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A thermoplastic is a material that is Plastic or deformable, Melt s to a liquid when heated and freezes to a Brittle , Glass y state when cooled sufficiently. Most thermoplastics are high Molecular Weight Polymer s whose Chain s associate through weak Van Der Waals Force s ( Polyethylene ); stronger Dipole-dipole interactions and Hydrogen Bond ing ( Nylon ); or even stacking of Aromatic rings ( Polystyrene ). Thermoplastic polymers differ from Thermosetting polymers ( Bakelite ; Vulcanized Rubber ) as they can, unlike thermosetting polymers, be remelted and remoulded. Many thermoplastic materials are Addition Polymer s; e.g., Vinyl chain-growth polymers such as polyethylene and Polypropylene . TEMPERATURE DEPENDENCE Thermoplastics are elastic and flexible above a Glass Transition Temperature ''T''g, specific for each one — the midpoint of a Temperature range in contrast to the sharp Freezing Point of a pure Crystal line substance like Water . Below a second, higher melting temperature, ''T''m, also the midpoint of a range, most thermoplastics have crystalline regions alternating with Amorphous regions in which the chains approximate Random Coil s. The amorphous regions contribute Elasticity and the crystalline regions contribute strength and rigidity, as is also the case for non-thermoplastic Fibrous Protein s such as Silk . (Elasticity does not mean they are particularly stretchy; e.g., nylon Rope and Fishing Line .) Above ''T''m all crystalline structure disappears and the chains become randomly inter dispersed. As the temperature increases above ''T''m, Viscosity gradually decreases without any distinct Phase change. Thermoplastics can go through melting/freezing cycles repeatedly and the fact that they can be reshaped upon reheating gives them their name. This quality makes thermoplastics recyclable. The processes required for recycling vary with the thermoplast. The plastics used for pop bottles are a common example of thermoplastics that can be and are widely recycled. Animal Horn , made of the Protein α-keratin , softens on heating, is somewhat reshapable, and may be regarded as a natural, quasi-thermoplastic material. Some thermoplastics normally do not crystallize: they are termed "amorphous" plastics and are useful at temperatures below the ''T''g. They are frequently used in applications where clarity is important. Some typical examples of amorphous thermoplastics are PMMA, PS and PC. Generally, amorphous thermoplastics are less chemically resistant and can be subject to Stress Cracking . thermoplastics will crystallize to a certain extent and are called "semi-crystalline" for this reason. Typical semi-crystalline thermoplastics are PE, PP, PBT and PET. The speed and extent to which crystallization can occur depends in part on the flexibility of the polymer chain. Semi-crystalline thermoplastics are more resistant to solvents and other chemicals. If the crystallites are larger than the wavelength of light, the thermoplastic is hazy or opaque. Semi-crystalline thermoplastics become less brittle above ''T''g. If a plastic with otherwise desirable properties has too high a ''T''g, it can often be lowered by adding a low-molecular-weight Plasticizer to the melt before forming ( Plastics Extrusion ; Molding ) and cooling. A similar result can sometimes be achieved by adding non-reactive Side Chain s to the Monomer s before Polymerization . Both methods make the polymer chains stand off a bit from one another. Before the introduction of plasticizers, Plastic Automobile parts often cracked in cold Winter Weather . Another method of lowering ''T''g (or raising ''T''m) is to incorporate the original plastic into a Copolymer , as with Graft Copolymer s of polystyrene, or into a Composite Material . Lowering ''T''g is not the only way to reduce brittleness. Drawing (and similar processes that stretch or orient the molecules) or increasing the length of the polymer chains also decrease brittleness. Although modestly vulcanized natural and synthetic rubbers are stretchy, they are Elastomer ic thermosets, not thermoplastics. Each has its own ''T''g, and will crack and shatter when cold enough so that the Crosslinked polymer chains can no longer move relative to one another. But they have no ''T''m and will decompose at high temperatures rather than melt. Recently, Thermoplastic Elastomer s have become available. LIST OF THERMOPLASTICS
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