Polyphenylene sulfide (PPS) is a semi crystalline, high temperature engineering thermoplastic. It is rigid and opaque polymer with a high melting point (280°C). It consists of para-phenylene units alternating with sulfide linkages. Regular PPS is an off-white, linear polymeric material of modest molecular weight and mechanical strength. It is produced by reaction of sodium sulphide and dichlorobenzene in a polar solvent such as N-methylpyrrolidone and at higher temperature [at about 250° C (480° F)]. When heated above its glass transition temperature (Tg ~85°C), it crystallizes rapidly.
PPS possesses many desirable properties:
- Exceptional Mechanical Strength
- High Temperature Resistance
- Dimensional Stability
- Inherent Flame Resistance
- Good Chemical Resistance
- Electrical Insulation Properties
And, it can be easily processed as well as its toughness increases at high temperatures.
These assets make PPS a chosen alternative to metals and thermosets for use in automotive parts, appliances, electronics and several others applications.
To produce molding grades, PPS is cured (chain extended or crosslinked) around the melting point of the polymer in the presence of a small amount of air. This curing process results in:
- Increase in molecular weight
- Increased toughness
- Loss of solubility
- Decrease in melt flow
- Decrease in crystallinity
- A darkening in color (a brownish color in contrast to this linear PPS grades are off-white)
PPS is an ideal material of choice to produce complex parts with very tight tolerances. The polymer exhibits an excellent dimensional stability even when used under high temperature and high humidity conditions.
PPS has excellent electrical insulation properties. Both the high-volume resistivity and insulation resistance are retained after exposure to high-humidity environments. It has a less pronounced O2 sensitivity and can be conveniently doped to get high conductivity.
PPS has high strength, high rigidity and low degradation characteristics even in high temperature conditions. It also shows excellent fatigue endurance and creep resistance.
PPS has good chemical resistance. If cured, it is unaffected by alcohols, ketones, chlorinated aliphatic compounds, esters, liquid ammonia etc. however, it tends to be affected by dilute HCl and nitric acids as well as conc. sulphuric acid. It is insensitive to moisture and has good weatherability.
PPS has however, a lower elongation to break, a higher cost and is rather brittle. Today, PPS is available in different forms and grades such as compounds, fibers, filaments, films and coatings.
PPS resin is generally reinforced with various reinforcing materials or blended with other thermoplastics in order to further improve its mechanical and thermal properties. PPS is more used when filled with glass fiber, carbon fiber, and PTFE.
Popular Applications of PPS
The excellent properties of PPS with its ease of production and moderate cost makes it one of the most suitable choices for various applications where cost and high performance are essential.
Automotive Applications/ Automobile Parts
PPS applications in automotive market have seen strong growth mainly due to its ability to replace metal, thermosets and other types of plastic, in more demanding applications. It is an ideal choice for automotive parts exposed to:
- High temperatures
- Automotive fluids, or
- Mechanical stress
PPS is a lighter weight alternative to metals, resistant to corrosion by salts and all automotive fluids. The ability to mold complex parts to tight tolerances and insert molding capability accommodate multiple component integration.
Electronic and Electrical Applications
Due to its high temperature resistance, high toughness, good dimensional stability and good rigidity, PPS becomes an ideal material of choice in E&E market.
- Offers excellent flow and low shrinkage for precision molding of connectors and sockets
- Provides superior stiffness and mechanical integrity for reliable assembly, and
- Is the most stable material choice for all soldering methods
PPS compounds also have UL94 V-0 flammability ratings without the use of flame retardant additives. Special low flash grades have been developed to meet the needs of high precision molding applications.
In the electrical / electronic sector, PPS is also used to manufacture a range of articles including bobbins and connectors, hard disk drives, electronic housings, sockets, switches and relays. The key trend influencing PPS growth in electrical / electronic applications is substitution of other lower temperature polymers.
Polyphenylene sulfide has been replacing metal alloys, thermosets, and many other thermoplastics in mechanical engineering applications. The thermal stability and broad chemical resistance of PPS make it exceptionally well suited to service in very hostile chemical environments.
- It finds uses in many heavy industrial applications, including some outside the arena of reinforced injection molding compounds
- It is used in fiber extrusion as well as in non-stick and chemical resistant coatings
- It is well suited to manufacture mechanically and thermally highly stressed molded parts
- In machine construction and precision engineering, PPS is used for various components such as pumps, valves and piping
- It can also be found in oil field equipment such as lift and centrifugal pump components, oil patch drop balls, rod guides and scrapers
- In the heating, ventilation and air conditioning (HVAC) equipment sector, PPS is used for compressors, mufflers/reservoirs, hot water circulation components, induced draft blower housing, motor relays and switches, power vent components and thermostat components
Medical and Healthcare Applications
Polyphenylene sulfide compounds (typically glass reinforced grades) are used in medical application such as surgical instruments and device components and parts that require high dimensional stability, strength and heat resistance. PPS fibers are also used in medical fibers and membranes.
Processing Conditions for PPS
PPS resins (base polymer, glass-reinforced, and mineral/glass-reinforced systems) are typically used for injection molding, extrusion blow molding, and extrusion applications.
PPS can be processed at temperatures in the range of 300 to 350°C. The processing of PPS can be hampered at times due to its high melting point.
If the PPS used is of the filled grade, the upper processing temperature should be used to avoid any kind of wear and tear of the barrel, screw and screw tip.
Extrusion PPS grades are commonly available for fiber and monofilament production as well as tubing, rod and slab.