Polyester resins arethermosetting; this means that the plastic softens when initially heated, but sets permanently rigid once it has cooled (as opposed to "thermoplastics", which re-soften with heat). Polyester resin is purchased in liquid form for the production of glass-reinforced plastic catalysed, typically with methyl ethyl ketone peroxide (MEKP) (also known as butanone peroxide) to initiate the polymerization reaction.