Party in the weave lab

Bizarrely, at the Textile School, there is an annual party in the weaving lab. It was one of the funnest nights i've had in Boras, and one of the strangest environments for a party, being surrounded by looms and yarns...

Dyeing, Printing and Finishing : the treatment of a chosen garment

The textile product that I am concerned with is a piece of outerwear and its primary specifications are lightweight and warmth. It is aimed at the 'active wear' market, so it is not intended for high energy sporting activities, but certainly it is to be used for light exercise and everyday regular activity. Another very important aspect of designing outerwear is colour fastness in light and when wet. Because this is a domestic product, it is important that the colour does not fade from washing it at home. It should not fade in the sunlight either so the dyeing process needs to be considered well. On the label there is a sort of marketing 'catchphrase', which reads 'Authentic Construction for Timeless Apparel', so this indicates that it is supposed to be a very durable piece of clothing.

This garment is of particular interest because it is a combination of various textile processes- the main body outer layer and the outer of the sleeves are all clearly knitted, the main body inner layer is a sheared fabric so it is very insulating, the sleeves' inner layer is fleece, and the neck toggles and trim are woven. So it is clear just from looking at this garment that a lot of different machines have been involved in its production.

So I have established that the shell is a knitted fabric. As this is from a huge company presumably producing a vast quantity of this particular fabric, it will be knitting on an industrial circular knitting machine, as this is the fastest way of producing jersey material. The backside of the shell is fleecy material; one side has been heavily brushed or raised to achieve this effect. I will discuss this in more detail later.

On looking at the label I see that the shell is '70% cotton, 30% polyester'. Therefore the fabric will be mercerized to improve the quality and increase the colour yield. Even though this is a blend, the wetting of cotton results in shrinkage of the material and thus it must be kept under tension to avoid a crinkled and wooly appearance. This treatment is carried out using caustic soda to control the contraction and the swelling of the fibres; during the mercerizing process these fibres become translucent and increase their tensile strength, but reduce their flexing resistance and torsional (ability to ‘twist’) strength. The caustic soda in this case will be at 28°-30° Bé. The bean like section of the fibre becomes at first elliptic and then circular, thus allowing for a better light reflection and in turn increasing the overall lustre. On tubular knitted goods, there is a slightly different road to mercerizing as with woven fabrics. Because knitted goods have slightly more affinity to lose shape, the fabric is wetted and then left in a padding mangle. The withdrawal of the fabric in width direction is controlled by means of an adjustable ring spreader, ‘slowing down’ the fabric before the final squeezing controls shrinkage in length direction. The sodium hydroxide concentration is brought down to approximately 4° Bé by means of circular showers. The fabric is then washed, neutralized and rinsed.

A bleaching treatment will also be carried out to eliminate any impurity and obtain a pure white tone; it prepares substrates for dyes and strips undesired tone variations. As the material contains cotton this is necessary as cotton and natural fibres are more prevalent to tone variation. Cellulosic fibres such as cotton require the bleaching agents sodium hypochlorite or hydrogen peroxide. They both require the addition of sodium hydroxide in the bleaching liquor, which turns it alkaline by ‘charging’ the formation of the bleaching ion, which with sodium hypochlorite is the hypochlorite ion and in the later it is the perhydroxyl ion. Continuous bleaching on a knitting product such as this can be carried out using a J-box. The bleaching products are applied by means of suitably positioned padding mangles; the fabric is introduced in the machine, where it remains for the time necessary to complete the bleaching process.

Now the material is ready for dyeing. It is a large company making a large amount of this material so the process should be continuous, so that it is more efficient. Pad dyeing is the method used for continuous or semi-continuous systems. It is carried out using mechanical means (padding) as opposed to exhaust dyeing (where the material is simply immersed in the dye until it has sufficiently penetrated into the fibre and fixed). Very reduced water quantities are required, resulting in lower energy consumption. For dye application the fabric is attached to spreading and stretching units, which prevent the formation of creases (which could prevent homogenous dyeing and fixing). The fabric then goes via guide cylinders into an ‘impregnation vat’ containing the dyestuffs, and finally to heavy rollers which squeeze out the excess liquor. This type of full width dyeing is also known as ‘Foulard dyeing’. It is important that the fabric feeding must be constant. The temperature, speed, pressure and pH are all controlled automatically.                                               

After this dye impregnation, the whole mangle can be wrapped in plastic and rotated for 8-20 hours. Then the unfixed dyes are all washed off in a trough with chemicals. The next part of this continuous dyeing machine involves immersing the fabric in a steam chamber to fix the dyestuffs to the cellulosics. The fabric is washed again to get rid of any final unfixed dye.

