Homogeneous Dispersion of Cellulose Nanofibers in Waterborne

Research Article pubs.acs.org/journal/ascecg

Homogeneous Dispersion of Cellulose Nanofibers in Waterborne Acrylic Coatings with Improved Properties and Unreduced Transparency Yao Tan, Yongzhuang Liu, Wenshuai Chen, Yixing Liu, Qingwen Wang, Jian Li, and Haipeng Yu* Key laboratory of Bio-based Material Science and Technology, Ministry of Education, Northeast Forestry University, Harbin 150040, China ABSTRACT: Cellulose nanocrystal has been widely used as a reinforcement filler in waterborne coatings, but the application of cellulose nanofiber (CNF) as a filler is difficult because of inhomogeneous dispersion. Herein, a facile and effective strategy to improve the dispersion of CNFs in the polymer matrix by mixing with γ-aminopropyltriethoxysilane (APS) is presented. The APS dosages 0.08−0.48 wt % to 0.3 wt % CNFs were investigated, and the dosage 0.16% was found to achieve a superior stability of CNFs in the aqueous solution. The APS(0.16%)-modified CNFs were then incorporated and demonstrated distributing uniformly in the waterborne acrylic coating. The as-prepared coatings retain high light transmittance around 90%, and display improved mechanical properties. The composite coatings show a maximum 500% improvement in Young’s modulus, two-level improvement in hardness, and 35% reduction in abrasion loss as compared with those of neat coating. These results reveal that APS modification induces the homogeneous dispersion of CNFs in aqueous solution, and turns the CNF into an ideal reinforced filler for waterborne coatings. KEYWORDS: Cellulose nanofiber, Homogeneous dispersion, Interface, Silane coupling agent, Waterborne coating

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lignocellulosic biomass.8−10 The advantages of its nanoscale size and high specific strength make it a promising reinforcing filler. CNC has been incorporated into a wide range of polymer matrices, including thermoplastic and thermosetting, such as polyethylene, polypropylene, poly(vinyl acetate), epoxy, polyacrylate, and polyurethane.11−15 The incorporation of CNCs showed higher mechanical properties compared with those of the unmodified polymer.16−21 Because CNCs are fabricated and dispersed in aqueous solutions, they show a high level of compatibility with WPS, and therefore eliminate the need of chemical modification. Recently, Veigel et al. reported that the addition of nanocellulose primarily improves the internal cohesion of the WPS coating, and cellulose nanofiber (CNF) filler was more effective than CNC.22 Compared with CNC, CNF has a higher aspect ratio, larger surface area, and more reactive groups.23−25 CNF is versatile and useful in a variety of applications.26−29 The WPS coatings are also expected to improve the performance and durability by CNF as a novel additive.30−32 The CNFs contain large amounts of hydroxyl groups, which promotes their dispersion in aqueous solution. However, owing to their long aspect ratio, large surface area, and high interface energy,

INTRODUCTION Solvent-based wood coatings are being replaced by ultraviolet hardening and waterborne coatings.1 Waterborne polymers and polymer blends system (WPS) can significantly reduce the emission of volatile organic compounds from coatings, improving air quality and reducing health risks. The film formation of the solvent-based resin system is homogeneous, but the film formation of WPS is a heterogeneous process. The mechanical properties of WPS are lower than those of solventbased resin systems and need improvement. The mechanical properties depend on the binder of the WPS materials, but can be improved using additives such as reinforcement fillers.2 Research has shown that the properties of WPS can be apparently improved by the addition of nanofillers.3 These fillers can be divided into two categories: inorganic and organic fillers. Inorganic nanoparticles such as SiO2, TiO2, and CaCO3, carbonates, silicates and sulfates of various metals, carbon nanotubes, and nanoclay have been applied in the formulation of WPS.4,5 The addition of these nanoparticles improves the stiffness as well as the photostabilization of the polymer matrix. However, these improvements can cause decreases in tensile strength and the break elongation point. There has been a growing trend in modifying WPS coatings with bioderived polymer fillers, predominantly cellulose nanocrystal (CNC).6,7 CNC has a nanoscale needlelike geometry and can be obtained from renewable © XXXX American Chemical Society

