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Five Methods of Polyvinyl Alcohol (PVA) Modification

Views: 262     Author: Site Editor     Publish Time: 2023-07-24      Origin: Site


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Polyvinyl alcohol (PVA) is a colourless, non-toxic, non-corrosive, biodegradable water-soluble organic polymer. At present, in addition to being used as raw materials for polyvinyl chloride, PVA is increasingly widely used in textile pastes, coatings, adhesives, emulsifiers, films and other industrial fields. However,

Polyvinyl alcohol molecule contains a large number of hydroxyl groups, resulting in PVA water resistance, stability is relatively poor, thus affecting its application. In the chemical fibre industry, PVA has shortcomings such as skinning, blistering and insufficient adhesion to fibres. Therefore, it is common to use chemical modification methods to improve its water resistance, PVA modification is mainly the use of vinyl acetate double bond, ester group and alcohol after the hydroxyl group of chemical activity, change the side chain group or structure, introduction of other monomers to become a PVA-based copolymer; or introduction of other functional groups, change the chemical structure of the PVA macromolecule. The following is a brief introduction to the five modification methods.

1. Epoxy resin modification

Polyvinyl alcohol (PVA) contains a large number of hydrophilic groups of hydroxyl groups, in the external dry and wet changes, showing a strong hydrophilic co-operation. Epoxy resin, which has high bond strength and good stability, is used as the modifier, mainly because the epoxy group in epoxy resin can react with the hydroxyl group in polyvinyl alcohol to form ether.

Measured amounts of polyvinyl alcohol and water were added into a three-necked flask equipped with stirring and refluxing devices. Start stirring, increase the temperature to 90 ℃, hold the reaction for 1h to obtain a certain concentration of polyvinyl alcohol aqueous solution. Cool down to 70 ℃, accelerate the stirring speed, take a quantitative epoxy resin in the glue, holding reaction 2h after the epoxy resin modified polyvinyl alcohol.

Through orthogonal experiments, it was determined that the optimum reaction conditions were 8% of polyvinyl alcohol, 2h of modification time, 2.4% of epoxy resin (mass fraction), and 60℃ of modification temperature. The modified poly(vinyl alcohol) produced under these conditions had excellent properties, with a degree of cure of 89.6%, which was substantially improved compared with that of the unmodified poly(vinyl alcohol) with a degree of cure of 64.5%.

2. Maleic acid modification

Water resistance is improved by cross-linking polyvinyl alcohol with maleic acid (commonly known as maleic acid (MA)).The esterification cross-linking of PVA film is achieved by the esterification of PVA molecules with MA at high temperature, which in fact introduces a carbonyl group between the polymer chains of the PVA, forming a new polymer again, but the main chain of the entire PVA polymer remains unchanged. The PVA film is modified with a polyvinyl alcohol (PVA) to improve water resistance by cross-linking with maleic acid (commonly known as maleic acid). The main chain of the PVA macromolecule remains unchanged.

After the reaction with MA esterification, only chemical cross-linking of PVA occurs, and does not produce a stable crystalline structure, so that the PVA film is easily dissolved by water and destroy the structure of the film. Therefore, the PVA film is subjected to heat treatment. Because the esterification reaction in the liquid phase is characterised by reversibility, and the cross-linking of macromolecules in the PVA membrane during the heating process, the esterification reaction becomes irreversible due to the complete volatilisation of the component water that has not yet been removed from the casting liquid as well as the water produced by the esterification reaction between PVA and MA, and the PVA membrane of the cross-linking structure is stabilised.

PVA is chemically cross-linked with MA, thus improving the disadvantages of PVA such as poor water resistance and mechanical properties. Proper concentration of cross-linking agent and heat treatment conditions can make PVA obtain better water resistance.

3. Nano-silica modification

The mechanical properties of composites (e.g., strength, stiffness, modulus of elasticity, etc.) can be significantly improved by adding nanoscale reinforcement to the composite matrix. The polyvinyl alcohol adhesive modified with silica nanoparticles is non-toxic, non-polluting, and will be widely used, so it is worthwhile to carry out systematic research.

Nanoparticle modification of the current research is mainly focused on the following two aspects: one is based on silica, with a modifier to modify its surface, and then grafted polymer; the second is based on polymer, and then grafted modifier, and then grafted silica. Modification of nanosilica is generally dispersed in organic solvents, and then add modifiers.

The introduction of inorganic nanoparticles in the adhesive for modification can improve its tensile bond strength and elongation at break. When the nanoparticles are doped in a certain range, the nanoparticles can be well dispersed into the adhesive matrix, and because of their huge specific surface area, they can have strong interaction with the adhesive matrix, thus improving the mechanical properties of the adhesive. However, if the doping amount exceeds a certain range, it will lead to serious agglomeration phenomenon, the particle interfacial area is reduced, the interaction between nanoparticles and adhesive matrix is also weakened accordingly, and the content of reactive nanoparticles is reduced, which leads to the reduction of its mechanical properties. The mechanical properties of nano-modified adhesive test results show that the doping amount of nanosilica particles is 4%, the modified adhesive performance indicators reach the maximum value.

4. Butenal modification

Polyvinyl alcohol (PVA) and butenal as the main raw material, hydrochloric acid (HCl) as a catalyst and acetaldehyde as a modifier, you can prepare polyvinyl alcohol shrinkage mixed aldehyde adhesive.

A certain amount of PVA into a mechanical stirrer and reflux condensing device equipped with three flasks, add deionised water, adjust the water bath temperature to 95 ℃, holding 2h, so that it is fully dissolved; to be the solution cooled to room temperature, while stirring, slowly add measured drops of HCl, fully mixed; water bath temperature to the specified temperature, according to the formula to add butenal and acetaldehyde solution, fully stirred. At the end of the reaction, the pH was adjusted to 8-9 with NaOH solution, and then appropriate amount of urea was added and stirred for 20 min.

The results showed that: when the reaction temperature was (90±2)℃, the reaction time was 4 h, the 8% PVA solution was 200 mL, HCl was 1 mL, butenal was 1.0-1.5 mL and acetaldehyde was 4 mL, the viscosity of the acetalisation product was moderate, the adhesive strength was relatively maximal (4.5MPa), and the water resistance was relatively good; under the premise of other conditions remaining unchanged, the final viscosity of the system could be further adjusted by changing the amount of butenal. Under other conditions, by changing the amount of butenal, the final viscosity of the system can be further adjusted to meet the requirements of wood adhesives.

5.Succinic acid modification

Succinic acid is used as a cross-linking agent, and through the reaction between COOH- and OH-, ester groups are generated to cross-link the PVA molecules and generate modified PVA glue which is difficult to dissolve in water. The introduced COOH- group improves its water resistance, hardness and adhesion.

Weigh the appropriate amount of PVA, add water, under electric stirring water bath heating, control the water bath temperature of 80-90 ° C, completely dissolved, stop heating, to obtain PVA glue. Under a certain temperature water bath, add an appropriate amount of succinic acid to the above PVA gum, stirring under closed conditions to make the reaction, cooling to room temperature, to obtain modified PVA gum.

Using succinic acid as a cross-linking agent, the PVA glue was modified, and the optimal modification conditions were determined as follows: PVA glue mass concentration of 7%, reaction temperature of 85 ℃, and the mass ratio of PVA glue to succinic acid of 5.6:1, under which the hardness, adhesion, viscosity, and impact resistance of the modified PVA glue obtained were significantly improved, and the water resistance performance was also improved. This method can be used to prepare PVA adhesives and coatings with high requirements for adhesion and water resistance.

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