Cellulose (2016) 23:941–953 DOI 10.1007/s10570-015-0820-y
ORIGINAL PAPER
Preparation and characterization of novel hydrophobic cellulose fabrics with polyvinylsilsesquioxane functional coatings Dongzhi Chen . Fengxiang Chen . Hongwei Zhang . Xianze Yin . Yingshan Zhou
Received: 30 August 2015 / Accepted: 8 November 2015 / Published online: 30 November 2015 Ó Springer Science+Business Media Dordrecht 2015
Abstract A series of novel hydrophobic cotton fabrics with polyvinylsilsesquioxane (PVS) polymer functional coatings were successfully prepared by solution immersion. The influence of the added amount of PVS polymer on the morphology, resistance to thermal and thermooxidative degradation, and hydrophobic properties of the treated cotton fabrics was studied by attenuated total reflection infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, and water contact angle measurements, respectively. The experimental results show that the PVS polymer formed a protective film on the surface of the cotton fibers; the resistance to thermal and thermooxidative degradation, and the water-repellent properties of the novel cotton fabrics were also improved with increasing added amount of PVS polymer, compared with that of reference material. The enhancement in the thermal properties
of the treated cotton fabrics can likely be attributed to synergistic carbonization between the PVS protective layer and the cellulose fibers during thermal degradation. Meanwhile, it was also found that, with increasing added amount of PVS polymer, the hydrophobicity of the treated cotton fabrics was greatly improved. The noticeable improvement in the hydrophobicity of the treated cotton fabrics is ascribed to the combination of low-surface-energy PVS film and the intrinsically rough surface of the woven cotton fabrics. This strategy for fabricating novel cellulose fabrics provides a guide for the development of high-performance functional cellulose fabrics with tunable properties in the textile industry.
Electronic supplementary material The online version of this article (doi:10.1007/s10570-015-0820-y) contains supplementary material, which is available to authorized users.
Introduction
D. Chen (&) F. Chen H. Zhang X. Yin Y. Zhou School of Materials Science and Engineering, Wuhan Textile University, Wuhan 430200, People’s Republic of China e-mail:
[email protected] F. Chen Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan 430062, People’s Republic of China
Keywords Cellulose fabrics Functional coating Morphologies Thermal properties Hydrophobicity
Cotton cellulose fibers, as one of the most important natural polymer materials, are usually spun into yarns or threads to produce a large quantity of various types of apparel, home textiles (carpets and upholstered furniture), and outdoor textiles, because cotton fabrics are soft, comfortable, breathable, and skin-friendly. However, the abundant hydroxyl groups on cotton fiber surfaces make such fabrics absorbent and easily stained by liquids. Moreover, cotton fabrics have some inherent
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deficiencies such as low thermal stability, weak ultraviolet (UV) protection capability, and rapid combustion. These defects greatly limit application of cotton cellulose fibers. To overcome these disadvantages, a variety of approaches, such as sol–gel processing (Alongi et al. 2012; Qi and Xin 2010), solution immersion (Li et al. 2010; Xu et al. 2011), phase separation (Wu et al. 2014), spin coating (Lee et al. 2011), atomic layer deposition (Hyde et al. 2010), electroless deposition (Preda et al. 2013), layer-by-layer assembly (Pan et al. 2015), ultrasound irradiation (Sadr and Montazer 2014), and plasma treatment (Caschera et al. 2013; Shahidi et al. 2007), have been developed for design and fabrication of multifunctional cotton fabrics. In recent decades, great progress has been made in developing novel multifunctional cotton fabrics by functional treatment, so varieties of functional cotton fabrics with desirable properties such as antibacterial resistance (Chen et al. 2011; Hong 2014; Kwak et al. 2015; Zhao et al. 2013), UV blocking (El-Shafei and Abou-Okeil 2011; Wang et al. 2009), flame retardancy (Alongi et al. 2011b; El-Shafei et al. 2015; Xie et al. 2013; Yang et al. 2012), and electrical