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.

Across the world, food processing is understandably one of the most critical industries. Along with the medical industry, food processing calls for factors of hygiene that would otherwise be overlooked in areas such as automotive, chemical or oil & gas. With the advent of automation, food processing is increasingly seeing the need for materials that are FDA approved and that will not in any way degrade during operation. In addition to this, food processing usually involves heat, which means the materials used cannot deform or melt during operation.

Whether we look at large-scale food processing or kitchen top processing, high-performance food safe polymers have found a way into nearly all aspects of this industry. Not only are polymers food-grade and non-contaminating, they also allow for a significant reduction in noise and weight – both of which are paramount, especially when dealing with home level food processing equipment. We take a look here at some of the key areas in which high-performance polymers find application.

1. PEEK valves for Coffee Machines
   High-end coffee machines are built keeping the final taste of the finished brew as the most critical parameter. Traditional machines used valves made of aluminium, which routed the high-temperature liquid within the machine. However, as consumer palates became more discerning, the manufacturers realized that aluminium valves caused a faint metallic taste to be left in the mouth.
   
   In the hunt for a high-temperature material that is compatible with coffee and FDA approved, PEEK was used as a replacement for the aluminium. It should be said that PEEK being an expensive material, the price of a PEEK valve is many multiples of what the aluminium component costs. However, with the PEEK Valve, the taste of the coffee is preserved. PEEK Valves are now a mainstay of any high-end coffee machine
    

2. PTFE and Acetal Rotary Seals and Shafts
   Most food processing equipment involves some rotary motion. Whether it is the gentle kneading of dough or the high RPM mixing and grinding of spices, all rotary motion causes some amount of friction and thereby, heat.
   
   Polymers in food equipment are essential in such cases. PTFE and Acetal (POM/Delrin) seals and shafts are preferred in these applications. Not only are these materials lightweight in comparison to metals, but they are also self-lubricating, implying no need for external lubrication and minimal noise creation.
   
   PTFE+Molybdenum-di-sulphide is possibly the most preferred material in rotary applications, as it possesses a low coefficient of friction, while also having superior wear resistance. In addition to this, parts can be machined to close tolerances, allowing a good fit between the polymer seal and the moving parts that minimize vibration.

3. PTFE Wiper Seals
   Many food processing applications involve foods that are sticky and may not easily separate. The equipment may process the food and place it on a non-stick pan; however, an additional member is needed to move the food either out of the equipment or to another part of the apparatus for further processing. In such cases, PTFE wiper seals are used to push or move the food around. Since PTFE is non-stick, the wiper seals do not themselves allow any food particles to adhere to themselves. This is beneficial not only because the food can be moved without deforming or affecting its shape in any way, but also because food particles that get stuck pose a hygiene issue.
   
   PTFE is also unique because it can withstand up to 250°C of temperature. This means that even if the food is hot, there is no chance it will affect the PTFE seal. With so many benefits, it is no wonder that PTFE is among the most valued polymers for food processing.

4. PTFE Tubes for liquids
   PTFE tubes are both chemically inert and have a service temperature of 250°C. The transfer of hot fluids is often essential for food processing. In addition to being able to take the temperature, the polymer tube needs to ensure that it does not react in any way with the food. In some cases, liquids may collect within the tube for long periods, if the equipment is not used often. PTFE not only stays non-reactive over long periods of time, but its non-stick nature ensures that once the equipment is re-started, there is little chance that any residual fluids will remain stuck within the tube.
    

5. PTFE and PEEK Stirrers and Impellers
   Much of food processing involves the mixing of various ingredients. While most stirrers are made using metals coated with a non-stick material, some applications do call for the stirrers themselves to be made of inert materials. This may be needed in applications where the material being mixed may be abrasive and cause the non-stick coating on the metals to chip. Since both PTFE and PEEK are FDA approved and will not chip or degrade when in contact with foods, they are preferred as stirrers and impellers in many mixing operations.
    

Fundamentally, the combination of chemical inertness, food grade, non-stick and high-temperature capabilities means that there are many more applications within food processing where high-performance polymers could find use. As food processing moves further into the realm of robotics and automation, the devices created will need an increasing number of food-safe polymers to be incorporated to ensure both hygiene and long-term performance can be guaranteed.
 

 

When you manufacture a specialised product, one of the biggest challenges lies in ensuring the end-user recognises the technical advantages of the same. This is especially true when there are cheaper substitutes that compare with your product visually, implying the end-user might have doubts on whether you are selling him anything special, or whether you are simply charging a premium over something commoditised.

