BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method and composition for whitening textiles.
In particular, the present invention relates to a method and composition for whitening
textiles where oxidative bleaching is not desirable.
[0002] Wet processing of fabrics is a multi-step process, generally including at least:
scouring, bleaching and finishing. Where the fabric is a stretch or elastomeric fabric,
the process also typically involves heat-setting prior to wet-processing. Selection
of the steps in processing a particular fabric depends upon several factors, including
desired appearance and performance of the finished fabric.
[0003] Bleaching is a typical part of the wet-processing because traditional fabrics made
with natural fibers are typically buff or off-white in color, not "blue-white" that
is, white without a yellow cast. On the other hand, synthetic fibers developed over
the recent decades are typically white, either because the material itself is white,
or because whitener is added to the fiber; however, during processing of fabric made
from such fibers, yellowing may occur. This yellowing occurs during heat setting,
a high temperature process wherein various properties such as resiliency, elasticity
and wrinkle-resistance are imparted to the fabric. When yellowing occurs, such fabrics
may also be subjected to reductive bleaching.
[0004] For the majority of fabrics, bleaching is typically accomplished with strong oxidizing
agents such as: hydrogen peroxide and other peroxide generators, or chlorine-containing
bleaches including sodium hypochlorite, calcium hypochlorite or sodium chlorite. With
certain fabrics, however, it is not desirable to use such strong bleaching agents,
as such agents could degrade the fabric.
[0005] Where fabrics are not suitable to bleaching with an oxidizing agent, a reducing scour
or reducing bleach (collectively, "reducing bath") can be used. The reducing agent
in such reducing baths is generally sodium dithionite (usually referred to as sodium
hydrosulfite or hydro) or thioureadioxide (also known as formamidinesulfinic acid
or FAS). Sodium hydrosulfite is very unstable, especially when it comes into contact
with even small amounts of water, and often spontaneously combusts. Sodium hydrosulfite
is typically provided as a powder containing 70-90 wt % active hydro, sodium carbonate
as a fire retardant, and a chelating or sequestering agent such as EDTA or STPP. Hydro
is also now available as an aqueous solution; however, such solutions are prone to
oxidation and must be stored under an inert atmosphere (and refrigerated for longer
term storage). Although hydro is widely used in the textile industry, there are severe
limitations to the use of this reducing agent. The most severe limitations include:
the high fail rate, even with multiple passes through the reducing bath; the low amount
of resultant whitening; the strong sulfur smell (caused by emission of SO
2); and the high flammability of hydro powder.
[0006] A typical conventional reducing bath contains sodium hydrosulfite, defoamer and wetting
agents. Other chemicals useful in the reducing bath include, without limitation: other
surfactants, sequestrants, caustic (usually soda ash or NaOH) and optical brighteners.
The amount and type of chemicals used in the reducing baths will depend on the fabric
being treated, method of treatment and end use of goods. In general, about 3% hydro
is added to the reducing bath at the beginning of the process, and then a second dose
of about 3% is added about halfway through the treatment (over about 10 mm), so that
the typical residence time of fabric in a conventional reducing bath is about 1 -
1.5 hrs. Wet-processing of grey fabric usually takes about 6-9 hrs, and multiple treatments
may be used. Sometimes the reducing bath step is repeated in order to increase whitening.
This can occur immediately after the first reducing bath step, or the whole wet-processing
procedure can be repeated. Even after such treatments, the differences in whitening
between the treated and untreated fabric will not be readily discernible to the naked
eye, and can only be determined by spectral analysis.
[0007] There is therefore a need for a reducing bleach for fabrics which is safe, easy to
use, cost-effective, and highly efficacious. As detailed below, the present invention
accomplishes all of these objectives.
STATEMENT OF THE INVENTION
[0008] The present invention is directed to a system for bleaching fabric comprising greige
fabric to be treated, a two component reducing solution, and means for heating said
reducing solution to 70 to 110 °C; wherein said first component of the reducing solution
comprises between 0.1 and 0.6 % owg borohydride and said second component of the reducing
solution comprises bisulfite in water said bisulfite being present in a stoichiometric
excess relative to the borohydride, and wherein the first component of the reducing
solution is added to the second component just prior to treatment of the greige fabric.
