NZ237321A - Bleaching process for natural fibres using electromagnetic radiation and alkaline peroxide oxidant - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £37321 237 321 Patents Form 5 .ntupivii^ J5 ii-.n'iinn'i' PATENTS ACT 1953 » 5 MAR 1992 COMPLETE SPECIFICATION After Provisional i ; \ ::1 No. 237203 2S>-33IX 5 March 1991 PHOTOBLEACHING PROCESS FOR NATURAL FIBRES WE, WOOL RESEARCH ORGANISATION OF NEW ZEALAND (INC.) of Springs Road, Lincoln, Canterbury, New Zealand being a Society Incorporated in New Zealand under the Incorporated Societies Act 1909, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement 237J2 The subject of the invention is a process of bleaching wool, other animal fibres, silk and cellulose fibres such as cotton. The invention combines two fours of bleaching, chemical and photo-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 previously possible process using current bleaching practices. To simplify the description of the process the term "wool" as is used herein represents the generic class of keratin fibres such as all animal fibres and silk.

The bleaching of wool has been the subject of a large number 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 effect is measured is given in the section below describing some experimental results.

Hydrogen peroxide (hereafter "peroxide") is the most common bleaching agent for wool. Many processes have been described for peroxide at various levels of concentration, pH, time and temperature, in conjunction with buffers, stabilisers and detergents. Of special relevance to the present invention is pioneering research by A.L. Smith and M. Harris (American Dyestuff Reporter (1936) 25 180P) whereby alkaline conditions up to pH 11 were tested and shown not to be viable on the grounds of chemical damage to the wool. u I ■' 237321 progressive factor, but experimentation over intervening years has confirmed that alkaline conditions above pH 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°C. 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 process by C. Garrow, E.P. Lhuede and C.M. 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 never 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 compounds are known to accelerate photobleaching. Further _ , ~K' OFFICE I 5 MAR 1392 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. Simpson, Proc. 8th Int. Wool Textile Res. Conf., Vol. 4, p279, 1990 pub. by Wool Research Organisation of NZ).

Experiments by the present inventor were aimed to catalyse photobleaching by the addition of a solution of peroxide. When these solutions were in all variations between strongly acidic and 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 was carried out under near-neutral pH conditions, and concluded both oxidation and reduction conditions combined with blue radiation gave a significant bleach on particularly yellow wool (J.L. Hoare and R.G.Stewart, Text.Res. J. 39^ 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 and in an optimum form of application will achieve whiteness superior to any other process no matter how vigorously 4 : ■ OFFICE .ilJ I KJ f~ ± applied. Analysis 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. m 1992 deceived 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 present invention although they are of doubtful merit as their stabilising power is slight at the optimum pH for the process which is the subject of this invention. The concentration of peroxide recommended is approximately 2.5% (w/v) as 2 H C>2/ but 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 being treated is wet. It is possible to entrain peroxide in wool and induce photobleaching at a faster than normal rate after the fabric is dry, but the full advantages of the present invention are not realised unless the wool is kept in a wet state during most of the photobleaching exposure. A suitable 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 at very high pH. It will absorb significant amounts over the range of interest for the current invention, 237321" which 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.

Addition of a metal ion sequestering agent to the aqueous solution is usually a significant benefit. Traces of iron, copper or other elements are often present in the water supply or already on the wool. They can both interfere with 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 apparent 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 solution of 2.5% (w/w) peroxide adjusted to pH .0 - 11.5, squeezing out and exposure to blue light, typically up to substantially 30 minutes. (c) Alternatively the wool can be fully immersed in a solution as in (b) and the whole container irradiated with blue light. There is no defined end-point for this. However, bleaching is rapid at first, and is very slow after exposure beyond one hour. (d) Following either (b) or (c) the wool should be rinsed and acidified prior to drying. , ; , - 7 - ' 23752 (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 rinsing and drying as in (d).

