Лако-красочные материалы — производство

Peel tests are quite similar to cleavage tests, except that at least one of the adherents is prepared from a flexible material which could be plastically deformed during the measurement. A typical peel test is illustrated in Figure 7.9; this is known as the ‘T-peel test’, and is described in ASTM D1876. Two adherents of equal thickness are

bonded with an adhesive, after which the ends or ‘tabs’ of the specimens are gripped by jaws of a tensile testing machine and subsequently separated at a defined deformation rate. The test can also be carried out below and above room temperature. If the adherents have the same thickness and bending modulus and yield strength, the resultant peel is symmetrical, while the crack front usually propagates down the center of the adhesive bond-line. If one of the adherents has thickness or bending properties substantially different from the other, the adhesive bond rotates so that the bond end bends towards the test fixture which is grasping the thinner adherent. The locus of failure also shifts towards the thinner adherent.

In the 90°-peel test a flexible adherent is adhesively bonded to a rigid adherent. The flexible adherent is peeled off the rigid adherent at a fixed rate so that the angle between the tab and the rigid adherent is maintained at 90° throughout the test. The fixture to which the rigid adherent is clamped must be movable so as to allow the peel front to stay in a constant position under the testing machine crosshead as the bond is peeled.

The 180°-peel test (Figure 7.11) is described in ASTM D903. As with the 90°-peel test, the flexible adherent is bonded to a rigid adherent, after which the specimen is placed in a tensile testing machine so that the tab is pulled off parallel to the rigid adherent. The flexible adherent undergoes substantial bending to conform to the stress to which it is being subjected. Thus, this adherent must be flexible enough not

to yield to failure by such a bend. This test is often applied to investigate the adhesion of thin films or sheets to an adhesive. It is also used to examine the adhesion of very soft adhesives, but these must be supported by a canvas during the test. In this case the sealant or rubber-based material is applied uniformly to the rigid adherent. Before the adhesive is allowed to cure, or the solvent allowed to evaporate, the canvas is pressed into the adhesive. When the adhesive has cured, the canvas forms an ideal flexible adherent for this test.

Another variation of peel tests is shown in Figure 7.12. In the 90°- and 180°-peel tests the bending radius of the adherent close to the peel front is mainly controlled by the bending stiffness ofthe adherent material. Depending on the type ofadhesive and the type of adherent, the crack could propagate either through the adhesive or close to the adherent’s interface.

In the floating roller peel test the radius of curvature near the peel front is guided by the radius ofthe mandrel over which the flexible adherent is peeled. This system will also control the locus of the crack in the adhesive layer. For example, if a flexible adherent is peeled over a mandrel of small radius, the locus of crack propagation shifts towards the thin adherent. This method is useful if the aim is to examine the effect of surface preparations on adhesion.

A modification of this peel test takes into account the sensitivity of the interfacial adhesion to moisture. The European standard EN 1967, Structural adhesives — Evaluation of the effectiveness of surface treatment techniques for aluminum using a wet peel test in association with the floating roller method, has been developed to assess pretreatments for aluminum substrates, and is intended for laboratory evaluations. The objective of this standard was to develop a rapid test procedure with a high sensitivity, which could differentiate between the pretreatment processes. The application of water containing a wetting agent leads to a change of the failure

mode from cohesive failure — which usually is found in dry joints — to interfacial failure when a wetting agent was injected into the peel zone. The change in failure mode corresponded to a reduction in peel resistance if the surfaces were not pretreated according to the complete CAA or PAA (Chromic Acid Anodizing, Phosphoric Acid Anodizing) (see Section 8.2.1.1) process. The failure mechanisms in the boundary layer zone initiated by this test are mainly explained by the diffusion of water, as the wetting agent essentially improves surface wetting. The main objective of this method is evaluation of the quality of a surface pretreatment used in the preparation of aluminum or its alloys.

7.3