Polyetherimide (PEI): A Review of its Properties, Processing, and Applications

Introduction to Polyetherimide (PEI)

Polyetherimide (PEI), often recognized by its trade name Ultem®, is a high-performance thermoplastic polymer. It belongs to the class of high-performance materials based on imides, amides, and their combinations. For instance, a common commercially available grade of PEI is Ultem-1000.

PEI is a critically important material, particularly for industries such as aerospace and electronics. It is also highly suitable for the creation of heat-resistant composite materials with high physical and mechanical properties.

Differentiating Polyetherimide (PEI) from Polyimides (PI)

PEI is essentially a structurally modified version of Polyimide (PI) developed to overcome a primary limitation associated with traditional PIs: poor processability. Polyimides, in general, are characterized by excellent thermal and heat stability, excellent mechanical properties, including high tensile strength and elastic modulus, good dielectric properties, high chemical and radiation resistance, inherent flame retardancy, and exceptional dimensional stability.

Traditional polyimides face a setback of poor processability because they possess highly rigid molecular chains. For example, Kapton (which is structurally close to a PI), has a rigid structure with a single ether bridge bond in its repeat unit, rendering the polymer insoluble in organic solvents and impossible to process from a molten form.

The crucial difference with Polyetherimides (PEI) lies in the molecular design methodology employed to synthesize PEI, which aims to make the polymer thermoplastic and soluble. This involves modifications intended to reduce the glass transition temperature (Tg) of the rigid-chain PIs by increasing the flexibility of the main polymer chain or by reducing intermolecular interaction.

Key strategies used in the polymerization of processable polyetherimides include:

1. Introducing chemical groups with larger rotational degrees of freedom into the main chain, such as ether links (–O–), as well as –S–, –SO₂–, –CO–, –C(CH₃)₂–, and –C(CF₃)₂. Synthesizing monomers containing –O– ether groups between aromatic groupings is the most convenient approach.
2. Incorporating asymmetrical structures, often using meta- or ortho-bonds in aromatic groups.
3. Introducing bulky groups (like –C₆H₅, -CH₃, and –CF₃) into main aromatic rings to reduce cross-molecular packing.

These modifications result in PEI having an highly amorphous structure that allows for easier processing from both molten and organically dissolved forms. For instance, Ultem polyetherimide is described as completely amorphous and possessing good workability from molten or dissolved form. Amorphous PEI (Ultem-1000) is contrasted with semicrystalline thermoplastics such as PEEK and PPS.

Properties of Polyetherimide (PEI)

PEI materials are characterized by a combination of high-performance attributes:

Thermal Properties: High thermal and heat stability are hallmark features of PEI offering an advantageous high service temperature of 340 °F (171 °C).

Mechanical Properties: PEI is an extremely strong material, offering high tensile strength and elastic modulus.

• PEI exhibits high tensile strength, a high elastic modulus, stiffness and hardness.
• Possesses superb creep resistance, high deformational stability, and exceptional dimensional stability
• It possesses high impact strength and toughness, although certain Ultem grades note reduced impact toughness compared to polyetherketones.
• It maintains rigidity up to 284 °F (140 °C) and toughness down to -4°F ( -20 °C).

Chemical and Environmental Resistance: PEI exhibits good chemical resistance, often demonstrating little to no change in weight or mechanical properties when exposed to various media.

