Introduction: With the increasing preference for environmentally friendly packaging, the demand for degradable materials is gradually rising. People are seeking and developing sustainable or ecologically cyclic materials, among which biodegradable plastics are one of the most environmentally friendly and promising green materials. This article briefly describes several common types of biodegradable plastics for reference.

Degradable Plastic:

Degradable plastic refers to plastics that undergo significant changes in chemical structure under specified environmental conditions over a period, leading to the loss of certain properties (such as integrity, molecular weight, structure, or mechanical strength) and/or fragmentation. Standard test methods reflecting performance changes should be used for testing, and their categories should be determined based on degradation methods and service life. Degradable plastics are classified into biodegradable plastics, compostable plastics, photodegradable plastics, and thermal oxidizable plastics based on their designed final degradation pathways.

Biodegradable Plastic:

Biodegradable plastics degrade in natural environments such as soil and/or sand, and/or under specific conditions such as composting or anaerobic digestion, or in aqueous culture solutions, by the action of naturally occurring microorganisms such as bacteria, fungi, and algae, ultimately completely degrading into carbon dioxide (CO2) and/or methane (CH4), water (H2O), mineral inorganic salts of its constituent elements, and new biomass. This is commonly referred to as biodegradable plastic.

Classification of Biodegradable Plastics:

Depending on the composition of raw materials and manufacturing processes, biodegradable plastics can be divided into three main types: natural polymers and their modified materials, microbial synthesized polymers, and chemically synthesized polymers. Currently, commonly used biodegradable plastics include Polyhydroxyalkanoates (PHA), Polylactic Acid (PLA), Polycaprolactone (PCL), and Polybutylene Succinate (PBS).

1. Polyhydroxyalkanoates (PHA):

Polyhydroxyalkanoates are different structured fatty acid esters synthesized by microorganisms through fermentation of various carbon sources. The most common ones are polyhydroxybutyrate (PHB), polyhydroxyvalerate (PHV), and their copolymer (PHBV). PHB, widely present in nature, is a thermoplastic polyester commonly found between bacterial cells. PHB exhibits similar physical and mechanical properties to polypropylene but possesses biodegradability and biocompatibility, degrading completely into β-hydroxybutyric acid, carbon dioxide, and water in biological organisms. Materials made from this bioplastic can be used in drug delivery systems, implants, and devices that decompose harmlessly in the human body after healing. However, compared to polypropylene, PHB is relatively hard and more brittle. Copolymerization of PHB and PHV (PHBV) can improve the high crystallinity and brittleness of PHB, enhancing its mechanical, thermal, and water resistance properties. PHB/PHV copolymers, marketed under the name Biopol, have been developed and can be used in various industries such as food packaging, cosmetics, pharmaceuticals, hygiene, and agriculture.

2. Polylactic Acid (PLA):

Polylactic Acid (PLA) is a polyester synthesized chemically from lactic acid, a microbial fermentation product.

PLA production starts with lactic acid as the raw material. Traditional lactic acid fermentation mostly uses starchy raw materials. Currently, countries like the USA, France, Japan, etc., have developed the fermentation production of lactic acid using crops such as corn, sugarcane, sugar beet, potatoes, etc., and subsequently, PLA production. Corn is the preferred raw material for producing biodegradable plastic PLA. The manufacturing process of biodegradable plastic PLA involves grinding corn into powder, separating starch, extracting raw glucose from starch, and then fermenting glucose into lactic acid similar to the beer fermentation process. The extracted lactic acid is then processed into the final polymer, PLA.

PLA is a biodegradable polymer produced from renewable resources such as grains. In the PLA production route, lactic acid monomers are first hydrolyzed from starch materials, fermented into sodium lactate through the fermentation process, and then used to prepare PLA. Lactic acid is further concentrated, followed by polymerization through condensation (forming prepolymers), thermal depolymerization (forming diester), ring-opening polymerization, and depolymerization in sequence. The molecular weight of PLA can reach up to 75000 g/mol.

The conventional method of lactic acid condensation only produces lactic acid oligomers. The most researched method for preparing high-molecular-weight PLA is through the ring-opening polymerization reaction of lactide, which is synthesized by the high-temperature pyrolysis of lactic acid oligomers. The reaction mechanism and conditions for the ring-opening polymerization of lactide have been extensively studied and reported. Recently, Mitsui Chemicals of Japan proposed a new technology to prepare PLA directly from lactic acid condensation without lactide. This technology uses highly active catalysts for solution condensation to obtain high-molecular-weight PLA. As lactic acid and lactide contain asymmetric carbon atoms, polymerization can produce PLA with different stereoregularity, such as L-PLA, D-PLA, and DL-PLA.

