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Nanocellulose composites with enhanced interfacial compatibility and mechanical properties using a hybrid-toughened epoxy matrix
Accepted Manuscript Title: Nanocellulose Composites with Enhanced Interfacial Compatibility and Mechanical Properties Using a Hybrid-Toughened Epoxy Matrix Authors: Pei-Yu Kuo, Luizmar de Assis Barros, Ning Yan, Mohini Sain, Yan Qing, Yiqiang Wu PII: DOI: Reference:
S0144-8617(17)30967-0 http://dx.doi.org/10.1016/j.carbpol.2017.08.091 CARP 12698
To appear in: Received date: Revised date: Accepted date:
26-5-2017 11-8-2017 19-8-2017
Please cite this article as: Kuo, Pei-Yu., Barros, Luizmar de Assis., Yan, Ning., Sain, Mohini., Qing, Yan., & Wu, Yiqiang., Nanocellulose Composites with Enhanced Interfacial Compatibility and Mechanical Properties Using a Hybrid-Toughened Epoxy Matrix.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2017.08.091 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nanocellulose Composites with Enhanced Interfacial Compatibility and Mechanical Properties Using a Hybrid-Toughened Epoxy Matrix Pei-Yu Kuoa, Luizmar de Assis Barrosb, Ning Yancde*, Mohini Saincd, Yan Qinge, and Yiqiang Wue a
Department of Forestry and Natural Resources, National Ilan University, No. 1, Section
1, Shennong Road, Yilan City, Yilan County, Taiwan b
Institute of Forestry, Department of Wood Chemistry, University Federal Rural Do Rio
de Janeiro. Rodovia BR 465-Km7 Campus Universitário, Seropédica RJ, 23851-970, Brazil c
Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON, M5S 3B3,
Canada d
Department of Chemical Engineering and Applied Chemistry, University of Toronto,
200 College Street, Toronto, ON, M5S 3E5, Canada e
College of Materials Science and Engineering, Central South University of Forestry and
Technology, Changsha, 410004, China *
Corresponding author E-mail: [email protected]
Highlights
Novel hybrid epoxy improved biofibre wettability and compatibility with the resin matrix
Nanocellulose fibres (NCFs) enhanced mechanical performance of neat epoxy
Incorporating bio-based epoxy into petro-epoxy increased toughness significantly
1
Abstract Although there is a growing interest in utilizing nanocellulose fibers (NCFs) based composites for achieving a higher sustainability, mechanical performance of these composites is limited due to the poor compatibility between fibre reinforcement and polymer matrices. Here we developed a bio-nanocomposite with an enhanced fibre/resin interface using a hybrid-toughened epoxy. A strong reinforcing effect of NCFs was achieved, demonstrating an increase up to 88 % in tensile strength and 298 % in tensile modulus as compared to neat petro-based P-epoxy. The toughness of neat P-epoxy was improved by 84 % with the addition of 10 wt% bio-based E-epoxy monomers, which also mitigated the amount of usage of bisphenol A (BPA). The morphological analyses showed that the hybrid epoxy improved the resin penetration and fibre distribution significantly in the resulting composites. Thus, our findings demonstrated the promise of developing sustainable and high performance epoxy composites combing NCFs with a hybrid petro-based and bio-based epoxy resin system.
Keywords: fibre/resin interface; bio-nanocomposite; nanocellulose fibres; bio-epoxy; hybrid-toughened epoxy
2
1. Introduction
The global concern for the depletion of non-renewable resources has resulted in significant efforts to develop new building blocks from renewable biological resources to synthesize biopolymers and functional chemicals.[1, 2] At the same time, the heightened awareness of potential health risks associated with bisphenol A (BPA), a widely used industrial chemical for making resins and plastics (such as epoxy resins and polycarbonates)[3] has led to strong interests in replacing BPA with natural compounds such as saccharides,[4] vegetable oils,[5] polyphenols,[6] lignin,[7] liquefied biomass[8], etc. Currently numerous bio-based epoxy resins are commercially available in market that include sugar-based epoxy resins (DENACOL™ and ERISYSTM) and oilbased epoxy resins (Vikoflex®, Super Sap™, and Senso™). Despite the strong promise of the biobased epoxies due to their improved sustainability, their lower mechanical properties are a deterrent for high performance applications. Nanocellulose fibres (NCFs) are renewable, lightweight, cost-effective with high aspect ratios and high specific modulus (100-160 GPa[9]). They can be obtained from various bio-resources such as algae,[10] tunicate,[11] bacteria,[12] agricultural residuals,[13] and wood.[14] NCFs have been shown to have strong mechanical reinforcing effects on polymer matrices in composites.[15-17] In addition to mechanical reinforcement, our previous work has also shown that NCFs accelerated the curing process of epoxy resins,[18] and that NCFs’ mildly-acidic surface could react with amine type epoxy curing agents.[19] Therefore, NCFs are well suited as reinforcement for application in epoxy composites. However, there is a significant process challenge in applying NCFs in epoxy composites that is related to the tendency of NCFs suspensions to form gels at rather low concentrations. As a result, most studies in literature only add 0.1-3 wt% concentrations of NCFs into epoxy matrices, which yield limited improvement to mechanical performance.[20-24] To address this issue, some 3
studies incorporated NCFs in sheet format into epoxy resins, which significantly improved mechanical performance due to the higher NCF content.[17] In addition, several studies also used (2,2,6,6-Tetramethylpiperidin-1-yl) oxyl (TEMPO)[21], natural rubber[25], silane,[26], acetic acid[27], fatty acid methyl ether[28] to pretreat NCFs to enhance fibre/matrix interphase prior to the application in matrix. However, untreated NCFs have not been used in bio-epoxy resins in the literature,[21] and to the best of our knowledge, no mechanical improvement of bio-epoxy by NCFs has been reported[15], and no study has looked into applying NCFs in a hybrid epoxy system. The major advantage of using a hybrid-epoxies system as proposed in this study is its potential to have a higher toughness than the conventional epoxy systems. The improvement is caused by the added epoxy monomers from the bio-epoxy reducing the cross linking density of the existing epoxy resin, and by the new bio-epoxy monomers forming a second network that can create interpenetrating phase in the composite. Furthermore, the presence of polar groups in the second phase bio-epoxy monomer phase has been shown to significantly improve the adhesion properties of epoxy resins.[29] Previous studies indicate that bio-epoxy resins can improve the toughness and adhesion properties of conventional epoxy systems.[30, 31] Among all types of bio-epoxy resins, tree bark extractive-based epoxy resin (E-epoxy) has a low cost, high mechanical performance, high thermal resistance, and contains significant amounts of hydroxy groups,[32] making it an attractive alternative of composite matrix. The aim of this study is to better understand the reinforcement effect that NCFs have on the hybrid epoxy system and the influence of the hybrid resin on the fibre/resin interface. Resin curing behaviour, fibre/matrix interface, mechanical performance, and thermal stability of the NCFs reinforced hybrid epoxy composites are also investigated in detail.
2. Materials and Methods 2.1 Materials 4
NCFs and E-epoxy monomer were both lab-made and the preparation procedure and characterization results were previously reported.[32, 33] NCFs used in this study were obtained via mechanical grinding and E-epoxy was synthesized using pine bark extractives reacting with epichlorohydrin with an epoxy equivalent weight of 442 g/eq. The commercial petroleum-based epoxy monomer (P-epoxy), Epon Resin 863 with an epoxy equivalent weight (EEW) of 169 g/eq, was used as the control sample. Epikure W was applied as the curing agent with amine hydrogen equivalent
weight
(AHEW)
of
44
g/eq,
and
its
major
component
is
diethylmethylbenzenediamine (DETDA). The mixing ratio between epoxies/curing agent was determined by their EEW and AHEW values to reach 1:1 stoichiometric ratio of amine hydrogen to epoxide groups. The P-epoxy monomer and the curing agent were obtained from Momentive Specialty Chemicals, OH, USA. Acetone (> 99.5%) was purchased from Caledon Laboratory Chemicals, ON, Canada and used without further purification.
2.2 Methods The manufacturing process of NCF/Epoxy composites is described in detail below and illustrated in supplementary figure 1. 2.2.1 Nanocellulose Sheets Preparation 10.7g gel-state NCFs (2.5 wt%) was diluted to reach 0.1 wt% in concentration and air bubbles were removed from the solution using an ultrasonic bath. To form a nanocellulose fiber sheet the suspension was vacuum-filtered through a Supor-100 membrane filter (pore size = 0.1 µm, Pall Corporation) in a Buchner funnel. After filtration, the wet fibre films were carefully removed from the membrane, and wet-pressed following a standard TAPPI process. The wet film was placed between two filter membranes and compressed by a standard TAPPI wet press with 50 psi pressure for seven minutes. The NCF sheet was then removed from the membrane, and dried in the oven (oven-dried NCFs; OD-NCFs) or immersed in acetone overnight in order to prepare 5
solvent exchanged NCF (solvent-exchanged NCFs; SE-NCFs). Both OD-NCFS sheets and SE-NCF sheets were cut into dogbone shapes with dimensions suggested by ASTM D638-5 for standard tensile test. Ten cut NCF specimens were dried in the oven at 55˚C for 24 hours to test the mechanical properties of NCFs and the rest 30 NCF specimens were used for the following composite preparations.