• It is resistant to hydrolysis when exposed to hot water and steam, and can withstand repeated sterilization cycles in a steam autoclave.
• PEI generally shows excellent resistance (score 7-9) to a wide array of chemicals including air, aliphatic amines, ammonium hydroxide, ammonium acetate, ammonium thiosulfate, benzyl chloride, 1,4-butanediol, camphor, carbon disulfide, carbon tetrachloride, chlorosulfonic acid (8-9), copper chloride (8-9), copper sulfate (8-9), corn oil, cyclohexane, developing solutions (8-9), diethylene glycol, diphenyl ether, dipropyl ether, distilled water, engine oils, ethane, ethylene carbonate, ethylene glycol, ethylene oxide, ethylene sulfate (8), ferric chloride (8-9), ferric nitrate, ferric sulfate, formaldehyde, Freon, glycerin, hexane (8-9), hydraulic fluids, hydraulic oils (2-9), hydrogen peroxide (8-9), inert gases, iron chloride, iron nitrate (7-9), isopropyl alcohol (8-9), isopropyl ether, Javelle water, jet aircraft fuels, kerosene, linseed oil, lubricating oils, methanol, methyl acetate, methyl ethyl ketone, methyl isobutyl ketone, mineral oils, N,N-dimethylaniline, N,N-dimethylformamide, naphtha, nitric acid (1-10%), nitrogen, nitromethane, octane, vegetable oils, oxygen, petrolatum, petroleum ether, phosphoric acid (8-9), phosphoric anhydride, plasticizers, potassium carbonate, potassium hydroxide, potassium salts, salad dressings, salt water, soap solutions, sodium carbonate, sodium chloride, sodium hydroxide, sodium hypochlorite, sulfuric acid, transformer oils, and turpentine.
• However, PEI shows poor resistance (score 0-3) to certain aggressive chemicals such as Antimony Trichloride (1), Benzene (0), Chloroform (3), Dibutyl Phthalate (2), Dimethylformamide (0), Ethylene Chloride (0), Fluorine (1), Iodine (aqueous solution) (2), Methyl Iodide (2), Methylene Chloride (3), Petroleum (0-9), Phenol (0-3), Phenylethyl Alcohol (3), Pyridine (2), and Uranium Hexafluoride (2).

Processing and Fabrication

Common processing techniques for PEI include injection molding and extrusion molding, although compression molding is also effective. It can be processed using all standard methods depending on its intended use, and is frequently employed for casting thin-walled products due to its classification as a superstructural material. For composite structures, PEI matrices can be thermoformed using methods that yield high-quality products with minimal voids and spring-back.

Applications

PEI's unique combination of properties makes it a material of choice across numerous demanding industries:

• Aerospace Applications:
  
  
• PEI is critically important for aerospace products, where it is used in applications that involve contact with atmospheric particulates and chemicals.
• Its use extends to interior aerospace applications that demand durability, flammability, and low smoke toxicity properties. PEI is fire-resistant without the addition of antipyrenes and releases little smoke when exposed to an open flame, making it suitable for use in airliners.
• Specific examples include high-temperature electrical conductive polymeric nanocomposite films for aerospace.
• PEI also serves as a matrix for glass fiber composites in complex structures like thermoplastic Composite Wing Leading Edges (CWLEs), which are increasingly used in aerospace structures for certain types of drones. The manufacturing of such parts involves multi-step thermoforming methods, where PEI's material properties, particularly near its glass transition temperature, are critical.

• Electrical and Electronic Components:
  
  
• PEI is widely applied in this sector due to its high dielectric strength, good electrical properties, and low moisture absorption. Its low dielectric constant of 2.58 and low dissipation factor of 0.00035 make it suitable for insulating matrices in electrical and electronic devices.
• Applications include electrical connectors, electrical insulation parts, circuit-board cores for semiconductor applications, and chip test sockets.
• PEI is also used for heat-resistant insulation of electric wires and high-voltage cables, and as carriers for aluminum hard disk plates and silicon plates.

• Medical and Scientific Equipment:
  
  
• PEI is a biocompatible polymer suitable for medical device applications.
• It is specifically used in medical instrument components and scientific equipment parts.
• A key advantage for these uses is its resistance to hydrolysis when exposed to hot water and steam, and its ability to withstand repeated sterilization cycles in a steam autoclave.

• Industrial and Mechanical Parts:
  
  
• PEI can serve as a lightweight, high-performance metal replacement solution.
• It is utilized in various industrial and mechanical components such as manifolds, heat-resistant gears, seals, hubs, mounting parts, reels, check valve spheres, slotted couplings, blades, wear-resistant strips, valve seats, sleeve bearings, and bearing retainers.
• For the chemical industry, it is used in pumps (blades, impellers, gears, sleeve bearings, rolling bearings, packing glands, rotor housing), filters (wheels and hubs), mesh tables, dispensers, and separators (friction assembly liners, housings).
• In mechanical engineering and machinery, PEI parts include carriage travel hubs, camshaft gears, rims for worm-gear gearboxes, clasp nuts of cross-feed carriages, movable guides, transmission box gears, drive unit gearbox worm gears, hubs, inserts in friction assemblies (skids, mandrels, slide blocks), spindle liners, and sprockets.

