EP3988710A1

EP3988710A1 - Method for alleviating discoloration of cellulosic fibers, and products manufactured thereby

Info

Publication number: EP3988710A1
Application number: EP21187351.8A
Authority: EP
Inventors: Lap Wah CHEUNG; Chun Fan Chan; Man Kit Chung; Wai Biu Mok; Kwong Wai Mok
Original assignee: Lenzing AG; Chemiefaser Lenzing AG
Current assignee: Lenzing AG
Priority date: 2021-07-23
Filing date: 2021-07-23
Publication date: 2022-04-27
Also published as: WO2023001849A1; TW202307308A; CN115679690A

Abstract

This invention relates to a method for preventing or alleviating the yellowing of cellulosic fibers during high-temperature treatments, wherein the fibers are treated with an aqueous solution with a pH controlled between 4.0 and 7.0 and containing a protective agent, subsequently dried, and subsequently high-temperature treated, wherein the protective agent contains an alkaline earth metal and zinc hydroxy(substituted)alkanoate and oxoalkanoate.

Description

Prior Art

Cellulosic fibers are among the most widely used fibers for textile and nonwoven applications worldwide. This includes natural cellulosic fibers such as cotton, hemp and linen, as well as regenerated cellulosic fibers such as viscose, modal, Lyocell, cupro, polynosic and the like. It further includes cellulose acetates, both cellulose diacetate and cellulose triacetate. Due to their chemical structure they offer very good wear comfort and other favorable properties.
However this chemical structure also has some disadvantages: Cellulosic fibers will be subjected to high temperature treatment such as above 170°C in order to achieve different performance, such as pre-setting to maintain the dimensional stability (190-195°C), or comprised of polyester fiber required to conduct a thermosoling process for disperse dye dyeing (210-220°C; a treatment by hot air for fixation of disperse dyes within polyester fibers), or molding processes for end-articles such as for example, bras or shoes (190°C). However, such treatment causes discoloration (yellowing) of the cellulosic fiber. Such yellowing is not only disadvantageous for non-dyed, i.e. fluorescent whitened cellulosic fibers but also for cellulosic fibers that are intended to be dyed with pastel or bright colours. Among others, that's why the insole of a bra's material is seldom using cellulose by molding except cut & sew make-up. The most troublesome fiber blends would be comprised of elastane/spandex and polyester (especially woven fabric) with cellulosic fibers or regenerated fibers.
A known solution to this problem is acidic treatment at pH 5 or at little bit lower pH (preferably at between pH 5 and pH 4) prior to high temperature treatment or after high temperature conducting an extra oxidative bleaching step.
However, acidic treatment or washing is not always possible due to the subsequent process steps or the sensitivity of cellulosic fibers to acidic conditions. E.g. overdosing of the acid can cause acid hydrolysis and thereby irrevocable weakening or even complete destruction of the fibers. Fibers containing polyester and dyed applying thermosoling cannot be treated at pH 4-5, due to the general instability of the disperse dyes under such acidic conditions.
Historically, the yellowing was alleviated by treating the cellulosic fibers with 1-5 g/l of borax, i.e. disodium tetra borate. However, today this is banned by Oko-Tex.

Problem

In view of this prior art the problem to be solved consisted in finding a solution to minimize the yellowing (discoloration) of cellulosic fibers or regenerated fiber and their blends regardless of its form (fiber, yarn, fabric, etc.) undergoing high temperature treatment which is required for drying or heat-setting or molding or thermosoling or curing process or combining treatments.

Description

It is an object of the present invention to provide a method for preventing or alleviating the yellowing of cellulosic fibers during high-temperature treatments (i.e. heat-treating at 180°C or higher, e.g. in heat-setting, thermosoling and molding processes for end-articles), wherein the fibers are

a. treated with an aqueous solution with a pH controlled between 4.0 and 7.0 and containing a protective agent, subsequently
b. dried, and subsequently
c. high-temperature treated.

