Dyeing fashionable bright blue shades on cellulosic fibres

1. Abstract

Often people ask about the problems encountered in dyeingbright blue shades on cellulosic substrates, particularly cotton and viscose. Thefollowing question comes from Dyehouses in Peru and Brazil.

"We are having problems dyeing bright blue shades. We can see red areas across and throughout the fabric. Doyou know what could be the problem?"

This article seeks to consider some of the critical issuesinvolved.

2. Full Article

2.1 The Solution

The solution comes from the Dye House Solution as follows

There are many examples of such patchy red areas in dyeingwith reactive bright blue dyes.

However, they have diagnosed different origins to each problem. Each case must be treated on its individual merits. Some clues as to the origin of the problem can be gleaned by circumstances surrounding the observed problem.

2.2 Most Widely Used Dyes

Table 1: Largest Global Dye Sales

2.3 Shade Sensitivities of Bright Blue Reactive Dyes

• The following comments relate to dyes provided by major European or Japanese suppliers. For those dyes sold on the basis of Colour Index number, there can be no guarantee of product quality, constitution, impurities or that they do not suffer additional sensitivities.

• Given the above reservation, a number of sensitivities have been observed:

• pH: Acid sends the shade red. Red patches can be due to uneven softening at acidic pH.
• Temperature: Higher temperature sends the shade redder - so allow fabric to fully condition (or stream calendar) before inspection.
• Alkaline Earth Metals: Ca and Mg (and to a lesser extent Mn) both cause a shade shift to red. (See Case Study 1).
• Gaseous oxides of Nitrogen: Gas fire stenters are notorious for producing such problems in the Dyehouse; gas fired heaters and stoves in fabric and garment storage and vehicle exhaust fumes in loading bays.

• The problem can therefore occur at different stages of wet processing and garment confection and delivery

2.4 Gas Fume Fading of Bright Blue Reactives

• There are internationally recognised colour fastness tests to screen dye selection against sensitivity to gas fume fading.

• However, the conditions in a dyeing operation and levels of oxides of nitrogen contamination can sometimes exceed the test conditions. (See Case Study 2).

• It is possible to observe different effects when nitrogen oxides meet wet and dry dyed fabric. In wet exposure, the nitrogen oxide can be converted to nitrous acid. The resultant shade shift is therefore simply due to pH sensitivity and may be recovered by an alkali scour. This is relatively rare however.

• The more usual contamination is on dry, or drying fabric. In this case, it is believed that the colour change occurs through nitrosofication of an amino group in the dye molecule.

This is demonstrated below for the anthraquinone blue Remazol Br Blue R special (Fig 1)

Fig 1: CI Reactive Blue 19

The degree of fading / shade change can vary according to the state of the fabric. As noted earlier, substrates with an acidic pH. (See Table 2). Dyehouse management, good housekeeping practices and effective (and informed) supervision are therefore important.

Table 2: Gas Fume Fading and pH Sensitivity

Colour Fastness (Effect on Pattern) under different conditions of Test.

CI Reactive Blue 221 is Sumifix Supra Blue BRF. CI Reactive Blue 222 is Sumifix Supra Navy Blue BF. Both are blue dulling components which represent a standard blue and Navy response and are used to indicate increased sensitivity.

2.5 Concluding Remarks

• With most (if not all) bright blue reactive dyes good housekeeping (particularly pH control) is very important. For dyeing application on manually operated machines TSP is often recommended as the fixation alkali. But it must be noted that TSP does have an environmental impact and, in this respect may be said to solve one problem at the expense of creating another. Sodium Acetate / Acetic acid is sometimes recommended as a buffer to control pH effects before softening and or drying and after finishing.

• Remazol Br Blue R Special does have additional level dyeing problems. This is because of the nature of its SERF profile. The offending features are displayed in Fig 2:

o        Low Substantivity (S)

o        High Secondary Exhaustion (E-S)

o        High Reactivity (R).

Fig 2: SERF of Remazol Br Blue R Special

• CI Reactive Blue 19, available as Remazol Br Blue R and Sumifix Br Blue R from major suppliers, is a dye which is widely used because of its:

o        high yield in cloth

o        brightness

o        fastness properties.

• This dye is vinyl sulfone-based, and is used in dyeing in the form of sulphate ethyl sulfone, but with the addition of alkali, it becomes vinyl sulfone and fixes to the fibre by reacting with it.

• Because Blue 19 in the form of sulfate ethyl sulfone has a high solubility and a very low affinity, it displays a low primary exhaustion rate, but in the form of vinyl sulfone, its solubility decreases dramatically and it displays a high affinity, and thus the exhaustion rate increases rapidly. Consequently, under typical dyeing conditions, dyeing advances rapidly after the addition of alkali, giving the use of this dye a high risk of uneven dyeing.

2.6 "A Funny Thing Happened On The way to The Dyehouse"

Circumstances are not always what they appear from a distance (and what we are told by the Customer).

Case Study 1

Production Type: Exhaust Dyeing Cellulosic Knitgoods and Blends

Location: Israel

Customer Report: Unlevel Red patches.

Customer Diagnosis: Dyestuff at fault.

Facts: This same customer reported the same problem in the same month for each of the previous three years.

Inspection of Customer Premises: Dyehouse used water sourced from a well. Problem occurred in the month following an extremely dry period.

Observations: Water level in the Well extremely low. Seasonal occurrence, repeated annually. Extremely high levels of Ca and Mg for relatively limited periods of time, during which many dyes suffered.

Case Study 2

Production Type: Exhaust Dyeing Cellulosic Knitgoods and Blends.

Location: Barcelona

Customer Report: Unlevel Red patches. Problem only occurred with one dye (Procion Blue H-EGN). All other bright blue reactives passed laboratory screening test for gas fume fading.

Customer Diagnosis: Dyestuff at fault.

Facts: On arrival, Barcelona (unusually) cold and covered in snow. Difficult to see anything in Dyehouse due to intense "fog".

Observations: Extractor fan above gas fired stenter not working. All windows closed because of cold conditions. Extremely high build up of nitrogen oxides, well in excess of laboratory gas fume fading test. Dyeings of all available dyestuff options hung on Dyehouse wall. All reduced to a virtual white in relatively short time.

Recommendation: Leave dye selection as it is and (in the short-term) open the windows. Replace extractor fan above stenter. Schedule production so that all bright blue shades dried on other stenter available (oil fired). Ensure that all trucks of wet fabric waiting for drying are effectively covered to exclude exposure to fumes (especially bright blues). All recommendations gratefully accepted.

2.7 Epilogue

Sometimes being right on top of a problem, can lead to restricted vision. And it can happen to any one of us. None of us is immune. There is nothing clever about a "fresh pair of eyes" to look at a problem. But the emphasis on fresh is important.

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