Production of crust (shoe upper) leather from wet blue leather
Production of crust (shoe upper) leather from wet blue leather:
★ Acid wash:
Water ----------------------------------- 200%
Oxalic acid ---------------------------- 0 8%
Wetting agent (LD-600) -------------- 0.3%
(Run for 30 minutes)
then drain and rinse
★ Re-chroming:
Water ----------------------------------- 150%
Formic acid ---------------------------- 0.3%
(Run for 10 minutes)
Basic chromium sulphate ------------ 4%
Chrome syntan ------------------------ 4%
Sodium formate ----------------------- 1%
LDW/CB/GS -------------------------- 1%
(Run for 45-60 minutes)
GTW ----------------------------------- 1.5%
(run for 30 minutes)
Neosyn 9p ----------------------------- 2%
(run for 20 minutes)
Relugan RF---------------------------- 2%
(run for 20 minutes)
Sodium bi carbonate (1:20)
Drain
★ Neutralization:
Water ---------------------------------- 100%
BS -------------------------------------- 2%
Sodium formate ---------------------- 1%
(Run for 45 minutes)
Check pH 4.8 to 5.5 BCG
Drain
★ Re-tanning and dyeing:
water at 50°c ------------------------- 150%
RE ------------------------------------- 3%
(run for 20 min.)
Relugan DP-100 -------------------- 4%
(run for 30 min)
Neosyn N/OT ----------------------- 2%
(run for 10 min)
Acid black dye ---------------------- 3%
(run for 30 min)
Check penetration
Mimosa ------------------------------ 5%
Quebracho --------------------------- 5%
DLE ---------------------------------- 3%
OS ------------------------------------ 4%
(Run for 30 min)
Mimosa ------------------------------ 5%
Quebracho --------------------------- 5%
Syntan A W ------------------------- 3%
DLE ---------------------------------- 3%
OS ------------------------------------ 3%
(Run for 45 min)
Formic acid (1:3)------------------- 1.5%
(Run for 10 min + 30 min)
Drain and rinse
Wash with 200% water at 55 to 60°C for 5-10 min
★ Fat-liquoring
water at 50°C ---------------------- 150%
B-40 --------------------------------- 4%
C-2 ---------------------------------- 4%
Synthol-O -------------------------- 0.5%
Busan 30L ------------------------- 0.2%
(Run for 45-60 min)
Add Formic acid (1:3) ------------ 1%
(run for 10 min + 30 min)
Drain and wash well
★ Top dye and fat:
water at 55°C --------------------- 150%
Acetic acid ------------------------ 0.5%
(run for 10 min)
Basic black dye ------------------ 1%
(run for 30 min)
Acid black dye ------------------- 2%
(run for 30 min)
Formic acid ---------------------- 1%
(run for 30 min)
LDW ----------------------------- 0.5%
Synthol-O ----------------------- 0.5%
RF -------------------------------- 0.5%
Busan 30L ----------------------- 0.2%
Drain and rinse
Horse up overnight
★ Mechanical operation:
01. Setting - (flatten the the leather surface, smooth without any distortion of grain pattern)
02. Vacuum dry
03. Hang to dry
04. Tannel dry
05. Vibrating Staking
06. Toggle dry
07. Trimming
08. Kissplate
Then finishing.
Finishing Defects
Finishing Defects:
★ Change of shade on finished leather
Causes:
- Faulty combination of pigment
Remedy:
- Use suitable combination and avoid using such pigments which fade on exposure.
★ Poor covering & showing of stained bottom
Causes:
- Dyed and finishing shade do not match.
- Improper brushing or spraying of the finish solution.
Remedy:
- Bottom dyeing must be matched to finish shade
- Check application of the finish solution.
★ Glazing difficulties
Causes:
- Use of excess or insufficient plasticizers.
- Insufficient drying.
Remedy:
- Adjust the quantity of plasticizers
- Dry properly.
★ Dry and wet rubbing of pigment film
Causes:
- Insufficient binder or plasticizer
- Excess of binder
Remedy:
- Increase the quantity as the case may be
- Reduce binder.
