Toughened adhesives rubbery phase

Many formulations are claimed to be capable of absorbing normal residues of rolling oil on steel surfaces, thus reducing the surface preparation requirements as compared with, say, epoxies. As with epoxies, acrylics can be toughened by the introduction of a rubbery phase within the matrix of the adhesive and they are particularly suitable for polymeric adherends because the surface is eompliant. 

Many of the entries in this book dealing with the science of a(fliesion and Theories of adhesion are relevant to composition materials in general. There are a number of articles that discuss specific aspects of composite materials that come within the scope of this book. Toughened adhesives, particnlarly Epoxide adhesives and Toughened acryiics, consist of polymers with a rubbery phase dispersed as small spheres within a more glassy matrix. Appropriate adhesion between the phases is crucial for effective toughening. 

Some important Structural adhesives are two-phase materials composed of dispersed rubbery particles in a glassy matrix (see Toughened adhesives). Optimum performance depends upon achieving the correct phase structure, which is achieved only by careful attention to the procedure for mixing, applying and curing the adhesive. 

Figure 1.2 In an overloaded toughened adhesive, crack propagation is stopped by the dispersed rubbery phase.

Toughened adhesives contain a dispersed, physically separate, though chemically attached, resilient rubbery phase. The toughened concept (see Section 1.2) - in the modern sense - has so far only been successfully applied to two adhesive families - anaerobics and epoxies described in Section 5.1.2 and 5.1.5 respectively. It has also led to the creation of an entirely new species of adhesive - the toughened acrylic - which is discussed in Section 5.1.12.2 below. 

Figure 5.2 Electron micrograph of the typical microstructure of a rubber-toughened epoxy adhesive showing the dispersed rubbery phase of particles in the matrix of a thermosetting epoxy polymer. (TTie wavy lines are artefacts due to the cutting of a very thin section of material.)...

This tendency for an overloaded joint to fail catastrophically when distorted has been substantially suppressed by the introduction of the toughened adhesives. These adhesives resist crack propagation by incorporating a dispersed rubbery phase within the harder load-bearing portion of the adhesive (Fig. 7.3). When the joint is overloaded... 

Fig. 7.3. Electron micrograph revealing the structure of a toughened epoxy-based adhesive the regular, evenly distributed rubbery phase is clearly seen.

Fig. 7.4. Crack stopping mechanism of a toughened adhesive, (a) Crack initiated due to overload, (b) Crack propagates and splits the glassy, load-bearing phase of the adhesive, (c) Crack stopped because the energy focussed at its tip is dissipated in the rubbery phase which distorts during redistribution and delocalisation of the destructive forces.

Without the addition of a fiexibilizing agent, epoxy resin adhesives have relatively poor characteristics due to the brittle nature of the epoxy matrix.Liquid functionally terminated nitrile rubbers have proved to be an excellent fiexibilizing agent for the epoxy resins. It has been theorized that toughening of the epoxy resin occurs by the formation of a rubbery second phase in the epoxy matrix. With nitrile rubbers over 20% acrylonitrile, order of magnitude increases in fracture toughness of the epoxy matrix have been reported. 

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