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Article Cite This: ACS Omega 2019, 4, 2540−2546
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Preparation of Citric Acid Fermentation Waste-Based Mulch Films with Hydrophobic Surface by Poly(styrene-co-acrylate) Coatings Jixiang Liu, Zhaonan Sun, Kai Wang, Xiaolei Chan, and Haijia Su* State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering (BAIC-SM), Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China
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S Supporting Information *
ABSTRACT: To improve the surface hydrophobicity of citric acid fermentation waste (CAFW)/poly(vinyl alcohol) (PVA) mulch films for their application in various environments, poly(styrene-coacrylate) (PSA) is used as the single or double sides coatings of CAFW/PVA mulch films. The structure and properties of the obtained CAFW-based mulch films with PSA coatings are investigated. Attenuated total reflection Fourier transform infrared spectroscopy reveals that hydrogen bondings are formed between O−H bonds of the CAFW-based mulch film and CO bonds of PSA coatings, which accounts for good interface adhesion between PSA coatings and CAFW-based mulch films. In addition, after PSA coatings, the elongation at break of the obtained films is obviously increased, whereas the tensile strength remains nearly unchanged, meanwhile the thermal property of CAFW-based mulch film is slightly improved. Importantly, the water contact angle of CAFW-based mulch films increases dramatically from 37 to 97° and the surface hydrophobicity of CAFW-based mulch films is expectedly enhanced.
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INTRODUCTION Citric acid is an important organic acid that is widely used in food, beverage, chemical, metallurgical, agriculture, and pharmaceutical field.1−4 Currently, fermentation is the primary way to produce citric acid and the production is over 1.7 million tons ever year.5 With the increase in demand of citric acid, a large number of citric acid fermentation waste (CAFW) as the byproduct of citric acid fermentation industry are discharged. It has been reported that CAFW production was nearly over 80 million tons per year.6 Nevertheless, CAFW is generally disposed by landfilling, ocean dumping, and incineration, which lead to serious resource-wasting and second environmental pollution. Therefore, it is preferred to develop a high-value added and pollution-free way to utilize CAFW. As we know, CAFW is composed of organic components including polysaccharides, proteins, and fats with active groups such as hydroxyl, carboxyl, and carbonyl, which have good compatibility with polymers to form functional films.7 Blending CAFW with polymers to produce functional composites may be a promising pathway. Mulch films is widely used in agriculture for increasing soil temperature, suppressing weed, enhancing the efficiency of fertilizers and water, reducing pesticide consumption, and improving the yield and quality of crops.8,9 However, traditional mulch films are produced by petroleum-based materials, such as polyethylene or low-density polyethylene that is nearly unable to degrade in natural environment and lead to serious environmental pollution.10 Therefore, many © 2019 American Chemical Society
types of biodegradable and renewable materials were used to prepare biodegradable mulch films, such as starch, polyesters, and industrial biomass. Such as, poly(vinyl alcohol) (PVA)/ lignocellulosic sugar cane bagasse films were cast from water suspensions for agricultural mulch films.11 Similarly, poly(vinyl alcohol) was also mixed with chitin,12 Kenaf fiber,13 orange fiber residue,14 and cellulose.15 In addition to PVA, poly(lactic acid) matrix was also used to blend with biomass to prepare biodegradable agricultural films.16,17 In this way, the presence of a large amount of the heterogeneous, water-insoluble biomass raw material led to the formation of voids and dishomogeneities that strongly affected the integrity of the film. Furthermore, the surfaces of biodegradable mulch films mentioned above are usually hydrophilic, and thus the structure integrity can hardly be maintained in moist environment. For example, over 50 wt % weight loss was observed for the starch-based film even within 20 days of application.18 As a result, these biodegradable mulch films would lose its function and be difficult to meet the demand of crop growth. Therefore, it is desirable to enhance the surface hydrophobicity of biodegradable mulch films for the long-term application in highly humid conditions. Poly(styrene-co-acrylate) (PSA) emulsion is widely used in coating, paints adhesive, and paper industries.19−21 It has been Received: December 5, 2018 Accepted: December 26, 2018 Published: February 1, 2019 2540
