Adhesive Vs Cohesive Bonds

Adhesion, or bondability, is key when discussing thermoplastic elastomer materials because they are often overmolded onto a rigid substrate.

The strength of the bond between the two materials is critical to ensuring a quality product that remains durable throughout its life cycle. But what’s the difference between adhesive and cohesive bonds and how do they relate to TPEs?Cohesive forces exist between molecules of the same substance. These intermolecular forces between like elements cause a tendency to naturally resist separation.

On the other hand, adhesive forces attract unlike molecules. They are a result of outside forces acting between the dissimilar substances, like mechanical or electrostatic forces.Understanding the difference between the two kinds of bonds is just the beginning. Successfully overmolding a soft TPE onto a rigid substrate depends on a variety of factors, including:
1.Chemical Compatibility-Physical blending between the TPE and rigid material depends on the chemical compatibility of the materials. Thermoplastic elastomers often need to be chemically modified in order to adhere to a specific material.
2.Heat-Enough heat needs to be transferred from the melted TPE to the rigid material in order to ensure sufficient diffusion of the polymer chains.
3.Processing Conditions – Melt temperature, injection speed and holding pressure are all key factors in determining the success of adhering a soft material to a rigid one.

Once you’ve balanced these factors your material will undergo testing to determine the resistance-to-peel strength of the bonds between the TPE and rigid substrate. Two types of failures are possible.
1.Adhesive Failure-Occurs at the interface of the TPE and rigid material.

2.Cohesive Failure-Occurs when the TPE or the rigid material break before the bond between them does. Of the two, this is the preferred failure mode.