Abstract
The application of cellulose hybrid biocomposites filled with calcium carbonate has attracted wide attention in packaging and other fields in recent years. In this study, regenerated cellulose (RC) films filled with calcium carbonate were successfully prepared by dissolution, regeneration, and in situ precipitation of CaCO3. The optical, mechanical, physical, and chemical properties of biocomposites were examined by UV-visible spectroscopy, tensile testing, scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analyses (TGA). The results showed that RC films with different CaCO3 contents exhibited good flexibility, optical properties, mechanical strength, and thermal stability. The RC biocomposite filled with calcium carbonate showed a tensile strength of 84.7 ± 1.5 MPa at optimum conditions. These RC biocomposites filled with CaCO3 may find application in packaging.
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Preparation and Characterization of Regenerated Cellulose Biocomposite Film Filled with Calcium Carbonate by in situ Precipitation
Qianqian Zhu,a,d Jingjing Wang,a Jianzhong Sun,a,* and Qianqian Wang a,b,c,*
The application of cellulose hybrid biocomposites filled with calcium carbonate has attracted wide attention in packaging and other fields in recent years. In this study, regenerated cellulose (RC) films filled with calcium carbonate were successfully prepared by dissolution, regeneration, and in situ precipitation of CaCO3. The optical, mechanical, physical, and chemical properties of biocomposites were examined by UV-visible spectroscopy, tensile testing, scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analyses (TGA). The results showed that RC films with different CaCO3 contents exhibited good flexibility, optical properties, mechanical strength, and thermal stability. The RC biocomposite filled with calcium carbonate showed a tensile strength of 84.7 ± 1.5 MPa at optimum conditions. These RC biocomposites filled with CaCO3 may find application in packaging.
Keywords: Organic-inorganic hybrid biocomposite; Calcium carbonate; Mechanical property; Characterization
Contact information: a: Biofuels Institute, School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013 China; b: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640 China; c: Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Key Laboratory of Biomass Energy and Material, Jiangsu Province, Nanjing 210042, China; d: Analysis and Testing Center, Jiangsu University, Zhenjiang, 212013 PR China; *Corresponding authors: jzsun1002@ujs.edu.cn; qianqian.wz@gmail.com
INTRODUCTION
As a renewable, biodegradable, and biocompatible polymer, cellulose offers tremendous opportunities for different products, including paper and paperboard, textiles, and drug tablets (Wang et al. 2018, 2019; Zhu et al. 2020a,b). Although it is generally considered as abundant and low-cost biomass, the price of the cellulose pulp is still high compared with inorganic fillers (Laukala et al. 2017; Xu et al. 2017). Inorganic fillers, such as calcium carbonate, are used in nearly every paper and paperboard product to save virgin pulp, dry energy, and production costs. The addition of CaCO3 also brings some special properties for the final paper products, such as optical, physical, mechanical, and surface property improvements (Cheng et al. 2016; Seo et al. 2017; Choi et al. 2018). Ground and precipitated calcium carbonate fillers (GCC and PCC, respectively) are normally added to the cellulose pulp suspension before the formation of paper or paperboard. In-situ CaCO3 formation can be also applied with the aim of maximizing filler retention (Ciobanu et al. 2010; Seo et al. 2017). Biomimetic design and synthesis of nanocellulose-CaCO3 hybrid materials to new green materials are also in the spotlight (Saito et al. 2014; Nakao et al. 2019). Biodegradable regenerated cellulose (RC) films can be prepared by coagulation and regeneration from various cellulose solvents, including ionic liquids (ILs), N,N-dimethyl-acetamide/lithium chloride (DMA/LiCl), and NaOH/urea solution (Sathitsuksanoh et al. 2013; Wang et al. 2016). The obtained RC films showed much better properties, such as better air oxygen permeability, thermal stability, and lower thermal expansion coefficient, compared to petroleum-based plastic substrates. The RC films show great potential in packaging and other applications. To further improve the performance of cellulose films, biodegradable cellulose copolymer films with organic fillers/coatings were prepared (Delhom et al. 2010; Yang et al. 2014).
