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A novel surface modification method for anhydrite whisker
Materials and Design 107 (2016) 117–122
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
A novel surface modification method for anhydrite whisker Tianzeng Hong a,b,c,1, Zhihui Lv a,b,1, Xin Liu c, Wu Li a,c, Xueying Nai a,⁎, Yaping Dong a,⁎ a b c
Qinghai Institute of Salt Lakes, Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Chinese Academy of Sciences, Xining, Qinghai 810008, China University of Chinese Academy of Sciences, Beijing 100049, China Key Laboratory of Salt Lake Resources Chemistry, Qinghai Province 810008, China
a r t i c l e
i n f o
Article history: Received 2 March 2016 Received in revised form 7 June 2016 Accepted 9 June 2016 Available online 11 June 2016 Keywords: Anhydrite whisker Surface modification Stearic acid Chemical adsorption
a b s t r a c t Calcium sulfate anhydrite whisker (CSAW, anhydrous-insoluble) was modified with inorganic-organic surface modification method. The modified-CSAW (M-CSAW) was investigated by field emission scanning electron microscope (FESEM), X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TG). The results indicate that trisodium phosphate and stearic acid have remarkable effects on CSAW. The contact angle of M-CSAW reaches up to 108.43°. Trisodium phosphate actives the surface of CSAW compared to traditional methods (acid or base activation approaches), and then chemical reaction occurs on the surface of CSAW between COO– and Ca2+. The MCSAW can be potentially applied to whisker/polymer composite materials. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Calcium sulfate whisker (CSW) has good thermal stability, chemical resistance, perfect structure, low cost, high strength and great stiffness [1]. It is widely used as a reinforcing agent in polymer [2–4], rubber [5], papermaking [6,7] and so on. CSW exists in three types: dehydrate, hemihydrates and anhydrite. The synthetic anhydrite whisker can be further divided into two types (anhydrous-soluble and anhydrous-insoluble). Among the three types, CSAW has the best physical and chemical performances, making itself a preferential filler than many other conventional fillers, such as calcium carbonate and natural fiber. However, the small dimension with high surface free energy leads to readily aggregation [8], which badly affects the performances of the target composites. It seems to be the dominated barrier for the interfacial interaction between whisker and polymer [9]. In order to overcome the aforementioned drawbacks, surface modification is utilized as an efficient way to manipulate the surface properties [10–12], enhance whisker dispersion, and reinforce the matrix [13,14]. The appropriate surface modification strategies and agents play an important role in making whisker/matrix more compatible.
⁎ Corresponding authors. E-mail addresses: [email protected] (X. Nai), [email protected] (Y. Dong). 1 These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.matdes.2016.06.034 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
Fatty acids (e.g. oleic acid, palm oil, and stearic acid) are often used as surface modification agents for fillers to provide a hydrophobic surface, aiming to increase the compatibility of filler and polymer. They act as a modifier in some particulate systems because of their intrinsic advantages, including low cost, ease of processing, etc. [15]. Meanwhile, modifying surface with fatty acids can reduce water absorption, prevent interaction from filler particles, and lower the surface energy. These advantages endow them more compatible and easily disperse in organic matrix. In addition, fatty acids provide a variety of benefits to the mechanical properties. K. Molnar et al. [16] have modified calcium sulfate with stearic acid, and investigated the characters of composite. The stearic acid coating gave rise to a drastic change of tensile properties and deformation behavior. J. Y. Liu et al. [17] have prepared polycaprolactone/CSW composite by coprecipitation methods. The flexural strength and impact strength were improved up to 21% and 22% with 15 wt% whiskers. CSAW has best mechanical property among the three types, but studies do not use CSAW as a filler so far, because CSAW is very stable, and the surface has low chemical activity. The inert surface is difficult to be modified with stearic acid directly, although stearic acid has lower surface energy and adhesive energy. If CSAW can be modified with stearic acid, the product could be expected as one of suitable fillers. The mechanical properties of as-prepare composite may be drastically improved accordingly. Finding new method to graft stearic acid onto the surface of CSAW is urgently needed and meaningful.
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Fig. 1. FESEM images of (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic M-CSAW.
