1 Introduction
In recent years, the use of disposable plastic products has been increasing, causing human survival has brought serious environmental problems. The current social requirements for industrial products and daily necessities environmental protection requirements are increasing, and the demand for green materials is growing. People We began to look for alternatives to the current plastic performance without causing white pollution. As a colorless, non-toxic, non-corrosive and biodegradable organic polymer, polyvinyl alcohol (PVA) is a good substitute for the existing plastics. PVA as a colorless, non-toxic, non-corrosive, biodegradable organic polymer, its environmental characteristics have been widely recognized by the world, is used in a large number of textile slurry, coatings, adhesives, emulsifiers and other industrial fields. It is widely used in textile slurry, coating, adhesive, emulsifier and other industrial fields. However, since polyvinyl alcohol film is a hydrophilic material, it has poor water resistance and stability. water resistance and poor stability, which largely limits the application of this material is largely limited by its poor water resistance and stability. At present, the modification of polyvinyl alcohol materials at home and abroad has improved the water-resistance of this material. The water-resistance and mechanical properties of this material have been improved, and it can be used in a wider range of fields. The application of polyvinyl alcohol material in a wider range of fields.
2 Modification Technology of Polyvinyl Alcohol
The modification methods of polyvinyl alcohol are mainly pre-modification, post-modification and co-modification. Modification. Pre-modification, i.e. copolymerization modification is mainly through the modifier and vinyl acetate copolymerization, by changing the chemical structure and regularity of polyvinyl alcohol molecular chain, reducing inter-and intra-molecular hydrogen bonding to modify. And the later modification is mainly modification by direct reaction between modifier and polyvinyl alcohol. According to the structure of polyvinyl alcohol molecular chain structure, it can be found that there are a large number of hydroxyl groups in its molecular chain. Therefore, the modification process is carried out by the reaction between the added reagent and the hydroxyl groups. Co Blending modification is mainly the blending of different polymers, where the subvalent forces between molecular chains (e.g. hydrogen bonding, van der Waals forces, etc.) interact to form molecular aggregates. bonding, van der Waals forces, etc.) interact to form molecular aggregates, resulting in changes in the distribution and structure of functional groups. The modification is carried out by changing the distribution and structure of functional groups.
2.1 Pre-modification Method
2.1.1 Ethylene/ Vinyl acetate modified
Modified polyvinyl alcohol can be prepared by copolymerization reaction of ethylene and vinyl acetate. When the ethylene content is high, ethylene/vinyl acetate copolymer (EVA) can be produced, and when the ethylene content is low, the prepared ethylene/vinyl acetate copolymer can be obtained by alcoholysis of modified polyvinyl alcohol.
2.1.2 Acrylic/vinyl acetate modified
By the copolymerization reaction of acrylic acid compounds with vinyl acetate, the products are then modified polyvinyl alcohol is prepared by alcoholysis. There are many types of acrylic compounds There are many types of acrylic acid compounds, including acrylic acid, acrylate, acrylonitrile, acrylamide, and unsaturated compounds including methacrylate. unsaturated compounds including methacrylic acid esters, so researchers have done a lot of research on these modifications. A lot of research work has been done on these modifications.
2.1.3 Epoxy compound/vinyl acetate modification
The method is to prepare modified polyvinyl alcohol by copolymerization of ethylene oxide, propyl alcohol, butyl alcohol and vinyl acetate by saponification. These products contain a certain amount of oxidized vinyl groups, the content of which is generally 0.1% to 20% of the main chain hydroxyl and carboxylate groups. If this product is thoroughly mixed with a certain proportion of PCl3 and boride salt, the hydrophilic modified antirust coatings for heat exchangers can be produced.
