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Dyeability of Nanofibers with Reactive , Vat and Disperse Dyes

Colorful nanofibers were prepared through electrospinning technique followed by dyeing with different reactive dyes by comparing different dyeing techniques such as ultrasonic as well as conventional (padding method). Excellent color yield and color fastness were achieved through proper optimization conditions in ultrasonic treatment. Nanofibers exhibited smooth and bead free morphology. Two different polymers were selected for electrospinning, i.e. cellulose acetate and nylon-6. Preparation of cellulose nanofibers included electrospinning of cellulose acetate following deacetylation under alkaline conditions. Results suggested that dyes conferred improved color yield on the basis of K/S values. Deacetylation was also confirmed through FTIR. The resulting nanofibers can serve better choice for advanced apparel applications.


I. INTRODUCTION
Nanofibers are drawing intense research interests because of ease of fabrication and wide variety of polymers used for electrospinning. They find wide variety of applications such as filters [1], biosensors [2], [3], wound healing [4], tissue scaffolds [5], drug delivery [6], [7], breathable membranes [8], [9] and nanocomposites [10]. Nanofibers are also considered as excellent water absorbents and offer greater flux for water vapors [11]. Different waterproof breathable nanofibers are reported [12]- [14]. Literature survey reveals broad application of nanofibers for apparel use but colorimetric properties are rarely discussed. The fundamental technique for production of colored nanofibers is to dye the nanofiber sample after electrospinning and color yield is the main criterion in selecting best quality of nanofiber. Therefore, present research focuses on the production of colored nanofibers after post electrospinning treatment through different techniques such as conventional as well as ultrasonic. In present work we have prepared two types of nanofibers, i.e. cellulose nanofibers and nylon-6 nanofibers followed by dyeing with reactive dyes and vat dyes. The dyeing procedure was compared through two techniques, i.e., conventional method and ultrasound assisted method. Different parameters such as color yield (K ⁄ S), color fastness to light and washing tests were assessed for dyed nanofibers. The characterization of resulting nanofibers was obtained by analytical techniques such as scanning electron microscopy (SEM) and FTIR.

B. Nanofiber Preparation and Dyeing Experiment
Polymer solutions for cellulose and nylon-6 were prepared by previously reported methods [15], [16]. Concisely, different polymer solutions were poured in syringes and voltages (around 10-15 kV) were applied to produce fine and thin nanofibers. Rotating metallic drum was used for the collection of electrospun nanofibers until the required thickness of web reached 40-60 μm.
Padding method was used for dyeing of CA nanofibers whereas; batch wise method was adapted for dyeing CNFs and nylon nanofibers. Briefly, padding of CA nanofibers using dye liquor at 65% pickup was carried out followed by drying at 120 o C and curing at 180 o C for 2 minutes. Liquor to good ratio of 50:1 was used in batch wise dyeing for CNFs at 60 o C for 30 min. In order to ultrasonicate nanofibers, sonication frequency was selected at 40 kHz. Samples were measured by K/S values, color fastness to light and washing. The morphologies of nanofibers before and after dyeing were analyzed by SEM.  A. Conventional Dyeing From the Fig. 2(a), it is clear that cellulose nanofibers dyed with reactive dyes showed color yield of 5.5. The K/S values of cellulose nanofibers when dyed with vat dyes was 19 that is 3 times greater than former. The possible reason behind this can be higher molar extinction coefficient of vat dye than reactive. On the other hand, low K/S values were noticed with disperse dyes dyed on nylon-6 nanofibers owing to greater crystalline domain that inhibit their diffusion. Fig. 2(b) presents ultrasonically treated nanofibers using reactive and vat dyes. Contrary to reactive dyes, vat dye showed improved K/S value (7.2) on cellulose nanofibers. This may be due to ultrasonic cavitation generated due to sonication. Ultrasonically dyed nylon-6 nanofibers also showed greater K/S values (2) that was also better than conventional method.

C. Effect of Temperature on Dyeing of Nanofibers.
As ultrasonic dyeing is reported for the very first time, therefore it was necessary to optimize dyeing parameter such as temperature. To begin with this, temperature was optimized using cellulose nanofibers at fixed time i.e. 80 min using C.I RB 5 and C.I RR 195. The color yield using RB% increased steeply from 40 o C to 60 o C that became constant on further rise in temperature as shown in Fig. 3. This could be due to substantivity ratio and hydrolysis of dye molecules on extremely higher temperatures. Irrespective to the dyeing temperature, the color yield of CI reactive black 5 obtained higher than the CI reactive red 195. This may be due to the difference of molar extinction coefficient, affinity and reactivity between both dyes. On the other hand, dye uptake of CI reactive red 195 slowly increased from 40 o C to 60 o C and reached higher yield at 70 o C. This steady profile of CI reactive red 195 is due to lowering of substantivity ratio caused by increasing temperature. Whereas, for nylon nanofibers using disperse dyes the dyeing at fixed time (60 min), the maximum color yield obtained was at 80C beyond which the K/S values decreased due to either dye hydrolysis or migration of dye molecules back to solution phase from nanofiber phase.

