WO1992015744A1

WO1992015744A1 - Photobleaching process for natural fibres

Info

Publication number: WO1992015744A1
Application number: PCT/GB1992/000394
Authority: WO
Inventors: William Stanley Simpson
Original assignee: Lappage, James
Priority date: 1991-03-05
Filing date: 1992-03-05
Publication date: 1992-09-17
Also published as: AU1367592A; NZ237321A

Abstract

A photobleach process requiring a combination of three components in order to effect a superior rapid bleach. The components are radiation, oxidant and an appropriate aqueous medium. When all three components are present within specified limits, the bleach is more rapid than conventional processes and in an optimum form of application will achieve whiteness superior to any other process no matter how vigorously applied. Specifications of the three essential components are: (1) Radiation Source: the preferred and optimum light source for the present invention should contain a high proportion of blue wavelengths. However, if the bleaching time is sufficiently short, wide spectrum light sources can be employed if there are advantages of availability or physical form. The rate or photobleaching increases with light intensity. (2) Oxidant: hydrogen peroxide is the oxidant of most practical importance. The concentration of peroxide recommended is approximately 2.5 % (w/v) as H2O2, but between 1 % and 10 % w/v can be viable depending on how the process is carried out. (3) Aqueous medium: the range of interest for the current invention is pH 9.0 - 11.5. The optimum is approximately pH 10.5 ± 0.5, and in this range the common alkaline buffers are only weakly effective. Tri-sodium phosphate has proven in experiments to be reasonably satisfactory.

Description

PHOTOBLEACHING PROCESS FOR NATURAL FIBRES

The subject of the invention is a process of beaching wool other animal fibres, silk and cellulose fibres such as cotton. The invention combines two forms of bleaching, chemical and pho chemical in a reaction medium more alkaline than present conventional processes applied to wool, silk and other animal fibres.

An object of this invention is to provide a combination bleach which is both faster and more extensive than any previou possible process using current bleaching practices. To simplif the description of the process the term "wool" as is used herei represents the generic class of keratin fibres such as all anim fibres and silk.

The bleaching of wool has been the subject of a large numb of research publications and patents. Only the salient points which reflect on the novelty of the present invention are described below. A more formal definition of how a bleach effe is measured is given in the section below describing some experimental results.

Hydrogen (hereafter "peroxide") is the most common bleachi agent for wool. Many processes have been described for peroxid at various levels of concentration, pH, time and temperature, i conjunction with buffers, stabilisers and detergents. Of speci relevance to the present invention is pioneering research by A. Smith and M. Harris (American Dyestuff Reporter (1936) 2_5 180P) whereby alkaline conditions up to pH 11 were tested and shown n to be viable on the grounds of chemical damage to the wool. Damage is a progressive factor, but experimentation over intervening years has confirmed that alkaline conditions above p 10 for peroxide bleaching were generally accepted as impractical. The primary reason is that when the temperature is raised to complete the bleach in an economic time, the oxidative damage to the wool increases to unacceptable levels. Peroxide itself is unstable at high pH particularly if the temperature is increased beyond 40^βC. Stabilisers are routinely employed for current peroxide bleaching processes carried out at pH 8.5 - 9.0, but these become ineffective at pH 10 and at higher levels.

Photobleaching of wool is not at present a commercial option Within the very large body of published work on the reactions which wool undergoes when subjected to irradiation from ultra¬ violet (UV) to visible wavelengths some research was devoted to specific bleaching effects when wool was irradiated with blue light. An attempt was made to define possible industrial proces by C. Garrow, E.P. Lhuede and CM. Roxburgh (Textile Inst. and Industry (1971) 9_ 286) based on passing air-dry wool through a very long passage filled with blue illumination but this was neve pursued. Blue light alone entails about 48 hours to achieve a full bleaching effect, and for lesser periods the value of the bleach obtained is outweighed by the cost of energy and equipment Certain reducing agents particularly thioglycollate compound are known to accelerate photobleaching. Further attempts to improve reductant-catalysed photobleaching by manipulating pH and including other additives confirmed that this type of process mainly improves the brightness rather than the whiteness of wool and commercial interest would therefore be limited (W.S. Simps Proc. 8th Int. Wool Textile Res. Conf. , Vol. 4, p279, 1990 pub Wool Research Organisation of NZ).