Received: February 29, 2016 Revised: May 30, 2016

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DOI: 10.1021/acssuschemeng.6b00415 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

dried, and the FTIR spectra were obtained using a Magna 560 FTIR instrument (Nicolet; Thermo Fisher Corp., Waltham, MA, USA). The data were recorded over the range of 400−4000 cm−1 at a resolution of 4 cm−1. X-ray Photoelectron Spectroscopy (XPS) Analysis. XPS analysis on the unmodified and APS-modified CNFs was performed using a KAlpha system (Thermo Fisher Scientific, Basingstoke, UK) operated at 14.0 kV. Binding energies were referenced to the C 1s neutral carbon peak at 284.8 eV. TEM Observation. TEM images of the unmodified and APSmodified CNFs were observed by a Tecnai G2 microscope (FEI, Hillsboro, OR, USA) operated at 80 kV. The diluted sample was deposited onto a glow-discharged carbon-coated grid and stained with 1% phosphotungstic acid to enhance the contrast of the image. Rheological Behavior Test. The rheological measurements of the neat and composite WAPs were performed on an AR2000 rotational rheometer (TA Instruments, New Castle, USA). The sample was placed between parallel plates with a diameter of 40 mm, and the gap was set at 1 mm. For the frequency sweep, measurements were performed from 0.1 to 100 rad s−1 at 25 °C. Viscosity Test. Viscosity of the neat and composite WAPs was carried out by a NDJ-5S rotational viscometer at 26 °C (Weide Instrument Co. Ltd., Ningbo, China). Differential Scanning Calorimetry (DSC). DSC measurements were carried out by a DSC 204F1 (Netzsch, Germany) at a heating rate of 5 °C min−1 in the range of 30−180 °C under a nitrogen atmosphere. SEM Observation. The surface and cross-sectional microstructure of the neat and composite coatings were observed using a Quanta 200 system (FEI) at an operating voltage of 15 kV. The cross section was obtained from breaking the sample after being frozen in liquid nitrogen. Light Transmittance. The light transmittance spectra of neat and composite coatings were recorded using a TU-1901 spectrophotometer (Purkinje General Instrument Co. Ltd., Beijing, China). The scanning wavelength was from 200 to 800 nm. Mechanical Tensile Test. Tensile tests on the neat and composite coatings were performed at room temperature using an Instron 5569 universal testing machine with a 1 kN load cell (Instron Corp., Canton, USA). The specimen dimensions were 20 mm in length, 5 mm in width, and 1 mm in thickness. The tests were conducted at a speed of 10 mm min−1. Hardness Test. The hardnesses of the coatings were examined by a commercial pencil hardness tester (QHQ-A; Jingke Material Testing Instrument Co. Ltd., Tianjin, China) using standard ISO 15184, with standard pencils with hardness from 2B to 5H. Abrasion Resistance Test. WAPs were applied to Lauan plywood specimens by brushing twice. The dimension of the specimens was 200 × 200 × 3 mm. The first coat was conducted with a total amount of wet coating (1.0 ± 0.1) g dm−2. After 24 h, it was sanded with a 400-grit sandpaper in the direction of the wood grain and the second coat with a total amount of wet coating (0.8 ± 0.1) g dm−2 was applied. The weight loss of abrasion was measured after 1000 cycles with a JM-IV abrasion tester (Yonglida Material Testing Instrument Co. Ltd., Tianjin, China).

