Preparation of High-Performance Polyethylene Composite Materials

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Preparation of High-Performance Polyethylene Composite Materials Reinforced with Cellulose Nanofiber: Simultaneous Nanofibrillation of Wood Pulp Fibers during MeltCompounding Using Urea and Diblock Copolymer Dispersant Keita Sakakibara, Yoshihito Moriki, and Yoshinobu Tsujii ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00071 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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ACS Applied Polymer Materials

Preparation of High-Performance Polyethylene Composite Materials Reinforced with Cellulose Nanofiber: Simultaneous Nanofibrillation of Wood Pulp Fibers during Melt-Compounding Using Urea and Diblock Copolymer Dispersant Keita Sakakibara,* Yoshihito Moriki and Yoshinobu Tsujii* Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. KEYWORDS. Cellulose nanofiber, Urea, Polymer dispersant, Nanofibrillation, Fragmentation, Dispersion

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ABSTRACT.

Production of nanocomposites from macro-scale materials in situ in a melting compounder is highly desirable yet challenging. In this study, we develop a highly efficient approach for the preparation of cellulose nanofiber (CNF)-reinforced high-density polyethylene (HDPE) composite materials from as-received wood pulp fibers, in which nanofibrillation-assisting plasticizers (urea and urea derivatives) and a diblock copolymer dispersant are used. The most effective plasticizer is urea, which plays a crucial role in producing CNF with less fragmentation during the kneading step owing to the plasticization/nanofibrillation and the reaction with hydroxyl groups of cellulose fibers into a carbamate. The diblock copolymer as a dispersant enables to stabilize appropriate dispersion of the produced CNF in non-polar HDPE. The resulting composites exhibit significantly improved mechanical properties, including a 6.9-fold increase in the Young’s modulus with 10 wt% loading of wood pulp fibers over that of neat HDPE.


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1. INTRODUCTION Recently, polymer composite materials have attracted considerable attention for use in lightweight structural components including automotive and construction.1-4 To increase their stiffness, rigid nanofibers such as carbon nanotubes are often incorporated into the polymer matrix. Among them, the recent discovery of cellulose nanofibers (CNFs),5-7 possessing low density, high aspect ratio, high Young’s modulus of around 140 GPa8-11 and strength of around 2 GPa,12 optical transparency,13 and low thermal expansion,14 have opened a new frontier as a reinforcement filler in the realization of a sustainable society.15-17 Several methods for the preparation of CNF composites have been developed, including solution casting and drying of dilute slurry of both polymer matrix and CNFs18 and resin impregnation to nonwoven CNF web/nanopaper.13, 19 Melt processing, especially extrusion, is a more practical and industrial method.16 A major hurdle in the uniform dispersion of CNFs in hydrophobic polymers has been overcome by various surface modification methods, including the chemical derivatization of hydroxyl groups on CNF surfaces by hydrophobic functionality20-24 and the physical adsorption of surfactants or polymer dispersants.25-28 However, these strategies often need multistep processes: production of CNF in aqueous systems through chemical/mechanical treatment of never-dried wood pulps, surface modification of CNF either by chemical or physical strategies, removal of water by drying, mixing with a thermoplastic polymer melt, and molding. Specifically, the production of CNFs and the removal of water from CNF slurry consume much energy and time. In addition, the latter step causes severe aggregation of CNFs because of the nature of high surface area, generating irreversible CNF aggregates. Therefore, a more reliable method for producing CNF-reinforced composites is needed.


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One approach is the production of nanocomposites from macro-scale materials in situ in a melting compounder. That is, it is significantly promising yet challenging that as-received wood pulp fibers are nanofibrillated into CNFs during the melt-compounding step by an extruder. This process can eliminates the production and drying of CNFs before extrusion, thus decreasing the total number of processing steps: just mixing cellulosic pulps with polymeric matrix and then meltcompounding. Although not too many reports in the literatures where a compounding process can achieve nano-scale morphology of composites from macroscale fillers, there are some prime examples for the preparation of polymer nanocomposite materials via melt compounding: one is that macroscopic clay particle is broken down into nanclays in an extruder,30 and the other is that unmodified, as-received graphite is compounded into graphene-like sheets via solid-state shear pulverization.31 Moreover, Yano et al. recently reported a pioneering method in CNF composites, named “pulp direct-kneading method,” where surface esterification of wood pulps successfully accelerates the nanofibrillation to yield dispersed CNFs in melt extrusion, providing highperformance CNF-reinforced polyolefin composite materials.29 Understanding and consideration of supramolecular structures in wood pulps provide an important basis for producing CNFs during melt processing. In wood cell walls, 30–40 cellulose molecules are arranged in parallel to form cellulose microfibrils with 3–4 nm width.32,

