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Dynamic mechanical properties of oil palm fibre (OPF)-linear low density polyethylene (LLDPE) biocomposites and study of fibre–matrix interactions
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Research Paper
Dynamic mechanical properties of oil palm fibre (OPF)-linear low density polyethylene (LLDPE) biocomposites and study of fibreematrix interactions S. Shinoj a, R. Visvanathan b,*, S. Panigrahi c, N. Varadharaju b a
Directorate of Oil Palm Research, Indian Council of Agricultural Research, Pedavegi, Eluru, Andhra Pradesh 534 450, India Department of Food and Agricultural Process Engineering, Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu 641 003, India c Department of Agricultural and Bioresource Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon SK, Canada S7N5A9 b
article info Article history:
The dynamic mechanical properties of oil palm fibre (OPF) linear low density polyethylene (LLDPE) composites in terms of storage modulus (E0 ), loss modulus (E00 ) and damping
Received 25 October 2010
parameter (tan d) in a temperature range of 150 C to 100 C is presented in this paper.
Received in revised form
The effect of fibre content, fibre size and fibre surface treatment on the dynamic
20 December 2010
mechanical properties was studied. The storage modulus and damping parameters were
Accepted 17 February 2011
predicted using different equations. The storage modulus and loss modulus increased with
Published online 30 March 2011
increase in fibre content and also upon alkali treatment on fibres. The glass transition temperature of pure LLDPE was 145 C, which increased to 128 C for composites with 40% fibre content. Fibre loading in composite and alkali treatment on fibre increased the loss modulus peak. Single tan d peaks were observed for all the composite samples tested. The tan d peak values decreased upon fibre addition whereas alkali treatment increased the tan d peak at all frequencies indicating better impact properties after alkali treatment. The interfacial strength indicator values (B) decreased upon alkali treatment and higher B values were observed for composites containing fibre size range 75e177 m followed by 425e840 m and 177e425 m indicating decreased order of bond strength. Activation energy was found to be high for composites with 425e840 m size fraction (80.7 kJ mol1) followed by 177e425 m size fraction (72.6 kJ mol1) and 75e177 m size fraction (68.1 kJ mol1). Storage modulus (E0 ) and damping parameters predicted using different equations were in agreement with experimental values. ª 2011 IAgrE. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years, there has been growing interest in natural fibre-polymeric composites due to increased public awareness and concern over environmental impact of artificial or manmade materials. Government legislations and regulations in
force in different parts of the world similar to the one implemented by the European Union envisaging use of 85% recyclable materials in automotives by 2015 (Holbery & Houstoun, 2006), further increase attention towards natural fibres and their composites. Natural fibre composites find applications in diverse fields ranging from the construction industry to
* Corresponding author. Tel.: þ91 422 6611272; fax: þ91 422 6611455. E-mail address: [email protected] (R. Visvanathan). 1537-5110/$ e see front matter ª 2011 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2011.02.006
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medical and aerospace applications. Natural fibres are advantageous in comparison to glass, carbon or similar inorganic fibres in a polymeric matrix due to their low energy cost, positive contribution to global carbon budget (Hill & Khalil, 2000), greater deformability, biodegradability (Rozman, Tay, & Abusamah, 2003), combustibility, ease of recycling (Sreekala, Kumaran, Geethakumariamma, & Thomas, 2004), less abrasiveness to processing equipment, lower cost (Raju, Ratnam, Ibrahim, Rahman, & Yunus, 2008), renewable nature, non-toxicity, high specific strength (Yousif & El-Tayeb, 2008), etc. Oil palm is an edible oil yielding crop cultivated on 11 Mha over 42 countries in the world (Khalil, Siti, Ridzuan, Kamarudin, & Khairul, 2008). The major source of fibre from oil palm is empty fruit bunches (EFB); a waste material obtained after stripping fruits from the fresh fruit bunch (FFB), yielding 73% fibre (Wirjosentono, Guritno, & Ismail, 2004). The palm oil industry has to dispose of about 1.1 tonne of EFB for every tonne of oil produced (Karina, Onggo, Abdullah, & Syampurwadi, 2008). The current uses of this highly cellulosic material are as boiler fuel, in the preparation of fertilisers or as mulching material (Singh, Manohan, & Kanopathy, 1982). When left in the field, these waste materials create considerable environmental problems (Law, Daud & Ghazali, 2007). Linear low density polyethylene (LLDPE) is a popular thermoplastic material used mainly for packaging applications (Zheng, Yanful, & Bassi, 2005). The low processing temperature (below 130 C) of LLDPE makes composite fabrication possible without partial melting or annealing of the fibres (Lee & Joo, 1999). Materials respond to an applied force by exhibiting either elastic or viscous behaviour or a combination of these called viscoelastic behaviour. Most polymers exhibit a viscoelastic behaviour that shows a marked timeetemperature dependence of their mechanical properties. The creep and stress relaxation experiments to study the viscoelastic behaviour of materials have two disadvantages (Mohsenin, 1980, chap. 4): i) in order to get complete information, it is necessary to make measurements over many decades of time scales: and (ii) it is impossible to have a truly instantaneous application of load or deformation at the beginning of the experiment. These disadvantages can be overcome by dynamic mechanical tests in which the specimen is deformed by a stress which varies sinusoidally with time. Since a periodic experiment at frequency f is quantitatively equivalent to tests carried out over time 0.5pf (Joseph, Mathew, Joseph, Groeninckx, & Thomas, 2003), it is possible to provide a considerable amount of information corresponding to very short times. According to the International Confederation for Thermal Analysis (ICTA), dynamic mechanical analysis (DMA) is defined as the thermo-mechanical technique in which the storage modulus (elastic response) and loss modulus (viscous response) of the sample under oscillating load are monitored against time, temperature, or frequency of oscillation, while the temperature of the sample in a specified atmosphere is programmed (Jacob, Francis, Thomas, & Varughese, 2006). The elastic storage modulus E0 is the component of the dynamic modulus E*, for which the strain is in phase with the applied stress, and the loss modulus E00 is the component of dynamic modulus E* for which the strain is 90 out of phase with the
applied stress. The ratio of E00 to E0 gives the tangent of the phase angle, d: tan d is known as the damping and it is a measure of energy dissipation (Aziz & Ansell, 2004). Natural fibre-reinforced polymers are used to a great extent for the interior lining of cars and commercial vehicles. For these applications, the strength and stiffness properties have to satisfy the requirements of low temperatures (Jacob et al., 2006). For establishing such a wide range of temperature-dependent material data, the DMA is excellent. Dynamic mechanical analysis is a sensitive technique compared to Differential Scanning Calorimetry (DSC) to determine the Glass Transition Temperature (Tg) and has been used in studying the effect of temperature on the mechanical properties of materials including polymers and composites (Amin & Badri, 2007). The properties of composites depends on the extent of fibreematrix interactions. Dynamic mechanical analysis has been used as a tool to quantify the fibreematrix interactions in a composites (Jacob et al., 2006; Li, Mai, & Ye, 2005). Hence this study was performed with following objectives: (i) to estimate the temperature-dependent dynamic mechanical properties of OPFeLLDPE composites: (ii) to predict the dynamic mechanical properties of the composites: and (iii) to employ the DMA parameters to assess the fibreematrix interactions and glass transition temperatures.
2.
Materials and methods
2.1.
Raw materials
The oil palm EFBs were obtained from a local palm oil mill at Pedavegi, Andhra Pradesh, India. The FFBs were steam treated at a pressure of 294 kPa for 1 h before stripping the fruits to yield EFBs, in a usual process in the palm oil milling sequence. The EFBs were dried in an electrically heated forced convection cabinet drier at 60 C from an initial moisture content of 65e70% (w.b) to 35e40%. Fibres were extracted from the dried EFB using a mechanical decorticator, where the EFBs were subjected to impact and shear through the rotating mechanical beaters. The extracted fibres were washed in distilled water to remove field impurities and dried in a hot air oven at 50 C for 48 h and stored in airtight containers for use in the experiments. Linear low density polyethylene in powder form was used in the experiments (procured from ExxonMobil Chemical Canada, Toronto, ON). The properties of LLDPE were: density, 0.938 g cm3; melt flow index, 3.3 g (10 min)1, melting point, 126.5 C.
2.2.