However, the research on in situ precipitation of calcium carbonate on RC films is limited. The precipitation behavior of CaCO3 on RC film from the ionic liquid (1-butyl-3-methylimidazolium chloride, BMIMCL) in ethanol-water was presented by Xiao et al. (2011), while the mineralization of CaCO3 on RC film from DMA/LiCl solvent was investigated by Rauch et al. (2012). The mechanism for in situ calcium carbonate precipitation in RC hydrogel was proposed by Rauch et al. (2012). It is well known that cellulose surfaces are negatively charged, thus leading to accumulation of Ca2+ ions in the charged surface and inducing the nucleation, aggregation, and crystallization. The crystallization of calcium carbonate is limited to the specific nucleation sites, because of the complexation of the Ca2+ ions and negatively charged groups in cellulose fibers. The nanocrystals then grow larger and the small aggregates are formed, and these are interconnected by the cellulose fibers. Subsequently, these crystals that are strongly attached to the cellulose network become assembled into large aggregates. To the authors’ best knowledge, there is no report on the in situ precipitation of calcium carbonate on regenerated cellulose made from NaOH/urea solution. Little is known about the effects of calcium carbonate when incorporated into the RC matrix by in situ precipitation. As an inorganic filler, the incorporation of calcium carbonate could lead to an improvement in biocomposite film properties and cost reduction. Furthermore, the precise mechanism for in situ CaCO3 precipitation on RC films is still ambiguous.
The properties of a biocomposite made of RC by NaOH/urea solution and in situ calcium carbonate precipitation remains to be elucidated. In this study, strong and translucent RC biocomposite films filled with calcium carbonate were developed. The method involved the coagulation and regeneration of cellulose films and in situ calcium carbonate precipitation in the regenerated hydrogel. The properties of RC and calcium carbonate biocomposite films were evaluated. The starting materials were also characterized. The effects of the precursor concentration on the properties of the biocomposite films are discussed.
EXPERIMENTAL
Materials and Chemicals
Cellulose powder from cotton linters was purchased from Hubei Chemical Fiber Group Co., Ltd. (Xiangyang, China) and milled to pass through a 40-mesh sieve with a laboratory grinder (FZ102; Tianjin Taipingyuanda Instrument Co., Ltd., Tianjin, China). The molecular weight of cellulose was estimated to be 1.07 × 105 kg/mol by using a viscometer. Calcium chloride and sodium carbonate were obtained from Sigma-Aldrich (Shanghai, China). Sodium hydroxide, urea, and sulfuric acid were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received.
Cellulose Dissolution with NaOH/Urea Solution
Cellulose powder was dried at 80 ℃ for 6 h in a vacuum drying oven (BPZ-6033; Shanghai Yiheng Scientific Instrument Co., Ltd., Shanghai, China). Cellulose solvent, NaOH/urea solution, was made by mixing NaOH, urea, and H2O at a weight ratio of 7 : 12 : 81. The NaOH/urea solution was precooled to -12 ℃ in a refrigerator (SC-276; Haier Company Limited, Qingdao, China). A 4.0 wt% suspension of cellulose powder in NaOH/urea solution was made by dissolving 4 g of cotton linters powder in 96 g NaOH/urea solution. The suspension was vigorously stirred for 5 min. The air bubbles and undissolved cellulose aggregates were removed by centrifuge with an Avanti J-E Centrifuge (JA-10 rotor; Beckman Coulter, Brea, CA, USA) at 8000 rpm for 10 min. A transparent and viscous cellulose solution was obtained.
Preparation of RC Film via Solution Casting
Cellulose solution was poured onto a glass plate and cast with a glass rod. The glass rod wrapped with a certain thickness tape evenly distributed the cellulose solution. A layer of hydrogel with thickness proportional to the tape thickness was formed. The glass plate with the cellulose hydrogel was gently placed into a coagulation bath (1 L 5% H2SO4) until the film was detached from the plate, which took approximately 5 min. The detached film was carefully washed with deionized water (DI) until the pH of water did not change. The film was then air-dried on a stainless steel plate surface. The obtained film was labeled as RC and kept for further analysis.
