Degradable plastics are generally divided into four categories: Plastics that can be completely decomposed into low molecular compounds under the action of microorganisms. Its characteristics are that it is easy to store and transport. As long as it is kept dry, it does not need to be protected from light. It has a wide range of applications. It can not only be used in agricultural mulch films and packaging bags, but also widely used in the medical field. With the development of modern biotechnology, biodegradable plastics have received more and more attention and have become a new generation hot spot in research and development.
PHA degradable plastic has the best performance among biodegradable plastics. At the same time, due to its high cost and complicated production process, it is still in the initial stage of the market. In 2010, the global PHA production capacity was less than 80,000 tons, of which Metabolix Company in the United States had a production capacity of approximately 50,000 tons, accounting for more than 60% of the market. Chinese companies are also ahead in the production technology and research and development of PHA. Tianjin Guoyun Biomaterials Co., Ltd. has a PHA production capacity of 10,000 tons, Ningbo Tianan has a production capacity of 2,000 tons, and Shenzhen Yikeman Biotechnology Co., Ltd. has a production capacity of 5,000 tons. tons of production capacity. Japan's Kaneka Company and Brazil's PHBIndustrial Company are also typical representatives of the PHA industry. These companies are promoters of the PHA industry. Although the application of PHA is relatively limited, the actual sales volume of Metabolix per year does not exceed 100 tons, but with the With the gradual expansion of downstream applications in the future, especially the further maturity of applications in film packaging, agricultural film, edible tableware, non-woven fabrics and other industries, PHA has huge market potential. Adding water-absorbent substances to plastics can dissolve them when discarded in water after use. They are mainly used in medical and sanitary appliances (such as medical gloves) to facilitate destruction and disinfection.
Starch-based plastics
So far, starch-based degradable plastics mainly include four categories: filled type, photo/biodegradable type, organic hybrid type and all-starch plastic.
1. Filled starch plastics. In 1973, Griffin obtained the first patent for starch surface-modified filled plastics. By the 1980s, some countries had developed starch-filled biodegradable plastics based on Griffin's patent. Filled starch plastic is also called biodestructive plastic. Its manufacturing process is to add a certain amount of starch and other small amounts of additives to general plastics, and then process them into shapes. The starch content does not exceed 30%. Filled starch plastic technology is mature, the production process is simple, and it can be produced with slight improvements to existing processing equipment. Therefore, most domestic degradable starch plastic products are currently of this type.
Natural starch molecules contain a large number of hydroxyl groups, which form extremely strong hydrogen bonds within and between molecules. The molecules are highly polar, while synthetic resins are less polar and are hydrophobic substances. Therefore, natural starch must be surface treated to improve hydrophobicity and compatibility with polymers. There are two main methods: physical modification and chemical modification.
2. Photo/biological dual-degradable biodegradable plastics are difficult to degrade in some special areas such as drought or lack of soil, and photodegradable plastics cannot degrade when buried in the soil. For this reason, the United States and Japan and other countries have taken the lead in developing a type of photo/biodegradable plastic that is both photodegradable and biodegradable. Photo/biodegradable plastics are made of photosensitizer, starch, synthetic resin and a small amount of additives (solubilizer, plasticizer, cross-linking agent, coupling agent, etc.), where the photosensitizer is an organic compound or salt of a transition metal. The degradation mechanism is that starch is biodegraded, which loosens the polymer matrix and increases the specific surface area. At the same time, sunlight, heat, oxygen, etc. trigger the photosensitizer, causing the polymer chain to break and the molecular weight to decrease.
3. Mixed type starch mixed plastic is a starch plastic made by mixing starch with synthetic resin or other natural polymers. The main component is starch (30% ~ 60% ), a small amount of PE synthetic resin, ethylene/acrylic acid (EAA) polymer, ethylene/vinyl alcohol (EVOH) polymer, polyvinyl alcohol (PVA), cellulose, lignin, etc., its characteristics It has high starch content and some products can be completely degraded.
Japan has developed modified starch/EVOH polymer mixed with LDPE ***, dimethylsiloxane epoxy modified starch, and then mixed with LDPE ***. Mster-Bi plastics from Italian Novamont Company and NoVon series products from American Warner-lambert Company also belong to this category.
Mster-Bi plastic is a polymer alloy formed by a physical cross-linked network of continuous EVOH phase and starch phase. Since both ingredients contain a large number of hydroxyl groups, the product is hydrophilic and its mechanical properties will decrease after absorbing water, but it is insoluble in water.