★ Top coat cracks:
Causes:
- Use of too thick top coat
- Too much of binder
- Inadequate plasticizer
Remedy:
- Dilute the top coat
- Reduce binder
- Increase plasticizer
★ Grayish undertone on finished leather
Causes:
- Optical effect due to low grain
- Too much use of chrome salts in fixing solution.
Remedy:
- Hot ironing before finishing
- Adjust the quantity in chrome in fixing solution,
★ Pigment coat peeling off:
Causes:
- Too much pigment or too little binder.
Remedy:
- Correct which is applicable.
★ Scratch marks on corrected grain finished leather
Causes:
- Buffing sores
Remedy:
-Changing abrasive paper to correct grit.
★ Streaks of color appearing after glazing operation (comet formation)
Causes:
Dust, soot, dye stuff powder or undissolved pigments on the pigmented leather
Remedy:
- Keep the finishing room clean and away from contamination with foreign particles.
★ Frequent change of shades with the same recipe.
Causes:
- Consistency of pigment past differs or the finish not sufficiently stirred before use.
Remedy:
- Check consistency before making finish solution.
-Stirring well before taking the solution every time.
★ Tackiness of finish surface with tendency to get finger marks.
Causes:
- Too much plasticizer and the finish surface hygroscopic
Remedy:
- Reduce plasticizer, wax emulsion may be used.
★ Poor rub-fastness
Causes:
- Improper fixing.
- Insufficient binder.
- Too much plasticizer.
Remedy:
- Fix with 10% formaldehyde & sprit solution.
- Increase binder.
- Reduce sulphated oil.
Machine processing after finishing
Machine processing after finishing:
★ Plating:
the leather may have been plated after applying the base coat to seal the leather & to reduce absorbency.Plating after completely finishing is to impart the leather a shiny surface. The higher the temperature the lower the pressure required to get average plating or embossing effect.
★ Embossing:
Greater pressure is normally used in embossing than is used for the normal plating process.
★ Glazing machine:
This process is mainly carried out upon leathers which have been protein finished. Its main function is to flatten the grain, makes the leather surface smooth as well as glaze. Full grain aniline leather, calf and kid leathers are usually glazed.
★ Polishing & ironing:
- Polishing machine sets the leather and gives it softness, a uniform gloss & remove any dust.
- Ironing is similar to glazing but heat from the iron draws grease to the surface goving
natural looking gloss to the leather.
★ Sorting or grading:
All the leather pieces in a lot are not alike. Some pieces may have grain defects. Some pieces may have perfect grain but with slight looseness. Few may be of sound and silky without any defect.
Grain defects may be slight or severe type. So on the basis of these, sorting can be done
on following grading:
Grade-A: Tight grained defect less leathers or aniline or pigment finish leather.
Grade-B: Loose grained defect less for pigment finish.
Grade-C: Slightly grain defected leathers for corrected grain finish
Grade-D: Leather with severe grain defects to be split finished.
★ Measuring machines:
A large variety of machines are available for the measurement of area of finished leathers. For instance -
1.The plano-meter:
May be used for manually for finding the surface area of small irregular shaped skin by following the outline of the skin anti-clockwise with a pointer and reading the area on the central dial after completing the outline to starting point
2. The pinwheel or plan wheel type
3. A range of machines:
Which work on the electron beam or magic eye principle i.e there is no physical movement of parts, that do the measuring only a breaking the light-beams. Most accurate and measured in square feet, square metres, etc.
Corrected grain finish
Corrected grain finish:
★ Impregnation:
Impregnating resin - 100 parts
Penetrator - 100 parts
Brilliant dyestuff - 600 parts
Apply pad coat and dry
★ Pigment coat:
Pigment paste - 100-200 parts
15% casein solution - 25-50 parts
Acrylic resin binder(medium soft) - 200-300 parts
Wax emulsion - 15-20 parts
Water to make - 1000cc
★ Top coat:
NC lacquer emulsion - 300 parts
Wax emulsion - 50 parts
water - 150 parts
Apply by spraying
★ Fixing coat:
Formaldehyde - 200cc
Acetic acid - 10cc
Water to make - 1000cc
Apply 1 or 2X and dry over night
Aniline finish
Aniline finish:
★ Clearing coat:
Lactic acid - 30gm
Water to make - 1000 cc
Apply IX coat and dry
★ Bottom coat:
Transparent pigment paste - upto 25 parts
Matching dye - 5-10 parts
15% casein solution - 100 parts
10% linseed mucilage - 50 parts
Wax emulsion - 10 parts
Water to make - 1000cc
★ Top coat:
10% casein solution - 100 parts
Egg albumen - 50 parts
Glazing binder - 50 parts
Formaldehyde - 40 parts
Water to make - 1000cc
★ Fixing:
Formaldehyde - 200cc
Acetic acid - 10cc
Water to make - 1000cc
spray two cross coat & dry.
