Process for increasing stain-resistance of cationic-dyeable modified polyamide fibers

This invention relates to an improved process for dyeing cationic-dyeable polyamide fibers, especially carpet fibers, with an acid dye to provide stain-resistant fibers. The polyamide fibers contain cationically dyeable sulfonate groups along the polymer chain. The improvement involves adding certain water-soluble salts to a dyebath at a concentration of at least 20% based on the weight of fiber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improved process for preparing stain-resistant 
polyamide fibers, especially carpet fibers, containing 
cationically-dyeable sulfonate groups along the polyamide polymer chain, 
wherein the fibers have been dyed with an acid dyestuff. 
2. Description of the Related Art 
As known in the art, polyamide fibers can be made to resist staining by 
acid dyes by copolymerizing in the polyamide polymer a small percentage of 
a cationically dyeable monomer such as an aromatic sulfonate, for example, 
1 to 4 weight percent of the sodium salt of 5-sulfoisophthalic acid. In 
Ucci, U.S. Pat. No. 4,579,762 nylon 6 and nylon 6,6 carpet fibers formed 
from polymers having aromatic sulfonate units in the polymer chain and 
having improved stain-resistance to acid dyes are disclosed. 
From Windley, U.S. Pat. Nos. 5,164,261 and 5,468,554, it is further known 
that such cationic-dyeable modified polyamide fibers can be made more 
stain-resistant upon being dyed with at least 0.0048 weight percent of an 
acid dyestuff. However, under some circumstances, especially when it is 
desirable to dye such fibers to only a light shade, it has been found that 
maximum stain-resistance (i.e., no staining) may not be obtained. 
Chao, U.S. Pat. No. 5,030,246 discloses a process for continuously dyeing 
polyamide fibers which do not contain cationic-dyeable monomeric units in 
the polymer chain. Rather, the fibers are coated with a stainblocking 
agent which may contain aromatic sulfonate groups. The fibers can be acid 
dyed to a deeper shade by adding certain ammonium and metal salts to the 
dye liquor (typically in an amount from 0.2 to 8% based on the weight of 
the fiber). Chao discloses that effective salts include salts of 
lithium,.calcium, and magnesium, as well as certain ammonium, sodium and 
potassium salts. The use of the salts results in greater uptake of the 
dyestuff from the dye liquor onto the fibers, and the excess dye not taken 
up by the fibers is subsequently rinsed off with water. 
In Jenkins, U.S. Pat. No. 5,466,527, cationic dyeable nylon fibers 
containing a sufficient amount of SO.sub.3 H groups or COOH groups within 
the polymer structure to render the nylon fiber dyeable with cationic dyes 
are disclosed. A process for improving the stain-resistance, 
lightfastness, and ozone-resistance of such fibers is disclosed, wherein 
the fibers are dyed with level acid dyes or premetallized acid dyes at a 
pH value less than 7.0. In Example 6 (columns 13-14), a carpet made with 
cationic dyeable nylon fiber is described as being dyed in a dyebath 
containing level acid dyes or premetallized acid dyes and 2% sodium 
sulfate (Glauber salt) based on weight of fiber. Tables I and II in 
Example 6 show that an improvement in dye exhaustion levels is obtained 
when 2% sodium sulfate is added to the dyebath over dye exhaustion levels 
obtained from dyebaths which do not contain sodium sulfate. Thus, it is 
known from Jenkins that sodium sulfate can be added to a dyebath and 
better exhaustion of the acid dye onto the fiber can be obtained. However, 
it would be desirable to have a process for acid dyeing cationic-dyeable 
modified polyamide fibers, wherein the stain-resistance of the fibers can 
be improved. 
The present invention provides such a process. Particularly, it has now 
been discovered that when certain salts are included in the acid dyebath 
at certain concentrations, the stain-resistance of the acid dyed fibers is 
improved. The stain-resistance of the acid dyed fibers is improved without 
having to increase the dye content on the fiber.. 
SUMMARY OF THE INVENTION 
The present invention provides an improved process for dyeing polyamide 
fibers with an acid dyestuff in a dyebath, wherein the fibers comprise a 
polyamide copolymer which contains cationic-dyeable aromatic sulfonate 
salt group substituents along the polymer chain. The improvement comprises 
adding a water-soluble salt selected from the group consisting of 
ammonium, potassium, and sodium salts, and mixtures thereof to a dyebath 
at a concentration of at least 20% based on the weight of the fiber to 
produce acid dyed fibers having a stain-resistance rating of at least 8.0 
on the AATCC Red 40 Scale. 
