Microwave-assisted synthesis and characterization of CaCO3 particles-filled wood powder nanocomposites

Abstract

The purpose of this study was to develop a rapid and green method for the synthesis of biomass-based materials. A microwave-assisted method was used for the preparation of CaCO3 particles-filled wood powder nanocomposites, which involve natural cellulosic materials, CaCO3, and microwave heating. Dewaxed wood powder was pretreated in the NaOH/urea solution. The urea acts as part of the CO3^2- source and provides a basic condition for the synthesis of CaCO3. The influences of reaction parameters such as the heating time and the addition of (NH4)2SO4 on the products were investigated by X-ray powder diffraction (XRD), Fourier transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential thermal analysis (DTA). The experimental results demonstrated that the heating time had an effect on the crystallinity and morphology of the nanocomposites. In addition, the presence of (NH4)2SO4 played an important role in the morphology and dispersion of CaCO3 in the nanocomposites.

Full Article

Microwave-assisted Synthesis and Characterization of CaCO₃ Particles-filled Wood Powder Nanocomposites

Ming-Guo Ma,* Fu Deng, Ke Yao, and Chun-Han Tian

The purpose of this study was to develop a rapid and green method for the synthesis of biomass-based materials. A microwave-assisted method was used for the preparation of CaCO₃ particles-filled wood powder nanocomposites, which involve natural cellulosic materials, CaCO₃, and microwave heating. Dewaxed wood powder was pretreated in the NaOH/urea solution. The urea acts as part of the CO₃²⁻source and provides a basic condition for the synthesis of CaCO₃. The influences of reaction parameters such as the heating time and the addition of (NH₄)₂SO₄ on the products were investigated by X-ray powder diffraction (XRD), Fourier transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential thermal analysis (DTA). The experimental results demonstrated that the heating time had an effect on the crystallinity and morphology of the nanocomposites. In addition, the presence of (NH₄)₂SO₄ played an important role in the morphology and dispersion of CaCO₃ in the nanocomposites.

Keywords: Wood powder; CaCO₃; Nanocomposites; Microwave

Contact information: Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, PR. China;

* Corresponding author: mg_ma@bjfu.edu.cn

INTRODUCTION

Since the first report of microwave-assisted synthesis in 1986 (Fimbel and Siffert 1986), microwave heating methods have received more attention and are accepted as rapid and green promising methodologies in the synthesis of materials due to their unique effects as compared with conventional processes. Advantages of microwave heating include rapid volumetric heating, higher reaction rates, shorter reaction times, enhanced reaction selectivity, environmental friendliness, and energy efficiency (Polshettiwar et al. 2009). The American Chemical Society monograph on green chemistry recommended that microwave radiation would minimize the energy required for a reaction to take place (Ryan and Tinnesand 2002). This has opened up the possibility of realizing fast preparation of materials. The application of microwave heating in the synthesis of nanomaterials and nanocomposites has been reported by using these advantages (Tsuji et al. 2005).

Previous studies have reported the synthesis of cellulose-based composites such as cellulose/hydroxyapatite nanocomposites (Ma et al. 2010), cellulose/silver nanocomposites (Li et al.2011), cellulose/calcium silicate nanocomposites (Jia et al. 2011), and cellulose/F-substituted hydroxyapatite (Jia et al. 2012) via microwave heating; however, there is no known prior report on the synthesis of CaCO₃ particles-filled wood powder composites by the microwave-assisted method. This novel preparation method is based on the simultaneous formation of the colloidal inorganic particles, the synthesis of regenerated cellulose, and the fabrication of cellulose-based composites.

As very promising and sustainable green materials, biomass-based composites have been extensively explored (Ashori 2008). Biomass-based composites can completely utilize the components of biomass via the dissolution of lignocelluloses in complete dissolution systems such as certain organic solvents and ionic liquids (Li et al. 2011; Maki-Arvela et al. 2010).

Researchers have discovered that the introduction of inorganic materials into the lignocellulose matrix may significantly improve the performance of a composite (Blanchard and Blanchet 2011; Mai and Militz 2004) and have a positive effect on the compression strength of lignocellulose/plastic composites. Chen et al. (2004) investigated the effect of temperature on the mechanical and surface properties of calcium carbonate filled bamboo fibers and polypropylene composites and discovered that the treatments improved compatibility between bamboo fibers and polypropylene matrix. Consequently, the tensile strength and modulus of the composites were increased. Moreover, the types of cellulose were found to play an important role in the microstructure and morphologies of the cellulose/CaCO₃ composites (Ma et al. 2012b).

