Abstract
In recent years, the production of polyester fibres has been dynamically increased, accompanied by the wide use of polyester-cellulose blends. Viscose is the best yet complement for polyester in blends. In polyester/viscose blends polyester contributes strength, durability and dimensional stability, and viscose contributes absorbency, softness and handle. But the coloration of such blend creates problems because of the differential dyeing affinity of both the fibres. However, some synthetic dyes can be used but the process is very complex and also not friendly to the environment. Nowadays, as everyone is concerned about environmental safety and also aware of the carcinogenicity of some synthetic dyes, the use of natural dyes has emerged as an eco alternative. Hence, the present research has been made to assess the compatibility of natural dye obtained from Himalayan rhubarb (Rheum emodi)root with polyester/viscose blend using a high temperature high pressure (HTHP) dyeing technique.
Introduction
The textile industry is not only one of the oldest but also continues to be one of the mainstays of the world economy. Textile products were created from natural, rapidly renewable and abundant sources for thousands of years, and then innovators introduced synthetic fibres to overcome some of the inherent limitations of natural fibres. Among the synthetics, polyester fibres fully dominated the market due to their versatility in use, excellent mechanical properties, cheaper price, ease of care, biodegradability, etc. Though it also suffers from certain drawbacks, the fact that the inherent drawbacks in its structure can be corrected by means of blending means it is still the most widely used textile fibre.
Blends of polyester with natural and regenerated cellulose have become very popular in apparel usage for the obvious reason of techno-economic considerations, including the low cost, durability, comfort and aesthetic values of such blended fabrics. Viscose rayon, also known as artificial silk, is a blend-friendly fibre and most frequently blended with polyester in blends. The selection of these two fibres ensures sufficient comfort, resulting mainly from the use of viscose fibres, and suitable mechanical properties such as the tensile strength characteristics of polyester fibre.
Due to the presence of two dissimilar fibres, coloration of such blends has always been a challenging job to chemical processors. Though a good number of synthetic dyes are produced commercially for dyeing this blend, the current worldwide awareness about pollution and effluent disposal problems created by the synthetic dyestuff industry has revived the interest of people in the age-old art of dyeing with natural dyes. There is an increasing desire in the textile industry, as well as among textile consumers, to develop and use eco-friendly methods of dyeing textiles. This concern for the environment has once again attracted attention towards natural dyes, which are eco-friendly as well as safe for human skin and have gained momentum not only for safety but also for the beauty of their colours and for their novelty.
Hence, keeping in mind the exceeding popularity and benefits of natural dyes, the present study was undertaken to dye polyester/viscose blended fabric with Himalayan rhubarb dye, using different natural mordants and testing the colour strength and colourfastness properties of dyed and mordanted samples.
Experimental Details
Materials
Fabric: Pure white plain weave polyester/viscose (67/33) blended fabric with 92/66 fabric count and 0.23 mm thickness was used for dyeing experiments. 
Natural dye material: The Himalayan rhubarb root was used as the dye source. It was purchased from the local market of Haldwani, Uttarakhand, India. The raw dye material was examined carefully to remove peripheral matter and then ground to coarse powder form in the grinder.
Natural mordants: Six natural mordants, namelyIndian gooseberry (Phyllanthus emblica) fruit, belliric myrobalan (Terminalia belerica) fruit, myrobalan(Terminalia chebula) fruit, pomegranate (Punica granatum) rind, tea (Camellia sinensis) leaves and walnut (Juglan regia) bark were used to obtain varying colour/shade and to improve the colourfastness of the Himalayan rhubarb dyed samples.
Procedure
1. Preparation of Fabric for Dyeing
The polyester/viscose (P/V) fabric was soaked in warm detergent solution containing 5ml liquid non-ionic detergent in 1 litre of water for 1 hour. The fabric was rubbed, kneaded and squeezed in detergent solution from time to time then rinsed with excess tap water to remove traces of detergent completely and dried in shade. Samples of 22cm x 11cm dimension were cut from the prepared P/V blended fabric for the dyeing experiments; each sample weighed 2.50g. The fabric sample was soaked in water prior to dyeing for 30 minutes to remove air from it for rapid dye penetration.
2. Extraction of Dye from Natural Resource
The dye was extracted using an optimised method of extraction based on visual evaluation and colourfastness of dyed samples. The optimised parameters used for the extraction of dye from raw material are shown in Table 1.
3. Dyeing of P/V Sample
Dyeing of polyester/viscose blended fabric was carried out in a high-temperature, high-pressure beaker dyeing machine using the optimised dyeing conditions given in Table 2:
4. Application of Mordants
A constant amount (2g) of all the six natural mordants was taken and mordant solutions were prepared by adding two gram of each natural mordant in 50ml of distilled water, allowed to soak overnight, then boiled for 15-20 minutes and filtered. Mordants were applied with the help of three methods of mordanting namely, pre-mordanting, simultaneous mordanting and post-mordanting.
5. Colour Strength (K/S) of Control and Mordanted P/V Samples
The K/S value of control (sample dyed using optimised conditions without mordants) and mordanted polyester/viscose samples was determined by measuringsurface reflectance of the samples using a computer-aided Macbeth 3100 reflectance Spectrophotometer and ColourLab plus software. The reflectance values (R) were converted to the corresponding K/S values using the Kubelka-Munk equation [K/S = (1-R)2/2R].
6. Colourfastness of Control and Mordanted P/V Samples
Washing fastness test:The washing fastness test was carried out as per the standard test method IS: 3361-1979in a launderometer in which the samples were subjected to a rotary washing movement using liquid non-ionic detergent solution.
