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Preparation of Nanolignocellulose/Chitin Composites with Superior Mechanical Property and Thermal Stability

  • To resolve the issues of special processing equipment, cumbersome process flow and high cost of the composite material. The poplar wood fiber was used as the raw material, which were effectively crosslinked with chitin by the simple mechanical thermal rubber milling method, then the high performance nanolignocellulose/chitin composite were obtained by the binderless hot-press method. The nanostructure, chemical structure, surface composition, and thermal stability of nanolignocellulose/chitin composites were investigated by the scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric/differential thermogravimetric (TG-DTG), respectively. Results turned out that the nanolignocellulose was laminated by the grinding and the composite material appeared layered structure after the binderless hot-pressing. Chitin/chitosan from crab shell powder can be effectively crosslinked with nanofibrillarized lignocellulose to increase the contact area of surface hydroxyl groups. The static bending strength (MOR), modulus of elasticity (MOE) and internal bonding strength of the nanolignocellulose/chitin composite were 34.13 MPa, 7072 MPa and 0.97 MPa, respectively. Meanwhile, the swelling value of thickness after water absorption was only 9.27%, demonstrating the dimensional stability. According to the profile density distribution, the density of nano-lignocellulose/chitin composites was relatively uniform, which indicates that the preparation process is reasonable. The nanolignocellulose/chitin composite has excellent thermal stability, since the mass loss of pyrolysis process is lower than the untreated binderless fiberboard. In this study, a new and effective methods for preparing composite materials was proposed, which provides some research ideas and theoretical guidance for the efficient development of new nanolignocellulose composite and waste marine arthropod materials.
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Preparation of Nanolignocellulose/Chitin Composites with Superior Mechanical Property and Thermal Stability

    Corresponding author: Jian QIU, qiujianswfu@foxmail.com
  • College of Materials Science and Engineering, Southwest Forestry University, Kunming 650224, China

Abstract: To resolve the issues of special processing equipment, cumbersome process flow and high cost of the composite material. The poplar wood fiber was used as the raw material, which were effectively crosslinked with chitin by the simple mechanical thermal rubber milling method, then the high performance nanolignocellulose/chitin composite were obtained by the binderless hot-press method. The nanostructure, chemical structure, surface composition, and thermal stability of nanolignocellulose/chitin composites were investigated by the scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric/differential thermogravimetric (TG-DTG), respectively. Results turned out that the nanolignocellulose was laminated by the grinding and the composite material appeared layered structure after the binderless hot-pressing. Chitin/chitosan from crab shell powder can be effectively crosslinked with nanofibrillarized lignocellulose to increase the contact area of surface hydroxyl groups. The static bending strength (MOR), modulus of elasticity (MOE) and internal bonding strength of the nanolignocellulose/chitin composite were 34.13 MPa, 7072 MPa and 0.97 MPa, respectively. Meanwhile, the swelling value of thickness after water absorption was only 9.27%, demonstrating the dimensional stability. According to the profile density distribution, the density of nano-lignocellulose/chitin composites was relatively uniform, which indicates that the preparation process is reasonable. The nanolignocellulose/chitin composite has excellent thermal stability, since the mass loss of pyrolysis process is lower than the untreated binderless fiberboard. In this study, a new and effective methods for preparing composite materials was proposed, which provides some research ideas and theoretical guidance for the efficient development of new nanolignocellulose composite and waste marine arthropod materials.

1.   Introduction
  • Chitin is a non-toxic, tasteless, heat-resistant and corrosion- resistant natural amino acid polysaccharide polymer, mainly from shrimp, crab, insects and other marine arthropods, the second largest biological resource on earth after cellulose (Ifuku et al., 2011; Ifuku et al., 2012). It is widely used in the field of polymer, medicine, food additives and cosmetics by virtue of the advantages of high annual output, easy availability, low cost, renewable, degradable, recyclable and good biocompatibility (Fan et al., 2008; Pillai et al., 2009; Nogi et al., 2010; Ifuku et al., 2012; Lu et al., 2013; Ifuku, 2014; Zargar et al., 2015). Under the new normal economic situation, with the continuous development of nanotechnology in various fields, the integration of structure and function will be an important branch of global economic development and sustainable development, and also a research hotspot in the field of world science.

