What is polyether ether ketone (PEEK) polymer?
Polyether ether ketone (PEEK) is a high-performance thermoplastic that engineers choose when they need to meet multiple critical requirements simultaneously—a feat difficult for most other materials to achieve. While metals, standard polymers, and composites may excel in specific areas, PEEK performs outstandingly across a range of attributes, making it the material of choice for complex, high-risk applications.
When metals, standard polymers, and composites fall short, PEEK solves three critical material challenges faced by engineers:
1.Extreme temperature resistance: A continuous service temperature of 260°C enables it to perform in applications such as automotive transmissions, sections of aircraft engines, and downhole oil and gas environments—settings where other polymers would soften or degrade.
2.Chemical inertness: Resists acids, bases, organic solvents, and hydrocarbons across a wide range of pressures and temperatures—critical for applications in chemical processing, medical sterilization, aerospace, and the energy industry.
3.Equivalent metal strength at a 40% weight reduction: several times higher than the specific strength and specific stiffness of metals and alloys, enabling lightweighting without compromising performance.
In the plastics industry, PEEK is widely regarded as a leading high-performance polymer (HPP). However, for a long time, metal has been the material of choice for the automotive, aerospace, oil and gas, and medical device industries. PEEK polymers are rapidly changing this perception.
PEEK, or polyether ether ketone, belongs to a family of polymers known as "aromatic polyketones" (or, more precisely, "polyaryletherketones" or PAEKs). This means it is composed of the following structural units:
Research and development of polyaryletherketones (PAEKs) began in the 1960s, but it was not until 1978 that Imperial Chemical Industries (ICI) filed a patent application for polyetheretherketone (PEEK); the product was first commercialized in 1981 as Victrex PEEK polymer.
The term "aromatic" typically refers to a distinctive or sweet scent—a usage that might seem odd in this context—but scientists employ it to describe molecules containing or composed of cyclic structures (such as the aryl structural units mentioned earlier). Small molecules of this type, such as toluene and naphthalene, possess distinctive odors, hence the name. However, polyether ether ketone (PEEK) itself—like most thermoplastics—is odorless under normal conditions. Chemically speaking, PEEK is a predominantly linear, semi-crystalline polymer. The combination of these structural units yields the ether-ether-ketone (EEK) structure.
The "repeating unit" shown in the brackets above is replicated multiple times—on average, 200 to 300 times—to form a complete PEEK polymer chain. The "P" is derived from the Greek word "poly," meaning "many," signifying that a multitude of EEK units constitute PEEK. The aryl and ketone groups are highly rigid, imparting stiffness to the material and resulting in a combination of excellent mechanical properties and a high melting point. The ether group provides a degree of flexibility to enhance toughness; like the aryl and ketone groups, it is chemically inert, thereby contributing to the material's chemical resistance. The regular structure of the repeating unit allows PEEK molecules to undergo partial crystallization, and this crystallinity endows the material with wear resistance, creep resistance, fatigue resistance, and chemical resistance—aspects that will be discussed in greater detail later.
The resulting polymer is widely regarded as one of the highest-performing thermoplastics in the world. Compared to metals, polyetheretherketone (PEEK)-based materials are extremely lightweight, easy to process, and corrosion-resistant, while also possessing a much higher specific strength (strength per unit weight).
During the production of polyether ether ketone (PEEK), a process is employed to control chain length—and thus molecular weight. Long-chain PEEK (high molecular weight or high MW) offers superior toughness and impact resistance compared to short-chain PEEK. However, high-molecular-weight polymers exhibit extremely high melt viscosity, which limits their ability to fill small molds. While low-molecular-weight PEEK has lower impact resistance, it offers better melt flow, making it easier to manufacture small, precision parts.
PEEK vs. Ultem (PEI): Which High-Performance Plastic Is More Suitable?
When application requirements exceed the temperature limits of standard engineering plastics, PEEK and Ultem PEI are the two most common choices. Both materials are FDA-compliant and meet USP Class VI standards; they achieve a UL 94 V-0 flammability rating without additives and offer high machining precision and excellent surface finish. However, they differ significantly, and selecting the wrong material can lead to serious engineering errors. PEEK is a semi-crystalline material suitable for continuous use at 480°F (250°C), whereas Ultem is an amorphous, amber-transparent material with a maximum service temperature of approximately 340°F (171°C). There is a two- to threefold difference in cost between the two.
1.Temperature: PEEK withstands continuous temperatures of 480°F, whereas Ultem has a maximum service temperature of 340°F—a 140°F difference that is critical for autoclave and structural applications.
2.Structure: PEEK is semi-crystalline (offering excellent chemical resistance and fatigue strength); Ultem is amorphous (offering superior dimensional stability under heat and a transparent amber appearance).
3.Strength: PEEK has a tensile strength of approximately 14,500 psi; Ultem 1010 is approximately 15,200 psi (slightly higher), though PEEK’s strength remains closer to its operational limits.
