Sulfones are amorphous engineering thermoplastics noted for high heat-deflection temperatures and outstanding dimensional stability. These strong, rigid polymers are the only thermoplastics that remain transparent at service temperatures as high as 400°F.
Three commercially important sulfonebased resins are: polysulfone (PSU), including Udel from Amoco Performance Products Inc., and Ultrason S from BASF Corporation Plastic Materials; polyarvlsulfone (PAS), including Radel from Ainoco Performance Products Inc.; and polyethersulfone (PES), including Ultrason E from BASF Corporation Plastic Materials. These materials are claimed to offer the highest performance profiles of any thermoplastics processible on conventional screw-injection and extrusion machinery. Processing temperatures, however, are higher than those of other thermoplastics; the sulfones are processed on equipment that can generate and monitor stock temperatures in the range of 650 to 720° F.
Natural color of the sulfone resins is transparent light amber. Continuous use in air or in steam at rated temperature does not cloud, craze, or otherwise destroy their transparency. These resins are also available in opaque colors and in mineral-filled and glass -reinforced compounds, which provide improved strength, stiffness, and thermal stability.
Properties: Heat resistance is the outstanding performance characteristic of the sulfones. Service temperature is limited by heat-deflection temperature, which ranges from 345 to 400°F. A high percentage of physical, mechanical, and electrical properties is maintained at elevated temperatures, within limits defined by the heat-deflection temperatures. The strength and stiffness of PSU and PES are virtually unaffected up to their glass-transition temperature. For example, the flexural modulus of molded parts remains above 300,000 psi at service temperatures as high as 320T. Even after prolonged exposure to such temperatures, the resins do not discolor or degrade. Thermal stability and oxidation resistance are excellent at service temperatures well.
Heat aging of the polymers increases tensile strength, beat-deflection temperature, and modulus of elasticity appreciably. However, prolonged heat aging (about a year) decreases toughness, tensile strength, and elongation. Unnotched specimens of the sulfones are extremely tough and do not break in standard impact tests. The continuous service temperature limit (CSTL) for PSU is 320°F, and 355 °F for PES. With respect to flammability, PES is rated V-0 per UL 94, and PSU is rated at V-2.
Electrical insulating properties are generally in the mid-range among those of other thermoplastics, and they change little after heat aging at the recommended service temperatures. Dissipation factor and dielectric constant - and thus, loss factor - are not affected significantly by increased temperature or frequency.
Creep of the sulfones, compared with that of other thermoplastics is exceptionally low at elevated temperatures and under continuous load. For example, creep at 210°F is less than that of acetal or heat-resistant ABS at room temperature. This excellent dimensional stability qualifies the sulfone resins for precision-molded parts.
The hydrolytic stability of these resins makes them resistant to water absorption in aqueous acidic and alkaline environments. The combination of hydrolytic stability and heat resistance result in exceptional resistance to boiling water and steam, even under autoclave pressures and cyclic exposure of hot-to-cold and wet-to-dry. PES resins have excellent resistance to hot lubricants, engine fuels. and radiator fluids, and they are resistant to gasoline. The aromatic resins are also resistant to aqueous inorganic acids, organic acids, alkalis, aliphatic hydrocarbons, alcohols, and most cleaners and sterilizing agents.
The sulfones also share a common draw-back: They absorb ultraviolet rays, giving them poor weather resistance. Thus. they are not recommended for outdoor service unless they are painted, plated, or UV stabilized.