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1b), indicating the formation of a lot of carboxyl groups after oxidation. Furthermore, the intensity of C−O stretching vibration (υC−O) located at 1018 cm−1 is evidently increased, suggesting the formation of carboxyl groups after oxidation again. As assumed, the modified CAFW can form a homogeneous water solution with PVA, indicating their good compatibility. In addition, to improve the toughness of CAFW/PVA films, the plasticizer glycerol was added into the films. On one hand, glycerol can increase the mobility of PVA chains as a lubricant; on the other hand, the glycerol with multiple hydroxyl groups can form hydrogen bonding with CAFW or PVA, which makes it stable in the CAFW/PVA films. In other words, the formed hydrogen bonding can effectively prevent glycerol from moving out the surface of films, which is desirable for long-term utility of the films. Moreover, 14 wt % glycerol in the CAFW/PVA films was found to display the best lubricating effect. And thus, 14 wt % glycerol was chosen in the following experiments. Regarding PSA coating, the thickness of 30 ± 3 μm can achieve the best surface hydrophobicity for the generally prepared CAFW/PVA films with a thickness of 130 ± 10 μm, and the increased thickness of PSA coatings cannot further decrease the surface hydrophilicity. Therefore, PSA coatings with a thickness of 30 μm was used. Furthermore, CAFW-based mulch films with single PSA coating and double coatings were prepared and compared. Figure 1c−f presents the spectra of CAFW-based mulch films, CAFW-based mulch films with single PSA coating, CAFW-based mulch films with double PSA coatings, and pure PSA films. From Figure 1c, the strong characteristic absorbance bands in the range of 3600−3100 and 2900− 2800 cm−1 can be observed. The absorption band in the range of 3100−3600 cm−1 is attributed to the υO−H or υN−H stretching vibration of PVA, CAFW, and glycerol. The broad peak width indicates the formation of hydrogen bonding among the O−H bonds of PVA, CAFW, and glycerol, which is the primary mechanism to form homogeneous film.24 Additionally, the absorption bands at 2850 and 2960 cm−1 are assigned to symmetric and asymmetric C−H stretching vibrations of PVA, CAFW, and glycerol.25 After coating PSA layer on the CAFW-based mulch films, in addition to characteristic absorption band of υO−H, υN−H, and υC−H, the absorption band at 1728 cm−1 can also be found as shown in Figure 1d,e. According to the literature,19 this typical absorption band is assigned to υCO of acrylate in PSA, which is different from υCO of CAFW at 1710 cm−1 (Figure 1b). Therefore, it is easy to confirm that the PSA is successfully coated on the surface of the CAFW-based mulch film. Moreover, the band of υCO with PSA-coated CAFW-based mulch films show an obvious shift to a low wavenumber at 1728 cm−1, which is about 7 cm−1 shift compared to that of CAFW-based mulch film as shown in the inset of Figure 1. Generally, the formation of hydrogen bond O−H···OC results in weakening of the CO bond, which is accompanied by bond elongation and a decrease of the CO stretch vibration frequency compared to the noninteracting species. And thus a shift to lower frequencies can be easily detected by IR spectroscopy.26 Therefore, the shift of υCO from 1735 to 1728 cm−1 is caused by the hydrogen bondings among the O− H bonds of PVA, glycerol, or CAFW and CO bonds of PSA. At the same time, the peak width at half-height of υO−H in the double PSA-coated films are larger than that in uncoated one, indicating the existence of several kinds of hydrogen bondings
reported that surface water resistance of composites can be dramatically improved after coating by PSA emulsion. For examples, core−shell fluorine-containing PSA film was synthesized and water contact angle (WCA) test showed excellent water repellency.22 Wan et al.23 prepared silanemodified PSA coatings with the good acid, base, and water resistances. Considering that the ester bonds in acrylate blocks of PSA can form hydrogen bonds with hydroxyl and carboxyl groups of biodegradable mulch films, PSA may be an ideal coating material to improve surface water resistance for mulch films. In the present study, CAFW-based mulch films were prepared by blending CAFW and PVA, and then PSA emulsion are coated on the surfaces of the CAFW-based mulch films to increase hydrophobic property. The structure and properties of the obtained CAFW-based mulch films with PSA coating were investigated by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC), scanning electron microscope (SEM), and water contact angle measurements. The mechanical properties of CAFW-based mulch films with PSA coating were also evaluated.
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RESULTS AND DISCUSSION Preparation of CAFW-Based Mulch Films with PSA Coatings. To increase the compatibility of CAFW with PVA to form uniform films, CAFW was first oxidized by 30 wt % hydrogen peroxide under acidic condition at 90 °C for 2 h. In this situation, modified CAFW contained more carboxyl groups than pristine CAFW, which will promote uniform blending with PVA. As shown in Figure 1a, the pristine CAFW
Figure 1. ATR-FTIR spectra of (a) pristine CAFW; (b) CAFW processed by hydrogen peroxide at 90 °C; (c) CAFW-based mulch films; (d) CAFW-based mulch films with single PSA coating; (e) CAFW-based mulch films with double PSA coatings; and (f) pure PSA films.
