Polyvinyl alcohol (Pva) is a colorless, non-toxic, non corrosive, biodegradable water-soluble organic polymer. At present, in addition to being used as raw material of vinylon, PVA is increasingly widely used in industrial fields such as textile size, coating, adhesive, emulsifier, film and so on. However, PVA contains a large number of hydroxyl groups, which leads to poor water resistance and stability of PVA, thus affecting its application. In the application of chemical fiber industry, PVA has shortcomings such as crusting, 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 chemical activity of double bond, ester group and hydroxyl group after alcoholysis of vinyl acetate to change the side chain group or structure and introduce other monomers into PVA based copolymers; Or introduce other functional groups to change the chemical structure of PVA macromolecules. Five modification methods are briefly introduced below.
1. Epoxy resin modification
Polyvinyl alcohol (PVA) contains a large number of hydrophilic hydroxyl groups, which show a strong affinity to water in the external dry and wet changes. The epoxy resin with high bonding strength and good stability is used as modifier, mainly because the epoxy group in the epoxy resin can react with the hydroxyl group in polyvinyl alcohol to form ether.
Take the measured polyvinyl alcohol and water, and add them to the three mouth bottle equipped with stirring and reflux devices respectively. Start the stirring, raise the temperature by 90 ℃, and prepare a certain concentration of polyvinyl alcohol aqueous solution after heat preservation reaction for 1H. Reduce the temperature to 70 ℃, speed up the mixing speed, take a certain amount of epoxy resin and put it in the glue. After the heat preservation reaction for 2h, the epoxy resin modified polyvinyl alcohol is obtained.
Through orthogonal experiments, the optimum reaction conditions were determined as the mass concentration of polyvinyl alcohol was 8%, the modification time was 2h, the addition amount of epoxy resin was 2.4% (mass fraction), and the modification temperature was 60 ℃. The modified polyvinyl alcohol prepared under this condition has excellent properties, and the curing degree is 89.6%, which is significantly higher than that of unmodified polyvinyl alcohol, which is 64.5%.
2. Maleic acid modification
Through the crosslinking reaction between polyvinyl alcohol and maleic acid (commonly known as maleic acid, MA), its water resistance is improved. The esterification crosslinking of PVA membrane is realized by the esterification reaction between PVA molecule and Ma at high temperature. The esterification crosslinking reaction actually introduces carbonyl between the polymer chains of PVA to form a new polymer, but the main chain of the whole PVA polymer remains unchanged.
PVA after esterification with Ma only has chemical crosslinking, and does not produce a stable crystalline structure. Such PVA film is easy to be swelled by water and destroy the structure of the film. Therefore, PVA film should be heat treated. Because the esterification reaction in the liquid phase is reversible, and the crosslinking of polymers in PVA film during the heating process, the esterification reaction becomes irreversible due to the complete volatilization of the component water that has not been removed in the casting solution and the water produced by the esterification reaction between PVA and Ma, and the PVA film with crosslinked structure is stable.
PVA is chemically crosslinked with Ma to improve its water resistance and poor mechanical properties. Proper crosslinking agent concentration and heat treatment conditions can make PVA obtain better water resistance.
3. Nano silica modification
The mechanical properties (such as strength, stiffness, elastic modulus, etc.) of the composite can be significantly improved by adding nano reinforcements to the composite matrix. The polyvinyl alcohol adhesive modified by nano silica particles is non-toxic and pollution-free, and will be widely used, which is worthy of systematic research.
At present, the research on the modification of nanoparticles mainly focuses on the following two aspects: first, based on silica, modify its surface with modifier, and then graft polymer; The second is based on polymer, then grafted with modifier, and then grafted with silica. The modification of nano silica is generally to disperse it in organic solvents, and then add modifiers.
The tensile bond strength and elongation at break can be improved by introducing inorganic nanoparticles into the adhesive for modification. When the content of nanoparticles is in a certain range, nanoparticles can be well dispersed into the adhesive matrix. Because of their huge specific surface area, they can have a strong interaction with the adhesive matrix, thereby improving the mechanical properties of the adhesive. However, if the content exceeds a certain range, it will lead to serious agglomeration, the particle interface area will be reduced, the interaction between nanoparticles and adhesive matrix will be correspondingly weakened, and the content of reactive nanoparticles will be reduced, resulting in the reduction of their mechanical properties. The test results of mechanical properties of nano modified adhesive show that when the content of nano silica particles is 4%, the performance indexes of the modified adhesive reach the maximum.
4. Butenal modification
Polyvinyl acetal adhesive can be prepared by using polyvinyl alcohol (PVA) and butyraldehyde as main raw materials, hydrochloric acid (HCl) as catalyst and acetaldehyde as modifier.
Put a certain amount of PVA into a three port flask equipped with a mechanical stirrer and a reflux condensing device, add deionized water, adjust the water bath temperature to 95 ℃, keep it for 2h, and make it fully dissolved; When the solution is cooled to room temperature, add the measured HCl slowly while stirring, and mix it well; Heat up the water bath to the specified temperature, add butenal and acetaldehyde solution according to the formula, and fully stir; After the reaction, adjust the pH value to 8 ~ 9 with NaOH solution, add an appropriate amount of urea, and stir for 20min.
The results showed that when the reaction temperature was (90 ± 2) ℃, the reaction time was 4 h, 8% PVA solution was 200 ml, HCl was 1ml, butyraldehyde was 1.0 ~ 1.5ml and acetaldehyde was 4ml, the viscosity of the acetalization product was moderate, the bonding strength was relatively maximum (4.5MPa) and the water resistance was relatively good; On the premise that other conditions remain unchanged, 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
With succinic acid as crosslinking agent, through the reaction of cooh- and oh-, ester group is generated, which makes PVA molecules cross-linked and generates modified PVA adhesive that is difficult to dissolve in water. The introduction of COOH group can improve its water resistance, hardness, adhesion and so on.
Weigh an appropriate amount of PVA, add water, heat in the electric stirring water bath, control the water bath temperature to 80-90 ℃, and stop heating after complete dissolution 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 make it react, and cool it to room temperature to obtain the modified PVA glue.
Using succinic acid as crosslinking agent, PVA adhesive was modified. The optimal modification conditions were determined as follows: PVA adhesive mass concentration 7%, reaction temperature 85 ℃, PVA adhesive and succinic acid mass ratio 5.6:1. Under these conditions, the hardness, adhesion, viscosity and impact resistance of the modified PVA adhesive were significantly improved, and the water resistance was also improved. This method can be used to prepare PVA adhesives and coatings with high requirements for adhesion and water resistance.
