Some plastic and metal materials are considered difficult to bond. One of them is nickel. Cases where adhesives still achieve high strengths on nickel and those cases where build-up of adhesion is particularly tricky are examined below.
There are many reasons why nickel and nickel-plated surfaces are difficult to bond: (1) Nickel is an inherently inert material. This makes it difficult to build up adhesive bonds with the substrate. (2) A roughness depth of 15 µm is considered optimal for bonding; nickel surfaces, however, tend to be smooth, resulting in less effective surface area. (3) The nickel layer interacts with humidity. As a result, adhesives lose strength over time, since the humidity migrates into the layer of adhesive from the outside. (4) In addition, both the quality of the material and the nickel plating process have an influence on the adhesive bond. Tests on the adhesion properties have even shown significant differences between two batches from the same supplier.
Good Strengths at High Curing Temperatures
However, the tests also showed that some adhesives that are cured at high temperatures adhere very well to nickel surfaces. The epoxy resin DELO MONOPOX AD286, for instance, initially reaches a compression shear strength of 55 MPa (see Diagram 1). 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 the results after the test period is related, among other factors, to the diffusion of water along the interface between adhesive and substrate.
Challenge at Low Curing Temperatures
Bonding of nickel becomes particularly challenging when adhesive curing must take place at +100 °C and below because of material constraints. The difficult build-up of adhesion in the case of epoxy resins that can cure in a lower temperature range is partly due to the lower glass transition temperature (Tg) of these adhesives. The glass transition temperature is a measure of 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 basic projects focused on this subject and has already developed some initial solutions.
As part of the projects, selected adhesives are being investigated for their adhesion characteristics on nickel. One acrylate that has been developed for curing in a lower temperature range and which bonds reliably with nickel-plated components is DELO DUALBOND SJ4750. The adhesive is suitable, for example, for bonding applications in mini-loudspeakers and cameras. The one-component, dual-curing acrylate achieves initial strengths of over 30 MPa (see Diagram 2). 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.
Demanding, But Possible
Bonding to nickel is extremely demanding. Not only because there are still many unknowns. For example, the question as to whether curing in a high temperature range overcomes the inertness of nickel or whether this "only" makes the adhesive stronger has not yet been clarified. So far, there is no methodology to fully explore this issue. However, extensive tests with selected adhesives show good strengths on this metal and provide reliable bonding in the corresponding applications.