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POLYARYLATES Introduction Polyarylate resins are heat-resistant thermoplastics with excellent toughness, uv stability, flexural recovery, dimensional stability, and electrical properties. Polyarylate polymer properties can be tailored by compositional variation, alloying with other high performance thermoplastics, and reinforcement. This flexibility renders polyarylate one of the more versatile high temperature polymers for current and emerging markets served by engineering thermoplastics.

Chemistry Polyarylate polymers are aromatic polyesters derived from aromatic dicarboxylic acids and diphenols. In contrast to liquid crystalline aromatic polyester (derived from dicarboxylic hydroxy acids), polyarylates exhibit amorphous character on molding; T g is ca 198◦ C. Polyarylates present a competitive cost/performance profile in the context of amorphous engineering thermoplastics, delivering an excellent balance of mechanical properties, particularly practical heat resistance which substantially exceeds that of polycarbonates (qv). The preferred aromatic diphenol based on current commercial offerings is bisphenol A; the aromatic dicarboxylic acid is proportioned between isophthalic acid and terephthalic acid (eq. (1)). The election of a particular isophthalic acid/terephthalic acid ratio is influenced by both the commercial manufacturing process and market orientation (1). The ratio of x, y in the product of equation (1) is close to one for U-Polymer, produced by Unitika, the only current commercial polyarylate. Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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O

O

O

C

C

Vol. 7 CH3 ⫹ HO

O

C

OH

CH3 CH3 O

C CH3

O

O

O

C

C

CH3 O x

C yCH3

O

O

O

C

C

(1)

Diphenols are scarcely reactive towards aromatic diacids (2) and only low molecular weight polymers can be achieved through their direct reaction. To increase the reactivity and promote the polycondensation, at least one of the monomers has to be properly activated. Thus, two processes have been conceived for polyarylate synthesis: interfacial polycondensation (preferred over the solution process) between diacyl chlorides and alkali metal diphenate, and the high temperature metal-catalyzed melt polycondensation between either diphenol acetates and aromatic diacids, or diphenols and diacid diphenyl esters.

Properties Polyarylates deliver excellent thermal performance with heat-deflection temperatures ranging from 154 to 174◦ C at 1.82 MPa (264 psi) and a UL thermal index of 130◦ C. In addition, a very low coefficient of thermal expansion [5.0–6.2 × 10 − 6 mm/(mm·◦ C) (60–130◦ C)] allows for superior performance in polyarylate/metal assemblies. The inherent uv resistance of polyarylate polymers results in excellent retention of mechanical properties under prolonged weathering conditions. As a coating or laminate, polyarylate provides a uv barrier for other performance plastics. Mechanical properties compare favorably with other high performance amorphous and semicrystalline thermoplastics, where strength and toughness requirements are rigorous (Table 1). The flexural recovery (resilience) of polyarylate is excellent over a wide temperature range, which suggests applications where snapfit action is required. Polyarylate resins exhibit good combustion resistance in terms of low flame spread, a high oxygen index, and low smoke generation. Unmodified polyarylates exhibit sufficient fire-retardant characteristics for many applications, whereas more stringent combustion requirements can be achieved with the proper selection of additives. The electrical properties of polyarylate polymers are reasonably constant over a broad temperature range and present a competitive profile for high performance electrical applications. The amorphous character of polyarylates renders this class of polymer susceptible to stress-cracking tendencies in solvents such as ketones, aromatic hydrocarbons, esters, and chlorinated hydrocarbons. In applications where chemical resistance is critical, alloying polyarylate with other high performance materials can often meet the requirement. Polyarylates are transparent and range in color from light yellow to amber. The transparent character can be maintained in alloying with other reactive

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Table 1. Properties of Polyarylates Property Density, g/mL Flammability Oxygen index (LOI), % Refractive index Water absorption (24 h, 23◦ C) Dielectric constant (1 MHz) Dielectric strength, kV/mm Dissipation factor (1 kHz) Volume resistivity (23◦ C, dry), ·cm Heat-deflection temperature at 1.8 MPa, ◦ C Thermal conductivity (23◦ C), W/(m·K) Elastic modulus, GPaa Tensile strength, MPab Flexural modulus, GPaa Flexural strength, MPab Compressive strength, MPab Elongation at break, % Notched Izod (3.2 mm), J/mc Hardness (Rockwell)

