Abstract
M-aramid fibers have very good flame-retardant properties and are mostly used in protective textiles such as racing or firefighter suits. The dyeing and fastness properties of m-aramid fibers are investigated in this article in a batch process to optimize dyeing parameters such as dye concentration, salt, swelling agent, and temperature. The exhaustion at an acidic pH, preferably in the range of 4–5, indicated good results. The color yield of m-aramid fibers was found to be dependent on the concentration of swelling agent, salt, and dye, as well as the dyeing temperature. The effect of swelling agent on the K/S of m-aramid fibers with cationic dyes, along with their fastness properties, is investigated and reported in detail.
M-aramid belongs to the aromatic polyamide family that contains high-performance characteristics such as high melting point, high strength, resistance to organic solvents, and high flame retardancy. These characteristics make this fiber well suited for high-performance protective and technical textiles (Preston & Hofferbert, 1979; Xie, Yang, Yan, & Yao, 2010). M-aramid, due to its nonflammable chemical structure, has many applications in the area of thermally resistant and fire-retardant textiles (Baeva, Manyukov, Sadova, Konovalova, & Negodyaeva, 2007; Hearle, 2001). This fiber can be used in protective garments for the sportswear, military, filtration, oil refinery, and racing wear industries, as well as for filter bags, wire coverings, electrical insulation, and ornamental materials such as floor coverings, blinds, carpets, ship tapestry, and so forth (E. Kim & Choi, 2011; Tajul, Aimone, Ferri, & Rovero, 2015).
It is difficult to dye aramid fibers (m-aramid and p-aramid) due to their highly crystalline structure and high glass transition temperature (Volokhina, 2003; Zheng & Zheng, 2014). It is established in the literature that aramid fibers are very difficult to dye with traditional techniques (Han & Jaung, 2009; Manyukov, Sadova, Baeva, & Platonov, 2005). Due to the presence of amide groups, aramid fibers have strong hydrogen bonding among their molecular chains, making the structure more compact and crystalline, and thus difficult to dye via conventional processes (E. Kim & Choi, 2011).
Diffusion of dye depends upon the polymer chains’ mobility, which further depends upon the T g (glass transition temperature) of a particular fiber (Nechwatal & Rossbach, 1999). In order to obtain the proper penetration of the dye into the amorphous region of aramid fibers, the chains need to become flexible, either by using swelling agents/carriers or dyeing at elevated temperatures—probably 190 °C, which is not feasible (E. Kim & Choi, 2011). These carriers are used to swell the fiber, making room for the dye molecules to penetrate without deteriorating the fiber’s properties such as mechanical strength, shrinkage, and so forth (E. Kim, Min, & Jang, 2011).
Beal et al. (1973) dyed aramid fibers with cationic and disperse dyes by using phenoxy ethanol as a dye carrier, and they claimed it showed excellent color yield. Holsten and Smith (1993) invented the process for dyeing aramid fibers with cationic dyes (Basacryl Blue X-3GL, Basacryl Red GL, Basacryl Golden Yellow X-GFL) and the dye carrier N,N-diethyl m-toluamide, achieving satisfactory color yield. Tajul, Aimone, Ferri, and Rovero (2015) studied the kinetics and equilibrium adsorption of Basic Blue 41 on m-aramid fibers by using the swelling agent N-methylformanilide. They found that the amount of dye adsorbed increased as the concentration of the swelling agent increased, and the fibers showed good washing and rubbing fastness properties.
Moore and Weigmann (1986) pretreated fibers with different swelling agents (dimethylformamide, dimethylacetamide, dimethyl sulfoxide) to make the m-aramid fibers dyeable with cationic dyes. They found that the color yield increased when the fibers were pretreated with a swelling agent, but the mechanical properties were substantially reduced. In another patent, Sapers (1972) explained the dyeing process of aromatic polyamides with cationic dyes by using a mixture of cyclohexanone (52%) and acetophenone (46%) as the swelling agent. He claimed satisfactory shade depth and good color fastness properties. Riggins (2005) invented the process of m-aramid dyeing with acid dyes by using dimethylbenzamide as the swelling agent. He claimed that this process showed good color yield as well as flame retardancy.