    Biomass nanocomposites are widely used and there is an inexhaustible high-performance nanolignocellulose in wood (O'brien et al., 1998; Lu et al., 2012). The biomass nanofibers not only show fine nanoscale, high strength, high specific surface area and high aspect ratio, but also have the advantages of renewable, recyclable and biodegradable (Aaltonen et al., 2009; Spinacé et al., 2009; Li et al., 2011; Lu et al., 2014; Zhu et al., 2015; João et al., 2016; Mussana et al., 2018). To date, the domestic and international researchers are still trying to prepare wood/carbon fiber and wood/metal composites (Ashori et al., 2009; Deepa et al., 2015; Gao et al., 2016; Dang et al., 2017; Chen et al., 2018b; Wang et al., 2018), ever increasingly development towards the direction of compounding, functionalization and environmental protection have been made to achieve high added value and special use of lignocellulose composites (Okuda et al., 2004; Quintana et al., 2009; Rybiński et al., 2018). For example, previous researchers developed a new functional integration material of biomimetic Pearl shellfish- structured lignocellulosic non-glued nanocellulose structure (Dang et al., 2018); lignocellulose-based CaCO3/polymethyl methacrylate particles/nanocellulose composite and biomimetic layered nanocellulose/graphene-oxide composite materials via mechanical-chemical method and hot pressing method (Dang et al., 2017; Chen et al., 2018a). In addition, layered densified nanolignocellulose/calcium hydrogen phosphate composites (Chen et al., 2019); nanolignocellulose/ polyvinyl alcohol/titanium dioxide composites (Chen et al., 2017), etc. were prepared by mechanical thermal rubber milling-water directional assembly and hot pressing. However, there still remains many problems in the preparation of existing composite materials, including harsh technical conditions, cumbersome preparation steps, low material strength, short service life and high cost, etc.

    In order to further study the lignin-based structure- function integrated composites, a simple, fast, non- polluting and low-cost method was developed to prepare lignin-based lignocellulose composites with excellent properties and wide applications for industrial use with high added value. The biocompatible lignocellulose and crab shell powder were mechanical-thermal rubber milling pretreated via the nano-dispersion technology, and then nanolignocellulose/chitin fiber composites with high mechanical properties were obtained by binderless hot- pressing. Results turned out that the nanolignocellulose/ chitin composite not only improves the mechanical strength of the product, but also has the functions of dimensional stability and thermal stability. The aim of this study is to create a variety of bio-matrix composites for wood-based panel industry in China, and provide scientific basis and technical guidance for new wood- based panel technology.

2.   Materials and Methods
  • Poplar lignocellulose (water content: 13%, length: 300–1000 μm, average diameter: 40 μm) was purchased from Zhejiang Ningbo World Group Co., Ltd. Crab shell powder was obtained from the collecting crab shell treated by KINKA portable multi-functional ultra-fine grinding machine.

  • Poplar fibers were dried and added into beaker mixed with crab shell powder with the ratio of 9:1. Deionized water was added to form a mixed float with concentration of 3%. After fully swelling for 1 h at 60℃, then the poplar fibers were put into JM-L80B colloid mill for 6 h. The mixed float was continuously transported to the grinding disc through a loop consisted by a peristaltic pump and plastic pipe. The speed of rubber mill was set as 2880 r/min and the distance between grinding discs was 0.15 mm. After grinding, the glue was dried to about 100% moisture content, and then placed in the die and hot pressing 20 min (hot pressing temperature 200℃, pressure 2.5 MPa, thickness 10 mm) to form blanks. The preparation process of nanolignocellulose/chitin composites by binderless hot pressing was shown in Fig. 1. The hot pressing process of untreated wood fiber hot pressing material, the preparation process of nano-fiber material by mechanical hot-rubbing and the preparation process of nanolignocellulose/chitin composite material were provided.