Preparation of RC Film with in-situ CaCO3 Precipitation
In-situ precipitation of calcium carbonate into RC film was performed by sequentially impregnating calcium chloride and sodium carbonate in the presence of RC film. In detail, the RC hydrogel film was firstly immersed in calcium chloride solution at concentrations of 0.2, 0.5, 0.8, or 1.0 mol/L for 24 h. Then, the hydrogel film was immersed in a sodium carbonate solution of the same concentration as the calcium chloride solution for another 24 h. Calcium carbonate can be obtained through the double-exchange reaction between sodium carbonate and calcium chloride. The RC with in-situ CaCO3 precipitation was gently washed with DI water and air-dried. The films were labeled as RC-C02, RC-C05, RC-C08, and RC-C10, depending on the concentration of the solution for CaCO3 formation.
Methods
Light transmittance and optical properties of RC were determined by a conventional UV-visible (UV-vis) spectrometer (DU800; Beckman Coulter, Brea, CA, USA) in the range of 200 to 800 nm with air as the background. A wavelength of 600 nm was used to determine the film transmittance. Each sample was scanned three times. The tensile strength of RC films was measured with a tensile tester (Model YG026MB; Fangyuan Instrument Co., Ltd., Wenzhou, China) with a 1000-N load cell at a speed of 5 mm/min at 26 ℃ and 65% relative humidity. At least five specimens were measured for each sample, and the average value was reported. Surface and cross-section morphologies of RC films were determined using an S-3400N scanning electron microscope (SEM) equipped with an EDAX energy-dispersive X-ray (EDAX LLC, Mahwah, NJ, USA). The specimens were carefully mounted on an SEM sample stub with double-sided sticky carbon conductive tape and then spray coated with gold (MSP-1S; Shinkku VD, Tokyo, Japan) for 60 s in a vacuum chamber before analysis. The energy-dispersive X-ray spectrum was conducted to confirm the presence of CaCO3 in RC film. The X-ray diffraction (XRD) spectra of RC films were analyzed using a D8 Advance diffractometer with a CuKα source (Bruker AXS, Billerica, MA, USA). Scans with an angle range of 2θ = 10 to 60° were collected with a speed of 2°/min. Fourier transform infrared spectroscopy (FTIR) spectra were determined using a Nexus 470 FTIR spectrometer (Thermo Electron Corporation, Waltham, MA, USA). The RC film was first dried and milled into fine powder in liquid nitrogen. Samples were then mixed with KBr and pelletized into a transparent pallet, and their spectra in the range of 4000 to 500 cm-1 with a resolution of 4 cm-1 were recorded. Thermogravimetric analyses (TGA) were conducted to examine the degradation behavior of the RC at various CaCO3 precipitation using a thermogravimetric analyzer (TGA 4000; Perkin Elmer, Waltham, MA, USA). Approximately 5 mg specimens were heated at 10 ℃/min from 30 ℃ to 800 ℃ in a nitrogen atmosphere (50 mL/min).
RESULTS AND DISCUSSION
Optical Transmittance and Tensile Strength of RC-CaCO3 Films
The RC biocomposite films prepared by solution casting and in situ CaCO3 precipitation had good flexibility. Figure 1a to 1e show optical images of the RC films with CaCO3 precipitation at concentrations of 0, 0.2, 0.5, 0.8, and 1.0 mol/L for precursor solutions. The films prepared under different conditions showed obvious differences.
