As people pay more and more attention to environmental protection, the “white pollution” caused by the use of plastics has attracted worldwide attention, so materials with biodegradable capability have been flourishing. Polyvinyl alcohol films have excellent microbial decomposition properties, can be decomposed into carbon dioxide and water, and do not pollute the environment, but also have some unique properties, such as: gas barrier, transparency, anti-static, toughness, resistance to organic solvents, water solubility, etc., to meet the various requirements of various industries for the film, so the development and application of domestic and foreign boom.
Polyvinyl alcohol is commonly produced industrially by the polymerization of vinyl acetate into polyvinyl acetate, followed by alcoholysis. The physical and chemical properties of polyvinyl alcohol vary significantly with the degree of saponification and polymerization. The solubility of PVA in water depends entirely on the degree of alcoholysis and polymerization, especially the degree of alcoholysis has the greatest impact.
In the process of alcoholysis of polyvinyl alcohol, the hydrophobic ethylene acetate group within the molecule decreases, while the hydrophilic hydroxyl group increases, so some of the alcoholysis of PVA shows water solubility. As the degree of alcoholysis increases, the hydrophilic hydroxyl groups increase, and the hydrogen bonding between the hydroxyl groups also increases, so that the crystalline part of the water is difficult to penetrate increases accordingly, so it does not show cold water solubility and shows warm water solubility. Conversely, when the alcohol is low, due to the proportion of non-polar groups in the molecule increases, PVA becomes insoluble in water. Therefore, for the processing of PVA film, according to its different requirements, the use of different alcohol solubility, crystallinity and degree of polymerization of PVA resin.
In recent years, Japan and some companies in Europe and the United States in the development of PV A film has made breakthroughs, creating favorable conditions for the widespread application of PVA films. China’s packaging with the special requirements of barrier, transparency, water solubility, PVA film industry has further developed.
2. Properties, types and applications of PVA film
2.1 Features of PVA film
Compared with other plastic films, PVA film has the following features:
1) Excellent biodegradability and non-toxic, it can be decomposed by microorganisms into carbon dioxide and water, and it is a new generation of environmentally friendly plastic film material, which can completely solve the problem of packaging waste disposal;
2) Excellent gas barrier to oxygen, nitrogen, etc. when the film is dried;
3) Excellent resistance to animal oil, mineral oil, vegetable oil, aliphatic hydrocarbon, aromatic hydrocarbon and other non-polar organic compounds;
4) Due to a large number of hydroxyl groups, it has excellent antistatic properties and dustproof performance;
5) Good water solubility;
6) excellent transparency and gloss;
7) good mechanical properties, with high strength and good tensile, tear, impact properties;
8) good heat sealing properties, suitable for resistance heat sealing and high frequency heat sealing;
9) good printability and low temperature crack resistance.
Disadvantages of PVA film:
1) moisture absorption, gas barrier after moisture absorption, poor processing adaptability;
2) poor moisture resistance;
3) poor sliding properties.
2.2 Types of PVA film
2.2.1 Water-soluble PVA film
The water-soluble PVA films are produced from partially alcoholic PVA resins with an alcohol solubility of about 88%. The large molecular chains contain a certain amount of bulky vinyl acetate groups, which prevent the molecular chains from approaching each other, and also weaken the hydrogen bonding between the hydroxyl groups on the molecular chains, so that more hydroxyl groups interact with water. The films also exhibit excellent water solubility. The resulting films can be obtained by mildly heat treating and fine-tuning their crystallinity to obtain a wide range of soluble films from cold water soluble to warm water soluble.
2.2.2 Insoluble PVA film
Insoluble PVA films are water-resistant unstretched films, usually containing about 10% polyol plasticizer and 20 to 70 μm thick, processed from polyvinyl alcohol resins with a polymerization degree of more than 1000 and an alcohol solubility of 98% and above. The film has good transparency, high gloss, less static electricity, strong, moisture permeability, good resistance to oil and organic solvents, and good printing performance.
2.2.3 Biaxially oriented PVA film
Due to the stretching of biaxially oriented PVA film intermolecular cohesion enhanced, mechanical strength, toughness and cellophane similar. It can be laminated with other films into two, three or more layers of composite film, to meet the needs of food and other packaging. In addition, biaxially oriented PVA film has the best gas barrier, including O2, CO2, N2, SO2 and H2S gas; dimensional stability, heat resistance, cold resistance, transparency, gloss and other aspects of the film than other production methods have significantly improved; suitable for vacuum aluminizing process, can be used for gas replacement packaging, vacuum packaging, deoxidizer packaging, etc.; can also be used for polyester molding release film and agricultural land film. Biaxially oriented PVA film also has excellent fragrance retention; oil and solvent resistance; good peelability; good anti-condensation; allows ultraviolet light transmission and infrared does not transmit; good printability and adhesion and other characteristics.
2.3 Current Application Status of PVA Film
Due to the many excellent characteristics of PVA films, they have been widely used in various fields, and the market is very broad, so they have been widely valued by the developed countries around the world.
2.3.1 Water-soluble PVA film applications
With its water solubility, it can be used in the packaging of products used in water, such as: pesticides, fertilizers, pigments, dyes, cleaning agents, water treatment agents, mineral additives, detergents, concrete additives, photographic chemicals and horticultural care chemicals.
1) Water-soluble PVA pesticide environmental protection packaging bags
Pesticides are generally highly concentrated, highly toxic, and extremely harmful to human beings. The problem of pesticide packaging has been paid more and more attention by the chemical pesticide department.
