by Dr. Tereza
Varnali
| Bogazici University Faculty of Arts and Sciences Department of Chemistry Bebek 80815 Istanbul - Turkey Tel: + 90 (0)212 2631540/ e-mail: @boun.edu.tr |
Primitive man covered himself with the skins of animals he killed.
They had three major defects:
1) they were damp,
2 )they would putrefy,
3) they lost their flexibility and softness upon drying. (They dried the skins to stop putrefaction).
The making of leather is one of the oldest crafts (>3000years). Hides and skins are turned into leather by tanning.
Tanning causes following changes:
1) putrefaction does not take place,
2) on drying, the skin remains flexible
Hides and skins ( hides- of larger animals; skins- of smaller animals ) become durable and capable of being used for a wide range of purposes. Many uses of leather demand different properties. These are obtained by choice of raw material and variation of processes. Skins of mammals, ox, cow, calf, buffalo, sheep, goat, pig and horse form the main raw material but kangaroo and camel may also be used. Marine animals (whales, seals, sharks and bony fish) and reptiles (alligators, snakes,lizards) are processed as well.
The technology of leather making is in its broadest sense, a series of operations which aim at isolating collagen by removing noncollagenous components of skin and then at making it resistant to physical, chemical and biological factors. First part is carried in tannery beamhouse, the second in tanning and finishing departments.
CHIEF PROCESSES IN LEATHER MANUFACTURE
1)Pre-tannage (beamhouse operations)
a) flaying- removing the skin from the animal
b) curing- preserving during transport or storage
c) washing(soaking)- restoring to raw condition
liming- loosening hair follicle, fat, etc. and "plump" up the skin for tanning, deterioration of epidermis
e) unhairing- removing hair
splitting and smootheningfleshing- cutting away unwanted fat and flesh
g) deliming- neutralizing the alkali (from d)
bating- enzymatic loosening of hide fibres.,claening the skin(softens)
pickling,drenching or souring- adjusting pH for tannage
The purpose of these operations is to increase the amount of water in the hide to the amount close the that of the "living" hide, remove foreign bodies and loosen structure. This loosening makes it easier for the tanning agents, fats, dyestuffs and other substances, to penetrate into the hide. In the beamhouse the non collagenous proteins are removed from the hide, so is its epidermis, hair and globular proteins, melamins, components of cell walls, while collagen fibre skeleton remains practically untouched.
2)Tannage (tannery operations)
tanning by appropriate method
3)After tannage (finishing operations))
a) shaving or splitting- to achieve uniform thickness
b) washing- discarding surplus chrome salts
c) neutralizing- adjusting pH
d) dyeing- to get required color
e) setting out- removal of wrinkles and flatten
f) stuffing- impregnating with oil and fat (ie. waterproofing)
g) oiling- making flexible and of good color
h) drying
i) rolling- compessing for firming and flattening
There are many variants on this simple outline. All these processes, their choice and control which determine the quality of the leather made form the basis of Leather Technology. In almost all stages, substances are moving either into or out of the skin or hide. Process of wetting back, conditioning and drying involve mainly the movement of water. During liming, deliming, pickling and neutralizing salts, acids and alkali are involved. In tanning, dyeing and fatliquoring, various chemicals move into the skin, while in liming and bating, unwanted materials migrate to the surface and pass into the surrounding liquor. There is a large variation in the pH .
The pH of processes:
Pickling 1-2
Cr tanning 2-4
Vegetable tanning 3-5
Neutralizing and Fatliquoring 4-6
Deliming 5-9
Bating 7.5-9
Oil tannage 6.5-10
Liming 12-13
Dyeing acid dyestuffs 3-4
direct dyes 4-6
special types 6-8.5
In a wet rawhide, the peptide groups and particularly acid and basic groups hydrate. Water molecules are attached to and bound to these groups. The more water attracted to the protein molecule, the more it becomes separated from the adjacent molecule, so that the molecules are pushed apart and the skin is said to swell. By increasing the ionization of either the acid or basic groups by the addition of alkali or acid respectively, the attraction for water is increased and the skin swells or "plumps" more.
In a wet rawhide containing 70% water, the bulk of water is mechanically held as free water and its loss doesn’t create a hardening effect, however when the drying has proceeded so that the hide contains ~25% water, the bulk of this 25% is chemically bound(hydrated) to the peptide and aminoacids of the skin and as this is removed by drying, hardening and stiffening will occur.
SKIN STRUCTURE
Mammalian skin is an organ fulfilling many physiological functions such as regulation of body temperature, storage of body requirements, protection, elimination of waste products, sensory detection and communication. Age, sex, diet, environment, stress and state of health reflect itself. Fresh hides consist of water, protein, fatty materials and some mineral salts.
The most important for leather making is the protein. This protein may consist of many types: Collagen- on tanning gives leather; keratin- constituent of hair, wool, horn and epidermal structures.
Approximate composition of a freshly-flayed hide:
Water 64 %
Protein 33 % (structural proteins and non-structural proteins)
Structural proteins:
Elastin 0.3 %
Collagen 29 %
Keratin 2 %
Non-Structural proteins:
Albumins, Globulins 1 %
Albumins are soluble in water and dilute salt solutions,acids and alkali. Coagulates by heat; Globulins are insoluble in water but dissolve in salt solutions of moderate concentrations. They are insoluble in strong salt solutions and coagulate by heat.
Mucins, Mucoids 0.7 %
Fats 2 %
Mineral salts 0.5 %
Other substances 0.5 % (pigments, etc. )
A cross section of the skin
FIGURE
Starting on the hair side:
a) the hairs- each in a hair follicle with a hair root at its end, fed by a tiny blood vessel. Hair consist of protein keratin.
b) epidermis- the interface between the delicate tissues within a body and the hostile universe.A protective layer of keratinous cells. Keratin gives the skin considerable mechanical strength and flexibility. It is quite insoluble and serves to waterproof the body surface. It is readily attacked by bacteria, easily disintigrated by alkalis, such as caustic soda, lime and sodium sulphide or hydrosulphide. This is the basis of the unhairing process in the limeyard when the lime and the sulphide destroy hair roots and soft underside of the epidermis.
c) sweat glands- discharge sweat through the pores of the grain
d) sebaceous glands- at side of hair follicles, discharge a waxy oily substance to protect hair.
e) corium- network of collgen fibres. Strongest part of the skin. Towards the center, fibres are coarser and stronger. Predominant angle at which they are woven can indicate properties of leather. If fibres are more upright and tightly woven, a firm hard leather with little stretch is expected. If they are horizontal and loosely woven, a soft stretcier leather is expected.
f)flesh- next to the meat, fibres are more horizontal, fatty (or adipose) tissue may also be present.
In living skin, all these collagen fibres and cells are in a watery jelly of protein-like structure. The living collagen fibres are formed from this subsance, which consequently ranges in constitution from the blood sugars to substances which are almost collagen- inter fibrillary proteins also known as pro-collagen or non-structural proteins. When dried, convert to glue like material to make skin hard. In making leather, which is to be soft or supple, it is important to remove these interfibrillary proteins.
Excessive growth of fat cells weakens the corium fibre structure.
Corium fibres are composed of rope-like bundles of smaller fibrils which consist of bundles of sub-microscopic micelles. These in turn are made of very long, thread like molecules of collagen twisted together. This gives a very strong, tough, flexible structure.
COLLAGEN
Collagen is an extracellular protein organized into soluble fibers of great tensile strength. A single molecule of Type I collagen has a width of ~14 A, and a legth of ~3000 A. It is composed of 3 polypeptide chains. It has the shape of a rod. If it had the thickness of a pencil, it would have the length of 1.5m. This rod is reinforced by crosslinking bonds.
A single chain of collagen is defined as an a -chain. Each collagen molecule consists of three a -chains usually identical. The only known exception is Type I collagen. Type I collagen consists of two identical chains (a 1) and one different chain (a 2) which is denoted as [ a 1(I)] 2 a 2. It is the only heteropolymer among collagens. Index I is used because the chains in particular collagen types differ slightly in their amino acid composition.
The amino acid sequence is a typical feature of protein, determining its structure as a whole. Collagen, contains 19 amino acids, among which are two that do not occur in other proteins i.e. hydroxyproline and hydroxylysine. Besides collagen contains more glycine than most other proteins, but it does not contain cysteine, cystine (with exception of collagen III) and tryptophan.
The unique shape and properties of the collagen molecule are due to its amino acid composition and sequence. Collagen has a distinctive amino acid composition and sequence: Gly-X-Y (Glycine, X is often Proline and Y is often 4-Hydroxyproline -with some 3-Hydroxyproline and some 5-hydroxylysine). Hyp confers stability upon collagen, probably through intramolecular hydrogen bonds that may involve bridging water molecules.
Pro residues are converted to Hyp in a reaction catalyzed by prolyl hydroxylase. If collagen is synthesized under conditions that inactivate prolyl hydroxylase, it loses its native conformation (denaturation) at 24 C, whereas normal collagen denatures at 39 C (denatured collagen is known as gelatin). Prolyl hydroxylase requires ascorbic acid (vit-C) to maintain activity. If there is Vit-C deficiency, disease scurvy, collagen can not form fibers properly, this results in skin lesions, poor wound healing.
The typical features of collagen are:
The number of glycine residues amounts to 1/3 of all amino acid residues.
The number of iminoacids residue is 1/5 of all amino acids residues in mammals and birds. (The name iminoacid is currently used in biochemistry though it is not quite correct since those compounds are derivatives of pyrollidine not imines. Systematic name of proline is pyrolidine a -carboxylic acid and that of hydroxyproline is b - hydroxyprolidine - a - carboxylic acid.)
The presence of two specific hydroxyamino acids: hydroxyproline, hydroxylysine.
The presence of certain amount of aldehyde groups (participating in crosslinking bonds).
The presence of hexoses bound to protein side chains.
The occurrence of characteristic hydrophilic and hydrophobic space groupings in a chain.
The average molecular weight of one residue 90.7.
The number of aminoacid in a chain amounting to about 1,000 on the average.
The average molecular weight of one chain amounting to about 90,000.
Collagen at present is a great protein of known sequence. Details regarding this sequence are given in monographs.
By generalizing, we can describe the discussed sequence as follows:
The collagen a -chain consists of a central helical part containing 1011-1047 aminoacid residues of which every third must be glycine.
The helical part contains ~ 20% iminoacids in the second or third positions, if we divide the molecule in tripeptides, each of which starts with glycine (G-X-Y). In mammals collagen about 2/3 of the iminoacids are hydroxylated and are always in the Y position (4-hydroxyproline). The only exception is 3- hydroxyproline which occurs in the X-position however once or twice in the chain only.
The nonhelical extensions are relatively rich in hyrophobic aminoacids and contain a lysine redisue which can be enzymatically oxidized and serves as a functional group for the formation of intra and intermolecular crosslinks.
Hydroxylysine is occuring exclusively in collagen. It is the only aminoacid glycosylated at several sites but not every residue in the chain. Lysine like proline is hydroxylated only when it is in the Y-position.
