The essence of biodegradation is that enzymes induce the oxidation and hydrolysis of plastics, resulting in the breakage of the main chain and the reduction of molecular weight, thus losing their original mechanical properties and making them more accessible to microbial uptake.
Therefore, based on the characteristics of biodegradable plastics and the elements of biodegradable plastics formation, the raw material sources and research mechanisms of biodegradable plastics are dissected for their different degradation technologies.
1.Natural-based biodegradable plastics
Natural-based biodegradable plastics are biodegradable plastics prepared from natural polymers. Natural polymers existing in plants and animals can be used as biodegradable materials after certain processing, including cellulose, lignin, starch and polysaccharide from plant sources, and chitin and gelatin from animal sources.
These natural polymer materials are abundant and inexpensive, plus they are easily degraded by microorganisms, and the products are safe and non-toxic, so they are increasingly valued. However, compared with petroleum-based polymers, the mechanical, thermal and processing properties of natural-based biodegradable plastics are poor, which cannot meet the requirements of material diversification and functionalization at this stage.
At present, natural polymer substances are mainly modified by certain processing means to meet the various performance requirements as engineering materials.
The key to develop natural-based biodegradable plastics lies in improving the modification technology and controlling the cost. Although natural polymer materials have excellent and completely biodegradable characteristics, its high temperature resistance is not good. Biodegradable plastics prepared from natural polymers are generally deformed at 50~55℃.
In addition, its tensile resistance and flexibility are also poor. Therefore, it must be chemically and physically modified, and a perfect process and special equipment centered on modification must be developed to obtain natural polymer degradable plastics with wide use value.
For example, the application of nano-modification technology to compound natural polymers with nano-additives to produce food packaging materials, can effectively solve the problem of poor mechanical strength and barrier properties of natural polymer materials; and nanomaterials can also give the new composite materials more functional properties.
2.Bio-based biodegradable plastics
Bio-based biodegradable plastics are biodegradable plastics made on the basis of natural polymers or polymers synthesized from natural monomers. These plastics are mainly aliphatic polyesters, including polyhydroxy fatty acid esters (PHA) [such as poly(β-hydroxybutyrate) (PHB), poly(hydroxyvalerate) (PHV), and copolymers of PHB and PHV (PHBV), etc.], poly(lactic acid) (PLA), poly(butylene succinate) (PBS), poly(ethylene glycol furandicarboxylate) (PEF), poly(propylene terephthalate) ( polyethyleneterephthalate (PTT), polycarbonate (PC), etc.
In addition, there are also polyamino acids and other CO2-based biodegradable materials. PLA is widely used in biomedical polymer materials, textiles, packaging, agricultural films, etc.
Using renewable natural biomass resources such as starch, polymers are directly synthesized through microbial fermentation, such as polyhydroxy fatty acid esters (PHA, including PHB, PHBV, etc.); or monomers such as lactic acid are produced through microbial fermentation, and then polymers are chemically synthesized, such as polylactic acid (PLA).
3.Petroleum-based biodegradable plastics
Petroleum-based biodegradable plastics are biodegradable plastics made by polymerization of monomers produced in the petrochemical industry chain. PCL has excellent biocompatibility, memory and biodegradability, which are mainly used in medical and daily use, such as orthotics, sutures, bandages, degradable plastics, etc. PBS has good heat resistance and is the best biodegradable plastic material, which can be used as various packaging materials and plastic products for environmental protection.
4.Co-hybrid biodegradable plastics
Blended biodegradable plastics are biodegradable materials prepared by blending and compounding two or more polymers (at least one of which is biodegradable), generally using natural polymers such as starch, cellulose and lignin as biodegradable components, among which starch is more commonly used. Blended biodegradable plastics can combine the excellent characteristics of each component, so that the material can be biodegradable and improve its performance in mechanical and thermal aspects.