The cellulose fibers and CaCO₃ particles were observed using alkali extraction cellulose; meanwhile, using microcrystalline cellulose instead of alkali extraction cellulose, a combination of cellulose particles having irregular shape along with CaCO₃ microspheres was obtained. The previous study included synthesizing wood powder/ CaCO₃ composites via a hydrothermal route using dewaxed wood powder (Ma et al. 2012a).

The synthesis of the CaCO₃ particles-filled wood powder nanocomposites was carried out by the microwave-assisted method using dewaxed wood powder, CaCl₂ solution, and Na₂CO₃ solution. The influences of reaction parameters such as the heating time and the addition of (NH₄)₂SO₄ on the products are discussed in detail. Compared with a previous report (Ma et al. 2012), this synthetic methodology for the preparation of CaCO₃ particles-filled wood powder nanocomposites is simpler, more rapid, and greener.

EXPERIMENTAL

Methods

All chemicals were of analytical grade and used as received without further purification. All experiments were conducted under an air atmosphere. The preparation of dewaxed wood powder solution is explained in a previous report (Ma et al. 2012). In a typical experimental, 7.00 g of NaOH and 12.00 g of urea were added to 81 mL of distilled water under vigorous stirring to form a NaOH-urea aqueous solution. Then, 1.00 g of dewaxed wood powder was added to the solution under vigorous stirring. The resulting solution was then cooled to -12 °C for 12 h. The obtained solution was used for the preparation of CaCO₃ particles-filled wood powder nanocomposites.

For the synthesis of CaCO₃ particles-filled wood powder nanocomposites, 10 mL of CaCl₂ solution (0.10 M) and 10 mL of Na₂CO₃ solution (0.10 M) were added to the above obtained solution (20 mL) under vigorous stirring. The flasks containing the solution were microwave-heated to 90 °C for 10, 30, and 60 min and then air cooled to room temperature. The microwave oven used for sample preparation was purchased from Beijing Xiang-Hu Science and Technology Development Reagent Co., Ltd. and was equipped with a magnetic stirring system and a water-cooled condenser outside the microwave cavity. The product was separated from the solution by centrifugation, washed with water and ethanol three times, and dried at 60 °C for further characterization.

For comparison, samples were also synthesized by a microwave-assisted method using (NH₄)₂SO₄solution (10 mL, 0.10 M) instead of Na₂CO₃ solution (10 mL, 0.10 M) at 90 °C for 10, 30, and 60 min while keeping the other conditions the same.

Characterization

X-ray powder diffraction (XRD) was performed in the 2θ range from 10º to 70º on a Rigaku D/Max 2200-PC diffractometer with Cu Kα radiation (λ = 0.15418 nm) and a graphite monochromator at ambient temperature.

Fourier transform infrared spectroscopy (FTIR) was carried out on a FTIR spectrophotometer (Nicolet 510) using the KBr disk method.

Scanning electron microscopy (SEM) images were recorded with a Hitachi 3400 N scanning electron microscopy. All samples were Au-coated prior to examination by SEM.

Thermal behavior of the samples was tested using thermogravimetric analysis (TGA) and differential thermal analysis (DTA) on a simultaneous thermal analyzer (DTG-60, Shimadzu) at a heating rate of 10 °C min^-1 in flowing air.

RESULTS AND DISCUSSION

Figure 1a shows the X-ray powder diffraction pattern of a typical sample prepared by microwave heating at 90 °C for 10 min. The sample consisted of mixed phases of well-crystallized calcite with a hexagonal structure (JCPDS 47-1743) and cellulose (marked with * in Fig. 1a).

No peaks from impurities such as aragonite were observed. The composites were successfully synthesized via the microwave-assisted route within only 10 min; however, the CaCO₃ particles-filled wood powder composites were obtained via a hydrothermal route at 160 °C for 2 h (Ma et al. 2012).

The samples prepared by the microwave heating at 90 °C for 30 and 60 min had similar XRD patterns (Figs. 1b and 1c) as those heated for 10 min. The slight difference was that the peak intensity of calcite increased with increasing heating time, indicating a better crystallinity of calcite.

