Salt free dyeing: A new method of dyeing of Lyocell fabrics with reactive dyes
by Aravin Prince Periyasamy and Dr (Mrs.) Bhaarathi Dhurai.

1. Introduction

Lyocell is the first in a new generation of cellulosic fibers. Lyocell has all the benefits of being a cellulosic fiber, as it is fully biodegradable, absorbent with excellent handle. It has a relatively high strength in both the wet and dry state, which allows for the production of finer yarns and lighter fabrics. As a result, fiber strength and modules of fiber are higher than those for regenerated cellulosic fibers as well as polyester staple fibers. Fabrics produced from Lyocell are breathable and moisture absorbent and have high dimensional stability.

With growing popularity of reactive dyes for dyeing of lyocell environmental problems associated with their use have received attention. Since lyocell have been moderate affinity for most reactive dyes, large quantities of electrolyte such as NaCl or Na2So4 (40-100 gpl) are normally required for exhaustion. Hence dye bath exhaustion and fixation can still be as low as 50% for some dyes. Wastewater therefore contains a significant quantity of dye and salt, leading to serious environmental problems.

It has been found that pretreatment of lyocell before dyeing can offer a simple and effective method of improving dye-fibre affinity, avoiding the need for salt as electrolyte in dye bath. It has been found that Poly (Vinylamine Chloride) [PVAmHCl] is a physical modifying agent. Its wide range of properties has found use in Catalysis, Chelating, liquid chromatography, treatment of wastewater, recovery of oil and in polymeric dyes. Previous studies have shown that a variety of compounds may be effective in this way, all involving chemical modification of Cellulosic. Non-reactive pretreatments including some polymers with affinity for cellulose tend to be desorbed during dyeing and inhibit uptake of dye or cause it to precipitate. This study has established the value of polymeric quaternary ammonium compounds, amines or amides, which may be attached to lyocell by non-chemical mechanisms. Despite the encouraging results obtained with non-reactive polymers in the salt free dyeing of lyocell, problems remain in dye selection and obtaining level results.

The aim of this work to determine the effectiveness of PVAmHCl as pretreatment agent of Lyocell blended fabrics in improving its dyeability with reactive dyes and in achieving evenness of dye uptake. It was also to determine the effectiveness of pretreatment of dyed fabrics on K/S value and fastness properties like wash fastness and Rub fastness.

Various physical properties like tensile strength, flexural rigidity, cloth crease recovery angle and aerial density were also determined to see the effect of PVAmHCl. Results obtained were analyzed to arrive at some advantages of the pretreatment.

1.1 Properties of Polyvinylamine Chloride (PVAmHCl)

PVAmHCl has been used as a physical modifying agent. Due to its wide range of properties, PVAmHCl has found use in catalysis, liquid chromography, treatment of wastewater, recovery of oil and in polymeric dyes. It has been used in application as diverse as paper making and biomedical research, but it’s used for modification of lyocell for salt free dyeing as not been previously reported. Interest in PVAmHCl arises from the presence of large number of cationic sites (NH+₃Cl-). Nucleophilic sites involving primary amino group within the PVAmHCl molecule are of particular value for achieving salt free dyeing of lyocell with reactive dyes. As the pH increases, the proportion of NH+₃Cl- groups in the molecule decreases and that of NH₂ groups increases.

2. Materials and Methods

2.1 Fabric

The lyocell yarns were spun and the fabric was woven. The geometrical properties of the fabric are given in Table-1

2.2 Dyes & chemicals

The details of the dye and the chemicals used are given in Table 2.

2.3 Preparation of Fabric

The fabric sample was desized by using acid desizing method. The fabric was scoured by alkali method using standard procedure. Then it was subjected to bleaching process using hydrogen peroxide as bleaching agent.

2.4 Pretreatment

Padding method was used for pretreatment of lyocell with PVAmHCl the pH of pretreatment liquor was maintained at buffer comprising potassium dihydrogen phosphate (7 gpl) and Sodium Hydroxide (1.45 gpl). Padding was carried out using 2 dips (4 min for each) and 2 nips. Fabric samples were pre dried at room temperature and then baked at 102^oC for 12 min in rapid baker. Padding was done at different concentration of PVAmHCl. Pretreatment process conditions are given in Table-3.

