Among all the natural fiber reinforced biocomposites, the flax based composite shows the best properties when compared to other natural composites, including glass-reinforced traditional composites. Flax fibers offer higher reinforcing prop­erties than hemp and kenaf natural fibers. Namely, comparison of the mechanical properties of the natural fiber reinforced composites has shown that composites based on flax fibers exhibited higher tensile strength relative to those based on hemp or kenaf fibers. Flax fibers exhibit a higher fineness and more unique distribution compared to hemp or kenaf. According to current theories, a higher fiber fineness should results in better fiber embedment during compression molding and conse­quently higher mechanical properties. Generally, mechanical properties of natural fibers are determined by the cellulose content and microfibrillar angle. The cells of the flax fibers consist mostly of pure cellulose cemented by means of noncellulosic incrusting such as lignin, hemicellulose, pectin or mineral substances, resins, tan­nins and small amount of waxes and fats. Flax cell wall consists of about 70-75% cellulose, 15% hemicellulose and pectin materials. The Young’s modulus of the nat­ural fibers decreases with the increase of diameter. The mechanical properties of the natural fibers are also closely related to the degree of polymerization of the cellulose in the fiber21. Basic physicochemical properties and cellulose content for flax fibers versus other natural reinforcing fibers are shown in Table 10.3.

Oksman et al.22 have studied the mechanical properties of PLA/Flax composites versus PP/Flax. The addition of flax fibers increase the modulus, but the higher fiber content has not improved the modulus in PLA composites as it has been ob­served for PP composites due to the fiber orientation in the polymer matrix. The test composites were compression molded and the fibers could be orientated differently from one sample to another. Because of the brittle nature of PLA, triacetin was used to plasticize the pure PLA and for the PLA/Flax composites. The addition of triac­etin has shown a positive effect on the elongation to break for pure PLA and PLA/ Flax composites, which was expected because of the softening effect. The highest triacetin addition (15%) clearly shows a negative effect for PLA/Flax composites, both the stress and stiffness were strongly decreased. As expected, it was shown that the addition of triacetin did not affect the impact properties of the PLA/Flax com­posites. The addition of 5% triacetin in PLA has shown the best results on impact strength. The authors also reported that thermal properties of PLA were increased with the incorporation of flax fibers. The softening temperature was increased from about 50°C for pure PLA to 60°C with flax fibers, and it is further increased if the composite is crystallized.