Preferably, the salt is selected from the group consisting of sodium and 
potassium chlorides; sodium, potassium, and ammonium sulfates; and sodium 
acetate. In some instances, the concentration of salt in the dyebath may 
be at least 100% based on weight of fiber. The acid dye may be a 
premetallized acid dye. The fibers may be acid dyed at a pH level greater 
than 6.5, although such a pH level is not necessary. Suitable polyamide 
copolymers for use in this invention include copolymers containing 1 to 4 
weight percent of monomer units derived from a salt or other derivative of 
5-sulfoisophthalic acid based on the final weight of the copolymer. The 
polyamide copolymer may also contain units selected from the group 
consisting of polyhexamethylene adipamide units, 
poly-.epsilon.-caprolactam units, and mixtures thereof. 
DETAILED DESCRIPTION OF THE INVENTION 
As known in the art, polyamide fibers can be visibly and permanently 
stained when left in contact with a solution containing acid dyes 
typically used as colorants for foodstuffs. Those skilled in the art have 
developed different methods for rendering polyamide fibers 
stain-resistant. For example, in the foregoing Windley, U.S. Pat. Nos. 
5,164,261 and 5,468,554, the entire disclosures of which are hereby 
incorporated by reference, the polyamide fibers are rendered 
stain-resistant by copolymerizing in the polyamide polymer a cationic dye 
modifier and by adding an acid dye to the polymer melt or by dyeing the 
fibers with an acid dye from a dyebath. The present invention is an 
improvement over the process disclosed in the foregoing patents and 
involves adding certain salts to the dyebath which selectively improve the 
stain-resistance of the dyed fibers without the need for increasing the 
dyestuff content on the fiber. 
More particularly, this invention provides an improved process for dyeing 
polyamide copolymer fibers containing a cationic dye modifier with an acid 
dye, producing dyed fibers which, when dried, have a stain-resistance 
rating of at least 8.0, preferably 9.0, on the AATCC Red 40 Stain Scale. 
The term, "acid dye" includes premetallized acid dyes. The improved 
process is accomplished by dyeing the cationic dyeable fibers with an acid 
dye in a dyebath which contains salt at a concentration to provide at 
least 20% salt based on the weight of fibers (% owl) being dyed. The 
process is particularly advantageous for fabrics which are dyed to light 
or medium shades. It is presumed that the improvement is equally effective 
for fibers dyed to deep shades, but stains are less evident on deep-dyed 
fibers, so the improvement is difficult to evaluate. The term "fiber" 
includes both continuous (e.g., bulked continuous filament) and short 
lengths (e.g., staple). Effective salts are ammonium, sodium and potassium 
salts, or combinations of any of these salts, at a concentration of at 
least 20% based on weight of fiber. When these salts are used in 
accordance with this invention, dyeing with acid dyes takes place rapidly 
and there is no need to adjust the pH of the dyebath at any stage of the 
dying process. These acid-dyed cationically-modified polyamide fibers have 
greater stain-resistance than cationically-modified polyamide fibers which 
have been dyed in a dyebath which does not contain these salts. 
Cationic Dyeable Modified Polyamides 
The cationic dye modifier used to form the polyamide copolymers useful in 
this invention have the formula: 
##STR1## 
where Y is H, Li, Na, K, or Cs and R is H or an alkyl group containing 1 
to 5 carbon atoms. The --OR groups are lost during polymerization. The 
preferred cationic dye modifiers are those containing two carboxyl groups 
with 5-sulfoisophthalic acid being especially preferred. Generally, 
sufficient cationic dye modifier is used to produce a copolymer containing 
from 1 to 4 weight percent (wt. %) of cationic dye modifier based on the 
final polymer weight of the polyamide copolymer with from 2 to 3 wt. % 
being the preferred range. 
The polyamide copolymers useful in this invention can be prepared by 
blending the salt of the base polyamide (e.g., nylon 6,6 salt or 
caprolactam) with the cationic dye modifier followed by polymerizing the 
blended composition in an autoclave using standard polymerization 
procedures, solidifying and fragmenting the polymer, increasing the degree 
of polymerization by further polymerizing the polymer in the solid phase 
while simultaneously super-drying the polymer, and further polymerizing 
the polymer in the molten state. In a preferred embodiment used for 
production of carpet fibers, the copolymer is prepared by polymerizing 
nylon 6,6 salt containing sulfonate modifier in an autoclave to a relative 
viscosity of about 35, increasing the relative viscosity to about 45 to 65 
by solid-state polymerization while super-drying the polymer, and melting 
the polymer in a screw extruder, transfer line and spin block to produce a 
copolymer with a relative viscosity between 45 and 70. In the production 
of textile fibers, the acceptable relative viscosity can be lower (about 
35 to 45). Preferably, the copolymers used in this invention contain from 
20 to 40 amine ends per 10.sup.6 grams of copolyamide. 
