Crust
The dried leathers have a series of mechanical operations for softness and general presentation so that they can be sorted for final top finishing, or for export.
1. CONDITION
The object is to give the leather a moisture content of 18-22% to allow mechanical softening. The dried leathers have different levels of moisture from drying at 8-14% because of the different thickness and conditions. In practice, leathers are dried to the lowest level at the first drying, so that subsequent conditioning can produce uniform moisture content and allow a uniform softening. Conditioning adds a controlled amount of water to the leather, usually on the flesh side. This is normally a simple spray application combined into a string conveyor, and a great improvement over the use of damp sawdust. The moistened leathers are piled flat and stand for 24 hours to allow the moisture to reach equilibrium. They should be covered with plastic sheets during this time to maintain the required microclimate.
2. STAKE
The object is to mechanically stretch the leather, separating the fibres, which have become attached to each other during drying. It is important that the moisture content is correct, in the 18-22% range. This is often recognised by touch and handle better than a moisture meter. If the leather has too much moisture, there is insufficient movement of the fibres and the resultant leather is not soft enough after drying out; if the leather is too dry, less than 18-22%, the fibres are damaged by the mechanical action. The actual extent of the fibres self-attachment varies with the wet processing, particularly fat-liquoring, and the drying conditions. The conveyor driven vibrating staking machine is excellent for most leathers, and causes less damage than earlier types. It also has an advantage that the operators need less training. The older jaw-type staking, Slocomb, machine is still suitable for special softness provided there are skilled operators; it has a poor safety record because the manual operations do not allow adequate guards to be fitted on all the moving parts.
3. OPTION TO DRY DRUM (OR MILL)
This option of a fast revolving (20 rpm) dry drum is increasingly used to produce very soft leathers with a relaxed surface appearance. It could be for upholstery, garments, or casual shoes. There are degrees of development, with dust extraction needed to clean any fibres from the atmosphere. It can be done on crust leather without finish, split, suede or on finished leather, provided the finish film will withstand the mechanical action. Several hours running are needed; the internal surface of the drum must be smooth to avoid any damage to the leather surface. The leather needs to have the toggling afterwards to restore the flat surface and the area.
4. DRY SECOND TIME
The object is to dry out the softened leather in a flat state, from the conditioned levels of 18-22% to the normal dry leather, which contains 14%. This level of 14% is the norm for all natural fibres and the leather should not feel damp at all. It can be stored in this condition, which is not the case for the conditioned leathers, which become harder and can develop mould. Toggle drying, or paste drying, is used and the times are short for this mild drying. The temperature used can be critical to prevent area loss. Yield is always better at as low a temperature as possible, for example 18°C is better than 30°C. It is feasible with a low amount of moisture to remove and reasonable air circulation.
5. TRIM - with by product of dry tanned leather waste
After the second drying, the leather will be flat but there may be folds, pleats or ragged edges in some areas, which either disfigure the appearance or will prevent further operations being done correctly and without damage. For example, the leather would not be able to pass between some revolving cylinders in an even manner. As always, it is important to control and supervise carefully trimming operations, because it is all too easy to trim away too much leather, lose the sales value for the piece and profit for the tannery.
6. CRUST SORT - (with an option to export)
This is the second quality control point, after the wet blue sorting control. It is an important stage and sorters need to have good experience and judgement, because so much of the grading is subjective. The standards have to be consistent.
The surface is assessed for potential cutting area and the extent to which defects and damages reduce that area and quality value. The break of the leather is also checked together with the actual thickness (in tenths of millimetres) and how the leather feels on handling. There will often be different dyeings, which have to be checked for shade correctness in finishing. A finishing load, or batch, starts here and is now going to be specifically for a certain type and colour of finished leather, and usually for a specific customer. The best qualities are for full grain and have different degrees of covering finish to improve their cutting value. The worst grades need to have a corrected finish, where the grain is removed by buffing.
The crust stock is an important logistic asset because it can be the means of making quick deliveries to customers, provided that there is the suitable crust leather available. This means the suitable grades, thickness, base dye colour and character to meet the requirements. Finishing can be the means to adjust the surface appearance.
The export option also involves having leathers with the required properties "the quality" and quantity required by the buyer. Selling in this way is only possible in established trade based on full confidence of sorting standards.
Contributed By Mr. Woodley, Michael International Consultant May, 2007
http://www.ab-corporategifts.com
6/03/2008
Leather Production - Retannage
The object is: to produce different types of leather from the one semi- finished leather, which is usually "wet-blue". The retannage optimises the serviceability of the leather, adapting it to meet fashion requirements and the demands of customers. It also creates a dried (crust) leather to allow satisfactory finishing of the dried leather surface.
The neutralization, retannage, dyeing and fat-liquoring wet operations are almost always carried out in that order in one operation, with a total time of 3-7 hours. They are done in a drum, or sectioned dyeing vessel, at a range of temperatures between 35-60° C. Drum speed is about 12 rpm. Re-tannage is often understood to include all the four stages.
The individual stages of the operation influence each other, so there are adjustments needed to obtain optimum effects for high-quality finished leather. The properties, which can be influenced by the re- tannage include fullness, grain tightness, softness, fat distribution, leather colour, levelness of the dyeing, light-fastness, grain fineness, smoothness, buffing, dry-drumming, embossing, buffing, water repellence and chemical and physical analytical results. It is obviously of great importance in determining the final quality.
Special treatments, such as water repellency, are an integral part of all leather process design but there will be a part of the retannage, which involves special chemicals. It has to be realised that such leathers have to be designed from the initial wet work to minimise all hydrophilic chemicals.
1. NEUTRALISE
The object is to remove strong free acids from the leather by using milder chemicals. This weakens the strong positive surface charge of the chrome leather so that anionic tanning materials, dyestuffs and fat-liquors agents can penetrate and are not restricted to the surface. The leather is said to be ‘de-acidified’ because it does not usually reach the neutral point of pH 7. Surface pH is below 5.0 externally and 4.5 internally. If a deep neutralisation is needed to allow other chemicals to penetrate deeper, the external and internal values are about 5.5. It controls the reactivity of wet blue leather and has to produce the same level in all the pieces of leather in one retanning load. It is, therefore, important for treating a selection of wet blue from different suppliers.
2. RETAN
The object is to control and adjust the properties of the resultant crust leather. It is the main use for the synthetic organic tanning materials, but vegetable tannins, polymeric, resin and mineral tanning agents (chrome included) are also used. The environmental concern for trivalent chrome has affected how this material is used. The character of leather is determined by the first tanning operation, but the retannages are an adjustment to that. There are 3 main types of result, depending on the crust required:
- Filling of the looser structure of wet blue by vegetable tannins, replacement syntans and resin tanning materials with a selective filling effect. This leather is also designed to have good tightness, buffing, embossing and finishing properties. This is for corrected grain leather, which is the lowest quality of the wet blue selection.
- The full grain selection has a good grain and cutting value. The retannage is designed to have less filling to retain that natural elegance with a good break, full colour shades from dyeing and an attractive feel from a full handle over all the hide or skin. Softness and an attractive look are often more important than the tightness of the lower grades. The eye appeal is the main sales factor in top quality leather.
- A compromise of the others, so that there can be further sorting in the crust to optimise quality, value and profit for the tannery. It is a valuable option to have a versatile retannage, suitable for finishing as either full grain or corrected grain.