33

The

microfibrils assemble further into a larger structural unit, called fibril aggregate, on the order of 15 nm and above. Inside an aggregate, the microfibrils are bounded tightly via intermolecular hydrogen bonding of hemicellulose such as glucomannan. Further assembly of cellulose fibril aggregates, which are bound to each other in the matrix of lignin and hemicellulose, comprises a cell wall. In chemical pulping, most lignin and a part of hemicelluloses are removed from the cell wall, softening wood fibers suitably for paper making. This creates a pore space of below 2 nm in


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between fibril aggregates in the wood pulps, located in the cavities formed mainly by hemicelluloses and amorphous cellulose.34, 35 This (hemi)cellulosic pore space is essential for the successful disintegration of wood pulps into cellulose microfibrils and fibril aggregates, both of which are called CNFs. Hence, we postulate that a plasticizer for amorphous cellulose and hemicelluloses is effective for producing CNFs during melt processing, allowing the (hemi)celluloses to melt flow. Low-molecular-weight molecules may be advantageous to penetrate the pores for assisting nanofibrillation. In this paper, we demonstrate the successful nanofibrillation and dispersion of wood pulp fibers during the melt-compounding process with HDPE by using urea as the plasticizer and a block copolymer as the dispersant (Figure 1), affording CNF-reinforced HDPE composite materials with high dispersion, and thereby, high mechanical reinforcement. Urea has been extensively used as a key chemical for the dissolution of cellulose36,

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and plasticization of

hemicelluloses,38 suggesting that it can interact with amorphous cellulose and hemicellulose in the cell wall. A polymer dispersant is also used to inhibit the aggregation of CNFs produced in situ in a kneader through adsorption onto the CNF surface.27, 28 In this study, we demonstrate a significant improvement in the mechanical properties, with a 6.9-fold increase in the Young’s modulus with 10 wt% CNF loading over that of neat HDPE. We disclose the mechanism that the nanofibrillated CNF from the pulp fibers is subjected to react with urea-derived cyanic acid (HCNO) to form a carbamate, which is also considered to contribute to the successful nanofibrillation.


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HDPE

Wood pulp fiber

Plasticizer

In twin-screw extruder

Plasticization

Nanofibrillation

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Polymer dispersant

Composite

Dispersion

CNF

Figure 1. Schematic for demonstrating the simultaneous nanofibrillating and compounding process with the aid of plasticizer and polymer dispersant in a twin-screw extruder.

2. EXPERIMENTAL SECTION 2.1. Materials. Never-dried needle-leaf bleached kraft pulp (NBKP) (18 wt%) was supplied by Daio Paper Corp. (Tokyo, Japan). The pulp was mechanically refined using a Niagara Beater until the value of the Canadian Standard freeness was below 150 mL. Two samples of high-density polyethylene (HDPE) were used: a sample named HE-3040 purchased from Sumitomo Seika Chemicals Co., Ltd. (Osaka, Japan) in the form of fine particles (diameter ~11 m) with a density of 0.961 g cm-3, and another named Suntec HD J-320 purchased from Asahi Kasei Corp. (Tokyo, Japan) in the form of pellets with a density of 0.959 g cm-3. Dicyclopentenyloxyethyl methacrylate (DCPMA) (96%), named FA-512M, was kindly supplied by Hitachi Chemical Co., Ltd. (Tokyo,


