Simultaneous reactive dyeing and surface modification of polyamide fabric with TiO 2 precursor finish using a one-step hydrothermal process

Abstract

In this article, the approach of dyeing polyamide (PA) fabric by using C.I. Reactive Blue 19 dye and simultaneously modifying it with titanium dioxide precursor under hydrothermal conditions is developed. The anthraquinone-based Reactive Blue 19 dye, which is more resistant to biodegradation owing to its fused aromatic structure compared to an azo-based one, is utilized as a model compound in this research to demonstrate the photodegradation effect of TiO₂ on reactive dyes. It is shown that a layer of TiO₂ nanoparticles is homogeneously coated on fiber surfaces and their particle sizes are smaller than those remaining in the residual dyeing liquors. The crystallinity and optical properties of the resultant PA fabrics are changed due to this hydrothermal dyeing process. In comparison with the dyed-only PA fabrics, the PA fabrics dyed and simultaneously modified with anatase TiO₂ nanoparticles exhibit better color fastness against artificial light (xenon) while maintain similar grades of color fastness against washing with soap, wet scrubbing, dry cleaning as well as dry/wet rubbing. While changes in tensile strength, elongation and water absorbency of the resultant PA fabrics were not found, the addition of tetrabutyl titanate in the dyeing liquor is proved to facilitate the reaction of reactive dye with PA fabric and the resultant PA fabric shade. More interestingly, it is noticed that the residual dyeing liquor can be photodegraded after 50 mins of either UV or visible light irradiation, and the dyeing wastewater can thus be released in an eco-friendly manner to the environment.

Keywords

dyeing hydrothermal polyamide (PA) fabric surface modification titanium dioxide

Among a variety of common dyestuffs, reactive dyes have been widely used in the textile industry mainly due to their color stability and brilliant hues.¹ Recent studies have reported that reactive dyes composed of reactive functional groups are highly soluble in water and have a high degree of reactivity with textile fibers by means of covalent bonds,² ionic bonds,³ hydrogen bonds,⁴ and van der Waals forces⁵ during dyeing processes. However, wastewater treatment of effluents from dyeing process is always a key concern for textile industries. The discharge of textile dyehouse effluents into aquatic environments can cause public health and safety concerns. This is because the unexhausted dyes in dyeing wastewater have intensive colors and are generally cytotoxic, mutagenic and carcinogenic chemicals.⁶ Up to now, traditional wastewater treatment techniques such as coagulation, flocculation, filtration and biodegradation have been expensive and ineffective in removal of the reactive dyes from textile wastewater.^7–9 Therefore, it is desirable to explore alternative methods for the degradation of reactive dyestuffs in a more effective, environmentally friendly and sustainable route.

The photocatalytic oxidation degradation of dyeing wastewater has increasingly gained interest because it is more effective and capable of degrading complex organic chemicals in comparison with other purification methods.^10–12 It has been demonstrated that the photodegradation conditions, such as catalyst loading, pH and initial concentration of the dyestuff, have a great effect on the decolorization and degradation of textile dyes,¹³ while the presence of some dyeing auxiliary chemicals inhibits photocatalytic decolorization and degradation.^14,15 Anatase titanium dioxide (TiO₂) has a wideband gap of 3.2 eV and is one of the most extensively used photocatalysts in the treatment of wastewater containing organic and dye pollutants^16–21 because of its low cost, nontoxicity, physical and chemical stability, and unique electronic and optical properties.^22–25 For instance, the photocatalytic degradation kinetics of reactive dyes including Reactive Red 11, Reactive Red 2, Reactive Orange 84, Reactive Orange 16 and Reactive Black 5 in aqueous suspensions of TiO₂ under visible light have been examined.²⁶ The photocatalysis mechanism of TiO₂ involves the adsorption of photons, the generation of electron–hole pairs and subsequently the production of reactive hydroxyl radicals (HO·) to completely destroy pollutants in wastewater.²⁷

Textile fibers or fabrics are ideal catalyst substrates for the deposition of TiO₂ particles due to their large specific surface area, high surface reactivity, and good mechanical properties.^28,29 Many studies have been carried out to modify polyamide (PA) fabrics by either dispersing nanoscale TiO₂ particles within polymeric matrices or coating TiO₂ on the surface of PA fabrics in order to produce potentially self-cleaning PA fabrics, as well as to improve their strength, wettability, dyeability and electrical conductivity.^30–34 The influences of TiO₂ quantity on nylon-6 granules and yarn properties has been investigated.³⁵ Also, TiO₂ finishing has been applied to cotton,³⁶ linen,³⁷ silk,³⁸ wool,³⁹ viscose,⁴⁰ aramid,⁴¹ polyester⁴² and polyester/wool blended⁴³ fabrics by the pad–dry–cure process to prepare functional textiles. However, very little has been reported on the textile technology to produce the dyed and TiO₂-modified textile fabrics in a single-step process.^44,45