conductivity (Hu et al. 2010; Shateri-Khalilabad and Yazdanshenas 2013) have been reported. Recently, in particular, superhydrophobic (Deng et al. 2014; Liu et al. 2012; Nateghi and ShateriKhalilabad 2015; Periolatto et al. 2013; Wang et al. 2011) and oleophobic (Duan et al. 2011; Gao et al. 2010) fabrics have attracted considerable interest from the points of view of both fundamental research and industrial applications due to their self-cleaning properties (Basu et al. 2012; Li et al. 2008). A superhydrophobic surface can not only prevent the material from getting dirty due to contact with a variety of liquids but also prolong the service lifetime of the material by avoiding degradation caused by water wetting. Generally, superhydrophobic surfaces can be easily obtained by a combination of low surface energy and high roughness of the material (Georgakilas et al. 2008). It is well known that fluorinated chemicals with low surface energy ranging from 5 to 28 mN/m (Brewer and Willis 2008; Liu et al. 2012), such as fluorocarbon polymers (Xiong et al. 2012) and fluorinated compounds (Ferrero and Periolatto 2013; Ma et al. 2013), have been widely employed to lower the surface free energy to construct superhydrophobic surfaces. Although cotton fabrics treated with fluorinecontaining polymers show superior hydrophobicity, these fluoride polymeric coatings are expensive, toxic,
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and bioaccumulative in many types of flora and fauna, representing a potential risk to human health and the environment (Muresan et al. 2013; Wang et al. 2014). Therefore, new functional finishes with low surface free energy to replace fluorinated chemicals are urgently sought by materials researchers for the design of superhydrophobic fabrics in the textile industry (Gao et al. 2009). To date, polysiloxane has attracted considerable attention owing to its unique properties, such as high flexibility and gas permeability, good thermal stability, low surface free energy (Park et al. 2013), hydrophobicity (Mohamed et al. 2014), excellent weather resistance, and low toxicity and chemical reactivity (Mark 2004), being widely applied in many important fields such as sealants, adhesives, fabric finishes, and coatings. In particular, polydimethylsiloxanes (PDMS) containing amine groups have been widely used as fabric softeners or smoothing agents to lubricate fibers in the textile industry (Reddy et al. 2008; Zia et al. 2011). In addition to the above-mentioned applications, some reactive PDMS have also been employed as coatings to impart durable hydrophobicity on cotton fabrics (Zhou et al. 2012). However, there has been little work on hydrophobic fabrics using silicone resin as finishing agent. It has only been found that fiber finishes formed of liquid methylsiloxane polymers containing reactive groups could impart excellent durable water repellency and resistance to aqueous-borne stains (Fortess 1954), but the morphology and thermal stability of these functional fabrics have not been investigated further. In our latest work (Chen et al. 2015), PVS polymer was applied to PDMS matrix to impart superior thermal stability and mechanical properties on PDMS composites. Considering the advantages of PVS polymer and disadvantages of cellulose fabrics, it is expected that coating PVS polymer on the surface of cotton fibers could confer the fabrics with some unique properties to overcome their defects. To the best of our knowledge, there are few reports on cotton fabrics functionalized with PVS. The aim of this work is to study the effect of adding different amounts of PVS polymer on the morphology, resistance to thermal and thermooxidative degradation, and hydrophobic properties of cotton fabrics. In this work, a series of novel cotton fabrics were successfully prepared using PVS polymer as a functional finish for the first time, and their chemical
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composition, morphology, thermal behavior, and hydrophobicity were studied by attenuated total reflection infrared (ATR-IR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and water contact angle measurements, respectively. The influence of the added amount of PVS polymer on the properties of the cotton fabrics is also discussed in detail.