According to the invention the protective agent contains an alkaline earth metal and zinc hydroxy(substituted)alkanoate and oxoalkanoate. The earth metal preferably is magnesium. A suitable protective agent is e.g. TP1740, commercially available from Brueggemann Chemical, Germany. Preferably the protective agent may be applied to the cellulosic fibers in a concentration of between 1.0 g/l and 50.0 g/l, more preferably between 1.0 and 20.0 g/l.
In a preferred embodiment of the invention (see Figure 1) the aqueous solution is applied to the fibers by the padding method or the exhaust method. Suitable devices for these application methods are in principle well-known in the textile industry. Preferably the application method may be a pad-dry treatment. The treatment temperature according to this embodiment may be between 20 and 40°C, preferably between 20 and 30°C. The padding process may be operated with a pickup of between 30 and 100 weight-%, preferably between 50 and 90 weight-%, calculated on the dry fibers.
In another preferred embodiment of the invention (see Figure 2) the aqueous solution of step a. further contains a bleaching agent, preferably a reductive bleaching agent, as well as an acid stable fluorescent whitening agent. This embodiment may usually be performed in the same way and with the same equipment as a conventional bleaching step, e.g. in a vessel, trough or other device suitable for the bleaching of fibers or textiles. A suitable fiber:liquor ratio for this embodiment of the invention may be 1:3 to 1:50 (weight), preferably 1:4 to 1:20 (weight). Suitable temperatures may range from 20 to 95°C, preferably from 80 to 90°C. The duration of the treatment may be between 15 and 90min, preferably between 30 and 60min.
In yet another preferred embodiment of the invention (see Figure 3) between step b. and step c. of the process described just above (i.e. with the bleaching agent and the acid stable fluorescent whitening agent according to Figure 2) the fibers are treated in an additional step d. by the padding method or the exhaust method with an aqueous solution with a pH controlled between 4.0 and 7.0 and containing a protective agent. Subsequently the fibers are dried before step c.. According to this embodiment of the invention for step b. a suitable fiber:liquor ratio for this embodiment of the invention may be 1:3 to 1:50 (weight), preferably 1:4 to 1:20 (weight). Suitable temperature in step b. may range from 20 to 95°C, preferably from 80 to 90°C. The duration of the treatment in step b. may be between 15 and 90min, preferably between 30 and 60min. For step d., preferably a pad-dry treatment, the treatment temperature may be between 20 and 40°C, preferably between 20 and 30°C. The padding process of step d. may be operated with a pickup of between 30 and 100 weight-%, preferably between 50 and 90 weight-%, calculated on the dry fibers.
Reductive bleaching agents are generally known to the skilled in the art. E.g. sodium hydrosulphite (also denoted as sodium dithionate or "Hydros") may be used as a reductive bleaching agent according to the invention. Reductive bleaching agents according to the invention may be applied in a concentration of between 0.5 and 4.0 weight-%, preferably between 1.0 and 3.0 weight-%
The whitening agent according to the invention described herein may be a fluorescent whitening agent, for example an oxidative, stilbene-based fluorescent whitener. Preferably it may be stable towards acids. A suitable agent is commercially available under the trade name Uvitex BHA liq, (from Huntsman/U.S.A). Preferably the whitening agent may be applied in a concentration of between 0.5 and 4.0 weight-%, preferably between 1.0 and 3.0 weight-%.
In a preferred embodiment of the invention described above the cellulosic fibers are bleached before step a. - see the optional step in Figure 3. This bleaching may be performed e.g. under oxidative conditions in an alkaline solution such as a solution containing between 0.1 and 12.0 g/l NaOH (flake), preferably between 0.5 and 3.0 g/l - for the application to man-made cellulosic fibers -, and between 1.0 and 12.0 ml/l hydrogen peroxide (50%)), preferably between 2.0 and 6.0 ml/l - for the application to man-made cellulosic fibers -, at a fiber:liquor ratio of e.g. 1:10 (weight:weight).
In the method according to the invention the cellulosic fibers may be one or more out of the group containing natural cellulosic fibers, such as cotton, hemp and kapok, and man-made cellulosic fibers, such as viscose, modal, Lyocell, cupro, polynosic, cellulose diacetate and cellulose triacetate fibers.
Preferably the fibers are treated in the form of staple fibers, filaments, yarns or woven or knitted fabrics, garments or other textile goods, including underwear, bras and any other garment and textile good that needs a high-temperature treatment during its manufacture.
Surprisingly the method according to the invention is able to improve the whiteness of the fluorescent whitening agent (fluorescent brightening agent) on cellulosic fibers or regenerated fibers and their blends which afterwards undergo high temperature processes. The method according to the invention further allows to dye a pastel and/or bright colour of the natural cellulosic fibers, regenerated cellulosic fibers and/or their blends which underwent high temperature processes without bleaching again. Furthermore, the method according to the invention saves time, water and other resources compared to rebleaching of cellulosic fibers or regenerated fibers and their blends which would be required after treatments at high temperature conditions according to the state of the art.
The invention will now be illustrated by examples. These examples are not limiting the scope of the invention in any way. The invention includes also any other embodiments which are based on the same inventive concept

