Curing aerospace structures adhesives

One of the most popular uses of radiant curing is the advancement (viscosity increase) or crosslinking of pressure-sensitive adhesives. These applications have been satisfied mostly with acrylate-based adhesive systems. With epoxy-based adhesives, the main applications are electrical and electronic components, the bonding of large aerospace structures such as composites, and the bonding of transparent substrates such as glass and plastic. 

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... 

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. 

The heart of any bond is, of course, the adhesive. Most bonding in the aerospace industry relies on the so-called structural adhesives, i.e. adhesives based upon monomer compositions which polymerise, or cure, to give high modulus, high strength bonds between relatively rigid adherends, such as those discussed above, so that a load-bearing structure is produced. 

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. 

Novolac resins are broadly used in electronics because their functionality higher than two increases the cross-linking 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... 

Composites Both UV and EB cures are employed for the production of wood composite materials and in fiber-reinforced composites for aircraft and aerospace applications. The EB technology has been successful in the manufacture of large structures that exceed the size of autoclaves, and in curing adhesive joints in cases where uniform radiation can be provided more easily than uniform heat. In industrial and consumer applications, multiple combinations of different reinforcing fibers can be co-cured in one cycle by EB with considerably lower residual stresses than those introduced by thermal cure.16... 

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