2.2.2 Composite Preparation Cut NCFs were immersed into either P-epoxy or hybrid epoxy overnight. The amount of curing agent added to the resins was adjusted from 16 wt% to 9 wt% based on the calculated EEW values for hybrid resins in order to match its stoichiometric ratio of amine and epoxy groups. To reduce the viscosity of both resins, P-epoxy and hybrid epoxy resin were diluted with acetone at a 1:1 ratio. Next, the resin-filled NCF composites were removed from the bath and cured in an oven with the following curing schedule: 1h at 65˚C, 1h at 80˚C, 1h at 121˚C, and 2h at 177˚C (post-cure). Same curing schedule was applied on the epoxies without NCFs to make control samples for comparison. The compositions for the cured epoxies and epoxy composite are listed in Table 1.
2.3 Characterization 2.3.1 Curing Activation Energy (Ea) and Glass Transition Temperature (Tg) The Ea and Tg of the epoxies and the epoxy composites were evaluated using a dynamic differential scanning calorimetry (DSC) model Q 100 (TA instruments, USA). Dynamic DSC measurements were carried-out at a ramp rate of 5, 10, 15, and 20 °C/min, from 30 to 300 °C in order to obtain the curing heat-flow curves of liquid P-epoxy and E-epoxy resins as well as their composites. The cured samples were heated to 300 °C at 10°C /min in order to detect the Tg of both epoxies and their composites.
6
2.3.2 Tensile Properties The tensile tests of the cured epoxies, composite specimens, and NCF sheets were performed using a universal testing machine, Model 3367 (Instron instruments, USA), equipped with a 2 kN load cell. Samples were cut into dogbone shaped specimens using an ASTM 638-5 specimencutting die. The sample thickness was determined using a bench top micrometer. The span length of the specimen was 25 mm, with a cross-head speed of 2.5 mm/min for tensile testing. Five specimens were measured for each composition.
2.3.3 Morphological Characterization Scanning electron microscope (SEM) images were used to study the morphology of NCFs and its composites. All the images were acquired using a JSM6610-Lv (JEOL, Japan) with an accelerating voltage 10-15 kV. To visualize the distinct features of fibres, NCF sheets were prepared using liquid nitrogen to freeze the water inside NCF sheets and then slowly dehydrolyzed to maintain their surface structure and avoid cell collapse. Composite samples were examined on their crosssectional areas after their tensile tests. All the samples were coated with gold prior to SEM imaging.
2.3.4 Thermal Stability The thermal stability of the cured neat epoxy resins and composites were investigated using a thermogravimetric analysis, TGA-Q500 (TA Instruments, USA). Approximately 10 mg of each sample was weighed in a platinum pan and operated under a continuous flux of air or nitrogen (60 cm3/min). Under dynamic analysis, all of the samples were heated from room temperature to 700 °C, with a 10 °C/min ramp. 2.3.5 Wetting Behaviour
7
The surface tension of resins, surface energy of NCF sheets, and contact angles between resins and NCF sheets were measured using an OCA 15 Plus goniometer (DataPhysics Instruments GmbH, Germany) at room temperature. The static contact angle was measured by a sessile drop method and the surface tension was measured by a pendant drop method with a microsyringe tip diameter of 6um. Images of contact angles were recorded with a microscopic video recording system and the values of these angles were determined from the image using OCA 15 software. At least 5 measurements were averaged to calculate one mean value. The surface energy of NCF sheets was calculated by contact angles of two types of liquids on the fibre surface, including ultrapure water (18.4 MΩ٠cm) and diiodomethane. 2.3.6 Surface Chemical Analysis and Element Mapping Time-of-flight secondary ion mass spectrometry was used to identify the interface between NCF sheets and epoxy resins using a ToF-SIMS5 (IonTof GmbH, Munster, Germany). Both positive and negative ion spectra were obtained with 60 keV Bi3++ primary ions operated in a high-current bunched mode. The scanned areas were at least 100 x 100 μm2 covered by 164 x 164 pixels and the primary ion dose density was between 1 x 109 - 1 x 1013 ions/cm2. In order to obtain high resolution mapping images, each composite sample was immersed in liquid nitrogen and then was cut by microtome to have a clear cross section without leaving any cutting mark. 2.3.7 Fourier Transform Infrared Spectroscopy (FTIR). Samples were sandwiched between two KBr crystals and studied using a Bruker Tensor 27 spectrometer in an environmental chamber. All spectra were recorded over 4000−600 cm−1 at a resolution of 4 cm−1 with 32 scans.
3. Results and Discussion 3.1 Morphological Characterization of Epoxy/NCF Composites 8
The structure of composite materials, especially fibre/matrix interfaces, plays an important role in determining its overall mechanical performance. In this study, the structure of PO-0 composite (reinforced by OD-NCFs) possessed a laminated sandwich layout (Figure 1a); the structure of PS-0 composite (reinforced by SE-NCFs) depicted a homogeneous well-mixed layer (Figure 1b). In Figure 1a, the face layers were epoxy resins and the core layer was made by OD-NCFs. This compact fibre sheet was approximately 15-25 μm thick and had a clear borderline between itself and the matrix due to the impenetrability of the dried fibre sheet. By contrast, the SE-NCF in composite swelled to approximately 90-100 μm due to resin penetration (Figure 1b). Since SE-NCFs never experienced a drying process with all water being replaced by acetone, SE-NCFs possessed a fibrous surface morphology, while OD-NCFs exhibited a compact and smooth surface (Figure 1c, d). This compact surface was caused by the collapse of nanocellulose fibrils driven by capillary force during water evaporation. The dense structure was coalesced by hydrogen bonds that formed between hydroxyl groups of the cellulose or hemicellulose,[34] producing a dense surface that inhibited the penetration of epoxy resins into the NCF network. Therefore, OD-NCFs were nearly impenetrable, and a clear boundary was observed between OD-NCFs and epoxy resins in the cross section of the composites. At a higher magnification, observation of the PS-0 showed a matrixcovered appearance with many white spots showing possibly the fracture surfaces of single or bundled NCFs. As reported by many studies,[35, 36] the individual fractured NCFs appear as white dots in SEM images. Their short pull-out length indicates strong interfacial adhesion and strong intermolecular interactions, likely due to either the surface hydroxyls of the NCFs reacting with the oxirane group through an etherification reaction or due to secondary bonding between these hydroxyl groups. 9
Compared to the P-epoxy, the E-epoxy resin had a higher amount of hydroxyl groups, which not only accelerated the curing reaction between amine and epoxy resins, but also had the potential to form strong hydrogen bonding with NCFs. The influence of adding E-epoxy resins on the diameter of NCF bundles was depicted in Figure 2 showing the bundle cross-sections in (a) Pepoxy/NCFs and (b) hybrid epoxy/NCFs. The diameter of NCF bundles was calculated by ImageJ software (1.48 version, National Institutes of Health, USA). The diameter of white dots in PS-0 ranged from 83 nm to 502 nm with an average of 253 nm ± 83 nm (n=50); the white dot diameters in HS-1 ranged from 72 nm to 626 nm with an average of 195 nm ± 117 nm (n=50), which is significantly smaller than that in PS-0 (p < 0.001, Mann-Whitney U test). This suggests that the hybrid epoxy resins can wet and penetrate NCFs better than neat P-epoxy resins can. To better understand the effect of E-epoxy on resin penetration, wetting behaviour and surface/interface characterizations are presented in the following sections.
