1. Introduction

Apocynum venetum is a wild plant and an environmentally friendly fiber, and it has become the subject of interest and gaining more and more attention due to its anti-hypertensive, anxiolyticlike activities, antibacterial function, health protection, etc. [1, 2].Besides, natural A. venetum fiber exhibit excellent fundamental roles in diverse applications, such as textile industry and daily life [3-6]. However, it is always blended with cotton to use due to its limited source, expensive price and poor spinning.

As we know, cellulosic fiber is commonly dyeing with reactive dye, and it can be hardly dyeing with acid dye. Because the acid dye is the relatively small size of the molecule with simple structure, and it is lack of longer conjugated double bond, that means it has low substantively and affinity for cellulose. Thus, up to date A. venetum/cotton blended fabrics dyeing with acid dye has rarely been reported.

The utilization of cationic copolymer nanoparticles has become a vital feature in coating industries, papermaking, paints, textile and fiber treatments, functional materials, and so on [7-10]. Since cationic nanoparticles with positive charge exhibit attractive characteristics such as preferentially adding/adsorbing the negative charge on surfaces, eliminating electrostatic repulsion, and improving properties of materials and subsequent process [11].

In most cases cellulose fiber surfaces have negative charges, thus, some researchers introduced positively charged sites on cotton fibers surface, and the cationized cellulosic fibers can be dyed with acid dyes in order to improve the affinity of anionic dyes toward the fabric by forming the electrostatic attraction [12-14].

The aim of this work to determine the effectiveness of cationic copolymer nanoparticles as a treatment agent of A. venetum/cotton blended fabrics in improving the dyeability of fabrics dyeing with Acid Red B. Thus, A. venetum/cotton blended fabrics were treated with different nanoparticles concentrations, treatment time, treatment temperature and other modification factors before dyeing with acid dye. And the color parameters L^*, a^*, b^* and C^* values of the fabrics were measured after the fabrics dyeing with Acid Red B. The morphology of the fiber surfaces modified by the nanoparticles was observed by a field emission scanning electron microscopy (FE-SEM).

2. Results and discussion 2.1. Scanning electron microscopy (SEM) analysis

Scanning electron microscopy (SEM) was used to determine the effects of the application of cationic copolymer nanoparticles on the topography of untreated and treated blended fabrics (Fig. 1). Fig. 1a shows the fiber has a rough surface before the adsorption, which will benefit the adsorption of nanoparticles. Fig. 1b demonstrates the SEM photographs of the nanoparticles adsorption onto the fiber after the copolymer cationic nanoparticles treatment. It is clear to see that the nanoparticles on fiber surfaces are uniformly distributed, and they have spherical morphology and form monolayer on the fiber surface, similar as the previous phenomenon [15-17]. Furthermore, it also showed that nanoparticles can be adsorbed on the surface of modified A. venetum or cotton fiber uniformly. Thus, treated A. venetum and cotton fibers have the same adsorption ability to anionic dye (Acid Red B).

2.2. Nanoparticles concentration

Firstly, we used different concentration nanoparticles to treat A. venetum/cotton blended fabrics ranging from 0 to 8 g/L, afterward the fabrics were dyed with Acid Red B at room temperature to evaluate the modification effect. As shown in Fig. 2, L^* value decreased from 90.1 to 77.9 with the nanoparticles concentration increasing from 0 to 0.5 g/L. However, further increasing the concentration does not evidently decrease L^* value. Furthermore, a^*, b^* and C^* values were increased from 6.9 to 27.4, 2.9 to 8.4 and 7.5 to 28.7, respectively. These indicated that the redness, yellowness and chroma of the dyed fabric increased. Thus, it was confirmed that modification of nanoparticles on blended fabrics had a better color performance compared with untreated samples.

2.3. Treatment temperature

A. venetum/cotton blended fabrics were modified by 0.5 g/L nanoparticles at different temperature for 120 min, and the obtained data were presented in Fig. 3. It revealed the temperature was increased from 30 ℃ to 40 ℃, and all the color parameters changed obviously. L^* value decreased from 84.9 to 74.7, which indicated that the color of the dyed fabric was darker. While a^*, b^* and C^* increased rapidly, these showed that the color light and chroma of dyed fabrics were better than before. It might be due to that the increasing of the temperature made the adsorption capacity of nanoparticles on the fiber surface increased. When the temperature was elevated to 60 ℃, the distribution of nanoparticles on the fiber is more uniform. So a^*, b^* and C^* values all increased, and the L^* value decreased slightly. A further increase of the temperature to 70 ℃, all the color parameters had not a significant change. Therefore, the treatment temperature was more suitable at 60 ℃.

2.4. Treatment time

As shown in Fig. 4, the a^*, b^*, and C^* values were increased with the time increasing, but L^* value decreased from 75.1 to 73.8. After the treating time reached to 60 min, the color parameters changed slightly. These could be explained that the adsorption and distribution of polymer nanoparticles on the surface of the fiber have been completed in the 60 min. While the time was prolonged to 150 min, the L^* was increased obviously, so the color of the fabrics became light. Besides, a^*, b^* and C^* values decreased sharply. It was revealed that the color and chroma of the dyed samples were poor. It might be due to that too long treatment time lead to the desorption of the particles from the fiber, which would greatly reduce the number of positive charges. Thus the treatment time in 60 min was more appropriate.