Experimental Materials Desized, scoured, and bleached cotton fabric was obtained from Jinqiu Textile Company, Shaoxing, China. Vinyltrimethoxysilane (97.9 %) was supplied by Wuhan University Silicone New Material Co. Ltd., Wuhan, China. CaCO3 (AR), anhydrous ethanol (AR), and concentrated hydrochloric acid (37 %) were obtained from Shanghai Reagent Plant, China. Toluene (AR) was purchased from Tianjin BoDi Chemical Reagent Co., Ltd. All of the above solvents were of analytical purity and used as received. Synthesis of PVS polymer Polyvinylsilsesquioxane (PVS) was prepared according to a modified approach (Abe et al. 2000; Takamura et al. 1999). The synthesis process was as follows: 10 mL ethanol or methanol, 250 mL toluene, and 1.00 mol vinyltrimethoxysilane or vinyltriethoxysilane were charged into a round-bottomed flask equipped with a magnetic stirrer, which was placed in an ice bath and stirred for 10 min. Afterwards, a mixture consisting of 10.35 g concentrated hydrochloric acid and 47.48 g deionized water was added dropwise into the cooling reaction. Then, the mixture was further stirred at room temperature for 10 min, followed by stirring and refluxing at 85 °C for 3 h. After the mixture had been cooled to room temperature, the liquid layer was separated and washed three times with 25 mL deionized water. Finally, the washed liquid layer was collected and transferred into a round-bottomed flask equipped with a magnetic stirrer, and 10.0 g anhydrous CaCO3 was added, stirring for half an hour and filtering to obtain a filtrate that was concentrated to provide a colorless oil product (PVS polymer). 1H NMR (CDCl3): d 5.96–6.07
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(m, H2C=CH–Si), d 3.48 (s, Si-OCH3), d 2.36 (s, Si– OH); 13C NMR (CDCl3, ppm): d = 50.6 (SiOCH3), d = 129.23 (CH=CH2), d = 137.18 (CH=CH2); 29Si NMR (CDCl3, ppm): d = -70.22 (H2C=CHSiO (OCH3)), d = -77.84 (H2C=CHSiO (OH)) and d = -79.26 (H2C=CHSiO1.5); The structure of PVS polymer is shown in Scheme 1, and the molar percent of siloxane units x, y, z was 21.5, 16.9, and 61.6, respectively. The resulting PVS product had the following characteristics: Mn = 5655 g/mol and Mw/ Mn = 1.38 (as determined by gel permeation chromatography), being used in the following steps without further purification. Preparation of hydrophobic cotton fabrics treated with PVS polymer The preparation of hydrophobic cellulose fabrics with PVS is shown in Scheme 1. The general procedure involved two steps. In the first step, various added amounts of PVS polymer and 100 mL anhydrous ethanol were respectively charged into a 250-mL conical flask with magnetic stirrer, and a clear finishing solution of PVS polymer in ethanol was obtained by stirring. In the second step, dried fabrics (6 cm 9 20 cm, 140 g/m2) were immersed in the finishing solution and magnetically stirred for 1 h; the soaked cotton fabrics were taken out of the flask and dried for 1 h in a convection oven at 120 °C to provide hydrophobic cotton fabrics. The hydrophobic cotton fabrics were washed with deionized water three times, and dried again at 120 °C for 30 min to afford the treated cotton fabrics. A series of hydrophobic cellulose fabrics treated with PVS were prepared according to the same approach. Only the concentration of PVS relative to the amount of ethanol was varied. The treated cotton fabrics are denoted as PVSC samples, where PVSC-0 is the reference material (cotton fabric without PVS). The add-on percentage for the treated cotton fabrics was calculated using Eq. 1: Add-on % ¼
ðW 2 W 1 Þ 100 %; W1
ð1Þ
where W1 is the weight of cotton fabric before treatment and W2 is the weight of cotton fabric after treatment. With increasing weight of PVS polymer, the added amount of the treated cotton fabrics also increased, as shown in Table 1.
