FIELD OF INVENTION
This invention relates to a process for dyeing and printing of leather and leather articles including made-ups with indigo.
BACKGROUND
When an animal is alive, its skin is soft, flexible and tough. It has ability to allow water vapor to pass out but it will not allow water in. when skin diejs, it loses these characteristics. If it is kept wet, it rots and if it is dried, it goes hard and brittle.
The skin / hide becomes leather after the removal of subcutaneous tissue (fleshing) as well as epidermis and hair (unhairing). The subsequent process of tanning leather aims at retaining the skin's natural properties, stabilizing its structure and chemically processing it to prevent putrefaction/rotting. Thus, leather is animal skin that is treated such that it stabilizes and resists putrefaction while retaining its natural properties. Most leathers are then subjected to steps of dyeing, fat-liquoring and finishing.
The main constituent of leather is collagen fiber network. Collagen is a protein with a characteristic structure. The fiber network is composed of collagen fibrils (100 nm thick), each containing about 7000 collagen molecules. About 50 fibrils are grouped together and slightly twisted to form elementary fibers. The elementary fibers form bundles which are visible under the microscope as a three dimensional network. The
irregular interwoven structure is the basis for the mechanical properties of animal skin viz., strength, elasticity, flexibility, porosity and resistance to piercing with sharp tools and edges.
Color and coloration are complicated phenomena and coloring leather is particularly so because leather is not a uniform substrate.
The color of leather may be derived from processes involving natural and synthetic dyes, pigments and combinations of pigments and dyes. Dyes suitable for leather coloring may be grouped into anionic and cationic types. Anionic dyes make up the greater portion of the dyes used for leather and include acid dyes, direct dyes, mordant dyes and metal complex dyes. Anionic dyes combine with chrome leather and become bound through primary and secondary valence forces. Cationic dyes, on the other hand, have little affinity for the chrome leather and require a mordant.
Leather may be colored by methods such as drum dyeing, paddle dyeing, tray dyeing, spray dyeing and brush dyeing.
Drum dyeing method is both efficient and economical. The liquor ratio could be up to 10, as leather should be covered with liquor when the drum is at rest and the drum rotates at 12-18 rpm. The pegs, shelves or combinations thereof inside the face of the drum agitate the leather during dyeing. The drums vary in size from 3 to 5 feet in width and from 8 to 12 feet in diameter. The solutions of dyes, chemicals and auxiliaries are fed through hollow axle during the dyeing process. The temperature necessary for the dyeing is used.
Paddle dyeing embodies the same principle as drum dyeing and is used for fragile leathers that would be damaged by the vigorous mechanical action of the drum. Considerably higher liquor ratios are used in this process than are used in drum dyeing resulting in less efficient utilization of the dyes. The advantage over drum dyeing is that the leather is visible during dyeing.
Spray dyeing can be used when mechanical action of drum dyeing is undesirable. The spray method is cheap and efficient. However, fastness may not be as good.
Brush and tray dyeing are used for firm leather, sensitive to mechanical action, and also when the dye application is to be done to only one surface.
OBJECTS OF THE INVENTION
An object of this invention is to propose a process for dyeing of leather with indigo.
Still another object of this invention is to propose a process for the dyeing of leather with indigo which provides a dark, fast and level indigo shade.
Yet another object of this invention is to propose a process for printing of leather with indigo.
A further object is to propose a leather dyed with indigo and thereby possessing added value.
DETAIL DESCRIPTION OF THE INVENTION
According to this invention there is provided a process for dyeing of leather with indigo dye comprising the steps of subjecting leather to the steps of scouring, cationization and subjecting the cationized substrate to the step of dyeing with indigo dye.
The present invention teaches cationization and indigo dyeing of leather by continuous as well as discontinuous processes.
The invention further teaches indigo printing of leather by direct, discharge and resist styles.
Leather is cationic in nature in acidic media and therefore all anionic dyes can be applied in acidic media. However, indigo dyeing is conventionally carried out under highly alkaline conditions. Under highly alkaline conditions, the leather would cease to be cationic (or may remain feebly cationic in nature), losing electrostatic affinity for anionic (leuco) indigo. The result would be lighter dyeing and poor crocking fastness. The cationization of leather to generate dye-combining sites and to develop affinity for the anionic indigo dye helps overcome the
shortcoming and facilitates indigo dyeing of leather under highly alkaline conditions. Thus the steps involved are as follows
(Table Removed)
In accordance with this invention, the step of cationization comprises in treating leather with a cationization recipe. The step of cationization may be continuous with the step of indigo dyeing, or discontinuous such as exhaust or spray, brush, or pad followed by batching.
