Degradable plastics

Degradable plastics, also known as degradable plastics, refer to a class of polymers that have the same performance as their corresponding ordinary plastic products during a certain service period, and after completing a certain functional service period, their physical and chemical structure changes significantly under specific environmental conditions, and can quickly decompose and assimilate with the natural environment.
According to different degradation mechanisms, it can be divided into: photodegradable plastics, biodegradable plastics, photo-biodegradable plastics, and hydrolyzed plastics. At present, the degradable plastics developed are mainly biodegradable plastics, photodegradable plastics and photo-biodegradable plastics.
1. Aliphatic polyester biodegradable plastics
This is the earliest, most widely researched, and practical application of a class of biodegradable plastics. It can be divided into: microbial synthetic polyesters, polylacides, aliphatic polyesters and poly ε-caprolactone
(1) Microbial synthesis of polyester biodegradable plastics
(1) Poly-3-hydroxysuccinate (PHB)
PHB is a degradable polymer formed by certain bacteria during fermentation in an environment lacking oxygen, phosphorus, nitrogen and sulfur and other nutrients, so it is also known as microbial synthetic biodegradable plastics.
PHB is a thermoplastic polyester material with properties between PVC and PET with a melting point of 180°C and can be extruded and compression-molded.
PHB is mainly used in three aspects: medicine, agriculture and consumer packaging. Especially in medicine, it shows unique properties. In corrective surgery, PHB is inserted into the body as a drug matrix to control the release of the drug. When the drug is released, PHB is naturally degraded in the human body, and the final product is 3-hydroxybutyric acid, which is an ordinary metabolite in the human blood and will not bring any side effects to the human body. PHB can be made into surgical sutures and can be used in ophthalmic surgery without the need for a suture removal process.
(2) Poly3-hydroxysuccinate/3-hydroxyglutarate (P3HB/3HV)
P3HB/3HV is made by fermentation with propionic acid and glucose as carbogen foods. It has good mechanical properties, from soft to hard, excellent heat resistance, can be used in hot water, equivalent to PP, water resistance, oil resistance, chemical resistance and barrier properties are very good, weather resistance is also good, can be used directly without antioxidants and light stabilizers.
P3HB/3HV showed good biodegradability under both aerobic and anaerobic conditions (e.g., in rivers, sea silt, deep water pipes, deep soils), and finally decomposed into carbon dioxide and water and disappeared. It is safe and stable in air and clean water, and does not degrade.
(2) Polylactide biodegradable plastics
This is a class of biodegradable plastics that are second only to microbial synthesis in terms of research activity.
(1) Polylactic acid (PLA)
The scientific name of polylactic acid is polylactide, or PLA in English. PLA is a biodegradable plastic variety that has been developed relatively late but has a more mature technology. At present, the main constraint on the development of PLA is the price. On the one hand, the price of lactic acid raw materials is high, and the purification process of PLA is complex and the production cost is high.
With the increase of the relative molecular weight of PLA, its mechanical properties improved. Therefore, PLA for plastics requires a relative molecular weight of at least 100,000.
PLA can be naturally degraded, such as CPLA becomes a low molecular weight polymer in 3~6 months, and decomposes into carbon dioxide and water in 6~8 months.
PLA has excellent processability and can be processed with universal plastic processing equipment, such as injection molding, extrusion, drawing, wire drawing, blow molding, etc. It has good bag-making, printing and secondary processing. The processing temperature of CPLA can reach 200°C.
(2) Polyethylene lactide (PLGA)
PLGA is actually formed by copolymerization of lactic acid (PLA) and glycolic acid (PGA, also known as glycolic acid). PLGA is easily degraded in the temperature range of 20~40°C and pH of 7~9, and can be degraded into small molecule compounds in the human body and excreted with body fluids. PLGA has good physiological compatibility, and due to its high price, it is currently mainly used for medical treatment, such as surgical sutures, orthopedic fixation, tissue repair, drug sustained-release materials, etc.
(3) Aliphatic polyester biodegradable plastics
Aliphatic polyester biodegradable plastics include polybutyl succinate (PBS, also known as polysuccinic acid) and polybutyl succinate/adipate (PBSA).
PBS and PBSA are biodegradable, and PBSA is better than PBS.
Degradation performance: (1) Under anaerobic conditions, it can be degraded by more than 90% in 30 days.
(2) Composting test, 90% degradation after 12 weeks.
(4) Poly ε-caprolactone (PCL)
PCL is a crystalline polymer with a melting point of 60°C and a glass transition temperature of -60°C. The softness and tensile strength of PCL are similar to those of nylon.
Due to its low melting point, PCL is rarely used alone and is often blended with other resins to improve heat resistance and improve mechanical properties and processability.
PCL can be degraded by 45%~75% in 20 days under suitable conditions.
2. Natural polymer biodegradable plastics
Natural polymer biodegradable plastics mainly refer to the modified varieties of natural polymer materials such as starch, cellulose and chitosan, and the purpose of modification is mainly to improve or endow them with processing properties.
The types of natural polymer biodegradable plastics that have been developed are: modified natural polymer materials such as modified starch, modified cellulose such as cellulose acetate, mixtures of natural polymer materials and synthetic biodegradable plastics, and mixtures between natural polymer materials.
(1) Modified starch Modified starch has a glass transition temperature in the range of 100~150 °C with different degrees of modification, so that it is plastic, and can be processed and formed by melting method. The water absorption of modified starch is very low, generally only a few percent, making it a biodegradable plastic with practical value.
The degradation performance of the modified starch is very good, and its tensile test piece is placed in 2501 compost, and 70% degradation is achieved after 50 days.
(2) Modified cellulose acetate is a modified cellulose biodegradable plastic, which has high transparency, high mechanical strength, good toughness and easy molding.
Soil burial biodegradation test with 300 μm cellulose acetate flakes showed a 43% decrease in 49 days, a 63% decrease in 70 days, and 100% disappearance in 140 days.
(3) Starch/synthetic degradable plastic blending
Starch can be blended with a variety of synthetic biodegradable plastics to improve their processability. For example, biodegradable plastics made of starch and polyvinyl alcohol have good fluidity, almost the same mechanical properties as polyethylene, good oil and solvent resistance, good antistatic properties, and good printability and colorability. Its degradation performance: 80% degradation after 20 days of composting test.
3. Polyurethane biodegradable plastics
Pure polyurethane plastics are not biodegradable, but natural polymer compounds containing multiple hydroxyl groups (-OH) can be used as one of the components of polyurethane polyols to make various polyurethane materials. This reduces the amount of polyols used and reduces costs, and gives them biodegradability.
At present, the biodegradable polyurethane plastics that have been developed are: oligosaccharide-derived polyurethane, lignin, tannins and bark-derived polyurethane, cellulose-derived polyurethane, starch-derived polyurethane, etc.
Biodegradable polyurethane plastics inherit the excellent properties of pure polyurethane, especially good physiological compatibility and anti-thrombosis. Therefore, biodegradable polyurethanes can be widely used in the medical field, such as artificial bones, artificial skin, surgical sutures and sustained-release drug materials.
4. Polyesterized amide biodegradable plastics
In the past, biodegradable plastics were mainly focused on aliphatic polyesters, but in recent years, polyesterized amide (PEA) biodegradable plastics have been developed, which are mainly used in agriculture.
PEA has a melting point of 125°C and a tensile strength comparable to PE. Under normal conditions it remains stable, but under the synergistic effect of moisture and humus, PEA can be biodegraded.