JOURNAL OF COMPOSITE M AT E R I A L S
Article
Oil palm fiber reinforced polypropylene composites: effects of fiber loading and coupling agents on mechanical, thermal, and interfacial properties
Journal of Composite Materials 46(11) 1275–1284 ! The Author(s) 2011 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021998311417647 jcm.sagepub.com
R Ramli, RM Yunus, MDH Beg and DMR Prasad
Abstract This study investigates the effects of fiber type, fiber loading, and coupling agent on the performance of oil palm biomass (OPB) fiber composites. Fiber composition and fiber morphology were evaluated by scanning electron microscope (SEM) and Energy-dispersive X-ray spectroscopy (EDAX). Grinded fiber was compounded into polypropylene by means of a twin-screw compounder. Maleated polypropylene (MAPP) was used as a coupling agent during compounding. The incorporated fiber contents for OPB composites were up to 40% (by weight). The compounded samples were prepared into test specimens by injection moulder. The composites were characterized by tensile testing, flexural testing, impact testing, melt flow index, SEM, thermogravimetric analysis (TGA), and differential scanning calorimeter (DSC). The most significant effect on strength and modulus were found by the addition of coupling agent. This was attributed to the thermodynamic segregation of the MAPP toward the interface, resulting in the formation of covalent bonding to the – OH groups of the fiber surface. Composites with MAPP also provided better thermal stability.
Keywords oil palm fiber, composites, coupling agent, interface, thermal properties
Introduction In recent years, the growing interest has focused on thermoplastic composites reinforced with lignocellulosic- and cellulosic-based materials.1–3 Lignocellulosic materials are considered new generation of reinforcing materials with thermoplastics since they are renewable natural resources. Furthermore, lignocellulosic fibers, due to the strong cellulose backbone structure, possess good strength properties and the favorable strength/ weight-ratio of the fibers is an advantage compared to other conventional reinforcing materials. The natural fiber-reinforcing thermoplastic composites provide attractive new value-added market for agricultural products. On the other hand, such composites are performing with versatile advantages, especially, reducing the consumption of petrochemical-based plastic resins and also increasing the biodegradable nature of the thermoplastic materials. Nowadays, the utilization of lignocellulosic materials in production of polymeric composites is attractive, particularly because of low cost and high volume
applications. Biodegradable lignocellulosic filler possess several advantages compared to inorganic fillers, such as lower density, greater deformability, smaller abrasiveness, high stiffness, reduced dermal and respiratory irritations, good thermal properties, enhanced energy recovery, and relatively cheap.4–9 These attractive composites are used from automotive interior components10 to geotextile.11 In addition, short fiber-reinforced polymeric composites have gained importance due to the considerable processing advantages and improvement in certain mechanical properties.12 At present, there are different types of plant-based natural fibers being used as reinforcing agents with thermoplastic resins. Among the plant-based fibers, Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Gambang 26300, Kuantan, Pahang, Malaysia Corresponding author: MDH Beg, Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Gambang 26300, Kuantan, Pahang, Malaysia Email:
[email protected]
1276 flax, hemp, jute, straw, wood, bamboo, baggase, kenaf, palm oil, betel nut, coir, sisal, and banana are most common.13–17 OPB (empty fruit bunch, front, trunk) is an industrial waste product and hence available at minimal cost. About 30 million metric ton of empty fruit bunch (EFB) produced in the world is considered as by product of agriculture material.18–20 Malaysia is one of the largest producers of palm oil. Abundance of oil palm cellulosic materials can be readily obtained from the agriculture byproducts and provides a new area for research and development. It is shown that incorporation of fillers as reinforcing materials significantly changes various properties of thermoplastics.21,22 Numerous studies carried out on other wood-based filler were also been reported by a number of researchers.23,24. This study investigates the effect of fiber loading and coupling agent on the properties of OPB/PP