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![• The first group of methods include:
- sweating [soaked skins hung up in a dark
humid room (22-27oC), to allow bacteria to
attack keratin cells of hair & epidermis]
- enzyme unhairing – enzymes that
preferentially attack cells at the base of
hair roots or epidermis are available](https://image.slidesharecdn.com/tanning-190305093116/75/Tanning-14-2048.jpg)
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Tanning
- 1. HIDES & SKINSTANNING
- 2. Beamhouse work • Theoperations that take place in the beamhouse include: soaking, liming & unhairing, fleshing, deliming & bating • Beamhouse work requires 18-24hrs
- 3. SOAKING (WASHING) • Thefirst process in tanning consists of soaking the H/S in water • The aim is to allow the hide to reabsorb any water that was lost after flaying, curing & transport • Soaking also removes blood, dirt, dung, preservation salts and soluble proteins
- 4. • Absorbed waterrehydrates any dried inter- fibrillary protein, loosening its cementing action on the fibres • Collagen fibres, keratin cells of the hair & epidermis also take up water & become more flexible
- 5. • Wet saltedhides may be soaked for 8-20hrs (10-16oC) while dried hides take several days • The amount of water used ranges from 3-5 times the weight of hides & 6-7 times for dried skins • Satisfactory soaking is judged by cleanliness & absence of salt (determined in juice squeezed out of the skin, using a pocket refractometer)
- 6. • Soaking isnot a simple process since putrefying bacteria act as soon as there is surplus water or curing agent is washed out • Common additions to the soak liquor as disinfectants (bactericides) are: Sodium hypochlorite & trichlorophenate (or formic acid & pentachlorophenate)
- 7. • During soakingwater uptake is speeded up by: 1. Mechanical action (rocking frames, paddles or rotating drums) 2. Temperature (the soak is warmed up to 38oC); If temperature exceed 38oC bacteria action increase, protein fibers shrink &/or gelatinize.
- 8. 3. Chemical additions (i)acid addition – when wool or hair is kept on (1-2 parts of formic, hydrochloric or sulphuric acid per 1,000 parts of water at 60oC)
- 9. (ii) alkali addition– more common as it loosens hair (1-3 parts of caustic soda, soda ash, washing soda or borax per 1,000 parts of water) Sodium sulphide, sodium polysulphide or ammonia liquor may also be used
- 10. • If toomuch acid or alkali is used the skin surface will rapidly absorb it & swell to the extent of distorting surface – loose grain on leather. (iii) 3% Salt solution dissolve unwanted inter-fibrillary protein, thus speeding up soaking.
- 11. (iv) Wetting agents(Detergents): 1-2 parts per 1000 parts of water particularly if hides are greasy (v) Enzyme preparations (proteolytic action on interfibre proteins)
- 12. UNHAIRING & LIMING •Unhairing may be considered to be an extension of soaking • The purpose of unhairing is to separate the two structural proteins keratin & collagen • Unhairing removes hair, epidermis & to some degree the inter-fibrillary proteins, preparing the skin for the fleshing process
- 13. • There are2 methods for unhairing: 1. Methods based on destruction or modification of the epidermal tissue surrounding the hair so that it can be loosened & removed 2. Methods that attack the hair itself to destroy its structure (use alkali – Ca(OH)2, NaOH, NaHS or Na2S)
- 14. • The firstgroup of methods include: - sweating [soaked skins hung up in a dark humid room (22-27oC), to allow bacteria to attack keratin cells of hair & epidermis] - enzyme unhairing – enzymes that preferentially attack cells at the base of hair roots or epidermis are available
- 15. - paint unhairing– after soaked skins have drained, ‘paint’ made from hydrated lime & sodium sulfide is painted or sprayed – it attacks keratin cells
- 16. • Liming chemicalseither destroy the hair & epidermis completely or loosen them to the extent that they can be removed mechanically without difficulty • Liming with lime and sodium sulfide takes 12-18hrs • Depending on Na2S conc. the hairs are either completely destroyed or only partially
- 17. Fleshing: • After dehairing,any subcutaneous connective tissue, flesh and fat adhering to the flesh side is removed mechanically using a fleshing machine (or hand fleshing - two handed knife ) • After fleshing the hide/skin is now called a ‘pelt’ (the pelt wt. form the basis for calculating amounts of tanning materials needed)
- 18. Trimming & Splitting: •After fleshing any loose, raggy ends of skin are trimmed (or cobbled) by a hand knife from the pelt • Then the pelt is split horizontally on the splitting machine to a grain split of uniform thickness & one or more flesh splits of varying thickness • Splitting may also be done after chrome tannage
- 19. Deliming & Bating: •These are done in the same vessels(milling drums, mixers etc.) that are to be used for tanning
- 20. Deliming: • After unhairing,the lime in the skin is no longer needed & if left it has detrimental effect on subsequent tannage • With chrome tanning, the lime give a hard green inflexible leather & prevents proper tannage.
