Aerospace structures adhesives primers

The application of corrosion-resistant primers has become standard practice for the structural bonding of aluminum in the automotive and aerospace industries. The adhesive-primer combinations are chosen to provide both maximum durability in severe environments and higher initial joint strength. Improved service life is typically achieved by establishing strong and moisture-resistant interfacial bonds and protecting the substrates surface region from hydration and corrosion. 

Epoxy structural adhesives are used in an extraordinarily wide range of applications. They are available in essentially all of the forms discussed above, except for primer—liquid combinations or as room temperature curing liquids. The liighest technology7 application for epoxies is in aerospace structural... 

The next major improvement in aerospace adhesives occured in the late 1950s with the introduction of adhesives based on epoxy resins. Since these adhesives crosslink via an addition reaction, no volatiles are released during heat cure. This made low pressure bonding possible and the use of nonperforated honeycomb feasible in sandwich structure. Other improvements followed that resulted in more durable bonded structure. These include the development of corrosion inhibiting adhesive primers in 1968, corrosion resistant aluminum honeycomb in 1969, and the phosphoric acid anodizing process for preparing aluminum for bonding in 1974. 

The third section devotes itself to an in-depth examination of the structural bonded joint the substrates, primers and the structural adhesives themselves. In this last area, a full appraisal is made of a typical range of commercially available structural adhesives. This examines their role in the bonded structure, the formats in which they are supplied, the basic chemistries employed with their relative cure cycles and generic formulations. This is augmented with key properties of selected adhesives from this range and a typical qualification package generated for one adhesive to meet typical aerospace specifications. The section concludes with brief outlines as to how the adhesives are made, how they are applied to the substrates to be bonded and file methods by which they can be cnred. 

Although many non-structural adhesives ean be solvent or water based, in aerospace applications, this format is usually the preserve of the surface protection and corrosion inhibiting primers. However, as indicated, there are exceptions and these are often those adhesives which are based on phenolic resoles (for example, Redux 775 Liquid) or on polyimides (for example, PMR -2). 

The tables and text below show the makeup of a typical range of commercially available structural adhesives (Redux from Hexcel Composites " ) and their supporting primers, syntactic pastes, and foaming adhesives. The whole range is considerably larger—to meet the demands of other industries such as road and marine transport, electrical, and electronics, and so on—but here only the key products for the aerospace market are shown. Table... 

In contrast to aluminum and titanium structural bonds where performance can be optimized for most aerospace applications, steel bonds are often designed to minimize cost as long as certain performance standards are met [47]. If feasible, many manufacturers prefer to select adhesives or primers that provide adequate strength and durability with untreated steel rather than to prepare the surface for bonding. 

Uses Corrosion inhibitor primer for use with structural epo) film adhesives in aerospace and general industries Features Contains adhesion promoter and latent resin hardener chromate-free curable reduced VOCs... 

Despite considerable work over many years, silane pre-treatments or primers prior to structural bonding are still relatively rare in Aerospace applications. Silanes are, though, regularly used as formulating ingredients in Epoxide adhesives (paste and film)... 

Epoxy adhesives are widely used in most industries for the structural bonding of most types of materials (metals, wood, plastics, composites, ceramics, glass, concrete, masonry, etc). Primers may be recommended for some materials. They are also used to repair structures bonding patches and crack-filling. Some adhesives are specifically designed for aerospace applications or specialised mechanical and electrical assembly. 

This means, therefore, that the chemistries associated with the primers could be as varied as the adhesives used with them. In aerospace applications, however, the predominant surface protection primer chemistries, for structural bonding, are those based on epoxy phenolic-, polyimide- and polyurethane-based primers are also encountered. 

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