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| High Performance Plastics Selection by Herwig Juster |
Hello and welcome to this blog post series on High Performance Thermoplastics selection.
Improper selection of plastics for the application is the leading cause for plastic part failure and since most parts fail along weld lines or knit lines, optimal mould design including filling and processing of the part are crucial too.
Furthermore, there are almost 100 generic “families” of plastics and additionally blending, alloying, and modifying with additives results in 1,000 sub-generic plastic types leads to the following crucial question: How should you choose the optimal polymeric material for your part? Especially, when there are high temperatures, high mechanical requirements, as well as high chemical resistance needs for your application are involved, selecting a high performance polymer will be the key to the solution.
I have chosen a holistic approach to answer the aforementioned questions and therefore structured this post series in five major parts (Figure 1):
1. Introduction to High Performance Polymers
2. Short profile of the "magnificent six" families:
-Polysulfides (Polyphenylene sulfide - PPS), Polysulfones (PSU, PESU, PPSU), and Polyarylates (PAR)
-Imide-Based Polymers (PEI, PAI, PESI, TPI, PI) and Polybenzimidazoles (PBI, PBI+PEEK, PBI+PEKK)
-Polyether (PPE, PAEK, PEEK, PEKK)
-Liquid Crystal Polymers (LCP)
-Semi- and Fully Aromatic Polyamides (PARA, PPA, Aramid)
-Polyhalogenolefins (PTFE, PCTFE, FEP, PVDF, ECTFE)
3. Key properties and design data for selection
4. Polymer Material Selection 4-stage funnel methodology (POMS-Funnel-Method)
5. Examples for Ultra- and high performance polymer selection
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| Figure 1: content of the high performance thermoplastics selection blog post series. |
Let us start with the introduction to high performance thermoplastics (HPTs).
Introduction to Ultra- and High Performance Polymers
Definitions and classification of high performance thermoplastics
High performance polymers, also often referred to as high heat polymers, can be defined over the continuous use temperature (CUT) by using the Underwriters Laboratory (UL) Relative Thermal Index (RTI). According to the UL 746B, high heat polymers need to withstand a continuous use temperature of 150°C for 100,000 hours (approx. 11 years), while retaining at least half of the initial properties afterwards.
Polymers such as PPS and PEEK inherently fulfill this requirement. Conversely, Polyphthalamides (PPA’s) need to be mechanically reinforced and thermal stabilized so that their continuous use temperature can rise from 130°C to 150°C. Most PPA’s have a continuous use temperature between 120°c and 130°C.
Figure 2 shows the continuous use temperature of commodity, engineering and high performance thermoplastics including the 150°C border.
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| Figure 2: Continuous Use Temperature (CUT) of thermoplastics with 150°C borderline in red for high heat plastics. |
Not only are high performance thermoplastics used for high temperature applications. There are low-temperature applications such as aircraft parts, oil rigs, industrial refrigeration, superconducting magnets, and liquid-helium devices, which are exposed to temperatures down to -270°C. Material selection becomes critical to prevent any part failure at such low service temperatures. At temperatures below -40°C, the choice for plastic materials becomes limited and fluoropolymers such as PTFE can be a solution. Most important property of fluoropolymers at low temperatures is their ductility: when reaching the absolute zero temperature point (-269°C), the ductility of these polymers holds at approximately 1%. All in all, fluoropolymers are a good material choice for static seals at low temperatures.
Although HPTs main purpose is to be used at elevated or low temperatures, they possess many other exploitable useful properties as well. For instance, crystalline polymers such as poly(ether ether ketone) and poly(phenylene sulfide) can be found in several room temperature applications due to their superior environmental resistance, in particular to organic solvents and acid and alkaline media.
Another definition is over the sales price. Due to their unique properties and added value, HPTs experience low-volume sales at a relatively high selling price. When you compare the ratio of sales price of aliphatic Polyamides to that of high heat polymers, this spreads from 1:3 to 1:20. These ratios vary with the markets the polymers are sold for i.e., automotive, aerospace, electrical-electronic and chemical process industries.
The classification of HPTs into amorphous and semi-crystalline polymers which is also known from commodity and engineering thermoplastics can be done with HPTs as well. Amorphous representatives are polysulfone (PSU), poly (ether sulfone) (PES), polyetherimide (PEI), thermoplastics polyimide (TPI), and poly(amide imide) (PAI). Semi-crystalline representatives are semi-aromatic Polyamide (PARA, PPA), poly (phenylene sulfide) (PPS), high performance polyesters (LCP, PCT), poly(benzimidazole) (PBI), fluoropolymers (PTFE, PFA/MFA), poly(ether ether ketone) (PEEK), and poly(ether ketone) (PEK). The latter, especially when filled with glass, carbon, and minerals keep useful mechanical properties above their glass transition temperature (Tg). PEEK, for example, has a Tg of 148 °C , however its continuous service temperature is 250 °C. Also, unfilled PAI is the polymer with highest tensile strength up to 260°C continuous use. PBI is the polymer with the highest Tg, 427°C. In addition, it does not burn. Overall, it is used in applications where highest demand in temperatures, harsh chemicals, and plasma environments are necessary, e.g. fire protection clothing. It is possible to cast PBI into a coating, film or membrane.