In our study, M-CSAW was fabricated via a novel simple method. First, trisodium phosphate was selected to active CSAW surface, and then stearic acid was utilized to modify the surface of CSAW. M-CSAW exhibits remarkable hydrophobic property. The resulting M-CSAW was studied by SEM, FT-IR, XPS and TG to reveal the preparation mechanism. 2. Experiment 2.1. Experimental materials The main raw materials used in this work including CaSO4·2H2O and stearic acid were purchased from sinopharm chemical reagent Co., Ltd.; trisodium phosphate and Ethanol were purchased from Beijing chemical reagent plant and Tianjin chemical reagent plant respectively. All chemicals in the experiment were of analytical grade and used directly without further purification.
2.4. Characterization Water contact angle instrument DSA30, Kruss, Germany. Drops of deionized water (3 μL) were deposited on five different spots of the surface. The values are averages of five measurements. The morphology of CSAW and M-CSAW was conducted with a SU8010/Aztec(X-MaxN) field emission scanning electron microscope (FESEM) from Hitachi Limited using an acceleration voltage of 2.0 kV. X-ray powder diffraction (XRD) pattern was determined by a Philips X'Pert X-ray spectrometer using Cu Kα radiation with a tube voltage of 40 kV and a tube current of 35 mA. FT-IR spectra were obtained on a Nexus infrared spectrometer (Thermo Nicollet. USA), the wave-number range was set from 4000 cm −1 to 400 cm −1. X-ray photoelectron spectroscopy (XPS) measurement was carried out by using Thermo escalab 250Xi multifunction electron spectrometer (Thermo Electron Corporation) equipped with an Al Kα X-ray source. The thermogravimetric analysis (TG) experiments were performed on a Netzsch STA-449 F3 apparatus under identical
2.2. Synthesis of CSAW 3.0 g CaSO4·2H2O was mixed with 60 mL distilled water in a 100 mL stainless steel autoclave. The autoclave was sealed, heated at 140 °C for 2 h, then cooled down to room temperature quickly, filtered, washed and dried in air at 100 °C. The product was dried, and calcinated at 800 °C for 4 h in muffle furnace. 2.3. Surface modification of CSAW 2 g CSAW was dispersed in 100 mL 0.015 mol/L trisodium phosphate solution and under ultrasonic for 2 min. The homogeneous solution was stirred for 10 min, filtered off and washed. The product was dispersed in 100 mL ethanol, and 0.12 g stearic acid was added into the slurry of CSAW heating at 90 °C in the oil bath with magnetic stirring for 5 min, filtered and washed with plenty of ethanol to remove excessive stearic acid. Then the M-CSAW was dried at 100 °C for 1 h.
Fig. 2. XRD patterns of (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic MCSAW.
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Fig. 3. XPS spectrum of (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic M-CSAW.
conditions, the measurements were conducted in alumina crucible using at about 5 mg of sample and a heating rate of 6 °C min−1. Each sample was heated from room temperature to 1000 °C under nitrogen atmosphere (60 cm min−1). 3. Results and discussion 3.1. Characterization of CSAW and M-CSAW The morphology of CSAW is investigated by SEM, as shown in Fig. 1. The original CSAW is smooth and integrated, with diameter 0.3–1 μm (Fig. 1a). The morphology of CSAW does not change obviously after inorganic modification (Fig. 1b). The surface of M-CSAW is coarse after inorganic-organic modification (Fig. 1c), indicating that a thin film is formed on the surface of CSAW, affecting the hydrophilic-hydrophobic quality of CSAW [18]. The contact angle of CSAW, inorganic M-CSAW and inorganic-organic M-CSAW are 0°, 11.23° and 108.43° respectively (Fig. 1 insert). The straight edge of whisker reveals that M-CSAW keeps a high crystallinity level as the same of CSAW (Fig. 2). Present diffraction peaks at 25°, 31°, 41°, 48°, and 56° can be assigned to (020), (210), (212), (230), and (232), respectively. The modification occurs only on the surface of CSAW without altering its crystal structure [19]. XPS survey scan of CSAW, inorganic M-CSAW and inorganic-organic M-CSAW are showed in Fig. 3. All samples exhibit main character peaks at 169 eV, 284 eV, 347 eV and 531 eV. They are assigned to S2p, C1s, Ca2p and O1s signals, respectively. The C content increases obviously
