AN IMPRINT OF COLOUR

Printing flax/polyester blends - * a new approach

Brian Hill and George Gray

Work at the University of Ulster has been carried out to investi- gate methods of achieving startling print colour effects. It has been realised that by arranging fibre blends and distribution, pleasing aesthetic results are made possible. In this article Brian Hill and George Gray give a brief account of their research.

The dyeing of fibre blends is a well recognised process and the different effects that can be achieved by judi- cious selection of the fibres and the dyeing method are well documented. Thus solid dyeing effects, reservation effects, tone-in-tone effects and cross- dyed effects can be obtained over a range of fibre blends. One of the methods used to achieve these effects on cellulose/polyester blends is by padding the fabric in a bath containing both reactive and disperse dyes, after which the dyes are fixed by baking. If carefully selected, these types of dyes can also be used in combination in a print paste process. It was this technol- ogy which was used at the University of Ulster to obtain different colour effects via a process of single screen printing

To begin, the colour effects that can be achieved are dependent upon the distribution of the constituent fibres within the yarns. Therefore, an initial investigation was undertaken by the research team to consider how the fibres were distributed within the different types of cellulose/polyester blend yarns.

Blend yarns spun from single or ultimate fibres are intimate fibre mixtures, with each fibre type being regularly distributed within the yarn. Provided that sufficient doublings take place during preparation the yarn should be regular and uniform in cross-section. Processing on a cotton system is an example of such a tech- nique and, therefore, cottodpolyester yarns and yarns spun on a cotton system are regular intimate blends of the two fibres. This intimacy of blend is an inherent property of these yarns and is the reason for their good spinning and weaving performance as well as their in-service properties.

However, this intimacy of blend precludes fabrics made from these types of yarn from exhibiting startling cross-dyed or cross-printed effects.

However, not all fibres occur as single or ultimate fibres. Flax fibres, and other bast fibres, occur naturally in bundles of ultimates held together by gums and waxes. These bundles vary in size from 12 to 40 ultimate fibres. When these fibres are dry spun the bundles do not break down but remain as groups within the spun yarn. Therefore, the intimacy of blend which is achieved during cotton spinning is not found in flax spun yarns. The cross-section of these yarns shows a different fibre distribution to that found in cotton spun yarns (Figure 1).

This grouping of fibres can be utilised to produce colour effects by using the dye selectivity properties of the constituent fibres. The fibre groupings can be accentuated by incorporating short fibres in the blend to produce slubs or by injecting clumps of fibres into the yarn during spinning. The processes involved to produce these effects, which could be described as scintillant, are the result of the combined use of known processes realisable with existing equipment. What is original is the realisation that

by varying the fibre blends and distribution, several aesthetically pleasing effects can be achieved.

Production Each stage of the production process plays a part in the overall colour effects which are achieved and each must be given careful consideration.

Both plain and slub flax/polyester yarns ranging in count from 10 tex to 175 tex were dry spun on ring spinning frames. Across this count range a number of different blend combina- tions were chosen which varied from 15% to 85% polyester. While the entire count and blend range can be used to produce fabrics which exhibit novel scintillant effects, those fabrics made from coarse dry spun yarns exhibit more colour effects and can be pro- duced with very pronounced slub effects. Tests carried out using the blend ranges described above indicated that it is necessary to have at least 30% of a fibre present to make a significant contribution to the differential colour effect. The particular blend is selected on the basis of the effect required together with a consideration of the processability of the yarn. Slub yarns are of great interest because the surface character can be used to produce

A B

Figure 1 Cross-section of intimate (A) and grouped (B) fibre blends

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AN IMPRINT OF COLOUR

Single screen print illustrating cross print effect on dub fabric

colour effects. Yarns with a short slub character are of greater interest than those of a long slub character because when printed these slubs create focal points of colour which can be utilised by the designer.