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Japan). 2-(Dimethylamino)ethyl methacrylate (DMAEMA) (99%), 2,2ʹ-azobis(2,4-dimethylvaleronitrile) (V65) (95%), 2,2ʹ-azobis(4-methoxy-2,4-dimethyl-valeronitrile) (V70) (95%), iodine (98%), diethylene glycol dimethyl ether (diglyme) (97%), methyl iodide (CH3I) (99.5%), urea (99%), and 1,3-dimethylurea (98%) were purchased from FUJIFILM Wako Pure Chemical Corp. (Osaka, Japan). Tetra n-butylammonium iodide (BNI) (98%), biuret (98%), 1-methylurea (97%), 1-ethylurea (95%), 1-allylurea (99%), 1-phenylurea (98%), 1,3-diethylurea (98%), 1,3diallylurea (98%), and 1,3-diphenylurea were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). All other chemicals were obtained from commercial sources and used without further purification. 2.2. Synthesis of Polymer Dispersant. The synthesis of the polymer dispersant was achieved via a two-step reaction, i.e., the synthesis of PDCPMA-b-PDMAEMA, followed by the quaternization of the tertiary amine moieties in the DMAEMA residue. First, the synthesis of PDCPMA-b-PDMAEMA was conducted by successive addition of two monomers in reversible complexation-mediated living radical polymerization (RCMP).39 A mixture of DCPMA (300 g, 1.15 mol), I2 (4.36 g, 17.2 mmol), BNI (2.11 g, 5.73 mmol), V65 (2.84 g, 11.4 mmol), and V70 (9.23 g, 30.0 mmol) in diglyme (200 g) was heated at 60 °C under argon atmosphere with mechanical stirring for 3 h. The conversion for DCPMA was 78%, determined by 1H-NMR, and the number-average molecular weight (Mn) and the polydispersity index (PDI) were 5.2  103 g mol-1 and 1.13, respectively, determined by GPC with THF as an eluent. After 3 h, a mixture of DMAEMA (180 g, 1.15 mol), V70 (1.32 g, 4.3 mmol), and N-iodosuccinimide (0.16 g, 0.72 mmol) in diglyme (180 g) was added, and the solution was heated at 60 °C under argon atmosphere with mechanical stirring for 20 h. The conversion for DMAEMA was 98%, determined by 1H-NMR, and the Mn and PDI values for the block copolymer were 6.8  103 g mol-1 and 1.47, respectively,


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determined by GPC with THF as an eluent. After the polymerization, the solution was poured into the mixture of distilled water and methanol (4:1, v/v) and the precipitate was collected and redissolved in THF. The reprecipitation procedure was conducted repeatedly, and then, the product was dried under vacuum to yield PDCPMA-b-PDMAEMA (457.5 g). After the purification, the Mn and PDI values for the block copolymer were 7.9  103 g mol-1 and 1.47, respectively. Subsequently, the methylation of the tertiary amino group of PDMAEMA was conducted. PDCPMA-b-PDMAEMA (442 g, 1.21 mol per DMAEMA unit, calculated in terms of the theoretical molecular weight by conversion) was dissolved in the mixture of anhydrous acetone and THF (1.5 L) (1:1, v/v), followed by the addition of CH3I (173.3 g, 1.22 mol). The reaction was stirred at rt under argon atmosphere overnight. The precipitate was collected by centrifugation, washed with acetone twice, and dried under vacuum overnight to yield PDCPMA-b-PMTAI as a white solid (487.9 g), where PMTAI is poly[2-(methacryloyloxy)ethyltrimethylammonium iodide]. A successful quaternization was confirmed by 1H-NMR measurement. 2.3. Characterization of Polymer Dispersant. GPC analysis was carried out at 40 C on a Shodex GPC-104-series high-speed liquid chromatograph (Shoko Co., Ltd., Kanagawa, Japan), equipped with a differential refractometer (Shodex RI-74S), a guard column (Shodex GPC KF-G), and two serial columns (Shodex GPC KF-404HQ). A calibration curve was obtained with poly(methyl methacrylate) standards. THF was used as the eluent at a flow rate of 0.3 mL min-1. 1H

NMR spectra were recorded on a JNM-ECA600 (JEOL, Tokyo, Japan) spectrometer in CDCl3

with tetramethylsilane as an internal standard. 2.4. Compounding and Injection Molding. Never-dried refined NBKP (56.5 g; 10 g absolute dry mass) was suspended in water in a concentration of about 3 wt%. HDPE (HE3040) was mixed into the pulp slurry thoroughly for 15 min with the predetermined weight ratio. In the case where


