Aluminium adhesion promoters

Abstract— The use of organosilanes as adhesion promoters for surface coatings, adhesives and syntactic foams is described and reviewed in the light of published work. Data are presented on the beneficial effect of silanes, when used as pretreatment primers and additives, on the bond strength of two pack epoxide and polyurethane paints applied to aluminium and mild steel. It is shown that silanes when used as additives to structural epoxide and polyurethane adhesives are less effective than when used as pretreatment primers on metals but are highly effective on glass substrates. The compressive properties of glass microballoon/epoxide syntactic foams are shown to be markedly improved by the addition of silanes. 

The recorded bond strengths clearly show that MAMS and ECMS were totally ineffective as adhesion promoters on glass. MPS, APES and AAMS were all effective and their use resulted in a marked improvement in the bond strength of both adhesives. All three silanes resulted in a change of the site or mode of failure, the locus of failure transferring from the glass surface to the aluminium test specimen or within the adhesive. 

As previously stated, for zirconium-based adhesion promoters, suitable functional groups on the substrate and the overcoat would be oxygenated species. Typically suitable oxygenated functional groups can be found on corona discharge-treated plastics, anodized aluminium, etc. [4]. 

Aluminium drinking carts. There are many patents [8] referring to the use of fluorozirconic acid (H2ZrF6)-based systems to treat the surface of aluminium cans to improve the corrosion resistance of the metal and the adhesion of the applied coatings. Typically, the zirconium fluoride will be used in conjunction with polyacrylic acid, presumably to form a complex in situ which acts as an adhesion promoter. Such surface treatment of aluminium is not restricted to zirconium fluorides, as ammonium zirconium carbonate displays similar properties in such application areas. 

Importantly, for a particular application, and where the metals to be joined have been specified, an overall metal bonding system should be considered. Typically, this will include selection of a suitable adhesive and possibly primer or coupling agent in combination with appropriate methods of surface preparation or pre-treatment. Organofunctional silanes (see Silane adhesion promoters) are commonly used coupling agents, which have been demonstrated to provide covalent chemical bonds with steel or aluminium surfaces and possess epoxide or amine functionality for reaction with Epoxide adhesives. 

Organotitanates, aluminates, zirconates and zircoaluminates can also act like silanes as adhesion promoters. They perform similar functions, but unlike silanes there is no need for water molecules to be eliminated. These other treatments bond the polymer to the filler surface by a chemical bond involving proton co-ordination, and they can also be used with fillers that are not receptive to silanes, such as calcium carbonate, carbon black and barium sulfate, as well as barium ferrite, magnesium hydroxide, aluminium trihydroxide, titanium dioxide, talc and the nanoclays. 

Aluminium strip is chromated in order to provide an intermediate, adhesion-promoting coating prior to painting and lacquering in the canning industry. 

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). 

The importance of a clean surface for painting is paramount (see Engineering surfaces of metals). For metallic substrates, either mechanical or chemical surface treatment is almost always needed to remove surface contamination and promote adhesion. Chemical pre-treatment of metals prior to painting is an effective method of surface conversion that can provide both corrosion resistance and improved paint adhesion. For ferrous metals, the most common chemical pre-treatment is a phosphate for aluminium (and many other non-ferrous metals), the most effective pre-treatment is chromate. Metals must first be cleaned with an alkaline cleaner and then rinsed before they can be converted chemically in an immersion bath or by spray. 

The Saret 633/peroxide-cure system also promotes good adhesion to other untreated metals, including aluminium, zinc, brass, and stainless steel. This is illustrated in Figure 8.9 for EPDM containing 10 phr Saret 633. In each case, shear adhesion increased with the addition of Saret 633, compared with peroxide alone. 

Indeed, it will be recalled from Section 8.3.2.3 that it has also been suggested that a reason for the often superior durabilities shown by phenolic-based adhesives and primers is that any extract is acidic rather than alkaline and does not, therefore, promote corrosion of the aluminium substrate. 

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