Abstract
Flax fabric was pretreated conventionally in such a way that different levels of residual noncellulosic content could be obtained on the material. These fabric samples were dyed with three structurally different direct dyes to study the effects of noncellulosic content on dyeing behaviour. The dyeing characteristics were analysed by various techniques, namely colour strength, fastness properties and evenness. Colour exhaustion, strength and fastness properties of dyed fabrics and the levelness of dyeing were found to be improved with the removal of noncellulosic impurities from the grey material, up to a certain level, beyond which they did not change significantly.
Keywords: Flax, residual noncellulosic content, dyeability, direct dye.
Introduction
Wet processing of flax has gained importance in recent years on account of its outstanding features and diversified uses. Flax, being a bast fibre, obtained from the plant ‘Linum Usitatissimum’, consists of cellulose as the major component, along with other noncellulosic compositions, namely hemicellulose (? 18%), pectin (? 3%) and lignin (? 2%) (1,2). Being cellulosic in nature, it can be dyed with direct dyes7 similar to other cellulosic fibres.
However, the presence of a high amount of noncellulosic impurities and highly oriented structure make it difficult to dye with the usual dyeing methods used for cotton. Dyeing behaviour of cotton and other cellulosic fibres with direct dyes have been studied by many researchers. However, very little literature is available pertaining to the dyeing behaviour of flax in general, and particularly with direct dyes. No scientific work has been carried out to study the effects of residual noncellulosic content (RNC) on flax fibre dyeing with any colours.
Direct dyes, being anionic in nature, have substantivity for cellulosic fibre and can be applied on flax fibre also. The removal of noncellulosic compositions at a particular level from grey flax is the prime factor which may decide the dyeing properties. The pretreatment of flax mainly consists desizing, scouring and sometimes bleaching.
The pretreatment of grey flax fabric is carried out in such way that the removal of noncellulosic impurities can be obtained in a level at which the best dyeing behaviour can be obtained. (3,4) In the present investigation, grey flax fabric was pretreated in one step, using caustic soda, and the amount of residual noncellulosic contents were determined. An attempt was made to evaluate the effects of residual noncellulosic content (RNC) on the dyeability of flax with direct dyes. The RNC vis-à-vis the individual constituents on flax fabric were also examined in order to co-relate them with the dye structure and their concentration on the substrate.
Experimental
Materials
Fabric
A grey flax fabric (warp & weft count 10s and wt/mt2 = 252) sample was obtained from Jayshree Textile Mills, W. Bengal, India. Fabric samples of various levels of RNC were obtained by a conventional pretreatment process using caustic soda. The details of the process conditions and the fabric samples are reported in Table 1.
Dyes and Chemicals
Three structurally different direct dyes were used in the present investigation after purification (Table 2)5. All the chemicals and auxiliaries used in the present study were of laboratory reagent grade.
Dyeing Procedures
Flax fabric samples (2g) were dyed with direct dyes (D1, D2 and D3) in various percentage shades (0.5%, 1% and 3% owf), at a temperature of 90°C for 2h using a liquor-to-goods ratio of 50:16. After the stipulated dyeing time, each sample was washed thoroughly with distilled water, soaped at 50°C with 0.5% (% w/v) anionic detergent for 10 minutes, washed with distilled water until neutral pH and dried.
Testing and Analysis
The percentage weight loss of the pretreated sample was determined by measuring the differences in weight of the sample before and after treatment and after conditioning (temperature – 27 ± 2°C and relative humidity – 65 ± 2%).
Hemicellulose content of the flax fabric sample was determined by the standard method of Turner and Doree. The same method was used to determine the total residual noncellulosic content of the fabric sample, and from the said values and percentage weight loss the residual non cellulosic content was calculated.
Moisture regain values, whiteness index (Hunter whiteness) and yellowness index (ASTM 313) of various samples were determined by the prescribed standard method stated earlier. (S D Bhattacharya & S R Shah, Color. Tech, 118 (2002) 295).
To determine the percentage dye exhaustion, aliquots was taken from the bath before and after dyeing. The exhaustion percentage of dyed fibre was determined from values of absorbance of the original and exhausted bath and using Formula 1 (below). Absorbance of the dye solution was measured at the respective wavelength of maximum absorption using UVVisible spectrophotometer (Model: 119, Systronic, India).
