Mercaptan and Polysulfide Curing Agents

Polymercaptans and poly sulfides are aliphatic oligomers containing sulfohydro (-SH) groups that will react with epoxy groups at room temperature to form cured epoxy structures and epoxy adducts. A generalized structure is shown in Fig. 5.11. 

FIGURE 5.11 Generalized structure of polysulfide used in epoxy technology. 

Polymercaptans, which cure at 0° to — 20°C, are attracting attention in low-temperature curing adhesive formulation. At normal room temperature, polymercaptan has a pot life of 2 to 10 min and reaches handling strength in 10 to 30 min. An example of a typical mercaptan and its reaction sequence with an epoxy group is shown in Fig. 5.12. 

An epoxy-polymercaptan reaction that is catalyzed with a tertiary amine is used in the standard two-component 5-min curing epoxy which can be found in the hardware stores. These fast-curing products, however, have a tendency to be somewhat brittle and may perform quite poorly under peel stress. The standard 5-min cure is obtained with the accelerated mercaptan, such as Capcure 3830-81 (Cognis Corporation). The fastest polymercaptan has a gel time of 40 s in a 25-g mass. 

The chemistry of epoxy/mercaptan systems involves the tertiary amine catalyst forming a salt with the mercaptan to generate a mercaptide anion, which is a strong nucleophile. The mercaptide will readily open the epoxy ring. Reaction with another mercaptan group can regenerate the mercaptide anion, as shown in Fig. 5.13. 

---