Article pubs.acs.org/IECR
States of Water and Pore Size Distribution of Cotton Fibers with Different Moisture Ratios Zhiping Mao,* Hong Yu, Yuanfeng Wang, Linping Zhang, Yi Zhong, and Hong Xu* Key Laboratory of Science & Technology of Eco-Textile (Ministry of Education), Donghua University, Shanghai 201620, People’s Republic of China ABSTRACT: Low add-on technologies have attracted more and more attention due to the shortage of water resources. In order to explore the mechanism of low add-on technologies, the states of water and the pore properties (pore size distribution, total pore volume, specific surface area, and the average of pore radius) of cotton fibers were investigated over a wide moisture range by using differential scanning calorimetry (DSC) based on the Gibbs−Thomson effect. A simple model of cotton fiber with various moisture ratios was given. Larger pores changed in size with the change of moisture ratio prior to smaller pores in both wetting and drying of cotton fibers. The information obtained could be used to explain the results of dyeing processes. Low addon technology (wet pick-up 30−40%) was a favorable choice for surface treatment. Excellent surface performance of fabric could be obtained, and large amounts of water and energy could be saved by low add-on technology.
1. INTRODUCTION During most textile manufacturing processes, hydrophilic fibers such as cotton are in the swollen state by water which is not only a good solvent for most dyestuffs or finishing agents but also acts as a transportation medium for those from the aqueous bath to the interior of fabrics. Along with gradual reduction of water resources, low add-on technology has been getting much attention. Foam dyeing has been successfully developed as a low add-on technology since the 1970s.1−3 Unlike conventional pad dyeing, it uses foam to apply colorants to textiles, resulting in saving large amounts of water and energy due to the replacement of water with air. It is wellknown that the pores in the cellulose fibers are significantly altered with the moisture ratio of fibers. Pore properties, such as pore volume, internal surface area, pore radius, and pore size distribution, are quite different in fibers with various moisture ratios. Therefore, investigating the state of water and pore properties of fibers at different moisture ratios is the primary and essential step in exploring the mechanism and diffusion model of reactants in low add-on technology. Measurement techniques of pore properties include nitrogen adsorption/desorption,4 mercury intrusion,5 differential scanning calorimetry,6 solute exclusion technique,7,8 inverse sizeexclusion chromatography,9 and NMR cryoporometry.10−13 However, the pore information only can be determined from dry porous materials by using nitrogen adsorption or mercury intrusion, and from porous materials under saturated conditions by using the solute exclusion technique and inverse size-exclusion chromatography. Differential scanning calorimetry (DSC) is the most favorable choice for the determination of states of water and pore properties of porous materials at different moisture ratios. States of water in a series of various fibers, including cotton,14 wool,15 pulp fibers,16,17 and acrylic fibers,18 have been investigated. Water adsorbed in porous materials is defined as free water (“bulk” water), freezing bound water, and nonfreezing bound water. Free water (FW) is the unbound water whose transition temperature, enthalpy, and peaks in DSC © 2014 American Chemical Society
curves are equal to those of pure water. This is interpreted as water outside the porous structure. Freezing bound water (FBW) is interpreted as water held within pores, with the melting temperature being depressed by the Gibbs−Thomson effect. Nonfreezing bound water (NFBW) cannot be detected from DSC experiments, which is considered as water adjacent to a surface and does not freeze because the motion of water structures is severely limited due to the association with the surface.19 Pore properties of various porous materials have been observed by DSC, such as controlled-pore glass,20 silica gels,21−23 compressed finely ground calcium carbonate,24 and polymer hydrogel membranes.25 An isothermal step melting procedure was applied to cellulose fibers to determine the pore size distribution by Maloney et al.16 and Park et al.,26 but not only the direct-viewing pore size distribution perspective but also the pore volume and internal surface area cannot be obtained by this method. A detailed introduction to calculating the pore size distribution of glass from a continuous melting procedure was provided successfully by Landry.20 The results were compared favorably with Hg intrusion data. Also, the pore volume, internal surface area, and average of pore radius can be evaluated. Cotton fiber is very common and is indispensable in textile machining processes because of its comfort, and the states of water and pore properties in cotton fiber are significant, especially for instructing parameter settings in textile processes. In this study, states of water in cotton fibers with different moisture ratios were determined and the pore properties (pore volume, internal surface area, pore radius, and pore size distribution) were observed by differential scanning calorimetry. A simple model of wetting for cotton fiber was introduced, Received: Revised: Accepted: Published: 8927
March 13, 2014 May 5, 2014 May 6, 2014 May 6, 2014 dx.doi.org/10.1021/ie501071h | Ind. Eng. Chem. Res. 2014, 53, 8927−8934
Industrial & Engineering Chemistry Research
Article
based on the assumption that the pore shape of cotton fibers was cylindrical.26
and the information on pore properties was used to instruct and comprehend the practical dyeing process, especially low add-on technology.
