Process for dyeing a textile material with indigo using indoxyl, and system for exploiting the process

The invention relates to a process for dyeing a textile fabric with indigo by using indoxyl which comprises the following stages: PA1 a. pre-washing a thread F by passing it thorough a pre-wash system (1) composed of two or more vats (2) containing a pre-wash solution (10,15); PA1 b. subsequently dyeing the thread by passing it through a dyeing system (30) composed of a series of eight vats (35), each containing an indoxyl solution, and, during its passage from one vat (35) to another, the thread undergoes oxidation by exposure to air, the indoxyl converting to indigo which then dyes the thread by impregnation, the oxidation by exposure to air being performed in such a way that, during the passage from the first vat (35) to the eighth in succession, if a value x is given to oxidation between the first vat (35) and the second, the following values will be obtained successively: PA1 oxidation value between 1st and 2nd vats=x PA1 oxidation value between 2nd and 3rd vats=2x PA1 oxidation value between 3rd and 4th vats=3x PA1 oxidation value between 4th and 5th vats=4x PA1 oxidation value between 5th and 6th vats=5x PA1 oxidation value between 6th and 7th vats=6x PA1 oxidation value between 7th and 8th vats=7x PA1 oxidation value after 8th vat=8x; PA1 d. sizing the indigo dye onto the thread fibers; PA1 e. drying the thread; PA1 f. collecting the warp thread by winding it onto the spool (72).

The present invention relates to a process for dyeing a textile material 
with indigo using indoxyl, and the system for exploiting the process. 
By means of this process, an indigo-dyed textile thread is produced from 
which a fabric called denim fabric can subsequently be prepared for the 
production of articles such as blue jeans, for example. 
Indigo is one of the oldest known dyes and has been used for centuries to 
dye textiles such as cotton. 
Patent EP-B-0 692 042 describes a process for dyeing a textile fabric 
containing cellulose using indigo during which an aqueous solution of 
leuco indigo prepared by catalytic hydrogenation is employed, this latter 
being converted to its pigmentary form by air oxidation following its 
uptake into the textile fabric. 
Patent U.S. Pat. No. 4,283,198 describes a continuous process for dyeing 
cellulose fibres with indigo in which the fibres are passed through a 
series of steeping vats containing a dye liquor comprising leuco indigo, 
sodium hydrosulfite and sodium hydroxide. 
Patent application U.S. Pat. No. 3,457,022 describes a process for dyeing 
cotton fibres with indigo in which indigo is applied onto the threads at a 
temperature between 65.degree. C. and 80.degree. C. and indigo is then 
applied at temperatures below 40.degree. C. 
Blue jeans dyed with indigo have the disadvantage of fading on washing. In 
recent years, there has even existed a fashion to wear faded blue jeans, 
and the industry developed processes and systems in order to follow this 
fashion, in which fading of the textile was accentuated and furthered by 
promoting fading of the warp thread. The method of use of the dye assisted 
the fading and ageing process. But all fashion is ephemeral and lasts only 
a certain time. 
Patent application EP 98630005.1 describes a dyeing process with indigo 
during which a denim fabric is produced which is resistant to wear and 
fading. By means of this process, the indigo penetrates more deeply into 
the thread. In processes prior to this patent application, the thread 
contained a white central core and only the external part was dyed with 
indigo. In the process according to patent EP 98630005.1, the white 
central core was much more slender. 
The object of the present invention is the production of a denim fabric 
which resists wear and fading and the thread obtained is dyed right 
through to such an extent that the white central core is practically 
non-existent and has even disappeared. 
By means of the process and system according to the present invention, the 
complete penetration of the indigo by the thread is attained by completely 
or virtually eliminating the internal white core and thus eliminating 
fabric ageing caused by fading due to washing. 
Until now, one of the reasons for the failure of indigo to penetrate right 
through the yarn has been its large molecule: 
##STR1## 
It is known, however, that the synthesis of indigo is obtained by oxidation 
of two indoxyl molecules according to the equation: 
##STR2## 
It is assumed that indoxyl reacts according to its ketonic tautomeric 
formula. By means of the process according to the present invention, the 
thread to be dyed is impregnated with indoxyl and the indoxyl is then 
oxidized in situ to obtain indigo. 