If the process is continuous, the leather is passed through a bath containing the cationizing compound and caustic soda wherein cationizing compound is 3-80 gms per litre and caustic soda is 3-25 gms per litre. Alternatively, caustic soda may be provided in a separate bath, and in which instance the leather is first passed through the bath containing the cationizing compound and then through the bath containing caustic soda.
The step of cationization is carried out at room temperature or even at temperature as low as 15°c. If the process is continuous, it may be on line with the main indigo dyeing apparatus. The cationized leather is passed through a plurality of dye boxes followed each time by squeezing and aeration. However, between the cationization bath and dye baths, a brief residence time of 10-180 sees is allowed for chemical reaction of the cationization of the leather substrate.
As described hereinabove, the process can also be a batch process. The leather is first impregnated with a cationizing chemical and caustic soda for 15-30 sec and then squeezed. The treated leather is then covered with a plastic sheet so as to prevent evaporation of water. The treated leather, upon cationization, is washed for removal of caustic soda.
The degree of cationization in a batch process is higher than in a continuous process. However, the cationization effected by a continuous process is still sufficient for enhanced dye uptake.
Cationization can also be done by an exhaust process. In such a process, leather is introduced in a cationization bath containing the cationization compound and caustic soda. The bath is agitated to allow a transfer of
the cationizing chemical onto the leather substrate. Such a step of cationization is carried out at room or at elevated temperature. At the end, leather is washed to neutrality.
The cationization recipe containing the cationizing chemical and caustic soda can also be applied by spraying or brushing followed by batching after covering the treated leather with plastic sheet. The cationized leather is then washed to neutrality.
Besides the step of cationization, the step of dyeing may be carried out by discontinuous methods such as impregnation-squeeze-air, exhaust, brush, spray or continuous method. In a discontinuous method, the cationized leather is saturated with indigo dyeing recipe, squeezed and oxidized singularly or repetitively. In a continuous step, the cationized leather, several of them stitched on to fabric, are passed continuously through a plurality of dye baths with aeration between each bath.
The step of exhaustion comprises in introducing the cationized leather into the dye bath under inert blanket and then subjecting the bath to agitation. This is followed by squeezing, oxidation and rinses to neutrality.
Still further, the (thickened) dye recipe can be applied by brush, if the dye is required only on one side. Further, some leathers are delicate and cannot take mechanical action. In such an instance, the cationized leather can be subjected to spray dyeing using inert gas as pressurizing agency, followed by oxidation and rinses to neutrality.

The present invention also envisages the step of printing leather with indigo. Indigo being a vat dye, the printing recipe and fixation conditions typical of vat dye can be employed.
Reference is now made to the examples herein below, but which are not intended to imply any limitation to the scope of the invention.
EXAMPLE 1 SCOURING
A typical treating recipe consists of
Any commercial nonionic
Wetting/ scouring/dispersing/emulsifying product
With good detergency 2% (on weight of material)
Di-sodium hydrogen phosphate 1% (based on liquor
volume)
Liquor ratio 10
Temperature 50°C
Time 20 minutes.
EXAMPLE 2
CATIONIZATION
A pad liquor consisting of, 60 gram per liter of cationizing chemical such as 3-chloro-2-hydroxy propyl trimethyl ammonium chloride and 20 gram per liter of sodium hydroxide is prepared, sodium hydroxide is dissolved first and after the solution is cooled down to ambient or even sub ambient say 15°C, the cationizing chemical is added just prior to the application to the leather/leather materials.
The application is by pad-squeeze-batch method or by spray, brush or exhaust method in suitable equipment. In the pad- squeeze method, the leather material is saturated with the aforesaid recipe, squeezed to remove excess recipe and batched for 6 to 24 hours at ambient or sub ambient temperature, well covered with plastic sheet to prevent any drying.
In spray or brush method, the aforesaid recipe is sprayed or brushed onto leather and the leather so treated is batched for 6-24 hours at ambient or sub ambient temperature, well covered with plastic sheet to prevent any drying.
The exhaust method uses aforesaid recipe, at ambient or elevated temperature, the liquor ratio being 10 or higher and exhaustion time upto 60 minutes.
