Super-absorbent fabric provide a comfortable microenvironment for personal protective equipment. We prepared a super-absorbent and strong water retention fabric by in situ polymerization of starch-sodium alginate-polyacrylate (St-SA-PAA) hydrogel on the surface of polyester fabric. The pore size distribution of gel on fabric was 30~91 μm. Benefiting from the capillarity of polyester fiber, the water absorption rate of St-SA-PAA gel could be increased from 2.8 to 10 g/min, when it was deposited on fabric. The water absorption of super-absorbent fabric reached 343 g/g, which exceeded the reported super-absorbent fiber. Also, the supporting role of fiber improved the poor mechanical property of gel after water absorption. Interestingly, super-absorbent fabric had isotropic characteristic, and its water absorption and softness could be adjusted by controlling the amount of aerogel deposition, as well as its swelling behavior conformed to second-order kinetic. This work provides new approach for the development of comfortable damp-heat management fabrics.
Research Article
Super-Absorbent Fabric: In Situ Polymerization of Macroporous Starch-Sodium Alginate Hydrogel on Fiber Surface
https://doi.org/10.21203/rs.3.rs-1969983/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 19 Jun, 2023
You are reading this latest preprint version
Super-absorbent fabric
hydrogel
Starch
Sodium Alginate
Macroporous
personal protective equipment (PPE) plays a crucial role in protecting personnel working in hazardous environments (Yang et al. 2022). Especially the recent pandemic coronavirus disease 2019 (COVID-19) has put intense pressure on the global medical system. To protect the medical staff from COVID-19 infection, they must wear protective clothing to prevent the invasion of the virus, blood, and toxic gas (Jin et al. 2022). Although a large number of PPE provide protection for medical staff, PPE are usually airtight and moisture tight, which makes sweat unable to evaporate and store, causing medical staff thermal discomfort and physiological tension, and even may also have dehydration, reduced professional judgment, fatigue, and other symptoms (Zhang et al. 2020a; Peng et al. 2021; Lin et al. 2022; Nguyen et al. 2022). Nevertheless, the present popular products with unidirectional moisture conduction function are difficult to show their superiority in airtight protective clothing (Lao et al. 2020; Zhang et al. 2020b; Li et al. 2022). Therefore, it is urgent to develop a super-absorbent material with high water retention to solve the problem of sweat cannot being evaporated and stored in the existing protective clothing.
The existing super-absorbent materials mainly exist in the form of resin particles, films, fibers, hydrogels, and so on, with a certain water absorption capacity (Elsaeed et al. 2022; Fang et al. 2018; Ai et al. 2021; Priya et al. 2021, Kassem et al. 2020; Bary et al. 2017; Petroudy et al. 2021; Baldino et al. 2020; Hou et al. 2020; Huang et al. 2019; Arafa et al. 2022; Rather et al. 2022). However, the resin particles can only be spread to the gap of fluff pulp by spraying, which is easy to slippage between the permeable and impermeable layer, resulting in local excessive water absorption. Moreover, the water absorption capacity of the super-absorbent fiber reported so far is less than 250 g/g. At present, some researchers have prepared a zinc-poly(vinyl alcohol) (Zn-PVA) composite film by embedding a super-hygroscopic zinc-ethanolamine complex in the PVA matrix, and combining Zn-PVA composite film with medical protective clothing, the internal relative humidity of protective clothing was decreased from 91.0–48.2%, as well as its maximum water absorption was 134 g/m2 (Yang et al. 2022). However, for medical staff who work for a long time, a large number of films or fibers are needed to absorb the sweat discharged, which exits the problems of large volume, heavy weight, and short working time and so on (Yang et al. 2020; Karim et al. 2020). Most importantly, the water retention capacity of these super-absorbent materials needs to be improved and cannot be widely used in protective equipment. Although the researchers have prepared cellulose-based hydrogels and wheat-based aerogels by graft cross-linking copolymerization, which have the advantages of light weight, excellent water absorption and retention capacity, they have the problems of slow water-absorption rate, poor forming ability after water absorption, and easy crushing after extrusion (Gorgieva, & Kokol, 2011; Kaya et al. 2017; Capezza et al. 2020).
