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wood flour composites
Materials and Design 58 (2014) 374–380
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Processing, characterization and modeling of recycled polypropylene/glass fibre/wood flour composites M.A. Al-Maadeed a, Yasser M. Shabana b, P. Noorunnisa Khanam a,⇑ a b
Center for Advanced Materials, Qatar University, Doha, Qatar Mechanical Design Department, Faculty of Engineering, El-Mataria, Helwan University, P.O. Box 11718, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received in revised form 27 October 2013 Accepted 19 February 2014 Available online 1 March 2014 Keywords: Polymers Mechanical properties Thermal properties Modeling
a b s t r a c t Polypropylene (PP) is one of the most common thermoplastic materials in the world. There is a need to recycle the large amount of this used material. To overcome the environmental problems, related to the polymer waste, PP was recycled and used as a matrix material in different composites that can be used in high value applications. In this paper, composites made of recycled polypropylene (RPP) reinforced by glass fibres and/or wood flour of the palm tree were prepared, characterized and modeled. The mechanical and thermal properties of these recycled polymer matrix composites (RPMCs) were measured experimentally and modeled theoretically. The mechanical properties included tensile modulus, tensile strength and hardness, whereas thermal properties included thermal stability, melting and crystallinity percentage content were studied. In addition we applied the functionally graded materials concept, the elastic finite element analysis of a layered functionally graded pressurized pipe, which is one of the practical industrial applications, was accomplished in order to have some insight on the performance of such RPMCs. The results reveal that the desired mechanical and thermal properties met the requirements of a wide range of practical applications which can be attained by adding the considered fillers. Also, the proper selection of the layers of the pressurized pipe, which was made of RPMCs, led to decrease of the induced stresses and accordingly increased the operational safety. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Recycled thermoplastic polymers, which include polypropylene (PP) alone frequently lack sufficient strength and stiffness for use in some engineering applications. This has led to the use of synthetic and natural fillers and the production of recycled polymer matrix composites (RPMCs) due to their desirable properties such as moderate stiffness, strength and large ductility [1]. RPMCs offer inherently durable products well suited for many structural applications such as in construction, in automobile industry and in soil conservation [2–5]. However, their wide spread use in engineered applications has been restricted due to a limited understanding of their mechanical and thermal behavior. Recycling with the use of cleaner technology contributes to reducing the impact of industrialization upon the environment [6,7]. Wood–plastic composites (WPCs) can be considered as sustainable materials, as the wood can be obtained from landfill agro waste material, and the plastics can be mainly derived from ⇑ Corresponding author. Tel.: +974 44035664. E-mail addresses: [email protected], [email protected] (P.N. Khanam). http://dx.doi.org/10.1016/j.matdes.2014.02.044 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.
consumer and industrial recycling efforts, as municipal solid wastes. Investigations for structural applications considering WPCs were reported [8]. Wolcott [9] investigated the effects of adding wood cellulose based fibres and found that these fibres not only improve the composite strength and stiffness, but also improve a number of end-use and processing properties such as thermal stability, ultraviolet resistance and workability. Flexural, compressive, tensile and dowel bearing performance of WPCs were documented [10]. Clemons [11] studied RPMCs and measured some of the mechanical properties and their variations with the volume fraction of wood flour (WF). Xu et al. [12] fabricated WPCs by introducing different wood fibres to improve the compatibility between them and the matrix. They measured some of the mechanical properties such as tensile strength and Impact toughness. The effects of different parameters including the WF content on the behavior of PP wood composites were modeled using the Burgers model and a power law equation [13,14]. Rogueda Berriet et al. [15] studied the effects of the recycling process on the mechanical behavior of RPMCs through different loading tests. They also theoretically verified the experimentally measured behaviors of RPMCs by applying different mechanical models. Also, WPCs were modeled analytically taking into account the polymer type and the form of
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Table 1 Description of samples. Sample
Ratio
Notes
RPP
100%
GF
100%
Chopped wood
100%
RPP/GF composite (GFRPP)
70/30%
Wood/GF/RPP composite (WGFRPP)
5/25/70
RPP granules with melt flow index of 12 g/10min Chopped Short Glass Fiber GF Converting chopped wood into wood flour by Cryo mill Twin Screw Extruder (TSE) with zones Temp. 190 200 210 220 230 (°C) (TSE) 190 200 210 220 230 (°C)
the reinforcements [16–18]. Testing and analysis were conducted on formulations composed of a variety of commercially available polymer types. In addition, glass fibres (GFs) are known to be suitable reinforcements in polymer composites as they are chemically resistant [19–21]. Hybrid polymer composites containing WF with GFs were manufactured and their properties such as tensile strength, tensile modulus and hardness were characterized [22,23]. In this paper, recycled polypropylene (RPP) was used as a matrix material and GFs and WF were used as reinforcements to produce RPMCs suitable for real world applications. The processed RPMCs were experimentally tested and theoretically modeled to better understand their thermo-mechanical behavior to gain insight into the effects of different microstructural parameters on their thermo mechanical properties and performance. These properties include tensile modulus, tensile strength, hardness, melt flow index and thermal stability. Additionally, finite element analyses incorporating the functionally graded materials concept were performed on a pressurized pipe consisting of different layers. A pressurized pipe was chosen because it represents a practical industrial application and thus provides information regarding the performance of RPMCs such as that considered in the study. The results reveal that the mechanical properties are, in general, enhanced by increasing the volume fraction of the reinforcements. 2. Experimental Details 2.1. Materials and processing Fig. 1. (a) SEM photos of pure glass fiber and (b) wood flour.