The leather after the cationization treatment is thoroughly washed free of alkali and in that neutralization uses organic acid such as acetic acid or formic acid. The leather is then squeezed and dried suitably in air or hot air oven or vacuum oven or in some such equipment.
Cationization is also done by continuous method on-line, intendem with indigo dyeing in indigo dyeing machine itself. The steps involved are stitching hundreds of leather pieces onto polyester fabric and treating the battery of leather with cationizing chemical by saturate-squeeze method followed by caustic soda treatment again by saturate-squeeze method. Allowing a brief online dwell time, the treated (cationized) leather can straightaway go for intendem online indigo dyeing by plurality of dip-squeeze-air steps.
EXAMPLE 3 INDIGO DYEING
The indigo stock vat preparation uses softened water, 4 gram per liter dispersing agent, 100 gram per liter indigo, 60 gram per liter sodium hydroxide and 80 gram per liter sodium hydro sulphite in that order. The recipe is then heated to 50°C for 30 minutes under stirring. It is then cooled to ambient temperature without stirring.
The stock recipe is diluted to prepare 1.5 gram per liter indigo dye bath using softened water. 15 gram per liter sodium carbonate, 2 gram per liter sodium hydro sulphite and calculated quantity of stock recipe are added to softened water in that order to prepare indigo dye-bath.
Composition of Indigo Stock Recipe
Softened water
2 to 4 gms per litre dispersing agent
80 to 100 gms per litre Indigo

60 to 80 gms per litre caustic soda
80 to 120 gms per litre sodium hydrosulphite
Indigo dyeing of leather follows several dip-squeeze-air steps in continuous or in a discontinuous manner. The continuous method involves stitching hundreds of leather pieces onto polyester fabric and running such battery of leather pieces through a conventional indigo dyeing machine.
The exhaust dyeing becomes possible by virtue of cationization and is desirably performed under inert blanket/nitrogen blanket followed by squeezing and aeration.
Indigo dyeing by spray method is possible using inert gas as pressurizing device for the spray of indigo recipe onto the leather. Brushing involves slight thickening of indigo recipe with inert viscosity builder.
Composition of Indigo dyeing recipe
Softened water
15 to 30 gms per litre sodium carbonate 1 to 4 gms per litre sodium hydrosulphite Required quantity of Indigo Stock recipe.
The aforesaid steps of dyeing are followed by water rinses including neutralization with organic acid such as acetic acid or formic acid. Squeezing and drying in air, hot air oven, vacuum oven or in such suitable equipment complete the indigo dyeing of leather.
EXAMPLE4 INDIGO PRINTING
Indigo printing uses typical vat printing recipes, methods, machines and post-printing treatments. The only difference being that indigo dye replaces the vat dye and the leather replaces the textile substrate.
For Direct printing, the printing recipe could be
2.0-5.0 Gm indigo powder in the form of 10.0-20.0 Gm paste*
5.0-7.5 Gm glycerine
3.0 Gm sodium benzyl sulphanilate
50.0 Gm british gum paste
8.0-12.0 Gm Potassium carbonate
6.0-10.0 Gm Sodium sulphoxylate formaldehyde
100 Gm
The printing recipe can also be
2 to 8 gms indigo powder in the form of 10 to 20 gms paste
4 to 8 gms glycerine
2 to 4 gms sodium benzyl sulphanilate
50 gm or quantity sufficient British Gum
to achieve printable viscosity Paste
8 to 14 gms potassium carbonate
6 to 12 gms sodium sulphoxylate formaldehyde
100 gms
*indigo pasted with TR oil/wetting agent or indigo vatted and precipitated to give fine particle size and fortified with dispersing agent.
Process Sequence:
Printing—controlled drying steaming (100-102°C/ 3-5 minutes)
oxidation (2-4 ml/liter hydrogen peroxide-50%) rinsing
neutralization rinsing squeezing drying at 60- 120°C.
DISCHARGE PRINTING:
The printing recipe, to be printed onto indigo ground, can be
16.0 Gm zinc oxide (50% aqueous dispersion)
10.0 Gm calcium salt of condensation product of benzyl chloride
sulphonic acid and dimethyl aniline sulphonic acid.
24.0 Gm Sodium sulphoxylate formaldehyde
4.0 Gm Anthraquinone paste (30%)
45.0 Gm British gum paste
100 Gm
The paste may additionally contain 10 Gm titanium dioxide (of 50% aqueous dispersion) and 3.0 Gm glycerins.