D. Effect of Dyeing Time on Color Yield
Considering maximum dye uptake by cellulose nanofibers at 70 o C using RB5, impact of dyeing time on ultrasonic dyeing on cellulose nanofibers was investigated. RB5 shows steep rise in dye uptake with increase in time from 10 min to 20 min, further increase in time did not affect dye uptake and rendered a plateau beyond 20 min in the graph. This may be attributed to ultrasonic cavitation that breaks dye aggregates and tends to increase dye uptake. On the basis of these results 30 min were selected optimum for RB5 dyeing using cellulose nanofibers. Nylon-6 nanofibers also show similar behavior while using disperse dyeing. Whereas conventional dyeing rendered modest dye uptake and reached its maximum capacity at 60 min shown in Fig. 4. It is further suggested that sonication promotes significant mass transfer and transient cavitation near fiber surface are responsible for improved results.  Table I summarizes the the results of color fastness to light and washing. ultrasonically treated dyes showed better light fastness tests (4) compared to conventional. Vat dyes had better light fastness (7). This improvement on light fastness can be due to bulkier molecule size of vat dyes than the reactive. The observed light fastness in case of nylon 6 nanofibers was only 2 regardless of the dyeing technique. Generally, washing fastness of nanofibers was extremely excellent in performance. For instance, shade change of cellulose nanofiber was ¾ and 4 with reactive dyes, however, vat dye showed 4/5 better than the later. However, except cellulose nanofibers, the staining was 4/5 to 5 with other samples. Cellulose nanofibers dyed with reactive dyes presented staining 3 only. Sonication obviously does not International Journal of Chemical Engineering and Applications, Vol. 10, No. 1, February 2019 influence color fastness of the nanofibers. Color fastness to hot pressing was evaluated for each dyed nanofibers, the shade change and staining ratings obtained 4/5 for nanofibers dyed with CI Disperse Red 167:1. The nanofibers dyed by ultrasonic method did not show any difference than the nanofibers dyed by conventional method. Since the CI Disperse Red 167:1 is high energy level dye, it showed better fastness (hot pressing) results  Fig. 5 reveals SEM images of dyed cellulose and nylon nanofibers. All nanofibers were smooth and bead free in morphlogy. It can also be observed that the diameter of cellulose nanofibers was greater (400 nm to 600 nm) than nylon-6 (80-150 nm. Dyeing procedure had no detrimental impact on nanofibers morphology. However, ultrasonic treatment caused a little detrimental impact.

A. FTIR Characterization
The successful conversion of CA nanofibers into CNFs and subsequent dyeing was confirmed through FTIR as shown in Fig. 6. The disappearance of characteristic peaks from cellulose acetate i.e. 1745 due to C=O, 1375 due to C-CH3, 1235 due to C-O-C confirms the successful conversion of cellulose acetate to cellulose. However, there was no significant change in the IR absorption bands of cellulose before and after dyeing that implies that ultrasonic treatment has no influence on chemical structure of cellulose.  Fig. 7 shows comparison of tensile strength between US dyed nylon-6 nanofibers and conventionally dyed nanofibers. Since cellulose nanofibers were not in condition and lost their strength after US dyeing, therefore, nylon-6 nanofibers were considered suitable for measuring mechanical properties. Nylon-6 nanofibers dyed with US technique showed a substantial improvement in mechanical strength (3 to 3.5 N tensile strength) in contrast to the conventional dyeing. The probable reason behind this can be cavitation during ultrasonic dyeing that imparted stiffness to the nylon-6 nanofibers. To confirm the influence of dye class on mechanical strength of nylon-6 nanofibers the nanofibers were subjected to US in water referred as blank. It was observed that dyeing class had no impact in imparting mechanical strength. Their greater stiffness and tensile strength can trigger its applications as surgical apparel.

IV. CONCLUSION
Cellulose and nylon-6 nanofibers were successfully prepared and dyed using different dyes. Excellent color yield was obtained using cellulose nanofibers than nylon-6. US dyeing of nanofibers showed better color yield than conventional dyeing. US dyeing completes within at 30 min, which is half of the conventional dyeing, which demonstrates substantial time reduction. Irrespective to the dye class used, the US dyeing showed a significant increase in %F as compared to conventional dyeing, however, the %F decreased upon dye concentration increased. Dyeing of nylon-6 nanofibers with conventional method showed low K/S values, however similar dyeing with ultrasonic method improved its color yield. SEM images confirmed smooth and beadles nanofibers. On the basis of the color fastness and light fastness tests, it can be suggested that nanofibers can be effectively used for apparel applications. Color fastness tests indicate good dye fixation based on good to very good rating in color fastness to washing and hot pressing. The nanofibers dyed with US dyeing method showed a significant increase of tensile strength in comparison to conventional dyeing method.