Main components of degradable plastics

Degradable plastics refer to plastics that are easily degraded when a certain amount of additives (such as starch, modified starch or other cellulose, photosensitizer, biodegradable agent, etc.) are added to the natural environment. ) is added in the production process. Degradable plastics are generally divided into four categories: photodegradable plastics mixed photosensitizers, and plastics gradually decompose under sunlight. It belongs to an earlier degradable plastic rather than degradable plastic. Its disadvantage is that it is difficult to predict and control the degradation time due to the influence of sunshine and climate change. Biodegradable plastics can be completely decomposed into plastics containing low molecular weight compounds under the action of microorganisms. Its characteristics are convenient storage and transportation, no need to avoid light as long as it is kept dry, and its application range is wide. It can be used not only in agricultural plastic films and packaging bags, but also in the medical field. Photodegradation/biodegradation is a kind of plastic that combines photodegradation with microorganisms. It has the characteristics of photodegradation and microbial degradation of plastics. Water-degradable plastics can be dissolved by adding water-absorbent substances into plastics and throwing them into water after use. Mainly used in medical and health instruments (such as medical gloves), which is convenient for destruction and disinfection. With the development of modern biotechnology, biodegradable plastics have attracted more and more attention and become a new generation of research and development hotspots. All kinds of plastic products have greatly enriched people's lives, but the decomposition of waste plastics in nature is slow, and it takes decades or even hundreds of years to completely decompose, so the degradation and reuse of plastics are in front of all environmental chemists today. Interestingly, however, degradable plastics are not the original intention of scientists to develop plastics. At present, the application scope of degradable plastics that scientists are developing or have successfully developed is still relatively narrow, and it is still unable to replace popular plastics. In this section, degradable plastics are not used as packaging materials for direct contact with food, and the safety of degradable plastics remains to be discussed. Compared with ordinary plastics, the performance of biodegradable plastics is unstable. For example, biodegradable plastics are biologically unstable and will decompose in a microbial environment, leading to the growth of bacteria. Therefore, as a packaging material, degradable plastics have limited application prospects in the field of food packaging. Irreducible plastics have stable chemical properties, and their safety has been proved to be reliable. Recycling is the most effective way to solve white pollution correctly. The use of degradable plastics has four disadvantages: first, it consumes a lot of food; Second, the use of degradable plastic products still cannot completely eliminate "visual pollution"; Third, due to technical reasons, the use of degradable plastic products can not completely solve the "potential harm" to the environment; Fourthly, degradable plastics are difficult to recycle because they contain special additives. Edit the two main types of degradable plastics in this paragraph. Up to now, starch-based degradable plastics mainly include filling, light/biodegradable, * * mixed and all-starch plastics. * filled starch plastic 1973, Griffin obtained the patent of starch surface modified filled plastic for the first time. In 1980s, some countries developed starch-filled biodegradable plastics based on Griffin's patent. Starch-filled plastics, also known as biodegradable plastics, are made by adding a certain amount of starch and other small additives to general plastics, and then processing and molding them. The starch content does not exceed 30%. Filled starch plastics are mature in technology and simple in production process, and can be produced by slightly improving the existing processing equipment, so most of the degradable starch plastics products in China are of this type at present. Natural starch contains a large number of hydroxyl groups, which makes it form strong hydrogen bonds within and between molecules, and the molecular polarity is large, while synthetic resin is hydrophobic and polar. Therefore, natural starch must be surface treated to improve its hydrophobicity and compatibility with polymers. At present, physical modification and chemical modification are mainly used. * Photodegradable/biodegradable plastics are difficult to degrade in some special areas such as drought or lack of soil, and photodegradable plastics cannot be degraded when buried in soil. Therefore, the United States, Japan and other countries have taken the lead in developing a class of photodegradable and biodegradable plastics. Photodegradable plastics are made of photosensitizer, starch, synthetic resin and a small amount of additives (solubilizer, plasticizer, crosslinking agent, etc.). ), wherein the photosensitizer is an organic compound or salt of a transition metal. Its degradation mechanism is that starch is biodegraded, which makes the polymer matrix of degradable plastic materials loose and increases the specific surface area. At the same time, sunlight, heat and oxygen trigger photosensitizers, which leads to polymer chain breakage and molecular weight decrease. * * * * Mixed starch * * * Mixed plastic is made by mixing starch plastic with synthetic resin or other natural polymers * * *, and its main components are starch (30% ~ 60%), synthetic