The average content of proline plus hydroxyproline is equal throughout the chain, except for the C-terminal, which terminates with 5 consecutive three peptides Gly-Pro-Hyp. This suggests an exceptional stability of the C-terminal helical region of the molecule.
Conformation of collagen chain:
X-ray studies show that collagen ‘s three polypeptide chains are parallel and wind around each other with a gentle right handed rope like twist to form a triple-helical structure. Every third residue of each polypeptide chain passes through the center of the triple helix, which is so crowded that only a Gly side chain can fit in there. Also the three polypeptide chains are staggered so that gly, X and Y residues from the three chains accur at similar levels. The staggered peptide groups are oriented such that the N-H of each Gly makes a strong H-bond with the carbonyl oxygen of an X residue on a neighboring chain. The bulky and relatively inflexible Pro and Hyp residues confer rigidity on the entire assembly.
As with the twisted fibres of a rope, the extended and twisted polypeptide chains of collagen convert a longitudinal tensional force to a more easily supported lateral compressional force on the almost incompressible triple helix. This occurs because the oppositely twisted directions of collagen’s polypeptide chains and triple helix prevent the twists from being pulled out under tension..
The repetitive sequence in collagen which is called the helical region consists of an infinite set of points, lying on a screw line and separated by a constant axial translation.
Constant axial translation h (unit height)
Angular separation t (unit twist)
Radius of helix r0
Pitch P = 2 p h / t
P/h may be expressed as the rational fraction n /V , which means that the discontinuous helix has n points in V turns.
Number of points N per turn is found from the expression
N = 2 p / r = P / n = n / V , N being negative for the left hand helix.
Freser 1979: h=2.98 A0 Ramachandran: h=2.91 A0
t = 107 0 t = 111 0
N= 3.36 N= 3.25
Synthetic polytripeptide (GlyProPro)n h=2.87 A0
t = 108 0
N= 3.33
The non-integer number of residues in one turn could not be explained until Ramachandran and Kanthen’s suggestion was accepted which states that the molecule has the form of a three-strand rope in which the individual chains have a left hand helical conformation and the three chains are twisted around a common axis with a right hand rope twist. In this model two H-bonds per tri peptide have been acceped.
Ramachandran and Chandrasekharan suggest that
"Collagen has one bonded structure which contain water bridges."
Rich-Crick suggest a model with t=108, N= -10/3, P is 30 units hights of the basic helix (86 A long). The water bound to the chains do not affect the symmetry if it is accepted that more than one water molecule is involved in a bridge.
Considering the optimal interactions of the adjacent a 1(I) chains, the molecules align with an axial stagger of 233 residues which is consistent with the quarter stagger hypothesis.
Many authors have approached the question of energetics of collagen molecule through investigation of its thermal stability and denaturation thermodynamics (shown for globular proteins). For the denaturation process involving over 30 residues, the micro process(micro unfoldind) has Gibbs energy of the order 7-11 kJ/mole, macro process(macro unfolding) energy of 200-400 kJ/mole. The total values for D H were found to be 4,000-6,500 kJ/mole. D S=14-21 kJ/mole.
In addition to the enthalpy D H, we have two main criteria for estimating the strength of H-bond in the A-H……B system: The A-H stretching frequency or its relative shift (n 0- n ) /n 0 (Where n 0 is stretching frequency of the free A-H group) and the distances ( R ) A-H and A………B. According to these criteria H-bonds may be regarded as weak, intermediate and strong. For the OH……….O bonds this approximate classification is as follows: .
|
H-Bond |
D n / n 0 |
RO…O |
D H |
D H |
|
(%) |
(A0) |
kcal/mol |
kJ/mole | |
|
weak |
12 |
2.7 |
5 |
21 |
|
medium |
12-22 |
2.7-2.6 |
6-8 |
25-33 |
|
strong |
25-83 |
2.6-2.4 |
8 |
33 |
The length of H-bonds in collagen is approx. 3A0 .
most occuring ones:
C=O………..H-N
also C-H…………O=C,
N-H…………N-
If AH………B has Potential Energy curve as shown the bond is strong or moderate. For A-……..HB+ system well II is deeper than I. Finally, the potential energy curve may be symmetric when the potential barrier is small or equal to zero a "hesitating proton" is involved. Thus we distinguish: an asymmetric double minimum, a symmetric double minimum, and asymmetric single minimum with RA-H = ½ RA……B (then usually A=B)
FIGURE
The knowledge of the character and properties of crosslinking bonds is of great importance to tanning chemistry. The splitting of these bonds increases solubility of collagen, which decreases the shrinkage temperature. Increase in the amount of these bonds, which is equivalent to tanning, has an opposite effect.
Crosslinking reducible covalent bonds (only 2 examples given here):
Dehydro-hydroxylysino-norleucine
COOH OH COOH
I I I
CH-CH2-CH2-CH-CH2-N=CH-CH2-CH2-CH2-CH
I I
NH2 NH2
hydroxylysine-5-keto-norleucine
COOH OH O COOH
I I II I
CH-CH2-CH2-CH-CH2-NH-CH2-CH2-C-CH2-CH2-CH
I I
NH2 NH2
are typical components of such bonds. The first of the above occurs in skin.
The second of the above occurs in cartilage.
Collagen is organized into distinctive banded fibrils that have periodicity 680 A (with hole zones and overlap zones). Collagen contains covalently attached carbohydrates in amounts that range from ~0.4 to 12 % by weight depending on collagen’s tissue of origin. The carbohydrates which consist mostly of glucose, galactose and their disaccharides are covalently attached to collagen at its 5-hydroxylysyl residues by specific enzymes. They are located in the "hole " regions of collagen fibrils.
The supposed existence of an ester-type bond, via hexose residue, probably derives from the fact that saccharide units have been found in collagen, which are attached to hydroxylysine by glycosidic linkage in the helical region of the molecule, either as galactosyl-hydroxylysine or glucosyl galactosyl hydroxylysine.
Type I and II collagens contain about 0.4% carbohydrates and type II contain about 4 %. The major sites of glycosylation are those involved in the intramolecular crosslink. To date no experimental evidence has been made that would demonstrate the function of these carbohydrates. It has been thought that they may regulate the formation of crosslinks and aggregation of collagen molecules into the quarter stagger arrangement.
Collagens insolubility in solvents is explained by the observation that it is both intramolecularly and intermolecularly covalently cross-linked. The cross-links cannot be disulfide links, as in keratin, because collagen is almost devoid of Cys residues. Rather, they are derived from Lys and His side chains. Up to four side chains can be covalently bonded to each other. The cross links do not form at random but tend to occur near the N- and C- termini of the collagen molecules. The aspects of crosslinking are closely related to molecule aging. Degree of crosslinking increases with the age of the animal (meat of older animals tougher)
In early postnatal tissues the amount of reducible crosslinks is high and decreases as the physical maturity progresses. The stable crosslinks replacing the reducible ones have not yet been determined with certainity. Alterations of the physical and chemical properties of collagen fibres due to aging are very distinct. The fibers become increasingly insoluble, their ability to swell in acid solution decreases and so does the susceptibility to enzyme attack, whereas their mechanical strength and stiffness increases. The stiffness increases through the whole lifetime, creating brittleness which results in the decrease of tensile strength. When artificially introduced crosslinks give rise to more than the optimum number of crosslinks, the connective tissue becomes brittle.
No position in the central part of the molecule is susceptible to proteolytic attack (Proteolytic enzymes: peptidases and proteinases) pronase, pepsin or tripsin.
KERATIN
Keratin is insoluble in water,dilute acid, and dilute base. Keratins such as hair, epithelium, wool, contain up to 20% of non keratins, are only slightly cross linked and easily degraded. Cystine is present in considerable amounts and forms disulfide bonds.
H-C-SH H2C-S-S-CH2
I I I
H-C-NH2 à NH2CH HCNH2
I I I
COOH HOOC COOH
Transition of disulfide groups into thiol groups is equivalent to transition of stable compound into unstable one. Under action of alkali substances Na2CO3, NaOH, Na2S cyanides and borates the disulfide group is transformed. Hydrolitic splitting of disulfide bond when sulfur is bound to peptides occur at pH 10.6.
R-CH2-S-S- CH2-R + OH- à R-CH2-S-OH + -S- CH2-R
NON-PROTEINOUS SKIN COMPONENTS
Glycosaminoglycans:
They are typical polyelectrolytes of cellular and exracellular organic fluids . They control the viscosity of those fluids , act as buffers in tissue , participate in transport of ions and influence the water economy of the system due to their hygroscopicity .
Soaking and liming of skin are probably controlled by function of glycoaminoglycans .These substances have a characteristic skeleton of molecules typical for carbohydrates , functional groups such as - NH+,-COO- and -SO3- and their specific distribution.
From physiochemical point of view , glycosaminoglycans are representatives of polyelectrolytes ; ie polymers in which ionizing functional groups are
(-CH2-CH-)n
I
SO3 -H+
In glycoaminoglycans monosaccharide molecules occur bound by a and b glycoside bonds . This structure gives the molecule some stiffness . The only possibility of rotation of the molecule is around these bonds. In the compounds discussed , many H-bonds , many intramolecular and inter molecular occur . Glycosaminoglycans usally occur in extra cellular spaces where they fulfill a structural function imparting plumpress and flexibility to animal tissue .
Glycosaminoglycans are polysaccharides of animal origin which contain hexosamines. These compounds usually form complexes with proteins (mucoids).
It seems that proteins are bonded to saccharide part through sugar hydroxylic groups as well as to serine and tryptophane side chain hydroxyles . The group of glycosaminoglycans includes both acidic and neutral polysaccharides .
All glycosaminoglycans of animal origin have in common the fundamental hyalobiuronic acid link composed of the D-glucuronic acid residue connected with 2-deoxy-Dglucose by a 1,3-b -glycoside bond . The residues of hyalobiuronic acid are connected in a chain by 1,4-b -glycoside bonds .
FIGURE
The hyalobiuronic acid molecule is long , nonbranching polysaccharide chain with a considerable degree of hydration .The hydrodynamic volume of the molecule ie. the volume occupied by it in an aqueous solution , is almost twice as large as the real one .
For leather producing operations , especially for soaking and liming , the most important is its interaction with water. Important from the techonological point of view are some glycosaminoglycans of plant origin , which are applied as thickening agents in leather finishing. Pectins , derivatives of a -D -galactopyranosyluronic acid esterified in various extents , are biopolymers which occur freguently . Pectins are components of intercellular spaces in higher plants .
We do not know exactly the behaviour of glycosaminoglycans in leather making as it has not yet been experimented .
Fats:
From the point of view of tanning chemistry , fat in the skin is a component giving it flexibility, softness and stability . Natural fat is removed from skin in leather making processes , then it is necessary to apply fat to in the finishing process .
A significant amount of fat in the raw skin makes their processing difficult , because hydrophobic spaces are then formed , repelling water during soaking and because insoluble calcium soaps are formed during liming . Raw skins containing much fat have to be degreased before processing .
