Improving glass transition temperature of unsaturated polyester thermosets: Conventional unsaturated polyester resins

Highly crosslinkable unsaturated polyester resins (UPR) have attracted many interests in the application as reinforced matrix materials. Here, we present a systematical study of the influence of different curing conditions and styrene concentrations on resin viscosity and dynamic‐mechanical properties of the thermoset. The pure maleic Palapreg® P18‐03 was selected as model UPR because of its broad industrial use. By applying newly developed thermal curing profiles (without thermal initiators) and by raising the styrene content, the Tg of the network could be increased up to 206/215°C (1/10 Hz). For the first time, a fast curable UPR based on propylene glycol and neopentyl glycol with a Tg of up to 215°C is described. A partial substitution of problematic styrene with methylmethacrylate, tert‐butylacrylate, and maleic anhydride (MA) was studied as well. MA leads to significantly improved resin reactivity. A resin containing 42 wt% styrene and 8 wt% MA yields thermosets with remarkably improved mechanical properties and with a narrower glass transition range compared to the original P18‐03. This study presents simple methods to increase the glass transition temperature (Tg) and heat distortion temperature of thermosets based on so‐called "pure maleic" unsaturated polyester resins up to 215 °C: A slower, multi‐stage thermal curing (A), an increase in the styrene content (B) and small amounts of maleic anhydride (C). Obviously, the potential of “pure maleics” is not fully exploited under common industrial curing conditions, which are optimized for fast cure supported by thermal initiator.