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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 A scanning electron micrograph (SEM) of the surface of a sheet of biodegradable plastic. The spherical object that dominates the image is one of many granules of starch embedded in the surface of the plastic. When the plastic is buried in soil the starch grains take up water and expand. This breaks the material into small fragments, increasing the contact area with the soil bacteria that digest plastic. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. 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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Science EncyclopediaScience & Philosophy: Planck mass to PositPlastics - History, Chemistry, Polymerization, Manufacture And Processing, Thermoplastics, Crystalline And Noncrystalline Thermoplastics, Thermosets - Molecular weight