Cleaning of leather

The manufacture and use of leather garments such as coats jackets skirts or suede has steadily increased over the past decades. As the materials go through a lot of wear and tear in use they require cleaning of freshening up just like textiles. considering that leather garment are manufactured in the most different types of leather which have been treated with a great variety of tanning agents dyes fat liquoring or finishing agents the cleaning of leather is much most difficult and labor-intensive then the cleaning of textiles moreover the garments include materials which are not fast of cleaning. Such as button of synthetic resin non-woven fabrics and adhesives which are sensitive to solvents. In addition, there are many different types of possible soiling and wear marks such as staining by difficult items of daily use, stains caused by rain drops, discoloration and greasiness of suede or nubuk fibers, perspiration marks, stripping of finishes or change of handle. As this requires broad specialist knowledge, special dry cleaning factories for leathers have developed.

Fastness of chrome tanned leather and chrome free leather

Importance of chrome free leather or vegetable tanned leather
Although chrome tanned leather because of its excellent heat and light fastness properties are being replaced by other means of tanning such vegetable or synthetic tanning agents.
Chrome-free leather has gradually gained commercial importance, particularly for automobile upholstery applications. In many respects, however, chrome-free leather is inferior to chrome-tanned leather. UV and heat are known to be more detrimental to chrome-free leather than to chrome-tanned leather, especially in regard to the colorfastness of dyestuff and mechanical properties. Temperature, UV radiation, and humidity are key environmental factors that affect leather properties. The role of humidity and its interaction with UV radiation and temperature on leather properties, however, are not clear to the leather industry, and this information is needed for formulation of antioxidants that will protect chrome-free leather from UV and heat damage. Therefore, a systematic study was performed to formulate the relationship between these three environmental variables and resultant colorfastness and mechanical properties. A second order regression equation was derived to plot response surfaces that clearly illustrate the relationship between the environmental variables and colorfastness, as well as the resultant physical properties. Observations showed an intriguing interaction between humidity and radiation dosage. Measurements revealed that an increase in humidity resulted in a decrease in colorfastness and mechanical strength. However, after the UV radiation dosage reaches a certain level, an increase in humidity may actually help maintain both properties. Observation showed the stiffness decreased steadily with humidity, whereas the toughness index slightly increased with humidity. This study also used differential scanning calorimetry (DSC) to determine the denaturation temperature as a function of various environmental conditions. We observed a correlation between colorfastness and the denaturation temperature.

Leather tanning technology

Which chromium metal is safe in use? Which if its compound or salt is better in leather tanning?

Chromium Cr is a transition metal which is found in different form as trivalent and hexavalent form chromium sulfate is commonly used in wet blue (tanning) leather production. There is much inaccurate information in circulation regarding the use of chromium salts and their associated safety for the tanning of leather.
Approximately 80% of global leather production is tanned with safe chromium III salts. It is a highly effective tanning agent producing leather with a flexible range of properties making it suitable for many end uses.
Chromium is a transition metal that can exist in a number of different oxidation states each with distinctive properties.
Metallic chromium
this is a steel grey, hard metal found as chromite ore (it does not occur in the metallic state naturally). It is used to harden steel, to manufacture stainless steel, and to form alloys. It is also used in plating to produce a hard corrosion resistant surface.
Trivalent chromium (Cr III)
Trivalent chromium compounds occur naturally in the environment. They are found in rocks, soil, plants and volcanic emissions. Chromium salts are present in foodstuffs and are a necessary nutrient for the human body as trivalent chromium is required for the normal metabolism of fats and sugars. Nutritional supplements are currently on sale containing chromium picolinate. Chromium (III) sulfate is considered safe to use in leather manufacture.
Hexavalent chromium (Cr VI)
Hexavalent chromium is the hazardous form of this element. It can be formed when trivalent chromium is oxidised. This usually occurs in the presence of oxygen combined with other factors such as extremes in pH. The salts have a characteristic yellow colour and are classified as carcinogens. Chromium VI is not used in the tanning of leather.

Green fleshing is essential for EId-UL-Adha sheep skin

Green fleshing for Eid-Ul-Adha sheep skin is necessary


As raw material is coming in Korangi tanneries in Karachi at large scale and tanneries now-a-days busy in wet blue production care should be taken that this raw sheep skin contain higher fat content and thick fatty flesh because these skin not properly flayed in Eid-Ul-Adha seasons while in slaughtering house they are properly flayed by expert people. So green fleshing or pre-fleshing is an essential for good quality wet blue. Green fleshing is done after soaking and before liming. It also helps in preventing valuable wool to damage or going in waste because it removed easily when lime properly penetrate through the entire skin. Raw material of Eid-Ul-Adha is also of good quality and firm in structure to convert it from raw to wet blue good fleshing both green fleshing and after liming fleshed must be clean from extra fatty substance so that chemical can easily penetrate through flesh and no fat stains appear after its crust and dyeing production

Award for Buckman in Businness Council 2009

Buckman has wide range of their leather chemical specially its preservative are very useful in wet net leather product.
Specialty leather chemical manufacturer Buckman have been named winner of The Greater Memphis Chamber International Business Council’s 2009 Global Business Award.

