In 2020, many companies have on agenda polylactic acid (PLA) projects that are planned or already under construction. Can PLA reduce environmental burden caused by white pollution? The main thing is that it should be accessible and popular with everyone, and quality of products should be high and stable. Earlier we explained in detail concept, classification and prospects of biodegradable plastics, today we will learn more about more “popular” biodegradable plastic - PLA.
1. What is this PLA material?
Polylactic acid is a polymer of aliphatic hydroxy acid. It is a solid material in a glassy state at room temperature. When decomposed by microorganisms, it turns into carbon dioxide, CH4 and water. It is a typical linear and fully biodegradable material. .
Figure 1-1 Polylactic acid molecular formula
PLA can be processed by injection molding, blow molding, compression molding, thermoforming and extrusion. The lactic acid monomer PLA is a chiral molecule, and there are two optical enantiomers L-lactic acid and D-lactic acid. PLA has three configurations. PDLLA and PLLA are two common polymers. The crystallinity of PLLA is related to content of monomer L. The crystallization rate and crystallinity depend on nucleating agent, time and temperature. The physical properties of PLA are related to enantiomeric purity of lactic acid stereopolymer, and homopolymerized PLLA and PDLA have high regularity and crystallinity.
Table 1-1 PLA Properties
Second, polylactic acid (PLA) synthesis method
Lactic acid is feedstock for synthesis of polylactic acid. At present, lactic acid producers in my country mainly use starch-rich food crops to produce lactic acid through microbial fermentation.
2.1 Direct method
At a temperature of 140-210°C and in presence of a dehydrating agent, intermolecular lactic acid undergoes thermal dehydration and polycondensation into oligomers, and then continues to heat up, and low molecular weight PLA is a chain. -extended to higher molecular weight polylactic acid.
2.2 Indirect method
It is difficult to obtain high molecular weight polylactic acid by direct polycondensation. One of reasons is that viscosity of system constantly increases during polycondensation process, so that formed water cannot be removed. In order to obtain high molecular weight polylactic acid, first step is oligomerization of lactic acid into low molecular weight PLA, and second step is cleavage to cyclic lactide lactide dimer at high temperature and high vacuum, and then separate and purify crude lactide. In presence of an initiator, ring-opening polymerization produces polymeric polylactic acid.
Figure 2-1 Process flow for indirect synthesis of polylactic acid ()
3. Modification of polylactic acid (PLA)
3.1 Physical modification
Nucleizer: Adding a nucleator can solve problem of imperfect crystallinity of PLA, increase crystallinity, and improve heat resistance and physical properties of PLA.
Plasticizer: The addition of plasticizers such as glycerol, sorbitol, PEG oligomer, etc. can improve processing and mechanical properties of PLA, and improve toughness and impact resistance of PLA.
Inorganic fillers. Commonly used PLA inorganic fillers include kaolin, montmorillonite, calcium carbonate, hydroxyapatite, etc. The addition of fillers can increase toughness and thermal decomposition temperature of PLA, and can also play a critical role. role in cost reduction.
Mixing with other biodegradable plastics: After mixing PLA with PCL, PBAT, PBS and other degradable materials, melting point and crystallinity of PLA can be changed to improve mechanical properties.
3.1 Chemical modification
Chemical modification is combination of active monomers or groups of certain compounds with PLA molecules in form of covalent bonds. Compared with physical modification, chemical modification has a stronger bonding power, mainly including copolymerization modification, properties, grafting modification, chain extension modification. and cross-linking modification.
4. Performance comparison of polylactic acid (PLA) and major degradable plastics
Table 4-1 Performance Comparison Table
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