A limited range of yarns was pro- duced in which the blending of the fibres was kept to a minimum. This was achieved by spinning the yarn from undoubled slivers or by spinning on a long staple machine capable of inject- ing fibres in selected places along the length of the yarn. This provides the potential for gross differential colour effects. The potential to vary the fibre distribution along the length of the yarn in predetermined areas gives great scope to the flat pattern designer.

The fabrics for the initial project were woven with the same yarns in the warp and weft. Only simple woven constructions were produced because it was considered that the colour effects, the yarn characteristics and the applied surface pattern should be the main design elements.

Later developments used different yarns in the warp and weft directions. The yarns were selected so that large differences in blend proportions between warp and weft were obtained. Weaving structures were selected to produce areas where there was a predominance of one yarn. This gave the potential to create larger areas for coloration with differential dyeing. The potential for these fabrics to produce scintillant effects was examined by cross-dyeing the fabrics. Fabrics giving distributional colour effects were selected for printing. However, after a

few trials it was discovered that the distribution and colour effects pro- duced by printing did not match those produced by dyeing. This was the result of the cross-dyeing giving an overall view of the blend proportions whereas the printing was more influ- enced by the fibre distribution on the surface of the fabric. Therefore the potential of these fabrics to produce differential colour effects could only be determined by printing.

NThis project has led to the pro- duction of fabrics which ex- hibit novel effects, with the printed fabrics showing scintillance and the devorke fabrics creating a new effectfor this process.

Fabric processing Fabrics were prepared in the normal manner prior to printing with the flax component being either boiled or bleached. The fabrics were printed with a paste containing both reactive dyes and disperse dyes. A typical recipe contained: 50 parts disperse dye, 50 parts urea, 535 parts water, 320 parts sodium alginate thickener, 10 parts Matexil PA-L (ICI), 10 parts sodium bicarbonate, and 25 parts reactive dye (total 1000 parts).

Only those reactive dyes which gave good colour yield in the presence of sodium bicarbonate were selected so as to minimise any premature hydrolysis of the disperse dye. For those fabrics which are required to have a high light fastness, care was taken in the selection of the reactive dye component. In some cases it was difficult to remove the last traces of disperse dye and where this is essential then the use of Dispersol PC (ICI) dyes overcame this problem. After printing, the fabrics were dried and the dyes fixed by baking. For fabrics containing viscose, fixation should be achieved by high-temperature steam- ing. As this was not readily available, samples were firstly treated in an atmospheric steamer to fix the dye on to the viscose component followed by high-temperature baking to fix the disperse dye component. Comparison made with samples treated commer- cially in a high-temperature steamer indicated that the effects were similar. The fabric was then washed to remove the unfixed dye and chemical assist- ants, treated with hot caustic soda to clear the last traces of disperse dye and then boiled in soap solution. The fabric was then finally treated with a fluores- cent brightener substantive to the cellulose component only.

Colour and design This, the authors considered, was the most important aspect of the project. In the majority of tests a single screen was used to produce the multicolour effects. The most marked effects were achieved by printing with contrasting colours. When plain yarns were used it made no difference which of the hues was applied to which component. However, with slub yarns the most effective results were achieved when the lighter colour was applied to the slub component so that it was high- lighted against the darker background.

The use of a single screen with a dye paste containing the mixture of dye classes produces a two- or three-colour effect on blend or union fabrics; one on the cellulose component, one on the polyester and a combined shade where the fibres are intimately blended. If required, more than one screen can be used or, if desired, colour can be applied to only one of the fibre compo- nents. The choice depends on the desired effect.

