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Composite micromechanics of hemp fibres and epoxy resin microdroplets
Composites Science and Technology 64 (2004) 767–772 www.elsevier.com/locate/compscitech
Composite micromechanics of hemp fibres and epoxy resin microdroplets S.J. Eichhorn*, R.J. Young Manchester Materials Science Centre, UMIST/University of Manchester, Grosvenor Street, Manchester M1 7HS, UK Received 21 January 2003; received in revised form 11 August 2003; accepted 15 August 2003
Abstract It is shown that the microdeformation of single hemp fibres can be monitored by following the peak shift of the 1095 cm1 Raman band with respect to strain and stress. This relationship is then used to monitor the deformation micromechanics of strained single hemp fibres with a microdroplet of epoxy resin attached along the gauge length. It is shown that it is possible to map the stress along the fibre both outside and inside the droplet. The profile of the stress distribution indicates the build up of shear stress at the interface. The stress distribution within the droplet can be explained in terms of the effect of surface tension and contraction of the matrix around the fibre. It is shown by using a force balance that one can obtain a maximum interfacial shear stress of the order of the shear yield stress of the resin, similar to conventional fibre-composite systems. The work of adhesion determined by surface energy calculations gives important information for the utilisation of natural cellulose fibres in composite materials. # 2003 Elsevier Ltd. All rights reserved. Keywords: Hemp fibres; B. Interfacial strength; C. Stress transfer; D. Raman spectroscopy
1. Introduction Cellulose is one of the most industrially used materials, particularly in the production of timber and pulp and paper. It is also the structural material that plant-life has adopted for structural reinforcement. Its structure is polymeric, with a repeat unit (cellobiose) containing a two anhydroglucose units joined by a b-d-1,4 glycosidic linkage [1]. In the unit cell of the native or natural cellulose, two chains are joined by hydrogen bonding to each other in a parallel conformation, which is called cellulose-I [1]. This crystal form of cellulose is made from two further polymorphs namely cellulose-Ia and cellulose-Ib [2]. These units are packed side-by-side to form microfibrils of cellulose, which also contain disordered or amorphous regions. These microfibrils make up the main cell wall (S2 layer) of natural fibres, of which there are many varieties depending on species (e.g. flax, hemp, cotton, jute, wood) [3].
* Corresponding author. Tel.: +44-161-200-5982; fax: +44-161200-3636. E-mail address: [email protected] (S.J. Eichhorn). 0266-3538/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2003.08.002
Before the turn of the century hemp plants were common agricultural crops grown in temperate climates for the production of ropes and shipping sails [3]. The process of retting, which releases the fibres from the plant stem is the first stage of the farming process. This usually involves cutting down the plant and allowing natural bacterial degradation to take place in the field [4], but other methods are chemical or biologically controlled [3]. More recent methods such as steam explosion are used nowadays to avoid losses in yield [4]. After these processes (with the exception of steam explosion) the plant stems are subjected to mechanical action (hackling and decortication) which helps release the technical fibres or fibre bundles [5], which are generally used in industrial applications [3]. These fibres comprise small units called ultimate fibres, and for hemp are usually 5–8 mm in length [5]. These fibres have a layered structure comprised mainly of cellulose, with some pectins and lignin species as binding domains [3]. Cellulose fibres have in recent times enjoyed a renewed interest, especially for their potential reinforcing capabilities in polymer composites as an alternative to glass fibres [6]. The key to making well-reinforced composites is by ensuring that there is a strong interface
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between fibre and matrix [7]. This is achieved by the addition of silanes, maleic anhydride and other coupling agents [6] albeit at the increased cost of the industrial process. Raman spectroscopy has been utilised extensively for the study of the deformation micromechanics of polymeric fibres and fibre-composites [8,9]. The technique relies on the fact that Raman bands, corresponding to the vibrational modes of bonds in the polymer fibre, shift towards a lower wavenumber upon the action of strain and/or stress, thought to be due to direct molecular straining/stressing [10]. This has been used to map stresses along fibres embedded in matrix resins to determine, amongst other parameters the interfacial shear stress [9]. Properties of the interface have been determined by a wide variety of testing regimes using this technique including fibre pull-out, push-in, fragmentation and microbond [9]. Raman spectroscopy has also been used in recent times to investigate the deformation micromechanics of natural and regenerated cellulose fibres [11,12]. Some work has also been done on composite materials, but it was found that bonding across the ends of the fibres gave rise good stress transfer and hence made it difficult to evaluate the properties of the interface [13]. Work has also been done to look at the effect of strain on regenerated cellulose fibre–droplet interfaces [14], but with no measurement of the interfacial shear stress. This paper seeks to address this difficulty by the use of a new testing regime for natural fibres.
