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Incorporation of tyre rubber into wood plastic composites to develop novel multifunctional composites: Interface and bonding mechanisms
Industrial Crops & Products 141 (2019) 111788
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Incorporation of tyre rubber into wood plastic composites to develop novel multifunctional composites: Interface and bonding mechanisms Yonghui Zhoua,b, Yuxuan Wanga,b, Mizi Fana,b, a b
T
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College of Materials Engineering, Fujian Agriculture and Forestry University, PR China Department of Civil and Environmental Engineering, College of Engineering, Design and Physical Sciences, Brunel University London, UB8 3PH, United Kingdom
A R T I C LE I N FO
A B S T R A C T
Keywords: Rubber-wood-plastic composites Interface structure Bonding mechanism Mechanical property Nanoindentation
This paper reports the novel formulation of rubber-wood-plastic composites (RubWPC) with the focus on their interfacial optimisation by using maleated and silane coupling agents. Spectroscopic techniques were used to examine the variation of chemical properties of RubWPC after coupling agent treatments. The unveiled chemical interactions between the coupling agents and substrates suggested the improvement of constituent compatibility and interfacial adhesion, promoting distribution and embedment of wood and rubber particles in polyethylene (PE) matrix, which were confirmed by Scanning Electron Microscope (SEM) analysis. Compared to untreated and single coupling agent treated composites, the composite treated with the combination of maleic anhydride grafted polyethylene (MAPE) and bis(triethoxysilylpropyl)tetrasulfide (Si69) coupling agents possessed superior tensile properties (i.e. tensile strength increased by 57.1% and tensile modulus increased by 20.7%) owing to the enhanced interfacial bonding and more efficient stress transfer. In addition, the nanomechanical property of MAPE&Si69 treated composite (i.e. elastic modulus increased from 15.93 GPa to 19.25 GPa) was also increased due to the penetration of matrix resin into wood cells as well as the reaction between wood cell walls and the coupling agents.
1. Introduction The rising environmental concern towards the disposal of problematic waste tyres has led to increasing interests in economical and sustainable recycling and reuse of tyre rubber (Formela et al., 2017; Wu et al., 2007; Zhao et al., 2010). The special characteristics of tyre rubber being vulcanised and unmolten unfavourably restrict its recyclability and application values (Alexandre-Franco et al., 2010; Sonnier et al., 2006). Wood plastic composite (WPC) has grown rapidly in recent years mainly due to its emerging applications in automotive and aerospace components, building and construction products, industrial and consumer goods (Bajwa et al., 2011; Bi et al., 2018; de Oliveira et al., 2017; Pandey et al., 2017; Ramires and Frollini, 2012). In consideration of consistent growth and massive market share of WPC materials, the incorporation of ground tyre rubber (GTR) into WPC might be a highly promising approach for realising the valorisation of used tyres, which would generate a new generation of WPC, namely rubber-wood-plastic composites (RubWPC). The additional use of GTR in WPC will provide the resulted RubWPC with many advantageous properties beyond WPC considering the unique vibration damping, slip resistance, wearing and acoustic performance rubber possesses (Formela et al., 2017; Ramarad
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et al., 2015). Furthermore, the incorporation of rubber indicates the reduction of the use of wood in the system, which benefits the mitigation of the strong competition of wood resources between various industries, i.e. energy, chemicals and materials (Bais-Moleman et al., 2018). In the literature, there have been few investigations of the application of waste tyres in the development of cellulosic polymer composites. Cosnita et al prepared recycled rubber-PET (polyethylene terephthalate)-HDPE (high density polyethylene)-wood composites. The addition of inorganic fillers (e.g. CaO and ZnO/fly ash) contributed to the increase of dimensional stability and mechanical properties of the composites, thus enabling their application as paving slabs, park carpets or application in wet outdoor working environment (Cazan et al., 2019; Cosnita et al., 2017). Filho et al developed the hybrid composites based on epoxy polymer, rubber tyre particles and sugarcane bagasse fibres and examined the energy absorption properties of the composites through the impact tests (Moni Ribeiro Filho et al., 2019). In RubWPC system, the compatibility and bonding of wood-plastic, rubber-plastic and wood-rubber phases are the indispensable issues to be dealt with and overcome in order to formulate a reasonable RubWPC and promote its speedy industrialisation. However, these issues remain the least
Corresponding author. E-mail address: [email protected] (M. Fan).
https://doi.org/10.1016/j.indcrop.2019.111788 Received 8 April 2019; Received in revised form 12 September 2019; Accepted 14 September 2019 Available online 30 September 2019 0926-6690/ © 2019 Published by Elsevier B.V.
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2.3. FTIR analysis
understood, and have not yet been specifically studied to the best of our knowledge. This work investigates the novel formulation of rubber-wood-plastic composites (RubWPC) with the focus on the optimisation of the constituent compatibility and bonding by employing MAPE, Si69 and vinyltrimethoxysilane (VTMS) coupling agents. In our previous studies, MAPE and VTMS were found to be very effective in refining the compatibility and bonding between wood flour and PE (Rao et al., 2018), whilst Si69 presented superior results in enhancing the bonding between rubber particles and PE (Zhou and Fan, 2017). Therefore, these coupling agents were selected to optimise the bonding of RubWPC in this work. Spectroscopic (Fourier Transform Infrared Spectroscopy (FTIR) and 13C Nuclear Magnetic Resonance Spectroscopy (NMR)) and microscopic (SEM) techniques were used to comprehensively scrutinise the chemical structure, constituent compatibility, filler distribution, and microstructure of both untreated and treated RubWPC, thus to unveil the bonding mechanisms. Furthermore, the bulk and in situ mechanical property of RubWPC was determined by conducting tensile property and nanoindentation analyses in order to establish the structure-performance relationship of the composites.
FTIR spectra of RubWPC samples with the dimension of 2 mm × 2 mm × 1 mm were recorded on a PerkinElmer Spectrum One Spectrometer. The spectrum acquisition was operated at following conditions: 4 cm−1 resolution, 16 scans and 4000 – 650 cm−1 wave number range.
2.4. Solid state
13
C NMR analysis
Solid state 13C NMR analysis of RubWPC was conducted on a Bruker spectrometer with a CPMAS (Cross-polarisation/magic angle spinning) probe. Samples were packed into 7 mm zirconia rotors and spun at 6 kHz for spectrum acquisition in the region from -130 ppm to 270 ppm.
2.5. SEM analysis All the composites with the dimension of 5.0 mm × 1.4 mm (cross section) were transversely cut using a sliding microtome and plated with gold for microstructure examination. The observation was conducted on a Leo 1430 V P SEM operating at 15 kV.
2. Materials and methods 2.1. Materials
2.6. Tensile property analysis
The materials and additives used in the work were summarised in Table 1. All the raw materials and additives were stored in a cool dry place before uses.
Tensile test was carried out on an Instron 5900 Universal Testing Instrument in accordance with BS EN ISO 527-2:2012. The crosshead speed of the test was 1 mm/min. Tensile strain is calculated by measuring the elongation the specimen undergoes during tensile testing, is expressed as a percentage (%). The average of six measurements for each composite was reported. The tensile properties of the recycled PE matrix were also measured and given for reference: tensile stress 23.05 ± 0.17 MPa, tensile strain 10.35 ± 0.32%, and tensile modulus 2385.4 ± 133.3 MPa.
