Fibre-matrix interfacial adhesion on composite properties in natural fibre composites Le Quan Ngoc TRAN 1,2,a, Xiaowen YUAN 1,b*, Debes BHATTACHARYYA 3,c, Carlos A. FUENTES 2,d, Aart Willem VAN VUURE 2,e, Ignace VERPOEST 2,f 1 Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research (A*STAR), Singapore. 2 Department of Materials Engineering (MTM), KU Leuven, Belgium. 3 Department of Mechanical Engineering, The University of Auckland, New Zealand a tranlqn@simtech.a-star.edu.sg, b xwyuan@simtech.a-star.edu.sg, c d.bhattacharyya@auckland.ac.nz, d carlos.fuentes@mtm.kuleuven.be, e aartwillem.vanvuure@mtm.kuleuven.be, f ignaas.verpoest@mtm.kuleuven.be *corresponding author Keywords: Natural fibres, Interfacial adhesion, Unidirectional composites, Mechanical properties. Abstract In this research, the interface between natural fibres and thermoplastic matrices is studied, in which fibre-matrix wetting analysis and interfacial adhesion are investigated to obtain a systematic understanding of the interface. In wetting analysis, the surface energies of the fibres and the matrices are estimated using their contact angles in test liquids. Then, work of adhesion is calculated for each composite system. For the interface tests, transverse three point bending (3PBT) tests on UD composites are performed to measure interfacial strength. XPS characterisation on the fibres is also carried out to obtain more information about the surface chemistry of the fibres. Unidirectional (UD) composites are examined to explore the correlation between the fibre-matrix interface and the final properties of the composites. The results suggest that the higher interfacial adhesion of the treated fibre composites compared to untreated fibre composites can be attributed to higher fibre matrix physico-chemical interaction corresponding with the work of adhesion. In agreement with the interface evaluation, the flexural properties of the composites are significantly influenced by their interfacial adhesion. 1. Introduction Recently, there has been an increasing interest in using natural fibres in composite materials, especially with thermoplastic and bio-based polymer matrices, due to their good mechanical properties in combination with environment-friendly characteristics. However, unlike synthetic fibres, most natural fibres are relatively hydrophilic, have a rough surface and are physicochemically heterogeneous. These characteristics strongly affect the fibre-matrix interfacial interactions in the composite. In general, the adhesion at the composite interface can be described by following main interactions: physical adhesion related to wettability and compatibility of the fibre and the matrix which are controlled by the surface energies of the materials, chemical bonding, and mechanical interlocking created on the rough fibre surface. Good interfacial adhesion initially requires a good wetting between the fibre and the matrix, to achieve an extensive and proper interfacial contact; for this the surface energies of the two materials are important parameters. The surface energy of a fibre
generally should be higher than that of the liquid resin for a good wetting to take place during composite processing. Moreover, the surface energies will play an important role for keeping a stable contact after consolidation of the composite. In this study, the interfacial adhesion of untreated and treated coir and flax fibres in thermoplastics and bio-based polymer are characterised by wetting analysis and interfacial mechanical tests. In wetting analysis, the surface energies of the fibres and the matrices are estimated using their contact angles in various test liquids. Then, work of adhesion is calculated for each composite system. For the interface tests, transverse three point bending (T3PB) tests on UD composites are performed to measure interfacial strength. XPS characterisation on the fibres is also carried out to obtain more information about the surface chemistry of the fibres. Flexural properties of the UD composites are also assessed by three point bending tests on fibre longitudinal direction. The correlation between the fibre-matrix interface and the final properties of the composites will be discussed. 2. Materials and Methods 2.1 Materials Natural fibres used in this research were flax and coir fibres. The flax fibres were provided by Lineo (Belgium), which were continuous long fibres and ready for use in composite materials. The coir fibres were extracted from the husk shell of coconuts with a purely mechanical extraction process, which can keep the fibre as long as possible. For the coir fibres, a fibre cleaning process was applied by soaking in hot water at 70 o C for 2h, and drying in a vacuum oven at 90 C for 2h. These fibres are named untreated fibres in this work. Alkali treatment was applied for the flax fibre in 3% NaOH for 20 minutes, and for coir fibres in 5% NaOH for 2h. The fibres were then washed thoroughly with de-ionized water and dried in a vacuum oven at 90 C for 2h. The alkali treatment was expected to remove wax and fatty substances on the surface of the untreated fibres. Polylactic acid (PLA) and Polyvinylidene fluoride (PVDF) were used as matrix for flax and coir fibres respectively. The PLA was supplied by NatureWorks. The PVDF was a homopolymer provided by Solvay. 2.2 Surface characterization Wetting measurement and surface energies of fibres and matrices Wetting measurements of the fibers and the matrices are carried out to obtain their static equilibrium contact angles in various test liquids. For coir fibres, the static equilibrium contact angles were determined from their advancing contact angles as described in a previous study [1]. The equilibrium contact angles ( 0 ) of the flax fibres and the matrix films were estimated from their advancing ( ) and receding angles ( ), which were measured using the Wilhelmy method. When considering the sample surfaces were smooth and chemically homogeneous, a model of the arithmetic mean of the corresponding cosines of advancing and receding angles as shown in Eq.1 was used for the calculation of the equilibrium angles [2-4]. cos 0 0.5cos adv 0.5cos rec (1) The contact angle measurements were carried out with measurement speed at 25 m/s, in three different test liquids.