- 21. • With vegetabletanning, the lime slows down or reduces tannage & gives a dark colour • Deliming can be done through washing or by use of chemicals
- 22. Deliming by washing: •The skins are put in a paddle or drum & run with continuous flow of cold clean water • Undissolved lime on the surface & some dissolved lime between the fibres is quickly removed • The lime that is chemically held by the fibres (about 0.4% of the skin wt) is removed very slowly necessitating prolonged washing
- 23. • Prolonged washingmay lead to lime blast if the water used is hard (soluble Ca or Mg bicarbonates or carbonic acid react with lime to precipitate CaCO3) • Prolonged washing allows further alkali breakdown of the skin giving loose leather particularly if the water is warm
- 24. Chemical deliming: • Theloose lime is removed by quick washing followed by addition of controlled amounts of weak acids or acid producing salts to neutralize the alkali • Too much acid damages the skin by causing violent swelling & dissolving protein, therefore weak acids are used
- 25. • Weak organicacids used include: Boric, lactic or acetic acid • Acid salts that can be used include: sodium bisulphite, salts of weak alkalis such as ammonium chloride or sulphate • The lime is converted into readily soluble salts that are rinsed out (deliming subsides the alkali swelling caused by the liming chemicals)
- 26. • Deliming istested by dropping a few drops of phenophthalein onto a cross-section of the pelt (if the process is complete – colourless; if not it becomes red) • Delimed pelts must be taken to the next process immediately since putrefying bacteria can cause a slimy feel & loose leather with damaged structure will result
- 27. Enzyme Bating: • Thisis additional removal of protein material loosened by liming using enzymes • Enzymatic bating agents are used which attack non-structured collagen & non- collagenic proteins in a controlled manner to make them soluble
- 28. • Bating opensup the pelt for the ensuing tannage, improves softness, grain elasticity & colour levelness of the leather • Intensive bating does not impair tensile strength of the leather & may even improve the tear strength by increasing elasticity of the fibres
- 29. • Originally enzymesused for bating were pepsin & trypsin from dog dung & fowl droppings (cause soft smooth & silky grain) • Today two main types of bates are used: 1. Pancreatic bates – digestive enzymes from the pancreas 2. Bacterial bates – digestive enzymes of bacteria
- 30. • Enzyme batesare prepared under sterile conditions & mixed with fine wood flour & NH4 salts (sulphate or chloride) • NH4 salts keep the pH at best level for the enzyme action • Alkali bating is done at pH 8.0-8.5 at 37oC with 1-2% powdered enzyme added
- 31. • Short batingtime (i.e. 1hr) lead to slight flattening of the grain & increase flexibility of leather • Long bating time lead to extreme stretch & suppleness • Bated pelts could be scudded if it is essential to produce very clean grain (hair roots, fat, epidermis & unwanted protein are squeezed out)
- 32. • Acid bateswork at pH 4-5 & these give rise to softer leather because at these pH the fibres are not swollen & allow easier access of enzyme to the fibre structure
- 33. Tannage • Converts theunstable natural fibre network of the hide collagen into a stable material (leather) • The tanning materials react with the polypeptide chains of the collagen, the individual tanning material particles each forming several bonds with the different reactive groups of the collagen molecules.
- 34.
