Rheological studies of mineral clay and its application in reactive dye printing of cotton

Abstract

We studied the rheological properties of mineral clay (OMC) and sodium alginate (SA) pastes using steady shear, dynamic oscillatory, and transient tests. The results showed that OMC was a viscoplastic fluid and had more obvious shear-thinning features than SA, which fitted well to the Herschel–Bulkley model. Under strain, OMC and SA exhibited elastic and viscous behavior, respectively, within the linear viscoelastic region. The viscosity of OMC returned to >60% of the initial viscosity after the removal of the high shear stress. In printing experiments, the color yield and penetration of OMC surpassed that of SA and offered excellent outline sharpness, suggesting that it has the potential to be used as a thickener for reactive dyes in cotton printing.

Keywords

printing chemistry performance materials testing fabrication

Textile printing is an important method of decorating textile fabrics. Coloration is achieved by using dyes or pigments in the printing paste. A successful print involves the correct color, sharpness of the mark, levelness, a good hand and the efficient use of dye (“color yield”): all of these factors depend heavily on the type of thickener used and the rheology of the resulting print paste.¹ Thickeners play an important role in the formulation of printing pastes and impart adequate rheological properties to the pastes under the different flow conditions encountered in the printing process.² High strain rates are present during the first step of the application process, whereas after being forced through the screen openings and deposited on the fabric, the paste continues to flow at very low shear rates.³ Elasticity affects both the flow behavior of the paste through the screen openings to the fabric and in the following step (flow through the fibers) by governing the possible recovery of the paste immediately after its application.⁴ The elasticity has a great influence on the flow behavior of the printing paste through the screen opening onto the fabrics and then into the fibers.⁵

Sodium alginate (SA) is widely used as a thickener for reactive dye printing.^1–4 The reaction between SA and reactive dyes is limited due to mutual anion repulsion between alginate's carboxylate groups and the sulfonic acid groups of dyes. This results in a better color yield and softer handling of the textile.⁶ However, the price of SA has now increased to 30,000–40, 000 RMB/ton as a result of limited supplies, leading to increased printing costs, spurring efforts to find alternative thickeners.⁷

Bentonite can meet the requirements for a printing paste. It has several advantages over SA: it can be applied to all cotton, polyester cotton and silk fabrics; there are large reserves of bentonite and it is low cost; and it can reduce water chemical oxygen demand due to absorption . The rheology of bentonite–water and bentonite-water–polymer systems has been widely studied,⁸ although it has not been widely used as a printing paste because of its poor water-holding ability.

OMC is a mixed-layer, nonmetallic, layered silicate mineral found in the Guangxi province of China. It is made up of two silica tetrahedral sheets with a central Al octahedral sheet and has a number of permanent negative charges resulting from the isomorphous substitution of Al³⁺ for Si⁴⁺ in the tetrahedral layer and Mg²⁺ for Al³⁺ in the octahedral layer.⁹ It is commonly used in many industrial fields, such as ceramic accessories, high-quality cosmetics and environmental protection. OMC exhibits good thickening, binding, permeability, stability, and thixotropy. OMC also has price advantages over other printing pastes at only 600 RMB/ton. OMC thus has broad potential applications in the printing field.

We investigated the rheological properties of OMC by steady shear, dynamic oscillatory, and transient tests and compared it with SA. The performance of both OMC and SA in the reactive printing of cotton were extensively explored.

Experimental

Materials

OMC was obtained from Guangxi Shangsi (China). SA was provided by Jie Crystal Chemical (China). Four monochlorotriazine reactive dyes were used: C.I. Reactive Red 245; C.I. Reactive Orange 13; C.I. Reactive Blue 49; and C.I. Reactive Black 39 +Brown 11. The other printing paste additives (urea and sodium bicarbonate) were supplied by Tianjin Yocom Chemical Reagent (China). Reserve salt (sodium-m-nitrobenzenesulfonate) was supplied by Guangzhou Chemical (China). All printing experiments were performed on 100% cotton fabric that had been desized, scoured, bleached and mercerized.