For a long time, there are 3 major shortcomings of pesticide packaging: (1) liquid pesticide packaging using glass bottles, glass bottle breakage rate is high; (2) the use of ordinary pesticide bags are very easy to produce a lot of packaging residues, resulting in waste; (3) abandoned with a lot of residues of pesticide bags easily discarded in rivers, streams, fields, fields and other places, contaminating the soil and water. Over time, it will become an invisible killer that poisons humans and pollutes the environment.
In recent years, Japan, the United States and other advanced countries have adopted a new technology of water-soluble pesticide packaging. The main principle is to use a new type of environmentally friendly water-soluble PVA film as packaging materials, the large packaging products into small packages of water-soluble film. Because of the unique performance and environmental characteristics of water-soluble PVA plastic film, it is widely valued by developed countries around the world.
Water-soluble PVA pesticide environmental protection bags have the following advantages:
(1) Water solubility, water solubility speed can be designed and selected. Non-toxic and non-polluting, soluble in cold water.
(2) It can greatly reduce the impact of pesticides, industrial chemicals and other toxic or irritating substances on human body and environment.
(3) It can effectively solve the problem of increasing production and transportation costs due to the heavy glass bottles, as well as the problem of waste caused by poorly sealed corks, easy to leak and difficult to recycle.
(4) Excellent oil resistance, fat resistance and organic solvent resistance.
(5) excellent antistatic properties, packaging powder, will not adsorb powder and dust.
(6) Accurate measurement, to avoid the use of pesticides easily caused by overdose or underdose, resulting in the strengthening of pests and germs resistance.
Based on the above characteristics, water-soluble PVA pesticide environmental protection packaging bag quickly recognized by the world’s pesticide manufacturers, and has been widely used.
2) Water-soluble PVA medical packaging laundry bag
Water-soluble environmentally friendly medical packaging bags are new environmentally friendly products developed in recent years in Europe and the United States. Its purpose is to avoid contamination and cross-infection of medical personnel and patients.
Over the past 30 years, the care of hospital patients has generated large amounts of infectious medical waste, and the packaging used to dispose of the waste has often come into contact with contaminants and is therefore likely to carry pathogenic bacteria and infectious materials. If the contaminants are dissolved in disinfection media, they can be easily disposed of, and the burden of landfills and incineration can be greatly reduced, while eliminating the contamination of the environment and human body by germs and infectious materials. Through the extensive use of Europe and the United States and other developed countries in recent years found that the use of water-soluble PVA environmentally friendly materials produced by water-soluble PVA environmentally friendly medical packaging bags with ideal wear resistance and environmental protection.
Water-soluble PVA laundry bags for medical packaging have the following advantages:
(1) Once the package is ready, the contents from handling, cleaning to drying process will not contact the outside world, completely isolated.
(2) the bag film completely dissolved in water, no residue, water quality is not polluted.
(3) due to water-soluble medical bags with high barrier, to ensure that health care workers in the handling process without germs and viruses infection.
(4) water-soluble medical packaging bags with anti-static, non-toxic and completely degradable performance, a large number of experiments have proved that germs and viruses and can not penetrate the film, extremely conducive to medical packaging.
3) Packaging of cement additives
Cement additives are strongly alkaline and acidic, with high concentrations, and are usually used outdoors, where they can easily touch workers’ eyes and skin and cause pollution to the environment due to wind and dust. In the past, workers wearing appropriate protective equipment to avoid contamination by cement additives, to the construction operation has brought great inconvenience, if the pollutants touch the body will easily cause harm to the human body.
In recent years, developed countries in the West have been widely used water-soluble PVA film packaging cement additives, completely avoiding pollution, easy to operate, accurate measurement.
2.3.2 Insoluble PVA film
Based on the excellent characteristics of good transparency, high gloss, less static electricity, strong, oil resistant, organic solvent resistant and good printing performance, the film can be used in the following fields:
1) Temporary protective packaging of conveyed products;
2) Packaging of electronic components to prevent dust pollution, such as: packaging of high performance dry cell;
3)Manufacture of polarizing film for liquid crystal display;
4) Packaging of textile products;
5) Metal surface protection film;
6) Production of laminated films with polyethylene, polypropylene, nylon, polyester, polycarbonate, etc.
7) in the use of unsaturated polyester, epoxy resin or other thermosetting resin production of automotive parts, tanks and other items, PVA film can be used as a good performance of the release agent ;
8) As PVA film has excellent oxygen barrier, breathable performance, can be used as a good performance of food preservation film.
2.3.3 Biaxially oriented PVA film
After the biaxial stretching treatment, the crystallinity is improved, the water resistance is improved, the toughness is good, and it can be used in food packaging, release film, oil and solvent resistant packaging, electronic parts packaging and agricultural materials, etc. Its main characteristics and applications are shown in the table below.
|PVA Film Features||Example of use|
|Anti-oxidation and anti-discoloration||Bonito flakes, seaweed, bean paste, pickled products and other plastic bag packaging products|
|Anti-oil and grease oxidation||Salted meat, sausage, ham, other meat products; peanuts, bean cake, nuts and oil snack packaging|
|Anti-mold||Chocolate semi-finished products, cakes, sweet beans packaging|
|Fragrance preservation, anti-sublimation||Coffee, spices, seasoning, tea, naphthalene, gasification rust inhibitor packaging|
|Anti-static||Powdered product packaging|
|Pest prevention||Chinese medicine, animal food, peanuts, textile packaging|
|Transparent and shiny||General food, textile packaging|
|Oil and solvent resistant||Wrapping paper and packaging containers|
|Metal spray coating||Coffee, tea, nori, powdered food, fast food, lining bags inside cartons|
|Anti-fog packaging||Fresh fruits, vegetables, cooked eggs, fresh mushroom packaging|
|Others||Electronic parts packaging|