Buckman vice chairman Kathy Gibson received the award on behalf of the company at a special ceremony held at the International Business Council’s Annual Fall Meeting on November 13 at the Memphis Hilton Hotel.
‘The Global Business Awards are designed to recognise companies based in the Memphis community that excel in international business, as well as being outstanding community leaders’, said Mike Demster, Greater Memphis Chamber Vice President, International and Technology Business Development. ‘Both Buckman and Luminetx exemplify the criteria for the awards through their growth and global sustainability and their strong values and ethical conduct in dealing with their employees and global partners.’
In her acceptance speech, vice chairman Kathy Gibson stated that: ‘We started out in Memphis in 1945 with one product, one employee and one customer. Today, with 1500 employees, 10 manufacturing sites, thousands of products, and hundreds of patents which allow us to serve customers in over 90 countries, we are still very proud to call Memphis our home and international headquarters. Today we’re not only committed to our global growth, we remain committed to Memphis and the tremendous opportunities that are here to grow business. I know I speak not only for my family but also for the 1500 associates of Buckman worldwide in thanking you for this recognition.’

Eid-ul-Adah great source of hide and skin in Islamic countries

Eid- UL-Adah brings great source of hide and skin for Korangi tanneries in Karachi
This season of Eid is a great source of hide and skin about million of skin and hide are brought in tanneries. Many tanneries purchased direct skin from houses or places of scarifies in raw condition without preserving. They have arranged of preserving skin and hide at their tanneries. Skins are more than hide sheep and goat is most common scarifying animal in Pakistan on Eid-UL-Adah. Three days of Eid-UL-Adah are busiest days of skin bringing to tanneries it will be helpful to run tanneries in dynamic.
First month wet-blue production will be at peak and tanneries will be busy day and night. Karachi is the largest leather industrial city in Pakistan where the skin and hide are brought from all over the country also from Afghanistan especially sheep skin are imported from Afghanistan. Although skin and hide are being gathered from all over the country but there is enough source in Karachi for tanneries to run immediately.

Seminar on ISO 1400 requirement and implantation Held at NILT Pakistan

Seminar on ISO 1400 requirement and implantation
Held at NILT Pakistan
CTP (cleaner technology program for Korangi tannery) held a seminar on environment management system was held on Saturday 21 November 2009 at National Institute of Leather Technology all Korangi tanneries were invited to learn more about environment management system and how to implement this system in tannery. The procedure was taught about getting certificate of ISO 1400.
Tanneries share their control about environment system and energy saving plan how they did in their tanneries also new technique were introduced to make more control on environment.
Only M.Shafi Company has succeeded to get this certificate. They have established a chrome recovery plant and they are measuring the flow of water into drum so that they can avoid excess use of water as normally used in other Pakistani tanneries. The major discussing was how to make optimum control of energy usage and chemical consumption. The participators were given training certificate for their training on this seminar.

Saytzeff `s rule (elimination reaction)

Saytzeff rule
In elimination reaction, alkene with the greatest number of alkyl groups on the doubly bonded carbon atoms predominates in the product mixture.
Example
Two alkenes are possible when 2-bromo butane is heated with alcoholic KOH.



OH¯ OHֿ
H H H H Two “R” in this product
I I I I
H- C - C - C – C- H CH2=CH.CH2.CH3 (2 Butene 80%)
I I I I
H Br H H CH2=CH.CH2.CH3 1 Butene (20%)
Explanation
According to Saytzeff rule the main product is 2-butene (80%) as there are more alkyl groups(two) are attached then 1-butene (one alkyl group). It has also been kinetically that more highly substituted alkenes are more stable.(due to low energy activation) than less substituted alkenes. Therefore E2 elimination reaction gives more stable alkenes.
Order of stability of alkenes
CH2=CH2<(CH3)2 C=C .(CH3)2 Increasing stability Elimination reaction The reactions in which two atoms or groups are removed from adjacent saturated carbon atoms to form C=C bond (pi bond) are called elimination reactions. Example : the dehydro dehalogenation of alkyl halides R.CH – CH2 + NaOH→ RCH =CH2 +H2O | | ALKENE H X E1 reaction Like SN¹ reaction the elimination also involves two steps. Step 1: ionization of molecule takes place into negative and positive ions. | | slow | | H -C –C –X → H- C- C+ X- | | | | Alkyl halide carbonium ion Step 2: carbonium ion loses a proton which is accepted by solvent which acts as base. | | \ / B¯: + H –C – C + > B : H + C =C Alkene
| | / \
Base carbonium ion
Rate of reaction depends on initials ionization of the halide and independent of concentration of OHֿ ion.
Rate = K [RX] (1ST Order)
Example
The hydrolysis of tertiary butyl bromide by KOH to form 2 methyl propene.