3. DYE
The object is to colour the leather as required by the customer, or sales forecasts. This should be an even colour and should cover any grain defects. If the leather will not have a covering finish, the colour should be light fast, and wash fast. It is usually done in drums, or sectioned dyeing vessels, with different levels of float and temperatures. Not all leathers are drum dyed. This may be done to leave more choice in deciding the product to make from the crust and to save the cost. Dry crust leather can be dyed by through feed continuous methods. At present, this does not produce the same qualities as the batch dyeing due to the limitations of time, temperature and dyestuff type. There are many controls to affect the results and there is a wide choice of dyestuffs, which are high cost materials. These are often in liquid form for easier, and healthier, handling. Health concerns have also banned the use of dyes containing known carcinogenic chemicals, and these are not produced by any reputable company.
Dyeing can be with anionic and cationic types but the whole retannage process needs to be designed according to the dyeing conditions. Colour matching is an old established human art, being replaced by instrumental colour measurement in larger tanneries. Wet white leathers are often required to be dyed with dyes free of heavy metals, to meet the ecological requirements.
4. FAT-LIQUOR
The object is to soften the leather, as required in the product, by lubricating the wet fibres, with a fatty emulsion, so that they do not stick together on drying. Without fat-liquors, the leather would dry hard and any mechanical action would damage the fibre and limit the quality potential. It controls the feel of the dry leather. It is normally the last operation in retannage and can be combined into that float. These emulsions of materials have different stabilities and care has to be taken that there is the required degree of penetration and fixation. Complete penetration produces leather with a cloth-like feel, suitable for garments, compared with a surface effect, which would feel greasy and be difficult to apply a top finish. Care and experience are needed to select the correct balance of materials. The method of drying also needs to be considered when deciding on the fat- liquors. Fat-liquors, with suitable stability, are also applied during tannage to obtain a deeper coating of the fibres. After the final wet operations (retannage etc.), the leather is generally horsed up or stacked on platforms overnight. The water content is about 70-75%, on the leather weight.
5. SAMM AND SET OUT
The object is to reduce the water content and to spread the leather out by stretching it in all directions. The helical blades spread the hide, or skin, into a flat shape and squeeze out the surplus water. Animal skin is three dimensional to cover the animal shape, so this operation now starts to change that into two dimensions. The shapes need to be positioned on the rollers to avoid any creases forming in the surface; this can be difficult for the shanks. After setting out, the leather should be easier to handle for subsequent drying.
6. DRY
The object is to eventually take the moisture level down to about 8-14 % for mechanical softening. Water evaporates from the surface in two stages:
- The first stage is at constant rate with the surface completely wet so that the water can migrate to the surface from the centre of the hide at the same speed that it evaporates. This is unbound water and the heat of the drying does not affect the leather temperature because of the evaporation effect.
- The second stage, the falling rate stage, is when the surface is only partially wet and the
temperature of the leather itself will start to rise. This is a critical stage and can damage the leather if moisture is trapped in the centre.
Uncontrolled drying is not advisable. Shrinkage of the leather also occurs during drying and is a factor in costing. Higher temperatures create higher shrinkage. However, this shrinkage should not be physically restricted because the leather would become hard and unacceptable, with the fibres unable to reposition themselves at different moisture levels. Slow drying, at a low temperature, produces the softest leather with the lowest shrinkage, but this is not normally economical. It is not normally possible to dry the leather to its required physical condition in a single operation. First there is an initial drying and there are 4 main methods used:
- Suspension, or hang, drying where the leather is simply hung up in the drying room or tunnel, with controlled conditions of temperature, relative humidity and air circulation. The resultant dry leather feels full and round with good softness, but has the serious disadvantage that area and smoothness is lost. All such leathers must be toggled after staking to recover sales value and profit.
- Paste drying has the set out leathers pasted onto cool glass, or non- corrosive metal, plates with an aqueous adhesive solution and then passed through a tunnel drier for 5-8 hours. These machines generally have a total of 100 to 200 of these vertical plates and up to 50 metres long. The leather passes through a number of sections, each of which have well controlled drying conditions enabling the drying rates to be progressively matched to the different stages described above. This resultant leather needs some softening, but it is flat and has a better area yield than suspension drying. The residual paste film means that full grain finishing is not normally possible, but it is ideal for such buffed leathers as corrected grain or splits. There also exists the possibility of special pastes, which can be removed easily for full grain.
- Vacuum drying is the best method for most full grain leathers, whilst it is also perfectly satisfactory for corrected types. The set out leathers are laid flat, grain down, onto a stainless steel table, where they are also stretched out further by hand slickers as their shape dictates. The table top and flattened leathers are then enveloped by a sealing hood. Reduced air pressure then allows drying to take place at a lower temperature, with the hide, or skin, being kept flat in position by applied pressure, reducing shrinkage. The temperature is about 75°C, and even less. After a few minutes, the leather is removed with the grain side dry and the flesh side slightly damp. It must not be dried out completely to the lower limit, but hung, without tension, in the ambient temperature to lose the final moisture freely. These driers have been developed into multi-table machines and the most sophisticated systems have a conveyor delivery from the setting out through vacuum driers, to staking and toggling operations. These have great potential for large productions. Process time and labour content is significantly reduced.
- Toggle drying stretches the leather manually onto perforated metal sheets, so that the shape is retained and flattened by the toggle clips, which have pincer grips. These hold the edge of the leather and are fixed into the perforations of the sheets by a small positioning foot, or lug. The drying conditions and control are as for hang drying. The frames are sometimes assembled as a type of bookcase or, much better, as a form of conveyor, which has greatly improved this whole operation and reduced the handling. Toggling is used for upholstery, side clothing, splits and lining leathers. It has 10% more area than hang drying but at the expense of quality, because of the tension produced from the toggles. Stretching prevents some shrinkage. It is better for vegetable tanned light leathers, which shrink less, and for re-toggling damp leathers after mechanical softening and dry milling. Here it keeps the leather soft and flatter.
These initial drying methods produce leathers, which are too hard and have an uneven moisture content, but below the normal 14%. They could not be finished in this state.
Contributed By Mr. Woodley, Michael International Consultant May, 2007
http://www.ab-corporategifts.com
The neutralization, retannage, dyeing and fat-liquoring wet operations are almost always carried out in that order in one operation, with a total time of 3-7 hours. They are done in a drum, or sectioned dyeing vessel, at a range of temperatures between 35-60° C. Drum speed is about 12 rpm. Re-tannage is often understood to include all the four stages.
The individual stages of the operation influence each other, so there are adjustments needed to obtain optimum effects for high-quality finished leather. The properties, which can be influenced by the re- tannage include fullness, grain tightness, softness, fat distribution, leather colour, levelness of the dyeing, light-fastness, grain fineness, smoothness, buffing, dry-drumming, embossing, buffing, water repellence and chemical and physical analytical results. It is obviously of great importance in determining the final quality.
Special treatments, such as water repellency, are an integral part of all leather process design but there will be a part of the retannage, which involves special chemicals. It has to be realised that such leathers have to be designed from the initial wet work to minimise all hydrophilic chemicals.