Composite preparation
Alkali treatment was done by immersing the fibres in 5% (w w1) NaOH (technical grade) solution for 1 h and washing thereafter in distilled water till such time as traces of alkali had been removed. NaOH of 5% concentration was used as the mechanical properties of the resulting composite have been reported as maximal at this concentration (Joseph, Joseph, & Thomas, 2006). The washed fibres were further conditioned at 50 1 C for 48 h in a hot air oven and stored in airtight containers. The fibres were shredded in a laboratory grinder and sieve analysis was done to separate the shredded fibres
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into different fractions in a sieve shaker using ASTM standard test sieves. The fibres were separated into three size fractions viz ASTM sieve size 20e40 (425e840 m), 40e80 (177e425 m) and 80e200 (75e177 m). Mixing of LLDPE and OPFs was done for 10 min in a ribbon blender (Baker Perkins Saginaw, Michigan, USA) rotating at 40 rpm. The composite mixture was poured into a mould of 200 200 mm size for 3 mm board thickness and compression moulded in a hydraulic press equipped with water cooling facility. The compression cycle was: a) preheat the press to 150 C, b) heat the mix in the mould for 12 min under 850 kPa pressure, c) cool the mould under the same pressure for 15 min and d) cool the mould under atmospheric pressure for 15 min. Formulation of composite mixes and sample coding are presented in Table 1.
2.3.
Dynamic mechanical analysis (DMA)
Rectangular specimens of 55 10 3 mm size were scanned in a dynamic mechanical analyser (model DMA-242 of NETZSCH, Selb, Germany) at ambient air atmosphere. Experimental conditions were as follows: temperature range, 150 C to 100 C; frequency sweeps, 1 Hz, 10Hz, and 20 Hz; heating rate, 5 C min1; deformation mode, three point bending; deformation amplitude, 120 mm. The storage modulus (E0 ), loss modulus (E00 ) and damping parameter (tan d) values were measured.
3.
Results and discussion
3.1.
Storage modulus (E0 )
Storage modulus (E0 ) is a measure of the maximum energy stored in a material during one cycle of oscillation. It also gives an idea of temperature-dependent stiffness behaviour and load-bearing capability of the composite material. The storage modulusetemperature curve of OPFeLLDPE composites with 40% fibre loading (size range-177e425 m), at different frequency levels is presented in Fig. 1. As the frequency increased, storage modulus values also increased at all temperatures. The modulus values decreased with increase in temperature. The reduction in E0 is associated with softening of the matrix at higher temperatures (Faria, Cordeiro, Belgacem, & Dufrense, 2005). The storage modulusetemperature curves of OPFeLLDPE composites at different fibre loadings, fibre sizes and treatments, obtained at 1 Hz in the temperature range 150 C to 100 C are presented
Fig. 1 e Storage modulus and damping parameter of OPFeLLDPE composites (fibre loading 40% and fibre size range 177e425 m) as a function of temperature at indicated frequency levels (>: E0 20 Hz, ,: E0 10 Hz, 6: E0 1 Hz, 3: tan d 20 Hz, J: tan d 10 Hz, B: tan d 1 Hz).
in Fig. 2. This curve is useful in assessing the molecular basis of the mechanical properties of materials since it is very sensitive to structural changes such as molecular weight, degree of cross linking, and fibreematrix interfacial bonding. The E0 values of pure LLDPE observed in this study were very low compared to the values reported by Min, Chuah, and Chantara (2008). This is due to the fact that the melt flow index of LLDPE used in this study was higher (3.3 g (10 min)1) than that used by them (0.9 g (10 min)1). The E0 decreased sharply in the temperature range 80 C to 150 C. It was observed that E0 increased with an increase in fibre loading as reported earlier by Jacob et al. (2006) for sisaleOPFenatural rubber composites, except for the composite with 10% fibre loading. A similar trend has been observed for LLDPE on addition of glass fibres (Hashmi, Kitano1, & Chand, 2003). The pure LLDPE is more flexible resulting in low stiffness and low E0 . Addition of fibres increases the stiffness of the material causing an increase in storage modulus. Greater stress transfer at the fibreematrix interface (Huda, Drzal, Mohanty, & Misra, 2008; Jacob et al., 2006) for composites also causes an increase in E0 . The increase in E0 with fibre addition has
Table 1 e Formulation of composite samples and coding. Specimen code M00 AT12 AT32 AT42 AT31 AT33 UT32
Fibre loading, % 0 10 30 40 30 30 30
Fibre size range, m e 177e425 177e425 177e425 425e840 75e177 177e425
Treatment e Alkali treated Alkali treated Alkali treated Alkali treated Alkali treated Untreated
Fig. 2 e Storage modulus of OPFeLLDPE composites as a function of temperature at 1 Hz frequency (>: M00, ,: AT12, 6: AT32, 3: AT42, J: AT31, B: AT33, D: UT32).
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been attributed to the decrease in molecular mobility of LLDPE contributed by the stiff fibres (Hashmi et al., 2003). Higher values of E0 indicates an increased ability to store energy (Amin & Badri, 2007). It is also interesting to note that the increase in modulus with fibre loading was of the same order in both glassy and rubbery regions. It was found that E0 increased with alkali treatment of fibre, indicating better fibreematrix adhesion. A similar trend has been observed for alkali treatment of fibre in long kenafepolyester composites, long hempepolyester composites (Aziz & Ansell, 2004), OPFesisal fibreenatural rubber composites (Jacob et al., 2006) and polylactic acidepineapple leaf fibre composites (Huda et al., 2008). The increase in modulus upon alkali treatment is due to greater interfacial adhesion and bond strength between matrix and fibre (Aziz & Ansell, 2004). Improvement in interfacial adhesion between matrix and fibre upon alkali treatment of OPF has been reported by Yousif and El-Tayeb (2008) while conducting single fibre pull-out tests on OPF embedded in a polyester matrix. It was interesting to note that fibre size did not make any difference in E0 values at temperatures above 90 C, whereas the difference was prominent at temperatures below 90 C. Higher modulus was exhibited by fibre in the size range of 425e840 m and the modulus successively decreased with fibre size. The high stiffness offered by the large fibres and resulting decreased molecular mobility is probably the reason for increased E0 for composites with 425e840 m size fibres. However, the effect was not prominent at higher temperatures as the system is more flexible with increased molecular mobility at high temperatures, wherein the difference in fibre size could not make much difference.
3.2.
Loss modulus (E00 )
Loss modulus (E00 ) is proportional to the amount of energy that was dissipated as heat by the sample. It is a measure of the viscous response of that portion of the material that will flow under conditions of stress (Green & Wilkes, 1995). The loss modulusetemperature curve of OPFeLLDPE composites with 40% fibre loading (size range-177e425 m), at different frequency levels is presented in Fig. 3. The loss modulus
Fig. 4 e Loss modulus of OPFeLLDPE composites as a function of temperature at 1 Hz frequency (>: M00, ,: AT12, 6: AT32, 3: AT42, J: AT31, B: AT33, D: UT32).
increased with frequency at temperature below 10 C, and a reverse trend was observed beyond this temperature. A similar trend was observed for matrix samples. The E00 of OPFeLLDPE composites at different fibre loadings, fibre sizes and treatments obtained at 1 Hz frequency is plotted against temperature in Fig. 4. The E00 values increased upon fibre loading, but at temperatures below 50 C, the composite with 10% fibre content had E00 values lower than that of matrix. This finding partially agrees with the observation made by Jacob et al. (2006) for OPFesisal fibreenatural rubber composites, where the E00 increased with fibre loading with maximum E00 at 50% fibre loading. It is observed in the present study that the loss modulus values decreased sharply between respective glass transition temperatures and 80 C, indicating a sharp decrease in the viscosity in this region. This decrease in modulus was less abrupt at higher fibre loadings. It was also observed that the E00 values increased for alkali treatment at all temperatures (Fig. 4). While comparing the composites fabricated with different size fibres, it was found that the lowest values of E00 were exhibited by the lowest size fraction, viz., 75e177 m. Lower size fibres get mixed well with the matrix offering a more homogeneous system. However a consistent trend could not be observed between the other two size fractions throughout the temperature range. The E00 peaks and corresponding transition temperatures at different frequencies are summarised in Table 2. The
Table 2 e E00 max and corresponding glass transition temperature (Tg) values at different frequencies. E00 max, MPa
Sample
Fig. 3 e Loss modulus of OPFeLLDPE composites (fibre loading 40% and fibre size range 177e425 m) as a function of temperature at indicated frequency levels (6: 20 Hz, >: 10 Hz, B:1 Hz).
M00 AT12 AT32 AT42 AT31 AT33 UT32
Tg from E00 max, C
1 Hz
10 Hz
20 Hz
1 Hz
10 Hz
20 Hz
683 566 652 655 616 548 452
680 564 653 651 607 552 455
696 577 670 670 621 566 469
145 142 132 128 129 136 130
131 128 122 119 119 125 121
128 125 120 116 117 122 119
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3.3.