Classification of finishes
Classification of finishes:
1. Classification according to the finishing effect:
- Fancy finish
- Aniline finish
- Semi-aniline finish
- Two or multi-tone finish
- Antique finish
- Invisible finish
- Opaque finish
(Short key---
according to first alphabet---
"FAST AIO" )
2. Classification according to finishing technique:
- Corrected grain
- Glaze finish
- Plate finish
- Glaze/plate finish
- Roller coating finish
- Embossed finish
- Spray finish
- Pad finish
(Short key---
according to first alphabet---
high "CGP" নিয়ে "GRE" দিয়ে "SP" হয়েছে")
C. Classification according to main finishing material used:
- Casein finiah
- Nitro-cellulose finish
- Polymer on binder finish
- Polyurethane finish
- Patent finish
(Short key---
according to first alphabet---
"CN PPP")
Different layers in finish coat
Different layers in finish coat:
The final film on leather is composed of mainly three layers which can be named as-
1. Ground or bottom layer
2. Intermediate or middle layer
3. Top or final layer
1.Ground layer:
The layer which remains in direct contact with the leather surface. Generally, a solution of a suitable film forming materials which may or may not be colored with acid dye is used for this purpose.
Objectives:
- To prepare a suitable base for the successive intermediate and top layers.
- To make the leather surface impermeable both ways to different materials.
- To provide better adhesion between leather surface and the film layer i.e. intermediate layer.
- To fill up the empty spaces between the corium major and corium minor for better break and tightness of the grain.
- To make, as far as possible, the leather surface smooth and uniform.
- To cover up the defects on leather surface to some extent.
2. Intermediate layer or Pigment layer:
The layer just above the ground coat is the intermediate layer or Middle coat and it
represents the major portion of the film thickness. This layer can also be called "the
pigment coat" as most of the pigment particles remain embedded in the film forming
materials of this layer. Like stone chips embedded in the slab pigment particles give the
body to the finish layer bear the load or stress applied on finish film. The pigment
particles also provide.
3. Top layer:
This is the final top-most layer resting on the intermediate one.
Objectives:
- To give required degree of gloss.
- To protect intermediate layer from scratch, frictional damages and abrasion.
- To make the finish film waterproof or at least water resistance
- To protect the leather from damages due to sweat, acids, alkalies and other injurious materials and vapours with which the leather may come in contact.
- To provide extra resiliency, feel and smoothness to the leather.
- To make the top surface of the leather ideal for embossing.
- To prevent mould or fungal growths.
Application of Finish
Application of Finish:
The final look & qualities of finished leather largely depend upon how the finish was applied on the leather surfaces. Finishes are applied on leather by three methods-
1. Pad coating
2. Roller coating
3. Spray coating
Here,
- Bottom coat is generally applied by "Pad coating".
- Intermediate coat is applied on both "Pad & Spray coating".
- Top coat is always applied by "Spray coating ".
1. Pad coating:
Plush pad, with or without the plastic foam padding underneath the soft velvet like cloth, soft hair brush & sponge are generally used.
Advantages:
- Low labour cost & economical.
- Low or no maintenance cost.
- Uneven substance hide/skins receive even coats.
- Copes well with light-weight skins.
Disadvantages:
- Labour should be skilled.
- Uneven coating & streaking is possible if labour is unskilled.
- flesh side is dirty.
2. Roller coating:
The roller coating machine consists of an application roller which is grooved or has cavities which hold the liquid materials, excess liquid being wiped off by a doctor blade. The leather is introduced to the machine on a rubber conveyor.
Advantages:
- Even coating of higher density material
- Clean flesh
- Continuous production
- Suitable for long runs of single colors
- Production rate high i.e 2000 sq ft of leather coated once take 15-20 mins only
Disadvantages:
- Will only cope with small variation in substance (0.3-0.4mm)
- Good maintenance is necessary for consistent results.