Inorganic components and their significance :
Lyotropic Hofmeister series :
Hofmeister found that cations and anions can be arranged according to their influence on protein solubility .
Anionic series:
citrate > tatrate > sulfate > acetate > Cl- > NO3- > Br- > I - > CNS->
cationic series:
Al+3>H+>Ba+2>Sr+2 >Ca+2>K+>Na+>Li+
The reason for such an order is the intensity of electrostatic field around the ions; small ions have more intensive fields than large ions of same valency . The intensity of the field of small ions is the reason of greater hydration , which is an immediate reason of ordering .
The tanner should remember that the ability of particular ions to solubilize proteins is equal to their peptidizing ability in leather making . This rule is important in soaking , liming , and bating as a part of non-collagenous proteins becomes dissolved in aprocess which is parallel to softening and swelling, if the ionic strength and the kind of ions are appropriate. Peptidizing in this case is not equivalent to dissolving only: In this process a part of the weaker peptide bods is split and the native proteins are thus converted into peptides with smaller molecules, which are easier soluble. This is due to the properties of the ions introduced..
SKIN COMPONENTS and WATER
Energy of ion hydration depends on charge and kind. For H+ approx. 176 kcal / mole ( 1156 kJ/mole ).
Ions of small radii and multivalent ones Li+, Na+, H3O+, Ca+, Al+3, OH-, F- increase the viscosity of water -they show a structure making ability . They produce, apart from polarization , the immobilization and electrostriction ( a dielectric deformation of molecules in the external electric field proportional to the square field intensity) of water molecules as well as the decrease of entropy ( due to additional ordering ) in the second hydration layer .
Large monovalent ions generally give a structure - breaking effect ( entalpy increase ) K+ , NH4+, Cl-, Br-, I-, NO3-, IO3, ClO4- ion increase mobility of water .
Nonpolar substances have a very strong structure forming influence on water (only observed in the first layer of water molecules). The water coordination number is increased to 5 and happens spontaneously. Water-water interaction does not change but hydrocarbon-hydrocarbon interaction decreases as hydrocarbon-water interaction is established.
Collagen -water system :
Water bound to collagen forms a kind a of chain , parallel to the collagen molecule chain .
There are two water molecules per tripeptide unit firmly bound by H-bonds to the helical part of the collagen molecule . Their residence time in their sites is about 0.1-1.0m s. This water accounts for more than 35 % of collagen weight. The remaining part of water , in a not strictly limited amount , which is in weak interactions with a number of different sites, forms a multilayer with liquid - like properties .
The first kind of water does not freeze at 0C0 . The strength of H bonds between `swelling` water of collagen is about 1-2 kcal /mol . There are no sharp limits between strongly and weakly bound water , nor between weakly bound completely free water .
Glycosaminoglycans and water :
Swelling of hyaluronic acid is greatest .
The swelling degree decreases with the increase of ionic strength of the NaCl soln . This may contribute to swelling on soaking .
Molecules penetrate the skin in a passive way - by diffusion . Three arguments speak in favour :
specific permeability remains unchanged even for a long time after skin is removed from animal .
diffusion obeys Fick’s law , an exception is Na and K ions which are actively absorbed by the skin
Fick’s law : p = r / c
p - permeability constant
r - penetration rate
c - concentration
Stratum corneum of epidermis is resistant to penetration and various compounds
( ie. Arrest alkyl phosphoric compounds) .
FLAYING
Butcher's job.
Important: bruises should be avoided
should be bled rapidly (cause blue-black marks)
should be removed immediately while warm (less chance of putrefaction)
FIGURE
ripping: ripping cuts must be located to give as square a skin as possible.
casing: small animals are not ripped but peeled off like a sock from the foot. These are kept in a dried condition (wool inside and flesh outside).
Saurians (lizards and crocodiles) are not ripped down the belly, as this is often the most valuable skin part. Ripping is carried out along the backbone.
CURING
For transport (from source to tannery) purposes, simple methods of stopping putrefaction arose. Drying is the most obvious method. Dry skin does not putrefy and can be soaked in water to return to the raw condition. Wet-salting, dry-salting, (or pickling with acid and salt) are other methods of preservation.
Wet-salting: the cold, flayed hide is spread out, flesh side up, on a concrete floor and well sprinkled with salt (coarse grained salt spreads better). A second hide is placed on top and also sprinkled with salt.
The salt dissolves in the moisture in the skin and the brine permeates the pile. Amount of salt (clean) used 25-30 % of raw hide weight.
Marine salt bacteria give rise to red or colored patches on the flesh. Their action can be stopped by mixing soda ash and napthalene with salt (for 44kg salt, 0.5kg napthanene and 1kg of soda ash is used).
Brining: more efficient. Hides are cleaned and hung in large paddles in a very strong salt solution (14kg salt to 45.5kg of cold water). Uniform salt penetration in 12-14 hours. Hides are then drained and piled.
Both brining and wet salting require large quantities of salt and the cured hide is still damp (50 % water).
Dry-salting: the flayed skin is salted by either or both of the above methods and then hung up to dry. This reduces weight and cost of transport.
important: drying should be gradual and even (parts getting too hot may gelatinize and dissolve away when put in water).
Drying: Activity of bacteria ceases when hide contains 10-14 % moisture.
important: drying should be gradual and even(parts getting too hot may gelatinize and dissolve away when put in water).
Ground dried- disadvantages: poor ventilaton on the ground side ,high temperature on exposed side.
Sun-dried- when laid or hung on poles or ropes, better ventilation and quicker drying but heat damage and rope marks may result .
frame dried-if put too tightly weakness and thinness may be caused
shade dried- dried open sided, covered shed, off the sun and well ventilated.
Dried hides require careful packing. Must not be bent or creased (cause cracks).
Dried hides are open to insect attack. Insectisides used for prevention.
Anthrax (sirpence) may be present on dry hides. May be fatal for workers that may be infected (destroys red blood cells). No danger after liming.
Pickling: Always used for hides after unhairing,liming and fleshing.
After unhairing, liming and deliming the skins are washed and then paddled or gently drummed in a 12 % salt solution (5.5kg per 45kg of cold water ~12%) 10 to 21 degrees C- to which 1 % or 1.2 % of sulphuric acid is added. Continued for 2 or more hours. Salt and acidity of the liquor should be checked to ensure salt concentration is still more than 10 % and acid concentration is still above 0.8 %.
May now be stored for several months (at above 320C, acidity may cause damage to skin).
All known putrefying bacteria stops activity at pH 2.0, but not mould growth.
Fungisides (at 1/1000 parts of liquor) used. ie. sodium trichlorophenate, sodium pentachlorophenate, beta-napthol, p-nitrophenol (may give yellow color).
Pickled skin should not be allowed to dry (acids or crystals may cause damage)
SOAKING (WASHING)
Important: Tannery water may be infected or may contain salts of calcium and magnesium bicarbonate (temporary hardness) or calcium and magnesium chloride, and sulphates (permanent hardness) plus carbonic acid. Precipitates cause patchy stains on the leather.
The first process consists of soaking the skins in water, the aim being to allow them to reabsorb any water which may have been lost after flaying, in the curing process or during transport. This absorbed water re-hydrates any dried inter-fibrillary protein, loosening its cementing action on the fibres. The collagen fibres and keratin cells of the hair and epidermis also take up water and become more flexible. Due to the water returning to interfibrillar spaces the fibers may slip one against the other and an adequate plumpness is imparted to the hide.
Wet salted hides may be soaked for 8-20 hours (10-160C). The amount of water used ranges from 3 to 5 times the weight of hides(6-7 times for dried skins). Satisfactory soaking is judged by cleanliness and absence of salt. Salt is determined in the juice squeezed out of the skin, using a pocket refractometer (refraction increases linarly with concentration).
This process is not simple, because putrefying bacteria may act as soon as there is surplus water or curing agent is washed out.
Common additions to the soak liquor as disinfectants (bactericides) are:
1 part sodium hypochlorite per 1000 parts water or 1.5 part to 1 part trichlorophenate per 1000 parts water.
Formic acid and pentachlorophenate may also be used. Speeding up the water uptake of the skin reduces the chance of putrefaction This can be done by
a) mechanical action - rocking frames, paddles, drums, green fleshing
b) temperature - as warmed up to 380C, the rate of bacterial action may increase, if temperature exceeds 380C, protein fibers tend to shrink, skins loose area, protein fibers gelatinize.
c) chemical additions :
- acid addition (used when hair or wool is kept on the skin), 1-2 parts of formic, hydrochloric or sulphuric acid per 1000 parts of water at 160C.
- alkali addition (more common as it looses hair),1-3 parts of costic soda, or soda ash or washing soda or borax per 1000 parts soak water. Sodium sulphide also gives alkali solution, and speeds up loosening of hair and epidermis. sodium polysulphide is less alkali and has a mild dispersing action on inter-fibre proteins giving smooth grains. Ammonia liquor has a safe, gentle swelling action, which opens up fibre structure. Ammonia and hydrogen peroxide, each about 2 parts per 1000 parts of water are favored for sheepskins, the wool not being loosened so much as with straight alkali.
There is the danger that if too much acid or alkali is used, the surface fibres of the skin will rapidly absorb it and swell so much that they distort the surface of the skin and block up the inter-fibrillary spaces,preventing the water from reaching the inside.This will give leather a loose grain.
- salt (NaCl) solutions of 3 % concentration dissolve unwanted inter-fibrillary protein, thus speed up soaking.
- wetting agents detergents 1-2 parts per 1000 parts of water (particularly if hides are greasy.
- enzyme preparations (proteolytic action on the interfibre proteins)
To controll the soaking properly, it is recommended to observe the following factors:
the pH of the solution, easing the swelling, and so the diffusion of bath components into the hide.Lowest degree of swelling is at isoelectric point.
Presence of salts(including NaCl), contained in soak water, as it influences water structure.
Surface tension at the water/hide interface, which is mainly depending on the fat content of the raw hide, and on the presence of surfactants in the solution.
THE PHYSICAL CHEMISTRY OF RAW HIDE AND CURING PROCESS
The hide of a live animal contains 62-78% water . Death causes dramatic change in metabolic process O2 and nutriton is cut out, removal of metabolites from the cell is stopped. Toxic accumulation enzyme controlled processes stop .
The process of self digesting ( autolysis ) of the cells starts intercellular enzymes cathepsins (peptide hydrolases ) .Autolysis does not cause change in flayed hide at r.t. even at 24 hours.
Autolysis of salted hides depend on temperature and amount of salt . The higher the temperature , the higher the autolytic process . However , the rate decreases with increasing salt concentration.
Common preservative like boric acid or sodium carbonate do not inhibit autolysis at all.
The yellow " salt " spots on hide arise from autolytic activity ( not from bacterial activity ) due to effect of alkaline phosphates in presence of calcium sulphate .
The secondary process accompanying autolysis is action of putrefactive bacteria for which autolysis products offer an excellent medium .
For bacterial growth certain humidity is required . Usually 30-35% , for molds it may be 12 -15 % .
