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Technology of soaping

1. Preface

Reactive dyestuff belongs to the group of dyestuffs widely used for dyeing/printing of cellulosic fiber. A number of reactive dyestuffs are sold and their product lines are diversified, from conventional ones to high-tech products by discharge and resist printing processes. In the case of reactive dyestuffs, new production technology has been established and reports have been made on various studies; basic technology, complex application technology, production control, control engineering, waste water treatment. The soaping process, as far as reactive dyes are concerned, is one part of the dyeing process which contains more complex factors than the soaping process used with dyes belonging to any other group of dyestuffs. Reactive dyestuff has many different types of reactive groups that range from mono-functional to poly-functional groups, and its chemical structure also varies in many ways. Equally, fixing rates also differ. What is common in all cases though is that they do not react with cellulose fiber completely. After dyeing or post-printing steaming, the fiber will contain the kinds of dyestuff shown below:
(a) Reactive dyestuff fixed on fiber
(b) Inactive, unfixed dyestuff
(c) Hydrolyzed dyestuff produced in the presence of water and alkaline or acid
Although the amount of such reactive dyestuff remaining on the fiber may even increase depending on the condition of pH level, temperature and processing time, it can all be removed completely by soaping. During high-temperature alkali dyeing and washing, hydrolyzed dyestuff is produced as the bond between the dyestuff and fiber breaks down. This hydrolyzed dyestuff holds so strong an affinity that it causes redeposition and adsorption, therefore, it is very difficult to remove the compounds (b) and (c). Deficient soaping often leads to bleeding, staining of white ground and redeposition, resulting in insufficient wet and washing fastness. What is required in soaping after reactive dyeing, therefore, is powerful soaping properties that can strike a balance between the removal of unfixed dyestuff and prevention of redeposition.

2. Recent studies

The common trend in research and studies on this reactive-dyestuff soaping, still actively carried out in Europe and the US, is to analyze the amount of wash-out at each soaper by using equipment simulating the actual production process. Temperature and time for hot/cold rinsing after dyeing and post-printing steaming are observed, as well as the process speed and rates of dyestuff removal in relation to the bath ratio. The point of these studies is to achieve a rational soaping process on a large scale by using a technology that incorporates the following practical factors:
(1) Saving water, energy and time
(2) Realizing a smart process with quick soaping in a small bath ratio
(3) Standardizing control system and reducing cost of production
(4) Controlling and reducing waste water for protection of the environment

More specific studies are listed below.

Wash-off properties can be affected by temperature difference between mono-functional and bi-functional dyestuff during continuous soaping.(1)(4)

Hot rinsing can be very effective in washing off unfixed and hydrolyzed dyestuff especially in the case of high affinity, and an observation of the effectiveness according to temperature, the relationships between pH, salt concentration and flow rate has been carried out, parametrically indicating optimum rinsing and soaping after package dyeing.(3)

Aspland(2) suggests, for the setting of appropriate conditions and temperature, that it is preferable to begin with hot rinsing for the removal of unfixed and hydrolyzed dyestuff which carries a high affinity, though he also points out extreme temperatures and high/low pH often cause discoloration due to hydrolyzing of dyestuff.

Thomsen(5) indicates in his graph that the possibility of removal of more than 50% of the total amount of washed-off dyestuff coming from hydrolyzed dyestuff with high affinity through hot rinsing at a high temperature between 70 and 95°C.

G. L. Bhalla(6) indicates alkali hydrolyzed in soaping through the use of a SN2 mechanism of nucleophilic displacement reaction to describe the condition of transition and sectional bond of dyestuff.

Kotani(7) discusses the appropriate use of a soaping agent for setting optimum soaping conditions and preventing staining of white ground.

The above recent studies mostly deal with the use of appropriate temperature for soaping.

3.Characteristics and performance

Originally, the goals of reactive-dyestuff soaping are:

 ●the removal of unfixed dyestuff
 ●the removal of hydrolyzed dyestuff

Setting of a high temperature seems preferable for removal, however, washed-off dyestuff often diffuses and penetrates into the yarn, resulting in re-adsorption and staining.

Moreover, it is not easy to achieve complete, quick soaping on printing paste since the concentration of dyestuff and the amount of paste deposition are high. Thus, the use of a special soaping agent, capable of producing maximum effects in a quick strong soaping at a small bath ratio, while preventing redeposition and staining of white ground regardless of rinsing temperature or any difference in process conditions, is considered most rational.

Though multiple soaping agent structures have been made public so far, it is very difficult for a single chemical structure to satisfy the above description. Fig. 1 shows some interesting chemical structures that are not only restricted to the idea of using surfactants as the base.