Experiments by the present inventor were aimed to catalys photobleaching by the addition of a solution of peroxide. Whe these solutions were in all variations between strongly acidic mildly alkaline, improvements in colour were modest or even negative, ie, the wool yellowed rather than bleached.

The only prior disclosure of an experiment of this kind w carried out under near-neutral pH conditions, and concluded bo oxidation and reduction conditions combined with blue radiatio gave a significant bleach on particularly yellow wool (J.L. Ho and R.G.Stewart, Text.Res. J. 3_9 884, 1969).

The present invention requires a combination of three components in order to effect a superior rapid bleach. The components are radiation, oxidant and an appropriate aqueous medium.

When all three components are present within specified limits, the bleach is more rapid than conventional processes a in an optimum form of application will achieve whiteness super to any other process no matter how vigorously applied. Analys of the treated products provides indirect evidence of a unique chemical modification of components of wool (as hereinbefore defined) not seen in other methods of bleaching.

Specifications of the three essential components are: 1. Radiation Source

Blue light (420-450nm) is known to be specially active in promoting photobleaching of wool, whereas UV and near UV light promote the opposite effect of yellowing. There is a gradual transition between bleaching and yellowing as the wavelength is decreased. However, the bleaching effect is much more rapid than yellowing so that wide spectrum radiation such as sunlight will often first induce a bleaching effect within a day or two followed by yellowing after weeks of exposure.

The preferred and optimum light source for the present invention should contain a high proportion of blue wavelengths. However, if the bleaching time is sufficiently short, wide spectrum light sources can be employed if there are advantages of availability or physical form. The rate of photobleaching increases with light intensity. 2. Oxidant

Hydrogen peroxide is the oxidant of most practical importance. However, peroxide is frequently employed in industrial practice in combination with stabilisers or as simple derivatives, typically unstable peroxy acids. Thus organic acids (e.g. formic, acetic) form peroxy acids. Such acids and their soluble salts acts in a similar fashion to the present hydrogen peroxide. Inorganic compounds such a pyrophosphates, borates and silicates are widely used in current bleaching practice to suppress auto-decomposition of peroxide. These variants are all compatible with the presen invention although they are of doubtful merit as their stabilising power is slight at the optimum pH for the proces which is the subject of this invention. The concentration

2 peroxide recommended is approximately 2.5% (w/v) as H O2, between 1% and 10% w/v can be viable depending on how the process is carried out.

Aqueous medium

Photobleaching occurs much more rapidly when the wool bein treated is wet. It is possible to entrain peroxide in woo and induce photobleaching at a faster than normal rate aft the fabric is dry, but the full advantages of the present invention are not realised unless the wool is kept in a we state during most of the photobleaching exposure. A suita wet state includes full immersion in an aqueous liquor, soaked and squeezed as in a pad/mangle, sprayed or coated with aqueous foam.

Wool is well known to absorb nearly 1 m mole/g of alkali a very high pH. It will absorb significant amounts over the range of interest for the current invention, which is pH 9.

- 11.5. The optimum is approximately pH 10.5 ± 0.5, and i this range the common alkaline buffers are only weakly effective. Tri-sodium phosphate has proven in experiments be reasonably satisfactory.

Addition of a metal ion sequestering agent to the aqueous solution is usually a significant benefit. Traces iron, copper or other elements are often present in the wa supply or already on the wool. They can both interfere wi the desired bleaching process and dull the wool colour.

Ethylene diamine tetra-acetic acid (EDTA) and its common salts under several proprietary names is a satisfactory sequestering agent.

