Gluing Metal to Metal

As metal to metal glues the polyurethanes are inferior to epoxy resin adhesives and to the best two polymer adhesives. Test on aluminum alloy using a number of polyurethane systems based on tolylene diisocyanate and saturated polyesters have shown the highest tensile shear strength that can be obtained is less than half that given by the best epoxy resin and two polymer adhesives when cured at 150oC. The strength of hot-cured metal to metal joints can be markedly improved by incorporating a relatively high proportion (up to 25%) of polyvinyl butyral, or formal, with which tolylene diisocyanate is compatible; peel strength, not usually very good is polyurethane adhesives is also substantially increased by such an addition. Increasing the proportion of polyester, thereby again increasing the flexibility also improves peel strength, but at a point where the gain becomes worthwhile a decrease in shear strength inevitably follows.

In examining as an adhesive for aluminum a polyurethane based on a triisocyanate and hydroxylated polyesters, it was found that the polyester having the lowest hydroxyl content gave the strongest bonds. This is rather surprising, but since the bonds were also reported to lack water resistance, it is interesting to consider whether the adhesive system contained hydroxyl groups in excess of the isocyanate requirement.

In a study of the effect of the ratio of molecular weight to isocyanate and others compared the adhesive strength of MDI and TDI. Using the same polyol mixture, it was bound that at room temperature (about 24oC) both systems had approximately equal maximum tensile shear strengths of about 5,300 psi, which is quite surprisingly high. But below this temperature the MDI system was considerably better (test made on aluminum): at -730C, for example, the MDI system had strength of 5,100 psi as against 3,900 psi for the TDI.

Because the polyurethane resins can be foamed in situ and adhere to metal their use as the core material in sandwich construction is of possible interest. Fabrication is not difficult, but the polyurethane having low moduli of elasticity compare unfavorably, for example, with aluminum honeycomb as a sandwich core material.

See the different adhesive Resin and Plastic.