Fig. 5 e Damping parameter of OPFeLLDPE composites as a function of temperature at 1 Hz frequency (>: M00, ,: AT12, 6: AT32, 3: AT42, J: AT31, B: AT33, D: UT32).
maximum heat dissipation occurs at the temperature where loss modulus is maximal, indicating the glass transition temperature (Tg) of the system (Huda, Drzal, Mohanty, & Misra, 2007). It was found that addition of fibre shifted Tg to a higher value. This trend is associated with the decreased mobility of the matrix chains and associated stress field surrounding the particles in the presence of fibres. As expected, the transition temperatures increased with frequency. Alkali treatment did not affect the transition temperatures obtained from the loss modulus peaks. However, the transition temperatures decreased with fibre sizes, showing a lower transition temperature for fibre size fraction 75e177 m. Addition of 10% OPF in the LLDPE matrix decreased the E00 peak values, whereas further fibre loading increased the E00 peak. The E00 peak corresponding to 40% fibre loading was close to that of pure LLDPE. The E00 peaks of the unfilled polyvinyl chloride (PVC)/epoxidised natural rubber (ENR) matrix reported by Saha, Das, Bhatta, and Mitra (1999) were higher than those of the OPFePVC/ENR composites in their study. It was also observed that the E00 peak increased upon alkali treatment at all frequencies studied. A similar trend has been observed for OPFesisal fibreenatural rubber composites (Jacob et al., 2006). The fibre size fraction of 75e177 m had lower loss modulus peaks than the other size fibres, however a similar trend could not be observed between the other two size fractions.
Damping parameter (tan d)
Damping parameter (tan d), the ratio of loss modulus and storage modulus, is an indication of impact properties of a material, i.e. the higher tan d, the better would be the impact properties (Aziz & Ansell, 2004). According to Jacob et al. (2006), tan d is also related to the degree of molecular mobility in the polymer material and the major contribution to composite damping is due to (a) the nature of the matrix and fibre, (b) the nature of the interphase, (c) frictional damping due to slip in the unbound regions between fibre and matrix interface or delaminations and (d) damping due to energy dissipation in the area of matrix cracks and broken fibres. Hence the fibreematrix interfacial bonding can also be quantified by the tan d value. The temperature corresponding to the tan d peak is also taken as the Tg of the composite. The tan d-temperature curve of OPFeLLDPE composites with 40% fibre loading (size range-177e425 m), at different frequency levels is presented in Fig. 1. It was observed that tan d increased with frequency at all temperatures. The tan d value of OPFeLLDPE composites at various fibre loadings, treatments and fibre sizes obtained at a frequency of 1 Hz, plotted against temperature is presented in Fig. 5. Only one peak was observed for all the composite samples tested, which according to Sreekala, Thomas, and Groeninckx (2005) is an indication of better interaction between fibre and matrix due to decreased heterogeneity in the composite. Higher tan d was observed for unfilled LLDPE except between temperatures of 100 C and 25 C. Alkali-treated composite exhibited higher tan d values at temperature below 25 C and interestingly, the trend reversed at higher temperatures. Lowest tan d values were observed for lower size fibres, however only in a temperature range of 40 C to 60 C. A consistent trend could not be observed at other temperatures scanned. The tan d peaks and corresponding transition temperatures are presented in Table 3. It was found that the tan d peak values decreased upon fibre addition as reported earlier by Jacob et al. (2006) and Aziz and Ansell (2004), however fibre addition beyond 10% did not cause a proportional reduction in tan d. This indicates that the vibration energy was dissipated to the same degree for all these fibre loadings, i.e. all composites possessed the same order of damping. Restriction to the movement of polymer molecules upon addition of stiff fibres is the cause for reduction in the tan d peak upon fibre
Table 3 e tandmax, corresponding transition temperature (Tg) values, interfacial strength indicator (B) at different test frequencies and activation energy (E ). Sample
M00 AT12 AT32 AT42 AT31 AT33 UT32
tan dmax, 102, MPa
B, 102
Tg from tan dmax C
E, kJ mol1
1 Hz
10Hz
20Hz
1 Hz
10Hz
20Hz
1 Hz
10Hz
20Hz
8.16 7.17 7.33 7.30 6.90 6.21 7.33
8.40 7.34 7.45 7.30 6.94 6.32 7.45
8.65 7.54 7.65 7.49 7.12 6.47 7.65
122 127 115 118 112 120 115
114 119 108 110 105 113 108
111 117 106 109 104 111 106
e 0.81 2.18 3.16 3.32 5.12 5.87
e 0.83 2.43 3.90 3.73 5.32 5.98
e 0.85 2.49 4.03 3.81 5.43 6.03
55.9 55.8 72.6 66.6 80.7 68.1 70.7
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addition (Liu, Dever, Fair, & Benson, 1997; Saha et al., 1999). Another reason for lower tan d upon fibre loading has been attributed to the availability of less matrix by volume in composites to dissipate vibration energy and fibre agglomeration at higher fibre loading (Jacob et al., 2006). Chemical treatment increased the tan d peak at all frequencies indicating better impact properties upon alkali treatment. However this trend contradicts the observations of Aziz and Ansell (2004) on long kenafepolyester composites. The decrease in tan d values upon alkali treatment in their study was explained as due to increased interfacial adhesion between the treated fibres and matrix. They have also reported that the tan d peaks were very close in magnitude for the treated and untreated hemp fibreepolyester composites. Higher tan d was exhibited by composites with 177e425 m size fraction comparing to the other two size fractions in the present study. The tan d peak increased with increase in frequency for LLDPE and all composite combinations tested (Table 3). The glass transition temperature corresponding to the peaks in the tan d plot were found to shift to higher temperatures as frequency increased for both composites and the pure matrix. However the glass transition temperatures corresponding to tan d peaks did not exhibit a consistent trend with change in fibre content. The glass transition temperature decreased with 10% fibre addition and thereafter increased upon further fibre loadings. Flexibility of groups, molecular polarity of components and presence of voids in composites affect the Tg of a composite (Sreekala et al., 2005). The chances of void formation are greater at higher fibre loadings which would explain the effect on transition temperature. The transition temperature decreased with fibre size at all frequencies.
3.4.
Theoretical approach to fibreematrix interactions
There are three parameters viz., interfacial strength indicator (B), fibre reinforcement efficiency (R) and activation energy (E ) derived from the DMA parameters to represent the fibreematrix interactions. The deformation energy in a natural fibreepolymeric composite is dissipated mainly in the matrix and at the interface. If matrix, fibre volume fraction and fibre orientation are identical, then the damping term can be used to assess the interfacial properties between fibre and matrix. Based on this, the interfacial strength indicator (B) can be defined to characterise the interfacial bonding properties (Li et al., 2005): 1 B¼
matrix, but also to the fibreematrix interactions at the interface that will tend to form layers of immobilised interface. The B values of OPFeLLDPE composites of various combinations studied at the three frequencies are presented in Table 3. The B values were found to increase as the frequency is increased. It was observed that alkali-treated fibre composites exhibited low B values, which was not expected by the theory proposed by Jacob et al. (2006) that higher B indicates better bond strength. Better bond strength upon alkali treatment was already proven from the increased E0 values in this study. Higher B values were observed for fibre size range 75e177 m followed by 425e840 m and 177e425 m indicating decreased order of bond strength. The activation energy, E for the glass transition of the composites can be calculated from the Arrhenius equation (Sreekala et al., 2005).