Textiles have the advantages of convenient processing, easily forming, excellent flexibility, and high porosity (Bernhard et al. 2022; Zhou et al. 2022). Using the fabric as the substrate, the gel with porous structure is loaded into the surface and pores of the fabric substrate, and it is expected to obtain comfortable fabric with the great forming ability and super water absorption capacity (Wang et al. 2021; Taghavi et al. 2021). In a recent report, Yu et al. prepared a flame-retardant fabric with hydrogel deposition thickness of 1.5 mm by immersing the cotton fabric into sodium alginate (SA)/N-isopropyl acrylamide (PNIPAAm)/silver nanoparticles (Ag NPs) gel polymerization solution, and further in-situ deposition in CaCl2 solution. The obtained flame-retardant fabric can be used in the field of fire protection, but there is a problem of weak combination ability between hydrogel and fabric (Yu et al. 2020).
Sodium alginate and starch are both natural macromolecules and have the advantages of being renewable and biodegradable. The abundant hydroxyl and carboxyl groups in their structures provide reactive sites for the preparation of super-absorbent polymers (Arafa et al. 2022; Xu et al. 2021; Lin et al. 2020; Kim et al. 2021). The polyester nonwoven has a fluffy fiber structure, which can release a large amount of -OH and -COOH under high temperature alkaline condition, it will provide rich space for physical loading and chemical bonding of super-absorbent polymers. There have been no published reports about the preparation of super-absorbent fabric by in-situ polymerization of starch-sodium alginate-polyacrylate (St-SA-PAA) gel on the surface of fabric. In this work, we hope to obtain ultra-light, soft and super-absorbent polyester fabric-based gel by graft cross-linking polymerization and freeze-drying strategy. Among them, the gel and fiber are closely combined through chemical bonding and hydrogen bonding interaction. Figure 1 shows the preparation flow chart of super-absorbent fabric-based gel. The group changes, crystalline structure, and microstructure of St-SA-PAA gel were studied. On this basis, the liquid absorption capacity and water retention capacity of fabric-based gel to deionized water, normal saline, and synthetic urine were analyzed. Also, the swelling dynamics of fabric-based gel is studied, and the possibility of using it as a liquid reservoir layer of medical protective clothing is explored. It is expected to provide a new method for the development of comfortable PPE fabric.
Materials
Sodium alginate (SA, 600–800 mPa•s, Mw = 2.7×105) was purchased from Qingdao Bright Moon Seaweed Group Co., Ltd. (Shandong, China). Corn starch was obtained from Xi'an Xiadian Corn Development Industry Co., Ltd. (Shaanxi, China). Acrylic acid (AA), Ammonium persulfate (APS), N,N'-Methylenebisacrylamide (MBA), NaOH, urea, MgSO4, Calcium chloride (CaCl2), and NaCl were provided by Sinopharm Chemical (Tianjin, China). Polyester nonwovens were supplied by Shandong Huifeng Adhesive-bonded Fabric Co., Ltd (Shandong, China).
Preparation of St-SA-PAA macroporous gels
The preparation of St-SA-PAA macroporous gel is as follows: Firstly, a 3 wt% starch and 2 wt% SA solution were mixed evenly to form a homogeneous system. Subsequently, acrylic acid neutralized by 25 wt% NaOH solution (neutralization degree 30%), APS, and MBA solution were added in sequence and stirred evenly. After that, transfer the above-mentioned homogeneous system to the petri dish and form St-SA-PAA hydrogel in a water bath at 75 ℃ for 1h. Then, the hydrogel was swelled in deionized water to remove unreacted small molecules. Finally, St-SA-PAA macroporous dried gel was obtained by freeze-drying. A series of St-SA-PAA dried gels was obtained by changing the mass ratio of starch to SA, and coded as St-SA-PAA-1, St-SA-PAA-3, St-SA-PAA-5, St-SA-PAA-7, and St-SA-PAA-9. The specific reaction parameters were shown in Table 1. Moreover, St-PAA and SA-PAA dried gels were prepared as blank control.