Further aspects of the present invention will become appare from the following description and examples. Typical parameters of the process of the invention are:

(a) The raw material can be loose wool, yarn, fabric, carpet or indeed any wool product.

(b) The wool can be pre-treated by wetting in a typical solutio of 2.5% (w/w) peroxide adjusted to pH

10.0 - 11.5, squeezing out and exposure to blue light, typically 20 - 30 minutes.

(c) Alternatively the wool can be fully immersed in a solution in (b) and the whole container irradiated with blue light f a similar time.

(d) Following either (b) or (c) the wool should be rinsed and acidified prior to drying.

(e) The process can include continuous treatment. This would entail impregnation with a solution as in (b) , passage through an apparatus illuminated with blue light, and rinsin and drying as in (d) .

The invention is applicable to wool and other fibrous materials in a physical form which allows adequate penetration of blue light radiation. For some consumer products only the visibl part need to be bleached, resulting in some economies over common industrial practice where the whole batch of fibre is chemically bleached. Typical examples are woolly sheepskins, carpets and rugs. Clumps of loose fibre such as scoured wool should be mechanically opened to form a loose mat or carded batting. Slivers and rovings are also of suitable density and thickness f the process. In any of these forms any uneveness of the bleach eliminated by the blending and mixing during subsequent manufacture into yarns.

Fabrics and felts are also in a form suitable for the bleaching process.

By way of example, two principle modes of operation have proved suitable for effecting the process. In a batch system a parallel array of blue fluorescent tubes was mounted in a horizontal position. The material to be bleached is wet with an aqueous bleaching solution as previously specified and placed under or over the array of lights. Reflectant materials such as polished stainless steel or aluminium sheet surrounding the equipment improves the efficiency of irradiation. After a specified time, usually in the range of 10-30 minutes, the bleached material is removed for rinsing and drying. This meth is particularly suited to rugs and woolly sheepskins.

A continuous system entails a conveyor for loose materials, but fabrics may be drawn from a feedroll through the bleaching zone to a takeup roll. Arrays of lights similar to those for a batch system are mounted in various configurations, but most conveniently as a stack of parallel arrays with the material to treated passing between each pair in succession. Fabric is therefore irradiated from both sides. A conveyor below of mesh construction achieves the same full exposure of loose materials.

A fully continuous system requires the material to be fed through an immersion bath of bleaching solution, a squeeze press to remove excess liquor, through the irradiation equipment, preferrably enclosed with reflectant material and into a rinsing bath, another squeeze press and finally a dryer. Examples 1. Bleaching a sheepskin rug.

A tanned woolly sheepskin was sprayed with a chemical solution so as to wet the outer woolly surface to a depth of about 5cm, then placed about 10cm below with the wet wool facing a parallel array of blue fluorescent tubes. The spray solution comprised the following: 25% w/v Hydrogen peroxide 0.1% Ethylene diamine tetra-acetic acid 1% Tri-sodium phosphate hydrate 0.1% Non-ionic detergent. The solution was adjusted to pH 10.5 with sodium hydroxide. The power consumption of the fluorescent tubes was equivalen to 1 Kw per square metre. Exposure time was 15 minutes. 2. Continuous bleaching of fabrics the photobleaching reactor consisted of four vertical arrays of blue fluorescent tubes with transport rollers mounted above and below the arrays, and the device was enclosed in a polished stainless steel compartment. Undyed wool fabric of

2 250 g/m manufactured in plain weave from 22 micron Merino wool yarns was padded with the solution described in Example

1 and squeezed to 110% liquor pickup based on air-dry weight It was passed continuously through the photobleaching react at various speeds corresponding to 10 to 30 min. exposure time. The first rinse bath contained dilute acetic acid to neutralise the fabric prior to a final water rinse and drying.

Undyed cotton duck fabric was treated in the same manner except that the padding solution was at pH 11.5. 3. Bleaching of loose fibre

The batch bleaching method and bleaching solution as in Example 1 was used to demonstrate the process.

