Biodegradable plastics have long been seen as a solution to the plastic pollution problem, but the reality is not as good as expected. Most so-called "compostable" plastics are made of polylactic acid (PLA), which cannot be completely decomposed during ordinary composting.
There are two completely different opinions on the degradation time of biodegradable plastic bags on the market: half a year and ten years. Which one is more accurate? This article will explore this issue and help everyone uncover the degradation time of biodegradable plastic bags.
In this article, we will explore the degradation time of PLA, the conditions required for it to break down, and what it means for both consumers and industries. We’ll also look at the factors that influence the rate of PLA breakdown, and how to handle PLA waste correctly.
Principle of Degradation of Polylactic Acid
The degradation of PLA is caused by the cleavage of ester bonds.The ester bonds in polylactic acid (PLA) are easily hydrolyzed and can be degraded into lactic acid by microorganisms in the body or soil. The final metabolic products are water and carbon dioxide, so it will not cause toxic side effects to the human body and is very safe to use.
Under the action of microbial activity (with the participation of enzymes), the enzyme enters the active position of the polymer and penetrates into the action point of the polymer, causing the polymer to undergo hydrolysis reaction. The polymer macromolecular skeleton structure gradually breaks into small chain segments, and finally breaks into stable small molecular products, thus completing the biodegradation process.
Degradation Process of Polylactic Acid (PLA)
1. In vivo degradation
The hydrolysis of PLA is a complex process. The main body erosion includes four phenomena: water absorption, ester bond breaking, diffusion of soluble polymers and decomposition of fragments.
Water absorption of the material occurs: the aqueous medium penetrates into the polymer matrix, causing the polymer molecular chain to relax, the ester bond to begin initial hydrolysis, the molecular weight to decrease, and gradually degrade into oligomers.
The terminal carboxyl groups of polylactic acid (produced by polymerization introduction and degradation) catalyze its hydrolysis. As the degradation proceeds, the amount of terminal carboxyl groups increases, the degradation rate accelerates, and thus a self-catalytic phenomenon occurs.
There will be more and more carboxyl groups inside the material to accelerate the degradation of the internal material, further increasing the difference between the inside and the outside. When the internal material is completely transformed into a soluble polymer and dissolved in an aqueous medium, a hollow structure with a surface composed of polymers that are not completely degraded will be formed. Further degradation causes the oligomers to hydrolyze into small molecules and finally dissolve in an aqueous medium.
The entire dissolution process is the transformation of a water-insoluble solid into a water-soluble substance.
The overall structure of the material is destroyed, the volume becomes smaller, and it gradually becomes fragments, and finally completely dissolves and is absorbed by the human body or excreted from the body.
2. In vitro degradation
After decomposition by hydrolysis reaction, it is decomposed by microorganisms. In the natural environment, hydrolysis occurs first, and the unstable ester bonds on the main chain are hydrolyzed into oligomers. Then, microorganisms enter the tissue and decompose it into carbon dioxide and water.
Under composting conditions (high temperature and high humidity), the hydrolysis reaction can be easily completed and the decomposition rate is also fast.
In an environment where hydrolysis reaction is not easy to occur, the decomposition process is gradual. Microorganisms are ubiquitous in nature, and polylactic acid can be degraded by a variety of microorganisms. Such as sickle enzyme Candida, Penicillium, humus, etc. Different bacteria have different degradation conditions for polylactic acid of different configurations.
The results show that sickle enzyme Candida and Penicillium can completely absorb D, L lactic acid, and some can also absorb soluble polylactic acid oligomers.
How Long Does It Take for PLA to Biodegrade
The time for complete biodegradation of polylactic acid is between 90-180 days under industrial composting at 50-60℃. The degradation rate is also affected by factors such as temperature, humidity, and microorganisms. The truly degradable material must be under composting conditions. Through the action of microorganisms, such as using specific enzymes and appropriate temperatures, the degradation rate of polylactic acid will be faster, and sometimes it may take several days.
The degradation time of biodegradable bags is not as short as possible. Too fast a degradation rate may cause the bags to break during use, thus affecting their normal use.