In terms of worldwide production, 400 million tons plastics were manufactured in 2022 [13], and 0.2% (0.8 million tons) can be related to high performance polymers.
History, manufacturing and first applications
The plastics boom time started back in the 1960s and in this time the movie hit “The Graduate” was launched. Mr. McGuire turned to Benjamin saying:” There's a great future in plastics. Think about it.”
And he was right. An early representative of high performance polymers was Poly Phenylene Sulfide (PPS), which was a byproduct of the chemical reaction of benzene and sulfur in the presence of aluminium chloride. It was discovered by Friedel and Crafts in 1888. However, in 1963 Edmonds and Hill developed a new way to produce PPS by using dihalogenated aromatics with sodium sulfide in a polar solvent. Industrialization of PPS succeeded in 1972 by using the Edmond and Hill process and Phillips Petroleum marketed it under the name Ryton®. Mr. Plunkette observed the formation of PTFE in 1938 in his laboratories at DuPont and this material, known as Teflon, quickly gained traction. Another example is the synthesis of poly(aryl ether ketones) (PAEK’s) by Johnson from Union Carbide in the late 1960’s. In 1962, DuPont introduced Polyimide (PI), sold under the name Kapton, to the markets and in 1965, Union Carbide launched the full scale production of Polysulfone (PSU), named Udel®. In the same year, the Amoco Chemical Corporation brought Poly Amide Imide (PAI), under the name Torlon®, to the plastics industry. Polyetehersulfone (PESU) followed in 1972 by ICI and 3M. PEEK was commercialised by ICI at their location in Hillhouse, United Kingdom in 1981. Also in 1981, Amoco introduced a PPA, under the brand name Amodel®. In 1982 General Electric Plastics, respectively J. Wirth introduced the polyetherimide (PEI) resin under the trade name Ultem®. One of the first applications of PEI was in head-lamp reflectors for cars. PEI can be easily metallized and has high dimensional stability at higher service temperatures. A remarkable application of Polysulfone was as sun protection helmet visor (outer visor) in astronauts’ helmets. Their main function was to protect the astronaut from sun exposure and high temperatures. For this, the inner surface of the polysulfone-based visor was also coated with gold for extended sun visor protection capabilities. The gold coating supported the protection against generated heat inside the helmet as well.
Base production principle for all the high performance thermoplastics mentioned before is the nucleophilic polycondensation, which is a chemical reaction between two functional groups and loosing low-molecular weight by-products such as water and alcohols. Important is to use high-purity monomers (>99%) in order to achieve high molecular mass polymers. Production of high performance plastics is technically more challenging compared to commodity polymers and therefore, for each polymer a dedicated reactor is needed. Manufacturing of compounds is done in-house for the major part, however chemical companies work together with toll-compounders to expand capacity in a fast way or to provide a local source of the compound.
Why can high performance handle high heat and harsh chemicals?
For better understanding the fundamental structure- property relationships of high performance polymers, we go back to the basic structure of a polymer such as a Polyethylene (PE). The main backbone consists of carbon-carbon bonds and on the carbons, hydrogens are also bonded. This linear macromolecule has a maximum temperature resistance of ca. 80°C and continuous use temperature of 50°C. PE with its aliphatic structure is prone to chain (unzipping) degradation reactions and therefore additives such as heat stabilizer need to be added. Exchanging the aliphatic elementary by aromatic, resistance to chain degradation can be achieved. Reason that the unzipping process is stopped with aromatic elements is that any free radicals generated are stabilized by the Pi-system of the aromatic ring.
If we replace the carbon in the main chain with a phenyl group, which is an aromatic cyclic group of atoms with the formula C6H5 we obtain the Polyparaphenylene (PPP) (Figure 3). In general, the Benzene ring will be appearing often in the chemical structure of high performance polymers since it forms the major backbone element. It is a ring of six carbon atoms and it is bonded by alternating single and double bonds, however the double is never clearly localized. This in turn makes it harder to be externally attacked.
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| Figure 3: Replacing carbon in the main chain with a phenyl group will result in Polyparaphenylene (PPP). |
PPP has a temperature resistance of 500°C and is a linear macromolecule made out of benzene building blocks. Aromatic structures result in high macromolecule stiffness. Aromatics in the backbone are a main driver to obtain high heat and chemical resistance. The detailed look at the structure of benzene reveals that the double bonds are not statically localized, i.e. electrons move along the carbon cyclic structure, which is expressed by the ring in the structural formula. This together with inherent molecular stiffness supports stability at high temperatures and in contact with chemicals. If we alternate benzene rings and amide groups, we will get a polymer called poly para-phenyleneterephthalamide (PPTA) or more common known as Aramid and has a heat resistance of 250°C (decomposition temperature between 430-480°C; peak temperature use up to 400°C). It can be processed to fibers and makes it a perfect material for personal protective equipment for firefighters and armed forces.