Table 1 The present chemical elements and their corresponding atomic concentrations of CSAW, inorganic M-CSAW and inorganic-organic M-CSAW surface. Sample
Ca2p (%)
S2p (%)
O1s (%)
C1s (%)
P2p (%)
CSAW inorganic M-CSAW inorganic-organic M-CSAW
11.41 12.19 11.39
12.72 7.51 5.95
49.57 48.77 43.06
26.3 27.59 34.86
– 3.94 4.74
after inorganic-organic modified (Table 1), demonstrating that the stearic acid has coated on the surface of CSAW. The M-CSAW have a new obvious peak located at 133 eV assigned to P2p (Fig. 3b and c), suggesting that phosphate has modified the surface of CSAW. The curvefitting analysis of high-resolution C1s spectra of CSAW reveals that the existence of C-C and C-H, because their signal locates at 284.75 ± 0.1 eV. There is a new signal appearing at 288.10 ± 0.1 eV for inorganic-organic M-CSAW (Fig. 4). This signal stands for O-C = O. Those outcomes could strongly evident that stearic acid coats on the surface of CSAW successfully [20]. Ca2p has two peaks located at 347.87 ± 0.1 eV and 351.46 ± 0.1 eV (Fig. 5a). Inorganic M-CSAW does not obviously change the binding energy of Ca2p (Fig. 5b), indicating that inorganic modification has little influence on the chemical state of Ca2+. However, the binding energy of Ca2p appears at 347.51 ± 0.1 eV and 351.11 ± 0.1 eV after inorganic-organic modification (Fig. 5c). The electron density of Ca2+ surrounding increased, since alkyl groups can offer electron, causing Ca2p binding energy decreased [21]. The decrease of Ca2p binding energy demonstrates that the stearic acid reacts with Ca2+ on the surface of CSAW [22]. Furthermore, FT-IR spectrum is conducted to investigate the interfacial reaction on the surface of CSAW. Single strong bands at 1636.82 cm− 1 and 1155.24 cm− 1 are ascribed to the stretching of Ca2 + and SO24 − (Fig. 6). The bands at 1055.64 cm− 1 and 1121.88 cm−1 can be attributed to the P-O stretching vibration peaks. P-OH vibration peak can also be found at 944.58 cm−1 (Fig. 6b and c). exists on the surface of inorganic MThe results indicate that HPO2− 4 CSAW. For organic-inorganic M-CSAW (Fig. 6c), there are new bands at 2919.73 cm−1 and 2851.26 cm−1 corresponding to –CH2– asymmetric and symmetric vibration [23]. The band of carboxylate at 1562.71 cm−1 indicates that stearic acid grafts to the surface of CSAW [24]. Fig. 7 shows the thermogravimetric (TG) curves of CSAW, inorganic M-CSAW and organic-inorganic M-CSAW. The first mass loss peaks below 200 °C can be ascribed to removal of water (Fig. 7). CSAW does not have obvious mass loss due to its good thermal stability (Fig. 7a).
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Fig. 4. High-resolution XPS spectrum of C1s peak: (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic M-CSAW.
Decomposition of phosphate causes slight weight loss of inorganic MCSAW (Fig. 7b). The decomposition of stearic acid temperature is about 260 °C, and salts derivative of the carboxylic acids present higher thermal stability [25]. The second mass loss begins at 420 ° C, indicating that calcium stearate exists on the surface of CSAW and decomposition [26] (Fig. 7c). Calcium carbonate was obtained as sub-product of the reaction and decomposes to calcium oxide and carbon dioxide at 642 °C with 0.56% mass loss [27]. The TG results indicate that organic-inorganic M-CSAW can meet the demand of thermal stability as filler.
3.2. The reaction mechanism of CSAW Fig. 8 shows the possible mechanism of surface modification, and the chemical reactions are as follows. Na3 PO4 þ H2 O→HPO4 2‐ þ OH‐ þ 3Naþ ‐
CaSO4 þ HPO4 2 →CaHPO4 þ SO4 2
‐
Fig. 5. High-resolution XPS spectrum of Ca2p peak: (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic M-CSAW.
ð1Þ
ð2Þ
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that there is carboxylate on the surface of CSAW (Figs. 6 and 7). The alkyl chain of stearic acid coated on the surface of CSAW forms a thin film and makes it become hydrophobic. 4. Conclusions
Fig. 6. FT-IR spectrum of (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic MCSAW.
