Particle Size Evolution of Melamine-formaldehyde Tanning Agent on Tanning Effect

Abstract

Melamine-formaldehyde [MF] resin is a tanning agent. Its averageparticle size [PS] is important for penetrating the skin and thusendowing performance of its resultant leather. The synthesisparameters for MF resin such as pH, molar ratio of formaldehydeto melamine, temperature and reaction time affect its PS. Weinvestigated in detail the growth of MF’s PS and its consequenteffects on hydrothermal stability, mechanical properties and theapparent properties of leather. The data showed that a broad rangeof average PS was formed at the different synthesis stages of MFresin. For example, the first reaction stage of methylolation gavean average particle diameter of 250 nm with a narrow sizedistribution under the conditions of 45°C, 3:1 molar ratio, a pH of8.5, and a reaction time of 3 hrs. In this case the resultant leatherwas poor as it had a low shrinkage temperature, lacked fullnessand showed a low-valued elongation at break. During the secondpolycondensation stage, the PS of MF resin increased withincreasing reaction temperature and the molar ratio offormaldehyde to melamine. Furthermore, PS increased morerapidly by lowering the pH of the polycondensation and extendingits reaction time. Thus, the new average diameter of MF resinparticles reached above 3000 nm when the polycondensationreaction was done inside the skin for 2 hours at a pH of 4.5.Consequently, the resultant leather was white and full. It had ahigh hydrothermal stability and mechanical strength as evidencedby its fully satisfactory elongation at break. Clearly, the cross-linkswere formed by multiple site interactions between MF resin withlarge PS and collagen fibers, which enhanced the tanning effect.