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Scheme 1 Preparation of hydrophobic cellulose fabrics treated with PVS polymer
Characterization of cotton fabrics treated with PVS polymer 1
H NMR and 13C NMR spectra were recorded with an Agilent DD2-500 spectrometer at 499.66 and 125.64 MHz, respectively, in CDCl3 with 0.05 % tetramethylsilane (TMS) as internal standard at RT. 29 Si NMR spectra were recorded with a Bruker Avance III 400 spectrometer in CDCl3 with 0.05 % Table 1 Recipes for cotton fabrics treated with PVS polymer
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Sample
Added amount of PVS (wt%)
TMS as internal standard. The infrared (IR) spectrum of PVS polymer was measured using the KBr pellet technique, and the IR spectra of the cotton fabrics were obtained using a Nicolet NEXUS 670 spectrometer by attenuated total reflection infrared (ATR-IR) spectroscopy at resolution of 2 cm-1. The structure and morphology of the cotton fabrics treated with PVS polymer were recorded using a JSM-6510LV SEM (JEOL Ltd., Japan) at voltage of 10 kV after coating Volume of ethanol (mL)
Added amount (%)
PVSC-0
0
100
0
PVSC-1
1.0
100
3.21
PVSC-2 PVSC-3
2.0 3.0
100 100
5.80 6.83
PVSC-4
5.0
100
9.92
PVSC-5
7.5
100
13.16
PVSC-6
10.0
100
14.28
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with platinum. The EDX unit connected to the SEM microscope was used to determine the percentage atomic contents of elements present on the surface of the coated fabrics. TGA was performed on a SETSYS1750 (SETARAM Instruments) under gas flow of 50 mL/min. About 10 mg of sample cut as small pieces was heated in an Al2O3 crucible in air atmosphere from ambient temperature to 700 °C at heating rate of 10 °C/min, and in nitrogen atmosphere from ambient temperature to 700 °C at a constant rise of temperature (10 °C/min). Static contact angles were measured utilizing deionized water droplets with a Kruss contact angle instrument (DSA 100, Germany) at room temperature (25 °C). Average values of water contact angle were obtained by measuring at least five different positions on the same sample. All of the reported values correspond to contact angles obtained under stationary conditions (i.e., 60 s after a 10-lL water drop was applied to the fabric).
Results and discussion FTIR characterization of novel cotton fabrics treated with PVS polymer At first glance, there was little difference in the FTIR spectra on PVS addition because the absorption peaks of PVSC-0 and PVSC-5 almost overlapped, as displayed in Fig. 1. For PVS polymer, common broad peaks at 3660 and 3426 cm-1 and a weak band at 2843 cm-1 were respectively assigned to stretching vibrations of separate silanol (Si–OH), H-bonded Si– OH and C–H in Si–O–CH3 of PVS polymer. Moreover, a small peak at 3062 cm-1 and a sharp peak at 1602 cm-1 were observed, due to stretching vibrations of H–C=C and C=C, respectively. The bands located at 1113 and 1052 cm-1 should be assigned to asymmetric and symmetric stretching vibrations of Si–O–Si from PVS, respectively, and a strong peak at 760 cm-1 is ascribed to stretching vibration of Si–C. For the reference sample (PVSC-0), a broad peak at around 3310 cm-1 and a weak peak at 1631 cm-1 can likely be assigned to stretching and deformation vibrations of H-bonded hydroxyl groups, respectively. It is hard to distinguish the difference between the reference material and the representative cotton fabric treated with PVS polymer (PVSC-5 sample), as shown in Fig. 1. Therefore, it is difficult to prove that the
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Fig. 1 FTIR spectra of PVS polymer, the reference sample (PVSC-0), and representative cotton fabric treated with PVS polymer (PVSC-5)
condensation reaction between Si–OH from PVS polymer and CH2–OH on the surface of cellulose fibers occurred, even though no noticeable absorption band at 3426 cm-1 attributable to Si–OH vibration was observed. The peaks located at 2918 cm-1 and 2854 cm-1 could be attributed to asymmetric and symmetric stretching vibrations of methylene (–CH2–) groups due to waxes remaining on the surface of the cotton fabric (Chung et al. 2004). It is extremely noteworthy that two weak absorption peaks are discernible at 1602 and 754 cm-1, being attributed to stretching vibrations of C=C and Si–C, respectively. In particular, the peak for stretching vibration of Si–C also shifted from 760 cm-1 for PVS polymer to 754 cm-1 for the PVSC-5 sample, as shown in Fig. 1. It is proposed that the slight change in this absorption peak likely provides direct evidence for a synergistic effect between the cotton fibers and the PVS polymer due to chemical cross-linking. A similar strong interaction between PDMS chains and aggregated particles has been reported in recent literature (Roy and Bhowmick 2010; Yilgor et al. 2011). Morphology and chemical composition of cotton fabrics treated with PVS polymer The morphology of the cotton fabrics treated with PVS polymer was studied by SEM. SEM images of the surfaces of treated cotton fabrics are provided in Fig. 2. Woven cotton fibers with diameter ranging from 6 to 25 lm and a few particles adhering to the