- Adhesion may be poor due to less mechanical action.
3. Spray coating:
It is the most common method and is applied by means of compressed air. It is suitable
for aqueous and solvent containing finishing float. Normally compressed air is forced
through a small nozzle and to this air finish is fed in the form of fine stream. When the
finish, mixed with air, is forced out through the nozzle, atomization of finish takes place
and the mixture moves towards the leather surface.
Advantages:
- Uniformity is easily achieved
- Spray season or coat dries up on leather very quickly
Disadvantages:
- Less adhesion, less rub fastness
- Materials loss is high
★ Different types of spraying:
- Hand spray gun
- Rotary spray gun
- Reciprocating spray gun
- Airless spray
Characteristics of the film/ Ideal film
Characteristics of the film/ Ideal film:
1. Flexibility & stretchiness:
Leather is a flexible material with certain degree of stretchiness. If the film doesn't posses these properties to the same extent as leather, it will make the leather hard & the film will crack in course of time.
2. Adhesion:
The film should be firmly fixed to the leather surfaces so that the former doesn't come out during use. The film should also adhere to the pigment particles & others very firmly.
3. Holding power:
The film should have sufficient capacity to hold in it other substances like pigment, plasticizer etc. During drying or fill formation or ingredient of the finish should precipitate out.
4.Gloss:
The film should glaze by itself or should aquire this quality after glazing under glazing machine or hot plating or brushing.
5. Abrasive resistance & fastness:
The film should have sufficient resistance to abrasion for longer life & it should hold the coloring materials so tightly that it doesn't come out when rubbed with a dry & wet cloth.
6. Water proof-ness & water vapour permeability:
The film should repeal water so that the leather under it doesn't come in contact with water but it should allow water vapour to pass through. This is important for shoe upper leather.
7. Thickness:
The film should be as thin as possible so that it doesn't spoil the leathery appearance but the film should hide all the defects in the leather.
8. Resistance to acid, alkali & chemicals:
During use the leather comes in contact with dirt, mud, acid, alkali, sweat etc. This is specially common for leathers used by air, navi & army personnel in the field. So, the film should have sufficient capacity to protect the leather from these.
Finishing materials
Finishing materials:
In leather finishes the finishing materials are suspended, dispersed, emulsified or dissolved in water or solvents. The important finishing materials are generally the followings:
1. Finely ground pigments with or without dyes of same shade to intensify, brighten & level up the shade.
2. Film forming materials having binding power.
3. Preservatives to prevent putrefaction.
4. Plasticizers to make the film soft, flexible & stretchy.
5. Gloss giving materials.
★Pigments:
A pigment is any particulate matter that is insoluble in & essentially physically & chemically unaffected by the media which it is dispersed. The particles of pigment are too large to penetrate into the substrate. They are usually present on the substrate surface. So the pigment is easily removed unless fixed with an adhesive.
Source of pigment powders:
1. White pigment:
Titanium di oxide, Zinc Oxide, Zinc sulphide, Lithopone, Alumina, China clay.etc.
2. Black pigments:
Carbon black or gas black, Lamp black or vegetable black, Bone black, Graphite, Black iron oxide, Manganese di-oxide, Aniline black, Black lakes, Cobalt oxide etc.
3. Yellow pigments:
Lead chromes, Barium chromate, Zinc chrome, Molybdenum chrome, Cadmium yellow, Cadmium lithopone, Hansa yellow, Napthol yellow, Chrome orange. etc.
4. Red pigments:
Iron oxide, Cadmium reds, Para reds, Toluidine reds, Lithol reds, Permanent reds, Lakes of phosphotungstic acid. etc.
5. Blue pigments:
Prussian blue, Cobalt blue, Ultramarine, Phthalocyanine blues. etc.
6. Green pigments:
Chromic oxide, Pigment green-B, Phthalocyanin green. etc.
★ Binder:
Pigment are insoluble materials & having no affinity for leather must be bonded onto leather surfaces. For this a wide range of binder is used in finishing.