Developments of the process One of the new developments of this project was the use of a modified squeegee with which, using this selective printing process, it is possible

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AN IMPRINT OF COLOUR

to achieve up to thirty colours using a single screen. The colours are repro- ducible along the length of the fabric. The squeegee has a rectangular cross- section and is relatively shallow in depth. The cross-sectional area is divided with spacers placed. These dividers are located so that they are flush with the screen mesh. When the squeegee is pulled across the screen the colours are transferred to the fabric giving a striped effect. This effect is somewhat disguised by the mixing of colours from adjacent compartments on the fabric and by the cross-printed effect of the dyes on the fabric. This mixing does not occur in the compart- ments, so the dyes do not become contaminated during the printing operation. The multi-striped effect can be further disguised by careful recon- sideration of the distribution of the applied pattern design on the screen. Here the design is not transferred to the screen as a single repeat, but is redistributed so that those areas which fall in line across the fabric width are relocated. Instead of printing two repeats of the design, the design is

divided so that only one repeat is printed for each pass of the squeegee. This reconsideration of the design image is only necessary when a single screen is used.

Another major development of this project was the production of devoree (burn out) fabrics. These fabrics were produced from spun staple yarns and not the core spun yarns which are normally used. Provided the design takes account of the type of fabric, then the staple polyester fibres, up to 150 mm in length, are sufficiently long to maintain the integrity of the yarn when the flax component is removed. This creates an effect which is entirely different from that found with core spun fabrics. In the latter case the fabric exhibits sheer areas where the continuous filament polyester has been left. In the former the removal of the flax component leaves an area where the yarn bulk is maintained but the density of the fibre within the yarn is reduced. This produces a translucent effect which makes these fabrics very suitable for vertical louver blinds, lamp shades and other products through

MBO brings independence for McConnell Dyeing and Weaving Ltd

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McConnell Dyeing and Weaving Ltd, the Glasgow-based company specialis- ing in the dyeing of textiles and manufacture of towels, has been bought by its senior managers from parent company M R Hadida in a six figure transaction.

buy-out was provided by investment capital group 3i’s Glasgow office.

Leading the MBO team is managing director David Kirkwood. He is supported by production director Del Porchetta and sales director Duncan Martin. The team was advised by Andy Allen of Ernst & Young.

main areas. The dyeing division operates as a commission-based dyer of yarn which is supplied by customers, dyed and stored by McConnell and despatched back to them when required. The yarn has many uses including upholstery fabrics and surgical yarns. McConnell is one of only two or three companies operating this type of business in Scotland.

The towel manufacturing division, now known as Thistletex Towels, weaves high-quality jacquard towels sold in the retail market to the John Lewis Partnership, to the House of

Equity funding for the management

The company specialises in three

Fraser and to the hotel contract market. With many weaving companies closing in the past few years there is virtually no UK-based competition.

The third division, Highland Con- nection, was set up to supply the golf club market with towel division products. Customers include many of the leading golf courses in the UK as well as Harrods and Dunlop.

Managing director David Kirkwood said that the MBO has enabled a local business to be brought back into Scottish ownership. ‘I see good growth prospects for the company - we have the support of our employees and a quality customer base. We are looking forward to running our own business with the support of 3i as a long term investor.’

Kevin Lyon of 3i’s Glasgow office said: ’In the past 18 months we have seen numerous buy-out opportunities arising as a result of the decision by many large parent companies to dispose of their non-core activities. As in the case of McConnell it is often possible for the managers of these divisions to buy them out and continue to run them as independent busi- nesses.’

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which light is transmitted. Colour can be added during printing or the fibre components can be dyed prior to processing with the burn out paste. In order to ensure that the structural integrity of the end product is main- tained during use, the treated fabric was laminated onto paper.

This project has led to the produc- tion of fabrics which exhibit novel effects, with the printed fabrics show- ing scintillance and the devoree fabrics creating a new effect for this process. The process treats design as a complete process from fibre selection through yarn and fabric production to the application of the flat pattern design. It is this combination of technology and design which produces these effects.

Brian Hill and George Gray are senior lecturers, Department of Design in Industry, University of Ulster, York Street, Belfast BT15 IED, UK.

Featured in the November issue:

Cotton Dyeing includes:

Cotton processing and the dreaded PCP

Exhaust application of phthalocyanines to cotton

Naturally coloured cotton

Also:

Reactive dyeing of cotton

at ultra-low liquor ratios

under neutral-fixation conditions

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