2.2. Formulation of RubWPC Table 2 summarised the formulations of untreated and coupling agent treated RubWPC. The sample preparation procedure was given as follows: the required amount of PE was first placed in a Plastograph twin-screw mixer (Brabender GmbH, Germany) and allowed to completely melt at 100 rpm and 190 °C for 2 min. Subsequently, rubber powder and wood flour were added into the mixer and mixed with PE for 3 min to obtain uniform mixtures, followed by mixing with lubricants and coupling agents for another 10 min to allow their reaction with the raw materials. Finally, the RubWPC blends were ground into pellets and compression moulded on an electrically heated hydraulic press at 190 °C and 9.81 MPa pressure for 10 min. The thickness of the samples was 1.4 mm, and an image of the samples was presented in Fig. 1.
2.7. Nanoindentation analysis Nanoindentation measurement was carried out on a Nano Indenter (Hysitron TI 950 TriboIndenter, USA) under load-controlled mode. The detailed description of sample preparation and testing procedure could be referred to our previous study (Zhou et al., 2017).
Table 1 Raw materials and additives used in the work. Material/Additive
Characteristics
Supplier
Recycled polyethylene pellet
JFC Plastics Ltd (UK)
Recycled tyre rubber 12-Hydroxyoctadecanoic acid (12-HSA)
MFI (melt flow index): 0.6 g/10 min at 190 °C Bulk density: 960 kg/m3 Particle size: 0.05-0.5 mm Bulk density: 285 kg/m3 Mean length: 0.55 mm Aspect ratio: 2.54 Bulk density: 360 kg/m3 Lubricant
Struktol TPW 709
Lubricant, a proprietary blend of processing aids
MAPE Si69
MFI: 1.9 g/10 min at 190 °C 0.5 wt% maleic anhydride (MA) > 95% purity, 538.95 g/mol, 250 °C boiling point
VTMS
> 98% purity, 148.23 g/mol, 123 °C boiling point
Recycled wood flour
2
Rettenmeier holding AG (Germany)
J. Allcock & Sons Ltd (UK) Safic Alcan UK Ltd (Warrington, UK) Safic Alcan UK Ltd (Warrington, UK) Sigma-Aldrich (Dorset, UK) Sigma-Aldrich (Dorset, UK) Sigma-Aldrich (Dorset, UK)
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Table 2 Formulation of RubWPC. Sample
Rubber (%)
Wood (%)
PE (%)
TPW 709 (%)
12HSA (%)
MAPE (%)
Si69 (%)
VTMS (%)
Untreated MAPE treated Si69 treated VTMS treated MAPE&Si69 MAPE&VTMS
20 20 20 20 20 20
30 30 30 30 30 30
43 40 40 40 40 40
3.5 3.5 3.5 3.5 3.5 3.5
3.5 3.5 3.5 3.5 3.5 3.5
0 3 0 0 1.5 1.5
0 0 3 0 1.5 0
0 0 0 3 0 1.5
introduction of MA moiety in MAPE, but it should also be resulted from the esterification occurred between the MA moiety and the −OH groups of wood particles (Rao et al., 2018). As shown in Fig. 2, the chemical distribution and structure of Si69 treated RubWPC appeared to be barely affected by the treatment since the FTIR spectrum did not show considerable difference from that of the untreated composite in regard to the band appearances and intensities. The most distinguishing characteristic would be the presence of the slightly stronger and sharper band at 1032 cm−1, which was an overlapped band originating from a number of vibrations including the CeSCe stretching of rubber and Si69, the SieOCe stretching of Si69, and the CeOC stretching, COee deformation and aromatic CeH deformation vibrations of wood flour (Abdelmouleh et al., 2004, 2007; Gunasekaran et al., 2007; Ihamouchen et al., 2012; Kotilainen et al., 2000; Yang et al., 2017). It had been previously explored that in the FTIR spectrum of Si69 treated WPC (Rao et al., 2018), the area of interest namely between 1010 cm-1 and 1100 cm-1 referring to CeOSi and SiOSeeei bonds was not seen much variation after the treatment due to its limited interactions with wood particles. Therefore, the slight intensity increase should be more predominantly contributed by the CeS linkages formed between Si69 and both rubber and PE molecules, which had also been observed in the previous study of Rubber-PE composites (Zhou and Fan, 2017). In addition, the band at 967 cm−1 assigning to the CHe wagging motion vibration from the butadiene units in rubber particles (Fernández-Berridi et al., 2006; Gunasekaran et al., 2007) disappeared after its crosslinking reaction with the coupling agent. With respect to the RubWPC treated with the combination of MAPE and Si69, the spectral band at 1032 cm-1 turned to be much sharper with increased intensity. This was due to the formation of
Fig. 1. Image of RubWPC sample.
3. Results and discussion 3.1. Chemical structure and bonding 3.1.1. FTIR analysis FTIR was used to investigate the chemical structure of untreated and coupling agent treated RubWPC, the result was presented in Fig. 2. In comparison to the untreated composite, the MAPE treated RubWPC showed sharper and more intense spectral bands at 1734 cm−1 and 1636 cm−1, which were assigned to C]O and CC] stretching vibrations respectively (Dai and Fan, 2015; Ihemouchen et al., 2013; Osman et al., 2010; Santos et al., 2015). On one hand, this was because of the
Fig. 2. FTIR spectra of untreated and coupling agent treated RubWPC. 3
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the aforementioned FTIR study of MAPE treatment of the composite. The reaction mechanism between MAPE and the raw materials was proposed in Fig. 4a, namely the multifunctional MAPE first covalently bonded to the large amount of OeH groups in wood to form ester linkages and thus crosslinked with PE and rubber macromolecules, acting as a bridge at composite interfaces. Si69 treated composite showed a strong and wide peak at 55.52 ppm in its NMR spectrum, which was originally ascribed to the methoxyl groups of lignin unit in wood flour (Martins et al., 2006; Santoni et al., 2015; Stael et al., 2000). The stronger appearance compared to that of the untreated was because of the presence of ethoxy groups within Si69. In addition, it should have also been overlapped with the signals from the C–Sx bonds formed between the dissociated Si69 and macromolecular polymer chains of the matrix (Zhou and Fan, 2017). The resonance peak at 65.08 ppm assigning to C6 of cellulose was widened and strengthened after the treatment, which suggested that the ethoxy groups (i.e. C2H50–) of Si69 had successfully reacted with the −OH groups linked to C6 of cellulose unit in wood to form a siloxane bond, as the reaction proposed in Fig. 4b. The similar phenomenon was observed in the spectra of VTMS treated composite due to the formation of new SieOCe bonds between the methoxy groups of VTMS and the OeH groups of wood, and the chemical interaction between VTMS and the compositions of the composite was proposed in Fig. 4c. In addition, the appearance of the peak at 153.33 ppm corresponding to aryl groups of lignin in the spectra of VTMS treated composite suggested that apart from the functional groups of carbohydrates (i.e. hydroxyl groups of cellulose and hemicellulose) in wood, the lignin units might also be involved in the reaction with the coupling agent. Furthermore, it was interesting to notice that the composites treated with the combinations of different coupling agents did not demonstrate as intense resonance as the composite treated with solitary coupling agent.
