22 августа, 2015 
Pokraskin Recently, it has become clear that new concepts are needed to explain the behavior of adhesive bonds, especially as the ‘standard theories’ initially proposed to help in our understanding of adhesion phenomena have been shown inadequate.
Polymer dynamics, which appear to be a dominant factor in the adhesion capacity of pressure-sensitive adhesives, are most likely also essential for the adhesion of not only contact adhesives but also hot-melt adhesives, which generally do not form crosslinks (or only very loose crosslinks) and are not considered to present any chemical reactivity.
The results of recent experiments [25, 26] have given rise to the assumption that, in addition to chemical bonds, polymer dynamic effects are also responsible for the overall adhesive behavior of reaction adhesives. For example, in epoxide resin adhesives, at the adherent interface there is a distinct influence of the surface condition on the kinetics and density of the crosslinking, respectively.
It is remarkable that on surfaces which attribute a good age resistance to an adhesive bonded joint (at least as far as we know), such as steel or aluminum surfaces blasted with corundum, the degree of crosslinking near the interface is less than on surfaces which attribute a poorer age resistance to the assembly.
This effect is not explained by surface morphology, but rather by the chemical properties that are influenced by contamination of the surface by the blast medium.
The fact that better, more durable adhesion results from a lower degree of crosslinking is contradictory to standard concepts, as the latter is probably expected to involve a higher rate of water transport.
It has long been assumed, and clearly demonstrated in some cases, that the material surface does have an effect on polymers, although the details of this phenomenon are not yet understood [26]. However, in the case of a lower degree of crosslinking a better polymer mobility can be postulated, which in turn has a stabilizing effect in terms of the dynamic adhesion mentioned above. The results of recent studies [27, 28] have indicated that the corrosive mechanisms in adhesive — bonded metal joints are also controlled by modifications ofthe polymer condition due to the effect of the surface. Indeed, it is becoming increasingly evident that not only in the non-aged and aged conditions but also in the dried condition, the deformation behavior of adhesive bond-lines is considerably influenced by the surfaces of the adherents.
Frequently, ‘softer’ (i. e. highly elastic) adhesives with plastic deformation behavior show a better durability than rigid systems, a property which speaks well for the importance of dynamic effects. Previously, adhesion was thought to be irreversibly weakened by humidity, but it is increasingly realized that this effect is in fact not absolutely irreversible. If, after an aging process, the recovery time is sufficiently long then there is a reimprovement ofthe adhesive bonded joints in many cases [28, 29].
At present, the modification of a polymer near the interface is attributed to chemical effects exerted by the surface [25, 26, 28]. The microscopically measurable roughness of the surface clearly has no influence either on these mechanisms or on the long-term durability ofthe adhesive bonds [25].
It is known, however, that nanostructured surfaces which can easily be produced on aluminum by pickling, or alternatively by anodizing on zinc or stainless steel, may have a considerable effect on adhesive strength, and particularly on durability when compared to non-nanostructured surfaces of an identical chemical composition. This phenomenon cannot be explained by simple concepts such as the postulate of micromechanical adhesion because, once again, the failure of adhesion occurs not within the nanostructures but rather within the polymer near the interface. Thus, it is safe to assume that there is an effect ofthe surface morphology that can be
explained not only by the steric hindrance of adsorption or segregation within the nanostructures, but also by orientation effects. The results of these investigations are presented in Section 7.5.
It becomes clear that, when attempting to understand the build-up and behavior of adhesion, one reaches a dimension which lies between the molecular one and the dimensions ofmatter, the characteristic technical properties ofwhich only take effect in case of a total quantity of approximately two orders of magnitudes higher than the molecular dimension. Although it has become possible to analyze this dimension only recently, the prevailing — albeit sometimes unsatisfactory — uncertainty with regard to the behavior of adhesive systems is expected to be overcome within the next few years.
Опубликовано в рубрике 