PLA-based composite

PLA has been used as package materials and other products However, the physical properties of PLA such as brittleness limit the application of PLA. A way to improve the mechanical and thermal properties of PLA is the addition of fibers or nanofiller materials. 

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However, for PLA, the disadvantages such as the inherent brittleness, poor heat resistance, and low melt strength limit the applications. Modifications of PLA and PLA-based composites are common ways to solve these problems. 

Three parts are covered in the chapter. First, the synthesis and properties of PLA are described. The modification and process of PLA are also discussed. Then, the composites with PLA as matrix and natural fiber or nanoparticles as reinforcement are reported in the second part. The processing and the properties of the composites are also given. The interface between PLA and the reinforcement and the surface treatment methods are discussed. Finally, the application and the development of PLA and PLA-based composites in the future are proposed. 

The commercial importance of polymers has been driving intense applications in the form of composites in various fields. Performance during use is a key factor of any composite material, which decides the real fate of products during services. Research and development in these areas may open up new opportunities for PLA for use as high performance biodegradable materials. It is predicted that PLA and PLA-based composites will be used in wide areas such as aerospace, automotive, marine, infrastructure, military, etc. 

Recently, nanostructured carbon-based fillers such as Ceo [313,314], single-wall carbon nanotubes, carbon nanohorns (CNHs), carbon nanoballoons (CNBs), ketjenblack (KB), conductive grade and graphitized carbon black (CB) [184], graphene [348], and nanodiamonds [349] have been used to prepare PLA-based composites. These fillers enhance the crystalUza-tion ofPLLA [184,313,314].Nanocomposites incorporating fibrous MWCNTsandSWCNTs are discussed in the section on fibre-reinforced plastics (section 8.12.3). 

PLA based compositions with a plasticizer and a crystal nucleating agent have been reported (18). Plasticizers are collected in Table 3.2. 

Wan Y.Z., Wang Y.L., Li Q.Y., Dong X.H., Influence of surface treatment of carbon fibers on interfacial adhesion strength and mechanical properties of PLA-based composites, J. Appl. Polym. Sci., 80, 2001, 367-376. 

PLA-based composites with polyhedral ohgomeric silsesquioxane (POSS) can be prepared via melt mixing in a twin-screw micro-compounder. The molten, compounded mixture was molded using an injection molding machine to form the composite material [69]. The addition of POSS acted as a plasticizer and enhanced energy absorption capacity. Maximum fracture strength was observed for 7 wt% addition of POSS in the composites [69]. 

Qian et al. conducted a study on bamboo particles (BP) that were treated with low-concentrations of alkali solution for various times and used as reinforcements in PLA based composites [35]. Characteristics of BP by composition analysis, scaiming electron microscopy, Brunauer-Emmett Teller test, and Fourier transform infrared spectroscopy, showed that low-concentration alkali treatment had a significant influence on the microstructure, specific surface area, and chemical groups of BP. PLA/treated-BP and PLA/untreated-BP composites were both produced with 30 wt% BP content. Mechanical measurements showed that tensile strength, tensile modulus, and elongation at break of PLA/BP composites increased when the alkali treatment time reached... 

The properties of PLA such as thermal stability and impact resistance are inferior to those of conventional polymers used for thermoplastic applications. Therefore, PLA is not ideally suited to compete against the conventional polymers [5]. In order to improve the properties of PLA and increase its potential applications, copolymers of lactic acid and other monomers such as derivatives of styrene, acrylate, and poly (ethylene oxide) (PEO) have been developed. PLA has also been formulated and associated with nanosized fillers. Modification of PLA, copolymerization with other monomers, and PLA composites are some approaches that have been used to improve the properties of PLA, such as stiffness, permeabiUty, crystallinity, and thermal stability [1-5]. Considerable research is being done to develop and study modified PLA, PLA-based copolymers, and PLA-based composites. 

As a packaging material, PLA film has been used to produce transparent films, trays, adhesives, and compostable tea bags (with PLA fibres) [37]. New technical improvements are likely to arise from the development of methods to improve stability during processing, blends and PLA-based composites, and new end-uses for the polymer in both packaging and non-packaging applications. 

Figure 3.13 Thermogravimetric curves of PLA and PLA-based composites (adapted from Tao et al., 2009).

In this study, M FC was used as an enhancer for a PLA-based composite using BF to increase mechanical properties such as bending strength and fracture toughness. The effect of weight content of MFC on those mechanical properties was examined. 

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. 

SEM micrographs of fractured surfaces during flexural testing are shown in Fig. 10.6. SEM images show that fiber bundle fracture, debonding, and cracks are observed on the fractured surface of the composite. SEM images clearly show lesser fiber-matrix debonding in PLA-based composites than that of PP-based composites, which indicates the better performance of PLA-based composites. 

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