Adhesive surface roughness

Many grades of interlayer are produced to meet specific length, width, adhesion, stiffness, surface roughness, color (93,94), and other requirements of the laminator and end use. Sheet can be suppHed with vinyl alcohol content from 15 to about 23 wt %, depending on the suppHer and appHcation. A common interlayer thickness for automobile windshields is 0.76 mm, but interlayer used for architectural or aircraft glaring appHcations, for example, may be much thinner or thicker. There are also special grades to bond rear-view mirrors to windshields (95,96) and to adhere the components of solar cells (97,98). Multilayer coextmded sheet, each component of which provides a separate property not possible in monolithic sheet, can also be made (99—101). 

Poor preparation of the substrate can result in loss of adhesion, pitting, roughness, lower corrosion resistance, smears, and stains. Because electroplating takes place at the exact molecular surface of a work, it is important that the substrate surface be absolutely clean and receptive to the plating. In the effort to get the substrate into this condition, several separate steps may be required, and it is in these cleaning steps that most of the problems associated with plating arise. 

Increase adhesion tension. Maximize surface tension. Minimize contact angle. Alter surfactant concentration or type to maximize adhesion tension and minimize Marangoni effects. Precoat powder with wettahle monolayers, e.g., coatings or steam. Control impurity levels in particle formation. Alter crystal hahit in particle formation. Minimize surface roughness in milhng. 

Once it is recognized that particles adhere to a substrate so strongly that cohesive fracture often results upon application of a detachment force and that the contact region is better describable as an interphase [ 18J rather than a sharp demarcation or interface, the concept of treating a particle as an entity that is totally distinct from the substrate vanishes. Rather, one begins to see the substrate-particle structure somewhat as a composite material. To paraphrase this concept, one could, in many instances, treat surface roughness (a.k.a. asperities) as particles appended to the surface of a substrate. These asperities control the adhesion between two macroscopic bodies. 

As of this time, no one has solved the problem of the effect of asperities on a curved surface nor has anyone addressed the issue of crystalline facets. Needless to say, the problem of asperities on an irregular surface has not been addressed. However, Fuller and Tabor [118] have proposed a model that addresses the effects of variations of asperity size on adhesion for the case of planar surfaces. Assuming elastic response to the adhesion-induced stresses, they treated surface roughness as a random series of asperities having a Gaussian height distribution (f> z) and standard deviation o. Accordingly,... 

A principal aim of the discussion thus far has been to set out a theoretical framework within which it is possible to rationalise the effects of surface roughness on adhesion. It may be useful to summarise this framework before examining practical examples taken from the literature. 

Fig. 7. Adhesion (critical energy release rate, Fc) of zinc coatings to steel substrates effect of steel surface roughness (after Ye et al. [68]).

McBain and Hopkins [2], in their classical scientific study, argued that the surface roughness of a porous material was the basis of mechanical adhesion , its being... 

One of the biggest challenges in this industry is the wide variety of substrates that can be encountered for any given application. Not only can the materials be substantially different in their chemical make up, but they may also be quite different in surface roughness, surface curvature and thermal expansion behavior. To help adhesion to these substrates, preparation of the surface to be bonded may be critical. This preparation may be as simple as a cleaning step, but may also include chemical priming and sanding of the surface. 

The scale of the microscopic surface roughness is important to assure good mechanical interlocking and good durability. Although all roughness serves to increase the effective surface area of the adherend and therefore to increase the number of primary and secondary bonds with the adhesive/primer, surfaces with features on the order of tens of nanometers exhibit superior performance to those with features on the order of microns [9,14], Several factors contribute to this difference in performance. The larger-scale features are fewer in number... 

Direct bonding. In many high-volume production applications (i.e., the automotive and appliance industries), elaborate surface preparation of steel ad-herends is undesirable or impossible. Thus, there has been widespread interest in bonding directly to steel coil surfaces that contain various protective oils [55,56,113-116], Debski et al. proposed that epoxy adhesives, particularly those curing at high temperatures, could form suitable bonds to oily steel surfaces by two mechanisms (1) thermodynamic displacement of the oil from the steel surface, and (2) absorption of the oil into the bulk adhesives [55,56]. The relative importance of these two mechanisms depends on the polarity of the oil and the surface area/volume ratio of the adhesive (which can be affected by adherend surface roughness). 

---