Aerospace bonding procedures

Adhesives and sealers can be an important part of a total corrosion protection system. Structural bonding procedures and adhesives for aluminum, polymer composites, and titanium are well established in the aerospace industry. Structural bonding of steel is gaining increasing prominence in the appliance and automotive industries. The durability of adhesive bonds has been discussed by a number of authors (see, e.g., 85). The effects of aggressive environments on adhesive bonds are of particular concern. Minford ( ) has presented a comparative evaluation of aluminum joints in salt water exposure Smith ( ) has discussed steel-epoxy bond endurance under hydrothermal stress Drain et al. (8 ) and Dodiuk et al. (8 ) have presented results on the effects of water on performance of various adhesive/substrate combinations. In this volume, the durability of adhesive bonds in the presence of water and in corrosive environments is discussed by Matienzo et al., Gosselin, and Holubka et al. The effects of aggressive environments on adhesively bonded steel structures have a number of features in common with their effects on coated steel, but the mechanical requirements placed on adhesive bonds add an additional level of complication. 

Composite materials can be prepared for bonding in several ways. Common procedures include mechanical abrasion of the cured composite surface and the use of a peel ply. This is a removable layer that is applied prior to curing the composite and then stripped to reveal a textured surface prior to adhesive bonding abrasion is sometimes used in addition to remove residues that may have been left by the peel ply (see Fibre composites - joining). There are also other surface treatments available, but aU aerospace bonding and pretreatment processes will require high standards of control. 

The rubber and resin are coreacted by heating the mixture at around 100 °C in the presence of a Lewis base. Most commonly tertiary amines are used for this purpose. The morphology of the rubber dispersion is important to bond performance and is governed by the coreaction procedure. This material may be formulated into a useful aerospace paste adhesive by the following combination of materials (pbw) ... 

Aluminum surfaces are prepared for adhesive bonding in aerospace applications by either etching or anodization in acid solutions. (For less stringent strength and durability requirements, mechanical abrasion can be adequate.(2)) Common preparations result in microrough adherend morphologies, which studies have shown yield the best overall bond durability. Three of these surface preparations are described in this section Forest Products Laboratory (FPL) etching procedure, phosphoric acid anodization (PAA), and chromic acid anodization (CAA). 

Anodising in chromic acid is the electrochemical pretreatment procedure favoured in Europe. European aerospace companies view this as giving a less friable oxide film than the phosphoric acid anodising process favoured in the United States and believe that it imparts much better durability to the bonded joint. This latter statement is somewhat debatable in the light of both experience in the laboratory and the actual chemical stmcture of the oxide films themselves. 

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