Fig. 1. Visual observation of (a) RC; (b) RC-C02; (c) RC-C05; (d) RC-C08; and (e) RC-C10 e films (thickness = 35 μm); (f) UV-Vis spectra of films; and (g) the tensile strength for different samples
The pure RC film had good transparency, such that the green plant underneath could be seen. The RC-CaCO3 films were not optically transparent. The transparency of RC-CaCO3 films decreased with increased precursor in the solutions. The green plant was almost invisible at the highest concentration of calcium carbonate. The RC-CaCO3 films can be considered as a three-dimensional network structure of RC with micro-nano calcium carbonate particles in between. Because of the inhomogeneous structure in the surface of the composite film, as shown in the SEM image in the following section (Fig. 2), the films exhibited light scattering due to the aggregation of the precipitated calcium carbonate microcrystals (Kumar et al. 2011). The change in opacity of the RC-CaCO3 films can be ascribed to the light absorption and scattering in the gaps between the cellulose fiber and the micro-nano calcium carbonate particles at the interface because the refractive indices of cellulose and CaCO3 are similar (1.56 to 1.60 vs. 1.58) (Kuo et al. 2018; Mohamadzadeh-Saghavaz et al. 2014). Translucent or almost opaque films can be achieved depending on the loading of calcium carbonate.
The DU800 UV-Vis spectrophotometer was used for quantitative analysis of the light transmittance of the films, as shown in Fig. 1f. The RC film had the highest transparency in the UV-visible region, which was above 70% transmittance. As the content of calcium carbonate increased, the transparency of films was gradually reduced in both UV and visible light range due to the light scattering (Kumar, et al. 2011).
The tensile strength of the films was measured, and the results are displayed in Fig. 1g. The films prepared from pure RC exhibited an average tensile strength of 76.8 MPa. It was reported that the degree of polymerization (DP) of RC did not change much after dissolution and regeneration processes, which may be responsible for the high strength (Cai et al. 2004, 2007). No obvious difference in tensile strength can be observed for the film at low calcium carbonate loading (RC-C02). The tensile strength of the films was greatly enhanced by the increasing calcium carbonate loading (Choi et al. 2018; Seo et al. 2014), which had an average tensile strength of 80.5 and 84.7 MPa for RC-05 and RC-08, respectively. Further increasing the calcium carbonate content not only did not increase the tensile strength but made it lower than the neat RC film. It was estimated that the voids of RC film were filled with calcium carbonate at low calcium carbonate loading, which may contribute to the tensile strength of films. However, with the further increase in calcium carbonate loading, the weak interactions between cellulose fibrils and calcium carbonate may undermine the tensile strength.
SEM Morphology of RC-CaCO3 Films
The morphological changes of surfaces and cross-sections of the films with different calcium carbonate loading were evident from SEM images in Fig. 2. The pure RC film exhibited a fibrous network structure with a smooth surface, as shown in Figs. 2a and 2a’. This porous RC network structure could provide large amounts of nucleation sites for the in-situ calcium carbonate formation process. The SEM images evidenced the presence of calcium carbonate crystals within both surface and cross-section of the films. The results indicated that the crystallization of CaCO3 occurred with cellulose surrounding. The small size of visible surface crystals of precipitated calcium carbonate appeared in the RC-C02 specimen, which was prepared with the lowest concentration for in situ precipitation. The precipitated calcium carbonate with the largest size was detected in the RC-C10 film. The SEM images of the cross-section of films showed that CaCO3 filled up the network structure.