Fabrication of super-absorbent fabric
The preparation of super-absorbent fabric is as follows: Firstly, polyester nonwovens were treated with NaOH solution at 100 ℃ for 1 h. The fabric was placed in the reaction solution of St-SA-PAA hydrogel according with certain bath ratio after washing and drying. Afterwards, the fabric-based hydrogel was formed in a water bath at 75 ℃ for 1 h, and it was swelled in deionized water to remove unreacted small molecules. Finally, super-absorbent fabric-based dried gel was obtained by freeze-drying.
Characterization
The micro-morphology of SA-PAA, St-PAA, St-SA-PAA, polyester fabric, and fabric-based dried gel was observed by scanning electron microscope (SEM) (Hitachi S-4800, Tokyo, Japan) after high-pressure gold spraying in an argon atmosphere. FTIR spectra of the gels and fabric were measured by Spotlight 400 & Frontier FTIR spectrometer (Perkin Elmer, USA). X-ray diffractometer (Rigaku Mini Flex 600 Japan) was used to conduct XRD analysis, and the scanning region of diffraction angle was from 10° to 80°. To observe the hydrophilicity of St-SA-PAA and fabric-based gel, the water contact angle was tested by the OCA 40 Micro goniometer (Data physics, Germany) through a pendant drop method.
Swelling property test
The St-SA-PAA and fabric-based dried gels were put into deionized water, normal saline, and synthetic urine solution to study the swelling properties, respectively. The swelling rate (SR) in different solutions was calculated by the following formula:
Where SR (g/g) is defined as the weight of water absorbed per gram of St-SA-PAA dry gels or fabric-based dry gels; Wt and Wd represent the weight at t time and the initial weight of St-SA-PAA and fabric-based dried gels, respectively.
Calculation of cross-linking density
According to the affine network model, the effective cross-linking density (ʋ*c) of St-SA-PAA and fabric-based dried gel can be calculated (Wei, Chen, Song, Wu, & Xu, 2021). The calculation formula is as follows:
ʋ*c=\(G/\left(RT{ʋ}_{2,r}^{2/3}{ʋ}_{2,s}^{1/3}\right)\)
Where G is the shear modulus of gel, R is the universal gas constant, T is the Kelvin temperature (k), ʋ2,r is the volume fraction of the polymer after drying, ʋ2,s is the volume fraction when the sample reaches swelling equilibrium. The calculation formula of ʋ2,r and ʋ2,s are as follows:
$${ʋ}_{2,r}={M}_{r}{C}_{0}/{\rho }_{p}$$
$${ʋ}_{2,s}=1/\left[\left(1+ESR{\rho }_{P}/{\rho }_{S}\right)\right]$$
Where Mr is the average molecular weight of polymer repeating unit, C0 is the total monomer concentration in the polymer, ρP and ρs are the density of dried gel and solvent, respectively. The calculation formula of ρP is as follows:
$${\rho }_{P}={m}_{p}/{V}_{p}$$
Where mp and vp are the mass and volume of dry gel respectively.
Tensile breaking strength test
YG(B)026D-500 electronic fabric strength tester was used to test the tensile breaking strength of the fabric and super-absorbent fabric according to the standard of GB/T3917.3-2009. Test conditions: relative humidity 65 ± 2%, temperature 20 ± 1 ℃.