Minimal temperature of possible growth is usually 5C0 higher or equal to to the freezing temperature of medium . The majority of bacteria find their optimal living conditions at neutral or slightly alkali pH , the majority of molds - at acidic ones ( approx. pH 5 ) .
Na2CO3 and naphthalene as antioxidant or trichlorobenzene is used for best prevention of bacteria.
Changes in collagen occur due to aging ( on storage ). Crosslinking in collagen is increasing (observed by phenomena: Ts , acid and base swelling and trypsin action) in vivo and post mortem.
HIDES AND SKINS CURING BY SALTING AND DEHYDRATING
The main problem in preservation of skins is to remove significant part of water and saturation of remaining water with salt usually NaCl . Also important are use of bactericides .
Cooling of raw skin to 25-30 C0 may be used as well but may easily be mechanically damaged (broken).
Saturation of the system with salt :
Flesh cattle contains 1.38 % of NaCl ( calculated on hide substances ). Dry salting, spraying dry salt on flesh side and flesh to flesh stacking on brining ( in saturated salt solution )., in both cases there is osmotic penetration of salt into hide .
Salt penetration at r.t. takes about 48 hours. Concentration remains lowest in middle layers . The rule is to use coarser salt for hides , finer salt for skins .A great amount of Mg and Ca sulfates in salt ( approx. 2 %) promotes appereance of `salt` spots . This is due to activation alkaline phosphatases in autolytic process . The salt spots do not arise when brine is used for preservation . This is due to precipitation of Ca+2 and Mg +2 compounds .
Amoung the efforts to replace salt by other chemicals that are less contaminating to waste water and used in smaller amounts is formaldehyde , a powerfull crosslinking agent and kills almost every microorganism . Authors recommend 0.25 % formaldehyde as preservant . In this concentration the leather is slightly firmer than usually obtained . This difficulty may be overcome by post-tanning tereatment . Excess formaldehyde may cause difficulties in unhairing . The amount proposed increases Ts from 64 to 68 C0 Addition of some ( 7 % ) salt makes hides mellower and with flatter grain . A Ts increase to 75 C0 is then observed .
Water removing :
The aim of curing is to remove water from tissue to such an extent that no irreversible changes in the collagen properties should take place .
When liquid is removed from the pores the porous body changes its shape . Interfibriller pores, during shrinkage ( walls approaching each other ) may be torn or be closed completely (due to high tension that may be occur in capillaries ) .
Curing of skin by drying is applied to fur skins as a primitive , uncontrolled way of preservation in hot climate .
Dehydration of hide by methyl alcohol or ethyl alcohol followed by ethyl ether is a process different from drying . This process is used in industry ( USA and CHECK ) as a process of quick introducing of tanning agents into hide , followed by quick tanning by water addition .
Freeze - drying : modern way of preserving skins .Skins are dried after freezing .Evaporation occurs in high vacuum and liquid state is omitted . Almost no changes in chemicaland phsical proporties are observed .
COLLAGEN SWELLING IN WATER SOLUTIONS
MELTING & SHRINKAGE TEMPERATURE
The purpose of soaking is to bring the hide to the same condition in which it was immediately after separation from the carcass. Recovered softness makes it easier to introduce small-molecule substances into the hide. During soaking the mechanical impurities: scud, blood, salt of other preservations used, are removed, a part of nonstructural proteins and remains of fat and meat. The hide becomes swollen in the process.
The collagen of glycosaminoglycons remain through the tanning process probably intact.
Mature crosslinked collagen is water insoluble but it swells. Extent of swelling is , in such a system, inversely dependent on the crosslinks number. In a fibre network the solvent may occupy the inter or intra fibrillar spaces, the general regularity however remains. Swelling of collagen depends on two factors. Osmotic and Lyotropic ones. Osmotic swelling ( Donnan swelling) occurs due to a high concentration of bound, nondiffusing ions located inside the structure. It takes place when pH of the solution is off the isoelectric point and the ionic strength of the solution is small. Greatest swelling effects may be observed at pH 2 and 12. It is reversible by straining of the fibres, changed pH or increase of ionic strength of the solution by increase of salt concentration.
Lytotropic swelling which is due to neutral salts at considerable ionic strength, decreases the cohesion of the fibres and is not completely reversible.
In heating the hide one observes the shrinkage of over 50% of the sample length. This is best observed if the sample is immersed in water. The temperature of shrinkage (Ts) depends on degree of crosslinking; it is lower for the raw hide, higher for the leather.
The non-swollen collagen is, a highky ordered polymer, which is synonymous with its crystallinity.
Osmotic swelling is due to pH change, when the ionic strength is small and temperature low. Changes of pH in the range 4 to 8 do not affect markedly the length and diameter of fibrils. Outside these pH values almost 10 fold increase of fibre volume may be observed. If pH drops below 2, when the volume decreases. The increase of ionic strength suppresses collagen swelling. The Donnan effect comes from the increase of charge bound at protein surface, as the pH is drifting away from the isoelectric point. According to Donnan’s theory occurrence of localized charges causes formation of excess ions having opposite charges inside the gel, which in turn initiates action of osmotic forces. Donnan effect does not elucidate satisfactorily the mechanism of attachment of solvent molecules to the biopolymer, although from the thermodynamic point of view it describes very well the influence of pH on the degree of swelling.
Lyotropic swelling may be observed in solutions of the salts, in which the forces, causing Donnan phenomenon are insignificant. One may be observe it at every pH if only salt concentration is high enough (over 0.5 molar) or in solutions having lower salt conc., and a pH neutral. Increasing salt concentration causes at first swelling increase (salting in) and then decrease (salting out) of swelling. Gelatin behaves like collagen. Comparing swelling effect of various salts have been ordered in a Lyotropic or Hofmeister series.
F-<Cl-<CH3COO-<NO3-<Br-<SCN-<SO4-2
K+<Mg+2<Na+<Cs+<Li+<Ca+2
HCl and H2SO4 have strongest swelling power. Maximum swelling occur in their 1.5% solutions.
Shrinkage - melting (phase transitions)
Swelling in nonaqeous solution and stabilization of structure of raw hide :
This may be observed if
Surface of tension of water in which leather is immersed is decreased by surfactants
The hide is treated with neutral salt soln.
The hide is dehydrated with organic solvents (ethanol, ether, acetone)
The hide is lyophilised.
The result of all operations or their combination is the removal of the substance which closes the pores during drying and considerable amounts of water, whereas the pore size do not change. The influence of dehydrating agents on hides is expressed by two factors: Volume decrease during drying and apparent specific gravity.
The characteristic property of a dehydrated hide is the loss of its properties after re-soaking in water. This process is reversible unlike the tanning. If however the dehydrated hide is impregnated i.e. by silicons it remains in this state even after multiple wetting and drying.
It is concluded that in a denaturation process besides intra molecular H-bonds, the bonds joining together the water molecules bound to protein become split. Thus the cooperative process of rebuilding the hydration layers is one of the peculiarities of "intra molecular" melting of the collagen macromolecule.
Hide behaves like an ionite resin, may behave like a molecular sieve or an inactive bulk polymer (depending on reference system).
Hide in the soaking process imbibes and binds more water, the lower the process temperature is. Simplest explanation is the increase of mobility of water molecules i.e. its entropy decrease with temperature. A substance or action increasing the distances among molecules has such an effect on an unordered system. Increase of ordering, however, is connected with entropy decrease. There is a temperature point (TO ot TS ) at which the long range order structure sharply decreases. Problems of water structure are related to hide-water system, in which the structure making function belongs to the functional groups of hide proteins and to structure making ions contained in the system. X-ray show long range interactions (7A0) disappear at higher temperature.
ENZYMOLOGY OF PROTEOLYTIC PROCESSES
Enzymatic processes after slaughter:
acetolytic process: cathepsins and other enzymes of hide itself are involved.
deterioration process: proteolytic activity of bacterial enzymes.
processes intentionally carried with enzymatic preparation soaking and unhairing
bating: enzymes attack on proteins "last one"
Proteolytic enzymes: peptidases and proteinases
Pepsin : active at low pH (formed in stomach mucosa) cleaves gelatin
renin : almost the same activity
Trypsin : (formed in intestine) most stable at pH ~3.
Chymotripsin : (formed in pancreas) most active at pH 8.
Cathepsin : intercellular enzymes pH 3.7 - 4.
Papain (from papaya) : optimum pH 5.5-6 but active in temperature 5-66 C0 of pH 2.8 to 10.8. Very broad.
Elastase : (formed in pancreas) pH 8.8 optimum activity
Collagenases : pH 8-9
Lipases : (hydrolyze fatty acid esters) attack ester bonds
UNHAIRING and LIMING
Loosening (depilation) or unhairing may be considered as an extension of soaking. Its purpose is to separate the two structural proteins keratin and collagen. The aim of unhairing (depilation) and liming is to remove the hair, epidermis and to some degree the inter-fibrillary proteins, and to prepare the hide for removal of loose flesh and fat by the fleshing process.
Unhairing:
Methods of hair removing can be divided into two groups:
methods based on destruction or modification of the epidermis tissue surrounding the hair, so that it can be loosened and removed
methods in which hair itself is attacked and its structure is destroyed(use of alkali CaOH or NaOH and Na2S).
-sweating : The earliest method was "sweating" and may still be used if the wool is of much greater value than the skins. Soaked skins are hung up in dark humid rooms (22-27 0C), bacteria attack keratin cells of hair and epidermis, until wool is loose. The wool is then "pulled" and sorted and the skins are rinsed and thrown in lime liquor to stop further putrefaction. The pulled wool is hydroextracted to remove water and dried to 16 % moisture in which condition it is marketed.
Cathepsins contained in the lysosomes of the cells of the skin are also participating in this process. For autolytic unhairing an optimal pH is about 4. In this ‘lysosomal’ unhairing probably proteins and gycosaminoglycans are equally attacked. In the products separated from the skin, hydroxy proline is found in some amounts, equivalent to 0.3% of skin collagen. This may be result of attacking of collagen-containing ‘lining of the hair pocket.
A development of sweating process is enzyme unhairing.
-enzyme unhairing: sweating is an incontrolled enzyme process. Enzyme preparations that preferentially attack the keratin cells at base of hair roots or epidermis are available.Enzymes are specific in their action and are active within a narrow range of temperature and pH. Since optimum temperature for unhairing enzymes are close to those for bacterial growth, it is essential to use some disinfectant to prevent bacterial putrefaction.
Enzymatic processes may give some difficulties in a tannery. Enzyme attack may be very vigorous, too much protein may be removed from the skin and collagen may be partly decomposed. The resultant hides are then thin and stiff. If the enzyme action is too weak, insufficient amount of protein will be digested, and additional operations necessary, like eg. Alkali swelling of hides before pickling(after liming). The enzymatically unhaired hides as a rule have to be tanned and dyed in a different way than hides processed by other techniques.
