Oxazolidines – the versatile leather tanning agents28 May 2008
Chrome tannage has been recognized as one of the most effective tanning agents and has been widely employed in the leather industry since its discovery more than 100 years ago. However, due to its environmental impact, the practice of tanning with chromium (Cr) compounds has been restricted or prohibited in developed areas such as the United States and Europe and certain goods made with chrome tanned leathers are no longer allowed to be manufactured or sold within these countries.
For the past several decades, considerable efforts have been made to develop chrome-free tannages or to reduce the discharge of chrome in the tannery effluent. Oxazolidines, a new class of tanning agents, have demonstrated versatile uses in leather tanning and have been shown to help eliminate or reduce chrome from the tannery effluent. Oxazolidines are considered heterocyclic compounds and their bi-functional behaviour is responsible for their cross-linking reaction with phenols, epoxies, proteins and other functional groups that can react with formaldehyde under alkaline conditions. Among the oxazolidine family, oxazolidine A and oxazolidine E, as shown in Figure 1, have found commercial acceptance and have been widely studied for leather tanning. Oxazolidine A and oxazolidine E are commercially sold as Zoldine ZA-78 Oxazolidine and Zoldine ZE Oxazolidine from ANGUS Chemical Company, a subsidiary of the Dow Chemical Company. Oxazolidines undergo an irreversible reaction with collagen over a wide pH range and at a rate of fixation that is readily controlled by operating conditions. Although useful with pickled pelts at a pH level as low as 2.6, success is best achieved when the pH at which oxazolidine is introduced is at 4.0 or higher. The fixation rates of oxazolidines are amplified when temperatures are increased and higher oxazolidine concentrations are present in the drum. Oxazolidines can be introduced into the tanning process as stand-alone tannages or in conjunction with other tanning agents, thus offering great latitude and flexibility in the process. This paper will give a brief overview of typical cases to illustrate oxazolidines versatile uses in leather tanning. Oxazolidines for crome (Cr)-free tanning Studies have shown that the highest hydrothermal stability, or shrinkage temperature, for leathers tannaged with oxazolidines alone to be about 85°C. This may be satisfactory for certain applications, but it is still not comparable with Cr-tannaged leather whose shrinkage temperature can exceed the boiling point of water (100°C). In the course of developing a Cr-free tanning process, additional efforts have focused on the combination of tannages, either vegetable tannage with oxazolidine or mineral tannage (eg aluminum (Al), titanium (Ti) and zirconium (Zr) salts) with oxazolidine. The leather produced with these combined tannages has comparable physical properties, including hydrothermal stability, with Cr-tanned leathers. For example, when lambskins initially tanned with 15% vegetable tannage (Mimosa ME) were re-tanned with oxazolidine E (Zoldine ZE Oxazolidine) at 45°C, the shrinkage temperature rapidly increased to more than 100°C. As shown in Table 1, when the oxazolidine offered was 2% or greater, the shrinkage temperature in relation to the boiling point was achieved in less than one hour. Oxazolidines for Cr-reduced tanning When chrome tanning is combined with either oxazolidine A or oxazolidine E, the chrome discharge in the effluent can be reduced by 50-60% compared to chrome tanning alone. The leathers produced with these combined tannages, while maintaining or increasing the hydrothermal stability, are often fuller and softer than conventional chrome-tanned leathers. DasGupta2 has reported that in a modified ThruBlu tanning process - a combination tanning process of oxazolidine and chrome that he developed - the chrome discharge was dramatically reduced, resulting in close to zero discharge of chrome in the effluent (shown in Figure 2). Also, the shrinkage temperature of the lambskin garment reached 110°C. The details of the modified ThruBlu tanning process were not disclosed, but 2% oxazolidine E was applied during the pre-tannage stage in the simultaneous degreasing process. Oxazolidines for salt-free pickling Pickling is a preliminary process for preparing hides and skins for tanning, largely by adjusting the pH with acid and controlling the swelling with salt. Higher salt concentrations in the tannery effluent resulting from the conventional pickling process have been an environmental concern, leading to a trend in developing a salt-free pickling process3. Additionally, failure to control the swelling of hides during this step would lead to damage or defects in the fibre. This not only impacts the subsequent tanning process, but also the final quality of the tanned leather. In a study of salt-free pickling processes4 oxazolidine E (Zoldine ZE oxazolidine) was used and compared to a conventional acid/salt pickling process. As shown in Table 2, the acid/oxazolidine E pickling process significantly outperformed the conventional acid/salt pickling process in controlling swelling. With a 2% oxazolidine E offer or higher in this novel salt-free pickling process, the swelling was well controlled by around 7% or more. Also, it was noted was that the chrome uptake in the subsequent tanning process was enhanced by more than 23%. This directly corresponds to more than 60% reduction of chrome discharge in the effluent compared to the conventional acid/salt pickling process. Summary Oxazolidines can be used as tanning agents in several different ways as illustrated in this paper. Combinations of oxazolidines with vegetable tannages can replace chrome tanning without sacrificing the physical and thermal properties of the tanned leather. When oxazolidines are used in conjunction with chrome tannage, chrome discharge in the tannery effluent is significantly reduced while the quality of the tanned leather is as good as or better than that tanned with chrome alone. Finally, with the use of oxazolidine, a more effective salt-free pickling process can be achieved and the environmental impact within leather manufacturing can be further reduced. N References: 1. A D Covington, Chrome Management, Proceedings of the 32nd LERIG, Chennai, 1997, pp. 97-127. 2. S Das Gupta, Modified LASRA ThruBlu Chrome Tanning of Lambskins, Private Communication. 3. ‘Pickle-free chrome tanning using a polymeric synthetic tanning agent for cleaner leather processing' Clean Technologies and Environmental Policy, Vol. 6 (No. 4), 2004. 4. Z H Shan, Leather Science & Engineering Dept, Sichuan University, Private Communication.