2. Experimental 2.1. Materials The natural fibres used for this study were field retted hemp which was supplied by Hemcore, UK. A coldcuring two-part epoxy resin provided by Sigma was also used (LY5052/HY5052 in ratio 5:2 by weight). 2.2. Equipment A Renishaw 1000 Raman imaging microscope was used to record the spectra of fibre monofilaments. A helium-neon (633 nm, 25 mW) laser was employed for all the studies. When focussed onto the fibre surfaces the spot size was roughly 2 mm in diameter and had a power of about 1 mW. Back scattered light was collected using a 50 lens and an Olympus microscope. The scattered Raman light was then filtered using a holographic notch filter and the resultant radiation split into to a spectrum by a diffraction grating. A highly-sensitive Peltier-cooled charge-couple device (CCD) detector recorded all spectra and these were collected on a computer.
2.3. Single fibre deformation The methods used to determine single-fibre mechanical properties of natural cellulose have previously been reported in depth [11]. Single fibres were extracted from the natural plant material by soaking fibres in hydrogen peroxide for 48 h and pulling out single ultimate fibres (8 mm) with tweezers under a dissection microscope. These fibres were then transferred and secured to a cardboard window using Ciba-Geigy HY/LY1927 twopart cold-curing epoxy resin. Mechanical testing of the fibres was performed using an Instron tensile testing machine housed in an environmentally conditioned room (at 23 1 C and 50 5% relative humidity). A crosshead speed of 1 mm/min was utilised for the testing and the load was recorded using a 1N capacity load cell. For statistical reasons 30 samples were tested, and average values of mechanical properties were calculated. To determine the stress on the fibres the load was converted to this parameter by measuring the fibre diameters using a calibrated FEG-SEM with an exciting voltage of 2 keV. By assuming that the fibres have a near-circular cross section the diameters were converted to the cross sectional area and hence stress could be determined by dividing the load values by this constant (again assuming that no change occurs in the cross-section during testing). The values for the mechanical properties and the fibre diameter are reported in Table 1. Single fibres were also deformed under the microscope in the Raman spectrometer and spectra obtained at each strain and stress level. These deformation studies were carried out using a custom-made stress/strain rig with a micrometer for increasing the strain and a high-precision load cell for measuring the load on the fibre. Spectra were collected at each strain and stress level using an exposure time of 120 s and in all 10 fibre samples were tested. 2.4. Composite micromechanics Single natural cellulose fibres were placed onto fibre cards with a drop of tissue tack at each end to secure the sample. Then epoxy resin was applied to each end and allowed to cure at room temperature for 48 h. Then each card was placed between two mounting points and by dipping the ends of a pair of tweezers into cold curing epoxy resin (LY5052/HY5052) and slowly opening them a thin strand of epoxy was produced. This was Table 1 Mechanical properties of hemp fibres, where df—fibre diameter (30 measurements), E—Young’s Modulus, f*—breaking stress and f*— breaking strain df (mm)
E (GPa)
f* (GPa)
f* (%)
31.24.9
19.1 4.30
0.270.04
0.800.10
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placed over the width of the fibres, whilst observing the process under a dissection microscope, to produce a microdroplet of epoxy resin along the gauge length. This droplet was allowed to cure at room temperature for 7 days prior to testing. It was possible to determine the stress profile of the system through the microdroplet by scanning along the fibre, at regular intervals of 5–10 mm, and taking Raman spectra over an exposure time of 120 s with 10 accumulations. However, spectra were unobtainable close to the point where the fibre first enters the microdroplet and over a distance of up to 80–100 mm into the resin. This is thought to be due to total internal reflection and one can see the result of this indicated by the dark areas of the droplet shown in Fig. 1. After each scan along the fibre through the microdroplet was completed, the strain in the sample was increased incrementally in steps of 0.2%, and the process repeated.
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which is about 2.5) [7], which suggests that they have potential for reinforcement in composite materials. Taking the maximum stress and dividing by the equivalent strain determines the secant modulus (32.5 2.5 GPa), which is a more representative value given the mixture of linear elastic and strain hardening effects. If one calculates the specific modulus of the hemp fibres (modulus/specific gravity) then one obtains a value of 22 GPa. A similar calculation for glass fibres using a modulus of 70 GPa [7] gives a specific modulus of 28 GPa. This indicates that hemp fibre mechanical properties are approaching the properties of glass fibres, and therefore could therefore be useful for low-weight and low-cost applications and where variability of mechanical properties is not an issue. A typical Raman spectrum for hemp fibres is shown in Fig. 3 indicating the strong 1095 cm1 peak the position of which was monitored during deformation.
