Polyphenylene Sulfide (PPS) - A robust polymer with multiple applications

Finding the right polymer solution for a given application can be tricky. Typically, an application will specify a certain load or temperature that the polymer component would need to withstand. Using these parameters, one usually sets out to find a polymer that is compatible and also cost effective.

When we talk of polymers that can withstand high pressures, there are many that come to mind. Indeed, if pressure is the only criteria, then most polymers – including Polypropylene, Polyethylene, PVC, PA6, Acetal or UHMWPE – are both cost effective and robust. But when temperature is added to the mix – especially anything in excess of 150-200°C, then the list stars thinning out considerably. For the longest time, PTFE and PEEK were the most obvious choices in this scenario. The only issue was that PTFE tends to deform, while PEEK is prohibitively expensive.

Polyphenylene sulfide (PPS) is a semi crystalline, high temperature engineering thermoplastic. It is a rigid and opaque polymer with a high melting point (280°C). In terms of properties, it can hold its own against PTFE, PEEK and even PI (Polyimide). Cost-wise, it rests somewhere in between PTFE and PEEK, making it a good balance between the two.

PPS offers an excellent balance of properties such as:
 

PPS can also be easily processed, using both injection moulding and compression moulding. Furthermore, its toughness increases at high temperatures and it is resistant to certain chemicals that affect PEEK, making it a material of choice in industries such as paper processing, where such chemicals are prevalent.

These assets make Polyphenylene sulfide a chosen alternative to metals and thermosets for use in automotive parts, appliances, electronics and several others applications.

Some of the key producers of PPS include:
   »  Toray Resin Company - TORELINA®, TORAYCA®
   »  RTP Company - RTP 1300 series
   »  Solvay - Ryton®, PrimoSpire®, Tribocomp®
   »  Celanese - FORTRON®, CoolPoly®, Celstran®
   »  Polyplastics - DURAFIDE®
   »  SABIC - LNP™ LUBRICOMP™, LNP™ STAT-KON™, LNP™ THERMOCOMP™ and more
   »  Lehman & Voss - LUVOCOM®

What is PPS Made From?

The first commercial process for PPS was developed by Edmonds and Hill (US patent 3 354 129, Yr. 1967) while working at Philips Petroleum under the brand name Ryton.

Today, all commercial processes use improved versions of this method. PPS 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)].
 

In the original process developed by Philips, the product obtained had a low molecular weight and could be used mainly for coating applications. To produce moulding 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:

• Increased molecular weight

• Increased toughness

• Loss of solubility

• Decrease in melt flow

• Decrease in crystallinity

• A darkening in colour (a brownish colour in contrast to this linear PPS grades are off-white)

Over time, modifications to the process have been reported to eliminate the curing stage and develop products with improved mechanical strength.
 

Key Properties of Polyphenylene Sulfide (PPS)

Crystal Structure and Physical Properties

PPS is a semi-crystalline polymer. Knowledge about the crystallization behaviour of PPS is very important to understand the recommended processing parameters. The following table shows the phase transition temperatures of PPS. Ranges depend on mol. weight and curing status (linear or crosslinked).

Glass Transition Temperature (Tg)	85 - 95 °C
Crystallization on Heating (Tc-h)	120 - 140 °C
Cristallite Melting (Tm)	275 - 285 °C
Recrystallization on cooling (T c-c)	255 - 225 °C
Density	1.35 g/cm3
Gamma Radiation Resistance	Good
UV Light Resistance	Good
HDT @0.46 Mpa (67 psi)	140 - 160 °C
HDT @1.8 Mpa (264 psi)	100 - 135 °C
Max Continuous Service Temperature	200 - 220 °C
Thermal Insulation (Thermal Conductivity)	0.29 - 0.32 W/m.K

Phase Transition Temperatures & Other Physical Properties of PPS

Dimensional Stability

PPS is an ideal material of choice to produce complex parts with very tight tolerances. The polymer exhibits excellent dimensional stability even when used under high temperature and high humidity conditions.

Coefficient of Linear Thermal Expansion	3 - 5 x 10-5 /°C
Shrinkage	0.6 - 1.4 %
Water Absorption 24 hours	0.01 - 0.07 %

Electrical Properties

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.

Arc Resistance	124 sec
Dielectric Constant	3 - 3.3
Dielectric Strength	11 - 24 kV/mm
Dissipation Factor	4 - 30 x 10-4
Volume Resistivity	15 - 16 x1015 Ohm.cm

Thermal Properties and Fire Resistance

PPS is a high-temperature specialty polymer. Most of the PPS compounds pass UL94V-0 standard without adding flame retardant. PPS can be resistance to 260°C for short time and used below 200°C for a long time.

