Leather Processing

Trimming, gathering

Soaking

Wash away and get rid of unwanted dirts, blood, fat on raw hide, returned to normal gelatin stage, create good condition for following work. Soaking results in clear and obvious grain, spready area, less wrinkle, vein, average area, full touch.

Material used:
* 20C to 25C water
* Soda
* Detergent
* Antibiotic agent
* Enzymes
* Uric auxiliary
* water softener

Liming

Releasing fibre structure, destroy some unwished protein, get rid of hair and corium, swelling

Material used:

Lime, Sulfide, Detergent & Vric axuiliary

In between mechanical action as fleshing and splitting

Operation time: soaking and liming 48 hours, drum speed 4RPM

Control of soaking and liming through:

* Mechanical action
* Temperature
* pH
* Float proportion
* Time

Deliming

Neutralized alkaline, clean away magnesium and calcium salt, desmelling and give way for further process.

Material used:
*Water 36° to 38°c
* Ammonium salt
* Organic acids
* Detergent

Operation time: 2-3 hours with drum speed 8-10RPM

Bating

Clean grain, get rid of remaining soluble protein, soften fibre and improve surface of leather, developing leather suppleness and stretchiness.

Material used: Enzymes

Condition:
* Time
* Amount of enzymes
* Temperature
* Mechanical action

Degreasing

Control the excessed amount of natural fat, prevent rugged touch and unpleasant smell of leather

Material used: Detergent or organic solvents

Pickling

Make pelt acid in order to create good condition for tanning. Certain degree open fibre

Material used:
* Formic acid
* Sulfuric acid
* Salt
* Electrolyte stable fat-liquors

Tanning

Transformation of protein to leather fibre. This is an irreversible chemical reaction.

Material used:
* Chrome tanning agent
* Vegetable tanning agent
* Aldehyde tanning agent
* Alum tanning agent
* Oil tanning agent

Chrome tanning

Chemical reaction between trivalent chromium and oxyl bond of protein. Strong combination, rot-proof, good suppleness, high shrinkage, temperature( up to 100°c) provide cationic bond to allow anionic chemicals to combine to leather fibre later such as dyes, fat-liquors and retanning agent.

Material used:
* Chromium sulfate
* Chromium chloride
* Masking agent
* Sodium bicarbonate
* Anti-fungi

Advantage of chrome tanned leather:
* Easily operated
* Cheaper cost
* Higher tensile
* Higher grinding resistance
* Higher shrinkage temperature
* Not easy putrefaction
* Not hardening when wetted
* Lighter
* More flexibility on further manufacturing
* Easily transported and stored

Disadvantage of chrome tanned leather:
* Stronger water absorption
* Slippery after wetted
* Looser fibre
* Less yield in area
* Rough edge
* Chromium is a polluting heavy metal.

Vegetable tanning

Traditional way of tanning using extract of different parts of plants like roots, bark, fruit, leaves. The tannin acid from the plant can react with protein and fill in between fibres forming stable leather

Commonly used vegetable tanning extracts:
* Mimosa
* Quebracho
* Chestnut
* Gelon
* Tara
* Oak
* GambierThe natural extracts have different colours, densities, astringencies allowing production of various appearance and quality vegetable tanned leather.Operation: Use pits or drums, time can be from three / four days to half a month.

Advantage of vegetable leather:
* Strong water resistance
* Not so slippery after wetting
* Tight fibre construction
* Minor tensile, not easily transformed
* Better yield
* Smooth edge
* Better embraced property
* Area changed less after wetting
* Sweat resistance

Disadvantage of vegetable leather:
* Production cost higher
* Operation more complicated
* Less tensile
* Less grinding resistance
* Lower shrinkage temperature

In general practice, there are many combinations to use chrome, vegetable and other tanning agents together to achieve the optimum result and ideal leather.

Chrome free tanning

Pretanned the pelt with aldehyde and synthetic tanning agent to wet white. Wet white can be stored, transported and mechanical operated.