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surface of the treated cotton fibers were easily observed. These particles could be due to dust adsorbed via static charges. Compared with untreated cotton fibers (Fig. 2g), many small downs on the surface disappeared after treatment with PVS polymer, and the surfaces of the treated cotton fibers were smooth, indicating that a uniform continuous layer could be formed, as shown in Fig. 2a–f. However, no much difference could be found between the treated cotton fabrics with increasing added content of PVS polymer. The chemical composition of a representative sample PVSC-5 was analyzed by EDX. On the surface of the treated cotton fibers, there were mainly three elements: C (68.51 %), O (28.74 %), and Si (1.74 %), further confirming that PVS polymer was successfully coated on the surface of the cotton fibers, as displayed in Fig. 2h. On the cross-section of cut cotton fabric, it is noteworthy that the same elements C, O, and Si were detected; in particular, the Si weight percent was 11.41 %, much higher than that of the surface of the treated cotton fibers, as shown in Fig. S1. The high content of Si element on the crosssection of the cotton fiber further verifies penetration of the PVS polymer due to the high absorption of cotton fabrics. Figure 3 shows magnified SEM images of the cotton fabrics. It is noteworthy that, after the PVS was cured, conglutination phenomena between cellulose fibers were clearly observed, which can likely be attributed to penetration of the PVS polymer, as displayed in Fig. 3b–f. A lot of characteristic parallel ridges, grooves,
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and some downs on the surfaces of pure cotton fibers (reference sample PVSC-0) were also observed, as shown in Fig. 3g. After coating, a film with particle protuberances was incorporated on the surfaces of the treated cotton fibers (Fig. 3a), as verified by a broken surface caused by high-energy electron bombardment in Fig. 3a0 . These protuberances can possibly be attributed to the deposition of PVS networks on the surfaces of cellulose fibers (Li et al. 2008). Although many downs on the surfaces of the virgin cotton fibers disappeared completely, the characteristic parallel ridges and grooves remained after PVS polymer treatment. Therefore, the texture of the cotton fibers remained after treatment, leading to increased surface roughness of the cotton fibers to some extent. From an overall point of view, after PVS treatment, the textures of the woven cotton fabrics remained and their overall topography was not changed much as compared with untreated cotton fabric. Thermal degradation of cotton fabrics treated with PVS polymer To investigate the effect of the added amount of PVS polymer on the thermal behaviors of the treated cotton fabrics, their thermal degradation in nitrogen atmosphere was evaluated by TGA. The TGA and DTG curves for the PVSC samples are depicted in Fig. 4A and B, respectively. It was clearly found that all the PVSC samples had two steps during the whole thermal degradation. The first degradative step, beginning
Fig. 2 SEM micrographs of novel cotton fabrics treated with PVS polymer: a PVSC-1, b PVSC-2, c PVSC-3, d PVSC-4, e PVSC-5, f PVSC-6, and g PVSC-0, and h EDX elemental analysis of the selected area of representative sample PVSC-5
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Fig. 3 SEM images (magnified 5 times) of cotton fabrics: a PVSC-1 and a0 2 times magnification of selected area of PVSC-1, b PVSC2, c PVSC-3, d PVSC-4, e PVSC-5, f PVSC-6, and g PVSC-0