composites. However, the main disadvantage of natural fiber is its hydrophilic nature that lowers the compatibility with hydrophobic polymeric matrices. Moreover, it also presents poor environmental and dimensional stability that prevents a wider use of natural fiber composites.25. For instance, cellulose fibers contain many hydroxyl groups (-OH) and readily interact with water molecules by hydrogen bonding. Therefore, swelling by water uptake can lead to micro-cracking of composites and degradation of mechanical properties.2 Furthermore, without using the expensive surface barriers, it is difficult to eliminate entirely the absorption of moisture in composites.26 Likewise, good wetting of the fiber by the matrix and adequate fiber– matrix bonding can decrease the rate and amount of water absorption in the interface region of the composite.27 In this context, it is mentioned that optimization of interfacial adhesion between cellulose-based fibers and thermoplastics has been the focus of a large number of research conducted during the last two decades.28 It is commonly observed that coupling agents in wood fiber-reinforced plastic composites play an important role for improving the compatibility and adhesion between polar wood fiber and non-polar polymer matrices by forming bridges of chemical bonds between the fiber and the matrix. So far, more than forty coupling agents have been used both in production and research purposes. Among them, maleated polypropylene (MAPP) is the most popular one.29 John et al.27 studied coupling agent performance in wood-fiber-reinforced high-density-polyethylene composites and found that the improvement on the interfacial bonding strength, flexural modulus, and other mechanical properties was mainly related to the types of coupling agent, functional groups, molecular weight, concentration, and chain structure. The maximum value of interfacial adhesion was achieved with
Journal of Composite Materials 46(11) 3 wt% concentration level for most maleated composites. Beg et al. also reported 3–5 wt% coupling agent is optimum for good adhesion between fiber and matrix.30 Although a great deal of research over the past decade has demonstrated the efficiency of the use of coupling agents in wood plastic composites,31–34 the work of that extent has not been conducted for oil palm fiber in composites. Therefore, it is necessary to optimize the coupling agents for oil palm fiber in composites. Therefore, the specific objectives of the work were to study the: . Effects of different types of fiber loading obtained from different OPB . Effects of coupling agents on the mechanical properties of composites . Thermal behaviour of composites
Experimental Materials OPB (empty fruit bunches, frond, and trunk) were obtained from Biomass Section, Malaysian Palm Oil Board (MPOB). The polypropylene was homo-polymer WH 101 from Cosmoplene with a density and melt index specified as 0.90 g/cm3 and 8 g/10 min, respectively. The used coupling agent was MAPP Epolene E43 produced by Eastman Chemical Company. Molecular weight and acid number of Epolene E43 was 9100 and 45, respectively.
Methods Characterization of fiber by SEM and EDAX. Analysis of X-ray (EDX) was carried out using LEO 1455 VP SEM and for variable pressure and environmental scanning electron microscope) SEM was carried out using Philips XL30 ESEM. SEM (Leica, S360) was used to analyze the morphological images of the fractured composite materials. The tensile fractured samples were gold puttered before viewing under the microscope. The magnification and the voltage are displayed on the microphotographs of the samples.
Preparation of composites. After drying, the OPB were compounded into polypropylene by means of a Brabender DSK 42/7 twin-screw compounder having barrel temperatures from 170 C to 190 C from the feeding zone to the die zone, respectively. The OPB were oven-dried at 105 C for 24 h prior to compounding in order to achieve a moisture content of less than 5%. Four levels of loadings were prepared for all
Ramli et al. composites. The incorporated fiber contents for OPB composites were 10, 20, 30, and 40% (by weight). The mixture was then extruded and pelletized. The compounded samples were prepared into test specimens by injection moulder using a 20-ton Battenfeld BA 200CD Plus machine, with a UNILOG 4000 control system (closed-loop control). A mould from Master mold Inc., having cavities for tensile specimens according to ASTM D638 Type 1 and rectangular bar, 125 mm 12.5 mm 3.13 mm, was used for production of test specimens.