displays the strong characteristic absorbance bands in the range of 3600−3100, which is assigned to the O−H (υO−H) or N−H (υN−H) stretching vibration. Moreover, the obvious bands at 2900−2800 cm−1 can be observed, which is attributed to the symmetric and asymmetric C−H stretching vibration (υC−H). In addition, a weak CO stretching vibration (υCO) at 1738 cm−1, indicating few amide bonds from proteins exist in CAFW. Compared to the pristine CAFW, a distinct υCO at 1710 cm−1 in the modified CAFW can be observed (Figure 2541
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Figure 2. SEM images of (a) CAFW-based mulch films; (b) CAFW-based mulch films with single PSA coating; and (c) CAFW-based mulch films with double PSA coatings.
Figure 3. Water contact angle of (a) CAFW-based mulch films and (b) CAFW-based mulch films with PSA coatings.
with various strengths, which would result in broad distribution of υO−H. Therefore, broad bands of υO−H confirm hydrogen bondings among O−H bonds of PVA, glycerol, or CAFW and CO bonds of PSA again. The hydrogen bondings among the O−H bonds of PVA, glycerol, or CAFW and CO bonds of PSA is beneficial to the compatibility of the PSA coating and CAFW-based mulch films. And thus PSA coatings could combine with CAFWbased mulch films tightly and no obvious separation of the double layers. Morphology of CAFW-Based Mulch Films with PSA Coatings. To further investigate the morphology of CAFWbased mulch films with PSA coatings, the cross section of fractured films was observed by SEM. The films were quenched and broken off by liquid nitrogen to avoid deformation of CAFW-based mulch films. From the cross section of CAFW-based mulch films (Figure 2a), homogeneous films are formed and no phase separation happens. The uniform phase structure is attributed to the formation of hydrogen bondings among PVA, glycerol, and CAFW, which has been confirmed by ATR-FTIR (Figure 1). In addition, the thickness of CAFW-based mulch films can be determined to be 130 ± 10 μm from Figure 2a. As shown in Figure 2b, an evident dark coating with 30 ± 3 μm on the surface of CAFWbased mulch films can be found. PSA coating shows a dark contrast to CAFW-based mulch films, which is due to the higher electron density of benzene rings in PSA coating. Furthermore, the PSA coating is tightly adhered to the CAFWbased mulch films. And no obvious interfacial separation between PSA coatings and CAFW-based mulch films is observed, indicating their good compatibility. As confirmed
by ATR-FTIR, the hydrogen bondings among the O−H or N−H bonds of CAFW-based mulch film and CO bonds of PSA account for the good adhesion between CAFW-based mulch films and PSA coatings. Moreover, for CAFW-based mulch films with double PSA coatings, similar phenomenon can also be found as shown in Figure 2c. Surface Hydrophobic Property. To check the surface wetting behaviors of CAFW-based mulch films with and without PSA coatings, water contact angle was measured. It is seen from Figure 3a that the water contact angle of CAFWbased mulch films is 37 ± 2°, indicating a high surface hydrophilicity. To manifest the contact angle hysteresis, dynamic advancing (θA) and receding angles (θR) were recorded. θA = 56 ± 2° and θR = 12 ± 2° for CAFW-based mulch films were obtained; the contact angle hysteresis Δθ = 44° may be caused by the big roughness and heterogeneity of CAFW-based mulch films. By contrast, water contact angle of CAFW-based mulch films with PSA coating increases to 97 ± 2° with θA = 103 ± 2° and θR = 78 ± 2° as shown in Figure 3b. This contact angle is similar to that of pure PSA as shown in Figure S3. This dramatic increase demonstrates the change of surface wetting behaviors from hydrophilicity to hydrophobicity. In addition, the smaller contact angle hysteresis Δθ = 25° than that of CAFW-based mulch films suggests the relatively uniform surface of CAFW-based mulch films with PSA coating, which can also be elucidated by AFM measurement. As shown in Figure S4, CAFW-based film shows a slight phase separation and a hard phase assigned to the CAFW is partly aggregated on the surface of CAFW-based films. However, a relatively smooth surface of CAFW-based film can be observed after PSA coating. Moreover, a much 2542
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Figure 4. Schematic structures and interactions of (a) CAFW-based mulch films and (b) CAFW-based mulch films with PSA coatings.