|
Samples |
Starch (%) |
SA (%) |
AA (g) |
APS (%) |
MBA (%) |
ESR (g/g) |
||
|---|---|---|---|---|---|---|---|---|
|
Deionized water |
Normal saline |
Synthetic urine |
||||||
|
St-SA-PAA-1 |
90 |
10 |
10 |
2.5 |
0.7 |
341 |
39 |
25 |
|
St-SA-PAA-3 |
70 |
30 |
10 |
2.5 |
0.7 |
788 |
55 |
41 |
|
St-SA-PAA-5 |
50 |
50 |
10 |
2.5 |
0.7 |
486 |
53 |
40 |
|
St-SA-PAA-7 |
30 |
70 |
10 |
2.5 |
0.7 |
254 |
43 |
29 |
|
St-SA-PAA-9 |
10 |
90 |
10 |
2.5 |
0.7 |
272 |
49 |
27 |
|
St -PAA |
100 |
0 |
10 |
2.5 |
0.7 |
258 |
41 |
30 |
|
SA-PAA |
0 |
100 |
10 |
2.5 |
0.7 |
362 |
45 |
34 |
| Note: the neutralization degree of AA is 30%, the reaction temperature is 75℃, and the polymerization time is 1 h. | ||||||||
Synthesis and characterization of St-SA-PAA gel
Figure 2a exhibits the synthesis mechanism of St-SA-PAA gel. Firstly, St-SA-PAA hydrogel was prepared from starch, sodium alginate and AA(Na) in aqueous solution by APS thermal initiation and MBA cross-linking polymerization, and then St-SA-PAA dried gel was obtained by further freeze-drying. Among them, the typical dual network structure was constructed by the stable chemical cross-linking and hydrogen bonding interaction among SA, starch polysaccharide chains and polyacrylic acid molecular chain. The chemical structure and crystal characteristic of St-SA-PAA gel were analyzed by FTIR and XRD, as shown in Fig. 2b-c. It can be seen that on the FTIR spectrum of starch, the strong broad absorption peak at 3297 cm− 1 was due to the stretching vibration of O-H, and the characteristic absorption peaks at 1152 cm− 1, 1073 cm− 1, and 994 cm− 1 were C-O-C groups (Lin et al. 2021). As well as, on the FTIR spectrum of SA, the absorption peaks at 3276 cm− 1, 1592 cm− 1, and 1409 cm− 1 were attributed to the -OH stretching vibration, the asymmetric and symmetric stretching vibration peaks of the -COO−, respectively (Arafa et al, 2022). Compared with the starch and SA, in addition to the characteristic peaks of starch and SA, the characteristic absorption peaks at 1545 cm− 1, 1447 cm− 1, 1698 cm− 1, and 2933 cm− 1 were significantly enhanced on the FTIR spectrum of St-SA-PAA. This is because the abundant N-H, C-N, C = O, and -CH2 groups were provided through graft cross-linking polymerization. Furthermore, for St-SA-PAA, the peak at 3391 cm− 1 belonged to -OH was weakened due to the graft cross-linking reactions mainly occurred on the -OH group. These changes provided evidences for the graft cross-linking polymerization among starch, SA, and AA.
Figure 2c shows the XRD patterns of starch, SA and St-SA-PAA. It can be found that the characteristic diffraction peaks appeared at 2θ = 15°, 17°, 18° and 23° on the XRD pattern of starch, and the diffraction peaks of SA appeared at 2θ = 13° and 21°. However, on the XRD pattern of St-SA-PAA, no characteristic diffraction peaks of starch and SA were observed, and a wide diffraction peak at 2θ = 19° was appeared, which may be due to the destruction of the crystal structure of starch and SA by graft cross-linking. According to Jade software, the crystallinity of starch, SA, and St-SA-PAA were 75.36%, 31.77%, and 47.19%, respectively (Fig. S1). The reduction of crystallinity is conducive to the permeation and transfer of water molecules in the microstructure of St-SA-PAA gel.
Figure 2d-k display the cross-sectional SEM morphologies of St-PAA, SA-PAA, and St-SA-PAA hydrogels in freeze-dried state. As can be seen all dried gels present interconnected random flake and porous honeycomb structures, which is related to the sublimation of ice crystal during freeze-drying. In the process of ice crystal sublimation, the continuous porous channels are formed in the macromolecular skeleton, and the intramolecular and intermolecular hydrogen bonding interactions make macromolecules aggregate to form sheet aggregates. From the microstructure of St-PAA dried gel, it can be observed that the pore size of St-PAA dried gel ranged from 45 ~ 99 µm, but the enlarged image showed that the network structure lacked three-dimensional support (Fig. 2d-e). However, in the microstructure of SA-PAA dried gel, a large number of random entangled sheet structure appeared with smaller pore size (Fig. 2f-g). Compared with the microstructures of St-PAA and SA-PAA dried gels, St-SA-PAA-3 and St-SA-PAA-5 exhibited a distinct three-dimensional network porous structure (Fig. 2h-k). It may be caused by the synergistic effect of chemical cross-linking and hydrogen bonding among multi-component structures. From their high magnification, the St-SA-PAA-3 dried gel presents a larger pore size of 34 ~ 114 µ m and thick pore wall (Fig. 2i), which are beneficial to enhance the liquid absorption capacity and improve the swelling firmness of the network structure after liquid absorption.
