Лако-красочные материалы — производство

Boerio and co-workers [32,39] showed that the average shear stress of epoxide/titanium lapshear joints primed with g-APS at either pH 10.4 or 8.0 showed almost no decrease in

strength after water immersion at 60°C for 60 days; unprimed controls lost 75% of their original strength. Epoxide/iron joints primed with APS at pH 8.0 retained 75% of their original strength after 60 days. Kaul and co-workers [24,42] investigated the strength of epoxide/aluminum single lapshear joints primed with g-APS and showed that the use of g-APS resulted in a lower dry bond strength than the unprimed control, and thicker films produced even lower bond strengths. The strength retention of unprimed joints after water immersion at 55°C was approximately 80% of the original, whereas the joints primed and standard dried (1 h at 25°C under vacuum) maintained only 50% of the dry strength. Joints primed and dehydrated (10 days at 110°C under vacuum) before bonding showed more than 95% retention.

In particularly useful paper, Gledhill and co-workers [43] investigated the effects of silane type, solution pH, solution age, and drying on the bond strength of a bisphenol A diglycidyl ether-based epoxide in a butt joint configuration. Several silanes were inves­tigated. Unprimed joints showed a fall in bond strength from 37 to 5.8 MPa after 1500 h of immersion in water at 60°C. Joints treated with an aqueous 1% solution of g-glycidoxy- propyltrimethoxysilane (GPMS) aged for periods up to 24 days before application showed a retained bond strength of between 17.5 and 34.4 after the same immersion time, with a peak retention between 30 and 90 min of aging. In a similar experiment using a solution of 95 parts of ethanol and 4 parts of water there was no solution age dependency, the recorded joint strengths were lower, and there was no evidence of increased water resistance. Attempts to accelerate the drying of the aqueous g-GPMS film resulted in a marked reduction in bond strength. In a study of bismaleimide adhesives the same authors showed that the use of an aqueous splution of g-APS increased the bond strength of the unprimed joint from 9.7 MPa-23 MPa, but was highly dependent on solution pH.

Kerr and Walker [28] investigated the bond strength of a two-pack polyamide — cured adhesive and a diphenylmethane diisocyanate-cured polyester adhesive on mild steel, stainless steel, and aluminum in a butt tensile configuration using a range of silanes as pretreatment primers and additives. It was shown that not all silanes were effective adhesion promoters on all substrates. The most effective silanes were g-mercaptopropyl — trimethoxysilane (MPS) on stainless and mild steel and N-p-(aminoethyl)-g-aminopro- pyltrimethoxysilane (AAMS) on degreased aluminum and stainless steel, with the urethane, where a 20% improvement in bond strength was achieved. AAMS was the most effective on degreased aluminum with the epoxide. On grit-blasted substrates, considerably higher bond strengths were achieved. In the comparative trials all the silanes were found to be more effective when used as pretreatments rather than as additives, a finding directly opposed to that found in the case of surface coatings, where the opposite was true. It was considered that this was a function of viscosity and curing time. After exposure to 100% relative humidity for periods up to 2 years, stainless steel specimens coated with MPS and g-GPMS showed an equilibrium bond strength retention more than double that of the uncoated controls. On glass the retention values were four to five times greater.

Hong and Boerio described a particularly interesting practical use of silanes in obtaining good adhesion to mineral oil-contaminated steel substrates [44]. They showed that the addition of 5 wt% of g-GPMS to amidoamine-cured epoxide adhesives [Epon 828 and V115 (both Shell Chemical Co.)] to oil-contaminated mild steel lapshear specimens increased the initial bond strength from 968 psi to 1556 psi. More surprisingly, the speci­mens immersed in boiling water for 12 h increased to 1681 psi, whereas the nonsilane control decreased to 665 psi.