A novel and simple method was used to modify CSAW. First, trisodium phosphate was used to active the CSAW surface. Second, stearic acid was applied to modify the surface of CSAW. The obtained M-CSAW has a good hydrophobic property. No obvious changes can be found on the morphology and crystalline degree of M-CSAW. The FT-IR and XPS results demonstrate that inorganic modification can active the surface of CSAW, and the organic (stearic acid) reacts with Ca2 + on the surface of CSAW to form –COOCa. According to TG, MCSAW has good thermal stability and can act as a promising filler in composites for future. This study provides a reaction mechanism model for a series of surface modification, which is a meaningful tool for whisker/polymer matrix composites. Acknowledgments This work was financially supported by Science and technology project Foundation of Qinghai Province, China (2015-ZJ-737) and the National Natural Science Foundation of China (No. 51402323). We thank Dr. Yabin Wang for his assistance during the revision process. References
Fig. 7. TG curves of (a) CSAW, (b) inorganic M-CSAW and (c) organic-inorganic M-CSAW.
ð3Þ It can be seen from Eq. (1) that sodium phosphate decomposes into sodium hydrogen phosphate and sodium hydroxide in aqueous solution. The pH of sodium phosphate solution (0.01moL/L) is 12.1, which confirm the decomposition of sodium phosphate. As shown in Eq. (2), the ion substitution reaction takes place on the surface of CSAW because the Ksp of calcium hydrogen phosphate (1.8 × 10−7, 25 °C) is far less than that of calcium sulfate (7.1 × 10−5, 25 °C) [28]. The reaction develops a thin calcium hydrogen phosphate film on the surface of CSAW, forming a large amount of hydroxyl in the surface of CSAW at the same time. The surface of CSAW is activated after inorganic modification. At the end of stage, C17H35COO– reacts with Ca2 + to form – possibly combines H+ to form H2PO− COOCa [24]. HPO2− 4 4 on the surface of CSAW. Hydroxyl group and alkyl chain make Ca2p binding energy diminished by shielding effect (Fig. 5). FT-IR and TG results indicate
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Fig. 8. The reaction mechanism model of M-CSAW.
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[12] Y. Huang, M. He, A. Lu, W. Zhou, S.D. Stoyanov, E.G. Pelan, L. Zhang, Hydrophobic modification of chitin whisker and its potential application in structuring oil, Langmuir 31 (2015) 1641–1648. [13] X. Liu, F. Chen, H. Yang, X. Weilin, Feasibility and properties of polypropylene composites reinforced with down feather whisker, J. Thermoplast. Compos. Mater. 28 (2015) 19–31. [14] L.M. Lu, K.H. Zuo, Y.-P. Zeng, Fabrication and properties of surface-modified β-Si3N4 whiskers reinforced dental resin composites, J. Polym. Sci. 128 (2013) 41–46. [15] C. Del Angel, A.B. Morales, F. Navarro-Pardo, T. Lozano, P.G. Lafleur, S. SanchezValdes, G. Martinez-Colunga, E. Ramirez-Vargas, S. Alonso, R. Zitzumbo, Mechanical and rheological properties of polypropylene/bentonite composites with stearic acid as an interface modifier, J. Appl. Polym. Sci. 132 (2015) 42264–42272. [16] K. Molnar, J. Moczo, M. Murariu, P. Dubois, B. Pukanszky, Factors affecting the properties of PLA/CaSO 4 composites: homogeneity and interactions, Express Polym Lett 3 (2009) 49–61. [17] J.Y. Liu, L. Reni, Q. Wei, J.L. Wu, S. Liu, Y.J. Wang, G.Y. Li, Fabrication and characterization of polycaprolactone/calcium sulfate whisker composites, Express Polym Lett 5 (2011) 742–752. [18] W. Yuan, J. Cui, Y. Cai, X. Shiai, A novel surface modification for calcium sulfate whisker used for reinforcement of poly(vinyl chloride), J. Polym. Res. 22 (2015) 173. [19] Q. Jiang, Y. Cheng, X.-Y. Cao, R. Wei, M. Zhao, Preparation and physical properties of chitosan-coated calcium sulphate whiskers, Chem. Pap. 68 (2014) 1400–1407.