    Figure 1.  Schematic illustration of synthesis of nanolignocellulose/chitin composites

  • The micro-morphologies of untreated lignocellulose, thermomechanical mechanical grinding lignocellulose and nanolignocellulose/chitin composites were observed by scanning electron microscopy (SEM, Quanta 200, FEI). The main groups and elemental changes of the surface and chemical components of the composites were analyzed by Fourier transform infrared spectroscopy (FT-IR, Nicolet iN10 MX, USA) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, USA). The thermal stability of the samples were characterized by thermogravimetric/differential thermogravimetric (TG-DTG, SDT Q600, USA) in nitrogen environment, the air velocity was 50 mL/min, and the change of heating rate was 10℃/min from room temperature to 700℃. Profile density was measured by GreCon X-ray Profile Densitometer (DAX 6000, Germany). According to GB11718-2009, The mechanical properties were measured by Shenzhen New Sansi 50 kN microcomputer controlled electronic universal capability testing machine according to the standard stated in GB11718-2009.

3.   Results and Discussion
  • The micro morphologies of the untreated lignocellulose are presented in Fig. 2a, the untreated lignocellulose was slender and smooth, with a few bifurcations at the end, and the pits are clearly visible at high magnification (Fig. 2b). The micro-morphology of wood fibers obtained by binderless hot pressing is shown in Fig. 2c, the slender strips of fibers interwoven in disorder. Figure 2d shows the micro-morphology of the nanolignocellulose/chitin composites, from which obvious stratification phenomenon could be observed. The compressive force, shear force and frictional force produced by the mechanical and thermal rubber milling of untreated lignocellulose by colloid mill resulted in the laminar structure of wood fibers. At high magnification (Fig. 2e), chitin filament was effectively combined with nanolignocellulose (marked as chitin filament in the figure). In the illustration, there are fewer pits on the wood fiber surface, and the fiber surface is not so smooth. There is also a layer of chitin/ chitosan attached to the surface, where calcium carbonate particles (marked as calcium carbonate particles in the illustration) could also be observed. Which could be ascribed to the increase of specific surface area of cellulose in the process of mechanical hot rubber milling, from which more hydroxyl groups were exposed outside, and then hydroxymethylated with chitin silk/chitosan, then the molecular chains of chitin and chitosan in nanolignocellulose were linked together. The cross- section morphology of nanolignocellulose/chitin composites after binderless hot pressing, showing as a dense layered structure, was shown in Fig. 2f.

    Figure 2.  The SEM image of untreated lignocellulose (a–b), SEM image of the binderless hot-pressed untreated lignocellulose (NC) (c), SEM image of nanolignocellulose/chitin composites (NLCC) (d–e), SEM image of binderless hot-pressed nanolignocellulose (f)

  • The XRD spectrum of the untreated wood fiber samples are presented in Fig. 3a, there are no obvious characteristic peaks could be observed except those of 16° and 22° (the characteristic peaks of cellulose). These two peaks are characteristic peaks of the typical reflective surfaces (101) and (002) of wood cellulose. Figure 3b shows the XRD spectrum of the thermomechanical mechanical grinding lignocellulose, and there is no obvious change between the XRD spectrum of the thermomechanical mechanical grinding lignocellulose and that of the untreated lignocellulose. The XRD spectra of nanolignocellulose/chitin composites are shown in Fig. 5c, the characteristic peaks of chitin can be found at 19°, 21° and 23°, while the mineral calcium carbonate can be found at 30°, 36°, 39°, 43°, 47°, 48° and 57°. Compared with the XRD peaks of the standard untreated crab shell powder (JCPDS76-1652), except for the characteristic peaks of wood, the XRD peaks here are in consistent of the standard XRD peaks (Fig. 3d). The results show that the nano fibrillated composites have high phase purity, consisting only of the products of crab shell powder and nanolignocellulose as well as the products after hydroxymethylation.