2.2 Post-modification Method
2.2.1 Acetalization modification
Polyvinyl alcohol and carbonyl compounds, under certain conditions, can undergo acetalization reactions to produce acetal compounds. For example, vinylon is prepared by reacting polyvinyl alcohol with formaldehyde as raw material, and finally obtaining a material with good water resistance and superior mechanical properties. Also polyvinyl alcohol acetal compounds are widely used in adhesives, coatings and other applications. However, due to the high content of free formaldehyde in these materials, the products have volatile and irritating odor, which pollute the environment and affect human health. Later, to solve these problems, people added ” formaldehyde trapping agents” to the materials, but this led to a decrease in the stability of the material storage, and it would increase the production cost. In recent decades, researchers have done a lot of research on the use of other modifiers. For example, Chang-Hua Zhang has prepared resins with low free formaldehyde content through the use of glyoxal to replace formaldehyde. The use of glutaraldehyde modifier caused a cross-linking reaction between polyvinyl alcohol molecular chains and formed a mesh structure, which not only improved the water-resistance of the material but also optimized the polyvinyl alcohol membrane separation performance. Therefore, acetalization modification is widely used in the modification of polyvinyl alcohol.
2.2.2 Esterification modification
The esterification reaction is a fundamental reaction that is well known to researchers and forms an ester bond through the interaction of hydroxyl and carboxyl groups. For example, Yang et al. prepared highly water-resistant polyvinyl alcohol film nanofiber membranes by using the esterification reaction occurring between maleic acid and polyvinyl alcohol. Li Baojun et al. modified poly(vinyl alcohol) with oxalic acid (OA) by esterification under sodium hydroxide catalytic conditions to produce a new PVA – OA homogeneous membrane, and investigated the trends of water resistance and mechanical properties of the homogeneous membrane with the degree of cross-linking. The esterification modification of polyvinyl alcohol is not only limited to the modification of polyvinyl alcohol materials by inorganic or organic acids, but also includes the modification reactions occurring with a series of modifiers such as acid anhydride and chloride derivatives. Researchers have modified polyvinyl alcohol by acid anhydride to obtain modified products with high water resistance and good thermal stability. The grafting modification of the material is carried out directly by the interaction of reagents such as chlorophosphoryl chloride or bromophosphate chloride with the hydroxyl group of polyvinyl alcohol, which improves the application properties of the material.
2.2.3 Etherification modification
The principle of etherification modification of polyvinyl alcohol is to use the chemical activity of polyvinyl alcohol hydroxyl group and introduce the corresponding monomer to etherify the hydroxyl group, so as to obtain the etherification modified polyvinyl alcohol. It has been reported abroad that the polyvinyl alcohol has been modified by etherification with aliphatic, cycloaliphatic or aromatic epoxy compounds in the anhydrous molten state, thus improving the permeability, thermal stability and processing properties of polyvinyl alcohol. Since the melting and decomposition temperatures of polyvinyl alcohol materials are relatively close to each other, the poor operability of polyvinyl alcohol in the molten state has led researchers in recent years to further research on etherification modification methods.
2.3 Blend Modification
2.3.1 Starch/Polyvinyl Alcohol Modification
Starch is a natural polymer that has a large number of hydroxyl groups similar to polyvinyl alcohol, so starch generally has 15% to 45% crystallinity. Due to the large number of hydroxyl groups contained in the molecules of starch and polyvinyl alcohol, the film properties can be improved by blending starch and polyvinyl alcohol, where the interaction of the large number of hydroxyl groups between the molecules breaks the hydrogen bonding within the starch molecule.
2.3.2 Chitosan/Polyvinyl Alcohol Modification
Chitosan is a polysaccharide obtained by deacetylation of chitin, which has good biocompatibility and biodegradability. good biocompatibility and biodegradability. Chitosan and polyvinyl alcohol The modified membrane material prepared by blending chitosan with polyvinyl alcohol has good air permeability and light transmission.
2.3.3 Polylactic acid/polyvinyl alcohol modified
Polylactic acid is generally produced by ring-opening polymerization of endo-ester or by polymerization of lactic acid. With good biocompatibility, biodegradability and non-toxicity, it is used for the modification of poly They are used for the modification of polyvinyl alcohol.
