Study on Modification of Polyvinyl Acetate Copolymer Adhesive (3)

3 solvent selection

Solvents are a must for most adhesives and coatings, and they are also responsible for the toxicity of these products. When the organic benzene system is selected as the main solvent, toxic and odorous gases are released during production and construction: the process even after construction for a long period of time, causing environmental pollution, harming human health, and prohibiting food and drug packaging materials. When water is used as a solvent to prepare polyvinyl acetate or multi-copolymer resin, there are shortcomings such as slow drying speed, easy gelation at a low temperature, and inapplicability. If industrial alcohol is used as a solvent, the problem of toxicity and drying speed is solved, and the problem of low temperature application is solved.

After a lot of experiments in this study, we found that the optimal amount of industrial ethanol for binary copolymerization is 90 parts. At this time, the viscosity is appropriate and the system is even and stable. For the terpolymer, when the amount of ethanol is 90 parts, the viscosity is large and insoluble. The substance precipitated out, but when the amount was 100 parts, the viscosity was suitable and the system was stable. Therefore, the optimal amount of ethanol for the binary polymerization solvent is 90 parts, and the optimal amount for the terpolymerization is 100 parts.

4 initiator selection and dosage

The choice and amount of initiator plays a role in the polymerization reaction and is the key to the success or failure of polymer synthesis. On the basis of considering the low cost, benzoyl peroxide was selected as the main initiator, which can be dissolved in the mixed solution of alcohol and monomer to ensure that the free radicals of the system and the household are relatively uniform, and the reactions in various parts of the system are relatively uniform, making The molecular weight distribution of the polymer is reasonable and has a price advantage. However, when developing each specific product, other initiators may be added as appropriate according to the specific circumstances. The activity and efficiency of the initiator should also be noted. For example, acrylonitrile can act quickly with an initiator to initiate chain growth, thus leading to high efficiency. In contrast, vinyl acetate-like low-activity monomers have a weaker ability to capture free radicals and therefore have lower initiation efficiency. The amount of initiator should be appropriate, the amount of initiator is small, the polymerization speed is slow, the monomer reflux is fast, the conversion rate is low, the polymer molecular weight is large, and the system is turbid and poor in solubility. When the amount of the initiator is too small, the reaction system cannot be properly initiated and the reaction is easily terminated. On the other hand, when the dose is increased, the number of free radicals in the system increases and the reaction rate increases. However, due to too many reaction centers, the average molecular weight of the polymer decreases, the viscosity is small, and the aggregation due to the heat of reaction can easily cause burst polymerization.

The initiator efficiency is related to factors such as monomers, solvents, and system viscosity. So far, the amount of initiator is often determined after a large number of experiments. Under the premise of ensuring the initiation, the amount of initiator used in the binary copolymerization was 2.7%-3.6% of the unit volume, and the amount of initiator used in the terpolymerization was 1.8%-2.7% of the monomer mass.

Effect of temperature

The temperature selected during the polymerization has a great influence on the polymerization rate, the average molecular weight of the polymer, and the like. The higher the reaction temperature, the faster the initiator decomposes, and the higher the free radical concentration. The chain growth and chain termination must also be accelerated. That is to say, the reaction rate is accelerated and the degree of polymerization is also reduced; on the other hand, the temperature is too high. The heat of polymerization is not easily discharged and the reaction is difficult to control. If the temperature is too low, the induction period of the chain initiation reaction will be prolonged, and once the reaction occurs, the chain growth will be rapidly accelerated, the exothermic reaction will be violent, the reaction temperature will rapidly increase, and the solvent will cause a sharp rise in the viscosity of the system due to the large amount of gasification, and even the explosion will occur. It is dangerous to cause an accident.

Therefore, when carrying out the polymerization reaction, the reaction temperature should be selected in consideration of the requirements of the product on the size of the molecular weight and the conversion rate and the like. At the same time, the reaction temperature should not be arbitrarily increased in order to accelerate the polymerization reaction, but the added initiator should be maintained in the reaction system for a certain period of time and continue to be effective. In this study, the effect of temperature on the binary copolymerization and terpolymerization of vinyl acetate was investigated. After repeated practice, the final reaction temperature was selected as the reflux temperature, and the operation was very convenient.

Effect of polymerization time

Under normal circumstances, the polymerization reaction time is prolonged, the monomer residual amount is small, and the conversion rate is high, which shows that the viscosity of the polymer glue liquid increases and the dissolution performance deteriorates. This experiment examines the relationship between polymerization time and monomer conversion (see Figure 2, C for binary polymerization, and D for terpolymerization).

It can be seen from Figure 2 that as the polymerization time increases, the monomer conversion rate increases, and when the time exceeds 7 hours, the conversion rate basically remains unchanged, but the conversion rate slightly decreases after 9 hours, so the polymerization time is selected as 7 - 9 hours.

(to be continued)