Toughened epoxy adhesives

Fig. 2. Morphology model of a core-shell, rubber-toughened epoxy adhesive.

Though toughened phenolic adhesives remain in use for specific applications, toughened epoxy adhesives have dominated metallic bonding on civil aircraft since their development in the 1960s. Advances since then have been incremental and mostly revolving around manufacturing issues such as handleability and allowed out-time. 

Kinloch A.J. and Shaw S.J. (1981). Fracture resistance of a toughened epoxy adhesive. J. Adhesion 12. 59-77. 

The first group, resins that are used primarily to toughen epoxy adhesive systems, is described in Chap. 8. This chapter focuses on the resinous modifiers that are used as alloy blends. Characteristics of commercially available epoxy alloy adhesives are presented in Table 7.1. Tensile shear and peel strengths that are typical of these hybrid adhesives are compared in Table 7.2. 

FIG URE 8.7 Morphology of a core-shell toughened epoxy adhesive. 

BR 127 (a) FM 73,94, 903 toughened epoxy adhesives 2.5-5.0 pm Air drying 30 min curing at 120°C for 30 min Corrosion-inhibiting protects metal oxide from hydrolysis... 

Epoxy-nitrile adhesives have also been developed for the purpose of toughening conventional epoxy adhesives. These early adhesives were made from homogeneous blends of nitrile elastomer and epoxy resin. They are not similar to the modern-day CTBN toughened epoxy adhesive where the nitrile phase consists of discrete particles. An advantage of the... 

Typical two-part CTBN and ATBN toughened epoxy adhesives... 

Typical formulations and properties of epoxy-terminated and nonterminated polysulfide-epoxy adhesive Formulation for a two-part CTBN modified epoxy adhesive system Starting formulation for a fast-setting, toughened epoxy adhesive... 

Fig. 13.9 Box beam bonding using rubber blend toughened epoxy adhesives.

The final paper, a contribution from The Institute of Chemistry in Beijing, covers the development and some of the properties of a CTBN-toughened epoxy adhesive cured with an imidazole. 

The use of elastomeric or flexibilizing modifiers occurred and grew with epoxy resins first. Various aspects of toughened epoxy adhesives have been covered in reviews by the present authors (2,3), where the elastomeric modifiers have essentially been carboxylic, liquid and solid butadiene/acrylonitrile polymers. There has not been a systematic review, however, of these and other reactive liquid polybutadiene/acrylonitriles in the burgeoning areas of acrylic, anaerobic and radiation-curable systems. Thus, this paper s intent. 

Low Curing Temperature CTBN-Toughened Epoxy Adhesive... 

LOW CURING TEMPERATURE CTBN-TOUGHENED EPOXY ADHESIVE... 

Secondly, the same rubber-toughened epoxy adhesive as was used for the T-peel tests [45] discussed in Section 3.3 has also been studied [51,57] using a LEFM test specimen, i.e. the standard tapered-double cantilever-beam specimen [58]. At the same rate of test and for the same locus of joint failure, a value of Gc of 2750 100 J/m was determined using the LEFM test, compared with a value 2900 400 J/m from the T-peel tests. Thus, here we have completely different test geometries giving the same value of Gc. So, again, a cross-check indicates the robustness of the above analytical approach for modelling the peel test. 

This practice continues to this day, for bonded patches over cracks, or other damage, in metallic structures. The preliminary design overlap for one-sided patches is 60 times the skin thickness, on each side of the crack. This ratio is not universal, of course it was developed for aluminum adherends bonded together by toughened epoxy adhesives. But the process by which the overlaps were established, which is described later, is universal and can be repeated for other materials. It should be noted, however, that nearly isotropic carbon-epoxy laminates have about the same Young s modulus as aluminum alloys and that the same factors can be applied directly. 

These three figures have been drawn to scale for aluminum adherends bonded together by typical 250 F-curing toughened epoxy adhesive. It is assumed that... 

Ratna, D., Banthia, A.K., 2000. Epoxidized soybean oil toughened epoxy adhesive. Journal of Adhesion Science and Technology 14, 15—25. 

The toughened epoxy adhesives previously discussed are single-phase systems in which the crosslinked epoxy resin is plasticized by a compatible elastomer. Because these are singlephase systems the glass transition temperature (Tg), which is roughly equivalent to the softening point, will fall somewhere between the... 

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