3 Application of PVA
3.1 Biodegradable Materials
It was found that the molecular weight and crystallinity of polyvinyl alcohol have a decisive role in its biodegradability. Therefore, the researchers have modified the chemical structure and regularity of polyvinyl alcohol chains by introducing copolymerization components to reduce inter- and intramolecular hydrogen bonding. chemical structure and regularity of poly(vinyl alcohol) molecular chains by introducing copolymerization components to reduce intermolecular and intramolecular hydrogen bonds to Therefore, researchers have introduced copolymerization components to modify the chemical structure and regularity of polyvinyl alcohol chains and reduce inter- and intra-molecular hydrogen bonds to improve the degradability of the material. Also, some researchers have blended multiple polymers their molecular chains to form aggregates through subvalent bonding forces, resulting in changes in their The functional group distribution and structure are changed to improve the degradability. So the modified polyvinyl alcohol is used to prepare degradable materials.
3.2 Fiber Materials
In China, research on vinyl acetate and polyvinyl alcohol was conducted in the 1950s, and the first fiber production plant was built in 1962. After the process and technology update, the production and quality of polyvinyl alcohol fiber have been very improved. In recent years, through a large number of modifications of polyvinyl alcohol research, the development of a variety of properties of fiber products, such as water-soluble fibers, high-strength and high-modulus fibers and fire-retardant fibers, etc., further expanded The application of polyvinyl alcohol in fiber industry has been further expanded.
3.3 Medical Materials
The researchers chemically cross-linked polyvinyl alcohol with a cross-linking agent and then rapid condensation to prepare polyvinyl alcohol hydrogels with chemically cross-linked network structures. Polyvinyl alcohol hydrogels can also be obtained by physically introducing crystalline regions gels. Since polyvinyl alcohol hydrogels have similar water content, elastic modulus, and low friction to natural human tissues The polyvinyl alcohol hydrogels have similar water content, elastic modulus, low friction coefficient and high mechanical strength, rich pore network structure, good biocompatibility, etc. and good biocompatibility, it is widely used in artificial cartilage material, drug They are widely used in the fields of biomedical engineerings, such as artificial cartilage materials, drug slow-release systems, and anticoagulant materials.
3.4 Membrane Materials
Polyvinyl alcohol itself is colorless, non-toxic, non-corrosive, biodegradable, and It is widely used in membrane materials for its high corrosion resistance. The water permeability of the material itself The material itself has poor water permeability and can be used for food packaging, Chinese herbal medicine packaging, etc. Later, researchers The material itself has poor water permeability and air permeability, so it can be used in food packaging and Chinese herbal medicine packaging. The composite ultrafiltration membranes with pollution resistance have been widely used in water treatment.
3.5 Coating Material
Polyvinyl alcohol is also used in a large number of applications in the field of coatings, such as polyvinyl alcohol by partial Polyvinyl alcohol can be used as a base material for water-based porcelain imitation coatings. Polyvinyl butyral resin is very resistant to non-polar gasoline and kerosene and can be used in oil-resistant coatings. Phosphate primers with polyvinyl butyral resin and chromate as the main components are used for oil-resistant coatings. Phosphate primers with polyvinyl butyral resin and chromate as the main components are used to coat the surfaces of various ships, bridges, instruments and other devices. Besides, modified polyvinyl alcohol materials are also widely used in functional polymer molecular materials, waterproofing agents, adhesives and many other fields.
4 Outlook
Polyvinyl alcohol has good use properties and environmental friendliness, and it is important to actively carry out related research that has important theoretical and practical significance. The modification of polyvinyl alcohol can not only improve the water solubility and mechanical properties of polyvinyl alcohol but also increase its biodegradability to increase its market potential. Along with the rapid development of various industries in China, there is a great demand for polyvinyl alcohol modified materials in the market. Currently, Polyvinyl alcohol materials with micro-and nano-structures and shape memory properties Polyvinyl alcohol materials with micro and nanostructures and shape memory properties are also widely studied and may be modified in micro and nanostructures in the future. The future possibilities of modifying polyvinyl alcohol materials in micro and nanostructures to improve various excellent properties and discover their potential use properties. This is important for improving the competitiveness of polyvinyl alcohol material products and promoting the development of the polyvinyl alcohol modified material industry. The development of the polyvinyl alcohol modified materials industry is of great significance.