Herms (1973) invented the process for the pretreatment of aramid with diethylene glycol to improve the dyeability of the fabric. Han & Jaung (2009) studied the acid-dyeing properties of m-aramid fibers pretreated with PEO45-MeDMA. In their study, they mentioned the color yield of pretreated fibers was greater than untreated fibers, but the maximum K/S value that could be achieved was 4. Nechwatal and Rossbach (1999) studied the effect of different dye carriers (benzyl alcohol, acetophenone, dimethyl sulfoxide, and dimethyl acetamide) on the cationic dyes’ performance on m-aramid fibers. They found that benzyl alcohol and acetophenone show better carrier effect with a loss in mechanical properties. E. R. Kim, Kim, and Lee (2013) used vat dyes to dye m-aramid fibers; they achieved good washing and perspiration fastness properties, but rubbing and light fastness were unsatisfied. The color yield on m-aramid fibers was found to be very low.
In the previous studies, most of the authors’ work is patented and not disclosed. Some researchers have worked on dyeing m-aramid fibers, but they could not achieve a substantial K/S value, of which the maximum 13 was mentioned in the literature. No one had optimized the process of dyeing m-aramid fibers, either. This study is focused on the process optimization of dyeing m-aramid fibers with cationic dyes using a swelling agent as well as the effect of different variables to achieve increased shade depth.
Materials and Method
Hundred percent m-aramid fibers were provided by Shahbaz Garments (Midas Safety), and the commercial cationic dye Red GTL 200 was obtained from CHT Bezema. Benzyl alcohol (99.5%) was purchased from Daejung Company, Korea. Sodium hydroxide was obtained from Riedel-de Haen, Germany. Hundred percent acetic acid was obtained from Merck, Darmstadt, Germany. Sodium carbonate and sodium nitrate were laboratory grade. All the chemicals were used as received.
Design of Experiment (DOE)
The DOE is meant to identify the parameters and processing conditions that affect the dyeing process and optimize the levels and their factors to obtain the best response. DOE helps us study the effects of input variables on output variables.
The DOE was made using the Box–Behnken method for dyeing m-aramid fibers in order to investigate the effect of levels of different input variables on response variables. When the operating zone is known, the Box–Behnken design is very useful. Every factor contains two levels in this design, as shown in Table 1.
Factors and Their Levels.
Table 1 shows two levels of each factor, which were studied to optimize the shade depth of m-aramid. The lower and upper levels of each factor were used for experimental design, and their selection was based on the pretrials and information provided in the established literature. Table 2 shows 27 designed experiments with different combinations of levels. All the experiments were performed according to run order.
Design of Experiment.
Process Flowchart of Fiber Dyeing
The basic steps involved in the dyeing process of m-aramid fibers are given in the flow chart, as shown in Figure 1. The fibers were first scoured with NaOH, followed by rinsing and scouring. After that, fibers were dyed in different steps and finally washed to remove unfixed dye from their surface. The detailed dyeing procedure is described in the Experimental Procedures section.
Flowchart of m-aramid processing.
Experimental Procedures
Hundred percent m-aramid fibers were scoured at 70 °C with a liquor to goods ratio of 70:1, containing 3 g/L of NaOH, 1 g/L sequestering agent, and 1 g/L wetting agent. After rinsing the fibers with cold water, they were washed at 70 °C, adding two drops of acetic acid in order to neutralize the effect of alkali used during scouring. After this, the fibers were rinsed and dried.
Dyeing experiments were carried out in an exhaust process using a high-temperature dyeing machine (Tsuji Dyeing Machine Mfg. Co. Ltd., Osaka, Japan). The machine used was equipped with eight beakers of 250 ml heated by three IR lamps. The dyeing profile developed is shown in Figure 2. A dye bath was prepared with cationic dye according to the DOE (1–6% on the weight of fabric), including the swelling agent (20–40 g/L) and salt (20–40 g/L), with a liquor to goods ratio of 70:1. Dyeing was started at 40 °C by maintaining pH at 4.5 using acetic acid. Then, fibers and swelling agent were added to the bath at Position A, and the process continued for 10 min at 50 °C. After 10 min, dye was added at Position B in the dye bath, and the sample was run for 10 min. After that, salt was added at Position C; the temperature was raised to 100 °C, 115 °C, and 130 °C according to DOE at Position D and run for 45 min. At Position E, the dye bath was cooled down, and fibers were washed thoroughly with 1 g/L standard soap at 80 °C for 20 min to remove unfixed dye and the benzyl alcohol deposited on the fiber surface. Fibers were rinsed with cold water and dried.