3.2 Wetting Behaviour The wetting behaviour of a solid by a liquid is a crucial factor in tuning interfacial bonding strength of composites. In this study, the solid phase is OD-NCFs and the liquid phase is either P-epoxy or hybrid P-Epoxy/E-epoxy. The surface energy of OD-NCFs was indirectly estimated through contact angle measurements using Owens-Wendt-Rabel-Kaelble plotting method. The surface energy of NCFs can be divided into two major components – polar and dispersive. The dispersion force dominates the interactions between the nanocellulose fiber and the hydrophobic matrix. The OD-NCFs used in this study showed a dispersive component of 36.8 mN/m within the range of reported values (27.4 to 50 mN/m)[37-39]. The surface energy of resins was measured through the pendant drop method. In order to match the processing procedure, both resins were diluted by acetone with 1:1 weight ratio. The surface tensions of both resin mixtures were close to the surface tension of acetone (26.3 mN/m). Although the surface 10
tension values of the two resins did not have statistically significant difference, their contact angles on OD-NCFs were varied. The contact angle of P-epoxy (27.6 mN/m) on OD-NCFs was significantly higher than that of E-epoxy (16.2 mN/m) as both contact angles were measured at approximately 30 seconds after droplet deposition (Suppl. Fig. 2). The smaller the contact angle is, the larger the wetting tendency will be. Therefore, our findings implied E-epoxy had a better wettability on NCFs than P-epoxy. 3.3 Surface Chemical Analysis and Mapping The surface chemistry of NCFs/epoxy composites was studied using FTIR and ToF-SIMS techniques. The general characterization of functional groups using FTIR is shown in supplementary figure 3. ToF-SIMS was applied to identify the molecules and their corresponding secondary ion distribution on the surface of the composite cross section with a depth resolution of 1–2 nm. Positive mass spectra of NCF sheets, cured resins, and composites are shown in supplementary figure 4. Using the information from mass spectra, two mass fragments were selected to visualize the morphology of the composite cross sections. C4H5O2+ was used to represent nanocellulose and C6H5NO+ was used to represent epoxy resin. The overall images were depicted by total ions (figure 3a-c) and the corresponding fragment ion images (figure 3df) are shown in two colors: red (C4H5O2+, signal from nanocellulose) and green (C6H5NO+, signal from epoxy). All the images were normalized by setting the highest intensity to 100% signal. Image 3d shows a distinctive boundary between NCF sheets and epoxy indicating an impenetrable surface on dried NCFs, which is consistent with the findings of SEM images shown in Figure 1(a). Image 3e shows the partial overlay of two colors, but the shape of solventexchanged NCFs is still visible in the image. Thus, solvent-exchange method can improve the resin penetration, but the fibre distribution is still heterogeneous. Image 3f depicts a homogeneous layer after introducing a hybrid E-epoxy/P-epoxy system, which suggests E-epoxy
11
enhanced the resin penetration and the fibre distribution. Combining these findings, E-epoxy resin shows an enhanced interfacial property.
3.4 Mechanical Performance of Cured Epoxies and Epoxy/NCF Composites The average tensile strength, tensile modulus, strain energy density, and Tg of P-0 and H-1 are summarized in Table 2. In this study, toughness, also called strain energy density, was estimated by the total area under the stress-strain curve during tensile strength testing. After adding 10 wt% E-epoxy monomers, the tensile strength of H-1 decreased slightly, from 76 MPa to 70 MPa, but the strain improved from 6.6 % to 12.3 %, which led to an overall toughness increase by 84 %. Student’s pair t-test analysis of P-0 and H-1 data revealed that tensile strength and tensile modulus were not statistically significantly different at 1% level (p = 0.087 and 0.111), but there was a significant increase in the toughness (p = 0.005). Thus, the E-epoxy monomer acted as a toughening agent to improve elongation. Compared to neat epoxy systems, incorporating the NCFs into resins shows a strong reinforcement effect on tensile properties (Table 2).
Based on the percolation theory, the percolation threshold of NCFs connectivity is less than 6 vol%,[40] but NCFs between 6 vol% to 18 vol% would not form a dense sheet network.[35] Thus, the fibre loading of this study was set at 25 wt% (equal to 20.4 vol% when NCF density was assumed to be 1.5 g/cc). The improvements to the tensile strengths of PO-0 alone and HO-1 after incorporating oven-dried NCFs were 6.0 % and 8.7 %, respectively. With SE-NCFs, the increase in the tensile strengths of PS-0 and HS-1 reached 47.4 % and 88.0 %, which was significantly higher than the oven dried NCFs. This is likely due to the reduction of the stress concentration on the interface of the solvent-exchanged NCF composites. Without solvent exchange, most of the stress is localized at the interfacial area between NCFs and epoxy resins, leading to low tensile strength. Therefore, solvent-exchange treatment can prevent stress 12
concentration and increase mechanical strength. By contrast, the tensile modulus of OD-NCF composites exhibited a 293% and 398% increase over neat epoxy due to their 2D structures. With a strong bonding between fiber/matrix interface, 2D reinforcements generally have higher enhancement in strength and modulus than 3D does. However, a poor interface between fiber/matrix results in 2D composites with a low strength and high modulus. Once the resin penetrated into NCFs, the structure of NCFs changed from 2D fiber sheet to 3D swelled network, which increased the tensile strength of composites but decreased its tensile modulus. HS-1 and H-1 showed improved toughness compared to PS-0 and P-0. There are two possible explanations for this improvement from the E-epoxy addition. The first reason is that the Eepoxy resin reduces the crosslinking density of the network by inserting some large molecules into the network structure. To support this reason, it was observed that the Tg of what decreased from 114 °C to 107 °C with the addition of E-epoxy. The second reason for the improved toughness is that the E-epoxy may form an interpenetrating network (IPN), which increases the size of physical cross-linked domains. The second network can provide space for microdeformation and increase the toughness. Thus, we tested the composites with various E-epoxy amounts (10-30 wt% of monomers) in order to create a more ductile composite, but the greatest toughness was achieved by adding 10 wt% E-epoxy. With more than 10 wt% E-epoxy, the tensile strength and toughness decreased. Many studies have also shown the optimal amount of bioepoxy to be added into conventional epoxy is approximately 10-20 wt%.[29, 40]
According to the literature,[17, 19, 20, 23, 34, 35, 41] the average tensile strength and modulus of conventional epoxy/NCF composites are 85 MPa and 4.6 GPa, respectively, for NCFs from a topdown mechanical grinding process. Our findings show an increased tensile strength, and a comparable tensile modulus compared to the literature report average value (Figure 4, our experimental results are labeled as red triangles). In contrast to surface modified NCFs epoxy 13
composites[42], our finding also exhibits better mechanical performance. Furthermore, we compared our experimental data to the values we predicted based on the rules of mixture (ROM). A previous study states that the mechanical properties can be better described by the ROM than by other micromechanical models,
[35]
such as the Halpin-Tsai model or the Cox-
Krenchel model for epoxy/NCF composite. Therefore, the predicted tensile properties of the nanocomposites used in this study were obtained using the ROM equations: Ecomposite = VfEf + (1-Vf)Em σcomposite = Vfσf + (1-Vf) σm
(1) (2)
where Ecomposite and σcomposite represent the predicted tensile modulus and strength, respectively, Vf denotes the fibre volume fraction, Ef, σf, Em and σm correspond to the tensile modulus and tensile strength of NCFs (f) and matrix (m), respectively. We utilized the highest tensile properties of nanocellulose sheet (σf = 243 MPa and Ef = 14.9 GPa),[34] reported in literature, and the tensile properties of matrix (σm = 70.2 MPa and Em = 1.24 GPa) measured in this study to fit into the prediction model. The results show that our experimental values are very close to the theoretical value. An alternative approach is to calculate a reasonable range of the mechanical properties of composites instead of a single value. Identifying a range of possible values is a practical way to evaluate the mechanical performance of composites since the performance can be affected by many factors such as geometry of reinforcements, micro-cracks, and adhesion force between two components. According to the equations derived from Christenson[43] and Watt and Peseinick[44], for tensile modulus, the lower bound is 1.72 GPa and the upper bound is 11.89 GPa, which encloses our experimental data (4.19-7.70 GPa).
3.5 Thermal Stability 14
Epoxy resins and composites mass loss as a function of temperature was measured by TGA shown in Figure 5. The maximum degradation temperature of the reference P-0 sample was 387°C; after incorporating NCFs, the maximum degradation temperature increased to 411°C, which is likely due to the formation of porous structures, or the formation of strong interactions between the fibres and the matrix.[45] The porous structures would be formed after thermal degradation of all NCFs that can decelerate thermal conductivity. Unlike the maximum degradation temperature, the onset decomposition temperature of composites decreased from 373 °C to 337 °C because of the inherent properties of the cellulose fibre. As reported by other research, cellulose dehydrates between 200 and 280 °C and extensive degradation of cellulose occurs when the temperature is around 300 °C, which is lower than epoxy resins.[46] Thus, even though the maximum degradation temperature of composite improved 24 °C compared to neat epoxy, the overall heat-resistance index temperature (Ts) does not show a significant difference. Additionally, the effect of E-epoxy resins on the thermal stability was explored and the results show that adding E-epoxy slightly decreased the thermal resistance of the composite, which formed a degradation shoulder at around 356 °C. Based on our previous work[32], the first degradation peak of cured E-epoxy was around 330 °C, which was due to the thermally sensitive compounds in the E-epoxy such as abietic acid and fatty acid. The second degradation temperature was around 416 °C, which was slightly higher than the degradation temperature of P-0 due to the cross-linked lignin. In an air environment (Figure 5c, 5d), the decomposition patterns were more complex than in a nitrogen environment, as the former involved oxidative reactions. There were three major peaks for neat epoxy decomposition, while there were five peaks for both composites. For the temperature region from 300 to 500 °C, these degradation peaks of composites all overlapped and the onset temperature was the same as that of P-0. However, the last degradation peak of composites at the 500-700°C region was 64 °C higher than that of neat P-epoxy resins. Adding 15
E-epoxy resins increased the degradation ratio at a low temperature range (300-350 °C), but the high temperature regions (550-650 °C) were almost intact. Furthermore, there were no cellulose degradation peaks that could be distinguished from the overlapping peaks.
