Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

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 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:
 

Key Properties of Polyphenylene Sulfide (PPS) Polymer


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)].
 

Synthesis of PPS


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

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 fibre, carbon fibre, and PTFE.

Key grades available include:

  • Unfilled Natural

  • 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 fibre 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 fibres are also used in medical fibres 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.

Polymers in Food Processing

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 polymers have found a way into nearly all aspects of this industry. Not only are polymers food-safe 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.

    PTFE and Acetal (POM/Delrin) seals and shafts are preferred in such applications. Not only are these materials light 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 seal and the moving parts that minimize vibration.

  3. PTFE Wiper Seals
    Many food processing applications involve food 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.
     

  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 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 polymers to be incorporated to ensure both hygiene and long-term performance can be guaranteed.
 

 

Polymers Are The Future For Electric Vehicles

The decline in sales for the auto industry has been pronounced and unprecedented.  While many point to short and medium factors, such as government policies and the non-availability of financing, the truth remains that most auto manufacturers remain woefully unprepared for the paradigm shift that is in the offing.

Electric vehicles are an inevitable mainstay of the future auto market both because of their economic and environmental impact. Thus far, fossil fuel-run vehicles have enjoyed the economic advantage, because EVs were both expensive to buy and had limited range and power. In addition to this, the limited infrastructure surrounding EVs meant that it was a hassle to own one, unless one was very inclined to shun fossil fuels. But as the technology has advanced, both these factors seem to be becoming less pronounced. Thanks to increased scale and large bets taken by the leaders in the EV space, the upfront costs of owning an EV have lowered significantly. In addition to this, continual improvements in the battery management systems have allowed the range to be increased to the point where a single charge may last over a week for someone doing only 30-40 kilometres a day. Further, government support for the industry has meant that the infrastructure has also moved ahead at a good pace. Many buildings – even in India – have mandatory EV charging points in all the parking spaces. Convenience-wise, this is even better than having to go to a fuel station once a week to fill up your tank with petrol or diesel!

Much of the technology of electric vehicles depends on high efficiency and a good strength to weight ratio. In such an endeavour, lightweight materials become essential. Polymers have long been known to provide long term performance and efficiency gains to any system. A rule of thumb in the auto industry has been that for a 10% reduction in weight, the fuel efficiency of the vehicle improves by 5%. For this reason, the quantum of polymers has increased from around 8Kgs to over 150Kgs over the last 40-50 years. 

The effectiveness of polymers in automotive applications has always been known. As polymer science has evolved, the range of application has also broadened. Polymers such as PEEK, PTFE, PEI (Ultem) and PI (Kapton) have exhibited tremendous resistance to heat, such that there seems a little argument for using metals (which would be at least 2-3 times heavier) in areas where these polymers can be used.

As electric vehicles gain in importance, we look at some of the areas in which polymers are especially useful in EVs.

1. Sensor shields and enclosures

The use of sensors is essential in ensuring safety. As autonomous vehicles see a rise in adoption, sensors will become possibly the single most important component set within a vehicle.

Polymer shields and connectors are important because unlike metals, they remain neutral to the signals and waves being sent and received by the sensors. PTFE and PEEK are already used extensively as Radomes in antennae. As the number of sensors in the vehicle grows, it is even more essential to ensure that there is no disruption to performance, in the event that all sensors are working at once. Polymers are unique in being able to offer protection from weather, heat, and additionally, do not interfere in any way with the signals.

2. Brackets

Brackets made from polymers are useful as they hold together other components and ensure that they do not get damaged during operation. Some of these components may generate heat, so the polymer would need to withstand this as well. Brackets made from Nylon have been used as replacements for metal even in conventional vehicles, as they offer a significant weight reduction and can be moulded to suit the exact shape of the component set that they are housing. Further, in the event that a component does come slightly loose, the potential noise from the rattling is minimised significantly when a polymer is involved.

3. Insulation

Much in an electric vehicle rides on the efficiency of the battery and the use of stored power. Anything that helps minimise the leakage of current from the system aids in improving the battery life and consequently the distance that can be traversed on a single charge. Materials like PTFE and Polyimide have proven highly effective as insulators in high-voltage-high-temperature applications.

4. EV charging stations

Electric Vehicles are gaining traction over traditional fuel powered vehicles. As their demand and prevalence grows, so too would the infrastructure needed to ensure that they can function smoothly. Investments in EV charging stations have increased significantly and new housing developments are increasingly required to ensure that there are charging stations for all parking slots.

As a superior insulation material, PTFE has been found effective in EV charging stations. PTFE insulation blocks can be used to improve the charging efficiency and ensure that there is minimal leakage of current.

5. Battery separators

One of the key factors with electric vehicles is that battery storage needs to be both ample and efficient. Both PTFE and PE (polyethylene) are seen as effective battery separators. These separators provide internal insulation to the battery, preventing the batteries from discharging when idle. Although PE separators are effective in most application, high-voltage applications need PTFE films, which possess higher breakdown voltage strengths and can remain effective over a much longer time period.