Why are biodegradable plastic bags “short-lived”?
“I only bought a reusable bag half a year ago, and it breaks into pieces when I pull it, like a cracker.” Recently, many netizens posted photos of their hoarded biodegradable plastic bags being “shattered to pieces” on social platforms, questioning the quality of the product. Some netizens said that such bags become more brittle the longer they are stored, and their service life is getting shorter and shorter.
It is obviously an "environmental pioneer" that is used to replace traditional plastics, but why is it "sacrificially sacrificed" in a drawer or car trunk before it is even "on duty"? Does it really get crispier and crispier the longer you put it on? A reporter from Science and Technology Daily interviewed Liu Shuang, an associate researcher at Shanxi Functional Food Research Institute of Shanxi Agricultural University.
Degradation mission determines vulnerable genes
To understand why biodegradable plastic bags are "short-lived", we must first understand the difference between them and ordinary plastic bags. According to "Biodegradable Plastic Shopping Bags" (GB/T 38082-2019) and "Definition, Classification, Labeling and Degradation Performance Requirements of Degradable Plastics" (GB/T 20197-2006), the main raw materials of biodegradable plastic bags are polylactic acid (PLA) and polybutylene adipate/terephthalate (PBAT). PLA is mostly extracted from plant starches such as corn and cassava, and is responsible for providing hardness and crispness; PBAT gives the material flexibility and elasticity similar to traditional polyethylene. Mixing the two in different proportions can adjust the texture from hard and crisp to soft. The main component of ordinary plastic bags is polyethylene, whose molecular structure is composed of stable carbon-carbon bonds and is extremely chemically inert. Few microorganisms in nature can "gnaw" it, so it can persist in the natural environment for hundreds of years.
The fragility of biodegradable plastic bags is the chemical price they pay for their degradable functionality.
Liu Shuang introduced that the molecular chain of biodegradable plastic bags contains a large number of ester bonds. The chemical activity of ester bonds is high, and its bond energy is lower than the carbon-carbon bonds of polyethylene. It is easily broken by external factors such as water, heat, and light. This process is called the aging of polymer materials. During the aging process, the molecular weight of polymers such as PLA and PBAT decreases significantly, and the mechanical properties of biodegradable plastic bags deteriorate, eventually manifesting as embrittlement, cracking and loss of strength.
"These performance attenuations cause netizens to think that it becomes more brittle as it is placed, but this is not a quality defect of the product." Liu Shuang said that in order for biodegradable plastic bags to be decomposed, the molecular chains of their raw materials must be designed to be relatively easy to break. Manufacturers will also design different plastic bags by adjusting the ratio of PLA and PBAT.
What needs to be corrected is that many consumers think that biodegradable plastic bags have begun to degrade when they break into pieces. This is actually a misunderstanding. Liu Shuang said that physical fragmentation in the home environment is only material aging and brittle fracture, and is by no means true biodegradation.
According to national standards, biodegradation refers to the process in which materials achieve a biodegradation rate of more than 90% within 180 days in an industrial composting facility under the action of constant temperature, high humidity and specific microbial flora.
"If biodegradable plastic bags are randomly discarded on the roadside or in ordinary trash cans, even if they are broken into tiny pieces, due to the lack of high temperature, high humidity and professional flora, these fragments may still remain in the soil or water for a long time." Liu Shuang said, therefore, fragmentation does not mean degradation, let alone harmless. Only if it is collected correctly and sent to an industrial composting facility can a true biodegradation cycle be completed.
Learning to store scientifically is the key
So, how to save biodegradable plastic bags?
The first thing is to try to use it as you buy it, and avoid purchasing large quantities at one time and storing it at home. If there is a need for a small amount of backup, the daily storage environment must be cool, dry, and protected from light. It is best to keep the temperature below 25 degrees Celsius and away from heat sources such as heaters and stoves. Keep the relative humidity below 60% and avoid placing it under the kitchen sink or in wet corners such as bathrooms. It is also necessary to absolutely avoid direct sunlight, especially do not leave it in the trunk of a car or a closed balcony for a long time, because the temperature inside the car in summer can exceed 70 degrees Celsius, and high temperatures will drastically accelerate the aging of the material. In addition, pay attention to rodent and insect prevention, because some biodegradable plastic bags with added starch may attract rodents to eat.
Regarding the shelf life, Liu Shuang reminded consumers to pay attention to the new national standard that will be implemented soon. The current national standards stipulate that the shelf life of biodegradable plastic bags is not less than one year, but the new national standards will no longer unify the rigid regulations, but require manufacturers to mark the recommended storage period on the packaging based on the aging test data of their own products.
"This means that the shelf life of biodegradable plastic bags of different brands and different formulas may range from several months to two years. Consumers should read the product packaging carefully when purchasing and use it according to the marked time." Liu Shuang said.
In terms of authenticity identification, methods such as "rubbing and listening to see if the sound is crisp" are circulated on the Internet. In this regard, Liu Shuang said that these methods are not very reliable because adding spices or changing the process can affect the sensory characteristics of the product. The most authoritative way is to check the product packaging label: regular biodegradable plastic bags must be marked with implementation standards, main material components, and the "double J" mark that represents industrial compostability.
(Science and Technology Daily reporter Han Rong)