Swelling behavior of St-SA-PAA gel and super-absorbent fabric
The swelling behavior of St-PAA, SA-PAA, and St-SA-PAA dried gel in different solutions was studied, as shown in Table 1. The absorption capacity of St-PAA dried gel to deionized water, normal saline (0.9 wt%), and synthetic urine was 258 g/g, 41 g/g, and 30 g/g, respectively. Moreover, the corresponding absorption capacity of SA-PAA dried gel was 362 g/g, 45 g/g, and 34 g/g, respectively. Compared with St-PAA and SA-PAA, St-SA-PAA showed higher liquid absorption capacity, with the absorption capacity of deionized water, normal saline and synthetic urine of 254 ~ 788 g/g, 39 ~ 55 g/g and 25 ~ 41 g/g, respectively. It shows that the synergistic effect of starch and SA can effectively improve the liquid absorption capacity of dried gel. This is because the abundant hydrophilic -OH and -COOH in the structure of starch and SA, as well as the three-dimensional macroporous dual network microstructure produced during the formation of gel, the synergistic effects improved the liquid absorption capacity of dried gel. From Fig. 3a, it can be seen that all initial samples have the same size, and the morphogenesis increases significantly after reaching the swelling equilibrium, especially for the absorption capacity of deionized water. This is mainly because the Na+, Ca2+, Mg2+ in normal saline and synthetic urine shield the hydrophilic anions -COO− in the polymer systems, which weakens the internal electrostatic repulsion force of St-SA-PAA dried gel after water absorption and reduces the expansion degree between molecular chains. In addition, the gels exhibited lower internal and external osmotic pressure during liquid absorption in normal saline and synthetic urine, resulting in a lower equilibrium swelling ratio (Wei et al. 2021). At the same time, it can be clearly observed that St-SA-PAA sample exhibited the phenomenon of partial gel breaking and poor mechanical strength (the dotted line in the figure indicates the position), after reaching the water absorption and swelling equilibrium. Figure 3b-c shows the swelling rate curve of St-SA-PAA-3 dried gel in different solutions. As can be seen that the time required for St-SA-PAA-3 to absorb 100 g of deionized water, 30 g of normal saline, and 30 g of synthetic urine is 35 min, 60 min, and 60 min respectively, and the calculated average absorption rate is only 0.5–2.8 g/min. This is because the high cross-linking density of gel tends to block the gel after water absorption, which hinders the water transmission channel and leads to slow water absorption rate.
St-SA-PAA is difficult to satisfy practical application requirements due to the slow water absorption rate and poor mechanical strength. Benefiting from the capillary and supporting effect of fabric matrix, the problems of slow water absorption rate and insufficient mechanical strength could be improved by loading gel into flexible fabric with porous structure. We impregnated the polyester fabric hydrolyzed in alkali into starch, SA and AA(Na) solution, and under the action of initiator APS and crosslinker MBA, the super-absorbent fabric-based gel was obtained by in-situ deposition polymerization on the fabric surface by graft cross-linking and freeze-drying strategy. Among them, SA, starch polysaccharide chain and polyacrylic acid molecular chain are covalently connected to polyester fiber through C-O to form a stable interface chemical bond. Besides, there is non-covalent hydrogen bonding interaction among polyester fiber, starch and SA macromolecular chain (Fig. 4a).