from 30 to 150 °C, should be due to evaporation of moisture absorbed by the treated cotton fabrics (Li et al. 2008). The second step, ranging from 200 to 400 °C, should be ascribed to the main depolymerization and carbonization of glycosyl units from the cellulose fabrics with formation of levoglucosan and its evaporation (Alongi et al. 2011b; Nehra et al. 2014). During the whole degradation, a broad peak located at around 66 °C was obviously observed, and the temperature of the maximum degradation peaks was slightly delayed from 359 °C for PVSC-1 to 360 °C for PVSC-6 with increase in the added amount of PVS polymer, as observed in Fig. 4B, being higher than the value of 349 °C for the reference sample (PVSC-0). This delay in the temperature of the maximum degradation peaks sufficiently confirms that the PVS polymer was favorable to improve the thermal stability of the cotton fabric. Meanwhile, all the cotton fabrics treated with PVS polymer exhibited higher decomposition temperatures than the reference material (PVSC-0) for the same weight loss percentage from their initial mass, indicating that the PVS polymer coating on the surface of the cotton fabrics was indeed favorable to improve the resistance to thermal degradation of the treated cotton fabrics, as displayed in Fig. 4A. The important degradation data for all the samples are plotted in Fig. 4C; For example, the initial characteristic temperatures at 5 % weight loss slightly increased from 265 °C for PVSC-1 to 283 °C for PVSC-5, then decreased to 278 °C for PVSC-6 with increasing added amount of PVS, most being higher than the
value of 265 °C for PVSC-0 (reference material). The temperatures at 30 % weight loss increased slightly from 343 to 350 °C with increasing added amount of PVS polymer, being higher than that of the reference cotton fabric (332 °C). Moreover, the temperatures for 65 % weight loss were delayed from 367 to 530 °C with increasing added amount of PVS, also being much higher compared with the value of 359 °C for PVSC-0. These increasing degradation temperatures further indicate that, with increasing added amount of PVS polymer, the resistance to thermal degradation of the treated cotton fabrics was indeed improved. Additionally, the residual yields of these treated cotton fabrics at 700 °C were improved from 15.0 % for PVSC-1 to 32.4 % for PVSC-6 with increasing amount of PVS polymer, while the residue for the reference sample PVSC-0 was 18.8 %, being higher than for some of the treated cotton fabrics (15 % for PVSC-1, 16.7 % for PVSC-2, and 18.5 % for PVSC3). In this case, it is reasonable that a trace amount of weak acid silanols remaining from the PVS polymer could catalyze cellulose decomposition and result in decreased residue after degradation. As the added amount of PVS was low, the PVS polymer film on the surface of the treated cotton fabrics was formed by chemical cross-linking between PVS and cellulose fibers during the curing process, and the film incorporated on the surface of the cotton fibers was too thin to facilitate carbonization of cellulose fibers, while the remaining silanols could decrease the residue of the treated cotton fabrics. As the amount of PVS used was further increased, the film on the treated cotton fabrics
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A
B
C
Fig. 4 Thermal degradation behaviors of all the cotton fabrics in nitrogen atmosphere A TGA curves, B DTG curves, and C degradation data of the cotton fabrics with different added
amounts of PVS polymer in nitrogen atmosphere: a temperature for 5 % weight loss, b temperature for 30 % weight loss, c temperature for 65 % weight loss, and d residue at 700 °C
became thicker, being able to form an effective protective layer and promote carbonization of the treated cotton fabrics, leading to improvement in their thermal stability. The final effect could be due to the competitive results of the remaining silanols and the carbonization resulting from the PVS film protective layer on the surface of the treated cotton fabrics. The results obtained here further demonstrate that coating with PVS polymer indeed improved the resistance to thermal degradation of the novel cotton cellulose fabrics.
The TGA curves are presented in Fig. 5A. It was obviously found that all the PVSC samples showed three decomposition steps during thermooxidative degradation. The first step between 40 and 150 °C had about 5 % weight loss from the initial mass, being mainly due to moisture in the cotton fabrics. The second degradation step between 280 and 400 °C had the greatest weight loss relative to the initial weight of the treated cotton fabrics, being mainly attributed to dehydration (producing aliphatic char) and depolymerization (creating levoglucosan) of the cotton fabrics and volatilization of degradative small molecules (aldehydes, ketone, ether, etc.) (El-Shafei et al. 2015; Yuan et al. 2012). The third step between 450 and 550 °C should be simultaneous carbonization and oxidation of carbonized cotton fabrics (Alongi et al. 2011a). The DTG curves for thermal oxidation of the
Thermooxidative degradation of the novel cotton fabrics with PVS polymer The thermooxidative behavior of the treated cotton fabrics in air atmosphere was also examined further.
