Polylactic acid is an environmentally friendly and biodegradable aliphatic material and it can be prepared from renewable resources such as corn.^[1,2] The raw material of PLA is lactic acid, starch can be decomposed into glucose, and then glucose can be fermented by bacteria to make lactic acid monomer with high purity.

Fig 1 The life cycle of PLA^[6]

Synthetic Method

PLA can be obtained by direct condensation polymerization and lactide formation polymerization.

(1) Direct condensation polymerization is direct dehydration of lactic acid monomer, PLA was obtained under high temperature and pressure. Direct polymerization is a relatively mature preparation method, but this method is difficult to obtain high molecular weight polylactic acid.^[3] and also the polylactic acid prepared has low strength and is easy to decompose.

(2) Lactide formation polymerization is ring-opening polymerization of propyl ester with metal catalysts in solution, which is simpler and cheaper.^[4] Ring-opening polymerization is to concentrate lactic acid dehydrogenation to prepare prepolymer, and then obtain ring dimer (i.e. Lactide) by internal ester exchange reaction in the form of "backbite". Lactide was then synthesized by ring - opening polymerization. Lactide formation polymerization is a more common way to prepare the PLA.

                                                          Figure 2 Preparation of polylactic acid^[5]

Research for Polymer blend toughening

   The purpose of blending modification is to improve the properties of the polymer by physical mixing. By blending modification, the polymer can improve other properties on the basis of keeping the original excellent properties. At the same time, some special properties of the blend material may appear due to the synergistic effect between the polymer components.

                                                Figure 3 Some polymers blended with PLA^[6]

   In order to maintain the good biodegradability of PLA based materials, many researchers also attempted to blend biodegradable materials with PLA. It not only maintains the biodegradability of PLA based materials but also greatly improves the toughness of them, which widens the application scope of PLA in the biological field. Matta^[7] et al. studied the rheological properties, thermal properties, mechanical properties and viscoelastic behavior of the prepared PCL blends with PLA, and observed the microstructure of the tensile section of the blend system by SEM. The results showed that the elongation at break and impact toughness of PLA/PCL blends were significantly improved compared with pure PLA materials, but the strength and modulus were decreased. Among the PLA/PCL blends with different proportions, the 80/20%PLA/PCL blends had the highest elongation at break and impact toughness

   For blending systems, compatibility is the key factor that affects the performance of blending systems. The good compatibility between the two phases is the prerequisite for the good properties (especially the mechanical properties) of the blends. Compatibility affects the difficulty of the blending process. Two-phase systems with good compatibility are easier to be dispersed in the blending process. Therefore, polymer systems with good compatibility should be generally preferred for blending.

   Polyethylene glycol(PEG)^[8,9], polypropylene glycol monobutyl ether(PPG)^[10], random poly-3-hydroxybutyric acid(PHB)^[11], polyester diol(PED)^[12], etc. all have good compatibility with PLA. In order to improve the mechanical properties of PLA, Hu^[10] et al. mixed PLA with PEG with low regularity, and found that PLA and PEG were still compatible when PEG content reached 30% at room temperature. Blending with PEG significantly reduced the Tg and modulus of PLA and increased the fracture strain of PLA. Okamoto^[12] et al. observed that PLA and PED are compatible by measuring the temperature dependence of the oscillation tensile modulus of PLA/PED blends in the solid state.

Manufacturing & Application

For powdered or granular poly (lactic acid), when mixed with natural fibers, the injection molding process is often used. The components were plasticized and dispersed by repeated shearing and extrusion with screw extruder and mixer.^[13]The products have a precise size and completed shapes, this process is suitable for mass production of complicated parts. K. alasinska et al. ^[14] used poly (lactic acid) as matrix and grinding sunflower shell as reinforcing components respectively, preparing biodegradable composite materials by injection molding.  

Compression molding is a process of solidifying premix into simple metal molds with certain temperature and pressure. The process is simple to operate, but the production cycle is long and the efficiency is low.  Mao et al. ^[15] mixed poly (lactic acid) and bamboo fiber by simple mixing, then paved and molded the sample to prepare of polylactic acid/bamboo fiber composites.

Melt processed PLA products include injection-molded disposable knives and forks, thermo-molded containers and cups, injection pull-blown bottles, extruded cast and stretched films, and melt spinning for non-woven fabrics, textiles, and carpets, etc. ^[16,17]