Bond strength is an overall indicator of how effectively an adhesive can join two surfaces together. It is also the amount of stress an adhesive joint can support before it breaks. This could be in shear, peel, tension, torsion, compression, impact or other modes of dynamic forces. Bond strength reflects the adhesive’s resistance to being pulled apart and is influenced by several factors, such as the adhesive type, substrate properties, how the surfaces are prepared before bonding, and the bonding technique employed.

Why does bond strength of an adhesive matter? The bond strength of an adhesive will provide critical information about the limits or ultimate capabilities of the material and the joint design. Knowing the maximum stress a joint can resist before failure can provide the boundaries and safety factors necessary to know the capability of the finished part.

To fully understand the bond strength or stress level of an adhesive, a variety of industry standards such as ASTM or ISO test methods, and customized or unique tests to an organization are the primary methods used to determine bond strength. Various modes of force are typically evaluated to find bond strength, which could be comprised of impact, compression, peel, shear, tensile, etc., or coupled with various dynamic and environmental tests. At ITW Performance Polymers, we use industry standard test methods to evaluate and provide unbiased test results.

These applied stresses are all related to the durability of adhesive joints, each highlighting different characteristics of the adhesive’s performance and strength. Below is a brief overview of each type.

• Impact strength:

Impact strength is the amount of directed kinetic energy an adhesive joint can absorb before it fails. This is important for understanding what type of crash or collision environments for which an adhesive will be suitable.

• Compression strength:

Compression strength is the amount of stress that can be exerted by pressing on an adhesive before it cracks, deforms, or shatters. This is most relevant to understand the types of forces that an adhesive, typically a coating, can withstand when being pressed or when placing heavy objects on top.

• Peel strength:

Peel strength is the amount of stress it takes to initiate and propagate a failure in an adhesive joint when a load is concentrated at a leading edge. Adhesives generally have the most difficulty resisting peel forces, so this property is important for understanding the worst-case scenario of a joint.

• Shear strength:

Shear strength is the amount of stress that an adhesive joint can support when loaded in shear. Shear strength is the most favorable mode of force to apply to an adhesive joint. This is important to understand the optimal performance of a properly designed adhesive joint.

• Tensile strength:

Tensile strength is the amount of stress that an adhesive, by itself, can support independent of a bonded joint or substrate. This property gives the “neat” internal strength of the adhesive itself. This property is important for understanding what general limits the adhesive will have independent of a joint or application.

For any application, most of these modes of force should be studied and understood so that the most favorable and worst-case limitations of the adhesive joint are understood. The specific modes of force can be identified by studying the design of a joint and analyzing the angles at which the adhesive is placed and from where the main stresses or loads will come. Many computer design or finite element programs can be used to identify these more clearly.

Depending on the various forces affecting an adhesive joint, different testing methods and adhesives will be used e.g. semi-structural or a structural adhesive. Therefore, adhering to the adhesive manufacturer’s guidelines for optimal curing time before testing is highly recommended. Testing the adhesive before it’s fully cured can yield both incorrect assessments of the adhesives effectiveness and deliver skewed results.

ITW Performance Polymers has a team of knowledgeable product experts available to offer guidance on selecting the appropriate adhesive that will provide the right bond strength for your needs. For assistance, please reach out to Technical Services today.

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Further reading: Thixotropy, Viscosity and Rheology … Oh my!, What is fixture time anyway?, and Is it strong enough?.  

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ITW Performance Polymers is an international leader in structural adhesives, grouting and chocking compounds, and wear-resistant coatings and repair products. Contact us to learn more about these products and their applications.