RPP with melt flow index of 12 g/10 min (230 °C/2.16 kg) and density of 0.9 g/cm3 was provided by Qatar Polymers, Qatar, in pellets form. Additionally, short chopped GFs of type E (silane treated), made in Belgium, were provided by the European OC Fiberglass Company. Fig. 1(a) shows the GFs which were long in nature with average aspect ratio of 360 with approximately 4 mm length. Chopped wood fibers were obtained from the landfill wastes of date palm trees in Qatar. These chopped wood fibers were converted to WF by Cryomil and then dried in the oven at 65 °C for 24 h. SEM images of the pure date palm WF are shown in Fig. 1(b), and it is comprised of fiber bundles, like flakes with average aspect ratio of 333. The used WF is cheaper and easier to process with plastics than wood fibers. Mixing process of the matrix material, RPP, and the reinforcements, WF and GF, were done in a five-stage Brabender twin screw extruder. The temperatures of the processing zones are in the range of 190–230 °C as listed in Table 1. Table 1 lists the materials compositions with weight ratios of the constituents and also the symbols which were used in this work. The mixtures were fed into the hopper of the extruder and then extruded. The samples were then cooled in water and followed by granulation process. The compounded samples were injected into a PE 5 injection moulding machine, model Fox 8 oxford U1743, and supplied by Tec equip-
ment (TI) to prepare different samples of the standard dog-bone shape of different composites and the length, width and thickness of each specimen were 20, 12.5 and 3 mm, respectively. 2.2. Experimental tests 2.2.1. Mechanical tests Tensile tests were carried out according to ASTM: D638-10 using an Instron 4301 universal testing machine and applying a deformation rate (crosshead speed) of 10 mm/min at room temperature. Five samples were tested for each composite and the average value was reported. The tensile stress was determined by dividing the tensile load by the initial sample cross-sectional area. Additionally, the tensile strain was calculated by determining the ratio between the increase in length and the initial gauge length between the clamps. Morphological analyses were carried out using Philips (SEM)– EDX. The fracture surfaces of the samples after rupture and the bonding between the reinforcements and the matrix were investigated. Micro-hardness tests were conducted on different samples using the standard Rockwell hardness F scale (HRF) including a
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1/16 in. (1.588 mm) diamond/steel ball penetrator and 60 kgf (588.6 N) load. ASTM: D785-08 and ASTM: E18-12 were applied to measure the hardness of different composites. For each sample, measurements corresponding to 10 indentations were carried out. All tests were conducted in a laboratory environment (room temperature and atmospheric pressure). 2.2.2. Thermal tests Differential scanning calorimetry (DSC) was carried out using the Perkin Elmer, Pyris 7 thermal analyzer. All measurements were taken under nitrogen atmosphere at the same heating and cooling rates of 10 °C/min. The following procedures were followed: (i) heating from 30 to 200 °C, (ii) cooling from 200 to 30 °C and (iii) heating from 30 to 200 °C. In addition, at the end of each heating or cooling process, there was a temperature hold for 5 min. The percentage of crystallinity (XC %) of a sample was calculated as follows [24]:
Xc % ¼
ðHf =W%Þ 100 Ho
Fig. 2. Experimental stress–strain relation of RPP, WGFRPP1 and GFRPP.
ð1Þ
where DHf is the heat of fusion of RPP in a composite determined from the DSC thermogram, DHo is the heat of fusion of 100% crystalline RPP which equals 209 J/g, [24] and W % is the weight percentage of RPP in the composite. The thermal decomposition of the samples was evaluated via thermo gravimetric analysis (TGA) using a Perkin Elmer thermal analyzer. The measurements were taken in nitrogen atmosphere at a heating rate of 10 °C/min from 30 to 700 °C. 2.2.3. Numerical characterization The radial and tangential stresses of a thick wall cylinder subjected to different boundary conditions of uniform applied pressure are given by [25]
rr ¼
E du u þm 2 1 m dr r
rh ¼
E 1 m2
m
du u þ dr r
ð2Þ
ð3Þ
where E, v and u are the Young’s modulus, the Poisson’s ratio and the displacement, respectively. The condition of equilibrium is given by
drr rr rh þ ¼0 dr r
ð4Þ
After evaluating the different stress components, the deviatoric stress components are used to calculate the von Mises stress as
re ¼
1=2 3 0 0 rij rij 2
ð5Þ
3. Results and discussion 3.1. Experimental characterization 3.1.1. Mechanical properties The stress–strain curves obtained from the tensile tests of RPP, GFRPP and WGFRPP are shown in Fig. 2. The figures show that RPP is the most compliant though its ductility is the highest due to its breaking strain, which is close to 16%. Therefore, necking was clearly observed while testing RPP. Adding GF resulted in an increase in stiffness but decreases the breaking strain. Therefore, GFRPP exhibits the highest strength and stiffness and lowest breaking strain. However, adding wood flour to GFRPP makes the material more compliant and decreases the strength whereas the
Fig. 3. (a) and (b) Tensile modulus and tensile strength of recycled polymer composites.
breaking strain is affected slightly as shown for WGFRPP. Linear stress–strain relations can be observed at low stress levels but they become nonlinear at high stress levels. Fig. 3(a) and (b) shows the tensile modulus and strength of these recycled polymeric materials. Each bar represents the average of five tests and each error bar indicates the standard deviation. It is clear that RPP has the lowest levels of both the tensile modulus and strength whereas GFRPP has the highest levels. It can be concluded that the mechanical properties of the composites
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M.A. Al-Maadeed et al. / Materials and Design 58 (2014) 374–380 Table 2 Hardness properties of recycled PP and WGFRPP, GFRPP composites. Samples
Hardness (HRF)
RPP WGFRPP1 GRPP
36.2 39.7 62.4
Fig. 5. DSC curves of RPP, WGFRPP and GFRPP.
Table 3 Melting temperature, crystallization temperature and % of crystallization of RPP and composites.
Fig. 4. SEM images of (a) WGFRPP1 and (b) GFRPP.
show efficient reinforcing effect by the reinforcements and the composites exhibit higher strength than pure RPP at realistic working strains. SEM micrographs of the tensile fractured samples of WGFRPP and GFRPP are shown in Fig. 4. Good fiber dispersion can be observed for both composites. Fig. 4(a) shows that fiber pull out and delamination at the fiber–matrix occur in WGFRPP because the adhesion and bonding limits between the fibers and the matrix are exceeded. These two fracture modes affect the tensile strength of the composites. Fig. 4(b) shows the SEM micrograph of a tensile fractured GFRPP sample. It can be seen that GFRPP exhibits less fiber pull out and more interfacial delamination compared to WGFRPP because the GF content is higher and it is expected that the adhesion between the fibers and the matrix in the case of GFRPP is high. Therefore, the tensile strength of GFRPP is expected to be higher than that of WGFRPP, which is in good agreement with the results shown in Figs. 2 and 3. Hardness is the resistance of a material to localized deformations. The hardness values of these three materials were measured experimentally using the Rockwell hardness F scale (HRF) and the measured values are listed in Table 2. It can be seen that adding either WF or GFs to RPP increases the hardness. However, GFRPP
Samples
TM (°C)
TC (°C)
XC %
RPP WGFRPP1 GFRPP
163.7 162.35 163.98
121.79 121.29 122.05
32.2481 29.6285 43.1724
has the highest hardness due to the properties of GFs. Additionally, the hardness shows the same trend as the tensile strength and the measured hardness values are acceptable in comparison with other estimates [26,27]. Islam et al. [26] reported that polymer hardness increased with the addition of fibre and also reported that treated fibre composites have higher hardness values than untreated fibre composites. This is due to better fibre matrix interfacial adhesion. The increase in the hardness of the composites with the fibre content was also reported by Tasdemir et al. [27].
