Aerospace structures adhesives formats

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). 

Compared with other polyimides, the adhesive strength is signifieantly enhanced because of the thermal reflow that occurs during the cure cycle. When cured at temperatures lower than 300 °C, polyimide 32 has a glass transition temperature of 230 °C, sufficiently high for the electronics industry. By contrast, the structural adhesives intended to he used in aerospace applications are prepared hy blending the oligomer mixture 31 with aromatic diamines to increase the formation of a cross-linked network. 

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