CHAN KA HEI 13057598D
CHUNG PUI WA 13088021D
WONG YAT LUN 13057927D
YIM MAN KEI 13059341D
Dyeing Cotton & Wool with Direct Dyes
Introduction
Direct dyes, also called substantive dyes, are popular dyes for dyeing cellulosic fibers such as cotton. It can also be used to dye protein fibers such as wool. The purpose of this experiment is to study the exhaustion and water fastness behavior of three classes of direct dyes (1% Solophenyl Yellow, 1% Cuprophenyl Red & 1% Cuprofix Blue) when used on cotton and wool fabrics.

Materials:
3 pieces of 2g bleached cotton fabric
3 pieces of 2g scoured wool fabric

Stock solutions: 1% Soda Ash | | 1% Solophenyl Yellow | (Class A dye) | 1% Cuprophenyl Red | (Class B dye) | 1% Cuprofix Blue | (Class C dye) |

Recipes:
The liquor ratio of the dye baths to be 50 : 1 and the final dyeing temperature to be 95°C. Dye/Chemical | Bath 1 | Bath 2 | Bath 3 | Class A dye(1%) | 2% | --- | --- | Class B dye(1%) | --- | 2% | --- | Class C dye (1%) | --- | ---- | 2% | Soda ash | 0.5% | 0.5% | 0.5% | Common salt | 20%(added in 1 portion) | 20%(added in 2 equal portions) | 20%(added in 4 equal portions) |

Calculation of the amount of chemicals for each dye bath: 1. | Volume of liquor (2g material with LR of 50:1) | == | 2 x 50100ml | | | | | 2. | Volume of Class A, B, C dyes | = | In 2 g, 2% is 0.04 g. As 3 classes are using 1% stock solution, it needs 4 mL. | | | | | 3. | Weight of common salt | = | In 2 g, 20% is 0.4 g. | | | | Bath 1 (Class A dye): in 1 portion of 0.4 gBath 2 (Class B dye): in 2 portions each of 0.2gBath 3 (Class C dye): in 4 portions each of 0.1g | | | | | 4. | Volume of soda ash solution | = | In 2 g, 0.5% is 0.01 g. As 3 classes are using 1% Soda Ash stock solution, it needs 1 mL. |

Procedure of the Dyeing Process: 1. Three 2g samples of bleached cotton and three 2g samples of wool were prepared. 2. Six dye baths (2 for each class of dye) were prepared according to the recipes described in the earlier section. The samples were then dyed according to the following steps: Dye Bath 1 for Class A dye (1% Solophenyl Yellow): a. The material sample was rinsed thoroughly with cold water. b. The dye bath with all chemicals (dye, soda ash & common salt) was heated to 40°C. c. The wetted material was immersed in the dye bath and the temperature was then raised to 95°C. d. The temperature of the dye bath was maintained at 95°C and the material was left in the bath to be dyed for a total of 25 minutes. Dye Bath 2 for Class B dye (1% Cuprophenyl Red): a. The material sample was rinsed thoroughly with cold water. b. The dye bath containing the dye and soda ash was heated to 40°C. c. The wetted material was immersed in the dye bath and the temperature was then raised to 95°C. d. After 8 minutes, the first portion of common salt was added to the dye bath. e. The temperature was maintained at 95°C, after another 8 minutes the second portion of common salt was added. f. The temperature of the dye bath was maintained at 95°C and the material was left in the bath to be dyed for a further 9 minutes. The total dyeing time was 25 minutes. Dye Bath 3 for Class C dye (1% Cuprofix Blue): a. The material sample was rinsed thoroughly with cold water. b. The dye bath containing the dye and soda ash was kept at room temperature. c. The wetted material was immersed in the dye bath and left to be dyed at room temperature for 8 minutes. d. The dye bath was then heated to 95°C . e. The common salt was added in 4 portions at 2-minute intervals. f. The temperature of the dye bath was maintained at 95°C and the material was there to be dyed for a further 9 minutes. The total dyeing time was 25 minutes. Scope of Study:
Results of the experiment were studied under 3 different aspects through visual observations: 1. Exhaustion of cotton & wool fabrics by comparing the dye baths upon completion of the dyeing process. 2. Color fastness to washing of the 6 dyed samples by immersing them in a bottle of 100ml pure water at 95°C for 1 minute and thereafter inspecting both the color of the samples and the tint of the water. 3. Color depth of dyed cotton & wool fabrics by comparing their color properties after the samples had been dried.

Results and Observation:
Aspect 1: Exhaustion
There was not much visible difference between the color depths of Bath 1 for cotton and wool. The color depth of Baths 2 & 3 for wool fabric was visibly more intense than the corresponding baths for cotton. This implied that the cotton samples in Bath 2 & 3 had taken in a larger quantity of dyes from the dye bath than the wool samples. It could be deduced that exhaustion of the cotton samples was generally higher than the wool samples.