The interaction and microstructure between St-SA-PAA and polyester fiber were analyzed by FTIR and SEM, as shown in Fig. 4b-f. On the FTIR spectrum of alkali-treated polyester fiber, a weak characteristic peak of -COO− appeared at 1502 cm− 1, which provided active sites for chemical bonding and physical loading of super-absorbent polymers (Fig. 4b). The characteristic peaks of polyester fiber and St-SA-PAA appeared on the FTIR spectrum of fabric-based gel, and the characteristic peak of C = O at 1698 cm− 1 enhanced, while the characteristic peak of -OH at 3391 cm− 1 had shift change compared with that of St-SA-PAA. The results demonstrated there was an interaction between St-SA-PAA and polyeste fiber. The micro-morphology of polyester, alkali-treated fabric, and fabric-based gel were observed by SEM. It can be seen that the surface of polyester fiber was smooth, while a large number of grooves appeared on the surface of alkali-treated fiber, which can provide loading space for polymer (Fig. 4c-d). From the surface of fabric-based gel (Fig. 4e), a large number of polymers were attached to the fiber surface, which still presented the reticular continuous pore structure of St-SA-PAA gel, with pore sizes ranging from 30 to 91 µm. It could provide space for absorption and storage of water molecules. Further, St-SA-PAA is closely combined with the fiber matrix, indicating that it has a good deposition effect. From the cross-section of fabric-based gel (Fig. 4f), it can be seen that the polymer ran through the loose and disordered fiber layer, which played a supporting role for the network expansion of gel, and could effectively improve the poor mechanical property of gel after water absorption. As shown in Fig. 4g, the tensile breaking strength of fabric-based gel could reach 155 N, which is far greater than that of fabric itself, so it could satisfy the practical application requirements as super-absorbent material.
Figure 5a illustrates the swelling behavior of fabric-based gel formed by in-situ deposition polymerization of St-SA-PAA-3 reaction solution onto the surface of alkali-treated polyester fiber (concentration 2.5%) under different bath ratios. It can be seen that with the increase of the bath ratio of polymerization reaction solution, the swelling rate (SR, g/g) of fabric-based gel increased first and then decreased. This is because the increase of polymerization reaction solution made more St-SA-PAA macroporous gel in situ polymerization on the surface of the fiber, thus improved the liquid absorption capacity. However, too high bath ratio increased the aggregation degree of gel on the fiber surface, the cross-linking density became higher, and the pore size of gel became smaller, resulting in a decreasing trend of liquid absorption. The illustration in Fig. 5a showed that the fabric-based gel obtained with a bath ratio of 1:30 exhibited a high hardness and cannot be bent; When the bath ratio is 1:25, the obtained fabric-based gel presented a soft appearance, and SR of deionized water could reach 189 g/g. It meant that the softness and water absorption of super-absorbent fabric can be adjusted by changing the bath ratio and the amount of gel deposition on the fiber surface.
Considering that the concentration of alkali hydrolysis has a significant difference on the degree of grooves for the surface of polyester fiber, which affects the distribution of polymer on the fiber surface and the ability of liquid absorption. We tested the SR of the super-absorbent fabric-based gel prepared when the alkali concentration was 5, 7.5, and 10%, respectively, as shown in Fig. 5b. It was found that when the bath ratio was 1:25 and the alkali concentration was 7.5%, super-absorbent fabric with SR of 343 g/g could be obtained, which exceeded the reported water absorption capacity of commercial super-absorbent fibers, super-absorbent poly (arylene ether ketone) fibers and super-absorbent cellulose nanofibers (Fig. 5c) ( Liu et al. 2015, Petroudy et al. 2021). But the SR of super-absorbent fabric in this work to normal saline and synthetic urine was only 36 g/g and 30 g/g. Compared with the liquid absorption rate of St-SA-PAA-3 gel, the corresponding super-absorbent fabric formed showed a faster liquid absorption rate when absorbing the same quality of deionized water, normal saline and synthetic urine, especially when absorbing 100 g of deionized water, it could increase the water absorption rate of St-SA-PAA-3 gel from 2.8 g/min to 10 g / min (Fig. 5d). As can be seen from their water contact angle (Fig. 5e), the fabric-based gel presented a lower water contact angle of 14°. It implied the porous fabric matrix had a significant contribution to improve the water absorption rate of St-SA-PAA gel. This was mainly because the large specific surface area of polyester fiber reduced the cross-linking density of gel, which prevented gel from blocking after water absorption. At the same time, the capillary action of the fiber accelerated the water absorption rate of the fabric-based gel. Table 2 shows that the crosslink density (vc*) of St-SA-PAA gel was decreased from 0.312 to 0.181 mol/L when it was in situ polymerized onto the fiber. In the future work, we will continue to explore new ways to further improve the liquid absorption rate of super-absorbent fabrics from the perspective of cross-linking density.
