1. Dyeing and Printing

2   Finishing

                     Dyeing with reactive dyes

Reactive dyeing of silk

Q:

We are considering a switch in methods of silk yarn dyeing, from fast-color dyeing with metal complex dyestuff to one that uses reactive dyestuff. What methods are there that use reactive dyestuff and what are the points to keep in mind?

A:

Because of a circumstance that chemical structures originally developed for acid dyestuff are often diverted as chromophore in reactive dyestuff, there are many reactive dye products that display a high degree of exhaustion on silk under acidic dyeing conditions. However, when using reactive dyestuff, setting of appropriate dyeing conditions for silk is necessary as it is important to ensure fast fixation through a chemical reaction with the fiber. The reaction that takes place between reactive dyestuff and silk fiber is similar to that of hydroxyl groups in cellulosic fiber and is believed to be a chemical bonding of the dyestuff's reactive groups with the hydroxyl groups of oxyamino acid, such as tyrosine and serine, contained in silk protein. In contrast to this, the reactive dyeing of wool, while being another kind of animal fiber, will require different conditions as the reaction is different in that it is with the amino terminal group of wool protein. The following is an example of a silk dyeing recipe using Al Amin selected dyes.   The most important point to remember is that the reactive dyeing of silk involves alkaline treatment and is therefore prone to problems such as hardening of fabric hand and the generation of friction marks. As the enquiry here concerns yarn dyeing, the generation of friction marks is less likely, however, it should be noted that the reserved use of alkali, as little as 3g/L in the recipe above, is still preferred. The dyeing temperature is set to 75°C on the premise that as little as 3g/L of soda ash is used. While 60°C is the temperature required for the case of using as much as 10g/L, dyeing at 75°C with less alkali is recommended in respect to the resultant fabric hand. The following are examples of dyestuffs that exhibit good applicability to silk in terms of dyeability and fastness.

Q:

Low-priced, non-traditional manufacturers' disperse dyes have increasingly become available recently and since the quality of such products seems to have improved considerably, we use a selection that has proved by a laboratory test to be free of trouble with color and fastness. However, concern remains with dispersibility, perhaps. Please give us some handling instructions with regard to disperse dyestuff's dispersibility.

A:

Even though traditional dyestuff manufacturers in Japan and Western Europe have been building up with meticulous care technological know-how for the improved dispersibility and dispersion stability of disperse dyestuff, there is a limit to how easy, even and stable a dispersibility can be achieved in an insoluble dyestuff product. While dyestuff synthesis technology for the increasing number of disperse dyestuffs manufactured in developing countries has reached a considerable level, dispersibility appears to fall short of being satisfactory in many products, because of circumstances that technologies required for commercialization of dyestuff including dispersing technology are underdeveloped. Moreover, the dispersibility and dispersion stability of disperse dyestuff may also suffer from deterioration during practical use in dyeing plants if what we call 'inappropriate conditions' are used in the preparation of dye solution or in dyeing procedure. As a result, trouble is often caused in the exhaustion method such as uneven dyeing due to aggregation of dye particles or deterioration in fastness, and in the continuous method, trouble with specking due to dye aggregation or the contamination of coarse dyestuff particles. The following are examples of trouble caused by insufficient preparation of dye solution. The generation of dye clots or secondary aggregation caused by insufficient preparation of dye solution when using powder dyestuff Specking due to poor dispersion of liquid dyestuff that has been dried and solidified during storage Dyestuff aggregation and resultant dye specks due to the addition of auxiliaries, such as a de-foaming agent or a thickener, to a highly-concentrated dye solution Dye specks due to the aggregation of thickeners, caused by the use of hard water or addition of acid Of those listed above, 1 and 2 are examples of trouble caused often depending on how dye solutions are prepared, or more specifically on how well-stirred they are. Careful handling is also advised for 3 and 4. While powder dyestuffs usually become dispersed down to primary particles as the preparation of dye solution involves stirring, any dyestuff particles remaining not so far dispersed due to inadequate stirring may cause, for instance, the generation of dye specks in continuous dyeing. Since insufficient dispersibility in disperse dyestuff at ambient temperatures can often be seen as problematic particularly in printing and continuous dyeing, extreme care is required especially against dye speck generation which can lead to mass faulty dyeing results. In order to obviate the occurrence of such trouble and to carry out dyeing with reduced risks, the filtration of dyestuff solution before dyeing is recommended, using an appropriate device. As for the method of filtration, while fabric is a common filtering material, the use of a synthetic resin type sponge filter is also effective. Since the dispersibility of some non-traditional manufacturers' disperse dye products can vary between lots, a dispersibility test is recommended before the use of each lot. In addition, preliminary filtration using cotton calico will be effective as a countermeasure against cases of adulteration with fragments of glass beads used to make dyestuff dispersible during manufacturing, or with insoluble substances such as rust, which are found in some of the said products, apart from trouble with dispersibility.