E f ¼ f0 e RT
(2)
where f is the frequency at which the measurements are made, T is the glass transition temperature corresponding to tan d peak expressed in Kelvin, f0 is the frequency at which T approaches infinity and R is the universal gas constant, 8.3 J kmol1. It was found that the activation energy of composites was higher than that of the pure LLDPE (Table 3), except for composites with 10% fibre loading, for which it was slightly less than that of the matrix. Similar results have been observed for short sisal fibreepolypropylene composites (Joseph et al., 2003). Activation energy of composites with 40% fibre loading in the present study was less than that of 30% loading. At higher fibre loading, the fibreematrix adhesion decreased due to the increased fibreefibre contact and phase separation, which decreased the activation energy (Sreekala et al., 2005). Activation energy is an indirect representation of fibre reinforcement. At higher fibre loading, reinforcement may not be complete due to fibre agglomeration. Activation energy was found to be high for composites with 425e840 m size fraction (80.7 kJ mol1) followed by 177e425 m size fraction (72.6 kJ mol1) and 75e177 m size fraction (68.1 kJ mol1). Similarly, alkali treatment on fibre increased the activation energy. The reinforcement efficiency factor (R), of OPFeLLDPE composites calculated from Eq. (3) (Sarasua & Pouyet, 1998) is provided in Table 4.
tand c
tandm Vf
(1)
where tan d is the damping parameter, Vf is fibre volume fraction and suffices c and m refers to composite and matrix, respectively. Better bond strength is indicated by higher B values (Jacob et al., 2006). According to Sarasua and Pouyet (1998), B is a parameter introduced to correct the volume fraction of reinforcement because of the formation of a layer of immobilised interface resulting from fibreematrix interactions. According to them, the fibreematrix interface adhesion can be characterised by DMA based on the assumption that the composite dissipation is attributed not only to the
Table 4 e Reinforcement efficiency factor of oil palm fibreeLLDPE composites at different temperatures. Sample code
Temperatures, C Tg
50
0
50
100
Reinforcement efficiency factor, R AT12 AT32 AT42 AT31 AT33 UT32
0.13 0.38 0.52 0.37 0.43 e
e 0.95 0.86 0.94 0.95 0.20
0.29 1.31 1.15 1.25 1.25 0.52
0.97 2.74 2.29 2.62 2.57 1.77
1.96 4.28 4.20 6.12 5.18 2.78
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E0c E0m
¼ 1 þ RVf
(3)
where E0 is the storage modulus, R is the reinforcement efficiency factor, Vf is the fibre volume fraction and suffices c and m represents composite and matrix, respectively. It is found that the fibre reinforcement becomes more prominent at temperatures beyond Tg.
3.4.1.
ColeeCole analysis
The ColeeCole plots for all the samples were drawn with E00 against E0 and presented in Fig. 6. It has been reported earlier that a homogeneous polymeric system exhibits a semicircle whereas two phase system exhibits two modified circles (Ibarra, Macias, & Palma, 1995). However a deviation from these trends is observed in the present study indicating interface effects and heterogeneity of the system as a whole.
3.5.
Models to predict DMA parameters
3.5.1.
Storage modulus (E0 )
Different equations were proposed by earlier researchers to predict the storage modulus of composites from the values of matrix and fibre fraction. The following equations were used earlier to predict the storage modulus of short sisal fibrereinforced polypropylene composites (Joseph et al., 2003). Einstein’s equation-I is: E0c ¼ E0m 1 þ 1:25Vf
(4)
0
where E is the storage modulus and Vf is the fibre volume fraction and suffices c and m represents composite and matrix, respectively. Einstein’s equation-II is: E0c ¼ E0m 1 þ Vf
(5)
The concepts were modified in Guth’s equation: E0c ¼ E0m 1 þ 2:5Vf þ 14:1Vf
(6)
Another modified equation is Cohan’s equation: E0c ¼ E0m 1 þ 0:675r þ 1:62r2 Vf þ 14:1Vf
(7)
Fig. 6 e ColeeCole plots of OPFeLLDPE composites (>: M00, ,: AT12, 6: AT32, 3: AT42, J: AT31, B: AT33, D: UT32).
105
where r is the aspect ratio of the fibre. In the present study, the aspect ratio of OPF was calculated as 1.95, 0.93 and 0.39 respectively for fibre fractions 425e840 m, 177e425 m and 75e177 m. Another equation is Mooney’s equation: 2:5V f 1 SV 0 0 f Ec ¼ Em e
(8)
where S is the crowding factor or relative sedimentation volume of the inclusion and is mathematically defined as the ratio of apparent volume occupied by the fibre to the true volume of the fibre. S is taken as 1 in the present study. The experimental and predicted values of the storage modulus at a frequency of 1 Hz and at respective glass transition temperatures (corresponding to the peaks of tan d-temperature plot) using these equations are presented in Fig. 7. Einstein’s equations-I and II were found to better predict the storage modulus of OPFeLLDPE composites at all fibre loadings, sizes and treatments in the present study, but resulted in deviation from experimental values for short sisal fibre-reinforced polypropylene composites (Joseph et al., 2003). Guth’s equation and Mooney’s equation better predicted storage modulus at 10% fibre loading as reported earlier for short sisal fibre-reinforced polypropylene composites (Joseph et al., 2003). It is interesting to note that Guth’s equation, Cohan’s equation and Mooney’s equation better approximated storage modulus of untreated fibre composite than treated composite. The Cohan model gave rise to extremely high modulus values as obtained previously for short sisal fibre-reinforced polypropylene composites (Joseph et al., 2003).
3.5.2.
Damping parameter (tan d)
Damping behaviour of a composite is of great practical interest for its long term utilisation. The rule of mixtures was applied to predict tan d by Neilson and Landel (1974). tandc ¼ Vf tandf þ Vm tandm
(9)
However Drzal, Rich, Koenig, and Llyod (1983) suggested that the first term in the above equation can be eliminated for rigid fillers and thus Eq. (9) becomes
Fig. 7 e The experimental and predicted values of the storage modulus at respective glass transition temperatures of OPFeLLDPE composites and at 1 Hz frequency (>: Experimental, ,: Einstein’s equation I, 6: Einstein’s equation II, 3: Guth’s equation, J: Cohans equation, B: Mooney’s equation (S [ 1)).
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composites at different fibre content, fibre size and alkali treatment were determined. Prediction of storage modulus and damping parameter using standard models were compared with experimental values. The specific conclusions of the study are:
Fig. 8 e The experimental and predicted values of the damping parameter at respective glass transition temperatures of OPFeLLDPE composites and at 1 Hz frequency (>: Experimental, ,: Drzal equation, 6: Tung and Dynes equation).
tandc ¼ Vm tandm
(10)
Tung and Dynes (1987) proposed that a stiffness term should be added, assuming that the matrix in presence of fibres offers a stiffness equivalent to the minimum elastic modulus of the composite. The resulting equation is tandc ¼ Vm
0 Em tandm E0c
(11)
A comparison of tan d values predicted using both Drzal equation (Eq. (10)) and Tung and Dynes equation (Eq. (11)) with experimental values is presented in Fig. 8. It is found that the Drzal equation better represented the tan d values than did the Tung and Dynes equation. Both these equations better represented the experimental values at fibre loading of 10% than at higher fibre loadings. It is interesting to note that the predicted values deviated from the experimental values when fibre loading increased and this deviation was more pronounced for the Tung and Dynes equation. In the case of short sisal fibreepolypropylene composites, the lack of a term representing localised constraints imposed by the fibres on matrix deformation was one of the reasons for deviation of predicted values from experimental results (Joseph et al., 2003). Greater deviation at higher fibre loading observed in the present study can be read in conjunction with this theory, as the localised constraint imposed by the fibres would be proportional to the fibre loading. However at 75e177 m fibre size, the models approximated experimental values. It is interesting to note that more deviation from the experimental values was observed in case of alkali-treated fibre composites than untreated fibre composites. Both the models did not consider the changes in the stiffness of interface due to treatment (Joseph et al., 2003).
4.
Conclusions
The dynamic mechanical properties viz., storage modulus, loss modulus and damping parameter of OPFeLLDPE
The storage modulus increased with increase in fibre content in the composite and also after alkali treatment of fibres. The loss modulus values also increased with fibre loading except for composite with 10% fibre content. The loss modulus values increased following alkali treatment on fibres. The glass transition temperature of pure LLDPE was 145 C, and this increased to 128 C for composites with 40% fibre content. Fibre loading increased the loss modulus peak values except for 10% fibre loading. Alkali treatment on fibres also increased the loss modulus peak. Single tan d peaks were observed for all the composite samples tested and lowest tan d values were observed for lower size fibres, between a temperature range of 40 C to 60 C. The tan d peak values decreased upon fibre addition, and alkali treatment increased tan d peak at all frequencies indicating better impact properties after alkali treatment. Higher tan d was exhibited by composites with 177e425 m size fraction. The interfacial strength indicator values (B) increased as the frequency increased. It was observed that alkali-treated fibre composites exhibited low B values. Higher B values were observed for the fibre size range 75e177 m followed by 425e840 m and 177e425 m indicating decreased order of bond strength. The activation energy of composites was higher than that of the pure LLDPE, except for composites with 10% fibre loading. Activation energy was found to be high for composites with 425me840m size fraction (80.7 kJ mol1) followed by 177e425 m size fraction (72.6 kJ mol1) and 75e177 m size fraction (68.1 kJ mol1). Storage modulus (E0 ) and damping parameters predicted using different equations were in agreement with experimental values.
