Polyetherimide

Polyetherimide (PEI; branded as Ultem ) is an amorphous, amber-to-transparent thermoplastic with characteristics similar to the related plastic PEEK. When comparing PEI to PEEK, the former is cheaper but has lower impact strength and a tighter temperature range.

Due to its adhesive properties and chemical stability it became a popular bed material for FFF 3D printers.

Structure
The molecular formula of the PEI repeating unit is C37H24O6N2 and the molecular weight is 592.61 g/mol. It contains phthalimide and bisphenol A sub-units.

Properties
The glass transition temperature of PEI is 217 °C (422 °F). Its amorphous density at 25 °C is 1.27 g/cm3(.046 lb/in³). It is prone to stress cracking in chlorinated solvents. Polyetherimide is able to resist high temperatures while maintaining stable electrical properties over a wide range of frequencies. This high strength material offers excellent chemical resistance and ductile properties suitable for various applications, even those involving steam exposure.