3.1.2. Thermal properties The differential scanning calorimetery (DSC) thermograms of the three materials are shown in Fig. 5. These curves were taken from the second heating of the materials from the DSC. The melting temperature (TM), crystallization temperature (TC) and percentage of crystallinity (XC %) of these recycled materials are listed in Table 3. The effect of adding GFs and/or WF as fillers on both TM and TC is very small and can be neglected because the differences between the listed values are within the experimental measurement error. These melting temperatures indicate that the chain dynamics of the considered composites are not affected by the fillers. For XC %, GFs act as nucleating agents and therefore improve the crystallinity, XC % increases by 33.87% relative to the percentage crystallinity of RPP when adding GFs. These results are consistent with the ones in literature [28–30]. Lozano et al. [28] reported that carbon fibre increase the crystallinity of the PP polymer matrix. Also, Lee et al. [29] results showed that crystallinity of the PP decreases with wood flour whereas it increases with clay and MAPP. Morever, Dai et al. [30] reported that fillers act as nucleating agent and improve the crystallinity of the composites.
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Fig. 6. TGA curves of RPP, WGFRPP and GFRPP.
However, wood hinders the nucleation and stops the growth of crystals. Therefore, the effect of WF is dominant when adding both GFs and WF as indicated by the 8.13% decrease in XC % as shown in Table 3. The results of the thermo-gravimetric (TGA) measurements are shown in Fig. 6. Thermal degradation in RPP starts with chain fragmentation processes [31] due to the formation of free radicals. Then, the volatile oligomers are produced causing the mass loss. GFs have high thermal stability, they are not easily degraded and they decrease the mobility of the chains due to their strong interaction with the RPP matrix. Therefore, adding GFs to RPP increases the decomposition temperature and the onset of the thermal degradation of GFRPP is delayed and shifted towards a higher temperature. This composite has only one degradation temperature at 457.73 °C, which is the highest degradation temperature when compared to the other materials. Similar results were achieved [32] in thermal decomposition, where glass fibre increased the thermal decomposition of the polymer composite. Adding WF, however, results in an onset temperature of 362.29 °C and final temperature step of 468.75 °C for WGFRPP. This is because adding WF expedites degradation, which starts early, due to the presence of ligno cellulosic material that decomposes at a lower temperature.
Fig. 7. Young’s modulus and Poisson’s ratio as functions of the fiber volume fraction.
Fig. 8. Shear modulus as a function of the fiber volume fraction.
3.2. Numerical characterization 3.2.1. Sensitivity analysis For the RPMCs made of RPP and WF, which were extracted from date palm trees, the effects of the reinforcement volume fraction and aspect ratio on the effective mechanical moduli and Poisson ratio were investigated based on the principles of micromechanics [33,34]. In the following analysis, the considered tensile moduli and Poisson’s ratios of RPP and wood fibers are 642.26 MPa, and 0.36 and 2170 MPa, and 0.3, respectively. The tensile modulus of RPP was experimentally measured through the tensile test. Fig. 7 shows the effect of the fiber volume fraction on the tensile modulus and Poisson’s ratio when the fiber aspect ratio equals 333. It can be seen that the tensile modulus is affected significantly as it increases by 71% when the fiber volume fraction reaches 0.3. Therefore, the beneficial effect of the fiber volume fraction is evident and is very promising from the stress reduction and high performance point of view. On the other hand, Poisson’s ratio decreases with the fiber volume fraction; therefore, the transverse strain becomes smaller when the longitudinal strain remains unchanged, and consequently, the overall deformations of structures composed of these composites generally decrease. The shear modulus, which is a measure of a material’s resistance to different shear loading conditions, is depicted in Fig. 8. As seen in the figure, the shear modulus increases by 53% when the fiber volume fraction reaches 0.3. The
Fig. 9. Variation of Young’s and shear moduli with the aspect ratio when the fiber volume fraction is 0.3.
general behaviors of increasing both the tensile and shear moduli with the fibre volume fraction (Fig.7 and 8) agree well with the results in [35,36] when the fibre is stiffer than the matrix. That is because the stiff fibres provide a reinforcing effect to the matrix and therefore the resistance to deformation increases. The effects of the fiber aspect ratio on the tensile and shear moduli are illustrated in Fig. 9. The tensile modulus remains nearly constant when the aspect ratio exceeds 30, whereas the shear modulus remains nearly unchanged beyond an aspect ratio of 20. The presented effect of the aspect ratio on the tensile modulus is consistent with the results in [37,38]. There is a critical fibre length at which the stress is efficiently transferred from the matrix to the fibre, resulting in a stronger composite. It can be concluded, based on this figure, that the recommended aspect ratio is close to 30 when a part, composed of these composites, is subjected to a normal load, however under a shear load, prolate fibers with an aspect
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Fig. 10. Stress variation for different materials.
379
Fig. 11. Strain variation for different materials.