The invention is applicable to wool and other fibrous materials in a physical form which allows adeguate penetration of blue light radiation. For some consumer products only the visible 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 for the process. In any of these forms any unevenness of the bleach is 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 principal 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 o O '' < * * d O . K).i± irradiation. After a specified time, usually in the range of 10-30 minutes, the bleached material is removed for rinsing and drying. This method 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 be 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 ecjuipment, 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. i \ r— t * ( ' * J • ^ ' I tj I u -L 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 equivalent 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. 2 Undyed wool fabric of 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 reactor 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 - 10 - ' . f£NT OFFICE -5 MAR 1992 RECEIVED 237321 Example 1 was used to demonstrate the process.

Clean mohair fibre was wet to approximately 120% wet pickup and laid as a random mat of average density 500 2 gm/m under the photobleaching array for 20 minutes before neuturalising and rinsing as for the fabric 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 degree of yellowness. Initial and final treated values of (Y-Z) are listed in the Table below for the materials bleached as described in Examples 2 and 3.

Effect of Treatment on Yellowness (Y-Z) of Various Materials Material Treatment (Y-Z) Initial (Y-Z) Final Fine-wool fabric Continuous min. 1.3 -3.0 Fine-wool fabric Continuous min. 1.3 -4.2 Fine-wool fabric Continuous min. 1.3 -5.0 Cotton duck fabric Continuous min. 2.5 -5.2 Mohair fibre Batch 20 min. 0.5 -4.9 Merino fibre Batch 20 min. 0.3 -6.8 Crossbred wool fibre Batch 20 min. 3.4 -2.8 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 - 11 - ...

I. PATENT OFFICE -5 MAR 1992 RECEIVED 237321 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. - 12 - ' - - •<-. rv, i: r OFFICE -5 MAS 1992 237321

Claims (14)

WHAT WE CLAIM IS:

1. A photobleaching process for natural fibres, the process including a combination of electromagnetic radiation containing a high proportion of blue light, oxidant ana an aqueous treatment in which, the radiation's wavelength is near UV or visible light, wherein the oxidant is hydrogen peroxide and/or its derivatives and the aqueous medium is an alkaline solution which has a pH of between 9.0 ana 11.5.

2. A photobleaching process as claimed in claim 1 wherein the radiation's 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 oxidant is 2.5% (w/v).

5. A photobleaching process as claimed in claim 1 wherein the natural fibre is wool ana wherein the wool is pre-treatea by wetting in a solution in the range 1.0% to 10.0% (w/v) peroxide oxidant adjusted to pH 10.0 - 11.5, squeezing it out: and exposing it to blue light, for up to substantially 30 minutes.

6. A photobleaching process as claimed in claim 1 wherein the wool is fully immersed in a solution in the range 1.0% to 10.0% (w/v) peroxide oxidant adjusted to pH 10.0 0 11.5 and the wool and the solution is irradiated with blue light. . - •• - 13 - 237

7. A photobleaching process as claimed in claim 5 wherein the rate of photobleaching is rapid initially, and is very slow after the container is irradiated for more than substantially one hour.

3. A photobleaching process as claimed in claim 5 or claim 5 wherein the wool is rinsed ana acidified prior to drying. 3.

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 in the range i.0% to 10.0% (w/v) hydrogen peroxide adjusted to pH 10.0 to 11.5, passage through an apparatus illuminated with blue light, and rinsing and drying for a period up to one hour.

11. A photobleaching process as claimed in any of claims 1 to 8 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 ana 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 to thereby improve the efficiency of irradiation so that after a specified time, in the range of 10-30 minutes, the bleached material is removed for rinsing and drying.

12. A photobleaching process as claimed in claim 11 wherein the reflectant materials are polished stainless steel or aluminium sheets. - 14 - 23732

13. A photobleaching process as claimed in claim 10 wherein the apparatus is suitable for a continuous processing system ana 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 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.

14. A photobleaching process substantially as hereinbefore described with reference to any one of the Examples. WOOL RESEARCH ORGANISATION OF NEW ZEALAND INC Bv Their Attorneys BALDWIN SON & CAREY ^ 5 SKP 1994 15