Production of syntan

Production of syntans
The base products are mononuclear and polynuclear phenols naphthalene and their derivatives cresols, naphtols, aromatic ethers and spent sulphite liquors. They are condensed, also as mixtures, mostly with formaldehyde and sulphonated by the introduction of water-solubilizing group, usually containing sulphuric acid. They exit mainly in the form of sodium or ammonium salts. By varying the products and procedure it is possible to manufacture the most diverse syntans having particular properties. Most syntans are anionic, a few are amphoteric. Technical information and leaflets concerning the exact properties of these products are available from specialized suppliers.
Classification and properties
1. Replacement tanning agent
As they possess an equivalent self-tanning capacity these products can replace vegetable tanning agents or portions of them without problems and, if desired, give the leathers to be tanned particular properties.
2. White tanning agent
In most cases this type of tanning agent can be classified as a replacement tanning agent. They generally exhibit a reduced filling capacity if they have a good white effect and high light fastness.
3. Shrinking tanning agent
These tanning agent are highly astringent and acidified in order to achieve an astringing on the grain and thus a high shrinking grain. Besides phenolic tanning agent glutaraldehyde has been used for many years to obtain line shrinking effects.
4. They have been developed to improve the diffusion of highly concentrated tanning agents with large –sized particles and thus to accelerate or reduce the tanning times.
5. Retanning agents
There is great variety of products. Mainly used for subsequent treatment of chrome-tanned leather in order to obtain special effects and properties such as fine grain firm grain and good handle softness or toughness fullness pastel shades or level dying properties good buffing properties light fastness or resistance to ageing and improved physical properties.
When phenol is condensed with formaldehyde, a novolac resin is formed. This is thermal setting resin and its hardness and molecular weight depend upon the ratio of formaldehyde to phenol. At molar ratios greater than one part formaldehyde to phenol, the theoretical molecular weight is infinite and a hard resin results. With the molar ratio adjusted properly, so that an average molecular weight is about 300-400, a thick syrupy material is obtained. This is insoluble in water and, for practical application, leather water dispersion is necessary. The resin is sulfonated by the addition of sulfuric acid. The route to the final product, a sulfonated phenol or by condensation followed by sulfonation.
The mole ratio of the condensing agent (formaldehyde) is important. The more formaldehyde, the higher the average molecular weight. If the molecule is too small there will be poor tanning action. If the molecule is too large, there will be poor penetration into the leather. The quantity of sulfuric acid, or the degree of sulfonation, also affects the tanning properties. Sufficient solubilization is needed to maintain a true solution, but excess sulfonation decreases tanning efficiency.
Two factors are in the exchange tannins are of prime consideration: the cost and specific characteristic. With phenol as a starting material, the manufacturing, cost makes it impossible to make a tanning agent competitive with natural vegetable tannins. If, however, there are incorporate into the tanning material lighnosulfonates or naphthalene sulfonic acid materials, the cost per “tan unit” goes down and a more competitive product results. The leather making properties of the syntan are also greatly altered.
One of the most successful of the lighnosulfonic acid type was the Tanigan Extra A which is described as exchange tannin based on dihydroxydiphenylsulfone, sulfite cellulose and the sulfonic formaldehyde resin. As a tanning agent it was successful and was capable of producing leather similar to that obtained from the vegetable tannins. A large number of exchange tannins are presently being offered to the leather industry based on combination of dihydroxydiphenylsulfone, Bisphenol A , direct sulfonation, co-condensation with naphthalene sulfonic acid and other systems.