ratio less than 2 are recommended. For a combined load (normal and shear) the optimum aspect ratio lies between 1 and 30, the exact value of which depends on the ratio between these two loads. Overall, these results show that both the RPP’s ability to sustain different loading conditions and its resistance to deformation are enhanced by adding wood fibers. Therefore, these composites can be used over a wide range of suitable applications in industry. 3.2.2. Practical example The aim of this part of the study was to investigate the mechanical behavior of RPMCs and their mixtures under pressure loading as one of the loading conditions in many engineering applications. Therefore, a pipe as a common final product in industry, subjected to internal pressure was elastically analyzed using the finite element method. The considered inner diameter, wall thickness, and internal pressure of the pipe were 50 mm, 3 mm and 0.5 MPa, respectively. The concept of functionally graded materials (FGMs) was applied by considering a pipe composed of three layers, which have the same thickness, with different properties. The FGM constituents were RPP, GFRPP and WGFRPP. Three FGMs were considered to demonstrate the effects on stress, strain and displacement when using different FGMs: FGM1 (RPP–WGFRPP–GFRPP), FGM2 (WGFRPP–RPP–GFRPP) and FGM3 (GFRPP–WGFRPP–RPP). The components of each FGM are listed in order of distance from the outer layer. For the finite element analyses, the general purpose commercial software ABAQUS version 6.10 was used and the numbers of elements and nodes of a quarter model were 6000 and 6241, respectively. Referring to Fig. 2 that shows the experimental stress–strain relations obtained from the tensile tests performed on the above mentioned constituent materials and considering a safety factor of 2.5, the allowable stress limits (ra) of RPP, WGFRPP and GFRPP are 5.9, 7.84 and 9.33 MPa respectively. Also, the corresponding allowable strain (ea) limits are 0.016, 0.013 and 0.01, respectively. At these values, the elastic analyses are valid and consistent with the behaviors illustrated in Fig. 2. Because fracture can be defined by not only stress but also strain and displacement, the presented results include the von Mises stress (rv), which can be considered as a measure of failure, the maximum principal strain (e1), and the magnitude of the displacement (u). Fig. 10 shows the variation in the stress ratio (ra/rv) as a measure of safety in the radial direction where the non-dimensional radial position n(r ri)/(ro ri) of the horizontal axis is a function of the number of layers n, which considered to be 3, and the inner and outer radii, ri and ro, respectively. It can be seen that the stress ratio of RPP is the lowest. If the pipe is composed of more than one layer, the stress distributions show sharp changes at the interfaces, due to the properties mismatch of the layers, as shown in the figure. This behavior is consistent
Fig. 12. Displacement variation for different materials.
with the basic principles of the mechanics of materials and solid mechanics [39,40]. When the pipe is composed of any of the three FGMs, the safety is generally higher than that of RPP because the stress ratio becomes high. It can be seen that the increases in safety at the outer and inner surfaces of FGM1 relative to RPP are 70% and 18%, respectively. On the other hand, the maximum and minimum increases in safety for FGM2 are 71% and 16% and those for FGM3 are 63% and 13%, respectively. Thus, the beneficial effects of using FGMs are evident and very promising from a stress reduction and high performance point of view. Fig. 11 shows the variation in the strain ratio (ea/e1) in the radial direction. Again RPP has the lowest safety because it has the lowest strain ratio whereas the FGMs show better performance and higher safety. The maximum and minimum increases in safety for FGM1 relative to RPP are 70% and 7%, while those for FGM2 and FGM3 are 64% and, 2.5% and 68% and, 6%, respectively. It is confirmed from these figures that the critical stresses and strains induced in these materials, can be reduced simultaneously by using FGMs. The displacement variation in the radial direction is shown in Fig. 12. It can be seen that the displacement is generally small. The RPP pipe has the highest displacement level, and which can be reduced by using any of the FGMs. When the inner layer of the FGM is stiff (GFRPP), its resistance to deformation is high and therefore the induced displacements of FGM1 and FGM2 under internal pressure were low. However, the second layer of FGM2 (RPP) is more compliant than that of FGM1 (WGFRPP) and therefore the displacement of FGM2 was higher than that of FGM1. FGM3 exhibited the highest FGM displacement as its inner layer is compliant (RPP). It should be noted that FGM1 results in the lowest displacement level and reduces the RPP displacement by 41%.
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4. Conclusions Recycled polymer–matrix composites (RPMCs) composed of recycled polypropylene as a matrix and glass fibers (GFs) and/or wood flour as reinforcements were processed, characterized and modeled. The thermomechanical properties of binary and ternary composites are presented and discussed. For the mechanical properties, it was found that adding GFs to RPP increased each of the stiffness, strength and hardness but decreased the ductility whereas adding WF as a second reinforcement has opposite effects. The tensile and shear moduli of binary composites were increased by 71% and 53%, respectively, when the fibre volume fraction reaches 0.3 while the optimum range of the fibre aspect ratio was from 1 to 30. Also, the SEM micrographs revealed that the adhesion and bonding limits between the fibres and the matrix are transgressed as fibre pull out and interfacial delamination are observed. However, the binary composite had fibre pull out less than that of the ternary composite, one possible reason is WF decreased the adhesion between the fibres and the matrix. For the thermal properties, on the other hand, GFs and WF had opposite effects on the crystallinity as GFs act as nucleating agents and increased the crystallinity but WF hinders the nucleation and stops the growth of crystals. Also, the binary composite had higher degradation temperature than that of ternary composite as GFs have high thermal stability but WF expedites degradation due to the presence of ligno cellulosic material that decomposes at a low temperature. A functionally graded layered-pipe, composed of RPMCs and subjected to internal pressure loading was numerically modeled using the finite element analysis. Different arrangements of the layers were investigated to quantify their effects on the behavioral parameters such as stress, strain and displacement. It was found that the functionally graded materials can be applied to reduce the values of these behavioral parameters and therefore increase the operational safety. The safety based stress, strain and displacement can be upto 71%, 70% and 41% respectively depending on the arrangements of the layers. Acknowledgements The authors would like to gratefully acknowledge both Qatar Science and Technology Park (QSTP) for financially supporting this project and the Center for Advanced Materials at Qatar University for completing the experimental tasks. References [1] AdrianNunez J, PabloSturm C, JoseKenny M, Aranguren MI, NormaMarcovich E, Maria Reboredo M. Mechanical characterization of polypropylene–wood flour composites J Appl Polym Sci 2003;88:1420–8. [2] Cheung Hoi-yan, Ho Meipo, Lau Kin-tak, Francisco Cardona, Hui David. Natural fibre reinforced composites for bioengineering and environmental engineering applications.. Compos Part B – Eng 2009;40(7):655–63. [3] DerekThompson W, EricHansen N, Knowles Chris, Lech Muszynski. Opportunities for wood plastic composites products in the U.S. highway construction sector. Bioresources. 