they tend to intertwine together and form agglomerates. Thus, the homogeneous dispersion of CNFs in WPS coatings is more difficult than that of CNCs. Uniform dispersion of CNFs within the WPS matrix is critical for reinforcement. CNFs require appropriate modifications to form a strong interfacial adhesion through chemical interactions at the molecular level.33,34 Surface modifications of CNFs such as TEMPO-mediated oxidation, esterification treatment, and grafting with macromolecules have been developed.35−37 Silane has bifunctional molecules possessing oxy alkyl groups and organic moiety, allowing it to react with OH groups of CNFs and functional groups of WPS. The modification of CNFs with silane coupling agents forms the hydrocolloid network, which generates a steric hindrance stability effect.38 The aim of this study is to investigate the modification of CNFs using γ-aminopropyltriethoxysilane (APS) and the dispersion of CNFs in commercially available waterborne acrylic resin/polyester polymer blend (WAP) coatings, which is expected to improve their mechanical properties. The interaction between APS and CNFs is controlled by adding APS. The rheological properties, morphology, and mechanical properties of the coatings were investigated by rotational rheometry, transmission electron microscopy (TEM), scanning electron microscopy (SEM), light transmission, tensile measurements, and abrasion testing.

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EXPERIMENTAL SECTION

Materials. Wood (Populus ussuriensis) flour sieved under 60 mesh was used as a raw feedstock and cellulosic source of CNFs. APS was purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Commercial WAP for wood furniture varnish was purchased from Carpoly Chemical Engineering Co., Ltd. (Jiangmen, China). The main component was acrylate/polyurethane (60:40 v/v) copolymer emulsion, and the additives included wetting agent, defoamer, dispersant, thickener and flatting agent. The solid content measured by mass according to standard ISO 3251:2008 was 36.6%. Sodium chlorite, potassium hydroxide, acetic acid, and hydrochloric acid were all of analytical grade and purchased from Kermel Chemical Reagent Co., Ltd. (Tianjin, China). Preparation of Samples. APS Modification of CNFs. The CNFs were prepared using a method reported previously.39 Wood flour was first subjected to 1 wt % sodium chlorite solution under an acidic condition (pH = 4−5) at 75 °C for 3 h to remove lignin, and the ratio of wood flour to sodium chlorite solution was 2 g/L (w/v). The sample was then treated in 5 wt % potassium hydroxide (ratio: 4:1 g/ L) at 90 °C for 2 h to remove hemicelluloses. The resultant purified cellulose was filtered and washed with distilled water. 0.096, 0.192, 0.288, 0.384, and 0.576 mL of APS were respectively added in 120 mL of distilled water under continuous stirring for 15 min, resulting in solutions with APS concentration of 0.08%, 0.16%, 0.24%, 0.32%, and 0.48% (wt %), respectively. Next, the purified cellulose was added in APS solution at 0.3% w/v, followed by ultrasonication treatment in an ultrasonic generator (JY99-IIDN; Scientz Biotechnology Co., Ningbo, China) at 1200 W for 30 min. The solution was then stirred for 3 h to obtain the APS−CNF suspensions. Modification of WAP with APS-Modified CNFs. 1.5, 3.0, 4.5, and 6.0 g of APS(0.16%)−CNF suspension were respectively added in 30 g of WAP, and the APS(0.16)-CNF suspension content of the mixtures was regarded as 5, 10, 15, and 20 wt %. The mixtures were stirred at room temperature for 3 h. The coatings were prepared by casting the mixtures on a PTFE Petri dish (90 mm diameter), and water evaporated at ambient conditions for 7 days. The resultant composite coatings were coded as WAC-5, WAC-10, WAC-15, and WAC-20. Characterization. Fourier Transform Infrared Spectroscopy (FTIR) Analysis. The unmodified and APS-modified CNFs were

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RESULTS AND DISCUSSION Dispersion and Stability of APS-Modified CNFs. APS Modification of CNFs. APS is a bifunctional monomer with triethoxyl and amine groups. The presence of water can first induce hydrolysis of the oxyalkyl to form silanol, which induces kinds of probable reactions (Figure 1). First, silanols can form Si−O−Si bonds via self-condensation, and the steric hindrance of grafted APS chains can prevent the collision and selfagglomeration of CNFs to some extent. Second, APS molecules can be introduced in situ to the reactive hydroxyl groups on CNFs by a silane coupling process. The interaction between Si−OH and C−OH on the CNFs can form hydregen bonds