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the effect of the polymer dispersant was investigated, PDCPMA-b-PMTAI dissolved in the mixture of 2-propanol and water (1:2, v/v) at a concentration of 20 wt% was added into the pulp slurry. The mixture was put onto a silicone tray and dried in an air-forced oven (ST-110, ESPEC Corp., Osaka, Japan) at 105 °C overnight to yield the dried sample, called premix. The typical concentration of NBKP in the premix was 30 wt%. The premix was diluted with HDPE pellet (J320) and mixed with urea or urea derivatives as a plasticizer with a predetermined weight ratio of NBKP/HDPE/plasticizer/dispersant (if any). The mixture obtained was kneaded by a twinscrew miniextruder (Xplore MC15, 15 mL total volume, Xplore Instruments BV, the Netherlands). The kneading was performed at 140 C for the predetermined time (mostly 60 min) at a screw speed at 200 rpm. An inert nitrogen was flowed during the kneading in order to protect the polymer from degradation. The molten mixture was extruded directly into a micro injection molder (Xplore IM12, 12 mL total volume, Xplore Instruments BV) and injected into the mold (1200202, ISO 527-2/1BA standard, Xplore), yielding dumbbell-shaped test pieces with dimensions of 40 mm (length)  5 mm (width, narrow)  2 mm (thickness). The injection and mold die temperatures were 150 and 50 °C, respectively, and the injection pressure was 100 bar for 5 s, which then increased to 13 bar for 32 s. 2.5. Mechanical Properties. The tensile strength and modulus of the samples were measured using universal mechanical testing equipment (model 3365; Instron Corp., MA, US). The load cell and crosshead speed were 1 kN and 10 mm min-1, respectively. The tensile deformation was monitored by a CCD camera. The average values of Young’s modulus, tensile strength, elongation at break, and work of fracture values were calculated with the standard deviation taken as the error. The work of fracture corresponds to the area below the stress-strain curve. Before mechanical testing, the samples were dried at rt overnight.


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2.6. Morphological Characterization. Polarized optical microscopy (POM) images were recorded on a polarized optical microscope BX60 (Olympus Corp., Tokyo, Japan). For POM, the dumbbell-shaped specimens were cut using a microtome (REM-710, Yamato Kohki Co., Saitama, Japan) with a thickness of approximately 20 m along the machine direction. The slice was placed between two quartz plates and heated by a Linkam 10002 hot stage (Linkam Scientific Instruments, Ltd., UK) for POM under melt state at 140 °C. For the count of individual un-nanofibrillated pulp fibers, each color image (1296  923 pixels) was converted to grayscale and the area of the fibers (A) was calculated using Image-J software (https://imagej.nih.gov). In this paper, the degree of nanofibrillation of pulp fibers (%) is defined as 100(1 - A1/A0), where A1/A0 is the ratio of the area of the un-nanofibrillated fibers in a composite against the area of the NBKP/HDPE composite (10/90, w/w; Table 1, sample 1) as reference. The field-emission scanning electron microscopy (FE-SEM) observations were carried out using a JEOL JSM-6700F operated at 1.5 or 3.0 kV. The dumbbell-shaped test samples were subjected to extraction in boiling xylene at 155 °C over 3 h, washed with toluene successively, and dried under vacuum at 100 °C overnight, resulting in the residual cellulosic sheets. Prior to SEM observations, the samples were sputter-coated with a thin layer of platinum to enhance their conductivity (Hitachi Ion Sputter E-1010). X-ray microcomputed tomography (micro-CT) scanning was performed using skyscan 1172 (Bruker-microct, Belgium) in order to observe the three-dimensional morphology of the samples with the following scanning conditions: a current of 100 A, voltage of 59 kV, pixel size of 0.79 m, 360 rotation, and 0.100 step. The dumbbell-shaped sample was cut into a piece of dimensions 10 mm  10 mm  2 mm.

2.7. Spectroscopic Characterization. Attenuated total reflection-infrared (ATR-IR) spectra were recorded on a Nicolet 670SX FT-IR spectrophotometer (Thermo Fisher Scientific K. K.,


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Tokyo, Japan) equipped with an MCT detector and a Specac Golden Gate single-reflection ATR accessory (germanium crystal with incident angle of 45°). All data were collected at a spectral resolution of 4 cm-1. 13C Solid-state dipolar decoupling/magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) measurements were performed with an NMR spectrometer Advance III 800 (Bruker Corp.) under a static magnetic field of 18.8 T.

