Polymer precision machining is challenging and throws up many problems that require a deep understanding of the plastic in question to solve. Unlike metals, which lend themselves to tolerances of within a few microns, machined polymers can rarely be expected to offer anything under 50 microns (0.05mm) on dimensional tolerances. In addition to this, there is unpredictability, both in the post-machining behavior of the part and in the long-term dimensional stability during operation.

For the most part – any post-machining dimensional issues can be easily resolved by observing the polymer for a few days and then working backwards to create the buffers needed such that once the part settles down, it conforms to the drawing requirements. Provided the polymer is processed correctly, there will be a degree of predictability on, say, the shrinkage of the part after machining.

Recently we machined an 8” PTFE Seal, which exhibited a shrinkage of 0.2-0.3mm within 24 hours of machining. When observed for another 24 hours, there was no additional shrinkage seen. In this case, we had to go back to the moulding and sintering cycle (heat cycle for curing the polymer) and make the necessary adjustments such that this shrinkage could be avoided. As an added measure, we ensured that the diameter we machined was on the very far end of the tolerance spectrum, such that even if a 0.1mm shrinkage did occur, the part – which had an overall tolerance of +/-0.25mm, would still be well within the criteria for acceptance.

Performance on the field is another issue altogether. While it is tough to ensure any level of dimensional stability on highly mechanical applications, where the polymer may be surrounded by metals and hence subject to loads and stresses from much harder materials, we do need to understand the reaction of the plastic to heat.

Understanding glass transition temperature

In an engineering application, a plastic will be analysed for load, wear, and temperature resistance. Usually, this third parameter will focus on the maximum service temperature as a measure for whether the polymer can survive in the given application. However, when we look at precision machined polymer components, a lot of thought must be given to the glass transition temperature of the polymer material.

Put simply, the glass transition temperature is the point at which the polymer moves from a crystalline state, to an amorphous state. The resulting shift can be highly significant for a polymer, since the molecules will try and realign to the new state and this can result in dimensional changes to the part itself. In a crystalline state, one would expect the links between molecules to be more rigid. As such, any machining done on the part would be both predictable and allow for a high level of dimensional accuracy at low tolerances. However, when amorphous, the unpredictability sets in and the part is likely to ‘resettle’ causing tolerances to relax.

Therefore, it is not only important to select a polymer that is capable of a certain service temperature, but to also ensure that during operation, the temperatures do not fluctuate in a manner that would cause the part to keep moving over the glass transition threshold. The table and charts below show the glass transition and service temperatures for some high-performance polymers.

If we take the example of PEEK, we can see that the glass transition temperature is 140°C. While this may be well above the service temperature of other polymers, it also causes some problems. PEEK parts that are used at room temperature (25-30°C), but where the application can call for temperatures in excess of 200°C, PEEK components may experience some issues. Even though PEEK can withstand the service temperature comfortably, the increase of temperatures beyond 140°C will cause the part to experience changes at the molecular level and may push its dimensions out of tolerance, causing a misalignment in the assembly.

In contrast to PEEK, we have PI (Polyimide). Here we see that the glass transition temperature is 340°C – very close to the maximum service temperature of 360°C. As a result, for high temperature applications, PI ensures that you can operate close to its limit and not sacrifice any dimensional integrity.

Other polymers, such as PTFE, LDPE, and HDPE, have glass transition temperatures well below zero. This implies that for any non-sub-zero application, the polymers are always amorphous. Increasing the crystallinity of these polymers is challenging and as a result the typical tolerances one can expect on these plastics would rarely be below 50-100 microns. PTFE, with fillers of glass, can be machined to a tolerance of as low as 10 microns (0.01mm), but this is only in special cases.

In general, polymers which have a smaller difference between their glass transition and service temperatures would offer higher predictability on dimensions in applications where the temperature would fluctuate.

	Glass Temperature	Transition Max Temperature	Service
Polymer Name	Value (°C)	Value (°C)	Diff.
ABS - Acrylonitrile Butadiene Styrene	102	89	-13
CPVC - Chlorinated Polyvinyl Chloride	110	100	-10
HDPE - High Density Polyethylene	-110	120	230
HIPS - High Impact Polystyrene	92	80	-12
LDPE - Low Density Polyethylene	-110	100	210
PA 11, Rigid	45	120	75
PA 12, Rigid	45	120	75
PA 6 - Polyamide 6	60	120	60
PA 66 - Polyamide 6-6	58	140	82
PAI - Polyamide-Imide	275	280	5
PC - Polycarbonate, high heat	200	140	-60
PEEK - Polyetheretherketone	145	280	135
PEI - Polyetherimide	215	170	-45
PESU - Polyethersulfone	230	180	-50
PET - Polyethylene Terephthalate	78	140	62
PFA - Perfluoroalkoxy	90	260	170
PI - Polyimide	340	360	20
PMMA - Polymethylmethacrylate/Acrylic	110	90	-20
POM - Polyoxymethylene (Acetal)	-50	105	155
PP (Polypropylene) Homopolymer	-10	130	140
PPS - Polyphenylene Sulfide	93	220	127
PPSU - Polyphenylene Sulfone	220	210	-10
PSU - Polysulfone	190	180	-10
PTFE (Teflon)	-110	250	360
PVC Rigid	100	80	-20
PVDF - Polyvinylidene Fluoride	-25	150	175
UHMWPE	-150	130	280

 

Processes to limit dimensional deviation

Coming back to the issue with PEEK, the fact remains that PEEK is used in many high-temperature applications. The method to ensure its stability is tricky and involves a lot of trial and error. Annealing the part at above its glass transition temperature and allowing it to cool gradually can help immensely in ensuring that the molecular realignment is minimised. In other words, by letting the plastic become amorphous and then re-crystallise, we limit the extent to which dimensional deviations can occur during operation. Annealing is highly critical to PEEK, since non-annealed parts will easily crack not only during operation, but during machining itself. PEEK is somewhat unique in that it may even require annealing between machining operations to ensure that the structure of the polymer is being allowed to relax each time stresses are placed on it. This repeated annealing needs to be done using trial and error – as each component would behave differently and react in its own way to a specific machining operation.

Ultimately, the choice of polymer would depend on several factors other than temperature. However, it is very important to note that when making the choice for a polymer in a high-temperature application, thought needs to be given not only to the maximum service temperature of the polymer, but also to the fluctuations in temperature during operation. Only by comparing this with the glass transition temperature can one be assured that the polymer fits the application.

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.