[20] A.J. de Menezes, G. Siqueira, A.A.S. Curvelo, A. Dufresne, Extrusion and characterization of functionalized cellulose whiskers reinforced polyethylene nanocomposites, Polymer 50 (2009) 4552–4563. [21] Y.M. Hua, J. Yuan, L.H. Yue, J.X. Cai, Minicrystal and XPS analysis of SiO2 coated superfine CaCO3 powder Chinese, J. Inorg. Chem. 17 (2001) 134–138. [22] J.-G. Liao, L. Zhang, Y. Zuo, Y.-L. Wang, Y.-B. Li, Surface modification of nano-hydroxyapatite with stearic acid, Chinese J. Inorg. Chem. 25 (2009) 1267–1273. [23] L. Dang, X. Nai, D. Zhu, Y. Jing, X. Liu, Y. Dong, W. Li, Study on the mechanism of surface modification of magnesium oxysulfate whisker, Appl. Surf. Sci. 317 (2014) 325–331. [24] Y.-B. Li, H.-D. Li, X. Cheng, W.-H. Liu, D.-X. Li, Z.-Z. Xu, Surface modification of amorphous calcium phosphates by stearic acid and their stability, Chinese J. Inorg. Chem. 24 (2009) 707–711. [25] A.P.D. Moreira, B.S. Souza, A.M.R.F. Teixeira, Monitoring of calcium stearate formation by thermogravimetry, J. Therm. Anal. Calorim. 97 (2009) 647–652. [26] N.B. Singh, N.P. Singh, Formation of CaO from thermal decomposition of calcium carbonate in the presence of carboxylic acids, J. Therm. Anal. Calorim. 89 (2007) 159–162. [27] M. Gonen, D. Balkose, F. Inal, S. Ulku, The effect of zinc stearate on thermal degradation of paraffin wax, J. Therm. Anal. Calorim. 94 (2008) 737–742. [28] Department of Inorganic Chemistry, Dalian University of Technology, Inorganic Chemistry, Higher Education Press, Beijing, 2006 710.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
A novel surface modification method for anhydrite whisker Tianzeng Hong a,b,c,1, Zhihui Lv a,b,1, Xin Liu c, Wu Li a,c, Xueying Nai a,⁎, Yaping Dong a,⁎ a b c
Qinghai Institute of Salt Lakes, Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Chinese Academy of Sciences, Xining, Qinghai 810008, China University of Chinese Academy of Sciences, Beijing 100049, China Key Laboratory of Salt Lake Resources Chemistry, Qinghai Province 810008, China
a r t i c l e
i n f o
Article history: Received 2 March 2016 Received in revised form 7 June 2016 Accepted 9 June 2016 Available online 11 June 2016 Keywords: Anhydrite whisker Surface modification Stearic acid Chemical adsorption
a b s t r a c t Calcium sulfate anhydrite whisker (CSAW, anhydrous-insoluble) was modified with inorganic-organic surface modification method. The modified-CSAW (M-CSAW) was investigated by field emission scanning electron microscope (FESEM), X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TG). The results indicate that trisodium phosphate and stearic acid have remarkable effects on CSAW. The contact angle of M-CSAW reaches up to 108.43°. Trisodium phosphate actives the surface of CSAW compared to traditional methods (acid or base activation approaches), and then chemical reaction occurs on the surface of CSAW between COO– and Ca2+. The MCSAW can be potentially applied to whisker/polymer composite materials. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Calcium sulfate whisker (CSW) has good thermal stability, chemical resistance, perfect structure, low cost, high strength and great stiffness [1]. It is widely used as a reinforcing agent in polymer [2–4], rubber [5], papermaking [6,7] and so on. CSW exists in three types: dehydrate, hemihydrates and anhydrite. The synthetic anhydrite whisker can be further divided into two types (anhydrous-soluble and anhydrous-insoluble). Among the three types, CSAW has the best physical and chemical performances, making itself a preferential filler than many other conventional fillers, such as calcium carbonate and natural fiber. However, the small dimension with high surface free energy leads to readily aggregation [8], which badly affects the performances of the target composites. It seems to be the dominated barrier for the interfacial interaction between whisker and polymer [9]. In order to overcome the aforementioned drawbacks, surface modification is utilized as an efficient way to manipulate the surface properties [10–12], enhance whisker dispersion, and reinforce the matrix [13,14]. The appropriate surface modification strategies and agents play an important role in making whisker/matrix more compatible.