3. Results and discussion 3.1. Single fibre deformation All fibres showed either Hookean linear elastic behaviour or strain hardening where the modulus of the fibre increases with strain. An example of each of these types of behaviour is shown in Fig. 2. The initial modulus of the fibres was found to be (19.1 4.3 GPa) where the quoted error is the standard deviation divided by the square root of the number of tests (30). However, the values of the modulus were found to vary from 10 to 50 GPa. The mean modulus is reasonably high given the low specific gravity of the fibres (about 1.5 compared to E-glass
Fig. 2. Examples of typical stress–strain curves for hemp fibres.
Fig. 1. Hemp fibre with epoxy resin droplet on surface indicating the regions where internal reflection (TIR) takes place and where the interface measurements (IM) are possible.
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Fig. 3. Typical Raman spectrum for a hemp fibre indicating the 1095 cm1 peak.
Fig. 4. Raman band shift in the 1095 cm1 peak for a hemp fibre.
This peak corresponds to the ring-stretching modes of the cellulose structure [15] with the glycosidic stretching as another possible assignment [16]. The peak is found to shift under the application of strain and stress and an example of a typical spectral shift is shown in Fig. 4. It was found that the failure strain was greater than that obtained on average for the single fibre tensile testing. This was thought to be due to the fact that slower strain rates (0.0016% s1) were being employed due to the long exposure times for recording spectra, and hence a decrease in band shift rate. The 1095 cm1 peak was found to shift approximately linearly with both strain and stress (Fig. 5a and b) as has been reported for other fibres [8,11,12]. The precise gradient of these shifts means that one can use the fibres as a stress gauge within a composite material as will be discussed in the next section. The band shift rate with respect to strain is of the order of the figure obtained for regenerated cellulose fibres from previous studies [11,12]. 3.2. Composite micromechanics Using the calibration obtained from the previous section it was possible to convert the Raman shift data
Fig. 5. Raman band shifts for the 1095 cm1 peak for a hemp fibre with respect to (a) strain and (b) stress. Dotted lines indicate 95% confidence bands.
obtained from the scanning of the microdoplet-fibre systems to stress. A typical plot of this conversion indicating the regions inside and outside the microdroplet is shown in Fig. 6. It is clear that the stress drops dramatically at the edges of the droplet and then there is a small symmetrical increase and decrease in the stress in the fibre through the centre. This increase and decrease is thought to be due to surface tension effects at the edge of the droplet where the gradient of the droplet to the fibre decreases. This will put the fibre into compression because of its relatively small modulus. The resin is cold cured so there should not be an appreciable shrinkage around the fibre, which would be another possible cause. However, the fibre diameter in the middle of the droplet appears to be 1.5 that of the fibre outside the droplet. This is thought to be due to the fact that there is a refraction of the light, due to the different refractive indices of air and epoxy resin. Therefore one is really sampling a different region of the fibre at each point along the specimen through this region. For large deviations of the light this distorts the length scale by a factor of 1.5 (the ratio of the refractive index of epoxy to air). Therefore, by multiplying the scale in Fig. 6 by this factor one can see that it would flatten the symmetrical
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Fig. 6. Typical stress profile as determined from the peak position of the 1095 cm1 Raman band through an epoxy microdroplet at elevated strain. Data are fitted with a 4th order polynomial function.
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maximum interfacial shear stress, which indicates the level of adhesion between the fibre and matrix. For all the samples tested (3) this was found to be of the order of the shear yield stress of the resin (40–45 GPa) [18] which compared to other systems such as aramid–epoxy (42 GPa) [19] and glass–epoxy (40–50 GPa) [20] is an indication of a reasonable interface between the fibre and matrix. Previous studies using thin films of epoxy and natural fibres [13] have shown that with short fibres bonding occurs across the ends giving rise to better stress transfer. The presence of defects in the fibres, due to both processing and naturally occurring damage, has also been shown to give rise to local stress concentrations within the fibres giving rise to the formation of cracks in composite materials [13,21]. If the interface is strong between the fibre and matrix then this could explain why the fracture toughness of natural fibre composites is generally reported as being lower than for equivalent glass–epoxy systems with improvements only made with higher fibre volume fractions of natural fibres [22]. The contact angles of the droplets investigated were measured from light microscope images. By using the equation [23]
RF FA cos ¼ ð2Þ
RA where is the contact angle, and RF, FA and RA are the surface energies between the resin and the fibre, the fibre and the air and the resin and the air respectively it was possible to determine the work of adhesion WA between the matrix and the fibre. Knowing, from literature values [23,24], that FA 35 mJ/m2 and RA 43 mJ/m2 and the equation [23] WA ¼ FA þ RA RF
Fig. 7. Interfacial shear stress (ISS) derived along the fibre/droplet system. Data calculated using a force balance [1].
