Fiber-Reinforced PLA

There are a number of reasons for the increasing interest in PLA as a matrix in natural fiber biocomposites. First, PLA is now arguably one of the most advanced biopolymers in terms of its commercialization. Second, PLA has good mechanical properties that are similar to those of PS. Third, PLA can be melt processed with standard processing equipment at temperatures below the point where natural fibers start to degrade [113]. Other factors that are being taken into consideration are biocompatibility and the reduction of material cost by replacement of a certain percentage of biopolymer with natural fiber [113]. 

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Table 23.1 Mechanical properties of natural fiber-reinforced PLA composites [47]...

Aydin et al. reported the influence of alkali treatment on the mechanical, thermal, and morphological properties of eco-composites made by short flax fiber reinforced PLA Scanning electron microscopy (SEM) revealed that the packed structure of the fibrils was deformed by the removal non-cellulosic materials. The mechanical tests indicated that the modulus of the untreated flax fiber/PLA composites was higher than that of PLA on the other hand, the modulus of alkali treated flax/PLA was lower than that of PLA. Thermal properties of the PLA in the treated flax fiber reinforced composites were also affected. Tg values of treated flax fiber reinforced composites were lowered by nearly 10°C for 10% NaOH treatment and 15°C for 30% NaOH treatment. A bimodal melting behavior was observed for treated fiber composites different than both of neat PLA and untreated fiber composites [60]. 

Improved fiber matrbc adhesion and thereby improved mechanical properties can be accomplished by engineering a superior processing condition in preparing the bio-composites, by altering the polymer architecture of the matrix or by the surface treatment on the fiber [47]. Table 23.1 showed mechanical properties of natural fiber-reinforced PLA composites. 

Laminated composites from kenaf fiber reinforced PLA were prepared by compression molding (29). The mechanical and thermal properties of these composites were assessed dependent on the modification of the kenaf fiber by alkaline and silane treatments, such as 3-aminopropyltriethoxysilane which acts as a coupling agent. 

Bajpai, P.K., Singh, L, and Madaan, J. (2013) Tribological behavior of natural fiber reinforced PLA composites. Wear, 297 (1-2), 829-840. 

Nuthong, W., Uawongsuwan, P., Pivsa-Art, W., and Hamada, H. (2013) Impact property of flexible epoxy treated natural fiber reinforced PLA composites. Energy Procedia, 34, 839-847. 

While not changing the chemical composition of the fiber extensively, physical treatments cause variations in structural and surface properties of the fiber and consequently affect the mechanical bonding to the polymer matrix. Thermal treatment, corona and plasma treatments can be given as examples to physical treatments applied on plant fibers [3]. Ragoubi et al. [33] reported an increase in mechanical and thermal properties of reed fiber-reinforced PLA and PP composites upon corona discharge treatment of fibers. 

Natural fiber-reinforced PLA composites are attractive because both the reinforcement (natural fiber) and matrix (PLA) are obtained from renewable resources. Natural fibers are considered as environment friendly alternatives to conventional reinforcing fibers such as glass, carbon, aramid, and so on. Natural fibers can be subdivided into three categories plant (cotton, jute, flax, hemp, etc.), animal (wool, silk, etc.), and mineral fibers (asbestos, inorganic whiskers, etc.). Generally, plant fibers are more popularly used as natural fiber reinforcements. Of these fibers, the most used are flax, jute, sisal, ramie, hemp, kenaf, and cotton. Plant fibers can generally be classified as nonwood (vegetable fibers) and wood fibers [20]. 

Several studies have been made to optimize the properties of natural fiber-reinforced PLA composites from the point of view of fiber-matrix adhesion. Pretreatment of fibers, such as chemical modification, seems to be the most promising approach, in which covalent bonds are formed between the fiber and matrix. One of the most common and efficient methods is alkali treatment (for example, with 2% sodium hydroxide aqueous solution) of fibers, which has been used to... 

While cellulose fiber reinforced polypropylene (PP) is already used by default for example in the automobile industry for interior parts (Karas and Kaup, 2005), the conventional use of cellulose fiber reinforced PLA is still at the beginning. But there are also some products such as biodegradable urns, mobile phone shells or prototypes of spare tyre covers made from natural fiber reinforced PLA at the market (Anonymous, 2007 Iji, 2008 Grashom, 2007). Maty studies deal with the use of natural fibers as reinforcements in PLA composites. An overview about the mechanical characteristics and apphcation areas of natural fiber-reinforced PLA can be foimd for example in Bhardwaj and Mohanty (2007), Avella et al. (2009), Ganster and Fink (2006), Jo-noobi et al. (2010), and Graupner et al. (2009). For the improvement of the composite characteristics it is still necessary to carry out optimization processes for fibers, PLA matrix and the interactions of both. Moreover the processing parameters, force elongation characteristics of fibers and matrix as well as the use of additives like plasticizers or adhesion promoters have decisive influences on the mechanical characteristics of the composites. 

The admixture of lyocell fibers as a force elongation modifier for kenaf fiber reinforced PLA composites as well as the optimization of the processing parameters and the use of lignin as a kind of natnral additive for improved fiber/matrix interactions will be described in the present chapter. The smdy is focnsed on compression molded lyocell and kenaf fiber reinforced PLA composites. 

Figure 3. Tensile strength of 40 mass% lyocell fiber reinforced PLA in dependence of the fiber fineness. Composites reinforced with needle felts (left) Composites reinforced with multilayer webs (right) (mean values, standard deviations are shown as error bars dots show the dimension of the fiber diameter).

Ibrahim et al. (2010) investigated similar trends. They investigated compression molded kenaf fiber reinforced PLA with fiber loads ranging between 10 and 50 mass%. The tensile strength decreased from 27 N/mm measured for the 20% kenaf PLA composites to 16 N/mm measured for the 40% kenaf PLA composites. And Oksman et al. (2003) described similar effects for compression molded flax-PLA. The tensile strength of 40% flax-PLA decreased compared to 20% flax-PLA. They partially attribute this behavior to poor adhesion between fiber and matrix. 

Figure 13. Tensile strength of kenaf, lyocell and mixed kenaf/lyocell fiber reinforced PLA composites (mean value as column, standard deviation as error bars).

Okubo, K., Fujii, T., and Thostenson, E.T. (2009) Multi-scale hybrid biocomposite processing and mechanical characterization of bamboo fiber reinforced PLA with microfibrillated cellulose. Composites Part A, 40, 469-475. 

Figure 7.8 Tensile load-strain curves of neat PLA and ramie fiber-reinforced PLA composites (a) neat PLA, (b) untreated fiber-PLA, (c) alkali-treated fiber-PLA,...

Oksman et al. [26] reported that the tensile strength of flax fiber-reinforced PLA composites was improved by 50% compared to similar PP/fiax fiber composite at fiber loading of 30-40%. Similarly, modulus of PLA-based composites was increased from 3.4 GPA for neat PLA to 8.4 GPa for the composite. PLA was also not degraded by the compounding process used during manufacturing of composites. 

Wan Y Z, Wang Y L, Xu X H, Li Q Y (2001b), /n vitro degradation behavior of carbon fiber-reinforced PLA composites and influence of interfacial adhesion strength , J. Appl. Polym. Sci., 82, 150-158. 

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