1. NEUTRALISE
The object is to remove strong free acids from the leather by using milder chemicals. This weakens the strong positive surface charge of the chrome leather so that anionic tanning materials, dyestuffs and fat-liquors agents can penetrate and are not restricted to the surface. The leather is said to be ‘de-acidified’ because it does not usually reach the neutral point of pH 7. Surface pH is below 5.0 externally and 4.5 internally. If a deep neutralisation is needed to allow other chemicals to penetrate deeper, the external and internal values are about 5.5. It controls the reactivity of wet blue leather and has to produce the same level in all the pieces of leather in one retanning load. It is, therefore, important for treating a selection of wet blue from different suppliers.
2. RETAN
The object is to control and adjust the properties of the resultant crust leather. It is the main use for the synthetic organic tanning materials, but vegetable tannins, polymeric, resin and mineral tanning agents (chrome included) are also used. The environmental concern for trivalent chrome has affected how this material is used. The character of leather is determined by the first tanning operation, but the retannages are an adjustment to that. There are 3 main types of result, depending on the crust required:
- Filling of the looser structure of wet blue by vegetable tannins, replacement syntans and resin tanning materials with a selective filling effect. This leather is also designed to have good tightness, buffing, embossing and finishing properties. This is for corrected grain leather, which is the lowest quality of the wet blue selection.
- The full grain selection has a good grain and cutting value. The retannage is designed to have less filling to retain that natural elegance with a good break, full colour shades from dyeing and an attractive feel from a full handle over all the hide or skin. Softness and an attractive look are often more important than the tightness of the lower grades. The eye appeal is the main sales factor in top quality leather.
- A compromise of the others, so that there can be further sorting in the crust to optimise quality, value and profit for the tannery. It is a valuable option to have a versatile retannage, suitable for finishing as either full grain or corrected grain.
3. DYE
The object is to colour the leather as required by the customer, or sales forecasts. This should be an even colour and should cover any grain defects. If the leather will not have a covering finish, the colour should be light fast, and wash fast. It is usually done in drums, or sectioned dyeing vessels, with different levels of float and temperatures. Not all leathers are drum dyed. This may be done to leave more choice in deciding the product to make from the crust and to save the cost. Dry crust leather can be dyed by through feed continuous methods. At present, this does not produce the same qualities as the batch dyeing due to the limitations of time, temperature and dyestuff type. There are many controls to affect the results and there is a wide choice of dyestuffs, which are high cost materials. These are often in liquid form for easier, and healthier, handling. Health concerns have also banned the use of dyes containing known carcinogenic chemicals, and these are not produced by any reputable company.
Dyeing can be with anionic and cationic types but the whole retannage process needs to be designed according to the dyeing conditions. Colour matching is an old established human art, being replaced by instrumental colour measurement in larger tanneries. Wet white leathers are often required to be dyed with dyes free of heavy metals, to meet the ecological requirements.
4. FAT-LIQUOR
The object is to soften the leather, as required in the product, by lubricating the wet fibres, with a fatty emulsion, so that they do not stick together on drying. Without fat-liquors, the leather would dry hard and any mechanical action would damage the fibre and limit the quality potential. It controls the feel of the dry leather. It is normally the last operation in retannage and can be combined into that float. These emulsions of materials have different stabilities and care has to be taken that there is the required degree of penetration and fixation. Complete penetration produces leather with a cloth-like feel, suitable for garments, compared with a surface effect, which would feel greasy and be difficult to apply a top finish. Care and experience are needed to select the correct balance of materials. The method of drying also needs to be considered when deciding on the fat- liquors. Fat-liquors, with suitable stability, are also applied during tannage to obtain a deeper coating of the fibres. After the final wet operations (retannage etc.), the leather is generally horsed up or stacked on platforms overnight. The water content is about 70-75%, on the leather weight.
5. SAMM AND SET OUT
The object is to reduce the water content and to spread the leather out by stretching it in all directions. The helical blades spread the hide, or skin, into a flat shape and squeeze out the surplus water. Animal skin is three dimensional to cover the animal shape, so this operation now starts to change that into two dimensions. The shapes need to be positioned on the rollers to avoid any creases forming in the surface; this can be difficult for the shanks. After setting out, the leather should be easier to handle for subsequent drying.
6. DRY
The object is to eventually take the moisture level down to about 8-14 % for mechanical softening. Water evaporates from the surface in two stages:
- The first stage is at constant rate with the surface completely wet so that the water can migrate to the surface from the centre of the hide at the same speed that it evaporates. This is unbound water and the heat of the drying does not affect the leather temperature because of the evaporation effect.
- The second stage, the falling rate stage, is when the surface is only partially wet and the
temperature of the leather itself will start to rise. This is a critical stage and can damage the leather if moisture is trapped in the centre.
Uncontrolled drying is not advisable. Shrinkage of the leather also occurs during drying and is a factor in costing. Higher temperatures create higher shrinkage. However, this shrinkage should not be physically restricted because the leather would become hard and unacceptable, with the fibres unable to reposition themselves at different moisture levels. Slow drying, at a low temperature, produces the softest leather with the lowest shrinkage, but this is not normally economical. It is not normally possible to dry the leather to its required physical condition in a single operation. First there is an initial drying and there are 4 main methods used:
- Suspension, or hang, drying where the leather is simply hung up in the drying room or tunnel, with controlled conditions of temperature, relative humidity and air circulation. The resultant dry leather feels full and round with good softness, but has the serious disadvantage that area and smoothness is lost. All such leathers must be toggled after staking to recover sales value and profit.
- Paste drying has the set out leathers pasted onto cool glass, or non- corrosive metal, plates with an aqueous adhesive solution and then passed through a tunnel drier for 5-8 hours. These machines generally have a total of 100 to 200 of these vertical plates and up to 50 metres long. The leather passes through a number of sections, each of which have well controlled drying conditions enabling the drying rates to be progressively matched to the different stages described above. This resultant leather needs some softening, but it is flat and has a better area yield than suspension drying. The residual paste film means that full grain finishing is not normally possible, but it is ideal for such buffed leathers as corrected grain or splits. There also exists the possibility of special pastes, which can be removed easily for full grain.
- Vacuum drying is the best method for most full grain leathers, whilst it is also perfectly satisfactory for corrected types. The set out leathers are laid flat, grain down, onto a stainless steel table, where they are also stretched out further by hand slickers as their shape dictates. The table top and flattened leathers are then enveloped by a sealing hood. Reduced air pressure then allows drying to take place at a lower temperature, with the hide, or skin, being kept flat in position by applied pressure, reducing shrinkage. The temperature is about 75°C, and even less. After a few minutes, the leather is removed with the grain side dry and the flesh side slightly damp. It must not be dried out completely to the lower limit, but hung, without tension, in the ambient temperature to lose the final moisture freely. These driers have been developed into multi-table machines and the most sophisticated systems have a conveyor delivery from the setting out through vacuum driers, to staking and toggling operations. These have great potential for large productions. Process time and labour content is significantly reduced.
