Adhesives and Adherends

Adhesive system Type Curing conditions Pretreatment Adherend 1 Adherend 2 

Kll Araldite 2015 2K-EP, rigid 7d 23°C/50% r.h. steel/flame steel/flame 

With respect to the manufacturing conditions in the railway industry, manual methods such as grinding and shot blasting should only be applied if the process is reproducible. Wet chemical surface pretreatments as known from the aircraft industry are too expensive and therefore not economical. As a consequence, dry chemical and physical methods were selected and optimized with respect to the process parameters which depend on the substrate. 

Two different methods, based on flame and laser pretreatments, were investigated. These methods are generally applicable under atmospheric conditions. The first is based on the surface reaction of low-molecular, silane-based precursors which are activated by pyrolysis in a propane gas flame. XPS analysis of the pretreated surface shows a thin silicate layer on the surface which remains active for about 10 days. After a careful parameter optimization, adhesion strength can be greatly improved, especially on steel surfaces. A similar method is commercially available as the Silicoater process. The second method, CLP, is based on a laser pretreatment in combination with a specific primer [4]. With parameters optimized for the specific substrate, adhesion strength can be greatly improved, especially on aluminum surfaces. 

For assessment of the surface pretreatment, lap shear tests were performed after 14 days of cataplasma storage for metals, and seven days of cataplasma storage for fiber-reinforced plastics (FRPs) cataplasma storage means that samples are stored at 70 °C and 100% r.h. The results indicate that CLP is most 

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Diffusion Theory. The diffusion theory of adhesion is mosdy appHed to polymers. It assumes mutual solubiUty of the adherend and adhesive to form a tme iaterphase. The solubiUty parameter, the square root of the cohesive eaergy deasity of a material, provides a measure of the iatermolecular iateractioas occurring within the material. ThermodyaamicaHy, solutioas of two materials are most likely to occur whea the solubiUty parameter of oae material is equal to that of the other. Thus, the observatioa that "like dissolves like." Ia other words, the adhesioa betweea two polymeric materials, oae an adherend, the other an adhesive, is maximized when the solubiUty parameters of the two are matched ie, the best practical adhesion is obtained when there is mutual solubiUty between adhesive and adherend. The diffusion theory is not appHcable to substantially dissimilar materials, such as polymers on metals, and is normally not appHcable to adhesion between substantially dissimilar polymers. 

The two issues that are dominant in determining the interfacial strength in the case of contact adhesion are (1) the completeness and intimacy of contact between the adhesive and adherend at the interface and (2) the strength of the intermolecular interactions across the interface. Methods for predicting both of these factors are discussed below. 

Initial intimacy of contact between the adhesive and adherend must of course precede the formation of a diffusion interphase, but in contrast to contact adhesion, the issue which is dominant is not the maximization of the work of adhesion but instead must be some appropriate measure of the phase compatibility, in the sense of mutual solubility. 

Regardless of which, or which combination, of the above mechanisms is responsible for adhesion in a given case, intimate molecular contact between the adhesive and adherend is required. This means that the contact angle of the liquid adhesive against the adherend surface should be as low as possible, and preferably 0°. For the case of contact adhesion, this is immediately evident, but in cases where mechanical interlocking is the primary mechanism for adhesion it is also the case because the adhesive must first be able to flow or wick into the pores of the... 

Fig. 21. Peel strengths of various adhesives against poly(ethylene terephthalate) adherends vs. the solubility parameter difference between adhesive and adherend. Drawn using data from ref. [72].

An apparently universal truth is that intimate contact between adhesive and adherend is a necessary (but not sufficient) requirement for good adhesion. 

Thanks to their multiphase constitution, block copolymers have the originality to add advantageously the properties of their constitutive sequences. These very attractive materials can display novel properties for new technological applications. In this respect, thermoplastic elastomers are demonstrated examples (l, 2, 3) they are currently used without any modification as elastic bands, stair treads, solings in the footwear industry, impact resistance or flexibility improvers for polystyrene, polypropylene and polyethylene whereas significant developments as adhesives and adherends are to be noted (5.). 

Ion beams provide useful information either as a diagnostic tool or as a precision etching method in adhesion research. The combination ISS/SIMS method used along with other techniques such as SEM provides a powerful tool for elemental analysis of surface composition. These results, as well as earlier work in this laboratory, indicate that the surface composition can be significantly different from the bulk due to contamination, selective chemical etching and segregation. These same techniques also provide an analysis of the mode of failure in adhesive joints. Many failures classified as "adhesive" on the basis of visual inspection are frequently mixed mode failures or failures at a new interface containing elements of both adhesives and adherend. 

