Hello and welcome to the second part of our High Performance Thermoplastics selection blog series. The second part will focus on the introduction of high performance polymers, their chemistry and production processes, their main properties, their processing methods, and last but not least, applications.
There are six major high performance thermoplastics families (“the magnificent six”) which we will discuss:
-Part 2A: Polysulfides (Polyphenylene sulfide - PPS), Polysulfones (PSU, PESU, PPSU), and Polyarylates (PAR)
-Part 2B: Imide-Based Polymers (PEI, PAI, PESI, TPI, PI) and Polybenzimidazoles (PBI, PBI+PEEK, PBI+PEKK)
-Part 2C: Polyether (PPE, PAEK, PEEK, PEKK)
-Part 2D: Liquid Crystal Polymers (LCP)
-Part 2E: Semi- and Fully Aromatic Polyamides (PARA, PPA, Aramid)
-Part 2F: Polyhalogenolefins (PTFE, PCTFE, FEP, PVDF, ECTFE)
All thermoplastics can be visualized by using the performance pyramid, as shown in Figure 1. Based on production volumes, performance, and price, thermoplastics can be categorized as commodity plastics such as Polyolefins, engineering plastics such as Polyamide, and high performance polymers. Polysulfides (PPS), Polysulfones (PSU, PESU, PPSU), and Polyarylates (PAR) are highlighted since they will be discussed in this post.
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| Figure 1: Plastics Performance Pyramid highlighting PPS, Polysulfones, and PAR. |
Polysulfides (Polyphenylene sulfide - PPS), Polysulfones (PSU, PESU, PPSU), and Polyarylates (PAR)
The first major family we discuss is the Polysulfide family which contains Polyphenylene sulfide (PPS), followed by Polysulfones (PSU, PESU, PPSU), and Polyarylates (PAR). Polyphenylene ether (PPE) can be added to this family too, however I placed it in the family of Polyethers together with Polyetheretherketone (PEEK).
Polyphenylene sulfide (PPS) - The polymer that thinks it’s metal…
Everyone who had already a part made out of PPS in his hand and let it drop will immediately recognize its metal-like sound. Thus, sometimes PPS is referred to as the “polymer that thinks it’s metal”.
Chemistry and Production
PPS is a semi-crystalline polymer and its backbone consists of aromatic rings (phenylene groups) linked by sulfide bridges. This unique structure grants PPS its remarkable properties. The production process typically involves step-growth polymerization, where para-dichlorobenzene reacts with sodium sulfide under specific conditions in a solvent like N-methylpyrrolidone (NMP). However, numerous modifications and post-treatments can be employed to create specific PPS grades with tailored properties. There are two main routes making PPS and depending which route was taken, a so-called cured PPS (“flash PPS”, referring to the flash evaporation of water and solvent at the end of the two-step process; first step: sodium hydroxide reacts with solvent N-Methyl-2-pyrrolidone (NMP); step two: hydrogen sulfide gas introduced and a suspension is the result; curing by air is needed to increase the molecular weight; it leads to chain elongation and branching; typical MW before curing: 18,000 – 22,000 and after: 45,000) or linear PPS (“quench PPS”, where the reaction takes place in a solvent and it is continuous without having to remove water; "quench" or "quenching" = process of rapidly heating and mixing the slurry from the reactor with a quench medium, usually water and/or steam, leading to polymerization determination). The flash process has a higher yield efficiency (>92 %), good operational stability with better quality control. The quench process results in higher MW directly ( MW ~50,000) and lower yield efficiency (~88%).
Cured (“branched”) PPS has a dark brown color and in the beginning, linear PPS grades showed superior toughness, less off-gasing, lower flash appearance, and improved weldline strength. Parts ade out of cured PPS show an improved dimensional stability and creep performance. However, over time and with production improvements, differences were minimized.
Well-known trade names for PPS include Ryton® (Syensqo), Torelina® (Toray), Fortron® (Celanese), Durafide® (Polyplastics), Xytron® (Envalior) and DIC PPS (DIC).
