Why adhesive strength is so important

What is adhesive strength?

We distinguish two components of strength:

1. Adhesion: the bond between the adhesive and surface after the adhesive has cured.
2. Cohesion: the internal strength of an adhesive. It’s the ability to hold itself together and resist failure from external forces.

Maximum bond strength is usually not achieved instantly, as strength develops over time. Strength development is the increase of the cohesive strength. Some adhesives reach full bond strength very quickly, whereas others need more time to develop cohesion strength.

How does an adhesive achieve final bond strength?

To further understand the strength of adhesives, it is easiest to separate the process in three stages: initial tack, adhesive strength building and final bond strength. Initial tack is the property of an adhesive that enables it to instantly form a bond after the adhesive and the substrate are brought into contact under low pressure. The adhesive then starts its strength build-up phase. After the adhesive has fully cured, the adhesive attains its final properties and builds up its final bond strength.

Water-based adhesives build strength when the water evaporates. Final bond strength is reached after the remaining water is fully evaporated. For solvent based adhesives, final bond strength is obtained after the remaining solvent evaporates. Hotmelt adhesives reach full bond strength once they cool down to room temperature.
 

How strong should an adhesive be?

When choosing an adhesive, people tend to look for the ‘strongest’ one. However, not every application requires the strongest adhesive. In some cases it is important that strength does not develop too fast. This gives operators the opportunity to reposition the foam while the adhesive is still wet. In other cases, strength is not the most important factor because a fabric cover will keep the foam in place. For high-tension bonding, high initial strength is the most important.

Measuring adhesive strength

Adhesive strength is usually measured by pull, shear and peel tests.

• Pull: the maximum stress that a material can withstand while being stretched or pulled before failing or tearing.
• Shear: the resistance of an adhesive to shearing stresses.
• Peel: the resistance when a substrate is peeled from the edges.