Tensile tests Tensile tests were carried out using an Instron machine with a load cell of 5 kN. Tests were performed as specified in ASTM D 638 – Type I: Test method for tensile properties of plastic. The gauge length was 50 mm and the crosshead speed of testing was 50 mm/min. Five specimens were tested for each batch. Tensile modulus (TM) and tensile strength (TS) were taken for analysis.
Flexural tests The flexural test was conducted according to ASTM D790-86: Test Method 1, Procedure A, that is threepoint loading system utilizing centre loading, using the Lloyd machine. The support span was 50 mm, the diameters of the loading nose and supports were 20 mm and 10 mm, respectively. Tests were run with the speed of 2 mm/min with 100 N load cell.
1277 attain the isothermal temperature. Each specimen weighed 10 mg (1 mg), with a scanning temperature range of 25 C–600 C.
Results and discussion Characterization of oil palm fiber using SEM and EDAX The OPB-fibers used in the study were mostly in fiber bundles of 10 to 50 individual fibers (see Figure 1). OPB-fibers were in irregular shape, with an uneven porous surface. This appearance was believed to aid in the mechanical interlocking of the fibers and the plastic matrix when composites are manufactured from them. From the SEM micrographs, it can also be observed that the gummy polysaccharides of lignin, pectin, and hemicellulose are localized on the surfaces of the fibers. However, the fibers of frond and trunk appeared to have clean but rough surfaces containing large numbers of etched striations. The rough surface morphologies of these fibers are expected to assist with mechanical interlocking bonding mechanisms when used in composites, and the clean surface expected to ameliorate the bonding between the fibers and the matrix material. The chemical composition of OPB (Table 1) did not vary greatly with other lignocellulosic fibers, cellulose, hemicelluloses, and lignin as the major components,35 which was determined from the EDAX. Five points per sample was shown in Figure 1.
Characteristics of oil palm fiber using TGA Izod impact tests Izod impact test was performed on a Ceast 6456 Izod pendulum impact tester. Notching (45o) was produced on the impact specimens using Davanport notch cutting apparatus. The test was conducted based on ASTM D256-88.
Melt flow index Melt flow index (MFI) of each sample was determined according to ASTM D1238 (230 C at 2.16Kg) using Zwick D7900 melt flow indexer. An average of three runs taken for each sample.
Thermogravimetric analysis The thermogravimetric analysis (TGA) was carried out by SDT 2960 Simultaneous DTA-TGA analyzer. Measurements were taken while maintaining a static air flow of 150 mL/min with a constant heating rate of 20 C/min in an open alumina crucible in order to
Fiber specimens were thermogravimetrically analyzed in a temperature interval of 25 C to 600 C under nitrogen purge and the heating rate was 20 C/min in order to attain the isothermal temperature. Typical DTA/ TGA traces for fiber (oil palm EFB) is shown in Figure 2. Two main stages of decomposition were observed for the fiber, starting with dehydration combined with emission of volatile components at a temperature of about 300 C followed by a rapid weight loss due to oxidative decomposition corresponding to formation of char as the temperature increased (Figure 2).36 The pattern was found to be similar for all the fiber (oil palm trunk, frond, and EFB). The thermal properties of the fibers are presented in Table 2. As the thermal stability of natural fibers depends on the constituents like hemicellulose and lignin, the EFB showed comparably better thermal properties compared with the OPF and OPT, which was attributed to the higher concentration of hemicellulose (29.60%) and a comparable concentration of lignin (17.84%) residues within EFB fibers.
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Journal of Composite Materials 46(11) (a)
(b)
Oil palm frond
Empty fruit bunch (EFB) (c)
Oil palm trunk
Figure 1. Micrographs of oil palm fibers (frond, trunk, empty fruit bunch [EFB]).
Thus, the order of thermal stability of the fiber was EFB > OPT > OPF.
