Peeling Process
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Peeling process (Author: Prof. Dr.-Ing. habil. Michael Nase)
Fundamentals
Peeling refers to the separation of two films (see: film testing) that are bonded or sealed together. In the peeling process, a distinction is generally made between interlaminar and translaminar crack propagation. In interlaminar crack propagation, the crack propagates along the interface between the two sealed films (Fig. 1). In general, interlaminar crack propagation is a prerequisite for a reproducible peel process. In translaminar crack propagation, the crack spreads across the cross-section of the sealed seam, leading to premature and uncontrolled failure (see Fig. 1).
| Fig. 1: | Schematic representation of an interlaminar crack path (a) and the corresponding environmental-SEM image at a 90° peel angle (b), as well as a schematic representation of a translaminar crack path (c) and the corresponding environmental-SEM image at a 180° peel angle (d) |
Peel behaviour using the example of the PE-LD/iPB-1 peel system
Using the example of the peel system low-density polyethylene blended with 10 m.-% isotactic polybutene-1 (abbreviation: PE-LD/iPB-1), the peel process is illustrated in Fig. 2 based on an in-situ peel test. environmental-SEM images can be taken at selected points during the peel process (Figs. 2b–i).
At the start of the peel process, the vertically positioned sealed seam (Fig. 2b) is completely closed. After approx. 0.7–0.9 mm of fracture path, a local opening of the sealed seam can be observed (Fig. 2c). This phenomenon is referred to as ‘peel initiation’, in reference to crack initiation or crack initiation behaviour in polymers. Crack initiation represents the beginning of stable crack growth (see: crack propagation). At the microscopic level, peel initiation is a local phenomenon caused by the non-linear seal seam boundary, which is caused by uneven thermal radiation during the sealing process. The force associated with peel initiation is referred to as the peel initiation force Fi and marks the first locally observable start of the peel process. For the PE-LD peel system with 10 m.-% iPB-1, the peel initiation force Fi = 0.2 N was determined based on the peel curve shown in Fig. 2a. After a fracture path of approx. 1.5 mm, the rise of the peel curve changes (indicated by the dotted lines in Fig. 2a). The change in the rise of the peel curve coincides with the end of the peel front formation process, i.e. the peel process is no longer initiated locally but globally (Fig. 2d). In addition, vertically structured areas can be seen in Fig. 2d (marked by a white arrow in the detailed view), the formation of which is due to local thermo-mechanical stress as a result of the sealing process. Once the peel process has started globally, the course of the peel curve transitions into a plateau area, which is evaluated for the calculation of the peel force. In this case, the peel force Fpeel = 1.15 N. At the beginning of the plateau area at approx. 20 % of the fracture path, the peel system shows a continuous peel process (Fig. 2e). Plastic deformations and a high degree of structuring within the sealed seam are visible.
| Fig. 2: | Peel force – fracture path diagram (peel curve) of PE-LD with 10 m.-% iPB-1 (a) and the corresponding environmental-SEM images (ESEM images) at different peel progressions (b–i); the scale shown in Fig. 2b also applies to Figs. 2c–i |
However, it is not possible to visually distinguish the PE-LD matrix from the iPB-1 peel component using the environmental-SEM. The sample (peel test specimen) cannot be selectively contrasted as in transmission electron microscopy (TEM), meaning that no material-specific contrast is produced on the peel system in question.
However, microscopic observation of the deformation processes enables a deeper understanding of the peel process and the damage mechanism that occurs, combined with an assignment of structural causes to mechanical property phenomena. The similarity of the plastic deformation areas occurring at the beginning (Fig. 2e), in the middle (Fig. 2f) and at the end (Fig. 2g) of the plateau area proves continuous and defined crack propagation through the seal seam of the peel system in question. Following the second local maximum of the peel curve, there is a steep drop in the peel curve. This significant decrease in force is accompanied by the dissolution of the peel front at a fracture path of approximately 12.5 mm (Fig. 2h). The presence of a peel front dissolution process is similar to the peel front formation process on a non-linear seal seam boundary and/or on a non-linear crack front due to locally varying crack velocities. After approx. 13 mm of fracture path, a complete separation of the two sealed peel films can be observed (Fig. 2i). The peel test in conjunction with the environmental-SEM enables simultaneous mechanical and microscopic characterisation of the highly complex peel process.
See also
- Peel behaviour – modelling
- Peel test
- Peel angle
- Peel properties of peel systems
- Peel force–fracture path diagram
References
- Nase, M., Rennert, M., Henning, S., Zankel, A., Naumenko, K., Grellmann, W.: Fracture Mechanics Characterisation of Peel Films. In: Grellmann, W., Langer, B.: Deformation and Fracture Behaviour of Polymer Materials. Springer Series in Materials Science 247, Springer, Berlin Heidelberg (2017) pp. 271–281 (ISBN 978-3-319-41877-3; E-Book: ISBN 978-3-319-41879-7; see AMK-Library under A 19); https://link.springer.com/book/10.1007/978-3-319-41879-7
- Nase, M., Zankel, A., Langer, B., Baumann, H. J., Grellmann, W., Poelt, P.: Investigation of the Peel Behavior of Polyethylene/Polybutene-1 Peel Films using In-Situ Peel Tests with Environmental Scanning Electron Microscopy. Polymer 49 (2008) 5458–5466; https://doi.org/10.1016/j.polymer.2008.10.004
- Nase, M.: Zusammenhang zwischen Herstellungsbedingungen, übermolekularer Struktur und Eigenschaften von Peelfolien. Shaker Publishing 2010 (ISBN 978-3-8322-9099-3; see AMK-Library under B 1-17)