The Measurement of Adhesion in Plastics
A combination of several factors is used to achieve adhesion between any two surfaces. The force of adhesion measures the efficacy of this combination. For instance, two solid blocks can be pulled apart either by
- applying forces perpendicular to the plane of interface, or
- applying forces parallel to the interface. Herein, the force of adhesion measures the force. Plastic films are thin and flexible and as such, measurement is more complex for plastic surfaces. In this case, for example, to measure the adhesion between two films heat sealed together, a block could be adhered to each of the opposite surfaces of this structure. Thus, the force measured is approximately the force of adhesion between the films, when the adhesion between the films is less than the adhesion of the film surfaces to the blocks. It is necessary to make a correction for the force deforming the various plastic layers to measure exactly the force of attraction. One can distinguish between these with separate measurements of the deformation and a precise control of conditions. For most packaging applications, in practice, since this is very important only the total force to separate is measured. As such, during the formulation of adhesives for plastic films increasing deformability is often adopted to maximize bond strength.
In comparison to other solid materials, plastics have several advantages, which specifically help in achieving high adhesion levels, like the ability to absorb energy by deformation. It is also possible to melt most plastics, and subsequently with heat scaling, two liquid surfaces are brought together in maximum contact without an air layer. The liquid form also allows for the absorption of other compatible surface contaminants.
On the same lines, with melt coating, a liquid can be brought into contact with a solid surface. Lastly, high polymers allow for the dissolution of the polymer coating in a solvent, as well as dispersal in water.
In the commercial environment, it is not only impractical to measure adhesion between two plastic films using two blocks, but also in film applications the forces at play are not accurately simulated by the measurement.
It is essential to reproduce the angle between the heat seal and the separated films, as the peel force changes with the peel angle, and during the measurement of peel force, despite carefully controlling the rate of peel. The interface presents random imperfections, which lead to sharp fluctuations of the peel force in a stick-slip fashion, and the average of these widely varying values, cumulatively form the peel force.
The adhesion between a thin coating and a film substrate is also of interest as in such an example, a second film is adhered to the coated surface. For instance, printing ink or a barrier coating. The two films are peeled apart in the stick-slip fashion. In most instances, this second film is pressure-sensitive tape. In case the adhesion of the coating to the tape is lesser than the adhesion of the coating to the substrate, the adhesions are considered to be adequate in many applications.
It has been assumed, in most of the cases, irrespective of if it is peeling apart pressure-sensitive tape from a coated film or two heat-sealed films, the breakdown of the film/film or coating/film interface leads to peeling though, one cannot negate cohesive failure within the films or coating layer for the peeling. This necessitates the need to identify the two surfaces that have been pulled apart.
Consider the following representation:
||Peeling Apart Outcome
||Clean break between both the surfaces.Adhesion between the two is the weakest link.
||Cohesive failure in the coating.Easy coating deformation to the applied force.
Cohesive strength is exceeded and the adhesion ruptures.
||Cohesive failure in the substrate film.Observed in oriented films since the strength in the thickness direction of the film is greatly diminished.
Highlights areas of improvement for increased adhesion.