The aim of this study is to develop a hydrothermal dyeing process to modify and dye PA fabric using titanium sulfate or tetrabutyl titanate precursor in conjunction with C.I. Reactive Blue 19 dye under hydrothermal conditions. The representative anthraquinone-based Reactive Blue 19 dye is utilized as a model compound because it is more resistant to biodegradation owing to its fused aromatic structure compared to an azo-based one.⁴⁶ The residual dyeing liquors were collected and irradiated with UV or visible light. The photoactivity of the remaining particles in the residual dyeing liquor was then estimated by measuring the absorbance of residual dyeing liquor against irradiation time.

The surface morphology, crystalline phase, chemical composition, microstructure and optical property of the resultant PA fabrics are characterized by using field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), sequential inductively coupled plasma emission spectroscopy (ICPS), transmission electron microscopy (TEM) and diffuse reflectance spectrum (DRS) techniques. The changes of the tensile strength, water absorption capacity, air permeability and color fastness of the resultant PA fabrics, as well as the particle size distributions of TiO₂ particles, are also measured. As a comparison, exhaustion of reactive dye and photodegradation of residual dyeing liquor are investigated.

Experimental

Materials

A plain woven fabric made of 100% PA [NH(CH₂)₅CO] filaments was purchased from Haiyan Jiaxia Chemical Fiber Co. Ltd, Jiaxing, China. The linear densities of its warp and weft yarns are 8.3 tex, and the numbers of threads in warp and weft directions are 420 and 300 ends per 10 cm, respectively. All of the chemicals used in this experiment were of analytical reagent grade, including titanium sulfate [Ti(SO₄)₂], urea [NH₂CONH₂], tetrabutyl titanate [Ti(OC₄H₉)₄], 25% ammonium hydroxide [NH₃·H₂O], sodium carbonate [Na₂CO₃], glacial acetic acid [CH₃COOH], acetone [CH₃COCH₃] and anhydrous ethanol [CH₃CH₂OH]. The C.I. Reactive Blue 19 dye [CAS No.2580-78-1, C₂₂H₁₆N₂Na₂O₁₁S₃] was kindly provided by Baden Aniline and Soda Factory (BASF) (China) Co. Ltd. (Shanghai, China). All solutions were prepared using deionized water.

One-step hydrothermal dyeing treatment of PA fabrics

Prior to treatment, about 1.6 g of PA fabrics was cleaned successively with 100 ml of acetone, anhydrous ethanol and deionized water at 40℃ for 15 min.

Two hydrothermal dyeing treatment schemes were designed for the purpose of comparing the effect of different TiO₂ precursors on fabric dyeing. Two TiO₂ precursors, titanium sulfate and tetrabutyl titanate, were used to modify the pretreated PA fabrics respectively. Unlike previous studies,^47,48 where tetrabutyl titanate (or titanium sulfate and urea) was solely used in the fabric modifications, the TiO₂ precursor in this study was mixed with the C.I. Reactive Blue 19 dye liquor to treat PA fabric under hydrothermal conditions. The time used for the hydrothermal dyeing process for the two different TiO₂ precursors were set as 5 and 3 h respectively based on previous hydrothermal-only experiments^47,48 in order to produce anatase TiO₂ particles.

First, 0.016 g of C.I. Reactive Blue 19 dye (1% of the weight of fabric (o.w.f)) was completely dissolved in 80 ml deionized water at a liquor ratio of 1:50 under constant stirring; the pH value of this dye bath was then adjusted to 4.5 by adding glacial acetic acid.

In Scheme 1, 0.5 g of titanium sulfate was added into the above dye bath under vigorous stirring at room temperature, followed by adding 0.25 g of urea. The pH value of the dyeing liquor decreased from 4.5 to 1.6. The pretreated PA fabric was then immersed in the dyeing liquor for 5 min. The dyeing liquor and the fabric were immediately transferred to a 100 ml PTFE sealed container which was placed in a stainless steel autoclave. The autoclave was placed in a furnace and run at a rotation rate of 60 revolutions per minute; it was heated to 120℃ at a heating rate of 2℃ per minute. After 3 h of hydrothermal reaction, the autoclave was cooled naturally to room temperature. The pH value of the resultant solution was 6.4.