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Figure 1. Schematic illustration of the interaction modes between CNFs and APS: (a) chemical cross-linking through forming hydrogen bonds; (b) forming ester bonds after dehydration condensation; and (c) forming Si−O−Si bonds which regenerate the steric hindrance influence. Figure 2. (a, b) FTIR spectra of CNFs and APS-modified CNFs. (c) XPS spectra of surface element distribution of CNFs and APSmodified CNFs. XPS high-resolution spectra of (d) Si 2p of APSmodified CNFs, (e) C 1s of CNFs, and (f) C 1s of APS-modified CNFs.

between the CNFs and APS, which reduces the free hydroxyl groups and weakens the interaction between CNFs. Sometimes they will be cross-linked by the C−O−Si bond formation through dehydration condensation.40,41 The FTIR spectra were used to compare the differences between unmodified and APS-modified CNFs (Figure 2a). The peaks at 1315 and 1429 cm−1 are attributed to the symmetric bending of CH2 and the bending vibrations of the C−H and C−O groups of the rings in cellulose, respectively, which were observed in the spectra of unmodified and APS-modified CNFs. The spectrum of APS-modified CNFs shows absorption bands nearby 1575, which are characteristic of −NH2 bending vibrations and out-of-plane bending absorptions of N−H. The broad absorption peak around 1000−1200 cm−1 is suggested to attribute to Si−O−Si and C−O−Si bonds (Figure 2b).40,41 In Figure 2c, the spectrum of CNFs consists of C 1s and O 1s peaks. In the spectrum of APS-modified CNFs, XPS survey confirms the presence of Si and N elements (Table 1). The C/ O ratio of the APS-modified CNFs is larger than that of CNFs, this is because the introduction of alkyl groups of APS chemically bonded to CNFs. The high-resolution spectrum of the Si 2p in Figure 2d reveals the presence of Si−O−Si and −CH2SiO3 (silicon bound to three oxygen atoms) at the binding energy of 101.8 and 103.62 eV, respectively. This is consistent with the schematic diagram in Figure 1. Selfcondensation occurred among the silanols; however, the −CH2SiO3 can be assigned to the chemical bonding of Si− OH or Si−O−C. Further identification of the Si−O−C formation is realized through the analysis of relative carbon composition C 1s in both CNFs (Figure 2e) and APS-modified CNFs (Figure 2f). The C 1s spectrum typically shows three types of carbon bonds: C−C at 284.3 eV, C−O at 285.8 eV, and O−C−O at 287.6 eV. The relative carbon compositions of CNFs were 18.81% for C−C, 69.11% for C−O and 12.08% for O−C−O. However, the carbon compositions in APS-modified CNFs of C−C and O−C−O decreased dramatically, but with

an increase of C−O. This could be due to the CNFs wrapped with the APS oligomers through hydrogen bonding or ester bonding, resulting in a relative decrease of C−C and O−C−O. The increase of C−O can only be attributed to the formation of C−O−Si. The chemical bonds of C−O−Si can also be formed between CNFs and APS oligomers. Thus, the relative C−O composition increased.42 Dispersion and Stability. The unmodified CNFs exhibited fibrous structures with 2−20 nm in widths and lengths exceeding 1 μm.43 Because of the high aspect ratio and high content of hydroxyl groups, the CNFs tend to intertwine with their neighbors and form agglomerations, resulting in poor stability and obvious precipitation (Figure 3a,e APS−0). The addition amount of APS is another key factor to affect the stable dispersion, so it is important to seek an optimum dosage of APS. With the dosage of 0.08% APS, the CNFs seem to not being fully modified by APS, and obvious sedimentation can be seen after period of time. When adding 0.16% APS, the ratio of CNFs and APS seems to reach the optimum, and the interaction of hydrogen bonds, steric hindrance and gravity got well balanced, thus exhibiting a superior stability in suspension, even after being stored for 7 months (Figure 3b and e APS−0.16%). While, if adding APS with excessive concentrations (e.g., 0.32%, 0.48%), too many silanols generate in the suspension, and silanols will be likely to transform further into oligomers. The CNFs that coated with APS tend to form the directional alignment structure (Figure 3d), whose stability property become weaker than that of the network structure (Figure 3e). C