- Toggle drying stretches the leather manually onto perforated metal sheets, so that the shape is retained and flattened by the toggle clips, which have pincer grips. These hold the edge of the leather and are fixed into the perforations of the sheets by a small positioning foot, or lug. The drying conditions and control are as for hang drying. The frames are sometimes assembled as a type of bookcase or, much better, as a form of conveyor, which has greatly improved this whole operation and reduced the handling. Toggling is used for upholstery, side clothing, splits and lining leathers. It has 10% more area than hang drying but at the expense of quality, because of the tension produced from the toggles. Stretching prevents some shrinkage. It is better for vegetable tanned light leathers, which shrink less, and for re-toggling damp leathers after mechanical softening and dry milling. Here it keeps the leather soft and flatter.
These initial drying methods produce leathers, which are too hard and have an uneven moisture content, but below the normal 14%. They could not be finished in this state.
Contributed By Mr. Woodley, Michael International Consultant May, 2007
http://www.ab-corporategifts.com
Leather Production - Tannage
Tannage
Tannage is the irreversible conversion of the natural fibre network of pelt into the material leather. The collagen structure is permanently cross-linked. This makes it resistant to bacteria, putrefaction and higher temperatures. When dry, it does not feel hard and can normally be rewetted.
1. PICKLE - (can be option to export from this stage)
The object is to prepare the pelt for tannage, or for export, by adjusting the level of acidity in salt solution. This controlled acidity is needed to allow penetration of tanning materials through all the thickness uniformly and to combine correctly. If this was not done, the tanning materials would react too strongly on the outer layers, not penetrate deeper and the appearance of the surface would be damaged. The leather would also not be tanned correctly. The pickle is varied according to the type of tannage to be made, with less acid conditions being used for vegetable tannage. However, salt concentrations are increased and essential to avoid any acid swelling. 6% salt on the total volume of water is a safety level for the most acid conditions. Temperature must not exceed 28°C. Vegetable tannage should start at pH 4, chrome at pH 3 and the export pickle needs to have a stronger pickle in terms of acid content (absorbed by the pelt) and a preservative to prevent mould. This pH is below 2. Storage is good if the pickled pelts are kept cool, below 32°C air temperature.
2. TANNAGE
The object is to convert the pelt into leather by creating a more stable structure, which will not putrefy and maintain an attractive appearance. The options are:
2.1 Chrome tannage
Chrome tannage is the most widely used and most important tannage, in more than 80% of leathers. This is used in the production of shoe upper, furniture upholstery, clothing, leather goods and gloving leathers. Basic chromium sulphate is the main chemical, penetrating at pH 3 in controlled stages and being fixed at pH 3.8-4.0. Process starts at about 24°C to finish at 39°C for better chrome exhaustion. The higher end temperature is achieved by increasing the drum speed from 5 to10 rpm. The leather will have a shrinkage temperature of 95-100°C. The cross-linkage is identified as being made with the carboxyl groups of the collagen. The process is done in a drum and takes about 10 hours for hides; skins are tanned faster because they are thinner. The colour is blue and it is often exported semi-finished as ‘wet blue’ with the use of preservative. Environmental protection is limiting the amount of trivalent chrome salts in an effluent for discharge into a public waterway, because the heavy metal affects the treatments in the waterworks. This has resulted in many developments to reduce the levels of chrome in effluent by increasing chrome uptake in the drums, and by various recycling methods. There are also ecological concerns that the trivalent blue chromium salts in leather could be converted into the carcinogenic hexavalent state, with its danger to health.
2.2 Vegetable Tannage
Vegetable tannage uses material obtained from parts of certain plants. These may be the bark, wood, fruit, root or leaf of the plant. The chemical compositions are complex and are mixtures of natural polyphenolic compounds. The shrinkage temperature is 70-85°C. The characteristic colour varies from pale yellow-brown to an intensive red-brown depending on the type of vegetable tanning material or mixture of tanning materials used and the application conditions. The nature of the materials limits the light fastness, and there is a considerable change in colour with time and exposure to sunlight. They have high molecular weights and probably form linkages due to a combination of hydrogen bonding and their size. The resultant leather is therefore, much fuller and heavier, than chrome-tanned leather. Vegetable is the original tanning method and used for heavier, more compact leathers from hides – sole, strap, belt, bag, harness, upholstery – but it can make soft light leathers such as linings and leather goods - particularly from skins. The process is slow, with limited mechanical action from pits or slower running drums. The time is about 4 days for lighter weight leathers and 12 days for sole leathers. The temperature normally starts at about 20°C for penetration, increasing later to 35°C. It has been largely replaced by chrome tannage, which is quicker and more economical.
2.3 Alternative Tannages
Alternative tannages now receive more attention because of environmental concern. There have been synthetic tannages in use for many years, known as syntans. These cover a wide range of organic chemicals, such as phenols, naphthalene, glutaraldehyde and various polymers. This is a drum process and takes about 10 hours, similar to chrome tannage, but at temperatures of 25-30°C and pH 3.0-3.8. The shrinkage temperature of the leather produced is 70-80°C. It was originally used to make speciality leathers, such as white, and to replace part of the established tannages. The current interest is to replace chrome tannage and ‘wet blue’. There are ‘wet white’ tannages for export and for further processing in the same tannery. These avoid the chromium but do not produce an exactly identical leather character to full chrome leathers at present. However, the auto-upholstery buyers are demanding ‘chrome-free’ leathers and such supplies are expected to increase. The colour of the leathers is white, or yellowish, and they have very good light fastness. These leathers are limited to drying temperatures of 40-50°C, compared with 70°C for chrome tanned wet blue.
2.4 Pre-Tannages
There are also many pre-tannages which are used to follow the pickle, but before the main tannage, so that there is an improved uptake or performance from the tannage itself. Such examples are formate before chrome tannage, phosphate before vegetable and syntans before ‘chrome- free’ tannage of wet white.
After tannage, leathers contain a lot of surplus tan liquor. Consequently, this excess is normally allowed to drain, with the leathers piled on a ‘horse’ or stacked on platforms, overnight. This allows further fixation of tanning material by the leathers whilst the drained liquid is collected, for recycling or for treatment in the effluent.
3. SAMM
The object is to remove the unbound water so that the hide can be packed, split or shaved, with consistent uniform moisture content and an exact thickness. The natural differences in the structure of the hide mean that the tanning material absorption and the liquid absorption also vary. Consequently, the leather is still not the same thickness all over after tannage, even if it was already levelled by splitting in the pelt. It is first sammed to reduce the water content from about 70% to about 60%. The hides are then squeezed between the moist felt rollers of a samming machine, which also flattens the shape. A setting out action, to further spread the hide, is often incorporated into the sequence on the machine, with extra rollers. The moist leather is then sorted for export or for further production in the tannery.
4. SIDING
This refers to cutting the hide into two halves and is done now, if it was not done before tanning. It may be done manually with the use of a cutting guide on a table, or by specialist equipment. It has to be accurately done down the backbone to produce the flatness in that area. Hides can also be cut into other shapes if required in the final leather.
5. SORTING - (can be an option to export at this stage)
The object is to grade the hides, and skins, according to their potential quality. Wet blue (or wet white) is normally exported without splitting, so that the full hide thickness is available to the buyer. Quality is sorted on an agreed basis. This will assess the degree of damage in the hide, or skin, and how it affects the cutting value. Each quality has a different value. Exports may specify particular grades. The wet leathers for export need to be carefully folded, and packed in plastic sheeting so that the packing is completely waterproof. This is to prevent permanent creasing of the leathers and any drying out in transit. The rewetting of such dried leathers is extremely difficult, and it is advantageous if a very small amount of a hydrophilic compound (fat-liquor type) can be added to the tannage, provided there is no effect on final quality. If it is not for export, a similar sorting is done for the tannery's own operations. This has the same assessments, and produces a range of qualities. Sorting also decides whether it can be full grain leather, or whether the grain needs to be buffed away and corrected in some way to disguise faults. The sorting figures are then compared with outstanding customer orders. Their requirements will show thickness, quality and quantity. In this way, the actual customer orders start from here, and should have a reliable completion date for a delivery schedule.