In Scheme 2, 0.5 g of tetrabutyl titanate was added into the dye bath under vigorous stirring at ambient temperature. The pH value of the dyeing liquor increased from 4.5 to 6.0 and the hydrothermal reaction time was set at 5 h. The other processing conditions were the same as in Scheme 1. The pH value of the resultant solution was 4.0.

In order to compare fabric properties, for example fabric tensile strength, water absorption, air permeability and color fastness, PA fabrics were also dyed with C.I. Reactive Blue 19 dye at 120℃ for 5 h without the addition of any TiO₂ precursor. The pH value of the resultant solution was 3.4.

The hydrothermal dyeing treated PA fabrics were then immersed in an aqueous ammonium hydroxide solution (pH = 8.5) at 85℃ for 20 min for the purpose of color fixation. They were then soaked with a suspension containing 2 g/l of soap and 2 g/l of sodium carbonate at 95℃ at a liquor ratio of 1:30 for 15 min, and subsequently washed with acetone, ethanol and deionized water for 15 min respectively, and finally dried at 80℃. The as-synthesized particles remaining in the suspension were collected by centrifugation and successively washed with acetone, anhydrous ethanol and deionized water, and dried at 120℃. The masses of as-obtained particles were measured by using an analytical balance (Sartorius Cubis MSE125P, Goettingen, Germany) for further analyses.

Characterization and measurement

Surface morphologies of both PA fabrics and as-synthesized particles were characterized by using FE-SEM (Quanta 450 FEG+X-MAX50, Oxford, UK). The chemical compositions of the PA fabrics from Schemes 1 and 2 were determined by EDX attached to the FE-SEM instrument.

The crystallinity of both PA fabrics and as-synthesized particles were analyzed using XRD (Shimadzu 7000S X-ray diffractometer system, Kyoto, Japan) and calculated using MDI Jade 5.0 software. The scanning angle 2θ was recorded from 5° to 80° using Cu Kα₁ radiation (λ = 0.1540562 nm) at 40 kV and 40 mA. The crystal sizes of as-synthesized particles were determined using the Scherrer equation D = Kλ/βcosθ (where D was the diameter of the particle, λ was the X-ray wavelength, β was the FWHM of the diffraction line, θ was the diffraction angle and K was a constant 0.89).⁴⁹

Contents of the Ti element on the resultant PA fabrics were identified using a sequential ICPS (ICP2060T, Skyray Instruments, Stoughton, USA), and the microstructures of the cross-section of the resultant PA fibers were investigated by using TEM (JEOL3010, Tokyo, Japan, 200 keV).

Particle size distributions of the as-synthesized TiO₂ particles were examined in a Zetasizer Nano ZS90 particle size analyzer (Malvern Instruments Ltd., Malvern, UK).

DRS of the resultant PA fabrics over the range 200–800 nm were performed in a UV–visible/NIR spectrophotometer (Hitachi U-3010, Tokyo, Japan) with an integration sphere ø150 mm. The fabric specimens were folded so that the light could not transmit through the tested fabrics.

Tensile properties of resultant PA fabrics were measured in a HD 026N electromechanical tester (Nantong Hongda Experiment Instruments Co., Ltd., Nantong, China) against standard GB/T3923.1-2013 (or ISO 13934-1:2013). The gauge length was 200 mm and the fabric width was 50 mm. The extension speed was 100 mm/min and the pre-tension was 2 N.

The water absorbency of the PA fabrics were evaluated according to GB/T 23320-2009 (or ISO 18696:2006) and were calculated by equation (1).

G_{w} = \frac{(m_{w} - m_{c})}{m_{c}} \times 100 %

(1)

where G_w was the water absorbency (%), m_w and m_c were the masses (g) of wet and dry PA fabrics respectively.

Air permeabilities of the PA fabrics were assessed against GB/T 5453-1997 (or ISO 9237:1995).

The color depths (K/S values) at the maximum absorption wavelength of 592 nm and CIE 1976 (L,* a*, b*) color coordinates of dyed PA fabrics were measured in an X-Rite CA22 spectrophotometer, Grand Rapids, USA under D65 illuminant and 10° standard observer. Color fastness against washing with soap, wet scrubbing, artificial light (xenon), dry cleaning and rubbing were tested according to GB/T 3921-2008 (or ISO 105-C10:2006), GB/T 420-2009 (or ISO 105-C07:1999), GB/T 8427-2008 (or ISO 105-B02:2013), GB/T 5711-2015 (or ISO 105-D01:2010) and GB/T 3920-2008 (or ISO 105-X12:2001), respectively. The changes in color and staining were judged by references set in GB/T 250-2008 (or ISO 105-A02:1993) and GB/T 251-2008 (or ISO 105-A03:1993), respectively. The binding strengths of the synthesized particles onto PA fabrics were examined by FE-SEM.

