Some thermoplastic polycondensates developed in the last thirty years contain sulfur atoms in the form of sulfide (S) or sulfone (SO2) in the building block of their main chain. The sulfur atom is linked to the aromatics in the main chain in the form of diphenyl sulfone or sulfide groups.
Diphenyl sulfide
1. Besides sulfur, all polysulfones contain a second characteristic link, the oxygen atom. They can thus be termed polyether sulfones.
2. The terms polysulfone, polyarylsulfone or polyphenylsulfone used for some polymers of this product family is not justified by the chemical structure as all the products contain SO2 ether and phenyl groups.
3. Union Carbide Co. introduced ‘Udel’ polysulfone in 1965.
Introduction-1
- Commercial Polysulfones are, like polycarboxylics and linear polyesters, monovalent polyaromatics.
- Polysulfone excels in many fluid-handling applications.
Introduction -2
- Transparent, opaque and glass-fiber reinforced grades of Polysulfone resin are available for injection molding into complex parts or extrusion into forms such as rod, film, sheet, profile and tubing.
- Shapes can be machined for prototype evaluations; film and sheet can be thermoformed on conventional equipment. Polysulfone resin can also be blow molded.
Manufacture
- Polysulfone is manufactured from bisphenol A and 4, 4-dichlorosulfonyl sulfone by a multistep condensation reaction.
General Description
- high strength, stiffness and hardness between - 100 and + 150 °C, short term to 180°C.
- high thermal stability & heat distortion temperature,
- crystal clear (slightly yellowish) transparency,
- high processing temperatures,
- high melt viscosity,
- high chemical resistance,
- susceptibility to stress cracking with certain solvents,
- high resistance to b -, g -, X- and IR- radiation,
- high transmittance for microwaves,
- high flame resistance and low smoke development.
Structure and General Properties
1. In order to utilize fully the valuable properties of the dimethyl sulfone group, it must be bonded with thermally and hydrolytically stable links.
2. High flexibility is provided by the ether linkage, the oxygen atom which improves thermal stability and processability.
3. The aliphatic (isopropylidene) group contributes limited flexibility but does not improve thermal stability.
Additives
- Polysulfones are degraded only by UV radiation of wavelength less than 320 nm.
- Photooxidation is caused mainly by the sulfone group (SO2) rather than by impurities. Because mainly hydroxyl groups are formed, the most suitable UV absorbers are hydroperoxide decomposers and radical interceptors (sterically hindered amines, HALS).
- PSU is inherently flame retardant.
Properties can be improved by mineral fillers, glass fiber and C-fiber.
Availability
- The PSU range includes crystal clear and colored transparent and opaque grades as well as mineral-filled and glass fiber reinforced grades.
- PTFE- and graphite-filled grades are supplied by compounders.
- Tube, sheet and film are available.
Mechanical Properties
Transition Temperatures
- Commercial polysulfone has an amorphous structure and is thus crystal clear.
- The benzene rings in the molecule chain result in a glass transition temperature of 190°C. A secondary maximum is found at -100°C.
- Over this wide temperature range of almost 300°C, PSU is hard, stiff and tough like PC. The rigid structure results in high heat resistance and the high processing temperature of 300 to 400 °C.
The impact strength of PSU is, as for all thermoplastics, dependent on the notch radius, temperature of the surroundings and moisture absorbed.
Thermal Properties -1
- The specific volume of PSU increases linearly between 20 and 170 °C from 0.8038 to 0.8253 cm-3g-1 and between 170 to 400°C more steeply to 0.94 cm-3g-1.
- PSU is suitable for applications involving temperatures up to 150°C (continuous) & 170°C (short-term) in air.
Thermal Properties -2
- The temperature index (to UL) is 160°C for electrical and mechanical applications in which the impact strength is not of prime importance.
- It is lower than that of polyethersulfone (180°C) but higher than that of PBT (140 °C), PC (130°C) and PA (120 °C).
Electrical Properties
- Although PSU is a polar plastic, the polarity loses much of its effect because of the high glass transition temperature and associated immobility of the chain segments.
- At temperatures above 150 °C, the dissipation factor increases rapidly from 0.001 to 0.01 at all usual frequencies.
Chemical Properties
Resistance to Chemicals
- PSU is resistant to aqueous inorganic acids, alkalis, salt solutions, aliphatic hydrocarbons, cleaners and paraffin oil; resistance to fuels is limited.
- It is not resistant to ketones, aromatics, chlorinated hydrocarbons, polar organic solvents, water at high temperatures (stress cracking).
