Unhairing

In the tanning industry the unhairing stage concerns the removal of animal hair from the skin by chemical burning of the hair root, or by chemical degradation of the hair shaft. The type of hair removal depends on the type and length of the hair itself. Two main groupings of hair removal exist: painting and drum/paddle/pit unhairing. Sheepskins—or animals with long hair (e.g. wool)—use the painting method to remove the hair from the follicle. Animals with shorter hair (e.g. cattle hides) can have their hair removed in a process vessel.

Historically, scalding and singeing were other methods used to remove hair from hides. These methods are still used today to some extent, in hides not destined to be used for leather.

Chemicals used

The chemicals used for unhairing are numerous. Traditionally, alkalis such as lime (calcium hydroxide) or soda ash (sodium carbonate) were used. These treatments were lengthy and the hair retention was usually high. This meant a manual scraping of the hair from the follicle was required. It is now known that lime (an alkali with a low solubility) and weak alkalis cause an immunisation (to the action of chemicals) of the keratin.

To increase the speed of unhairing and to lower hair retention rates, sharpening agents are added. The most common is sodium sulfide. Sodium sulfide, at pH values greater than 11.5, break the disulfide bridges of cystine (found in keratin) and result in the collapse of the hair or loosening of the hair resulting in depilation. Excessive sharpening agent result in hair pulping. Controlled exposure of sharpening agent, or other alkali, to the hair will result in hair-save unhairing (during which the hair can be removed intact from the process vessel).

Environmental considerations

Hair is a nitrogenous organic material, and as a result, increases the oxygen demand of effluents that contain it. Chemicals used in unhairing worsen the oxygen demand. It is also a nuisance pollutant in that it coats pipes, tanks, screens and pumps easily. It can also float on the surface of ponds, tanks or raceways and lower gas exchange. Hair (especially pulped hair) sludges at times and this decreases the tank volume, unless agitated. Solutions in the tanning industry exist for the removal of hair and the unhairing chemicals from effluent streams.

Poor unhairing

Hair retention is visualised as either intact hairs being present, hair roots being present or as epidermal scales/flakes still being seen. This is often referred to as scud. Scudding is thus the operation where unwanted hair or epidermis is scraped off the hide / skin.

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