The dyeing liquors were centrifuged on a TG16-WS high speed centrifuge (Hunan Xiang Yi Laboratory Instrument Development Co., Ltd., Changsha, China) at a speed of 12,000 rpm for 10 min. Dye exhaustion during the dyeing process was then estimated using the UV absorbance of dyeing liquor at 592 nm in a UV-1600 spectrophotometer (Beijing Rayleigh Analytical Instrument Corp., Beijing, China). The deionized water was used as the reference solution. The dye exhaustion E (%) was calculated by equation (2).³⁵

E (%) = (1 - A_{1} / A_{0}) \times 100 %

(2)

where A₀ was the absorbance of raw dyeing liquor before dyeing fabrics, A₁ was the absorbance of residual dyeing liquor after dyeing fabrics.

Photocatalytic degradation of residual dyeing liquor was conducted on a photochemical reactor (YM-GHX-V, Shanghai Yuming Instrument Co., Ltd., Shanghai, China) equipped with a 1000 W mercury lamp. About 50 ml of residual dyeing liquor was added into a quartz glass tube under magnetic stirring, and irradiated with UV and visible light irradiations, respectively. For UV light irradiation, the UV-passing filters were used to block off visible light and allow the UV light of 254 nm wavelength and of 2.35 mW/cm² radiation intensity, which was measured in an ST-512 UV illumination meter, to be used for the irradiation. UV cut-off filters were employed to simulate visible light irradiation and allow visible light of intensity much greater than 2 × 10⁷ lux measured by a TES 1332A digital light intensity meter for the irradiation experiment. The absorbance of residual dyeing liquor before and after irradiation, A₁ and A₂, for a time duration of t (minutes) were measured at 592 nm in the UV-1600 spectrophotometer. The decolorization grade D (%) due to photocatalytic degradation was calculated by equation (3).¹¹

D = (1 - A_{2} / A_{1}) \times 100 %

(3)

Furthermore, the transmission spectra of residual dyeing liquor in the range 200–800 nm were obtained by using the same spectrophotometer. All the above measurements were repeated three times and the average was given.

Results and discussion

Surface morphology observations

The FE-SEM images of surface morphologies of four PA fabrics – untreated, only dyed, titanium sulfate (Scheme 1) and tetrabutyl titanate (Scheme 2) treated PA fabrics – are shown in Figure 1. The surface of untreated PA fabrics is relatively smooth, without any attachments. Some small particles added in the spinning process are found (Figure 1(a)). In comparison with the untreated PA fabric, the surface of the dyed-only PA fabric has no distinct change except a film of organic matter (Figure 1(b)). But for both titanium sulfate and tetrabutyl titanate treated PA fabrics, a layer of aggregated substances is coated on their fiber surfaces. From the high-resolution FE-SEM images illustrated in the upper left corner, it can be seen that these tiny clusters are composed of nano- and submicrometer-sized particles (Figures 1(c) and 1(d)). The average size of the particles deposited on the titanium sulfate treated PA fabric is apparently larger than that deposited on the tetrabutyl titanate treated one. However, the morphologies of those particles attached to PA fabrics are similar to the particles formed in hydrothermal process treatment of polyamide fabric without dyeing.⁴⁸

Figure 1.

FE-SEM images of PA fabrics: (a) untreated; (b) only dyed with reactive dye; (c) Scheme 1; (d) Scheme 2.