    Figure 3.  The XRD patterns of untreated lignocellulose (NL) (a), thermomechanical of mechanical grinding lignocellulose (TMGL) (b), nanolignocellulose/chitin composites (NLCC) (c) and chitin(d) respectively

    Figure 5.  Survey XPS spectra of untreated lignocellulose (UL), thermomechanical of mechanical grinding lignocellulose (TMGL), and nanolignocellulose/chitin composites (NLCC) (a); C 1s (b); O 1s (c); Ca 2p (d); N 1s (e); and Si 2p (f)

  • The FT-IT spectra of the untreated lignocellulose, mechanical rubber grinding fiber, chitin and nanolig-nocellulose/chitin composites were carried out to reveal the chemical structure characteristics of the nanolig-nocellulose/chitin composites, as shown in Fig. 4. The FT-IR spectra of the mechanical grinding lignocellulose have little change compared with that of the untreated lignocellulose, but the absorption bands of nanolign-ocellulose/chitin composites at 1100–1230 cm–1, 1700– 1725 cm–1, 2500–2525 cm–1, 2850–2925 cm–1 and 3650– 3200 cm–1 have changed significantly. The FT-IT spectra of the nanolignocellulose/chitin composites were similar to those of the rubber milled crab shell powder, indicating that the chemical structure of crab shell powder after mechanical and thermal rubber milling is not destroyed in nanolignocellulose/chitin composites, and characteristics of natural crab shell powder maintained well. Comparing the FT-IT spectra of the four materials, the wide peak at 3440 cm–1 corresponds to the stretching vibration peak of —OH, which indicates that hydrogen bond appears in the rubber milling process; the stretching vibration peaks of C—H at 2908 cm–1 and —NH at 3255 cm–1 are characteristic peaks of cellulose; the bending vibration peaks at 1429 cm–1 are due to the bending vibration of CH2 in lignin and polysaccharide; and the peak at 1382 cm–1 is due to the bending vibration of C—H in cellulose and hemicellulose; C=C characteristic peak stretching vibration of aromatic ring at 1700 cm–1, indicating the existence of lignin; enhancement of absorption peak at 1230 cm–1 was due to phenolic substances formed during mechanical rubber milling of lignin; peak at 711cm–1, 870 cm–1, 1430 cm–1 was attributed to CaCO3 from crab shell powder, which proved that the powder contains carbonate. In addition, the vibration absorption peaks of amides at 1556 cm–1 and 3220 cm–1 are characteristic absorption peaks of chitin. The results showed that nanolignocellulose/ chitin composites retained the chemical components and the active hydroxyl groups of unspoiled crab shell powder.

    Figure 4.  The FT-IR patterns of untreated lignocellulose (NL) (a), thermomechanical of mechanical grinding lignocellulose (TMGL) (b), nanolignocellulose/chitin composites (NLCC) (c) and chitin (d)

  • The surface atomic composition and chemical bonds of the untreated lignocellulose, thermomechanical mechanical grinding lignocellulose and nanolignocellulose/chitin composites are shown in Fig. 5. The survey spectrum of untreated lignocellulose and nanolignocellulose/chitin composites scanned at low resolution are shown in Fig. 5a. Two strong peaks of C 1s and O 1s can be found in both the samples. The binding energy at 285.69 eV and 532.67 eV are corresponded to C and O elements in wood cellulose, respectively. At the same time, the C/O values of elements and the percentages of C in different chemical states are shown in Table 1. The C/O ratio of untreated lignocellulose is 2.12, while which increases to 3.15 in nanolignocellulose/chitin composites. Weak peaks of Si 2p, Ca 2p and N 1s were respectively observed at 103.26 eV, 348.56 eV and 400.68 eV in the survey spectrum of nanolignocellulose/chitin composites, indicating that the crab shell powder contains Si, Ca and S elements, with atomic contents of 0.78%, 4.26% and 2.47%, respectively. In Fig. 5b, the C1s spectra of the fibers are divided into three peaks, the binding energies are 277.98, 285.69 and 284.16 eV, respectively, which are corresponded to CC, C—O and C=O bonds. As shown in Fig. 5c, the O1s binding energies of the two samples are about 530.05 eV and 528.28 eV, which can be attributed to the absorbed water and OH groups. The peak position of O1s of nanolignocellulose/chitin composites shifts to low binding energy due to the Ca and Si elements in crab shells. The results show that the surface of nanolignocellulose adheres to crab shell powder, which is effectively combined with chitin/chitosan in crab shell powder. Figure 5d shows the high resolution XPS spectra of nanolignocellulose/chitin composites, two peaks at 348.56 eV were caused by Ca 2p3/2 and the peaks at 352.46 eV were cause d by Ca 2p1/2. Meanwhile, weak peaks of Si and N appeared at 117.98 eV and 409.98 eV, mainly from NH groups and silicon elements in crab shell powder, further confirmed that nanolignocellulose was effectively crosslinked with crab shell powder.

