Polyetherimide (PEI), AKA Ultem, is used as a build surface due to its adhesion properties

From PEI build surface

PEI (Polyetherimide), also known as Ultem, is a reusable "relatively maintenance free" build surface for both ABS (with a Heated Bed) and PLA (hot or cold) requiring no additional adhesives such as glue or tape. PEI is appealing due to its "no-surface-prep" conditions, which make it convenient to work with. Additionally, printing on PEI requires no support rafts as parts adhere directly to the surface with no issue.

PEI is sometimes incorrectly referred to as BuildTak, however they are not the same thing.