4. The all-starch type deforms and disorderes the starch molecules to form a thermoplastic starch resin, and then adds a very small amount of plasticizers and other additives, which is the so-called all-starch plastic. The starch content is more than 90%, and a small amount of other substances added are non-toxic and completely degradable, so all-starch is a truly completely degradable plastic. Almost all plastic processing methods can be used to process all-starch plastics, but traditional plastic processing requires almost no water, while the processing of all-starch plastics requires a certain amount of water to play a plasticizing effect. The water content during processing is 8% to 15%. % is appropriate, and the temperature should not be too high to avoid burning. Sumitomo Corporation of Japan, Wanlerlambert Company of the United States, and Ferruzzi Company of Italy claimed to have successfully developed all-starch plastics with a starch content of 90% to 100%. The products can be completely biodegraded within one year without leaving any traces. They are pollution-free and can be used for Manufacture of various containers, films, garbage bags, etc. Germany's Battelle Research Institute has developed degradable plastics using modified green pea starch with a high linear content, which can be processed and formed using traditional methods. As a substitute for PVC, it can be completely degraded in humid natural environments.
Oxidative degradation
This is a technology that has not been understood by most people in China. Additives are added to traditional plastic production raw materials, which is different from the general color masterbatch addition method. same. After plastic products are abandoned, two substances in the additives play a role: one is a pre-oxidant (mainly some non-toxic metal ions), and the other is a biodegradation promoting substance (mainly some natural plant cellulose). The pre-oxidant controls the plastic to maintain its proper life and function when it is not abandoned. After it is abandoned, it reduces the molecular weight through peroxidation reaction, 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 and easy to implement, reduces costs, and can be produced with general equipment. According to relevant verification, the performance of the plastic is also well maintained. Saved food. The British WELLS company adopts this method.
Simple identification method of common plastics Before using various plastic recycling methods to reuse waste plastics, most plastics need to be sorted. Since plastic consumption channels are many and complex, and some post-consumer plastics are difficult to distinguish simply by their appearance, it is best to indicate the material type on plastic products. China has formulated GB/T16288-1996 "Plastic Packaging Products Recycling Marks" with reference to the material variety marking proposed and implemented by the American Plastics Society (SPE). Although the above marking method can be used to facilitate sorting, there are still many unmarked products in China. Plastic products bring difficulties to sorting. In order to separate different types of plastics for classification and recycling, we must first master the knowledge of identifying different plastics.
Appearance Identification
By observing the appearance of plastics, the major categories of plastic products can be initially identified: thermoplastics, thermosetting plastics or elastomers. Generally, thermoplastics are divided into two categories: crystalline and amorphous. The appearance of crystalline plastic is translucent, opalescent or opaque. It is only transparent in the film state, and its hardness ranges from soft to horny. Amorphous is generally colorless, completely transparent when no additives are added, and has a hardness that is harder than horny rubber (plasticizers and other additives are often added at this time). Thermoset plastics usually contain fillers and are opaque, or transparent without fillers. The elastomer has a rubbery feel and a certain stretch rate.
Heating identification
The heating characteristics of the above three types of plastics are also different, and they can be identified through heating. Thermoplastics soften when heated, melt easily, and become transparent when melted. Filaments can often be drawn from the melt, and they are usually easy to heat seal. Thermosetting plastics are heated until the material chemically decomposes, maintaining its original hardness without softening and relatively stable dimensions, and carbonizing at the decomposition temperature. When the elastomer is heated, it does not flow until the chemical decomposition temperature. When the decomposition temperature reaches the decomposition temperature, the material decomposes and carbonizes.
The softening or melting temperature ranges of commonly used thermoplastics are shown in the table
Plastic varieties
Softening or melting range/c
Polyvinyl acetate
35~85
Polyoxymethylene
165~185
Polystyrene
70~ 115
Polypropylene
160~170
Polyvinyl chloride
75~90
Nylon 12< /p>
170~180
Polyethylene
110
Nylon 11
180~190
< p>Polychlorotrifluoroethylene200~220
Nylon 610
210~220
Poly-1-butene< /p>
125~135
Nylon 6
215~225
Polyvinylidene chloride
115~140 (Softening)
Polycarbonate
220~230
plexiglass
126~160
Poly -4-Methylpentene-1240 cellulose acetate
125~175
Nylon 66
250~260
Polypropylene Nitrile
130~150 (softened)
Polyethylene terephthalate
250~260