Table 1. Energy-dispersive X-ray spectroscopy (EDAX) quantitative elemental analysis of oil palm biomass Weight (%)
Effects of fiber type on the properties of composites
Component C O Na Mg Al Si P S Cl K Ca Fe Cu
Trunk
Frond
EFB
55.05 46.23 0.12 0.08 1.89 0.40 Nil Nil 0.23 0.11 0.41 1.19 4.45
56.51 44.63 Nil Nil 4.19 1.09 Nil Nil 0.07 0.70 0.79 1.53 3.51
54.82 38.09 Nil 0.08 7.42 3.57 0.12 Nil 1.42 7.41 0.21 1.64 5.51
Figures 3and 4 show the effect of fiber types of OPBpolypropylene composite on the physical and mechanical properties. Generally, composites’ TS were found to be lower than that of matrix; however, TM, flexural strength (FS), and flexural modulus (FM) were found to be higher than that of matrix PP. It can be seen that the difference of TS of composites for different types of oil palm fiber (frond, EFB, trunk) was within the range of 4%–5%. However, significant differences were observed for TM, FS, and FM, where these values of the trunk/PP composites were higher than that of frond or EFB composites. The reason of this difference may be due to chemical and physical characteristics of different type of fibers from different portion of OPB. Both impact strength and MFI was found to be lower for composites compared to PP.
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Figure 3. Tensile properties of composites (40 wt% fiber content).
Figure 2. Thermogravimetric analysis (TGA)/TGD thermograms of oil palm empty fruit bunch (EFB). Table 2. Thermogravimetric data for the OPT, OPF and EFB Formulation
Stage
Temperature range ( C)
Tmax
Empty fruit bunch (EFB)
1st 2nd 1st 2nd 1st 2nd
206-324 332-399 201-316 330-394 199-312 330-390
301 370 296 366 291 364
Oil palm trunk (OPT) Oil palm frond (OPF)
Residue (%) 16.47 17.56 17.96
Effects of fiber loading and coupling agents on the properties of EFB-PP composites To study of the effects of fiber loading and coupling agents, EFB fiber (with random fiber length of 0.5 mm-2 mm) was selected as a reinforcing agent. Figures 5–8 and Table 3 presents the properties of the EFB/PP composites with different fiber loading. The effect of fiber loading on the properties of the composites was significant for all of the evaluated properties. Modulus increased gradually as the fiber loading was increased for both FM and TM, and which is likely to be due to fibers with higher stiffness than the matrix.35 It was noted that the most prominent effect of fibers is the increase of modulus of the resultant composites. TS and MFI was decreased with increasing of fiber content, while FS and impact strength increase with increasing of fiber content. Results of this study when compared with some researchers’12 showed that the TSs of natural fiber/polyolefin composites decreases with an increase of fiber content, while results of other workers showed a reverse trend.37 The decrease in the TS in this study could be attributed to the weak interfacial adhesion or interaction between the PP matrix and the EFB fibers. Poor interfacial bonding between the fibers and the matrix cause inefficiency of stress transfer when stress is applied on a tensile specimen. These interfaces
Figure 4. Impact strength and melt flow index of composites (40 wt% fiber content).
are the weakest part of the composite and serve as failure part of the composite and as failure initiation points. Moreover, fibers with uniform circular cross section and a certain aspect ratio normally improved the strength. However, the capacity of irregularly shaped fibers like EFB fibers with shorter fiber length used in the study may not be able to support stresses transferred from the polymer matrix, thus there was a reduction of TS as the fiber loading was increased.35 Addition of coupling agents increased TS, TM, FS, and FM of composites; however, maximum improvement of TS and FS was found for the addition of 3 wt% MAPP with 30% fiber content. At highest fiber content, fiber agglomeration might occur, which results in reduction of strength at 40% fiber loading. FS and FM were found to be increased for 40 wt% fiber content. The improvement of mechanical properties by the addition of MAPP was likely to be due to the better interfacial bonding between the fiber and the matrix. Coupling agents did not show any dramatic effects on impact strength; however, there was a slight increase
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Figure 5. Tensile strength of composites with (10%–40%) wt fiber content and (0%–4%) wt coupling agent.