Fig. 2. SEM images of the RC-CaCO3 films prepared by in situ precipitation with different concentrations of precursor solutions (a, a’) RC; (b, b’) RC-C02; (c, c’) RC-C05; (d, d’) RC-C08; (e, e’) RC-C10; (f) EDS spectrum of RC-C02
Calcium carbonate particles had a smaller size and a high presence within the network pores, indicating that precipitation occurred mainly within the RC network structures. Thanks to the dense in-situ precipitation of micro-nanosized CaCO3 particles, fibrous structures could no longer be observed. It was reported that the regenerated hydrogel experienced crystallization pressure during the CaCO3 crystal growth (Rauch et al. 2012). The crystallization pressures were relatively weak compared to the strong RC network structure at a low concentration of precursor solution. When the concentration of the precursor solution was increased, the crystallization pressures pushed the RC fiber away, as shown in Figs. 2b’ and 2c’. Layered structures without voids were observed in RC-C02 and RC-C05, which may be responsible for the high tensile strength. Further increase in the crystallization pressures resulted in the densely packed RC-C08 structure. The RC-C10 films presented large voids due to the crystallization pressure that pushed the RC network fibril away as discussed above. The collapse of the densely packed structure resulted in a decrease in tensile strength. Furthermore, as shown in Fig. 2g, energy dispersive X-ray spectroscopy (EDS) signals of C, O, and Ca elements confirmed the presence of CaCO3 in the films (Fu et al. 2014; Hafez et al. 2020).
XRD Analysis of RC-CaCO3 Films
The incorporation of CaCO3 in the film was further recognized by the XRD pattern measurement, which is shown in Fig. 3. As a comparison, the pattern of the precipitated calcium carbonate was also supplied in which characteristic peaks corresponded to the structure of calcite. The XRD peaks located at 2θ values of 23.1°, 29.6°, 36.4°, 39.8°, 43.6°, 47.8°, and 48.9° can be assigned to (102), (104), (110), (113), (202), (108), and (116) planes of calcite (Saraya and Rokbaa 2016). The diffraction pattern of the RC film showed three major characteristic peaks of cellulose II structure at 2θ = 12.0°, 20.0°, and 22.5°, which corresponded to the (), (110), and (020) planes, respectively (Qi et al. 2009; Li et al. 2015). The XRD pattern of the RC-C05 film was quite similar to the pattern of the RC film. The differences between them were evident in weakened peaks at 2θ = 20.0° and 22.5°, corresponding to the crystalline cellulose II (110) and (020) plane, and a new peak for (104) plane of calcite (Rauch et al. 2012). The less intense and broader peaks for cellulose II in the RC-CaCO3 composite indicated that the precipitated CaCO3 generated a further disordered arrangement of cellulose chains. Only the (104) plane of calcite was found in the XRD pattern. Other characteristic peaks of calcite were missing, which may be due to the low crystallinity of calcite in the composite (Niu et al. 2014; Xu et al. 2017). Overall, XRD results indicated the successful in-situ precipitation of calcite into the RC film.
Fig. 3. X-ray diffractograms of RC film, RC-C05 films, and CaCO3
To gain an insight into the effect of in-situ CaCO3 precipitation on the modulation of the RC film structure, the precipitated CaCO3 crystal structure, RC film, and RC-CaCO3 films were also characterized by FTIR and the spectra are shown in Fig. 4. From the FTIR spectrum of the precipitated CaCO3, it can be seen that CaCO3 had adsorption bands at 1459, 875, and 714 cm−1 corresponding to the stretching vibration of the O-C-O bond, the bending vibration in the O-C-O plane, and the out plane bending vibration, respectively. They all were characteristic peaks in the most stable polymorph of CaCO3 (calcite) (Nelson and Featherstone 1982; Swain et al. 2014).
FTIR Analysis of RC-CaCO3 Films
As shown from the spectrum of pure RC film, typical characteristic peaks of cellulose were observed at 1060, 1426, 2900, and 3403 cm−1 (Wang et al. 2015). The band at 1060 cm-1 corresponded to the stretching vibration peak of the ether bond COC. The presence of the band at 1426 cm-1 was due to the shear vibration of the -CH2 group. The band at 1659 cm-1 may attributed to the OH bending of absorbed water. The band at 2900 cm−1 was attributed to the stretching vibration of CH in the -CH2 group. A wide characteristic absorption peak appearing at a position near 3403 cm-1 was the stretching vibration peak of -OH. A moderate intensity band at 1459 cm−1, assigned to calcite, indicated the successful CaCO3 incorporation in the biocomposite network. Moreover, the absorption peak at 714 cm−1 also confirmed the presence of CaCO3 in the films. For RC-CaCO3 films, the -OH peaks near 3340 cm−1 broadened and moved to the low wave direction, which may indicate the formation of new hydrogen bonding between cellulose and calcium carbonate (Ma et al. 2016).