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Atomic Concentration and Relative Carbon Composition of CNFs and APS-Modified CNFs CNFs APS-CNFs

C 1s (%)

O 1s (%)

C/O

35.19 39.72

64.81 56.68

0.54 0.70

N 1s (%) 2.63

Figure 3. TEM images of (a) unmodified CNFs, (b) APS(0.16%)− CNFs, (c) APS(0.24%)−CNFs, and (d) APS(0.48%)−CNFs. (e) Photographs showing the dispersion stability of APS-modified CNFs in aqueous solution after standing for 0 h, 24 h, 1 month, and 7 months. The labels on the bottles correspond to the APS addition concentration.

Si 2p (%)

C−C (%)

C−O (%)

O−C−O (%)

0.97

18.81 13.26

69.11 81.01

12.08 5.73

Figure 5. DSC curves of the neat WAP and the WAP with APS(0.16%)−CNFs.

of order. The cross-linking between semirigid APS(0.16%)− CNFs and the hard segments of WAP restricts the motion of soft-segment molecules, and hinders the degree of phase separation of the segments. Additionally, no clear melting peaks or crystallization peaks were observed by DSC, implying that the WAPs form amorphous polymers. Light Transmittance and Gloss. The neat WAP coating exhibited high transparency, and the light transmittance at 600 nm was above 90%. The composite WAP coatings with APS(0.16%)−CNFs retained optically transparency, and the light transmittance at 600 nm was maintained at 90% nearby (Figure 6a). The content of APS(0.16%)−CNFs did not

Modification of WAP with APS-Modified CNFs. Rheological Behavior and Viscosity. The effect of APS(0.16%)−CNFs on the rheological behavior of suspension was investigated (Figure 4a). The suspension of APS(0.16%)−

Figure 4. (a) Rotational rheological behavior of APS-modified CNF suspension compared with that of the unmodified CNF suspension. (b) Viscosity of the neat WAP and the WAPs with different contents of APS(0.16%)−CNF suspension. Figure 6. (a) Photographs and UV transmittance spectra, and (b) glossiness of the neat and composite WAP coatings.

CNFs had higher storage (G′) and loss (G″) moduli compared with those of unmodified CNF suspension. With the increase of shear frequency, the suspension system showed a shear thickening behavior. In comparison with conventional solvent-based paint systems, which show Newtonian flow behavior, WPS exhibits different flow behaviors such as pseudoplasticity, thixotropy, or plastic behavior. Increasing the content of APS(0.16%)−CNF suspension from 5% to 20% conduced to the viscosity of WAP a maximal 40% increase from 200 MPa (Figure 4b). These results suggest that the curing reaction between alkyd and amine groups occurred, and the addition of APS(0.16%)−CNFs in WAP increases the viscous flow activation energy, improving the thixotropy of the system. The change in glass transition temperature of WAP with the addition of APS(0.16%)−CNFs was observed using DSC (Figure 5). An endothermic peak appeared at higher temperatures for the WAP with APS(0.16%)−CNFs. This result may be attributed to the formation of domains with different degrees

influence the transparency because CNFs are substantially smaller than the wavelengths of visible light. Moreover, they dispersed homogeneously in the WAP, which did not obstruct the transmission of light. The 60° gloss level of the composite coatings were slightly lower compared with that of the neat coating (Figure 6b). Microstructure. The distribution of APS(0.16%)−CNFs in the composite coatings was characterized by SEM. Figure 7 shows the surface and cross-sectional micromorphologies of neat and composite coatings. The neat coating presented a smooth surface at all magnifications. The composite coating with unmodified CNFs showed aggregates throughout the matrix. However, the APS-modified CNFs were uniformly distributed in the coating matrix, and no more aggregates were observed. The “sea−island structures” uniformly distributed D