Dyeing Procedure 
A dyebath is prepared by adding the desired amount of acid dye and salt to 
water. Dye concentrations can vary from 0.01 to 0.05%, based on weight of 
fiber. The salt concentration must be sufficiently high to exhaust the 
dyestuff from the dyebath onto the fiber. As stated earlier, it has been 
found that this salt concentration must be at least 20%, based on weight 
of fiber (% owf), and can be as high as 200%, or even higher, for the 
salts which are useful in this invention. The fibers may be dyed at a pH 
level greater than 6.5, but such a pH level is not necessary. If desired, 
a lower pH level can be used. 
The results reported in the following examples were obtained by placing 10 
grams of fiber in 200 ml of dyebath containing 0.0037 grams of dye and 
salt at various concentrations. The dyebath was heated to boiling in the 
shortest possible period and held at the boil for 5-30 minutes, usually 
about 10 minutes. The dyebath was then cooled to 70.degree.-80.degree. C. 
Cold water may be added to accomplish the cooling. The fibers were 
removed, washed and dried, either at room temperatures or by heating. The 
fibers were dyed to a light gray shade. Comparable results were obtained 
with fibers dyed to dune and beige shades. Equivalent results were 
obtained for fibers in the form of yarns, or woven, knitted, or pile 
fabrics. This procedure can be readily adapted to a continuous process. 
The following examples are illustrative of this invention but should not be 
construed as limiting the scope of the invention. 
TEST METHODS 
Stain Test Method 
The following test procedure was used to determine the stain-resist 
performance of the fiber samples. 
A solution of staining agent was prepared by dissolving 45 grams of 
cherry-flavored, sugar-sweetened "Kool-Aid" premix powder in 500 cc. of 
water. The solution allowed to reach room temperature, i.e., 75.degree. 
F..+-.5.degree. F. (24.degree. C..+-.3.degree.), before using. The 
colorant used in the "Kool Aid" solution was Red Acid Dye 40. 
A specimen, approximately 1.5.times.3 inches (3.8.times.7.6 cm.) for carpet 
samples and approximately 2.times.4 inches (5.1.times.10.2 cm.) for 
knitted fabrics was cut from each sample and placed on a flat 
non-absorbent surface. The solution of staining agent was poured onto each 
specimen through a cylinder to form a 1 to 2 inch (2.5 to 5.1 cm.) 
circular stain, using 20 cc. for carpet samples and 10 cc. for textile 
samples. The samples were lagged in the laboratory for twenty-four hours 
and then rinsed thoroughly with cool tap water and squeezed dry, using an 
extractor to remove excess solution. 
The stain-resistance of the specimen was determined visually according to 
the color left on the stained area of the sample. The color depth was 
determined by comparison with a series of ten transparent plastic 
rectangles in accordance with the AATCC Red 40 Stain Scale, in which 10 
represents no staining, 9 very light staining, with increasing color as 
the scale decreases to 1, which represents heavy staining. 
In each of the following examples, 200 ml. of dyebath and 10 grams of fiber 
sample were used. Concentrations of salt and dye are expressed as 
percentages based on the weight of fiber (% owf). The same weight of dye 
(0.0037 gram) was used in each example. In all but one of the examples, 
the cationic dyeable polyamide copolymer contained 3% by weight of 
5-sulfoisophthalate. The dyed samples were light gray in color. The dye 
formulation was composed of the following ingredients: 0.015% owf Tectilon 
yellow 3R KWL 200 (acid yellow 246), 0.0075% owf Tectilon red 2B KWL 200 
(acid red 361), and 0.005% owf Tectilon blue 4RS KWL 200. The pH levels of 
the solutions were measured with a Fisher Accumet pH Meter Model 610A 
equipped with a Fisher glass electrode.

EXAMPLES 
Fiber Preparation 
A polyamide copolymer was prepared by blending nylon 6,6 salt and the 
sodium salt of 5-sulfoisophthalic acid and polymerizing in an autoclave. 
The polymer melt was solidified, fragmented, and polymerized further in 
the solid state in an inert atmosphere at a temperature of 185.degree. C. 
The copolymer was then fed to a twin screw extruder and discharged into a 
transfer line at a temperature of 290.degree. C. It was extruded through a 
spinneret to produce yarns in which each of 128 filaments had four 
symmetrically-placed voids. After application of finish, the yarns were 
drawn 2.7.times. at 190.degree. C. in a continuous process. The drawn 
filaments were passed through a jet where they were impinged with air at 
240.degree. C. and 120.degree. C. and collected on a screen drum. The yarn 
was removed by a take-up roll and wound onto tubes. The knitted and tufted 
fabrics used in the following Examples were prepared from these yarns. 
