Adhesion durability chemical bonding

Formation of durable chemical bonds is an obvious means to stabilize the interface and has been demonstrated for phenolic/alumina joints [25] and for silane coupling agents [26,27]. However, for most structural joints using epoxy adhesives and metallic adherends, moisture-resistant chemical bonds are not formed and mechanical interlocking on a microscopic scale is needed between the adhesive/primer and adherend for good durability. In these cases, even if moisture disrupts interfacial chemical bonds, a crack cannot follow the convoluted interface between the polymer and oxide and the joint remains intact unless this interface or the polymer itself is destroyed. 

Moisture acts as a debonding agent through one of or a combination of the following mechanisms 1) attack of the metallic surface to form a weak, hydrated oxide interface, 2) moisture assisted chemical bond breakdown, or 3) attack of the adhesive. (2 ) A primary drawback to good durability of metal/adhesive bonds in wet environments is the ever present substrate surface oxide. Under normal circumstances, the oxide layer can be altered, but not entirely removed. Since both metal oxides and water are relatively polar, water will preferentially adsorb onto the oxide surface, and so create a weak boundary layer at the adhesive/metal interface. For the purposes of this work, the detrimental effects of moisture upon the adhesive itself will be neglected. The nitrile rubber modified adhesive used here contains few hydrolyzable ester linkages and therefore will be considered to remain essentially stable. 

Apparently, the chemical bonding present at the paint/adhesive interface is much stronger than that occurring at either the phosphate/adhesive or the phosphate/topcoat interfaces. In the case of ZM, phosphating to improve durability is not necessary, and in fact, was proven to be detrimental. The paint provides a moisture resistant barrier layer which reduces the activity of water at the interface providing for a surface receptive to the chemical and physical bonds necessary to promote good adhesion and enhance durability. 

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. 

Epoxy-amine liquid prepolymers are extensively appHed to metallic substrates and cured to obtain painted materials or adhesively bonded stractures. Overall performances of such systems depend on the interphase created between the organic layer and the substrates. When epoxy-amine Hquid mixtures are appHed to a more or hydrated metaUic oxide layer (such as Al, Ti, Sn, Zn, Fe, Cr, Cu, Ag, Ni, Mg, or E-glass), amine chemical sorption concomitant with metaUic surface dissolution appear, leading to the organometaUic complex or chelate formation [1, 2]. Furthermore, when the solubility product is exceeded, organometaUic complexes may crystaUize. These crystals induce changes of mechanical properties (effective Young s modulus, residual stresses, practical adhesion, durability, etc.). 

On that basis, the book intends to bridge current issues, aspects and interests from fundamental research to technical apphcations. In seven chapters, the reader will find an arrangement of latest results on fundamental aspects of adhesion, on adhesion in biology, on chemistry for adhesive formulation, on surface chemistry and pretreatment of adherends, on mechanical issues, non-destructive testing and durability of adhesive joints, and on advanced technical applications of adhesive joints. Prominent scientists review the current state of knowledge about the role of chemical bonds in adhesion, about new resins and nanocomposites for adhesives, and about the role of macromolecular architecture for the properties of hot melt and pressure sensitive adhesives. Thus, insight into detailed results and broader overviews as well can be gained from the book. 

From the above discussion, it follows that it should be possible to improve the durability of bonded joints by the introduction of suitable coupling agents at the interface. If the coupling agent is capable of chemically interacting with the metal or its oxide, displacement of the adhesive at the interface will be prevented. 

Properties of this material have been reported. It has been found to bond to enamel and dentine, and to do so reliably and with good durability [107]. Results from X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FTIR) show that this bonding is the same as for conventional glass-ionomers, and involves the formation of chemical bonds to calcium in the mineral phase of the tooth. There is also evidence of micromechanical adhesion in this material [107]. 

In the former technique the degree of abrasion is known to affect subsequent bond strength and durability, heavy abrasion to expose surface fibres being recommended. Other technologists suggest that wet sanding should be carried out below a reactive primer , such as a silane solution, which promotes chemical bonding with the adhesive and carries away the dust from abrasion. 

Optimisation of surface pretreatment is the key to maximising joint durability. The adhesive influences the surface oxide layer and the surface oxide layer influences the boundary layer polymer matrix the whole must therefore be viewed as a unique system for every adherend-adhesive combination. The interplay of chemical bonding... 

Covalent chemical bonds can form across the interface and are Ukely to occur in cross-Unked adhesives and thermoset coatings. This type of bond is usually the strongest and most durable. However, they require that mutually reactive chemical groups should exist. Some surfaces, such as previously coated surfaces, wood, composites, and some plastics, contain various functional groups that under appropriate conditions can produce chemical bonds with the adhesive material. There are ways to intentionally generate these conditions, such as by surface treatment of plastics with techniques like corona or flame treatment. 

Primers for adhesive bonding chemically functionalize the substrate surface to provide pathways for chemical bonding with a selected silicone cure system. The increase of chemical bond improves adhesion durability. The main disadvantage of priming the substrate is the addition of an extra step in the whole process of adhesive application. The primers are usually Silane adhesion promoters, a reactive alkoxy silane molecule, oligomer, or a mixture of two or more different silanes. 

In summary, it can be stated that a large number of different surface treatments exist to improve the adhesional properties of PP. It should be stated here that some manufacturers of cyanoacrylate adhesives recommend special chemical primers which produce adhesion on PP without other treatment. In the experience of the author, this type of primer improves the adhesion but produces bonded joints that are insufficiently durable, especially under humid enAuronmental conditions. 

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