Rubber-toughened epoxy adhesives

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

Adhesives. Two single part, rubber toughened hot cured structural epoxy adhesives were used throughout the current research. Both adhesives were provided by Dow Automotive,... 

Table I. Rubber-Blend-Toughened, Structural Epoxy Adhesives...

Fig. 13.8 The T-peel resistance as a function of the crosshead speed for nitrile rubber/phenolic polyurethane liquid rubber blend toughened epoxy (upper curve) and conventional nitrile rubber-toughened epoxy structural adhesives (lower curve).

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

Fig. 2.3. Schematic representation of failure mechanisms in rubber toughened adhesives, (a) Shear yielding, b) Crazing (not necessarily applicable to epoxies).

In a contribution from B. F. Goodrich, Drake and Siebert extensively review the journal and patent literature since 1975 on reactive butadiene/acrylonitrile liquid and solid elastomers used in formulating epoxy structural adhesives. Areas reviewed include the preparation of elastomer-modified epoxy resins, the characterization of rubber-toughened epoxy resins, fracture mechanics and adhesive formulation and testing. 

An intense interest, currently without abatement, in the characterization of rubber-toughened thermosets in general, and rubber-toughened epoxy resins in particular, has surfaced over the last several years. Riew (18) summarized work leading to the optimization of toughness in an epoxy/CTBN/piperidine model. Concurrently, Kaelble (19) reported on such systems from a block copolymer orientation and examined properties related to adhesive... 

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) ... 

Figure 2 shows fracture surfaces for bonds between zinc and rubber-toughened epoxy resin (see Toughened adhesives). In Fig. 2(a), there is a region of cohesive failure within the resin the sites of bubbles, which may have initiated the fracture, can be seen. In Fig. 2(b), a piece of resin is seen adhering to what appears to be the bare zinc substrate. 

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. 

Reactive liquid polymers (RLPs) The Uquid rubbers most often used in epoxy adhesives are those based on acrylonitrile-butadiene copolymers or on long-chain polyethers. To act as effective tougheners, however, they cannot be directly added to the formulation but have to be prereacted with some of the epoxy resin, or sometimes with the amino hardeners in two component paste adhesives (adduct fonnatiou). To achieve this, only RLPs with reactive eud groups are used by the formulator. 

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