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A
B
C
Fig. 5 Thermooxidative degradation of all the cotton fabrics in air atmosphere A TGA curves, B DTG curves, and C degradation temperatures of the treated cotton fabrics with different added
amounts of PVS polymer in air atmosphere: a temperature for 5 % weight loss, b temperature for 10 % weight loss, c temperature for 65 % weight loss, and d residue at 700 °C
representative PVSC samples are provided in Fig. 5b. All the PVSC samples displayed three degradation peaks. It is noteworthy that the temperature at the greatest degradation peaks decreased from 369 °C for PVSC-0 to 362 °C for PVSC-6, but the temperature at the third degradation peaks increased from 509 °C for PVSC-0 to 514 °C for PVSC-6 with increasing added amount of PVS polymer. In theory, the amounts of silanol groups increased with increasing added amount of PVS polymer, which likely resulted in the decreasing temperature at the greatest degradation peaks. As the temperature increased, the carbonized layer from the PVS film contributed to the temperature at the third degradation peak, shifting it to higher values with increasing added amount of PVS polymer. The degradative temperatures and residues for all the samples are plotted in Fig. 5C; For example, for
5 % weight loss from the initial mass, the initial decomposition temperature decreased to 276 °C with increasing added amount of PVS in all the samples, which could be due to a combination of evaporation of surface moisture and oxygen catalyzing decomposition of vinyl groups from the cotton fabrics treated with PVS. The characteristic temperatures at 10 % weight loss were slightly delayed from 337 °C for PVSC-1 to 342 °C for PVSC-6, showing little difference from the reference material (338 °C for PVSC0). It is interesting to note that the temperature for 65 % weight loss of the treated cotton fabrics was improved from 370 °C (PVSC-1) to 458 °C (PVSC-6) with increasing added amount of PVS, being higher than for the reference cotton fabric (377 °C for PVSC0), except for sample PVSC-1. This increasing degradative temperature implies that the PVS film
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was favorable to improve the resistance to hightemperature degradation of the treated cotton fabrics. Additionally, it was found that the reference cotton fabric was degraded completely in air atmosphere, and no residue was left. However, with increasing added amount of PVS, the residual yields of the treated cotton fabrics obtained at 700 °C in air were obviously improved from 1.58 to 20.14 %, as shown in Fig. 5d. These increasing degradative temperatures and residues also provide direct evidence that the increasing added amount of PVS polymer played an apparent role in improving the resistance to thermooxidative degradation of the treated cotton fabrics as compared with the reference sample (PVSC-0). The above improvement in the thermooxidative stability of the treated cotton fabrics could be due to the synergistic effect between the PVS layer and cellulose fibers, being a competitive result of several factors, such as oxygen catalysis and carbonization between the PVS film and cellulose fibers. In air, oxygen can catalyze thermal oxidation of PVS film and cellulose fibers, which could decrease the decomposition temperatures of the treated cotton fabrics. In theory, as the added amount of PVS was increased, a lot of weak acid silanols probably remained, which could catalyze depolymerization of cellulose fibers and contribute to early weight loss of the treated cotton fabrics. When the treated cotton fabrics were at low temperature (about 400 °C), oxygen-catalyzed oxidation governed the whole degradation process; as the temperature was increased further, carbonization between the PVS polymer and cellulose fibers dominated the whole thermooxidative degradation. With increasing added amount of PVS, the thickness of the PVS film increased, the carbonization of the treated cotton fabrics became apparent, and hence the resistance to high-temperature degradation of the treated cotton fabrics was gradually improved. Hydrophobic properties of the novel cotton fabrics with PVS It is well known that surface hydrophobicity is closely correlated with surface energy and the multiscale roughness of materials. In this work, the hydrophobicity of the treated cotton fabrics was examined by water contact angle measurements. The static water contact angles on the surface of the treated cotton fabrics increased from 118° for