⁎ Corresponding authors. E-mail addresses: [email protected] (X. Nai), [email protected] (Y. Dong). 1 These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.matdes.2016.06.034 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
Fatty acids (e.g. oleic acid, palm oil, and stearic acid) are often used as surface modification agents for fillers to provide a hydrophobic surface, aiming to increase the compatibility of filler and polymer. They act as a modifier in some particulate systems because of their intrinsic advantages, including low cost, ease of processing, etc. [15]. Meanwhile, modifying surface with fatty acids can reduce water absorption, prevent interaction from filler particles, and lower the surface energy. These advantages endow them more compatible and easily disperse in organic matrix. In addition, fatty acids provide a variety of benefits to the mechanical properties. K. Molnar et al. [16] have modified calcium sulfate with stearic acid, and investigated the characters of composite. The stearic acid coating gave rise to a drastic change of tensile properties and deformation behavior. J. Y. Liu et al. [17] have prepared polycaprolactone/CSW composite by coprecipitation methods. The flexural strength and impact strength were improved up to 21% and 22% with 15 wt% whiskers. CSAW has best mechanical property among the three types, but studies do not use CSAW as a filler so far, because CSAW is very stable, and the surface has low chemical activity. The inert surface is difficult to be modified with stearic acid directly, although stearic acid has lower surface energy and adhesive energy. If CSAW can be modified with stearic acid, the product could be expected as one of suitable fillers. The mechanical properties of as-prepare composite may be drastically improved accordingly. Finding new method to graft stearic acid onto the surface of CSAW is urgently needed and meaningful.
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Fig. 1. FESEM images of (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic M-CSAW.
In our study, M-CSAW was fabricated via a novel simple method. First, trisodium phosphate was selected to active CSAW surface, and then stearic acid was utilized to modify the surface of CSAW. M-CSAW exhibits remarkable hydrophobic property. The resulting M-CSAW was studied by SEM, FT-IR, XPS and TG to reveal the preparation mechanism. 2. Experiment 2.1. Experimental materials The main raw materials used in this work including CaSO4·2H2O and stearic acid were purchased from sinopharm chemical reagent Co., Ltd.; trisodium phosphate and Ethanol were purchased from Beijing chemical reagent plant and Tianjin chemical reagent plant respectively. All chemicals in the experiment were of analytical grade and used directly without further purification.
2.4. Characterization Water contact angle instrument DSA30, Kruss, Germany. Drops of deionized water (3 μL) were deposited on five different spots of the surface. The values are averages of five measurements. The morphology of CSAW and M-CSAW was conducted with a SU8010/Aztec(X-MaxN) field emission scanning electron microscope (FESEM) from Hitachi Limited using an acceleration voltage of 2.0 kV. X-ray powder diffraction (XRD) pattern was determined by a Philips X'Pert X-ray spectrometer using Cu Kα radiation with a tube voltage of 40 kV and a tube current of 35 mA. FT-IR spectra were obtained on a Nexus infrared spectrometer (Thermo Nicollet. USA), the wave-number range was set from 4000 cm −1 to 400 cm −1. X-ray photoelectron spectroscopy (XPS) measurement was carried out by using Thermo escalab 250Xi multifunction electron spectrometer (Thermo Electron Corporation) equipped with an Al Kα X-ray source. The thermogravimetric analysis (TG) experiments were performed on a Netzsch STA-449 F3 apparatus under identical
2.2. Synthesis of CSAW 3.0 g CaSO4·2H2O was mixed with 60 mL distilled water in a 100 mL stainless steel autoclave. The autoclave was sealed, heated at 140 °C for 2 h, then cooled down to room temperature quickly, filtered, washed and dried in air at 100 °C. The product was dried, and calcinated at 800 °C for 4 h in muffle furnace. 2.3. Surface modification of CSAW 2 g CSAW was dispersed in 100 mL 0.015 mol/L trisodium phosphate solution and under ultrasonic for 2 min. The homogeneous solution was stirred for 10 min, filtered off and washed. The product was dispersed in 100 mL ethanol, and 0.12 g stearic acid was added into the slurry of CSAW heating at 90 °C in the oil bath with magnetic stirring for 5 min, filtered and washed with plenty of ethanol to remove excessive stearic acid. Then the M-CSAW was dried at 100 °C for 1 h.