Dye exhaustion percentage = ( 1 – A1/A0 ) X 100 (Formula 1) where A0 & A1 are the dye concentration in dyebath before and after dyeing respectively. Colour strength of samples after dyeing was also determined in terms of K/S strength on a UV-Visible spectrophotometer, using the Kubelka-Munk equation.(8)
Fastness properties of pretreated flax fabric to residual noncellulosic content of 9.37% dyed in 1% (owf) shade was only determined. Washing fastness of dyed sample was determined using Launder-o-meter of EEC make (India) as per ISO – II specification. Colour fastness to rubbing of dyed sample was determined in dry as well as in wet conditions as per IS: 766 – 1956 method. Fastness to light was determined as per IS: 2454 – 1967 using Xenon arc continuous illumination on Fade-o-meter.9,10
Results and Discussion
Flax fabric, of various levels of residual noncellulosic contents vis-àvis hemicellulose contents, was prepared using conventional sodium hydroxide pretreatment method. Flax can be dyed with direct dyes like cotton and other cellulosic fibres. However, this fabric in its grey state has a high proportion of noncellulosic impurities (27.4% owf) and does not exhibit even and good dyeing performance. Chemical analysis of grey fabric showed that the largest constituent of noncellulosic impurities of fibre was hemicellulose (17.83%), followed by pectin (2.35%), lignin (2.19%), fats and waxes (1.20%). (28)Therefore, this fibre was pretreated to different extents of noncellulosic content (RNC), ie. hemicellulose content (HC), and its dyeing behaviour was evaluated.
Flax fabric of different levels of RNC vis-à-vis hemicellulose, along with grey fabric, was dyed with three structurally different direct dyes and the exhaustion percentage (in 1% shade) at equilibrium was determined (Table 3) and colour strength (K/S) in three different percentages (0.5, 1 & 3%) was also determined (Table 4).
It can be seen that the dye-exhaustion percentage of direct dye was found to increase with the removal of noncellulosic impurities from the flax fabric. For example, in the case of dye D1 the equilibrium percentage exhaustion in 1% depth of SL 1, SL 2, SL 3 and SL 4 was 31%, 48%, 61% and 74% respectively. Similar trends can also be seen for other two dyes (D2 and D3).
The colour-strength values (K/S) of all the direct dyes in three different percentages shows the same behaviour, ie. an increase in K/S value with the decrease in RNC (Figure 1). During dyeing, direct dye is adsorbed by the flax fibre from an aqueous solution. Noncellulosic constituents present in flax form a hydrophobic coating on the surface, which prevents the adsorption and diffusion of dye molecules inside the fibre. Dye uptake is proportional to the accessible region in the fibre (15,16). With removal of the noncellulosic material, the area accessible to dye molecules increases. This can be confirmed from the increase in the values of wicking length, ie. from SL 1 (2.5cm) to SL 4 (25.4cm) (Table 1).This is the probable reason for the increase in the equilibrium dye exhaustion or K/S on progressive removal of noncellulosics.
The removal of noncellulosic impurities also confirms the increase in whiteness and decrease in moisture regain and yellowness index (Table 1). Further, hemicellulose, the main constituent of the noncellulosic component present in flax, is mainly attached to the cellulosic component of the fibre by hydrogen bonds. With the subsequent removal of noncellulosic impurities vis-à-vis hemicellulose, hydrogen bonding capacity of cellulosic molecules increases (17,18). As a result a greater amount of dye attaches to the main cellulosic fibre with the decrease in RNC.
Dyeability of flax was also found to be influenced by the dye structure, similar to other cellulosic fibres (11). The results indicate that, in identical conditions of dyeing, colour strength of the dye on flax varies with type of direct dye. The equilibrium dye exhaustion percentages in 1% shade of D1, D2 and D3 are 74%, 68% and 61% respectively for SL 4 (RNC = 9.37%). The direct dye, having low molecular weight with linear structure, gives higher exhaustion on cellulosic fibre. Further, absorption of direct dye on cellulose is partly due to the H-bonding between primary hydroxyl group (–OH) of cellulose in the amorphous region and the electron donating or proton donating groups of the dye molecule (12,13). Exhaustion is also favoured if H-bonding groups in the dye are spaced by the repeat distance of the cellulosic unit (10.3 A°) (14).
All these structural factors may affect the exhaustion of dye in the case of flax also. In the present study the three dyes selected have different structures. Dye D1 has the lowest molecular weight (485 g/mole) and a lower number of sulphonic groups, followed by dyes D2 (634 g/mole) and D3 (1030 g/mole), and as a result differential dyeing behaviour can be observed.
