An acrylic gel dyeing stage

1. Outline

In the dyeing of acrylic fiber, development of bright hues can be expected from the use of cationic dyestuff. It is thus broadly used, in garments (e.g. sweaters), bedding (e.g. blankets), soft furnishing (e.g. carpets), soft toys and more.

The following are methods for dyeing acrylic fiber.

1) Tow dyeing: The method most commonly used. Acrylic tows are stuffed into the dyebath and dyed with temperature raised gradually. Carpets are dyed in this way.

2) Garment dyeing (inc. semi piece dyeing): After being knit, substrates, such as blankets, undergo dipping and nipping in cationic dye liquor, then fixing (steaming).

3) Dope dyeing: A method of dyeing based on ionic bonding between sulfonic acid groups in acrylic polymer dissolved in solvent (dope) and dyestuff or pigment added to the dope, from which acrylic fiber is wet or dry-spun.

4) Gel dyeing: A method in which cationic dyeing is incorporated in the spinning process

Table 1 summarizes features of the dyeing methods above.

Table 1 Features of acrylic dyeing methods

NO	Method	Advantage	Disadvantage
1)	Tow	• Easy color matching	• High cost • Effluent treatment necessary
2)	Garment	• Small-lot compatible	• Low productivity • High cost
3)	Dope	• Minimal burden on the environment • Easy color matching	• Production other than large lots difficult • Switching of colors difficult • Piping and cleaning of spinning bath difficult
4)	Gel	• Low cost • Medium lot (larger than approx. 5t) • Practically no burden on the environment	• Less piping and cleaning of spinning bath than dope dyeing

2. Gel dyeing

The following are details of gel dyeing which has advantages in respect to environmental concerns and costs.

a) Inclusion of the dyeing process
n the wet spinning of acrylic fiber, dope is prepared first for extrusion into a solvent solution, then the drawing, rinsing, oiling and drying stages follow for eventual densification. The process is completed with subsequent crimping, crimp-setting and cutting.

Before being dried for densification, acrylic fiber remains gel-swollen, or in other words, spongy, a state which allows the absorption of cationic dye liquor. In short, gel dyeing is the inclusion of the dyeing and fixing processes after the rinsing stage of spinning.

-	Conventional method: Spinning-Drawing-Rinsing-Oiling-Drying-Aftertreatment
-	Gel dyeing: Spinning-Drawing-Rinsing-Dyeing-Fixing-Oiling-Drying-Aftertreatment

b) Dyebath
What is required in a dyebath is a function that allows quick and thorough penetration of the dyestuff inside the tow. Although the time required varies depending on the type of spinning machine used, in general, uniform penetration into the tow interiors should be achieved within 1-2 seconds. The prefered dyebath used is of a vibratory type. In addition, as dyestuff is often only physically adsorbed into the fiber in this stage, fixing is indispensable to induce ionic bonding with the fiber, as described in the following. Moreover, the use of a vibratory dyeing machine is a key to uniform dyeing of the tow.

c) Fixing
Normally, the fixing of the absorbed cationic dyestuff onto fiber only takes several seconds of 100°C saturated steaming. The use of saturated steam of above 100°C or boiling water is also possible. Alternatively, this fixing stage could be dispensed with in the case of some raw yarns, depending on the outer and inner structure of fiber and the number of sulfonic acid groups contained. (e.g. Kanebo's Lufnen falls into this category.)

d) Dyestuff
The dyestuff used should be cationic and have a uniform dyeing rate. Combinations of only the trichromatic colors, red, blue and yellow, are used. The preferred form of dyestuff is liquid. Each of the trichromatic colors is individually prepared in a pre-determined ratio and concentration for production of any desired hue before being added to the dyebath.

Steaming/fixing unit	Gel inlet

Simultaneous dyeing of black and white 'double front'	Gel dyeing trough

3. Dye recipe determination and color matching

a) Dye recipe determination
Beaker dyeing is carried out once a desired hue is selected, using the same trichromatic colors of cationic dyestuff and tows of the same type, denier, luster as the fiber to be spun/gel-dyed, for determination of an exact recipe. Sample spinning and gel dyeing then follow, using this recipe and a miniature machine, to make further adjustments. The concentration of dyebath and recipe at the point that dyeing reaches equilibrium are also verified. In this way, recipes for the initial bath as well as for sequential additions are determined in practical production.

b) Color matching
A computerized color matching system (CCM) is an obvious device for the color matching of the desired hue. As soon as the primary gel-dyed results of practical production come out, they are compared with the desired hue. The recipe is re-adjusted if the resultant hue differs from that desired, according to the CCM measurement. This process is repeated until the recipe is stable, or ultimately, until required adjustments can be made successfully in a single attempt.

c) Color kitchen - automatic dyestuff dispenser
Improved efficiency can be attained by adopting a fully-computerized, automatic dyestuff dispenser.

d) Switching of colors
For switching of colors, the dyebath is washed after an old liquor is removed for replacement with a new one. Thorough preparation may result in a swift switching within 3 minutes. Production should be carried out in the order of the depth of shade and concentration starting with the paler, thereby facilitating immediate, subsequent start of usual spinning by the removal of the dyeing trough, bypass and fixing units.

4. Advantages

To conclude, the features of gel dyeing are summarized below.

-	Production of any hue, from pale to deep, is possible (only with cationic dyestuff)
-	Production possible from a minimum of 5t
-	Quick color switching
-	Costs one tenth to one fifth of tow dyeing
-	Environment-friendly as it uses little water