According to Felicjaniak, who invested in detail the unhairing activity of pancreatic enzymes, there is a distinct difference in unhairing and proteolytic action of those enzymes.To increase unhairing activity, the pelt has to be prepared by applying an inorganic chemical before enzyme is used. The compounds giving optimal results are ammonium chloride, thicyanate, sodium thiosulphate and some others. These substances increase the unhairing activity when used in 1% per pelt weight. Sodiumthiosulphate increases proteolytic(not unhairing) activity of the enzyme. Optimal unhairing activity was reacheed at ph 8-9 (somewhat higher than optimal proteolytic activity pHopt=7.5).
Soaked skins are paddled or drained in a water float 28-300C with 1-2 % of specific enzyme preparation at pH 8-9 for ~4 hours. Bacterial contamination may be prevented by adding 0.2 % sodium chlorite.
-paint unhairing: The washed or soaked skins are piled to drain off surplus water and then painted, or sprayed on the flesh side with a "paint" which may be made from approximately 50 parts hydrated lime, 50 parts water and 5-2- parts sodium sulphide (fused). The sodium sulphide and lime dissolves in the water and penetrate through the corium and dissolve the keratin cells which enclose the hair roots. The process may take 5-12 hours depending on thickness of the skin, tightness of fibre structure, and amount of fat and flesh left on the skin. Green fleshing before painting can be good.
Green fleshing is a method of giving some mechanical action. May be done by hand by scraping the flesh with a curved knife on a wooden beam or by a fleshing machine. Apart from the squeezing action loose fat, flesh or muscle tissue is removed, aiding entry of water from the flesh side. It also flattens and stretches the skin and has a cleaning action.
Advantages of the process:
-strong alkali (lime) and sodium sulphide prevent putrefaction therefore give better skins than sweating.
-with reasonable control, hair loosening is reliable.
-unhairing is quicker and owing to shortage of water in painted skins, the strong alkali cannot cause undue swelling, buckling and distortion of the skin.
-the amount of paint can be varied over the area of the skin, giving more to thicker backbone and less to thinner, loose flanks and bellies.
Disadvantages of the process:
-requires more labour
-wool yield is less (owing to the disintigration of wool roots)
lime and sodium sulphide damage hair and wool, causing a harsness to the touch or weakening of strength and eventually complete disintigration. This action is a function of sulphide concentration.
Variations of the process:
Sodium sulphide + water à caustic soda + sodium hydrosulphide
too much sodium sulphide may weaken the skin as it produces caustic soda in water. Adding wetting agents does not improve penetration unless used in large quantities (excessive spread down the wool shaft, with loss of wool or quality). On greasy skin, excessive soluble lime produce lime soaps which give water resistance to the skin and may result in poor tan.
The painted skins are usually given a modified liming after unhairing, to remove unwanted protein and prepare them for fleshing or splitting.
Alternative materials:
Sodium hydrosulphide when used instead of sodium sulphide gives no alkaline swelling, does not weaken the skin, causes less damage to the wool. The penetration power is not as good as sodium sulphide, especially on greasy skins.
Calcium hydrosulphide is even milder and gives very good wool and skins.
Arsenic sulphide was popular(before the above chemicals were introduced), poisonous.
If sharpened lime is used, pH is higher than in the absence of alkali, but the swelling is greater, despite the fact that the pH is higher than that of maximum swelling due to
( i) an increased breakdown of structural restrainsts
( ii)replacement of divalent calcium ions by monovalent sodium ions.
When Na2S is used for depilation the swelling effect is the same as that for NaOH, since H2S is a very weak acid.
When calciumhydrosulphide is used, the depilatory process can take place without any additional swelling. The tanner can vary independently depilatory and swelling effects.
LIMING
The major chemical modification of collagen during liming is the hydrolysis of some of the amido groups attached to aspartic and glutamic acid residues.
-CONH2 + HOH à -COOH + NH3 (pHdecrease)
as the carboxyl group can ionize:
-COOH COO- + H+
more potential negatively charged centers are introduced so that the isoelectric pH of collagen is invariably reduced by liming.
A small portion of arginine residues is also converted to ornithine and urea.
-CH2CH2CH2NH-C=NH à -CH2CH2CH2NH2 + CO(NH2)2
I
NH2
Apart from these reactions, some modifications of covalent cross-links may occur, especially of ester type, which are thought to join chains together.
The total effect is that liming produces a pelt which swells more at all pH values than does native, unlimed skin. Forces of swelling lead to a general loosening of the fibre- network layer and to the splitting of larger collagen fibres.
In normal tannery liming, apart from breakage of a few peptide main chains by hydrolysis, the fibrils swell and not show any marked changes in their general appearance. However they are cleaner, as interfibrillary material (mostly globular proteins and mucopolysaccharides) is removed. Thus liming helps to prepare a clean system of fibrils.
Liming may be enhanced by duration process, raising temperature,and raising pH of liquor.
Unhairing and liming may be carried out at the same time by immersing the hides and skins completely in lime and water mixtures, often with the addition of other chemicals called sharpeners.
Liming process may be carried out without lime in certain cases, e.g., with greasy skins the lime is replaced by other alkali such as caustic soda.
Straight lime liquors:
Water dissolves a relatively small amount of lime.And lime is unusual because less dissolves with increasing temperature. Approximately 1/8 parts lime per 100 parts water gives a clear solution.
The alkali solution causes the collagen fibre of the corium to swell by absorbing more water. The hair and epidermis swell to a lesser extent, and the interfibrillary proteins become more soluble and are loosened from the structure.
These effects occur with all soluble alkalis and the stronger the alkalinity, the greater the effect. As lime has a limited solubility compared with other alkalis, it is considered safe for hides and skins.
Under very alkali conditions, some of the young keratin decomposes to produce sulphur compounds, these, in conjuction with lime, accelerate the break-down of further keratin.Thus, the lime causes unhairing and the more keratin break-down impurities it contains, the more rapidly it unhairs. This is why old lime liquors are more effective for unhairing. All these reactions are accelerated by increase in time, temperature and high alkalinity.
The alkali also modifies and breaks down the collagen fibre of the skin, but much more slowly than the keratin. Therefore, if skins are limed too long they suffer from thinness, looseness and weakness.
For skins that heve already been unhaired: straight lime liquors of 2 parts lime per 100 parts water are used. The skins are immersed in about 5-6 times their weight of this liquor (in paddles or slowly revolving drums-agitated), for 12-60 hours at 13-18 C.
This quantity of lime is in excess of that needed to get the necessary alkalinity (pH 12-13) plus the lime chemically fixed to the hide. The surplus may be spent by adsorption on grease, loose protein or by carbonation. It also makes handling the slippery hides easier. However this excess lime often requires expensive effluent treatment.
One Pit Liming system:
Hides are laid or suspended in a pit in a similar lime solution. The undissolved lime tends to settle to the bottom of the pit and therefore not available for dissolving to maintain the 1/8 % solution. It may be agitated by moving the frame the hides are suspended from or by roocking a scraper arm moving along the pit bottom.
After 1-2 days in this liquor to allow the lime to start swelling the hides uniformly
and gradually, 1/8 - ¼ % sodium sulphide may be added. This is a sharpener that speeds up the process:
- by producing sodium hydrosulphide, which very quickly attacks the keratin, giving hair loosening;
- by producing caustic soda, which increases the alkalinity and therefore the rate of swelling.
On the third or fourth day, in the case of ox hides, a further similar addition of sulphide may be made to finish the process. If too much sulphide is added too quickly, however, rapid unhairing results, but this is accompanied by excessive swelling of the surfaces of the hide, while the interior is unswollen.
The used liquor may be drained away but as it has become stronger in unhairing power and less alkaline, it has good properties for starting the liming of the next pack of hides.
According to old technologists ‘art of tanning’ was the knowledge how to mix the fresh and used lime liquors. In fresh liquor swelling is stronger and opening up weaker, old liquors (often infected by microorganisms) are better hair looseners. A similar effect may be obtained by adding methyl amine to lime liquor.
The "modified one pit" or the "three pit" system may be used.
Modified One Pit system:
About half of the previously used lime liquor is left in the pit, which is then topped up with water. 1 % lime is added and the goods limed in this as before, except that the time and the amount of sodium sulphide added may be reduced.
Three Pit system:
This system is more thorough development of the same idea, the pack of hides being placed succesively in three pits for 2-3 days each.
The first pit contains a twice used lime liquor.
The second pit contains a once-used lime liquor with an addition of ½ % lime and ¼ % sodium sulphide.
The third pit contains a fresh liquor of 2% lime and ¼ % sodium sulphide.
All these percentages are based on the volume of the water. This is called a "counter current" system, the hides moving from pit to pit one way, whilst the oldest liquor A is run away and replaced with a fresh liquor C.
disadvantage: laborious
One liquor system more common (can controlled more accurately).
Drum Unhairing:
When the hair is of little value and the hides are of a quality which will not suffer from the process, they may be drummed in a relatively strong sodium sulphide solution, for example: 300 % water on hide wt. At 16 C, 2-5 % sodium sulphide (fused), 12 % salt.
After 6 hours the hair and epidermis are reduced to a pulp (lapa), which can be washed off, and the hides are well swollen. The salts limit the swelling, which may also be reduced by replacing part of sodium sulphide with sodium hydrosulphide.
1 % lime is often added to the sulphide solution .
NO LIME TREATMENTS are favored for thinner skins(hair, sheep,goat). Advantages are absence of lime soap formation which may cause uneven tanning and dyeing.
Hair or wool may be previously removed by a "lime free" sulphide paint. Usually sulphide or hydrosulphide is used adjusted to pH 12.6-13 at a liquor concentration of about 0.2 % caustic soda. As the temperature increases to 28-30 C less swelling occurs giving a finer flatter grain and although hydrolysis of the skin occurs this is slower and can be controlled by time(6-8 hours).
Sharpening Agents:
sodium sulphide- speeds up unhairing and alkaline swelling
sodium hydrosulphide and arsenic sulphide-speeds up unhairing only(not alkaline swelling)
caustic soda-sodium hydroxide-causes increased swelling only.
Sodium carbonate-mild alkali, reacts with lime to give caustic soda.
Ammonia- has a gentle swelling action on the skin, particular in early stages. It is formed naturally in old lime liquor.
Amines-dimethyl amines are added to lime liquors, having a gentle swelling action and helping hair removal.
Salt: at low concentration (below 2 % ) it increases the swelling or plumping action of the lime liquors, but at high concentration it tends to decrease the swelling or plumping and to give a softer, spongier type of leather.
Temperature: temperature control is important. An increase from 16-27 C in the liming temperature will halve the time required for loosening the hair; more significantly, it will double the rate of solution of collagen. Normally temperatures 10-16 are used in Europe but some modern processes go higher.
Limed skin should never be unduly exposed to the air, as carbondioxide in the air may convert the lime to hard, insoluble calcium carbonate.
Use of amines in lime liquors:
In recycling lime liquors, aliphatic amines were found to be in increasing amounts with time and enhance unhairing. Perhaps the unhairing effect is due to their properties as reductors. They bleach pelts which may be explained by a reduction process. Unhairing by dimethylamine sulfate(DMAS) in alkali (NaOH) was found to satisfactory and two significant remarks were done:
1)DMAS may be used for unhairing as main component (suitable for purposes where hair saving is required).
