Dyeing Properties of Berberine on Cationic Dyeable Polyester

Abstract

The natural cationic colorant berberine was used to dye cationic dyeable polyester (CDP) fibers. Its dyeing properties, namely color strength (K/S) of dyed CDP and percent exhaustion (E%) of berberine, on CDP were explored at various dyeing pH levels, temperatures, time, and concentration of berberine. Furthermore, dyeing kinetic and thermodynamic studies were also carried out to evaluate the adsorption properties and mechanism. It was found that the E% of berberine on CDP fibers and the rate of adsorption increased with increasing dyeing temperature, while the influence of pH was extraordinarily small. In addition, the adsorption isotherm of berberine on CDP fibers was in line with the Langmuir model.

Introduction

The application of natural dyes for textile dyeing has a long history over thousands of years. For instance, it is believed that cochineal colorant was applied to leather materials and silk more than 4000 years ago in China. ¹ However, synthetic dyes have been developed at an astonishing tempo over the past 150 years. Recently, due to the increase of worldwide health and environmental awareness, a resurgence of interest in natural dyes has occurred to provide eco-friendly textile materials. Thus, there is a renewed interest in the study of natural dyes for textile dyeing (e.g., almond shell, ² annatto, ³ hazelnut, ⁴ cochineal, and Monascus purpureus colorants ⁵ ). Compared to synthetic dyes, natural dyes, derived from plants, minerals, and animal sources, possess environmental advantages (e.g., nontoxicity, biodegradability, antimicrobial^6-9 and anti-UV properties ¹⁰ ).

Berberine, the extract of Coptis chinensis Franch. roots, has received extensive attention and been widely used both in medical science and in the textile dyeing and finishing industries. As shown in Scheme 1, there is a positively-charged nitrogen atom in the chemical structure of berberine. Berberine can be used in antibacterial finishes as a natural functional agent due to the presence of its quater­nary ammonium salt. In addition, a series of studies indicate that berberine dyed substrates exhibited excellent antibacte­rial ability.^11-13 Berberine, one of the few natural cationic dyes, can be used in the dyeing of anionic and amphoteric fibers as a result of ionic interactions with the fiber. Previ­ously, the focus of most research was the study of dyeing and antibacterial properties of berberine on polyamide fibers,^14,15 wool, ¹⁶ acrylic fibers, and modified cotton fibers. Addition­ally, the dyeing properties of modified berberine has been studied on various fabrics. ¹⁷

Scheme 1

Chemical structure of berberine chloride under different pH conditions.

In this study, the effects of dyeing pH, temperature, time, and berberine concentration on the dyeing properties of berberine on CDP fibers were evaluated. Simultaneously, its adsorption properties were investigated in terms of dyeing thermodynamics and kinetics.

Experimental

Materials

Cationic dyeable polyester (CDP) loose fiber (1.5 dtex/38 mm) was obtained from Jiangsu Green Peafowl Chemical Fiber Co. Ltd. Berberine chloride (99%) was purchased from Aladdin Reagent Co. Ltd. Glacial acetic acid, phosphoric acid, and boric acid (Sinopharm Chemical Reagent Co. Ltd.) were analytical grade and were used to prepare buffer solutions.

Dyeing Methods

All dye solutions were prepared using berberine powder, Britton-Robinson buffers, and distilled water. The liquor ratio (LR) was maintained at 50:1. The pH was adjusted using Britton-Robinson buffers (H₃PO₄-HOAc-H₃BO₃/ NaOH). The dyeing process has been presented in Scheme 2. After immersing CDP fibers in dye solutions at room temperature (RT), the system was heated to 40 °C at a max­imum rate. The dye bath temperature was then increased to 100 °C at 1 °C/min, held there for 60 min, and then cooled to 70 °C at 3 °C/min. At the end of dyeing, the dyed samples were removed, rinsed thoroughly in tap water, and dried in the open air.

Scheme 2

Dyeing process of (a) various levels of dyeing pH, temperatures, time, and concentrations of berberine, (b) kinetic dyeing conditions, and (c) thermodynamic dyeing conditions.

To evaluate the effect of pH and temperature on the color strength (K/S) and percent exhaustion (E%) values of dyed CDP, the samples were dyed in the dye bath containing 1% owf berberine at various pH (2-12) and temperature (70-130 °C) values. To estimate the effect of dyeing time on the K/S and E% values of dyed CDP, the samples were dyed in the dye bath containing 1% owf berberine at pH 5 and removed at different times. To assess the effect of dye concentration on the dyeing properties of berberine on CDP fibers, samples were dyed at varying concentrations (0.25-5% owf) of berberine at pH 5.

To determine the kinetics adsorption properties, a series of experiments at a constant berberine concentration (1% owf) and constant temperature (85, 90, 95, or 100 °C) at pH 5 over different time periods were carried out. To determine the thermodynamics adsorption properties, a series of experi­ments at various berberine concentrations (0.25-10% owf) and constant temperature (85, 90, 95, or 100 °C) at pH 5 for 12 h were carried out.