These contact angles of the fibres and the matrices are then used to estimate the surface energies comprising of different components [5]. Using the surface energies, the work of adhesion is calculated for each composite system, accordingly. Characterization of fibre surface chemistry The fibre surface chemistry is studied by X-ray photoelectron spectroscopy (XPS) to obtain more information about functional groups at the fiber surface. XPS analyses were performed on a Kratos Axis Ultra spectrometer (Kratos Analytical Manchester UK) equipped with a monochromatic aluminium X-ray source (powered at 10 ma and 15 kv). More information regarding the XPS analysis procedure can be found in [1]. 2.3. Preparation of UD composite samples UD flax fibres and coir fibres composites were prepared by compression moulding. The UD fibres and polymer films with a certain stacking sequence were placed into a closed mould. Processing parameters were 185 C for coir/pvdf and 165 C for flax/pla, and 10 bar pressure for all samples. 2.4 Investigation of interface and mechanical properties of UD composites by three-point bending tests Flexural three point bending tests were performed on an Instron universal testing machine based on ASTM D790, both in transverse direction to obtain a value for the interface strength and in longitudinal direction to evaluate composite strength. The fracture surfaces of test samples in transverse direction were also investigated using SEM images to understand the interface failure. 3. Results and discussions 3.1. Fibre surface properties Fibre surface chemistry Using the XPS technique, the surface chemistry of the fibres is characterised. Table 1 presents the relative atomic percentages of the elements, together with the oxygen to carbon atomic ratio of untreated and alkali treated fibres. It can be seen that a high proportion of carbon in untreated coir and flax fibres may represent a hydrocarbon rich waxy layer on the surface. In the same fashion, the low oxygen-carbon ratio also indicates a high proportion of aliphatic and aromatic carbons[6]. After the alkali treatment of the fibres, the carbon percentage decreases while the O/C ratio increases. The O/C value of alkali treated coir and flax fibres is close to that reported for dioxane lignin (range of 0.31-0.36), but still far different from cellulose having an O/C ratio of 0.83 [7,8]. Therefore, the surface of both treated coir and flax likely has a greater proportion of lignin. It is probable that waxes and fatty substances on the coir surface are washed away by alkali, to expose the lignin which binds the elementary fibres. Table 1 also shows the decomposition of the C 1s peak into four sub-peaks C1 C4. These represent: carbon solely linked to carbon or hydrogen C C or C H (C1), carbon singly bound to oxygen or nitrogen C O or C N (C2), carbon doubly bound to oxygen O C O or C=O (C3) and carbon involved in ester or carboxylic acid functions O=C O (C4). For untreated fibres, a high proportion of C1 and low proportion of C2-C4 suggests a combination of hydrocarbon rich waxes and lignin existing on the fibre surface. In case of alkali treated fibres, C1 decreases while C2-C4 increase,
which shows that more lignin as binder of elementary fibres is exposed on the surface of the treated fibres after mainly the waxes are removed [9]. Table 1. Relative atomic percentages and decomposition of C 1s peaks obtained by XPS on untreated and alkali treated coir and flax fibres Fibres C O Si O/C Binding energy (ev) (%) (%) (%) 284.8 286.3 287.5 288.8 C1 (%) C2 (%) C3 (%) C4 (%) (C-C/C-H) (C-O) (C=O/O-C- O) (O-C=O) Untreated coir 74.9 21.8 0.9 0.29 66.2 23.1 6.2 4.5 Untreated flax 88.1 10.7 1.2 0.12 61.7 22.3 4.1 - Alkali 5% treated coir 72.9 23.2 1.3 0.32 48.0 34.2 11.5 6.4 Alkali 3% treated flax 78.9 18.8 2.3 0.24 58.4 12.9 7.6 - Fibre surface energies The equilibrium contact angles of the coir, flax fibres and the matrices in the three test liquids (water, diiodomethane, and ethylene glycol) are shown in Table 2. Table 2. Contact angles and surface energies estimated following the Owens-Wendt approach of fibres and matrices Static/equilibrium contact angle Surface energy (mj/m 2 ) Fibre/Matrix (degrees) disperse-polar (Owens-Wendt) water Diiodo Ethylene methane glycol Untreated coir 77.3 48.2 47.1 39.4 34.0 5.4 Untreated flax 59.1 48.7 48.5 43.7 29.2 14.5 Alkali 5% treated coir 70.9 51.5 41.9 40.8 31.9 8.9 Alkali 3% treated flax 53.9 52.6 48.1 45.2 26.6 18.6 PVDF 77.7 55.2 44.3 37.7 31.3 6.4 PLA 64.0 52.8 53.6 40.1 27.4 