3.6 The Effect of NCFs on the Cure Activation Energies (Ea) of Epoxy Resins The kinetic characterization of curing behaviour is important to better understand the influence of NCFs' surfaces on network formation of the epoxy resins. To avoid any premature assumptions about the reaction mechanism and to reduce noise, the Friedman differential isoconversional method was applied to observe the evolution of the cure Ea at a given conversion rate. Compared to the Kissinger-Akahira-Sunose and Ozawa– Flynn–Wall methods, the Friedman method offers a significant improvement in the accuracy of the Ea value.[47] The basic assumption of this analysis is that the reaction rate at a constant conversion depends only on temperature. In a kinetic analysis, it is generally assumed that the reaction rate can be described by two functions, k(T) and f(α):
𝑑𝛼/𝑑𝑡=k(𝑇)𝑓(𝛼)=𝐴𝑒𝑥𝑝(−𝐸𝑎/𝑅𝑇)𝑓(𝛼)
(3)
where 𝛼 is the conversion rate, t is the time, T is the temperature, k(T) is the rate constant, A is the pre-exponential factor, R is the gas constant, and f(α) is the reaction model. When the heating rate (𝛽) is constant, Equation (1) can be rewritten as:
(𝑑𝛼/𝑑𝑇) = 𝐴𝑒𝑥𝑝 (−𝐸𝑎/𝑅𝑇)(𝛼)
(4)
Then, it is assumed that the function 𝛼 is constant at a particular degree of conversion and the logarithmic derivative of this equation can be written as:
16
ln [𝛽(𝑑𝛼/𝑑𝑇)]= ln (𝐴) −𝐸𝑎, α/𝑅𝑇
(5)
where 𝐸𝑎, α is the activation energy at a given conversion rate. Figure 6 shows 𝐸𝑎 at different curing conversions rates, for both neat epoxy (solid lines) and NCF reinforced epoxy (dotted lines) systems. The DSC results showed that 𝐸𝑎 decreased as a result of the catalytic effect of NCFs. For the P-epoxy, the value of 𝐸𝑎 decreased from 60.6 to 36.6 kJ/mol, while the value of 𝐸𝑎 for E-epoxy decreased from 45.4 to 35.8 kJ/mol. The E-epoxy monomer had a lower 𝐸𝑎 than the P-epoxy monomer because of the hydroxyl groups in the Eepoxy resins, which have been demonstrated in our previous research.[32]In this study, our results show that NCFs could still act as a catalyst for E-epoxy resins to reduce Ea to approximately 36 kJ/mol. Additionally, when there are excess epoxy groups the etherification reaction between epoxy and hydroxyl groups at elevated temperatures are shown in supplementary figure 5. The possibility of this etherification reaction has been previously reported[48]and those newly formed links can promote a good interface between the resins and the fibres.
4. Summary This study investigated the influence of incorporating NCFs into P-epoxy and hybrid P-epoxy/Eepoxy resins. Adding 10 wt% E-epoxy improved toughness of P-epoxy significantly from 2.34 to 4.48 MJ/m3, while adding SE-NCFs (25 wt%) increased the tensile strength and modulus of Pepoxy by 88 % and 298 %, respectively. Although E-epoxy resin can improve toughness, excess amounts of E-epoxy can have an adverse effect on its mechanical performance and Tg. Thus, the highest mechanical strength was obtained with 10 wt% E-epoxy replacement in the matrix. The mechanical performance of bio-epoxy composites obtained here was above the literature 17
reported average values (85 MPa for tensile strength and 4.6GPa for tensile modulus), and close to the values predicted using the rule of mixture. This indicates a strong interface between the fibre and the matrix, which was further supported by the SEM/ToF-SIMS results. In addition, the nanocellulose fibre diameter in the neat commercial epoxy composites was around 253 nm, and the nanocellulose fibre diameter in the hybrid epoxy composite systems was around 195 nm, which suggests that E-epoxy resins have a better penetrating ability and compatibility with the nanocellulose. The addition of NCFs slowed the maximum thermal degradation rate of epoxies in both inert and air environments. Therefore, this study successfully produced strong and ductile NCF-reinforced hybrid epoxy composites with good thermal properties and showed its competitive mechanical performance compared to the commercial products. In order to bring NCF-reinforced hybrid epoxy composites to market, cost-analysis should be performed in the future and its long-term durability should be tested. Acknowledgements The authors would like to acknowledge ORF-Bark Biorefinery Partners for the financial support. Yan would also like to acknowledge support from NSERC DG and DAS program. Appreciation is extended to Andrew Paton and Mariam Khan for their generous help and support. Finally, we would also like to thank Momentive Inc. for providing the commercial resin samples.
18
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22
Figure 1. SEM images in cross-sections of composites (a)(b) and in surfaces of NCF sheets (c)(d). (a) PO-0 shows a clear boundary between epoxy and OD-NCFs, which suggests the surface of OD-NCFs is impenetrable as shown in (c). (b) the SE-NCFs of PS-0 has swelled to approximately 100 µm by resin penetration due to its porous surface(d).
(a) PS-0
(b) HS-1
Figure 2. SEM images of fracture surfaces in cross-sections of (a) PS-0 (b) and HS-1, showing that the diameter of NCFs (white spots) in HS-1 is smaller than that in PS-0.
PO-0
PS-0 23
HS-1
Figure 3. ToF-SIMS mapping images: (a-c) total ion images, and (d-f) fragment ion distribution images [Red: C4H5O2+, signal from cellulose; Green: C6H5NO+, signal from epoxy]. (a,d) PO-0 composite, (b,e) PS-0 composite. (c,f) HS-1 composite.
Figure 4. Tensile properties of SE-NCF reinforced composites compared to literature data (Dashed lines are the average mean from previous studies [17, 19, 20, 23, 34, 35, 41] )
24
Figure 5. Thermal degradation of P-0 and Epoxy/NCFs with various E-epoxy replacement (a)(b) in nitrogen environment (c)(d) in air environment
Figure 6 Dependence of 𝐸𝑎 on the extent of conversion evaluated from dynamic DSC data.
25
Table 1 Compositions of Cured Epoxies and Epoxy/NCF Composites
Sample Name
Monomer weight ratio (P-epoxy : E-poxy)
EEW (g/eq)
OD-NCF
SE-NCF
P-0
10 : 0
169
-
-
H-1
9:1
196
-
-
PO-0
10 : 0
169
✓
-
PS-0
10 : 0
169
-
✓
HO-1
9:1
196
✓
-
HS-1
9:1
196
-
✓
HS-2
8:2
224
-
✓
HS-3
7:3
251
-
✓
*
The amount of curing agent added to the resins was adjusted based on the calculated EEW values for hybrid resins in order to match its stoichiometric ratio of amine and epoxy groups.
