Peel testing is used to assess bond quality and is a simple way of determining the adhesion strength of a material, especially in the case of a thin film deposited on a substrate.
By measuring the tension characteristics between the adhesive and the adherend surface, it is extremely useful in assessing the effects of processes (e.g. surface treatment) or real-life performance ( e.g. environmental exposure).
There are various methods of peel testing but they all require at least one flexible adherend to be held in a tensile grip and separated from the other surface. The tensile force-extension values are continuously recorded to obtain precise data for calculating peel values. The main differences between the methods are the angles of peel and whether the peel angle remains constant during the test.
- 90 degree - requires a sliding table or peel wheel fixture to ensure the peel angle remains constant.
- 135 degree - simulates the angle at which a person may peel a lid from a container (eg. a yoghurt pot).
- 180 degree - one of the adherends must be flexible enough so that it can essentially fold back on itself.
- Climbing drum - used when a flexible material is bonded to a very rigid adherend (eg. composite honeycomb aerospace panel).
- Floating roller - used when a flexible material is bonded to a comparatively rigid adherend.
- T-peel - used for flexible-flexible adherends. Typically returns lower peel values than other methods since the entire load applied is transmitted to the bond.
The result quoted for peel tests is the 'force per unit width' required to continue peeling the joint after initiation and is expressed typically as N/mm. It is determined only from the flat portion of the force-extension curve without consideration for the first and last portions of the curve, where the load climbs and falls respectively.
However, the maximum tensile force, corresponding to the initiation of adhesive failure, is sometimes used, as it can be a more relevant indicator particularly when adhesives & laminates are sensitive to environmental degradation.