Journal of Composite Materials 46(11)
Figure 7. Flexural strength of composites with (10%–40%) wt fiber content and (0%–4%) wt coupling agent.
Figure 6. Tensile modulus of composites with (10%–40%) wt fiber content and (0%–4%) wt coupling agent.
with increasing coupling agent. Fiber weight fraction also increased impact strength (IS) and 20 wt% fibers provided the maximum IS. MFI of composites was found to be increased with increasing coupling agent content but decreased with increasing the fiber loading, which is shown in Table 3.6,38,39
Effects of coupling agent and fiber loading on the interfacial properties of composites The surface morphology of virgin polymer was found to be smooth and crack-free, shown in Figure 9(a). It was also observed that certain protrusions are
Figure 8. Flexural modulus of composites with (10%–40%) wt fiber content and (0%–4%) wt coupling agent.
present on the surface of composite materials. Some holes were observed in Figure 9(b) and (c) that indicated the occurrence of fiber pull-out. This indirectly implies that there was a poor filler-matrix interaction between untreated fiber and matrix. It can also be seen that from the micrograph there was an occurrence of feathering, which indicated the ductility of PP matrix. However, these protrusions were lesser in case of composites with 30 wt% of loading of the filler. Whereas the
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Table 3. Impact strength (kJ/m) and melt flow index (MFI; g/10 min) Coupling agent (%) 0 2 3 4
Impact strength (J/m) Fiber loading %
MFI (g/10 min) Fiber loading as %
10
20
30
40
10
20
30
40
37.98 38.40 38.98 39.04
43.77 43.00 44.50 44.20
41.14 41.23 42.16 41.70
41.14 41.50 42.20 42.60
4.90 4.43 5.34 5.14
3.37 3.94 3.61 3.83
3.50 3.41 3.18 3.26
2.27 2.30 2.46 3.04
(a)
(b)
PP surface
Fracture surface of 30 wt% fibre composites without coupling agent
(c)
Hole
Fracture surface of 40 wt% fibre composites without coupling agent
Figure 9. Scanning electron micrographs of PP (a) and composites (b and c).
40 wt% fiber composites showed undulating and cracked topology. Thus, 30 wt% loading of the filler appeared to be the critical loading percentage of the filler into the matrix material on account of better dispersion and interfacial adhesion. In addition to pull-out of fiber, fiber breakage was also been seen in the sample (Figure 9(b)), at a lesser extent than the former. Due to hydrogen bonding between fibers and the wide difference in polarity between untreated lignocellulosic fibers and the matrix, the fibers tend to agglomerate into bundles and become unevenly
distributed throughout the matrix. It is obvious that fiber pull-out was one of the main modes of failure. Fibers were shown to be oriented in a random fashion. The existence of voids as shown in Figure 9(b) and (c) may created the stress concentration points which is in turn reduce the strength of the samples. It can also be seen that the surface of composite materials with 30 wt% filler percentage was relatively smooth compared to other PP composites, which indicated the former was less ductile than the latter.
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(a)
Fibre Break Fibre Break
Fracture surface of 30 wt% fibre composites with 3% MAPP
Fracture surface of 40 wt% fibre composites with 3% MAPP
Figure 10. Scanning electron micrographs of composite fracture surface.
Figure 11. TGD thermogram of PP and composites (with 40 wt% empty fruit bunch [EFB]).
