Talking about the development and application of food packaging film in green (2)

Biodegradable:

Degradation process of commonly used polyolefin materials for film production in soil is very complicated, and the main factor influencing its biodegradability is the excessive molecular weight of polyolefin. After years of research, foreign researchers pointed out that straight-chain hydrocarbon polymers only have obvious biodegradability when the molecular weight falls below 500. After the molecular weight exceeds 500, such polymers cannot promote the growth and growth of fungi. Starch-added biodegradation is currently the main method of biodegradation, that is, a film obtained by graft-copolymerizing or blending starch and polyolefin of corn, rice, potato, cereals, etc. treated with hydrophilic hydrophobized surface. The agricultural film produced by this method undergoes biodegradation under the influence of microorganisms in the soil after an agricultural production cycle, and the agricultural film is decomposed into small pieces and finally becomes part of the soil. The key technology for producing degradable films by blending starch with polymers is to solve the compatibility problems between the two.

High calcium carbonate filling:

The research results show that by adding more than 30% inorganic minerals such as CaCO3 in plastics, the burning speed of plastic packaging materials can be improved, and the burning temperature can be reduced. At the same time, the waste can be indirectly promoted to burn more fully and rapidly. In addition, general garbage bags and plastic products may be exposed for a period of time after they are discarded. During this period, the calcium carbonate-containing film will undergo degradative behavior, that is, the breakage of polymer macromolecular chains and the formation of carbonyl groups. It can be seen that the large amount of calcium carbonate filled in the PE film can make the film have an initial degradation function, so that the film can be thermally degraded and thermally decomposed ahead of time at the same furnace temperature, which lays the foundation for the full combustion in the later stage.

After more than 10 years of research and development, biodegradable films have achieved some gratifying results, but there are still some deficiencies. In starch-added degradable film materials currently being developed, the content of starch is less than 20%. According to the report of Modern Plastics, only starch content of more than 50% can achieve good biodegradation. Under the action of microorganisms, the membrane is decomposed by starch at most, and the residual polyolefin membrane still exists in a low-strength porous structure, so the starch-added degradable membrane is only an incomplete biodegradable membrane. .

3. Water soluble film

A water-soluble film refers to a film that is soluble in water at room temperature. As a novel green packaging material, the water-soluble film has environmental characteristics such as thorough degradation and safe and convenient use. Therefore, it has received extensive attention from developed countries in the world, and is widely used in various countries such as Europe, America, Japan, etc. The packaging of products, such as pesticides, fertilizers, pigments, dyes, detergents, water treatment agents, mineral additives, detergents, concrete additives, photographic chemicals, and chemical agents for gardening, facilitate the application of these drugs. Typical water-soluble films are methylcellulose films, polyethylene oxide films, polyvinyl alcohol films. The methylcellulose film is soluble in cold water at room temperature and a 1 mil (25.4 μm) thick methyl cellulose film dissolves in water within 20 seconds. The polyethylene oxide film is the most soluble of all water-soluble films.

Polyvinyl alcohol film is currently the most widely used water soluble film in the packaging field. Water-soluble polyvinyl alcohol films often use polyvinyl alcohol resins with a low average degree of polymerization and an alcoholysis degree of only about 88%. The production process of polyvinyl alcohol film includes two different process routes of aqueous solution casting (aqueous solution method) and extrusion film forming (dry method). Aqueous solution method is polyvinyl alcohol and plasticizers, surfactants, anti-adhesive agent, quick-drying agent, release agent, etc. mixed into a glue, the glue is filtered and evenly coated on the mirror steel, dried After peeling off the rolled product. Dry method is to vacuum dry polyvinyl alcohol and plastic modifiers, film forming agents and other additives mixed in a certain proportion of plastic, and then extruded into the film, after finishing, winding and other processes that get products.

As far as the degradation mechanism is concerned, polyvinyl alcohol has both water and biological degradation characteristics. It first dissolves in water to form a glue solution and penetrates into the soil, which increases soil stickiness, air permeability and water retention, and is particularly suitable for sand soil remodeling. The PVA in the soil can be decomposed by bacteria isolated from the soil, the strain of M. monocytogenes. A symbiotic system composed of at least two bacteria can degrade polyvinyl alcohol: one is an active bacterium of polyvinyl alcohol, and the other is a bacterium that produces substances required for PVA-active bacteria. The oxidation of secondary alcohol catalyzes PVA, and then the hydrolase cleaves the oxidized PVA backbone and further degrades, eventually reducing to CO2 and H2O. At present, water-soluble films have a wide range of applications in chemical packaging, washing and packaging, textile packaging, food packaging, electronic and electrical product packaging, and water transfer printing and agriculture. However, due to the unique production process of water-soluble films, various defects such as bubbles, uneven thickness, “fish eyes”, perforation, wrinkles, “craters”, etc. are produced in the production, and the causes thereof are very complicated. The occurrence of defects directly affects the application of water-soluble films. Therefore, it is necessary to conduct in-depth research. In addition, the productivity of water-soluble films is very low (4 to 7 m/min) compared to the production of water-insoluble thin-film devices using conventional processes. This is an important reason for the higher cost of water-soluble films. How to improve the solubility of water-soluble films? Productivity will be an important research topic in the future. Green packaging is a requirement for the development of international environmental protection. It also represents the development trend of packaging industry in various countries, and is one of the important contents of improving product market competitiveness and avoiding new trade barriers. During the entire life cycle of the green packaging, it follows the principles of economic activities of “reduction, reuse, and resource recycling” of the circular economy so that the impact of packaging on the environment is minimized.

The development of green packaging does not mean that the packaging waste has solved the environmental pollution, but it only provides technical support for changing the status quo. Strengthening recycling and reuse, and achieving ecological recycling of packaging waste are the most powerful measures to reduce environmental pollution.

(Sun Yaoqiang Han Yongsheng)

Source: Plastic Packaging Committee