), but a slight decrease after the autoclave treatment.
Fractographic examinations corroborated the plasma effect: the composite reinforced with untreated flax fibres is characterised by a poor fibre-matrix interfacial bond, as shown by the presence of some holes and fibre ends in the left-hand photograph in Figure 1. In the case of composites reinforced with plasma-treated flax fibres, the fibre-matrix interfacial bond is comparatively stronger, as seen in the right-hand photograph in figure 1: fibres broken in the failure, with almost no holes.
by a poor fibre-matrix interfacial bond, as shown by the presence of some holes and fibre ends in the left-hand photograph in Figure 1. In the case of composites reinforced with plasma-treated flax fibres, the fibre-matrix interfacial bond is comparatively stronger, as seen in the right-hand photograph in figure 1: fibres broken in the failure, with almost no holes.
For the materials exposed to water, there was a noticeable degradation in properties within a very disturbed medium. Water appears to cause simultaneous effects on all the components, which makes it very difficult to analyse separately. We do observe the plasticization of the components, however, with an increase in the strain at failure. It can be seen that the protection afforded by the autoclave treatment does not extend to preservation of the initial properties, but the decrease in stiffness is less than with the plasma treatment.
Testing two unidirectionally reinforced composites (E-glass fibres and flax fibres) showed the advantages of flax fibres in terms of stiffness and low weight, with a better specific modulus (E/ρ) for the flax/UPR composite (+55%).
In conclusion, this work showed that both the recycling of scutched flax tow and environmentally friendly fibre treatments can be used with standard composites. Associating plasma and autoclave treatments allows a good compromise between improvement of the mechanical and permeametric properties. The mechanical results obtained on laminates give an interesting, although general indication about material performance. The effect of the treatments on the performance of fibres without resin remains to be studied. The authors would like to thank Jean-Jacques Malandain for SEM photomicrograps, Alain Baillivet for making nonwoven and Poncin-Epaillard for her collaboration.