PLA exhibits good moisture resistance, oil resistance, and sealing properties. It is stable at room temperature but undergoes automatic degradation at temperatures above 55°C or under the action of oxygen and microorganisms. After use, it can be completely degraded by microorganisms in nature, ultimately generating carbon dioxide and water without polluting the environment, which is very beneficial for environmental protection.

The degradation of PLA occurs in two stages: 1) first, it undergoes chemical hydrolysis into lactic acid monomers; 2) lactic acid monomers degrade into carbon dioxide and water under the action of microorganisms. PLA-made food cups can be completely degraded in just 60 days, achieving both ecological and economic benefits.

3. Polycaprolactone (PCL):

Polycaprolactone (PCL) is a low melting point polymer obtained by ring-opening polymerization of ε-caprolactone, with a melting point of only 62°C. Research on the biodegradability of PCL has been underway since 1976, and it can be completely degraded by microorganisms in both anaerobic and aerobic environments. Compared to PLA, PCL has better hydrophobicity but slower degradation rates. Additionally, its synthesis process is simple and cost-effective. PCL has excellent processing properties and can be processed into films and other products using conventional plastic processing equipment. Moreover, PCL exhibits good compatibility with various polymers such as PE, PP, PVA, ABS, rubber, cellulose, and starch. Blending or copolymerizing with these polymers can produce materials with excellent properties. Especially, blending or copolymerizing PCL with starch maintains its biodegradability while reducing costs, making it highly desirable. PCL-starch blends can produce biodegradable plastics with good water resistance at a price comparable to paper. By using in-situ polymerization, PCL can be grafted onto starch to produce thermoplastic polymers with excellent properties.

4. Polyester – PBS/PBSA:

Compared to similar products, the advantages of polyester biodegradable plastics are:

1) One of the fatal weaknesses of the above biodegradable plastics (such as polylactic acid, polycaprolactone, and polyhydroxyalkanoates) is their poor heat resistance, which affects their application promotion in the food service industry.

2) The processing conditions for the above biodegradable plastics (such as polylactic acid, polycaprolactone, and polyhydroxyalkanoates) are stringent, and there are some difficulties in industrialization.

3) Polylactic acid is a water-degradable bioplastic that cannot accept water molecules during storage. Performance cannot be guaranteed during normal storage and use.

Polybutylene Succinate (PBS) is a typical polyester biodegradable plastic. It has become a leader in biodegradable plastic materials by overcoming the above weaknesses. It has extensive applications in packaging, tableware, cosmetic bottles and pharmaceutical bottles, disposable medical supplies, agricultural films, slow-release materials for pesticides and fertilizers, biomedical polymers, and other fields. PBS has excellent comprehensive properties, reasonable cost-effectiveness, and a promising application prospect. Compared to other biodegradable plastics such as polycaprolactone, polyhydroxyalkanoates, and polylactic acid, PBS has a similar price, with no significant advantage. However, compared to other biodegradable plastics, PBS has excellent mechanical properties close to polypropylene and ABS plastics. It exhibits good heat resistance, with a heat distortion temperature close to 100°C. After modification, it can be used at temperatures exceeding 100°C, making it suitable for preparing packaging for hot and cold beverages and food containers, overcoming the low heat resistance of other biodegradable plastics. PBS has excellent processing properties and can be processed into various molded products using existing plastic processing equipment. It is currently the best-performing biodegradable plastic in terms of processing properties. Additionally, it can be blended with a large amount of calcium carbonate, starch, and other fillers to produce low-cost products. PBS production can be achieved by slightly modifying existing general polyester production equipment. Currently, there is a severe overcapacity in domestic polyester equipment, and the transformation of PBS production provides new opportunities for surplus polyester equipment.

Furthermore, PBS only degrades under specific microbial conditions such as composting, making its performance stable during normal storage and use.

PBS is primarily produced from aliphatic dicarboxylic acids and diols. It can meet demand through petrochemical products and can also be produced from renewable agricultural products such as starch, cellulose, and glucose through biological fermentation, achieving green cyclic production from nature. Moreover, using raw materials produced by biological fermentation processes can significantly reduce raw material costs, further reducing the cost of PBS.