# Thermo-Mechanical Properties of PLA/Short Flax Fiber Biocomposites
## 1 Introduction
Natural fibers, containing a large amount of cellulose, hemicellulose, lignin, and pectin, tend to be active polar hydrophilic material while polymer matrices are not polar and exhibit hydrophobic properties. So there is typically a weak interfacial bonding between the highly polar natural fiber and non-polar organophilic matrix and this leads to a loss of the mechanical properties of the resultant composites [24]. Different approaches have been investigated to improve the interfacial compatibility and bond strength as the use of surface modification techniques [4; 25]. The strength of the interfacial bond depends on different parameters as surface energy, chemistry, and fiber roughness that can be modified by different treatments [26; 27]. In particular, chemical treatments with sodium hydroxide, peroxides, organic, and inorganic acids, silane, anhydrides, and acrylic monomers have been proposed to remove non-cellulosic components (in cellulosic fibers) and to add functional groups that provides better chemical bonding between the treated fibers and the polymeric matrix [28]. A weak interface adhesion decreases the composite tensile strength but increases the impact resistance, due to the fiber pull-out mechanism. On the other hand, a strong interface adhesion generally leads to a stiff composite having high tensile strength but brittle impact behavior (fiber pull-out is prevented) [29]. The fabrication methods of composites based on PLA and natural fiber such as flax fibers are the same as those used for the thermoplastics in which short fibers are added and traditional molding methods such as extrusion, compression, and injection molding are used [22]. The adopted processing conditions (screw speed, profile temperature, pressure, residence time, etc.) influence the final mechanical properties of PLA/fiber composites. The fibers damaging due to the long permanence at high temperature or to an excessive screw speed must to be considered. In particular, it was observed that compression molding technique does not damage the natural fiber during processing. However, this technique is time consuming and unsuitable for mass production. On the contrary, for high productions, the injection molding is used, showing easy processability that allows the prevention of fiber degradation during the process and generally it improves the fiber dispersion in the matrix [23]. In terms of fiber length, compression and hot press molding can be used for both long and short fibers, while injection molding can process only short fibers. [PERSON] et al. [30] manufactured PLA/flax fiber composites with a twin-screw extruder up to 40 wt.% of long fibers, followed by compression molding. [PERSON] and [PERSON] [31] have produced PLA/flax fiber composites up to 30 wt.% fibers, using hot-pressing followed by pelletizing and injection molding. In the present study, biocomposites based on PLA containing un-treated short flax fibers, up to 40 wt.% of fibers, were produced by melt extrusion followed by injection molding. The developed composites were characterized in terms of tensile properties, thermal properties by differential scanning calorimetry (DSC), and thermo-mechanical properties by dynamic mechanical thermal analysis (DMTA). Scanning electron microscopy (SEM) analysis was carried out to evaluate the fiber/matrix interface and the dispersion/structure of the fibers in the PLA matrix. Analytical models were used to predict the tensile properties of the developed composites and to assess the interfacial matrix/fiber adhesion.