Adhesive transfer, mechanisms

Repair. Repairs for damaged bonded structure can be either mechanical or adhesively bonded. Mechanical repairs are metallic doublers on one or both sides of a damaged component, held on by fasteners. The fasteners transfer the load through the doubler around the damaged site and restore part functionality. Although common for metal-to-metal bonded structure, mechanical repair of sandwich structure is rare because of the risk of further delamination. Unless the doubler and fasteners are perfectly sealed, water can travel into the honeycomb core eventually causing freeze-thaw damage and delamination. 

The chemistry of a structural adhesive is designed to do at least two important things. First, the adhesive must at some time pass through a fluid state in order to wet the adherends. Second, the adhesive in its final state in the bond line must be a solid, high-molecular-weight polymer that is able to carry and transfer mechanical forces. In almost all cases, the polymer matrix of a structural adhesive will be crosslinked. The chemistry must... 

Generally, there are four steps are included in the mass transfer mechanisms of the adsorption process. These steps are fluid-film transfer, pore diffusion, surface adhesion, and surface diffusion. The rate of surface adhesion for physical adsorption on the surface of porous adsorbents is very rapid, enough to be assumed instantaneous relative to the other transfer rates [5]. 

Such being the case, further inferences about the nature of the wear process follow. A disrupted fluid film allows localized contacts at the rubbing surfaces, and it is the mechanistic processes at these contacts that determine the course of lubricated wear. When the wear process is abrasive, it is most likely influenced directly by fluid film thickness and surface roughness, whereas processes such as adhesion, transfer, oxidation, additive reaction and the like are responsive to surface conditions at the contacts as well as to the number of contacts. These are the aspects of lubricated wear that are emphasized in this chapter, from the viewpoint of phenomenology, mechanisms and modeling. 

The wear of polymers Is general believed to be caused by one or more of the following mechanisms adhesive transfer, abrasive cutting,... 

P(2) When determining the load transfer mechanism, i.e. whether the loads are transmitted through the bondline or through the rivets, special attention shall be paid to the type of adhesive used. See clauses P(3) and P(4) of 5.4.1. 

The primary categorisation is based on the load transfer mechanism, i.e. whether the loads are directed through an adhesive material, mechanical fasteners or both. The most descriptive title for this top level of categorisation would be joint mechanism . As this implies an immediate reference to mechanical joints, this term is excluded from the EUROCOMP Design Code. Therefore the three main groups listed above are entitled joint categories . 

The polymer matrix resin caruiot perform its fundamental duties in the composite unless the fibers have been weU wetted to the matrix [23]. Dry fibers do not transfer external loads effectively. Therefore, a critical function of a polymer matrix resin is simply to wet out and to bond it to the fibers and to form a continuous phase that intercoimects all constituents of the composite. The hydrophihc polar character of natural fibers results in inherently low compatibility with hydrophobic nonpolar character of polymer matrices. It may cause insufficient wetting of a polymer resin to the fiber surfaces, providing poor interfacial adhesion, low mechanical properties. 

As discussed in Chapter 2, for the formation of intrinsic adhesion forces across the adhesive/substrate interface it is a necessary, though sometimes insufficient, requirement that intimate interfacial contact occurs between the adhesive and substrate. This means that at some time in the bonding operation the adhesive must be in a liquid form, ideally with a relatively low viscosity. However, in service most adhesive joints carry some mechanical loads, even if only those resulting from the weight of the substrates, and to be able to transfer mechanical loads efficiently from one substrate to the other requires that the adhesive is a solid with an adequate stiffness and strength. These conflicting requirements mean that once the adhesive has established interfacial contact, i.e. completely wet the substrate, it must then harden or set . In this chapter the methods by which the adhesive may be transformed from a liquid state to a solid state will be considered and this classification will also be used to introduce briefly some of the main chemical types of adhesive to those unfamiliar with this aspect of adhesives technology. 

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