6. SPLIT OPTION - (tanned state) producing a split for further processing and tanned waste
by-product
The object is to obtain a more even thickness for processing and a more uniform final leather, if it were not done in the limed condition. At this stage, the leather has a more stable structure. The tanned hide is less swollen and so it is easier to handle. The actual levelling is more accurate. The thickness is determined by the final product to be made. It will allow for some final adjustment by shaving. The machine and operators are critical to a successful operation, from quality and profitability aspects. A good machine is a valuable investment for the tannery.
7. TRIM - with by product of damp tanned leather waste
The object is to produce an economic shape for sale or processing further. The grain layer (top split) needs to have any ragged edge cut away to facilitate other machine work, whilst the lower flesh split has to be trimmed to such a regular shape that can have a uniform thickness. Trimming should be to retain, or improve, value. The quantity of trim should be controlled to see that it is not excessive, because it loses profit. The actual flesh split is larger than a flesh limed split.
8. SHAVE - with by product of tanned shavings
The object is to make the final thickness adjustment and have an even cutting through leather with consistent moisture. The moisture content should be 30-45% Splitting leather can never be accurate enough and so a shaving machine is needed to refine the produced thickness and leave the flesh side smooth. The shaved thickness is determined by the customer requirements, allowing for the loss in processing between the semi-moist condition and the final despatch.
9. PREPARING LOADS for retannage and dyeing
The object is to plan the production of crust leather to meet the demands of the customers’ and sales forecasts. Customer orders define the quantity and quality of finished leather. The tannery identifies the type of retanned leather (the crust), which corresponds to the finished leather. If there is not sufficient quantity and quality available in crust stock, there has to be more produced from the wet blue. In this way, the work tickets from the wet blue are planned to supply the crust stock as required, from orders or sales forecasts. The loads will be in standard sizes for the retanning drums and correspond to established processes.
Contributed By Mr. Woodley, Michael International Consultant May, 2007
http://www.ab-corporategifts.com
Tannage is the irreversible conversion of the natural fibre network of pelt into the material leather. The collagen structure is permanently cross-linked. This makes it resistant to bacteria, putrefaction and higher temperatures. When dry, it does not feel hard and can normally be rewetted.
1. PICKLE - (can be option to export from this stage)
The object is to prepare the pelt for tannage, or for export, by adjusting the level of acidity in salt solution. This controlled acidity is needed to allow penetration of tanning materials through all the thickness uniformly and to combine correctly. If this was not done, the tanning materials would react too strongly on the outer layers, not penetrate deeper and the appearance of the surface would be damaged. The leather would also not be tanned correctly. The pickle is varied according to the type of tannage to be made, with less acid conditions being used for vegetable tannage. However, salt concentrations are increased and essential to avoid any acid swelling. 6% salt on the total volume of water is a safety level for the most acid conditions. Temperature must not exceed 28°C. Vegetable tannage should start at pH 4, chrome at pH 3 and the export pickle needs to have a stronger pickle in terms of acid content (absorbed by the pelt) and a preservative to prevent mould. This pH is below 2. Storage is good if the pickled pelts are kept cool, below 32°C air temperature.
2. TANNAGE
The object is to convert the pelt into leather by creating a more stable structure, which will not putrefy and maintain an attractive appearance. The options are:
2.1 Chrome tannage
Chrome tannage is the most widely used and most important tannage, in more than 80% of leathers. This is used in the production of shoe upper, furniture upholstery, clothing, leather goods and gloving leathers. Basic chromium sulphate is the main chemical, penetrating at pH 3 in controlled stages and being fixed at pH 3.8-4.0. Process starts at about 24°C to finish at 39°C for better chrome exhaustion. The higher end temperature is achieved by increasing the drum speed from 5 to10 rpm. The leather will have a shrinkage temperature of 95-100°C. The cross-linkage is identified as being made with the carboxyl groups of the collagen. The process is done in a drum and takes about 10 hours for hides; skins are tanned faster because they are thinner. The colour is blue and it is often exported semi-finished as ‘wet blue’ with the use of preservative. Environmental protection is limiting the amount of trivalent chrome salts in an effluent for discharge into a public waterway, because the heavy metal affects the treatments in the waterworks. This has resulted in many developments to reduce the levels of chrome in effluent by increasing chrome uptake in the drums, and by various recycling methods. There are also ecological concerns that the trivalent blue chromium salts in leather could be converted into the carcinogenic hexavalent state, with its danger to health.
2.2 Vegetable Tannage
Vegetable tannage uses material obtained from parts of certain plants. These may be the bark, wood, fruit, root or leaf of the plant. The chemical compositions are complex and are mixtures of natural polyphenolic compounds. The shrinkage temperature is 70-85°C. The characteristic colour varies from pale yellow-brown to an intensive red-brown depending on the type of vegetable tanning material or mixture of tanning materials used and the application conditions. The nature of the materials limits the light fastness, and there is a considerable change in colour with time and exposure to sunlight. They have high molecular weights and probably form linkages due to a combination of hydrogen bonding and their size. The resultant leather is therefore, much fuller and heavier, than chrome-tanned leather. Vegetable is the original tanning method and used for heavier, more compact leathers from hides – sole, strap, belt, bag, harness, upholstery – but it can make soft light leathers such as linings and leather goods - particularly from skins. The process is slow, with limited mechanical action from pits or slower running drums. The time is about 4 days for lighter weight leathers and 12 days for sole leathers. The temperature normally starts at about 20°C for penetration, increasing later to 35°C. It has been largely replaced by chrome tannage, which is quicker and more economical.
2.3 Alternative Tannages
Alternative tannages now receive more attention because of environmental concern. There have been synthetic tannages in use for many years, known as syntans. These cover a wide range of organic chemicals, such as phenols, naphthalene, glutaraldehyde and various polymers. This is a drum process and takes about 10 hours, similar to chrome tannage, but at temperatures of 25-30°C and pH 3.0-3.8. The shrinkage temperature of the leather produced is 70-80°C. It was originally used to make speciality leathers, such as white, and to replace part of the established tannages. The current interest is to replace chrome tannage and ‘wet blue’. There are ‘wet white’ tannages for export and for further processing in the same tannery. These avoid the chromium but do not produce an exactly identical leather character to full chrome leathers at present. However, the auto-upholstery buyers are demanding ‘chrome-free’ leathers and such supplies are expected to increase. The colour of the leathers is white, or yellowish, and they have very good light fastness. These leathers are limited to drying temperatures of 40-50°C, compared with 70°C for chrome tanned wet blue.
2.4 Pre-Tannages
There are also many pre-tannages which are used to follow the pickle, but before the main tannage, so that there is an improved uptake or performance from the tannage itself. Such examples are formate before chrome tannage, phosphate before vegetable and syntans before ‘chrome- free’ tannage of wet white.