Examples

All examples were performed on a fabric consisting of 94% Modal EcoClean fibers / 6% Spandex 20D fibers in a single jersey construction.
The whiteness was determined as the CIE whiteness index method, according to the AATCC TM110 2e standard. The higher the whiteness number the better the whiteness. When taking a measurement for CIE Whiteness Index (W10) the instrument must be set to illuminant/observer combinations D65/10° (per ASTM E313-98).

Example 1 (Comparative example): Acidic treatment

The fabric was padded with either tap water (pH ~7) or diluted acetic acid (pH4-5) (see Table 1) and then dried at 100°C for 90 seconds. Pickup was always about 78% (weight/weight, relative to the dry fabric weight). Thereafter the fabric was submitted to heat setting for 120s at a temperature as outlined in Table 1.

Table 1:

Example	1.a	1.b	1.c	1.d	1.e	1.f	1.g	1.h
pH of washing	pH 7				pH 4-5
Heat Setting Temperature, °C	180	190	200	210	180	190	200	210
Grey Scale/Color Change	4	3-4	3	2-3	4-5+	4-5	4	3-4
CIE whiteness, W10	24.44	10.35	-2.85	-21.75	27.23	23.95	19.73	14.13

Table 1 shows that if the temperature can be kept at about 180°C, the yellowing is acceptable regardless of pH. If higher temperatures are required, then the pH shall be kept at a medium acidic value, about pH 4 to 5, to achieve a better whiteness.

Example 2: Padding with protective agent

The fabric was padded with pH 7 with a solution additionally containing 0.5 g/l of the protective agent TP 1740 (from Brueggemann Chemical/Germany) and pickup at 78%. Then it was dried at 100°C for 60 second and submitted to heat setting for 120s at a temperature of 200°C. Table 2:

Example 2.a 2.b

TP1740 [g/l| 0 0.5

Grey Scale/Color Change 3 3-4

CIE whiteness, W10 -2.85 19.23

Example 3: Fluorescent Whitening Article Resisted to High temperature Treatment

First, the fabric was bleached at 90°C for 30 min in a solution containing 1 g/l NaOH (flake) and 4 ml/l hydrogen peroxide (50%) at a liquid/fabric ratio of 1/10 (weight/weight). Subsequently this fabric was whitened at 90°C for 30 min in a solution containing 3 g/l sodium hydrosulphite, 0.85 weight-% Uvitex BHA liq (acid-stable fluorescence whitening agent from Huntsman/USA), 3 g/l Glauber's salt and a quantity "X" (g/l) TP1740 ("X" see Table 3) at a liquid/fabric ratio of 1/10 (weight/weight). The fabric was bleached with hydrogen peroxide followed by reductive bleaching and whitening at the same time. The fabric was treated according to the above recipe. In Example 3.b the whitening bath additionally contained 2 g/l of the protective agent TP 1740 (from Brueggemann Chemical/Germany) and submitted to heat setting for 120s at 180°C. Table 3:

Example 3.a 3.b 3.c

Content TP1740 [g/l] 0 2.0 8.0

Grey Scale/Color Change 3 3-4 4-5

CIE Whitening, W10 73.23 76.66 78.61
Table 3 shows that adding the protective agent TP1740 further reduces the yellowing and results in an improved whiteness after heat setting.

Example 4: Neutral washing with protective agent and fluorescent whitening agent, followed by pad-drying with protective agent

The fabric was bleached in the same way as in Example 3. Thereafter the fabric was whitened at 90°C for 30 min in a solution containing 3 g/l sodium hydrosulphite, 0.85 weight-% Uvitex BHA liq (from Huntsman/USA), 3 g/l Glauber's salt and 8 g/l of the protective agent TP 1740 (from Brueggemann Chemical/Germany). Subsequently the fabric was washed at pH 6 and another 8 g/l protective agent TP1740 was applied by pad-drying. Finally the fabric was submitted to heat setting for 120s at 180°C. The resulting fabric finally showed a Grey Scale/Color Change value of 5. This kind of treatment showed the highest performance in preventing the yellowing of the cellulosic fibers during heat-treatments at elevated temperatures like heat-setting.

Example 5: Padding with protective agent followed by pad-thermosoling

This example shall simulate the conditions occurring during a polyester continuous dyeing process, wherein fabrics are treated that contain blends of polyester fibers with man-made cellulosic fibers. The fabric was treated with 2g/l of the protective agent TP1740 at pH 6 and dried as usual. Subsequently the resulting fabrics were treated by thermosoling for 60 seconds at 210°C resp. 220°C (see Table 4, examples 5.b and 5.d). For comparison same fabrics, but without protective agent treatment were thermosoled under the same conditions (see Table 4, examples 5.a and 5.c). The result shows that the protective agent is alleviating the yellowing of the cellulosic fiber under such high temperature. As an additional advantage of the method according to the invention there is no or minimum impact toward disperse dyes. Table 4:

Example 5.a 5.b 5.c 5.d

Content TP1740 [g/l] 0 2.0 0 2.0

Thermosoling temperature [°C] 210 210 220 220

CIE Whitening, W10 18.84 23.43 4.06 18.33

Claims

A method for preventing or alleviating the yellowing of cellulosic fibers during high-temperature treatments, wherein the fibers are
a. treated with an aqueous solution with a pH controlled between 4.0 and 7.0 and containing a protective agent, subsequently

b. dried, and subsequently

c. high-temperature treated,
characterized in that the protective agent contains an alkaline earth metal and zinc hydroxy(substituted)alkanoate and oxoalkanoate
A method according to claim 1, wherein the aqueous solution is applied to the fibers by the padding method or by the exhaust method.
A method according to claim 1, wherein the aqueous solution further contains a bleaching agent, preferably a reductive bleaching agent, as well as an acid stable fluorescent whitening agent.
A method according to claim 3, wherein between step b. and step c. the fibers are treated by the padding method or by the exhaust method with an aqueous solution with a pH controlled between 4.0 and 7.0 and containing a protective agent, and subsequently dried before the step c..
A method according to claim 1, wherein the cellulosic fibers are bleached before step a..
A method according to claim 1, wherein the cellulosic fibers are one or more out of the group containing natural cellulosic fibers, such as cotton, hemp or kapok, and man-made cellulosic fibers, such as viscose, modal, Lyocell, cupro, polynosic, cellulose diacetate and cellulose triacetate fibers.
A method according to claim 1, wherein the fibers are treated in the form of staple fibers, filaments, yarns or woven or knitted fabrics, garments or other textile goods.