Resistance to Stress Cracking
- PSU is susceptible to stress cracking in certain solvents. The critical level of stress (internal stress plus external tensile stress) differs according to solvent. Ethylacetate, acetone, trichloroethane, 2-ethoxyethanol, CCl4
- It is therefore important that internal stresses in moldings are low. Where moldings are likely to come into contact with cleaning agents, they should be conditioned in a glycerine bath at 166°C or in air at 165°C.
Weathering Resistance
- Because of the aromatic bonds, weathering causes chemical degradation of PSU.
- Stabilization with active carbon black, lacquering or metallizing (electroplating or in high vacuum) are required for long-term protection.
Resistance to High Energy Radiation
- The high resistance to b-, g- and IR -radiation contrasts with damage caused by UV in sunlight. PSU is very resistant to microwave (2.45 GHz) range radiation even at high temperatures and is thus suitable for cooking utensils for use in microwave ovens, a major use in the USA and Great Britain.
Flammability
- PSU is of low flammability.
- It burns with a luminous, sooty flame without dripping and extinguishes after the ignition source is removed.
- The smell is choking.
- Classification varies depending on grade from V-2 to V-0 (from 1.02 mm).
- The self-ignition temperature is 621°C.
- Only small amounts of smoke are given off.
Toxicological Assessment
- PSU has FDA approval for repeated contact with foodstuffs as well as for pipe fittings and other accessories for drinking water supplies.
- PSU is certificated for applications in dairy equipment, particularly for pipes for transporting milk.
Sterilization
- PSU is an ideal material for manufacturing medical equipment because of its transparency, sterilizability and freedom from taste and smell. The elongation at break, tensile strength and tensile modulus of elasticity remain almost unchanged after 10,000 cycles.
1. Polysulfone is hygroscopic and must therefore be dried prior to processing at 120 °C for at least 5 hours.
2. The high melt viscosity can easily cause orientation and mold temperatures must therefore be high (see the section on resistance to stress cracking).
3. Molding shrinkage is 0.7%; it is independent of direction in the case of unreinforced material.
Surface Finishing
1. Conditioned moldings can be electroplated or vacuum metallized.
2. The coatings are cured at 163 °C.
3. Printing does not require pretreatment.
4. A formulation developed for printing on PVC can be used for flexographic printing on film.
Joining -2
Welding
- PSU moldings and semi-finished products can be welded by the heated tool, hot gas, ultrasonic or induction methods.
- Temperature of the hot plate in the heated tool method is 370°C, contact time 10 secs.
- Increasing contact pressure between the surfaces to be joined should be used.
- Molded PSU usually contains some moisture and should therefore be dried for 3 to 6 hours at 130 °C in a circulating air oven.
Bonding
- PSU can be bonded with, for example, a 5% solution of PSU in methylene chloride. The parts are pressed together for 5 minutes using a pressure of 35 N/mm2.
- Elastomeric contact adhesives are suitable for joining PSU with rubber, textiles and felt.
- Two-pack Epoxide adhesives and Polyurethane adhesives are best for high strength joins.
Typical Applications
- PSU resin can be injection molded, vacuum formed or machined. Current applications include a wide assortment of medical trays and containers as well as surgical instruments, a binocular ophthalmoscope and bellow housing for anesthetics.
- For severe applications requiring repeated sterilization or uncompromising toughness, PSU stands out. With its high heat deflection temperature of 207°C, it can withstand continuous exposure to heat and still absorb tremendous impact without deflecting or breaking. Plus its chemical resistance surpasses that of most amorphous polymers.
- In electrical engineering and electronics, PSU is used for connector track, coil cores, instrument cases, insulators, condensers, brush holders, components for alkaline batteries, printed circuits and light fittings.
- Many consumer products contain PSU parts, e.g., Food service components, coffee machines, air humidifiers, opera glasses, cookers, cameras, projectors, hair driers, egg boilers, microwave utensils, beverage vending machines, sterilizable medical equipment. In the dairy industry it is used in parts for milking machines, Plumbing fixtures, faucet components, pipes and fittings.
- PSU resins offers unique properties - such as the ability to easily create porous filaments or flat sheet from stable solutions - that allow it to be used in micro and ultra filtration and reverse osmosis membranes. These membranes can be used in applications ranging from artificial kidneys for blood purification to wastewater recovery as well as food and beverage processing.
- Plus, PSU resins are used to create water purification membranes that can filter harmful bacteria spores such as cryptosporidium from municipal drinking water.
In the automotive and aerospace industries PSU is used for switch and relay components, brush holders, coil cores for suppressors, insulating fittings, seals, fuses, aircraft interiors, visors for astronauts.
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