Without wishing to be bound by any particular theory, it is assumed that 
the indoxyl molecule, which is smaller than the indigo molecule, 
penetrates the thread more deeply, and the disappearance of the white core 
is thus obtained due to oxidation in situ of indoxyl to give indigo, which 
will fix onto the thread throughout its thickness. 
By means of the process according to the invention, the operations normally 
carried out for dyeing thread with indigo blue are modified by the 
inclusion of additional operations, both at the vat stage and the 
oxidation and preliminary mercerization stages, to assist the absorption 
and impregnation ability of the thread. 
During the oxidation process according to the invention, the first stage 
consists of very rapid exposures to air at the beginning of the process in 
the initial dye baths and subsequently the exposure period is 
progressively increased after each bath in order to aid indoxyl 
penetration into the thread and achieve oxidation in situ. 
The thread is then steamed under specific temperature and pressure 
conditions, before being washed and re-mercerized. 
The second stage of the process consists of forming insulation around the 
now dyed thread to render it fast to washing, and it is then ready for 
use. To achieve this, the thread must first be rendered neutral by 
successive runs through acid baths. 
The thread is then completely dried, and the indigo will permanently fixed 
and will possess the required characteristics. As has been stated above, 
the thread obtained by the known processes of the art is composed of a 
white, i.e. non-dyed, central core and an external part impregnated with 
indigo dye. By means of the process according to the invention, the 
central core is dyed right through with indigo.

The process according to the invention will now be described by reference 
to FIGS. 1 to 5 successively. According to this process, firstly a 
pre-wash stage is carried out by passing the thread F proceeding from the 
delivery roller on which it is wound through a pre-wash system (1) 
composed of two or several vats (2) containing a pre-wash solution 
(10,15). The thread is guided by rollers (3 and 4) into the first vat (2) 
containing the pre-wash solution and then, on leaving this solution, it 
passes between two squeeze rollers to remove the excess liquid, and then, 
guided by roller (6), it is again immersed in the pre-wash solution. On 
leaving this bath, the thread passes between two squeeze rollers (7). 
Then, guided by the guide rollers (8) and (9), the thread is immersed in 
the pre-wash solution (15) contained in the second vat (2). The thread 
travels through the second vat following an identical course to that 
followed through the first vat by running successively between the two 
squeeze rollers (12), under the guide roller (11), then between the 
squeeze rollers (13). 
The pre-wash solution contains an absorbent product such as FINBIL 
(produced by the Bozzetto Company) (trade description), which is a fatty 
alcohol phosphoric acid ester derivative. It is used in the aqueous 
solution at a concentration of 5 to 8 g/litre. This pre-wash bath is used 
to impregnate the thread to aid subsequent penetration by indoxyl. 
After this pre-wash stage, the thread reaches the actual dyeing stage, 
which is carried out in the dyeing system shown in FIG. 2. 
The dyeing system (30) is composed of a series of 8 vats (35) containing an 
indoxyl solution. The thread F proceeding from the pre-wash system is 
guided by the guide roller (31) into the dye solution contained in the 
first vat (35), turns round roller (32) and leaves the bath to run between 
the first squeeze rollers (34) of the first vat (35), turns round roller 
(33) and returns to the dye solution in the first vat, turns round the 
second roller (32) of the first vat to then leave the bath, passes between 
the second set of squeeze rollers (34) of the first vat. Then the thread 
passes round roller (36). During the journey between the second set of 
squeeze rollers (34) and roller (36), the impregnated thread is oxidized 
and indigo is formed which dyes the thread. After turning round roller 
(36), the thread runs into the dye solution contained in the second vat, 
in which it follows the same course as in the first vat by means of 
rollers (31), (32) and (34) of this second vat. On exit from the second 
vat, the thread no longer turns round one roller (36), as it did on exit 
from the first vat, but round two rollers (36) and one roller (37) so that 
the passage exposed to air on leaving the second vat will be twice as long 
as the passage on leaving the first vat, and oxidation will thus be twice 
as great. After turning round rollers (36) and (37), the thread enters the 
third vat, and so forth, until it reaches the eighth vat, and on leaving 
each vat the thread turns round the rollers (36). It should be noted, as 
shown in FIG. 2, that between the third and fourth vat there are three 
rollers (36), between the fourth and the fifth vat there are four rollers 
(36) and so forth until the exit from the eighth vat, where there are 8 
rollers (36). Consequently, the period of exposure to air increases 
progressively with the passage from the first vat to the eighth, so that 
if the value x is given to the oxidation process between the first vat and 
the second, the result will be successively: 
oxidation value between 1st and 2nd vats=x 
oxidation value between 2nd and 3rd vats=2x 
oxidation value between 3rd and 4th vats=3x 
oxidation value between 4th and 5th vats=4x 
oxidation value between 5th and 6th vats=5x 
oxidation value between 6th and 7th vats=6x 
oxidation value between 7th and 8th vats=7x 
oxidation value after 8th vat=8x 
The dye baths contain an aqueous solution of indoxyl containing 6 to 8% by 
weight of pure indoxyl. 
