Alternatives to sulfide dehairing25 February 2008
Work on alternatives to sulfide dehairing through the use of oxidative agents was presented at the XXIX congress of the IULTCS/103rd annual meeting of ALCA in Washington DC earlier this year. The authors are William N Marmer and Robert L Dudley of the Eastern Regional Research Center, USDA Agricultural Research Service. This is an abridged version and the copyright of the full paper rests with ALCA.
ABSTRACT The dehairing of cattle hides with sodium sulfate generates large quantities of waste added to the fact that sodium sulfide is a potential hazard in the workplace. The authors worked with industry to develop a rapid dehairing process for application in the abattoir which used concentrated sodium sulfide as the depilatory. The hazards of using sodium sulfide are possibly more so in the abattoir, since the agent is applied by spraying. Because of this a sulfide-free, oxidative process for rapid dehairing was developed. Several oxidative systems were screened for their effectiveness and a system composed of alkaline hydrogen peroxide and potassium cyanate was selected. Some oxidative chemicals were rejected because they did not dehair a hide in less than six minutes. One example, sodium perborate, reduces both the amount of solid and liquid wastes. However, the byproduct is borate, which may accelerate the degradation of wooden drums. In a paper cited for the Lollar Award (JALCA, 100:419-431, 2005), the results of a study that used sodium percarbonate as the dehairing agent for use in the beamhouse were reported. The dehairing conditions were similar to the conditions developed for sodium perborate but the byproduct is benign sodium carbonate. Although oxidatively dehaired hides exhibit a slight resistance to acid dye uptake, leather produced from percarbonate hides in all other studied aspects - shrinkage temperature, chromium uptake, elongation at break, and toughness - was comparable to leather produced from sulfide-dehaired hides. Introduction The use of oxidative chemicals for hair removal has been the subject of a number of papers and the current presentation provides an overview of the authors' research into replacing sulfide in the rapid dehairing of carcases in the abattoir, taking the seminal paper by Gehring et al2 as a starting point. Dehairing with alkaline sodium perborate, a common bleaching agent used in laundry detergents, yielded hides that took up greater quantities of both chromium and acid dye than Na2S-dehaired hides. The perceived problem with perborate dehairing was the conversion to borate during its use. Concern was expressed that the uptake of borate by wooden process drums might damage the wood over time, although there was no experimental evidence to support this hypothesis. The focus was then turned to another common and inexpensive bleaching agent: sodium percarbonate. This compound exists in the solid state as the sesquiperhydrate, 2Na2CO3·3H2O2, in which the H2O2 is a discrete entity. Dehairing experiments were carried out into reducing the amount of chemicals required for the removal of hair from the hide and shortening the time required for dehairing. A low float protocol, using sodium percarbonate as the oxidative chemical, was developed which both reduces the amount of water used for the dehairing and reduces the dehairing time. In addition to developing new dehairing formulations for the beamhouse, an old formulation was revisited - alkaline hydrogen peroxide/potassium cyanate - that was developed for rapid dehairing in the abattoir. The base concentrations used for the rapid dehairing may be too high for use in the beamhouse but formulations using alkaline hydrogen peroxide and alkaline hydrogen peroxide/potassium cyanate were developed which show great promise for use in the beamhouse. Results Optimal parameters of 5% percarbonate and 7% sodium hydroxide (w/v in a 200% float) were obtained. For the perborate dehairing, the dehairing solution was composed of 5% perborate and 5% sodium hydroxide (w/v in a 200% float)1. All subsequent results were obtained with a 7% NaOH and 5% percarbonate dehairing mixture. With the exception of a few short hairs, almost all of the hair was removed in four hours by the percarbonate procedure in a hairburn process. The residual hair was removed during relime. Typically the leather from the butt region of the hide is stiffer, has the lowest % elongation and a high toughness index. Likewise leather from the shoulder has a larger % elongation and the highest toughness