4. Emill

An optimum soaping agent for reactive dyestuff will require:

●Insusceptibility to water quality and hardness
●Soaping properties with minimal fluctuation due to temperature, capable of producing sufficient effects even in low temperatures
●High affinity to dyestuff, properties of forming additional chemical compounds with hydrolyzed dyestuff component but without affinity to fiber
●Causing no color alteration after soaping
●Properties of promoting diffusion, dispersion, solubility of washed-off dyestuff and further washing off supported by appropriate water temperature, mechanical movement of cloth, flow rate, agitation and frequency of squeezing and rinsing
Emill has been developed based on the above conditions and can promise the following performances:
●Quick, powerful soaping
●A great soaping result even in a small bath ratio
●Prevention of color redeposition from wash-off
●Reduction in the frequency of rinsing (easy rinsing)
●Low foam (no foam)
Emill can be divided into 4 different groups based on equally high and distinctive performances:
Group (A)
G-170, G-100 and G-50 potently prevent redeposition of wash-off and staining besides removing it. White-ground soaping agents suitable for prints. Versatile and inexpensive.
Group (B)
2G conc, 2G conc new and BZ conc are conc-type products with superior soaping properties and good stain removal.
Group (C)
R-60, YK and 2G can balance the performance between wash-off removal and prevention of staining (the characteristics of A and D). Most conventional, versatile and inexpensive products.
Group (D)
Powerful soaping with anti-staining/redeposition properties, outstanding effects in the removal of staining 5G conc.
SK-D: Also suitable for deep-printing G-A: Good for deep-dyeing (black)

Table 1 is the summary of the performances, purposes and characteristics of above products. Table 1 Emill group characteristic, performance and method

The following are specific data on practical use.

When dyeing only the cotton side of polyester/cotton union cloth (border pattern) with reactive black dyestuff (VS type), the polyester side becomes stained and is no longer available for reserve dyeing. The graph in Fig.2 shows that the amount of reactive dyestuff remaining on the polyester part and the amount of unfixed dyestuff on the cotton after soaping this one-side dyed material at 60°C, 75°C and 90°C.

In ②, the high-temp (90°C) rinsing removed more than 50% of unfixed dyestuff on the cotton part without using any soaping agent, but it proved impossible to remove the stain on the polyester.

In ③ and ④, a competitor's soaping agent was added. Removal of stains on polyester was incomplete, without any improvement observed even with the use of STPP (sodium tripolyphosphate).

In ⑤ and ⑥, EMILL SK-D was used. Deposition on both fibers was removed effectively with soaping above 75°C. The addition of STPP proved effective and brought about improved results.

Fig.3 shows comparison of anti-staining effects and colorfastness to washing according to the kind of soaping agent in 60°C, 75°C and 90°C soaping of reactive-dyed cotton.

Soaping was carried out directly after dyeing 100% cotton knit with C. I. Reactive Red 120, without undergoing cold or hot rinsing. To test the anti-staining of white ground, stains on white cloth were observed with a white cloth added to the soaping bath. To test fastness to washing, bleeding on an attached white cloth was measured after the soaping and drying of a dyed cloth, using the launderometer, based on JIS A-4 method.

② is a conventional type with a balance between removing and anti-staining properties.

③ causes no staining on white ground because of low removing properties and has very low fastness to washing.

④ is potent in removal but inferior in preventing staining, resulting in more redeposition of dyestuff and low washing fastness.

As described above, the performance of soaping agents differ according to the kind. Emill G-170 (⑤) has a very low concentration of 0.5 g/L and shows great results in both anti-staining of white ground and washing fastness.

Fastness examination of dyed, soaped cloth (dyestuff bleeding test)

Test method

Dyeing of cotton cloth with CI Reactive Black 5

→Soaping (80°C x 10min) and drying
→The dyed cloth and cotton white cloth sewn together vertically
→The bottom of the dyed cloth dipped in a 0.5g/L solution of nonionic surfactant, 20°C x 10min.
→Determination of the bleeding rate of the reactive dye remaining unfixed on the white cloth

5. Conclusion

We have observed some examples of soaping effects produced by Emill after reactive dyeing, printing, and in removing high-affinity unfixed and hydrolyzed dyestuff. Depending on the kind of dyestuff used (fixing rate, affinity), the dyeing depth (pale to deep), the kind of substrate (dyed or printed) or soaping equipment, bath ratio, and time and temperature (for soaping and rinsing before/after soaping), the result of soaping will differ considerably. The article mainly dealt with soaping effects which do not alter with any of the above conditions. Since circumstances are that reactive dyestuff is to be used more and more frequently, careful selection is expected for soaping agents according to the desired effect.

References:

(1) Luttringer: Tex chemis, col 25 (5) 27 (1993)
(2) Aspland: Tex chemist & col 24 (5) 36 (1992)
(3) Technical paper: Tex chemist & col 23 (No11) 21 (1991) 23 (No1) 13 (1991) 
(4) Luttringer: Textilveredlung 25 (No10) 313 (1990)
(5) Thomsen: Mell Textilber 75,220, 3 / 1994
(6) G. L. Bhalla: Am Dyestuff Repter 81, (No7) 38 (1992)
(7) Kotani: Dyeing & Finishing 46 (No13) 693 (1994)
(8) CA 115 (18) 185801w
(9) CA 121 (22) 258574k
(10) CA 98 (10) 74296w
(11) CA 93 (4) 27692j
(12) CA 109 (26) 232705f
(13) CA 114 (26) 249244f
(14) CA 90 (26) 206270e
KOTANI CHEMICAL
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