Expanded PTFE (ePTFE) Tapes are one of the most specialised variants of PTFE. Their uses go from simple gasket applications, all the way to intricate membranes for use in high-end filtration and medical membranes. However, the similarity between ePTFE Tapes and Thread Sealant Tapes (also called: Plumber’s Tape), can sometimes confuse clients, who might question whether they are not one and the same.

Thread Sealant PTFE Tape is a highly commoditized PTFE tape that is cheaply available in nearly any hardware store. As its name suggests, it is mainly used in plumbing, where it is wrapped around the threads of a pipe, before a mating pipe is tightened over it. The ability of the tape to easily take the shape of the existing threads means that it creates a tight fit, thereby preventing water leakage. Plumber’s Tape is a highly useful material in its designated application. However, it has grave limitations when compared with expanded PTFE (ePTFE) Tape.

Differences in Production
There exist many key differences in the production process. Thread Sealant Tape is made by extruding a bead of PTFE, which is then passed through various calendaring, slitting and spooling operations. The end result is an unsintered PTFE tape with a thickness of ~0.075mm (75µm). Sintering is the process by which PTFE is cured at high temperatures to let it attain its final properties. Unsintered PTFE tape is basically still ‘raw material’ which has been drawn and flattened into a tape form.

Expanded PTFE (ePTFE) Tape is also made by first extruding the tape. However, this tape is then put through a drying process, after which it is passed through a stretching machine at elevated temperatures. The stretching process needs to be CNC controlled in order to ensure the stretch rate, speed and temperature are maintained as per strict parameters. The resulting tape would usually have a thickness ranging from 0.25mm to 15mm

The fact that Thread Sealant Tape is calendared, unsintered tape, while ePTFE Tape goes through a stretching and heating process is the reason the tapes exhibit such different properties. In truth, the rapid speed at which ePTFE tape is stretched and heated means that it too is not what one might call ‘fully sintered’. However, the stretching process intentionally does not cure the PTFE above its melting point in order to ensure that certain properties are preserved.

Differences in End Properties
As mentioned above, the key purpose of Plumber’s Tape (PTFE thread seal tape)  is to seal leaks in piping. The unsintered PTFE material is soft and easily takes the shape of the threads that it is wrapped around.

In contrast, ePTFE Tapes exhibit a range of properties, in addition to sealing, which make them vital across a number of industries. To start with, as a sealing material, ePTFE is used in areas where you not only require a seal, but where the seal needs to be capable of taking high pressures (up to 100Bar), high temperatures (up to 250°C), and be resistant to a range of corrosive chemicals. Trying to use simple Thread Sealant Tape in such demanding environments will cause the tape to degrade almost instantly, as it lacks the mechanical strength to withstand the same.

Expanded PTFE (ePTFE) also has high dielectric properties. The tape can resist immense levels of voltage and is used in high-end cable wrapping to improve efficiency and insulation. Again, some cable manufacturers do try to use Thread Sealant Tape to wrap around cables, after which the cables are sintered in order to fuse the tape. However, the resulting cable has a much lower insulative capacity and may tend to fail in higher intensity applications.

Finally, expanded PTFE (ePTFE) exhibits micro porosity. The calendering process used in making Thread Sealant Tapes ensures that there are no pores in the material. However, because ePTFE is stretched under high temperatures, it attains a level of porosity. Most notably, ePTFE exhibits hydrophobic and oleophobic properties, meaning it repels water and oil respectively. At the same time, the material will allow the passage of vapours. This unique characteristic makes it an invaluable material in venting and filtration applications. It also allows ePTFE Membranes to find use as grafts and stents for use in the medical industry.

The combination of these properties ensures that ePTFE is demanded not only in fluid sealing systems, but in filtration, medical, heavy electricals, chemical plants and even aerospace applications.

When you consider the above points, it is easy to see that ePTFE Tape and Thread Sealant tapes are worlds apart in terms of their effectiveness and their breadth of application. Nonetheless, it should be noted that many applications are basic enough that using Thread Sealant Tapes might suffice. Commercially speaking, ePTFE would cost many multiples of what a simple Thread Sealant Tape costs. Hence, the decision to use ePTFE rests on the end application itself and on whether the required properties of the material need to extend to as high as what expanded PTFE offers.