Waterborne Dimethylolpropionic Acid-diisocyanate Adducts with Alkali-deblockable Isocyanate Groups as Pretanning Agent for Chrome Tanning

Abstract

Diisocyanates qualify as tanning agent by virtue of their strong tendency to crosslink collagen molecules by reacting with amino groups. However, due to their sensitivity to water, these compounds cannot be used directly in aqueous environment, which is the basis for leather processing. To address this problem, waterborne dimethylolpropionic acid-diisocyanate adducts (WDDAs) with temporarily-blocked isocyanate terminals were prepared in the present paper by using NaHSO3 as blocking agent, and further evaluated as a pretanning agent for chrome tanning. Fourier transform infrared (FTIR) spectra revealed that the isocyanate terminals in WDDAs were successfully blocked by NaHSO3. The blocked isocyanates were stable under ambient temperature, but prone to deblocking under alkaline condition. The regenerated isocyanates were found still capable of crosslinking collagen molecules to impart hydrothermal stability. In leather tanning, a high shrinkage temperature (Ts>110 oC) was achieved by successively treating goat skin with 5 wt% WDDAs and 4 wt% chrome powder. Stereomicroscope and SEM observation further indicated that the WDDA-chrome tanned leather exhibited tight grain surface and well opened up fiber structure in comparison with semi-chrome tanned leather. Due to the presence of carboxyl group in dimethylolpropionic acid, it was also found that WDDAs enhanced the absorption of chromium in the resultant leathers (% chrome uptake> 90%), resulting in significantly less Cr residual in the tanning wastewater. In addition, it is important to note that the usage of salt in pickling process was avoided owing to previous crosslinking of amino groups, which is very helpful for overcoming the problem of total dissolved solid (TDS) related to neutral salts. The tanning effect of WDDAs and their auxiliary function for chrome tanning allow them to be applicable as a pretanning agent, which efficiently alleviates the environmental impact of traditional chrome tanning for sustainability.