Fig. 3. Solid state
13
3.1.3. Interface structure and bonding mechanism SEM was employed to investigate the interface microstructure and bonding of RubWPC, the results were shown in Figs. 5 and 6. The untreated composite (Fig. 5a) displayed incompatible surface with the presence of microcracks and voids throughout the wood-matrix and rubber-matrix interfaces. There also existed noticeable agglomeration of rubber particles on the surface, which prevented the intimate contact and thus interaction with wood flour. These observations indicated the inadequate compatibility between the constituents of the untreated composites, leading to a discontinuous and inhomogeneous interfacial structure. With the application of MAPE or Si69 coupling agents, wood and rubber particles were well distributed and embedded in the PE matrix and firmly bonded to it (Fig. 5b and c). It was also seen that more wood cell lumens were entirely or partially filled by the matrix polymer in these treated composites (marked in Fig. 5b and c), substantiating the enhanced wood-PE compatibility and wettability of wood flour by matrix. Furthermore, although there still existed a few microcracks between wood flour and rubber particles, the observed close interfacial contact and compact structure confirmed the positive impact of the treatments on refining the inherent constituent immiscibility. These phenomena suggesting the improved interfacial adhesion and bonding after MAPE or Si69 treatments were discerned in the composite treated with the combination of these two coupling agents as well (Fig. 6b). The interface structure and bonding scenario in VTMS treated composite (Fig. 6a) was found to be distinct from those in MAPE or Si69 treated composites. The absence of physical contact between rubber and wood phases in VTMS treated composite gave rise to the generation of voids and gaps, and the rubber surface seemed to be clean, intact and free from adhering wood flour and matrix (marked in Fig. 6a). This result was in accordance with the chemical structure analysis (Section 3.1.1), namely that VTMS exerted superb compatibilisation impact on wood-matrix phase in the composite through intermolecular
C NMR spectra of RubWPC.
CeOCe bonds between MA groups and wood flour and the crosslinking reaction between the CeSCe linkages in rubber and the PE macromolecules in MAPE. With respect to VTMS treatment, it was observed more prominent band appearance at 1031 cm−1 in its FTIR spectrum. Our previous study suggested that VTMS was not an effective coupling agent in bridging the constituents of Rubber-PE composite (Zhou and Fan, 2017). The intensity strengthening was primarily due to the SieOCe linkages formed between the functional groups in wood flour (hydroxyl groups) and VTMS (methoxyl groups) along with the introduced SieOCe groups within VTMS (Rao et al., 2018). Similar to MAPE&Si69 treated composite, MAPE&VTMS treated RubWPC demonstrated much more outstanding peaks at 1031 cm-1 and 1740 cm-1 than both untreated and VTMS treated RubWPC, which was again due to the reactions between MAPE and wood and rubber particles.
3.1.2. NMR analysis The comparison of NMR spectra of RubWPC was shown in Fig. 3 for the further investigation of the variation of their chemical properties. Compared to the untreated composite, the most noticeable difference in MAPE treated composite was the appearance of the resonance peak at 173.47 ppm in its spectrum, which was attributed to the MA groups in the incorporated MAPE and the formation of ester bonds between the MA groups and the OeH groups in raw materials. This result confirmed 4
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Fig. 4. Possible chemical interactions between the coupling agents (a: MAPE; b: Si69; c: VTMS) and the constituents of RubWPC.
crosslinking and covalent bonding, but it was unfortunately not the suitable coupling agent for refining the rubber-PE or rubber-wood interfaces owing to its relatively limited physical and chemical
interactions with rubber particles, which were evident in the chemical and interfacial structure analyses. However, as shown in Fig. 6c, the addition of MAPE into the system provided the composite with decent 5
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Fig. 5. Microstructures of cross section of untreated (a), MAPE treated (b), and Si69 treated (c) RubWPC.
resin into the voids, valleys, and crevices of the substrates, and then anchored itself through solidification. Figs. 5b, c and 6b showed that MAPE and Si69 treated RubWPC had much more noticeable resin penetration into the fillers and resin impregnation at rubber-PE and woodPE interfaces compared to the untreated counterpart (Fig. 5a), denoting the existence of more contact areas for mechanical interlocking of the substrates. It was undoubted that the interface improvement was a mutual result of the above bonding mechanisms (i.e. interdiffusion, chemical reaction, mechanical interlocking, etc.), which should be dependent, namely an increase in any bonding would benefit the enhancement of other bonding. Furthermore, since it was observed reduced agglomeration of rubber particles in the treated composites (Fig. 6), the coupling agents, especially MAPE and Si69, might also act as dispersing agents between rubber and PE matrix to form hydrogen bonds, promoting the dispersion of rubber particles in the resin (Osman et al., 2010). The influence of the enhanced interfacial bonding on the mechanical properties and its relation with the performance of the composites were presented in the next section.
interfacial bonding, which again confirmed that MAPE multifunctionally and effectively promoted the adhesion between rubber particles and either nonpolar matrix or polar wood flour. The improvement of the interfacial bonding of the treated composites might be proposed in Fig. 7. As unveiled in Section 3.1.1, the coupling agents reacted with the substrates through the inherent multifunctional molecules, i.e. the hydrophilic moieties reacted with the large amount of OeH groups in wood to form strong covalent bonds (as demonstrated in Fig. 7), while the nonpolar groups chemically crosslinked with rubber and PE macromolecules. These chemical interactions clearly verified the enhanced chemical compatibility and adhesion after the treatments, which in turn provided the basis for intimate contact between constituents and thus good wetting of fillers. Hence, these chemical interactions along with other molecular attractions, such as electrostatic adhesion and Van der Waals forces, triggered a greater extent of molecular interdiffusion between the constituents as graphically demonstrated in Fig. 7. In addition, mechanical interlocking (marked in Fig. 7) might be another mechanism concerning the outstanding bonding, which occurred through the penetration of polymer 6
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Fig. 6. Microstructures of cross section of VTMS treated (a), MAPE&Si69 treated (b), and MAPE&VTMS treated (c) RubWPC.
by showing much higher tensile strain (Zhou and Fan, 2017). In order to further investigate the variation of the rigidity and ductility of the composites, the tensile moduli were also determined and listed in Table 3. MAPE and VTMS treated composites showed marginally lower tensile moduli (4.39% and 0.07%) than untreated composite. These results along with their subtle decrease of tensile strain denoted that the treatments did not alter the overall rigidity or ductility of the composites. In other words, the nature of the treated composites was not dominated by either stiffness increase from wood-matrix moiety or the ductility increase from rubber-matrix moiety within the composites. Instead, it was a result of mutual impacts. Contrary to the comparatively unremarkable differentiations, the tensile modulus of Si69 treated composite was increased by 18.00%, which seemed to indicate that the stiffening impact was more pronounced in its system. The composites treated with the combination of distinct coupling agents, especially the combination of MAPE and Si69, showed superior tensile properties (i.e. higher tensile strength and tensile modulus) than the composites treated with single coupling agent. This result indicated that the combination of coupling agents was capable of equipping the
3.2. Mechanical property analysis 3.2.1. Tensile property analysis Table 3 summarised the tensile properties of the composites. The addition of MAPE and VTMS coupling agents led to a subtle increment (i.e. 6.40%) of the tensile strength, while the addition of Si69 resulted in an increase of 31.07%. It was apparent that the strengthening effect was a result of the aforementioned enhanced interfacial adhesion and bonding between the fillers and matrix after the treatments, which in the meantime provided the composites with better stress distribution and more efficient stress transfer throughout the interfaces (Joffre et al., 2017; Luedtke et al., 2019; Raju et al., 2008). It can be seen from Table 3 that the tensile strain of the treated composites were decreased by 6.34% (MAPE), 17.61% (Si69), and 4.58% (VTMS) respectively, indicating that the coupling agent might impart a stiffening impact on the composites by restricting the mobility of the polymer chains (Osman et al., 2012, 2010). However, the previous study of Rubber-PE composites suggested that the coupling agent treatments made the composite more ductile with enhanced resistance to crack propagation 7
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Fig. 7. Proposed interfacial bonding mechanisms of untreated (a) and coupling agent treated (b) RubWPC.
