Abstract
The application of cellulose materials for packaging usage has attracted a large amount of attention in recent years. In this study, cellulose was dissolved in a cold NaOH/urea solution. Transparent, strong, and flexible regenerated cellulose/polyvinyl alcohol (PVOH) films were fabricated via a solution blending and casting process. The physicochemical properties of the neat RC films, PVOH films, and RC-PVOH blend films were characterized via scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, tensile tests, gas permeability, and ultraviolet-visible spectroscopy. The results of the physicochemical characterizations indicated that the RC-PVOH composite films with various PVOH contents yielded strong properties in terms of optical transparency, thermal stability, mechanical strength, and oxygen barrier performance. The RC-PVOH films with 8% PVOH loading exhibited a peak degradation temperature of 362 °C, a tensile strength of 80.8 MPa ± 0.3 MPa, and an oxygen permeability of 2.40 cm3 x μm/m2 x 24 h x kPa. The RC-PVOH composite films could be a competitive alternative as a packaging material to replace petroleum-based plastics.
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Preparation, Characterization, and Oxygen Barrier Properties of Regenerated Cellulose/Polyvinyl Alcohol Blend Films
Qianqian Zhu,a,b Jingjing Wang,a Jianzhong Sun,a,* and Qianqian Wang a,c,d,e,f,*
The application of cellulose materials for packaging usage has attracted a large amount of attention in recent years. In this study, cellulose was dissolved in a cold NaOH/urea solution. Transparent, strong, and flexible regenerated cellulose/polyvinyl alcohol (PVOH) films were fabricated via a solution blending and casting process. The physicochemical properties of the neat RC films, PVOH films, and RC-PVOH blend films were characterized via scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, tensile tests, gas permeability, and ultraviolet-visible spectroscopy. The results of the physicochemical characterizations indicated that the RC-PVOH composite films with various PVOH contents yielded strong properties in terms of optical transparency, thermal stability, mechanical strength, and oxygen barrier performance. The RC-PVOH films with 8% PVOH loading exhibited a peak degradation temperature of 362 °C, a tensile strength of 80.8 MPa ± 0.3 MPa, and an oxygen permeability of 2.40 cm3 x μm/m2 x 24 h x kPa. The RC-PVOH composite films could be a competitive alternative as a packaging material to replace petroleum-based plastics.
Keywords: Cellulose solution; Blend casting; Mechanical property; Oxygen permeability
Contact information: a: Biofuels Institute, School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013 China; b: Analysis and Testing Center, Jiangsu University, Zhenjiang, 212013 PR China; c: Hunan Province Key Laboratory of Engineering Rheology, Central South University of Forestry and Technology, Changsha 410004 China; d: Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry; Key Laboratory of Biomass Energy and Material, Jiangsu Province, Nanjing 210042, China; e: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640 China; f: State Key Laboratory of Bio-Fibers and Eco-Textiles, Qingdao University, Qingdao 266071 China;
* Corresponding authors: Email: jzsun1002@ujs.edu.cn (SJZ); qianqian.wz@gmail.com (WQQ)
INTRODUCTION
Packing materials can provide effective barriers to light and oxygen, as well as to other chemicals, biological, and mechanical damage (Hubbe et al. 2017; Wang et al. 2018a). Petroleum-based plastics and synthetic plastic materials are ideal packaging materials, as the primary desirable features of plastic materials are their low cost, lightweight nature, easy processability, high stability and flexibility, and excellent physicochemical properties. However, plastic packaging materials are mostly permeable to gases and vapors, and thus they fail to prevent oxidation damage, which limits their application when impermeable barrier protection is required. In addition, considering the massive amount of plastic pollution, there has been a major increase in the interest of biodegradable polymers, e.g., cellulose, starch, chitosan, and polylactic acid (PLA).
