High temperature thermoplastics are a class of polymers that can withstand extreme temperatures without compromising their structural integrity. They are becoming increasingly important in various industries, including aerospace, automotive, and electronics, due to their ability to maintain their properties under extreme conditions. This article aims to provide an overview of different types of high temperature thermoplastics, their properties, advantages, disadvantages, and applications.
What Makes Polymers Thermally Resistant
The exceptional thermal resistance of most high temperature thermoplastics stems from the integration of rigid aromatic rings within their molecular architecture. Unlike aliphatic structures where severing the polymer chain requires breaking only a single chemical bond, these aromatic rings necessitate the rupture of two chemical bonds before the chain can be broken (Figure 1). This fundamental structural distinction creates a significantly higher energy barrier for chain degradation, thereby enhancing the polymer’s ability to withstand high temperatures.
Figure 1. This figure shows the structural differences between nylon 6/6, an engineered thermoplastic, and polyphthalamide, a high-temperature thermoplastic. The key distinction is the presence of an aromatic ring in the polyphthalamide structure.
Other high temperature thermoplastics, such as fluoropolymers, derive their exceptional heat resistance from the incorporation of fluorine molecules replacing hydrogen along the carbon backbone (Figure 2).
Figure 2. This figure shows the chemical differences between polyethylene, a commodity thermoplastic, and ethylene tetrafluoroethylene, a fluoropolymer. Fluoropolymers, achieve their remarkable heat resistance by substituting hydrogen atoms along the carbon backbone with fluorine molecules.
Types of High Temperature Thermoplastics
The category of high temperature thermoplastics encompasses a diverse range of materials, including polyaryletherketones (PAEK), sulfones, polyphenylene sulfide (PPS), polyphthalamides (PPA), imide-based polymers, liquid crystal polymers (LCPs), and melt-processable fluoropolymers. A more detailed examination of these specific polymer families and individual materials within them will follow.
Before diving into these high-temperature thermoplastics, let’s review the fundamental differences between amorphous and semi-crystalline polymers, as these distinctions will be relevant to our discussion of high-temperature materials.
Generally, amorphous polymers offer lower chemical resistance and maximum use temperatures but exhibit higher toughness and tighter tolerance capabilities. Conversely, semi-crystalline polymers demonstrate superior chemical resistance and higher maximum use temperatures but tend to have lower toughness and experience more mold shrinkage (Figure 3). Among the high-temperature thermoplastics we will discuss today, sulfones and imides are amorphous, while the rest are semi-crystalline.
Figure 3. General differences between amorphous and semi-crystalline polymer types.
Polyaryletherketones (PAEKs)
PAEKs are a class of semi-crystalline high temperature thermoplastics that include several different polymers with similar but distinct structures. They use aryl, ether, and ketone functional segments. The specific type of polyarylketone depends on the use and placement of these function segments (Figure 4).
Figure 4. Illustrates the differences in chemical structures of the various polyaryletherketones.
Their thermal resistance is determined by the ketone/ether ratio: the higher the ratio, the higher the glass transition and melting point. The strength, stiffness, and creep resistance of PAEKs increase with their ketone/ether ratio, while their ductility and impact resistance decrease. The key advantages and disadvantages of PAEKs are outlined in the table below.
Sulfones
Sulfones are amorphous high temperature thermoplastics that represent an upgrade to acrylics, stryenics and polycarbonates. This family includes various polymers with similar but distinct structures, such as sulfone groups, aromatic rings, ether linkages, and methyl groups (Figure 5).
Figure 5. Illustrates the differences in chemical structures between the different types of sulfones.
Sulfones, being amorphous, derive their thermal resistance from their glass transition temperature. Polysulfone exhibits the lowest glass transition temperature at around 187°C. Polyethersulfone and polyphenylene sulfone are the same at 220 C. In general, PSU and PES are stiffer and stronger than PPSU, while PPSU is tougher. PPSU has the highest chemical resistance, especially to cleaning and disinfecting agents. The key advantages and disadvantages of sulfones are outlined in the table below.
Polyphenylene Sulfide (PPS)
PPS is a semi-crystalline high temperature thermoplastic known for its high modulus, hydrolysis resistance, high chemical resistance, high thermal stability, low viscosity, and flame retardancy. It is commercially available in two varieties: branched and linear. PPS must be filled with fibers and fillers to overcome its inherent brittleness. The glass transition temperature of PPS is 110 °C, while its melting point is 290 °C. PPS exhibits excellent stiffness and strength up to ~100 °C. Above the Tg, these properties begin to experience large decreases. The key advantages and disadvantages of PPS are outlined in the table below.
Polyphthalamide (PPA)
PPA, commonly used in reference to semi-aromatic nylons, is a semi-crystalline high temperature thermoplastic with good chemical resistance and high thermal resistance. These materials are produced through a condensation reaction involving aliphatic diamines and aromatic diacids (like isophthalic acid or terephthalic acid), unlike aliphatic nylons like nylon 6 or nylon 6/10, which use aliphatic diacids (Figure 6).
