Degradable materials refer to a class of materials that can meet the requirements of application during use and storage, and can change chemical structure in a short period of time under certain environmental conditions and lose material use performance. Degradable materials are classified into common degradation materials and biodegradable materials. Biodegradable material refers to a degradation material that can be converted into carbon dioxide and water within 180 days by the action of microorganisms under composting conditions. Not all degradation materials are biodegradable materials.

Such as photo-oxidative degradation plastics, which are not real biodegradable plastics, but this photodegradable material is the first to popularize the concept of degradation, probably in the 1960s and 1970s. Photooxidation plastics, which are ordinary petroleum-based plastics, add some additives that can cause the breakage of petroleum-based plastic molecular chains by the action of sunlight, so they can become plastic fragments under the action of light. This degradation has relatively large defects: the first photodegradation does not really achieve degradation, and the second decomposes into plastic fragments, which have a very serious impact on plants and microorganisms, and also produce microplastics.

This type of destructive biodegradable plastic is a natural appearance of natural substances such as starch, wheat flour, rice flour or bamboo fiber, which is added to conventional plastics to achieve so-called biodegradation. Although natural materials can degrade, the rest of the plastic is still plastic. Therefore, such destructive bioplastics are not completely biodegradable plastics. Some people say that it is partially degraded, but partial degradation is the real harm, because the so-called partial degradation refers to the degradation of natural materials, so that the remaining petroleum-based plastics are destroyed, but the recovery value and use value are lost. It gradually ages in nature and becomes plastic. Its hazard is similar to the photodegradable plastics. It causes traditional petroleum-based plastics to become debris and remains in the soil, causing soil compaction, microbial death, soil erosion and so on. This material has a more terrible place because it looks environmentally friendly and even smells environmentally friendly. For example, some starch-based bioplastics smell even starchy, very comfortable, which makes people mistakenly think that this is really environmentally friendly to use, but it is not.

There is a material that is truly biodegradable, called compostable biodegradable materials. the real biodegradable material must be compostable. Those that are degraded by the sun, the sun, and the rain are not really biodegradable materials. By definition, the real degradable material must be a material that is completely converted to carbon dioxide and water within 180 days by microbial action under composting conditions. This 180 days is artificially prescribed. Why do you have to do it for half a year? Because it needs to be processed together with the kitchen waste. It is difficult to handle three hundred days and five hundred days, so people make this rule. In fact, depending on the thickness of the material, the degradation time of compost will be different. If it is a plastic film, it can be completely degraded in 30 or 50 days. If it is made into a relatively thick injection molding product, it may take 180 days or even not enough.

Compostable biodegradable materials fall into two categories, one being bio-based compostable materials and the other being petroleum-based compostable materials. The bio-based compostable material is further divided into materials directly derived from plants, such as pulp, polylactic acid (PLA), aliphatic polyester (PHA), and blends of natural materials with the above two types of polymers. Petroleum-based compostable materials derived from petroleum, such as polybutylene terephthalate (PBAT), polyethylene glycol (PCL) etc.

Compostable biodegradable materials also have drawbacks because they are dependent on composting facilities. The most suitable conditions are 60℃ (40℃ - 80℃ can be), 90% humidity conditions can be quickly degraded. However, in the natural environment, although it can still be completely degraded, it will take at least several years. The degradation time needs to be longer under conditions of low temperature, drought or microbial inactivity.

Polylactic acid (PLA) is a polymer obtained by polymerizing lactic acid as a raw material, it takes annually renewable resources–corn, cassava and other plants as raw material. After fermentation by microorganisms, lactic acid will be extracted, then PLA will be produced through a process of refining, dehydration polymerization, high temperature pyrolysis and final polymerization. PLA has excellent biodegradable property. After disposal, within one year, it can be degraded into carbon dioxide and water by microorganisms in the soil, and do no harm to our environment. Polylactic acid products are 100% biodegradable in the composting environment after being discarded, and realize the ecological carbon cycle originating from nature and returning to nature. Therefore, it is an ideal green polymer material.

Polylactic acid is a relatively mature and widely used fully biodegradable compostable plastic. In addition to the biodegradable properties, this material has several different features.

One of the important characteristics is that polylactic acid is a safer material for food contact. Its monomer is lactic acid, lactic acid is a food additive, and lactic acid is present in yogurt, cosmetics or condiments. Its own level of safety is safer than that made with petroleum monomers. Moreover, polylactic acid was used as a medical material in the 1960s and 1970s for surgical sutures, fixation of bone rods, etc., and later, because of technological breakthroughs and reduced costs, it entered the ranks of civilian use.

Second, polylactic acid is a biobased material derived from plants. The more commonly used raw materials are corn, cassava and sucrose. The corn, cassava and other crops are extracted into starch, fermented into lactic acid, and then chemically polymerized into polylactic acid, which is a relatively clear material for bio-based materials.

Again, the polylactic acid material is low in carbonity. The so-called low carbon refers to the concept of relatively low carbon dioxide consumption or release. For example, a material, in the whole life cycle from its production to demise, from monomer, polymerization, transportation, storage to use, the total amount of carbon dioxide absorbed and emitted is combined, by comparison, if it is carbon dioxide in plastic materials. The relatively low total emissions, we call it low carbon materials.

Taking polylactic acid as an example, from the start of corn or cassava cultivation, to the process of glucose sugar extraction, lactic acid fermentation, polylactic acid polymerization, and processing of polylactic acid products, the total amount of carbon dioxide emissions is one kilogram of polylactic acid film. The amount of carbon dioxide released is about 1.274 kg. To make a comparison, make a kilogram of PET material mineral water bottle, which will emit 4 kg of carbon dioxide. A kilogram of PP material shampoo bottle is produced to discharge 2.5 kg.

Finally, the degradation composting characteristics of polylactic acid materials. In a composting environment, it can degrade 100% of carbon dioxide and water without polluting the environment. In the lab, we have some clear data, such as 0.3 mm thick polylactic acid tablets, which are invisible to the naked eye after six weeks. Of course, it does not mean that it is not equal to degradation. It is not completely degradable in six weeks and takes time.

In fact, in addition to composting, compostable materials have other treatments, such as burning. Compared with plastics, the products of complete combustion of polylactic acid are carbon dioxide and water, which are not polluted. It can also be landfilled. Although the degradation rate is relatively slow, it does not pollute the groundwater at least, does not damage the plant growth, and does not waste farmland. Of course, it can be recycled, but the cost of recycling is relatively high.

If we really have nothing to eat, it is inevitable to ensure food security in advance. It is impossible to do polymer materials first. But from an objective point of view, this worry is superfluous. I will share several sets of data for you.

Global corn production is 1 billion tons / year, 80% of which is used for feed and fuel, fuel ethanol is burned about 50%, feed is about 30%. It is only about 10% used in food, about 100 million tons. Assume that the PLA production is 2 million tons per year, which is 6 million tons of corn, accounting for 0.6%. Our food production is over-supplied, and there are still 200 million tons of grain in China. In addition to corn, other crops, such as cassava, have an annual global production of 300 million tons, 60% of which is used in industry, not human food. These sets of data are sufficient to demonstrate that the use of plant starch to produce polylactic acid materials does not affect the food industry.