EXAMPLE 1 
Certain salts, such as calcium salts, are effective in exhausting the 
dyestuff from the dyebath onto the fiber at low and high salt 
concentrations. However, the stain-resistance of fiber samples dyed to 
light and medium shades by methods using such salts is unacceptable, as 
shown in Table 1. In comparison, the process of this invention employs 
certain salts at a concentration of at least 20% on weight of fiber (% 
owf) and provides fibers having good stain-resistance, as shown in Table 
1. 
TABLE 1 
______________________________________ 
Salt pH of % Dyestuff on 
Stain 
Composition 
% OWF Soln Fiber Rating 
______________________________________ 
*Ca(H.sub.2 PO.sub.4).sub.2 
2 5.3 0.037 6.0 
*Ca(H.sub.2 PO.sub.4).sub.2 
20 5.3 0.037 6.0 
*(NH.sub.4).sub.2 SO.sub.4 
10 7.2 Less than 0.037 
7.0 
*Na.sub.2 SO.sub.4 
10 7.8 Less than 0.037 
6.0 
Na.sub.2 SO.sub.4 
20 7.4 Less than 0.037 
8.0 
(NH.sub.4).sub.2 SO.sub.4 
40 6.8 0.037 8.0 
______________________________________ 
*Comparative Examples 
EXAMPLE 2 
When the salts of this invention are used in the dyebath at concentrations 
above 20%, based on weight of fiber (% OWF), they are effective in 
imparting even greater stain-resistance to cationic-dyeable modified 
polyamide fibers which are dyed with an acid dyestuff. Such fibers are 
made from polyamide copolymer containing 2% and 3% 5-sulfoisophthalate. 
This effect is illustrated in Table 2 for polyamide fibers taken from 
knitted fabrics which have been dyed to a light gray color, wherein the 
fibers are made from polyamide copolymer containing 2% and 3% 
5-sulfoisophthalate. 
TABLE 2 
______________________________________ 
% Sulfoisoph- 
Salt % Dyestuff 
Stain 
thalate Composition 
% OWF pH on Fiber 
Rating 
______________________________________ 
2 NaCl 185 7.4 0.037 9.0 
3 KCl 185 7.4 0.037 9.0 
______________________________________ 
EXAMPLE 3 
The process of this invention is useful for dyeing and imparting good 
stain-resistance to the above-described polyamide fibers in any form e.g., 
yarns or nonwoven, knitted, woven or pile fabrics. This effect is 
illustrated in Table 3 for fibers dyed to a light gray shade. 
TABLE 3 
______________________________________ 
Salt % Dyestuff 
Stain 
Substrate 
Composition 
% OWF pH on Fiber 
Rating 
______________________________________ 
Yarn NaCl 200 7.4 0.037 9.0 
Knitted NaCl 185 7.4 0.037 9.0 
Fabric 
Pile Fabric 
(NH.sub.4).sub.2 SO.sub.4 
40 6.8 0.037 8.0 
(carpet) 
______________________________________ 
EXAMPLE 4 
The data in Table 4 illustrate the unique effectiveness of the sodium and 
potassium salts for producing polyamide fibers which have good 
stain-resistance after being dyed with the above-described dyes, as 
compared to similar salts of lithium, magnesium, zinc, and calcium. All of 
the polyamide fibers were made from copolymers containing 3% by weight of 
5-sulfoisophthalic acid and were dyed to a light gray shade. The tests 
were run on fabric taken from knitted socks. 
TABLE 4 
______________________________________ 
Salt Stain 
Composition 
% OWF pH % Dyestuff on Fiber 
Rating 
______________________________________ 
Na.sub.2 SO.sub.4 
200 7.4 0.037 9.0 
K.sub.2 SO.sub.4 
200 7.9 0.037 9.0 
*Li.sub.2 SO.sub.4 
200 8.3 0.037 6.0 
*MgSO.sub.4 
200 7.3 0.037 6.0 
*ZnSO.sub.4 
200 6.8 0.037 6.0 
*Ca(H.sub.2 PO.sub.4).sub.2 
20 5.3 0.037 6.0 
Na.sub.2 SO.sub.4 
185 7.8 0.037 9.0 
NaOAc 110 7.8 0.037 9.0 
KC1 185 7.4 0.037 9.0 
______________________________________ 
*Comparative Examples