- 35. • The collagenmolecules become cross-linked and strengthened both individually & collectively leading to great stability • Differences of leather from untanned hide:(i) Increased resistance to hot H2O (ii) Resistance to microorg. when wet (iii) Less swelling in H2O (iv) Better shape retention (v) Dries leather-like (i.e. not ‘bony’)
- 36. • Tannage canbe effected by means of very different types of tanning material which react poly-functionally with the hide collagen in different ways • Therefore there is a vast array of tanning methods and materials
- 37. Different methods oftanning 1. Vegetable Tanning: Extracts from certain plants (leaves, barks, roots, fruits) are used to convert skins into leather Vegetable tanned leathers have a characteristic colour (vary from pale yellow- brown to intensive red-brown depending on the type of vegetable tanning material used & conditions applied); the colour darkens in daylight
- 38. • Vegetable tannedleather is used for making soles, sandals, straps, belts, upholstery, bags, decorations, shoe lining & book binding leather etc. • Characteristics of vegetable tanned leather: (i) good strength (ii) shape retention (iii) low heat conductivity
- 39. 2. Mineral Tannages •Basic salts of chromium, zirconium & aluminium are used • Very soft leather is produced compared to vegetable tanning • Leather colour is white for aluminium & zirconium but pale green or blue for chromium tannage • Little change of colour in daylight • Chromium tan does not wash out but Al does wash out • Alum tans are used for gloves or fur-skins
- 40. Chrome tanning: • Thelimed, delimed, bated & pickled pelt is converted into leather by treating with chrome sulphates & basifying agents • Chrome leather is highly resistant to hot H2O & boiling • The most important process for production of shoe upper leather, furniture upholstery & clothing leather
- 41. • Advantages ofchrome over vegetable tannage a) Chrome leather has favourable properties (high mechanical strength, outstanding dyeing properties, absolute light fastness) b) Ideal as semi-finished product (wet blues) with good storage stability for subsequent processing into all types of leather c) Heat resistant (can withstand modern drying & steaming processes used in finishing & shoe manufacture)
- 42. d) Chrome tannageoffers wide scope for streamlining e) Less capital is required because of quicker turn-over of raw materials & more modest space requirements f) Production times are shorter & tanning material costs lower Note: ‘semi-chrome’ tannage refers to complete vegetable tanning followed by retannage with chrome ‘chrome-retan’ is a full chrome tannage
- 43. 3. Synthetic tans(Syntans): • Include substances which were manufactured to replace partially or completely, the natural vegetable extracts, to accelerate production & make it cheaper. They may be of various chemical structures • Syntans give leather that is paler than veg tans, may darken in daylight, does not washout with water • Uses are white leathers & specialty leathers
- 44. 4. Aldehyde tannage: •Uses formaldehyde or gluteraldehyde • Produces soft white leather that does not wash out • The leather is water absorbent • Used for water washable gloves & clothing
- 45. 5. Oil tannage: •Cod liver oil is used • Produce light, soft, air-permeable leather that is resistant to washing • The leather is pale yellow that bleaches in sunlight • Used for wash leathers & washable gloves
- 46. Other tanning processes: 1.Oil & sulphur tannage 2. Isocyanate tannage 3. Sulphochloride tannage 4. Quinone tannage
- 47. Dyeing • Leather dyeingis a transition process between tanning & finishing • Some leathers require penetration of dyestuff into collagen network while others need surface application • Synthetic dyestuffs are used & any colour shade can be applied
- 48. • Leather isdyed discontinuously in rotating vessels (drums or Y-shaped dyeing units) or continuously (through-feed dyeing, dyeing by roll coating or spray dyeing) • Anionic or cationic dyestuffs are also available
- 49. Fatliquoring • The lastoperation in the aqueous phase of tanning before drying • The process involves the introduction of oil into the leather so that individual fibres become uniformly coated • The oil should be emulsified to ensure uniform distribution over the leather (soap is normally used to emulsify)
- 50. • This isthe process that determines the mechanical & physical properties of the leather • Leather dried before or with little fatliquoring becomes hard & bony; such leather may crack in use • Over lubrication result in excessive softness & raggy leather
- 51. • Fatliquoring separatethe fibres in the wet state so that they do not stick together too much during drying • It gives the leather softness, suppleness & handle required for a particular application
- 52. • Improves tensilestrength, water permeability & wetability of the leather • Improper fatliquoring can lead to poor adhesion of the finish coats & also impair the adhesion of shoe soles
- 53. Finishing • This refersto further processing of the leather after drying • The aim of finishing is to suit the fashion demands of the consumer with regard to colour, surface effect etc.
- 54. • Involves treatmentwith dyestuff solutions, pigment preparations, top coats & lacquers or by means of mechanical treatment (e.g. plating, embossing or dry drumming) • The poorer the raw material the more complex the finishing treatment required
- 55. Finishing of differentgrades leathers
- 56. Application of finishproducts: • Curtain coating • Padding • Spraying • Roll coating
- 57. Impregnation: • Could meanwaterproofing of leather or tightening the grain • If leather is given a relatively heavy finish, the result is frequently loose grain or a frizzy, unnatural grain break
- 58. • Loose grainis corrected by impregnation with preparations of acrylic or polyurethane dispersions or with polycyanate prepolymers • The impregnating products penetrate deep into the leather & stick the grain layer to the reticular layer beneath it.
- 59. Other special processes/finishing effectsinclude: • Glazing • Polishing • Coating • Special effects • Tipping
- 60. Waterproofing: 1. Closed waterproofing 2.Open waterproofing