Mercaptoester coupling agents

The serious drop in strength of epoxy bonded carbon steel structural joints exposed to humid environments for extended periods of time is greatly reduced by surface treating the bond surfaces with polyfunctional mercaptoester coupling agents. 

Figure 2. Ethylene mercaptoester polymeric coupling agent (EME) the reaction product of random ethylene/viny alcohol co-polymers with mercaptoacetic acid. The number after the EME (e.g. EME 47) refers to the weight percent of mercaptoester units along the chain. Essentially no vinyl alcohol units remain.

Figure 3. Effect of EME 58 (58 wt% mercaptoester units co-polymer) coupling agent concentration on the peel strength of flexible epoxy (amine-cured)/AD = acetone-degreased steel test panels following (a) I day and (b) 3 day exposure to 57°C condensing humidity. See Appendix 4 for epoxy resin and cure description.

Effect of the coupling agent mercaptoester concentration on the peel adhesion and corrosion protection of epoxy/EME/steel test panels [7]... 

Figure 6. Influence of EME co-polymer coupling agent mercaptoester unit concentation on (a) the dry peel strength and (b) the time in 57 C water until the presence of visible corrosion products was observed for epoxy/steel peel test panels. From ref. 6.

Figure 8. Shear strength durability in a 57°C water immersion of epoxy/steel torsional joints with and without EME 90 (90 wt% mercaptoester unit co-polymer) coupling agent pretreatment. From ref. 6. Adhesive diglycidyl ether of bisphenol A (Epon 828) cured with a stoichiometric amount of methylene dianiline for I h at 120°C followed by 2 h at 150°C.

Figure 10. Peel strength durability in a 5TC condensing humidity of acetone-degreased and EME 67 (67 wt% mercaptoester unit copolymer) coupling agent treated 1010 carbon steel/epoxy peel test panels. The epoxy top-coat formulation is given in Appendix 4.

Polyfunctional mercaptoester chelators have been synthesized and evaluated as potential coupling agents for the adhesion of epoxy resins to steel. The adhesive systems were evaluated by subjecting torsional joints to shear failure after accelerated immersion testing in distilled water. 

DeNicola and Bell have reported attempts to improve durability of epoxy-bonded steel joints using coupling agents containing )8-diketone and polyfunctional mercaptoester chelating groups. The success of this approach again depends on the hydrolytic stability of the chemical bonds formed with the oxide surface. 

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