Polyvinyl alcohol, the most familiar water-soluble polymer, is a white, powdered resin derived from the alcoholysis of polyvinyl acetate. It has good water solubility due to the presence of a large number of side groups and light groups in its molecular chain. It also has good film-forming, adhesion and emulsification properties, and has excellent resistance to oils and solvents. Therefore, polyvinyl alcohols are widely used as adhesives, paper finishing agents and sizing agents, textile pastes, temporary adhesives in the ceramic industry, emulsifiers and protective colloids for emulsion polymerization, quenching fluids for steel, cosmetics, oilfield chemicals and automotive safety glass. Polyvinyl alcohol in paper applications abroad is more common, not only for general paper surface sizing, but also for oil-resistant paper, cellophane, copy paper and other special paper surface sizing; not only for high-grade paper, but also widely used in white paper, such as ordinary paper.
1. Preparation of Polyvinyl Alcohol
Polyvinyl alcohol cannot be obtained by direct polymerization of vinyl alcohol because it is extremely unstable and free ethylene glycol monomer cannot be present. In the preparation of polyvinyl alcohol, the first step is the polymerization of ethylene acetate to form polyvinyl acetate. Then, polyvinyl acetate is alcoholized to produce polyvinyl alcohol. The corresponding production processes are: polymerization of vinyl acetate, alcoholysis of polyvinyl acetate and recovery of ethanoic acid and methanol, etc.
The polymerization of vinyl acetate is often done by solution polymerization, which is accomplished by four basic processes: chain initiation, chain growth, chain transfer and chain termination. The initiators include benzene peroxide phthalate, hydrogen peroxide, azoisobutylene and so on. Azoisobutyric acid is mostly used as initiator in industrial production, because it is safer and cheaper. The solvents are methanol, toluene, benzene, propanone, ethanoic acid, etc. Methanol is often used as a solvent in industry because of its small chain transfer constant, and the production of polyvinyl alcohol does not require the separation of methanol and can be directly alcoholized.
The alcoholysis of polyvinyl acetate can be carried out using acid as the catalyst, called acid alcoholysis. It can also use alkali as a catalyst, called alkali alcoholysis. In the industry, acid alcoholysis is rarely used because of its unstable and dark products. There are two types of alkali alcoholysis: wet and dry. Because the wet alcoholysis produces a large amount of sodium acetate, part of which is carried into the product, affecting the quality of the product, and part of which remains in the alcohol solution and needs to be neutralized with a large amount of sulfuric acid into sodium sulfate, there is a possibility of pollution of the environment. In the recovery of acetic acid and methanol, due to the large amount of acetic acid and methanol in the final production liquid, the azeotropes of acetic acid and methanol are usually first distilled out in the azeotropic distillation column, and then mixed with water in the water extraction distillation column to obtain separation. The ethanoic acid and methanol are hydrolyzed by ion exchange resin in the hydrolyzer to obtain a mixture of ethanoic acid and methanol, which is then separated by a water extraction distillation column. The methanol from the azeotropic distillation column and the water extraction distillation column can be reused.
2. Polyvinyl Alcohol Aqueous Solution Properties
The properties of PVA aqueous solutions include water solubility, viscosity, salt tolerance, gelation and adhesion.
2.1 The Water Solubility of PVA
The solubility of polyvinyl alcohol varies greatly with the degree of alcoholysis, and the best water solubility is achieved at 87% to 89%.
It dissolves quickly in both cold and hot water, and shows the best solubility. Products with an alcohol solubility of 89% to 90% or more generally need to be heated to 60 to 70 ℃ to dissolve. Polyvinyl alcohols with an alcohol solubility of 99% or more can only be dissolved in hot water at 95 ℃. And the alcohol solubility of 7 5% to 80% of the product only dissolved in cold water, not in hot water. When the alcohol solubility is less than 6%, the water solubility decreases due to the increase of the water-repellent ethyl phthalate group. Until the degree of alcoholysis reaches below 50%, polyvinyl alcohol is no longer soluble in water. For the products of the above varieties, the solubility of polyvinyl alcohol is reduced on one day. If the solution is made into an aqueous solution, it will not precipitate out of the solution when it is cooled.
2.2 PVA Aqueous Solution Viscosity
The viscosity of aqueous polyvinyl alcohol solutions varies with species, concentration and temperature. The viscosity value increases sharply with increasing concentration, and decreases significantly with increasing temperature. For different species of polyvinyl alcohols, the viscosity of aqueous solutions with high alcoholic solubility increases over time until they become gels. In general, the rate of change is proportional to the concentration of the solution and inversely proportional to the temperature. The stability of the solution can be improved by extending the dissolution time or increasing the stirring. The higher the degree of polymerization, the higher the concentration, and the lower the stability of the viscosity of aqueous solutions of polyethylene intoxication.
2.3 Salt Tolerance and Gelation of PVA in Aqueous Solution
Polyvinyl alcohol aqueous solutions show high tolerance to ammonium hydroxide, acetic acid and most inorganic acids. High tolerance to salts, such as sodium nitrate, ammonium chloride, sodium chloride, zinc chloride, potassium iodide and potassium thiocyanate.