The threads pass through the baths at a speed of 20 to 60 m/minute 
according to the type and quality of the thread. 
As indoxyl reacts easily with oxygen in the air, the stability of the 
indoxyl baths in the vats (35) should be fixed before the solution is 
applied to the thread. This is achieved by adding to the bath a reducing 
mixture of sodium hydrosulphite (Na.sub.2 S.sub.2 O.sub.4) and sodium 
hydroxide (NaOH). At the interface between the bath and the atmospheric 
air, the reducing bath reacts with the oxygen according to the equation 
Na.sub.2 S.sub.2 O.sub.4 +2NaOH+1/2O.sub.2 .fwdarw.H.sub.2 O+2Na.sub.2 
SO.sub.3 
If necessary, if the oxidation period in the open air is too long, a forced 
air circulation may be applied by means of ventilation systems. 
The example shown in FIG. 2 comprises 8 vats, but the operation can 
obviously be repeated two, three or more times by using two, three or more 
sets of 8 vats, thus achieving greater indoxyl penetration within the 
thread. 
By means of this dyeing process, a rich, dark indigo coloration of the 
thread is obtained. 
Once the thread has been dyed, it is washed and pre-dried by means of the 
system (40) shown in FIG. 3. The thread is passed successively through 
vats (41) to (45), guided by rollers (46), (47) and (48). Vat (41) 
contains water, and 50% acetic acid (CH.sub.3 --COOH) is introduced into 
vats (42), (43) and (44) so that vat (42) contains a 20 cc/l proportion, 
vat (43) contains 10 cc/l and vat (44) contains 5 cc/l. Vat (45) again 
contains water, the final pH being between 6 and 7, preferably 6.7. 
On leaving vat (45), the thread is pre-dried by turning round rollers (50), 
as can be seen from FIG. 3. 
After washing and pre-drying the thread, it is sized with the indigo dye to 
increase washing and crocking fastness of the fabric to be subsequently 
prepared, by passing the thread through a sizing system (60), shown in 
FIG. 4. The thread passes through a vat (62) containing a blend of resins 
(63) and is guided through the vat by guide rollers (64) and squeeze 
rollers (61). 
The composition of the resin blend is as follows: 
10 to 50 g/l polyalkylamine quaternary salts 
10 to 50 g/l fluorocarbon resin 
10 to 200 g/l acrylic resin 
1 to 20 g/l autocatalysed polysiloxane 
10 to 200 g/l polyurethane resin 
During this sizing operation, a transparent film is formed on the thread 
fibres which fixes the indigo onto the thread, thereby increasing its 
washing fastness. 
After this sizing stage, the thread is dried by means of the system (70) 
shown in FIG. 5, by running the threads round rollers (71). This drying is 
effected at a temperature between 150-180.degree. C. 
Subsequently, the warp thread is wound round the roller (72). 
The process according to the invention can be exploited using various types 
of thread, for example, cotton, cotton and nylon, cotton and polyester, 
etc. 
The process and the system for exploiting the process are the preferred 
embodiments. It should be noted that modifications can be made, it being 
understood that these modifications are also within the scope of the 
following claims.