Research Article


Figure 9. (a) Hardness and (b) abrasion resistance of the neat coating and the composite coatings. Figure 7. SEM images showing the microstructures of the (a) surface and (d) cross section of neat WAP coating; (b) surface and (e) cross section of WAP coating containing unmodified CNFs; and (c) surface and (f) cross section of WAP coating containing APS(0.16%)−CNFs.

film was 2H. The pencil hardness for the composite coatings increased to 3H after the addition of 5, 10, or 15 wt % APS(0.16%)−CNF suspension, and reached 4H at 20 wt %. The abrasion resistance of the composite coatings is shown in Figure 9b. Grinding was performed for 1000 rotations, and the mass loss was recorded. The mass loss of the composite coatings showed a decrease with the increasing APS(0.16%)− CNF suspension content, and the decreased magnitude ranged from 7% to 35%. Because the grafted silicane groups from the APS is compatible with the WAP matrix, the hardness and abrasion resistance are enhanced.

over the cross section indicated the good dispersion of CNFs. The fracture surface of the WAP coating with APS(0.16%)− CNFs showed different texture with that of the neat WAP coating. Mechanical Test. The mechanical properties of the neat and composite coatings were studied. The neat coating showed a low Young’s modulus of 70.2 MPa and a high elongation breakpoint of 143.3%. The reinforcement effect of the APS(0.16%)−CNFs on the WAP coating was demonstrated, which was closely related to the content of APS(0.16%)−CNF suspension (Figure 8). The highest modulus was the 20 wt %

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CONCLUSIONS A facile approach was developed to improve the dispersion and stabilization of CNFs in WAP coatings. The APS was selected to disperse CNFs into a homogeneous suspension through chemical cross-linking and steric hindrance effect. After a trace amount (0.16 wt %) of APS was added, the CNF suspension became stable, and no sedimentation occurred for at least 7 months. The uniform dispersion of CNFs allows the fabrication of reinforced WAP coatings. The addition of APS−CNFs causes a 30 °C increase in the glass transition temperature of WAP by hindering the mobility of macromolecular chains. In comparison with the neat coating, the composite coatings retain a high transparency around 90%, and display improved mechanical performance. The composite coatings show a 500% improvement in Young’s modulus, 2-fold improvement in hardness, and 35% reduction in abrasion loss.

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Figure 8. Breakpoint elongation and Young’s moduli of the neat coating and the composite coatings.

AUTHOR INFORMATION

Corresponding Author

* H. Yu, E-mail: [email protected].

sample, which reached 6 times than that of the neat WAP coating. Therefore, the APS−CNFs showed improved toughness and reinforcing effects than those of CNCs.16−18 This finding may be explained because APS(0.16%)−CNFs can act as a chain extender to facilitate transfer stress through the coating, promoting an additional stiffening effect because of the reduced mobility of the WAP chain attached to the CNFs. The composite coatings did not display a substantial loss in ductility compared with that of the neat coating. These results can be attributed to the hydrogen bonds formed preferentially between groups of hard segments dissolved in the soft domain of WAP. The reinforcement was achieved without hindering chain slippage in the soft segments, and the elongation at breakpoint of the matrix was maintained. Hardness and Abrasion Resistance. The hardness of the composite coatings was evaluated using pencil-scratching methods (Figure 9a). The pencil hardness of the neat coating

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the Program for New Century Excellent Talents in University (NCET-10-0313), and was also supported by the Natural Science Foundation of Heilongjiang Province of China (JC2016002).

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Homogeneous Dispersion of Cellulose Nanofibers in Waterborne

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