Polyvinyl alcohol is particularly sensitive to borax and can lose its fluidity even when mixed with small amounts of borax or boric acid in aqueous solution. The minimum amount of borax and boric acid required for the gelation of aqueous polyvinyl alcohol decreases as the alcohol solubility decreases. In addition to borax and boric acid, some chromium compounds are available. Such as potassium chromate and dichromate can also cause polyethylene aqueous solutions to gel.
2.3 Adhesive Strength of PVA Aqueous Solution
Polyvinyl alcohol is highly soluble on porous, hydrophilic surfaces such as paper, textiles, wood and leather. It is also an effective adhesive for pigments and other fine solid particles. For smooth, non-absorbent surfaces, such as glass and metal, the bonding power decreases with increasing drunkenness. As an adhesive, the wet adhesion of polyvinyl alcohol is usually used.
3. The Application of PVA in Paper Industry
Polyvinyl alcohol is mainly used in the paper industry as a pigment binder for paper coating, as a sizing agent for paper and board, and as a binder for paper and board converting.
3.1 Pigmented Adhesives for Paper Coating and Processing
Polyvinyl alcohol is a very good pigment binder. In the past, the most commonly used pigment binder was animal glue. Later on, casein was widely used. Because casein is derived from milk, only 3k g of casein is extracted from 100k g of milk, limiting its use in coated paper both in terms of resources and cost. The use of polyvinyl alcohol instead of casein results in significant savings in the cost of binders and reduces the cost of paper. Since the use of polyvinyl alcohol as a binder can be lower than other binders, and it provides a coating with stronger coverage and better gloss, it is possible to replace all or part of the expensive titanium dioxide with inexpensive raw materials such as clay.
Some paper mills now use starch instead of casein, which also has some effect. However, polyvinyl alcohol also has many advantages over starch. For example, 1,000 kg of porcelain clay requires 3 to 5 kg of polyvinyl alcohol, while with starch, it takes 14 to 20 kg to achieve the same stain resistance and hiding power. One part of polyvinyl alcohol can generally replace 4-6 parts of starch, because the adhesion of polyvinyl alcohol is 3-4 times that of casein and starch. In addition to lower dosage, better whiteness, color, thickness and gloss, polyvinyl alcohol coating has improved stain resistance, ink absorption, softness and water resistance than casein and starch coating. Moreover, the quality of polyvinyl alcohol is stable and not easy to rot and deteriorate.
In order to make the viscosity of the finishing agent meet the processing requirements and to improve the quality of the paper product, pigment binders, especially for grease-resistant coated paper, are often used in combination with polyvinyl alcohol and synthetic emulsions. In this case, the specification of polyvinyl alcohol and the ratio of the two have an impact on the fluidity of the coating, the surface strength of the coated paper and the water resistance of the coating. The higher the degree of polymerization of polyvinyl alcohol, the higher the surface strength of the coated paper, but the flowability of the coating decreases. The higher the degree of polyvinyl alcohol dissolution, the better the water resistance of the coating, but the flowability of the coating decreases. The degree of polymerization has a greater influence in this respect. The higher the emulsion ratio, the better the flow of the coating and the water resistance of the coating, but the surface strength of the paper decreases. For this reason, polyvinyl alcohols used as binders for paper pigments are selected for their high degree of polymerization and alcohol solubility.
3.2 Sizing Agents for Paper and Board
The film of polyvinyl alcohol is strong, tough, abrasion resistant, adhesive and oil resistant. Therefore, it can be used for both internal and surface application. Polyvinyl alcohol is applied in the form of an aqueous solution by roller pressing, spraying or calendering.
(1) Internal sizing
Small amounts of polyvinyl alcohol effectively increase the strength and toughness of high grade paper such as securities paper, ledger paper, index paper, and graph paper. In cellophane, polyvinyl alcohol improves strength, transparency, gloss, grease resistance and dimensional stability of the paper. Polyvinyl alcohol can be used with general plasticizers and transparency agents, after the paper is formed on the glue calendering. The common concentration of polyvinyl alcohol in aqueous solution is 5%.
(2) Surface sizing
To improve the physical strength of the paper, or to improve the surface tension and printability of the paper, oxidized starch can be used as a surface sizing agent. Polyvinyl alcohol instead of starch as a surface sizing agent has been successful, after sizing can greatly improve the paper’s oil resistance, resistance to organic chemicals, tear strength, folding strength and other properties. As a surface sizing agent, polyvinyl alcohol can be used alone or in combination with oxidized starch. In particular, polyvinyl alcohol and starch oxide are often used in the production of high-quality paper. This combination provides thixotropic properties to the stock, thus reducing the penetration of the stock into the paper and providing greater compressibility and reproducibility of the web during printing. Polyvinyl alcohols used as sizing agents are generally 1700 degree of polymerization and fully alcoholized grades.
3.3 Adhesives for Paper and Board Processing
Polyvinyl alcohols are widely used as industrial adhesives for paper and cardboard, to bond paper or cardboard. These include adhesives for bag and box making and paper laminating adhesives, the latter for fiberboard, linerboard, etc. Partially alcoholized polyvinyl alcohols are used as rewetting adhesives, while fully alcoholized polyvinyl alcohols and starch are used as adhesives for water-resistant paper. Suitable starch materials are: pearl buckwheat starch and hydroxyethyl starch. Polyvinyl alcohol/clay can also be used as a paper binder. Mixtures of polyvinyl alcohols and clays are effective as laminating binders. They are highly viscous, fast curing, water resistant and inexpensive.