Acknowledgement Authors acknowledge Canadian Bureau of International Education (CBIE) for providing Graduate Student Exchange Program (GSEP) fellowship to the first author. The study leave granted to the first author by Indian Council of Agricultural Research is also thankfully acknowledged.
references
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Research Paper
Dynamic mechanical properties of oil palm fibre (OPF)-linear low density polyethylene (LLDPE) biocomposites and study of fibreematrix interactions S. Shinoj a, R. Visvanathan b,*, S. Panigrahi c, N. Varadharaju b a
Directorate of Oil Palm Research, Indian Council of Agricultural Research, Pedavegi, Eluru, Andhra Pradesh 534 450, India Department of Food and Agricultural Process Engineering, Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu 641 003, India c Department of Agricultural and Bioresource Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon SK, Canada S7N5A9 b
article info Article history:
The dynamic mechanical properties of oil palm fibre (OPF) linear low density polyethylene (LLDPE) composites in terms of storage modulus (E0 ), loss modulus (E00 ) and damping
Received 25 October 2010
parameter (tan d) in a temperature range of 150 C to 100 C is presented in this paper.
Received in revised form
The effect of fibre content, fibre size and fibre surface treatment on the dynamic
20 December 2010
mechanical properties was studied. The storage modulus and damping parameters were
Accepted 17 February 2011
predicted using different equations. The storage modulus and loss modulus increased with
Published online 30 March 2011
increase in fibre content and also upon alkali treatment on fibres. The glass transition temperature of pure LLDPE was 145 C, which increased to 128 C for composites with 40% fibre content. Fibre loading in composite and alkali treatment on fibre increased the loss modulus peak. Single tan d peaks were observed for all the composite samples tested. The tan d peak values decreased upon fibre addition whereas alkali treatment increased the tan d peak at all frequencies indicating better impact properties after alkali treatment. The interfacial strength indicator values (B) decreased upon alkali treatment and higher B values were observed for composites containing fibre size range 75e177 m followed by 425e840 m and 177e425 m indicating decreased order of bond strength. Activation energy was found to be high for composites with 425e840 m size fraction (80.7 kJ mol1) followed by 177e425 m size fraction (72.6 kJ mol1) and 75e177 m size fraction (68.1 kJ mol1). Storage modulus (E0 ) and damping parameters predicted using different equations were in agreement with experimental values. ª 2011 IAgrE. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years, there has been growing interest in natural fibre-polymeric composites due to increased public awareness and concern over environmental impact of artificial or manmade materials. Government legislations and regulations in
force in different parts of the world similar to the one implemented by the European Union envisaging use of 85% recyclable materials in automotives by 2015 (Holbery & Houstoun, 2006), further increase attention towards natural fibres and their composites. Natural fibre composites find applications in diverse fields ranging from the construction industry to
* Corresponding author. Tel.: þ91 422 6611272; fax: þ91 422 6611455. E-mail address: [email protected] (R. Visvanathan). 1537-5110/$ e see front matter ª 2011 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2011.02.006
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medical and aerospace applications. Natural fibres are advantageous in comparison to glass, carbon or similar inorganic fibres in a polymeric matrix due to their low energy cost, positive contribution to global carbon budget (Hill & Khalil, 2000), greater deformability, biodegradability (Rozman, Tay, & Abusamah, 2003), combustibility, ease of recycling (Sreekala, Kumaran, Geethakumariamma, & Thomas, 2004), less abrasiveness to processing equipment, lower cost (Raju, Ratnam, Ibrahim, Rahman, & Yunus, 2008), renewable nature, non-toxicity, high specific strength (Yousif & El-Tayeb, 2008), etc. Oil palm is an edible oil yielding crop cultivated on 11 Mha over 42 countries in the world (Khalil, Siti, Ridzuan, Kamarudin, & Khairul, 2008). The major source of fibre from oil palm is empty fruit bunches (EFB); a waste material obtained after stripping fruits from the fresh fruit bunch (FFB), yielding 73% fibre (Wirjosentono, Guritno, & Ismail, 2004). The palm oil industry has to dispose of about 1.1 tonne of EFB for every tonne of oil produced (Karina, Onggo, Abdullah, & Syampurwadi, 2008). The current uses of this highly cellulosic material are as boiler fuel, in the preparation of fertilisers or as mulching material (Singh, Manohan, & Kanopathy, 1982). When left in the field, these waste materials create considerable environmental problems (Law, Daud & Ghazali, 2007). Linear low density polyethylene (LLDPE) is a popular thermoplastic material used mainly for packaging applications (Zheng, Yanful, & Bassi, 2005). The low processing temperature (below 130 C) of LLDPE makes composite fabrication possible without partial melting or annealing of the fibres (Lee & Joo, 1999). Materials respond to an applied force by exhibiting either elastic or viscous behaviour or a combination of these called viscoelastic behaviour. Most polymers exhibit a viscoelastic behaviour that shows a marked timeetemperature dependence of their mechanical properties. The creep and stress relaxation experiments to study the viscoelastic behaviour of materials have two disadvantages (Mohsenin, 1980, chap. 4): i) in order to get complete information, it is necessary to make measurements over many decades of time scales: and (ii) it is impossible to have a truly instantaneous application of load or deformation at the beginning of the experiment. These disadvantages can be overcome by dynamic mechanical tests in which the specimen is deformed by a stress which varies sinusoidally with time. Since a periodic experiment at frequency f is quantitatively equivalent to tests carried out over time 0.5pf (Joseph, Mathew, Joseph, Groeninckx, & Thomas, 2003), it is possible to provide a considerable amount of information corresponding to very short times. According to the International Confederation for Thermal Analysis (ICTA), dynamic mechanical analysis (DMA) is defined as the thermo-mechanical technique in which the storage modulus (elastic response) and loss modulus (viscous response) of the sample under oscillating load are monitored against time, temperature, or frequency of oscillation, while the temperature of the sample in a specified atmosphere is programmed (Jacob, Francis, Thomas, & Varughese, 2006). The elastic storage modulus E0 is the component of the dynamic modulus E*, for which the strain is in phase with the applied stress, and the loss modulus E00 is the component of dynamic modulus E* for which the strain is 90 out of phase with the
applied stress. The ratio of E00 to E0 gives the tangent of the phase angle, d: tan d is known as the damping and it is a measure of energy dissipation (Aziz & Ansell, 2004). Natural fibre-reinforced polymers are used to a great extent for the interior lining of cars and commercial vehicles. For these applications, the strength and stiffness properties have to satisfy the requirements of low temperatures (Jacob et al., 2006). For establishing such a wide range of temperature-dependent material data, the DMA is excellent. Dynamic mechanical analysis is a sensitive technique compared to Differential Scanning Calorimetry (DSC) to determine the Glass Transition Temperature (Tg) and has been used in studying the effect of temperature on the mechanical properties of materials including polymers and composites (Amin & Badri, 2007). The properties of composites depends on the extent of fibreematrix interactions. Dynamic mechanical analysis has been used as a tool to quantify the fibreematrix interactions in a composites (Jacob et al., 2006; Li, Mai, & Ye, 2005). Hence this study was performed with following objectives: (i) to estimate the temperature-dependent dynamic mechanical properties of OPFeLLDPE composites: (ii) to predict the dynamic mechanical properties of the composites: and (iii) to employ the DMA parameters to assess the fibreematrix interactions and glass transition temperatures.
2.
Materials and methods
2.1.
Raw materials
The oil palm EFBs were obtained from a local palm oil mill at Pedavegi, Andhra Pradesh, India. The FFBs were steam treated at a pressure of 294 kPa for 1 h before stripping the fruits to yield EFBs, in a usual process in the palm oil milling sequence. The EFBs were dried in an electrically heated forced convection cabinet drier at 60 C from an initial moisture content of 65e70% (w.b) to 35e40%. Fibres were extracted from the dried EFB using a mechanical decorticator, where the EFBs were subjected to impact and shear through the rotating mechanical beaters. The extracted fibres were washed in distilled water to remove field impurities and dried in a hot air oven at 50 C for 48 h and stored in airtight containers for use in the experiments. Linear low density polyethylene in powder form was used in the experiments (procured from ExxonMobil Chemical Canada, Toronto, ON). The properties of LLDPE were: density, 0.938 g cm3; melt flow index, 3.3 g (10 min)1, melting point, 126.5 C.
2.2.
