Epoxy toughening, success

It is believed that this particular two phase morphology is the key to the toughening mechanism of the host matrix (4-9). The success of the epoxy toughening principle by RLP depends vitally on the interaction of the rubber with the epoxy matrix, and on the phase separation mechanism. Control of these two major variables is complex and only partially understood (8,9). Experimental results of the mechanical properties of RLP modified epoxy resins are frequently conflicting, and depend on the preparation techniques of the polymer composites (9,10,11). 

The butadiene-acrylonitrile tougheners that have been especially successful in both DGEBA and DGEBF types of epoxy resins are... 

The use of rubbers (particularly epoxy-terminated butadiene nitrile, ETBN, rubber or carboxy-termi-nated butadiene acrylonitrile, CTBN, rubber) to toughen thermoset polymers is perhaps the most widely explored method and has been applied with some measure of success in epoxy resins. Phase separation of the second rubbery phase occurs during cure and its incorporation in the epoxy matrix can significantly enhance the fracture toughness of the thermoset. Although the rubber has a low shear modulus, its bulk modulus is comparable to the value measured for the epoxy, ensuring that the rubber inclusions introduced... 

The rubber-toughening process has been one of the most successful methods for modifying polymer toughness. The incorporation of small amounts of rubber into polymer matrices has resulted in significantly improved fracture resistance (1). Particularly interesting improvements in the toughness of epoxies have been accomplished by the addition of carboxyl-terminated butadiene/acrylonitrile (CTBN) elastomers (2,3). 

It is possible to bond PVC and GRP pipes successfully with toughened acrylics, and GRP pipes can also be bonded with cold-curing, two-part epoxies. These adhesives generally cope well with most fluids likely to be carried in such pipes. Solvent welding may also be used to bond PVC provided that the gaps are small. 

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. 

One drawback with addition-type polymers is that the cured adhesive tends to be brittle. Attempts to toughen addition polyimides have met with some success, though improvements in this area are needed. The development of systems exhibiting the same level of toughness as today s state-of-the-art epoxy adhesives is a goal for the future. 

Another approach for toughening UP and VE resins which has had success with epoxy resins is the use of liquid rubber (or elastomer) additives. The chief benefit of emplojdng liquid rubbers (LR) versus a flexibilized resin is that decreases in hardness, stiffness and heat-deflection properties can be minimized. During cure, the liquid rubber phase separates from the resin and is concentrated in a particulate phase. Very little of the rubber remains in solution with the cured resin so the resin s heat deflection temperature is for the most part unaffected. The toughness of the two-phase, or composite, material will be a function of the microstructure, which in turn will depend on processing and cure conditions. The subject of epoxy resin toughening is covered in separate articles in this book. 

The major factor that has limited the penetration of epoxies into plastics bonding applications has been their brittleness. Rubber-toughened systems have improved this problem to a certain extent, and some successful applications have been reported in bonding... 

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