Fire Resistance (LOI)	43 - 47 %
Flammability UL94	V0

Mechanical Properties

PPS has high strength, high rigidity and low degradation characteristics even in high temperature conditions. It also shows excellent fatigue endurance and creep resistance.

Elongation at Break	1-4%
Elongation at Yield	1-4%
Flexibility (Flexural Modulus)	3.8-4.2 GPa
Hardness Rockwell M	70-85
Hardness Shore D	90-95
Stiffness (Flexural Modulus)	3.8-4.2 GPa
Strength at Break (Tensile)	50-80 MPa
Strength at Yield (Tensile)	50-80 MPa
Toughness (Notched Izod Impact at Room Temperature)	5 - 25 J/m
Young Modulus	3.3 - 4 GPa
Click here to compare the mechanical properties of reinforced grades vs. unfilled neat polymer

Chemical Properties

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.

Optimizing the properties of PPS material

There are a great number of PPS compounds in the market. Due to the chemical robustness of the polymer, a great variety of fillers and reinforcing fibres and combinations of these can be applied.

PPS resin is generally reinforced with various materials or blended with other thermoplastics in order to further improve its mechanical and thermal properties. Key fillers include glass fiber, carbon fibre, and PTFE.

Key grades available include:

• Unfilled Natural

•  

  PPS with glass fiber - 25%, 30% and 40% glass filled

• Glass mineral filled

• Conductive and Anti-Static Grades

• Internally lubricated bearing grades

• PPS+20% PTFE

The mechanical properties of reinforced grades differ significantly from the unfilled neat polymer. The typical property values for reinforced and filled grades fall in the range as shown in the table below.

Property (Unit)	Test Method	Unfilled	Glass Reinforced	Glass-Mineral Filled*
Filler Content (%)		-	40	65
Density (kg/l)	ISO 1183	1.35	1.66	1.90 - 2.05
Tensile Strength (Mpa)	ISO 527	65-85	190	110-130
Elongation at Break (%)	ISO 527	6-8	1.9	1.0-1.3
Flexural Modulus (MPa)	ISO 178	3800	14000	16000-19000
Flexural Strength (MPa)	ISO 178	100-130	290	180-220
Izod notched Impact Strength (KJ/m2)	ISO 180/1A		11	5-6
HDT/A (1.8 Mpa) (°C)	ISO 75	110	270	270

Typical Mechanical Properties of PPS and PPS Compounds
Data from Product brochures: DURAFIDE®, Polyplastics; Ryton®, Solvay
* depending on filler ratio Glass / Mineral

Typically neat polymer grades are used for fibres and films, whereas filled/reinforced grades are used for a great variety of applications in thermally and/or chemically demanding environment.

Further PPS-based nanocomposites can also be prepared using carbon nanofillers (expanded graphite (EG) or ultrasonicated EG (S-EG), CNTs) or inorganic nanoparticles. Due to insolubility of PPS in common organic solvents, most PPS-nanocomposites have been prepared by melt-blending approach. One of the main reasons for adding nanofillers to PPS is to improve its mechanical properties to meet the increasingly high demand of certain applications.

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

Polyphenylene Sulfide 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 

• Mechanical stress

PPS is a lighter weight alternative to metals, resistant to corrosion by salts and all automotive fluids. The ability to mould complex parts to tight tolerances and the insert moulding capability accommodate multiple component integrations.

Under-the-hood is the largest application area for PPS followed by electrical parts. PPS applications in automotive include fuel injection systems, coolant systems, water pump impellers, thermostat holder, electric brakes, switches, bulb housing and so on.

It is rarely used for the manufacture of interior or exterior auto parts.

Electronic and Electrical Applications

Owing 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 moulding of connectors and sockets

• Provides superior stiffness and mechanical integrity for reliable assembly

• 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 moulding applications.

In the electrical / electronic sector, Polyphenylene Sulfide 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.

Appliances

Thanks to its exceptional dimensional stability, low density, corrosion and hydrolysis resistance, PPS can be used to manufacture heating and air conditioning components, fry pan handles, hair dryer grills, Steam iron valves, toaster and dryer switches, microwave oven turntables etc. in electric appliances.

Industrial Applications

PPS has been replacing metal alloys, thermosets, and many other thermoplastics in mechanical engineering applications. The thermal stability and broad chemical resistance of Polyphenylene Sulfide 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 moulding 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 moulded 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, Polyphenylene Sulfide 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

PPS 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.

Conclusion

As processing techniques advance and scale develops, it is likely that PPS would become the material of choice many industrial, automotive, and electrical applications. It remains to be seen where else this polymer can find use, since its versatility lends itself to such a wide range of usability.