Final answer:
Thermoforming is the process of heating plastic above its glass transition temperature or melting temperature to make it malleable for molding or shaping. The glass transition temperature is the softening point of thermoplastics, and the melting temperature is where the crystalline parts of a polymer liquefy.
Step-by-step explanation:
Thermoforming is done by heating the plastic above its glass transition temperature (Tg) or melting temperature (Tm). The glass transition temperature is a specific point at which thermoplastics soften and become more flexible and deformable. For example, hard plastics like polystyrene and poly(methyl methacrylate) become moldable above their Tg, which is around 100°C (212°F), transitioning from a brittle, glassy state to a more malleable form. Additionally, the melting temperature (Tm) is observed in polymers with crystalline regions; heating above this temperature leads to the material becoming a viscous liquid which can then be shaped or molded.
Polymers like polyisoprene and polyisobutylene are used above their Tg in the rubbery state to retain flexibility. For instance, a plastic liquid crystal thermometer utilizes the color change properties of heat-sensitive liquid crystal materials to indicate temperature changes, showcasing the practical applications of understanding these thermal properties. Conversely, thermosets, which make up about 20% of commercially-produced polymers, cannot be reshaped once cured; they harden permanently. Understanding the differences between these polymer types is critical for their application in manufacturing and other industries.