Additives are incorporated into polymers to alter and improve their basic mechanical, physical or chemical properties. Similar to the relationship between a raw and a cooked egg, once heated, a thermoset polymer can't be softened again and once cooked, the egg cannot revert back to its original form. Thermosets are valued for their durability and strength and are used primarily in automobiles and construction, although applications such as adhesives, inks, and coatings are also significant. A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding or pressing.
Ice cubes are a common household item which exemplify the thermoplastic principle.
Ice will melt when heated but readily solidifies when cooled. Like a polymer, this process may be repeated numerous times. Thermoplastics offer versatility and a wide range of applications. They make up the greatest share of plastics used in food packaging because they can be rapidly and economically formed into any shape needed to fulfill the packaging function. Examples include milk jugs and soda bottles.