[0009] The present invention is also directed to a method for bleaching greige fabric, comprising
the steps of contacting greige fabric with a two-part reducing solution for a time
sufficient to cause bleaching of the greige fabric to occur, wherein: such two-part
reducing solution comprises a first component comprising between 0.1 and 0.6 % owg
borohydride and a second component comprising bisulfite in water, said bisulfite being
present in a stoichiometric excess relative to the borohydride; and the first component
of the reducing solution is added to the second component just prior to contacting
the greige fabric with the reducing solution; and the two part reducing solution is
maintained at a temperature between 70 and 110 °C during bleaching of the greige fabric.
DETAILED DESCRIPTION OF THE INVENTION
[0010] As used in this specification, the following terms have the following definitions,
unless the context clearly indicates otherwise. "Greige" or "grey" fabrics are those
just off the loom or knitting machine and which have only been partially finished
(e.g., not yet dyed). "Fabric" or "textile" means, without limitation, yarns in skein
or package form, and all woven, non-woven or knitted goods made from spun fibers,
filamentary fibers or yarn and selected from natural and synthetic materials. Such
natural and synthetic materials include but are not limited to: cellulosic acetates
such as cellulose acetate and cellulose triacetate; silk, wool, and other protein
fibers, flax and other bast fibers; nylon and other polyamide fibers; elastomeric
or rubber fibers; acrylics; polyethylene terephthalate; and cotton and rayon fibers.
"Spandex" fibers are elastomeric fibers in which the fiber forming substance is a
long-chain synthetic polymer of at least 85% of a segmented polyurethane, and generically
included both polyether- and polyester-segmented polyurethanes. "Yarn" means a continuous
strand of fibers, filaments or other material in a form suitable for knitting weaving
or otherwise intertwining to form a textile fabric. "Elastomeric" fibers or yarn are
those which at room temperature can be stretched repeatedly to at least twice their
original length, and which, upon immediate release of the stretch, will return with
force to approximately the original length. "Elastomeric fabrics" are fabrics comprising
some elastomeric yarn in either the warp or weft (filling) orientation of the fabric.
The following abbreviations are used throughout the specification: min = minutes;
hr = hours; % owg = percent of the textile weight (on the weight of the goods) % owb
= percent on weight of bath; wt % = percent by weight; g = grams; L = liters; g/L
= grams per liter. Unless otherwise specified, temperatures are in degrees centigrade
(°C) and ranges specified are to be read as inclusive.
[0011] The reducing bleach solution of the present invention is a two-component solution,
typically aqueous. The first component contains a borohydride, and the second component
contains a bisulfite in aqueous medium. The first component is added to the second
component just prior to fabric treatment. In general, the reducing bath will also
contain defoamer and wetting agent, usually added to the second component. Other chemicals
useful in the reducing bath include, without limitation: other surfactants, sequestrants,
caustic (usually soda ash) and optical brighteners. Such ingredients and their amounts
are known to those skilled in the textile arts.
[0012] Borohydride and bisulfite react in water (at about pH 5-8) to produce hydrosulfite
in situ according to the following equation 1:

In addition, it has been suggested that the radical anion ·SO
2-, also a strong reducing agent, is a possible intermediate in this reaction. See,
for example, Cook, M. M. in
Environmental Chemistry of Dyes and Pigments, pp 33-41 (Reife et al., ed; J. Wiley, 1996). Although the exact mechanism of this
reaction has not been fully characterized and this invention is in no way limited
by any particular theory or mechanism of action, it is believed that this radical
anion also contributes to the reducing ability of the reducing bleach of the present
invention.
[0013] The amount of borohydride useful in the system of the present invention is between
0.1 and 0.6 % owg, preferably between 0.1 and 0.5 % owg, and most preferably between
0.1 and 0.3 % owg. Any borohydride may be used so long as the counterion does not
interfere with fabric wet-processing. It is preferred to use a borohydride of formula
T(BH
4)
y, wherein T is an alkaline metal, alkali earth metal or ammonium, and y is 1 or 2,
depending on the valence of T. It is particularly preferred to use sodium or potassium
borohydride. Borohydride is readily available as an aqueous alkaline solution containing
12 wt % sodium borohydride and 40 wt % sodium hydroxide. Such aqueous solution is
available as COLORSTRIP™ 2000 from Rohm and Haas Company (N. Andover, MA). If such
an aqueous borohydride solution is used, the amount suitable for purposes of the present
invention is between 10 to 30 wt %, preferably no more than about 25 wt %, of the
amount of 70% active anhydrous hydrosulfite recommended for conventional reducing
bleaches, which will depend on the fabric to be treated.