R p (nm) =
2. EXPERIMENTAL PROCEDURES 2.1. Materials. A scoured and bleached, plain-weave cotton fabric of 117 g m−2 was obtained from Hua Fang Co. Ltd., Shandong, China. The dry fibers had an average cross-section diameter of 10 μm. The samples were purified by extracting contaminants with ether successively using a Soxhlet extractor. Commercial CI Reactive Red 120 was kindly supplied by Zhenyang Dyes Ltd., Jiangsu, China. 2.2. Preparation of Samples. The samples were dried in a vacuum desiccator for about 1 week, and then 3−5 mg was weighed in an aluminum pan. The aluminum pan had been previously heated at 100 °C in an autoclave with water to eliminate any reaction between the aluminum surface and water. Samples with different moisture ratios were prepared in two ways to represent wetting and drying, respectively. For wetting, a certain amount of water was added to each sample by a microsyringe. For drying, fully saturated cotton fibers were dried at 90 °C and the drying was interrupted at different times. Then the pan was sealed at once. The moisture ratio was expressed as grams of water per grams of dry sample, as follows: moisture ratio =
wt of water (g g −1) wt of dry sample
ΔHpeak ΔHf Wdry
ρΔHf ΔT
(3)
where T0 is the melting temperature of water (273.15 K) and γsl is the surface energy at the ice−water interface (12.1 mJ m−2).25,28 ρ and ΔHf are the density of the water (1 g cm−3) and the specific heat of fusion of water (334 J g−1), respectively, which are assumed to be independent of temperature.14,16 The differential pore volume dVp/dRp was calculated by eq 4.20 dVp dR p
d(ΔT ) dQ dt 1 dt d(ΔT ) dR p mΔHf ρ
=
(4)
This required a baseline subtraction step that effectively removed the underlying heat capacity contribution to the DSC signal. d(ΔT)/dt is the scanning rate of the DSC experiment (0.1 K min−1), and m is the mass of dry cotton fibers (g). The total pore volume Vp, internal surface area Sp, and average pore radius Rave were calculated as follows.20
(1)
2.3. DSC Measurements. The melting curves of the water in the cotton fibers were determined using a DSC 240 F1. A continuous melting was adopted and the temperature change was executed as low as possible during DSC measurements to avoid thermal and time delays in the DSC curve. The temperature was dropped rapidly to −30 °C, and then held for 30 min to freeze the water, as indicated by a large exothermic response in the real-time signal. The samples were then heated to 10 °C at a scanning rate of 0.1 K min−1. The heat fluxes of the melting curves were calibrated using the heat of melting of pure water. The melting procedure was exactly the same as that used in testing the samples. The temperature at which the ice starts to melt was calibrated to 0.00 °C (onset of the melting peak) and the temperature reproducibility was about ±0.02 °C. 2.4. Calculation of Water Contents and Pore Properties. The amount of freezable water (FW and FBW) in a melting peak was calculated according to water content =
2T0γsl cos θ
Vp =
∫0
Sp =
∫0
R ave =
∞
dVp dR p
∞
dR p
2 dVp dR p R p dR p
(5)
(6)
2Vp Sp
(7)
