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Avoiding the Stresses of CTE Mismatches When Choosing Adhesives

Posted on 9/17/2020 9:07:06 AM By ASC
  

Boosted by the burgeoning switch-over from mechanical fasteners to adhesives in assemblies, adhesives manufacturers are coming up with more and more adhesives designed to meet the different needs of specific manufacturers. However, one problem is still raising its head, and that’s what happens when the coefficient of thermal expansions (CTE) of the adhesives and the substrate/s don’t match each other.

This can place thermo-mechanical stress on the joints when either the adhesive itself has a CTE which differs from that of a single substrate, or when two substrates are bonded that don’t have the same coefficient, as would be the case when bonding aluminum to glass, ceramic or glass to plastics, and rubbers to metals.

In the first scenario, the adhesive layer might well expand and contract at a different speed to that of the substrate even when the CTE difference is only slight, and in the second, CTE mismatches of different substrates can result in even more stress on the joints. Finding a solution is a problem in itself, because it involves some trade-offs. Solutions are available, but often bring about unwanted changes in other parts of the process.

Solving the mismatches may seem to involve no more than selecting an adhesive with as low a thermal expansion coefficient as possible, and, if necessary, reducing it even more by using a ceramic or specialty filler that has a negative CTE, which could lower the values considerably. However, there is a catch. Using fillers can increase the adhesive’s modulus resulting in it becoming stiffer or more rigid, and the same end result is likely when using ultra-low CTE adhesives.

Using rubber toughened adhesive systems to reduce stress on the joint is another option. These “flexibilized”adhesive systems offer a toughened cure, sometimes show relatively high lap shear strength, and have a tensile modulus which isn’t too high. Some also provide relatively high elongation values, and offer a higher thermal expansion coefficient. However here, too, accepting that when one side rises the other will drop, remains an important factor for consideration at the engineering design stages of adhesive product development or selection.