12.7 Using the contact angles, the surface energies of the fibres and matrices are then estimated following the Owens-Wendt approach, and shown in Table 2 and Figure 1. It can be seen that the untreated coir fibres seem to be hydrophobic with a low polar fraction of the surface energy, while the untreated flax has higher both total surface energy and polar component. After treatment of the fibres with alkali, the surface energies of the treated coir and flax fibres increase with more polarity. The results show a good correlation with the results of fibre surface chemistry, where the removal of waxes on the fibre surface by alkali treatment may help to expose more lignin which has higher polar properties compared to waxes. For the matrices, the surface energy of PVDF and PLA are as expected with high polar fraction.
Surface energy (mj/m 2 ) 50 dispersive polar 40 30 34.0 31.9 29.2 26.6 31.3 27.5 20 10 0 18.6 14.4 12.7 5.4 8.8 6.4 U-coir A-coir U-flax A-flax PVDF PLA materials Figure 1. Surface energies of untreated coir (U-coir), alkali treated coir (A-coir), untreated flax (Uflax) and alkali treated flax (A-flax) fibres and matrices described by the Owens-Wendt approach 3.2. Fibre-matrix interfacial adhesion Physical adhesion determined by work of adhesion The physico-chemical interaction at the fibre-matrix interface is quantitatively characterised by work of adhesion, which can be calculated from the surface energies of fibre and matrix. Work of adhesion and interfacial energy for different composite systems based on the coir and flax fibres in PVDF and PLA were calculated and are shown in Table 3. The results show that flax/pla composite systems have higher work of adhesion in comparison with coir/pvdf systems mainly thanks to the high surface energies of flax and PLA. It also can be seen that alkali treatment somewhat improves the work of adhesion of all fibre-matrix systems, which can be partially attributed to the higher surface energy and polar component of the fibres. Table 3. Work of adhesion, interfacial energy, transverse strength and efficiency factor of longitudinal strength of different fibre matrix systems Composite W a (mj/m 2 ) Interfacial energy (mj/m 2 ) Transverse strength (MPa) Efficiency Factor of Longitudinal strength U-Coir/PVDF 77.0 0.1 16.6 ± 2.7 0.66 A-Coir/PVDF 78.3 0.2 21.5 ± 2.8 0.85 U-Flax/PLA 83.7 0.1 19.2 ± 1.6 0.77 A-Flax/PLA 84.7 0.6 22.1 ± 6.7 0.82 Interfacail adhesion measured by transverse 3-point bending test
Transverse strength (MPa) Efficiency factor of Strength Transverse Strength Eff. Factor of Long. Strength 50 45 40 35 30 25 20 15 10 5 0 0.85 0.82 0.77 0.66 U-Coir/PVDF A-Coir/PVDF U-Flax/PLA A-Flax/PLA Composites 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 Figure 2. Transverse bending strength and efficiency factor of longitudinal strength of different composites The transverse bending strength of UD composites measured by 3PBT are presented in Table 3 and Figure 2. To understand more about fracture mechanism of the composite samples, the fracture surface of tested samples was investigated by SEM images, as shown in Figure 3. The images indicate that the failure occurred at the fibre-matrix interface with clean surface of fibres. Thus the transverse strength can be considered representative for interfacial strength of the composites. From the results; it can be seen that there is an improvement in interfacial strength for alkali treated coir and flax fibres composites in comparison with that of untreated fibres. It seems that the change of fibre surface properties by alkali treatment affects fibre-matrix physical adhesion which results in higher interfacial adhesion of the final composites. Since there is likely no chemical bonding involved in the interface interactions, physical adhesion is proposed as the main interaction leading to the difference in the interfacial adhesion in the composite systems. The modification of fibre surface energy by alkali treatment can contribute to better adhesion thanks to increasing fibre surface energy of the fibres and the matrices. Figure 3.Typical SEM images of the facture surface of coir fibre composites in transverse 3PBT. The efficiency factor is the ratio of experimental longitudinal strength over the calculated value following the rule of mixtures. The results of efficiency factors in Table 3 and Figure 2 also show the improvement of the interface and composite strength when the fibres are treated. This is highly consistent with the results of the interfacial adhesion.