26
Table 2 Tensile properties and Tg of the cured epoxies and NCF reinforced epoxy composites
P-0
Tensile strength (MPa) 76 ± 2.9
Tensile Modulus (GPa) 1.96 ± 0.06
2.34 ± 0.02
Tg (°C) 131 ± 1.4
H-1
70 ± 3.2
1.24 ± 0.14
4.43 ± 0.06
121 ± 1.3
PO-0
81 ± 8.0 (↑6 %)
7.70 ± 0.80 (↑293 %)
2.35 ± 0.38
110 ± 0.3
PS-0
112 ± 3.8 (↑47 %)
4.19 ± 0.97 (↑114 %)
2.44 ± 0.08
129 ± 2.9
HO-1
76 ± 9.3 (↑9 %)
6.18 ± 1.42 (↑398 %)
2.52 ± 0.14
87 ± 0.7
HS-1
132 ± 2.4 (↑88 %)
4.94 ± 0.26 (↑298 %)
4.36 ± 0.23
114 ± 1.0
HS-2
100 ± 14.0 (↑42 %)
4.52 ± 0.06 (↑265 %)
2.17 ± 0.04
112 ± 0.0
HS-3
91 ± 5.0 (↑30%)
3.55 ± 0.10 (↑186 %)
2.14 ± 0.26
107 ± 0.5
27
Strain Energy Density (MPa)
S0144-8617(17)30967-0 http://dx.doi.org/10.1016/j.carbpol.2017.08.091 CARP 12698
To appear in: Received date: Revised date: Accepted date:
26-5-2017 11-8-2017 19-8-2017
Please cite this article as: Kuo, Pei-Yu., Barros, Luizmar de Assis., Yan, Ning., Sain, Mohini., Qing, Yan., & Wu, Yiqiang., Nanocellulose Composites with Enhanced Interfacial Compatibility and Mechanical Properties Using a Hybrid-Toughened Epoxy Matrix.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2017.08.091 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nanocellulose Composites with Enhanced Interfacial Compatibility and Mechanical Properties Using a Hybrid-Toughened Epoxy Matrix Pei-Yu Kuoa, Luizmar de Assis Barrosb, Ning Yancde*, Mohini Saincd, Yan Qinge, and Yiqiang Wue a
Department of Forestry and Natural Resources, National Ilan University, No. 1, Section
1, Shennong Road, Yilan City, Yilan County, Taiwan b
Institute of Forestry, Department of Wood Chemistry, University Federal Rural Do Rio
de Janeiro. Rodovia BR 465-Km7 Campus Universitário, Seropédica RJ, 23851-970, Brazil c
Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON, M5S 3B3,
Canada d
Department of Chemical Engineering and Applied Chemistry, University of Toronto,
200 College Street, Toronto, ON, M5S 3E5, Canada e
College of Materials Science and Engineering, Central South University of Forestry and
Technology, Changsha, 410004, China *
Corresponding author E-mail: [email protected]
Highlights
Novel hybrid epoxy improved biofibre wettability and compatibility with the resin matrix
Nanocellulose fibres (NCFs) enhanced mechanical performance of neat epoxy
Incorporating bio-based epoxy into petro-epoxy increased toughness significantly
1
Abstract Although there is a growing interest in utilizing nanocellulose fibers (NCFs) based composites for achieving a higher sustainability, mechanical performance of these composites is limited due to the poor compatibility between fibre reinforcement and polymer matrices. Here we developed a bio-nanocomposite with an enhanced fibre/resin interface using a hybrid-toughened epoxy. A strong reinforcing effect of NCFs was achieved, demonstrating an increase up to 88 % in tensile strength and 298 % in tensile modulus as compared to neat petro-based P-epoxy. The toughness of neat P-epoxy was improved by 84 % with the addition of 10 wt% bio-based E-epoxy monomers, which also mitigated the amount of usage of bisphenol A (BPA). The morphological analyses showed that the hybrid epoxy improved the resin penetration and fibre distribution significantly in the resulting composites. Thus, our findings demonstrated the promise of developing sustainable and high performance epoxy composites combing NCFs with a hybrid petro-based and bio-based epoxy resin system.
Keywords: fibre/resin interface; bio-nanocomposite; nanocellulose fibres; bio-epoxy; hybrid-toughened epoxy
2
1. Introduction
The global concern for the depletion of non-renewable resources has resulted in significant efforts to develop new building blocks from renewable biological resources to synthesize biopolymers and functional chemicals.[1, 2] At the same time, the heightened awareness of potential health risks associated with bisphenol A (BPA), a widely used industrial chemical for making resins and plastics (such as epoxy resins and polycarbonates)[3] has led to strong interests in replacing BPA with natural compounds such as saccharides,[4] vegetable oils,[5] polyphenols,[6] lignin,[7] liquefied biomass[8], etc. Currently numerous bio-based epoxy resins are commercially available in market that include sugar-based epoxy resins (DENACOL™ and ERISYSTM) and oilbased epoxy resins (Vikoflex®, Super Sap™, and Senso™). Despite the strong promise of the biobased epoxies due to their improved sustainability, their lower mechanical properties are a deterrent for high performance applications. Nanocellulose fibres (NCFs) are renewable, lightweight, cost-effective with high aspect ratios and high specific modulus (100-160 GPa[9]). They can be obtained from various bio-resources such as algae,[10] tunicate,[11] bacteria,[12] agricultural residuals,[13] and wood.[14] NCFs have been shown to have strong mechanical reinforcing effects on polymer matrices in composites.[15-17] In addition to mechanical reinforcement, our previous work has also shown that NCFs accelerated the curing process of epoxy resins,[18] and that NCFs’ mildly-acidic surface could react with amine type epoxy curing agents.[19] Therefore, NCFs are well suited as reinforcement for application in epoxy composites. However, there is a significant process challenge in applying NCFs in epoxy composites that is related to the tendency of NCFs suspensions to form gels at rather low concentrations. As a result, most studies in literature only add 0.1-3 wt% concentrations of NCFs into epoxy matrices, which yield limited improvement to mechanical performance.[20-24] To address this issue, some 3
studies incorporated NCFs in sheet format into epoxy resins, which significantly improved mechanical performance due to the higher NCF content.[17] In addition, several studies also used (2,2,6,6-Tetramethylpiperidin-1-yl) oxyl (TEMPO)[21], natural rubber[25], silane,[26], acetic acid[27], fatty acid methyl ether[28] to pretreat NCFs to enhance fibre/matrix interphase prior to the application in matrix. However, untreated NCFs have not been used in bio-epoxy resins in the literature,[21] and to the best of our knowledge, no mechanical improvement of bio-epoxy by NCFs has been reported[15], and no study has looked into applying NCFs in a hybrid epoxy system. The major advantage of using a hybrid-epoxies system as proposed in this study is its potential to have a higher toughness than the conventional epoxy systems. The improvement is caused by the added epoxy monomers from the bio-epoxy reducing the cross linking density of the existing epoxy resin, and by the new bio-epoxy monomers forming a second network that can create interpenetrating phase in the composite. Furthermore, the presence of polar groups in the second phase bio-epoxy monomer phase has been shown to significantly improve the adhesion properties of epoxy resins.[29] Previous studies indicate that bio-epoxy resins can improve the toughness and adhesion properties of conventional epoxy systems.[30, 31] Among all types of bio-epoxy resins, tree bark extractive-based epoxy resin (E-epoxy) has a low cost, high mechanical performance, high thermal resistance, and contains significant amounts of hydroxy groups,[32] making it an attractive alternative of composite matrix. The aim of this study is to better understand the reinforcement effect that NCFs have on the hybrid epoxy system and the influence of the hybrid resin on the fibre/resin interface. Resin curing behaviour, fibre/matrix interface, mechanical performance, and thermal stability of the NCFs reinforced hybrid epoxy composites are also investigated in detail.
2. Materials and Methods 2.1 Materials 4
NCFs and E-epoxy monomer were both lab-made and the preparation procedure and characterization results were previously reported.[32, 33] NCFs used in this study were obtained via mechanical grinding and E-epoxy was synthesized using pine bark extractives reacting with epichlorohydrin with an epoxy equivalent weight of 442 g/eq. The commercial petroleum-based epoxy monomer (P-epoxy), Epon Resin 863 with an epoxy equivalent weight (EEW) of 169 g/eq, was used as the control sample. Epikure W was applied as the curing agent with amine hydrogen equivalent
weight
(AHEW)
of
44
g/eq,
and
its
major
component
is
diethylmethylbenzenediamine (DETDA). The mixing ratio between epoxies/curing agent was determined by their EEW and AHEW values to reach 1:1 stoichiometric ratio of amine hydrogen to epoxide groups. The P-epoxy monomer and the curing agent were obtained from Momentive Specialty Chemicals, OH, USA. Acetone (> 99.5%) was purchased from Caledon Laboratory Chemicals, ON, Canada and used without further purification.
2.2 Methods The manufacturing process of NCF/Epoxy composites is described in detail below and illustrated in supplementary figure 1. 2.2.1 Nanocellulose Sheets Preparation 10.7g gel-state NCFs (2.5 wt%) was diluted to reach 0.1 wt% in concentration and air bubbles were removed from the solution using an ultrasonic bath. To form a nanocellulose fiber sheet the suspension was vacuum-filtered through a Supor-100 membrane filter (pore size = 0.1 µm, Pall Corporation) in a Buchner funnel. After filtration, the wet fibre films were carefully removed from the membrane, and wet-pressed following a standard TAPPI process. The wet film was placed between two filter membranes and compressed by a standard TAPPI wet press with 50 psi pressure for seven minutes. The NCF sheet was then removed from the membrane, and dried in the oven (oven-dried NCFs; OD-NCFs) or immersed in acetone overnight in order to prepare 5
solvent exchanged NCF (solvent-exchanged NCFs; SE-NCFs). Both OD-NCFS sheets and SE-NCF sheets were cut into dogbone shapes with dimensions suggested by ASTM D638-5 for standard tensile test. Ten cut NCF specimens were dried in the oven at 55˚C for 24 hours to test the mechanical properties of NCFs and the rest 30 NCF specimens were used for the following composite preparations.