7 basic building blocks of high performance thermoplastics
The high thermal resistance of PPP has one major downside, i.e. it makes it unsuitable for all melt-based processing techniques such as injection moulding and extrusion. However, the integration of heteroatoms such as Oxygen, Nitrogen, and Sulfur in a polymeric aromatic-based structure can change this. Following, common chemical groups which are used to make melt-processable high performance polymers are described.
1. Diphenyl ether group (Figure 4): In this case, oxygen is the linkage of two phenyls. Diphenyl ether groups are used for example in Polyaryletherketones (PAEK’s). Also, as Phenyl ether group it is used to make Phenyl ether polymers such as PPE+PS (Noryl®).
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| Figure 4: Diphenyl ether group. |
2. Diphenylsulfone group (Figure 5): Here, sulfur is double-bonded to oxygen as well as bonded to phenyls. Diphenylsulfone groups are the main building block for Polysulfone (PSU), Polyethersulfone (PESU) and Polyphenylsulfone (PPSU).
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| Figure 5: Diphenylsulfone group. |
3. Diphenylketone group (Figure 6): Oxygen is bonded over a double bond to carbon resulting in a carbonyl group. Together with the diphenyl, it forms the ketone group. The ketone group is the second crucial element for obtaining Polyetheretherketones (PEEKs).
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| Figure 6: Diphenylketone group. |
4. Diphenylsulfide group (Figure 7): Here, sulfur is linked to phenyls and forming the sulfide group. It forms the basis of Polyphenylensulfide (PPS).
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| Figure 7: Diphenylsulfide group. |
5. Imide group (Figure 8): It consists out of two acyl groups (R-C=O) bounded to nitrogen. It is the base element of Polyimides (PIs), Polyamideimides (PAIs), and Polyetherimides (PEIs).
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| Figure 8: Imide group. |
6. Terephthalic acid (TPA) and Isophthalic acid (IPA) (Figure 9): It is used as precursor for making Polyethylene terephthalate (PET). It also forms the monomer for Polyphthalamides (PPAs). Two carboxyl groups are attached to a benzene in a 1,4 or 1,3 configuration.
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| Figure 9: Terephthalic acid (TPA) and Isophthalic acid (IPA). |
7. Fluor-carbon group (Figure 10): the fluor-carbon bond is the most stable single bond with 485 kJ/mol bonding energy (in comparison, carbon-carbon bond has 350 kJ/mol). Additionally, the fluor atom is much larger compared to the carbon forming a protecting layer around the carbon-carbon main chain. This explains to the same extent the high chemical and thermal stability of fluoropolymers such as PTFE and PVDF.
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| Figure 10: Fluor-carbon group. |
What is their market size and role within the plastics industry?
In 2021, globally 390 million tons of plastics were produced and this represents 1 vol% (0.4 wt%) of the 90 billion tons of materials which include ceramics, metals, and many more. 90% of the 390 million tons are commodity plastics (351 million tons) and 9.8 % are engineering plastics (38 million tons). High performance thermoplastics represent around 0.2% (78.000 tons) of the global plastics production. Demand growth of high performance thermoplastics is driven by several industries such as automotive (transformation to e-mobility), semiconductor (ensuring the next generation of chips), air, space, and defence industry, as well as electric and electronics.
Research & Innovation, sales, and marketing of high performance thermoplastics
Research and development, together with innovation play a key role in the business of high performance polymers. Growth is driven by presenting new material solutions to the market and therefore, an important part of the budget is dedicated to such activities.
Basically, there are three major routes the high performance polymer manufacturer are taking to drive innovate material growth forward:
1. New composition: new base polymer combined with additives to have a new compound
2. Extension of existing product line: using existing base polymers and combine it with new additives to achieve new functions such as thermal conductive or blend to existing base polymers to lower costs or have for example better mechanical properties
3. Synthesis of base monomers and polymers: focus to improve the synthesis of monomers in order to lower costs and improve process technology too.
There are combinations of the three paths and management can steer depending on which economical cycle we are, to focus on one or the other path. Base for all developments is an in-depth understanding of the end-customer requirements which need to be fulfilled with the material solution. Application development, together with customers can range between 6 months in industries such as mobile devices, three to five years in automotive industry, and up to 10 years in medical and aircraft/aerospace applications due to regulatory fulfilment.
Sales and marketing of high-performance plastics is technically intensive, especially in the early phases of plastic product development. Specialized sales teams with a focus on the end user market such as automotive industry, consumer, medical technology are needed. And also, high performance plastics sales teams have a greater technical orientation compared to commodity plastic sales teams. For example, material manufacturers which provide solutions to extend the injection tool life, improve yield and provide support in product development generate more value for the customer than a material manufacturer who provides just a discount on the material.
Stay tuned for part 2 “Short profile of the "magnificent six" families” where we will start with Polysulfides (PPS, Polysulfones) and Polyarylates.
Thanks for reading and #findoutaboutplastics
Greetings,
Herwig
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Literature:
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[8] http://pbipolymer.com/about/celazole-pbi-advantage/
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[12] Kaiser: Kunststoffchemie für Ingenieure
[13] https://plasticseurope.org/knowledge-hub/plastics-the-fast-facts-2023/
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