Peel-Cling Test Extented
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Peel-Cling test extended
Fundamentals of the test method
The peel-cling test for stretch films in the packaging industry is used to evaluate self-adhesion (i.e., the adhesion of films to themselves) [1].
This test method measures the force required to separate a strip of film with a specified area from a fixed backing film made of identical material. The base film lies on a 20° inclined plane without bubbles or wrinkles. The film strip (1 inch = 25.4 mm) is then placed in the centre of this plane and rolled on with a brush or roller, also without wrinkles or bubbles, with a clamp attached to the lower end with a thread. This thread is guided by means of a return roller to the clamping device (see also: specimen clamping) of the universal testing machine. During the peel process, the thread lifts from its horizontal starting position until it reaches the cling line, whereby the diagram is recorded as shown in Fig. 1 [2–4].
| Fig. 1: | Registered force–deformation curves in the peel-cling test |
With cling force FCling, peeling causes an increase in deformation at constant force, but a clear start-up behaviour is observed, which can be attributed to the elastic deformation and bending effects of the peel film. However, the expected constant level of cling force is not determined; instead, there is an increase in force, the course of which is characterised by oscillations. These oscillations are comparable to a “stick-slip process,” in which crack propagation, or in this case further peeling, only occurs after a certain amount of energy has been applied.
Correction of cling force
The increase in the average force is obviously due to the non-constant test conditions, as the actual peel angle changes continuously during the test. It therefore makes sense to correct this angle change mathematically, as only the horizontal force FH is of interest for evaluating the cling process (Fig. 2) [4, 5]. In this case, the measured variables are the force F and the variable path x, which is designated L in Fig. 1.
| Fig. 2: | Correction of cling force in the peel-cling test |
The horizontal force is calculated according to Eq. (1),
| (1) |
where the angle α is calculated from the current path x according to Eq. (2), given the thread length L and the cling length s.
| für 0 ≤ x ≤ l | (2) |
However, practical tests on various film systems show that the correction only results in a minor improvement in the force curve. A measurement-based correction is therefore required to keep the cling angle constant, for which there are basically two implementation variants (Fig. 3).
| Fig. 3: | Extended peel-cling test: a) with adjustable deflection roller, b) with rotatable support for the cling film |
The first variant works with a height-adjustable return roller for the thread, whereby the adjustment is carried out by a stepper motor (Fig. 3a). After the peeling process has started, the angle of the thread changes, which is measured by sensors. This value corresponds to the actual value of the measurement signal and, when compared with the target value (horizontal at 0°), gives the control deviation, which is compensated by the stepper motor.
Further development of the peel-cling test
Another technical solution is to use a rotatable cling table (Fig. 3b), whereby the thread inclination is also compensated by means of a stepper motor, but in this case via the angle of the rotary table. This variant also allows start angles other than 20°, which means that the peel conditions for the cling film can be changed. Both solutions require the thread angle to be measured, either using a CCD camera or a shadow image method (Fig. 4). A technical solution to this problem is currently being developed in collaboration with Coesfeld GmbH, Dortmund.
| Fig. 4: | Arrangement of sensors in the peel-cling test for measuring the thread angle |
See also
References
| [1] | ASTM D 5458 (1995; reapproved 2025): Standard Test Method for Peel Cling of Stretch Wrap Film |
| [2] | Rennert, M., Fiedler, S., Nase, M., Menzel, M., Günther, S., Kressler, J., Grellmann, W.: Investigation of the Migration Behavior of Polyisobutylene with various Molecular Weights in Ethylene/α-olefin Copolymer Blown Stretch Films for Improved Cling Properties. Journal of Appl. Polymer Sci. 131 (2014) 4861–4874; https://doi.org/10.1002/app.40239 |
| [3] | Rennert, M., Nase, M., Reincke, K., Arndt, S., Lach, R., Androsch, R., Grellmann, W.: Influence of Low-Density Polyethylene Blown Film Thickness on the Mechanical Properties and Fracture Toughness. Journal of Plastic Film and Sheeting 29 (2013) 4, 327−346; https://doi.org/10.1177/8756087913483751?urlappend=%3Futm_source%3Dresearchgate |
| [4] | Rennert, M.: Fracture Mechanics Investigation of Autohesive Interfacial Interactions of Polyethylene Stretch Wrap Films. Promotion. Martin-Luther-University Halle-Wittenberg. 16.11.2018. (see AMK-Library under B 1-29) (Content) |
| [5] | Reincke, K.: Testing of Polymeric Films. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022). 3rd. Edition, p. 643-678 (ISBN 978-1-56990-806-8; E-Book ISBN 978-1-56990-807-5; ePub ISBN 978-1-56990-802-2; see AMK-Library under A 22) |