Preparation of printing pastes

The thickener and distilled water were stirred in a magnetic stirrer and left to swell for 24 h at room temperature. The printing pastes were prepared as in Table 1. The printing pastes were adjusted to give a viscosity in the range 9 ± 0.1 Pa·s at a shear rate of 10 s⁻¹ at 25 ℃. The soaping recipe was 3 g/L standard soap and 2 g/L sodium carbonate at a bath ratio of 1:50.

Table 1.

Printing paste recipes

Mass of auxiliary components (g)	Amount (g)
Sodium bicarbonate	3
Reserve salt	1
Urea	5
Reactive dye	1
Stock paste	70
Water	20
Total	100

Rheological tests

Rheological measurements of all stock pastes were performed using a torque rheometer (MCR102, Anton Par, Austria) with a cone-plate geometry (diameter 50 mm, angle 0.04 rad, gap 0.0489 mm) at 25 ±0.01 ℃. The stock paste was placed into the rheometer and balanced for 120 s at the test temperature before measurement.

Steady shear test

The experiment consisted of a steady flow test with continuous shear rates ranging from 0.1 to 1000 s⁻¹ to obtain flow curves for the printing pastes.

Dynamic tests

Dynamic strain sweep tests were carried out in the strain range 0.1–1000% at a fixed frequency of 1 Hz at 25 ℃. The relevant viscoelastic parameters were obtained as a function of strain. Dynamic frequency sweep tests were carried out in the frequency range 0.1–100 rad·s⁻¹ at a fixed strain of 0.1% at 25 ℃. The relevant viscoelastic parameters were obtained as a function of frequency.

Transient tests

At time t = 0, the first stage, a constant shear rate of 1 s⁻¹ was applied to the printing paste for 180 s; in the second stage, a constant shear rate of 400 s⁻¹ was applied to the printing paste for 180 s; and in the third stage, a constant shear rate of 1 s⁻¹ was applied to the printing paste for 150 s.

Effect of temperature on apparent viscosity of printing paste

The experiment consisted of a steady flow test with a continuous temperature ramp of 2 ℃ s⁻¹ in the range 5–85 ℃.

Screen printing process

Printing experiments were carried out using a flat screen 180 mesh with a 10 mm diameter magnetic rod (MINIMD/767, Austria) at grade 3 (6 m/min). Printed samples were dried at 80 ℃ for 2 min, steamed with a Type DHE steam machine at 102 ℃ for 10 min, washed in boiling water with a soaping ratio of 1:50 for 10 min, and then washed with water, dried, and ironed.

Quality-determining parameters

The amount of printing paste was determined gravimetrically from the differences in the mass of cotton fabric samples determined before printing and immediately after the application of the printing paste. The “paste add-on” amount was used to evaluate the “screenability” of the paste, i.e. the ability of the paste to pass through the screen openings onto the fabric.

Color yield

The surface color yield (K/S) was measured on a SF600 pulscolor spectrophotometer (Datacolor, USA).

The penetration of the samples was determined as follows¹⁰:

Penetration (%) = \frac{(K / S) b}{0.5 [(K / S) f + (K / S) b]} \times 100

(1)

where (K/S)_f and (K/S)_b are the K/S values of the face and back of the printed samples, respectively.

The color unevenness for 13 K/S values of the face was calculated as follows³:

Colorunevenness (%) = \frac{\sqrt{\frac{1}{12} \sum_{i = 1}^{13} (K / S_{i} - \bar{K} / \bar{S}) 2}}{\bar{K} / \bar{S}}

(2)

where K/S_i represents the K/S values of the face of the printed samples and K/S is the average K/S value.

Fabric handling

The fabric was cut as a Φ = 120 mm round pie. The softness, stiffness, and smoothness were measured by Fabric Style Meter (PhabrOmeter, USA).

Colorfastness to crocking

The dry and wet colorfastness to crocking were measured according to the AATCC 8-2007 standard.

