Preparation of High-Performance Polyethylene Composite Materials

Production of nanocomposites from macro-scale materials in situ in a melting compounder is highly desirable yet challenging. In this study, we develop...
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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*

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Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan S Supporting Information *

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 highdensity polyethylene (HDPE) composite materials from asreceived 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 the stabilization of the appropriate dispersion of the produced CNF in nonpolar 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. KEYWORDS: cellulose nanofiber, urea, polymer dispersant, nanofibrillation, fragmentation, dispersion

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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, © XXXX American Chemical Society

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. 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 eliminate 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 there are not too many reports in the literatures where a compounding process can achieve nanoscale morphology of composites from macroscale fillers, there are some prime examples for the preparation of polymer nanocomposite materials via melt compounding. One is that the macroscopic clay particle is broken down into nanclays in Received: October 30, 2018 Accepted: December 18, 2018 Published: December 18, 2018 A

DOI: 10.1021/acsapm.8b00071 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

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

an extruder,30 and the other is that unmodified, as-received graphite is compounded into graphene-like sheets via solidstate 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 high-performance 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 a 3−4 nm width.32,33 The microfibrils assemble further into a larger structural unit, called the fibril aggregate, on the order of 15 nm and above. Inside an aggregate, the microfibrils are bounded tightly via intermolecular hydrogen bonding of a hemicellulose such as glucomannan. Further assembly of the 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 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. Lowmolecular-weight molecules may be advantageous to penetrate the pores for assisting nanofibrillation. In this article, we demonstrate the successful nanofibrillation and dispersion of wood pulp fibers during the meltcompounding process with high-density polyethylene (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 cellulose36,37 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 (HNCO) to form a carbamate, which is also considered to contribute to the successful nanofibrillation.

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EXPERIMENTAL SECTION

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 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 μm) 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 FA512M, was kindly supplied by Hitachi Chemical Co., Ltd. (Tokyo, Japan). 2-(Dimethylamino)ethyl methacrylate (DMAEMA) (99%), 2,2′-azobis(2,4-dimethyl-valeronitrile) (V65) (95%), 2,2′-azobis(4methoxy-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 Wako Pure Chemical Corp. (Osaka, Japan). Tetra-nbutylammonium iodide (BNI) (98%), biuret (98%), 1-methylurea (97%), 1-ethylurea (95%), 1-allylurea (99%), 1-phenylurea (98%), 1,3-diethylurea (98%), 1,3-diallylurea (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. 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 B

DOI: 10.1021/acsapm.8b00071 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials Table 1. Tensile Properties of NBKP/HDPE Composites Prepared with Urea as a Plasticizera sample

urea loading (wt %)

kneading time (min)

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 4 4 4 4

60 60 60 60 60 60 30 90 120 180

degree of nanofibrillationb (%) 0 21 ± 35 ± 52 ± 56 ± 46 ± 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

vibration of carbonyl (CO) in the base of urethane.40 The carbonyl absorption gradually increased with an increase in the loading of urea. To elucidate the chemical structure deeply, solid-state 13C CP/MAS NMR spectroscopy was conducted (Figure 5b). All characteristic resonances of the anhydroglucose units were assigned, i.e., at 105.63 (C1), 89.15 and 83.37 (C4), 72−75 (C2, C3, and C5), and 65.23 ppm (C6), which are almost similar to those of the starting NBKP (cellulose I).41,42 The resonance of the residual PE was assigned at 32.81 ppm. The resonance at 158.83 ppm was assigned to the carbonyl carbon.40 In addition, the resonance of urea at 163.4 ppm40 was undetected. These results indicate that urea was converted to a carbamate. The mechanism of the carbamate formation is well interpreted on the basis of the industrial production of cellulose carbamate, known as an environmentally friendly cellulosic man-made fiber (CarbaCell).43−45 In this production, a large amount of urea at temperatures above the melting temperature of 132.7 °C decomposes into ammonia (NH3) and cyanic acid (HNCO).46,47

R‐NH‐CO‐NH 2 → NH3 + R‐NCO R‐NH‐CO‐NH 2 → R‐NH 2 + HNCO R‐NH‐CO‐NH‐R′ → R‐NH3 + R′‐NCO

NH 2CONH 2 → NH3 + HNCO

These isocyanate derivatives react with the surface hydroxyl groups on the cellulose fibers.