NaOH was successfully used.
Further observations of sulfideless unhairing have led(Somerville) to the conclusions that it is possible to apply short liquors (1:2)in unhairing without sulfides and a (1:1) liquor in case of liquors containing 3% NaOH, 1% Na2SO4, 0.5% NaSH, 1%DMAS calculated on hide weight. Operation time 24 hours.
Lyotropic agents act by structure making of the solution, they contribute to the dissolving of the protein. Urea and NaCl affect hide in the same way. Unhairing affect is a property of compounds dissolving protein molecule through transferring H-bonds stabilizing protein molecules on the solute molecules thus forming protein-solute bonds.
Thermal unhairing(scalding): used for pigskins. Epidermis is more sensitive to thermal decomposition than other layers and short time scalding doesn’t influence collagen. Pigskin is passed very quickly between rollers, one of them heated to 160-200 C, contact time is 0.05-1.0 s. In this process the grain proteins are denatured, however denaturation is a very shallow, and nubuck skin is obtained without buffing. Denaturated layer dissolves very easily or may be seaparated spontaneously.
Oxidative unhairing(Rosenbusch):
5 NaClO2 + 4 HCl à 4 ClO2 + 5 NaCl + 2 H2O
4 keratin-S-S-keratin + 10 ClO2 + 4 H2Oà 8 keratin-SO3H + Cl2
Chlorine dioxide reacts with keratin and splits disulfide bonds. Glycolic acid is used to maintain pH 3.0-3.5, t<40 C, time:24 hours.
When the process is finished excess ClO2 decomposed by use of sodiumthiosulfate, inorder to prevent oxidizing of chrome tanning agents(later).
This process replacing all the beamhose operations: degreasing,pickling and alumpretanning by binding of chlorine to collagen, has some disadvantages, therefore not used in industry.
1)has to be carried in acidic medium. All the materials used in construction of machines in the beamhouse need to be replaced by acid resistant ones.
2)expensive
3)toxic gases are formed
4)leathers obtained are stiff and spready.
Measurements used in control processes:
In many processing the quantity of chemicals to be used are based on the percentages on skin weight. It is always necessary to check whether this is dry skin weight oe wet skin weight. It is often specified "limed weight", "shaved weight", "crust dry weight", where these refer to the weight of the leather passing through these processes, which may have caused it to absorb more water, or cut away some of its thickness, or reduced its weight by drying.
Leather is normally sold on the basis of a price per unit weight or per unit area.
FLESHING AND SPLITTING
Fleshing operation consists of removal of unnecessary fragments of tissue, excess water containing salt,soluble proteins, impurities and bacteria. It may be considered as squeezing away solution from the solid.
If hair is not completely removed by liming, it must be removed by machine or a hand knife.
Trimming (cobbing): Any loose, raggy ends of skin are removed by a hand knife.
Hand fleshing: The hide or skin is placed on a beam flesh up, the unwanted flesh, connective tissue and fat is removed by slicing and pushing action with a two handed knife.
Figure
Machine fleshing:
Splitting: When hides or skins are plump in the limed state it is appropriate to split them into a grain layer and one or more flesh layers.
Blue Crome Splitting : can also be done on wet hides after chrome tannage. They should be well sammed (60 % moisture) for accurate uniform splitting.
Acid Blown Splitting: pickled sheepskins are washed in water to remove salt which then allows the skins to acid swell. After these acid blown skins are split they should be repickled as soon as possible.
Splitting is also done on crust vegetable tanned leather, which may be damped before splitting or dry split.
Splitting causes loss of strength. Sum of the strengths of the two splits will only be about 80 % of that of the original whole.
Lime scudding: done with a blunt curved knife which squeezes and pushes the grain removing the loose protein, hair roots, muscle tissue, pigment and some loose fat allof which are called scud. Commonly carried out on a machine similar to an unhairing machine.
Rounding of hides: subdivision
FIGURE
DELIMING and BATING
they depend on the unhairing method used. They are done in one bath. According to technologists this process is done properly if:
beside the removal of Ca +2 ions from pelt it meets the following requirements:
1)the pH inside the collagen network is regulated and the bath is buffered
swelling is reduced to the expected degree
3)fibers are separated by washing out matrix.
4)products of protein degradation are removed from pelt.
Points 1 and 2 are chemical
points 3 is chemical and enzymatic
point 4 is enzymatic only.
Approximately 2/3 of the Ca(OH)2 in the pelt after liming may be removed by washing. Further washing does not remove more. Higher temperature may help but pelt will be less plump then. Ca+2 may form the sulfate in tanning therefore should be removed.
Deliming should be carried gradually, slowly due to the pH inside the pelt. Buffered systems from slowly dissociating acids and appropriate salts are used.
Problems of removing iron ions (from blood or water) from the hide are combined with deliming and are removed by EDTA.
EDTA-Cr+3 complexes penetrate in pelt, react with Ca and Fe ions making Cr+3 ions free(tanning properties).
EDTA-Cr+3 stable < pH 4.5 at pH=7 half is set free
EDTA-Ca+2 stable < pH 9 at pH=7 half is set free
In this way, putting a limed pelt, rinsed with warm water in float pH 2.5 one has two effects, deliming and tanning. Tanning is low at start then due to changes in masking, it becomes faster when going deeper into the pelt. (not used in industry).
Bating is done in the same operation.It means final removal of non-collagenous hide components, keratin, degradation products, globular proteins, elastin and cell structure residues. A condition of success is avoiding negative action of deliming on enzymes used for bating.
DELIMING
After liming, the lime or other alkali in the skin is no longer required, and in most cases it has detrimental effect on subsequent tannage. With chrome tanning it gives a hard green inflexible leather and prevents proper tannage, whilst with vegetable tanning it also slows down or reduces tannage and gives a dark color.
Washing: The easiest way of removing the lime is to put the skins into a paddle or drum and to run them, whilst a continuous flow of cold clean water is fed in. Washing readily removes undissolved lime from the surface, and some dissolved lime held between the fibres. Some of the lime or other alkali such as caustic soda, is chemically held by fibres (about 0.4 % on the weight of skin) and this is only very slowly removed by washing. The process becomess progressively slower and slower as the lime is removed.
Dangers associated with prolonged washing: hard water may cause Lime Blast
(soluble Ca or Mg bicarbonates or carbonic acid react with lime to ppt CaCO3).
Prolonged washing allows further alkali breakdown of the skin to occur, giving loose leather, particularly if the water is warm. Warm water (35 0C) will reduce lime plumping of the fibre structure, thus allowing easier access of water to wash out interfibre protein and lime. Maximum permissible temperature is 38 0C.
Chemical deliming: Speeds up washing process and avoids difficulties. The loose lime is removed by wasing, as above, the supply of water is turned off, and controlled amounts of acids or acid producing salts are added to the water. The acids neutralize the alkali. Too much acid damages the skin by causing violent swelling and solution of protein. As it is impossible to estimate accurately the amount of alkali in a pack of limed goods, it is usual to use either the weak organic acids- boric (or boracic), lactic,acetic acid- or acid salts such as sodium bisulphite, or salts of week alkalis such as ammonium chloride, ammonium sulphate. All of these give less danger of over-deliming with consequent acid swelling than the strong, cheap mineral acids, hydrochloric and sulphuric acid. The ammonium salts while reducing alkalinity, therefore reducing alkali swelling, are incapable of bringing the skin into acid condition.
Weak acids (and weak bases) and their salts give buffer systems. I it is required to adjust the pH of a skin to a certain figure it is practical to choose a weak acid(or base) with pK value near to the required pH.
Formic acid 3.7
Acetic acid 4.7
Boric acid 9.2
Ammonia 8.6
Some modern systems of deliming use non-swelling acids. These may be quite strong acids but due to the potential dipoles in their structure they do not swell the protein.
May contain phthallic acid types or complex meta phosphates.
Extent of deliming is estimated by making a clean cut in the skin and checking the pH.
Degree of deliming given depends on the process which is to follow. One of these the bating process is carried out at pH 8 and use of ammonium salts is particularly suitable.
Where this process is omitted it is still necessary to reduce the alkalinity for tannage.
In the case of many vegetable tannages for heavy leather, this is achieved by the acidity of the tan liquors themselves. For vegetable tannage, oil tannage or formaldehyde tannage of lighter leathers, such as sheep or goat, further deliming or acidification is given by drenching or souring process withweak organic acids. For the minneral tannages, such as chromium or alum, this process may also be used but it is much more common to use a pickling process employing mineral acids, such as sulphuric acid or hydrochloric acid, and salt. This can give a much more acid skin with a pH of 2-4, at which these tannages commonly start.
Delimed skins must be taken to the next process immediately, as the alkali has been removed and putrefying bacteria can cause a slimy feel and loose leather with damaged structure.
BATING
Additional removal of protein material loosened by liming is achieved by enzymatic digestion- the operation of bating.
Based on sterile enzymes (origins :pepsin and trypsin in dog dung and fowl droppings were used once upon a time; causing a soft smooth and silky grain) .
Two main types:
pancreatic bates: digestive enzymes from pancreatic glands.
bacterial bates: digestive enzymes of bacteria.
They are prepared in sterile conditions, mixed with fine wood flour and ammonium salts(sulphate or chloride). Ammonium salts keep the pH at best level for the action of the enzyme.
Method:
The hides and skins are delimed to a pH of 8.0-8.5 and washed. This is the degree of alkalinity at which most enzymes show greatest digestive power. The goods are then usually treated in 300-500 % water at 37 0C with a 1-2 % addition of the powdered enzyme mixture. It is important to maintain the pH and temperature accurately, as slight variations give great loss of bating power.
When slight flattening of the grain or increase in flexibility is required the time of bating is short (ie.1 hr).Longer bating times are necessary for extreme stretch and suppleness. The digestive action of bating is stopped by making conditions unsuitable for enzyme activity (ie. Cooling to 16 0C, make more acid, or commence tannage).
Care should be taken not to immerse skins directly from bating to cool water or remains of the erector pili muscles will contract giving "goose pimple" effect.
Enzyme is not used up by the process and old bate liquors can be as strong as fresh ones. There also is the probability of contamination or infection by other putrefying bacteria.
Most important points to control:
pH of the skin(alkalinity on the cross section of the skin), temperature and time.
If a good bating on the grain is desired to get a smooth, flat and scud-free surface but to bate the corium is undesired, the hide or skin is lightly delimed with ammonium salts to give surface pH 8.5 while the corium is still pH 11. With a short bating time one will get good bating on the grain and very little in the corium. With time the difference in pH will be uniform through the skin structure.
Other bating preparations are made where optimum pH for enzyme activity is different,(ie.acid bates)pH 4 or 5. They may give softer leather because at these pH the fibres are not swollen and allow easier access of enzyme and carrier to enter fibre structure. They are often useful for bating already pickled stock whan the acidity is neutralised with about 1 % sodium acetate to raise the pH to 4.7- 5.0, the goods rinsed to remove some salt and bated as specified.
