A method of decreasing aldehyde content in a polymeric material uses a compound (Z) which includes a moiety of formula (I): wherein each R1 and R2 independently represents a substituent, n1 is 0 to 4 and n2 is 0 to 4; wherein X is selected from the group comprising C, N, P, O and S atoms; and each of the two benzene rings includes a moiety formula (A) and a moiety formula (B): NH wherein the carbon atom of moiety (A) and the nitrogen atom of moiety (B) are separated by at least one and not more than two atoms.