Peel angle

Fig. 1, Schematic of commonly u.sed methods for testing the strength of adhesive joints, (a) Peel test. Note that the peel angle can be changed depending on the test requirements, (b) Double overlap shear test. In this test, the failure is predominantly mode II. (c) Single overlap shear test. In this test the failure mode is mixture of mode I and mode II. (d) Blister test.

Peel testing Peel angle is important with 180° and 90° peel angles being the most common. In general, a small amount of the adhesive is peeled away and the force necessary to continue the peel measured. 

The basic method is a closely specified test intended for quality control or the comparison of bonding systems, but could readily be extended to investigate the effects of test piece dimensions, peeling angle, test speed, etc. 

The metallic layers were examined either by conventional or cross-section TEM in a Jeol 200 Cx microscope. For the cross section preparation a sandwich of two laminates is made, glued face to face with an epoxy, cut in small pieces, mechanically polished, and then ion milled to a final TEM observation thickness. The plane section TEM sample are prepared by dissolving the PET in trifluoroacetic acid for 5 to 10 mn. The area observed, on plane section TEM, for the grain size calculation is close to 0.2 urn. For the adhesion measurements, test pieces consist of aluminum support (1 mm thick) double sided tape (Permacel P-94) PET (12pm) / evaporated aluminum/ ethylene acrylic acid (EAA) copolymer film. These laminates are prepared for the peel test by compression under 1.3 105 N.m2 at 120°C for 10 seconds. The peel test is performed by peeling the EAA copolymer sheet from the laminate in an INSTRON tensile tester at 180° peel angle and 5 cm min peel rate. 

Peel tests were conducted on universal testing machines at 23°C at a test speed of 10 mm/min. In the fixed arm peel test there was a requirement that the peel fixture was attached to the test machine through a fnction-free linear bearing system. This enabled the direction of force on the peel arm to remain vertical whilst the peel fracture was propagating. A number of peel angles were used in the fixed arm test, but all laboratories included a configuration where the peel angle was 90°. 

Figure 2 Fixed arm peel results with a peel angle of 90 from all the participating laboratories.

Five of the laboratories conducted fixed arm peel tests for a range of peel angles. The characteristics of these results are illustrated in Figure 4 from Laboratory 2. 

Figure 4 Fixed arm peel data from Laboratory 2 at five different peel angles for the PP laminate system.

The important features of Figure 4 are the dependence of both G/f" and Ga on peel angle. This suggests that these parameters cannot be objective material (or laminate) properties. However, Ga can be seen to be independent of peel angle and hence this quantity is considered to be an objective measurement of the adhesive fracture toughness. The Ga values for all five laboratories are shown in Figure 5. 

Figure 5 Adhesive fracture toughness as a function of peel angle for 5 laboratories for fixed arm peel tests on PP laminate system.

There is some scatter in these adhesive fracture toughness data. However, there are no reasons for excluding any of the results. The mean value is 206 jW with a standard deviation of 42 jW. With consideration to the overall level of scatter, this gives good agreement with the results for the data at a peel angle of 90" (214 J/m ). 

Average Ga (J/m ) Small peel angle Number of tests Average Ga (J/m ) Small peel angle Number of tests... 

It is concluded that the adhesive fracture toughness is independent of configuration. However, the peel strength and peel angles do depend on configuration. 

Aluminum panels, which had a thickness of 0.61 and 1.63 mm, were etched with chromic acid. ASTM procedure D3167—76 (reapproved 1981) was followed for 135° peel tests. The adhesive film was placed between the aluminum panels and press-laminated at a temperature of 177 °C for 1 h at a pressure of approximately 5 psi (34.5 x 103 Pa). The temperature was then increased to 220 °C, and the joints were kept under pressure for another hour. The heaters in the hydraulic press were then switched off and the platens air-cooled and then water-cooled until the platen temperature was down to 100 °C. The bonded panels were cut into 12.7-mm-wide joints and tested at a rate of 20 mm/min and at a peel angle of 135°. 

Tests were carried out by adding lead shot slowly to the container until peeling proceeded at a rate of 2 to 3 mm. per minute. As the films were peeled, the substrate panels slid in the tracks under a second small retaining roller. The peel angle was set by the nature of the system and was observed to be somewhat less than 90°. 

One method occasionally used evaluate the relationship of silicone release liners or other materials of very low surface energy to a pressure sensitive, is to bring a coated pressure sensitive into contact with the liner under pressure, to carry out an adhesion test. This is of little value, as the adhesive is unable to wet out and so come into intimate contact with the release liner. The adhesive must first be coated onto the liner, dried, or cooled in the case of a hot melt, and then a carrier laminated to it, as is standard practice for transfer coating. As low values of adhesion can be expected, the force required to bend the backing may dominate, and so a thin flexible backing should be used, to maintain a constant peel angle, 25 pm polyester being satisfactory. Then a standard 180° peel test can be carried out with the release liner secured to a test panel. 

Figure 14.11. (a) Reduction in peel angle causes elastic stretching of the film (b) results showing how the peel force is retJuced by the stretching median ism. ... 

Fig. 2. Difference between the formal and actual peel angle...

The experimental conditions selected may well enable Eqn. 4 to be simplified. If the extension of the peeled strip is negligible, as is often the case, k is unity and is zero 90 and 180° are commonly chosen peel angles. Under these circumstances, Eqn. 4 reduces to... 

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