increase and decrease in fibre stress through the centre of the droplet making it less apparent. A simple force balance approach [17] was then used to calculate the interfacial shear stress ( i), which is related to the rate of change of stress with position (d f/dx) by df 4 ¼ i dx d
ð1Þ
where d is the diameter of the fibre. By using the curve fit to the experimental data, as shown in Fig. 5 for example, then one can calculate dsf/dx and plot this as a function of position (x). A typical result of this calculation is shown in Fig. 7. It is clear that the interfacial shear stress is zero in the centre of the droplet, as would be expected. However, this parameter goes through maxima with the droplet and these points determine the
ð3Þ
the work of adhesion was found to be 77 8 mJ/m2. The work of adhesion between a glass and epoxy surface has been determined [25] as 89 9 and 95 4 mJ/m2 for two types of epoxy resin. These values are of the same order as those obtained for hemp/epoxy although it is thought that this parameter will be much more variable for natural fibres than glass due to uneven surfaces, defects and variable surface chemistry.
4. Conclusions Raman spectroscopy has been demonstrated to be a powerful tool in the analysis of single natural fibres and composite systems. The 1095 cm1 cellulose Raman band has been shown to shift under the application of strain and stress towards a lower wavenumber indicative of molecular deformation. It has also been shown that it is possible to use Raman spectroscopy to map the stress of a hemp fibre inside an epoxy resin droplet on
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the surface. This technique has also shown that an interfacial shear stress for epoxy and hemp fibres is comparable to aramid/epoxy and glass/epoxy systems, but coupled with good adhesion of the fibre ends (in a short fibre composite) may lead to low fracture toughness of a natural fibre composite. From the geometry of the droplet on the surface it is also possible to determine the work of adhesion of the matrix to the fibre, which gives an indication of good adhesion between the fibre and resin. It was found that the upper range of values are comparable to glass–epoxy interfaces. However the values are variable and this has implications for composite reinforcement leasing to an inherent variability in strength.
Acknowledgements The authors wish to thank Hemcore, UK for providing the field retted hemp samples. This work was completed using funding via EPSRC grant No. GR/ M82219.
References [1] Klemm D, Phillip B, Heinze T, Heinze U, Wagenknecht W. Comprehensive cellulose chemistry, fundamentals and analytical methods, vol. 1. Weinheim: Wiley–VCH; 1998. [2] Atalla RH, van der Hart DL. Native cellulose. A composite of two distinct crystalline forms. Science 1984;223:283–5. [3] Gordon-Cook J. Handbook of textile fibres. Merrow Publishing Ltd, 1984. [4] Kessler RW, Becker U, Kohler R, Goth B. Steam explosion of flax—a superior technique for upgrading fibre value. Biomass and Bioenergy 1998;14:237–49. [5] Vincent JFV. A unified nomenclature for plant fibres for industrial use. Applied Composite Materials 2000;7:269–71. [6] Eichhorn SJ, Baillie CA, Zafeiropoulos N, Mwaikambo LY, Ansell MP, Dufresne A, Entwistle KM, Herrera-Franco PJ, Escamilla GC, Groom L, Hughes M, Hill C, Rials TG, Wild PM. Review: current international research into cellulosic fibres and composites. J Mat Sci 2001;36:2107–31.