Crystalline phase analysis

XRD patterns of both PA fabrics and as-synthesized particles are shown in Figure 2. The intense diffraction peaks at around 2θ = 21.5°, which is ascribed to the reflection of (200)/(001) of the γ crystal form of PA, for the dyed-only, titanium sulfate and tetrabutyl titanate treated PA fabrics are significantly weakened after one-step hydrothermal treatments. Furthermore, the diffraction peaks at about 2θ = 20.2° and 23.6°, corresponding to the reflection of (200) and (002)/(202) of the α crystal form of PA, are obviously strengthened. The results indicate that the transformation of meta-stable γ-phase to more stable α-phase⁵⁰ occurs during hydrothermal treatment at high temperature and high pressure. The crystallinities of PA fibers increase from 27.8% for untreated PA fabric to 43.8% for fabric solely dyed with C.I. Reactive Blue 19. The crystallinities further increase to 45.4% and 46.5%, respectively, when PA fabrics are treated with titanium sulfate and tetrabutyl titanate in hydrothermal dyeing processes. The results suggest that the hydrolysis and recrystallization primarily occur in the amorphous region of PA fibers.⁵¹ However, the diffraction peaks of TiO₂ are not observed in the XRD patterns of PA fabrics treated with titanium sulfate and tetrabutyl titanate precursors. This might be because the amount of TiO₂ particles immobilized on PA fabrics is so small that it cannot be detected by the XRD system. Thus, the crystal structures of as-synthesized TiO₂ particles are characterized. A series of typical peaks centered at 25° (101), 38° (004), 48° (200), 54° (105), 55° (211) and 62° (204) are observed in the XRD patterns of as-synthesized particles, which are in accord with the anatase structure of TiO₂ (JCPDF Card No.21-1272).⁵² Based on the width of the peaks at 2θ = 25°, 38° and 48°, and calculation using Scherrer's equation, the mean crystallite sizes of as-synthesized particles are found to be 10.1 nm for Scheme 1 and 10.2 nm for Scheme 2.

Figure 2.

XRD patterns of (a) PA fabrics and (b) as-synthesized particles.

Cross-section of PA fibers deposited with TiO₂ particles

TEM and high-resolution images, and selected area electron diffraction (SAED) patterns of the cross-sections of PA fibers having TiO₂ particles deposited on the fabric surfaces are shown in Figure 3. It is noted that a thin layer of TiO₂ particles is uniformly coated on the surface of PA fibers treated with titanium sulfate and tetrabutyl titanate precursors, respectively. The thicknesses of the aggregates of nano-scaled TiO₂ particles deposited on the fiber surface are about 200 nm for both schemes, and hardly any TiO₂ particles are distributed beneath the PA fiber surface. From the high-resolution TEM images, the size of the TiO₂ particles obtained from both of the schemes is around 10 nm. The distances between adjacent lattice planes are found to be 0.354 nm and 0.356 nm for Schemes 1 and 2 respectively, which are very close to the d-spacing 0.35 nm of the (101) plane of anatase TiO₂. The corresponding SAED patterns of TiO₂ particles indicate that the TiO₂ nanoparticles are crystalline for both schemes. Moreover, the diffraction rings in the corresponding electron diffraction patterns are assigned to the lattice planes (101), (004), (200), (105) and (211) of anatase TiO₂.

Figure 3.

TEM and high-resolution TEM images, and SAED patterns for PA fibers obtained in (a) Scheme 1 and (b) Scheme 2.

Particle sizes of the remaining TiO₂ particles

Particle size distributions of the remaining TiO₂ particles in residual solutions for both schemes measured using the particle size analyzer (ZS90, Malvern Instruments Ltd.), as well as their FE-SEM images, are shown in Figure 4. It can be seen in the FE-SEM images that all of the particles aggregate together. The TiO₂ particle aggregates produced in Scheme 1 are in the range 1100–2350 nm; they are slightly larger than those (in the range 800–1500 nm) produced in Scheme 2. The coagulation of the submicrometer- and micrometer-sized TiO₂ particles might be mainly caused by reactive dye and dyeing agents. The size and surface topography of TiO₂ particles have a considerable influence on their extent of aggregation. It is common for individual nanoparticles to aggregate together as clusters due to both their large surface area and greater van der Waals attraction forces between them. Additionally, it is noticed that the TiO₂ particle aggregates coated on PA fibers are significantly smaller than those remaining in the residual dyeing liquors. This may be because the growth of TiO₂ nanoparticles on PA fibers is somehow restricted by the matrix of polyamide polymers.

Figure 4.

Particle size distribution (a) and FE-SEM images of TiO₂ particles for (b) Scheme 1 and (c) Scheme 2.

