TEXTILE FINISHING

Finishing techniques for woollen and worsted fabrics

Bill Parker

The basic principles of finishing both woollen and worsted fabrics have changed very little in 50 years, but the speed and control of many processes have changed beyond recognition in many cases. In this article, Bill Parker looks at the tech- niques in use today.

Woollen finishing is usually carried out 'in house' as one stage in a vertically integrated business on the same premises, whereas worsted finishing is usually carried out by specialist companies of dyers and finishers. The diversity of worsted fabrics means that worsted finishers require a much wider range of machinery than their woollen counterparts.

It is essential that the finisher inspects the fabric before commencing processing to ensure that the fabric is fit to be processed - it is easier to remove heavy loom stains at this stage than trying to remove insoluble residues after finishing. It is also necessary to measure the width and length of the pieces of fabric, usually 60-70 m in length, and weigh each piece so that at intermediate stages of processing the optimum length and width will be obtained to give, ultimately, the correct finished weight in grams per finished metre at the correct width, usually 150 cm.

At this stage, delicate fabrics may require dry-cleaning, either in a drum machine or at open width in a solvent scouring machine if they are prone to rubbing or being permanently creased if cleansed by aqueous scouring methods in rope form.

Worsted fabrics being made from two-fold high twist yarns need to be stabilised (or set) before being subjected to aqueous processes. Setting may be carried out by various methods, the simplest being blowing, where the fabric is wrapped under tension between layers of cotton or cotton/ nylon wrapper onto a perforated steel cylinder (Figure 1). Steam is passed into the cylinder and hence through the fabric and wrapper and subsequently cooled by closing off the steam supply and drawing cold air back through the package by means of a vacuum pump. This process is often applied to wool/ polyester mixtures where the combination of heat and moisture plasticises the fabric and removes spinning and weaving tensions. Subsequent cooling, still under tension, then sets the fabric flat and free from cockling and distortion.

to the atmosphere and consequently the temperature of he fabric cannot exceed about 95"C, the process is known as atmospheric decating or decatising.

adequate stability as in the case of, say, worstedmohair panamas, then high-

Because the blowing machine is open

Where this temperature does not give

Singeing and setting Singeing, where the fabric is passed at high speed over burning gas jets, is another process that can be applied at this stage to remove fibre from the surface of the fabric. Low-pill polyester fibre in wootlpolyester mixture fabrics has now overcome the need for this process, which used to be necessary to prevent pilling in garments made from this type of fabric. However, worsted crepe dress cloths, where a crisp handle is desirable, do benefit from singeing.

pressure decatising may be used. The fabric is again rolled under tension between layers of wrapper onto a perforated steel cylinder and the whole package is placed in an autoclave for steaming where temperatures above 100°C can be obtained. Although much better stability is obtained at these higher temperatures, care must be taken to avoid subjecting wool to them for more than a few seconds because of the risk of yellowing the wool and tendering the fabric.

where the fabric is passed though boiling water and wrapped under tension onto a heavy steel cylinder. The fabric is then reversed onto a second cylinder, again through boiling water, to ensure that each end if the piece of fabric receives equal treatment. The fabric is cooled by passing through cold water under tension onto a removable cylinder and allowed to cool for several hours.

Crabbing is another setting process

Milling and scouring Woollen fabrics, which rarely require setting, commence their finishing processes with scouring and milling. Scouring processes cleanse the fabric, facilitate relaxation shrinkage and with

Wrapper Y 6 Fabric

I Figure 1 Blowing of worsted fabrics, with cotton as cottonhylon wrapper

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TEXTILE FINISHING

a slight milling action consolidate the fabric to a lesser degree than the milling process itself. Milling utilises the DFE of the wool fibre and alternately compressing and relaxing the fabric 'felts' it until the desired consolidation, cover of surface fibre, handle and the dimensions of the fabric are obtained. By varying the tensions on the rollers (for width milling) and on the board (for length milling) the ratio of the loss of width in relation to the loss of length can be controlled.

