Dispersion of an Organic Second Phase in the Thermoset Precursors

3 DISPERSION OF AN ORGANIC SECOND PHASE IN THE THERMOSET PRECURSORS 

Another procedure for the preparation of modified thermosets consists of introducing preformed particles in the initial formulation. This technique is also well documented for modified thermoplastics (Paul and Bucknall, 2000). In Chapter 7 different macromolecular architectures such block copolymers, crosslinked microparticles, hyperbranched polymers, and den-drimers, were presented (Fig. 7.11). All these compact molecules can be used as thermoset modifiers. Thermoplastic powders and core-shell polymers are the more accessible preformed molecules. Some examples are given below. 

Semicrystalline polyamide fine powders have been used as toughening agents for epoxy networks. The powders can be obtained by grinding granules, or directly by anionic polymerization of lactams, 6 or 12, in an organic solvent from which the formed semicrystalline polymer precipitates. Microporous powders with an average particle size in the range of 10 pm and a narrow particle-size distribution, are obtained. 

At their melting temperature these powders can be dissolved in the epoxy monomers and they are able to react and participate in the cross-linking reaction through the amide groups (Lennon et al., 2000). For this reason, the cure cycle must be selected in order to keep the polyamide particles below their melting point (in the range 170°C or 220°C, depending on the type of polyamide used), and thus keep their initial shape and size. But in some cases a partial dissolution of the powder surface can improve the particle-polymer network interactions. 

Amorphous polyimide powders prepared by dissolution/precipitation processes, can be used to toughen thermosetting polymers. Polyethylene powders are frequently used in low-shrink unsaturated polyester formulations. 

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Dispersion of an Organic Second Phase in the Thermoset Precursors... 

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