Optical property analysis

DRS of the untreated, dyed-only, titanium sulfate and tetrabutyl titanate treated PA fabrics are shown in Figure 5. The optical images of dyed PA fabrics are inset in Figure 5. It is clear that the untreated PA fabric has a great capability to absorb UV light in the range of wavelengths smaller than 350 nm. There are two characteristic absorption peaks at 230 nm and 320 nm, which are attributed to the electronic π → π* transitions of the carbonyl amide group and benzene of PA, respectively.⁵³ When PA fabrics are only dyed with C.I. Reactive Blue 19, a characteristic absorption band at around 310 nm is ascribed to the chromophore components of the dye molecules,⁴⁶ and another broad absorption band is observed primarily in the wavelength range of 550–650 nm, confirming the conjugated system of C.I. Reactive Blue 19 dye with anthraquinone structure.⁶ The DRS of the titanium sulfate and tetrabutyl titanate treated PA fabrics are greatly influenced by the dye used, which has an intense absorption peak in the visible light region. At the same time, they are also influenced by the coating of TiO₂ nanoparticles on the fabric surface, which is caused by the electron promotion of TiO₂ from the valence band to the conduction band.¹⁰ In comparison with the untreated PA fabric, more than 22.5% and 10.2% of UV lights for the titanium sulfate treated PA fabric are absorbed in UVA and UVB wavebands respectively, while more than 35.3% and 13.2% of UV lights for the tetrabutyl titanate treated PA fabric can be absorbed in UVA and UVB wavebands, respectively. The average reflectance for Scheme 1 is much greater than that for Scheme 2, indicating the color of the tetrabutyl titanate treated PA fabric is darker than that of the titanium sulfate treated one. This is confirmed by the optical images.

Figure 5.

Diffuse reflectance spectra of PA fabrics.

Tensile properties

The changes of fabric structural parameters and tensile properties of the untreated, dyed-only, titanium sulfate and tetrabutyl titanate treated PA fabrics are shown in Table 1. It can be seen that the numbers of yarns in both warp and weft directions of the PA fabrics increase slightly, which indicates the occurrence of fabric shrinkage during dyeing and hydrothermal processes. The fabric shrinkage appears to be intensified due to the interactions between titanium precursor and PA fabrics,⁴⁸ as the presence of TiO₂ nanoparticles on the surface of PA fibers leads to greater surface roughness in PA yarns. During the fabric-stretching process, the friction resistance between PA yarns or fibers increases in comparison with the smooth PA fibers without TiO₂ particles deposited on their surfaces. Therefore, the greater tensile strengths and elongations at breakage of the fabrics from both of the schemes are due to both the presence of titanium particles in the fabrics and the shrinkage of the PA fabrics in both directions during the hydrothermal dyeing treatment.

Table 1.

The results of fabric structural changes and tensile properties of PA fabrics (standard deviations are in parentheses)

PA fabric	Number of yarn picks (ends/10 cm)		Tensile strength (N)		Elongation (%)
PA fabric	Warp	Weft	Warp	Weft	Warp	Weft
Untreated	420	300	629.0 (± 12.6)	474.0 (± 11.2)	21.8 (± 0.8)	23.4 (± 1.1)
Dyed-only	425	303	635.7 (± 11.8)	482.4 (± 11.1)	22.4 (± 0.7)	24.1 (± 0.9)
Scheme 1	445	315	681.0 (± 13.7)	518.0 (± 12.3)	24.6 (± 1.0)	25.2 (± 0.8)
Scheme 2	448	317	687.0 (± 12.5)	525.0 (± 12.0)	25.7 (± 0.9)	26.1 (± 1.0)

PA, polyamide.

Air permeability and water absorbency

The results of air permeability and water absorbency of PA fabrics are shown in Table 2. Due to the significant shrinkages of PA fabrics in both warp and weft directions after dyeing or hydrothermal dyeing processes, the fabric air permeabilities decrease sharply from 329.9 mm/s for untreated PA fabrics to 168.3 mm/s for dyed-only PA fabrics, 148.9 mm/s for fabrics from Scheme 1 and 125.8 mm/s for fabrics from Scheme 2. This indicates lower fabric porosities after dyeing and hydrothermal dyeing processes.

Table 2.

The results of air permeability and water absorbency of PA fabrics

PA fabric	Air permeability (mm/s)	Water absorbency (%)
Untreated	329.9	14.2
Dyed-only	168.3	15.8
Scheme 1	148.9	16.7
Scheme 2	125.8	16.5

PA, polyamide.

The water absorbency of the untreated PA fabric is 14.2%, while after hydrothermal treatments the water absorbencies of the treated fabrics are 15.8% for the solely dyed PA fabrics, 16.7% for fabrics from Scheme 1 and 16.5% for fabrics from Scheme 2. Considering the reduced fabric porosities, the increased fabric absorbency might be due to the improved fabric wettability owing to both the interaction of the water molecules with the hydrophilic TiO₂ particles coated on the fabric surface and the increased fiber surface roughness.^54,55