If an olein lubricant is used in carding and spinning together with sodium carbonate (soda ash) solution solution a soap is formed during milling which lubricates the fabric (soap milling). When previously scoured fabric is to be milled in acid, then solutions of sulphuric, formic or acetic acids may be used to facilitate milling and help prevent running creases being formed whilst the cloth is being processed in rope form.

Whilst milling and scouring can be two separate processes it is more usual today for both processes to be carried out in sequence in a combined scour/ milling machine (Figure 2).

Many worsted and worstedpolyester fabrics do not require milling, but do need a little consolidation in addition to cleansing. This can be carried out on a traditional scouring dolly or on a modern high-speed scouring machine (Figure 3) where the optional use of the corrugated baffle plate on the high- speed ropes of cloth batters the cloth and so softens and improves the handle, particularly of worsted polyester fabrics.

Delicate fabrics of either worsted or

woollen yarns which may be prone to rubbing marks or to running creases if scoured in rope form may be scoured at open width where cleaning and relaxation only can be obtained. The traditional soap and soda ash scouring agents to saponify the spinning and weaving lubricants have been superseded to a large extent by modern detergents.

After final rinsing it is usual to acidify the fabric using a weak acid such as acetic to neutralise any traces of alkali in the fabric and leave it in a slightly acidic state so that subsequent drying and any high-temperature finishing will not degrade the wool fibre. At this stage, it is also usual to add any softening or anti-static agent which may be required. Certain flame- retardant or showerproofing agents can be added at this stage unless it is to be applied at open width by padding as a separate process.

Drying The next stage is to remove most of the mechanically held water in the fabric by centrifuge in the case of rope fabrics or by mangle or vacuum slot for open- width fabrics.

Dyeing is the next process for piece- dyed fabrics, but colour woven fabrics will then be dried.

Woollen and worsted fabrics are usually dried on a multi layer tenter where air is fan-driven through steam- heated radiators through slots and thence through the fabric. High-speed, high-temperature drying on a single- layer stenter would yellow and degrade the wool fibre even in an acid state. Also, a better response to subsequent

finishing processes is obtained if wool has a l0-12% regain, rather than being totally dry. It is much easier to control this regain with 150 m of cloth in the drying machine than 25 m in a single- layer stenter. In addition to drying the cloth, the tenter may have a weft straightening machine in tandem prior to entry in the tenter and the tenter will be set wider than the ultimately required finished width of 150 cm to allow for relaxation in dry finishing. As most dry finishing processes put some tension on the warp of the fabric, it is possible to compensate for this pull by an overfeed device on the tenter, in effect over shrinking the fabric so that the final weight will be correct.

After drying, it is usual to inspect the fabric from a quality control aspect to ensure that the dimensions are correct at that stage and the amount of milling (if applicable) is correct, that the colour is level and on shade in the case of piece dyed fabrics, that there are no stains, rubbing marks or running creases, that there are no outstanding yarn or loom faults to be remedied and that there is no contamination to be removed.Contamination by cellulosic matter can be removed by carbonising, which is usually carried out by specialist commission carbonisers and which entails impregnating the fabric with concentrated sulphuric acid, followed by baking and crushing to remove the carbonised material.

Fabric may be contaminated with other fibres such as polypropylene or polyester and these have to be picked out of the fabric by hand.

of these faults can cause further Finishing fabric containing a number

4 Pneumatic 1

A re n

I

Y " (or milling)lagent Y

Fabric in rope form

Corrugated 'baffle' $

plate 3

Pneumatic 4 pressure A

High pressure jets of scouring liquor

Figure 2 Combined scouring and milling machine Figure 3 High-speed scouring machine

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TEXTILE FINISHING

problems when the fabric is finally checked, as some finishing processes will preclude remedial processes being carried out, or at least remedial work will disturb the final finish.