Figure 6. Illustrates the differences in the polymerization of an aliphatic nylon versus a semi-aromatic nylon.
These semi-aromatic polymers have high melting points and are difficult to process, so manufacturers can modify their chemistry by adding comonomers. The term “PPA” can refer to different chemistries depending on the manufacturer. For example, Amodel® and Akromid® T5 are based on PA6T, while Akromid® T9 is based on PA9T.
The glass transition temperatures and melting points of PPA are dependent on their final polymer chemistry. Glass transition temperatures can range from 82 to 165°C, while melting points can fall between 229 and 325°C. It’s essential to remember that not all PPAs are identical, so paying close attention to these thermal transitions is crucial when considering these materials. Like many semi-crystalline polymers, PPA exhibits excellent stiffness and strength up to the Tg. Above the Tg, these properties begin to experience large decreases.
Like all polyamides, PPAs are hygroscopic, meaning they have an affinity for water. This affinity leads to water absorption, which can negatively impact the material’s mechanical properties. The extent of water absorption depends on the exposure environment and the specific polymer chemistry. The key advantages and disadvantages of PPA are outlined in the table below.
Imides
The imide family of amorphous resins includes polyamide-imide (PAI), polyetherimide (PEI) and polyimide (PI). Polyimide itself cannot be melt-processed so it will be excluded from further discussion. Since these materials are amorphous, their thermal resistance will be largely determined by their glass transition temperature. As shown in the table, glass transition temperature of PAI at 280 °C is significantly greater than that of PEI a 217 °C.
PAI exhibits remarkable strength and rigidity at elevated temperatures, even nearing its glass transition point, exhibiting significantly greater strength than PEI across all temperatures. It also displays exceptional resistance to wear, a wide range of chemicals, and high-energy radiation. PEI is known for its good chemical resistance, heat resistance and high strength. PEI is inherently flame resistant and resistant to UV light. The key advantages and disadvantages of imides are outlined in the table below.
Liquid Crystal Polymer (LCP)
LCPs are unique in that they remain well-ordered even in the melt and slide past each other easily under shear. The result is very low melt viscosity making it easy to fill extremely thin walls and to replicate intricate features. LCPs exhibit little or no shrinkage in the flow direction and require very little time to solidify. This enables increased productivity due to quick fill and shorter cycle times. The thermal properties of LCP will be determined by its chemistry. The higher percentage of hydroxybenzoic repeating units (Figure 7), the higher its heat resistance, including its melting point.
Figure 7. Illustrates the chemical structure of LCP where the percentage of hydroxybenzoic and hydroxynaphthenoic repeating units can vary.
The mechanical properties of the LCP will also be dependent on its chemistries. LCPs are resistant to most acids, bases, organic solvents, alcohols, ketones, esters, and chlorinated hydrocarbons. Strong oxidizing agents and very strong alkalis can attack the polymer. The key advantages and disadvantages of LCPs are outlined in the table below.
Fluoropolymers (Melt Processable)
Melt-processable fluoropolymers, such as perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), and ethylene tetrafluoroethylene (ETFE) (Figure 8), offer a range of beneficial properties.
Figure 8. Illustrates the chemical structure of melt-processable fluoropolymers.
PFA, a copolymer of tetrafluoroethylene (TFE) and a perfluoroalkyl vinyl ether, effectively combines the best attributes of PTFE and FEP, exhibiting a higher service temperature than FEP, greater tensile strength than PTFE, and a smoother surface finish than both. FEP itself is a copolymer of hexafluoropropylene (HFP) and tetrafluoroethylene (TFE). Lastly, ETFE, a copolymer of ethylene and tetrafluoroethylene, is recognized for its durability and resilience.
These fluoropolymers are considerably weaker, although more flexible and tougher than the high-temperature thermoplastic materials reviewed thus far. These materials have exceptional chemical resistance, meaning they are highly resistant to nearly all chemicals, including strong acids, alkalis, organic solvents, and most corrosive substances. They retain their chemical resistance even at high temperatures. The key advantages and disadvantages of melt-processable fluoropolymers are outlined in the table below.
Conclusion
In conclusion, the diverse landscape of high temperature thermoplastics offers a remarkable array of solutions for applications demanding performance under extreme thermal conditions. From the rigid aromatic structures of polyaryletherketones and sulfones to the unique fluorine-based chemistry of melt-processable fluoropolymers, each material family provides a distinct profile of thermal stability, mechanical strength, chemical resistance, and processability. As detailed, understanding these individual properties, along with their respective advantages and disadvantages—whether amorphous or semi-crystalline—is crucial for engineers and designers. The optimal selection ultimately hinges on a careful evaluation of specific application requirements, balancing performance needs with economic and manufacturing considerations, ensuring these advanced polymers continue to drive innovation across demanding industries.