Bate scudding: bated skins are scudded when it is essential to get very clean grain.The material squeezed out consist of hair roots,pigments, fat epidermis and unwanted protein.
PICKLING
Aim is to prepare pelt for tanning by stopping enzymatic bating and adjust acidity and pH or its water content for subsequent process.
Excessive and unwanted swelling on the acid side of the isoelectric point is prevented by pickling, ie. the controlled addition of strong acids and their salts, prior to tannage. Dibasic acids (sulphuric) are more effective.
Pickling: The use of liquors containing acid and salt is referred as pickling.
The skins are paddled in the salt and water until all salt has dissolved and diffused evenly. Sulphuric acid (diluted at least 10 times) is added and run (about 2 hrs) until good penetration. Pickling will give pH 1-2 and there is ample salt present not only to prevent acid swelling, but to cause some decrease in thickness of the skins, which are now white, soft and flaccid. 3 % salt on the total volume of water present will prevent acid swelling..
The limed or bated skins must be adjusted to a suitable acidity for tannage by pickling. To pickle the pelt means to acidify it in such a way as to prevent it from simultaneous swelling under the action of acid. This is usually done by salt addition. The presence of acid in the medium suppresses dissociation of carboxylic grous of collagen hide chains. Solution of acid and salt outside the pelt is hypertonic relative to the water contained in the pelt, so it has a higher osmotic tension. This is a reason for the coming over of water molecules from pelt to the solution. Solution in the pelt surrounding becomess less concentrated, and owing to this the electrolytes in it become more dissociated, and diffuse into the interfibrillary spaces in collagen. This process stops when concentration of ions in pelt and in solution become equilibrated, and at the same time an interaction occurs between salt cations and collagen carboxyls, and between anions of salts and basic groups of collagen side chains. Ions remaining in the pelt keep their solvation water, thus the pickled pelt is hydrophilic and mellow.
Pickling slows down the chrome tanning process, does not allow the tanning agents to be bound to the external pelt layers, as this would stop their deeper penetration. Acid, contained in the pickle, reacts with the basic functional groups of collagen side chains and imparts to it a pH of about 1.5.
The first contact of chromium float with leather should occur at pH of the order 1.5-2.5. the pH of the common chromium floats is 2.5-3.5, so the acid used for pickling shall decrease the pH of the pelt to the lower value than that of the chromium float. The purpose of the salt, contained in the pickle in concentration of about 10% is to quench the tendency of collagen to swell in acidic medium. Accordingly, the salt is to be chosen which shall decrease osmotic swelling; that is, such a salt whose a cation and anion have possibly the lowest place in the lyotropic series, where ‘structural breaking’ is a prevailing feature. This breaking action of salts and acids, manifesting itself by dehydration of pelt, ie. removal of the structured water from it, makes the start of tanning easier.
Introduction of formaldehyde to the pickling bath, still containing organic acids, causes only slight tannage due to a low pH. Other tanning agents for pretanning pelts, may be as well introduced to the pickling. Introduction of organic acids into the pelt may be considered as a method of chrome salt masking.This method is based on introduction of a tanning agent into pelt as a nonactive molecule.
PARAFFIN DEGREASING
Paraffin degreasing:excessive amounts of grease may interfere with uniform penetration of tan or dye, show difficulties or greasy patches in the finished leather.
Degreasing is particularly important before chrome tannage, where chrome salts can react with some greases to produce chrome soaps.
In the case of paraffin greasing, well drained pickled skins are drummed with half their weight of paraffin(for 1-2 hrs). The paraffin loosens the grease.A small amount of wetting agent may be added to the paraffin( 5 % non ionic wetting agent).At the end the greasy paraffin is drained off. A considerable amount is held by the skin and may be removed by squeezing process(expensive). It is more usuall to wash the skins in a drum with a 5 % salt solution at 27 0C for approx.30 mins. Salt solution must be used as water alone would result in acid swelling. This washing is repeated until wash solution remains clear. Many skin greases or fats are semi-solid cold (15 0C) and even at the maximum temperature of 38 0C permissible on raw skins to avoid heat damage or shrinkages, such fats or greases are still only melted to a viscous pasty mass (particularly if they contain water in oil emulsion). By light pretannage of the skins with 1 % formalin in short float, enough to raise shrinkage temperature by 10 0C, the skins can be safely treated at temperature 45 0C when these pasty fats are quite fluid and easily emulsified.
Dry degreasing process: This is done on dry tanned leather and consist of treating the leather with fat or grease solvents such as white spirits (inflammable) or chlorinated hydrocarbons (non-inflammable but toxic) such as trichloroethylene or perchloroethylene.
TANNAGE
The tanning process converts the protein of the raw hide or skin into a stable material, which will not purefy and is suitable for a wide variety of purposes. There is a vast array of tanning methods and materials
(table pg 107 Sharphouse)
Vegetable Tans: extracted from plant leaves, barks etc. , consist of large polyphenol molecules with some acidic groups and numerous secondary functions(dipole or hydrogen bond). The acidic groups may combine with the basic groups of the protein displacing the water of hydration. Vegatable tannage could be considered as replacing water molecules by vegetable tan molecules. Generally acid conditions favor vegetable tan fixation in increasing the ionization of the protein basic groups.
Synthetic Tans: may be of various chemical structure. It is common for them to be made water soluble by the sulphonic acid group. This group is highly ionized and has strong attraction for the protein basic group with a consequent dehydrating effect.
Syntans with high secondary functions will have more pronounced effect and give fuller leather(replacement syntans), those with greater proportion of sulphonic groups give a thinner less flexible leather(auxiliary syntans).
Minneral Tannages: the basic salts of chromium, zirconium and aluminum behave in a rather different way. Their initial fixation is on the acid groups of the protein where they displace some of the bound water, but they may form cross links between adjacent acid groups, which will stabilize the wet hydrated skin structure.
The dehydration effect of theses tannages and the quantity fixed is less than with vegetable tannages and therefore the shrinkage and hardening on drying is more pronounced. Invariably some type of oil is applied to the wet fibres before drying. Its effect on softening the dried leather should be more pronounced on chrome leather than on vegetable tanned leather.
Aldehyde Tannages: formaldehyde, gluteraldehyde or the aldehydes produced in chamois tannage, combine with basic groups of the protein and form cross links with basic groups on adjacent molecules in the wet protein. Quite small amounts of aldehyde are sufficient to produce a significant effect.
Oil Tannage:A very old way of imparting properties of finished leather to skins. Oil tanned leathers are light, soft, air-permeable, and resiatant to washing.Usually cod liver oil used.
Dehydration methods: a method of rendering the skin soft when dry; not real tanning, effect is lost if the skin is re-wetted.
Treatment of skin with high salt concentrations will dehydrate the skin protein.
Pickled and well dry salted skins dry out white, flexible,and have apparence of tannage. Solvent dehydration, ie.washing wet skin with acetone, will produce the same white leather. A white flexible dry product is produced by "freeze drying" whereby the wet skin is frozen and submitted to vacuum, the water volatilizes off in gas form without going through liquid phase.
General conditions that give rapid tan fixation(astringency) give a poor rate of tan penetration and vice versa. Rapid fixation of vegetable tan is favored by acid conditions (low pH), low non-tan content, whilst in the minneral tannages rapid fixation is caused by higher pHs (5-7) and lower pHs will give penetration.
It is common to give "combination tannages" using two or more types . Vegetable tans are often added to chrome leathers to improve fullness or firmness in flanks or grain, whilst the bulk of syntans are used in conjuction with other tannages to give a whiter leather or to speed up the tannage.
Note: "semi-chrome" refers to complete vegetable tannage followed by retannage with chrome; "chrome-retan" is a full chrome tannage followed by a vegetable or synthetic tannage.
THE CHEMISTRY OF MINERAL TANNAGE
Delimed pelt- outcome of pre-tannage- is still raw material. When moist it is soft and pliable. The aim is to make it durable, soft, porous opaque, together with stability over a wide range of physical and chemical conditions (pH, T, humidity).
The tanning agent must be able of crosslinking the molecules of collagen (has to be multifunctional). Degree of crosslinking needs careful consideration: if too much crosslink than harsh and brittle product(mobility of fibrils are restricted).
Apart from introducing a limited number of crosslinks, a tanning agent should not at the same time lead to undue fibril modification (reduction in fibril length or solution of protein material).
In addition to number of crosslinks introduced by tanning, their general character is of importance. Could be: H-bonds, ionic bonds, covalent bonds.
Chromium : atomic number 24, wt 52, configuration [Ar]3d54s1 .
common states Cr+3 and Cr+6 ( ie. CrO4-2)
Chromium tanning is between collagen and Cr+3. 6 coordination positions(octahedron), stereoisomers possible. In solution chromium III nitrate is tought to give a complex ion of the form [Cr(H2O)6]+3 . 3NO3-. Upon storing color changes from violet to blue, to green. Two primary stages(may occur simultaneously):
release of H+ from the hydrated cation to give a salt
[Cr(H2O)6]+3 . 3Cl- à [Cr(H2O)5 OH]+2 . 3Cl- + H+
this reaction accounts for the acidity of solutions of Cr salts (ie. chromium chloride has pH<2). Addition of mineral acids reverse this reaction but addition of alkali promotes it.
entry of the anion into the complex with displacement of H2O
[Cr(H2O)6]+3 . 3Cl- à [Cr(H2O)5 Cl]+2 . 2Cl- + H2O
in presence of neutral salt ie. KCl, anionic complexes may be formed
[Cr(H2O)2Cl4]- K+. chloride ions held in complex are not precipitated by addition of silver nitrate.
Anions vary in ability to enter into complexes. Usually the stronger the acid formed by the anion the less tendency it shows to form complexes.
Other reactions:
Olated polynuclear complexes: here the metal ions are linked through OH bridges (the OH groups are not free for titration by acid - resistant to de-olation).
With increasing hydroxyl content or basicity there is a tendency to insolubility (olation).
Basicity: it is defined as percentage fraction of OH combined with chromium relative to the hydroxide Cr(OH)3 , which is 100 % basic. Therefore [Cr(H2O)5 OH]+2 is 33 % basic.
Oxolation: conversion of OH groups to oxo groups
these are even more resistant to acids.
Mixed bridge formation: takes place when other anions are present.
Masking and masked solutions:
Anions which are firmly held in a complex retard penetration of OH ligands. Therefore they may prevent formation of large olated and insoluble complexes. This action is known as masking. Entry of masking agents into the chrome complexes in solutions of basic chromium sulphate appears to depend on:
relative amouns of masking agent and Cr
absolute concentration of Cr
presence of other competing ligands(sulphate, chloride, hydroxyl)
whether competing ligands are added together or separately
pH
T
time
whether ligand is added as free acid or salt
with monocarboxylic acid
with dicarboxylic acids
if less than 2 carbons separate COO groups (ie. oxalic acid chelate ring structure of extreme stability, therefore use of oxalate ions in quantitative estimation of Cr ions)
tartrate complex
Masking action of geometrical isomers (cis/trans) maleic and fumaric acid:
Addition of sodium fumarate to a basic chromium liquor leads eventually to formation of an insoluble polymer.
whilst maleic anions give
When used under controlled conditions, the chain forming dibasic acids are of great technical importance, eg. Where large amounts of fixed chromium are needed to fill the loose flank regions of a hide.