Raising and cutting Raising is a process more usually applied to woollen fabrics that have been milled at an earlier stage. Raising produces more cover or pile which is laid one way on the fabric. Double-action raising machines are the most common, although single-action machines, where all the raising rollers rotate in the same direction, are used and teasel machines, where natural teasels are mounted on the surface of a large raising roller, are still used for certain high-quality fabrics. With double-action machines, one set of raising rollers will raise the fibre from the fabric while alternate rollers will comb or flatten down the pile. The speed of these rollers can be varied as can the speed of the whole raising barrel in relation to the speed of the cloth and its tension when passing over the barrel. Hence, the type of raising and the amount of cover obtained can be vaned according to the desired finish for that fabric. Raising can be carried out on wet or dry fabrics.

Worsted flannels usually receive some raising although the bulk of worsted fabrics have little or no surface cover when finished.

Cutting, cropping and shearing are different names for the same process: the removal of the surface cover from most worsted fabrics and levelling the cover or pile of woollen fabrics by cutting back over-long wild fibre which has occurred in raising or milling. The fabric, under tension, is first subjected to rotary brushing to raise up any fibre flattened to the surface of the fabric and is passed over the bed of the machine. This presents the fibre to be cut at the best angle for the high-speed rotating cylinder, which has a shearing action against its blade (Figure 4).

The machine may have as many as three cutting parts arranged to cut the back of the fabric once and the face twice in one pass through the machine. Sensors can control the bed electronically so that the height of the cover or pile is constant regardless of any variation in the thickness of the fabric.

The process of raising and cutting, both wet and dry, can be alternated several times if necessary, particularly with fabrics such as billiard cloths.

Steaming, brushing and decatising The next process of steaming and

Figure 4 Brushing and shearing of worsted and woollen fabric surface

brushing is common to most woollen fabrics. This straightens the surface cover so that all the fibres lie parallel and hence it improves the light reflection, giving the fabric some sheen.

The fabric may be blown at this stage and/or rotary pressed, where it is pressed against the surface of a heavy, steam-heated rotating steel cylinder by a highly polished stainless steel plated bed, with a pressure on the bed of several tonnes. The combination of moisture in the cloth and heat and pressure of the press even for only a few seconds imparts a sheen or slip to the fabric.

This press sheen can be made more permanent, to withstand subsequent steaming in garment form, by subjecting the fabric to high-pressure decatising. As mentioned earlier, dimensional stability is given to the fabric during decatising but in addition the sheen will also be stabilised together with a softer, kinder handle.

pressure decatising which is quicker than the batch processing of conventional machines and also it ensures that there is no variation from end to end of a piece of fabric, which does sometimes occur in batch processing.

Although the finish after high- pressure decatising is permanent, it sometimes needs to be topped up by further pressing. This can be carried out by steaming and rotary pressing or by damping and paper pressing, particularly on worsted fabrics.

spray damping (dewing) machine

Ekofast is a continuous form of high-

Damping is usually obtained on a

where the fabric is passed over a fine mist of water droplets and aged for several hours to allow the moisture to be absorbed by the fabric. It is then folded (or plaited) between pre-heated press papers (highly glazed sheets of cardboard) and placed in a hydraulic press where 1-2 tonnes of pressure are applied. After several hours the package is removed from the press, turned (i.e. replaited so that the cloth passing round the edge of the press paper is then in the middle of the press paper, and the whole package is returned to the press. After a total of 24 hours the cloth is removed from the press and the resultant sheen and handle is deemed to be superior to the rotary press finish.

Whilst three of the four important factors of pressing may be the same for both processes, i.e. heat, moisture and pressure, the fourth factor (time) is merely seconds for the rotary press, but 24 hours for the paper press.

A final check is then made to ensure that the quality standards set for that particular fabric have been met, i.e. colour, finish, handle, dimensions, weight and outstanding blemishes or stains, the fabric is ready for despatch to the customer.

Bill Parker spent his working life in the wool industry. He retired as finishing manager at Joshua Ellis & Co. Ltd, Dewsbury, in 1992.

384 JSDC VOLUME 109 DECEMBER 1993