Fig. 1. XRD patterns of the CaCO₃ particles-filled wood powder nanocomposites prepared by microwave heating at 90 °C for different times: (a) 10 min; (b) 30 min; and (c) 60 min

Fig. 2. FTIR spectra of the CaCO₃ particles-filled wood powder nanocomposites: (a) 10 min; (b) 30 min; and (c) 60 min

The crystal structure and phase of the nanocomposites were further examined by FTIR analysis (Fig. 2). The band at 1131 cm^-1 can be assigned to the C-O of cellulose. The band at 1447 cm^-1 is characteristic of ν_3-3 CO₃^2- and ν_3-4 CO₃^2- (Nelson and Featherstone 1982). The band at 871 cm^-1 is characteristic of calcite (Donners et al. 2000). There were differences among the FTIR spectra of all the samples. One can see that only a shoulder was observed at 715 cm^-1, which was also characteristic of calcite, when the heating time was 10 min (Fig. 2a). When the heating time was increased to 30 min, a peak at 715 cm^-1 appeared (Fig. 2b). When the heating time was increased to 60 min, the peak intensity at 715 cm^-1 increased (Fig. 2c). From Fig. 2, one can see the peak intensity at 715 cm^-1increased with increasing heating time, demonstrating the increasing crystallinity of calcite. These results were in accordance with the XRD results.

The morphology of the samples was investigated with SEM, as shown in Fig. 3. When the heating time was 10 min, some of the CaCO₃ congregates grew on wood powder as substrate (Fig. 3a). The magnified micrograph of the CaCO₃ congregates is shown in Fig. 3b, from which one can see the CaCO₃ with a rice-like shape. This result was different from a previous report (Ma et al. 2012), in which CaCO₃ with a six-side-like shape was dispersed on the surface of the wood powder. When the heating time was increased to 30 min, CaCO₃ congregates and particles were also observed on the substrate of wood powder (Fig. 3c).

Fig. 3. SEM images of the CaCO₃ particles-filled wood powder nanocomposites: (a, b) 10 min; (c, d) 30 min; and (e, f) 60 min

The sizes of CaCO₃ congregates were larger, as shown in Fig. 3d, compared with those in Fig. 3b. When the heating time was increased to 60 min, wood powder fibers loading with CaCO₃ congregates and particles were observed (Fig. 3e). From the magnified micrograph, one can see that the size of CaCO₃ congregates decreased, while the size of CaCO₃ particles increased (Fig. 3f), compared with Figs. 3a-d. Furthermore, one can clearly see that the number of CaCO₃ particles increased with increasing heating time. When the heating time was 10 min, CaCO₃ particles of relatively small size were obtained. When the heating time was increased to 60 min, CaCO₃ particles were dispersed on the surface of the wood powder fibers and embedded in the wood powder fibers. These results demonstrated that the heating time played an important role in the morphology and dispersion of CaCO₃ in the nanocomposites. From Fig. 3, one can observe that the CaCO₃ shape changed from rice-like to congregate/particles to individualized particles with increasing heating time.

An investigation was conducted into the effect of the addition of (NH₄)₂SO₄ on the CaCO₃ particles-filled wood powder nanocomposites. The samples were synthesized using (NH₄)₂SO₄ instead of Na₂CO₃. The XRD patterns indicated that the samples consisted of the mixed phases of calcite and cellulose (Fig. 4A); however, the peak intensity of calcite increased with increasing heating time. All the samples had similar FTIR spectra, compared with Fig. 2. The peak intensity at 1447 cm^-1 increased with increasing heating time, which was characteristic of ν_3-3 CO₃^2- and ν_3-4 CO₃^2-. The peak intensity at 1124 cm^-1 decreased with increasing heating time. Moreover, a shoulder at 715 cm^-1 was observed (Fig. 4B (a and b)). When the heating time was increased to 60 min, the peak at 715 cm^-1was observed (Fig. 4B (c)). These results indicated that the peak intensity of CaCO₃ increased, while the peak intensity of wood powder decreased with increasing heating time.

Fig. 4. (A) XRD patterns and (B) FTIR spectra of the CaCO₃ particles-filled wood powder nanocomposites in the presence of (NH₄)₂SO₄ at 90 °C for different times: (a) 10 min; (b) 30 min; and (c) 60 min

The morphology and dispersion of the CaCO₃ particles-filled wood powder nanocomposites are displayed in Fig. 5. These specimens were prepared by microwave heating in the presence of (NH₄)₂SO₄ at 90 °C for 10, 30, and 60 min. When heating time was 10 min, the CaCO₃ particles were dispersed on the surface of the wood powder fibers (Fig. 5a). The magnified micrograph of the sample is shown in Fig. 5b, from which one can see the CaCO₃ with a cube-like shape. Increasing the heating time to 30 min, the CaCO₃ particles and CaCO₃ congregates were dispersed on the surface of the wood powder fibers (Figs. 5c and 5d). When the heating time was increased to 60 min, serious CaCO₃aggregation of the cube-shaped particles was observed, as shown in Fig. 5e and f. From Fig. 5, one can conclude that the degree of the CaCO₃ agglomeration increased with increasing heating time and that better dispersion of CaCO₃ in the wood powder matrix was obtained by using a short heating time.