After tannage, leathers contain a lot of surplus tan liquor. Consequently, this excess is normally allowed to drain, with the leathers piled on a ‘horse’ or stacked on platforms, overnight. This allows further fixation of tanning material by the leathers whilst the drained liquid is collected, for recycling or for treatment in the effluent.
3. SAMM
The object is to remove the unbound water so that the hide can be packed, split or shaved, with consistent uniform moisture content and an exact thickness. The natural differences in the structure of the hide mean that the tanning material absorption and the liquid absorption also vary. Consequently, the leather is still not the same thickness all over after tannage, even if it was already levelled by splitting in the pelt. It is first sammed to reduce the water content from about 70% to about 60%. The hides are then squeezed between the moist felt rollers of a samming machine, which also flattens the shape. A setting out action, to further spread the hide, is often incorporated into the sequence on the machine, with extra rollers. The moist leather is then sorted for export or for further production in the tannery.
4. SIDING
This refers to cutting the hide into two halves and is done now, if it was not done before tanning. It may be done manually with the use of a cutting guide on a table, or by specialist equipment. It has to be accurately done down the backbone to produce the flatness in that area. Hides can also be cut into other shapes if required in the final leather.
5. SORTING - (can be an option to export at this stage)
The object is to grade the hides, and skins, according to their potential quality. Wet blue (or wet white) is normally exported without splitting, so that the full hide thickness is available to the buyer. Quality is sorted on an agreed basis. This will assess the degree of damage in the hide, or skin, and how it affects the cutting value. Each quality has a different value. Exports may specify particular grades. The wet leathers for export need to be carefully folded, and packed in plastic sheeting so that the packing is completely waterproof. This is to prevent permanent creasing of the leathers and any drying out in transit. The rewetting of such dried leathers is extremely difficult, and it is advantageous if a very small amount of a hydrophilic compound (fat-liquor type) can be added to the tannage, provided there is no effect on final quality. If it is not for export, a similar sorting is done for the tannery's own operations. This has the same assessments, and produces a range of qualities. Sorting also decides whether it can be full grain leather, or whether the grain needs to be buffed away and corrected in some way to disguise faults. The sorting figures are then compared with outstanding customer orders. Their requirements will show thickness, quality and quantity. In this way, the actual customer orders start from here, and should have a reliable completion date for a delivery schedule.
6. SPLIT OPTION - (tanned state) producing a split for further processing and tanned waste
by-product
The object is to obtain a more even thickness for processing and a more uniform final leather, if it were not done in the limed condition. At this stage, the leather has a more stable structure. The tanned hide is less swollen and so it is easier to handle. The actual levelling is more accurate. The thickness is determined by the final product to be made. It will allow for some final adjustment by shaving. The machine and operators are critical to a successful operation, from quality and profitability aspects. A good machine is a valuable investment for the tannery.
7. TRIM - with by product of damp tanned leather waste
The object is to produce an economic shape for sale or processing further. The grain layer (top split) needs to have any ragged edge cut away to facilitate other machine work, whilst the lower flesh split has to be trimmed to such a regular shape that can have a uniform thickness. Trimming should be to retain, or improve, value. The quantity of trim should be controlled to see that it is not excessive, because it loses profit. The actual flesh split is larger than a flesh limed split.
8. SHAVE - with by product of tanned shavings
The object is to make the final thickness adjustment and have an even cutting through leather with consistent moisture. The moisture content should be 30-45% Splitting leather can never be accurate enough and so a shaving machine is needed to refine the produced thickness and leave the flesh side smooth. The shaved thickness is determined by the customer requirements, allowing for the loss in processing between the semi-moist condition and the final despatch.
9. PREPARING LOADS for retannage and dyeing
The object is to plan the production of crust leather to meet the demands of the customers’ and sales forecasts. Customer orders define the quantity and quality of finished leather. The tannery identifies the type of retanned leather (the crust), which corresponds to the finished leather. If there is not sufficient quantity and quality available in crust stock, there has to be more produced from the wet blue. In this way, the work tickets from the wet blue are planned to supply the crust stock as required, from orders or sales forecasts. The loads will be in standard sizes for the retanning drums and correspond to established processes.
Contributed By Mr. Woodley, Michael International Consultant May, 2007
http://www.ab-corporategifts.com
Leather Production - Pre-tannage
Pre-tannage
The principal chemical component in leather is the protein collagen and the pre-tannage operations are aimed to remove other proteins as required and to prepare the collagen chains for subsequent cross- linkage by the various tannages.
The first stage in pre-tannage is to restore the original moisture to the skin, which will allow all the subsequent processing to be done correctly. The alkaline chemical treatments, normally as lime, then clean the hide structure by removing some types of proteins and giving a degree of swelling. There is a loosening, or destruction, of the epidermis including the hair. The fibre structure is opened up and fats are partially removed as soaps. After the hair is removed, the alkaline swelling is removed and there is a further opening-up of the fibre structure by enzymes. The hide, or skin, is often call ‘pelt’ in these pre-tannage stages.
PREPARE INPUT LOAD
The variety of shapes and sizes of hides and skins reflect the history and health of the animals themselves. It is a good practice to select similar weight, condition and sizes of the raw material so that the chemical and physical processes can be more uniform and efficient. If this is not done, then some pieces will have too much treatment and others will have too little.
SOAK
The object is to restore the hide to its natural moisture content and degree of swelling. There is also the removal of dirt, soluble proteins and curing agents (mainly salt). It may be either be done in pits (for pre-soaking dried material), paddle (for careful soaking of delicate skins in long floats) or, more usually, in a drum. The drum speed is low and intermittent. Chemicals to aid re-hydration, such as biodegradable surfactants are often included and slight alkalinity helps to achieve a limited swelling. The salt concentration should not be allowed to fall below 2° Beaumé. Bactericide is needed to avoid any putrefaction damage. Ideal temperature is 26°C and pH 9-10.
GREEN FLESH OPTION - with a by-product of green fleshings and residual fat
The mechanical operation of fleshing is an option, for hides, to cut away flesh tissue so that the chemicals in the subsequent operations can penetrate easier. All chemicals penetrate faster from the flesh side of a hide or skin, compared with the grain side.
UNHAIR (lime) - with a waste by product of hair or wool
The object is to remove the hair and to open up the fibre structure. This is normally done by lime based, sulphide containing, liquors, in drums or paddles. The drums are slow moving at 2-4 rpm, with intermittent running. The method can also be done by surface application (painting); this is normally on the flesh side, by hand or machine, to leave the hair or wool without serious attack. The grain side is used to obtain a particularly smooth grain, when the hair is completely destroyed. In the main drum or paddle operation, the hair structured is destroyed. Fats are made into soaps and there is strong swelling, or plumping, due to the high alkalinity (about pH 12-13). Temperature of 26°C is ideal, but is not to be above 30°C. A thorough washing is needed when liming is completed, with the temperature 4°C above the liming temperature. This allows better fleshing and smoother necks.
Liming for heavy leather to be vegetable tanned has an extra day in a weak lime solution. This will increase the opening-up of the structure to permit more filling by the larger vegetable tanning molecules. Some sheepskins are not put into liquors for unhairing. They are kept in rooms to produce a controlled bacterial attack. This ‘sweating’ is used to improve the quality of the wool but does not improve the leather quality.