resulted composite with better interfacial bonding and stress transfer by taking the advantage and avoiding the disadvantage of individual coupling agent. For instance, MAPE was exceptionally effective in refining wood-PE interface, but not rubber related interfaces. By contrast, Si69 was not as efficient as MAPE in terms of the wood-PE interface refinery, but it seemed to be the optimal candidate for addressing the interfacial bonding between rubber and other constituents (i.e. wood and PE) in the composite. Therefore, the combination of MAPE and Si69 coupling agents generated a complementary influence, maximising their capability of interface refining and bonding strengthening of each coupling agent. Furthermore, compared to the previously reported study on WPC materials (Zhou et al., 2017), the incorporation of GTR was able to enhance the ductility of the resulted RubWPC. For instance,
Table 3 Tensile properties of untreated and coupling agent treated RubWPC. Sample
Tensile stress (MPa)
Tensile strain (%)
Tensile modulus (MPa)
Untreated MAPE treated Si69 treated VTMS treated MAPE&Si69 treated MAPE&VTMS treated
5.31 5.65 6.96 5.64 8.34
2.84 2.66 2.34 2.71 2.74
1405.6 1343.9 1658.6 1402.1 1696.6
± ± ± ± ±
0.17 0.21 0.26 0.20 0.23
6.15 ± 0.24
± ± ± ± ±
0.12 0.14 0.08 0.16 0.09
2.56 ± 0.12
± ± ± ± ±
63.7 85.5 101.7 60.3 102.1
1554.9 ± 78.3
8
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Gindl, 2006). In addition, MAPE and Si69 coupling agents might be able to diffuse into wood cell wall and form hydrogen bonding and covalent bonding with the structural components of the cell wall especially hemicellulose owing to its greater accessibility (Frihart, 2005), thus increase the nanomechanical property of the treated RubWPC. 4. Conclusions The effect of the incorporation of MAPE, Si69 and VTMS coupling agents on the chemical structure, interfacial bonding and mechanical property of RubWPC material has been determined. FTIR and NMR analyses unveiled the chemical interactions of the coupling agents with the substrates through their intrinsic multifunctional molecules. These chemical reactions substantiated the improvement of chemical compatibility and adhesion of the composites after coupling agent treatments, which benefitted the macromolecular interdiffusion and the wetting of the fillers by the matrix. Microstructure analysis (SEM) suggested that the addition of MAPE and Si69 coupling agents promoted the intimate contact between the constituents of the composites and the distribution and embedment of rubber and wood particles in the matrix with reduced filler agglomerations. MAPE and Si69 treated RubWPC also showed more resin penetration into the fillers and resin impregnation at rubber-PE and wood-PE interfaces, providing a rigorous basis for the mechanical interlocking among the substrates, and thus enhancing the bonding quality and nanomechanical properties. Chemical interactions and mechanical interlocking were mutually responsible for the improvement of interfacial bonding of the RubWPC treated with MAPE and Si69 coupling agents, leading to the increase of tensile strength (by 57.1%) and tensile modulus (by 20.7%) along with nanomechanical properties. It is envisaged that the developed RubWPC materials could be used in building and construction sectors, such as decking, fencing, cladding, outdoor furniture and noise barriers. Acknowledgement The authors gratefully acknowledge the financial support from the European CIP-EIP-Eco-Innovation-2012 (Project number: 333083). References
Fig. 8. Nanomechnical properties of untreated and MAPE&Si69 treated RubWPC.
Abdelmouleh, M., Boufi, S., Belgacem, M.N., Duarte, A.P., Ben Salah, A., Gandini, A., 2004. Modification of cellulosic fibres with functionalised silanes: development of surface properties. Int. J. Adhes. Adhes. 24, 43–54. https://doi.org/10.1016/S01437496(03)00099-X. Abdelmouleh, M., Boufi, S., Belgacem, M.N., Dufresne, A., 2007. Short natural-fibre reinforced polyethylene and natural rubber composites: effect of silane coupling agents and fibres loading. Compos. Sci. Technol. 67, 1627–1639. https://doi.org/10.1016/j. compscitech.2006.07.003. Alexandre-Franco, M., Fernández-González, C., Alfaro-Domínguez, M., Palacios Latasa, J.M., Gómez-Serrano, V., 2010. Devulcanization and demineralization of used tire rubber by thermal chemical methods: a study by X-ray diffraction. Energy Fuels 24, 3401–3409. https://doi.org/10.1021/ef901523t. Bais-Moleman, A.L., Sikkema, R., Vis, M., Reumerman, P., Theurl, M.C., Erb, K., 2018. Assessing wood use efficiency and greenhouse gas emissions of wood product cascading in the European Union. J. Clean. Prod. 172, 3942–3954. https://doi.org/10. 1016/j.jclepro.2017.04.153. Bajwa, S.G., Bajwa, D.S., Holt, G., Coffelt, T., Nakayama, F., 2011. Properties of thermoplastic composites with cotton and guayule biomass residues as fiber fillers. Ind. Crops Prod. 33, 747–755. https://doi.org/10.1016/j.indcrop.2011.01.017. Bi, H., Ren, Z., Guo, R., Xu, M., Song, Y., 2018. Fabrication of flexible wood flour/thermoplastic polyurethane elastomer composites using fused deposition molding. Ind. Crops Prod. 122, 76–84. https://doi.org/10.1016/j.indcrop.2018.05.059. Cazan, C., Cosnita, M., Isac, L., 2019. The influence of temperature on the performance of rubber - PET-HDPE waste -based composites with different inorganic fillers. J. Clean. Prod. 208, 1030–1040. https://doi.org/10.1016/j.jclepro.2018.10.045. Cosnita, M., Cazan, C., Duta, A., 2017. The influence of inorganic additive on the water stability and mechanical properties of recycled rubber, polyethylene terephthalate, high density polyethylene and wood composites. J. Clean. Prod. 165, 630–636. https://doi.org/10.1016/j.jclepro.2017.07.103. Dai, D., Fan, M., 2015. Preparation of bio-composite from wood sawdust and gypsum. Ind. Crops Prod. 74, 417–424. https://doi.org/10.1016/j.indcrop.2015.05.036. de Oliveira, F., da Silva, C.G., Ramos, L.A., Frollini, E., 2017. Phenolic and lignosulfonatebased matrices reinforced with untreated and lignosulfonate-treated sisal fibers. Ind.
