2.4. Adhesion Tests

The adhesive properties were evaluated through a shear, peel and tack test. Using a motorized laboratory coater (RK K Control Coater provided by Lumaquin S.A., (Montornès del Vallès, Spain) equipped with a bar of 50 µm, 50 g/m² of polymer was applied onto the substrates, which were subsequently dried in the oven for 1 min at 100 °C, leaving a layer of polymer of approximately 25 g/m². Standard sized tapes were cut for each type of test.

Dynamic shear tests were performed at 5 mm/min with a Zwick/Roell Z 2.5 machine on PET tapes adhered on untreated steel panels at 25 °C with a contact area of 25 × 25 mm². The tape was applied 20 min before the test by means of a rubber roller with a mass of 2 Kg [15]. The shear stress vs. strain curves were recorded and the elastic modulus (G), the yield stress (τ_y) and the maximum stress (τ_m) values were determined as shown in Figure 1. The shear modulus was determined as the initial slope of the curve with the linear correlation coefficient (r²), which in all cases was higher than 0.99, and the yield stress as the value of shear stress for a plastic shear deformation of 2%.

Experimental curve stress vs. strain, registered from dynamic shear testing, with indications of shear modulus (G) and yield stress (τ_y) determination.

The peel resistance, defined as the force required to remove a tape from a test panel, was evaluated by means of the 180° peel test after 20 min and 24 h from the tape application. Tapes of PET and paper of 275 × 25 mm² were applied onto glass panels. A Zwick/Roell Z 2.5 tensioner was used at a constant speed of 300 mm/min. The average force to remove the tape and the failure mode were recorded [16].

The tack is the capacity of the adhesive to form bonds with a substrate with a brief contact under slight pressure. The tack was determined by the loop tack test with an AT1000 tensile tester equipment. A loop was formed with a PET tape of 175 × 25 mm² and held with the upper clamp. A controlled contact was made at a constant speed of 300 mm/min onto glass panels. The maximum force required to peel off the tape from the panel and the failure mode were recorded [17,18].