Лако-красочные материалы — производство

As has been noted above, in order for adhesive systems containing methacrylated monomers to cure, a free radical must be generated. A typical means of generating a
free radical is shown in Fig. 3. This method of initiating the cure of acrylic adhesives is also known as a redox reaction. Redox reactions for acrylics typically involve the use of a chemical substance, often an aromatic amine (as shown in Fig. 3). These amines are present in one portion or component of the adhesive. They typically react with another chemical substance—typically a peroxide (also shown in Fig. 3). The result of the reaction of amine and peroxide is the formation of a free radical which is capable of reacting with methacrylated monomers to begin the polymerization process and subsequent cure.

The curing reaction will generally continue as long as there are unreacted monomer molecules present and there are no chemical or other factors present that could interfere with the reaction’s completion. With some formulations, certain surfaces and certain contaminants can interfere with the reaction resulting in incomplete cure of the adhesive, and low bond strength. Surfaces and contaminants to avoid are usually mentioned in the manufacturer’s literature. Barring any unwanted interfering reactions as mentioned above, the polymerization and curing process, once begun, will ultimately lead to the formation of the high molecular weight polymers useful as adhesives.

The complex details of properly initiating and appropriately controlling the chain reactions in acrylic adhesives have been carefully worked out by adhesive chemists (with the most effective ways of achieving this kept secret by companies that supply these adhesive materials). When accomplished skillfully, the final result is a tough high molecular weight complex polymer, useful as an adhesive. A full and complete review of even the published information on this cure chemistry is beyond the scope of this chapter. The reader can gain further insight by consulting the bibliography at the end of this chapter.

It should be noted that in addition to the use of amines and peroxides there have been other initiating reactions reported as methods to generate free radicals and cure acrylic adhesives. One example is shown in Fig. 4. In addition to this example, literature searches will reveal numerous other mechanisms to initiate cure. As seen in Fig. 4, organic compounds used to initiate the reaction can vary considerably; however, they all tend to result in the formation of critically important free radicals. These radicals are capable of

— C — SO/

1

Sulfonate

radical

Figure 4 Amine plus sulfonyl chloride initiation of the cure of acrylic adhesives.

reacting with monomeric species found in the composition, ultimately resulting in poly­merization and cured high molecular weight polymers.