2010;5(3):1336–52. [4] Ashori Alireza. Review paper. Wood plastic composites as promising green composites for automotive industries. BioResour Technol 2008;99:4661–7. [5] Wambua Paul, Ivens Jan, Ignaas Verpoest. Natural fibres: can they replace glass in fibre reinforced plastics? Compos Sci Technol 2003;63(9):1259–64. [6] Coutinho FMB, Costa THS. Performance of polypropylene–wood fiber composites. Polym Test 1999;18:581–7. [7] Esport Ana, Camacho Walker, Sigbritt Karlson. Thermal and thermo mechanical properties of bio composites made from modified recycled cellulose and recycled poly propylene. J Appl Polym Sci 2003;89:2353–60. [8] Edward Slaughter Andrew. Design and fatigue of a structural wood–plastic composite. Master thesis. Department of Civil and Environmental Engineering. Washington State University; 2004. [9] Wolcott MP. Wood–plastic composites. Encyclopaedia of Materials Science and Technology; 2001.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Processing, characterization and modeling of recycled polypropylene/glass fibre/wood flour composites M.A. Al-Maadeed a, Yasser M. Shabana b, P. Noorunnisa Khanam a,⇑ a b
Center for Advanced Materials, Qatar University, Doha, Qatar Mechanical Design Department, Faculty of Engineering, El-Mataria, Helwan University, P.O. Box 11718, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received in revised form 27 October 2013 Accepted 19 February 2014 Available online 1 March 2014 Keywords: Polymers Mechanical properties Thermal properties Modeling
a b s t r a c t Polypropylene (PP) is one of the most common thermoplastic materials in the world. There is a need to recycle the large amount of this used material. To overcome the environmental problems, related to the polymer waste, PP was recycled and used as a matrix material in different composites that can be used in high value applications. In this paper, composites made of recycled polypropylene (RPP) reinforced by glass fibres and/or wood flour of the palm tree were prepared, characterized and modeled. The mechanical and thermal properties of these recycled polymer matrix composites (RPMCs) were measured experimentally and modeled theoretically. The mechanical properties included tensile modulus, tensile strength and hardness, whereas thermal properties included thermal stability, melting and crystallinity percentage content were studied. In addition we applied the functionally graded materials concept, the elastic finite element analysis of a layered functionally graded pressurized pipe, which is one of the practical industrial applications, was accomplished in order to have some insight on the performance of such RPMCs. The results reveal that the desired mechanical and thermal properties met the requirements of a wide range of practical applications which can be attained by adding the considered fillers. Also, the proper selection of the layers of the pressurized pipe, which was made of RPMCs, led to decrease of the induced stresses and accordingly increased the operational safety. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Recycled thermoplastic polymers, which include polypropylene (PP) alone frequently lack sufficient strength and stiffness for use in some engineering applications. This has led to the use of synthetic and natural fillers and the production of recycled polymer matrix composites (RPMCs) due to their desirable properties such as moderate stiffness, strength and large ductility [1]. RPMCs offer inherently durable products well suited for many structural applications such as in construction, in automobile industry and in soil conservation [2–5]. However, their wide spread use in engineered applications has been restricted due to a limited understanding of their mechanical and thermal behavior. Recycling with the use of cleaner technology contributes to reducing the impact of industrialization upon the environment [6,7]. Wood–plastic composites (WPCs) can be considered as sustainable materials, as the wood can be obtained from landfill agro waste material, and the plastics can be mainly derived from ⇑ Corresponding author. Tel.: +974 44035664. E-mail addresses: [email protected], [email protected] (P.N. Khanam). http://dx.doi.org/10.1016/j.matdes.2014.02.044 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.
consumer and industrial recycling efforts, as municipal solid wastes. Investigations for structural applications considering WPCs were reported [8]. Wolcott [9] investigated the effects of adding wood cellulose based fibres and found that these fibres not only improve the composite strength and stiffness, but also improve a number of end-use and processing properties such as thermal stability, ultraviolet resistance and workability. Flexural, compressive, tensile and dowel bearing performance of WPCs were documented [10]. Clemons [11] studied RPMCs and measured some of the mechanical properties and their variations with the volume fraction of wood flour (WF). Xu et al. [12] fabricated WPCs by introducing different wood fibres to improve the compatibility between them and the matrix. They measured some of the mechanical properties such as tensile strength and Impact toughness. The effects of different parameters including the WF content on the behavior of PP wood composites were modeled using the Burgers model and a power law equation [13,14]. Rogueda Berriet et al. [15] studied the effects of the recycling process on the mechanical behavior of RPMCs through different loading tests. They also theoretically verified the experimentally measured behaviors of RPMCs by applying different mechanical models. Also, WPCs were modeled analytically taking into account the polymer type and the form of
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Table 1 Description of samples. Sample
Ratio
Notes
RPP
100%
GF
100%
Chopped wood
100%
RPP/GF composite (GFRPP)
70/30%
Wood/GF/RPP composite (WGFRPP)
5/25/70
RPP granules with melt flow index of 12 g/10min Chopped Short Glass Fiber GF Converting chopped wood into wood flour by Cryo mill Twin Screw Extruder (TSE) with zones Temp. 190 200 210 220 230 (°C) (TSE) 190 200 210 220 230 (°C)
the reinforcements [16–18]. Testing and analysis were conducted on formulations composed of a variety of commercially available polymer types. In addition, glass fibres (GFs) are known to be suitable reinforcements in polymer composites as they are chemically resistant [19–21]. Hybrid polymer composites containing WF with GFs were manufactured and their properties such as tensile strength, tensile modulus and hardness were characterized [22,23]. In this paper, recycled polypropylene (RPP) was used as a matrix material and GFs and WF were used as reinforcements to produce RPMCs suitable for real world applications. The processed RPMCs were experimentally tested and theoretically modeled to better understand their thermo-mechanical behavior to gain insight into the effects of different microstructural parameters on their thermo mechanical properties and performance. These properties include tensile modulus, tensile strength, hardness, melt flow index and thermal stability. Additionally, finite element analyses incorporating the functionally graded materials concept were performed on a pressurized pipe consisting of different layers. A pressurized pipe was chosen because it represents a practical industrial application and thus provides information regarding the performance of RPMCs such as that considered in the study. The results reveal that the mechanical properties are, in general, enhanced by increasing the volume fraction of the reinforcements. 2. Experimental Details 2.1. Materials and processing Fig. 1. (a) SEM photos of pure glass fiber and (b) wood flour.
