Silanes adhesive bond durability

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Other aspects of interfacial science and chemistry are examined by Owen and Wool. The former chapter deals with a widely used chemistry to join disparate surfaces, that of silane coupling agents. The latter chapter describes the phenomenon of diffusion at interfaces, which, when it occurs, can yield strong and durable adhesive bonds. Brown s chapter describes the micromechanics at the interface when certain types of diffusive adhesive bonds are broken. The section on surfaces ends with Dillingham s discussion of what can be done to prime surfaces for adhesive bonding. 

The corrosion resistance and polymer-bonding compatibilities of the lonizable organophosphonates and the neutral organo-silanes are directly related to their inherent chemical properties. Specifically, NTMP inhibits the hydration of AI2O2 and maintains or Improves bond durability with a nitrile-modified epoxy adhesive which is cured at an elevated temperature. The mercaptopropyl silane, in addition to these properties, is compatible with a room temperature-cured epoxy-polyamide primer and also exhibits resistance to localized environmental corrosion. These results, in conjunction with the adsorbed Inhibitor films and the metal substrate surfaces, are subsequently discussed. 

The ionic phosphonates like NTMP are effective hydration inhibitors because they can form an insoluble complex with the oxide surface. They are useful as epoxy adhesive couplers in cases where the adhesive and its curing cycle are compatible with the adsorbed phosphonate molecule. (14) Wedge test results indicate that in two epoxy-aluminum systems studied, certain organosilanes tend to both increase the epoxy-metal bond durability and maintain hydration resistance. The results of anodic polarization experiments further suggest that these silane films are effective against localized pitting. 

FIGURE 10.3 Effect of silane adhesion promoter on the durability of a mild steel joint bonded with epoxy adhesives.16... 

Conversion coatings offer many of the features identified as being desirable on a properly pre-treated metal surface consequently, they have been applied to a range of metals for the enhancement of structural adhesion. A number of studies have demonstrated that properly optimized conversion coatings are comparable to the anodic oxides in terms of providing durability with aluminium.Other studies have demonstrated variable adhesion performance with conversion coated aluminium adherends. A recent study has shown phosphated medium carbon steel to give better bond durability than grit-blasted and silane-treated material. ... 

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. 

Shear bond or tensile tests determine the dental adhesive bond strength, with thermocycling in water effectively evaluating the bond durability. The monomer VBATDT is effective for bonding Ag-Pd alloy, while 4-META is suitable for Ni-Cr alloy and titanium. For Ni-Cr alloys, the chromium content influences bond durability. Ferric chloride combined with a silane coupler significantly improves the bond strength of 4-META/MMA-TBB-O resin joined to dental porcelain. 

Some alternative chromate free pretreatment methods for aluminium surfaces have already been described [14 16], Commercially available products mainly consist of titanium and zirconium compounds. Hybrid polymer sol-gel materials are potential substitutes for hexavalent chromium-based surface treatments as well. Due to the chemical characteristics, in particular, the presence of hydroxy and alkoxy groups, the hybrid materials are qualified to coat metal as well as metal oxide surfaces. These groups can react with OH-groups on the surfaces of both metals and metal oxides. Water and alcohols are eliminated, while bonds between the hybrid polymer and the metal surface are created (Fig. 6.6), thus leading to good durable adhesion of the layers to the metal substrates [17,18]. Similar to silane adhesion promoters, the hybrid sol-gel materials can also link to organic polymer paint systems (Fig. 6.7). 

Adhesion is a complex phenomenon based on a number of different mechanisms. In contrast to simple mechanical adhesion, silanes enable a sealant to bond chemically to a substrate, resulting in more durable adhesion. The bond is less susceptible to the negative effects of moisture and temperature. The nature of the substrate surface also plays an important role in achieving good adhesion. The more chemically active sites (preferably hydroxyl groups) the substrate has, the... 

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. 

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