Dyeing procedure of m-aramid fibers.
The tensile strength of fibers was obtained by a single fiber strength tester (Testometric 2.5) using a testing speed of 10 mm/min and a gauge length of 25 mm. The samples were tested in the laboratory at standard conditions of temperature and relative humidity (25 °C and 65%, respectively). The strength of the fibers was calculated before and after dyeing by taking the average of five fibers from each sample. The color depth of the fibers was determined using K/S values with a spectrophotometer that had illuminant (D65) at angle (D/0) standard observer. The washing fastness test was performed according to ISO 105-C 03 standard (International Organization for Standardization, 1989), and the changes in shade and staining were rated in accordance with ISO 105-A02 gray scale (International Organization for Standardization, 1993). The rubbing fastness was investigated using ISO 105-X12 standard procedure (International Organization for Standardization, 2016).
Result and Discussion
The dyeing uptake mechanism of cationic dyes follows the Langmuir adsorption, which states that “the adsorption of dye occurs due to the ion-ion interaction between substrate and the dyes” (E. Kim & Choi, 2011). The effect of different parameters on responses will be discussed in this section.
Effect of Temperature and Dye % on Fiber’s K/S Value
The dependence of K/S value on dyeing temperature is shown in Figure 3, which illustrates that the increase in color depth is caused by increase in temperature. The effect of increasing temperature can be explained as follows: The movement of chains is enhanced as the fiber structure swells, causing more dye molecules to penetrate into the fiber, thus improving the K/S value. Higher temperatures can also promote the migration of dye molecules, which increases their rate of diffusion into the fiber.
K/S value depending upon the temperature and shade %.
Effect of Swelling Agent and Temperature on Fiber’s K/S Value
Ingamells, Peters, and Thornton (1973) explained that the plasticization of synthetic fibers increases when the concentration of phenol is increased in an aqueous solution. Benzyl alcohol acts as a plasticizer, which disturbs the polymeric chain packing; as a result, the mobility of fibers increases as the intermolecular forces are reduced (Aitken, Burkinshaw, & Price, 1992).
Figure 4 shows K/S value, which is directly affected by the swelling agent. This can be explained by the previously mentioned hypothesis; furthermore, as the swelling agent has a low molecular size compared to the dye molecule, it easily penetrates and swells the fiber, allowing it to be dyed at a lower temperature than expected. It is also shown in Figure 4 that by increasing the temperature, the K/S value is improved. This is again due to the enhanced mobility of molecular chains in the fiber, which enables more molecules to penetrate, thus increasing the K/S value.
Dependence of K/S value on swelling agent and temperature.
Effect of Salt on Fiber’s K/S
From Figure 5, it is obvious that there is no significant effect of salt on K/S value. Initially, increasing salt concentration delivers no change in shade depth, but beyond a certain level, a slight increase in K/S value occurs. This might be due to the fact that salt acts as an exhausting agent when transferring the dye into the fiber, due to its solubility in solution; this forces the dyestuff to leave the solution approaching the substrate. Once the dye is adsorbed into the fiber, it diffuses into it.
Dependence of K/S value on shade % and salt.
Analysis of Variance (ANOVA)
ANOVA for color strength (K/S) is given in Table 3. It is clear from Table 3 that all the experimental variables have statistically significant impacts on the color strength (K/S) of the dyed fiber. The effects of temperature (X1) and dye (X2) are not strictly linear, as indicated by the statistically significant terms X1 × X1 and X2 × X2 in Table 3 and Figure 6. There is a statistically significant interaction between dye and swelling agent concentrations (X2 × X4), as shown in Table 3.
Analysis of Variance for Color Strength (K/S).
Note. X1 = temperature (°C); X2 = dye (%); X3 = salt (g/L); X4 = swelling agent (g/L).
Main effects plot for K/S.
