Plastic bags that can be degraded by the sea are all liars?

Plastic waste mainly comes from land applications, such as consumer goods packaging made of high durability polyolefin. These traditional plastics have ideal properties, including high chemical stability, moisture resistance and thermoplastic properties. Unfortunately, when they reach the marine environment, they split into microplastics that are not bioabsorbable.

In order to minimize the adverse effects of polyolefin packaging on marine ecosystems, it is considered that these materials should be replaced by marine degradable polymers. The ideal method is to develop flexible films with the characteristics of polyolefin materials, which can be biodegraded rapidly if they enter the marine environment. At present, many reports claim that biodegradable plastics have been developed in the marine environment, and their product stability and moistureproof properties are similar to those of polyolefins.

But is that really the case?

What is the degradation performance of biodegradable plastics in marine environment?

A packaging material manufacturer in Mitsubishi Chemical and Nagoya, Japan.

A kind of plastic bag that can be degraded in sea water was jointly developed.

Unfortunately, after extensive research and development and commercialization of biodegradable plastics, there are still many limitations.

For example, some biodegradable plastics are sensitive to water, which is a key obstacle to their application to liquids and some dry products, such as emulsions and coffee; in addition, these materials must maintain a balance between stability and biodegradability. in order to maximize the shelf life and quality of closed products, researchers have not been able to completely eliminate this technical problem. In order to achieve high-speed manufacturing and reduce production costs, these materials must be compatible with thermoforming processes. Finally, if these materials reach the marine environment, they must be able to quickly degrade into biosafety materials.

However, the degree of degradation and mineralization of biodegradable plastics also depends on temperature, relative humidity, oxygen level, number and diversity of microorganisms. The role of these environmental factors is often ignored because people mistakenly think that biodegradable plastics can be degraded rapidly in any natural environment.

In fact, the degradation tests of biodegradable plastics are mostly carried out at high temperature (more than 30 ℃), aerobic conditions and the presence of a variety of microorganisms. However, it is difficult for the marine environment to achieve this degradation condition.

Experiments show that even in areas where marine life is highly enriched, the degradation rate of biodegradable plastics is slower than that in terrestrial or managed environments. In other words, you can't expect biodegradable plastics to degrade as quickly in seawater as they do in a perfect experimental environment.

The appearance of seawater degradable plastic products is not significantly different from that of ordinary plastics.

Seawater degradable plastic samples soaked in seawater for 3 months (part I)

Comparison with well-preserved samples (below)

In addition to biodegradable plastics, the emergence of a soluble plastic film has also attracted people's attention. The main component of the soluble plastic film is polyvinyl alcohol (PVA), a polymer that is both soluble in water and biodegradable-that is, when it is thrown into the sea or moist soil, it can be dissolved and eventually biodegraded.

In 2018, the team of Cui Yuefei, a senior engineer at South China University of Technology, demonstrated the magic of water-soluble plastic bags: Cui Yuefei put it in a glass of water and put it into a plastic bag. After a few minutes, the plastic bag dissolves in water and disappears. Coincidentally, in order to prove the safety of water-soluble plastic bags at a news conference in Santiago, Chile, the demonstrator drank a glass of water dissolved in plastic bags, and the video became a sensation all over the world.

But in addition to the eye-catching demonstration, Cui Yuefei responded that this argument is unscientific and unrigorous, and even if water-soluble plastic bags are non-toxic and harmless, they are not recommended to eat and drink, because they still exist in the water in the form of macromolecules after they are dissolved in water.

In fact, soluble plastic film has the same limitations as biodegradable plastics: at what temperature and how long it takes to dissolve completely, it is not only related to the degree of polymerization and alcoholysis of polyvinyl alcohol, but also depends on the specific composition of the soluble film. Therefore, this kind of material can not be generalized as "rapidly soluble in water".

The evaluation standard of marine biodegradation is worth studying.

So, who will check whether the seawater biodegradable performance of biodegradable plastics is up to standard? At present, the International Organization for Standardization (ISO), the Organization for Economic Cooperation and Development (OECD), the American Society for testing and Materials (ASTM) and TUV of Austria have released laboratory methods for quantifying physical and enzymatic degradation in the aquatic environment, which are used to detect the degradation rate of plastics claimed to be degradable in seawater and the composition of substances differentiated after degradation.

However, many scientists and engineers believe that the scope of these test standards is limited and can not effectively replicate the dynamic conditions and ecosystems of the ocean. They believe that the standard method is to effectively biodegrade the sample based on an environmental data point by exposing the sample to a temperature, oxygen level and a microbial complex. In order to increase the credibility of the results of marine biodegradation, samples should be tested at various temperatures, oxygen levels and the presence of multiple microbial populations.

The stability of the material is poor and the production is difficult.

Traditional multi-layer structural plastics are usually formed by laminating films with sealants or adhesives. Sealants can also be used as compatibilizers to connect hydrophobic and low surface energy materials. However, sealants can affect the marine biodegradability of these multilayer materials.

In addition, unlike polyolefins, aliphatic polyesters and starches are particularly prone to thermal degradation and pollution in the manufacturing process. For example, on packaging converters, these polymers experience high temperatures due to friction and are likely to degrade incorrectly. Even if there is only a small amount of water vapor in the production process, these plastics will begin to hydrolyze, resulting in poor performance and even packaging failure. These materials may also be contaminated by exposure to microbial populations during transportation or storage.

Another assessment method is to test it in the marine field environment. Field testing in the marine environment will expose the sample to complex factors, synergies and competitive reactions, thus providing a more accurate study of the degradation of polymers in the ocean. Unlike laboratory studies, marine field tests include biotic and abiotic factors, such as the mechanical effects of microorganisms, light, waves and seasonal temperature fluctuations.

However, field testing can not measure the evolution of carbon oxides during immersion of polymer samples. Therefore, it is uncertain whether the sample can be mineralized into benign by-products or broken into microplastics. Another challenge is that these field tests are often very expensive.

Biodegradation does not mean zero pollution

Beijing Institute of Petrochemical Technology and Natural Resources Conservation Association jointly released a report entitled "Environmental footprint Assessment of Plastics in China" in Decemb