Fig. 4. FTIR of RC and RC-CaCO3 films
Thermal Analysis RC-CaCO3 Films
The thermal properties of RC-CaCO3 were examined by TGA. The TGA results of RC film, CaCO3, and RC-CaCO3 in the temperature range from 30 to 850 ℃ are compared in Fig. 5. Only one weight loss region in the range of 600 to 750 ℃ was observed during the decomposition of calcium carbonate. The measured value of weight left for CaCO3 after decomposition (56.5%) was almost the same as the theoretical value (56.1%). The TGA curve of the RC film exhibited a small weight loss in the region from room temperature to 150 ℃ due to the loss of moisture in the films. Following this, a two-stage degradation of RC film was located at roughly 250 ℃ to 380 ℃ and 400 ℃ to 700 ℃, respectively (Zhu et al. 2020c). The first steep decomposition region was attributed to the loss of the glycosidic bonds of cellulose, while the second stage, for the RC-CaCO3 film, it was related to further degradation of cellulose and CaCO3 in the films (Jia et al. 2012; Fu et al. 2014). The thermal decomposition curve of RC-CaCO3 displayed the same trend as that of pure RC films. However, some visible differences between the neat RC film and RC-CaCO3 films in the two major decomposition regions were detected. It was noteworthy that the decomposition temperature for the RC-CaCO3 film was slightly lower than that of pure RC film. The reason for TGA curve deviation may arise from the differences in the film structures and processing histories. Further studies are required to explain this early decomposition. A similar trend for the thermal degradation curve of the composite with low CaCO3 loading was also reported (Ummartyotin et al. 2016).
In the pure RC film, char residue was approximately 3.7% at 800 ℃. The RC-CaCO3 films presented residues of 4.6%, 8.8%, and 12.7% for RC-C05, RC-C08, and RC-C10, respectively, confirming the CaCO3 incorporation within the RC matrix. The CaCO3 content in the films could be estimated as the difference between the residue from pure RC film and RC-CaCO3 films (Mohammadkazemi et al. 2016). Taking this into account, the estimated CaCO3 contents for RC-C05, RC-C08, and RC-C10 films were 1.7%, 9.7%, and 17.0%, respectively.
Fig. 5. TGA curves of the CaCO3, RC film, and biocomposites
CONCLUSIONS
- Fully renewable RC-CaCO3 biocomposite films were successfully synthesized by in situ precipitation of CaCO3. The results indicated that CaCO3 was successfully introduced into the RC matrix. The XRD results indicated that the most stable CaCO3 phase, calcite, was formed during in situ precipitation.
- The properties of the biocomposite films changed as the calcium carbonate content varied. The biocomposites exhibited partial declines in transparency due to the light scattering of CaCO3 particles. The agglomerations of calcium carbonate on the surface of the films resulted in low transparency.
- As calcium carbonate content increased, the tensile strength was first increased and then decreased. The RC-C08 film achieved the highest tensile strength of 84.7 MPa. The condensed layer structure may explain the increase in the tensile strength. The RC-CaCO3 biocomposite films fabricated in this study can be regarded as a green biomaterial with large possibilities of being widely used in packaging and other applications.
ACKNOWLEDGMENTS
This work was supported by the National Key R&D Program of China (2018YFE0107100), the State Key Laboratory of Pulp and Paper Engineering (202003), South China University of Technology, the Key Laboratory of Biomass Energy and Material of Jiangsu Province (JSBEM202012), Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, and Jiangsu University self-made experimental equipment funding (5623080004).
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Article submitted: June 20, 2020; Peer review completed: August 15, 2020; Revised version received: August 17, 2020; Accepted: August 18, 2020; Published: August 31, 2020.
DOI: 10.15376/biores.15.4.7893-7905