| Samples | ρp (g/cm3) | ʋ2s | ʋ 2r | ʋ*c (mol/L) | Compressive strength (kPa) |
|---|---|---|---|---|---|
| St-SA-PAA-3 | 0.30 | 0.0043 | 229.52 | 0.312 | 71.4 |
| Super-absorbent fabric | 0.45 | 0.0065 | 197.09 | 0.181 | 260.5 |
Figure 5f shows that the initial sample size of all super-absorbent fabric-based gels is the same. After reaching the swelling equilibrium, the appearances increased significantly, especially in deionized water, and the fabric has isotropic characteristic after water absorption. In addition, no liquid was escaped from the fabric-based St-SA-PAA gels after reaching the swelling equilibrium under the pressure of 100 g weight (Fig. 5g), showing excellent water storage capacity and mechanical properties. Considering the trade-off between softness and water absorption capability, in this work, the super-absorbent fabric-based gel was prepared by in-situ polymerization of St-SA-PAA-3 reaction solution onto the alkali treated (concentration 7.5%) polyester fiber surface with the bath ratio of 1:25. However, the super-absorbent fabric prepared in this work showed low absorption capacity of normal saline and synthetic urine, which was less than 100 g/g. In the future work, we will explore the methods to improve the absorption capacity of super-absorbent fabric to electrolyte solution from the perspective of electrostatic effect.
Based on the ultra-light, soft, super-absorbent, high water retention, and isotropic characteristics of the super-absorbent fabric, it can be used as the liquid absorption and reservoir layer of medical protective suits by laminating composite (Fig. 5h). For the medical staff who work for a long time, the super-absorbent fabric can quickly absorb and store sweat. At the same time, the surface temperature of the human body could be effectively reduced after the sweat is absorbed, achieving the effect of damp heat management, which provide a lasting comfortable micro-environment for medical staff. Otherwise, the super-absorbent fabrics have potential applications in the fields of soil moisture preservation (such as crop growth in arid areas), sanitary products (such as disposable diapers), and oil-water separation due to its strong water absorption and storage capacity.
Swelling dynamics mechanism of super-absorbent fabric
To verify the swelling behavior of super-absorbent fabric on liquid, the swelling kinetics of fabric-based gel prepared in this work on deionized water and normal saline was studied, as shown in Fig. 6. The super-absorbent material will swell When the liquid diffuses into its interior, and the swelling process conforms to Fickian or non-Fickian swelling mechanisms, which are usually investigated by formula (1) ( Chen, Ni, Shen, Xiang, & Xu, 2020):
$$\text{F=}\frac{{\text{Q}}_{\text{t}}}{{\text{Q}}_{\text{e}}}{\text{=Kt}}^{\text{n}}$$
1
Where F represents the swelling ratio, Qt and Qe represent the SR at time t and equilibrium, respectively (g/g); t means the swelling time, K means the characteristic constant, and n is the diffusion index.
According to the limit of n, three theoretical models can be assumed: (1) n < 0.5, Fickian diffusion; (2) 0.5 < n < 1, non-Fickian diffusion; (3) n > 1, macromolecular chain relaxation diffusion process. According to formula (1), the fitting curve of fabric-based gel to deionized water and normal saline was drawn, as shown in Fig. 6a-b. It could be found that the diffusion index n of deionized water and normal saline of fabric-based gel was 0.19217 and 0.01717, respectively, which were less than 0.5, indicating that the diffusion behavior of liquid in fabric-based gel conformed to Fickian mechanism.