1. Casein
2. Shellac
3. Egg albumin
4. Blood albumin
5. Glues
6. Isinglass
7. Methyl-cellulose
8. Nitro-cellulose emulsions
9. Nitro-cellulose lacquer
10. Linseed
11. Locust bean
12. Resin emulsion
★ Diluents:
For better application on leather the viscosity of lacquer should be reduced considerably & that is done by dilution. Diluents are liquid which don't themselves dissolve cellulose esters but can be blended with out interfering with film forminf properties of the lacquer, eg: alcohol - methyl, ethyl, propyl, butyl alcohol. etc.
★ Plasticizers:
Plasticizers are usually heavy, non-volatile liquids with high boiling point compound (ester). Primary function is softening the film. Other functions are ease of glazing or provide the film additional water resistance. For instance- Castor oil, Dibutyl-phthalate, butyl stearate, olive oil. etc.
★ Waxes:
Waxes are generally used to provide the finish film polishing, which gibe very good shine, also provide very good feel, water proof-ness & good-gloss on brushing to the finished leather surfaces. Commonly used waxes are- carnuba wax, bees wax, paraffin wax, montan wax, synthetic wax, etc.
★ Antibacteriacides:
Mercuric chloride, Sodium fluoride, thymol, phenol or carbolic acid or beta napthol, tri-chloro-phenol, etc are used as antibacteriacides in finishing.
★ Wetting agents:
Soap, Sulphated oil etc are used.
★ Anti-foams:
Wax emulsion is used.
★ Leveling agents:
These are largely surfactants which enable the finish to have good flow out over the tight finishes.
★ Fillers:
They are used to fill any open or loose grain & prevent the finish sinking into the grain of the leather.
★ Thickener:
Used on heavily buffed or split leathers in order to get a good thick film or to achieve a reasonably good film very quickly in one coat. Sodium or ammonium poly-acrylates are chiefly used.
★ Matting agents or dullers:
silica.
★ Fluorescent materials:
Zinc sulphide.
★ Fixing agents:
Formaldehyde, acetic acid, Chromium salts are used as fixing agent in finishing.
Specific need for an ideal lining material
Specific need for an ideal lining material:
1. It must be easily adaptable to all types of footwear.
2. It must be thin enough to prevent bulkiness.
3. It must remain smooth, mellow & soft with wear.
4. It must be light weight & flexible.
5. It must be non-cracky & non-shrinking.
6. It must absorb & transmit foot moisture easily & quickly.
7. It must resist foot & shoe odours caused by perspiration.
8. It must be resistant to mould or fungus growth the shoe.
9. It must have aesthetic appeal - color fastness, a look of quality, fine surface texture. etc.
10. It must be economical for use by the shoe manufacturer.
(Short key---
মনে করো, তুমি প্রথমে এমন একটি লাইনিং কিনতে গেলে যা ব্যবহার করা যায় - (1)"all types of footwear" -এ। তারপর তুমি এমন একটি লাইনিং নির্বাচন করলে যা (2)"thin" এবং এর ভালোই (3)"soft and smooth" প্রোপার্টি /বৈশিষ্ট্য আছে . যেহেতু soft তাই এটি (4)"light weight and flexible"। যেহেতু, flexible তাই অবশ্য (5)"non-cracky".
বাইরের বৈশিষ্ট্যগুলো দেখার পর, তুমি এর ভিতরের প্রোপার্টি / বৈশিষ্ট্য দেখলে - (6)"absorbs and transmits foot moisture "। তাই, অবশ্যই এটি (7) "resistance to foot & shoe bad odour" এবং "resistance to growth of mold or fungus". শেষবার এটিকে ক্রয় করার পূর্বে তুমি এর (9) "aesthetic appeal" দেখলে এবং তুমি (10)"economical price" এর মধ্যে এটিকে ক্রয় করলে।
Finishing properties of shoe upper leather, sole leather, lining leather
Finishing properties of shoe upper leather:
1. Attractive appearance.
2. Easily cleaned.
3. Flexible.
4. Good adhesion.
5. Good break.
6. Scuff resistance.
7. Water resistance.
8. Resistance to solvent used in shoe making.
(Short key----
মনে করো তুমি প্রথমে নিজের জন্য জুতো কিনতে গেলে। জুতো কিনার পূর্বে তুমি প্রথমে অবশ্যই দেখবে সেই জুতোর "attractive appearance"। এটি দেখার পর সেই জুতোটি যখন তুমি হাতে নিলে তখন এর উপর তুমি কিছু ধূলে পড়ে থাকতে দেখলে। তাই তুমি নিজ হাত দিয়েই জুতোর উপর লেগে থাকা ধূলোবালি "easily clean" করলে। এরপর তুমি এর "flexibility" টেস্ট করলে। তারপর টেস্ট করলে দুটি good-টেস্ট - "good adhesion" & "good break"। তারপর তুমি তিনরকমের রিসেস্টেন্ট টেস্ট করলে-
" scuff resistance", "water resistance" এবং "solvent resistance")
Finishing properties of sole leather:
1. Give an attractive pale look.
2. Impart gloss or shine.
3. Not crack on flexing.
Finishing properties of shoe lining leather:
1. Should totally harmless to the human skin.
2. Should give maximum comfort to the user.
3. Should durable & cheap.
Finishing & Importance of leather finishing
Finished/Finishing:
The treatment of the surface or top section of the leather with material which will enhance the appearance of the leather & improve its properties for the purpose to which it is to be used.
Importance of leather finishing/ why leathers are finished:
1. Imparting color to the undyed leather.
2. Changing the color to that which is required.
3. Uniformity of shade from skin to skin & pack to pack.
4. Levelness of color on the skin.
5. Giving a surface to the leather varying from matt to gloss.
6. Adding a transparent film through which the natural appearance of the leather may be viewed.
7. Adding a transparent color film to the leather.
8. Covering the leather with an opaque film.
9. Alter the surface of the material i.e. splits.
10. Improve the scuff resistance of leather.
11. Improve the water resistance of the leather.
12. By filling the surface of loose leather improve the break of the grain.
13. Seal the leather surface so that it remains clear.
14. Render the leather light-fast, heat-fast & fast to acids & alkalis.
15. Render the leather resistant to a light pull-up on lasting.
Pigment
Pigment:
Higher molecular substance which is not soluble in water, has good hiding power opacity & adhering characteristics are known as pigment.
Properties of Pigment:
- Good light fastness
- Good hiding/covering power.
- Resistance to acid, alkali & other chemicals.
- Resistance to heat & cold.
- Soluble in water.
- Interfere with other pigments.
- Can change with atmospheric condition.
Classification of Pigment:
1. Inorganic pigment:
(a) Earth color pigment
(b) Prepared pigment
2. Organic pigment:
(a) Lakes pigment.
(b) Toner pigment.
Dye
Dye:
Chemical substance/colorants that can be fixed with material, when fixed isn't fugitive.
Properties of Dye:
- Fast to light.
- Intense in color
- Solubility in water.
- Not washable with water, dil acid and alkali.
Classification of Dye:
1. Based on Chemical constitution.
2. Based on application.
1. Based on chemical constitution:
- Azo dye.
- Anthraqunoid dye.
- Di-phenyl-amine dye.
- Di-phenyl-methane dye.
- Tri-phenyl-methane dye.
- Nitro dye.
- Nitroso dye.
- Heterocyclic dye.
- Xanthen dye.
- Phthalocyanin dye.
2. Based on application:
- Acid dye
- Basic dye
- Direct dye
- Vat dye
- Sulphur dye.
- Natural dye.
★ Acid-dye:
These dyes are generally sodium or potassium salts of organic coloured acid & ionise as follows:
NaA (dye) —> Na+ + A- (coloured acid)
It is negatively charged acid protein A- which imparts colour to the fabric. For full & uniform shades the dyes are completely ionised in the solution by addition of acids.
★ Basic dye:
These dyes are generally chlorides or hydro-chlorides of coloured bases & ionise in the solution as follows:
BCl (dye) —> B+ (coloured base) + Cl-
The positively charged coloured bases actually react & imparts colour to the substances.
★ Direct dye:
Like acid dye this dye are also Na/K salts of coloured acid & behave in the same way as acid dye. This dyes can be directly used (without any pretreatment) to impart color to the fabric.
★ Vat-dye:
This group of dyes are suitable for cotton dyeing. They are insoluble in water but when reduced in an alkaline medium become soluble but practically colourless.
