Лако-красочные материалы — производство

The strength of a bonded joint will be determined by the strength of its weakest component, which is generally designed to be the adhesive. It follows that the requirements for satisfactory joint performance are (a) good contact between the adhesive and substrate (b) absence of weak layers in the joint (c) that the adhesive should possess appropriate mechanical properties. These basic requirements are implicit in the essential elements of bonding, and are related to many factors which affect the performance of bonded assemblies as summarised in Table 4.2.

Table 4.2. Factors affecting joint ‘strength’

Given that the adhesive itself should determine the strength of a bonded joint, the stress required to rupture a joint is, nevertheless, not a well-defined materials constant. When two materials are bonded, the resultant composite has at least five elements, namely the adhesive itself, two adhesive/adherend interfaces, and two adherends. If a primer is applied to both substrate surfaces, the number of elements increases to at least nine. These elements involving a metallic adherend are depicted schematically in Fig. 3.2. Note that the adhesive (or primer in this case) is in contact with the metal surface oxide layer, and not with the metal itself.

Kinloch(8) suggests that the measured bonded joint strength almost always reflects the value of two parameters:

(1) the intrinsic adhesion

(2) the energy dissipated visco-elastically and plastically in the highly strained volume around the tip of the propagating crack and in the bulk of the joint.

The latter term generally dominates the measured joint strength, and also gives rise to the test rate and temperature dependence of joint strengths. Brittle fracture may be initiated at the interface with unmodified (rigid) adhesives if the energy of fracture cannot be dissipated within the adhesive layer.

Real joints do not of course consist of simple, separate, elastic materials with a clear mathematical geometry. Metal adherend surfaces are micro-rough, possessing oxide layers, while concrete surfaces are macro-rough comprising aggregates and cement paste, and both surfaces readily adsorb air-borne contamination. The thickness and modulus of primer layers, if employed, is often unknown, and the thickness and properties of the adhesive layer are difficult to regulate and to determine.

In order to develop interfacial strength the adhesive must be involved in wetting, adsorption and inter-diffusion reactions with the adherend. Its chemical composition will influence the extent of interaction and its ability to displace and absorb surface contam­ination. The adherend surface topography can affect joint strength in several ways. Irregular surfaces have a greater potential bonding area than smooth surfaces, and mechanical keying may play an important role. However, wetting may be far from complete with viscous adhesives, particularly if the irregularities are deep and narrow; resultant voids could act as stress raisers. The viscosity of the adhesive may be lowered by increasing the temperature, and wetting will also be enhanced. Primers may also be used to overcome the wetting problem. Rheological aspects of adhesion, together with the simultaneous influence of adherend surface chemistry on wetting and adhesion, are considered in Chapter 3. Adherend surface pretreatment is also described in the previous chapter, in particular because of its profound influence on bond durability.

The reader will appreciate that a large number of factors can affect joint strengths, and hence the caveat advanced in the introduction that extreme care must be exercised in the interpretation of bonded joint performance.