more uniform phase structure than that without PSA coating is obtained. As expected, PSA coatings can greatly increase the surface hydrophobic behavior of CAFW-based mulch films. It is reported that hydrophobicity of materials is affected significantly by its surface chemical composition.27 In these cases, schematic structures and interactions of CAFW-based mulch films and CAFW-based mulch films with PSA coatings are shown in Figure 4a,b. CAFW-based mulch films are comprised of PVA, glycerol, and CAFW. All compounds contain abundance of hydrophilic groups, such as hydroxyl, amide, and carboxyl groups, which are also inclined to aggregate on the surface of CAFW-based mulch films due to high surface energy (Figure 4a). This accounts for the poor hydrophobic property of CAFW-based mulch films. After coating by PSA, acrylate blocks of PSA prefer to move toward the interface between CAFW-based mulch films and PSA coatings because of the potential formation of hydrogen bonding, meanwhile styrene blocks of PSA migrate to the surface of the coating after the drying process (Figure 4b). Therefore, the coating has a low surface energy, helping it to possess a hydrophobic property. The surface hydrophobicity of PSA-coated films is obviously improved, suggesting PSA is a kind of effective hydrophobic coating for CAFW-based mulch films. DSC Analysis. The thermal properties of the CAFW-based mulch films with and without PSA coatings were evaluated by DSC. The second heating curves of pure PVA film, CAFWbased mulch films, CAFW-based mulch films with single PSA coating, and CAFW-based mulch films with double PSA coatings are shown in Figure 5. The shapes of all DSC curves are similar, displaying the glass transition temperature (Tg) only and no melting peaks, indicating the amorphous nature of all the films. According to the composition of different films, the exclusive Tg is assigned to PVA. As shown in Figure 5a, pure PVA displays a distinct Tg at 76 °C, which is consistent
Figure 5. Differential scanning calorimetry curves of (a) pure PVA film; (b) CAFW-based mulch films; (c) CAFW-based mulch films with single PSA coating; and (d) CAFW-based mulch films with double PSA coatings.
with the literature.28 For CAFW-based mulch films, a lower Tg at 65 °C than pure PVA can be found (Figure 5b). On one hand, the addition of CAFW and glycerol into PVA can effectively reduce the interactions (hydrogen bonding) between PVA chains because of the formation of hydrogen bondings among CAFW, glycerol, and PVA; on the other hand, glycerol as a small molecular plasticizer can also increase the flexibility of PVA chains. Eventually, the mobility of PVA chains increases, which results in a significantly low Tg. Compared with CAFW-based mulch films, an increased Tg at 71 °C can be obtained for CAFW-based mulch films with single PSA coating. As discussed above, acrylate blocks of PSA coating are able to form new hydrogen bonding with PVA chains. In this situation, the mobility of PVA chains would be restrained by long PSA chains. As a result, Tg of CAFW-based mulch films with single PSA coating exhibits a slight increase. This explanation can also be confirmed by Tg of CAFW-based 2543
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bonds of the CAFW-based mulch film and the CO bond of PSA coatings. After PSA coatings, the thermal and mechanical properties of CAFW-based mulch films were improved. Furthermore, the increase of water contact angle (WCA) from 37 to 97° revealed that the surficial hydrophobicity of CAFW-based mulch films was effectively enhanced. A facile way to prepare the hydrophobic surface of CAFW-based mulch films was provided in our work and the remarkable improvement in surficial hydrophobicity of CAFW-based mulch films will promise its further application in humid environment for long-term growth crop.