Value 1.21 V0 36 1.64 0.26 1.64 0.4 0.004 2×1014 174 0.21 2.1 70 2.1 75 84 60 205 R125

a To

convert GPa to psi, multiply by 145,000. convert MPa to psi, multiply by 145. c To convert J/m to lbf·ft/in, divide by 53.38. b To

polyesters while mechanical properties can be tailored to end-use applications. Additional property modification is achieved by targeting mechanically compatible blends.

Processing Polyarylates can be processed by most conventional melt processes, including injection molding, extrusion, blow molding, and thermoforming. Processing characteristics are similar to that of polycarbonate with the exception that higher melt temperatures are required. Injection molding is the processing technique that is most widely employed, and resins are available that fill complex mold designs. Recommended melt temperatures range from 260 to 382◦ C depending on the composition, and mold temperatures can range from 65 to 149◦ C. Additionally, extrusion and blow-molding grades offer good melt strength combined with high throughput rates that are essential for blow molding, profile extrusions, and thermoforming operations. Successful processing of polyarylates requires moisture levels below 0.02%, which is similar to requirements for other polyesters (ie, PET). All polyarylates are unstable in the presence of moisture at the processing temperatures described earlier; unsatisfactory part properties result from processing poorly dried resin.

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Applications Polyarylates are presently used in a wide variety of end-use applications and also show potential for future applications. Presently, polyarylates find use in the automotive, safety, electrical and electronics, and exterior construction and lighting areas. Automotive. Properties of polyarylates, such as high heat resistance, dimensional stability, and capability for high gloss surface metallization of parts, allow these resins to be used in headlight housings and brake light reflectors. For exterior trim parts such as mirror housings, window trim, and door handles, the good uv resistance and weatherability of polyarylate eliminate the need for painting. In brackets and fasteners, the resin’s creep resistance and dimensional stability are advantageous. Polyarylate alloys are being evaluated for use in body panel applications. Safety Equipment. Polyarylates are used in fire helmets and fire shields because of their inherent resistance to hot and cold temperatures, impact and flammability resistance, colorability, and transparency. Polyarylates can also be used in other safety-related applications such as mining light covers and lenses for traffic signals. Electrical and Electronic. Polyarylates provide the excellent temperature resistance and flame retardancy needed for electrical connectors, relay housings, coil bobbins, and switch and fuse covers. Exterior Applications. Because of the excellent uv resistance of polyarylates, end use is found in such outdoor applications as glazing, skylights, and transparent panels. Polyarylates can be used alone or in multilayer constructions with other polymers. Properties such as transparency, impact and heat resistance, and low haze levels after long-term exposure have suited this resin for use in globes and other lighting components.

Producers Currently, Unitika Corp. Ltd. is the only producer of this material in the world. The trade name of Unitika’s polyarylate is U-Polymer. Polyarylate is marketed in North America by Toyota Tsusho America, Inc. World production is about 1000 t/year.

BIBLIOGRAPHY 1. A. Bettelheim, Plast. Technol. 31, 20, 22 (1985). 2. B. D. Dean, M. Matzner, and J. M. Tibbitt, in G. Allen, C. Bevington, G. C. Eastmond, A. Ledwith, S. Russo, and P. Sigwalt, eds., Comprehensive Polymer Science, Vol. 5, Pergamon Press, Oxford, 1989, Chapt. “18”.

YUTAKA SHIRAHAMA Unitika America Corporation FUMIO OHAMA Unitika Corporation Ltd.

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POLYCARBONATES

POLYBENZIMIDAZOLE POLYMERS (PBI).

See RIGID ROD

POLYMERS.

POLYBENZOTHIAZOLES (PBT). POLYBENZOXAZOLES (PBO). POLYBUTADIENE. POLYBUTENES.

See RIGID ROD POLYMERS. See RIGID ROD POLYMERS.

See BUTADIENE POLYMERS. See BUTENE POLYMERS.

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