Outline sharpness

Fine patterns were evaluated by taking photos using a DigiEye system (Digiful, UK). Ocular estimation was used to examine the sharpness of the outline.

Results and discussion

Flow properties of printing paste

The variation of the shear stress as a function of shear rate at different concentrations of OMC and SA is shown in Figure 1. SA was a pseudoplastic fluid, whereas OMC was a viscoplastic fluid with a yield stress. The experimental data were accurately fitted to the Herschel–Bulkley model¹¹:

τ = τ_{0} + K_{\overset{\cdot}{γ}}^{n}

(3)

where

τ_{0}

is the yield stress in Pa, K is the consistency index in Pa·sⁿ, and n is the flow index.

Figure 1.

Shear stress as a function of shear rate at different concentrations of OMC and SA; full lines represent the curve fitting of Herschel–Bulkley model.

The flow parameters obtained from the Herschel–Bulkley data are shown in Table 2. The yield stress of the OMC solution increased with increasing concentration. The OMC solutions showed more shear-thinning features than SA, which benefits the uniform flow of the printing paste through the mesh, onto the fabric, and penetration into the fibers.

Table 2.

Flow parameters fitted to the classical Herschel–Bulkley model

Mass fraction of OMC (%)	$τ_{0}$ (Pa)	K	n	R ²
52%	35.99	52.95	0.124	0.98368
54%	46.17	62.72	0.144	0.97945
56%	101.30	58.16	0.223	0.99350
58%	178.12	62.84	0.297	0.99603

Viscoelastic properties

Dynamic strain sweep test

Printing pastes show both viscous and elastic behavior. The elasticity affects the flow behavior of pastes through the screen openings to the fabric and through the fibers by controlling the recovery of the paste after application. If G′ > G″ and δ < 45 °, then the paste has solid-like features. If G″ > G′ and δ > 45 °, then the paste has liquid-like features.

The viscoelastic curves of OMC and SA are shown in Figure 2. OMC and SA show the completely different viscoelastic behaviors in the strain sweep tests. At lower strains, OMC primarily shows elastic behavior. With increasing strain, the viscosity of OMC increases and the elasticity decreases, i.e. the viscoelasticity of OMC changes from elastic to viscous. SA shows stable viscoelasticity and retains its viscous behavior.

Figure 2.

Storage modulus G′, loss modulus G″, phase angle δ, as a function of strain, for OMC and SA at 25 ℃. (a) Angular frequency (rad·s⁻¹). (b) angular frequency (rad·s⁻¹).

Table 3 lists the viscoelastic parameters for OMC and SA. Within the linear viscoelastic region, OMC shows strong elastic behavior, but SA mainly shows viscous behavior.

Table 3.

Viscoelastic parameters of printing pastes using OMC and SA

Printing pastes	G′ (Pa)	G″ (Pa)	δ (°)
OMC	3310	409	7.03
SA	11.2	30.6	69.9

Dynamic frequency sweep test

Figure 3 clearly shows that OMC and SA have completely different viscoelastic behavior at different frequencies. With increasing frequency, the values of G′ and G″ for SA also increased. If G″ > G′ and δ > 45 °, then SA mainly shows viscous behavior in the frequency range 0.1–100 rad·s⁻¹. The values of G′, G″, and δ of OMC remain stable. G′ is always higher than G″ and δ much less than 45 °, suggesting strong elasticity.

Figure 3.

Storage modulus, G′, loss modulus, and G″ and phase angle δ, as a function of frequency, for OMC and SA at 25 ℃.

Thixotropic properties

Thixotropy is associated with the internal structure of the printing paste. The thickener solution forms a network constructed by hydrogen bonding interactions and entanglement between molecular chains. When the system is stimulated by a shear stress, the network is damaged. As soon as the shear stress disappears, the network is gradually restored. Figure 4 shows that the viscosity of OMC returns to >60% of the initial viscosity after the removal of the shear stress.

Figure 4.

