Sunday, 30 September 2018

High Heat Plastics (HHP) Demystified incl. Cheat Sheet

Introduction to high heat plastics
High heat plastics (HHP’s), as part of the specialty polymers group, found their ways in several industries from automotive to medical devices. As their name already suggests, these are able to continuously withstand high heat conditions. Generally, thermoplastic and/or thermoset polymers which maintain useful mechanical properties at temperatures in the range of 150°C and above [1, 2] can be defined as HHP’s. Furthermore, HHP’s exhibit high strength, toughness and long-lasting properties even when several doses of different types of radiation for sterilization are applied [4].
Due to their unique properties and added value, HHP’s experience low-volume sales at a relatively high selling price [2]. When you compare the ratio of sales price of aliphatic nylons to that of high heat polymers, this spread 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. Although HHP’s main purpose is to be used at elevated temperatures, they possess many other exploitable useful properties as well. For instances, 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 [2].
Nowadays, specialty polymers account for approximately 0.3% of the global polymer production volume. For examples, in 2016, the global production of plastics summed up to approximately 335 million metric tons of which one million metric tons were specialty polymers.

A bit of background on how it all started
A good example of how researchers learned and applied the aforementioned properties and principles was the replacement of natural silk by using nylon 6, 6 and nylon 6. Nylon was introduced by Wallace H. Carothers of DuPont and Paul Schlack of I.G. Farben in the late 1920’s. Silk is an expensive material which has superior quality and performance. The first synthetic fibers were expensive too, since polymer science and engineering was still in its children shoes. Nevertheless, challenges were progressively overcome which paved the way to produce synthetic fibers in high quality, quantity and at low cost. Several more high performance plastics were one after the other explored and commercialized over the following decades. An early representative was poly(phenylene sulfide), which was a byproduct of the chemical reaction of benzene and sulfur in the presence of aluminum chloride by Friedel and Crafts in 1888. In 1982 General Electric Plastics, respectively J. Wirth introduced the polyetherimide (PEI) resin under the trade name Ultem which was. Another example is the synthesis of poly(aryl ether ketones) (PAEK’s) by Johnson from Union Carbide in the late 1960’s.

High temperature plastics grew up – Classification of HHP’s
The classification into amorphous and semi-crystalline polymers which is also known from commodity and engineering thermoplastics can be done with HHP’s as well. Amorphous representatives are polysulfone (PSU), poly (ether sulfone) (PES), polyetherimide (PEI) and poly(amide imide) (PAI). Semi-crystalline representatives are semi-aromatic Nylons (PARA, PPA), poly (phenylene sulfide) (PPS), high performance poylesters (LCP, PCT), 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 but its continuous service temperature is 250 °C. An overview of classification by a plastics thermometer is shown in Figure 1.
Figure 1: High heat plastics thermometer.

Why can certain thermoplastics withstand high temperature loads?
The answer can be found in the chemical composition. Key building blocks are, for example, aromatic rings and carbon-oxygen double bonds. In this context, polymer performance can be tailored during synthesis by balancing the ratio of rigid, non-contorted units such as aromatic rings to flexible, easily-contorted units such as aliphatic bonds. HHP’s are usually produced by means of step-growth polymerization processes, i.e., polycondensation and polyaddition [3]. These allow greater design freedom and properties control than chain-growth polymerizations.
The replacement of aliphatic units with aromatic ones imparts increased resistance to chain degradation by heat and associated oxidation in the resulting polymers. While eventually formed free radicals cannot be stabilized by surrounding bonds in aliphatic polymers, these are easily stabilized by resonance in aromatic polymers. As a result, chain scission and degradation is prevented. Accordingly, a complete aromatic polymer such as polyparaphenylene should show an optimum in stability. Investigations have shown that it is thermally stable above 500°C [4]. The biggest downside is its inherent unprocessability, a result of the high stiffness of its chains. To keep up with processability demands, flexible linkages such as C-O, C-S, C-C are incorporated in HPP’s.
Figure 2 shows the continuous use temperature of commodity, engineering and high heat plastics [2].

Figure 2: Continous Use Temperature (CUT) of thermoplastics with 150°C borderline in red for high heat plastics.

“Ultra polymers” – hidden champions among HHP’s?
On the very upper end of our plastics thermometer you can find the thermoplastics polyimide (TPI) and poly(amide-imide) (PAI) as amorphous representatives and poly(benzimidazole) (PBI) as a semi-crystalline representative. These polymers are regarded as “Ultra polymers” due to their outstanding thermal and mechanical properties. For examples, 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 [6, 7].

Who are the main suppliers of HHP?
In the table below, an overview of the major suppliers of HHP’s is given. It should be seen as a living document which can change over the years since chemical companies merge or sell certain portfolios.

My HHP cheat sheet – what you find in there
I tried to capture the most interesting and important material data and transformed it into a cheat sheet which allows you to have all in one infographic. It has three property sections (mechanical, thermal and processing) and represents the following polymer groups:
• Polysulfones
• Polyimides
• Polyphylensulfides
• Semi-aromatic Nylons
• Polyaryletherketones
• Liquid Crystal Polymers
• Fluoropolymers

This time it was a longer post and I hope you have enjoyed this blog post. The cheat sheet can be useful e.g. for a first comparison in the material selection phase.

What are your experiences with HPP’s? Leave a comment below!

Thank you for reading and till next time!
Greetings, Herwig Juster

P.S. New to my blog – check out the start here section

[2] Vinny Sastri: Plastics in medical devices
[3] D. Parker, J. Bussink, H. van de Grampel, et al., Polymers, High-Temperature, Ullmann’s Encyclopedia of Industrial Chemistry, DOI: 10.1002/14356007.a21_449.pub3
[4] Raymond B. Seymour and Gerald S. Kirshenbaum: High Performance Polymers: Their Origin and Development
[6] Johannes Karl Fink High Performance Polymers, Second Edition (Plastics Design Library)Jul 1, 2014

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