mulch films with double PSA coatings (74 °C). More PVA chains in double PSA coatings are restricted than that of single PSA coating because of large contact probability between PVA and PSA, and a high Tg is also produced as shown in Figure 5d. The high Tg after PSA coating indicates that PSA coating can improve the thermal stability of CAFW-based mulch films or the practical application temperature. Mechanical Properties. Figure 6 shows the tensile strength and elongation at the break of CAFW-based mulch
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EXPERIMENTAL SECTION Materials. Poly(vinyl alcohol) (PVA) is purchased from Beijing Yili Fine Chemical Co., Ltd., China. The average degree of polymerization is 1750 ± 50. Citric acid fermentation waste (CAFW) is supplied by COFCO Biochemical (Anhui) Co., Ltd., China. Poly(styrene-coacrylate) (PSA) emulsion is obtained from Beijing Mengtaiweiye Building Materials Co., Ltd., China. PSA is a statistical copolymer and the molar ratio of styrene and acrylate blocks is 0.32:0.68, as determined by 1H NMR as shown in Figure S1. The particle size of PSA emulsion is ca. 290 ± 10 nm measured by laser particle sizer as shown in Figure S2. Glycerol (AR), hydrogen peroxide (30 wt %), and nitric acid (68 wt %) are purchased from Beijing Chemical Works, China. Deionized water is used in all experiments. Preparation of CAFW-Based Mulch Films with PSA Coating. PVA, 0.6 g, was dissolved in 15 mL deionized water. Three grams of dry CAFW was modified by 0.3 g of 30 wt % hydrogen peroxide under acidic condition at 90 °C for 2 h. Modified CAFW, PVA solution, and 0.6 g of glycerol were blended by magnetic stirring (500 rpm) and the blend was heated at 90 °C for 2 h. Then, the homogeneous blend was neutralized and poured onto a polypropylene plate (28 cm × 28 cm) and CAFW-based mulch film was obtained after drying at room temperature overnight. CAFW-based mulch film was coated by 3 mL of 15 wt % PSA emulsion on one side and 6 mL of 15 wt % PSA emulsion on double sides. After drying at room temperature overnight, CAFW-based mulch films with single PSA coating and double coatings were obtained. Characterization. ATR-FTIR spectra were recorded on a Perkin-Elmer SP100 FT-IR spectrometer with a PIKE Technologies Horizontal ATR attachment between 4000 and 400 cm−1. Sixteen scans were taken with a resolution of 4 cm−1 for each sample. The DSC was performed on a TA Q2000 Differential Scanning Calorimeter at a heating rate of 10 °C min−1 from −50 to 200 °C under nitrogen atmosphere. The glass transition temperature (Tg) was determined from the second heating scan. The SEM was conducted with a Hitachi S-4300 (Japan). Each sample was sputter-coated with the gold for analysis. Before SEM analysis, the sample was quenched by liquid nitrogen and then broken off for cross-sectional scanning. Contact angle measurements were performed using a tensiometer (Krüss DAS 30, Future Digital Scientific Co., Garden City, NY) and a syringe with a 24-gauge flat-tipped needle. Dynamic advancing (θA) and receding angles (θR) were recorded while Milli-Q water was added to and withdrawn from the drop, respectively. Mechanical properties were evaluated in accordance with ASTM D638 by electronic universal testing machine (UTM-2502, Shenzhen Suns Technology Co., Ltd., China) at room temperature. Each sample was measured five times to get an average value. 1H
Figure 6. Tensile strength and elongation at break for (a) CAFWbased mulch films; (b) CAFW-based mulch films with single PSA coating; and (c) CAFW-based mulch films with double PSA coatings.
films, CAFW-based mulch films with single PSA coating, and CAFW-based mulch films with double PSA coatings. The tensile strength of CAFW-based mulch films is 2.1 MPa and a low tensile strength may reflect the amorphous nature of the films. In addition, we found that there is a slight decrease in tensile strength after PSA coatings. CAFW-based mulch films with double PSA coatings possess a tensile strength of 1.9 MPa. This observation may be attributed to the lower strength of PSA coatings than that of the CAFW-based mulch films. Contrary to the tensile strength, elongation at break increases obviously from 92.3% for CAFW-based mulch films to 135.8% for CAFW-based mulch films with single PSA coating and 173.6% for CAFW-based mulch films with double PSA coatings. On one hand, the high flexibility of PSA may provide a high elongation at break. On the other hand, the high physical cross-linking density through the hydrogen bonding between CAFW-based mulch films and PSA coatings also gives an increased elongation at break.29 Therefore, we can effectively improve the elongation at break of the CAFWbased mulch films and keep its tensile strength simultaneously by PSA coatings. This is meaningful for practical application of CAFW-based mulch films as well.
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CONCLUSIONS To overcome the drawbacks of poor hydrophobicity of CAFWbased mulch films for long-term application in moist environment, CAFW-based mulch films with hydrophobic surface were prepared by blending CAFW and PVA and PSA coatings. The SEM images showed that PSA coatings were tightly adhered to the CAFW-based mulch films and no obvious interfacial separation between PSA coatings and CAFW-based mulch films was observed, which indicated their good interface adhesion. Good adhesion between CAFWbased mulch films and PSA coatings was observed on account of the formation of hydrogen bondings between the O−H 2544
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NMR spectra were recorded on a Bruker AV400 (400 MHz) spectrometer. Chemical shifts (δ) are given in parts per million relative to tetramethylsilane (δ = 0) as the internal reference. The morphologies of the films were measured using atomic force microscopy (Agilent-5500 AFM) under the tapping mode.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03420. 1 H NMR spectrum, particle size and its distribution, and water contact angle of PSA, and AFM topography of CAFW film and CAFW film with PSA coating (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Haijia Su: 0000-0002-4266-7212 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors express their thanks for the support from the National Natural Science Foundation of China (21525625 and 21838001).
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REFERENCES
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