Colorfastness

Influence of peroxide-based washing on reactive dyestuff

Q:

We have been requested to provide not the usual washing fastness but a high level of fastness to peroxide-based washing in dyed cotton goods for export to Europe. What are the precautions to bear in mind in order to pass fastness tests when using reactive dyestuff in dyeing?

A:

Fastness to peroxide-based washing concerns fastness to washing with the use of detergents which contain oxidizing components such as sodium percarbonate or sodium perborate. These detergents make possible bleaching without color alteration and are thus effective with prints in which both colored and blank areas are present, because their oxygen-based oxidizing component produces relatively mild oxidizing effects and hardly interacts with the chromophore of the dyestuff. Detergents that contain percarbonate-based oxidants are increasing mainly in Europe, and so are cases of problems arising in the washing fastness concerned. Due attention has accordingly been drawn, and compatible testing methods have also been established. The following shows ISO conditions for testing fastness to peroxide-based washing.   ISO 105 ISO 105-C09 C06/"2" C08 with TAED with SNOBS Washing agent ECE test detergent (B) with phosphate 4g/L ECE test detergent (A) without phosphate 4g/L ECE test detergent (B) without phosphate 10g/L AATCC test detergent without phosphate 12g/L Sodium percarbonate Tetrahydrate 1g/L Tetrahydrate 1g/L Tetrahydrate 12g/L Monohydrate 3g/L Activator - TAED 0.15g/L TAED 1.8g/L TAED 4g/L pH 10.5 10.3 9.8 9.5 Liquor ratio 25:1/75:1 50:1 100:1 100:1 Temperature 60°C 60°C 60°C 60°C Time 30 min 30 min 30 min 30 min Since resistance to oxidation in reactive-dyed goods varies according to the kind of reactive group of the reactive dyestuff used, cleavages may sometimes take place in chemical bonds between fiber and dyestuff under conditions for peroxide-based washing. The following are examples of test results that show relationships between reactive groups and levels of fastness to peroxide-based washing. Reactive group Liquor staining White staining Grade 4 Grade 4-5 Grade 2-3 Grade 4 Grade 1 Grade 3-4 Grade 4-5 Grade 5 In all tests above, the following chromophore was commonly used so that differences in fastness could be observed according to the kind of reactive groups. The chromophore used Conditions used in the testing of fastness to peroxide-based washing are as follows; Liquor staining: 5g/L sodium percarbonate liquor staining observed after a 2-hour wash at 40°C White staining: (after the process above) 4-hour treatment at 37°C with a perspirometer staining observed in attached white cloth Though washing fastness levels are usually determined by observing the degree of white staining with the use of a gray scale, in the tests above, liquor staining was simultaneously observed so as to determine the kind of dyestuff that becomes stripped due to cleavages in chemical bonds between fiber and dyestuff. While inferior fastness to peroxide-based washing may be found in substrates dyed with dyestuffs which possess pyrimidine or quinoxaline-based reactive groups, a relatively good fastness level is displayed in cases of using monochloro triazine or vinylsulfone-based dyestuffs. In particular, those with bi-functional reactive groups that combine both kinds exhibit superior fastness, the chemical bonding being highly stable between fiber and dyestuff.

AL AMIN BROTHERS

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