Clean mohair fibre was wet to approximately 120% wet pickup

2 and laid as a random mat of average density 500 gm/m under the photobleaching array for 20 minutes before neuturalisin and rinsing as for the f bric in Example 2. Merino and crossbred fleecewool was bleached in the same manner.

Effect of the Bleach

The extent of the bleach was determined by the New Zealand

Standard Method of Measurement of wool colour. For the CIE

Tristimulus values recorded, (Y-Z) is taken as an index of degre of yellowness. Initial and final treated values of (Y-Z) are listed in the Table below for the materials bleached as describe in Examples 2 and 3. Effect of Treatment on Yellowness (Y-Z) of Various Materials Material Treatment

Fine-wool fabric Continuous 10 min.

Fine-wool fabric Continuous 30 min.

Cotton duck fabric Continuous 30 min.

Mohair fibre Batch 20 min.

Merino fibre Batch 20 min.

Crossbred wool fibre Batch 20 min.

In making these claims it is recognised by the applicants that present wool bleaching procedures have well known constraints. Fibre damage must not exceed certain practical limits or the bleached product is unservicable. The treatment which is the subject of this invention is also shown to meet this practical criteria and create a viable product.

Particular examples of the invention have been described with reference to wool and it is to be appreciated that with minor modifications and variation the invention is applicable to other natural fibres without departing from the scope of the appended claims.

Claims

CLAIMS :

1. A photobleaching process for animal fibres, the process including a combination of radiation, oxidant and an aqueous treatment in which the radiations wavelength is near UV and visible light, the oxidant is peroxide and/or its derivatives and the aqueous medium is an alkaline solution.

2. A photobleaching process as claimed in claim 1 wherein the radiations wavelength contains a high proportion of blue wavelengths of between 420-450nm.

3. A photobleaching process as claimed in claim 1 or claim 2 wherein the concentration of peroxide oxidant is between 1% and 10% w/v.

4. A photobleaching process as claimed in claim 3 wherein the concentration of peroxide is 2.5% w/v.

5. A photobleaching process as claimed in any one of the preceding claims wherein the aqueous medium has a pH of between 9.0 - 11.5.

6. A photobleaching process as claimed in claim 1 wherein the _fi_br_{es are} ρre-treated by wetting in a typical solution of 2.5% (w/w) peroxide adjusted to pH 10.0 - 11.5, squeezing out and exposure to blue light, typically for 20-30 minutes.

7. A photobleaching process as claimed in claim 6 wherein the fibres are fully immersed in a solution and the whole container irradiated with blue light for a similar time.

8. A photobleaching process as claimed in claim 6 or claim 7 wherein the fibres are rinsed and acidified prior to drying.

9. A photobleaching process as claimed in any one of the preceding claims wherein the process is a continuous treatment.

10. A photobleaching process as claimed in claim 9 which entails impregnation with a solution, passage through an apparatus illuminated with blue light, and rinsing and drying.

11. A photobleaching process as claimed in claim 10 in which the apparatus is suitable for effecting the process in a batch process with a parallel array of blue fluorescent tubes mounted in a horizontal position and in which the material to be bleached is wet with an aqueous bleaching solution before being placed under or over the array of lights which are surrounded by reflectant materials such as polished stainless steel or aluminium sheet to thereby improve the efficiency of irradiation so that after a specified time, usually in the range of 10-30 minutes, the bleached material is removed for rinsing and drying.

12. A photobleaching process as claimed in claim 10 wherein the apparatus is suitable for a continuous processing system and includes a conveyor for loose materials or in the case of fabrics they may be drawn from a feedroll through a bleaching zone to a takeup roll, the arrays of lights being mounted in various configurations such as a stack of parallel arrays with the material to be treated passing between each pair in succession so that the fabric is irradiated from both sides.

13. A photobleaching process substantially as hereinbefore described with reference to any one of the Examples.