Composite preparation
Alkali treatment was done by immersing the fibres in 5% (w w1) NaOH (technical grade) solution for 1 h and washing thereafter in distilled water till such time as traces of alkali had been removed. NaOH of 5% concentration was used as the mechanical properties of the resulting composite have been reported as maximal at this concentration (Joseph, Joseph, & Thomas, 2006). The washed fibres were further conditioned at 50 1 C for 48 h in a hot air oven and stored in airtight containers. The fibres were shredded in a laboratory grinder and sieve analysis was done to separate the shredded fibres
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into different fractions in a sieve shaker using ASTM standard test sieves. The fibres were separated into three size fractions viz ASTM sieve size 20e40 (425e840 m), 40e80 (177e425 m) and 80e200 (75e177 m). Mixing of LLDPE and OPFs was done for 10 min in a ribbon blender (Baker Perkins Saginaw, Michigan, USA) rotating at 40 rpm. The composite mixture was poured into a mould of 200 200 mm size for 3 mm board thickness and compression moulded in a hydraulic press equipped with water cooling facility. The compression cycle was: a) preheat the press to 150 C, b) heat the mix in the mould for 12 min under 850 kPa pressure, c) cool the mould under the same pressure for 15 min and d) cool the mould under atmospheric pressure for 15 min. Formulation of composite mixes and sample coding are presented in Table 1.
2.3.
Dynamic mechanical analysis (DMA)
Rectangular specimens of 55 10 3 mm size were scanned in a dynamic mechanical analyser (model DMA-242 of NETZSCH, Selb, Germany) at ambient air atmosphere. Experimental conditions were as follows: temperature range, 150 C to 100 C; frequency sweeps, 1 Hz, 10Hz, and 20 Hz; heating rate, 5 C min1; deformation mode, three point bending; deformation amplitude, 120 mm. The storage modulus (E0 ), loss modulus (E00 ) and damping parameter (tan d) values were measured.
3.
Results and discussion
3.1.
Storage modulus (E0 )
Storage modulus (E0 ) is a measure of the maximum energy stored in a material during one cycle of oscillation. It also gives an idea of temperature-dependent stiffness behaviour and load-bearing capability of the composite material. The storage modulusetemperature curve of OPFeLLDPE composites with 40% fibre loading (size range-177e425 m), at different frequency levels is presented in Fig. 1. As the frequency increased, storage modulus values also increased at all temperatures. The modulus values decreased with increase in temperature. The reduction in E0 is associated with softening of the matrix at higher temperatures (Faria, Cordeiro, Belgacem, & Dufrense, 2005). The storage modulusetemperature curves of OPFeLLDPE composites at different fibre loadings, fibre sizes and treatments, obtained at 1 Hz in the temperature range 150 C to 100 C are presented
Fig. 1 e Storage modulus and damping parameter of OPFeLLDPE composites (fibre loading 40% and fibre size range 177e425 m) as a function of temperature at indicated frequency levels (>: E0 20 Hz, ,: E0 10 Hz, 6: E0 1 Hz, 3: tan d 20 Hz, J: tan d 10 Hz, B: tan d 1 Hz).
in Fig. 2. This curve is useful in assessing the molecular basis of the mechanical properties of materials since it is very sensitive to structural changes such as molecular weight, degree of cross linking, and fibreematrix interfacial bonding. The E0 values of pure LLDPE observed in this study were very low compared to the values reported by Min, Chuah, and Chantara (2008). This is due to the fact that the melt flow index of LLDPE used in this study was higher (3.3 g (10 min)1) than that used by them (0.9 g (10 min)1). The E0 decreased sharply in the temperature range 80 C to 150 C. It was observed that E0 increased with an increase in fibre loading as reported earlier by Jacob et al. (2006) for sisaleOPFenatural rubber composites, except for the composite with 10% fibre loading. A similar trend has been observed for LLDPE on addition of glass fibres (Hashmi, Kitano1, & Chand, 2003). The pure LLDPE is more flexible resulting in low stiffness and low E0 . Addition of fibres increases the stiffness of the material causing an increase in storage modulus. Greater stress transfer at the fibreematrix interface (Huda, Drzal, Mohanty, & Misra, 2008; Jacob et al., 2006) for composites also causes an increase in E0 . The increase in E0 with fibre addition has
Table 1 e Formulation of composite samples and coding. Specimen code M00 AT12 AT32 AT42 AT31 AT33 UT32
Fibre loading, % 0 10 30 40 30 30 30
Fibre size range, m e 177e425 177e425 177e425 425e840 75e177 177e425
Treatment e Alkali treated Alkali treated Alkali treated Alkali treated Alkali treated Untreated
Fig. 2 e Storage modulus of OPFeLLDPE composites as a function of temperature at 1 Hz frequency (>: M00, ,: AT12, 6: AT32, 3: AT42, J: AT31, B: AT33, D: UT32).
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been attributed to the decrease in molecular mobility of LLDPE contributed by the stiff fibres (Hashmi et al., 2003). Higher values of E0 indicates an increased ability to store energy (Amin & Badri, 2007). It is also interesting to note that the increase in modulus with fibre loading was of the same order in both glassy and rubbery regions. It was found that E0 increased with alkali treatment of fibre, indicating better fibreematrix adhesion. A similar trend has been observed for alkali treatment of fibre in long kenafepolyester composites, long hempepolyester composites (Aziz & Ansell, 2004), OPFesisal fibreenatural rubber composites (Jacob et al., 2006) and polylactic acidepineapple leaf fibre composites (Huda et al., 2008). The increase in modulus upon alkali treatment is due to greater interfacial adhesion and bond strength between matrix and fibre (Aziz & Ansell, 2004). Improvement in interfacial adhesion between matrix and fibre upon alkali treatment of OPF has been reported by Yousif and El-Tayeb (2008) while conducting single fibre pull-out tests on OPF embedded in a polyester matrix. It was interesting to note that fibre size did not make any difference in E0 values at temperatures above 90 C, whereas the difference was prominent at temperatures below 90 C. Higher modulus was exhibited by fibre in the size range of 425e840 m and the modulus successively decreased with fibre size. The high stiffness offered by the large fibres and resulting decreased molecular mobility is probably the reason for increased E0 for composites with 425e840 m size fibres. However, the effect was not prominent at higher temperatures as the system is more flexible with increased molecular mobility at high temperatures, wherein the difference in fibre size could not make much difference.
3.2.
Loss modulus (E00 )
Loss modulus (E00 ) is proportional to the amount of energy that was dissipated as heat by the sample. It is a measure of the viscous response of that portion of the material that will flow under conditions of stress (Green & Wilkes, 1995). The loss modulusetemperature curve of OPFeLLDPE composites with 40% fibre loading (size range-177e425 m), at different frequency levels is presented in Fig. 3. The loss modulus
Fig. 4 e Loss modulus of OPFeLLDPE composites as a function of temperature at 1 Hz frequency (>: M00, ,: AT12, 6: AT32, 3: AT42, J: AT31, B: AT33, D: UT32).
increased with frequency at temperature below 10 C, and a reverse trend was observed beyond this temperature. A similar trend was observed for matrix samples. The E00 of OPFeLLDPE composites at different fibre loadings, fibre sizes and treatments obtained at 1 Hz frequency is plotted against temperature in Fig. 4. The E00 values increased upon fibre loading, but at temperatures below 50 C, the composite with 10% fibre content had E00 values lower than that of matrix. This finding partially agrees with the observation made by Jacob et al. (2006) for OPFesisal fibreenatural rubber composites, where the E00 increased with fibre loading with maximum E00 at 50% fibre loading. It is observed in the present study that the loss modulus values decreased sharply between respective glass transition temperatures and 80 C, indicating a sharp decrease in the viscosity in this region. This decrease in modulus was less abrupt at higher fibre loadings. It was also observed that the E00 values increased for alkali treatment at all temperatures (Fig. 4). While comparing the composites fabricated with different size fibres, it was found that the lowest values of E00 were exhibited by the lowest size fraction, viz., 75e177 m. Lower size fibres get mixed well with the matrix offering a more homogeneous system. However a consistent trend could not be observed between the other two size fractions throughout the temperature range. The E00 peaks and corresponding transition temperatures at different frequencies are summarised in Table 2. The
Table 2 e E00 max and corresponding glass transition temperature (Tg) values at different frequencies. E00 max, MPa
Sample
Fig. 3 e Loss modulus of OPFeLLDPE composites (fibre loading 40% and fibre size range 177e425 m) as a function of temperature at indicated frequency levels (6: 20 Hz, >: 10 Hz, B:1 Hz).
M00 AT12 AT32 AT42 AT31 AT33 UT32
Tg from E00 max, C
1 Hz
10 Hz
20 Hz
1 Hz
10 Hz
20 Hz
683 566 652 655 616 548 452
680 564 653 651 607 552 455
696 577 670 670 621 566 469
145 142 132 128 129 136 130
131 128 122 119 119 125 121
128 125 120 116 117 122 119
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3.3.
