Nov. 15, 2002
Polycarbonates are amorphous engineering thermoplastics that offer exceptional toughness over a wide temperature range.

Polycarbonates are amorphous engineering thermoplastics that offer exceptional toughness over a wide temperature range. The resins are produced in the U.S. by Dow Chemical Co. (Calibre), General Electric Co. (Lexan), and Mobay Chemical Corp. (Makrolon). The natural resins are waterclear and transparent.

Polycarbonate resins are available in general-purpose molding and extrusion grades and in special grades that provide specific properties or processing characteristics. Ilese include flame-retardant formulations as well as grades that meet Food and Drug Administration regulations for parts used in food-contact and medical applications. Other special grades are used for blow-molding, weather and UV-resistance, glass-reinforcement, EMI, RFI, ESD. shielding, and structural-foam application& Polycarbonate is also available in extruded sheet and film.

Properties: Polycarbonates are characterized by a combination of toughness, transparency, heat and flame resistance, and dimensional stability. In unnotched Izod impact tests and falling-dart tests, the unreinforced material resists breakage at very low temperatures (-65°F) and in thick sections. In a 1/8-in. section with a standard 10. mil notch radius, notched Izod impact strength is 12 to 16 ft-lb/in. of notch. This value drops to 2 to 3 ft-lb/in. of notch in sections over 0.160 in. Grades are available, however, that do not have this transition behavior and maintain high impact strength to 1/4,in. thick. A properly designed polycarbonate part, even in a thick section, has more impact resistance at -65 ° F than most plastics have at room temperature.

Humidity changes have little effect on diLuensions or properties of molded parts. Even boiling water exposure does not cbange dimensions more than 0.001 in./in. after parts are returned to room temperature. Creep resistance is excellent throughout a broad temperature range and is improved by a factor of two to three in glass-reinforced compounds.

The insulating and other electrical characteristics of polycarbonate are excellent and almost unchanged by temperature and humidity conditions. One exception is arc resistance, which is lower than that of many other plastics.

Polycarbonates are generally unaffected by greases, oils, and acids. Nevertheless, compatibility with specific substances in a service environment should be checked with the resin supplier. Water at room temperature has no effect, but continuous exposure in hot (65°C) water causes gradual embrittlement. The resins are soluble in chlorinated hydrocarbons and are attacked by most aromatic solvents, esters, and ketones, which cause crazing and cracking in stressed parts. Grades with improved chemical resistance are available, and special coating systems can be applied to provide additional chemical protection.

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