Thixotropic curve of printing pastes.

Effect of temperature on apparent viscosity of printing paste

Figure 5 shows that OMC and SA have completely different viscoelastic behavior with temperature. The temperature has little impact on the viscoelastic properties of OMC. The G′ of OMC is larger than G″; G′, G″, and the complex viscosity remained constant with increasing temperature, suggesting elastic behavior. However, for SA, G″ > G′, suggesting viscous behavior. G′, G″, and the complex viscosity decrease with increasing temperature. This indicates that the viscoelasticity of SA is sensitive to temperature, which may be because higher temperatures destroy the physical network of the SA solution.

Figure 5.

Effect of temperature on OMC and SA.

Printing performance of printing pastes using SA and OMC

The printing paste add-on, color yield (K/S), and color unevenness are given in Table 4. The screenability and color unevenness of OMC are higher than those of SA. The color yield of OMC was 67.8% higher than SA and the penetration of OMC was better than that of SA, which may be a result of the differences in the nature of the chemicals and the composition, molecular weight, and rheological properties. Tables 5 and 6 show that the handling properties of the fabric using OMC were similar to those with SA and that the colorfastness to crocking of the two print pastes was excellent. The patterns of printed textile are shown in Figure 6. OMC had excellent screenability, color yield, penetration, colorfastness to crocking, and handling properties. It has extensive potential for development and application and could be used as a substitute for SA in cotton printing with reactive dyes.

Table 4.

Printing performance for large patterns using OMC and SA

Reactive dye	Thickener	Screenability (g·m⁻²)	Color yield	Penetration (%)	Color unevenness (%)
Red	OMC	175.9	14.33	60.60	3.70
	SA	113.4	8.54	40.36	4.51
Yellow	OMC	161.1	16.20	28.93	2.29
	SA	105.8	11.75	32.09	1.47
Blue	OMC	103.7	2.56	76.22	5.02
	SA	165.4	5.40	64.66	2.93
Black	OMC	161.7	13.38	55.76	1.40
	SA	102.2	13.99	19.65	3.32

Table 5.

Printing performance of reactive dyes for fabric handling using OMC and SA

Reactive dye	Thickener	Stiffness score (%)	Softness score (%)	Smoothness score (%)
None	None	45.67	79.01	81.65
Red	OMC	44.83	80.52	81.59
	SA	45.89	79.68	80.84
Yellow	OMC	45.3	80.52	81.10
	SA	45.88	80.03	80.94
Blue	OMC	46.84	80.74	81.29
	SA	47.45	79.25	80.81
Black	OMC	44.72	80.77	81.74
	SA	45.5	80.93	81.99

Table 6.

Fastness properties of printed cotton using four kinds of reactive dyes with OMC and SA

Reactive dye	Thickener	Dry fastness	Wet fastness
Red	OMC	5	4
Red	SA	5	4-5
Yellow	OMC	4-5	4-5
Yellow	SA	5	5
Blue	OMC	5	4
Blue	SA	4-5	4-5
Black	OMC	5	4-5
	SA	5	4-5

Figure 6.

Digital photographs of printed textiles with (a) OMC and (b) SA.

Conclusions

Printing pastes using OMC and SA as thickeners show completely different rheological properties. Steady shear results revealed that OMC showed obvious shear-thinning features, which is well fitted to the Herschel–Bulkley model. In the dynamic strain sweep test, OMC mainly shows elastic behavior at lower strains and experiences a transition from elasticity to viscosity while SA always shows viscous behavior. In the dynamic frequency sweep test, OMC has relatively stable viscoelasticity and solid-like features while SA shows liquid-like features. The viscosity of OMC returns to >60% of the initial viscosity after the removal of high shear stress.

The color yield and penetration of OMC are better than that of SA and offered excellent outline sharpness, which has enormous potential to be used as the replacement for SA on cotton printing with reactive dyes.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Young Researchers Foundation of Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Zhejiang Sci-Tech University (ZYG2015010).

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