2.2.2 Composite Preparation Cut NCFs were immersed into either P-epoxy or hybrid epoxy overnight. The amount of curing agent added to the resins was adjusted from 16 wt% to 9 wt% based on the calculated EEW values for hybrid resins in order to match its stoichiometric ratio of amine and epoxy groups. To reduce the viscosity of both resins, P-epoxy and hybrid epoxy resin were diluted with acetone at a 1:1 ratio. Next, the resin-filled NCF composites were removed from the bath and cured in an oven with the following curing schedule: 1h at 65˚C, 1h at 80˚C, 1h at 121˚C, and 2h at 177˚C (post-cure). Same curing schedule was applied on the epoxies without NCFs to make control samples for comparison. The compositions for the cured epoxies and epoxy composite are listed in Table 1.
2.3 Characterization 2.3.1 Curing Activation Energy (Ea) and Glass Transition Temperature (Tg) The Ea and Tg of the epoxies and the epoxy composites were evaluated using a dynamic differential scanning calorimetry (DSC) model Q 100 (TA instruments, USA). Dynamic DSC measurements were carried-out at a ramp rate of 5, 10, 15, and 20 °C/min, from 30 to 300 °C in order to obtain the curing heat-flow curves of liquid P-epoxy and E-epoxy resins as well as their composites. The cured samples were heated to 300 °C at 10°C /min in order to detect the Tg of both epoxies and their composites.
6
2.3.2 Tensile Properties The tensile tests of the cured epoxies, composite specimens, and NCF sheets were performed using a universal testing machine, Model 3367 (Instron instruments, USA), equipped with a 2 kN load cell. Samples were cut into dogbone shaped specimens using an ASTM 638-5 specimencutting die. The sample thickness was determined using a bench top micrometer. The span length of the specimen was 25 mm, with a cross-head speed of 2.5 mm/min for tensile testing. Five specimens were measured for each composition.
2.3.3 Morphological Characterization Scanning electron microscope (SEM) images were used to study the morphology of NCFs and its composites. All the images were acquired using a JSM6610-Lv (JEOL, Japan) with an accelerating voltage 10-15 kV. To visualize the distinct features of fibres, NCF sheets were prepared using liquid nitrogen to freeze the water inside NCF sheets and then slowly dehydrolyzed to maintain their surface structure and avoid cell collapse. Composite samples were examined on their crosssectional areas after their tensile tests. All the samples were coated with gold prior to SEM imaging.
2.3.4 Thermal Stability The thermal stability of the cured neat epoxy resins and composites were investigated using a thermogravimetric analysis, TGA-Q500 (TA Instruments, USA). Approximately 10 mg of each sample was weighed in a platinum pan and operated under a continuous flux of air or nitrogen (60 cm3/min). Under dynamic analysis, all of the samples were heated from room temperature to 700 °C, with a 10 °C/min ramp. 2.3.5 Wetting Behaviour
7
The surface tension of resins, surface energy of NCF sheets, and contact angles between resins and NCF sheets were measured using an OCA 15 Plus goniometer (DataPhysics Instruments GmbH, Germany) at room temperature. The static contact angle was measured by a sessile drop method and the surface tension was measured by a pendant drop method with a microsyringe tip diameter of 6um. Images of contact angles were recorded with a microscopic video recording system and the values of these angles were determined from the image using OCA 15 software. At least 5 measurements were averaged to calculate one mean value. The surface energy of NCF sheets was calculated by contact angles of two types of liquids on the fibre surface, including ultrapure water (18.4 MΩ٠cm) and diiodomethane. 2.3.6 Surface Chemical Analysis and Element Mapping Time-of-flight secondary ion mass spectrometry was used to identify the interface between NCF sheets and epoxy resins using a ToF-SIMS5 (IonTof GmbH, Munster, Germany). Both positive and negative ion spectra were obtained with 60 keV Bi3++ primary ions operated in a high-current bunched mode. The scanned areas were at least 100 x 100 μm2 covered by 164 x 164 pixels and the primary ion dose density was between 1 x 109 - 1 x 1013 ions/cm2. In order to obtain high resolution mapping images, each composite sample was immersed in liquid nitrogen and then was cut by microtome to have a clear cross section without leaving any cutting mark. 2.3.7 Fourier Transform Infrared Spectroscopy (FTIR). Samples were sandwiched between two KBr crystals and studied using a Bruker Tensor 27 spectrometer in an environmental chamber. All spectra were recorded over 4000−600 cm−1 at a resolution of 4 cm−1 with 32 scans.
3. Results and Discussion 3.1 Morphological Characterization of Epoxy/NCF Composites 8
The structure of composite materials, especially fibre/matrix interfaces, plays an important role in determining its overall mechanical performance. In this study, the structure of PO-0 composite (reinforced by OD-NCFs) possessed a laminated sandwich layout (Figure 1a); the structure of PS-0 composite (reinforced by SE-NCFs) depicted a homogeneous well-mixed layer (Figure 1b). In Figure 1a, the face layers were epoxy resins and the core layer was made by OD-NCFs. This compact fibre sheet was approximately 15-25 μm thick and had a clear borderline between itself and the matrix due to the impenetrability of the dried fibre sheet. By contrast, the SE-NCF in composite swelled to approximately 90-100 μm due to resin penetration (Figure 1b). Since SE-NCFs never experienced a drying process with all water being replaced by acetone, SE-NCFs possessed a fibrous surface morphology, while OD-NCFs exhibited a compact and smooth surface (Figure 1c, d). This compact surface was caused by the collapse of nanocellulose fibrils driven by capillary force during water evaporation. The dense structure was coalesced by hydrogen bonds that formed between hydroxyl groups of the cellulose or hemicellulose,[34] producing a dense surface that inhibited the penetration of epoxy resins into the NCF network. Therefore, OD-NCFs were nearly impenetrable, and a clear boundary was observed between OD-NCFs and epoxy resins in the cross section of the composites. At a higher magnification, observation of the PS-0 showed a matrixcovered appearance with many white spots showing possibly the fracture surfaces of single or bundled NCFs. As reported by many studies,[35, 36] the individual fractured NCFs appear as white dots in SEM images. Their short pull-out length indicates strong interfacial adhesion and strong intermolecular interactions, likely due to either the surface hydroxyls of the NCFs reacting with the oxirane group through an etherification reaction or due to secondary bonding between these hydroxyl groups. 9
Compared to the P-epoxy, the E-epoxy resin had a higher amount of hydroxyl groups, which not only accelerated the curing reaction between amine and epoxy resins, but also had the potential to form strong hydrogen bonding with NCFs. The influence of adding E-epoxy resins on the diameter of NCF bundles was depicted in Figure 2 showing the bundle cross-sections in (a) Pepoxy/NCFs and (b) hybrid epoxy/NCFs. The diameter of NCF bundles was calculated by ImageJ software (1.48 version, National Institutes of Health, USA). The diameter of white dots in PS-0 ranged from 83 nm to 502 nm with an average of 253 nm ± 83 nm (n=50); the white dot diameters in HS-1 ranged from 72 nm to 626 nm with an average of 195 nm ± 117 nm (n=50), which is significantly smaller than that in PS-0 (p < 0.001, Mann-Whitney U test). This suggests that the hybrid epoxy resins can wet and penetrate NCFs better than neat P-epoxy resins can. To better understand the effect of E-epoxy on resin penetration, wetting behaviour and surface/interface characterizations are presented in the following sections.
