Polyether ether ketone (PEEK) is a high-performance polymer increasingly replacing traditional materials like metals and thermoset plastics in demanding environments. It is recognized for its unique combination of mechanical strength, thermal stability, and chemical resistance, allowing it to function where conventional plastics would fail. PEEK provides engineers with a lightweight yet robust alternative for highly stressed components. Its exceptional characteristics have made it a material of choice across industries requiring reliability under extreme operating conditions.
Defining PEEK
PEEK is an acronym for Polyether ether ketone, belonging to the family of polyaryletherketone (PAEK) polymers, often classified as “super engineering plastics.” This material is a semicrystalline thermoplastic, possessing both organized, crystal-like regions and amorphous regions within its structure. As a thermoplastic, PEEK can be repeatedly melted and reformed without chemical degradation, which facilitates its manufacturing into complex parts.
The polymer’s name reflects its molecular structure, which consists of aromatic rings connected by repeating ether and ketone linkages. These specific chemical groups—three aromatic rings, two ether linkages, and one ketone group in each repeating unit—provide the molecular backbone with inherent stability and rigidity. The aromatic rings contribute to thermal and mechanical strength, while the ether groups introduce flexibility and toughness.
PEEK was first developed in 1978 by researchers at Imperial Chemical Industries (ICI) in the United Kingdom, with commercialization following in the early 1980s. The polymer is synthesized through step-growth polymerization, where monomers are reacted at high temperatures, typically around 300°C, to form the long polymer chains.
Unique Physical and Chemical Properties
Mechanically, PEEK exhibits excellent stiffness and high tensile strength, with unfilled grades often showing approximately 90 to 100 megapascals (MPa). It also has high resistance to creep, which is the tendency of a material to permanently deform under mechanical stress over extended periods, especially at elevated temperatures.
PEEK has a high glass transition temperature of 143°C and a sharp melting point of 343°C. This allows it to remain structurally stable and functional at continuous operating temperatures up to 260°C. Furthermore, the material releases very low levels of smoke and toxic gases when exposed to fire, contributing to its safety profile in enclosed applications.
Chemically, PEEK offers exceptional inertness, resisting degradation from a vast array of organic and inorganic solvents, acids, and bases. This resistance includes hydrolysis, allowing components to withstand prolonged exposure to hot water or steam without significant property loss, making PEEK highly sterilizable. Additionally, PEEK exhibits excellent wear resistance and a low coefficient of friction, especially when compounded with reinforcing fillers like carbon fiber or PTFE, making it suitable for dynamic, load-bearing applications.
Primary Industrial Applications
In the aerospace and automotive sectors, PEEK’s combination of low density and high mechanical strength is leveraged to produce lightweight components, often replacing aluminum and other metals. Replacing metal with PEEK in parts like brackets, fasteners, and engine components helps reduce overall vehicle weight, contributing to fuel efficiency and lower emissions. Its high continuous use temperature and resistance to jet fuel and hydraulic fluids make it ideal for demanding under-the-hood and structural applications.
In the medical field, PEEK is valued for its biocompatibility, which means it does not provoke a harmful response when in contact with living tissue. This property, combined with its resistance to sterilization methods like steam and radiation, makes it suitable for surgical instruments and implantable devices. PEEK is commonly used for spinal fusion cages and dental components, partly because its mechanical properties—specifically its flexural modulus—are similar to that of human cortical bone, which can promote better integration.
The oil and gas industry utilizes PEEK for components that must endure exceptionally harsh downhole conditions. Its resistance to high pressure, corrosive chemicals, and high temperatures makes it ideal for seals, compressor plates, and electrical connectors in exploration and extraction equipment. The electronics and semiconductor manufacturing industries also rely on PEEK for its purity, electrical insulating properties, and resistance to aggressive processing chemicals. It is used in components like wafer carriers and insulation films, where minimizing contamination is paramount.
Processing and Fabrication Methods
PEEK is a melt-processable polymer, allowing it to be shaped using manufacturing techniques common to other thermoplastics, despite its high melting point. The most common method for high-volume production of intricate parts is injection molding, where the molten polymer is injected under high pressure into a mold cavity. This process requires specialized machinery capable of reaching and maintaining temperatures up to 400°C to handle PEEK’s high melt temperature.
For larger, simpler shapes, or when working with reinforced grades, compression molding is frequently employed. In this technique, PEEK powder or pellets are placed in a mold and consolidated under heat and pressure.
Extrusion is another standard method, primarily used to create continuous profiles such as rods, sheets, and tubes. These shapes are often used as stock material for subsequent machining.
PEEK components often require high precision, meaning parts are commonly finished using computer numerical control (CNC) machining after the initial molding or extrusion process. More recently, PEEK has become a focus in additive manufacturing, or 3D printing, specifically using fused filament fabrication (FFF). This technique allows for the creation of highly complex geometries and custom parts, benefiting low-volume, high-value applications like medical implants.