Color fastness

The K/S values, CEI L*a*b* color coordinates, and ratings of color fastness for the treated PA fabrics are shown in Table 3. Interestingly, the K/S value of the dyed-only PA fabric is much greater than that of the titanium sulfate treated PA fabric (Scheme 1) but smaller than that of the tetrabutyl titanate treated PA fabric (Scheme 2). The existence of tetrabutyl titanate in dyeing liquor is known to make the reactive dye molecules diffuse more easily into PA fibers,⁵⁶ probably due to its neutral pH and special affinity to dyes. The color fastness against artificial light (xenon) for the fabrics from both of the schemes are grade 6, which is better than the grade 5 for the dyed-only PA fabric, mainly ascribed to the coating of TiO₂ particles on the fabric surfaces. The color fastness against washing with soap, wet scrubbing, dry washing, dry and wet rubbing are grade 5 for the dyed-only, titanium sulfate and tetrabutyl titanate treated PA fabrics. The results indicate the reactive dyes can penetrate into the matrix of PA fibers. It is demonstrated by FE-SEM observation that the fiber surfaces of PA fabrics treated in both schemes are still coated with some of the aggregated particles even after being wet scrubbed for 20 cycles, suggesting strong binding forces between the PA fibers and TiO₂ particles.^57,58 It is found through EDX element analysis that, on the fabrics after 20 cycles of wet scrubbing, the atom percentages of the Ti element decrease from 1.49% to 0.74% for the titanium sulfate treated PA fabric and from 1.44% to 0.44% for the tetrabutyl titanate treated PA fabric. Moreover, the results of ICPS indicate that the atomic masses of the Ti element decrease from 920.41 μg/g to 653.54 μg/g for Scheme 1 and from 367.98 μg/g to 284.4 μg/g for Scheme 2, respectively. The reductions of the Ti element for both schemes are caused by the loss of TiO₂ particles attached to the surfaces of PET fabrics during the process of being wet scrubbed for 20 cycles.

Table 3.

K/S values, CIE L*a*b* coordinates and color fastness of dye PA fabrics

PA fabric	K/S value	CIE Lab*			Rating of color fastness
		L*	a*	b*	Washing	Wet scrubbing	Light: xenon	Dry cleaning	Rubbing
		L*	a*	b*	Washing	Wet scrubbing	Light: xenon	Dry cleaning	Dry	Wet
Dyed-only	8.47	38.75	6.82	−41.03	5	5	5	5	5	5
Scheme 1	4.83	46.48	5.04	−40.94	5	5	6	5	5	5
Scheme 2	11.16	36.17	10.94	−45.13	5	5	6	5	5	5

PA, polyamide.

Dye exhaustion in the hydrothermal dyeing process

It is known that tetrabutyl titanate belongs to an organometallic chemical compound with colorless or pale-yellow color. It can decompose in water at near-neutral pH to form TiO₂ particles, which are deposited on the surface of PA fabrics during the hydrothermal dyeing process. As a result, there is not a considerable change in dye exhaustion of PA fabric treated with tetrabutyl titanate. On the other hand, titanium sulfate is an inorganic chemical compound with strong acidity and affects the dyeing behavior of reactive dyes with PA fabrics. When PA fabric is solely dyed with C.I. Reactive Blue 19 dye, dye exhaustion is 98.5%, implying almost all dye molecules can be absorbed by the PA fabric under hydrothermal conditions. When a TiO₂ precursor is added into the dyeing liquor under hydrothermal conditions, the dye exhaustions are 92.8% and 98.4% for Scheme 1 and Scheme 2 respectively. Thus the deposition of TiO₂ particles on the fabric surface using titanium sulfate influences the affinity of reactive dyes for the PA matrix.

The transmission spectra and optical photographs of residual dyeing liquors are shown in Figure 6. It is evident that the transmission spectra of raw dyeing liquors correspond well to the DRS of dyed PA fabrics. The characteristic absorption peaks at about 250, 310 and 345 nm in the UV region are ascribed to the chromophore components of anthraquinone dye molecules. The major absorption peak at 592 nm in the visible light region corresponds to the blue color.⁴⁶ The residual dyeing liquor of the dyed-only PA fabric is basically colorless, while the residual dyeing liquors of both hydrothermal schemes are turbid. The mass percentage concentrations of TiO₂ particles remaining in the residual dyeing liquors are calculated as 0.183 g/l for Scheme 1 and 0.146 g/l for Scheme 2. As is shown in the optical photographs (see Figure 6(b)), the residual dyeing liquor following centrifugation for Scheme 2 is transparent and for Scheme 1 is light blue. The residual liquors for both titanium sulfate and tetrabutyl titanate precursors without addition of dye and auxiliaries are colorless (not shown).

Figure 6.

Transmission spectra (a) and optical photographs (b) of dyeing liquors.