the untreated and MAPE treated WPC demonstrated a tensile strain of 2.36% and 1.78% respectively, which were 20% and 49% lower than that of untreated and MAPE treated RubWPC (Table 3), and the stiffer nature of WPC was confirmed by their considerably higher tensile modulus than RubWPC. 3.2.2. Nanomechanical property analysis The examination of mechanical properties at nanoscale benefits the understanding of interface characteristics of composites. Fig. 8 demonstrated the nanomechanical properties of the wood cell walls of untreated and MAPE&Si69 treated RubWPC determined by nanoindentation. The hardness of the cell walls in the treated composite was nearly equal to that of reference cell walls in untreated composite. In contrast, the elastic modulus of the treated cell walls was significantly increased by 20.84% compared to that of untreated cell walls, which was consistent with the tensile properties of the composites. This result might be associated with the considerable penetration of polymer resin into the more deformed and accessible cell lumens and vessels after the treatment, which were detected in its microstructure analysis (i.e. Fig. 6b). Although it is widely accepted that the indentation modulus in damaged cell walls were lower than that in intact cell walls, the resin filling in cell lumens was a mechanical interlock that provided additional strength, recovering the loss of elastic behaviour due to mechanical processing (Frihart, 2005; Gindl et al., 2004; Konnerth and 9
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Crops Prod. 96, 30–41. https://doi.org/10.1016/j.indcrop.2016.11.027. Fernández-Berridi, M.J., González, N., Mugica, A., Bernicot, C., 2006. Pyrolysis-FTIR and TGA techniques as tools in the characterization of blends of natural rubber and SBR. Thermochim. Acta 444, 65–70. https://doi.org/10.1016/j.tca.2006.02.027. Formela, K., Hejna, A., Zedler, Ł, Przybysz, M., Ryl, J., Saeb, M.R., Piszczyk, Ł, 2017. Structural, thermal and physico-mechanical properties of polyurethane/brewers’ spent grain composite foams modified with ground tire rubber. Ind. Crops Prod. 108, 844–852. https://doi.org/10.1016/j.indcrop.2017.07.047. Frihart, C.R., 2005. Adhesive bonding and performance testing of bonded wood products. J. ASTM Int. 2, 1–10. Gindl, W., Schöberl, T., Jeronimidis, G., 2004. The interphase in phenol–formaldehyde and polymeric methylene di-phenyl-di-isocyanate glue lines in wood. Int. J. Adhes. Adhes. 24, 279–286. https://doi.org/10.1016/j.ijadhadh.2003.10.002. Gunasekaran, S., Natarajan, R.K., Kala, A., 2007. FTIR spectra and mechanical strength analysis of some selected rubber derivatives. Spectrochim. Acta A: mol. Biomol. Spectrosc. 68, 323–330. https://doi.org/10.1016/j.saa.2006.11.039. Ihamouchen, C., Djidjelli, H., Boukerrou, A., Krim, S., Kaci, M., Martinez, J.J., 2012. Effect of surface treatment on the physicomechanical and thermal properties of highdensity polyethylene/olive husk flour composites. J. Appl. Polym. Sci. 123, 1310–1319. https://doi.org/10.1002/app.34172. Ihemouchen, C., Djidjelli, H., Boukerrou, A., Fenouillot, F., Barres, C., 2013. Effect of compatibilizing agents on the mechanical properties of high-density polyethylene/ olive husk flour composites. J. Appl. Polym. Sci. 128, 2224–2229. https://doi.org/10. 1002/app.38434. Joffre, T., Segerholm, K., Persson, C., Bardage, S.L., Luengo Hendriks, C.L., Isaksson, P., 2017. Characterization of interfacial stress transfer ability in acetylation-treated wood fibre composites using X-ray microtomography. Ind. Crops Prod. 95, 43–49. https://doi.org/10.1016/j.indcrop.2016.10.009. Konnerth, J., Gindl, W., 2006. Mechanical characterisation of wood-adhesive interphase cell walls by nanoindentation. Holzforschung 60, 429–433. https://doi.org/10.1515/ HF.2006.067. Kotilainen, R.A., Toivanen, T., Alén, R.J., 2000. FTIR monitoring of chemical changes in softwood during heating. J. Wood Chem. Technol. 20, 307–320. https://doi.org/10. 1080/02773810009349638. Luedtke, J., Gaugler, M., Grigsby, W.J., Krause, A., 2019. Understanding the development of interfacial bonding within PLA/wood-based thermoplastic sandwich composites. Ind. Crops Prod. 127, 129–134. https://doi.org/10.1016/j.indcrop.2018.10.069. Martins, M.A., Forato, L.A., Mattoso, L.H.C., Colnago, L.A., 2006. A solid state 13C high resolution NMR study of raw and chemically treated sisal fibers. Carbohydr. Polym. 64, 127–133. https://doi.org/10.1016/j.carbpol.2005.10.034. Moni Ribeiro Filho, S.L., Oliveira, P.R., Panzera, T.H., Scarpa, F., 2019. Impact of hybrid composites based on rubber tyres particles and sugarcane bagasse fibres. Compos. Part B: Eng. 159, 157–164. https://doi.org/10.1016/j.compositesb.2018.09.054. Osman, H., Ismail, H., Mariatti, M., 2012. Polypropylene/natural rubber composites filled with recycled newspaper: effect of chemical treatment using maleic anhydridegrafted polypropylene and 3-aminopropyltriethoxysilane. Polym. Compos. 33, 609–618. https://doi.org/10.1002/pc.22178. Osman, H., Ismail, H., Mustapha, M., 2010. Effects of maleic anhydride polypropylene on
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Incorporation of tyre rubber into wood plastic composites to develop novel multifunctional composites: Interface and bonding mechanisms Yonghui Zhoua,b, Yuxuan Wanga,b, Mizi Fana,b, a b
T
⁎
College of Materials Engineering, Fujian Agriculture and Forestry University, PR China Department of Civil and Environmental Engineering, College of Engineering, Design and Physical Sciences, Brunel University London, UB8 3PH, United Kingdom
A R T I C LE I N FO
A B S T R A C T
Keywords: Rubber-wood-plastic composites Interface structure Bonding mechanism Mechanical property Nanoindentation
This paper reports the novel formulation of rubber-wood-plastic composites (RubWPC) with the focus on their interfacial optimisation by using maleated and silane coupling agents. Spectroscopic techniques were used to examine the variation of chemical properties of RubWPC after coupling agent treatments. The unveiled chemical interactions between the coupling agents and substrates suggested the improvement of constituent compatibility and interfacial adhesion, promoting distribution and embedment of wood and rubber particles in polyethylene (PE) matrix, which were confirmed by Scanning Electron Microscope (SEM) analysis. Compared to untreated and single coupling agent treated composites, the composite treated with the combination of maleic anhydride grafted polyethylene (MAPE) and bis(triethoxysilylpropyl)tetrasulfide (Si69) coupling agents possessed superior tensile properties (i.e. tensile strength increased by 57.1% and tensile modulus increased by 20.7%) owing to the enhanced interfacial bonding and more efficient stress transfer. In addition, the nanomechanical property of MAPE&Si69 treated composite (i.e. elastic modulus increased from 15.93 GPa to 19.25 GPa) was also increased due to the penetration of matrix resin into wood cells as well as the reaction between wood cell walls and the coupling agents.
1. Introduction The rising environmental concern towards the disposal of problematic waste tyres has led to increasing interests in economical and sustainable recycling and reuse of tyre rubber (Formela et al., 2017; Wu et al., 2007; Zhao et al., 2010). The special characteristics of tyre rubber being vulcanised and unmolten unfavourably restrict its recyclability and application values (Alexandre-Franco et al., 2010; Sonnier et al., 2006). Wood plastic composite (WPC) has grown rapidly in recent years mainly due to its emerging applications in automotive and aerospace components, building and construction products, industrial and consumer goods (Bajwa et al., 2011; Bi et al., 2018; de Oliveira et al., 2017; Pandey et al., 2017; Ramires and Frollini, 2012). In consideration of consistent growth and massive market share of WPC materials, the incorporation of ground tyre rubber (GTR) into WPC might be a highly promising approach for realising the valorisation of used tyres, which would generate a new generation of WPC, namely rubber-wood-plastic composites (RubWPC). The additional use of GTR in WPC will provide the resulted RubWPC with many advantageous properties beyond WPC considering the unique vibration damping, slip resistance, wearing and acoustic performance rubber possesses (Formela et al., 2017; Ramarad
⁎
et al., 2015). Furthermore, the incorporation of rubber indicates the reduction of the use of wood in the system, which benefits the mitigation of the strong competition of wood resources between various industries, i.e. energy, chemicals and materials (Bais-Moleman et al., 2018). In the literature, there have been few investigations of the application of waste tyres in the development of cellulosic polymer composites. Cosnita et al prepared recycled rubber-PET (polyethylene terephthalate)-HDPE (high density polyethylene)-wood composites. The addition of inorganic fillers (e.g. CaO and ZnO/fly ash) contributed to the increase of dimensional stability and mechanical properties of the composites, thus enabling their application as paving slabs, park carpets or application in wet outdoor working environment (Cazan et al., 2019; Cosnita et al., 2017). Filho et al developed the hybrid composites based on epoxy polymer, rubber tyre particles and sugarcane bagasse fibres and examined the energy absorption properties of the composites through the impact tests (Moni Ribeiro Filho et al., 2019). In RubWPC system, the compatibility and bonding of wood-plastic, rubber-plastic and wood-rubber phases are the indispensable issues to be dealt with and overcome in order to formulate a reasonable RubWPC and promote its speedy industrialisation. However, these issues remain the least
Corresponding author. E-mail address: [email protected] (M. Fan).