3. Results and Discussion.　 3.1. Preparation of Composite Materials using Plasticizer. The composite materials from the tertiary mixture of NBKP, HDPE, and a plasticizer were prepared. The key step is the kneading process, in which the following three important events should be considered: i) nanofibrillation of wood pulp fibers into CNF, ii) dispersion of the produced CNF in HDPE, and iii) fragmentation of CNF into small pieces. Successful nanofibrillation and dispersion while avoiding fragmentation are needed for obtaining a high mechanical reinforcement. Under a fundamental tradeoff between the nanofibrillation and fragmentation, the plasticization played a central role in making the composites tough, so that the concentration of urea, kneading time, and types of plasticizers were optimized from the viewpoints of macroscopic and microscopic structures and mechanical properties. Finally, the role of the plasticizer was discussed. 3.1.1. Dependence of Urea Concentration. Figure 2a–c shows POM images of the melt-state composite materials of NBKP/HDPE/urea with different weight percentages of urea. This observation has an advantage to see individual un-nanofibrillated pulp fibers on micrometer scale as bright objects owing to their optical anisotropy derived from the high crystallinity, whereas the semi-crystalline HDPE becomes dark in a molten state in the images. Figure 2a shows an NBKP/HDPE composite without the plasticizer, indicating many individual un-nanofibrillated


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fibers. On the contrary, the addition of urea effectively decreased the amount of un-nanofibrillated or original pulp fibers (Figure 2b, 2c). The degree of nanofibrillation (see the definition in the Experimental Section 2.6) thus increased (Table 1, samples 2–6) from the reference (sample 1). Figure 2d and 2e shows the reconstructed 3D images of the X-ray micro-CT observation of NBKP/HDPE/urea with the weight ratio of 10/90/0 and 10/86/4 (Table 1, samples 1 and 5), respectively, over a sample volume cross-section of approximately 600  600  150 m3 (length  width  height). The blue areas represent the resin matrix, while the white ones indicate cellulose because of its higher density. At the 0.7 m resolution used, this observation mainly shows the unnanofibrillated fibers and the aggregation of CNFs, whereas the dispersed CNFS are invisible. This observation also indicates that fewer un-nanofibrillated fibers appeared in the composites with urea.

Figure 2. (a–c) Polarized optical microscope images and (d,e) reconstructed 3D X-ray CT images of NBKP/HDPE/urea with weight ratio of (a, d) 90:10:0, (b) 88:10:2, and (c, e) 86:10:4.


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Table 1. Tensile properties of NBKP/HDPE composites prepared with urea as a plasticizer.a

Sample

Urea Loading (wt%)

Kneading Time (min)

Degree of nanofibrillationb (%)

Young's modulus (GPa)

Tensile strength (MPa)

Elongation at break (%)

Work of Fracture (MJ m-3)

1

-

60

0

1.13 ± 0.04

27 ± 0.13

14 ± 0.94

3.3 ± 0.30

2

1

60

21 ± 1

1.19 ± 0.02

31 ± 0.12

15 ± 1.4

4.2 ± 0.38

3

2

60

35 ± 14

1.84 ± 0.01

37 ± 0.09

8.8 ± 2.4

3.2 ± 0.31

4

3

60

52 ± 8

3.11 ± 0.10

46 ± 0.46

3.2 ± 0.78

1.4 ± 0.09

5

4

60

56 ± 6

3.76 ± 0.05

51 ± 0.22

2.5 ± 0.13

0.86 ± 0.05

6

5

60

46 ± 23

3.97 ± 0.14

50 ± 0.69

2.4 ± 0.12

0.82 ± 0.05

7

4

30

biuret (E = 2.62 GPa) ~ monosubstituted urea (E = 1.52–2.62 GPa) > disubstituted urea (E = 1.41–1.82 GPa). This suggests that a free amino group is desirable in the plasticizer structure. Analogously to urea, the substituted urea derivatives are considered to proceed thermal decomposition in a similar way, giving corresponding organic amines and isocyanate derivatives.48 R-NH-CO-NH2  NH3 + R-NCO R-NH-CO-NH2  R-NH2 + HNCO R-NH-CO-NH-R’  R-NH3 + R’-NCO These isocyanate derivatives react with the surface hydroxyl groups on the cellulose fibers. Cellulose-OH + R-NCO  Cellulose-OCONH-R The presence of the substituted carbamate was suggested by the ATR FT-IR spectrum of the isolated cellulose fibers from NBKP/HDPE/1-phenylurea composites (Figure S5), exhibiting the stretching vibrations of both carbonyl (C=O) and alkene (C=C) in the base of phenyl carbamate at 1714 and 1602 cm-1, respectively. Nevertheless, the Young’s modulus and tensile strength of the composites prepared using biuret, mono and disubstituted urea derivatives did not exceed those of the composites prepared using urea, suggesting that the plasticization as well as the introduction of carbamate moieties for theses was not high enough to accelerate the nanofibrillation of pulp fibers as compared to urea.


