Thermoplastics are plastics that become soft and malleable when heated, and then become hard and solid again when cooled. Examples of thermoplastics include acetal, acrylic, cellulose acetate, nylon, polyethylene, polystyrene, vinyl, and nylon. When thermoplastic materials are heated, the molecular chains are able to move past one another, allowing the mass to flow into new shapes. Cooling prevents further flow. Thermoplastic elastomers are flexible plastics that can be stretched up to twice their length at room temperature and then return to their original length when released.
The state of a thermoplastic depends on the temperature and the time allowed to measure its physical properties. At low enough temperatures, amorphous, or noncrystalline, thermoplastics are stiff and glassy. This is the glassy state, sometimes referred to as the vitreous state. On warming up, thermoplastics soften in a characteristic temperature range known as the glass transition temperature region. In the case of amorphous thermoplastics, the glass transition temperature is the single-most important factor determining the physical properties of the plastic.
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