RPP with melt flow index of 12 g/10 min (230 °C/2.16 kg) and density of 0.9 g/cm3 was provided by Qatar Polymers, Qatar, in pellets form. Additionally, short chopped GFs of type E (silane treated), made in Belgium, were provided by the European OC Fiberglass Company. Fig. 1(a) shows the GFs which were long in nature with average aspect ratio of 360 with approximately 4 mm length. Chopped wood fibers were obtained from the landfill wastes of date palm trees in Qatar. These chopped wood fibers were converted to WF by Cryomil and then dried in the oven at 65 °C for 24 h. SEM images of the pure date palm WF are shown in Fig. 1(b), and it is comprised of fiber bundles, like flakes with average aspect ratio of 333. The used WF is cheaper and easier to process with plastics than wood fibers. Mixing process of the matrix material, RPP, and the reinforcements, WF and GF, were done in a five-stage Brabender twin screw extruder. The temperatures of the processing zones are in the range of 190–230 °C as listed in Table 1. Table 1 lists the materials compositions with weight ratios of the constituents and also the symbols which were used in this work. The mixtures were fed into the hopper of the extruder and then extruded. The samples were then cooled in water and followed by granulation process. The compounded samples were injected into a PE 5 injection moulding machine, model Fox 8 oxford U1743, and supplied by Tec equip-
ment (TI) to prepare different samples of the standard dog-bone shape of different composites and the length, width and thickness of each specimen were 20, 12.5 and 3 mm, respectively. 2.2. Experimental tests 2.2.1. Mechanical tests Tensile tests were carried out according to ASTM: D638-10 using an Instron 4301 universal testing machine and applying a deformation rate (crosshead speed) of 10 mm/min at room temperature. Five samples were tested for each composite and the average value was reported. The tensile stress was determined by dividing the tensile load by the initial sample cross-sectional area. Additionally, the tensile strain was calculated by determining the ratio between the increase in length and the initial gauge length between the clamps. Morphological analyses were carried out using Philips (SEM)– EDX. The fracture surfaces of the samples after rupture and the bonding between the reinforcements and the matrix were investigated. Micro-hardness tests were conducted on different samples using the standard Rockwell hardness F scale (HRF) including a
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1/16 in. (1.588 mm) diamond/steel ball penetrator and 60 kgf (588.6 N) load. ASTM: D785-08 and ASTM: E18-12 were applied to measure the hardness of different composites. For each sample, measurements corresponding to 10 indentations were carried out. All tests were conducted in a laboratory environment (room temperature and atmospheric pressure). 2.2.2. Thermal tests Differential scanning calorimetry (DSC) was carried out using the Perkin Elmer, Pyris 7 thermal analyzer. All measurements were taken under nitrogen atmosphere at the same heating and cooling rates of 10 °C/min. The following procedures were followed: (i) heating from 30 to 200 °C, (ii) cooling from 200 to 30 °C and (iii) heating from 30 to 200 °C. In addition, at the end of each heating or cooling process, there was a temperature hold for 5 min. The percentage of crystallinity (XC %) of a sample was calculated as follows [24]:
Xc % ¼
ðHf =W%Þ 100 Ho
Fig. 2. Experimental stress–strain relation of RPP, WGFRPP1 and GFRPP.
ð1Þ
where DHf is the heat of fusion of RPP in a composite determined from the DSC thermogram, DHo is the heat of fusion of 100% crystalline RPP which equals 209 J/g, [24] and W % is the weight percentage of RPP in the composite. The thermal decomposition of the samples was evaluated via thermo gravimetric analysis (TGA) using a Perkin Elmer thermal analyzer. The measurements were taken in nitrogen atmosphere at a heating rate of 10 °C/min from 30 to 700 °C. 2.2.3. Numerical characterization The radial and tangential stresses of a thick wall cylinder subjected to different boundary conditions of uniform applied pressure are given by [25]
rr ¼
E du u þm 2 1 m dr r
rh ¼
E 1 m2
m
du u þ dr r
ð2Þ
ð3Þ
where E, v and u are the Young’s modulus, the Poisson’s ratio and the displacement, respectively. The condition of equilibrium is given by
drr rr rh þ ¼0 dr r
ð4Þ
After evaluating the different stress components, the deviatoric stress components are used to calculate the von Mises stress as
re ¼
1=2 3 0 0 rij rij 2
ð5Þ
3. Results and discussion 3.1. Experimental characterization 3.1.1. Mechanical properties The stress–strain curves obtained from the tensile tests of RPP, GFRPP and WGFRPP are shown in Fig. 2. The figures show that RPP is the most compliant though its ductility is the highest due to its breaking strain, which is close to 16%. Therefore, necking was clearly observed while testing RPP. Adding GF resulted in an increase in stiffness but decreases the breaking strain. Therefore, GFRPP exhibits the highest strength and stiffness and lowest breaking strain. However, adding wood flour to GFRPP makes the material more compliant and decreases the strength whereas the
Fig. 3. (a) and (b) Tensile modulus and tensile strength of recycled polymer composites.
breaking strain is affected slightly as shown for WGFRPP. Linear stress–strain relations can be observed at low stress levels but they become nonlinear at high stress levels. Fig. 3(a) and (b) shows the tensile modulus and strength of these recycled polymeric materials. Each bar represents the average of five tests and each error bar indicates the standard deviation. It is clear that RPP has the lowest levels of both the tensile modulus and strength whereas GFRPP has the highest levels. It can be concluded that the mechanical properties of the composites
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Hardness (HRF)
RPP WGFRPP1 GRPP
36.2 39.7 62.4
Fig. 5. DSC curves of RPP, WGFRPP and GFRPP.
Table 3 Melting temperature, crystallization temperature and % of crystallization of RPP and composites.
Fig. 4. SEM images of (a) WGFRPP1 and (b) GFRPP.
show efficient reinforcing effect by the reinforcements and the composites exhibit higher strength than pure RPP at realistic working strains. SEM micrographs of the tensile fractured samples of WGFRPP and GFRPP are shown in Fig. 4. Good fiber dispersion can be observed for both composites. Fig. 4(a) shows that fiber pull out and delamination at the fiber–matrix occur in WGFRPP because the adhesion and bonding limits between the fibers and the matrix are exceeded. These two fracture modes affect the tensile strength of the composites. Fig. 4(b) shows the SEM micrograph of a tensile fractured GFRPP sample. It can be seen that GFRPP exhibits less fiber pull out and more interfacial delamination compared to WGFRPP because the GF content is higher and it is expected that the adhesion between the fibers and the matrix in the case of GFRPP is high. Therefore, the tensile strength of GFRPP is expected to be higher than that of WGFRPP, which is in good agreement with the results shown in Figs. 2 and 3. Hardness is the resistance of a material to localized deformations. The hardness values of these three materials were measured experimentally using the Rockwell hardness F scale (HRF) and the measured values are listed in Table 2. It can be seen that adding either WF or GFs to RPP increases the hardness. However, GFRPP
Samples
TM (°C)
TC (°C)
XC %
RPP WGFRPP1 GFRPP
163.7 162.35 163.98
121.79 121.29 122.05
32.2481 29.6285 43.1724
has the highest hardness due to the properties of GFs. Additionally, the hardness shows the same trend as the tensile strength and the measured hardness values are acceptable in comparison with other estimates [26,27]. Islam et al. [26] reported that polymer hardness increased with the addition of fibre and also reported that treated fibre composites have higher hardness values than untreated fibre composites. This is due to better fibre matrix interfacial adhesion. The increase in the hardness of the composites with the fibre content was also reported by Tasdemir et al. [27].