Fig. 5 e Damping parameter of OPFeLLDPE composites as a function of temperature at 1 Hz frequency (>: M00, ,: AT12, 6: AT32, 3: AT42, J: AT31, B: AT33, D: UT32).
maximum heat dissipation occurs at the temperature where loss modulus is maximal, indicating the glass transition temperature (Tg) of the system (Huda, Drzal, Mohanty, & Misra, 2007). It was found that addition of fibre shifted Tg to a higher value. This trend is associated with the decreased mobility of the matrix chains and associated stress field surrounding the particles in the presence of fibres. As expected, the transition temperatures increased with frequency. Alkali treatment did not affect the transition temperatures obtained from the loss modulus peaks. However, the transition temperatures decreased with fibre sizes, showing a lower transition temperature for fibre size fraction 75e177 m. Addition of 10% OPF in the LLDPE matrix decreased the E00 peak values, whereas further fibre loading increased the E00 peak. The E00 peak corresponding to 40% fibre loading was close to that of pure LLDPE. The E00 peaks of the unfilled polyvinyl chloride (PVC)/epoxidised natural rubber (ENR) matrix reported by Saha, Das, Bhatta, and Mitra (1999) were higher than those of the OPFePVC/ENR composites in their study. It was also observed that the E00 peak increased upon alkali treatment at all frequencies studied. A similar trend has been observed for OPFesisal fibreenatural rubber composites (Jacob et al., 2006). The fibre size fraction of 75e177 m had lower loss modulus peaks than the other size fibres, however a similar trend could not be observed between the other two size fractions.
Damping parameter (tan d)
Damping parameter (tan d), the ratio of loss modulus and storage modulus, is an indication of impact properties of a material, i.e. the higher tan d, the better would be the impact properties (Aziz & Ansell, 2004). According to Jacob et al. (2006), tan d is also related to the degree of molecular mobility in the polymer material and the major contribution to composite damping is due to (a) the nature of the matrix and fibre, (b) the nature of the interphase, (c) frictional damping due to slip in the unbound regions between fibre and matrix interface or delaminations and (d) damping due to energy dissipation in the area of matrix cracks and broken fibres. Hence the fibreematrix interfacial bonding can also be quantified by the tan d value. The temperature corresponding to the tan d peak is also taken as the Tg of the composite. The tan d-temperature curve of OPFeLLDPE composites with 40% fibre loading (size range-177e425 m), at different frequency levels is presented in Fig. 1. It was observed that tan d increased with frequency at all temperatures. The tan d value of OPFeLLDPE composites at various fibre loadings, treatments and fibre sizes obtained at a frequency of 1 Hz, plotted against temperature is presented in Fig. 5. Only one peak was observed for all the composite samples tested, which according to Sreekala, Thomas, and Groeninckx (2005) is an indication of better interaction between fibre and matrix due to decreased heterogeneity in the composite. Higher tan d was observed for unfilled LLDPE except between temperatures of 100 C and 25 C. Alkali-treated composite exhibited higher tan d values at temperature below 25 C and interestingly, the trend reversed at higher temperatures. Lowest tan d values were observed for lower size fibres, however only in a temperature range of 40 C to 60 C. A consistent trend could not be observed at other temperatures scanned. The tan d peaks and corresponding transition temperatures are presented in Table 3. It was found that the tan d peak values decreased upon fibre addition as reported earlier by Jacob et al. (2006) and Aziz and Ansell (2004), however fibre addition beyond 10% did not cause a proportional reduction in tan d. This indicates that the vibration energy was dissipated to the same degree for all these fibre loadings, i.e. all composites possessed the same order of damping. Restriction to the movement of polymer molecules upon addition of stiff fibres is the cause for reduction in the tan d peak upon fibre
Table 3 e tandmax, corresponding transition temperature (Tg) values, interfacial strength indicator (B) at different test frequencies and activation energy (E ). Sample
M00 AT12 AT32 AT42 AT31 AT33 UT32
tan dmax, 102, MPa
B, 102
Tg from tan dmax C
E, kJ mol1
1 Hz
10Hz
20Hz
1 Hz
10Hz
20Hz
1 Hz
10Hz
20Hz
8.16 7.17 7.33 7.30 6.90 6.21 7.33
8.40 7.34 7.45 7.30 6.94 6.32 7.45
8.65 7.54 7.65 7.49 7.12 6.47 7.65
122 127 115 118 112 120 115
114 119 108 110 105 113 108
111 117 106 109 104 111 106
e 0.81 2.18 3.16 3.32 5.12 5.87
e 0.83 2.43 3.90 3.73 5.32 5.98
e 0.85 2.49 4.03 3.81 5.43 6.03
55.9 55.8 72.6 66.6 80.7 68.1 70.7
104
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addition (Liu, Dever, Fair, & Benson, 1997; Saha et al., 1999). Another reason for lower tan d upon fibre loading has been attributed to the availability of less matrix by volume in composites to dissipate vibration energy and fibre agglomeration at higher fibre loading (Jacob et al., 2006). Chemical treatment increased the tan d peak at all frequencies indicating better impact properties upon alkali treatment. However this trend contradicts the observations of Aziz and Ansell (2004) on long kenafepolyester composites. The decrease in tan d values upon alkali treatment in their study was explained as due to increased interfacial adhesion between the treated fibres and matrix. They have also reported that the tan d peaks were very close in magnitude for the treated and untreated hemp fibreepolyester composites. Higher tan d was exhibited by composites with 177e425 m size fraction comparing to the other two size fractions in the present study. The tan d peak increased with increase in frequency for LLDPE and all composite combinations tested (Table 3). The glass transition temperature corresponding to the peaks in the tan d plot were found to shift to higher temperatures as frequency increased for both composites and the pure matrix. However the glass transition temperatures corresponding to tan d peaks did not exhibit a consistent trend with change in fibre content. The glass transition temperature decreased with 10% fibre addition and thereafter increased upon further fibre loadings. Flexibility of groups, molecular polarity of components and presence of voids in composites affect the Tg of a composite (Sreekala et al., 2005). The chances of void formation are greater at higher fibre loadings which would explain the effect on transition temperature. The transition temperature decreased with fibre size at all frequencies.
3.4.
Theoretical approach to fibreematrix interactions
There are three parameters viz., interfacial strength indicator (B), fibre reinforcement efficiency (R) and activation energy (E ) derived from the DMA parameters to represent the fibreematrix interactions. The deformation energy in a natural fibreepolymeric composite is dissipated mainly in the matrix and at the interface. If matrix, fibre volume fraction and fibre orientation are identical, then the damping term can be used to assess the interfacial properties between fibre and matrix. Based on this, the interfacial strength indicator (B) can be defined to characterise the interfacial bonding properties (Li et al., 2005): 1 B¼
matrix, but also to the fibreematrix interactions at the interface that will tend to form layers of immobilised interface. The B values of OPFeLLDPE composites of various combinations studied at the three frequencies are presented in Table 3. The B values were found to increase as the frequency is increased. It was observed that alkali-treated fibre composites exhibited low B values, which was not expected by the theory proposed by Jacob et al. (2006) that higher B indicates better bond strength. Better bond strength upon alkali treatment was already proven from the increased E0 values in this study. Higher B values were observed for fibre size range 75e177 m followed by 425e840 m and 177e425 m indicating decreased order of bond strength. The activation energy, E for the glass transition of the composites can be calculated from the Arrhenius equation (Sreekala et al., 2005).
