Abstract: The invention relates to a polyester composition for use in mirror optic systems such as headlights for motor vehicles and reflector lamps comprising energy saving lamps, comprising a polybutylene terephthalate resin (PBT) and having a cyclic dimer content of less than 0.35 wt. %, relative to the weight of the PBT, and which composition further comprises a second polymer selected from the group consisting of polyethyleneglycol terephthalate (PET), PET copolymers, polybutylene naphthanate (PBN), PBN-copolymers, polytrimethylene terephthalate (PTT), PTT-copolymers, polyethyleneglycolnaphthanate (PEN), PEN-copolymers, polycyclohexanedimethylene terephthalate (PCT), PCT-copolymers, aromatic polycarbonates and aromatic polyester carbonates (APEC), in an amount of 1-40 wt. %, relative to the total weight of PBT and the second polymer. The invention also relates to a process for preparing the composition, use of the composition for making moulded parts and to a moulded part made of the composition.

Abstract: ABSTRACT OF THE DISCLOSURE The invention relates to a polyester composition comprising a polybutylene terephthalate resin (PBT) and at least one additive, wherein the composition has a cyclic dimer content of less than 0.35 wt. % relative to the weight of the PBT. The invention also relates to moulded parts, prepared from this polyester composition, and mirror optic systems such as headlights for motor vehicles or garden reflector lights, comprising these moulded parts. The invention further relates to a PBT having a cyclic dimer content of less than 0.35 wt. %, a process for preparing this PBT and the use of the PBT for preparing the polyester composition according to the invention.