Production and Application of α-cyanoacrylate Adhesives (I)

Alpha-cyanoacrylate as an adhesive was occasionally found in a laboratory in the United States in 1955. Originally selected as a raw material for synthetic fibers, the prisms were glued instantly when the refractive index was measured. Since then, cyanoacrylates have evolved into a special gelling agent today for more than 30 years. It is widely used due to its advantages such as convenient use, fast curing, and strong permeability. Since 1962, China's production as a commodity has now reached a production volume of more than 20 tons, and the number of varieties has increased. Compared with Japan's annual output of more than 400 tons, the gap is very large.

I. Rapid synthesis of α-cyanoacrylates

The synthesis method we introduced was the addition condensation reaction of aqueous formaldehyde solution and ethyl cyanoacetate in the presence of a basic catalyst, and the use of dichloroethane as the reaction medium and dehydration to simplify the operation steps and shorten the reaction time. Improve equipment utilization. There are several methods for synthesizing d-cyanoacrylates from the reaction of cyanoacetate and formaldehyde. But also with the catalyst, temperature, acid medium, pH value, time has a great relationship, the yield is also different.

1. The choice of formula

Table 1 shows the relationship between the amount of formaldehyde and the yield. In actual production, it is better to use approximately equimolar formaldehyde. Otherwise, the output will be reduced.

In addition, the purity of ethyl cyanoacetate is required to be over 94%, and the content of organic acid is less than 0.1%. The reason is that if the impurities are mainly the presence of diethyl malonate, they may play a role in the reaction. reaction. This consumes the amount of formaldehyde, which affects the quality and yield of the product. Organic acids can cause the alkaline catalyst to lose its effect and affect the reaction rate.
RCOOH+OH-=RCOO-+H2O

Condensation reaction can also be caused by the formation of by-products from the condensation reaction intermediate product, which affects the polymer formation and reduces the yield.

2. Rapid synthesis of reaction medium

The method of adding less and post-adding the solvent is because the dilution effect will not occur due to the excessive addition of the solvent, the concentration of the reactants will be reduced, the speed of the addition and shrinkage reaction will be significantly reduced, and the utilization of the equipment will be improved. In the reaction, we used dehydration while adding formaldehyde, so that the reaction proceeded to the production of polymers, reducing the hydrolysis reaction. Accelerate the reaction and shorten the reaction time. According to our actual production conditions, the polymerization reaction and dehydration can be completed in 2 to 2.5 hours, which is half the time compared to the traditional process.

3. The role and amount of catalyst

Basic catalysts such as hexahydropyridine, sodium hydroxide, diethylamine, and the like. Add formaldehyde, mix and react with cyanoacetic acid vinegar. The amount of catalyst should be appropriate. Excessive amount can accelerate the reaction, but it can also cause side reactions and reduce the yield, because it will cause a large amount of hydrolysis of ethyl cyanoacetate. However, the amount of the reaction is too slow, and the amount of formaldehyde used should be 0.025-0.5g per mol of formaldehyde for multiple tests. At the same time, the method of supplementing the catalyst is used in the process, so that the yield is significantly improved.

4. The role of plasticizers

We commonly used in the reaction of plasticizing cultivation of tricresyl phosphate vinegar, butyl phthalate, phthalic acid dioctyl vinegar and so on. In the addition condensation reaction, the addition of the plasticizer can avoid the viscosity increase at the later stage of the reaction, and it is difficult to stir, resulting in difficulty in dehydration and prolonging the reaction time. During the cracking reaction, the plasticizer brush acts as a heat transfer medium, lowers the cracking temperature, has fewer side reactions, and has a lighter color and a higher yield. In the finished product, a quantity of plasticizer is present, which can increase the adhesion and softness. According to our production experience, its dosage is appropriate, and it is more unfavorable to the reaction, and it will increase the cost and reduce the yield loss ( Refer to Table 2).

5. Effect of phosphorus pentoxide

After the condensation reaction is completed, the solvent is added for dehydration. However, it is impossible to complete the dehydration process, and the product requirements cannot be satisfied.

Containing more than 0.5% of water, it is very important to choose the proper dehydrating agent. The more application of phosphorus pentaoxide, it can remove the remaining small amount of water. In addition, P2O5 is also used as a catalyst for the cracking of crude monomers, making the cleavage reaction easy.