[7] Harris B. Engineering composite materials. IOM Communications Ltd 1999. [8] Young RJ. Monitoring deformation processes in high-performance fibres using Raman spectroscopy. J Text Inst 1995;86:360–81. [9] Young RJ. Evaluation of composite interfaces using Raman spectroscopy. Key Engin Mat 1996;116-117:173–92. [10] Batchelder DN, Bloor D. Strain dependence of the vibrational modes of the diacetylene crystal. J Polym Sci Polym Phys Ed 1979;17:569–81. [11] Eichhorn SJ, Sirichaisit J, Young RJ. Deformation mechanisms in cellulose fibres, paper and wood. J Mater Sci 2001;36:3129–35. [12] Eichhorn SJ, Young RJ, Yeh W-Y. Deformation processes in regenerated cellulose fibers. Text Res J 2001;71:121–9. [13] Eichhorn SJ, Young RJ. Deformation micromechanics of natureal cellulose fibre networks and composites. Comp Sci Technol 2003;63:1225–30. [14] Tze WTY, Gardner DJ, Tripp CP, Shaler SM, O’Neill SC. Raman micro-spectroscopic study of cellulose fibers at fiber/ polymer interface. Abstracts of Papers of the American Chemical Society 2002; 223: 031-CELL Part 1. [15] Wiley JH, Atalla RH. Band assignments in the Raman-spectra of cellulosics. Carbohydrate Research 1987;160:113–29. [16] Edwards HGM, Farwell DW, Webster D. FT Raman microscopy of untreated natural plant fibres. Spectrochimica Acta Part A 1997;53:2383–92. [17] McCrum NG, Buckley CP, Bucknall CB. Principles of polymer engineering. Oxford University Press, 1997. [18] Andrews MC. Stress transfer in aramid/epoxy model composites. PhD thesis, UMIST, Manchester Materials Science Centre, 1994. [19] Andrews MC, Bannister DJ, Young RJ. The interfacial properties of aramid/epoxy model composites. J Mat Sci 1996;31:3893–913. [20] Chen F, Tripathi D, Jones FR. Determination of the interfacial shear strength of glass-fibre-reinforced phenolic composites by a bimatrix fragmentation technique. Comp Sci Technol 1996;56: 609–22. [21] Hughes M, Sebe G, Hague J, Hill C, Spear M, Mott L. An investigation into the effects of micro-compressive defects on interphase behaviour in hemp-epoxy composites using half-fringe photoelasticity. Composite Interfaces 2000;7:13–29. [22] Bledzki AK, Gassan J. Composites reinforced with cellulose based fibres. Prog Polym Sci 1999;24:221–74. [23] Gassan J, Gutowski VS, Bledzki AK. About the surface characteristics of natural fibres. Macromol Mater Eng 2000;283:132– 9. [24] Hull D. An introduction to composite materials. Cambridge University Press, 1990. [25] Swadener JG, Liechti KM, de Lozanne AL. The intrinsic toughness and adhesion mechanisms of a glass/epoxy interface. Journal of the Mechanics and Physics of Solids 1999;47:223–58.
Composite micromechanics of hemp fibres and epoxy resin microdroplets S.J. Eichhorn*, R.J. Young Manchester Materials Science Centre, UMIST/University of Manchester, Grosvenor Street, Manchester M1 7HS, UK Received 21 January 2003; received in revised form 11 August 2003; accepted 15 August 2003
Abstract It is shown that the microdeformation of single hemp fibres can be monitored by following the peak shift of the 1095 cm1 Raman band with respect to strain and stress. This relationship is then used to monitor the deformation micromechanics of strained single hemp fibres with a microdroplet of epoxy resin attached along the gauge length. It is shown that it is possible to map the stress along the fibre both outside and inside the droplet. The profile of the stress distribution indicates the build up of shear stress at the interface. The stress distribution within the droplet can be explained in terms of the effect of surface tension and contraction of the matrix around the fibre. It is shown by using a force balance that one can obtain a maximum interfacial shear stress of the order of the shear yield stress of the resin, similar to conventional fibre-composite systems. The work of adhesion determined by surface energy calculations gives important information for the utilisation of natural cellulose fibres in composite materials. # 2003 Elsevier Ltd. All rights reserved. Keywords: Hemp fibres; B. Interfacial strength; C. Stress transfer; D. Raman spectroscopy
1. Introduction Cellulose is one of the most industrially used materials, particularly in the production of timber and pulp and paper. It is also the structural material that plant-life has adopted for structural reinforcement. Its structure is polymeric, with a repeat unit (cellobiose) containing a two anhydroglucose units joined by a b-d-1,4 glycosidic linkage [1]. In the unit cell of the native or natural cellulose, two chains are joined by hydrogen bonding to each other in a parallel conformation, which is called cellulose-I [1]. This crystal form of cellulose is made from two further polymorphs namely cellulose-Ia and cellulose-Ib [2]. These units are packed side-by-side to form microfibrils of cellulose, which also contain disordered or amorphous regions. These microfibrils make up the main cell wall (S2 layer) of natural fibres, of which there are many varieties depending on species (e.g. flax, hemp, cotton, jute, wood) [3].