Fig. 5. SEM images of the CaCO₃ particles filled wood powder nanocomposites in the presence of (NH₄)₂SO₄ at 90 °C for different times: (a, b) 10 min; (c, d) 30 min; and (e, f) 60 min.

In view of the XRD, FTIR, and SEM results, differences clearly existed in the samples prepared by varied reactants between Na₂CO₃ and (NH₄)₂SO₄. When the sample was synthesized in the presence of Na₂CO₃, Na₂CO₃ was used as the CO₃²⁻ source for the synthesis of CaCO₃. Moreover, the urea also acts as part of the CO₃²⁻ source for the synthesis of CaCO₃ and provides a basic condition by decomposing with increasing heating time; however, when using (NH₄)₂SO₄ instead of Na₂CO₃, the urea acts as the only CO₃²⁻ source for the synthesis of CaCO₃. Both the urea and (NH₄)₂SO₄ provide a basic condition. Therefore, the variety of the CO₃²⁻ source and base source induced the difference of phase, microstructure, and morphology in the nanocomposites.

Fig. 6. TGA and DTA curves of the CaCO₃ particles-filled wood powder nanocomposites in the presence of (A) Na₂CO₃ and (B) (NH₄)₂SO₄ at 90 °C for 30 min.

The thermal stability of the CaCO₃ particles-filled wood powder nanocomposites was also investigated by TGA and DTA. TGA and DTA curves for the samples synthesized in the presence of Na₂CO₃ and (NH₄)₂SO₄ at 90 °C for 30 min are shown in Fig. 6. The TGA curve of the sample exhibited two stages of weight loss in the presence of Na₂CO₃; these weight losses were accompanied by endothermic peaks at about 364 °C and 449 °C in the DTA curve (Fig. 6A). The peak around 364 °C was caused by thermal degradation of the wood powder, while the other peak at around 449 °C resulted from the complete decomposition of the wood powder in the nanocomposites (Zheng et al. 2002). The total weight loss of the sample was 58.5 %. On the other hand, the sample that was synthesized in the presence of (NH₄)₂SO₄ displayed two stages of weight loss at about 330 °C and 403°C (Fig. 6B). The total weight loss of the sample was 72.0 %. From Fig. 6, one can see that the thermal degradation temperature of the sample synthesized in the presence of (NH₄)₂SO₄ decreased, compared with that of the sample synthesized in the presence of Na₂CO₃. If the total weight loss was assigned to the complete decomposition of wood powder, the residual amount (41.5 %) of CaCO₃ in the sample prepared by using Na₂CO₃ was more than that (28.0 %) of the sample synthesized by using (NH₄)₂SO₄. As mentioned, both Na₂CO₃ and urea were used as the CO₃²⁻ source for the formation of CaCO₃ when the sample was prepared in the presence of Na₂CO₃, while only urea was used as the CO₃²⁻ source for the synthesis of CaCO₃ in the presence of (NH₄)₂SO₄. This might lead to the production of different amounts of CaCO₃ in the two composites.

CONCLUSIONS

1. A rapid and green microwave-assisted route was used for the synthesis of the CaCO₃ particles-filled wood powder nanocomposites.
2. Both Na₂CO₃ and urea were used as the CO₃²⁻ source for the synthesis of CaCO₃. XRD patterns showed increasing peak intensity of CaCO₃ with increasing heating time, indicating a higher crystallinity of calcite.
3. The heating time played an important role in the morphology and dispersion of CaCO₃ in the nanocomposites. The CaCO₃ shape changed from rice-like to congregate/particles to particles with increasing heating time.
4. Using (NH₄)₂SO₄ instead of Na₂CO₃, the peak intensity of CaCO₃ in XRD patterns increased with increasing heating time; meanwhile, the peak intensity of CaCO₃ increased and the peak intensity of wood powder decreased with increasing heating time in FTIR spectra. The total weight losses of the samples synthesized using Na₂CO₃ and (NH₄)₂SO₄ were 58.5 % and 72.0 %, respectively.
5. This synthetic strategy reported in this study opens a new window to the manufacture of wood-based composites as well as high value-added applications of wood powder.
6. These nanocomposites utilized the main valid components of lignocelluloses and could be a candidate for the biomedical applications.

ACKNOWLEDGMENTS

Financial support from Beijing Nova Program (Z121103002512030) and the Program for New Century Excellent Talents in University (NCET-11-0586) is gratefully acknowledged.

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Article submitted: January 15, 2014; Peer review completed: April 23, 2014; Revised version received and accepted: May 6, 2014; Published: May 12, 2014.