• Other Notable Applications:
  
  
• Has established a strong track record in 3D printing.
• PEI has good chemical resistance to a variety of substances, including most aqueous media like acids, bases, alcohols, and detergents, and is resistant to hydrolysis. However, its resistance can be limited with certain strong alkalis or specific solvents like methylene chloride. PEI demonstrates resistance to gamma-ray and weather.
• PEI exhibits high deformational stability and superb creep resistance, making it suitable for long-term use under stress.
• Maintains its elastic modulus well at elevated temperatures, though it can show a dramatic loss of properties above its glass transition temperature in compression tests. Its low friction coefficient and high plasma stability also contribute to its utility in demanding conditions.

Compounding and Special Additives

Glass fiber reinforced Polyetherimide (PEI) is a high-performance material that leverages the inherent strengths of PEI while enhancing specific properties, making it suitable for demanding applications. The reinforcement with glass fibers contributes significantly to its robust mechanical and thermal performance, as well as its dimensional stability.

• Enhanced Mechanical Strength and Stiffness:
  
  
• Glass fiber filled PEI (specifically, 30% Glass Fiber Filled PEI) is characterized as an extremely strong material.
• It exhibits a minimum tensile strength of 16,000 PSI (110 MPa).
• The tensile modulus is remarkably high, with a minimum of 800,000 PSI (5516 MPa). This indicates a very high stiffness.
• The elongation at break is relatively low, with a minimum of 2%, which is typical for highly rigid, reinforced plastics.
• The hardness, measured by Shore D, ranges from 84 to 94.

• Excellent Thermal Properties and Stability:
  
  
• Glass-filled PEI materials are known for their excellent thermal properties.
• They have a high service temperature of 340 °F (171 °C), making them suitable for applications exposed to elevated temperatures.
• The glass transition temperature (Tg) is 423 °F (217 °C), and the second melt peak temperature is 426 °F (219 °C). The properties of the PEI resin, including effective density, heat capacity, and thermal conductivity, vary significantly near its Tg, showing "knee points" at this temperature.
• The material exhibits good heat and thermal stability.

• Exceptional Dimensional Stability:
  
  
• A key characteristic of glass-filled PEI is its exceptional dimensional stability.
• The temperature-dependent Coefficient of Linear Thermal Expansion (CTE) curve exhibits an orthotropic property and a sudden change at Tg. This is crucial for maintaining part shape across temperature variations, especially important for complex structures like composite wing leading edges.
• After thermoforming, parts made from a woven fabric glass fiber and PEI matrix show few existing voids and small spring-back after demolding, indicating excellent shape retention post-processing.
• The effective relaxation stiffness also exhibits apparent orthotropic properties and a complex time dependency, reflecting its ability to maintain form under stress over time.

• Other Notable Properties:
  
  
• Good Chemical Resistance: Glass-filled PEI offers good resistance to various chemicals. The broader class of Polyimides (which includes Polyetherimide) are known for high chemical resistance, showing resistance to many substances such as various concentrations of nitric acid, phosphoric acid, and sulfuric acid, as well as detergents and milk. However, some chemicals like butyl acetate and methylene chloride can cause severe effects or dissolve it.
• Inherent Flame Retardancy: The material possesses inherent flame retardancy.
• Good Electrical Properties: PEI materials generally have good dielectric properties.
• Biocompatibility and Sterilization Resistance: It can withstand repeated sterilization cycles in a steam autoclave and is used in medical instrument components.
• Anisotropy: For woven fabric glass fiber composites with a PEI matrix, the effective thermal conductivity exhibits anisotropy, meaning its thermal conduction properties vary with direction, largely due to the warp and weft directions of the woven fabric.

Conclusion

Polyetherimide (PEI), particularly the Ultem® series, stands out as a crucial high-performance engineering thermoplastic. Its highly amorphous structure, coupled with exceptional thermal, mechanical, and chemical resistance properties, enables its use in demanding environments. From aerospace and electronics to medical and automotive applications, PEI's versatility, ease of processing from melt or solution, and ability to be enhanced through compounding with additives make it an indispensable material for advanced technological solutions.