HNCO begins to react with the hydroxyl group of cellulose fibers to produce cellulose carbamate.40,43−45

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 these was not high enough to accelerate the nanofibrillation of pulp fibers as compared to urea. 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 resincompatible 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

Cellulose‐OH + HNCO → Cellulose‐OCONH 2

HNCO also reacts with intact urea, yielding biuret and cyclic ureide derivatives, such as cyanuric acid and ammelide (2amino-4,6-dihydroxy-1,3,5-triazine).46,47 Furthermore, autocondensation reactions associated with biuret decomposition yield urea and HNCO at higher temperature.47 According to the above-mentioned data and considerations, the role of urea can be explained as follows. First, urea decomposed into HNCO, which then reacted with cellulose to produce the carbamate. Subsequently, the reacted cellulose broke down into nanofibers. In this sense, the solid-state 13C CP/MAS NMR spectroscopy of the composite (Figure S3) exhibits a hardly detectable peak of free urea, indicating that most of the urea decomposed during the kneading process. This is beneficial because of less degradation of the mechanical properties by the remaining additive, which was almost consumed via a chemical reaction. Other Plasticizers. To obtain additional insight into the effect of the plasticizer, biuret, monosubstituted, and disubstituted ureides were used as a plasticizer. Figure S4 and Table S1 show the characteristic tensile stress−strain curves and the data of the mechanical properties, respectively. Except 1,3-diphenylurea, all of these urea derivatives played the role of a plasticizer of pulp fibers leading to nanofibrillation, similar to the case of urea. Since 1,3-diphenylurea has a high F

DOI: 10.1021/acsapm.8b00071 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials 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/PDCPMA-b-PMTAI was optimized to be 10:80:4:6 (Table 2, sample 14), corresponding to a 6.9-fold 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 ductile-brittle transition, as described in the Dependence of Urea Concentration section. 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. 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. Figure 7 summarizes EC/EM and σC/σM for urea-containing composite materials against the degree of nanofibrillation. The addition of urea increased the degree of nanofibrillation and both EC/EM and σC/σM (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 σC/ σM significantly. This is obviously due to the dispersion effect (arrow 2). The prolonged kneading time also increased the degree of nanofibrillation; however, EC /EM and σC /σM

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

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); and (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; and (3) fractionation of the produced CNFs by prolonged kneading. G

DOI: 10.1021/acsapm.8b00071 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

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; σC and σM, 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, highdensity polyethylene; 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(2-hydroxyethyl methacrylate); PDCPMA, poly(dicyclopentenyloxyethyl methacrylate); and PMTAI, poly[2-(methacryloyloxy)ethyltrimethylammonium iodide].

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 highperformance CNF-based composite materials.

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 CNF-reinforced composite materials with superior reinforcement efficiency. Comparison of the Mechanical Properties of CNFReinforced HDPE Composites. Figure 8 compares EC/EM and σC/σM 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 cellulose nanocrystal (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 a 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 all of the concentrations investigated in this study.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.8b00071.

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Polarized optical microscopic images, mechanical properties and stress−strain curves, solid-state 13C spectra, ATR FT-IR spectrum, chemical structure of PDCPMA-b-PMATI, and thermal properties of plasticizers (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Keita Sakakibara: 0000-0002-6013-695X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 17H06238). 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. Xray 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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CONCLUSIONS We developed a novel procedure for preparing CNF-reinforced HDPE composite materials from wood pulp fibers along with urea and a polymer dispersant via a simple melt-compounding 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 H

DOI: 10.1021/acsapm.8b00071 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

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