RIGHT-FIRST-TIME

R@t-first-time dyeing-the dye manufacturer’s role

Keith Parton

The manufacturer of dyes can help the dye user to achieve right- first-time dyeings in a number of ways. Keith Parton looks at some aspects of the manufacturer’s role in this area.

The textile chain consists of a number of operations which follow a definite route, depicted in Figure 1. The process begins with the forecaster who, up to two years in advance, innovates with new fabrics, designs or finishes and, not least, colour palettes. Much later, the selector who represents a given chainstore or fashion house decides which styles or colounvays will be manufactured. The decision is left as late as possible in order to minimise the risk. This means that the processors, including the dyer and the garment manufacturer, are under constant pressure to reduce their lead times to the retailer.

This manufacturing route is well established. The links between each operator are strong (at least until something goes wrong) and close cooperation and partnership between each stage are vital.

this chain at various stages, and right- first-time dyeing can contribute to the partnership. There are ranges of products

The dye manufacturer interlinks with

that have been developed through research and development, and the manufacturers continue to develop application methods for new and existing products.

Dye manufacture is a complex global business, and it can take up to six months to manufacture a dye. This includes an allowance for ordering and manufacturing intermediates, shipping them where necessary and then synthesising the final product. This time delay means that the dyestuff manufacturer has to rely on the colour forecasters to ensure that the right product is available at the right time.

There are also links between manufacturers and retailers, but the strongest link is at the point of sale with the processor, where process recommendations and recipe formulations are supplied by the manufacturer. The aim of the manufacturer has to be to supply the processor with the right product for the job.

Dyestuff - Process manufacture recommendations

- Recipe formulation

- Application development

Figure 1 The textile chain

In order to achieve right-first-timc dyeing, the dyer has to consider a number of technical factors:

Fabric preparation Fabric contamination (attributable to poor housekeeping or poor control of raw materials such a5 water) Accuracy of dye recipe formulation Accuracy of dye standardisation Accuracy of calibration: balances, process controllers and so on Reproducibility of recipe (no matter how accurately the above variables arc, controlled, if the recipe is sensihve to dyebath variations, then it is unlikely to be reproducible).

This paper will consider the accuracy ot dye standardisation and technical factors affecting the reproducibility of a recipe.

Dye standardisation The average size of a manufacturing batch for a dyestuff is around 10 tonnes. The dye synthesis is monitored from the addition of the quality-tested intermediates through to the unfinished product. The yield is typically? 10% of the average yield for that particular product. At this stage, the unfinished product (known as conc. or presscake) contains a high number of contaminants, mainly water. For example, a product such as CI Acid Red 57, with a selling concentration of 100% or 200% quality, has an average presscake concentration of 260%.

The presscake is dried and diluted with a suitable diluent such as common salt, Glauber’s salt or dextrine, to the required selling strength.

In theory, it should be possible at this stage to standardise the product accurately to the desired strength, but in practice the actual strength attained is affected by accuracy of standardisation, the homogeneous nature of the mix of product and diluent and the moisturt,

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RIGHT-FIRST-TIME

content of the product. In fact, it is generally accepted that dyestuffs are manufactured to a tolerance of k 2.5%.

A strength deviation of 2% will give a colour difference value of approximately A€ 0.4 (strength). This deviation for a single product is acceptable, but where the product is used as part of a mixture, the knock-on effect between the two colours can give a colour difference value up to AE 0.8 (hue).

A A€ value of 0.8 that is mainly attributed to a difference in hue will hinder the prospect of a guaranteed right-first-time dyeing process. Further, in some circumstances, a dyestuff can gain up to 1-2% moisture during storage under certain atmospheric conditions.

Dye powders are more prone to moisture adsorption than liquid or granular forms. Liquid versions are also prone to settlement, and granular forms are the preferred compromise.

To overcome the influences associated with dye standardisation and the nature of the physical form on the ability to achieve right-first-time dyeing, it is recommended that it is necessary to determine nwre accurately the strength of each dye component. This information is available at the time of dye manufacture and it is perfectly feasible for a dyehouse to make an allowance for the strength of a particular dyestuff.

This information is available from the dyestuff manufacturer in the form of a quality assurance certificate of analysis. An important requirement, if right-first- time dyeing is to be achieved, is to allow for these small strength discrepancies in all laboratory and bulk processes.

Reproducibility All textile substrates are dyed with their own individual class of dye. Within each class, there are sub-classes which offer their own individual benefits. The type of

dye selected is determined by the nature of the fibre to be dyed, and the processing and in-service fastness requirements. Wool fabric, for example, is typically dyed with levelling acid dyes, to ensure that there is adequate migration to achieve a level dyeing.

Selecting dye from one particulare sub- class does not guarantee that each component will behave in the same way, and hence does not guarantee reproducibility. This can be emphasised by examining the migration and build-up properties of individual dyes.

The reproducibility of a recipe depends on the following factors.

Accuracy of the recipe 0 The behaviour of the wool ground @ The compatibility of the components

in the recipe @ The pH sensitivity of the components @ The temperature sensitivity of the

components.

Accuracy of recipe This is a fundamental requirement to the success of any dyehouse, and is determined by the quality of the matchings produced by the dyehouse laboratory.

Behaviour of wool ground Wool is unique amongst textile fibres in its ability to change shade during processing. The change in shade of a substrate is most noticeable in pale bright shades. The use of reductive dyebath bleaching products can minimise this effect, but still if the process is prolonged, it becomes more difficult to achieve the desired shade.

formulated in the dyehouse laboratory are prepared on the correct substrate.

Compatibility of recipes If a range of milling acid and metal-

Also, it is critical that all recipes

complex dyes is considered, the rate of exhaustion of each dyestuff can be considered. This rate is expressed as a CV value. Dyestuffs with different CV values will exhaust at differing rates, and the possibility of good reproducibility is reduced. Dyes with identical CV values build up on-tone and are more reproducible.

pH and temperature sensitivity The need to control pH accurately during all dyeing operations is well known. With certain dye classes, the control of temperature, including the rate of cooling, have to be specified.

greatly with both pH and cooling temperature. Dyes such as those in the Sandolan MF range are more robust in this respect.

To illustrate the effect of time, pH and temperature variations, the effect of shade variation of selected dye recipes in terms of colour difference is looked at. A shorter dyeing time is critical in improving fibre quality.

The benefits of achieving the right shade first time are enormous. They can be summarised as

Reduction in water consumption Reduction in energy consumption Time and colour saving Improved fibre quality.

The yield of levelling acid dyes varies

Keith Parton is laboratory manager of the textile dyestuff laboratory, Sandoz Chemicals UK, Horsforth, Leeds.

In the February issue of the Journal, a special feature on:

Management including articles on:

People development, by Kevin Hull (personnel director, Coats Viyella Yarns &

Efficient innovation -the key to success, by John Hansford (Zeneca Colours) Fabrics Division)

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