Properties of PPS
- High-temperature performance: PPS boasts a melting point around 280°C (536°F) and can withstand continuous use (UL 746B) at temperatures exceeding 200°C (424°F). This makes it ideal for applications that encounter extreme heat.
- Chemical resistance: PPS exhibits exceptional resistance to a wide range of chemicals, including strong acids, bases, and solvents. This allows it to function in harsh chemical environments. Concentrated nitric acid is one of the few chemicals which can dissolve PPS, apart from that there are no other chemicals which can dissolve PPS at a temperature below 200°C.
- Dimensional stability: PPS maintains its shape remarkably well at elevated temperatures, minimizing warping or shrinkage. For medium sized parts, 0.1% of its given dimension can be held and for large parts, 0.2% of the set dimension can be held.
- Flame retardant: PPS is inherently flame retardant and self-extinguishing, making it a safe choice for applications requiring fire resistance. It can achieve the UL94 V0 at 0.75 mm and has a limiting oxygen index (LOI) which is estimated according to ISO 4589, of 45%.
- Electrical insulator: The non-polar structure of PPS makes it a good electrical insulator.
- Excellent flow properties: allowing to fill thin wall thicknesses. Amount of crystallization is around 50%.
Table 1 summarizes selected mechanical properties of non reinforced PPS, cured PPS, and linear PPS moulding compounds. Often questions around the part marking code for highly filled PPS compounds arise. According to ISO 11469, the 65 wt% PPS compound R-7-120NA has following part marking code, which is in line with the VDA 260 too: >PPS-(GF+MD)65<.
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| Table 1: Selected mechanical properties of non reinforced PPS, cured PPS, and linear PPS moulding compounds [1]. |
Processing Methods
PPS can be processed using various techniques:
- Injection Moulding: This is the most common method for shaping PPS into complex parts. Mould temperatures of minimum 135°C are needed to have a proper crystallization of the polymer.
- Extrusion: PPS can be extruded into sheets, rods, tubes, and pipes for diverse applications.
- Machining: PPS exhibits good machinability, allowing for the creation of precise components.
- Coating: PPS powders can be used to coat metal sureafce such as kitchen pans.
Applications of PPS
Due to its exceptional properties, PPS finds applications in a variety of industries:
- Automotive: PPS is used for pump components, engine parts close to the combustion engine, and electrical connectors due to its heat and chemical resistance. Also for e-mobility, PPS plays a key role, especially for high temperature electronics. Currently, a typical internal combustion engine (ICE) has around 700 grams of PPS polymer on board. New numbers form Asia reveal that there will be 3 to 4 kg of PPS in electric vehicles (EVs) and hybrid electric vehicles (HEVs).
- Chemical Processing: PPS is ideal for pipes, valves, and pump housings due to its excellent chemical resistance.
- Electrical & Electronics: PPS is a valuable material for connectors, circuit boards, and other electrical components because of its electrical insulating properties.
- Aerospace: PPS finds use in aircraft components due to its lightweight nature and high-temperature performance.
Economic Aspects
PPS is a high-performance polymer, and its production costs are typically higher than those of some commodity plastics. However, PPS prices are still much lower compared to other high performance polymers such as PEEK. PPS and its exceptional properties often translate into longer lifespans, reduced maintenance needs, and improved performance, justifying the initial investment.
PPS stands out as a versatile and high-performance thermoplastic. Its unique combination of properties makes it a valuable material for demanding applications across various industries. As polymer engineering continues to evolve, PPS is certain to remain a key player for years to come.
Polysulfones (PSU, PESU, PPSU) - Take me to the moon
Polysulfones (PSFs) are an amorphous class of high-performance thermoplastics renowned for their exceptional properties, including heat resistance, chemical resistance, and dimensional stability. These attributes make them ideal for applications requiring materials that can withstand harsh conditions and maintain their integrity over time. Polysulfone was used as an external sun protection helmet visor for the Apollo 11 crew which landed on the moon on July 20th 1969.
Chemistry and Production
The chemical structure of polysulfones is characterized by a repeating ether sulfone unit (-O-SO2-). This unique structure contributes to the material's excellent properties.