Echological effect in leather production

Eco
Efforts to switch to other efficient long-term preserving agents have not been successful yet. Salt used in preserving hide and skin are of different type sea salt rock salt they are different in impurity. Short term preservation without salt or processing of fresh hides is only of local importance.
Remedy ecological treatment
Before processing (soaking) remove salt from raw hide and skin by intensive sweeping and dispose of separately.
Soaking float s
Soaking float make up about 10-15% of the total effluent. The floats may contain common salts, soil by blood and dung, natural fatty substances, small amounts of soluble skin protein and sometime preserving agents, wetting or enzymic soaking auxiliaries, depending on the type of preservation of the raw stock.

tanning with aldehyde more environment friendly

Reduce the chrome waste in effluent by aldehyde tanning agent introduced by TFL
Chrome free leather is now easier to make for all quality of chrome tanned leather.
The demand for chrome-free tanning continues to increase - not only for automotive leathers but also for footwear and leather goods. To answer demand TFL, one of the leading companies for ‘free of chrome’ technology, have launched Sellatan CFX liq.
This polyaldehyde based special tanning agent is making the pretanning of full-substance articles easier than is currently available glutaraldehyde based pretans.
As a new solution for the pretanning of wet-white and vegetable tanned articles the innovative tanning agent improves and simplifies the process. Sellatan CFX liq penetrates easily through the cross section due to its masking effect and is a good pretanning agent for full substance pelts. Furthermore it ensures very good water release during sammying, allowing accurate shaving with better substance control.
Also, because of its low astringency it imparts to leather a smooth, fine and tight grain with good dye ability.
Sellatan CFX liq is electrolyte stable and increases the shrinkage temperature when applied in pickle. For this reason it can be used in the chrome tannage and/or chrome retannage in order to improve softness and help reduce the amount of chrome offered.
Its free phenol and formaldehyde content is negligible and therefore it can be used to produce leathers, which pass the TFL White Line specifications

what is leather?

WHAT IS LEATHER?

leather and fur making is one of the oldest trades of mankind.

Primitive man covered himself with the skins of animals he killed for food.

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 through metallic salt like chrome.

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

fleshing- cutting away unwanted fat and flesh

splitting and smoothening

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-<40 ph="7" ph="7" zr="O," ph="1.5." ph="4-5.These" ph="5)." ph=" 4.8" ph=" 4.8" ph=" 4.8" ph=" 4.9" ph=" 4.9" ph=" 4.0" ph=" 4.5" ph=" 3.5" ph="4.0" ph=" 3.2" ph=" 2.8" ph=" 3.6" ph=" 4.0" ph=" 3.4">4. They are more anionic and therefore more sensitive to pH change and can be adjusted to give stronger surface shades, sometimes with the sacrifice of a little wash-fastness.



Amphoteric Dyes

These premetallised dyes has the potential to give the molecule a negative charge while the metal has the potential to give cationic charge. In this respect they resemble the skin protein, having a pH giving no charge(isoelectric-point). At pHs below this they will be cationic and above it anionic.

Sulphonated Basic Dyes

These are basic dyes sulphonated. They are useed in a similar way to acid dyes but have some of the brilliance of shade (and poor light-fastness) of basic dyes. They are amphoteric .At ion.They are applied in warm water solution to the leather. Salt is added to the water to favour absorption on the leather fibre and then sodium carbonate is added to give an alkaline condition of pH 8.0-9.0. Under these conditions, the chlorine from the triazinyl chloride dye is split off to give sodium chloride with the alkali and the free bond created covalently links the dye to the leather.

They are of particular merit on glove or clothing leather which has to stand washing, and on woolskins. There are a limited number of shades and only pastel shades are obtainable. Light fastness is only average. They can be used on mordanted chrome but are unsuitable on vegetable tanned leathers which will not withstand the alkali dyeing conditions. They are particularly good for dyeing washable aldehyde leathers.

Sulphur Dyes

These are made by fusion of aromatic amines or phenols with sulphur or alkaline polysulphide. They are only soluble in alkaline solutions of sodium sulphide(pH 9-12).

This alkalinity seriously damages most tannages with the exception of chamois leather and aldehyde leathers, for which these dyestuffs can be used. After application of the dyestuff, acidification and oxidation, the sodium sulphide is destroyed and the remaining dyestuff is quite insoluble in water or soap solution-hence giving good wash-fastness. The range of shades available is rather limited.

Cationic Dyes-Basic Dyes

These were the original "aniline" dyestuffs and are made from tar distillation products by similar methods to those used for acid dyestuffs, except that the sulphonation processes are omitted. Water solubility is conferred by the presence of amine groups which form strongly ionizing salts with acids. A typical member of this group is





FIGURE pg 203 Sharphouse and pg 204



The colloidal dye ion carries a positive charge. Consequently there is a strong ionic attraction between these cations and colloidal anions.

Basic dyes give strong surface shades, no penetration and often unlevel dyeings on vegetable/syntan tanned leathers. In a similar way they will give a strong surface shade on leather previously dyed with anionic dyes. This fixation is rapid hot or cold and is relatively independent of pH between 3-9.

