A polymer material that becomes pliable with heat, and with sufficient temperature, a liquid. When cooled, thermoplastics return to solid. There are more than 40 types, including acrylic, polypropylene, polycarbonate and polyethylene. Dating back to the mid-1800s, celluloid was the first thermoplastic, which remained popular until the mid-1900s. See
thermoset and
3D printing.