Polyvinyl alcohol (PVA) is a colorless, non-toxic, non-corrosive, biodegradable, water-soluble organic polymer. Currently, besides being used as a raw material for vinylon, PVA is increasingly widely used in textile sizing, coatings, adhesives, emulsifiers, and films. However, the large number of hydroxyl groups in the PVA molecule leads to relatively poor water resistance and stability, thus affecting its applications. In the chemical fiber industry, PVA suffers from drawbacks such as skinning, blistering, and insufficient adhesion to fibers. Therefore, chemical modification methods are commonly used to improve its water resistance. PVA modification mainly utilizes the chemical reactivity of the double bonds, ester groups, and hydroxyl groups after alcoholysis of vinyl acetate to change the side chain groups or structure, introduce other monomers to form PVA-based copolymers, or introduce other functional groups to change the chemical structure of the PVA macromolecule. Five modification methods are briefly introduced below.
1. Epoxy Resin Modification Polyvinyl alcohol (PVA) contains a large number of hydrophilic hydroxyl groups, exhibiting a strong affinity for water under external wet and dry conditions. Epoxy resin, known for its high bonding strength and stability, was chosen as the modifier primarily because the epoxy groups in epoxy resin can react with the hydroxyl groups in polyvinyl alcohol (PVA) to form ethers.
Measured amounts of PVA and water were added separately to a three-necked flask equipped with a stirrer and reflux device. Stirring was initiated, and the temperature was raised to 90°C. After reacting at this temperature for 1 hour, a PVA aqueous solution of a specific concentration was obtained. The temperature was then lowered to 70°C, and the stirring speed was increased. A measured amount of epoxy resin was added to the adhesive solution, and after reacting at this temperature for 2 hours, epoxy resin-modified PVA was obtained.
Through orthogonal experiments, the optimal reaction conditions were determined to be a PVA mass concentration of 8%, a modification time of 2 hours, an epoxy resin addition of 2.4% (mass fraction), and a modification temperature of 60°C. The modified PVA prepared under these conditions exhibited excellent properties, with a curing degree of 89.6%, significantly higher than the 64.5% curing degree of unmodified PVA.
2. Maleic Acid Modification
The water resistance of PVA membranes is improved by crosslinking polyvinyl alcohol (PVA) with maleic acid (MA). Esterification crosslinking of PVA membranes is achieved through a high-temperature esterification reaction between PVA molecules and MA. This esterification reaction introduces carbonyl groups between the PVA polymer chains, reforming a new polymer, while the overall PVA polymer backbone remains unchanged.
After esterification with MA, PVA only undergoes chemical crosslinking and does not produce a stable crystalline structure. Such PVA membranes are easily damaged by water swelling. Therefore, heat treatment is necessary for PVA membranes. Because esterification reactions in the liquid phase are reversible, the crosslinking of polymers in the PVA membrane during heating becomes irreversible due to the complete evaporation of residual water in the casting solution and water generated during the PVA-MA esterification reaction. This stabilizes the crosslinked PVA membrane.
Through chemical crosslinking with MA, PVA improves its poor water resistance and mechanical properties. Appropriate crosslinking agent concentration and heat treatment conditions can enable PVA to achieve better water resistance.
3. Nanoscale Silica Modification
Adding nanoscale reinforcements to the composite matrix can significantly improve the mechanical properties of the composite material (e.g., strength, stiffness, elastic modulus). Polyvinyl alcohol adhesives modified with nanoscale silica particles are non-toxic and pollution-free, and will undoubtedly be widely used, warranting systematic research.
Current research on nanoparticle modification mainly focuses on two aspects: one is using silica as a base, modifying its surface with a modifier, and then grafting polymers; the other is using a polymer as a base, then grafting a modifier, and then grafting silica. Modification of nanoscale silica generally involves dispersing it in an organic solvent and then adding the modifier.
Introducing inorganic nanoparticles into adhesives can improve their tensile bond strength and elongation at break. When the nanoparticle content is within a certain range, the nanoparticles can be well dispersed in the adhesive matrix. Due to their large specific surface area, they can interact strongly with the adhesive matrix, thereby improving the mechanical properties of the adhesive. However, if the content exceeds a certain range, it will lead to severe agglomeration, reducing the particle interface area, weakening the interaction between the nanoparticles and the adhesive matrix, and decreasing the content of reactive nanoparticles, thus reducing its mechanical properties. Mechanical property test results of nano-modified adhesives show that when the content of nano-silica particles is 4%, the various performance indicators of the modified adhesive reach their maximum values.
4. Butenal Modification Polyvinyl alcohol (PVA) and butenal are used as the main raw materials, hydrochloric acid (HCl) as the catalyst, and acetaldehyde as the modifier to prepare polyvinyl alcohol acetal condensate adhesives.
A certain amount of PVA was placed in a three-necked flask equipped with a mechanical stirrer and a reflux condenser. Deionized water was added, and the water bath temperature was adjusted to 95℃ and maintained for 2 hours to ensure complete dissolution. After the solution cooled to room temperature, a measured amount of HCl was slowly added dropwise while stirring, ensuring thorough mixing. The water bath temperature was then raised to the specified temperature, and butenal and acetaldehyde solutions were added according to the formula, followed by thorough stirring. After the reaction was complete, the pH was adjusted to 8–9 with NaOH solution, and an appropriate amount of urea was added. The mixture was stirred for 20 minutes.
The results showed that when the reaction temperature was (90±2)℃, the reaction time was 4 hours, the amount of 8% PVA solution was 200 mL, the amount of HCl was 1 mL, the amount of butenal was 1.0–1.5 mL, and the amount of acetaldehyde was 4 mL, the viscosity of the acetalization product was moderate, the bonding strength was relatively high (4.5 MPa), and the water resistance was relatively good. Under the condition that other factors remained constant, the final viscosity of the system could be further adjusted by changing the amount of butenal to meet the requirements for wood adhesives.
5. Succinic Acid Modification
Using succinic acid as a crosslinking agent, ester groups are generated through the reaction of COOH- and OH-, crosslinking PVA molecules to produce a modified PVA adhesive that is poorly soluble in water. The introduced COOH- groups improve its water resistance, hardness, and adhesion.
Weigh an appropriate amount of PVA, add water, and heat in a water bath with electric stirring, controlling the water bath temperature at 80-90℃. After complete dissolution, stop heating to obtain the PVA adhesive. Add an appropriate amount of succinic acid to the above PVA adhesive in a water bath at a certain temperature, stir under sealed conditions to allow the reaction to proceed, and cool to room temperature to obtain the modified PVA adhesive.
Using succinic acid as a crosslinking agent, the optimal modification conditions for modifying the PVA adhesive were determined to be: PVA adhesive mass concentration 7%, reaction temperature 85℃, and PVA adhesive to succinic acid mass ratio 5.6:1. Under these conditions, the hardness, adhesion, viscosity, and impact resistance of the obtained modified PVA adhesive are significantly improved, and its water resistance is also enhanced. This method can be used to prepare PVA adhesives and coatings that require high adhesion and water resistance.
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