A sheet containing a base layer containing polycarbonate and a flame retardant, wherein the flame retardant contains a compound of formula 1 wherein R1, R2, R3 and R4, represents an optionally halogen-substituted C1-C8 alkyl, C5-C6 cycloalkyl, C6-C10 aryl or C7-C12 aralkyl, R5 and R6, represents a C1-C4 alkyl, and Y represents a C1-C7 alkylidene, C1-C7 alkylene, C5-C12 cycloalkylidene, —O—, —S—, —SO—, —SO2—, —CO— or a residue of formula (2) or (3) wherein Z represents carbon and R21 and R22 can be selected for each Z individually and, represent hydrogen or C1-C6 alkyl, with the proviso that, on at least one Z atom, R21 and R22 are simultaneously alkyl and wherein the polycarbonate comprises a mixture of linear and branched polycarbonate and wherein the linear polycarbonate content is at least 40 wt. %, based on the total polycarbonate weight.