Liming (leather processing)
Encyclopedia
In Liming
for parchment
or leather
processing, the hides are soaked in an alkali
solution by a drum/paddle or pit-based operation where four main objectives are met. The objectives are :
Liming operations of cattle hides usually last 18 hours and are generally associated with the alkaline phase of beamhouse operations.
by the presence of alkali (particularly sodium sulfide), rendered soluble, facilitating their removal from the leather. Removal is done by the mechanical action of liming or reliming, but more prominently when the pelt is deswelled (during deliming). Failure to remove these proteins results in a hard, tinny leather (due to fibre glueing upon drying) that is brittle and inflexible.
that is present in the hair, scales and in the epidermis of the skin is hydrolyzed
in the presence of alkali
(at pH values greater than 11.5). The disulfide
bridges found in keratin protein are cleaved but can be reformed. Long periods of liming will result in hair removal. The main removal of keratin is performed using the unhairing operation. In traditional processing liming/unhairing
was indivisible and took place at the same time. Modern liming methods, and in particular, the processing of sheepskins the hair is removed first and then limed in a liming drum. In hair-save technology, the hides are unhaired first and then limed for a further 12-18 hours.
results in the alkaline swelling of skin. The result is the influx of water into the hide/skin and a marked increase in fibre diameter and fibre shortening. The thickness of the skin increases, but the surface area of the pelt decreases. The weight increase, due to the uptake of water results in a doubling of the hide/skin weight. But this weight also needs to take into consideration that proteins (especially the hair) have been removed and the fleshing operation is often performed after liming.
, retanning, dyeing
and fatliquoring.
Liming (leather processing)
In Liming for parchment or leather processing, the hides are soaked in an alkali solution by a drum/paddle or pit-based operation where four main objectives are met...
for parchment
Parchment
Parchment is a thin material made from calfskin, sheepskin or goatskin, often split. Its most common use was as a material for writing on, for documents, notes, or the pages of a book, codex or manuscript. It is distinct from leather in that parchment is limed but not tanned; therefore, it is very...
or leather
Leather
Leather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.-Forms:...
processing, the hides are soaked in an alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
solution by a drum/paddle or pit-based operation where four main objectives are met. The objectives are :
- Removal of interfibrillaryFibrilFibril is a fine fiber approximately 1 nm in diameter.Cytoplasmic fibrils are observed on the protoplasmic cylinders found in most spirochetal species, although no function of the cytoplasmic fibrils has been ascribed....
proteinProteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s. - Removal of KeratinKeratinKeratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
proteins. - CollagenCollagenCollagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
swelling due to the alkaline pHPHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
. - Collagen fibre bundle splitting.
Liming operations of cattle hides usually last 18 hours and are generally associated with the alkaline phase of beamhouse operations.
Removal of interfibrillary proteins
The interfibrillary proteins are denaturedDenaturation (biochemistry)
Denaturation is a process in which proteins or nucleic acids lose their tertiary structure and secondary structure by application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent , or heat...
by the presence of alkali (particularly sodium sulfide), rendered soluble, facilitating their removal from the leather. Removal is done by the mechanical action of liming or reliming, but more prominently when the pelt is deswelled (during deliming). Failure to remove these proteins results in a hard, tinny leather (due to fibre glueing upon drying) that is brittle and inflexible.
Keratin removal
KeratinKeratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
that is present in the hair, scales and in the epidermis of the skin is hydrolyzed
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
in the presence of alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
(at pH values greater than 11.5). The disulfide
Disulfide
In chemistry, a disulfide usually refers to the structural unit composed of a linked pair of sulfur atoms. Disulfide usually refer to a chemical compound that contains a disulfide bond, such as diphenyl disulfide, C6H5S-SC6H5....
bridges found in keratin protein are cleaved but can be reformed. Long periods of liming will result in hair removal. The main removal of keratin is performed using the unhairing operation. In traditional processing liming/unhairing
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:...
was indivisible and took place at the same time. Modern liming methods, and in particular, the processing of sheepskins the hair is removed first and then limed in a liming drum. In hair-save technology, the hides are unhaired first and then limed for a further 12-18 hours.
Alkaline collagen swelling
The presence of calcium hydroxideCalcium hydroxide
Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...
results in the alkaline swelling of skin. The result is the influx of water into the hide/skin and a marked increase in fibre diameter and fibre shortening. The thickness of the skin increases, but the surface area of the pelt decreases. The weight increase, due to the uptake of water results in a doubling of the hide/skin weight. But this weight also needs to take into consideration that proteins (especially the hair) have been removed and the fleshing operation is often performed after liming.
Collagen fibre bundle splitting
The action of liming, in particular the swelling of the skin, results in the splitting of the fibre bundle sheath. Due to the fibre diameter increasing, the bundle sheath cannot contain the thicker fibres and it bursts open. This allows increased access to the fibres which allows better tanningTanning
Tanning is the making of leather from the skins of animals which does not easily decompose. Traditionally, tanning used tannin, an acidic chemical compound from which the tanning process draws its name . Coloring may occur during tanning...
, retanning, dyeing
Dyeing
Dyeing is the process of adding color to textile products like fibers, yarns, and fabrics. Dyeing is normally done in a special solution containing dyes and particular chemical material. After dyeing, dye molecules have uncut Chemical bond with fiber molecules. The temperature and time controlling...
and fatliquoring.