2.5 Dyeing

The fabric dyed with reactive dye using procedure recommended by dye manufacturer. This fabric sample is considered as control sample. Exhaust dyeing was carried out at liquor ratio 1:30. Dyeing of fabric pretreated with different concentrations of PVAmHCl was carried out at 80^oC for 60 min. Fixation was conducted for 20 min. using 8 to 11 gpl of Na2CO3. Process conditions for dyeing are given in Table-4.

2.6 Testing

The details of various tests conducted on the fabric are as follows.

2.6.1 Color strength (K/S Value)

Color strength K/S was measured on Minolta Spectrophotometer. These values are calculated using the following “KUBELKA-MUNK” equation.

                                                                    
Where.

K means Absorption co-efficient.

R means Reflectance of the dyed sample.

S means Scattering co-efficient at the wavelength of maximum absorption.

2.6.2 Physical properties

The physical properties of the dyed fabric samples and the instruments used are given in Table 5.

Note: Tensile strength, crease recovery, flexural rigidity, aerial density were tested only for control sample and sample treated with 10 gpl concentration of PVAmHCl.

3. Results and Discussion

3.1 Effect of pretreatment on K/S value

Results of K/S value are presented in Table-6.

From the table following observations are made.

• K/S value an untreated sample is comparatively lower than the K/S value of samples treated with 5 & 10% concentration of Polyvinylamine Chloride.
• K/S value is found to be less than conventional method when 2.5% Polyvinylamine Chloride was used.
•  Maximum K/S value found at 10% Polyvinylamine Chloride.
• As the concentration of Polyvinylamine Chloride increases above 10% K/S value found to be decreasing.

Above observations indicate that the pretreatment of lyocell fabric increases dye uptake. The decrease in K/S value may be due to the following reasons. When excess of Polyvinylamine Chloride is padded on the fabric, the bonding between the fiber and some cationic polymer become weak, and repulsion force also existed within the cationic Polyvinylamine Chloride. This would lead to presence of unbound polymer in the dye bath, thereby hindering the absorption of dye and possibly causing it to flocculate. It can be seen from Table-6 that dye reactivity on pretreated fabric was greater, due to presence of primary amino groups provided by the Polyvinylamine Chloride. This confirms the effectiveness of pretreatment in enabling the fabric to be dyed without salt.

3.2 Effect of pretreatment on wash and rubbing fastness properties

Results of wash and rubbing fastness are presented in Table-7.

The fastness properties of dyed Lyocell fabrics pretreated with PVAmHCl were determined. The results were compared with those of conventional dyeing. The wash fastness was excellent for all samples of the salt-free dyeing, confirming the effectiveness of dye fixing due to the pretreatment with PVAmHCl. Rubbing fastness was also observed to be good when compared with that obtained by conventional dyeing.

3.3 Effect of pretreatment on physical properties

Results of Tensile strength, flexural rigidity, crease recovery angle, and aerial density, are presented in Table-8.

From the table-8 following observations can be made:

• Tensile strength of conventional dyed fabric and pretreated samples is found to be almost same.
• There is increase in crease recovery angle of fabric when treated with PVAmHCl.
• Flexural rigidity of sample increases as a result of pretreatment.

Increase in crease recovery angle, as a result of pretreatment may be explained as follows. There might be cross-linking of PVAmHCl between the cellulosic molecules. These cross-links hinder the molecular and fibrillar slippage and stabilize the structure, thereby increasing the crease recovery angle. Increase in Flexural rigidity shows that fabric becomes slightly stiff as a result of treatment of fabric with PVAmHCl.

4. Conclusion

When the Lyocell fabrics were pretreated with PVAmHCl, the reactivity of reactive dyes on fiber increases, whereas, wash fastness and rubbing fastness of pretreated sample are better than that for conventional dyed sample. Fabric Crease recovery and Flexural rigidity increased as a result of pretreatment. There is no change in the tensile strength of fabric as a result of pretreatment. By using pretreatment method, the following advantages are observed.

• Elimination of salt as an electrolyte,
• Maximum fixation of dye,
• Minimum hydrolysis of dye,
• Low volume of water requirement during wash off process,
• Significant saving in process cost,
• Environmental friendly.

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4. 2 About Authors:

Aravin Prince Periyasamy is Assistant Professor, Dept of Textile Chemistry, D.K.T.E Society’s Textile Engineering College, Ichalkaranji, Maharashtra and Dr (Mrs.) Bhaarathi Dhurai is Associate Professor, Dept of Textile Technology, Kumaraguru College of Technology, Coimbatore -641049, T.N, India.  E-mail: aravinprince@gmail.com.