Aerospace structures adhesives cure temperatures

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... 

Their J-39 adhesives are room-temperature curable, second-generation acrylics (16). Methyl methacrylate is the monomer, and a nitrile rubber and Dow ABS resin are used as toughening agents. This family of adhesives have been used in automotive, machinery and aerospace structures. Their J-50 adhesives (17) are two-component, fast-cure acrylics. In addition, they also developed a special adhesive, J-15 structural adhesive, for hydroplanes. 

This was one of the earliest types of structural adhesive used by the aerospace industry. Vi-nyl-phenolics are still in use today because they are reasonably low in price and are excellent for applications involving bonding metal skins to wood. FM 47 adhesive is representative of this type. The major disadvantage of this type is that crosslinking occurs via a condensation reaction. As a consequence the volatiles given off during heat cure result in porous bond lines. This type of adhesive may be stored at room temperature and is cured at 350°F. Service temperature is limited to slightly above 180°F. 

Film adhesives, as their name implies, are solid at ambient temperature. Once the bonded joint is closed, heat is applied to affect cure and pressure is employed to aid the wetting of the pretreated substrate by the adhesive (for further information see Section 5.3.2.3 in Chapter 5 Aerospace A Pioneer in Structural Adhesive Bonding ). As the temperature rises, and before gelation begins, the adhesive matrix melts and begins to flow across the adherend to ensure that intimate wetting... 

Novolac resins are broadly used in electronics because their functionality higher than two increases the crossimkmg density and yields cured resins exhibiting enhanced chemical and physical properties. Mixtures of epoxy resins and phenol novolacs 61 are excellent structural adhesives in the aerospace industry. However, the phenolic hydroxyl groups are not very reachve at moderate temperatures and most systems include catalysts or accelerators. Classical adhesive compositions are prepared by mixing a solid epoxy resin, t5rpicahy an epoxidized phenol novolac resin (60 parts), a phenol novolac resin (40 parts), a solvent such as 2-butoxyethanol or butylcehosolve acetate, an imidazole catalyst, and silver flakes. 

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. 

Aromatic polybenzimidazoles were synthesized by H. Vogel and C. S. Marvel in 1951 with anticipation, later justified, that the polymers would have exceptional thermal and oxidative stability. Subsequently, NASA and the Air Force Materials Laboratory (AFML) sponsored considerable work with polybenzimidazoles for aerospace and defense applications as a non-flammable and thermally stable textile fiber and as high temperature matrix resins, adhesives and foams. The route to fiber used solutions of high molecular weight polymer. Structural applications used low temperature melting pre-polymers that were cured (polymerized) in place. Applications of polybenzimidazoles were not implemented in the 60 s and 70 s since the polymers tetraamine precursors were not commercially available. 

---