Conversely cationic dyes have very little affinity on cationic chrome leather, giving only the faintest tints. However, if the chrome leather have been heavily masked, dried out and vegetable retanned or syntan mordanted, and fatliquored with highly sulphated or sulphited oils, or is anionic dyed, these anions may increase the affinity for basic dyestuffs.

Basic dyestuffs give a range of strong briliant shades of yellow, orange, red, blue, violet and brown. Rich blacks are made from a mixture of these. Unfortunately they fade badly in sunlight, although some may be better. Some improvement can be made by an after-treatment with salts of phosphomolybdo-tungstic acids.

Basic dyes tend to be soluble in some oils, greases, waxes and solvents. The free bases or their oleates are very soluble and are used for coloring oils, waxes, etc( eg. Boot polishes, carbon paper, typewriter ribbons, etc.). This property is associated with their propert of "marking off",ie. by simple contact, color may be transfered to another surface.

This must be avoided on many clothing leathers, etc.Their solubility in non-aqueous solvents may cause trouble in dry cleaning or where a leather finish contains such solvents.

The free base obtained under alkaline conditions is of poor water solubility. Hard water or alkaline conditions may cause precipitation. Basic dyestuffs are often dissolved by pasting with a little acetic acid, before adding boiling water to avoid this happening.

Nonionic dyestuffs(e.g. azoic ones) require longer dyeing times.Binding occurs at very broad pH. Nonionic dyestuffs are resistent to washing and abresion. However surfactants of glycols of polyether type remove them, probably due to formation of new hydrogen bonds. These dyestuffs may be completely washed out from pelt by acetone; in chrome-tanned leather, however, 1/3 of them will remain. One may conclude that in the binding of dyestuffs to chromium complexes strong bonds participate. Behavior of dyestuff in solution depends primarily on dissociation of its functional groups being responsible of its solubilty.

The effect of individual substituents on the pH has been known for long. The following are some rules worth mentioning:

Sulfonic group is strongly dissociated when attached to an aromatic ring or in the presence of amino groups; its pK is 0.5.

Carboxylic group is more dissociated when attached to benzene ring, than when linked to an aliphatic chain.Attacment of other groups to the ring changes the degree of dissociation of the group itself; direction of this change depends on the kind of group and on its position; nitro group in ortho position increases dissociation the strongest.

Hydroxylic group is affected by other groups; nitro groups and halogens in phenols increase significantly the degree of dissociation.

Amino groups dissociate to a small extent, other groups influence it like,e.g.,hydroxyl.

Two groups of dyestuffs of very specific way of action have gained importance recently: metal complex and reactive dyestuffs.

Standardization of dyestuffs

Manufacturers standardize their dyestuffs so that they are of uniform strength and that repeat dyeing of a certain recepie give similar shades. The strength of dyestuff is adjusted by addition of common salt in case of anionic dyes and starch or dextrin it the case of basic dyes.

Standard shades may be designated by the letter S or 100 after their name. A stronger quality might be "200" or 200% stronger, in which case only half the quantity should be used in a standard recipe. In the same way 50% would indicate that this quality is only half standard strength.

For leather,they usually standardise on chrome leather or vegetable tanned leather.

FATLIQUORING OF LEATHER

Leather, at the time of completion of the tannage does not contain sufficiant lubricants to prevent it from drying into a hard mass.

Almost all light leathers need a greater softness and flexibility than is imparted by tannage. This is attained in the fatliquoring process by introducing oil into the leather, so that the individual fibres are uniformly coated. The percentage of oil on the weight of leather is quite small, from 3-10 %. The precise manner in which this small quantity of oil is distributed throughout the leather materially affects the subsequent finishing operations and the character of the leather. Proper lubrication or fatliquoring greatly affects the physical properties of break,stretch, stitch tear, tensile strength, and comfort of leather. Over lubrication will result in excessive softness and raggy leather in the bellies and flanks. Under lubrication, or improper penetration, results in hard bony leather that may crack in use.

To allow a small amount of oil to be spread uniformly over a very large surface of the leather fibres it is necessary to dilute the oil. Although this could be done with a true solvent such as benzene, it is cheaper, safer and more convenient to use the method of emulsification. In an emulsion with water, the oil is dispersed in microscopically small droplets, giving it a white, milky appearance. It is important that the oil drops in water should remain as an emulsion until they penetrate the leather, and should not separate out as large drops or as a layer of oil, which could not penetrate the leather fibre and would only give a greasy surface layer.

