Polyvinyl alcohol (PVA) is a colorless, non-toxic, non-corrosive and biodegradable water-soluble organic polymer. At present, in addition to being used as a raw material for vinylon, PVA is increasingly widely used in industrial fields such as textile pulp, coatings, adhesives, emulsifiers, and films. However, PVA molecules contain a large number of hydroxyl groups, resulting in poor water resistance and stability of PVA, which affects its application. In chemical fiber industrial applications, PVA has shortcomings such as skinning, foaming, and insufficient adhesion to fibers. Therefore, chemical modification methods are widely used to improve its water resistance. The modification of PVA mainly uses the double bond of vinyl acetate, the chemical activity of ester group and hydroxyl group after alcoholization to change the side chain group or structure, introduce other monomers to become PVA-based copolymers; or introduce other functional groups to change the chemical structure of PVA macromolecules. Five modification methods are briefly described below.
Polyvinyl alcohol (PVA) contains a large number of hydrophilic hydroxyl groups, which show strong hydrophilicity in the external dry and wet changes. Epoxy resins with high bonding strength and good stability are used as modifiers, mainly because the epoxy groups in the epoxy resin can react with the hydroxyl groups in the polyvinyl alcohol to form ethers.
Take the metered polyvinyl alcohol and water, and add them to the three-necked bottle equipped with the stirring and reflux device respectively. Start stirring, raise the temperature to 90 ° C, and obtain a certain concentration of polyvinyl alcohol aqueous solution after the thermal insulation reaction for 1h. Lower the temperature to 70 ° C, speed up the stirring speed, take a quantitative epoxy resin and place it in the glue, and then the epoxy resin modified polyvinyl alcohol is obtained after the thermal insulation reaction for 2h.
Through orthogonal experiments, it was determined that the mass concentration of polyvinyl alcohol was 8%, the modification time was 2 hours, the amount of epoxy resin added was 2.4% (mass fraction), and the modification temperature was 60 ° C. The modified polyvinyl alcohol prepared under these conditions had excellent properties, and the curing degree was 89.6%, which was greatly improved compared with the curing degree of unmodified polyvinyl alcohol 64.5%.
The water resistance is improved by cross-linking polyvinyl alcohol with maleic acid (MA). The esterification and cross-linking of PVA membranes is achieved through the esterification reaction between PVA molecules and MA at high temperature. The esterification and cross-linking reaction actually introduces a carbonyl group between the polymer chains of PVA to form a new polymer, but the main chain of the entire PVA polymer remains unchanged.
The PVA after esterification with MA only chemically crosslinked and did not produce a stable crystalline structure. Such a PVA film is easily swollen by water and destroys the structure of the film. Therefore, the PVA film needs to be heat treated. Because the esterification reaction in the liquid phase is characterized by reversibility, the crosslinking of polymers in the PVA film during the heating process makes the esterification reaction irreversible due to the complete volatilization of the component water in the casting liquid and the water produced by the esterification reaction between PVA and MA, and the crosslinked PVA film is stable.
PVA improves the water resistance and poor mechanical properties of PVA through chemical crosslinking with MA. Appropriate concentration of crosslinking agent and heat treatment conditions can make PVA obtain better water resistance.
By adding nano-scale reinforcements to the composite matrix, the mechanical properties (such as strength, stiffness, elastic modulus, etc.) of the composite can be significantly improved. The polyvinyl alcohol adhesive modified with nano-silica particles is non-toxic and pollution-free, and will be widely used, which is worthy of systematic research.
The current research on the modification of nanoparticles mainly focuses on the following two aspects: One is to use silica as a base, modify its surface with a modifier, and then graft the polymer; the other is to use the polymer as a base, and then graft the modifier, and then graft the silica. Modification of nano-silica is generally to disperse it in an organic solvent and then add a modifier.
The addition of inorganic nanoparticles to the adhesive for modification can improve its tensile bond strength and elongation at break. When the amount of nanoparticles is within a certain range, the nanoparticles can be well dispersed into the adhesive matrix, because it has a huge specific surface area, which can strongly interact with the adhesive matrix, thereby improving the mechanical properties of the adhesive. However, if the amount exceeds a certain range, it will lead to serious agglomeration phenomenon, the particle interface area is reduced, the interaction between the nanoparticles and the adhesive matrix is correspondingly weakened, and the content of reactive nanoparticles is reduced, resulting in a decrease in its mechanical properties. The test results of mechanical properties of nano-modified adhesives show that when the content of nano-silica particles is 4%, the performance indicators of the modified adhesives reach the maximum value.
The polyvinyl acetal mixed aldehyde adhesive can be prepared using polyvinyl alcohol (PVA) and butenal as main raw materials, hydrochloric acid (HCl) as catalyst and acetaldehyde as modifier.
Put a certain amount of PVA into a three-mouth flask equipped with a mechanical stirrer and a reflux condensing device, add deionized water, adjust the temperature of the water bath to 95 ° C, and keep it warm for 2h to fully dissolve it; when the solution is cooled to room temperature, slowly add a dosing amount of HCl while stirring, and mix it well; the water bath is heated to the specified temperature, add butenaldehyde and acetaldehyde solutions according to the formula, and stir well; after the reaction is completed, adjust the pH value to 8~ 9 with NaOH solution, then add an appropriate amount of urea, and stir for 20 minutes.
The results show that when the reaction temperature is (90 ± 2) ℃, the reaction time is 4 h, the 8% PVA solution is 200 mL, the HCl is 1 mL, the butenaldehyde is 1.0~ 1.5 mL and the acetaldehyde is 4 mL, the viscosity of the acetalated product is moderate, the bonding strength is relatively maximum (4.5 MPa) and the water resistance is relatively good; under the premise of other conditions remaining unchanged, by changing the amount of butenaldehyde, the final viscosity of the system can be further adjusted to meet the requirements of wood adhesives.
Using succinic acid as the crosslinking agent, the ester group is formed through the reaction of COOH- and OH-, and the PVA molecule is crosslinked to form a modified PVA glue that is insoluble in water. The introduced COOH- group can improve its water resistance, hardness, adhesion, etc.
Weigh an appropriate amount of PVA, add water, heat the water bath with electric stirring, control the water bath temperature to 80-90 ° C. After it is 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 glue, stir it under closed conditions to react, and cool to room temperature to obtain a modified PVA glue.
With succinic acid as the crosslinking agent, the PVA glue was modified, and the optimum modification conditions were determined as: PVA glue mass concentration 7%, reaction temperature 85 ℃, PVA glue and succinic acid mass ratio 5.6:1. Under these conditions, the hardness, adhesion, viscosity and impact resistance of the modified PVA glue were significantly improved, and the water resistance was also improved. This method can be used to prepare PVA adhesives and coatings with high adhesion and water resistance requirements.