Polysulfones are typically produced through a nucleophilic aromatic substitution reaction between bisphenol A and a dichlorosulfone. The reaction is carried out in a polar aprotic solvent, such as dimethyl sulfoxide (DMSO), in the presence of a base.
Main Properties
- Heat resistance: Polysulfones have a glass transition between 190°C (374 °F) for PSU and 220°C (428 °F) for PESU and PPSU, leading to a high heat deflection temperature, making them suitable for applications in hot environments.
- Chemical resistance: They are resistant to a wide range of chemicals, including acids, bases, and solvents.
- Dimensional stability: Polysulfones exhibit excellent dimensional stability, maintaining their shape and size even under varying conditions.
- Flame retardancy: Many polysulfone grades are inherently flame-retardant and fulfil the UL94 V0.
- Hydrolytic stability: They are resistant to hydrolysis, making them suitable for applications in contact with water. PSU and PPSU polymers exhibit excellent resistance to hydrolysis, remaining stable under high-temperature steam and water exposure. This characteristic makes them suitable for medical instruments requiring repeated autoclaving. For example, PSU performs well even after 100 autoclave cycles at 134°C. PPSU on the other hand, maintains its original physical properties like rigidity and ductility up to 1000 cycles at the same temperature.
- Biocompatibility: There are polysulfone (PSU) and polyphenylene sulfone (PPSU) grades which have been developed for usage in medical devices that require long-term biocompatibility. For example, there are PSU and PPSU grades such as Syensqo's Eviva PSU and Veriva PPSU grades which are biocompatible polymers developed for long-term medical implants. They meet regulatory standards for devices in contact with bodily tissue/fluids for 30+ days and are manufactured following ISO 13485 and cGMP guidelines. In addition, standard grades of polysulfone and PPSU are used in medical devices that are in contact with bodily fluids and tissue short-term, or less than 24 hours.
Differences between polysulfone (PSU), polyethersulfone (PESU) and polyphenylene sulfone (PPSU)
While all three materials - polysulfone (PSU), polyethersulfone (PESU), and polyphenylene sulfone (PPSU) - are high-performance thermoplastics with similar properties, there are some key differences between them.
- Polysulfone (PSU): The repeating unit in PSU is a bisphenol A sulfone. PSU offers good heat resistance, chemical resistance, and dimensional stability.
- Polyethersulfone (PESU): PESU has a repeating unit of bisphenol S ether sulfone. It exhibits excellent heat resistance, hydrolytic stability, and chemical resistance, making it suitable for applications in harsh environments.
- Polyphenylene Sulfone (PPSU): PPSU has a repeating unit of biphenyl sulfone. It offers exceptional heat resistance, mechanical strength, and flame retardancy. It has the highest stress crack resistance and notched impact strength of all three types of Polysulfones.
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| Table 2: Comparison of the the different properties of PSU, PESU, and PPSU. |
- Injection molding: This is the most common method for producing polysulfone parts.
- Extrusion: Polysulfones can be extruded into sheets, films, and profiles.
- Blow molding: This process is used to produce hollow objects from polysulfone.
- Thermoforming: Polysulfones can be thermoformed into complex shapes.
- PSU: Automotive components, electronics, medical devices, and food processing equipment.
- PESU: Medical devices, electronics, water filtration, and chemical processing.
- PPSU: Aerospace components, electronics, medical devices, and high-temperature applications.
- BASF: Ultrason® E PESU; Ultrason® P PSU; Ultrason® S PPSU;
- Syensqo: Udel® (PSU), Veradel® (PESU), Radel® (PPSU)
- Sumitomo Chemical: Sumikaexcel® PESU
- Structure: Typically composed of only aromatic hydroxycarboxylic acid. Most common Type 1 PAR is the Poly-4-hydroxybenzoate (PHB) which consists of aromatic rings linked by ester groups.
- Properties: Known for their excellent heat resistance, dimensional stability, and chemical resistance. They often exhibit a balance of properties, making them suitable for a wide range of applications.
