A deeper exploration of the relation between sulfonation degree and retanning performance of aromatic syntans

It is well‑known that the sulfonation degree(DS)of aromatic syntan is an important factor affecting its retanning performances.But the quantitative relation between DS and syntan property and the influencing mechanism of DS on syntan property are not clarified.In this work,five phenolic formaldehyde syntans(PFSs)with the same polym‑erization degree but varying DS were prepared to investigate the effect of DS on the properties of syntan and crust leather.It was found that the absolute value of zeta potential and the particle size of PFS decreased with increasing DS in aqueous solution.Molecular dynamic simulation results proved that the DS of PFS was a major contributor to electrostatic interaction and hydrogen bonding in the PFS–water system and greatly affected the aggregation and dispersion of PFS in aqueous solution.The PFS with a low DS was prone to aggregate to large particles in aqueous solution because of low intermolecular electrostatic repulsion and less hydrogen bonds and therefore can be used to increase the thickness and tightness of leather.The PFS with a high DS presented a small particle size with more anionic groups in aqueous solution,thereby sharply decreasing the positive charge of leather surface and facilitat‑ing the penetration of the post‑tanning agents into the leather.These results might be scientifically valid for rational molecular design of syntans and more productive use of syntans in leather making.