E f ¼ f0 e RT
(2)
where f is the frequency at which the measurements are made, T is the glass transition temperature corresponding to tan d peak expressed in Kelvin, f0 is the frequency at which T approaches infinity and R is the universal gas constant, 8.3 J kmol1. It was found that the activation energy of composites was higher than that of the pure LLDPE (Table 3), except for composites with 10% fibre loading, for which it was slightly less than that of the matrix. Similar results have been observed for short sisal fibreepolypropylene composites (Joseph et al., 2003). Activation energy of composites with 40% fibre loading in the present study was less than that of 30% loading. At higher fibre loading, the fibreematrix adhesion decreased due to the increased fibreefibre contact and phase separation, which decreased the activation energy (Sreekala et al., 2005). Activation energy is an indirect representation of fibre reinforcement. At higher fibre loading, reinforcement may not be complete due to fibre agglomeration. Activation energy was found to be high for composites with 425e840 m size fraction (80.7 kJ mol1) followed by 177e425 m size fraction (72.6 kJ mol1) and 75e177 m size fraction (68.1 kJ mol1). Similarly, alkali treatment on fibre increased the activation energy. The reinforcement efficiency factor (R), of OPFeLLDPE composites calculated from Eq. (3) (Sarasua & Pouyet, 1998) is provided in Table 4.
tand c
tandm Vf
(1)
where tan d is the damping parameter, Vf is fibre volume fraction and suffices c and m refers to composite and matrix, respectively. Better bond strength is indicated by higher B values (Jacob et al., 2006). According to Sarasua and Pouyet (1998), B is a parameter introduced to correct the volume fraction of reinforcement because of the formation of a layer of immobilised interface resulting from fibreematrix interactions. According to them, the fibreematrix interface adhesion can be characterised by DMA based on the assumption that the composite dissipation is attributed not only to the
Table 4 e Reinforcement efficiency factor of oil palm fibreeLLDPE composites at different temperatures. Sample code
Temperatures, C Tg
50
0
50
100
Reinforcement efficiency factor, R AT12 AT32 AT42 AT31 AT33 UT32
0.13 0.38 0.52 0.37 0.43 e
e 0.95 0.86 0.94 0.95 0.20
0.29 1.31 1.15 1.25 1.25 0.52
0.97 2.74 2.29 2.62 2.57 1.77
1.96 4.28 4.20 6.12 5.18 2.78
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E0c E0m
¼ 1 þ RVf
(3)
where E0 is the storage modulus, R is the reinforcement efficiency factor, Vf is the fibre volume fraction and suffices c and m represents composite and matrix, respectively. It is found that the fibre reinforcement becomes more prominent at temperatures beyond Tg.
3.4.1.
ColeeCole analysis
The ColeeCole plots for all the samples were drawn with E00 against E0 and presented in Fig. 6. It has been reported earlier that a homogeneous polymeric system exhibits a semicircle whereas two phase system exhibits two modified circles (Ibarra, Macias, & Palma, 1995). However a deviation from these trends is observed in the present study indicating interface effects and heterogeneity of the system as a whole.
3.5.
Models to predict DMA parameters
3.5.1.
Storage modulus (E0 )
Different equations were proposed by earlier researchers to predict the storage modulus of composites from the values of matrix and fibre fraction. The following equations were used earlier to predict the storage modulus of short sisal fibrereinforced polypropylene composites (Joseph et al., 2003). Einstein’s equation-I is: E0c ¼ E0m 1 þ 1:25Vf
(4)
0
where E is the storage modulus and Vf is the fibre volume fraction and suffices c and m represents composite and matrix, respectively. Einstein’s equation-II is: E0c ¼ E0m 1 þ Vf
(5)
The concepts were modified in Guth’s equation: E0c ¼ E0m 1 þ 2:5Vf þ 14:1Vf
(6)
Another modified equation is Cohan’s equation: E0c ¼ E0m 1 þ 0:675r þ 1:62r2 Vf þ 14:1Vf
(7)
Fig. 6 e ColeeCole plots of OPFeLLDPE composites (>: M00, ,: AT12, 6: AT32, 3: AT42, J: AT31, B: AT33, D: UT32).
105
where r is the aspect ratio of the fibre. In the present study, the aspect ratio of OPF was calculated as 1.95, 0.93 and 0.39 respectively for fibre fractions 425e840 m, 177e425 m and 75e177 m. Another equation is Mooney’s equation: 2:5V f 1 SV 0 0 f Ec ¼ Em e
(8)
where S is the crowding factor or relative sedimentation volume of the inclusion and is mathematically defined as the ratio of apparent volume occupied by the fibre to the true volume of the fibre. S is taken as 1 in the present study. The experimental and predicted values of the storage modulus at a frequency of 1 Hz and at respective glass transition temperatures (corresponding to the peaks of tan d-temperature plot) using these equations are presented in Fig. 7. Einstein’s equations-I and II were found to better predict the storage modulus of OPFeLLDPE composites at all fibre loadings, sizes and treatments in the present study, but resulted in deviation from experimental values for short sisal fibre-reinforced polypropylene composites (Joseph et al., 2003). Guth’s equation and Mooney’s equation better predicted storage modulus at 10% fibre loading as reported earlier for short sisal fibre-reinforced polypropylene composites (Joseph et al., 2003). It is interesting to note that Guth’s equation, Cohan’s equation and Mooney’s equation better approximated storage modulus of untreated fibre composite than treated composite. The Cohan model gave rise to extremely high modulus values as obtained previously for short sisal fibre-reinforced polypropylene composites (Joseph et al., 2003).
3.5.2.
Damping parameter (tan d)
Damping behaviour of a composite is of great practical interest for its long term utilisation. The rule of mixtures was applied to predict tan d by Neilson and Landel (1974). tandc ¼ Vf tandf þ Vm tandm
(9)
However Drzal, Rich, Koenig, and Llyod (1983) suggested that the first term in the above equation can be eliminated for rigid fillers and thus Eq. (9) becomes
Fig. 7 e The experimental and predicted values of the storage modulus at respective glass transition temperatures of OPFeLLDPE composites and at 1 Hz frequency (>: Experimental, ,: Einstein’s equation I, 6: Einstein’s equation II, 3: Guth’s equation, J: Cohans equation, B: Mooney’s equation (S [ 1)).
106
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composites at different fibre content, fibre size and alkali treatment were determined. Prediction of storage modulus and damping parameter using standard models were compared with experimental values. The specific conclusions of the study are:
Fig. 8 e The experimental and predicted values of the damping parameter at respective glass transition temperatures of OPFeLLDPE composites and at 1 Hz frequency (>: Experimental, ,: Drzal equation, 6: Tung and Dynes equation).
tandc ¼ Vm tandm
(10)
Tung and Dynes (1987) proposed that a stiffness term should be added, assuming that the matrix in presence of fibres offers a stiffness equivalent to the minimum elastic modulus of the composite. The resulting equation is tandc ¼ Vm
0 Em tandm E0c
(11)
A comparison of tan d values predicted using both Drzal equation (Eq. (10)) and Tung and Dynes equation (Eq. (11)) with experimental values is presented in Fig. 8. It is found that the Drzal equation better represented the tan d values than did the Tung and Dynes equation. Both these equations better represented the experimental values at fibre loading of 10% than at higher fibre loadings. It is interesting to note that the predicted values deviated from the experimental values when fibre loading increased and this deviation was more pronounced for the Tung and Dynes equation. In the case of short sisal fibreepolypropylene composites, the lack of a term representing localised constraints imposed by the fibres on matrix deformation was one of the reasons for deviation of predicted values from experimental results (Joseph et al., 2003). Greater deviation at higher fibre loading observed in the present study can be read in conjunction with this theory, as the localised constraint imposed by the fibres would be proportional to the fibre loading. However at 75e177 m fibre size, the models approximated experimental values. It is interesting to note that more deviation from the experimental values was observed in case of alkali-treated fibre composites than untreated fibre composites. Both the models did not consider the changes in the stiffness of interface due to treatment (Joseph et al., 2003).
4.
Conclusions
The dynamic mechanical properties viz., storage modulus, loss modulus and damping parameter of OPFeLLDPE
The storage modulus increased with increase in fibre content in the composite and also after alkali treatment of fibres. The loss modulus values also increased with fibre loading except for composite with 10% fibre content. The loss modulus values increased following alkali treatment on fibres. The glass transition temperature of pure LLDPE was 145 C, and this increased to 128 C for composites with 40% fibre content. Fibre loading increased the loss modulus peak values except for 10% fibre loading. Alkali treatment on fibres also increased the loss modulus peak. Single tan d peaks were observed for all the composite samples tested and lowest tan d values were observed for lower size fibres, between a temperature range of 40 C to 60 C. The tan d peak values decreased upon fibre addition, and alkali treatment increased tan d peak at all frequencies indicating better impact properties after alkali treatment. Higher tan d was exhibited by composites with 177e425 m size fraction. The interfacial strength indicator values (B) increased as the frequency increased. It was observed that alkali-treated fibre composites exhibited low B values. Higher B values were observed for the fibre size range 75e177 m followed by 425e840 m and 177e425 m indicating decreased order of bond strength. The activation energy of composites was higher than that of the pure LLDPE, except for composites with 10% fibre loading. Activation energy was found to be high for composites with 425me840m size fraction (80.7 kJ mol1) followed by 177e425 m size fraction (72.6 kJ mol1) and 75e177 m size fraction (68.1 kJ mol1). Storage modulus (E0 ) and damping parameters predicted using different equations were in agreement with experimental values.
Acknowledgement Authors acknowledge Canadian Bureau of International Education (CBIE) for providing Graduate Student Exchange Program (GSEP) fellowship to the first author. The study leave granted to the first author by Indian Council of Agricultural Research is also thankfully acknowledged.
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