Toughened epoxy adhesives energy

Figure 1.14 The British Leyland Energy Conservation Vehicle Mark 3 (ECV3) is probably one of the most innovative cars ever built. A significant and unique feature is its aluminium alloy monocoque body which relies entirely on a toughened epoxy adhesive for its structural integrity.

Figure 7.16 The effect of joint width upon the maximum adhesive fracture energy, Clem, for a rubber-toughened epoxy adhesive bonding steel substrates using a TDCB geometry [106].

Figure 7.25 Effect of temperature on the fracture energy, Gic, for a toughened epoxy adhesive bonding steel substrates using a TDCB specimen [154]. x, Bulk behaviour of adhesive O, adhesive joint behaviour 3, predicted joint behaviour, from Equation 7.62. The plastic zone size, 2r yc, calculated from Equation 7.61 is given in parentheses. 

Sprenger and coworkers used a room-temperature curing rubber-toughened epoxy adhesive with a Tg of 70°C (Kinloch et al. 2003 Sprenger et al. 2003 2004). The addition of only 4 wt% of 20 nm diameter silica nanoparticles doubled the fracture energy, fi om 1,200 to 2,300 J/m. The addition of the nanoparticles also increased both the lap-shear and peel strength, as shown in Pig. 55.10. 

In contrast to polyethylene and EVAs, Epoxide adhesives are thermosets and are much stiffer and less ductile. The adhesion fracture energy (Gc, see Fracture mechanics), between both unmodified and rubber-toughened epoxies (see Toughened adhesives) and several metals has generally been found to be much higher when microfibrous surfaces were involved (Table 2) ... 

Structural adhesives are normally categorized or subdivided into subclasses based on the resin chemistries used in their formulation, which can be in the form of solids, liquids, pastes or films. The most important categories of structural adhesives are the thermoset cured adhesives based on phenolic, epoxy and polyurethane or acrylic resins and include phenolic adhesives. Epoxide adhesives, Toughened epoxide adhesives, Polyurethane adhesives, Acrylic adhesives and Toughened acrylic adhesives. Several of the categories can be further subdivided into both one- and two-component adhesives. The one-component structural adhesives, which can be liquids, pastes or solids (films), usually require the inclusion of added energy for activation or to effect cure in the... 

A further important parameter is the adhesive thickness within a joint. In certain adhesive systems such as rubber-toughened epoxies, there is an optimum thickness of adhesive within which energy-dissipation processes can take place. Above and below this critical size, the adhesive fracture energy will be lower. 

It was noted in the previous section that the carboxyl end groups on the CTBN elastomer affected the final performance of the material as a toughener since these groups would co-react with the epoxy resin and facilitate stress transfer from the brittle matrix to the phase-separated elastomer. Without this adhesion the particles could debond prematurely, which would lead to poor dissipation of the energy of the growing crack. It has also been noted that excessive adhesion between an epoxy resin and a thermoplastic could be deleterious to the performance (Williams et al, 1997). The process of toughening of a thermoset... 

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