The development of environmentally friendly, renewable, and biodegradable cellulose-based packaging materials that also exhibit high transparency and excellent barrier properties has exponentially expanded, but there are still many challenges (Wang et al. 2018a). Cellulose-based packaging materials, e.g., regenerated cellulose films, surface-functionalized cellulose coatings, and cellulose nanocomposites, have been extensively investigated in order to improve the barrier strength and mechanical properties of paper and paperboard (Hubbe et al. 2017; Wang et al. 2018b; Zhu et al. 2020a,b). The application of thin coating layers of cellulose nanomaterials is a powerful tool for improving the mechanical and barrier properties of packaging materials. Spraying cellulose nanofibrils onto paper caused it to exhibit excellent oxygen barrier properties but increased its sensitivity to water vapor (Mirmehdi et al. 2018). As a low cost and non-toxic solvent, low-temperature NaOH/urea solution can dissolve cellulose with a high molecular weight (114,000 g/mole) rapidly (Wang et al. 2016). The cellulose solutions can be kept stable for one week. The low-temperature NaOH/urea solvent is recognized as a milestone in the history of cellulose processing technology. Biodegradable cellulose copolymer films were prepared by the NaOH/urea method followed by coating with chitosan and PVOH solution. The prepared biocomposites exhibited good physical properties as potential packaging materials. The composition-process-structure-property relationship of the resulting biocomposites was systematically investigated (Cazón, et al. 2018a,b, 2019). Optical transparent regenerated cellulose films that had a high oxygen permeability (0.003 cm3 x μm/m2 x 24 h x kPa to 0.03 cm3 x μm/m2 x 24 h x kPa) at 0% relative humidity (RH) were prepared from NaOH/urea solutions (Yang et al. 2011a). To improve the moisture barrier properties of cellulose films, multiple studies have incorporated other polymers or fillers into the films (Wu et al. 2012; Wang et al. 2018a). As such, the mechanical and water barrier properties of the altered cellulose films have been substantially improved without undermining their original valuable properties (Wang et al. 2018a).
As a promising packaging material, PVOH is water-soluble, has high film-forming ability, is biodegradable, and is biocompatible, but has a relatively low mechanical strength and high water uptake capacity (Dalei et al. 2019). Thus, PVOH packaging materials may be applicable for usage as packaging for hot food products that require high water vapor permeability (Jain et al. 2018). Cellulose/PVOH based biocomposites have recently been fabricated via different strategies (Abdulkhani et al. 2013; Cazón et al. 2018a,b 2019). Homogeneous dispersion of cellulose and plastic polymer is a huge challenge. Solution blending is widely used to achieve a good dispersion. To our best knowledge, there has been a lack of research concerning RC-PVOH composite films prepared by solution blending and casting. One such method, crosslinking, occurs when various crosslinking agents are introduced into the cellulose/PVOH composites.
Cellulose and PVOH, with their crystalline or semi-crystalline structures, could be interlinked via the hydrogen bonds between the hydroxyl groups. To take advantage of the properties of both cellulose and PVOH, various ratios of cellulose and PVOH solutions were blended together in aqueous form. The main goal of this study was to evaluate regenerated cellulose (RC) and PVOH composites films prepared via solution casting for their potential application as a packaging material. First, the cellulose was dissolved in a cold NaOH/urea (aq) solution, and the PVOH was dissolved in hot water. A series of RC-PVOH composite films were produced by changing the ratios of the blending solutions of cellulose and PVOH. Flexible, transparent, and oxygen impermeable cellulose-based composite films were fabricated and characterized in the present study.
EXPERIMENTAL
Materials and Chemicals
Cotton linters with a viscosity average molecular weight of 1.07 × 105 kg mol−1 were obtained from Hubei Chemical Fiber Group Co., Ltd. (Xiangfan, China) and milled in a laboratory grinder to pass through a 40-mesh sieve. The flour obtained was oven-dried and used in the subsequent experiments. The polyvinyl alcohol 124 (PVOH-124, DP 2400), sodium hydroxide, urea, and concentrated sulfuric acid were all purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received.