The nonfreezing bound water adjacent to the pore walls was ignored when the total pore volume was calculated with eq 5, and a cylindrical pore shape was assumed when the internal surface area and average pore radius were calculated with eqs 6 and 7. 2.5. Dyeing Processes for Cotton Fabric. The dyeing process of various wet pick-ups (30, 40, 80, and 100%) for cotton fabric with CI Reactive Red 120 was carried out. Each dyeing process of a different wet pick-up was repeated strictly at least 3 times. Pad dyeing (one-bath pad steam dyeing) was carried out when the wet pick-up was required to be 80 or 100%. Cotton fabric was padded with dyeing solution containing 3.75 or 3 g L−1 CI Reactive Red 120, 12 g L−1 Na2CO3, and 80 g L−1 NaCl. Different wet pick-ups of fabric were achieved by adjusting the pressure between rubber rollers. Steaming fixation was carried out at 148 °C and a relative humidity 65% for 4 min, followed by soaping (2 g L−1 soap flakes, 2 g L−1 Na2CO3, liquor ratio 1:20, 95 °C, 10 min) and washing (warm rinse and cold rinse). Foam dyeing was carried out by a Neovi-foam system (Neowin Chemicals Co., Ltd., China) when wet pick-up was required to be 30 or 40%, because such a low wet pick-up could not be reached with pad dyeing.29 To keep the dye quantity on the cotton fabric the same as with conventional pad dyeing, the concentration of CI Reactive Red 120 in the dye solution was 10 and 7.5 g L−1, respectively. Also, the dyeing liquor contained 12 g L−1 Na2CO3, 2 g L−1 foaming agent (sodium dodecyl sulfate), and 0.7 g L−1 stabilizer (dodecanol and guar gum 3:4 by weight). The Neovi-foam system was composed of a foam
(g g −1) (2)
where ΔHpeak is the energy transferred according to the peak (J) and Wdry is the mass of cotton fibers (g). The overlapping peaks were separated by splitting the integrated areas at the temperature of the local minimum or the point of inflection between the peaks.17 The amount of NFBW was calculated by subtracting the total freezable water in the sample from the moisture ratio in the initial sample. The relationship between a pore radius (Rp) and the depressed melting temperature (ΔT) was described by eq 3, which reduced to the Gibbs−Thomson equation when the contact angle was assumed to be 180°.27 This calculation was 8928
dx.doi.org/10.1021/ie501071h | Ind. Eng. Chem. Res. 2014, 53, 8927−8934
Industrial & Engineering Chemistry Research
Article
generator and a foam applicator. The foam generator produced tiny dyeing foam (blow ratio 1:8), and the foam applicator evenly applied a certain amount of dyeing foam onto the fabric according to the requirement of the wet pick-up. The next steps were exactly the same as those in pad dyeing. 2.6. Measurements of Dyed Cotton Fabric. The color strength (K/S) of each dyed fabric was measured using a Datacolor SP600+ spectrophotometer (Datacolor Co., USA). The wavelengths measured ranged from 400 to 700 nm. The dye fixation of each dyed sample was determined by using eq 8: %F =
M − M1 ·100 M
(8)
where F is the dye fixation percent; M is the dye quantity on the cotton fabric before washing, which can be calculated from the concentration of dyestuff in the dyeing solution and the wet pick-up of the fabric. M1 is the dye quantity in the total liquor of the soap bath and wash bath, which can be calculated by measuring the absorbance using a UV−vis 3310 (HITACHI Co., Japan).