Fig. 2. XRD patterns of (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic MCSAW.
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Fig. 3. XPS spectrum of (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic M-CSAW.
conditions, the measurements were conducted in alumina crucible using at about 5 mg of sample and a heating rate of 6 °C min−1. Each sample was heated from room temperature to 1000 °C under nitrogen atmosphere (60 cm min−1). 3. Results and discussion 3.1. Characterization of CSAW and M-CSAW The morphology of CSAW is investigated by SEM, as shown in Fig. 1. The original CSAW is smooth and integrated, with diameter 0.3–1 μm (Fig. 1a). The morphology of CSAW does not change obviously after inorganic modification (Fig. 1b). The surface of M-CSAW is coarse after inorganic-organic modification (Fig. 1c), indicating that a thin film is formed on the surface of CSAW, affecting the hydrophilic-hydrophobic quality of CSAW [18]. The contact angle of CSAW, inorganic M-CSAW and inorganic-organic M-CSAW are 0°, 11.23° and 108.43° respectively (Fig. 1 insert). The straight edge of whisker reveals that M-CSAW keeps a high crystallinity level as the same of CSAW (Fig. 2). Present diffraction peaks at 25°, 31°, 41°, 48°, and 56° can be assigned to (020), (210), (212), (230), and (232), respectively. The modification occurs only on the surface of CSAW without altering its crystal structure [19]. XPS survey scan of CSAW, inorganic M-CSAW and inorganic-organic M-CSAW are showed in Fig. 3. All samples exhibit main character peaks at 169 eV, 284 eV, 347 eV and 531 eV. They are assigned to S2p, C1s, Ca2p and O1s signals, respectively. The C content increases obviously
Table 1 The present chemical elements and their corresponding atomic concentrations of CSAW, inorganic M-CSAW and inorganic-organic M-CSAW surface. Sample
Ca2p (%)
S2p (%)
O1s (%)
C1s (%)
P2p (%)
CSAW inorganic M-CSAW inorganic-organic M-CSAW
11.41 12.19 11.39
12.72 7.51 5.95
49.57 48.77 43.06
26.3 27.59 34.86
– 3.94 4.74
after inorganic-organic modified (Table 1), demonstrating that the stearic acid has coated on the surface of CSAW. The M-CSAW have a new obvious peak located at 133 eV assigned to P2p (Fig. 3b and c), suggesting that phosphate has modified the surface of CSAW. The curvefitting analysis of high-resolution C1s spectra of CSAW reveals that the existence of C-C and C-H, because their signal locates at 284.75 ± 0.1 eV. There is a new signal appearing at 288.10 ± 0.1 eV for inorganic-organic M-CSAW (Fig. 4). This signal stands for O-C = O. Those outcomes could strongly evident that stearic acid coats on the surface of CSAW successfully [20]. Ca2p has two peaks located at 347.87 ± 0.1 eV and 351.46 ± 0.1 eV (Fig. 5a). Inorganic M-CSAW does not obviously change the binding energy of Ca2p (Fig. 5b), indicating that inorganic modification has little influence on the chemical state of Ca2+. However, the binding energy of Ca2p appears at 347.51 ± 0.1 eV and 351.11 ± 0.1 eV after inorganic-organic modification (Fig. 5c). The electron density of Ca2+ surrounding increased, since alkyl groups can offer electron, causing Ca2p binding energy decreased [21]. The decrease of Ca2p binding energy demonstrates that the stearic acid reacts with Ca2+ on the surface of CSAW [22]. Furthermore, FT-IR spectrum is conducted to investigate the interfacial reaction on the surface of CSAW. Single strong bands at 1636.82 cm− 1 and 1155.24 cm− 1 are ascribed to the stretching of Ca2 + and SO24 − (Fig. 6). The bands at 1055.64 cm− 1 and 1121.88 cm−1 can be attributed to the P-O stretching vibration peaks. P-OH vibration peak can also be found at 944.58 cm−1 (Fig. 6b and c). exists on the surface of inorganic MThe results indicate that HPO2− 4 CSAW. For organic-inorganic M-CSAW (Fig. 6c), there are new bands at 2919.73 cm−1 and 2851.26 cm−1 corresponding to –CH2– asymmetric and symmetric vibration [23]. The band of carboxylate at 1562.71 cm−1 indicates that stearic acid grafts to the surface of CSAW [24]. Fig. 7 shows the thermogravimetric (TG) curves of CSAW, inorganic M-CSAW and organic-inorganic M-CSAW. The first mass loss peaks below 200 °C can be ascribed to removal of water (Fig. 7). CSAW does not have obvious mass loss due to its good thermal stability (Fig. 7a).