The properties of the finished leather can be varied by controlling the degree to which the emulsion penetrates the leather before it "breaks" depositing the oil on the fibres. By such a technique, in the case of chrome-tanned leather, it is possible to concentrate the bulk of the fatliquor in the surface layers, leaving the middle containing relatively little oil. This yields a leather which is soft but resilient, with a tight break. In contrast if the fatliquor is allowed to penetrate uniformly, the leather will be soft and stretchy, with any natural grain looseness accentuated.

The commonest material used as a surfactant is soap. However in the presence of hard water, calcium or mineral salts or acid, the hydrophilic nature of he soap is reduced and it loses its surfactant powers.Most leather is acid; sulphated or sulphited, alcohols or oils, have much better resistance to these conditions and thus much better wetting action, and emulsions formed with their aid are much more stable to these conditions.

They are all classed as colloids, anionic or cationic surfactans, depending on the charge of the ionic group carried.Anionic surfactants are more effective at high pHs and on anionic materials, eg.vegetable tanned leather.Cationic surfactants are more effective at lower pHs and on cationic materials,eg. Chrome leather. Non Ionic surfactants in which the hydrophilic group does not ionize(consists of several hydroxyl groups) are used as auxiliaries in parafin degreasing, as wetting agents, and to stabilize fatliquors to obtain emulsion penetration into the leather.

Location of the oil: If we consider a cross section of the hide upon bending, we see that on the outside of the bend the fibers must stretch, and on the inside of the bend must compress.In the center of the skin there is very little motion of the fibers over one another during bending. Therefore both the grain and the flesh surfaces must be lubricated(to prevent break or grain wrinkle), but less lubrication is necessary in the center.

OILS, FATS and WAXES

Mineral Oils and Waxes:

Simplest type is mineral oil, obtained from crude oil from oil wells. They are mixtures of many substances which are separated by distillation. They are relatively cheap and chemically stable and are not affected by mould or bacteria.Can be obtained in pale color. Mineral oils do not mix with water therefore give waterproof properties and can be obtained at any viscosity. Despite the advantages they have only limited use in leather manufacture. Relative to other oils:

a) they are more difficult to incorporate thoroughly without giving a slightly oily or waterproof surface, which is a disadvantage for many leathers which are to be dyed or finisked

they have a poor "feeding action", and used alone they give leathers which feel thin or empty but may be quite flexible

if the resultant leather is heated, the oil may migrate to the surface, which becomes oily or discoloured.

These oils do not appear to be as firmly held by the leather fibres as other oils; they are saturated hydrocarbons (unsaponifiable).

Paraffin wax(mp 35-36 C), Montan wax(mp 76-84 C), Ceresine wax(mp 60-85 C).

Natural oils and fats: Most of the oils and fats in animals, fish and plants are fatty acid glycerides. When boiled with caustic soda, they decompose to give soap and glycerine (saponification). By adding acid to the soap the free acid is formed. These fatty acids are water insoluble and range from very fluid oily liquids to greasy pastes and hard waxy materials. The property of the natural oil is largely governed by which of these fatty acids are combined with the glycerine.

All these glycerides can be split into glycerine and free fatty acid (rancidity) by acids and by action of enzymes(produced by moulds). It may happen to the oil or fat in the leather and if the solid type fatty acids are liberated they may crystallize on the surface of the leather spoiling the appearance of the leather giving a whitish dusty appearance known as "fat spue". Another trouble due to rancidity of the oil is that free fatty acids form compounds with chromium, alum or zirconium salts used in tanning, which make the leather water-repellent and difficult to wet back uniformly for dyeing or finishing purposes. Fatty acids may be classified according to their chemical reactivity that is their degree of unsaturation. Saturated fatty acids are usually more viscous or solid, do not darken with sunlight, unaffected by damp,warm air, do not combine with sulphur or iodine, difficult to sulphate. Unsaturated fatty acids are morefluid, darken with sunlight, become sticky or gummy on oxidation by air, readily combine with sulphur or iodine, easily sulphated. Thus highly unsaturated oils may cause trouble on aging of the leather. In the paint trade they are classified as semi-drying(ie. castor oil) because they become gummy on exposure to air, and drying oils( ie. linseed oil) which on exposure "dry" to a hard, non-oily or non-tacky varnish.

Practically all naturally occurring fatty acids have an even number of C atoms. Shorter chain saturated fatty acids C-6,C-8, and C-10 are found in coconut and palm oils, milk fat and other softer oils. C-12, lauric acid, is found in sperm oil. Saturated fatty acids of C-16 and C-18 are common to animal fats and many vegetable oils.

The C-24 and C-25 category are found in waxes.ie.carnauba wax and bees wax.