Rubbing fastness test: The fastness of dyed samples to dry and wet rubbing was assessed using a crockmeter as per the standard test method IS: 766-1956.
Perspiration fastness test:The fastness of dyed samples to acidic and alkaline perspiration was estimated using perspirometer as per the standard test method IS: 971-1956.
Light fastness test:Light fastness of dyed samples was evaluated by exposing the samples to day light under prescribed conditions and fading was carried out as per the standard test method IS: 686-1957.
Results and Discussion
Colour Strength (K/S) of Dyed and Mordanted Samples
The K/S values of the samples dyed with Himalayan rhubarb dye and treated with different natural mordants using three methods of mordanting are given in Table 3.
It is evident from Table 3 that the highest K/S value (13.06) among all the mordanted samples was exhibited by walnut bark post-mordanted sample and second highest value (12.45) was exhibited by teapre-mordanted sample. In general, it can be observed from the results that the colour strength of the samples dyed with Himalayan rhubarb dye improved after mordant treatment using pre and post-mordanting methods. In case of simultaneous mordanting method K/S values were found to be decreased in comparison with control sample.
Colourfastness of Dyed and Mordanted Sample
The mordanted samples were subjected to various colourfastness tests. The colourfastness ratings of P/V samples dyed with Himalayan rhubarb dyes after mordant treatment are shown in Table 4.
Washing Fastness
It can be seen from Table 4 that the ratings for change in colour were found to decrease in all the mordanted samples as compared to control sample having rating 5, ie. no change in colour. Slight (4) or slight to no change (4-5) in colour was exhibited by most of the mordanted samples except myrobalan simultaneousmordanted and walnut bark pre and post-mordanted samples that exhibited noticeable to slight (3-4) change in colour. Colour staining ratings on cotton fabric were found to be improved in all the mordanted samples, with slight to no staining (4-5) or no staining (5) as compared to control sample with slight (4) staining. No staining (5) on P/V fabric was exhibited by both control and mordanted samples. According to Singh (2000), good washing fastness of the samples may be due to the reason that some of the natural dyes have a tendency to aggregate inside the fibre, thereby increasing the molecular size and hence exhibit good washing fastness. Complexing with mordants also has the effect of insolubilising the dye and making it colourfast.
Rubbing Fastness
Both the control and mordanted samples were found to be fast towards dry rubbing, with fastness ratings being in the range of 4 to 5. On wet rubbing, noticeable to slight (3-4) change in colour was exhibited by Indian gooseberry, belliric myrobalan, myrobalan, tea and pomegranate simultaneous mordanted and tea post-mordanted samples however slight change (4) in colour was exhibited by Indian gooseberry, belliric myrobalan, pomegranate rind, walnut bark post mordanted and pomegranate simultaneous and myrobalan pre-mordanted samples as compared to control sample with slight to no change (4-5) in colour. Noticeable to slight staining on cotton fabric was exhibited by Indian gooseberry and belliric myrobalan pre mordanted samples similar to that of control sample however in other mordanted samples the colour staining ratings were improved. It is clearly evident from Table 3 that dry rubbing tests showed better results than wet rubbing test as it was having better fastness ratings. This may be because of the breakage of dye metal complexes into simple particles during wet rubbing (Gulrajani, 1993). Perspiration Fastness
It can be observed from Table 4 that in case of acidic perspiration fastness the ratings of change in colour of the mordanted samples either remained same or decreased to slight (4) or noticeable to slight (3-4) change in colour as compared to control sample with slight to no change (4-5) in colour. The ratings of colour staining on both the adjacent fabrics were found to be either similar or improved as compared to control sample with noticeable staining (3) on cotton and slight staining (4) on P/V fabric. It is also evident from the results that in case of alkaline perspiration fastness, the control sample exhibited slight change (4) in colour and noticeable and noticeable to slight staining on cotton and P/V fabrics respectively that either remained as such or slightly improved in all the mordanted samples.
Fastness to Sunlight 
The control samples dyed with Himalayan rhubarb dye exhibited very good light fastness (6) which was further improved to excellent to outstanding (7-8) in tea pre-mordanted and walnut bark post-mordanted samples. In general, the light fastness of samples dyed with Himalayan rhubarb dye improved after mordant treatment. This may be attributed to the presence of anthraquinone structure in Rheum emodi thatshows an increased resistance to light fading. According to Gupta (2000), the quinone based dyes exhibit exceptional fastness properties though dyes based on other chromophoric groups may not be so fast.
The colour obtained by Himalayan rhubarbdyed and mordanted P/V blended fabric under optimized conditions ranged from brown to yellowish-brown.
Conclusion
It can be concluded from the findings of the study that polyester/viscose blended fabric can be successfully dyed with Himalayan rhubarb dye with acceptable fastness properties. Application of mordants further improved the colourfastness to washing, light, rubbing and perspiration. The mordanted P/V blended samples exhibited good to excellent fastness to washing, rubbing, and light and moderate fastness to perspiration. Majority of the mordanted samples exhibited high K/S values in post mordanting method as compared to control sample. The colour shade developed on Himalayan rhubarb dyed P/V samples after mordanting ranged from brown to yellowish-brown. The colour obtained by Himalayan rhubarb dye was fast even in the absence of mordants, hence has a tremendous potential in textile dyeing industry and may gain significant place in the market on the basis of its good fastness properties, and plentiful availability in India.