Then can proceed further to various tanning and retanning. This process created much less polluted elements and can be named as clean production.

Wet end

Neutralization, retanning, dyeing, fat – liquoring, fixing

Neutralization

Enrich anionic charge in fibre to enable dye, retanning agent, fat- liquor better penetration, elimination free chrome salts and chrome six, adjusting degree of opening in fibre.

Material used:
* Mild acidic
* Salts like sodium acetate sodium formate
* Sodium bicarbonate
* Some syntans

Operation:

* 38°C-40°C water
* 60 – 90 min
* pH 4.8 – 6.5
* Cross section through
* Rinse well

Retanning

* To improve better tanning effect.
* Give leather right properties
* Repair defect from past process
* Achieve better yield, thickness and physical resistance

Material used:

* Mineral tanning agents: Chrome, Alum, Zirconium
* Vegetable tanning agents
* Aldehyde tanning agents
* Aromic synthetic tanning agents
* Acrylic resins
* Meloaic resins
* Amino resins
* Protein fillers
* Super molecular resins

B2h and Neutralization & B2i cover tanning Retanning_1

Operation:

Combination of retanning agent and working procedure designed according to the request of end leather crust. Operating simultaneously with dyeing and fat-liquoring with 8-12 hours

Consideration:

* Wet blue condition
* Crust requested
* Ingredient characteristics

Dyeing

Gives leather wished shade, correcting defects, even cut look, ready for followed finishing and creates all kind of special effects.

Dyestuffs: to give dyed object (leather) wished colour
* Natural dyes
* Basic dyes
* Acid dyes
* Direct dyes
* Metal complex dyes
* Sulphur dyes
* Reduction dyes
* Reactive dyes

Leather dyeing commonly used acid dyes and metal complex dyes. Colour matching: possible least dyestuffs mixed, dyestuff dyeing properties possibly closed, dyestuff with same tone

Operation:
* 12 RPPM drum
Temperature:
* Higher fixation
* Lower penetration
Water proportion:
* Higher fixation
* Lower penetration
pH:
* Lower penetration
* Higher fixation
Mechanical action:
The higher the better penetration

Fat liquoring

Soften leather
Lubricating fibres to achieve better tensile strength, stretch, tearing and flexing properties
Adjust touching
Introduce special effects such as washable, water resistance, heat retarding, pull-up

Fat liquors:

* Animals fats: Sperm oil, Neat foot oil, Lanolin, Phospholipids
* Vegetable oils: Soya, Colza, Palm
* Mineral oil
* Synthetic oil
* Resin softeners
Operation:
* Water temperature 45°C – 55°C
* Time 60 – 120 min

Drying

To control moisture content in leather stretch leather fibre, improve fixation of retanning, dyeing and fat- liquoring. Unifying construction of fibre mess resulting at spreading and good touch leather. Effecting shrunken or mill grain. Keep area yield.

Operation:

* Vacuum dry
* Hang dry
* Hot plate dry
* Toggle dry

Finishing

To cosmetic leather surface, correct some of the defects. Imply shade, gloss and special effects and pattern as required. To level out-look of leather increasing cutting value. Finishing combination.

Binder: to cover the grain fix the colour protecting the surface, improve physical properties.

* Casein
* Butadiene
* Acrylic resin
* Polyurethane
* Nitro-cellulose
* Complex resin

Colourants: Pigment and dyes to give wished colours

Solvents: Water and organic solvent to dilute the mixture

Filler: Waxes, oils for filling and mixing binders and pigments, better floating and avoid sticking

Other ingredients: Impregnation fillers, cross-linkers, metallic or pearl powers

Operation:

* Hand brushing
* Gum spraying
* Roller coating
* Curtain coating
* Printing
* Transfer lamination

Effects example:

* Anilin
* Glazing
* Polishing
* Stone washing
* Cracking
* Oil pull-up
* Wax pull-up
* Antique
* Brushing
* Metallic
* Embossing