Preparation of the RC Films and RC-PVOH Blend Films
A 4% cellulose solution was prepared by dissolving 4 g of cellulose flour in 96 g of a precooled NaOH and urea solution (in a ratio of 7 parts NaOH to 12 parts urea to 81 parts deionized water, by wt%) at -12 °C. The suspension was vigorously mixed for 5 min until a transparent solution was obtained. The air bubbles and small undissolved cellulose aggregates were then removed via centrifugation (Avanti-JE, Beckman Coulter, Brea, CA) at 8000 rpm for 10 min. A 4% PVOH solution was prepared by dissolving 4 g of PVOH in 96 g of deionized water at 90 °C for 2 h. The air bubbles were removed with an ultrasonic bath. The RC-PVOH composite films were produced by blending the cellulose and PVOH solutions at different ratios (100:0, 98:2, 95:5, 92:8, 90:10, and 0:100, which were labeled as RC, RC-PVOH2, RC-PVOH5, RC-PVOH8, RC-PVOH10, and PVOH, respectively). The resulting transparent solutions were degassed and cast on a glass plate and then coagulated in 1 L 5% H2SO4 bath for 5 min at room temperature. The obtained hydrogel films were washed with deionized water until a neutral pH was reached, and then they were dried on a stainless steel plate at room temperature.
Characterization
The surface and cross-section morphologies of the RC-PVOH films were observed via scanning electron microscopy (S-3400N, Hitachi, Tokyo, Japan). The specimens were sputter-coated with a thin layer of gold and then loaded in the S-3400N SEM at a low acceleration voltage (1 kV) for observation and photographing. The transmittance of the film was acquired from 200 nm to 800 nm using a DU-800 UV-Vis spectrophotometer (Beckman Coulter, Brea, CA), in triplicate. The final optical transmittance of the films was normalized with the thickness of the RC and RC-PVOH films of 0.036 mm. Fourier-transform infrared spectroscopy (FTIR) experiments were performed with a Nicolet Nexus 470 FTIR spectrometer (Thermo Nicolet Corporation, Waltham, MA) in a region range of 4000 cm-1 to 500 cm-1 with a resolution of 4 cm-1. First, the composite films were ground into a fine powder in an agate mortar under a liquid nitrogen environment. The transparent pellets were then prepared by hydraulic pressing 2 mg of the sample mixture with 200 mg of KBr before FTIR viewing. Thermogravimetric analysis (TGA) of the films was performed with a Perkin Elmer TGA 4000 (PerkinElmer Inc., Waltham, MA) at a heating rate of 10 °C from 30 °C to 850 °C under nitrogen with a flow rate of 50 mL/min. Approximately 5 mg of the sample was used in each run. The X-ray diffraction (XRD) patterns of the composite films were recorded using a D8 Advance diffractometer (Bruker, Billerica, MA) from 2θ = 5° to 2θ = 35° at a scan speed of 2°/min at room temperature. The CuKα radiation source was operated at 30 kV and 15 mA. The tensile strength was determined via a tensile tester (Model YG026MB, Fangyuan Instrument Co. Ltd., Wenzhou, China) with a 1000 N load cell at a speed of 5 mm/min and at least 5 specimens were tested for each sample. The film oxygen permeability was measured using a VAC-V1 gas permeability tester (Labthink Instruments Co., Ltd., Boston, MA) according to standard GB/T1038 (2000). The oxygen permeability was determined at 23 °C, 0% RH, and with an area of 4.9 cm2 in triplicate. The oxygen permeability was normalized with the thickness of the film, and the corresponding oxygen permeability was reported in cm3 x μm/m2 x 24 h x kPa.
RESULTS AND DISCUSSION
The images of the surface morphology of the blended films with various PVOH content are shown in Fig. 1a to Fig. 1e. As shown in the SEM images with different magnifications, i.e., 370 x (Fig. 1e) to 3200 x (Fig. 1b to Fig. 1d), all the films exhibit relatively flat surfaces. Figure 1a’ to Fig. 1e’ show the cross-sections of the neat RC film and the RC-PVOH blend films. The microfibril structures of the RC films could be clearly seen in Fig. 1a’. Relatively smooth cross-sections with voids were observed in the RC-PVOH blend films, which was attributed to the incorporation of micro-scale PVOH fillers in the RC matrix. The SEM images of RC-PVOH2 and RC-PVOH5 reveal large cracks in the layered structures, whereas the image for the RC-PVOH8 film show relatively small and homogenously distributed voids. When the PVOH content increased to 10%, the layered structure with larger cracks was observed again in the RC-PVOH10 blend films. This phenomenon could be attributed to the phase separation of the PVOH in the RC matrix (Cano et al. 2015).