https://doi.org/10.1016/j.indcrop.2019.111788 Received 8 April 2019; Received in revised form 12 September 2019; Accepted 14 September 2019 Available online 30 September 2019 0926-6690/ © 2019 Published by Elsevier B.V.
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2.3. FTIR analysis
understood, and have not yet been specifically studied to the best of our knowledge. This work investigates the novel formulation of rubber-wood-plastic composites (RubWPC) with the focus on the optimisation of the constituent compatibility and bonding by employing MAPE, Si69 and vinyltrimethoxysilane (VTMS) coupling agents. In our previous studies, MAPE and VTMS were found to be very effective in refining the compatibility and bonding between wood flour and PE (Rao et al., 2018), whilst Si69 presented superior results in enhancing the bonding between rubber particles and PE (Zhou and Fan, 2017). Therefore, these coupling agents were selected to optimise the bonding of RubWPC in this work. Spectroscopic (Fourier Transform Infrared Spectroscopy (FTIR) and 13C Nuclear Magnetic Resonance Spectroscopy (NMR)) and microscopic (SEM) techniques were used to comprehensively scrutinise the chemical structure, constituent compatibility, filler distribution, and microstructure of both untreated and treated RubWPC, thus to unveil the bonding mechanisms. Furthermore, the bulk and in situ mechanical property of RubWPC was determined by conducting tensile property and nanoindentation analyses in order to establish the structure-performance relationship of the composites.
FTIR spectra of RubWPC samples with the dimension of 2 mm × 2 mm × 1 mm were recorded on a PerkinElmer Spectrum One Spectrometer. The spectrum acquisition was operated at following conditions: 4 cm−1 resolution, 16 scans and 4000 – 650 cm−1 wave number range.
2.4. Solid state
13
C NMR analysis
Solid state 13C NMR analysis of RubWPC was conducted on a Bruker spectrometer with a CPMAS (Cross-polarisation/magic angle spinning) probe. Samples were packed into 7 mm zirconia rotors and spun at 6 kHz for spectrum acquisition in the region from -130 ppm to 270 ppm.
2.5. SEM analysis All the composites with the dimension of 5.0 mm × 1.4 mm (cross section) were transversely cut using a sliding microtome and plated with gold for microstructure examination. The observation was conducted on a Leo 1430 V P SEM operating at 15 kV.
2. Materials and methods 2.1. Materials
2.6. Tensile property analysis
The materials and additives used in the work were summarised in Table 1. All the raw materials and additives were stored in a cool dry place before uses.
Tensile test was carried out on an Instron 5900 Universal Testing Instrument in accordance with BS EN ISO 527-2:2012. The crosshead speed of the test was 1 mm/min. Tensile strain is calculated by measuring the elongation the specimen undergoes during tensile testing, is expressed as a percentage (%). The average of six measurements for each composite was reported. The tensile properties of the recycled PE matrix were also measured and given for reference: tensile stress 23.05 ± 0.17 MPa, tensile strain 10.35 ± 0.32%, and tensile modulus 2385.4 ± 133.3 MPa.
2.2. Formulation of RubWPC Table 2 summarised the formulations of untreated and coupling agent treated RubWPC. The sample preparation procedure was given as follows: the required amount of PE was first placed in a Plastograph twin-screw mixer (Brabender GmbH, Germany) and allowed to completely melt at 100 rpm and 190 °C for 2 min. Subsequently, rubber powder and wood flour were added into the mixer and mixed with PE for 3 min to obtain uniform mixtures, followed by mixing with lubricants and coupling agents for another 10 min to allow their reaction with the raw materials. Finally, the RubWPC blends were ground into pellets and compression moulded on an electrically heated hydraulic press at 190 °C and 9.81 MPa pressure for 10 min. The thickness of the samples was 1.4 mm, and an image of the samples was presented in Fig. 1.
2.7. Nanoindentation analysis Nanoindentation measurement was carried out on a Nano Indenter (Hysitron TI 950 TriboIndenter, USA) under load-controlled mode. The detailed description of sample preparation and testing procedure could be referred to our previous study (Zhou et al., 2017).
Table 1 Raw materials and additives used in the work. Material/Additive
Characteristics
Supplier
Recycled polyethylene pellet
JFC Plastics Ltd (UK)
Recycled tyre rubber 12-Hydroxyoctadecanoic acid (12-HSA)
MFI (melt flow index): 0.6 g/10 min at 190 °C Bulk density: 960 kg/m3 Particle size: 0.05-0.5 mm Bulk density: 285 kg/m3 Mean length: 0.55 mm Aspect ratio: 2.54 Bulk density: 360 kg/m3 Lubricant
Struktol TPW 709
Lubricant, a proprietary blend of processing aids
MAPE Si69
MFI: 1.9 g/10 min at 190 °C 0.5 wt% maleic anhydride (MA) > 95% purity, 538.95 g/mol, 250 °C boiling point
VTMS
> 98% purity, 148.23 g/mol, 123 °C boiling point
Recycled wood flour
2
Rettenmeier holding AG (Germany)
J. Allcock & Sons Ltd (UK) Safic Alcan UK Ltd (Warrington, UK) Safic Alcan UK Ltd (Warrington, UK) Sigma-Aldrich (Dorset, UK) Sigma-Aldrich (Dorset, UK) Sigma-Aldrich (Dorset, UK)
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Table 2 Formulation of RubWPC. Sample
Rubber (%)
Wood (%)
PE (%)
TPW 709 (%)
12HSA (%)
MAPE (%)
Si69 (%)
VTMS (%)
Untreated MAPE treated Si69 treated VTMS treated MAPE&Si69 MAPE&VTMS
20 20 20 20 20 20
30 30 30 30 30 30
43 40 40 40 40 40
3.5 3.5 3.5 3.5 3.5 3.5
3.5 3.5 3.5 3.5 3.5 3.5
0 3 0 0 1.5 1.5
0 0 3 0 1.5 0
0 0 0 3 0 1.5
introduction of MA moiety in MAPE, but it should also be resulted from the esterification occurred between the MA moiety and the −OH groups of wood particles (Rao et al., 2018). As shown in Fig. 2, the chemical distribution and structure of Si69 treated RubWPC appeared to be barely affected by the treatment since the FTIR spectrum did not show considerable difference from that of the untreated composite in regard to the band appearances and intensities. The most distinguishing characteristic would be the presence of the slightly stronger and sharper band at 1032 cm−1, which was an overlapped band originating from a number of vibrations including the CeSCe stretching of rubber and Si69, the SieOCe stretching of Si69, and the CeOC stretching, COee deformation and aromatic CeH deformation vibrations of wood flour (Abdelmouleh et al., 2004, 2007; Gunasekaran et al., 2007; Ihamouchen et al., 2012; Kotilainen et al., 2000; Yang et al., 2017). It had been previously explored that in the FTIR spectrum of Si69 treated WPC (Rao et al., 2018), the area of interest namely between 1010 cm-1 and 1100 cm-1 referring to CeOSi and SiOSeeei bonds was not seen much variation after the treatment due to its limited interactions with wood particles. Therefore, the slight intensity increase should be more predominantly contributed by the CeS linkages formed between Si69 and both rubber and PE molecules, which had also been observed in the previous study of Rubber-PE composites (Zhou and Fan, 2017). In addition, the band at 967 cm−1 assigning to the CHe wagging motion vibration from the butadiene units in rubber particles (Fernández-Berridi et al., 2006; Gunasekaran et al., 2007) disappeared after its crosslinking reaction with the coupling agent. With respect to the RubWPC treated with the combination of MAPE and Si69, the spectral band at 1032 cm-1 turned to be much sharper with increased intensity. This was due to the formation of
Fig. 1. Image of RubWPC sample.