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Fig. 4. High-resolution XPS spectrum of C1s peak: (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic M-CSAW.
Decomposition of phosphate causes slight weight loss of inorganic MCSAW (Fig. 7b). The decomposition of stearic acid temperature is about 260 °C, and salts derivative of the carboxylic acids present higher thermal stability [25]. The second mass loss begins at 420 ° C, indicating that calcium stearate exists on the surface of CSAW and decomposition [26] (Fig. 7c). Calcium carbonate was obtained as sub-product of the reaction and decomposes to calcium oxide and carbon dioxide at 642 °C with 0.56% mass loss [27]. The TG results indicate that organic-inorganic M-CSAW can meet the demand of thermal stability as filler.
3.2. The reaction mechanism of CSAW Fig. 8 shows the possible mechanism of surface modification, and the chemical reactions are as follows. Na3 PO4 þ H2 O→HPO4 2‐ þ OH‐ þ 3Naþ ‐
CaSO4 þ HPO4 2 →CaHPO4 þ SO4 2
‐
Fig. 5. High-resolution XPS spectrum of Ca2p peak: (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic M-CSAW.
ð1Þ
ð2Þ
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that there is carboxylate on the surface of CSAW (Figs. 6 and 7). The alkyl chain of stearic acid coated on the surface of CSAW forms a thin film and makes it become hydrophobic. 4. Conclusions
Fig. 6. FT-IR spectrum of (a) CSAW, (b) inorganic M-CSAW and (c) inorganic-organic MCSAW.
A novel and simple method was used to modify CSAW. First, trisodium phosphate was used to active the CSAW surface. Second, stearic acid was applied to modify the surface of CSAW. The obtained M-CSAW has a good hydrophobic property. No obvious changes can be found on the morphology and crystalline degree of M-CSAW. The FT-IR and XPS results demonstrate that inorganic modification can active the surface of CSAW, and the organic (stearic acid) reacts with Ca2 + on the surface of CSAW to form –COOCa. According to TG, MCSAW has good thermal stability and can act as a promising filler in composites for future. This study provides a reaction mechanism model for a series of surface modification, which is a meaningful tool for whisker/polymer matrix composites. Acknowledgments This work was financially supported by Science and technology project Foundation of Qinghai Province, China (2015-ZJ-737) and the National Natural Science Foundation of China (No. 51402323). We thank Dr. Yabin Wang for his assistance during the revision process. References
Fig. 7. TG curves of (a) CSAW, (b) inorganic M-CSAW and (c) organic-inorganic M-CSAW.
ð3Þ It can be seen from Eq. (1) that sodium phosphate decomposes into sodium hydrogen phosphate and sodium hydroxide in aqueous solution. The pH of sodium phosphate solution (0.01moL/L) is 12.1, which confirm the decomposition of sodium phosphate. As shown in Eq. (2), the ion substitution reaction takes place on the surface of CSAW because the Ksp of calcium hydrogen phosphate (1.8 × 10−7, 25 °C) is far less than that of calcium sulfate (7.1 × 10−5, 25 °C) [28]. The reaction develops a thin calcium hydrogen phosphate film on the surface of CSAW, forming a large amount of hydroxyl in the surface of CSAW at the same time. The surface of CSAW is activated after inorganic modification. At the end of stage, C17H35COO– reacts with Ca2 + to form – possibly combines H+ to form H2PO− COOCa [24]. HPO2− 4 4 on the surface of CSAW. Hydroxyl group and alkyl chain make Ca2p binding energy diminished by shielding effect (Fig. 5). FT-IR and TG results indicate
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