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3.2. Effects of Polymer Dispersant. In our previous study, dispersion of CNF in HDPE was successfully achieved using the adsorption of a block copolymer (BCP)-type dispersant.27, 28 The BCP was designed to comprise both resin-compatible and cellulose-interactive blocks, called dispersion and immobilization blocks, respectively, in order to suppress the aggregation of CNF during melt compounding and to provide sufficient interfacial strength. The strongly and stably adsorbed BCP inhibits the aggregation of CNF due to the suppression of CNF-CNF hydrogen bonds through the overlap of polymer chains. The adsorption mechanism is based on the interaction-multiplier effect, where weak segment-level interaction is multiplied by the large number of segments in BCP to generate a large intermolecular effect. In this study, PDCPMA was selected as the dispersion block, enabling a uniform dispersion of CNF in HDPE as well as significantly improving the mechanical properties.28 For the immobilization block, PMTAI was chosen for the first time. PMTAI is a polyelectrolyte, and thus, polar, showing a stronger attraction to the polar CNF surface than the nonpolar PDCPMA block. In addition, BCP is soluble in aqueous solvent. Both blocks have been successfully synthesized via the living radical polymerization technique.39 For preparing composite materials, a predetermined amount of the dispersant PDCPMA-b-PMTAI (the chemical structure, see Figure S6) was added during the premix preparation. Figure S7 and Table 2 show the effect of the polymer dispersant on the mechanical properties. The ternary composites of NBKP/HDPE/PDCPMA-b-PMTAI (10/80/10, w/w/w; sample 11) exhibited less mechanical reinforcement. On the contrary, the quaternary composites of NBKP/HDPE/urea/PDCPMA-b-PMTAI demonstrated an extremely high Young’s modulus of above 5 GPa and tensile strength of around 55 MPa. The highest Young’s modulus and tensile strength were 5.2 GPa and 58 MPa, respectively, when the weight ratio of NBKP/HDPE/urea


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/PDCPMA-b-PMTAI was optimized to be 10:80:4:6 (Table 2, sample 14), corresponding to a 6.9fold higher Young’s modulus and 2.8-fold higher tensile strength than the neat HDPE. Note that the degree of nanofibrillation increased through the addition of the dispersant, correlating the increment in the mechanical properties, especially Young’s modulus (Figure S7b). Figure S8 and Table S3 show the effect of the weight percentage of NBKP in the quaternary systems, where the weight ratio of NBKP, urea, and PDCPMA-b-PMTAI was maintained constant to the optimized 10:4:6. As the loading of NBKP increased, the Young’s modulus and tensile strength increased linearly, whereas the elongation at break significantly decreased and stayed constant at a strain of approximately 2%. The small work of fracture is again related to a ductilebrittle transition, as described in the section 3.1.1. Since the interfacial strength between CNF and HDPE was further improved by the addition of the polymer dispersant,28 it is assumed that the mobility of the produced CNF could be restricted and thus the composite became more brittle.


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Table 2. Effect of BCP PDCPMA-b-PMTAI as a polymer dispersant on the NBKP/HDPE composite materials prepared with urea.a

Sample

Urea loading (wt%)

BCP loading (wt%)

Degree of nanofibrillation b (%)

Young's modulus (GPa)

Tensile strength (MPa)

Elongation at break (%)

Work of Fracture (MJ m-3)

11

0

10

12 ± 21

1.96 ± 0.05

26 ± 0.12

4.7 ± 0.20

1.0 ± 0.05

12

4

2

71 ± 5

4.71 ± 0.33

55 ± 1.3

1.7 ± 0.16

0.56 ± 0.07

13

4

4

72 ± 7

5.01 ± 0.06

56 ± 0.24

1.7 ± 0.08

0.57 ± 0.04

14

4

6

78 ± 7

5.20 ± 0.14

58 ± 1.1

1.7 ± 0.05

0.60 ± 0.03

15

4

8

73 ± 15

5.00 ± 0.19

56 ± 1.2

1.7 ± 0.04

0.56 ± 0.01

16

4

10

76 ± 1

5.15 ± 0.04

56 ± 1.2

1.7 ± 0.03

0.56 ± 0.003

aWeight

percentage of NBKP: 10 %. bSee Experimental section 2.6.