Fig. 1. SEM images of the surface and cross-sections of neat RC film and RC-PVOH blend films: (a, a’) RC; (b, b’) RC-PVOH2; (c, c’) RC-PVOH5; (d, d’) RC-PVOH8; and (e, e’) RC-PVOH10
The adhesion that occurred between the RC and PVOH components enhanced the mechanical strength of the blend films. In summary, the concentration of PVOH could be an important parameter that affects the physio-chemical and structural properties of the RC-PVOH blend films.
The FTIR spectra of the neat RC film, PVOH film, and RC-PVOH composite films with various PVOH ratios are shown in Fig. 2a. The broad bands near 3430 cm−1 were assigned to the O-H stretching, whereas the bonds at approximately 2900 cm−1 were assigned to the C-H stretching in both the RC and PVOH films. The O-H band tends to form hydrogen bonding, which is likely to affect the mechanical properties of the films (Cazón et al. 2019). The band at 2900 cm−1 reflects the change in the cellulose crystal structure (Abdulkhani et al. 2013). The absorption bands at 1720 cm−1 assigned to the C=O bending vibration in the PVOH film were absent in the RC-PVOH blend films, which was likely due to overlapping signals from the nearby 1630 cm−1 C=O stretching peak. Other characteristic bands were observed at 1165 cm−1, 1060 cm−1, and 899 cm−1. The band at 1165 cm−1 was assigned to the C-O-C vibration. The bands at 1060 cm−1 and 899 cm−1 were recognized as the C-O stretching and the C-H deformation, respectively (Wang et al. 2015).
As shown in Fig. 2a, the ratio of PVOH in the film had little effect on the overall infrared spectra of RC-PVOH composite films, since the FTIR spectra of the cellulose and PVOH composites had strong overlapping signals. Although the interfacial interactions between cellulose and PVOH are still not fully understood, the abundance of hydroxyl groups of RC and PVOH suggests a tendency to form intermolecular hydrogen bonding. FTIR results indicated that there are no other chemical interactions between the cellulose and PVOH molecules besides hydrogen bonding (Abdulkhani et al. 2013; Voronova et al. 2015; Salehpour et al. 2018).
Fig. 2. The FTIR and XRD spectra: (a) the FTIR spectra of the RC film, RC-PVOH composite films, and PVOH film; (b) the X-ray diffractograms of the RC-PVOH10 film compared to the cotton linters, neat RC, and PVOH films
Figure 2b shows the XRD patterns of the neat RC film, PVOH film, and RC-PVOH blend films. The cotton linters had typical cellulose I diffraction peaks at 2θ = 15.4°, 2θ = 17.1°, and 2θ = 23.2° for (), (110), and (200), respectively. Peaks at 2θ = 12.0°, 2θ = 20.0°, and 2θ = 22.6° were attributed to the (), (110), and (020) plane for cellulose II (Li et al. 2015; Qi et al. 2009). It was reported that cellulose I and II coexisted in the RC films regenerated from a NaOH/urea solution. It was noted that the () plane was stronger than the (110) and (200) peaks for cellulose II. This unusual phenomenon had previously been reported by Li et al. (2015) and Qi et al. (2009). The neat PVOH films exhibited two peaks at 2θ = 12.9° and 2θ = 19.3°.
It was reported that the increase in PVOH content would decrease the cellulose crystallinity significantly (Zhang et al. 2012). On the other hand, the increase in cellulose content could also undermine the crystallinity of PVOH as reported by Lam, et al. (2017). The RC-PVOH blend films displayed a mixed diffraction pattern of characteristics from cellulose I, cellulose II, and PVOH.