MODELO DE NEGOCIO FREEMIUM

The short liquor ratio of exhaust method of dyeing favours the semi-fully continuous dyeing. In exhaust dyeing, slow fixation helps to improve the penetration, levelness and controlled diffusion, while rapid fixation is favourable for economy and productivity. The excellent solubility, moderate substantivity, and versatile reactivity of the various ranges of reactive dyes give great potential in continuous dyeing. Reactive dyes have all the attributes required for semi-fully continuous dyeing [117, 121].

Continuous dyeing is a simple process for dyeing textile goods, and can use either one dip and nip or double dips, and nips in the dye and chemical bath solution together or separately, with or without intermediate drying steps. The single dip and nip is preferred because it minimizes the tailing problem. Tencel fabric gives a negative tailing behaviour meaning it has a 7% shade variation [11].

In continuous dyeing methods fabric can be dyed at low liquor ratio, so there is less need of changing the trough containing dye liquor. The mangle or padder pressure is set with respect to pick-up 60 - 80%. Continuous dyeing has high productivity; because of less dwell time after impregnation of fabric and less time (few minutes) is required to fix the dye in the fabric.

Dye fixation and colour yield depends on the fixation method such as steaming, curing and batching and also on intermediate drying. Drying by infrared radiation [124], particularly of shorter wavelengths, enable the dye to penetrate into wet cellulose fibres. This effect is at least partly responsible for the suppression of dye solution migration to the yarn surfaces during the initial stages of drying. Infrared fixation produced dyeing with higher fixation yields (80 to 98%) than when using hot air alone and gave colour yields close to those obtained by cold fixation (pad-batch process) without any drying, for which migration is

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presumably absent. It is therefore tempting to relate the higher fixation yields obtained using infrared radiation to the suppression of the dye migration, indicated by the lower colour yields. It seems obvious that migration of the initial dye solution out of the cotton fibres pores toward the yarn surfaces, where most of the water evaporation is occurring during the constant rate drying period, would result in less dye reacting with the cotton and a higher proportion of the fixed dye at the yarn surface. In a preliminary dyeing trial with Drimarene Red X-6BN, using infrared pre-drying followed by hot air fixation, the fixation yield was 76%, intermediate between those for fixation exclusively by infrared (82%) or hot air (72%). This result suggests that migration suppression is, in fact, at least partly responsible for the higher fixation yields obtained by infrared heating [124].

The following are the advantages of continuous dyeing process:

 Low capital cost of equipment.

 Low consumption of energy and water.

 Excellent reproducibility.

 Easy to control the dye-cellulose and dye-water reaction rates.

 Can be batch 1000 - 10000 m of fabric more than the exhaust dyeing.

 More economical.

3.3.3.1 Pad-Batch

Tencel fabric when dyed by the pad-batch method of dyeing using dyes from Ciba and DyStar, had high dye uptake as compared to the modal and the viscose fabrics [11].

The pad-batch method is very simple and consists of following steps:

 Impregnate the pre-treated fabric in the dye bath liquor at certain temperature.

 Pass the fabric between the mangle nip in order to squeeze any surplus liquor.

 Wrap the wet fabric on a rod or beam and cover it with a polythene bag for a specified time, at particular temperature, depending up on the reactivity of dye.

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 The prolonged exhaustion allows the dye to fix on to the fabric, the hydrolysed dye is then washed out during washing process [118].

Dyes of high reactivity are preferred, which helps to minimized the dwell time to as short as 2 - 4 hrs. The dyes are applied at low pH, they are easy to wash-off, have high stability in dye liquor, easy to mix bis-functional (chlorodifluoropyrimidines, dichloroquinoxalines [125] and aminofluorotriazine-vinylsulphone reactive dye [126]) and have excellent fixation and fastness. Dyes of low reactivity require longer dwell time (16 - 24 hrs) at higher pH. The dwell time depends on the swelling behaviour of textile fibre.

3.3.3.2 Pad-Dry-Thermosol

The pad-dry-thermosol method is used occasionally in place of pad-steam process. Intermediate drying at 110 to 120 ⁰C for 1 - 2 min, and baking at 150 ⁰C for 3 min is necessary for the formation of the dye-fibre bond [117]. Dichlorotriazine was the first dye type used for pad-dry-bake process with 10 g l-1 sodium bicarbonate. Highly substantive dyes containing highly reactive systems are preferred because it is easier to migrate the dye molecules into the fabric.

Urea is a dissolving agent, disaggregating agent and swelling agent for cellulose fibre, and is also hygroscopic agent. It enhances colour yields, speed up diffusion, and improves the levelness of dyes, particularly with viscose fabric. It is essential for pad-dry-bake process because it minimizes the migration of dye molecules during drying stage. The cheap and easily available urea has a disadvantage of being non-environmental friendly. It is being replaced by dicyandiamide [127], tri-methyl or tetramethylurea, or poly(ethyleneglycol).

3.3.3.3 Pad-Steam and Pad-Dry-Steam

Pad steam and pad-dry-steam methods of dyeing are suitable for aminofluorotriazine and selected sulfatoethylsulfone dyes. The steaming temperature and time depend on the type of reactive dyes. Steaming at 102 ⁰C for 1 min is suitable for minimizing the migration problems and level dyeing. Pad-steam process is suitable for dyeing the light shade cellulosic fabrics.

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