Rate of reaction will depend on the nature of the ligands already present in the chrome complex. If large amounts of very stable masking ligands such as oxalate ions are present, no tannage will occur, since these can not be displaced by carboxyl groups (a small amount is O.K.). When using masked liquors in industrial level, the aim is to prevent excessive and rapid reaction in the grain and flesh regions of the pelt, allowing adequate amounts of Cr to penetrate into central regions where collagen carboxyl groups can react with complexes.
Pretannage operations of liming and deliming leaves the pelt collagen at a pH 5-6. Not far removed from the isoelectric condition.
With basic chromium sulphates, reaction with the pelt would be very rapid and lead to overtanning of outermost surface unless special precaution is taken.One approach is to used masked tanning salt. Another approach is to discharge the carboxyl groups of pelt collagen by back titration with strong acid. Unionized carboxyl groups are inactive in forming complex with the Cr (tanning action completelt prevented) and hence penetration of the pelt by the chrome liquor may be achieved.The subsequent addition of alkali or highly basic Cr salt raises the pH value and tannage takes place. Excessive swelling of the pelt by acid is prevented by adding neutral salt to the pickle liquor (Balanced conditions require skill).
Particle size of Cr complexes are also of importance. It is thought that polynuclear complexes of 2 to 7 Cr atoms are present in solutions of chromium sulphate of basicity 33-50. It was found that at 40 % overall basicity the addition of carboxylic acid masking agents could increase the particle size twofold. At higher basicities, insoluble masked complexes and aggregation takes place. Particle sizes are obtained by rates of diffusion of complex ions.
The chromium sulphates used in leather industry are predominantly cationic at concentrations normally (2 % Cr2O3 solutions ) used. Concentrated stock liquors (11-15 % Cr2O3 ) and dry powders (25-33 % Cr2O3 ) when freshly diluted may be non-ionic or even anionic in character. Conversion to cationic character is always favored by aging of dilute solutions. With more easily displaced sulphate ligands, the chromium complexes will revert to the cationic form more rapidly than when organic anions are involved.
Little is known, of the rate of reaction between particular chromium complex ions and competing ligands, whether in solution or in actual tannage of pelt. Following reaction sequence suggested:
Conditions are likely to be most complex, overall reaction rate being influenced by factors such as:
compactness of pelt structure (affect diffusion rates)
sizes of Cr complexes
rate of over-all-coordination
pH
T
relative concentration of reactants
nature of ligands in complexes
nature and addition sequence of competing ligands
ZIRCONIUM TANNAGE
Basic Zirconium salts have definite tanning action with a shrinkage temperature of 90-95 0C. Such leather is of firm, full substance and has excellent white apparence. As with chromium, the sulphate, rather than the chloride, is the prefered starting material for making the tanning salt. It would find wide application provided high cost of tannin salts could be diminished. Large amounts of zirconium salts are needed (more than double the amount required for chromium tannage) to produce satisfactory leather, this is particularly with solutions of 33 % basicity and may be due to large particle size involved. Zirconium tannage most probably is a salt formation involving anionic zirconium complexes and basic groups in collagen.
Zirconium is Zr+4 and has coordination number 8. Neither the zirconyl group, Zr=O, nor the Zr-Zr group is found in solution.
Single OH s may be relaced by acid residues or carbonate residues. Formation of insoluble zirconium compounds starts when NaOH is added already at pH=1.5.
Masking: monocarboxylic acids have no effect, hydroxy acids show masking effect. Mechanism is thought to be a ‘multipoint attachment of zirconium to collagen.
1)Binding of anionic sites of zirconium complexes to amino groups
2)Polar binding of cationic sites of complexes to carboxyl groups
3)Covalent bonding of neutral sites and oxgen atoms of nonpolar carboxyl groups of collagen.
.
ALUMINUM TANNAGE
For a long time aluminum tanning has been known as tawing, with a paste containing NaCl, egg yolk, flour, water and potassium alum(white crystalline solid containing aluminum sulphate, potassium sulphate).Mechanism of tanning is expected to be like that of cromium but with much less stable complexes.
One reason for the pre-emminance of chromium as a tanning agent is its ability to form stable basic sulphates.Al+3 can not bind sulfate residues inside the complex formed.
Al2(H2O) 12 (SO4) 3 à 2H+ + SO4-2 + [Al2(OH) 2(H2O) 10 ] +4 + 2SO4-2
Work on aluminum tannage has shown that reasonable stable basic salts may be obtained with aluminum sulphate or chloride, by introducing organic acid ligands such as tartaric, oxalic or gluconic acids. They attack aluminum complexes at pH=4-5.These have only little tanning action, the rise in shrinkage temperature being of the order 15 0C, compared to 50 0C expected from chromium tannage.The low tanning property is due to instability of their inner sphere. Alum salts may be used in conjunction with other tanning agents to obtain specific effects (filler, dye precipitant to give intensive shades). In fur and wool-skin, pre-treatment with aluminum salts ensures minimum swelling during chromium tannage.
As a rule aluminum tanning is done in floats of zero basicity, at high concentration, in presence of NaCl(to prevent swelling), at pH about 2.5-3.5.
Aluminum tanned leathers are more resistant to hydrolysis after aging. They are to be finished after 3 months of tanning.
Aluminum tanned leathers are white, soft extensible but they are sensitive to water and high temperature (highest Ts achieved 75-85 0C).
TANNING WITH IRON SALTS
Leathers tanned with Fe salts are thin, brittle and do not resist aging. Oxidation and weak binding of complex to the hide is the reason for lack of resistance to aging.
When pH of tanning is 1.8-3.0, Ts from 65 to 90 0C may be obtained.
TITANIUM TANNAGE
Titanium may be used alone or with Cr and Zr compounds.
TiO2 SO4 (NH4 ) 2 SO4 2H2O (stable, water soluble salt)
Mechanism of tanning suggests attachement to amino and imino groups.
Ts obtained is up to 100 0C. Tanning time is reported to be 6-9 hours done at acid pHs. Masking agents recommended are citric, tartaric and lactic acids. Acetic, formic and oxalic acids are ineffective. (Knowledge mostly of Soviet origin)
ALDEHYDE TANNAGE
Formaldehyde (a pungent smelling gas) is water soluble and its solution is known as formalin(toxic and may develop acidity). Presumably tanning was observed for the first time when meat together with the skin were smoked. Formaldehyde is probably the only tanning gas. Stabilized formalin(containing 8-10 % MeOH) contains 40 % formaldehyde and is used for tanning white, washable leathers with the grain split or shaved off.
The skins are prepared to a pH 4 or 5 and drummed in 3% formalin with the least possible amount of water. A temperature of 30 C is beneficial. After runtime of 4-5 hours, they are left in the closed drum overnight and then ‘ashed up’ (1-1.5 % soda ash, 50-100% water) until pH is not less than 8. At this pH formaldehyde rapidly fixes to the skins. At higher pHs over tannage of grain side (with no penetration) occurs. If production of grain leather is intended various(modified) alkali systems are recomended (use of Mg salts) to avoid this danger.
The amount of aldehyde being attached to the hide is small, from 0.2 to 2 %. Part of this may remain unbound. Aldehydes combine with the basic amino group of skin protein. In alkali some condensation(aldol) to larger molecules give fullness to the leather.
Aldehyde tanned leathers have reduced ability to fix acid (basic groups have reacted).Similarly they can reduce fixation of some vegetable tans and dyes. Aldehyde tannage reduces isoelectric point of hides, so that at any pH it has a lower cationic charge than raw skin, and mineral tanned leather. This can reduce fixation of anionic sulphated oils so that such fatliquors penetrate better, but may washout more easily.
Ts is raised only to 70 C. Leather becomes whiter as exposed to light and readily absorbs water.
RNH3+ + CH2O à R-NH-CH2OH + H+
RNH-CH2OH + NH2CO-R à RNH-CH2 - NH-CO-R + H2O
Gluteraldehyde: (OCH-CH2CH2-CHO)
Under equivalent conditions it can give higher degree of tannage and increase of Ts than formaldehyde at lower pHs. Attention has been given to the phenomena that some degree of gluteraldehyde particularly on mineral tannages improves leathers resistance to perspiration.
Gluteraldehyde forms semiacetal bonds with hydroxyls of hydroxyproline, hydroxylysine and serine. With phenols it yields insoluble compounds, so can not be used with vegetable tannins.
With amino groups it my react in 3 ways:
Gluteraldehyde in 25-50% aqueous solution is found to oligomerize (3-5 molecules).This may be prevented by addition of alcohol(at low temperature).
OIL TANNAGE
As tanning agent unsaturated glycerides are used. The favored ones are found in cod-liver oil.These fatty acids may have one up to six double bonds in the aliphatic chain, but 15 % should have at least four to give the necessary reaction products from oxidation and polymerization to give characteristic ‘chamois’ leathering effect under normal conditions of tanning. Oil tanned leathers are light, soft air-permeable and resistant to washing.
In oil tannage for chamois leather, the flesh splits of sheepskins after usual beamhouse processes are brought to the iso-electric point(ie. pH=5). This makes it easier to bring them to a moisture content of 50% by pressing, samming, or squeezing, to expell all the interfiber water, leaving only damp, hydrated fiber structure. This is important in making the skins porous to air and oil. The skins are then drummed with 40%(of their weight) of the cod-liver oil, which should be spread over the surfaces of the fibers and be almost completely absorbed by interfacial tension forces.The skins may then be hung up or drummed in warm air. Oxidation reactions of the oil will now occur with exothermic liberation of heat, increase of proxide value of the absorbed oil, acrid fume liberation, yellowing of color and leathering or tanning of the skins.
Durplus oil is removed by warm damp pressing and then washing with warm solutions of sodium carbonate in water at pH 8-9 to saponify the greases. This surplus oil has no longer tanning properties.
Of the 40% oil offered only 5-7 % remains fixed to the fibers.Modern systems remove the surplus oil by solvent degreasing system hpwever the resultant leather is waterproof unless treated with alkaline surfactants. Resultant chamois leather is very soft, very stretchy, rapidly absorbs 600% water, which can be readily wrung out and makes it suitable for window cleaning.
Chamois leather when adjusted to suitable pHs shows poor affinity for anionic dyes but good affinity for basic, cationic dyes and strong affinity for reactive triazinyl dyes, which may fix also on oxidized oil residues.
VEGETABLE TANNING
The most important organic tanning agents are the vegetable tannins present in tanning liquors.They are prepared from certain parts of plants by aqueous extraction. Their tanning power has been appreciated for a long time and Babylonian texts have recorded their use.
Vegetable tannin