Figure 6 gives the main effects plot for K/S. It is evident from Figure 6 that increasing dyeing temperature (X1) from 115 °C to 130 °C results in a sharp increase in color strength (K/S), whereas increasing temperature from 100 °C to 115 °C does not have any significant effect on K/S. The effect of increasing temperature can be explained by the increase in kinetic energy and the movement and swelling of molecular fiber chains, which causes more dye molecules to penetrate the fibers, leading to an improvement in K/S value. Higher temperature can also promote the migration of dye molecules, which increases their rate of diffusion into the fiber. The results imply that higher temperatures are needed when dyeing m-aramid, compared to polyamides such as Nylon 6, which can be satisfactorily dyed under 100 °C.
Figure 6 shows that the swelling agent (X4) has a much more pronounced effect on K/S than the salt (X3). Ingamells et al. (1973) explained that the plasticization of synthetic fibers increases when the concentration of phenol is increased in an aqueous solution. Benzyl alcohol acts as a plasticizer, which disturbs the polymeric chain packing; as a result, the mobility of fibers increases as their intermolecular forces are reduced (Aitken et al., 1992). The swelling agent has a lower molecular size compared to the dye molecule, so it easily penetrates and swells the fiber.
Figure 7 gives the interaction plot between dye concentration (X2) and swelling agent concentration (X4), which was found statistically significant as shown in Table 3 (p value for X2 × X4 = .033). It is clear in Figure 7 that for higher dye concentrations (X2), higher concentrations of swelling agent (X4) are required to achieve better color strength (K/S).
Interaction plot for K/S.
Figure 8 shows the images of different dyed samples that were taken from the spectrophotometer. Rows show the effect of different concentrations of swelling agent on the same dye percentage. From A, B, and C, it can be seen that shade depth is enhanced by increasing the concentration of swelling agent at different dye percentages.
Spectrophotometer images.
Dyeing Mechanism of M-Aramid Fibers With Cationic Dye
Figure 9 is a schematic diagram indicating the dyeing mechanism of the m-aramid fibers with cationic dyes. After a swelling agent is added to the dye bath, it penetrates and swells the fiber. After that, the dye dissolves in the solution and is adsorbed in the fiber surface. The addition of salt improves the surface adsorption due to dye aggregation, and the remaining dye molecules from the solution exhaust into the fiber, enhancing the shade depth.
Dyeing mechanism of m-aramid fibers with cationic dye.
Measurement of Strength by Single Fiber Strength
The tensile strength of the fibers was measured before and after dyeing using a single fiber strength tester, and slight decrease in the strength occurred. This may be due to the fact that dyeing fibers at a high temperature causes their morphology to change, which affects the physical properties of fibers such as strength. The reduction in strength with respect to swelling agent and temperature is shown in Figure 10.
Effect of temperature on single fiber strength.
Fastness Properties
M-aramid fibers dyed with cationic dyes have overall very good washing fastness. Table 4 shows the rating of color staining to different fibers during washing, indicating very good results. The acrylic fiber was more stained compared to others, owing to its affinity with cationic dyes. The rating for fastness to crocking was also excellent.
Fastness Properties.
FTIR Analysis of M-Aramid Fiber
FTIR spectra of the dye and dyed fiber are given in Figure 11. In the spectrum of dyed fiber absorption, peaks are at 1,700 cm−1 and 3,300 cm−1, indicating the carbonyl group (CO) and secondary amine (–NH), respectively. The spectrum of dye shows various absorption peaks in Figure 11. As the spectra of dye and dyed fibers are comparable, it is clear that the dye penetrates the fiber.
FTIR spectra of dye and dyed fibers.
Conclusion
The optimization of the dyeing process of m-aramid fibers with cationic dyes is reported in this research work. Dyeing and color fastness properties of m-aramid fibers with cationic dyes have also been investigated. The shade depth of cationic dyes was found to be dependent on the amount of salt, dyeing temperature, concentration of swelling agent, and dye %. Excellent color yield was achieved with cationic dyes, and the K/S value was found proportional to the temperature and dye percentage. Auxiliaries like salt and swelling agent increase the adsorption and shade depth of cationic dyes with enhanced color fastness. The optimal process for obtaining a good shade depth with minimal loss of strength is to dye the fiber at 115 °C, containing 6% shade of dye, 40 g/L swelling agent, and 30 g/L of electrolyte in accordance with the response optimizer.
Footnotes
Acknowledgments
The authors acknowledge Midas Safety (Shahbaz Garments Ltd.), Pakistan, for funding this research.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research and/or authorship of this article: This study received funding from Midas Safety (Shahbaz Garments Ltd.).