3.3. Mechanical properties of UD composites The flexural properties of coir and flax fibre composites were measured by 3PBT in longitudinal direction on UD composites, as presented in Table 4. Higher flexural strength and stiffness are found in treated fibre composites, probably thanks to better interfacial wetting and adhesion which can be seen by higher efficiency factors (Table 3). Comparing coir and flax fibre composites, the flax fibre PLA composites are stiffer and stronger which are attributed to the higher mechanical properties of the flax fibres. Table 4. Mechanical properties of UD coir and flax fibre composites Composite E-Modulus(GPa) Strength(MPa) U-Coir/PVDF 2.3 ± 0.4 82.8 ± 1.8 A-Coir/PVDF 2.8 ± 0.4 103.4 ± 2.4 U-Flax/PLA 23.5 ± 1.1 288.6 ± 14.8 A-Flax/PLA 30.1 ± 1.9 307.9 ± 27.7 4. Conclusions The interfacial adhesion of coir fibre PVDF and flax fibre PLA composites was investigated. Using physical-chemical analysis, the results of the characterization of fibre surface chemistry using XPS are consistent to those of wetting measurements, showing the higher polarity of flax fibres compared to coir fibres. The alkali treatment on both fibres helps to increase the fibre surface energies and polarity. The work of adhesion of different fibre-matrix systems calculated based on their surface energies showed higher values of alkali treated fibre composites as compared to untreated fibres. Practical adhesion of UD composites was evaluated using transverse 3PB tests. The results showed an improvement in interfacial adhesion of alkali treated fibre composites. The influence of the interfacial adhesion was also reflected in the composite strength, which was expressed by the longitudinal efficiency factor. In this study, the mechanical properties of UD fibre composites also reflect well the results from the composite interface characterisations. References [1] Tran, L.Q.N., et al., Wetting analysis and surface characterisation of coir fibres used as reinforcement for composites. Colloids and Surfaces a-physicochemical and Engineering Aspects, 2011. 377(1-3): p. 251-260. [2] De Jonghe, V., et al., Contact angle hysteresis due to roughness in four metal/sapphire systems. Journal de chimie physique, 1990. 87(9): p. 1623-1645. [3] Andrieu, C., C. Sykes, and F. Brochard, Average spreading parameter on heterogeneous surfaces. Langmuir, 1994. 10(7): p. 2077-2080. [4] Dettre, R.H., Wetting of low-energy surfaces. Wettability, 1993. 49: p. 1. [5] Owens, D.K., Estimation of the surface free energy of polymers. Journal of Applied Polymer Science, 1969. 13(8): p. 1741. [6] Panthapulakkal, S. and M. Sain, Agro-residue reinforced high-density polyethylene composites: Fiber characterization and analysis of composite properties. Composites Part A: Applied Science and Manufacturing, 2007. 38(6): p. 1445-1454.
[7] Dorris, G.M. and D.G. Grey, The surface analysis of paper and wood fibers by ESCA. I. Application to cellulosics and lignin,. Cellulose Chem. Technol., 1978. 12: p. 9-23. [8] Dorris, G.M. and D.G. Grey, The surface analysis of paper and wood fibers by ESCA. II. Surface composition of mechanical pulp. Cellulose Chem. Technol., 1978. 12: p. 721-734. [9] Van Dam, J.E.G. and D. van, Process for production of high density/high performance binderless boards from whole coconut husk: Part 1: Lignin as intrinsic thermosetting binder resin. Industrial crops and products, 2004. 19(3): p. 207.