The fibers isolated from the composite sample 14 by xylene extraction have a diameter of around 200–500 nm, which indicates CNF production (Figure 6). The solid-state

13C

CP/MAS NMR

spectroscopy exhibited additional peaks at 177.64 and 132.88 ppm, corresponding to the carbonyl and dicyclopentenyl sp2 carbon, respectively (Figure S9). In addition, there were many peaks around 40–60 ppm, corresponding to the sp3 carbon derived from the polymethacrylate. This clearly indicates that the dispersant was tightly adsorbed onto the produced CNFs.


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a

10 m

b 490 nm

200 nm 200 nm

1 m

Figure 6. FE-SEM images of xylene-etched NBKP/HDPE/urea/PDCPMA-bPMTAI (10/80/4/6, w/w/w/w; Table 2, sample 14).

Figure 7 summarizes EC/EM and C/M for urea-containing composite materials against the degree of nanofibrillation. The addition of urea increased the degree of nanofibrillation and both EC/EM and C/M (green circles). This indicates that urea played a role in the nanofibrillation of pulp fibers (arrow 1) owing to the effect of plasticization through carbamation. Further addition of BCP increased the degree of nanofibrillation (red triangles), and thus, increased EC/EM and C/M significantly. This is obviously due to the dispersion effect (arrow 2). The prolonged kneading


24

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time also increased the degree of nanofibrillation; however, EC/EM and C/M decreased. This can be considered the fragmentation of the produced CNFs (arrow 3). These findings re-emphasize that both urea and BCP are necessary for the melt-compounding process to provide CNFreinforced composite materials with superior reinforcement efficiency.

a 7

2

EC/EM

6 5 4

3

1

3 2 1 0

20

40

60

80

100

Degree of nanofibrillation (%)

b 3

2

C/M

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

2

1

1

0

20

40

60

80

100

Degree of nanofibrillation (%)

Figure 7. Summary of (a) relative Young’s modulus and (b) tensile strength of the composite materials against degree of nanofibrillation under three compounding conditions: green circles: addition of urea concentration (Table 1, samples 1-5); blue squares: kneading time (Table 1, samples 8-10); red triangles: addition of BCP (Table 2, samples 12-16). The black arrows indicate the key phenomena during melt compounding: 1: nanofibrillation of pulp fibers plasticized by urea; 2: dispersion of the produced CNFs using BCP; 3: fractionation of the produced CNFs by prolonged kneading.


25


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3.3. Comparison of the Mechanical Properties of CNF-Reinforced HDPE Composites. Figure 8 compares EC/EM and C/M for the NBKP/HDPE/urea/BCP systems and other types of cellulosic filler-reinforced polyethylene reported in the literature,20-22, 25, 27-29, 49-57 where CNC,22, 49-53

microfibrillated cellulose (MFC),21, 55, 56 or CNF20, 23, 25, 27-29, 57 were used as a filler, being

chemically modified on the surface20-23,

29, 52, 55

or surface-activated by additives or

compatibilizers,25, 27, 28, 53, 56, 57 and prepared through melt-compounding processes such as screw extrusion20, 23, 27-29, 52, 53, 56 or templating sol-gel approach.49 The Young’s moduli of the composites of NBKP/HDPE/urea/PDCPMA-b-PMTAI systems with a cellulose content of above 5% were the highest. Below 5%, these values were comparative to those found by Sapkota et al., where an ideal CNC network in LDPE was achieved through the templating sol-gel approach.49 Their approach has led to the ideal nanocomposite structures because of the well-defined percolating network. In addition, the tensile strength values for NBKP/HDPE/urea/PDCPMA-b-PMTAI systems were higher than those found by others, except Sapkota et al.49 Most importantly, our composites exhibited linear dependence of the mechanical properties on the filler content. This indicates that CNF dispersion was maintained at the concentrations investigated in this study.


