5.2.3 Adhesive Chemical-Specific Properties

The adhesion, cohesion and overall strength of bonded assemblies can vary depending on the chemistry of the adhesive product and the specific chemicals that the cured adhesive is subjected to over the life span of the assembly. Each specific chemistry or technology has its respective benefits and considerations that should be considered, especially when selecting an adhesive for an application that will be subjected to specific chemicals. Although adhesives and sealants are often formulated from the same types of base materials, they are specifically engineered to have different properties; therefore, each adhesive should be evaluated individually based on its specific properties.

As Table 21 shows, there are a large number of technologies, which each have specific benefits and considerations, and with each chemistry having typical representative characteristics. This is a high-level overview; each chemistry is covered in depth in the following section.

It is important to understand that application requirements dictate what bond strength is ideal and what level of strength deterioration, when exposed to different stress or environmental factors, is still adequate for bonded assemblies over the life cycle of finished goods. While a specific adhesive may exhibit a reduced strength when exposed to a particular chemical, it may still meet the strength requirements after the initial reduction in strength. Assemblies with high criticality should be manufactured, exposed to representative and controlled chemical conditions, and strength tested.

Typically, when assemblies are subjected to long-term chemical exposure, adhesive selection will generally be limited to one or two families (chemistries) of adhesives that perform well in these environments. It may be necessary to sacrifice other properties or alter process considerations to accommodate adhesives which will perform well in the respective environmental factors.

If an assembly is subjected to long-term chemical exposure, bond strength may gradually decrease to the point of failure with specific chemistries. However, some chemistries, such as epoxies or acrylates, can typically withstand long-term chemical exposure but may have process limitations, substrate limitations or temperature limitations that will need to be sacrificed or altered to meet the chemical exposure requirements.

The TDS will commonly list many of the most common chemicals that an adhesive may be exposed to, such as water, ethylene glycol, motor oil and gasoline. If data for a particular chemical is not listed, an adhesives engineer should be consulted for chemical/adhesive specific data and/or testing.