* Corresponding author. Tel.: +44-161-200-5982; fax: +44-161200-3636. E-mail address: [email protected] (S.J. Eichhorn). 0266-3538/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2003.08.002
Before the turn of the century hemp plants were common agricultural crops grown in temperate climates for the production of ropes and shipping sails [3]. The process of retting, which releases the fibres from the plant stem is the first stage of the farming process. This usually involves cutting down the plant and allowing natural bacterial degradation to take place in the field [4], but other methods are chemical or biologically controlled [3]. More recent methods such as steam explosion are used nowadays to avoid losses in yield [4]. After these processes (with the exception of steam explosion) the plant stems are subjected to mechanical action (hackling and decortication) which helps release the technical fibres or fibre bundles [5], which are generally used in industrial applications [3]. These fibres comprise small units called ultimate fibres, and for hemp are usually 5–8 mm in length [5]. These fibres have a layered structure comprised mainly of cellulose, with some pectins and lignin species as binding domains [3]. Cellulose fibres have in recent times enjoyed a renewed interest, especially for their potential reinforcing capabilities in polymer composites as an alternative to glass fibres [6]. The key to making well-reinforced composites is by ensuring that there is a strong interface
768
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between fibre and matrix [7]. This is achieved by the addition of silanes, maleic anhydride and other coupling agents [6] albeit at the increased cost of the industrial process. Raman spectroscopy has been utilised extensively for the study of the deformation micromechanics of polymeric fibres and fibre-composites [8,9]. The technique relies on the fact that Raman bands, corresponding to the vibrational modes of bonds in the polymer fibre, shift towards a lower wavenumber upon the action of strain and/or stress, thought to be due to direct molecular straining/stressing [10]. This has been used to map stresses along fibres embedded in matrix resins to determine, amongst other parameters the interfacial shear stress [9]. Properties of the interface have been determined by a wide variety of testing regimes using this technique including fibre pull-out, push-in, fragmentation and microbond [9]. Raman spectroscopy has also been used in recent times to investigate the deformation micromechanics of natural and regenerated cellulose fibres [11,12]. Some work has also been done on composite materials, but it was found that bonding across the ends of the fibres gave rise good stress transfer and hence made it difficult to evaluate the properties of the interface [13]. Work has also been done to look at the effect of strain on regenerated cellulose fibre–droplet interfaces [14], but with no measurement of the interfacial shear stress. This paper seeks to address this difficulty by the use of a new testing regime for natural fibres.
2. Experimental 2.1. Materials The natural fibres used for this study were field retted hemp which was supplied by Hemcore, UK. A coldcuring two-part epoxy resin provided by Sigma was also used (LY5052/HY5052 in ratio 5:2 by weight). 2.2. Equipment A Renishaw 1000 Raman imaging microscope was used to record the spectra of fibre monofilaments. A helium-neon (633 nm, 25 mW) laser was employed for all the studies. When focussed onto the fibre surfaces the spot size was roughly 2 mm in diameter and had a power of about 1 mW. Back scattered light was collected using a 50 lens and an Olympus microscope. The scattered Raman light was then filtered using a holographic notch filter and the resultant radiation split into to a spectrum by a diffraction grating. A highly-sensitive Peltier-cooled charge-couple device (CCD) detector recorded all spectra and these were collected on a computer.
2.3. Single fibre deformation The methods used to determine single-fibre mechanical properties of natural cellulose have previously been reported in depth [11]. Single fibres were extracted from the natural plant material by soaking fibres in hydrogen peroxide for 48 h and pulling out single ultimate fibres (8 mm) with tweezers under a dissection microscope. These fibres were then transferred and secured to a cardboard window using Ciba-Geigy HY/LY1927 twopart cold-curing epoxy resin. Mechanical testing of the fibres was performed using an Instron tensile testing machine housed in an environmentally conditioned room (at 23 1 C and 50 5% relative humidity). A crosshead speed of 1 mm/min was utilised for the testing and the load was recorded using a 1N capacity load cell. For statistical reasons 30 samples were tested, and average values of mechanical properties were calculated. To determine the stress on the fibres the load was converted to this parameter by measuring the fibre diameters using a calibrated FEG-SEM with an exciting voltage of 2 keV. By assuming that the fibres have a near-circular cross section the diameters were converted to the cross sectional area and hence stress could be determined by dividing the load values by this constant (again assuming that no change occurs in the cross-section during testing). The values for the mechanical properties and the fibre diameter are reported in Table 1. Single fibres were also deformed under the microscope in the Raman spectrometer and spectra obtained at each strain and stress level. These deformation studies were carried out using a custom-made stress/strain rig with a micrometer for increasing the strain and a high-precision load cell for measuring the load on the fibre. Spectra were collected at each strain and stress level using an exposure time of 120 s and in all 10 fibre samples were tested. 