The interest in using natural flax fibers as reinforcement in biocomposites has in­creased dramatically, and in the same time, it also represents one of the most impor­tant uses. However, flax fibers are hygroscopic in nature; moisture absorption can result in swelling of the fibers, which may lead to microcracking of the composite and degradation of mechanical properties. This problem can be resolved by treating these fibers with suitable chemicals to decrease the hydroxyl groups; these groups may be involved in the hydrogen bonding within the cellulose molecules or by dif­ferent type of pretreatments.232425 Chemical treatments may activate these groups or can introduce new moieties that can effectively interlock with the matrix.

Kozlowski et al. modified flax fiber with plasma, boiling and bleaching for PLA/ Flax composite production. Modified flax gives composites with better mechanical properties that are more resistant to flame than unmodified ones25.

As mentioned, the type of pretreatment has a strong influence on the natural composite properties. Susheel Kalia et al.26, have reported numerous treatments of natural fibers. Mercerization, bleaching, grafting, coupling, treatment with silane, benzoylation as well as plasma treatment to fibers improve most of the usable prop­erties of natural fibers composites.

B. Wang et al. presented results of the modification of short flax fibers, which were derived from Saskat-chewan-grown flax straws use in fiber-reinforced com­posites, performed by mercerization, silane treatment, benzoylation and peroxide treatment27. SEM analysis has shown that physical microstructure changes occurred at the fiber surface. Silane treatment provided surface coating to the fibers. Benzo — ylation treatment produced a smooth fiber surface, while after dicumyl peroxide treatment the fibrillar structure of the individual ultimate fibers was observed. This may be due to the leaching out of waxes and pectic substances. Micropores, par­ticles adhering to the surface, groove like portions and protruding structures made the fiber surface very rough. Application of the coupling agents has been shown to be effective improving the surface properties of flax fibers, forming a mechanically interlocked coating on its surface. The results have shown that silane and peroxide treatment on flax fiber bundles lead to a higher tensile strength than that of the untreated fiber bundles. Comparatively lower tensile strength was observed in ben — zoylation treated fibers27.

Recently, introduction of enzymes in numerous wet processing of natural fibers, such as desizing, bio-scouring, bio-polishing, etc., are powerful methods for surface modification. Enzymes alone, or with combination with mercerization gives cottons that have less hydrophilic surface relative to traditionally treated fibers, but with increased numbers and types of functional groups.2829

A. Grozdanov et al. have worked on comparison of Flax/PLA biocomposites versus Kenaf/PLA biocomposites.3031 Natural fibers as a nonwoven preforms, flax and kenaf were kindly supplied by KEFI-Italy. Surface modification was carried out as follows: (a) dewaxing: the fibers were treated with 1:2 mixture of ethanol/ benzene for 72 h at 50 °C, followed by washing with distilled water and air drying to get defatted fibers; (b) vinyl monomer grafting: acrylonitrile (ACN) graft copoly­merization onto dewatted fibers was carried out using 0.01 M Ce4+/0.1 M HNO3 as initiator at temperature of 50°C; (c) alkali treatment: the defatted fibers were treated with 2 and 10% NaOH solution for 1 h at 30 °C; (d) acetylation; dewatted fibers were placed in 100 mL flask and covered with the appropriate amount of acetic an­hydride for 0.5 h at 20 °C, followed by Soxhlet extraction and drying. Biocompos­ites based on PLA matrix reinforced with flax and kenaf nonwoven preforms (20% wt. fiber content) have been prepared by compression molding at 170°C under the pressure of 50 bars. The flexural strength and the flexural modulus were measured in three-points bending mode using an Instron machine (model 5564), at a cross-head speed of 1 mm/min and at room temperature. The test span was 48.0 mm. For each sample 10 specimens were tested and the average values of the flexural strength and modulus were calculated. Thermogravimetrtic analysis (TGA) was performed in the range of 25 to 800°C, which had heating rate of 20 K/min (under nitrogen), using the Perkin Elmer DIAMOND system. The morphology of chemically treated and dried (12 h in vacuum) flax and kenaf fibers, as well as of their composites was examined by SEM (JEOL, model JSM-T20 (Uw = 20 kV).

Characteristic TGA data for different treated flax and kenaf nonwoven preforms are presented in Table 10.4. Comparison of the decomposition temperatures of flax and kenaf nonwoven preforms has shown that higher thermal stability (about 70°C) exhibited flax nonwoven forms probably as a result of higher crystallinity as well as higher cellulose and hemicellulose content.

Due to the chemical treatments, overall morphology of the flax and kenaf fibers has been changed. Characteristic SEM photos of alkali treated flax and kenaf fibers are shown in Fig. 10.1 (kenaf fib.) and 10.2 (flax fib.). The untreated fibers represent the bundles with relatively smooth surface (Figs. 10.1a and 10.2a), although small particles attached to the surface are also seen. The alkali treated fibers have a rough surface topography with significant defibrillation of individual fibers (Figs. 10.1b and 10.2b).

The flexural modules and flexural strength for the obtained PLA/Flax biocom­posites with various treated Flax preforms were shown in Table 10.5. It is evident that flax fibers improved the flexural modulus and flexural strength of the polymer matrix compared to neat PLA.

Mechanical properties of the obtained biocomposites reinforced with flax and kenaf nonwoven preforms have been shown in Table 10.6. Evidently, flax and kenaf nonwoven preforms increased the mechanical properties of the PLA neat polymer. Higher flexural modulus was obtained for the composites reinforced with kenaf fibers, while higher flexural strength was measured for PLA/Flax biocomposites.

Characteristic morphology in the obtained PLA/Flax biocomposites have shown that flax fibers were well covered with the polymer matrix, which resulted in good stress transfer between the flax fibers and PLA polymer matrix. SEM photographs of the studied PLA/Flax biocomposites with various treated flax performs are shown in Fig. 10.3. Surface treatments have resulted in defibrillation of the flax fibers and increased the possibility for mechanical interlocking of flax fibers and polymer ma­trix, which consequently increased the fiber/matrix interfacial strength.