PLA is a bio-based biodegradable plastic. Poly Lactic Acid (PLA) or Polylactide is a polymeric polyester type material obtained by polymerizing lactic acid or lactic acid dimer propylene glycol as monomer, which is a synthetic polymer material with bio-based and degradable characteristics. Since lactic acid is a chiral molecule with two spin isomers, L-lactic acid (L-lactic acid) and D-lactic acid (D-lactic acid), both lactic acids can be polymerized into polylactic acid with different stereo structures through the process of "lactic acid-propylene-polylactic acid", and there is no significant difference in the manufacturing process.
The most widely used is L-Polylactic acid. Generally, PLA made from L-lactic acid is called "L-Polylactic acid", and accordingly, PLA made from D-lactic acid is called "D-Polylactic acid". Since initially lactic acid was mainly used in the food and beverage manufacturing industry and L-lactic acid can be completely metabolized and digested by the human body, L-lactic acid is the most demanded and produced lactic acid worldwide, while D-lactic acid is used in the production of agricultural pesticides and herbicides, which has a narrower application and less market demand.
PLA, as an aliphatic thermoplastic polymer, is prominent in several scenarios of replacing petroleum-based plastics with the use of degradable plastics. It is comparable to conventional petroleum-based polyethylene terephthalate (PET) and polystyrene (PS) in terms of mechanical properties. Compared with existing fully biodegradable plastics, PLA has higher heat resistance and mechanical strength; in terms of applicable processing processes, it can be applied to existing extrusion, injection molding, extrusion blow molding, spinning, foaming and other processing methods, and is compatible with traditional plastic processing processes; at the same time, PLA also has the characteristics of complete biodegradability, degradation products are harmless to humans, environmental friendliness, etc. Therefore, it is widely used in tableware, packaging, 3D printing, textile and other fields. In addition, because of its good mechanical properties, PLA is often blended with other biodegradable plastics in certain proportions to enhance the strength of the material. For example, 5%-20% PLA is often added to PBAT in film bag products, which further increases the downstream market space for PLA. Specifically, PLA has the following properties.
1. Capable of forming a biomass resource recycling system. As a bio-based, fully biodegradable material, PLA is able to incorporate the raw material end into the resource regeneration and recycling system, and has advantages that petroleum-based materials that cannot incorporate raw materials into the recycling system do not have.
The natural and fully biodegradable nature of PLA drives a regenerative and circular system of biomass resources. From raw materials, PLA is made from lactic acid fermented from sugar extracted from deep processing of crops such as corn, sugar cane and sugar beets. From the point of view of product disposal, PLA products can be degraded to carbon dioxide and water and participate again in the regeneration and recycling process of biomass through photosynthesis of crops.
2. It is in line with the direction of the "carbon neutral" policy. The carbon in PLA is mainly formed by the absorption of carbon dioxide from the atmosphere during the growth of crops such as corn and sugar cane, and is returned to the atmosphere in the form of carbon dioxide during the degradation process, and is again re-involved in the regeneration and recycling of biomass resources through the photosynthesis of crops. Compared to petroleum-based materials, PLA materials can achieve a greater degree of "break-even" in atmospheric carbon content, which is more conducive to achieving "carbon neutrality" in the plastics industry.
3. The manufactured products are disposed in various ways. The main disposal methods for conventional plastics are recycling, landfill and incineration. Among them, multiple recycling will lead to a reduction in the performance of plastics, while landfills and incineration will inevitably lead to a variety of long-term, deep-seated environmental problems. The fully biodegradable nature of PLA makes it a compostable plastic, allowing it to be disposed of in an environmentally friendly manner using degradation. PLA has a large number of ester bonds in its main chain, which are easily hydrolyzed and broken upon contact with water or moist air, and the broken products are completely decomposed into carbon dioxide and water by microorganisms, which is the degradation principle of PLA materials at the microscopic level. At the macroscopic level, the degradation of PLA products is characterized by the destruction of the overall structure, the decrease in volume, the gradual transformation into fragments, and finally into carbon dioxide and water.
4. Good performance. The melting point of pure PLA is about 120℃-180℃, the melt flow rate (MFR) is 3-50g/10min, with a wide range of indicators; the tensile strength can reach 65MPa, the bending strength can reach 100MPa, and it has the characteristics of high transparency, high gloss, easy color matching and easy printing. The range of these indicators can be further extended by compound modification with the addition of other materials. The good mechanical and physical properties of PLA make it a new material that can replace traditional plastics. With the development of plastic modification and blending technology in recent years, compound modified PLA has reached or even surpassed some traditional petroleum-based plastics in terms of hardness, mechanical strength and heat resistance. At present, PLA has been able to partially replace PET materials in the manufacture of packaging materials, as well as partially replace PS and PP materials, and is suitable for extrusion molding, injection molding and other processing methods.
5. Environmentally friendly. PLA is mainly composed of carbon, hydrogen and oxygen elements, and if disposed of by incineration, PLA itself will not release toxic gases such as nitrides and sulfides. Moreover, even if PLA products are discarded in the natural environment, they will be biodegraded to carbon dioxide and water within a few years, and no "white pollution" will be formed. Therefore, PLA materials are environmentally friendly. Biodegradable plastics are an effective way to combat the pollution problem of traditional plastics, of which bio-based materials are more advantageous. Since the environmental pollution problems caused by traditional plastics mainly stem from their non-degradability, the industry has gradually formed a consensus to replace non-degradable plastics with degradable ones. One of the main sources of raw materials is the partial replacement of petroleum-based raw materials with bio-based ones.
After years of development, two major types of "petroleum-based degradable plastics" and "bio-based degradable plastics" have been gradually formed. In contrast, biomass feedstocks are derived from natural products and can be technically enhanced, thus freeing the manufacture of plastic materials from dependence on non-renewable resources at the feedstock end and reducing the pollution generated during the production of petroleum-based plastics. Therefore, the use of "bio-based biodegradable plastics" is more conducive to alleviating the contradiction between human social development and natural environmental protection, and realizing the sustainable development of human economy and society.
PLA, as a bio-based biodegradable material, is the most industrialized biodegradable material available. According to Eurostat data, the global production of plastics has reached 367 million tons in the year 2020. According to the European Bioplastics Association, the global production capacity of biodegradable plastics was 1.2259 million tons in the same year, which did not reach 1% of the global plastic production and still belonged to emerging materials. At present, the main biodegradable plastics that have reached a certain scale of commercial application are PLA, PBAT, PBS and PHA.
In terms of material properties, PLA has the characteristics of high hardness and good mechanical properties, and the price can be accepted by the downstream market, making it irreplaceable among the above materials. In recent years, biodegradable plastics in general are in a period of rapid growth, and PLA is the material with higher capacity and faster growth, and is the mainstream material in current biodegradable plastics. According to the European Bioplastics Association, the global capacity of PLA is 394,600 tons in FY2020, which is the highest capacity material among biodegradable plastics, accounting for nearly 50% of the total capacity of biodegradable materials in that year, with a CAGR of 35% in 2018-2020.