Thus, SEM examination of fractured surfaces of the PP-based fiber composites (without coupling agent) revealed the poor interfacial bonding. Micrographs indicated fiber pull-out, deboning, delimination, and fiber breakage. At high fiber loading, fiber-to-fiber contact was greater and dispersion problems were evident. Figure 10 shows the fracture surface of composites with coupling agent. The fibers did not agglomerate when MAPP was used as a coupling agent. It could also be observed that the wood fibers were dispersed within the matrix. Fiber distribution seems to be more uniform in the PP matrix with MAPP. As compared to Figure 10(a), the reinforcing fibers appeared to be impregnated uniformly due to the presence of 3 wt% MAPP with PP, which should have increased the reinforcing fibers–matrix compatibility. Overall, this would affect the mechanical and thermal performance of the substrate, where the fibers will potentially be used. It could also be seen from Figure 10(a) and (b) that the polypropylene adhered to most fiber surfaces, as a result of the presence of MAPP. Composite fracture was thus thought to have occurred by means of fiber fracture and shear failure of the polypropylene matrix.
Figure 12. Thermogravimetric analysis (TGA) thermogram of PP and composites (with 40 wt% empty fruit bunch [EFB]).
It was also observed that the reinforcing fibers in both composites were well dispersed throughout the matrix and separated from their fiber bundles.
Effects of fiber loading and coupling agents on thermal properties of composites DTA and TGA curves of PP and the composites are shown in Figures 11 and 12 and the properties are tabulated in Table 4. It can be seen from the figure that PP showed single major decomposition peak from 267– 471 C, with a maximum peak (Tmax) at 420 C, while composites showed two major peaks. With the addition of fiber, the final decomposition temperature increased from 420 C for PP to 464 C for composites with 10 wt% EFB. Further increment was observed on further addition of fiber (from 464 C for 10 wt% fiber composites to 470 C for 40 wt% fiber composites). The increasing of decomposition temperature was attributed to the hindered diffusion (i.e., barrier
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Table 4. Thermal properties of composites Formulation
Stage
Temperature range ( C)
Tmax
PP 10% EFB þ PP
1st 1st 2nd 1st 2nd 1st 2nd
267-472 210-351 377-490 214-361 389-496 223-371 392-499
420 325 464 331 470 339 472
40% EFB þ PP PP þ 40% EFB þ MAPP
Residue (%) 0.012 2.8 5.78
fibers and which occurred in the composite matrix as a result of extrusion and injection molding. 4. SEM analysis of composite fracture surfaces showed that composites with MAPP showed minimum fiber debonding and tensile failure of the matrix material. Composites containing 3 wt% MAPP showed excellent morphology comparatively mostly by better penetration of fibers within the matrix and showed less tensile failure of the fibers or fiber debonding.
5.47
References effect) of volatile decomposition products caused by the dispersion of fibers in the PP matrix.40 It can be seen from Table 4 that composites with coupling agent start to lose weight at higher temperatures when compared to composites without coupling agent. This apparent improvement in thermal stability of the treated fiber composites could be attributed to the presence of the MAPP coupling agent. MAPP is able to bond with the hemicellulose in the fiber, thus stabilizing its structure and improving the thermal stability of the composite. Several authors also reported the thermal stabilization of noncellulosic fiber constituents by means of a coupling agent.41 The poor thermal stability of hemicelluloses and pectins in a composite can thus be negated by the inclusion of a coupling agent. This indicates the complex interactions/reactions occurred between the fiber constituents, the MAPP coupling agent, and the polypropylene matrix of each composites.
Conclusion 1. In this investigation, it was found that an injectionmolded oil palm fiber-reinforced polypropylene composite containing 30 wt% fiber provided the best combination of TS, Young’s modulus, and ease of processing. 2. TGA and DTA analysis showed that oil palm fiber composites were less thermally stable than the polypropylene matrix alone; this was thought to be due to polymer chain breakages that occur in the composite matrix as a result of extrusion and injection molding. The thermal stabilities of composites containing 30% fiber loading showed enhancement as compared to the virgin polymer and other composite materials. 3. TGA analysis showed that treated oil palm fiberbased composites, and with a matrix of 3% MAPP and polypropylene, was thermally more stable than the polypropylene matrix alone; this was thought to be due to the better polymer chain adhesion with the
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