Analysis

Percent Exhaustion

To calculate the E% values for berberine on CDP fibers, a UV-Vis TU-1800 spectrophotometer (Beijing Pgeneral Co. Ltd.) was used to measure the absorbance of the original dye bath and the exhausted dye bath at the λ_max of berberine. E% values were calculated using Eq. 1.

$$E\text{\%}=100\times \frac{A_0-A_1}{A_0}$$

Eq. 1

A₀ is the absorbance at the λ_max of berberine in the original bath before exhaustion, and A₁ is the absorbance at the λ_max of berberine in the final bath after exhaustion.

Color Strength

The K/S value of the dyeings was determined using an X-Rite SCT Color-Eye 7000A spectrophotometer. Measurements were performed under the conditions of D₆₅ illuminant and 10° observer.

Berberine Concentration on CDP Fibers

A standard concentration plot of berberine was obtained (Eq. 2).

$$y=0.015489x+0.00305$$

Eq. 2

y is the absorbance at λ_max of berberine and x is the concen­tration of berberine.

The concentration of berberine on CDP fibers (C_f) was calcu­lated using Eq. 3.

$$C_f=C-C_s$$

Eq. 3

C_f is the concentration of berberine on CDP fibers, C is the concentration of berberine in the original bath, and C_s is the concentration of berberine in the final bath.

As the dyeing procedure finished, the absorbance at the λ_max of berberine (425 nm) in the original bath (A₀) and in the final bath (A₁) can be measured. C_s and C can be calculated from the standard curve of berberine.

Colorfastness

Washfastness of the dyed samples was tested using AATCC TM61-2013. ¹⁶ Sample pieces were prepared with a 50 × 150 mm multifiber test fabric sewn, stapled, or suitably attached centered along one 50-mm edge of the test specimen and in contact with the face of the material. Using the 3A version of the test method, the test specimens were washed at 71 °C for 45 min. Color change and staining of the test specimens were evaluated by comparison with the AATCC Gray Scale.

Results and Discussion

Effect of pH on K/S and E%

A set of experiments were designed to evaluate the effect of pH on K/S values of dyed CDP and E% values of berberine on CDP. The experiments were carried out at various pH values, namely, in the range of 2-12. As shown in Fig. 1, the K/S and E% values remained relatively constant when dyed within the above pH range. In general, the influence of pH on K/S and E% values of dyed CDP was extremely small.

Fig. 1

Effect of pH on K/S and E% of dyed CDP.

These results may be due to the electropositivity of berberine at various pH values and the dissociation of sodium sulfonate salts on the CDP fibers in the dye bath. Higher pH dyeing conditions are favorable to the dissociation of the sodium salts. The dissociation of sulfonic acids is much weaker than the sodium salts of sulfonates. Therefore, the resulting number of available anionic sites on CDP fibers under alkaline condi­tions is relatively greater than those under acidic conditions. The cationic dye berberine in its carbinol form above pH 9 is not colorless, and decreasing the pH would normally increase the electropositivity of berberine.

Effect of Dyeing Temperature on K/S and E%

The next step was to assess the effectiveness of temperature (T) on the dyeing properties of berberine on CDP fibers. CDP fibers were dyed at various temperatures (70 to 130 °C) and E% and K/S values were determined as well. The data are graphically shown in Fig. 2.

Fig. 2

Effect of dyeing temperature on K/S and E% of dyed CDP.

The glass transition temperature (Tg) of CDP fibers was analogous to that of PET fibers (67 to 125 °C) due to hav­ing similar supramolecular structures. Normally, the Tg values of amorphous, crystalline, and crystalline-oriented PET fibers are 67, 81, and 125 °C, respectively. The results showed that the E% and K/S values increased dramatically with increased temperatures when greater than 70 °C. Above the Tg, much more free volume was provided for berberine diffusion since the amorphous regions of CDP fibers swelled significantly with increased temperature. As a result, greater E% and K/S values occurred.

Meanwhile, a plateau of E% values were observed between 90 and 100 °C, followed by a decrease beyond 100 °C. As the dyeing temperature increased, the number of dyeing sites, namely sulfonate anions, reached equilibrium, resulting in the plateauing values of E% and K/S observed at higher temperatures between 90 and 100 °C. However, when the dyeing temperature was above 100 °C, the decreased E% value may be attributed to lower affinities of berberine for CDP fibers, resulting in dye desorption at higher tempera­ture. Alternatively, the dye or fiber may be degrading at these high temperatures.

Sorption Rates

The effect of dyeing time (t) was investigated to provide a theoretical guidance for actual production. As shown in Fig. 3, there was very little dye adsorption when the temperature was less than 70 °C due to the compact structure of CDP fibers at temperatures less than its Tg value. However, above 70 °C, when the temperature exceeded the Tg value, E% and K/S values increased sharply. Finally, the E% and K/S values plateaued within 90 min at 100 °C, which resulted from the adsorption equilibrium of berberine on CDP fibers.

Fig. 3

Effect of dyeing time on K/S and E% of dyed CDP.

