"Popular Science" How much do you know about plastics?

Degradable plastics refers to addition of a certain amount of additives (such as starch, modified starch or other cellulose, photosensitizers, biodegradables, etc.). Degradable plastics. So let's talk about degradable plastic today.

1.What is biodegradable plastic?

"Popular Science" How much do you know about plastics?

Biodegradable plastics refer to plastics that can degrade through biological erosion or natural metabolism. It can be divided into two processes: biophysical degradation and biochemical degradation. When microorganisms degrade plastic products, continued growth of biological cells hydrolyze, ionize, or protonate polymer components, which causes mechanical damage such as swelling and cracking of plastics, which is biophysical degradation; water-insoluble polymer decomposes or oxidatively degrades to water-soluble fragments, forms new low molecular weight compounds, and finally decomposes into carbon dioxide and water, which is biochemical decomposition.

2.What is photodegradable plastic? What substances are commonly used to create lighting effects?

"Popular Science" How much do you know about plastics?

Photodegradable plastics means that after absorption of light by polymer, light binding energy weakens, and long chain is split into smaller molecular weight fragments, which are oxidized by air, and free radical chain splitting reaction is further decomposed into low molecular weight compounds, which finally turn into into carbon dioxide and water. At present, carbonyl groups are mainly introduced into polyethylene chains, or photodegradation additives are added to plastics. plastic processing. , such as dissolving or melting so that it can be depolymerized by exposure to ultraviolet light. Commonly used photosensitizers include zinc oxide and piperidine, phthalates and epoxy plasticizers, iron salts and polyvinyl chloride, and metal salts such as nickel, molybdenum and cobalt.

3.Water-degradable plastic

"Popular Science" How much do you know about plastics?

Add water absorbents to plastics that can dissolve in water after use. They are mainly used in medical and sanitary devices (such as medical gloves) for easy destruction and disinfection.

Starch-based plastics

At moment, starch-based degradable plastics are mainly divided into four categories: filling type, photo/biodegradable type, mixed type, and whole starch plastic.

1. Filled starch plastics In 1973, Griffin first received a patent for filled starch-surface-modified plastics. By 1980s, some countries had developed biodegradable starch-filled plastics based on Griffin patent. Filled starch plastics, also known as biodegradable plastics, are produced by adding a certain amount of starch and other small amounts of additives to general purpose plastics and then processing them into molds. The starch content does not exceed 30%. The technology of filled starch plastic is mature, production process is simple, and existing processing equipment can be slightly improved, so most of domestic degradable starch plastic products are of this type.

Natural starch molecules contain a large number of hydroxyl groups to form extremely strong hydrogen bonds within and between molecules, molecules being more polar, while synthetic resins are less polar and hydrophobic. Therefore, surface treatment of native starch is necessary to improve hydrophobicity and its compatibility with polymers. There are basically two methods of physical modification and chemical modification.

2. Photo/Biodegradable Biodegradable plastics are difficult to decompose in some special areas such as drought or lack of soil, and photodegradable plastics do not decompose when buried in soil. For this reason, US, Japan, and other countries have adopted lead A class of photo/biodegradable plastics has been developed that has both photodegradability and biodegradability. Photo/biodegradable plastics are made from a photosensitizer, starch, synthetic resin and a small amount of additives (solubilizer, plasticizer, crosslinker, coupling agent, etc.) where photosensitizer is an organic compound or transition metal salt. The degradation mechanism is that starch biodegrades, which makes polymer matrix loose and increases specific surface area. At same time, sunlight, heat, oxygen, etc., trigger photosensitizers, which leads to polymer chain breakage and a decrease in molecular weight.

3. Starch-blended plastics are plastics mixed with starch and synthetic resins or other natural polymers.hop (30%~60%), a small amount of PE synthetic resin, ethylene/acrylic acid (EAA). ) copolymer, ethylene/vinyl alcohol copolymer (EVOH), polyvinyl alcohol (PVA), cellulose, lignin, etc., which are characterized by high starch content, and some products may be completely decomposed.

Japan has developed a modified starch/EVOH copolymer blended with LDPE, dimethylsiloxane epoxy modified starch, and then blended with LDPE. This type of product also includes Mster-Bi plastics from Italian company Novamont and products from NoVon series from American company Warner-lambert. Mster-B plastic is a polymer alloy formed by a physical cross-linked network of a continuous EVOH phase and a starch phase. Since both ingredients contain a large amount of hydroxyl groups, product is hydrophilic and its mechanical properties will decrease after water absorption, but it is insoluble in water.

4. The whole type of starch changes structure and disorder of starch molecules to form a thermoplastic starch resin, and then adds a very small amount of plasticizer and other additives, which is so-called whole starch plastic. Among them, starch content is more than 90%, and a small amount of other added substances are also non-toxic and fully degradable, so whole starch is a truly fully degradable plastic. Almost all plastic processing methods can be applied to processing of whole starch plastics, but traditional plastic processing requires almost no water, while processing of whole starch plastics requires a certain amount of water to plasticize, and water content during processing is Suitable from 8% to 15% and temperature must not be too high to avoid fire.