[0014] The amount of bisulfite used in the present invention will depend in part on the
amount of borohydride. As shown above in Eq. 1, 8 moles of bisulfite are required
for each mole of borohydride; however, it is desired to use a stoichiometric excess,
particularly a molar excess of at least 10 %. In general, the bisulfite will be of
formula M(HSO
3)
x, wherein M is an alkaline metal, alkali earth metal or ammonium, and x is 1 or 2,
depending on the valence of M. It is preferred to use sodium or potassium bisulfite.
Where the borohydride is used in the form of an aqueous alkaline solution, a greater
amount of bisulfite is needed. For example, such a borohydride solution containing
40 wt % caustic such as sodium hydroxide requires an additional 3.2 mole of bisulfite;
thus in this case it is preferable to use up to about 12 moles of bisulfite per mole
of borohydride. If the borohydride solution includes caustic, up to 40 wt % of caustic
can be used. Such caustic is generally selected from Q(OH)
z, wherein Q is an alkaline metal, alkali earth metal or ammonium, and z is 1 or 2,
depending on the valence of Q. It is preferred to use sodium hydroxide.
[0015] A typical wet-processing of elastomeric fabric using the system of the present invention
includes the steps of: scouring, reducing bleach, whitening (optical brightening),
dyeing, and acid washes. A typical amount and type of fabric used in such processing
is as follows:
Weight of Goods → 275 kg of 80% nylon/20% Lycra® warp knit fabric.
[0016] The initial step consists of contacting the fabric with a scouring bath (scouring
agent and wetting agent) for about 1 to 2.5 Firs.
[0017] The second step consists of contacting the fabric with the reducing bath such as
detailed in Table 1 below for about 30 to 90 min at a temperature between 70 and 110
°C.
Table 1
Sample Reducing Bath |
Ingredient |
Amount in bath |
Defoamer |
0.4 %owb |
Surfactant/Scouring Agent |
1.0 % owg |
Optical Brightener |
1.5 % owg |
Bisulfite |
16 % owg |
12% Liquid Borohydride |
4.0 % owg |
[0018] The third step consists of contacting the fabric with the whitening bath such as
detailed below in Table 2 for about an hour at about 70 - 110 °C.
Table 2
Whitening Bath |
Ingredient |
Amount in bath |
Citric Acid |
0.35 % owb |
Leveling Agent* |
1.0 % owg |
Ammonium Sulfate |
1.0 % owb |
Acetic Acid (56%) |
0.25 % owb |
Optical Brightener |
0.5 % owg |
Violet Acid Dye |
0.001 % owg |
[0019] This step may be followed by at least one dyeing step, followed by several washing
baths, the first of which contains 0.35% owb citric acid (or any mild acid). After
the last such bath, the fabric is checked for shade.
[0020] The method of contacting the fabric with the baths in the above steps can be done
at any point during wet processing of the grey fabric, using any of a number of methods
known to those skilled in the art. Such methods include, without limitation: producer
dyeing; stock dyeing; yarn dyeing; and piece or garment dyeing such as by pad baths,
jigs, becks (pressurized and non) and jets. It is not required that the same method
be used for each such bath.
[0021] The following examples are presented to illustrate further various aspects of the
present invention, but are not intended to limit the scope of the invention in any
respect. In the examples, the color evaluation readings are abbreviated as follows.

[0022] The a and b values determine hue, and shifts of either of these values impact on
the chroma (depth of hue). For purposes of the present invention, changes in L and
WI are most significant in determining whitening of the treated fabric. Shifts in
the value of b are also important, however, as white is often translated to mean a
blue-white.
Example 1
[0023] The following example demonstrates the bleaching capability of a reducing solution
of the present invention.
[0024] Warp knit fabric (21.5 g of 80% nylon and 20% Lycra fabric) was added to about 500
mL of water in a bath vessel. To this bath was added 1% owg of surfactant, with stirring.
The bath was heated to boiling (about 95 °C) with stirring.
[0025] In a separate container, 0.67 g (3.11% owg) of sodium metabisulfite was mixed with
about 10 g of water, and 0.18 g (0.83% owg) of 12% liquid sodium borohydride added
to the mixture. Immediately upon addition of the borohydride to the bisulfite, that
mixture was added to the fabric bath containing surfactant. The fabric was treated
in this reducing bleach for up to about a half hour. Results are detailed in Table
3, below.
