High temperature adhesives: forming a strong bond
17 November 2017
A lot of development work has gone into creating epoxies that can withstand 250°C for long periods and which also have a good bond strength at temperature, as Eamonn Redmond, sales manager with Inseto (UK), explains
Epoxy adhesives, whether one or two-part solutions, are extremely popular in most industries. They afford high bond strengths and are resistant to many chemicals. They also have relatively short curing times, which is a bonus in manufacturing scenarios. However, most epoxies only work up to about 180°C, after which the bond fails. Also, longevity at high temperatures for extended periods of time is important, as this prolongs the life of the product that uses the adhesive.
However, in many industry sectors, temperatures can exceed 180°C. In the oil and gas industry, for example, electronics are used down-hole for drill bit monitoring. Temperatures often exceed 200°C and, for this reason, 250°C is fast becoming a new survivability benchmark for harsh-environment applications. Elsewhere, in the aerospace and automotive sectors, the mounting of monitoring electronics close to engines to help improve efficiency is also driving the need for materials that can withstand high temperatures.
For high temperature applications, other bonding solutions are available. For instance, silicones have traditionally been used in high temperature applications; sometimes as high as 400°C. However, their inherently poor bond strength has always been a drawback, making them suitable as sealants but not as adhesives. Many silicones also take a long time to cure (e.g. 12h/1mm depth) and their high flexibility can lead to significant movement of the bonded parts during the excursion to temperature.
Best of both worlds
High temperature adhesives have been designed to be somewhat more flexible than traditional epoxies without compromising their cross-linking stability. This slightly enhanced flexibility compensates for stress peaks during bonding, especially in the case of peeling loads. In addition, it ensures the balance between the joining materials, which is required for both good temperature resistance and good thermal stability due to frequent yet varying thermal expansion coefficients.
DELO MONOPOX HT760, for example, is a high temperature, heat-cured adhesive from DELO Industrial Adhesives. During its development, this adhesive was subjected to a variety of tests. It demonstrated a bond strength (compression shear strength on ceramics) of 43MPa when tested at room temperature. After 500h at 250°C and subsequent cooling, the bond strength had increased to 52MPa. Subsequent tests followed to establish bond strength at high temperatures (Figure 1).
DELO MONOPOX HT760 is suitable for extremely high temperatures and withstands aggressive chemicals. It was subjected to compression shear strength tests (FR4 on FR4) after 20min of curing at 150°C, subsequent cooling and exposure to typical automotive fluids. Figure 2 shows the test results – excellent adhesion even after immersion in chemicals such as AdBlue, petrol, diesel, and DI water.
Now that survivability and bond integrity have been achieved, the chemistry can be modified to adjust viscosity (for applications that require greater ease of dispensing and stencil printing, for example) or for the adhesive to be electrically conductive. Its thermal conductivity can also be adjusted.
To illustrate this, new high temperature adhesives are available for potting / encapsulation applications. These include a high viscosity adhesive that can be used to create a ‘wall’ when the potting area is large, and a complementary low viscosity adhesive that is used to fill the space enclosed by the wall. This process can be used when height restrictions apply; as using just a single adhesive can lead to a very high doming effect.
High temperature adhesives are under development that can be light-fixed in place to prevent any movement (of the adhesive, or the parts being bonded) during the heat-cure process. Illuminating the surface of the adhesive with high-intensity light causes the surface of the adhesive to solidify. Heat curing can then be carried out at a more convenient time and/or location.