Polymercaptans

Tertiary amine accelerated polymercaptan/ epoxy systems, 10 410 Tertiary amine-carbonate technology,... 

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. 

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 polymercaptans can also be used to accelerate the curing of epoxy resins systems blended with polyamines, amidoamines, or amines. The other curatives serve as the base to accelerate mercaptans, and the mercaptans react rapidly, generating the heat to accelerate the cure with the other hardener. 

The major disadvantages of polymercaptan curing agents are their odor, skinning, and low heat deflection temperature. Progress has been made in the areas of odor and skinning through additives to the adhesive formulation. However, the low heat resistance is an artifact of the epoxy-mercaptan chemistry. 

FIGURE 5.13 Polymercaptan-epoxy chemistry accelerated with a tertiary amine.20... 

A fast-curing polymercaptan-epoxy adhesive formulation is shown in Table 11.11. The gel time for the mixed adhesive is about 8 min at room temperature. Tensile shear strength on aluminum is reported to be 2270 psi at 23°C. Note that the different color pigments used in parts A (carbon black) and B (titanium dioxide) are to assist by visually indicating when the two components are well mixed. 

The primary disadvantages of polymercaptans over other fast-reacting curing systems are (1) skin irritation caused by aromatic mercaptans (aliphatic mercaptans are less der-matitic than common polyamides) and (2) an objectionable sulfur odor, especially when the vapor pressure of the mercaptan is relatively high. 

Accelerated polymercaptan (Epi-Cure 861) Accelerated polymercaptan (Epi-Cure 862) Accelerated aliphatic amine (Epi-Cure 874) 92 99 87 19 46 ... 

Quick-curing epoxy adhesive cured with polymercaptan Starting formulations for rapid-setting, room temperature curing epoxy adhesives... 

Accordingly, the structure of the cured resin with polymercaptans having more than three functional groups would be close to that shown in Fig. 1 a. Thioglycolate of pentaerythritol (PnEThGE) (structure [I] below) is a hardener with four functional groups, and the physical properties of the cured resin can be compared with those cured with diamine or dihydrazide. 

Cp aliphatic dihydrazide, A anhydride, O diamine, polymercaptan, aromatic dihydrazide, A anhydride, diamine... 

Two types of epoxy coatings are formulated those cured at ambient temperature and those that are heat cured. The first type uses amine hardening systems, fatty acid polyamides, and polymercaptans as... 

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