Chemical Processes Involved and the Effect on the Environment of This Tanning Process
Animal’s skin is soft, flexible, very tough and hard when it’s alive. And does not allow water in but allows it to easily pass. For the reaction to occur with fibrous protein collagen which is the main building block of skins and hides, these chemicals have to enter the pores. The pores will have many different sizes as the skin surface is uneven.
Step 1 – Unhairing and Liming http://wwwchem.uwimona.edu.jm/courses/CHEM2402/Textiles/Leather.html In this process the animal skins are steeped into an alkali solution which then breaks down the structure of the hair bringing it to its weakest point which is the root which makes the removal of hair easier. For the hair to weaken, the disulphide link of the amino acid called cysteine needs to be broken down. This is a characteristic of a protein that gives strength to hair and wools. After soaking the hides and skins are taken for liming which is a treatment with milk of lime (calcium Hydroxide) Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l) as basic agent which involves the use of sharpening agents also known as disulphide reducing agents. Disulphide reducing agent weakens the cysteine molecular link as it supply’s hydrogen atoms which weakens the covalent disulphide bonds as it’s ruptured.
Liming is done to weaken the fibres which allows the animal skin to absorb chemicals which are later used in the tanning process. The main objectives of liming is the removal of fibril proteins, removal of keratin proteins, collagen swelling due to the alkaline pH, collagen fibre bundle splitting and removal of natural grease and fats.
Sodium sulphide (Na2S) denatures the fibril proteins meaning it renders it soluble which facilitates in the removal of hair from leather. During Keratin removal the keratin present in the hair is hydrolysed in the presence of alkali mainly at pH values greater than 11.5. Hair loss is introduced by the destruction of the cementing substances prokeratines and glycoproteins in the root of the hair (proteins). The amino acid cysteine is part of the prokeratine structure and features a disulphide bond which is broken apart due to the addition of the liming agent.
Using calcium hydroxide ((Ca(OH)2) causes the alkaline swelling of skin. This causes an increase in the thickness of the skin but a decrease in the surface area of the pelt. Because of the increasing fibre diameter the sheath bursts open as it cannot contain the thicker fibres. This gives more access to the fibres and hence allowing for better tanning, re-tanning, dyeing and fat liquoring.
Hydrolysis of amide groups
http://www.leathernet.com/tanning.htm http://www.blcleathertech.com/images/db/dt_leather-journal/36/JournalJulAUG08.pdf http://sci-pak.org/LinkClick.aspx?fileticket=DMT84oFYGUo=&tabid=76 http://www.theseus.fi/bitstream/handle/10024/58540/FINAL%20FINAL%20THESIS%20APPROVED1.pdf?sequence=1 Step 2 – Deliming and Bating
This is the process of neutralizing the calcium hydroxide (alkali) used in liming and unhairing hides and skin. Basically this step is carried out to reduce the excess liming agent in the fluid stream. The pH reduction is necessary of the hide/pelt from 12.5 for a suitable pH for it to be ready for bating. As the pH reduced and is as a result a reduction in swelling. And hence becomes less sensitive to high temperatures. There are several deliming agents that can be used. The conventional deliming agents are ammonium sulphate and ammonium chloride. They follow the following chemistry:
Now the ammonium ion will penetrate the pelt cross-section and further ionise, and now acts as an acid:
The advatnages of this method of deliming is that its cheaper then other methods and buffer at pH suitable for typical bating enzymes. But there are disadvanatges of this method, which is that ammonia is