Adhesive joints substrate surface pretreatment

Due regard should be paid to the practical difficulties of achieving a high standard of substrate surface pretreatment in repair and strengthening works on site. The stability of the adherend/adhesive interface is probably the most important factor in the durability of bonded joints. 

Primers are often used to help ensure an adequate service life and, indeed, the presence, and the chemical type, of any primer may play a crucial role in ensuring a long service life for the adhesive joint. For example, it will be shown later that the use of organometallic primers may be extremely beneficial and many commercial corrosion-inhibiting primers have been developed for use with epoxy aerospace adhesives following the above comments, it is probably not just a coincidence that many of the latter types of primers contain phenolic-based constituents. Examples of the effect of substrate surface pretreatments on joint durability were shown in Figs 4.16 and 4.17 and Table 4.15 further examples are illustrated in Fig. 8.8. 

Thus, in many applications, which may involve some of the most critical uses of adhesives technology, the bonded joints are exposed to an environment which also happens to be one of the most potentially damaging. Indeed, the aspect of the durability of adhesive joints to aqueous environments is undoubtedly one of the most important challenges that the adhesives community faces. In particular, there are two main challenges to develop (a) adhesive systems (i.e. combinations of an adhesive/primer/surface pretreatment/substrate type, all of which may interact to affect the joint durability) which possess excellent long-term durability, and which are environmentally friendly and cost effective and (b) test methods and models to accurately rank and predict the service-life from short-term experiments, and thereby convince the potential user that an adequate durability will be realised. Clearly, these two aims are strongly inter-linked. 

The materials being joined are commonly referred to as the substrates or adherends. The latter term is sometimes employed when the materials are part of a joint, but in the present text only the term substrate will be used. In the following chapters the importance of considering the adhesive joint from the viewpoint of being an "adhesive system" will be emphasized. The term adhesive system is employed to indicate that, for many aspects of the performance of adhesive joints, other factors besides the physical and chemical properties of the selected adhesive may be of importance. For example, parameters such as the type of alloy selected for the substrate and the form of any surface pretreatment or primer chosen for the substrates may be extremely important. Indeed, in many instances, such parameters may be the main factors in determining whether the adhesive joint will perform adequately, especially with respect to attaining the desired service-life from the bonded structure. 

Absorbed moisture in certain substrates, e.g. fibre composites, may also adversely affect the performance of the adhesive joint, but these aspects are best considered in Section 4.2.5 when surface pretreatments for such materials are reviewed. 

Although in many applications no form of pretreatment is employed for the substrate materials prior to adhesive bonding, to attain the maximum in joint performance some form of surface pretreatment for the substrate materials being joined is almost always necessary. Adhesives are available which will join substrates which are covered in light machine or protective oils or low molecular weight plasticizers but some control over the state of the surface is still usually necessary. For example, the type of protective oil needs to be matched to the adhesive and its thickness needs to be controlled, so that the adhesive can dissolve and displace the oil. Further, the reproducibility and service life of such adhesive joints is still invariably greatest when some form of surface pretreatment is employed. 

Various reviews [1 6] have given details of the common surface pretreatments that have been developed, mainly empirically, both for non-metals and metals to give adhesive joints which possess the maximum strength, a low coefficient of variation on the strength values (i.e. are reproducible) and have long service lives in hostile environments. It is not the purpose of the present chapter to repeat the many recipes that are listed in these reviews but rather to indicate the types of pretreatment that are employed for different substrates and identify the scientific principles involved. 

As mentioned earlier, other plastics such as poly(vinyl chloride), poly (ethylene terephthalate), polyacetals, nylons and polyimides do not present such a severe problem to the adhesives technologist as do fluorocarbon polymers or polyolefins. Nevertheless, in order to obtain very high joint strengths approaching the cohesive strength of the substrate, some form of surface pretreatment is often necessary. 

Finally, mention should be made of the concept of a sacrificial primer which enables metals to be readily bonded underwater [210,211]. From the comments in Section 2.6.2 it is obvious that when trying to apply an adhesive to a substrate underwater it will be difficult for the adhesive to completely displace the water adsorbed on the surface and establish interfacial contact with the substrate a layer of water would remain adsorbed onto the substrate and act as a weak boundary layer. The idea of the sacrificial primer is that the substrate is firstly cleaned, typically by an abrasion pretreatment, but simultaneously a primer is applied which is formulated so that it displaces any water and is more readily adsorbed onto the substrate surface. This primer, although water repellant, is also formulated so that it is compatible with the adhesive to the extent that it, in turn, is displaced and dissolved by the adhesive when it is applied. Thus, the concept of a sacrificial primer enables the adhesive to wet the substrate completely without a weak boundary layer of water being retained at the interface. The adhesive is also especially formulated to aid these processes and contains silanes which will diffuse to the interface and establish strong and stable interfacial bonds to ensure a long service life for the joint in its underwater environment [212]. 

We have so far considered the establishment of interfacial contact between the adhesive and substrate, the nature of the intrinsic adhesion forces which hold the interface together, and the role of surface pretreatments to assist in the above processes. The next stage in the formation of an adhesive joint is usually, but not always, the hardening of the adhesive so that it is capable of... 

Two other points of interest are that it appears that such hydration inhibitors are only really effective on an aluminium alloy substrate which has already been pretreated by the CAE process. It would obviously be useful for the technologist if such simple primers were effective on a degreased or abraded surface. Secondly, the above work [82] also suggested that some silane-based primers could function as hydration inhibitors, so adding another possible mechanism whereby such organometallic primers might increase the service life of adhesive joints. 

Polyolefins, such as polyethylene, polypropylene and polymethyl pentene, as well as polyformaldehyde and polyether, may be more effectively treated with a sodium dichromate-sulfuric acid solution. This treatment oxidizes the surface, allowing better wetting by the adhesive. Flame treatment and corona discharge have also been used. Table 7.20 shows the relative joint strength of bonded polyethylene and other plastic substrates pretreated by these various methods. 

It should, however, be remembered that surface protection primers essentially donate nothing to the bonded joint apart from the protection of the pretreatment applied to the substrate and acting as an adequate key between adhesive and adherend. Enhancement of the mechanical performance of the adhesive being used is rarely seen. Indeed, in a few cases a reduction in bonding strength occurs. 

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