Polylactic acid (PLA) has good mechanical properties and biocompatibility and is one of most promising bio-based degradable materials that has great potential for applications in packaging, clothing and medicine. However, inherent brittleness of PLA itself, poor toughness, low elongation at break, and other disadvantages greatly limit its use and limit its scope. Therefore, it is necessary to strengthen and modify polylactic acid. Today, we will summarize and introduce you some common methods of strengthening and modifying PLA.
1. PLA hardening classification
To date, main methods of modification are composite, copolymerization, plasticization and mixing of four types.
Table 1-1 Classification of PLA Strengthening Methods
1.1 PLA/Physical blending of elastomers
Physical mixing refers to direct mixing of PLA and elastomer using shear force of processing equipment so that elastomer is evenly distributed within PLA matrix. The TPU elastomer is dispersed in PLA matrix in form of "spherical" particles forming a "sea island" structure. Under external influence, TPU in form of a dispersed phase can act as a stress concentration point, absorb energy, increase impact strength of the material, and elongation at break increases with increasing TPU content.
Fig. 1-1. Effect of TPU content on tensile strength and elongation at break of a TPU/PLA blend
Chen Zhiping, Feng Yufeng, Lu Chunli, Huang Jing, Liao Wenjing. Preparation and characterization of thermoplastic polyurethane/polylactic acid mixtures[J]. Insulation materials, 2020.53(05):13-17.< /p>
When physically blended, type of elastomer, its dispersibility, compatibility and size of dispersed particles in matrix have a decisive influence on properties of mixture. In PLA-blended elastomers, most elastomers need to incorporate a third compatibilizer or inorganic nanoparticles to improve dispersion and compatibility of elastomers in PLA matrix due to their poor compatibility with PLA. The compatibilizer is a polymer grafted with maleic anhydride.
1.2PLA/elastomer reactive blend
Reactive blending of PLA/elastomer is a simple and effective method of increasing strength. In order to improve immiscibility of PLA/PU elastomer, polyol, isocyanate and PU elastomer chain extender are added in steps using reactivity of -NCO- group in PU elastomer, and mixing process is carried out at same temperature. time Implement a polyurethane elastomer dispersion and create a PLA-PU block or graft copolymer. Partially cross-linked polyurethane can form a network structure that reduces size of dispersed phase during melt processing and provides in-situ compatibility.
2. Biobased/Blend of degradable elastomer and PLA
2.1 Physical mixing
When bio-based/degradable elastomers are directly blended with PLA, polyester or polyamide elastomers with better compatibility with PLA are often used as raw materials to achieve best compatibility between them, or small molecules such as bio-based diols and diacids are used to interact with PLA to obtain graft or block copolymers to improve their compatibility, in order to achieve purpose of mixing with PLA to impart rigidity.
2.2 In-Place PLA Reactivity
In situ reactivity means that carboxyl, amino or hydroxyl groups must participate in reaction during melt mixing process and at same time generate a compatibilizer. The reaction usually takes place at interface, so that PLA/elastomer is formed. There is a stronger interfacial bond, thereby increasing strength of PLA.
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