Лако-красочные материалы — производство

3.1 Introduction

The successful performance of many every-day products, and many common materials and construction techniques, is dependent upon adequate adhesion between two or more constituents. Most engin­eers, however, have only the haziest of ideas about the whole concept of adhesion. For to know ‘how’ to prepare substrate surfaces for bonding does not necessarily require a knowledge of ‘why’ adhesive materials should stick to them. It is the intention of this chapter to connect theory with practice, to enable the reader to appreciate ‘why’ before discussing aspects of surface pretreatment pertinent to applications of adhesives in construction.

The strength of bonded assemblies depends not only on the cohesive strength of the adhesive, but also on the degree of adhesion to the bonding surface. One of the disadvantages of adhesive bonding as a method of fastening is that the surfaces need to be clean and, whatever their chemical nature, coherent if a satisfactory degree of contact, and therefore adhesion, is to be obtained. Frequently the adhesive itself is wrongly blamed for ‘not sticking’, but the general source of trouble lies with the surface pretreatment. The use of cold-cure epoxies generally necessitates the careful preparation of metallic adherends in particular, to ensure satisfactory long-term performance. Indeed, whatever the nature of the substrate, or the adhesive to be used with it, its pretreatment is probably the single most important aspect of the bonding operation. Inadequate surface pretreatment is usually the main cause of joints failing in service.

High initial bond strength is generally not as important as the bond durability, as dictated by the environmental stability of the adherend-adhesive interface. Surface pretreatments, whilst greatly affecting bond durability, generally have less effect on initial strength. Water is the substance which usually gives rise to problems in joint durability, with failure often being exhibited at, or near, the adherend-adhesive interface. The most important factor is the environmental stability of that interface, and appropriate surface pretreatment is viewed as the best means of maintaining adhesion under adverse conditions. Among several publications relevant to this crucial aspect of structural adhesive bonding are those by Comyn(l), Kinloch(2), Brockmann(3) and Hutchinson(4). The deleterious effects of water on joint strength, especially in combin­ation with an applied tensile stress, may be appreciated with reference to Figs. 3.1 and 4.20.

The purpose of surface preparation is to remove contamination and weak surface layers, to change the substrate surface geometry, and/or introduce new chemical groups to provide, at least in the case of metals, an oxide layer more ‘receptive’ to the adhesive. An appreciation of the effects of pretreatments may be gained from surface analytical or mechanical test techniques. Experimental assessments of the effects of surface pretreatment, even when using appropriate mechanical tests, are of limited value unless environmental exposure is included. Self-stressed fracture mechanical cleavage specimens, as discussed in Chapter 4 and in the texts edited by Kinloch(2,5) for example, are therefore referred to wherever possible.