Effect of reactive polyurethane on toughness of unsaturated polyester resin

Unsaturated polyester (UP) resin is one of the major thermosetting resins and is very useful as a matrix resin of composite material for its processibility. UP resin, however, has several shortcomings: it is weak in alkalis, volume shrinkage occurs during the crosslinking reaction of the oligomeric UP resin with a styrene monomer, and it is also brittle. The mechanical properties of UP resin can be enhanced by blending it with various materials. In this study, polyurethane (PU) was used as a modifier to improve the toughness of the UP resin. The effect of the polyol molecular weight as a PU soft segment and the PU contents on the toughness of PU‐modified UP resins were studied. A UP/PU polymer network may occur through a reaction between an isocyanate group in the methyl diisocyanate (MDI) and a hydroxyl one in the UP molecules. The maximum toughness was observed at approximately 2 wt % of the PU content. These results can be rationalized by the incorporation of a rubbery PU segment into a brittle UP resin. © 2002 John Wiley & Sons, Inc. J Appl Polym Sci 84: 735–740, 2002; DOI 10.1002/app.10169