Fastness Properties of Direct Dye on Flax Fabric
Flax fabric pretreated to a level of noncellulosic content of 9.37% (HC = 7.35%, SL 4) was dyed in a 1% shade using three different direct dyes and evaluated for fastness properties. The fastness properties of dyed materials were determined in terms of washing, rubbing and light and reported in Table 5.
The washing fastness of D1 and D2 on flax fabric was average (3 to 4 ratings), while that of D3 was poor (2 to 3 ratings). The fastness to change in colour on washing was less compared to staining for all the direct dyes. Poor washing fastness on cellulosic fibre of direct dyes has been reported by many workers. The fastness to washing can be improved to a certain extent by aftertreatment with a dye-fixing agent or other treatments.
Rubbing fastness ratings of direct dyes on flax fabric is in the range of 2 to 3–4, which is considered as inferior. The higher coefficient of flax fibre, along with a more irregular surface structure, may produce a higher abrasive force on fibre during rubbing. This may cause breaking of some dye-fibre bonds and result in poor rubbing fastness. Light fastness of direct dye on flax fabric is comparable to other cellulosic fibres. The light fastness is mainly influenced by dye-fibre interaction, ie. the stronger the dye-fibre interaction, the higher will be the light fastness, because of a low rate of fading, and vice-versa.19,20 In flax, like other cellulosic fibres, dye-fibre interaction is weak and hydrogen bonding causes poor light fastness.
Conclusions
Flax fabric dyeings are not as simple as other cellulosic fibres because they are greatly influenced by the presence of noncellulosic content. The dyeability of flax fabric increases with the progressive removal of noncellulosic impurities with direct dye. The exhaustion equilibrium of direct dyeing on flax fabric is obtained at the specific level of RNC (9.37%) vis-à-vis hemicellulose (7.35%). The fastness to washing, etc, of direct dyes – as on other cellulosic fibres – is average to poor on flax fibre but can be improved by suitable aftertreatments. The dye structure also plays an important role in the dye exhaustion of direct dye on flax, ie. dye having a low molecular weight and/or linearity of structure shows higher exhaustion, and vice-versa.
References
1. Turner A. J., J. Text. Inst., 40 (9), 972, (1949).
2. Grayson M., ‘Encyclopedia of Textiles, Fibres and Nonwoven Fabric’, (John Wiley & Sons, NY, 1987), 174.
3. Zhou L. M., Yeung K. W., Yuen C. W. M. and Zhou X., Color. Technol., 119, 170, (2003).
4. Chrastil J., Reinhardt M. R. and Blanchard J., Text. Res. J., 60, 441, (1990).
5. Robinson C. and Mills, Proc. Roy Soc; A131, 596, (1931).
6. Chakravartty R.R. and Trivedi S.S., “Technology of Bleaching and Dyeing of Textile Fibres”, 1, Part I, (Mahajan Brothers, Ahmedabad, 1979), 50.
7. Samanta A.K., Agarwal P. and Datta S., Ind. J. Fibre Text. Res., 32, 466, (2007).
8. David G. Duff and Roy S. Sinclair, “Giles’s laboratory course in dyeing”, 4th Edn., (Society of Dyers and Colourists, Bradford, UK, 1989), 131-147.
9. Okada Y., Hirose M., Kato T., Motomura H. and Morita Z., Dyes and pigments, 14, 113, (1990).
10. ISI (BIS), “Handbook of Textile Testing” (Bureau of Indian standards, N.Delhi) 1986.
11. Crank J., J. Soc. Dyer. Color., 63, 293, (1947).
12. Daruwala E. H., Kangle P. J. and Naber G. M., Text. Res. J., August 1961, 712-721.
13. Rose F. L. quoted by T. Vickerstaff, “The Physical Chemistry of Dyeing”, 2nd Edn., (Oliver & Boyd, London, 1955) 179.
14. Shore J., ‘Cellulose Dyeing’, (Soc. of Dyers and Colourists, Oxford, 1995), 163.
15. Chrastil J., Text. Res. J., July 1990, 413-416.
16. Davis E. and King R. R., J. Soc. Dyer. Color., 109, 342, (1994).
17. Sarkar P. B., Majumadar A. K. and Pal K.B., J. Text. Inst., 39, T44, (1948).
18. Gill R., J. Soc. Dyer. Color., 71, 380, (1955).
19. Suganuma K. and Kuno H. J., J. Soc. Dyer. Color., 102, 100, (1986).
20. Teli M. D., Adivarekar R. V. and Pardeshi P. D., Ind. Text. J., Nov 2000, 13.