3. Results and discussion 3.1. Chemical structure and bonding 3.1.1. FTIR analysis FTIR was used to investigate the chemical structure of untreated and coupling agent treated RubWPC, the result was presented in Fig. 2. In comparison to the untreated composite, the MAPE treated RubWPC showed sharper and more intense spectral bands at 1734 cm−1 and 1636 cm−1, which were assigned to C]O and CC] stretching vibrations respectively (Dai and Fan, 2015; Ihemouchen et al., 2013; Osman et al., 2010; Santos et al., 2015). On one hand, this was because of the
Fig. 2. FTIR spectra of untreated and coupling agent treated RubWPC. 3
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the aforementioned FTIR study of MAPE treatment of the composite. The reaction mechanism between MAPE and the raw materials was proposed in Fig. 4a, namely the multifunctional MAPE first covalently bonded to the large amount of OeH groups in wood to form ester linkages and thus crosslinked with PE and rubber macromolecules, acting as a bridge at composite interfaces. Si69 treated composite showed a strong and wide peak at 55.52 ppm in its NMR spectrum, which was originally ascribed to the methoxyl groups of lignin unit in wood flour (Martins et al., 2006; Santoni et al., 2015; Stael et al., 2000). The stronger appearance compared to that of the untreated was because of the presence of ethoxy groups within Si69. In addition, it should have also been overlapped with the signals from the C–Sx bonds formed between the dissociated Si69 and macromolecular polymer chains of the matrix (Zhou and Fan, 2017). The resonance peak at 65.08 ppm assigning to C6 of cellulose was widened and strengthened after the treatment, which suggested that the ethoxy groups (i.e. C2H50–) of Si69 had successfully reacted with the −OH groups linked to C6 of cellulose unit in wood to form a siloxane bond, as the reaction proposed in Fig. 4b. The similar phenomenon was observed in the spectra of VTMS treated composite due to the formation of new SieOCe bonds between the methoxy groups of VTMS and the OeH groups of wood, and the chemical interaction between VTMS and the compositions of the composite was proposed in Fig. 4c. In addition, the appearance of the peak at 153.33 ppm corresponding to aryl groups of lignin in the spectra of VTMS treated composite suggested that apart from the functional groups of carbohydrates (i.e. hydroxyl groups of cellulose and hemicellulose) in wood, the lignin units might also be involved in the reaction with the coupling agent. Furthermore, it was interesting to notice that the composites treated with the combinations of different coupling agents did not demonstrate as intense resonance as the composite treated with solitary coupling agent.
Fig. 3. Solid state
13
3.1.3. Interface structure and bonding mechanism SEM was employed to investigate the interface microstructure and bonding of RubWPC, the results were shown in Figs. 5 and 6. The untreated composite (Fig. 5a) displayed incompatible surface with the presence of microcracks and voids throughout the wood-matrix and rubber-matrix interfaces. There also existed noticeable agglomeration of rubber particles on the surface, which prevented the intimate contact and thus interaction with wood flour. These observations indicated the inadequate compatibility between the constituents of the untreated composites, leading to a discontinuous and inhomogeneous interfacial structure. With the application of MAPE or Si69 coupling agents, wood and rubber particles were well distributed and embedded in the PE matrix and firmly bonded to it (Fig. 5b and c). It was also seen that more wood cell lumens were entirely or partially filled by the matrix polymer in these treated composites (marked in Fig. 5b and c), substantiating the enhanced wood-PE compatibility and wettability of wood flour by matrix. Furthermore, although there still existed a few microcracks between wood flour and rubber particles, the observed close interfacial contact and compact structure confirmed the positive impact of the treatments on refining the inherent constituent immiscibility. These phenomena suggesting the improved interfacial adhesion and bonding after MAPE or Si69 treatments were discerned in the composite treated with the combination of these two coupling agents as well (Fig. 6b). The interface structure and bonding scenario in VTMS treated composite (Fig. 6a) was found to be distinct from those in MAPE or Si69 treated composites. The absence of physical contact between rubber and wood phases in VTMS treated composite gave rise to the generation of voids and gaps, and the rubber surface seemed to be clean, intact and free from adhering wood flour and matrix (marked in Fig. 6a). This result was in accordance with the chemical structure analysis (Section 3.1.1), namely that VTMS exerted superb compatibilisation impact on wood-matrix phase in the composite through intermolecular
C NMR spectra of RubWPC.
CeOCe bonds between MA groups and wood flour and the crosslinking reaction between the CeSCe linkages in rubber and the PE macromolecules in MAPE. With respect to VTMS treatment, it was observed more prominent band appearance at 1031 cm−1 in its FTIR spectrum. Our previous study suggested that VTMS was not an effective coupling agent in bridging the constituents of Rubber-PE composite (Zhou and Fan, 2017). The intensity strengthening was primarily due to the SieOCe linkages formed between the functional groups in wood flour (hydroxyl groups) and VTMS (methoxyl groups) along with the introduced SieOCe groups within VTMS (Rao et al., 2018). Similar to MAPE&Si69 treated composite, MAPE&VTMS treated RubWPC demonstrated much more outstanding peaks at 1031 cm-1 and 1740 cm-1 than both untreated and VTMS treated RubWPC, which was again due to the reactions between MAPE and wood and rubber particles.
3.1.2. NMR analysis The comparison of NMR spectra of RubWPC was shown in Fig. 3 for the further investigation of the variation of their chemical properties. Compared to the untreated composite, the most noticeable difference in MAPE treated composite was the appearance of the resonance peak at 173.47 ppm in its spectrum, which was attributed to the MA groups in the incorporated MAPE and the formation of ester bonds between the MA groups and the OeH groups in raw materials. This result confirmed 4
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Fig. 4. Possible chemical interactions between the coupling agents (a: MAPE; b: Si69; c: VTMS) and the constituents of RubWPC.
crosslinking and covalent bonding, but it was unfortunately not the suitable coupling agent for refining the rubber-PE or rubber-wood interfaces owing to its relatively limited physical and chemical
interactions with rubber particles, which were evident in the chemical and interfacial structure analyses. However, as shown in Fig. 6c, the addition of MAPE into the system provided the composite with decent 5
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Fig. 5. Microstructures of cross section of untreated (a), MAPE treated (b), and Si69 treated (c) RubWPC.