Table 3
Color Evaluation of warp knit fabric |
Color Value |
Standard (pretreatment) |
15 Minute Reduction |
22 Minute Reduction |
L |
94.48 |
94.80 |
94.46 |
ΔL |
--- |
0.32 |
-0.02 |
a |
-1.78 |
-1.82 |
-1.60 |
Δa |
--- |
-0.04 |
0.17 |
b |
8.27 |
6.76 |
6.36 |
Δb |
--- |
-1.50 |
-1.92 |
WI |
44.03 |
52.27 |
53.72 |
ΔWI |
--- |
8.25 |
9.69 |
ΔE |
--- |
1.54 |
1.93 |
[0026] The results demonstrate that the reducing bleach of the present invention significantly
whitens fabric subjected to such treatment. In fact, there is a difference in whiteness
between the untreated and treated fabric can be seen by the naked eye when the reducing
bleach of the present invention is used, whereas use of conventional reducing bleaches
results in such slight differences in whiteness that no noticeable difference can
be observed unaided.
Example 2
[0027] The following example further illustrates the whitening ability of the reducing bleach
of the present invention.
[0028] The procedure of Example 1 was followed, except that the reducing bath contained
a total of 1.0% surfactant, 1.0% owg 12% liquid sodium borohydride and 3.66% owg of
sodium metabisulfite. In addition, the fabric was treated for up to about 1 hour in
the reducing bath.
Table 4
Color Evaluation of Warp Knit Fabric |
Color Value |
Standard (pretreatment) |
30 Minute Reduction |
60 Minute Reduction |
L |
94.48 |
94.94 |
94.59 |
ΔL |
--- |
0.46 |
0.11 |
a |
-1.78 |
-1.74 |
-1.61 |
Δa |
--- |
0.04 |
0.17 |
b |
8.27 |
7.01 |
6.81 |
Δb |
--- |
-1.26 |
-1.46 |
WI |
44.03 |
51.20 |
51.58 |
ΔWI |
--- |
7.18 |
7.55 |
ΔE |
--- |
1.34 |
1.48 |
[0029] The results further demonstrate the whitening ability of the reducing bleach of the
present invention.
1. A system for bleaching fabric comprising greige fabric to be treated, a two component
reducing solution, and means for heating said reducing solution to 70 to 110 °C; wherein
said first component of the reducing solution comprises between 0.1 and 0.6 % owg
borohydride and said second component of the reducing solution comprises bisulfite
in water said bisulfite being present in a stoichiometric excess relative to the borohydride,
and wherein the first component of the reducing solution is added to the second component
just prior to treatment of the greige fabric.
2. The system of claim 1, wherein the bisulfite is of formula M(HSO3)x, wherein M is an alkaline metal, alkali earth metal or ammonium, and x is 1 or 2,
depending on the valence of M.
3. The system of claim 1, wherein the first component of the reducing solution comprises
between 0.1 and 0.5 % owg borohydride.
4. The system of claim 1, wherein the first component of the reducing solution comprises
between 0.1 and 0.3 % owg borohydride.
5. The system of claim 1, wherein the borohydride is of formula T(BH4)y, wherein T is an alkaline metal, alkali earth metal or ammonium, and y is 1 or 2,
depending on the valence of T.
6. The system of claim 1, wherein the reducing solution is maintained at a temperature
between 70 and 110 °C while said reducing solution is in contact with the fabric to
be treated.
7. A method for bleaching greige fabric, comprising the steps of contacting greige fabric
with a two-part reducing solution for a time sufficient to cause bleaching of the
greige fabric to occur, wherein:
such two-part reducing solution comprises a first component comprising between 0.1
and 0.6 % owg borohydride and a second component comprising bisulfite in water, said
bisulfite being present in a stoichiometric excess relative to the borohydride; and
the first component of the reducing solution is added to the second component just
prior to contacting the greige fabric with the reducing solution; and
the two part reducing solution is maintained at a temperature between 70 and 110 °C
during bleaching of the greige fabric.
8. The method of claim 7, wherein the first component of the reducing solution comprises
between 0.1 and 0.5 % owg borohydride.
9. The method of claim 7, wherein the bisulfite is of formula M(HSO3)x, wherein M is an alkaline metal, alkali earth metal or ammonium, and x is 1 or 2,
depending on the valence of M.
10. The method of claim 7, wherein the borohydride is of formula T(BH4)y, wherein T is an alkaline metal, alkali earth metal or ammonium, and y is 1 or 2,
depending on the valence of T.