Figure 1. DSC melting curves for water in cotton fibers with different moisture ratios.
overlapping phenomenon was also confirmed in the research of pulp fibers by Weise et al.17 and acrylic fibers by Hori et al.18 For some porous materials containing smaller size pores, such as controlled-pore glass20 and hydrogel-hollow-fiber membrane,25 the peaks of FBW and FW were separated completely. Compared with the endothermic peak of pure water (not shown here), the endothermic peak of FW appears at a little lower temperature after temperature calibration, which indicates that there is a subtle distinction between pure water and free water in cotton fibers. However, this phenomenon has not been mentioned and discussed in past literature. This is caused likely by the interaction of cellulose fibers and bound water. The amount of freezable water (FW and FBW) was calculated by eq 2. Park et al.26 compared the freezing bound water content calculated by this method with that measured by the isothermal step melting method. The two measurements showed a strong correlation, which meant that the results measured by continuous melting were credible. In addition, continuous melting is more easily operated during DSC measurements. During the wetting procedure of cotton fibers, the content of NFBW first increases to the maximum 0.21 g g−1 (average value) followed by the contents of FBW and FW increasing successively, as shown in Figure 2. Wetting of cotton fibers is
3. RESULTS AND DISCUSSION 3.1. States of Water in Fibers with Various Moisture Ratios. During most textile continuous processes, hydrophilic fabric is treated in a wet environment. Differential scanning calorimetry (DSC) was used to detect the states of water in wet cotton fabrics. In this method, three types of water in the fibers can be defined: free water, freezing bound water, and nonfreezing bound water. The content of each type of water is varied by the moisture ratio of the fibers. Free water is assumed to be the water in the interspace between fibers and yarns or on the surface of fabric, which can move freely in fabric carrying reactants, such as dyestuffs in textile processes, although the water located in much larger pores cannot be separated from free water due to the intrinsic limitation of thermoporosimetry. Bound water (sum of freezing bound water and nonfreezing bound water) is considered as water absorbed in pores of fibers, which is restricted by pore walls and cannot move freely, but can carry dyestuffs or finishing agents with different molecules into suitable pores with different sizes. These different types of water, as the transportation medium, lead to the diffusion of dyestuff in the fabric. Different contents of each type of water may result in diverse dyeing performances. Liquid held in the capillaries has a depressed melting temperature because of the lower pressure at a curved interface in cavities.26 For the water held in the pores, the melting temperature depression has a reciprocal relationship with the pore radius. The endothermic peaks of water in cotton fibers are shown in Figure 1. The endothermic enthalpy increases with increasing moisture ratio. There is no endothermic peak detected when the moisture ratio is 0.22 g g−1, which indicates that the water in the cotton fiber is nonfreezing bound water (NFBW). NFBW has no liquid−solid phase transition during the freezing and melting process, due to hydrogen-bond formation to cellulose chains and being restricted by the inner face of the pore. When the moisture ratio is 0.29 g g−1, only bound water exists in the fiber. Free water (FW) turns up when the moisture ratio increases to 0.45 g g−1. As pores in cotton fibers are not very small, the depressed melting temperatures are near the melting point of free water, leading to overlap of the peaks of freezing bound water (FBW) and FW. The
Figure 2. Effect of moisture ratio on free water (FW) and bound water (BW) content in cotton fibers. Bound water includes freezing bound water (FBW) and nonfreezing bound water (NFBW). 8929
dx.doi.org/10.1021/ie501071h | Ind. Eng. Chem. Res. 2014, 53, 8927−8934
Industrial & Engineering Chemistry Research
Article
(>100 nm) appear when the moisture ratio of cotton fibers increases and a majority of pores around 50 nm radius is observed. When the moisture ratio is 0.29 g g−1, there are no pores larger than 50 nm in the fibers. All the water contained in the cellulose fibers is held in pores, so the fibers are still in unsaturated state. Small pores become larger when the moisture ratio increases. From the pore distributions of cotton fibers with moisture ratios of 0.67 and 0.77 g g−1, it is found that the amount of larger pores (>60 nm) is independent of the moisture ratio, but the proportion of smaller pores (
