Chemical curing can cause reproducibility problems. Materials always glassy. Glass transition must be below curing temperature, max. Brittle because cross-linked network with short chain lengths between crosslinks. Desire for higher Tg results in stiffer chains, more brittle polymers.
Unsaturated polyester is cheap, epoxies can be expensive.
Thermoplastics Very tough because long chain lengths between entanglements lead to crazing on fracture. Very viscous because of high molecular weight. Can be cheap commodity plastic or very expensive engineering thermoplastic. Crystalline polymers more resistant to solvent stress cracking and give a higher operating temperature relative to processing temperature. Glasses High flow temperature and very brittle.