- Structure: Typically composed of terephthalic acid (TPA) and bisphenol A (BPA) or other aromatic dicarboxylic acids or bisphenols, such as isophthalic acid or bisphenol S. Most common Type 2 is the Polybisphenol-A terephthalate (PBAT).
- Properties: May exhibit enhanced properties in specific areas, such as higher toughness, lower water absorption, or improved flame retardancy. The choice of monomers can be tailored to meet specific application requirements.
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| Table 3: Structure and property comparison of Type 1 and Type 2 PAR. |
- High heat resistance: They have excellent thermal stability, allowing them to be used in high-temperature applications. BPAT has a Tg of 196°C (384.8 F), allowing for a short term usage up to 170°C. The heat deflection temperature (1.8 MPa) of BPAT is 174°C (345.2 F) and 40°C higher compared to PC (Tg=148°C; HDT = 135°C).
- Chemical resistance: They are resistant to a wide range of chemicals, including acids, bases, and solvents. They are especially resistant towards oils and alcohols when blended with PET, still keeping their transparency. Resistance towards alkali, ketones, and aromatic hydrocarbons is slow. Also, neat PAR resins are more sensitive to stress cracking.
- UV stability: The UV-stability of PARs is high, since UV radiation causes the formation of a protective layer which in turn serves as UV protection. PARs are able to prevent the passage of ultraviolet light at and below 350 nm and transmit almost 90% light at a wavelength of 400 nm or more. It could be shown in various 8,000-hour long-term tests that PAR, in contrast to PS and PC, retains its almost untarnished shine even under the influence of UV.
- Why is there such a high ultraviolet absorption capacity and high weathering durability? PAR absorbs UV energy and causes a so-called Fries rearrangement reaction. This reaction produces a benzophenone structure on the resin surface. As a consequence, PAR is able to block light of 400 nm or less. The tone of color (yellowing) changes of PAR too due to the Fries rearrangement.
- Mechanical strength: They have good mechanical properties, including high tensile strength, flexural strength, and impact resistance. In general, they can be placed between Polycarbonate and Polysulfones. Strength and stiffness are better compared to PC. Impact strength is high at lower temperatures (-40°C), however not as high as with PC. Additionally, the elongation at break is not as good as that of PC.
- Dimensional stability: They exhibit low shrinkage and excellent dimensional stability. PARs have excellent elastic recovery which makes them a suitable material for spring applications.
- Electrical properties: They have good electrical properties, such as high dielectric strength and low water absorption.
- Optical properties: almost as transparent as Polycarbonate and PMMA. PARs have a slight yellow color and transmit almost 90% light.
- Good for blending with other polymers:
- with PET and PETG, to decrease shrinkage and warpage of PET;
- with PC and PBT, in order to increase the thermal heat resistance.
- with PC, PET and PETG, to integrate a permanent UV protection.
- Injection moulding: This is the most common method for processing polyarylates, allowing for the production of complex parts. PARs have a high melt viscosity and processing is harder. As a result, flow enabler such as adding special groups or atoms, mineral filler/glass fiber reinforcement, and alloying with other polymers, resulting PAR/PET, PAR/PA, PAR/ PC is done. Since the structure of PAR is similar to PC, it can be processed by the same injection moulds.
- Extrusion: Polyarylates can be extruded into sheets, films, and profiles.
- Thermoforming: This process involves heating a sheet of polyarylate and forming it into a desired shape.
- Compression moulding: This method is suitable for producing large, thick-walled parts.
- Automotive industry: Components such as engine covers, under-hood parts, and interior trim.
- Electrical and electronics: Connectors, circuit boards, and housings.
- Medical devices: Surgical instruments, implants, and diagnostic equipment.
- Aerospace: Structural components and protective coatings.
- Industrial equipment: Pump housings, valves, and pipes.
- Unitika: U-Polymer®
- Westlake Plastics: Ardel® (only semi-finished products)
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Literature:
[1] https://www.syensqo.com/en/brands/ryton-pps
[2] https://www.plastics.toray/products/torelina/
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