As this is the most polluting of all the tannery operations, a lot of efforts have been made to reduce the toxic effects. This includes hair- saving instead of hair destruction, reduced sulphide dosages, recycling and alternative chemicals. Enzyme unhairing is another option being developed to avoid sulphide but some alkaline treatment is still needed to open the fibre structure.
FLESH - with a waste by-product of fleshing
The limed pelts are in a swollen state and the cutting action of the fleshing machine is more effective here than in the earlier green fleshing operation. Handling is difficult because of the slippery nature of the limed hides and skins. If limed pelt is exposed to the atmosphere for several hours, there is the risk of damage to the surface by the formation of lime-blast. This describes the formation of calcium carbonate films, when the carbon dioxide in the air reacts with the calcium hydroxide of the lime solution. In practice, it can be minimised.
TRIM - with a waste by-product of the trimmings
The cutting action of the fleshing machine blades on hides can cause strings of material, which need to be trimmed to give a clear shape. Skins are cleaner.
SPLIT OPTION (pre-tanned state) with by-products of splits for further processing or for Gelatine and glue stock
The final leather has to be as even as possible in thickness although this is not the case in the original hide or skin. The object in splitting is to obtain a more even thickness for processing and a more uniform final leather. Hides are much thicker than skins and need to be split either now, or later, in the tanned state. The grain (top) is levelled by passing the hide across an endless band-knife to an accuracy of a few millimetres; the bottom layer, known as the ‘flesh split’, is of irregular shape and thickness. Splitting is a skilled operation and needs experienced operators and a well-maintained reliable machine. Although, splitting at this stage is more difficult, and less accurate, than splitting in the tanned state, the advantage is that the tanning chemicals penetrate easier and are absorbed more efficiently.
Splits are processed separately and should be an important contributor to profitability. Several layers can be produced from an exceptionally thick hide, such as buffalo. However, in such cases, the middle layers are weaker in structure than the outer layers.
TRIM - with a waste by-product of trimming
The object is to produce an economic shape for sale or processing further. The grain layer (top split) needs to have any ragged edge cut away to facilitate other machine work, whilst the lower flesh split has to be trimmed to such a regular shape that can have a uniform thickness. Trimming should be to retain, or improve, value. The quantity of trim should be controlled to see that it is not excessive, because it loses potential leather to sell and represents profit. Hides can be kept as whole hides until after tannage, but there can be an advantage if the hide is segmented for some specialised productions; the best quality butt can be processed for one product and the shoulder and/ or belly pieces for other products. In this way, more value is added to the one hide.
DELIME AND BATE
The object of deliming is to remove the strong alkalinity of the pelts by the use of weaker alkalis, and weak acids, so that swelling is reduced. Bates are enzymes and the object of bating is to produce a smooth clean grain and remove non-structured collagen and other proteins. It is done at specific conditions of temperature and pH and continues the deliming. The enzyme action improves the softness, grain elasticity and colour levelness of the leather. The work is normally done in a drum at a temperature of 28-30°C and pH to come below 8.5. The drum speed is faster at 10-12 rpm. Maximum temperature is 35°C for deliming.
OPTION OF DEGREASE (sheepskins)
The object is to remove excess grease from the skins to allow proper processing. The percentage content of natural fat depends on the type and origin of the rawhide or skin. Based on dry material, hides have 2-10%, goats 5-10%, hair sheep 8-15%, wool sheep 20-30% and pigs 30-40%. Processing does remove some of the lower levels and leathers need to have some fat for softness, which is also added later. Gloving and nappa skins can tolerate 5% but the excess in wool sheepskins (and pigskins) needs to be degreased. Surfactants to emulsify the fat have been used in combination with fat solvents, which produce satisfactory leathers but are environmentally damaging. Kerosene has been used with solvent recovery, but is also not acceptable in modern practice. Enzymes are being increasingly used for degreasing together with biodegradable surfactants. Temperature is 35-38°C.
Contributed By Mr. Woodley, Michael International Consultant May, 2007
Compiled by: http:www.ab-corporategifts.com
The principal chemical component in leather is the protein collagen and the pre-tannage operations are aimed to remove other proteins as required and to prepare the collagen chains for subsequent cross- linkage by the various tannages.
The first stage in pre-tannage is to restore the original moisture to the skin, which will allow all the subsequent processing to be done correctly. The alkaline chemical treatments, normally as lime, then clean the hide structure by removing some types of proteins and giving a degree of swelling. There is a loosening, or destruction, of the epidermis including the hair. The fibre structure is opened up and fats are partially removed as soaps. After the hair is removed, the alkaline swelling is removed and there is a further opening-up of the fibre structure by enzymes. The hide, or skin, is often call ‘pelt’ in these pre-tannage stages.
PREPARE INPUT LOAD
The variety of shapes and sizes of hides and skins reflect the history and health of the animals themselves. It is a good practice to select similar weight, condition and sizes of the raw material so that the chemical and physical processes can be more uniform and efficient. If this is not done, then some pieces will have too much treatment and others will have too little.
SOAK
The object is to restore the hide to its natural moisture content and degree of swelling. There is also the removal of dirt, soluble proteins and curing agents (mainly salt). It may be either be done in pits (for pre-soaking dried material), paddle (for careful soaking of delicate skins in long floats) or, more usually, in a drum. The drum speed is low and intermittent. Chemicals to aid re-hydration, such as biodegradable surfactants are often included and slight alkalinity helps to achieve a limited swelling. The salt concentration should not be allowed to fall below 2° Beaumé. Bactericide is needed to avoid any putrefaction damage. Ideal temperature is 26°C and pH 9-10.
GREEN FLESH OPTION - with a by-product of green fleshings and residual fat
The mechanical operation of fleshing is an option, for hides, to cut away flesh tissue so that the chemicals in the subsequent operations can penetrate easier. All chemicals penetrate faster from the flesh side of a hide or skin, compared with the grain side.
UNHAIR (lime) - with a waste by product of hair or wool
The object is to remove the hair and to open up the fibre structure. This is normally done by lime based, sulphide containing, liquors, in drums or paddles. The drums are slow moving at 2-4 rpm, with intermittent running. The method can also be done by surface application (painting); this is normally on the flesh side, by hand or machine, to leave the hair or wool without serious attack. The grain side is used to obtain a particularly smooth grain, when the hair is completely destroyed. In the main drum or paddle operation, the hair structured is destroyed. Fats are made into soaps and there is strong swelling, or plumping, due to the high alkalinity (about pH 12-13). Temperature of 26°C is ideal, but is not to be above 30°C. A thorough washing is needed when liming is completed, with the temperature 4°C above the liming temperature. This allows better fleshing and smoother necks.
Liming for heavy leather to be vegetable tanned has an extra day in a weak lime solution. This will increase the opening-up of the structure to permit more filling by the larger vegetable tanning molecules. Some sheepskins are not put into liquors for unhairing. They are kept in rooms to produce a controlled bacterial attack. This ‘sweating’ is used to improve the quality of the wool but does not improve the leather quality.