26

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a

b 7

3.5

6

3.0

5

2.5

 C / M

EC/EM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 3

Tunicate CNC/LDPE (ref. 49) MCC CNC/LDPE (ref. 50) Wood CNC/LDPE (ref. 51) UPy-modified cotton CNC/LDPE (ref. 22) UPy-modified cotton CNC/LLDPE (ref. 22) Stearoyl ramie CNC/LDPE (ref. 52) Curaua CNC/bio-HDPE/castor oil (ref. 53) Corona-treated cellulose fiber/LLDPE (ref. 54) Allyl MFC (DS=0.29)/LDPE (ref. 55) ASA-modified MFC (DS=0.18)/LDPE (ref. 21) MFC/HDPE/MAPP/CPPA (ref. 56) ASA-modified CNF (DS=0.44)/HDPE/MAPP (ref. 20) Pivalated CNF (DS=0.40)/HDPE (ref. 23) ASA-modified NBKP (DS=0.43)/HDPE/MAPP/CaCO3 (ref. 29) Soybean CNF/LDPE/ethylene-acrylic acid dispersant (ref. 57) CNF/LLDPE/PE-b-PEO (ref. 25) CNF/HDPE/PLMA-b-PHEMA (ref. 27) CNF/HDPE/PDCPMA-b-PHEMA (ref. 28) NBKP/HDPE/urea/PDCPMA-b-PMATI (this study)

2.0 1.5

2

1.0

1 0

10

20

30

0.5

Filler content (%)

0

10

20

30

Filler content (%)

Figure 8. Comparison of (a) relative Young’s modulus and (b) tensile strength of cellulosic (nano)filler-reinforced polyethylene composite materials against filler content; EC and EM: Young’s modulus for composite and matrix, respectively; C and M: tensile strength (or yield stress) for composite and matrix; CNC: cellulose nanocrystal; LDPE: low-density polyethylene; MCC: microcrystalline cellulose; UPy: ureidopyrimidinone; LLDPE: linear low-density polyethylene; MFC: microfibrillated cellulose; DS: degree of substitution; ASA: alkenyl succinic anhydride; HDPE: high-density polyethyelen; MAPP: maleic anhydride grafted polypropylene; CPPA: cationic polymer using primary amine; NBKP: needle-leaf bleached kraft pulp; PEO: poly(ethylene oxide); PLMA: poly(lauryl methacrylate):

PHEMA:

poly(dicyclopentenyloxyethyl

poly(2-hydroxyethyl methacrylate);

methacrylate); PMTAI:

PDCPMA: poly[2-

(methacryloyloxy)ethyltrimethylammonium iodide].

Conclusions We developed a novel procedure for preparing CNF-reinforced HDPE composite materials from wood pulp fibers, along with urea and polymer dispersants, via a simple melt-compounding


27


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process. As a low-molecular-weight plasticizer, urea effectively plasticized (hemi)celluloses of cellulose microfibrils, and hence, yielded well-fibrillated pulp fibers with less fragmentation. The carbamation contributed to the nanofibrillation as well as less degradation of the mechanical properties through the remaining additive. Moreover, the polymer dispersant reduced the reaggregation of the produced CNFs with a minimal amount. In this system, three important steps, plasticization/nanofibrillation, carbamation, and dispersion, were achieved in a stepwise manner, avoiding unfavorable fragmentation. Our success will provide a new direction for the production of high performance CNF-based composite materials.


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ASSOCIATED CONTENT Supporting Information. Polarized optical microscopic images; Mechanical properties and stress–strain curves; solid-state 13C

spectra; ATR FT-IR spectrum; Chemical structure of PDCPMA-b-PMATI; Thermal

properties of plasticizers.


29


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AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed. E-mail: [email protected] (K.S.); [email protected] (Y.T.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was supported by grants from the project of National Agriculture and Food Research Organization (NARO) Bio-oriented Technology Research Advancement Institution (integration research for agriculture and interdisciplinary field) and JSPS KAKENHI (Grant Number 17H06238). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank DIC Corporation and Daio Paper Corporation for their support and valuable discussion regarding this work. We also thank the technical support provided by Ms. Y. Nakajima. X-ray CT observation and tensile tests were performed with help from Prof. H. Yano (RISH, Kyoto Univ.). Solid-state 13C CP/MAS NMR spectra were acquired with the NMR spectrometer in the Joint Usage/Research Center (JURC) at the Institute for Chemical Research, Kyoto University.


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Table of Contents Graphic

Wood pulp fiber

PE

Plasticizer Polymer (urea) dispersant

In twin-screw extruder

Plasticization

Nanofibrillation

Dispersion

HDPE

CNF


39

Preparation of High-Performance Polyethylene Composite Materials

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