★ Sulphur dye:
The dye-stuff of this group contain sulphur & their chemical nature is more or less undetermined.
★ Natural dye:
Before the discovery of coal-tar dye the leather colourist had to rely upon the coloring materials derived from the vegetable substance & some of them are used even today by some tanners.
Fat-liquoring
Fat-liquoring:
It is an important operation in leather manufacture in that the application of fatty emulsion enable it to surround the individual leather fibre with fatty substances or softening compounds which are firmly fixed to prevent their migration.
Objectives of Fat-liquoring:
1. Deposit of fatty substances in the Interfibrillar spaces in order to give the leather desire softness & handle properties.
2. Primary function is to prevent the fibre structure resticking during drying.
3. Correction or control of physical properties like tensile strength, split tear strength, wetting properties or water repelling properties, waterproofness, permeability to air & water vapour, water absorption & moisture storage-capacity, thermal & electrical conductivity.
Properties of Fat-liquor:
1. Raw oil:
Which not get fixed to the leather fibre but lubricate the latter.
2. Emulsifier:
Which emulsify the raw oil & carry them inside of leather & at the same time will get fixed to the leather fibres & slightly prevent the free movements of raw oil by their power of adhesion.
3. Continuous phase:
Acts as a vehicle for the oil-droplets surrounded with emulsifier.
Principle of Fat-liquoring:
1. Fats & oils of highest lubricating power should only be selected for the preparation of fat-liquors, so that small amount can produce sufficient softness to leather.
2. The emulsifier may or may not process any lubricating power but carry the lubricating oil deep into inside of the leather without facing any resistance during penetration into the leather.
3. The strength of both hydrophobic & hydrophilic ends of emulsifier should be as high as possible but between them hydro-phobic side should be slightly stronger.
4. Develop the opposite charge into the neutralized leather.
Mechanism of Fat-liquoring:
1. The neutral oil is transported into the pelts as an oil in water emulsion.
2. The emulsifying agent interact with the leather, reducing or eliminating its emulsifying power.
3. The neutral oil is deposited over the fibre structure - the level of the hierarchy of structure depends on the degree of penetration.
4. The water removed by drying, allowing the neutral oil to flow over the fibre structure.
Neutralization
Neutralization:
Neutralization is the removal of protein bound acid from the surface as well as interior of leather up to certain depth depending on penetration of re-tanning material used in re-tanning system.
pH range 4.3 to 5.5.
During neutralization the free & protein bound acids are efficiently removed either by repeated washing with water or by chemical treatment.
Neutralization is necessary to stop acid hydrolysis
& to remove salt spue.
Objectives of neutralization:
The objective of neutralization is not to remove totally all acidity present in the hide or skin but only to remove harmful acidity. By harmful acidity means the pH of the leather is below 3.3. So, it is necessary to take the pH of the leather a little over this value so that even if a little or more of acid if produced after some time no harmful effect will be observed.
Parameters of Neutralization:
1. Temperature:
Carried out in a range of 30-40°c. If use sodium-bi-carbonate it will be <38°c.
2. Duration:
About 30 minutes to 2 hours depending on the thickness of leather & texture, type of neutralizing agent & type of leather to be produced.
3. Intensity:
Depends on the type of leather to be produced. For soft leathers the pH range is 5.0-6.0, for firm upper leathers the pH range is 4.2-5.0. The outer zones are neutralised in the higher pH range, the inner zones are slightly neutralised.
Controls of Neutralization:
Cut off a small piece of leather & then let an 0.1% indicator solution of bromocresol green (BCG) onto cross section. The change of color indicates the progressive depth of penetration of neutralizing agents & the pH value on the leather. Transition interval of BCG:
Yellow = pH 3.4 & below.
Yellow green = pH 4.0
Green = pH 4.5
Blue green = pH 5.0
Effects of Neutralization:
Acid or direct dye, both these dye are sodium or potassium salts of certain colored acid when dissolved in water.
(-) charged acid fraction x- react with the (+) charged group of protein & thus color is imparted.
* Result of no neutralization is uneven dyeing.
* Result of over neutralization is impossible dyeing.
Neutralizing chemicals/agents:
1. Strong products:
Products of this category are seldom used, in combination of mild products. Used in small quantities neutralizing action is superficial. Larger amounts give good penetration, however over neutralization of the grain & simultaneous de-tanning effect. Example: Sodium carbonate (Soda).