After intimate contact is achieved between adhesive and adherend through wetting, it is believed that permanent adhesion results primarily through forces of molecular attraction. Four general types of chemical bonds are recognized as being involved in adhesion and cohesion electrostatic, covalent, and metallic, which are referred to as primary bonds, and van der Walls forces, which are referred to as secondary bonds. 

Promotion of the chemical reaction between adhesive and adherend... 

Residual stress resulting from thermal expansion or contraction is due to the differences in the thermal expansion coefficient between the adhesive and adherend and to temperature distribution in the joint due to differences in thermal conductivity. 

It is necessary that the adhesive retain some resiliency if the thermal expansion coefficients of the adhesive and adherend cannot be closely matched. At room temperature, a standard low-modulus adhesive may readily relieve stress concentration by deformation. At cryogenic temperatures, however, the modulus of elasticity may increase to a point where the adhesive can no longer effectively release the concentrated stresses. At low service temperatures, the difference in thermal expansion is very important, especially since the elastic modulus of the adhesive generally decreases with falling temperature. 

Strong chemical bonds between the adhesive and adherend help stabilize the interface and increase joint durability. Aluminum joints formed with phenolic adhesives generally exhibit better durability than those with epoxy adhesives. This is partially attributable to strongly interacting phenolic and aliphatic hydroxyl groups that form stable primary chemical bonds across the interface. 

The trialkoxysilane compounds thus satisfy two requirements indicated by Zisman (21) for useful finishing agents. They are able to displace water from the substrate surface and to prevent its readsorption. However, it was pointed out (21) that the applied film of finishing agent must be resin-wettable—i.e., yc > 40 dynes/cm., to allow good contact between adhesive and adherend. The results with the p-chlorophenyl-... 

Molecular Forces Between Adherend and Adhesive. The various theories of adhesion invoke the occurrence and interplay of physical and chemical interactions across the adherend-adhesive interface, as well as the deformation behavior of the adhesive (6, 7). Therefore, bond formation depends upon the development of intermolecular attraction, both within the bulk of the polymer and between adhesive and adherend. 

The molecular forces are secondary or van der Waals forces. It is also conceivable that primary valence forces form chemical bonds, either covalent or ionic, between adhesive and adherend. The contribution of covalent bonds to bond strength is a subject of great, if sometimes controversial, interest (6). 

A unified science of adhesion is still being developed. Adhesion can result from mechanical bonding between the adhesive and adherend and/or primary and/or secondary chemical forces. Contributions through chemical forces are often more important and illustrate why nonpolar polymeric materials such as polyethylene are difficult to bond, although polycyanoacrylates are excellent adhesives. Numerous types of adhesives are available such as solvent-based, latex, pressure-sensitive, reactive, and hot-melt adhesives. 

The combination of an adhesive and adherend is a laminate. Commercial laminates are produced on a large scale with wood as the adherend and phenolic, urea, epoxy, resorcinol, or polyester resins as the adhesives. Many wood laminates are called plywood. Laminates of paper or textile include items under the trade names of Formica and Micarta. Laminates of phenolic, nylon, or silicone resins with cotton, asbestos, paper, or glass textile are used as mechanical, electrical, and general purpose structural materials. Composites of fibrous glass, mat or sheet, and epoxy or polyester resins are widely employed as reinforced plastic (FRP) structures. 

Adhesive Bonding - A method of joining two plastics or other materials in which an adhesive is applied to the parts surfaces. Bonding occurs through mechanical or chemical interfacial forces between the adhesive and adherend and/or by molecular interlocking. Surface preparation of the adherends and curing of the adhesive may be required. 

This current state has to do with the discovery that the action of fundamental adhesion forces is not restricted to the interface. They not only fix some layer of adhesive molecules on the surface of the adherend but they can exert strong influence on the formation of chemical and morphological structure as well as on molecular mobility in the adjacent region of the adhesive during solidification. Hence an interphase is formed. These interphases depend on the combination of adhesive and adherend surface and on the process of contact formation as well. Due to its distinct structure, the interphase possesses properties that can be much different from the behavior of the bulk adhesive. Compared to the complexity of the problem, we just start understanding what is behind interphases. 

To obtain maximum adhesion, the adhesive bond strength between the adhesive and adherend should be greater than the cohesive bond strength of the adhesive, as indicated in Fig. 6.2. (Of course, the overall strength is also limited by the cohesive strength of the substrates.)... 

The wettability characteristics of an adhesive/adherend pair are determined by the relative values of surface tension of the adhesive and adherend. Surface tension of a liquid is a direct measurement of intermolecular forces and is half of the free energy of molecular cohesion. Surface tension is commonly represented by -q (gamma), and is measured in dynes/cm. The value of the surface tension of the solid substrate, or adherend, is called the critical surface tension, To ensure that the surface of the adherend will be wetted by an adhesive, an adhesive whose surface tension is less than the critical surface tension should be selected, so that... 

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