The unsaturated fatty acids, primarily of C-18 type are quite common in animal and vegetable oils. Fatty acids with more than 1 double bond are classified as drying oils such as linseed, cottonseed oils. Some contain OH groups such as lanopalmic (C-16 hydroxy, saturated) found in wool fat and ricinoleic (C-18 hydroxy, unsaturated) found in castor oil. Both wool fat (lanolin) or wool grease and castor oil are common fatliquoring materials when sulfated.

Typical natural oils used :

Animal oils and fats:

Beef tallow (mp 35-38 C)

Mutton tallow (mp 40-45 C)

Wool fat and grease

Stearine (mp 49-55 C)

Stearic acid(mp 71 C)

Neatsfoot oil (I value=85)

Vegetable oils:

Coconut oil. (I value=10)

olive oil, palm oil,palm kernel oil (I value=53)

castor oil-contains large quantity of C-18 ricinoleic acid, has OH groups that render it water soluble and is easily sulfonated.

linseed oil

soybean oil (I value=135)

3)Fish oils:

cod oil-(I value=150)high degree of unsaturation, drying properties, may be sulfated

Newfoundland Cod Liver Oil

Coast Cod, British Cod, etc.

Degras or Moellon- oxidized raw cod-liver oil

Herring oil, Salmon oil,Sardine oil, jap fish oil, menhaden oil

Whale oil

Sperm oil- rich in fatty alcohols and upon sulfonation becomes a very strong emulsifier.

Fatliquors:

Waxes:

Carnauba wax (mp 78-81 C)

Candelilla wax (mp 68 C)

Beeswax (mp 60-63 C)

Spermaceti-sperm oil (mp 42-49 C)

Wool fat and grease (Yorkshire grease) ( mp 30-40 C)



FINISHING (AFTER TANNAGE)

The finishing of leather is probably the most complicated and least understood phases of the industry. It has been more of an art than science. Finishing leather is not simply a matter of painting the surface to cover up the mistakes of the previous operations or to improve it by concealling scratches; it contributes to the durability and beauty of the leather and must be an integral part of the process. The compatibility of materials, tannage coloring and fatliquoring all play an important role in the character of the leather and the kind of finish it will take.

The finish system is a compromise between conflicting effects. If coverage of defects is the main problem some of the grain beauty will be lost. If resistance to scuffing is desired there is a danger of having a stiff varnished look. Each of these conflicting properties must be balanced in the final finish system.

Adhesion

When the finish is applied, it must stick; for this reason the leather surface must be "wettable".A top finish need not adhere to the leather itself, but it must stick to the base coats of the finish to prevent peeling. Leather finishes require extreme flexibility and stretch. Finishes that do not have good adhesion and good flexibility will peel and crack.

Stability

Leather may be exposed to extreme heat during the manufacture of a shoe. When the shoe is worn in cold weather, extremely low temperatures may be encountered. Thus the film must have a wide range of temperatures over which it is soft and pliable; it must also be hard to maintain the high gloss which is required.

The leather must be able to stand up to a reasonable amount of both wet and dry abrasion and to be refinishable with ordinary shoe polishing methods applied by the consumer.

Coating technology

Coatings may be classified as:

1)lacquer systems

2)drying oil systems

3)condensation systems

4)latex systems

Lacquer systems: The formation of the film is based on the evaporation of the solvent containing a film-forming material (nir\trocellulose dissolved in an organic solvent is an example).

Drying oil system:These are natural drying oils such as lnseed which will undergo polymerization upon drying. This is different from lacquer in that the setting up of the film is not simply a deposition of a high molecular weight material; rather; it is a chemical reaction taking place between the dissolved film-forming materials and atmospheric oxygen. In the drying oils the film forming material(a binder) is an organic chemical having a high degree uf unsaturation.As the oil absorbs oxygen from the air, the unsaturated material is oxidized and reactive portions of the fatty acid molecule develop which can then polymerize with other fat molecules to form a continuous film on the surface.

Condensation Systems:The formation of the film is due to a chemical reaction between the various components of the finish after application.The reaction may form a plastic or polymer in water between two molecules. Such systems are usually heat activated and may be baked, glazed, or hot pressed. Condensation or polymerization is used in the leather industry through protein-aldehyde reactions and with other resin systems.In this kind of finish the reactive components are usually mixed shortly before application, due to the limited pot life of the components.

Latex systems:

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Leather produced by tannage must now be prepated for sale. Obviously it must be dried, but the color may need be made uniform or changed by dyeing, the thickness may need modification, oils or fats may be needed to improve suppleness or water resistance, and the "handle" of the leather may be modified by drying methods or by squeezing, rolling, plating and flexing. The grain may be required smooth or pebbled, shiny or dull or the leather may be finished on the flesh side as suede.

