Rubber-toughened adhesives, failure

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

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

Rubber toughening is the most often used method of improving the impact resistance of polymers (Bucknall 1977). The impact modified materials are usually the blends of a rigid matrix polymer with an elastomer. The composition of the constituents, their miscibility, and the morphology influence the deformation and failure mechanism in the blend. Particle size of the elastomer, its dispersion, and its adhesion with matrix are also the important factors determining the toughness. 

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. 

Matrix-rubber particle adhesion is an important parameter for rubber toughening. For effective rubber toughening, rubber particles must be well bonded to the thermoset matrix. The poor intrinsic adhesion across the particle-matrix interface causes premature debonding of particles, leading to catastrophic failure of the materials. Nearly all the studies [9, 193, 2-10] have been concerned with reactive rubbers as toughening agents, and showed that dispersed particles have interfacial chemical bonds as a consequence of chemical reactivity. 

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. 

Typical failure surfaces observed in as-produced specimens are shown in Fig. 24. Specimen a was bonded using adhesive A, which contains no rubber toughener and is the most brittle adhesive in the series. The failure surfaces... 

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