4.Flame Retardancy/Food Contact Certification: Both meet the UL 94 V-0 standard; standard grades comply with FDA 21 CFR and USP Class VI standards.
5.Cost: Ultem sheets and rods typically cost 40%–60% of equivalent PEEK products, offering significant cost savings for high-volume production or large-cross-section parts.
6.Transparency: Ultem sheet is transparent amber; PEEK is opaque tan/beige.
7.Chemical Resistance: PEEK is resistant to almost all organic solvents, whereas Ultem is susceptible to attack by halogenated solvents and strong acids—a key distinction.
Specification Comparison
1.Thermal properties
The glass transition temperature of PEEK is approximately 289°F (143°C); however, as a semi-crystalline material, it retains good mechanical properties even well above this temperature. Its melting point is approximately 644°F (340°C). Ultem PEI has a glass transition temperature of approximately 419°F (215°C)—higher than that of PEEK—but because Ultem is an amorphous material, it gradually softens as the temperature approaches the glass transition point, lacking the structural support provided by a crystalline phase. This difference is also reflected in their continuous service temperature ratings: 480°F for PEEK and 340°F for Ultem.
2.Mechanical Properties
At room temperature, the tensile strength of Ultem 1010 is slightly higher than that of natural PEEK (15,200 psi versus 14,500 psi), while their impact resistance is comparable. Both materials exhibit similar notched Izod impact strength values, hovering around 1.0 ft-lb/in. PEEK has a flexural modulus of 600,000 psi, which exceeds Ultem 1010’s 480,000 psi; this indicates that PEEK possesses greater rigidity—a factor to consider for thin-walled structures subjected to bending loads.
At high temperatures, this disparity reverses sharply. Polyetheretherketone (PEEK) components retain their structural utility at 400°F, whereas Ultem components at the same temperature are already approaching their practical service limits.
3.Electrical Properties
Both materials are excellent electrical insulators, with dielectric strengths exceeding 400 V/mil. At a frequency of 1 MHz, Ultem exhibits a slightly superior dielectric constant (3.15 versus approximately 3.3 for PEEK), making it better suited for high-frequency electrical applications. In applications where electric arcing is a potential failure mode, PEEK offers superior arc resistance (180 seconds) compared to Ultem.
4.Overview of Chemical Resistance
PEEK exhibits excellent corrosion resistance: it withstands virtually all organic solvents, dilute and concentrated inorganic acids (with the exception of concentrated sulfuric acid above 150°F), hydrocarbons, hydraulic fluids, and chlorinated solvents. Its semi-crystalline structure is inherently more resistant to solvent attack than that of amorphous polymers, as the crystalline regions act as barriers to solvent diffusion.
Ultem is susceptible to attack by dichloromethane, trichloroethylene, concentrated acids, and halogenated solvents. It exhibits good resistance to alcohols, water, dilute aqueous solutions, and mild process chemicals. For applications involving chemical exposure, please request a chemical resistance chart from the material supplier to obtain comprehensive, fluid-specific compatibility data before finalizing specifications.
5.Machinability and Manufacturing Processes
Both PEEK and Ultem can be readily machined using carbide tools. PEEK is the more commonly machined of the two materials; it offers well-established machining recommendations, excellent chip evacuation, and predictable dimensional stability. Ultem shares similar machining characteristics but is slightly more notch-sensitive in thin-walled sections; load-bearing Ultem components should be designed to avoid sharp internal corners. Both materials achieve standard surface finishes and can be tapped and threaded without specialized tooling. Neither material requires post-processing after machining.
If your application involves contact between the polymer and any halogenated cleaning solvents or concentrated acids, please verify the chemical resistance of Ultem prior to specification. For scenarios involving extensive chemical exposure, PEEK is the safer choice.
6.Cost and Availability
The price of standard natural-grade PEEK sheet and rod is typically two to three times that of equivalent Ultem 1010 stock. While this price gap may narrow slightly when considering machined parts—since machining times for both materials are similar—material cost remains the dominant factor for most parts requiring stock removal. This price premium reflects the more demanding synthesis processes and processing conditions required for PEEK. Major plastic distributors stock both materials; sheets are typically available in thicknesses ranging from 0.125 to 4 inches, and rods in diameters from 0.25 to 6 inches, whereas tubing is available but less commonly stocked.
Standard sizes are typically available from distributors with short lead times—just a few days rather than weeks. For custom profiles and large-diameter profiles, however, factory lead times for both materials can range from 4 to 8 weeks.
Common Alternatives
If neither PEEK nor Ultem meets your needs, you might consider:
PEEK vs. Torlon (PAI)—Although post-curing adds processing complexity, Torlon offers superior tensile strength and wear resistance compared to PEEK.
Ultem vs. Polysulfone—Polysulfone offers similar autoclave performance at a lower cost; PPSU raises the service temperature to 360°F.
PEEK vs. PTFE—PTFE becomes a viable option when chemical resistance and non-stick properties are more important than structural strength.