2. Mild products:
- Sodium-bi-carbonate:
Frequently used product. Over-neutralization is possible if large amounts are used.
- Ammonium-bi-carbonate:
Rapidly penetrating product. Very suitable for leathers of dense texture (pig leathers, reptiles)
- Sodium formate:
Over-neutralization is not possible. Mostly used in conjunction with sodium-bi-carbonate.
- Calcium formate:
No risk of over-neutralization but formation of calcium sulphate may produce stain.
Beside sodium acetate, sodium sulphite, sodium thiosulphate, sodium aluminium silicate, polyphosphate etc are used.
Ideal condition of Neutralization/'N':
Is to make the leather surfaces electrically neutral & the acidity should increase as the middle layer is approached.
*****figure*****
The leather surface will be electrically neutral if the pH of the leather surfaces are kept in range i.e pH=5-6 & at middle layer pH=4.1 to 5.0. The leather will be properly neutralised if the pH value reaches its ISO-electric point.
Acid hydrolysis, Case hardening, Salt spue
The acid present in the chrome tanned leather can broadly be classified under the following heads:
- Free acid
- Protein bound acid.
- Acid present inside the co-ordination sphere of Cr-complex.
- Acid fixed with Cr-complex.
Causes:
- Acid hydrolysis of the skin protein.
- Leading ultimately to grain crackiness.
- Tearing up the leather.
Acid hydrolysis:
Generally acid hydrolysis provide some poor properties to leather. As they are-
- Poor tear resistance
- Poor overall strength.
- Grain crackiness.
Case hardening:
When limited pelt is added this solution the alkali reacts with the pelt & fixed both sides of the leather. Then the other tanning can't penetrate through the leather. This is known as case hardening.
Salt spue:
Salt spue are the salts in the leather becoming de-stabilised, when liquid is applied to the leather it causes an osmosis effect drawing the salt up to the surface.
Place a gentle heat over the white patch using a hairdryer if it doesn't disappear under heat it determines it may be salt spue (not Fat-spue).
Applying the more liquid to areas of salt spue will make the problem worse - as a test a apply wet/damp cloth to a small area around the white patch, if it more white spue appears it is a salt spue problem.
Salt spue can be difficult to stabilize, Someone can attempt to re-colour the area & hope it doesn't rear its ugly head again.
- Clean the area (don't soak the leather) with using the "Buffalo Mould Cleaner".
- Seal the are using " Gloss solvent sealant"
- Re-color & seal again.
Ageing & Conditioning
Ageing:
After chrome tanning tanned leather are taken out from tanning bath & piled-up on a wooden horse & kept for 2-3 days.
It allows greater fixation of chrome complex & uniform distribution of Cr throughout the leather.
The chrome complex goes on a changing by liberating more & more acid. Some of the acid fixed to basic side of the protein. Actually the acid is increased due to hydrolysis of the chrome-complex.
The basicities of Cr-complex/compound present inside the leather are increased. So, Cr-fixation & tendency to form new cross linkage are increased.
Conditioning:
After Cr-tanning & ageing the leather are conditioned to the required moisture content & to make them suitable for certain mechanical operations to achieve the desired thickness. The conditioning of the leather is known by the term "sammying" - which aims at re-tanning certain moisture content about 40-45% of its weight. This is done by partial drying to reduce moisture content either by open drying or by mechanical means using a machine called sammying machine. It should be noted that uniform moisture content is just enough to make them suitable for further mechanical operations to obtain the required thickness.
The sammying machine are used in tannery to squeeze the hides/skins. The combine action of cylinders covered by absorbent material & pressure the liquid of the bath etrappend into the fibres are removed. The sammying machine model are alternative type (two action) or continuous (single action).
After sammying, the leather are further processed in machines to obtain the correct thickness uniformity throughout the leather. This achieved by using mechanical means-using splitting & shaving machines.
Splitting:
Usually splitting is done in the splitting machine, keeping a slightly higher thickness uniformity through out the hide/skin.
Shaving:
Shaving is done later to obtain the uniform desired thickness in the shaving machine & checked with the thickness gauge for uniformity.
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