A Deeper Look inside the Leather Industry
Leather is a material from animal skin or hide that has undergone various physical and chemical processes to make it immune to microbes. The immunity is needed to prevent the microbes from consuming the skin and causing it to rot. The skin itself is subjected to bacteria attack even before the animal is flayed. Due to daily contact with its environment, as much as 1-2 billion bacteria/cm2 inhabit the skin. The number increases when the flayed skin is being processed to leather, due to contact with various equipment during tanning processes. Without proper treatment the skin will experience putrefaction, which is the process of decay in body, and loses its value.
Figure 1. Leather
To create fine leather, hide and skin need to be processed through multiple steps. The steps of this process can be divided into 3 major categories: pre-tanning, tanning and finishing. Pre-tanning prepares the skin and hide in order to ease penetration by tanning agents during tanning steps. During tanning, the tanning agent penetrates into the collagen structure and bounds to it. Finishing step is for manufacturers to apply different chemicals to tanned skins to achieve their desired properties.
Within 2 hours after the skin is flayed, proper treatment to preserve the skin is necessary to stop its rapid putrefaction. Introducing solution such as salt with biocides helps its preservation and to stop the skin to lose its commercial value. The salt helps to draw water out from the skin, avoiding bacteria to develop inside.
The dehydrated skin is then soaked with water to keep it hydrated. Water is used as a vehicle for both chemical penetration and removal, also as a prerequisite for future processes. Soaking can also be used to remove non-structural protein and fat.
The skin is then steeped into alkali solution to break down its hair structure. The hair root is being targeted together with keratin (an important hair protein) by breaking down sulphur-sulphur bond inside its cysteine linkage.
The hairless skin is then soaked in alkali and lime to remove keratin protein. During the liming process, the skin structure swells because water is drawn into collagen fibre network and forms a turgid, open structured, translucent and jelly-like material. The bond between epidermis and dermis also weakens and characteristic pattern is created after the liming process
Weak acid is introduced during this step to lower the pH of the skin. The swell reduces and the water flush out any impurities with it.
The bateing process makes hides pliable and prepare them for tanning process. Enzyme is introduced here to further open skin structure and to make pelt smooth and silky. Proteases are used to remove scud and unwanted proteins. The process also deswells swollen pelts.
Bated pelt is later treated with acid to obtain a pH for optimal penetration of tanning agent.
The prepared skin is tanned lightly to improve the penetration and distribution of tanning chemical by adding specific properties into the leather
Allowing tanning agent to enter skin and penetrates into the collagen structure and bound to it through basification process. The tanning stabilizes skin structures by replacing collagen with ions such as chromium.
Figure 2. Tanned Hides
The excess acid on the leather is neutralized to allow anionic chemicals such as dyeing agent to react with the leather substrate. Subsequent reactions such as dyeing utilize anionic chemicals for their colour. The removal of unwanted acid therefore becomes a necessary course of action.
Chromium tanning turns the hide It must be dyed to obtain desired colour.
- Fat liquoring
Tanned fibre is treated with reactive oil to serve as a lubricant.
To make the chemical binds better by drying the tanned leather inside heated tunnel for 4-6 hours.
Several finishing chemicals are added to ensure the compatibility of the skin and to hide defects and improve its display.
By : Vincent Kiathadi