Formaldehyde resins are present in textile, leather and wood industries. Due to the harmful character of formaldehyde, different alternatives have been found to exclude or reduce its content on processed goods. However, the effect of dyestuffs on the formaldehyde content of goods containing formaldehyde-synthesized resins has not been studied up to date. The aim of this work is to check if the presence of free amino groups in the structure of dyestuffs exerts an influence on the formaldehyde content on leathers treated with formaldehyde-synthesized resins. Six dyes, belonging to three different families (acid dyes, direct dyes and basic dyes), have been taken as examples to evaluate how their structures affect the reaction with formaldehyde present in leather. The variation of the formaldehyde content in dyed leathers with respect to control samples (treated with resin only) and its evolution with time have been also considered. It has been found that the ability of dyes in reducing the formaldehyde content in leather depends on the amount of amino groups amenable to reaction with formaldehyde. Those amino groups that in their vicinity have other functionalities, with which to form relatively stable structures, have a reduced reactivity with formaldehyde. The reduction ability of dyes also depends on the formaldehyde content in leather. The lower the formaldehyde content is in the leather, the higher this reduction ability. Acid Black 234 dye caused a formaldehyde content reduction of approximately 84% in leathers treated with melamine-formaldehyde resin of low formaldehyde content in the analysis carried out after 90 days of leather processing whereas the reduction was approximately 20% when the resin was of high formaldehyde content. The highest reduction ability of basic dyes corresponded to the dye that has the greatest amount of amino residues amenable to reaction with formaldehyde (Basic Orange 2). Basic Orange 2 dye exhibited higher reduction ability (90% of reduction in leathers treated with resin of high formaldehyde content after 90 days of leather processing) than the Acid Black 234 dye (approximately 20%), both containing similar amount of free amino residues. Thin layer chromatography analysis revealed that the Basic Orange 2 dye is mainly a single major component, while the Acid Black dye 234 is a mixture of components that can have a reduced reactivity with formaldehyde. Further experiments are required to investigate if the surface leather dyeing (Basic Orange 2 dye) have a higher influence on formaldehyde content reduction than the through-dyeing (Acid Black 234 dye).

Peer Reviewed