# Thermo-Mechanical Properties of PLA/Short Flax Fiber Biocomposites
## 1 Introduction  
The development of biocomposites, produced using a biopolymer matrix and a reinforcement from renewable resources, is currently an extensive research area due to the promising mechanical properties, recyclability after service [1], and biodegradability [2] of these materials. These composites have potential applications in different fields such as automotive, packaging, and household goods [3]. Clearly, the application of natural fibers is being driven not only by environmental reasons, but also by economical ones. Natural fiber-reinforced composites, in fact, can be used as low-cost materials having at the same time different structural properties [4]. The use of natural fibers has many advantages. Natural fibers are renewable, biodegradable, and less abrasive to tooling. Furthermore, they can be formed into light composites leading to weight reductions and, especially in automotive field, to fuel saving [5]. Among natural fibers, lignocellulosic fibers are investigated as promising substitutes of the synthetic fibers in polymeric composites and several studies have examined drawbacks and advantages of the most significant lignocellulosic fibers and their related polymeric composites [6; 7; 8; 9]. One of the strongest natural fibers is the flax fiber. Their structure is very complex and can be compared to a composite structure. In fact, the flax fibers are constituted by a series of polyhedron shape elementary fibers overlapped over a considerable length; they are held together by an interphase that mainly consists of hemicellulose and pectin. Each elementary fiber consists of a very thin primary cell wall, a secondary cell wall (dominating the cross section), and an open channel at the fiber center called \"lumen\" [10; 11; 12]. Typical diameter for an elementary flax fibers are around 10-15 \(\upmu\)m; on the other hand, technical flax fibers have a diameter that varies between 35-150 \(\upmu\)m [11]. Different biopolymers such as poly(lactic acid) (PLA), polyhydroxyalkanoate (PHAs), and cellulose esters have been used as matrices coupled with natural fibers for various applications [13; 14; 15]. Among these biopolymers, PLA is currently one of the most used and promising biopolymers because of its lower production cost compared to that of other biopolymers such as PHAs, poly-butylene succinate (PBS), poly-butylene succinate-\(\alpha\)-adipate (PBSA). PLA exhibits, at room temperature, very good mechanical properties: Young modulus of 3 GPa, tensile strength between 50 and 70 MPa, and impact resistance of 2.5-3 kJ/m\({}^{2}\)[16; 17]. However, PLA is very stiff and brittle with poor thermal properties and a very slow crystallization rate; often it is necessary to use a plasticizer or introduce rubber particles to improve its break elongation and impact properties [18; 19; 20], while nucleating agents are added to increase its crystallinity and enhance its thermal stability [21]. Some of PLA drawbacks may be overcome by fiber-reinforcement and blending. Mechanical properties of natural fiber-reinforced PLA composites have been investigated by different researchers and different types of natural fibers were used in place of synthetic fibers [22]. However, the addition of natural fibers to polymeric matrices requires special attention in order to obtain biocomposites with good mechanical properties. In fact, compared to the synthetic composites, the composites containing natural fibers generally show lower mechanical properties, higher moisture absorption, and lower durability [23]. Many researchers have been working to address these issues, with particular attention to the surface treatments of the natural fibers and the improvement of the fiber/matrix adhesion. In fact, the fiber/matrix adhesion plays an important role in the final mechanical properties of the composites since a good adhesion guarantees an efficient transfer of the stress between the matrix and the fiber, obtaining the desired reinforcement.