The swelling process of super-absorbent material is usually divided into two stages: the initial phase of rapid diffusion of solvent molecules into the pores of polymer networks and the subsequent stage of slow slackness and water absorption of polymer chains. The quasi first-order and second-order kinetic models were used to fit the experimental data to study the swelling behavior of fabric-based gel in deionized water and normal saline. The formulas are as follows:
The quasi first-order kinetic model:
\(\text{ ln}（{\text{Q}}_{\text{e}}\text{-}{\text{Q}}_{\text{t}}）\text{=ln}{\text{Q}}_{\text{e}}\text{-}{\text{k}}_{\text{a}}\) t (2)
The quasi second-order kinetic model:
$$\frac{\text{t}}{{\text{Q}}_{\text{t}}}\text{=}\frac{\text{1}}{{\text{k}}_{\text{b}}{\text{Q}}_{\text{e}}^{\text{2}}}\text{+}\frac{\text{t}}{{\text{Q}}_{\text{e}}}$$
3
Where the ka and kb are represent rate constant.
The swelling kinetics fitting curves are fitted according to the quasi first-order and second-order kinetic models, as shown in Fig. 6c-f. Figure 6c-d are the first-order kinetic fitting curves of fabric-based gel swollen in deionized water and normal saline. According to the first-order kinetic Eq. (2), the theoretical absorption of deionized water and normal saline by fabric-based gel are 297.6 g/g and 32.2g/g, respectively, while the linear correlation in the fitting curve of normal saline absorption was poor (R2 = 0.70471). Figures 6e-f are the second-order kinetic fitting curves of fabric-based gel swollen in deionized water and normal saline. According to the second-order kinetic Eq. (3), the theoretical absorption of deionized water and normal saline by fabric-based gel were 281.7 g/g and 34.0 g/g, respectively, which were close to the actual absorption rate, and R2 was greater than 0.9, showing a good linear correlation. Therefore, the swelling process of fabric-based gel generally conformed to the second-order kinetic model.
Herein, a ultra-light, soft, super-absorbent, and high water retention fabric was obtained by in-situ polymerization of macroporous St-SA-PAA gel on the surface of alkali-treated polyester fiber. Benefiting from the stable chemical bonding and hydrogen bonding interaction, St-SA-PAA gel was closely bound to the fiber matrix and exhibited a continuous network structure with a pore size distribution ranging from 30 to 91 µm. Compared with St-SA-PAA gel, the super-absorbent fabric-based gel showed a faster liquid absorption rate, this was mainly due to the large specific surface area and capillarity of the polyester fiber, which can effectively reduce the cross-linking density of gel. Moreover, polyester fiber played a supporting role in the network expansion of gel, which can effectively improve the poor mechanical property of gel after water absorption. Most importantly, the absorbability of the super-absorbent fabric to deionized water could reach 343 g/g, which exceeded the reported commercial super-absorbent fibers, super-absorbent poly(arylene ether ketone) fibers and super-absorbent cellulose nanofibers. Na+, Ca2+, Mg2+ ions in normal saline and synthetic urine shielded the hydrophilic anion -COO−, which weakened the internal electrostatic repulsion and expansion, resulting in low equilibrium swelling ratio of super-absorbent fabric to electrolyte solution. The swelling behavior of super-absorbent fabric conformed to the second-order kinetic model. This work provides a new way to develop comfortable personal protective equipment required for long-time work in extreme environments. In addition, super-absorbent fabrics have broad application prospects in the fields of soil moisture preservation, sanitary products, and oil-water separation.
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The authors declare no conflicting interests.
Funding
We acknowledge support from Scientific Research Program Funded by Shaanxi Provincial Education Department (21JK0664), and Doctoral Research Program of Xi'an Polytechnic University (107020533).
Authors' contributions
Yijun Yao and Yanqin Shen authors have contributed equally to this work. Yijun Yao: Investigation, Conceptualization, Methodology, Writing-Original draft preparation; Yanqin Shen: Conceptualization, Methodology, Data curation; Chengmeng Hu: Writing-Original draft preparation, Software, Validation; Hailiang Wu: Project administration, Funding acquisition, Supervision; All authors read and approved the final manuscript.
Acknowledgements
The financial support by the innovation team of ShaanXi Provincial Tri-Qin Scholars is gratefully acknowledged.
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No competing interests reported.
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