As the name implies, degradable plastics are plastics that can be degraded. Under certain conditions, they can be completely converted into carbon dioxide, water and other small-molecule biomass, and will not remain in the environment and cause white pollution.
So why can degradable plastics be degraded, and what is the principle of its degradation?
Take the simplest plastic bag as an example, we can see its product description, such as "photodegradation". The degradation method of photodegradable plastic bags is to add a photosensitizer to polyethylene, the main component of the plastic bag. After being exposed to light, the energy absorbed by the photosensitizer is transferred to polymer molecules, causing the plastic film to rupture and disintegrate, and finally turned into plastic fragments, visually eliminating white pollution.
But the problem with photodegradable plastics is that after the plastic products are broken, the basic composition has not changed, but the volume has changed from large to small. Therefore, some people propose to change the name of degradation in "photodegradation" to "disintegration", and strictly distinguish between photodegradation and complete degradation in the definition. At the same time, some scientists have proposed that the "incomplete degradation" of plastics may cause more serious microplastics problems. The disintegrated plastic particles may enter the interior of animals and human bodies, posing huge safety risks. At this time, a new technology appeared in front of people, namely biodegradable plastics.
Biodegradable plastics make use of microorganisms that are abundant in nature, which are digested and absorbed by microorganisms and converted into carbon dioxide, water and other biomass, and the degradation is more thorough. So, why do microorganisms only prefer biodegradable plastics, but for traditional plastics, they linger?
This must start from the structure of biodegradable plastics. Traditional plastics, such as polyethylene, the main component of plastic bags, are polymers of ethylene monomers, with high molecular weights and close connections between molecular bonds, making it difficult for microorganisms to digest and absorb. Biodegradable plastics often contain heteroatoms that are more preferred by microorganisms such as N and O, which can be digested and absorbed by microorganisms quickly. In addition, biodegradable plastics mostly contain ester bond structures, which are prone to hydrolysis, which also accelerates the overall degradation rate of the plastic.
We can divide the degradation of biodegradable plastics into two processes:
The hydrolysis process of polymer chains into small molecular fragments. Degraded plastics change from large plastic films to small plastic fragments in physical form under various actions such as microorganisms and natural environment, which is more conducive to the attachment of microorganisms. At the same time, the long-chain structure of degraded plastics becomes short-chain, which is easier for microorganisms to digest. Changes in physical form can significantly accelerate the degradation rate of degradable plastics. If a scientist conducts an experiment on degrading enzymes in cattle stomachs, adding a whole piece of degradable plastic film and powdered plastic particles to the enzyme solution, and comparing them, it turns out that the degradation rate of degradable plastic in powder form is faster. The latest research: beef gastric juice is too good! It can degrade plastics such as PET and PBAT. Seawater-degradable plastics that are currently being studied are polycondensed into new polymers by adding easily hydrolyzable fragments such as glycolic acid to the molecular chain. Among them, the hydrolysis of easily hydrolyzed fragments can effectively accelerate the transformation process of degraded plastics from macromolecules to small molecules, and ultimately accelerate the overall degradation rate of plastics.
For the degradation of degradable plastics, the process of turning large films into small fragments and polymer chains into small molecular fragments is the decisive step in the overall degradation rate of degradable plastics. How to speed up the hydrolysis process is the key to improving the degradation rate of degradable plastics.
The degradation process of small molecules under the action of microorganisms. After being hydrolyzed, microorganisms can better "eat" these small degrading plastic molecules. Microbes secrete specific degrading enzymes, and after digestion, small molecular fragments are absorbed by the microbes and converted into carbon dioxide, water, and biomass used to synthesize the microbes themselves. At this time, the degradable plastic has completed its entire cycle in nature, from petroleum or plants to molecular monomers, and then polymerized from monomer materials into degradable plastics and processed into various plastic products.