    Sample C (%) O (%) Ca (%) N (%) Si (%) C/O ratio
    Untreated lignocellulose 66.07 31.21 2.72 2.12
    Nanolignocellulose /chitin composites 67.51 21.43 6.15 2.37 2.55 3.15

    Table 1.  Element content and O/C ratios of untreated lignocellulose, nanolignocellulose/chitin composites

  • The profile density analysis patterns of the untreated lignocellulose, thermomechanical mechanical grinding lignocellulose, and nanolignocellulose/chitin composites are shown in Fig. 6. The peak density of the three samples all appeared on the two sides of the plate surface, and the valley value appeared in the middle and fluctuated to a certain extent. The peak value of the untreated lignocellulose fiberboard is on the side, the valley value is in the middle, and the densities ratio is 1.6:1, indicating that the middle part has larger porosity and uneven density distribution. However, the curve density distribution trend of thermomechanical of mechanical grinding lignocellulose and nanolignocellulose/chitin composite material is inconsistent with that of untreated lignocellulose. The fluctuation of valley value region is small, the curve is smooth, and the density difference is not significant, which directly affects the internal bonding strength of the board. The ratio of peak density to valley value of nanolignocellulose/chitin composite is about 1.1:1, which reflects that the actual density of the material is high and the density distribution is very uniform. This may be attributed to the lamination of wood fibers after rubber grinding, and the laminated structure formed by hot pressing become denser inside the board. The micro-fluctuation of density in the curve may be caused by the small voids left where nanofibers overlap with each other.

    Figure 6.  Profile density analysis patterns of untreated lignocellulose (UL), thermomechanical of mechanical grinding lignocellulose (TMGL), and nanolignocellulose/ chitin composites (NLCC)

  • Figure 7a shows the flexural strength and modulus of elasticity of untreated lignocellulose, thermomechanical of mechanical grinding lignocellulose, and nanolignocellulose/ chitin composites. The flexural strength and modulus of elasticity of the nanolignocellulose/chitin composites are 34.13 MPa and 7072 MPa, which are much higher than those of untreated lignocellulose and thermomechanical of mechanical grinding lignocellulose. This is due to the enhancement caused by chitosan and chitin filament from crab shell powder. Compared with untreated lignocellulose, static bending strength (MOR) and modulus of elasticity (MOE) values increased by 229% and 257%, respectively. The stress-displacement curves of three kinds of materials are shown in Fig. 7b, the maximum loads of three samples are 50.03 N, 217.44 N and 279.57 N, respectively. The flexural strength of nanolignocellulose/chitin composite is the highest. The mechanical properties of nano-lignocellulose/chitin composites are better than those of other biomass materials (Fig. 7c). Internal bonding strength value of nanolignocellulose/chitin composite was 0.974 MPa, which was 136% higher than that of untreated wood fiberboard, while water absorption thickness was only 9.27%, which was 52% lower than that of untreated wood fiberboard (Fig. 7d). Compared with other biomass materials of the same grade, the mechanical properties of the composites are superior to those of other materials (Laemsak et al., 2000; Altaner et al., 2014; Sitz et al., 2015; Govindan et al., 2017). According to GB/T11718-2009 standards, the physical and mechanical properties of thermomechanical of mechanical grinding lignocellulose, and nanolignocellulose/ chitin composites are far greater than those of the national standards. This is mainly due to the delamination and branching of lignocellulose during mechanical grinding process, which makes lignocellulose have more ester bonds and hydrogen bonds, enlarge the specific surface area, expose more hydroxyl groups, and increase the cross-linking degree between lignocellulose and chitin filament and chitosan from crab shell powder, thus enlarging the nanolignocellulose/chitin ratio. This corresponds to the profile density curve distribution and improves the physical and mechanical properties of the sheet. The results demonstrate that the mechanical properties of nanolignocellulose/chitin composites are superior to those of other biomass materials.