3.2 Wetting Behaviour The wetting behaviour of a solid by a liquid is a crucial factor in tuning interfacial bonding strength of composites. In this study, the solid phase is OD-NCFs and the liquid phase is either P-epoxy or hybrid P-Epoxy/E-epoxy. The surface energy of OD-NCFs was indirectly estimated through contact angle measurements using Owens-Wendt-Rabel-Kaelble plotting method. The surface energy of NCFs can be divided into two major components – polar and dispersive. The dispersion force dominates the interactions between the nanocellulose fiber and the hydrophobic matrix. The OD-NCFs used in this study showed a dispersive component of 36.8 mN/m within the range of reported values (27.4 to 50 mN/m)[37-39]. The surface energy of resins was measured through the pendant drop method. In order to match the processing procedure, both resins were diluted by acetone with 1:1 weight ratio. The surface tensions of both resin mixtures were close to the surface tension of acetone (26.3 mN/m). Although the surface 10
tension values of the two resins did not have statistically significant difference, their contact angles on OD-NCFs were varied. The contact angle of P-epoxy (27.6 mN/m) on OD-NCFs was significantly higher than that of E-epoxy (16.2 mN/m) as both contact angles were measured at approximately 30 seconds after droplet deposition (Suppl. Fig. 2). The smaller the contact angle is, the larger the wetting tendency will be. Therefore, our findings implied E-epoxy had a better wettability on NCFs than P-epoxy. 3.3 Surface Chemical Analysis and Mapping The surface chemistry of NCFs/epoxy composites was studied using FTIR and ToF-SIMS techniques. The general characterization of functional groups using FTIR is shown in supplementary figure 3. ToF-SIMS was applied to identify the molecules and their corresponding secondary ion distribution on the surface of the composite cross section with a depth resolution of 1–2 nm. Positive mass spectra of NCF sheets, cured resins, and composites are shown in supplementary figure 4. Using the information from mass spectra, two mass fragments were selected to visualize the morphology of the composite cross sections. C4H5O2+ was used to represent nanocellulose and C6H5NO+ was used to represent epoxy resin. The overall images were depicted by total ions (figure 3a-c) and the corresponding fragment ion images (figure 3df) are shown in two colors: red (C4H5O2+, signal from nanocellulose) and green (C6H5NO+, signal from epoxy). All the images were normalized by setting the highest intensity to 100% signal. Image 3d shows a distinctive boundary between NCF sheets and epoxy indicating an impenetrable surface on dried NCFs, which is consistent with the findings of SEM images shown in Figure 1(a). Image 3e shows the partial overlay of two colors, but the shape of solventexchanged NCFs is still visible in the image. Thus, solvent-exchange method can improve the resin penetration, but the fibre distribution is still heterogeneous. Image 3f depicts a homogeneous layer after introducing a hybrid E-epoxy/P-epoxy system, which suggests E-epoxy
11
enhanced the resin penetration and the fibre distribution. Combining these findings, E-epoxy resin shows an enhanced interfacial property.
3.4 Mechanical Performance of Cured Epoxies and Epoxy/NCF Composites The average tensile strength, tensile modulus, strain energy density, and Tg of P-0 and H-1 are summarized in Table 2. In this study, toughness, also called strain energy density, was estimated by the total area under the stress-strain curve during tensile strength testing. After adding 10 wt% E-epoxy monomers, the tensile strength of H-1 decreased slightly, from 76 MPa to 70 MPa, but the strain improved from 6.6 % to 12.3 %, which led to an overall toughness increase by 84 %. Student’s pair t-test analysis of P-0 and H-1 data revealed that tensile strength and tensile modulus were not statistically significantly different at 1% level (p = 0.087 and 0.111), but there was a significant increase in the toughness (p = 0.005). Thus, the E-epoxy monomer acted as a toughening agent to improve elongation. Compared to neat epoxy systems, incorporating the NCFs into resins shows a strong reinforcement effect on tensile properties (Table 2).
Based on the percolation theory, the percolation threshold of NCFs connectivity is less than 6 vol%,[40] but NCFs between 6 vol% to 18 vol% would not form a dense sheet network.[35] Thus, the fibre loading of this study was set at 25 wt% (equal to 20.4 vol% when NCF density was assumed to be 1.5 g/cc). The improvements to the tensile strengths of PO-0 alone and HO-1 after incorporating oven-dried NCFs were 6.0 % and 8.7 %, respectively. With SE-NCFs, the increase in the tensile strengths of PS-0 and HS-1 reached 47.4 % and 88.0 %, which was significantly higher than the oven dried NCFs. This is likely due to the reduction of the stress concentration on the interface of the solvent-exchanged NCF composites. Without solvent exchange, most of the stress is localized at the interfacial area between NCFs and epoxy resins, leading to low tensile strength. Therefore, solvent-exchange treatment can prevent stress 12
concentration and increase mechanical strength. By contrast, the tensile modulus of OD-NCF composites exhibited a 293% and 398% increase over neat epoxy due to their 2D structures. With a strong bonding between fiber/matrix interface, 2D reinforcements generally have higher enhancement in strength and modulus than 3D does. However, a poor interface between fiber/matrix results in 2D composites with a low strength and high modulus. Once the resin penetrated into NCFs, the structure of NCFs changed from 2D fiber sheet to 3D swelled network, which increased the tensile strength of composites but decreased its tensile modulus. HS-1 and H-1 showed improved toughness compared to PS-0 and P-0. There are two possible explanations for this improvement from the E-epoxy addition. The first reason is that the Eepoxy resin reduces the crosslinking density of the network by inserting some large molecules into the network structure. To support this reason, it was observed that the Tg of what decreased from 114 °C to 107 °C with the addition of E-epoxy. The second reason for the improved toughness is that the E-epoxy may form an interpenetrating network (IPN), which increases the size of physical cross-linked domains. The second network can provide space for microdeformation and increase the toughness. Thus, we tested the composites with various E-epoxy amounts (10-30 wt% of monomers) in order to create a more ductile composite, but the greatest toughness was achieved by adding 10 wt% E-epoxy. With more than 10 wt% E-epoxy, the tensile strength and toughness decreased. Many studies have also shown the optimal amount of bioepoxy to be added into conventional epoxy is approximately 10-20 wt%.[29, 40]
According to the literature,[17, 19, 20, 23, 34, 35, 41] the average tensile strength and modulus of conventional epoxy/NCF composites are 85 MPa and 4.6 GPa, respectively, for NCFs from a topdown mechanical grinding process. Our findings show an increased tensile strength, and a comparable tensile modulus compared to the literature report average value (Figure 4, our experimental results are labeled as red triangles). In contrast to surface modified NCFs epoxy 13
composites[42], our finding also exhibits better mechanical performance. Furthermore, we compared our experimental data to the values we predicted based on the rules of mixture (ROM). A previous study states that the mechanical properties can be better described by the ROM than by other micromechanical models,
[35]
such as the Halpin-Tsai model or the Cox-
Krenchel model for epoxy/NCF composite. Therefore, the predicted tensile properties of the nanocomposites used in this study were obtained using the ROM equations: Ecomposite = VfEf + (1-Vf)Em σcomposite = Vfσf + (1-Vf) σm
(1) (2)
where Ecomposite and σcomposite represent the predicted tensile modulus and strength, respectively, Vf denotes the fibre volume fraction, Ef, σf, Em and σm correspond to the tensile modulus and tensile strength of NCFs (f) and matrix (m), respectively. We utilized the highest tensile properties of nanocellulose sheet (σf = 243 MPa and Ef = 14.9 GPa),[34] reported in literature, and the tensile properties of matrix (σm = 70.2 MPa and Em = 1.24 GPa) measured in this study to fit into the prediction model. The results show that our experimental values are very close to the theoretical value. An alternative approach is to calculate a reasonable range of the mechanical properties of composites instead of a single value. Identifying a range of possible values is a practical way to evaluate the mechanical performance of composites since the performance can be affected by many factors such as geometry of reinforcements, micro-cracks, and adhesion force between two components. According to the equations derived from Christenson[43] and Watt and Peseinick[44], for tensile modulus, the lower bound is 1.72 GPa and the upper bound is 11.89 GPa, which encloses our experimental data (4.19-7.70 GPa).
3.5 Thermal Stability 14
Epoxy resins and composites mass loss as a function of temperature was measured by TGA shown in Figure 5. The maximum degradation temperature of the reference P-0 sample was 387°C; after incorporating NCFs, the maximum degradation temperature increased to 411°C, which is likely due to the formation of porous structures, or the formation of strong interactions between the fibres and the matrix.[45] The porous structures would be formed after thermal degradation of all NCFs that can decelerate thermal conductivity. Unlike the maximum degradation temperature, the onset decomposition temperature of composites decreased from 373 °C to 337 °C because of the inherent properties of the cellulose fibre. As reported by other research, cellulose dehydrates between 200 and 280 °C and extensive degradation of cellulose occurs when the temperature is around 300 °C, which is lower than epoxy resins.[46] Thus, even though the maximum degradation temperature of composite improved 24 °C compared to neat epoxy, the overall heat-resistance index temperature (Ts) does not show a significant difference. Additionally, the effect of E-epoxy resins on the thermal stability was explored and the results show that adding E-epoxy slightly decreased the thermal resistance of the composite, which formed a degradation shoulder at around 356 °C. Based on our previous work[32], the first degradation peak of cured E-epoxy was around 330 °C, which was due to the thermally sensitive compounds in the E-epoxy such as abietic acid and fatty acid. The second degradation temperature was around 416 °C, which was slightly higher than the degradation temperature of P-0 due to the cross-linked lignin. In an air environment (Figure 5c, 5d), the decomposition patterns were more complex than in a nitrogen environment, as the former involved oxidative reactions. There were three major peaks for neat epoxy decomposition, while there were five peaks for both composites. For the temperature region from 300 to 500 °C, these degradation peaks of composites all overlapped and the onset temperature was the same as that of P-0. However, the last degradation peak of composites at the 500-700°C region was 64 °C higher than that of neat P-epoxy resins. Adding 15
E-epoxy resins increased the degradation ratio at a low temperature range (300-350 °C), but the high temperature regions (550-650 °C) were almost intact. Furthermore, there were no cellulose degradation peaks that could be distinguished from the overlapping peaks.