Photodegradation of residual dyeing liquor in Scheme 1

Figure 7 shows the decolorization grades against the increases of irradiation time when the residual dyeing liquors for Scheme 1 are irradiated with UV and visible lights respectively. It is apparent that the residual dyeing liquors without TiO₂ particles for Scheme 1 cannot be photodegraded under either UV or visible light irradiation. The decolorization grades of residual dyeing liquors with TiO₂ particles gradually increase with time under both UV and visible light irradiation. There might be two photodegradation mechanisms of C.I. Reactive Blue 19 dye:⁵⁹ (1) ‘direct’ degradation on the TiO₂ surface, which is more likely in acidic and neutral conditions; and (2) ‘indirect’ degradation mediated by photogenerated OH·, which is more rapid at high pH values. As the irradiation time increases, some large molecules like Uniblue A sodium salt, sodium naphthalene sulfonate and vinylsulfonyl aniline gradually break down to smaller intermediates.⁵⁹ The rate of photodegradation under UV light irradiation is greater than that under visible light irradiation. This is because visible light has less quantum energy than UV rays. After 50 min of irradiation, the decolorization grades are beyond 95% under both UV and visible light irradiation. The transmission spectra of residual dyeing liquors after exposure to UV and visible light irradiation are shown in Figure 8. The characteristic absorption peaks of dye molecules greatly decease after exposure to UV light for 20 min or visible light for 30 min; this implies that the dye molecules in the residual dyeing liquors are almost completely photodegraded to smaller fragments.⁶⁰ As a result, the practical application of TiO₂ precursor in dyeing wastewater treatment process for color removal in dyeing liquors of reactive dyes is of great significance.

Figure 7.

The dynamic decolorization grade of residual dyeing liquors for Scheme 1.

Figure 8.

Transmission spectra of residual dyeing liquors for Scheme 1 before and after exposure to UV and visible light irradiation.

Conclusions

In this study, a one-step hydrothermal dyeing process was employed to modify PA fabrics using titanium precursors (either titanium sulfate or tetrabutyl titanate) and dye the fabrics simultaneously with C.I. Reactive Blue 19 dye. It was found that a layer of TiO₂ nanoparticles is homogeneously coated on the fiber surfaces, and the mean particle size of the TiO₂ anchored on fiber surfaces by using titanium sulfate is larger than that by using tetrabutyl titanate. However, their particle sizes are smaller than those remaining in the residual dyeing liquors.

In comparison with the dyed-only PA fabrics, the PA fabrics dyed and simultaneously modified with anatase TiO₂ nanoparticles have different crystallinity and optical properties due to the TiO₂ nanoparticles deposited on the surface of their constituent fibers. They also exhibit better color fastness against artificial light (xenon), while maintain similar grades of color fastness against washing with soap, wet scrubbing, dry cleaning as well as dry/wet rubbing. However, significant changes in the tensile strength, elongation and water absorbency of the resultant PA fabrics were not found.

It was interesting to note that the addition of tetrabutyl titanate in the dyeing liquor is proved to facilitate the reaction of reactive dye with PA fabric and the resultant PA fabric shade. On the contrary, the addition of titanium sulfate in the dyeing liquor would have deleterious effects for the dyeing process and the resultant PA fabric shade. Further research on such phenomena is ongoing. It was also found that the photocatalytic degradation of residual dyeing liquor could be over 95% after 50 min of irradiation exposure to UV or visible light due to the as-synthesized anatase TiO₂ particles remaining in residual dyeing liquors.

Footnotes

Acknowledgements

The authors acknowledge the support of the Science and Technology Program (No. 2015049) of the China National Textile and Apparel Council, the Sanqin Scholar Foundation (2017) and Handred Talent Programme of Shaanxi Province, and the China Scholarship Council.

Declaration of conflicting interests

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

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

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Simultaneous reactive dyeing and surface modification of polyamide fabric with TiO 2 precursor finish using a one-step hydrothermal process

Abstract

Keywords

Experimental

Materials

One-step hydrothermal dyeing treatment of PA fabrics

Characterization and measurement

Results and discussion

Surface morphology observations

Crystalline phase analysis

Cross-section of PA fibers deposited with TiO2 particles

Particle sizes of the remaining TiO2 particles

Optical property analysis

Tensile properties

Air permeability and water absorbency

Color fastness

Dye exhaustion in the hydrothermal dyeing process

Photodegradation of residual dyeing liquor in Scheme 1

Conclusions

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

References

Cross-section of PA fibers deposited with TiO₂ particles

Particle sizes of the remaining TiO₂ particles