    Figure 7.  Mechanical strength histogram (a); stress-deformation curve (b); comparison of mechanical properties of of untreated lignocellulose (UL), thermomechanical of mechanical grinding lignocellulose (TMGL), nanolignocellulose/chitin composite (NLCC) and other materials (c, d)

  • The thermogravimetric and differential thermogravimetric curves of untreated lignocellulose, thermomechanical mechanical grinding lignocellulose, and nanolignocellulose/ chitin composites are shown in Fig. 8. According to thermogravimetric curves (Fig. 8a), the degradation of untreated lignocellulose and thermomechanical mechanical grinding lignocellulose can be divided into three stages: the first stage is in the temperature range of 20℃–121℃ and 20℃–129℃, the pyrolysis rate is relatively low, the mass loss is about 4.02% and 5.79% respectively, mainly ascribing to the evaporation of free water in the samples; the second stage start at the temperature range of 121℃–382℃, 129℃–387℃ and 147℃–309℃. The highest pyrolysis rates were found at 367℃ and 375℃, with mass loss of 80.05% and 77.53%, respectively; in the third stage, the loss rates were 10.57% and 8.02% from 382℃ to 700℃ and from 387℃ to 700℃, respectively. This is due to the gradual degradation and carbonization of all components of the samples. The thermogravimetric curves of nanolignocellulose/chitin composites are divided into four stages: the first stage is water evaporation (20℃–136℃), the loss rate is about 6.37%; at the second stage (136℃–292℃), hemicellulose begins to decompose, and the mass loss rate accelerates to 14.76%; the third stage starts at 192℃, the mass loss rate reached 45% and reached the maximum at 450℃, mainly due to cellulose decomposition; the final stage occurred at 510℃–700℃, and the mass loss rate reached 5.43%. Thermogravimetric analysis showed that the residual carbon of the three samples was 5.36%, 8.66% and 28.44%, respectively. In DTA curve (Fig. 8b), the maximum degradation rate of nanolignocellulose/chitin composites was lower than that of the untreated lignocellulose. The mass loss of the three samples in the whole thermogravimetric and differential thermogravimetric analysis process was 94.64%, 91.34% and 71.56%, respectively. This may be attributed to the carbonate which was introduced into crab shell powder to prevent decomposition after mechanical thermal rubber milling.

    Figure 8.  The TG-DTG curves of untreated lignocellulose (a), thermomechanical of mechanical grinding lignocellulose (b), and nanolignocellulose/chitin composites (c)

4.   Conclusions
  • Nanolignocellulose/chitin composites with high strength and thermal stability were successfully prepared by a simple and efficient mechanical thermal rubber milling hot pressing method. The SEM, FT-IR and XPS analysis showed that the nanolignocellulose/chitin composite exhibited a layered structure and the chitin filament was effectively crosslinked with the nanofibrous cellulose. The physical and mechanical properties test showed that the core layer of nano-lignocellulose/chitin composites had good compactness, little difference in density and uniform distribution, which effectively improved the flexural strength and elastic modulus of the composites. Compared with untreated lignocellulose, MOR, MOE, internal bonding strength values increased by 229%, 257%, 136%, water absorption thickness value decreased by 52%. Compared with other biomass materials of the same grade, the mechanical properties of nanolignocellulose/ chitin composites are superior to those of other materials. At the same time, the nanolignocellulose/chitin composites contain carbonate, which is the product of crab shell powder grinding. The superior thermal stability can expand its application field and service life. The preparation method of this study is simple, green and environmentally-friendly. With the development of compounding, wood-based panels can be applied to more fields, broaden the development space of wood-based panels industry, and it is expected that lignocellulose will become an advanced structural-functional integrated new material.

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