It is important to note that you can also dye a cotton/polyester blend fabric with both reactive and disperse dyes. If this is the case, then in the dyeing range the natural fibre needs a reaction chamber with steam, and the synthetic fibre needs a heat chamber for fixing (Pad-steam/Thermofix).

The fabric is then dried. As I mentioned earlier, the backside of the fabric is fleecy, and therefore it needs to be given the effect of a softer hand. This can be done with an old technique known as raising. Before passing through the raising machine it is advisable to treat the fabric with softening-lubricating agents so that the mechanical stress does not damage the fabric. Raising creates a fuzzy surface by pulling the fibre ends out of the yarns with sharp metal ‘hooks’ that scrape the fabric surface. There are needles attached to the hooks at 45°, and these are spiral-wound on the raising rollers. There is usually more than one roller, and they will alternate between one with hooks facing the direction of the fabric feed (pile roller) and one with hooks in the opposite direction (counterpile roller). The final raising effect can be adjusted depending on fabric tension and the speed and the roller rotation direction.                                                                                                                        

Now the material for the shell of the garment is ready. This is the main material and it makes up the sleeve and hood as well as the outer layer. It is dyed using either only disperse or a combination of disperse and reactive dyes. It has a soft fleecy lining that provides warmth due to the insulating air cells in the nap.

The inner of the garment is made from 100% polyester. First of all, if you look closely, you can quickly identify that it is knitted. But the interesting thing is that it has a sort of false fur effect and it very soft and very warm. This material will have a slightly different range of processes carried out on it, as it is completely synthetic. For one it is probably knitted on a flat bed electronic knitting machine rather than a circular knitting machine, as it has to be made with big loops for a shearing process later on. It is not necessary to desize knitted fabric (it is knitted on flat-bed electronic so it is not warp knitted) so the pretreatment of this synthetic begins with scouring. The scouring process applied to man-made fibres removes oils, lubricants, anti-static substances, dust and dirt and can be carried out on fabrics or yarns. It is carried out by means of surfactants, detergents and emulsifying agents.

   

Next is the heat setting process; this is frequently applied to scoured fabrics. This process produces excellent dimensional stability and good crease-proof properties. The fabric goes into a machine known as a stenter by way of needles on chains holding the selvedges. Dry heated air circulates in the stenter and is usually at around 200°. With polyester the minimum temp. the stenter can be at is 170° and the maximum is 210°. It needs to be exposed to this temperature for between 15 and 50 seconds.                                    

After heat setting, there will be the decortication process. This treatment is specifically for polyester and it is aimed at providing a silky-smooth hand to the garment, a lustrous effect and an enhanced drape. There are different machines that can be used for the open-width process: jiggers or beam dyeing machines (batch systems), or special tensionless open-width continuous machines.                                                                                                                         

The material is cream coloured and very bright so it will have been treated with optical whiteners before dyeing. These are essentially dyes which absord UV-rays and reemit them within the visible spectrum. A surface containing Fluorescent Whitening Agents (FWA) can emit more than the total amount of daylight that falls on it, giving an intensely bright white. This prepares the material well for dyeing although sometimes no dyeing is required after this stage. The material is synthetic so it will be perfect to use disperse dyes. These have the best fastness for polyester. They are insoluble in water and nonionic but when a combination of heat and water are added they can begin to fix to the material. Pad-Termofix is the term given to dyeing of full width polyester fabric. The same process as before is carried out in the Foulard and afterwards dry air is heated to about 200° in a stenter to fix it.

Finally the most noticeable finishing effect for the inner material. The material was knitted with big loops on one side. These will be cut and this is known as shearing. Helical blades rotate at very high speed and cut the hairy surface. The height of the ‘hair’ can be controlled. In this jumper it is quite long so very thermal. The material is fed through a machine with a cutting bed and also a rotating cutter. There will usually be some kind of vacuum to get rid of loose fibres.

So now the outer layer material which is polyester/cotton blend and the hairy inner material layer are both ready to be used. There are fully fashion machines that will put the garments together almost entirely. All of the demands of the product have been met: the disperse dyes are very good combined with polyester and the fastness will be excellent, the garment is very warm from the fleecy and the hairy shearing effect, and it does not have any affinity to shrink or change form.