2.4. Composite micromechanics Single natural cellulose fibres were placed onto fibre cards with a drop of tissue tack at each end to secure the sample. Then epoxy resin was applied to each end and allowed to cure at room temperature for 48 h. Then each card was placed between two mounting points and by dipping the ends of a pair of tweezers into cold curing epoxy resin (LY5052/HY5052) and slowly opening them a thin strand of epoxy was produced. This was Table 1 Mechanical properties of hemp fibres, where df—fibre diameter (30 measurements), E—Young’s Modulus, f*—breaking stress and f*— breaking strain df (mm)
E (GPa)
f* (GPa)
f* (%)
31.24.9
19.1 4.30
0.270.04
0.800.10
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placed over the width of the fibres, whilst observing the process under a dissection microscope, to produce a microdroplet of epoxy resin along the gauge length. This droplet was allowed to cure at room temperature for 7 days prior to testing. It was possible to determine the stress profile of the system through the microdroplet by scanning along the fibre, at regular intervals of 5–10 mm, and taking Raman spectra over an exposure time of 120 s with 10 accumulations. However, spectra were unobtainable close to the point where the fibre first enters the microdroplet and over a distance of up to 80–100 mm into the resin. This is thought to be due to total internal reflection and one can see the result of this indicated by the dark areas of the droplet shown in Fig. 1. After each scan along the fibre through the microdroplet was completed, the strain in the sample was increased incrementally in steps of 0.2%, and the process repeated.
769
which is about 2.5) [7], which suggests that they have potential for reinforcement in composite materials. Taking the maximum stress and dividing by the equivalent strain determines the secant modulus (32.5 2.5 GPa), which is a more representative value given the mixture of linear elastic and strain hardening effects. If one calculates the specific modulus of the hemp fibres (modulus/specific gravity) then one obtains a value of 22 GPa. A similar calculation for glass fibres using a modulus of 70 GPa [7] gives a specific modulus of 28 GPa. This indicates that hemp fibre mechanical properties are approaching the properties of glass fibres, and therefore could therefore be useful for low-weight and low-cost applications and where variability of mechanical properties is not an issue. A typical Raman spectrum for hemp fibres is shown in Fig. 3 indicating the strong 1095 cm1 peak the position of which was monitored during deformation.
3. Results and discussion 3.1. Single fibre deformation All fibres showed either Hookean linear elastic behaviour or strain hardening where the modulus of the fibre increases with strain. An example of each of these types of behaviour is shown in Fig. 2. The initial modulus of the fibres was found to be (19.1 4.3 GPa) where the quoted error is the standard deviation divided by the square root of the number of tests (30). However, the values of the modulus were found to vary from 10 to 50 GPa. The mean modulus is reasonably high given the low specific gravity of the fibres (about 1.5 compared to E-glass
Fig. 2. Examples of typical stress–strain curves for hemp fibres.
Fig. 1. Hemp fibre with epoxy resin droplet on surface indicating the regions where internal reflection (TIR) takes place and where the interface measurements (IM) are possible.
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Fig. 3. Typical Raman spectrum for a hemp fibre indicating the 1095 cm1 peak.
Fig. 4. Raman band shift in the 1095 cm1 peak for a hemp fibre.
This peak corresponds to the ring-stretching modes of the cellulose structure [15] with the glycosidic stretching as another possible assignment [16]. The peak is found to shift under the application of strain and stress and an example of a typical spectral shift is shown in Fig. 4. It was found that the failure strain was greater than that obtained on average for the single fibre tensile testing. This was thought to be due to the fact that slower strain rates (0.0016% s1) were being employed due to the long exposure times for recording spectra, and hence a decrease in band shift rate. The 1095 cm1 peak was found to shift approximately linearly with both strain and stress (Fig. 5a and b) as has been reported for other fibres [8,11,12]. The precise gradient of these shifts means that one can use the fibres as a stress gauge within a composite material as will be discussed in the next section. The band shift rate with respect to strain is of the order of the figure obtained for regenerated cellulose fibres from previous studies [11,12]. 3.2. Composite micromechanics Using the calibration obtained from the previous section it was possible to convert the Raman shift data
Fig. 5. Raman band shifts for the 1095 cm1 peak for a hemp fibre with respect to (a) strain and (b) stress. Dotted lines indicate 95% confidence bands.