resin with a small amount of PE, ethylene/acrylic acid (EAA)*** polymer, ethylene/vinyl alcohol (EVOH) * * polymer and polyvinyl alcohol. Japan has developed modified starch /EVOH * * * polymer mixed with LDPE***, dimethylsiloxane epoxy modified starch, and then mixed with LDPE***. Mster-Bi plastics from Novamont Italy and NoVon series products from Warner-Lambert USA also belong to this kind of products. MSTER-BI plastic is a polymer alloy formed by a continuous physical cross-linking network of EVOH phase and starch phase. Because both components contain a lot of hydroxyl groups, the product is hydrophilic, and its mechanical properties will decrease after water absorption, but it is insoluble in water. * All-starch starch molecules are deformed and disordered to form thermoplastic starch resin, and a very small amount of plasticizer and other additives are added, which is the so-called all-starch plastic. Among them, the starch content is above 90%, and a small amount of other substances added are non-toxic and can be completely degraded, so the whole starch is a truly completely degradable plastic. Almost all plastic processing methods can be applied to the processing of all starch plastics, but the traditional plastic processing hardly needs water, and the processing of all starch plastics needs a certain amount of water to play a plasticizing role. The water content during processing should be 8% ~ 15%, and the temperature should not be too high to avoid scalding. Sumitomo Corporation of Japan, Wanlerlambert Company of the United States and Ferruzzi Company of Italy claim to have successfully developed all-starch plastics with starch content of 90% ~ 100%. The product can be completely biodegradable within 1 year, leaving no trace and pollution, and can be used to manufacture various containers, films and garbage bags. The Battelle Institute in Germany developed a degradable plastic with improved green pea starch with high linear content, which can be processed and molded by traditional methods. As a substitute for PVC, it can be completely degraded in humid natural environment. Oxidative degradation of plastics is a technology that has not been understood by most people in China. Adding additives to traditional plastic raw materials is the same as adding general masterbatch. After plastic products are discarded, there are two substances in the additives: one is pre-oxidant (mainly some non-toxic metal ions), and the other is biodegradable promoting substance (mainly some natural plant cellulose). Pre-oxidant controls the life and function of plastics when they are not discarded, and reduces the molecular weight through peroxide reaction after being discarded, making the polymer brittle and easy to be decomposed by microorganisms. Biodegradation promoting substances mainly promote the growth of microorganisms. Compared with starch-based plastic technology, this technology is simple, low cost and can be produced by general equipment. According to the relevant verification, the properties of plastics have also been well maintained. Food saved. This method was adopted by WELLS Company in Britain. Simple identification method of common plastics Before using various plastic recycling methods to reuse waste plastics, most plastics need to be classified. Because there are many and miscellaneous channels for plastic consumption, it is difficult to distinguish some consumed plastics by simple appearance. Therefore, it is best to indicate the material variety on plastic products. China has formulated GB/T 16288- 1996, referring to the material variety identification proposed and implemented by SPE. Although the above marking method can be used to facilitate sorting, there are still many unmarked plastic products in China, which brings difficulties to sorting. In order to separate different kinds of plastics for recycling, we must first master the knowledge of identifying different plastics. By observing the appearance of plastics, we can initially identify the main categories of plastic products: thermoplastics, thermosetting plastics or elastomers. Generally speaking, there are two types of thermoplastics: crystalline and amorphous. The appearance of crystalline plastics is translucent, opaque or opaque, and it is transparent only in the film state, with hardness ranging from soft to horny. Amorphous rubber is generally colorless, completely transparent without additives, and its hardness ranges from hard to horny rubber (plasticizers and other additives are often added at this time). Thermosetting plastics usually contain fillers, which are opaque, and are transparent if they do not contain fillers. Elastomer has the feeling of rubber and a certain stretching rate. The heating characteristics of the above three plastics are also different and can be identified by heating. Thermoplastics soften when heated, melt easily and become transparent after melting. They can usually draw filaments from the melt and are usually easy to heat. Thermosetting plastics maintain their original hardness and stable size until the material is chemically decomposed and carbonized at the decomposition temperature. The elastomer is heated and does not flow until the chemical decomposition temperature, and the material is decomposed and carbonized to the decomposition temperature. See Table /C for softening or melting temperature range of common thermoplastics/Oc Polyvinyl Acetate 35~ 85 POM/KOOC-0/65 ~/KOOC-0/85 Polystyrene 70 ~/KOOC-0//KOOC-0/5 Polypropylene/KOOC-0/60 ~/KOOC-0/. Cm3 1 10 nylon11180 ~190 density 0.94/ cm3 about 120 polytrifluoride 200~220 density 0.96/ Cm3 about. 0~ 220 poly 65438 126~ 160 poly -4- methylpentene-1 240 cellulose acetate 125~ 175 nylon 66 250~260 polyacrylonitrile/kloc-0.