Modifiers for unsaturated polyester and vinyl ester resins

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. 

Microstructural features that are required to improve fracture toughness in thermosets have been identified however, exact requirements may be material dependent. In general, improvements in fracture toughness are dependent on the volume fraction of the rubbery particulate phase, good dispersion of the particles within the matrix, proper particle 

Microstructural features such as volume fraction of rubbery particles and the particle size distribution will depend on the initial resin/modifier compatibility and the cure scheme. From the processor s standpoint, the blend needs to be stable and produce the desired morphology upon cure consistently. Ideally, one would like the liquid rubber modifier and the unreacted resin to form a miscible blend at room temperature. Cure conditions will then control rubbery particle formation. Factors that affect phase separation include cure temperature and time to gelation, which is a function of temperature and catalyst system. 

In the case of UP and VE resins complete miscibility with liquid rubbers or elastomers is unusual. Initial compatibility of two polymer components, i.e., the resin and the liquid rubber, will depend on a number of factors such as the molecular weight of each component, the molecular weight distribution of each component, the chemistry of each component. 

Other elastomers which have been used to modify polyesters include EPDM (ethylene-propylene-diene-methylene), poly(epichlorohydrin) with hydroxyl end groups (Crosbie and Phillips, 1985b Ullett and Chartoff, 1995), organic siloxane elastomers, polyacrylates (Ullett and Chartoff, 1995), and polyurethane elastomers (IQm and Chan-Park, 1994). 

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Impact modifiers (5) Modifiers for unsaturated polyester and vinyl ester resins... 

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