Good Strengths at High Curing Temperatures

However, these tests also showed that some adhesives which are cured at high temperatures adhere very well to nickel surfaces. A long-standing DELO MONOPOX epoxy resin, for instance, initially reaches a compression shear strength of 55 MPa. To evaluate the adhesion to nickel after simulated aging, the cleaned and bonded components were subsequently stored for seven days at +85 °C and 85% relative humidity. This test is typical for automotive aging simulations. Here, the adhesive still achieved strength values of 40 MPa, with the epoxy resin curing at +150 °C in 60 minutes. Due to these high curing temperatures, the adhesive crosslinks reliably with the surface. The difference between the initial values and test results is related—among other factors—to the diffusion of water along the interface between adhesive and substrate.

Challenging at low curing temperatures

Nickel bonding becomes particularly challenging with curing temperatures at or below 100 °C due to material constraints. The difficult build-up of adhesion in the case of epoxy resins that cure in lower temperatures is partly due to the lower glass transition temperature (Tg) of these adhesives. The glass transition temperature measures the stability of the polymer network; the lower the Tg, the less dense is the polymer network, which influences the bond's resistance.

The reasons why bonding of nickel-plated surfaces is difficult even with acrylate adhesives, which are based on a different chemistry and a different curing mechanism, are largely unknown. To gain insights here, DELO is actively conducting initial studies focused on this subject and has already developed some initial solutions.

As part of these projects, select adhesives are being researched for their adhesion characteristics on nickel. One DELO DUALBOND SJ acrylate that has been developed for curing in a lower temperature range and which bonds reliably with nickel-plated components is particularly suitable for bonding applications such as mini-loudspeakers an cameras. The one-component, dual-curing acrylate achieves initial strengths of over 30 MPa. After standardized aging tests (storage for seven days at +85 °C and 85 % humidity), stable strengths of more than 20 MPa are obtained. The adhesive can be prefixed under light and subsequently heat-cured to its final strength. Curing with light or heat alone is also possible. For the test, curing took place at +100 °C in 60 minutes.