Aggregation behavior of cationic amphiphilic polymers and their application in metal-free leather retanning

The manufacture of metal-free leather is considered to be favorable for environment compared to conventional technology of chrome tanning. However, metal-free leather crust presents some disadvantages for further processing, like low isoelectric point (pI) and high hydrophilicity. Applying cationic amphiphilic polymer retanning agent is expected to overcome the disadvantages. The aggregation behavior of this polymer, which is influenced by its charged nature, will greatly affect the leather properties. However, the structure-function relationship has not been clarified yet. Here we synthesized a series of cationic amphiphilic acrylic polymers, Poly(SMA-co-DMA)s (PSDs), with increasing protonation degrees (PSD-40 %, PSD-60 %, PSD-80 %, PSD-100 %) and applied them to metal-free leather retanning. Experiments and molecular dynamics simulations show that PSD with higher protonation degree exhibits smaller aggregate size and better dispersion in water. PSD retanning significantly promotes the uptake of anionic fatliquors by leather (>90 %) because PSD enhances the pI of leather. PSD-40 % cannot be uniformly dispersed in water. PSD-60 %, PSD-80 % and PSD-100 % possess suitable particle size (45-90 nm) to penetrate into leather fibrils and form hydrophobic films, thereby greatly improving the hydrophobicity of leather. The hydrophobicity of leather increases with decreased protonation degree of PSD. Moreover, the fiber dispersion and organoleptic properties of leather are positively correlated with leather hydrophobicity because of the separating and lubricating effects of the amphiphilic PSD.