3.1.2. Thermal properties The differential scanning calorimetery (DSC) thermograms of the three materials are shown in Fig. 5. These curves were taken from the second heating of the materials from the DSC. The melting temperature (TM), crystallization temperature (TC) and percentage of crystallinity (XC %) of these recycled materials are listed in Table 3. The effect of adding GFs and/or WF as fillers on both TM and TC is very small and can be neglected because the differences between the listed values are within the experimental measurement error. These melting temperatures indicate that the chain dynamics of the considered composites are not affected by the fillers. For XC %, GFs act as nucleating agents and therefore improve the crystallinity, XC % increases by 33.87% relative to the percentage crystallinity of RPP when adding GFs. These results are consistent with the ones in literature [28–30]. Lozano et al. [28] reported that carbon fibre increase the crystallinity of the PP polymer matrix. Also, Lee et al. [29] results showed that crystallinity of the PP decreases with wood flour whereas it increases with clay and MAPP. Morever, Dai et al. [30] reported that fillers act as nucleating agent and improve the crystallinity of the composites.
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Fig. 6. TGA curves of RPP, WGFRPP and GFRPP.
However, wood hinders the nucleation and stops the growth of crystals. Therefore, the effect of WF is dominant when adding both GFs and WF as indicated by the 8.13% decrease in XC % as shown in Table 3. The results of the thermo-gravimetric (TGA) measurements are shown in Fig. 6. Thermal degradation in RPP starts with chain fragmentation processes [31] due to the formation of free radicals. Then, the volatile oligomers are produced causing the mass loss. GFs have high thermal stability, they are not easily degraded and they decrease the mobility of the chains due to their strong interaction with the RPP matrix. Therefore, adding GFs to RPP increases the decomposition temperature and the onset of the thermal degradation of GFRPP is delayed and shifted towards a higher temperature. This composite has only one degradation temperature at 457.73 °C, which is the highest degradation temperature when compared to the other materials. Similar results were achieved [32] in thermal decomposition, where glass fibre increased the thermal decomposition of the polymer composite. Adding WF, however, results in an onset temperature of 362.29 °C and final temperature step of 468.75 °C for WGFRPP. This is because adding WF expedites degradation, which starts early, due to the presence of ligno cellulosic material that decomposes at a lower temperature.
Fig. 7. Young’s modulus and Poisson’s ratio as functions of the fiber volume fraction.
Fig. 8. Shear modulus as a function of the fiber volume fraction.
3.2. Numerical characterization 3.2.1. Sensitivity analysis For the RPMCs made of RPP and WF, which were extracted from date palm trees, the effects of the reinforcement volume fraction and aspect ratio on the effective mechanical moduli and Poisson ratio were investigated based on the principles of micromechanics [33,34]. In the following analysis, the considered tensile moduli and Poisson’s ratios of RPP and wood fibers are 642.26 MPa, and 0.36 and 2170 MPa, and 0.3, respectively. The tensile modulus of RPP was experimentally measured through the tensile test. Fig. 7 shows the effect of the fiber volume fraction on the tensile modulus and Poisson’s ratio when the fiber aspect ratio equals 333. It can be seen that the tensile modulus is affected significantly as it increases by 71% when the fiber volume fraction reaches 0.3. Therefore, the beneficial effect of the fiber volume fraction is evident and is very promising from the stress reduction and high performance point of view. On the other hand, Poisson’s ratio decreases with the fiber volume fraction; therefore, the transverse strain becomes smaller when the longitudinal strain remains unchanged, and consequently, the overall deformations of structures composed of these composites generally decrease. The shear modulus, which is a measure of a material’s resistance to different shear loading conditions, is depicted in Fig. 8. As seen in the figure, the shear modulus increases by 53% when the fiber volume fraction reaches 0.3. The
Fig. 9. Variation of Young’s and shear moduli with the aspect ratio when the fiber volume fraction is 0.3.
general behaviors of increasing both the tensile and shear moduli with the fibre volume fraction (Fig.7 and 8) agree well with the results in [35,36] when the fibre is stiffer than the matrix. That is because the stiff fibres provide a reinforcing effect to the matrix and therefore the resistance to deformation increases. The effects of the fiber aspect ratio on the tensile and shear moduli are illustrated in Fig. 9. The tensile modulus remains nearly constant when the aspect ratio exceeds 30, whereas the shear modulus remains nearly unchanged beyond an aspect ratio of 20. The presented effect of the aspect ratio on the tensile modulus is consistent with the results in [37,38]. There is a critical fibre length at which the stress is efficiently transferred from the matrix to the fibre, resulting in a stronger composite. It can be concluded, based on this figure, that the recommended aspect ratio is close to 30 when a part, composed of these composites, is subjected to a normal load, however under a shear load, prolate fibers with an aspect
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Fig. 10. Stress variation for different materials.
379
Fig. 11. Strain variation for different materials.
ratio less than 2 are recommended. For a combined load (normal and shear) the optimum aspect ratio lies between 1 and 30, the exact value of which depends on the ratio between these two loads. Overall, these results show that both the RPP’s ability to sustain different loading conditions and its resistance to deformation are enhanced by adding wood fibers. Therefore, these composites can be used over a wide range of suitable applications in industry. 3.2.2. Practical example The aim of this part of the study was to investigate the mechanical behavior of RPMCs and their mixtures under pressure loading as one of the loading conditions in many engineering applications. Therefore, a pipe as a common final product in industry, subjected to internal pressure was elastically analyzed using the finite element method. The considered inner diameter, wall thickness, and internal pressure of the pipe were 50 mm, 3 mm and 0.5 MPa, respectively. The concept of functionally graded materials (FGMs) was applied by considering a pipe composed of three layers, which have the same thickness, with different properties. The FGM constituents were RPP, GFRPP and WGFRPP. Three FGMs were considered to demonstrate the effects on stress, strain and displacement when using different FGMs: FGM1 (RPP–WGFRPP–GFRPP), FGM2 (WGFRPP–RPP–GFRPP) and FGM3 (GFRPP–WGFRPP–RPP). The components of each FGM are listed in order of distance from the outer layer. For the finite element analyses, the general purpose commercial software ABAQUS version 6.10 was used and the numbers of elements and nodes of a quarter model were 6000 and 6241, respectively. Referring to Fig. 2 that shows the experimental stress–strain relations obtained from the tensile tests performed on the above mentioned constituent materials and considering a safety factor of 2.5, the allowable stress limits (ra) of RPP, WGFRPP and GFRPP are 5.9, 7.84 and 9.33 MPa respectively. Also, the corresponding allowable strain (ea) limits are 0.016, 0.013 and 0.01, respectively. At these values, the elastic analyses are valid and consistent with the behaviors illustrated in Fig. 2. Because fracture can be defined by not only stress but also strain and displacement, the presented results include the von Mises stress (rv), which can be considered as a measure of failure, the maximum principal strain (e1), and the magnitude of the displacement (u). Fig. 10 shows the variation in the stress ratio (ra/rv) as a measure of safety in the radial direction where the non-dimensional radial position n(r ri)/(ro ri) of the horizontal axis is a function of the number of layers n, which considered to be 3, and the inner and outer radii, ri and ro, respectively. It can be seen that the stress ratio of RPP is the lowest. If the pipe is composed of more than one layer, the stress distributions show sharp changes at the interfaces, due to the properties mismatch of the layers, as shown in the figure. This behavior is consistent
Fig. 12. Displacement variation for different materials.