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PVSC-1 to 136° for PVSC-3 initially, and then decreased to 132° with increment of the added amount of PVS, being higher than the value of 0° for PVSC-0, thus indicating that the PVS coating had good water repellency, as depicted in Fig. 6a. Although the water static contact angles of the treated cotton fabrics were not very high, hydrophilic cotton fabrics were completely converted into hydrophobic cotton fabrics by coating with the PVS polymer. To further study the effect of the added amount of PVS polymer on the wettability stability of the treated cotton fabrics, water droplets on the surface of the treated cotton fabrics were kept for 5 min, and their static water contact angles measured again. Compared with the first measurement, the water contact angle of all the PVSC samples slightly decreased after air exposure for 5 min under ambient, which could be due to decreasing size of the water droplets due to evaporation. However, it was interesting to note that, with increasing added amount of PVS polymer, the water contact angles of the treated cotton fabrics increased from 114° for PVSC-1 to 123° for PVSC-3 initially, and then decreased to 121° to remain unvaried, suggesting that no capillary action took place through the fabric, so that the water droplet could not penetrate from one face to the other. Therefore, PVS polymer was favorable to impart good hydrophobic stability on the treated cotton fabrics, as demonstrated in Fig. 6b. Consideration of the increasing hydrophobic property of the treated cotton fabrics should take into account two important factors: the surface properties of the treated cotton fabrics, and the surface roughness of the woven cotton fabrics (Liu et al. 2012). Woven cotton fabric is composed of micrometer-sized cellulose fibers placed perpendicular to each other, producing a rough and porous surface of interlacing threads. After PVS curing, the hydroxyl groups on the cellulose fiber surface were likely depleted by condensation reaction between Si–OH and C–OH, and the resulting PVS film further prevented water from contacting hydroxyl groups, simultaneously lowering the surface energy remarkably and thereby enhancing the hydrophobicity of the treated cotton fabrics. To prove this hypothesis, pure PVS film was prepared by coating on the surface of a slide, and it was found that the water contact angle of the PVS film reached 113°, which could be comparable to fluorinated polymers,
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Fig. 6 Effect of added amount of PVS polymer on hydrophobicity of treated cotton fabrics: a water contact angle after water droplets were kept on the surface of treated cotton fabrics for
60 s, and b water contact angle after water droplets were kept on the surface of treated cotton fabrics for 5 min
as shown in Fig. S2. Therefore, PVS polymer can be used to develop hydrophobic surfaces as an alternative to fluorinated polymers.
polymer has great potential for forming hydrophobic surfaces as an alternative to fluorinated polymers.
Conclusions A series of novel hydrophobic cotton fabrics were successfully prepared using PVS polymer functional coating. The influence of the added amount of PVS polymer on the morphology, resistance to thermal and thermooxidative degradation, and hydrophobic properties of the treated cotton fabrics is discussed. It was found that, after curing, the PVS polymer formed a PVS film layer on the surface of the cotton fibers, and the resistance to thermal and thermooxidative degradation and the water-repellent properties of the novel cotton fabrics were improved with increasing added amount of PVS polymer, compared with reference material. The enhancements in the thermal properties of the treated cotton fabrics can likely be attributed to synergistic carbonization between the PVS protective layer and the cellulose fibers during thermal degradation. Meanwhile, we also found that, with increasing added amount of PVS polymer, the hydrophobicity of the treated cotton fabrics was obviously enhanced. This obvious improvement in the hydrophobicity of the treated cotton fabrics is ascribed to the combination of the low-surface-energy PVS film and the rough surface of the woven cotton fabrics. Hence, PVS
Acknowledgments This research received financial support from China Scholarship funding (No. 201308420571), the Natural Science Foundation of Hubei Province (No. 2014CFB756), the Discipline Innovation Group Foundation of Wuhan Textile University (No. 201401010), the Foundation of Wuhan Textile University (No. 153023), and the National Natural Science Foundation of China (Nos. 51503161, 51203123, and 51403165). Prof. Huang from Wuhan University is thanked for help with TGA.
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