3.6 The Effect of NCFs on the Cure Activation Energies (Ea) of Epoxy Resins The kinetic characterization of curing behaviour is important to better understand the influence of NCFs' surfaces on network formation of the epoxy resins. To avoid any premature assumptions about the reaction mechanism and to reduce noise, the Friedman differential isoconversional method was applied to observe the evolution of the cure Ea at a given conversion rate. Compared to the Kissinger-Akahira-Sunose and Ozawa– Flynn–Wall methods, the Friedman method offers a significant improvement in the accuracy of the Ea value.[47] The basic assumption of this analysis is that the reaction rate at a constant conversion depends only on temperature. In a kinetic analysis, it is generally assumed that the reaction rate can be described by two functions, k(T) and f(α):
𝑑𝛼/𝑑𝑡=k(𝑇)𝑓(𝛼)=𝐴𝑒𝑥𝑝(−𝐸𝑎/𝑅𝑇)𝑓(𝛼)
(3)
where 𝛼 is the conversion rate, t is the time, T is the temperature, k(T) is the rate constant, A is the pre-exponential factor, R is the gas constant, and f(α) is the reaction model. When the heating rate (𝛽) is constant, Equation (1) can be rewritten as:
(𝑑𝛼/𝑑𝑇) = 𝐴𝑒𝑥𝑝 (−𝐸𝑎/𝑅𝑇)(𝛼)
(4)
Then, it is assumed that the function 𝛼 is constant at a particular degree of conversion and the logarithmic derivative of this equation can be written as:
16
ln [𝛽(𝑑𝛼/𝑑𝑇)]= ln (𝐴) −𝐸𝑎, α/𝑅𝑇
(5)
where 𝐸𝑎, α is the activation energy at a given conversion rate. Figure 6 shows 𝐸𝑎 at different curing conversions rates, for both neat epoxy (solid lines) and NCF reinforced epoxy (dotted lines) systems. The DSC results showed that 𝐸𝑎 decreased as a result of the catalytic effect of NCFs. For the P-epoxy, the value of 𝐸𝑎 decreased from 60.6 to 36.6 kJ/mol, while the value of 𝐸𝑎 for E-epoxy decreased from 45.4 to 35.8 kJ/mol. The E-epoxy monomer had a lower 𝐸𝑎 than the P-epoxy monomer because of the hydroxyl groups in the Eepoxy resins, which have been demonstrated in our previous research.[32]In this study, our results show that NCFs could still act as a catalyst for E-epoxy resins to reduce Ea to approximately 36 kJ/mol. Additionally, when there are excess epoxy groups the etherification reaction between epoxy and hydroxyl groups at elevated temperatures are shown in supplementary figure 5. The possibility of this etherification reaction has been previously reported[48]and those newly formed links can promote a good interface between the resins and the fibres.
4. Summary This study investigated the influence of incorporating NCFs into P-epoxy and hybrid P-epoxy/Eepoxy resins. Adding 10 wt% E-epoxy improved toughness of P-epoxy significantly from 2.34 to 4.48 MJ/m3, while adding SE-NCFs (25 wt%) increased the tensile strength and modulus of Pepoxy by 88 % and 298 %, respectively. Although E-epoxy resin can improve toughness, excess amounts of E-epoxy can have an adverse effect on its mechanical performance and Tg. Thus, the highest mechanical strength was obtained with 10 wt% E-epoxy replacement in the matrix. The mechanical performance of bio-epoxy composites obtained here was above the literature 17
reported average values (85 MPa for tensile strength and 4.6GPa for tensile modulus), and close to the values predicted using the rule of mixture. This indicates a strong interface between the fibre and the matrix, which was further supported by the SEM/ToF-SIMS results. In addition, the nanocellulose fibre diameter in the neat commercial epoxy composites was around 253 nm, and the nanocellulose fibre diameter in the hybrid epoxy composite systems was around 195 nm, which suggests that E-epoxy resins have a better penetrating ability and compatibility with the nanocellulose. The addition of NCFs slowed the maximum thermal degradation rate of epoxies in both inert and air environments. Therefore, this study successfully produced strong and ductile NCF-reinforced hybrid epoxy composites with good thermal properties and showed its competitive mechanical performance compared to the commercial products. In order to bring NCF-reinforced hybrid epoxy composites to market, cost-analysis should be performed in the future and its long-term durability should be tested. Acknowledgements The authors would like to acknowledge ORF-Bark Biorefinery Partners for the financial support. Yan would also like to acknowledge support from NSERC DG and DAS program. Appreciation is extended to Andrew Paton and Mariam Khan for their generous help and support. Finally, we would also like to thank Momentive Inc. for providing the commercial resin samples.
18
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Figure 1. SEM images in cross-sections of composites (a)(b) and in surfaces of NCF sheets (c)(d). (a) PO-0 shows a clear boundary between epoxy and OD-NCFs, which suggests the surface of OD-NCFs is impenetrable as shown in (c). (b) the SE-NCFs of PS-0 has swelled to approximately 100 µm by resin penetration due to its porous surface(d).
(a) PS-0
(b) HS-1
Figure 2. SEM images of fracture surfaces in cross-sections of (a) PS-0 (b) and HS-1, showing that the diameter of NCFs (white spots) in HS-1 is smaller than that in PS-0.
PO-0
PS-0 23
HS-1
Figure 3. ToF-SIMS mapping images: (a-c) total ion images, and (d-f) fragment ion distribution images [Red: C4H5O2+, signal from cellulose; Green: C6H5NO+, signal from epoxy]. (a,d) PO-0 composite, (b,e) PS-0 composite. (c,f) HS-1 composite.
Figure 4. Tensile properties of SE-NCF reinforced composites compared to literature data (Dashed lines are the average mean from previous studies [17, 19, 20, 23, 34, 35, 41] )
24
Figure 5. Thermal degradation of P-0 and Epoxy/NCFs with various E-epoxy replacement (a)(b) in nitrogen environment (c)(d) in air environment
Figure 6 Dependence of 𝐸𝑎 on the extent of conversion evaluated from dynamic DSC data.
25
Table 1 Compositions of Cured Epoxies and Epoxy/NCF Composites
Sample Name
Monomer weight ratio (P-epoxy : E-poxy)
EEW (g/eq)
OD-NCF
SE-NCF
P-0
10 : 0
169
-
-
H-1
9:1
196
-
-
PO-0
10 : 0
169
✓
-
PS-0
10 : 0
169
-
✓
HO-1
9:1
196
✓
-
HS-1
9:1
196
-
✓
HS-2
8:2
224
-
✓
HS-3
7:3
251
-
✓
*
The amount of curing agent added to the resins was adjusted based on the calculated EEW values for hybrid resins in order to match its stoichiometric ratio of amine and epoxy groups.
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Table 2 Tensile properties and Tg of the cured epoxies and NCF reinforced epoxy composites
P-0
Tensile strength (MPa) 76 ± 2.9
Tensile Modulus (GPa) 1.96 ± 0.06
2.34 ± 0.02
Tg (°C) 131 ± 1.4
H-1
70 ± 3.2
1.24 ± 0.14
4.43 ± 0.06
121 ± 1.3
PO-0
81 ± 8.0 (↑6 %)
7.70 ± 0.80 (↑293 %)
2.35 ± 0.38
110 ± 0.3
PS-0
112 ± 3.8 (↑47 %)
4.19 ± 0.97 (↑114 %)
2.44 ± 0.08
129 ± 2.9
HO-1
76 ± 9.3 (↑9 %)
6.18 ± 1.42 (↑398 %)
2.52 ± 0.14
87 ± 0.7
HS-1
132 ± 2.4 (↑88 %)
4.94 ± 0.26 (↑298 %)
4.36 ± 0.23
114 ± 1.0
HS-2
100 ± 14.0 (↑42 %)
4.52 ± 0.06 (↑265 %)
2.17 ± 0.04
112 ± 0.0
HS-3
91 ± 5.0 (↑30%)
3.55 ± 0.10 (↑186 %)
2.14 ± 0.26
107 ± 0.5
27
Strain Energy Density (MPa)