As this is the most polluting of all the tannery operations, a lot of efforts have been made to reduce the toxic effects. This includes hair- saving instead of hair destruction, reduced sulphide dosages, recycling and alternative chemicals. Enzyme unhairing is another option being developed to avoid sulphide but some alkaline treatment is still needed to open the fibre structure.
FLESH - with a waste by-product of fleshing
The limed pelts are in a swollen state and the cutting action of the fleshing machine is more effective here than in the earlier green fleshing operation. Handling is difficult because of the slippery nature of the limed hides and skins. If limed pelt is exposed to the atmosphere for several hours, there is the risk of damage to the surface by the formation of lime-blast. This describes the formation of calcium carbonate films, when the carbon dioxide in the air reacts with the calcium hydroxide of the lime solution. In practice, it can be minimised.
TRIM - with a waste by-product of the trimmings
The cutting action of the fleshing machine blades on hides can cause strings of material, which need to be trimmed to give a clear shape. Skins are cleaner.
SPLIT OPTION (pre-tanned state) with by-products of splits for further processing or for Gelatine and glue stock
The final leather has to be as even as possible in thickness although this is not the case in the original hide or skin. The object in splitting is to obtain a more even thickness for processing and a more uniform final leather. Hides are much thicker than skins and need to be split either now, or later, in the tanned state. The grain (top) is levelled by passing the hide across an endless band-knife to an accuracy of a few millimetres; the bottom layer, known as the ‘flesh split’, is of irregular shape and thickness. Splitting is a skilled operation and needs experienced operators and a well-maintained reliable machine. Although, splitting at this stage is more difficult, and less accurate, than splitting in the tanned state, the advantage is that the tanning chemicals penetrate easier and are absorbed more efficiently.
Splits are processed separately and should be an important contributor to profitability. Several layers can be produced from an exceptionally thick hide, such as buffalo. However, in such cases, the middle layers are weaker in structure than the outer layers.
TRIM - with a waste by-product of trimming
The object is to produce an economic shape for sale or processing further. The grain layer (top split) needs to have any ragged edge cut away to facilitate other machine work, whilst the lower flesh split has to be trimmed to such a regular shape that can have a uniform thickness. Trimming should be to retain, or improve, value. The quantity of trim should be controlled to see that it is not excessive, because it loses potential leather to sell and represents profit. Hides can be kept as whole hides until after tannage, but there can be an advantage if the hide is segmented for some specialised productions; the best quality butt can be processed for one product and the shoulder and/ or belly pieces for other products. In this way, more value is added to the one hide.
DELIME AND BATE
The object of deliming is to remove the strong alkalinity of the pelts by the use of weaker alkalis, and weak acids, so that swelling is reduced. Bates are enzymes and the object of bating is to produce a smooth clean grain and remove non-structured collagen and other proteins. It is done at specific conditions of temperature and pH and continues the deliming. The enzyme action improves the softness, grain elasticity and colour levelness of the leather. The work is normally done in a drum at a temperature of 28-30°C and pH to come below 8.5. The drum speed is faster at 10-12 rpm. Maximum temperature is 35°C for deliming.
OPTION OF DEGREASE (sheepskins)
The object is to remove excess grease from the skins to allow proper processing. The percentage content of natural fat depends on the type and origin of the rawhide or skin. Based on dry material, hides have 2-10%, goats 5-10%, hair sheep 8-15%, wool sheep 20-30% and pigs 30-40%. Processing does remove some of the lower levels and leathers need to have some fat for softness, which is also added later. Gloving and nappa skins can tolerate 5% but the excess in wool sheepskins (and pigskins) needs to be degreased. Surfactants to emulsify the fat have been used in combination with fat solvents, which produce satisfactory leathers but are environmentally damaging. Kerosene has been used with solvent recovery, but is also not acceptable in modern practice. Enzymes are being increasingly used for degreasing together with biodegradable surfactants. Temperature is 35-38°C.
Contributed By Mr. Woodley, Michael International Consultant May, 2007
Compiled by: http:www.ab-corporategifts.com
5/19/2008
2008 China Leather Trade Fairs List
34th China Fur & Leather Products Fair 2007
From: 15th January, To 18th January, 2008
Venue: China International Exhibition Centre (Bejing)
City: Beijing
Organisers Website: Fur & Fair http://www.fur-fair.com/
The 103rd China Import and Export Fair
From: 15th April, To 20th April, 2008
Venue: China Import and Export Fair (Pazhou) Complex
City: Guangzhou
Organisers Website: http://www.cantonfair.org.cn/en/
The 7th China Dongguan International Fair for Shoes, and Leather Goods Manufacturing Technology and Material
From: April
Venue:
City: Dongguan
Organisers Website: www.adsale.com.hk/en/exhs-index.asp
The 2nd Fashion Focus Guangzhou Int’l Fair on Leather products and Footwear
From: July 10th, To July 12th 2008
Venue: Guangzhou Jin Han Exhibition Center
City: Guangzhou
Organisers Website: http://www.good-exhibition.com/dgpj/shishangen/index.htm
All China Leather Exhibition (ACLE)
From :3rd September to 5th September 2008
Venue: Shanghai New International Expo Centre
City: Shanghai
Organisers Website: http://www.aplf.com/ , http://www.china-leather.com , http://www.messe-duesseldorf.de
The 13th China (Wenzhou) International Leather Fair 2008
From: 24th September, to 26th September 2008
Venue: Wenzhou Int’l Convention & Exhibition Center P.R.China
City: Wenzhou
Organisers Website: http://www.donnor.com/leather/fair/index.asp
The 104th China Import and Export Fair
From: October 15th – November 6th, 2008
Venue: China Import and Export Fair (Pazhou) Complex
City: Guangzhou
Organisers Website: http://www.cantonfair.org.cn/en/
From: 15th January, To 18th January, 2008
Venue: China International Exhibition Centre (Bejing)
City: Beijing
Organisers Website: Fur & Fair http://www.fur-fair.com/
The 103rd China Import and Export Fair
From: 15th April, To 20th April, 2008
Venue: China Import and Export Fair (Pazhou) Complex
City: Guangzhou
Organisers Website: http://www.cantonfair.org.cn/en/
The 7th China Dongguan International Fair for Shoes, and Leather Goods Manufacturing Technology and Material
From: April
Venue:
City: Dongguan
Organisers Website: www.adsale.com.hk/en/exhs-index.asp
The 2nd Fashion Focus Guangzhou Int’l Fair on Leather products and Footwear
From: July 10th, To July 12th 2008
Venue: Guangzhou Jin Han Exhibition Center
City: Guangzhou
Organisers Website: http://www.good-exhibition.com/dgpj/shishangen/index.htm
All China Leather Exhibition (ACLE)
From :3rd September to 5th September 2008
Venue: Shanghai New International Expo Centre
City: Shanghai
Organisers Website: http://www.aplf.com/ , http://www.china-leather.com , http://www.messe-duesseldorf.de
The 13th China (Wenzhou) International Leather Fair 2008
From: 24th September, to 26th September 2008
Venue: Wenzhou Int’l Convention & Exhibition Center P.R.China
City: Wenzhou
Organisers Website: http://www.donnor.com/leather/fair/index.asp
The 104th China Import and Export Fair
From: October 15th – November 6th, 2008
Venue: China Import and Export Fair (Pazhou) Complex
City: Guangzhou
Organisers Website: http://www.cantonfair.org.cn/en/
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