2.5. pH value

In the condition of 0.5 g/L nanoparticles, treat 60 min at 60 ℃, the influence of pH value on the color parameters of A. venetum/ cotton blended fabrics was given in Fig. 5. It is clear that the pH value is in the range of 2.3-6.4, the L^* decreased from 70.6 to 68.7 and the a^*, b^*, C^* were increased with the increase of pH, which indicated that the color deepened, the red and yellow light increased, and the chroma also increased. Further increasing the pH value, the L^* was increased, while the a^*, b^* and C^* values showed a decreasing trend, especially when the pH was 12.0, the L^* increased to 72.9. It was revealed that the adsorption of nanoparticles on the fiber decreased remarkably in strong alkaline conditions. It made the color of the fabric lighter, so the suitable pH value was 6-8.

2.6. NaCl concentration

The results showed that the a^*, b^* and C^* were decreased but L^* value was increased with increasing the concentration of NaCl (Fig. 6). It can be described as increasing electrolyte concentration had an influence on the nanoparticle adsorption onto the fiber causing the color of the dyed fabrics was worse, particularly when the concentration of NaCl was above 0.01 mol/L. It can be explained that the electrostatic force is the main driving force between cationic nanoparticles and the fiber. If there exists a higher NaCl concentration in the system, Na⁺ will weaken the negative charge on the surface of the fiber through double layer effect. Meanwhile, Cl^- also will weaken the positive charge of the nanoparticles. Thus, the electrostatic attraction was reduced, which lead to the decrease of the adsorption capacity of the nanoparticles on the fiber. When the concentration of NaCl reached 0.1 mol/L, the dispersion of nanoparticles got unstable, and the number of particles adsorbed on the fiber decreased. It resulted in a significantly change on the color of the dyed fabrics.

3. Conclusion

In summary, we present a novel approach for A. venetum/cotton blended fabrics modification by cationic copolymer nanoparticles, and the nanoparticles treatment has influences on color performance. Eventually, it is necessary to optimize the process parameters of nanoparticles treatment, such as nanoparticles concentration, temperature, time, pH and NaCl. The results demonstrated that the proper modification conditions are 0.5 g/L nanoparticles, treating 60 min at 60 ℃, and pH value was 6-8, without NaCl. In addition, the morphological investigation by SEM reveals that a uniform monolayer of nanoparticles is presented on the modified fiber surfaces, and the two different fibers could have the same adsorption ability to Acid Red B.

4. Experimental

The materials used in this study and their sources are as follows: HCl was purchased from Zibo Aoda Chemical Co., Ltd., China. NaHCO₃ and NaOH were purchased from Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd., China. Acid Red B (C.I. Acid Red 14) was purchased from Tianjin Juhua Chemical Co., Ltd., China. Deionized, scoured and bleached A. venetum/cotton blended fabrics (blending ratio of A. venetum/cotton is 35/65, 40^s × 40^s, 44 ends/cm and 27 picks/cm, 125 g/m², thickness: 0.31 mm) were supplied by Xinjiang Gaubaukinder Co., Ltd., China. The cationic copolymer nanoparticles poly(styrene(St)-buty lacrylate(BA)-vinylbenzyl trimethyl ammonium chloride (VBT))were prepared according to Ref. [18]. Other materials and solvents were used as received.

Pre-treatment of A. venetum/cotton blended fabrics: A. venetum/cotton blended fabrics were treated with 0.1 mol/L HCl for 30 min at room temperature in order to remove metal ions. Afterward the fabrics were washed with double distilled water for 5 times. Subsequently, the samples were immersed in the concentration of 1 mmol/L NaHCO₃ solution for 10 min, which made the acid into the form of salt. After that, the samples were rinsed in double distilled water for 3 times and dried at 30 ℃.

Modification of A. venetum/cotton blended fabrics: The cationic copolymer nanoparticles P(St-BA-VBT)were applied to the treated A. venetum/cotton blended fabrics as described below: the fabrics were immersed into the dispersion of nanoparticles using a liquor ratio of 100:1 at 60 ℃ for 120 min. Afterwards these samples were thoroughly washed with double distilled water for three times at room temperature. Subsequently the samples were drying at 30 ℃ for other use and test.In this paper, Cp refers to the concentration of nanoparticles.

Dyeing: A. venetum/cotton blended fabrics were dyed with Acid Red B at 0.1% on mass of fabric (omf) in an infrared dyeing machine at 30 ℃ for 60min, using a liquor ratio of 30:1, the structure of Acid Red B is shown in Fig. 7. Then the dyeing samples were rinsed with double distilled water using a liquor ratio of 30:1 at 25 ℃ for 30min for three times.

Color measurement: The color parameters L^*, a^*, b^*, and C^* values of dyed fabrics were performed using a Datacolor SF-600 spectrophotometer (Datacolor, Co., USA) under D65 illumination, 10° standard observer with 100% UV filter off and a specular component included in reflectance mode. The samples were folded to six thicknesses and the average of six measurements was taken for each sample.

Scanning electron microscopy: FE-SEM images were obtained using a Hitachi S-4800 field emission scanning electron microscope. All samples were coated with Au, prior to SEM observation.

Acknowledgment

This work is supported by National Natural Science Foundation of China (No. 51173086), National Key Technology R & D Program, (Nos. 2014BAC13B02 and 2014BAE01B01), Industrialization Projects of Major Independent Innovation Achievements of Shandong Province (No. 2012ZHZX1A0914), and Application Basis and Cutting-edge Technology Research Project of Tianjin (No. 14JCZDJC37200).