with the basic principles of the mechanics of materials and solid mechanics [39,40]. When the pipe is composed of any of the three FGMs, the safety is generally higher than that of RPP because the stress ratio becomes high. It can be seen that the increases in safety at the outer and inner surfaces of FGM1 relative to RPP are 70% and 18%, respectively. On the other hand, the maximum and minimum increases in safety for FGM2 are 71% and 16% and those for FGM3 are 63% and 13%, respectively. Thus, the beneficial effects of using FGMs are evident and very promising from a stress reduction and high performance point of view. Fig. 11 shows the variation in the strain ratio (ea/e1) in the radial direction. Again RPP has the lowest safety because it has the lowest strain ratio whereas the FGMs show better performance and higher safety. The maximum and minimum increases in safety for FGM1 relative to RPP are 70% and 7%, while those for FGM2 and FGM3 are 64% and, 2.5% and 68% and, 6%, respectively. It is confirmed from these figures that the critical stresses and strains induced in these materials, can be reduced simultaneously by using FGMs. The displacement variation in the radial direction is shown in Fig. 12. It can be seen that the displacement is generally small. The RPP pipe has the highest displacement level, and which can be reduced by using any of the FGMs. When the inner layer of the FGM is stiff (GFRPP), its resistance to deformation is high and therefore the induced displacements of FGM1 and FGM2 under internal pressure were low. However, the second layer of FGM2 (RPP) is more compliant than that of FGM1 (WGFRPP) and therefore the displacement of FGM2 was higher than that of FGM1. FGM3 exhibited the highest FGM displacement as its inner layer is compliant (RPP). It should be noted that FGM1 results in the lowest displacement level and reduces the RPP displacement by 41%.
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4. Conclusions Recycled polymer–matrix composites (RPMCs) composed of recycled polypropylene as a matrix and glass fibers (GFs) and/or wood flour as reinforcements were processed, characterized and modeled. The thermomechanical properties of binary and ternary composites are presented and discussed. For the mechanical properties, it was found that adding GFs to RPP increased each of the stiffness, strength and hardness but decreased the ductility whereas adding WF as a second reinforcement has opposite effects. The tensile and shear moduli of binary composites were increased by 71% and 53%, respectively, when the fibre volume fraction reaches 0.3 while the optimum range of the fibre aspect ratio was from 1 to 30. Also, the SEM micrographs revealed that the adhesion and bonding limits between the fibres and the matrix are transgressed as fibre pull out and interfacial delamination are observed. However, the binary composite had fibre pull out less than that of the ternary composite, one possible reason is WF decreased the adhesion between the fibres and the matrix. For the thermal properties, on the other hand, GFs and WF had opposite effects on the crystallinity as GFs act as nucleating agents and increased the crystallinity but WF hinders the nucleation and stops the growth of crystals. Also, the binary composite had higher degradation temperature than that of ternary composite as GFs have high thermal stability but WF expedites degradation due to the presence of ligno cellulosic material that decomposes at a low temperature. A functionally graded layered-pipe, composed of RPMCs and subjected to internal pressure loading was numerically modeled using the finite element analysis. Different arrangements of the layers were investigated to quantify their effects on the behavioral parameters such as stress, strain and displacement. It was found that the functionally graded materials can be applied to reduce the values of these behavioral parameters and therefore increase the operational safety. The safety based stress, strain and displacement can be upto 71%, 70% and 41% respectively depending on the arrangements of the layers. Acknowledgements The authors would like to gratefully acknowledge both Qatar Science and Technology Park (QSTP) for financially supporting this project and the Center for Advanced Materials at Qatar University for completing the experimental tasks. References [1] AdrianNunez J, PabloSturm C, JoseKenny M, Aranguren MI, NormaMarcovich E, Maria Reboredo M. Mechanical characterization of polypropylene–wood flour composites J Appl Polym Sci 2003;88:1420–8. [2] Cheung Hoi-yan, Ho Meipo, Lau Kin-tak, Francisco Cardona, Hui David. Natural fibre reinforced composites for bioengineering and environmental engineering applications.. Compos Part B – Eng 2009;40(7):655–63. [3] DerekThompson W, EricHansen N, Knowles Chris, Lech Muszynski. Opportunities for wood plastic composites products in the U.S. highway construction sector. Bioresources. 2010;5(3):1336–52. [4] Ashori Alireza. Review paper. Wood plastic composites as promising green composites for automotive industries. BioResour Technol 2008;99:4661–7. [5] Wambua Paul, Ivens Jan, Ignaas Verpoest. Natural fibres: can they replace glass in fibre reinforced plastics? Compos Sci Technol 2003;63(9):1259–64. [6] Coutinho FMB, Costa THS. Performance of polypropylene–wood fiber composites. Polym Test 1999;18:581–7. [7] Esport Ana, Camacho Walker, Sigbritt Karlson. Thermal and thermo mechanical properties of bio composites made from modified recycled cellulose and recycled poly propylene. J Appl Polym Sci 2003;89:2353–60. [8] Edward Slaughter Andrew. Design and fatigue of a structural wood–plastic composite. Master thesis. Department of Civil and Environmental Engineering. Washington State University; 2004. [9] Wolcott MP. Wood–plastic composites. Encyclopaedia of Materials Science and Technology; 2001.
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