Build-Up Properties and Effect of Dye Concentration on Exhaustion

A further study investigated the build-up properties and effect of berberine concentration (0-5% owf) on the E% value of dyed CDP. As shown in Fig. 4, the K/S value of dyed CDP, as a whole, increased with increased berberine concen­tration. There was a manifest increase in K/S values as the berberine concentration increased in the 0-3% owf range. However, when the concentration was greater than 3% owf, the increase in K/S was very small. E% values increased with increased berberine concentration within 0-1.5% owf and remained constant when the concentration was greater than 1.5% owf. In addition, the concentration of berberine on the CDP fibers (C_f) increased linearly with increased initial dye bath concentration. The adsorption equilibrium was not reached when dyed using berberine concentrations of 0-5% owf, resulting from the presence of unoccupied dye sites.

Fig. 4

Effect of berberine concentration on K/S and C_f of dyed CDP.

Kinetic Studies

Kinetic curves at varying temperatures (85-100 °C) were generated to analyze the dye adsorption properties (Fig. 5, Table I). The t_1/2 and C_∞ values sharply declined with increased temperature, but k values increased. Variables t_1/2 and C_∞, and constant k, obtained from the slope and intercept of the t/C_t-t linear curve, are half-dyeing time, equilibrium absorp­tion capacity, and dyeing rate constant, respectively. The initial rates of dyeing were high and enhanced with increased tem­perature. The dyeing rates decreased slowly with the extension of dyeing time (Fig. 5). As the temperature increased, C_∞ gradually decreased. Higher temperature led to lower affinities of berberine for CDP fibers.

Fig. 5

Dyeing kinetic curves at various temperatures.

Table I

Dyeing Kinetic Parameters of Berberine on CDP Fibers

T (°C)	k (kg/g/min)	t1/2 (min)	C∞ (g/kg)
85	0.0071	14.23	9.83
90	0.0296	3.59	9.42
95	0.1246	0.89	9.03
100	0.1425	0.78	8.96

Adsorption Isotherms

To investigate the dyeing mechanism of berberine on CDP fibers, adsorption isotherms at different temperatures were prepared (Fig. 6). The adsorption isotherms were analogous to the Langmuir type. However, a comparison of Langmuir and Freundlich models was carried out to confirm. The dye concentrations on the fibers at equilibrium (C_f) and the dye concentrations in the dye bath at equilibrium (C_s) were applied for nonlinear least square fitting according to the adsorption equations of Langmuir (Eq. 4) and Freundlich (Eq. 5) models, separately.

$$C_f=\frac{S\cdot K_L\cdot C_S}{1+K_L\cdot C_S}$$

Eq. 4

$$C_f=K_F\cdot C_S^n$$

Eq. 5

Fig. 6

Adsorption isotherms of berberine for CDP.

K_L and S are the Langmuir affinity constant and Langmuir adsorption saturation of berberine on CDP, respectively. K_F and n are the Freundlich affinity constant and heterogeneous factors, respectively.

Furthermore, the R ² values of the two types at varying temperatures are tabulated in Table II. The R ² value of the Langmuir model is always greater than that of the Freundlich model. The adsorption isotherms in this study were in good accord with the Langmuir type, indicating that the berberine interacts with the anionic sites of the CDP fibers in the form of ionic bonds.

Table II

R ² of Freundlich Type and Langmuir Type Adsorption Isotherms, and K_L and S Values of Berberine on CDP at Varying Temperatures

T (°C)	R 2		KL (L/g)	S (g/kg)
	Freundlich	Langmuir
85	0.77234	0.95993	57.8487	25.1730
90	0.82388	0.97428	34.7365	32.4588
95	0.78466	0.93177	14.8964	71.1694
100	0.81556	0.95407	16.5649	65.3981

The Langmuir affinity constant and Langmuir adsorption saturation of berberine on CDP at varying temperatures are also presented in Table II. As the temperature increased, K_L decreased and S increased. This is related to the avail­ability of dye sites at higher temperatures, resulting from the increase in amorphous areas.

Washfastness

The berberine dyed CDP had excellent washfastness (Table III). The staining fastness of cotton, wool, and polyester fabrics was all grade 5. And the fading fastness and staining fastness of dyed CDP was also grade 5. The results showed that stable ionic bonds were formed between berberine and CDP fiber.

Table III

Washfastness of Dyed Fibers

Fading Fastness	Staining Fastness
	Cotton	Wool	Polyester
5	5	5	5

Conclusions

Berberine, a natural cationic dye, was used to dye CDP fibers successfully. The E% (or C_f) and K/S values of CDP fibers increased with increased dyeing temperature, dyeing time, and berberine concentration. The dyeing tempera­ture should be greater than 90 °C and the influence of pH was extraordinarily small. The dyeing kinetics results showed that the dyeing rate of berberine on CDP fibers increased with increased dyeing temperature, the half dyeing time decreased with increased dyeing tempera­ture, and the equilibrium dyeing quantity showed a slight reduction. The dyeing thermodynamics results showed that the adsorption model of berberine on CDP fibers ft the Langmuir model.