Aspect 2: Color fastness to washing
The water in the 3 flasks that had been used to soak cotton samples dyed in Bath 1, 2 & 3 were correspondingly less tinted than the water used to soak the wool samples. The fastness contrast between water and the dyed samples was also compared individually. Between the cotton samples, the one dyed in Bath 3 (Class C dyes) showed a higher contrast while the samples in Bath 1 (Class A dye) and Bath 2 (Class B) were comparable. For the wool samples, the one dyed in Bath 1 (Class A dye) and Bath 3 (Class C dye) were comparable and exhibited a higher contrast than the one dyed in Bath 2 (Class B dye). For each bath type, the cotton sample displayed a higher contrast than the wool sample. A scale of contrast was assigned to the 6 samples as below; a higher number implies a higher contrast. It could be deduced that color fastness of the cotton samples was generally better than the wool samples. Class A - Wool (3) | Class B - Wool (2) | Class C - Wool (3) | Class A - Cotton (4) | Class B - Cotton (4) | Class C - Cotton (5) |

Aspect 3: Color depth of the samples when dry
The colour of the cotton samples was deeper, brighter and more vibrant than that of the wool samples. It could be also deduced that the higher exhaustion of the cotton samples.

Discuss:
Direct Dyes
Direct dyes are the simpler dye classes. They are soluble in water and can be applied to natural and regenerated cellulosic fibers. The rate of absorption of the dye and its final exhaustion depends on various factors. It is affected by the substantivity of the dye for the particular fiber, the amount of salt present, the liquor ratio and the dyeing temperature.

Level of exhaustion and water fastness between cotton and wool
The results of the experiment demonstrated that exhaustion of direct dyes is higher for cotton than wool. Also, the water fastness of dyed cotton samples was better than that of wool. This implies that cellulosic fibers have a stronger affinity for direct dyes than protein fibers, the direct dyes can therefore penetrate and level better on the cotton fabric than on wool fabric. However, the water fastness of the direct dye in both cotton and wool samples is not ideal. This can possibly be explained by the lack of polar groups in the dye bath solution to form salt linkages. The binding forces between the dyes and the fabric fibers are thus purely physical. Physical binding forces are weak and tend to break easily. When they do, the dyes come off the fabric.

Salt as an electrolyte
In the experiment, salt functions as an electrolyte and is used to improve dye exhaustion. From a molecular point of view, several steps are involved in the dyeing process. First, the dye in the dye liquor diffuses from the liquor to the substrate. Second, the dye accumulates on the surface of the substrate; this step depends on the affinity (or substantivity) of the dye for the fiber. Third, the dye diffuses or migrates into the interior of the fiber; this step requires that the fiber itself be accessible to the dye. Access to the fibers is hindered by an electrostatic barrier formed on their surface. This comes about because when fibers and dyes are immersed in water, negative charges are formed; as like charges repel, an electrostatic barrier is formed. This barrier can be overcome by the addition of common salt because salt ions help to reduce the electrostatic charges on the fiber ions, thereby facilitating the approach of the dye ions to within a distance at which Van der Waals and other short range forces as hydrogen bonding can be formed.

Classification of Direct dyes:
Class A dye (1% Solophenyl Yellow) - the dye ions migrate well and therefore possess high leveling power. They dye unevenly at first but the distribution soon become even with further boiling. Class A dyes are insensitive to salt, so the full amount of the common salt can be added to the dye bath at the initial stage and the temperature raised to 95°C afterwards. The results of the experiments showed that exhaustion of both cotton and wool samples in Class A dyes is better than Class B and C dyes. It is demonstrated that the leveling power of Class A dye is higher than Class B and C.

Class B dye (1% Cuprophenyl Red) - the leveling power is poorer than Class A’s and exhaustion must be improved by the addition of salt. If these dyes are not taken up uniformly in the initial stages, it is extremely difficult to correct the unlevelness later on. The amount of salt in the system need to be carefully controlled. In the experiment, the common salt is divided into two portions and added in 2 steps after the liquor has reached 95°C in order to achieve a better degree of levelness in the initial stage.

Class C dyes (1% Cuprofix Blue) – the dye is not self-leveling and is highly sensitive to the concentration of salt. Exhaustion of these dyes cannot be controlled solely by the manipulation of the salt content alone, careful temperature regulation is also necessary for good results. Because of their sensitivity to salt, salt has to be added in quantities even smaller than that for Class B dyes. In the experiment, the wetted samples were put in the dyeing solution without common salt at room temperature and left to dye for 8 minutes. The common salt is divided into four portions and added separately after the liquor has reached 95°C to achieve a better degree of levelness.

Conclusion:
The experiment demonstrated that exhaustion of direct dyes for cotton fabrics (cellulosic fibers) is better than wool fabrics (protein fibers), implying that cellulosic fibers have a stronger affinity for direct dyes than protein fibers. It at the same time showed that water fastness of both cotton and wool dyed in direct dyes is not ideal. The experiment also revealed that the dyeing behavior of different dyes varies; to cope with their different levelling power and to improve exhaustion and water fastness, the conditions of the dyeing process need be carefully controlled.

Reference:
Trotman, E. R. (1970). Dyeing and Chemical Technology of Textile Fibres. London: Charles Griffin & Company Limited.
Kulkkarni, S. V. & (1986). Textile Dyeing Operations: Chemistry, Equipment, Procedures, and Environmental Aspects. USA: Noyes Publications.
Lewis, D. M. & Rippon, J. A. (2013). The Coloration of Wool and Other Keratin Fibres. UK: John Wiley & Sons