index. The mechanical properties of the leather from both the butt and shoulder are not dependent on the dehairing protocol. The biggest variations in mechanical properties occur in the belly region where the mechanical properties are dependent on the dehairing protocol. Overall the results show that the mechanical properties of the leather produced from hides dehaired with percarbonate are similar to those obtained from hides dehaired with sulfide. It is not possible to directly compare the mechanical test results with those for a perborate-dehaired hide. Most of these results were not statistically different from the sulfide controls but the percentage elongation for the perborate butt was very large and significantly different from the sulfide-dehaired butt. Likewise the toughness index for the perborate dehaired belly was significantly different from that of the sulfide-dehaired belly. The observations are probably more a reflection of the difference between the hides than the differences arising from the use of the different dehairing formulations1. The previous study used a 200% float of 5% NaOH. This time the base concentration was reduced to 4% NaOH and obtained a satisfactory dehairing in roughly five hours. If the temperature is maintained at 35°C the dehairing time dropped to four hours. Raising the temperature even higher shortened dehairing time but without proper temperature control and careful timing of the reaction, grain damage occurred. The formulation was modified and the float reduced to 100%. After four hours the hair was well pulped and more dehairing occurred at four hours than with the 200% float. The increase in the dehairing efficiency was due to the increase of mechanical action. In a low float there is more contact between the hide and the inner surface of the drum. By increasing the NaOH concentration to 6% (100% float) it was possible to reduce dehairing time to two hours, although with some modifications to subsequent steps. Modifications to the rapid alkaline hydrogen peroxide/ potassium cyanate formulations were made so that it could be applied to the beamhouse. The rapid oxidative dehairing formulation uses 20% NaOH, which can have very deleterious effects on the hide in very short periods of time. Neither 4% NaOH/ 1% hydrogen peroxide or 4% NaOH/ 2.4% KOCN/1% hydrogen peroxide did a very good job of removing hair after four hours but by increasing the hydrogen peroxide concentration to 2% the dehairing times were less than four hours. The formulation using KOCN had the shorter dehairing time. A number of experiments to optimise the KOCN concentration, size of float as well as the temperature of the dehairing reaction were carried out. This set of experiments aims to decrease dehairing time from four hourrs to approximately one hour. Conclusions The authors were able to demonstrate that sodium percarbonate is a useful reagent for the replacement of either sulfide or perborate in the beamhouse. The leather produced from percarbonate dehaired hides has similar mechanical properties to that produced from matched sides dehaired with sulfide. The only difference observed was that the dye uptake by the side dehaired with percarbonate was not as great as for the sulfide-dehaired side. Minor modifications can be made to the formulation to yield a low float formulation that has a shorter dehairing time. Both an alkaline hydrogen peroxide and an alkaline hydrogen peroxide/ KOCN formulation that can be used in the beamhouse have also been developed. The results are only preliminary but both of these formulations appear to have the same advantages as the percarbonate dehairing. References 1. W N Marmer and R L Dudley. Oxidative dehairing by sodium percarbonate, JALCA, 100, 427-431, 2005. 2. A G Gehring, D G Bailey, D L DiMaio, R L Dudley, W N Marmer and C E Mazenko. Rapid oxidative unhairing with alkaline calcium peroxide, JALCA, 98, 216-223, 2003. 3. W N Marmer and R L Dudley. The use of oxidative chemicals for the removal of hair from cattle hides in the beamhouse. JALCA. 99: 386-392, 2004. 4. E Heidemann. Fundamentals of leather manufacturing, Eduard Roether, KG, Darmstadt, 1993, p188. 5. L F Cabeza, M M Taylor, G L DiMaio, E M Brown, W N Marmer, R Carrio, P J Celma and J Cot. Processing of leather waste: Pilot scale studies on chrome shavings. Part II. Purification of chrome cakes and tanning trials. JALCA, 93, 83-98, 1998. 6. A G Gehring, D G Bailey and G L DiMaio. Evaluation of urea sulfate as a replacement for sulfuric acid in chrome tanning. JALCA, 96, 268-274, 2001.