Compostable biodegradable materials have defects and rely on composting facilities. The most suitable conditions are 60℃ (40℃-80℃ is OK) and 90% humidity to degrade rapidly. But in the natural environment, although it can still be completely degraded in the end, it will take at least several years.
Under conditions of low temperature, drought or inactive microorganisms, the degradation time will take longer. For example, in the Arctic, I estimate that it will be there even after being buried for decades or even 500 years.
Factors Affecting the Degradation of PLA Plastic Biodegradable
Molecular structure factors
1. Relative molecular weight
The higher the relative molecular weight of PLA, the slower its degradation rate. PLA with a large molecular weight takes longer to degrade into small molecules;
2. Crystallinity of polylactic acid
PLA materials with higher crystallinity degrade more slowly because the crystalline structure makes it difficult for microorganisms to penetrate and decompose.
3. Stereotacticity:
The degradation rates of PLAs with different stereotacticities, such as PDLLA, P(LDL)A, and PDLA(PLLA), show an upward trend.
Material thickness
Thinner PLA materials degrade faster, while thicker materials degrade slower.
Environmental factors
Conditions such as temperature, humidity, and light can affect the growth and activity of microorganisms.
The temperature is proportional to the degradation reaction of PLA. The increase in temperature accelerates the vibration frequency of the carbonyl group. The higher the temperature, the stronger the intermolecular forces, the stronger the mobility, the more intense the thermal motion, and the faster the degradation reaction rate.
Both acid and alkalinity can promote the hydrolysis reaction of esters. Compared with acid, PLA degrades faster in alkaline environments.
In warm and humid environments, microbial activity is high and PLA degrades faster; in dry and cold environments, microbial activity is reduced and PLA degrades slower.
In certain environments, such as ecosystems such as oceans or soil, the degradation process of PLA may be subject to certain restrictions, and its impact on the environment should be carefully considered when using it.
Types and quantities of microorganisms
Different microorganisms have different abilities to degrade polylactic acid, and some microorganisms are more effective at degrading polylactic acid. In an environment rich in suitable microorganisms, polylactic acid will degrade faster.
FAQ
Can polylactic acid be completely degraded?
PLA is an environmentally friendly biodegradable material. Its degradation effect is affected by the environment and requires a specific temperature, humidity and microbial environment. When PLA products on the market are degraded, they need to be equipped with complete recycling and industrial composting facilities to give full play to their environmental advantages.
Why PDLA and PLLA degrade slowly
The hydrolysis process requires hydrolysis of the amorphous region and hydrolysis of the crystalline region, so the hydrolysis rate is lower than that of PDLLA. PDLLA degrades faster than PDLA and PLLA because its methyl group is in a syndiotactic or atactic state with a faster absorption rate of water.
Is PLA really biodegradable?
Yes, but only under specific composting conditions. In natural environments, degradation is slow.
Can I compost PLA at home?
Most PLA products do not degrade in home composting setups.
Does PLA break down in landfills?
No. Landfills lack the oxygen and temperature needed for PLA to biodegrade.
What’s the best way to dispose of PLA?
Use industrial composting if available. Otherwise, reduce PLA use or seek certified collection services.
Is PLA food safe for the human body?
Polylactic acid has excellent biocompatibility, and its degradation product, L-lactic acid, can participate in human metabolism. It has been approved by the US Food and Drug Administration (FDA) and can be used as medical surgical sutures, injection capsules, microspheres and implants, etc.
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Biocompatibility: PLA is widely used in the medical field (such as surgical sutures, bone screws, etc.) because it can slowly degrade into lactic acid in the body for months to years, and is eventually excreted through metabolism, and generally does not cause toxic reactions. A small number of patients may experience local inflammation or rejection reactions, and strict postoperative monitoring is required.
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Food contact grade applications: PLA products that meet the standards (such as tableware and packaging) are stable at room temperature, without the migration of harmful substances, and are suitable for short-term contact with food. PLA has limited heat resistance (usually below 60°C). High temperatures may accelerate degradation and release trace amounts of lactic acid or oligomers. Long-term intake may cause gastrointestinal irritation to sensitive people.
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