The printing recipe can also be
15 to 20 gms zinc oxide (50% aqueous dispersion)
8 to 12 gms calcium salt of condensation product of benzyl chloride
sulphonic acid and dimethyl aniline sulphonic acid
20 to 25 gms sodium sulphoxylate formaldehyde
2 to 5 gms Anthraquinane paste (30%)
45 gms or quantity sufficient British Gum
to achieve printable viscosity Paste
Process Sequence:
Printing on indigo dyed leather controlled drying
steaming (100-102°C/ 4-5 minutes) treating with hot 1-2 %
sodium carbonate solution rinsing neutralization —
rinsing squeezing drying at 60-120°
RESIST PRINTING:-
Leather is first printed with a chemically or mechanically dye-resisting agent, drying at 60-120°C followed by indigo dyeing. Some suitable resists are potassium ferricyanide, potassium dichromate, copper sulphate, sodium m-nitro benzene sulphonate.
It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-

WE CLAIM;
1. A process for dyeing of leather with indigo dye comprising steps
of: scouring, cationization and subjecting the cationized
substrate to the step of dyeing with indigo dye.
2. A process as claimed in claim 1 wherein the cationization and
indigo dyeing of leather are performed by continuous or
discontinuous process.
3. A process as claimed in claim 1 or 2 wherein in case of the
continuous process, the leather is passed through a bath
containing the cationizing compound such as 3-chloro-2-
hydroxy propyltrimethyl ammonium chloride and caustic soda
or the leather is passed through a bath containing the
cationizing compound followed by a bath containing caustic
soda.
4. A process as claimed in claim 3 wherein the cationization is
carried out at room temperature or at sub ambient temperature
such as 15°C followed by indigo dyeing by passing through a
plurality of dye boxes with a time interval between the
cationization and dyeing.
5. A process as claimed in claim 4 wherein each dyeing is followed
by squeezing and aeration.
6. A process as claimed in claim 1 wherein in case of the discontinuous process, the cationization comprising steps of impregnation of the leather with a cationizing chemical and caustic soda followed by squeezing and coverage with a sheet, which is followed by cleaning for removal of the caustic soda.
7. A process as claimed in claim 1 wherein the cationization can
be carried out by an enhaust process by introducing the leather
in a cationization bath containing the cationization compound
and caustic soda followed by agitation at least at room
temperature and washing.
8. A process as claimed in claim 7 wherein the cationization bath
can also be applied by spraying or brushing followed by
batching after covering the treated leather with sheet, which is
then subjected to washing to neutrality.
9. A process as claimed in any of the preceding claims wherein the
dyeing may be carried out by discontinuous method such as
impregnation-squeeze-air, exhaust brush, spray or continuous
method.
10. A process as claimed in claim 9 wherein in a discontinuous
method, the cationized leather is saturated with indigo dyeing
recipe which is followed by squeezing and oxidization singularly
or repetitively.
11. A process as claimed in claim 9 wherein in a discontinuous
method, the cationized leather, several of which stitched onto
fabric, are passed continuously through a plurality of dye baths
with aeration between each bath.
12. A process as claimed in claim 9 wherein the method of
exhaustion comprises in introducing the cationized leather into
the dye bath under inert blanket and subjecting the bath to
agitation followed by squeezing, oxidation and rinsing to
neutrality.
13. A process as claimed in claim 9 wherein the dye recipe can be
applied by brush or spraying using inert gas as pressurizing
agent followed by oxidation and rinsing to neutrality.
14. A process for printing of leather and leather articles comprising
steps of:

- printing textile substrate with a vat dye
- steaming followed by oxidation
- rinsing, neutralization, rinsing, squeezing and drying.
15. A process as claimed in claim 14 wherein the steaming is
carried out at 100-102°C/3-5 minutes and oxidation is
performed with H2O2.
16. A process as claimed in claim 14 wherein in case of discharge
printing, the steaming is carried out at 100-102°c/4-5 minutes
followed by treatment with hot 1-2% sodium carbonate solution.
17. A resist printing of leather comprising steps of:
- printing of leather with a chemically or mechanically dye-resisting agent selected from potassium ferricyanide, potassium dichromate, copper sulphate, sodium m-nitro benzene sulphonate followed by indigo dyeing.
18. A process as claimed in any of the preceding claims wherein
indigo dyeing and printing may be carried out without
cationization, sacrificing some colour yield.