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.
