From Lab to Bulk

Poor shade reproducibility is one of the main causes of loss in productivity and profits. It frequently occurs when a shade does not repeat properly and requires corrective action such as dye or chemical additions, extra run time, boiling down, stripping, re-dyeing and/or over-dyeing. Corrective/repair procedures require extra time and processing, increasing the risk of physical damage. Practices such as stripping or additions, increase also the risk of uneven dyeing and bath instability. All this not only adds to the cost of production but also causes loss in the productivity and profits. Just a small corrective addition may entail up to 10% increase in the cost, a 20% decrease in productivity and slash the profits to about a half as compared to the right first time dyeing.

There are three key areas where a good degree of reproducibility is a pre-requisite to avoid the extra cost of dyeing, the loss in the productivity and, thereby, the loss in the profits.

1. A recipe should give the same shade, every time it is repeated in the lab.
2. A recipe developed in the lab should give the same shade in the bulk dyeings as it gives in the laboratory dyeings.
3. A recipe should give same shade; all the time it is repeated in the bulk dyeings.

In order to attain the desired degree of reproducibility, there are quite a large number of factors which must be taken into account, thoroughly observed and carefully controlled. It might be convenient to describe these factors with reference to “materials and inputs”, “machinery and equipment”, “process conditions” and “methods and practices”

Most common discrepancies between lab and bulk

• Bulk works on greige weight and laboratory on prepared weight.
• Fabric may not be the same; often matching will have been carried out six months earlier.
• The process is likely to be different e.g., no alkali dosing in the laboratory.
• Dyes and chemicals used in the laboratory are often not representative of those in the bulk.
• Liquor ratio is often different.

Some important points be checked for lab to bulk reproducibility

• Is the liquor ratio in the laboratory the same as in the dyehouse?
• Is the same substrate being used?
• Is the same salt quality being employed?
• Is the soda as quality acceptable?
• Is the lab washing off representative of the bulk?
• Is the temperature indicator on the dyeing machine and the lab equipment accurate?
• Is the water of same quality?
• Has the caustic soda solution the same concentration?

Some recommendations for dyeing

• Storage, weighing and mixing of dyes should be in cool dry area. Most dyes in powder form are hygroscopic; the moisture content will vary from day to day depending on the ambient relative humidity. Hence when the same amount of dye is weighed out on different days, the moisture content can cause a drastic variation in actual weight of dye. In this way an unacceptable color difference may occur during dyeing.
• Proper ventilation and air circulation are important for worker protection and also to prevent air-borne particle from flying or drafting to area where they might deposit in in-process goods.  
• Reactive dyes tend to degrade during storage. All drums of dyes should be dated when opened. Any drums which are nearing the end of their shelf-life should be retested using standard raw material quality control procedure.
• It is advantageous to weigh the dye on printing scales for a permanent record of amounts, and to dip the corner of a piece of filter paper in the final paste for diagnostic chromatography test later, if necessary.
• If water is hard, it should be softened before pasting up the dye.
• If chlorine is present in the water, 1g/1 of the thiosulphate should be added.
•  If iron and copper are present in water, water treatment, not chelates, should be employed.