Removal of surplus tan liquor: after tannage it is common to allow excess tan liquor to drain off the hides or skins, and to let them stand in a damp condition for a day or more. With most tannages further fixation of tan and setting of the fibres occur. When a flat leather is required, the skins are drained flat to avoid any tendency for the fibres to set in a creased condition. Common methods are horsing up or cessing or piling. The pack may be covered to prevent surface drying or soiling.

Figure 32 , 33 Sharphouse pr 163.

Washing: The aim is to remove the loose, surplus tan. To chrome leather and vegetable tanned light skins rigorous washing may be applied. In the case of heavy leathers rigorous washing with large quantities of water is avoided.

Neutralizing: Chrome leather is acid and develops acidity on standing. Consequently neqatively charged colloids such as dyestuffs, vegetable tans, sulphated oils, etc. will readily precipitate on the skin surface. The washed chrome leather is therefore neutralized by mild alkalis (drumming in 1-2% sodiumbicarbonate or borax, 200-300% water, ~30 mins.). The leather is then washed again and the next process ( dyeing or fatliquoring, etc.) should be carried out immediately.

It follows that degree of neutralizing needed will depend on the tannage. Thus masked chrome tannages will give improved dye penetration, as will, combination or retannages with vegetable tans or syntans.

Adjustment of thickness:

splitting: if a hide is thick enough (3mm), it may be split into two layers(grain layer2mm, and flesh layer 1mm).

Shaving: levelling off of thicker areas.

Removal of excess water: before drying, the leather may in many cases be bleached, or treated with oil by stuffing or fatliquoring. Many chrome leathers are neutralized, dyed and fatliquored before drying; whilst most vegetable-tanned leathers are dried out after tannage and are then wetted back for dyeing, because fresh vegetable tannage tends to wash out in the dye-bath, giving a thinner, emptier leather.

Chrome-tanned leather for suedes and gloving is often dried out before dyeing, in the former case to allow buffing of the dry leather to be carried out before dyeing, and in the latter case to allow the skins to be staked and very carefully sorted before dyeing.

Light leather: as much water as possible is squeezed out before drying (sammying machine)

Heavy Leather: also sammed but less easy. Setting out is performed to give a flat and wrinkle-free finish.

DRYING

Hide protein has associated with it a large amount of water and is in fact a hydophilic colloid.. Under slow drying conditions, evaporation from the surface proceeds at a slow enough rate for the water being removed from the surface to be replaced by that migrating from the inside.With high speed evaporation, however, the water from the inside cannot migrate rapidly enough, and the surfaces become dehydrated. The outer surface becomes a different material from the inside and it becomes a hard mass.

The attraction of the leather fibers for one another will result in some stiffness upon drying and a physical shrinkage of the leather.Drying methods that involve mechanically holding the leather in an extended position will result in a larger area. Tacking, pasting, toggling and vacuum drying all employ this principle.

Leather is dried to a very low moisture content so as to bring about permanent fixation of the materials within it.Drying, therefore, is a chemical as well as a physical activity.

It is very important to adjust the temperature in accordance with the moisture content of the leather at stages of drying when using a dryer.

Leather has a characteristic moisture content in accordance with its equilibrium with the air around it. The curve has a significant S- shape for all types of leather.The moisture content of the leather will be very low at very low relative humidities and will increase as the relative humidity increases.With a gradual increase in humidity, the moisture content of the leather will level off at a fairly constant level until the humidity of the air reaches approximately 80% relative humidity. At 80% and above, additional moisture will be taken up by the leather. At low moisture contnt the leather is stiff and will shrink in size.As the relative humidity is incresed, the moisture content, flexibility and area of the leather is increased. The normal characteristics of the leather are achieved near 50% relative humidity.

REFERENCES:

Leather Technician’s Handbook J.H.Sharphouse Vernon Lock Ltd. London 1971.

A Modern Course in Leather Technology Vol 1. Science for students of leather technology Ed: R.Reed Pergamon Press 1966 London.

The Chemistry and Technology of Leather Vol 2.Types of tannages Ed: Fred O’Flaherty, William T.Roddy, Robert M. Lollar Reinhold Pub. Corp. N.Y. 1958.

Deri Teknolojisi Ahmet Toptas Sade Ofset Matbaacilik. Istanbul 1993.

Physical Chemistry of Leather Making Krysztof Bienkiewicz Robert E. Krieger Pub. Co. Malabar, Florida 1983.

Biochemistry D.Voet, J.G Voet J.Wiley & Sons 1990 USA

Practical Leather Technology Thomas C. Thorstensen 4th ed. Krieger Pub. Co. Malabar, Florida 1993.

8.M.J.Osgood "Leather Finishing" JOCCA 4,(1987) 104-110.

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