resin into the voids, valleys, and crevices of the substrates, and then anchored itself through solidification. Figs. 5b, c and 6b showed that MAPE and Si69 treated RubWPC had much more noticeable resin penetration into the fillers and resin impregnation at rubber-PE and woodPE interfaces compared to the untreated counterpart (Fig. 5a), denoting the existence of more contact areas for mechanical interlocking of the substrates. It was undoubted that the interface improvement was a mutual result of the above bonding mechanisms (i.e. interdiffusion, chemical reaction, mechanical interlocking, etc.), which should be dependent, namely an increase in any bonding would benefit the enhancement of other bonding. Furthermore, since it was observed reduced agglomeration of rubber particles in the treated composites (Fig. 6), the coupling agents, especially MAPE and Si69, might also act as dispersing agents between rubber and PE matrix to form hydrogen bonds, promoting the dispersion of rubber particles in the resin (Osman et al., 2010). The influence of the enhanced interfacial bonding on the mechanical properties and its relation with the performance of the composites were presented in the next section.
interfacial bonding, which again confirmed that MAPE multifunctionally and effectively promoted the adhesion between rubber particles and either nonpolar matrix or polar wood flour. The improvement of the interfacial bonding of the treated composites might be proposed in Fig. 7. As unveiled in Section 3.1.1, the coupling agents reacted with the substrates through the inherent multifunctional molecules, i.e. the hydrophilic moieties reacted with the large amount of OeH groups in wood to form strong covalent bonds (as demonstrated in Fig. 7), while the nonpolar groups chemically crosslinked with rubber and PE macromolecules. These chemical interactions clearly verified the enhanced chemical compatibility and adhesion after the treatments, which in turn provided the basis for intimate contact between constituents and thus good wetting of fillers. Hence, these chemical interactions along with other molecular attractions, such as electrostatic adhesion and Van der Waals forces, triggered a greater extent of molecular interdiffusion between the constituents as graphically demonstrated in Fig. 7. In addition, mechanical interlocking (marked in Fig. 7) might be another mechanism concerning the outstanding bonding, which occurred through the penetration of polymer 6
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Fig. 6. Microstructures of cross section of VTMS treated (a), MAPE&Si69 treated (b), and MAPE&VTMS treated (c) RubWPC.
by showing much higher tensile strain (Zhou and Fan, 2017). In order to further investigate the variation of the rigidity and ductility of the composites, the tensile moduli were also determined and listed in Table 3. MAPE and VTMS treated composites showed marginally lower tensile moduli (4.39% and 0.07%) than untreated composite. These results along with their subtle decrease of tensile strain denoted that the treatments did not alter the overall rigidity or ductility of the composites. In other words, the nature of the treated composites was not dominated by either stiffness increase from wood-matrix moiety or the ductility increase from rubber-matrix moiety within the composites. Instead, it was a result of mutual impacts. Contrary to the comparatively unremarkable differentiations, the tensile modulus of Si69 treated composite was increased by 18.00%, which seemed to indicate that the stiffening impact was more pronounced in its system. The composites treated with the combination of distinct coupling agents, especially the combination of MAPE and Si69, showed superior tensile properties (i.e. higher tensile strength and tensile modulus) than the composites treated with single coupling agent. This result indicated that the combination of coupling agents was capable of equipping the
3.2. Mechanical property analysis 3.2.1. Tensile property analysis Table 3 summarised the tensile properties of the composites. The addition of MAPE and VTMS coupling agents led to a subtle increment (i.e. 6.40%) of the tensile strength, while the addition of Si69 resulted in an increase of 31.07%. It was apparent that the strengthening effect was a result of the aforementioned enhanced interfacial adhesion and bonding between the fillers and matrix after the treatments, which in the meantime provided the composites with better stress distribution and more efficient stress transfer throughout the interfaces (Joffre et al., 2017; Luedtke et al., 2019; Raju et al., 2008). It can be seen from Table 3 that the tensile strain of the treated composites were decreased by 6.34% (MAPE), 17.61% (Si69), and 4.58% (VTMS) respectively, indicating that the coupling agent might impart a stiffening impact on the composites by restricting the mobility of the polymer chains (Osman et al., 2012, 2010). However, the previous study of Rubber-PE composites suggested that the coupling agent treatments made the composite more ductile with enhanced resistance to crack propagation 7
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Fig. 7. Proposed interfacial bonding mechanisms of untreated (a) and coupling agent treated (b) RubWPC.
resulted composite with better interfacial bonding and stress transfer by taking the advantage and avoiding the disadvantage of individual coupling agent. For instance, MAPE was exceptionally effective in refining wood-PE interface, but not rubber related interfaces. By contrast, Si69 was not as efficient as MAPE in terms of the wood-PE interface refinery, but it seemed to be the optimal candidate for addressing the interfacial bonding between rubber and other constituents (i.e. wood and PE) in the composite. Therefore, the combination of MAPE and Si69 coupling agents generated a complementary influence, maximising their capability of interface refining and bonding strengthening of each coupling agent. Furthermore, compared to the previously reported study on WPC materials (Zhou et al., 2017), the incorporation of GTR was able to enhance the ductility of the resulted RubWPC. For instance,
Table 3 Tensile properties of untreated and coupling agent treated RubWPC. Sample
Tensile stress (MPa)
Tensile strain (%)
Tensile modulus (MPa)
Untreated MAPE treated Si69 treated VTMS treated MAPE&Si69 treated MAPE&VTMS treated
5.31 5.65 6.96 5.64 8.34
2.84 2.66 2.34 2.71 2.74
1405.6 1343.9 1658.6 1402.1 1696.6
± ± ± ± ±
0.17 0.21 0.26 0.20 0.23
6.15 ± 0.24
± ± ± ± ±
0.12 0.14 0.08 0.16 0.09
2.56 ± 0.12
± ± ± ± ±
63.7 85.5 101.7 60.3 102.1
1554.9 ± 78.3
8
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Gindl, 2006). In addition, MAPE and Si69 coupling agents might be able to diffuse into wood cell wall and form hydrogen bonding and covalent bonding with the structural components of the cell wall especially hemicellulose owing to its greater accessibility (Frihart, 2005), thus increase the nanomechanical property of the treated RubWPC. 4. Conclusions The effect of the incorporation of MAPE, Si69 and VTMS coupling agents on the chemical structure, interfacial bonding and mechanical property of RubWPC material has been determined. FTIR and NMR analyses unveiled the chemical interactions of the coupling agents with the substrates through their intrinsic multifunctional molecules. These chemical reactions substantiated the improvement of chemical compatibility and adhesion of the composites after coupling agent treatments, which benefitted the macromolecular interdiffusion and the wetting of the fillers by the matrix. Microstructure analysis (SEM) suggested that the addition of MAPE and Si69 coupling agents promoted the intimate contact between the constituents of the composites and the distribution and embedment of rubber and wood particles in the matrix with reduced filler agglomerations. MAPE and Si69 treated RubWPC also showed more resin penetration into the fillers and resin impregnation at rubber-PE and wood-PE interfaces, providing a rigorous basis for the mechanical interlocking among the substrates, and thus enhancing the bonding quality and nanomechanical properties. Chemical interactions and mechanical interlocking were mutually responsible for the improvement of interfacial bonding of the RubWPC treated with MAPE and Si69 coupling agents, leading to the increase of tensile strength (by 57.1%) and tensile modulus (by 20.7%) along with nanomechanical properties. It is envisaged that the developed RubWPC materials could be used in building and construction sectors, such as decking, fencing, cladding, outdoor furniture and noise barriers. Acknowledgement The authors gratefully acknowledge the financial support from the European CIP-EIP-Eco-Innovation-2012 (Project number: 333083). References
Fig. 8. Nanomechnical properties of untreated and MAPE&Si69 treated RubWPC.
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