obtained from the scanning of the microdoplet-fibre systems to stress. A typical plot of this conversion indicating the regions inside and outside the microdroplet is shown in Fig. 6. It is clear that the stress drops dramatically at the edges of the droplet and then there is a small symmetrical increase and decrease in the stress in the fibre through the centre. This increase and decrease is thought to be due to surface tension effects at the edge of the droplet where the gradient of the droplet to the fibre decreases. This will put the fibre into compression because of its relatively small modulus. The resin is cold cured so there should not be an appreciable shrinkage around the fibre, which would be another possible cause. However, the fibre diameter in the middle of the droplet appears to be 1.5 that of the fibre outside the droplet. This is thought to be due to the fact that there is a refraction of the light, due to the different refractive indices of air and epoxy resin. Therefore one is really sampling a different region of the fibre at each point along the specimen through this region. For large deviations of the light this distorts the length scale by a factor of 1.5 (the ratio of the refractive index of epoxy to air). Therefore, by multiplying the scale in Fig. 6 by this factor one can see that it would flatten the symmetrical
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Fig. 6. Typical stress profile as determined from the peak position of the 1095 cm1 Raman band through an epoxy microdroplet at elevated strain. Data are fitted with a 4th order polynomial function.
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maximum interfacial shear stress, which indicates the level of adhesion between the fibre and matrix. For all the samples tested (3) this was found to be of the order of the shear yield stress of the resin (40–45 GPa) [18] which compared to other systems such as aramid–epoxy (42 GPa) [19] and glass–epoxy (40–50 GPa) [20] is an indication of a reasonable interface between the fibre and matrix. Previous studies using thin films of epoxy and natural fibres [13] have shown that with short fibres bonding occurs across the ends giving rise to better stress transfer. The presence of defects in the fibres, due to both processing and naturally occurring damage, has also been shown to give rise to local stress concentrations within the fibres giving rise to the formation of cracks in composite materials [13,21]. If the interface is strong between the fibre and matrix then this could explain why the fracture toughness of natural fibre composites is generally reported as being lower than for equivalent glass–epoxy systems with improvements only made with higher fibre volume fractions of natural fibres [22]. The contact angles of the droplets investigated were measured from light microscope images. By using the equation [23]
RF FA cos ¼ ð2Þ
RA where is the contact angle, and RF, FA and RA are the surface energies between the resin and the fibre, the fibre and the air and the resin and the air respectively it was possible to determine the work of adhesion WA between the matrix and the fibre. Knowing, from literature values [23,24], that FA 35 mJ/m2 and RA 43 mJ/m2 and the equation [23] WA ¼ FA þ RA RF
Fig. 7. Interfacial shear stress (ISS) derived along the fibre/droplet system. Data calculated using a force balance [1].
increase and decrease in fibre stress through the centre of the droplet making it less apparent. A simple force balance approach [17] was then used to calculate the interfacial shear stress ( i), which is related to the rate of change of stress with position (d f/dx) by df 4 ¼ i dx d
ð1Þ
where d is the diameter of the fibre. By using the curve fit to the experimental data, as shown in Fig. 5 for example, then one can calculate dsf/dx and plot this as a function of position (x). A typical result of this calculation is shown in Fig. 7. It is clear that the interfacial shear stress is zero in the centre of the droplet, as would be expected. However, this parameter goes through maxima with the droplet and these points determine the
ð3Þ
the work of adhesion was found to be 77 8 mJ/m2. The work of adhesion between a glass and epoxy surface has been determined [25] as 89 9 and 95 4 mJ/m2 for two types of epoxy resin. These values are of the same order as those obtained for hemp/epoxy although it is thought that this parameter will be much more variable for natural fibres than glass due to uneven surfaces, defects and variable surface chemistry.
4. Conclusions Raman spectroscopy has been demonstrated to be a powerful tool in the analysis of single natural fibres and composite systems. The 1095 cm1 cellulose Raman band has been shown to shift under the application of strain and stress towards a lower wavenumber indicative of molecular deformation. It has also been shown that it is possible to use Raman spectroscopy to map the stress of a hemp fibre inside an epoxy resin droplet on
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the surface. This technique has also shown that an interfacial shear stress for epoxy and hemp fibres is comparable to aramid/epoxy and glass/epoxy systems, but coupled with good adhesion of the fibre ends (in a short fibre composite) may lead to low fracture toughness of a natural fibre composite. From the geometry of the droplet on the surface it is also possible to determine the work of adhesion of the matrix to the fibre, which gives an indication of good adhesion between the fibre and resin. It was found that the upper range of values are comparable to glass–epoxy interfaces. However the values are variable and this has implications for composite reinforcement leasing to an inherent variability in strength.
Acknowledgements The authors wish to thank Hemcore, UK for providing the field retted hemp samples. This work was completed using funding via EPSRC grant No. GR/ M82219.
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