Abstract
The effect of filler modification using methacrylic acid (MAA) on polypropylene (PP)/cocoa pod husk (CPH) composites was studied. The performances of unmodified and modified PP/CPH composites were analyzed for torque development, tensile properties, and thermal properties. The presence of MAA increased the stabilization torque of the PP/CPH composites. The tensile strength and modulus of the modified PP/CPH composites were improved compared to unmodified PP/CPH composites, but the elongation at break was reduced. The crystallinity and thermal stability of the PP/CPH composites increased after modification with MAA. All the composite property changes were due to the improvement in filler-matrix adhesion and this was confirmed by scanning electron microscopy (SEM).
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Modified Cocoa Pod Husk-Filled Polypropylene Composites by Using Methacrylic Acid
Koay Seong Chun, Salmah Husseinsyah,* and Hakimah Osman
The effect of filler modification using methacrylic acid (MAA) on polypropylene (PP)/cocoa pod husk (CPH) composites was studied. The performances of unmodified and modified PP/CPH composites were analyzed for torque development, tensile properties, and thermal properties. The presence of MAA increased the stabilization torque of the PP/CPH composites. The tensile strength and modulus of the modified PP/CPH composites were improved compared to unmodified PP/CPH composites, but the elongation at break was reduced. The crystallinity and thermal stability of the PP/CPH composites increased after modification with MAA. All the composite property changes were due to the improvement in filler-matrix adhesion and this was confirmed by scanning electron microscopy (SEM).
Keywords: Cocoa Pod Husk; Polypropylene; Composites; Methacrylic Acid
Contact information: Division of Polymer Engineering, School of Materials Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia; *Corresponding author: irsalmah@unimap.edu.my
INTRODUCTION
Natural filler-based composite materials, which are often called biocomposites, have garnered interest among researchers and industries due to today’s ecological problems and economic factors including the accumulation of agricultural waste and cost of products. Nowadays, there are numerous green and eco-friendly products made from natural filler and thermoplastic materials that have been successfully produced and marketed. Recently, natural filler has become a popular choice of filler in thermoplastic materials. This is due to the fact that natural fillers exhibit some excellent properties compared to conventional fillers. Natural fillers are inexpensive, are obtained from renewable resources, present a minimal health hazard, have low density, are less abrasive to processing machinery, and are eco-friendly (Chun et al. 2012; Salmah et al. 2012a; Chun et al. 2013b). The most well-known example of such a material is IKEA injection-moulded furniture, which is produced from polypropylene (PP)/wood flour biocom-posites (Niskanen 2011). In Malaysia, Melsom Biodegradable Enterprise has produced a series of eco-friendly tableware from rice husk-filled thermoplastic biocomposites (Chun et al. 2013a).
Many natural fillers found in Malaysia are obtained from crop residues and by-products of the agricultural industry, such as the coconut shell (Chun et al. 2012; Salmah et al.2012a; Chun et al. 2013a,b), palm kernel shell (Salmah et al. 2013; Salmah et al. 2012b), oil palm empty fruit bunch (Hassan et al. 2010), corn cob (Chun and Husseinsyah 2013; Yeng et al. 2013), and rice husk (Premala Hattotuwa et al. 2003). Cocoa (Theobroma cacao) is an important agricultural crop in several tropical countries, including Malaysia (Lucia et al. 2012). Cocoa pod husk (CPH) is a non-food part of the cocoa pod, and it usually accounts for 52 to 76% of the cocoa pod wet weight.
In the cocoa industry, every ton of dry cocoa bean produced will generate ten tons of cocoa pod husks as waste (Lucia et al. 2001). The CPH is readily abundant but does not have any marketable value; therefore, the utilizations of CPH as natural filler in thermo-plastic materials will provide a new application route for CPH into useful resources for the thermoplastic industry. Meanwhile, the utilization of CPH can bring economic benefit and reduce the ecological impact.
In general, compounding natural filler in thermoplastic materials would not produce good composite properties due to the weak interfacial adhesion between the hydrophilic natural filler and the hydrophobic matrix. Filler modification is one of the methods used to modify the hydrophilic properties of natural filler. Thus, the filler disper-sion, wettability, and filler-matrix interaction can be enhanced via filler modification. In previous studies it was reported that natural filler modified with silane coupling agent (Chun et al. 2012; Xie et al. 2010), acrylic acid (Salmah et al. 2012a), maleic acid (Chun et al. 2013b), sodium dodecyl sulphate (Chun et al.2013a), and modified fatty acid (Chun and Husseinsyah 2013) significantly improved the properties of the composite. In the present work, PP/CPH composites were developed and methacrylic acid (MAA) was used to modify CPH to enhance the properties of the PP/CPH composites. The effects of MAA on the torque development, tensile properties, thermal properties, and morphology of the PP/CPH composites were studied.
EXPERIMENTAL
Materials
Cocoa pod husk (CPH) was collected from cocoa plantations, Perak. The CPH was first dried in an oven at 80 oC for 24 h. The dried CPH was crushed into small pieces and further ground into fine powder. The CPH powder was sieved, and the average particle size of the CPH powder was 22 µm, measured using a Malvern Particle Size Analyzer Instrument.
Polypropylene (PP) type co-polymer, grade SM 340 was used as the matrix and was supplied by Titan Petchem (M) Sdn. Bhd. Methacrylic acid (MAA) and ethanol were obtained from Sigma Aldrich, Penang.
Table 1. Formulation of PP/CPH Composites
Filler Modification
The MAA solution was prepared by dissolving 3% MAA into ethanol. CPH was added slowly into MAA solution and stirred continuously for 1 h. The CPH was soaked in MAA solution and left overnight (12 h). The soaked CPH was filtered and dried in an oven at 80 oC for 24 h.
Melt Compounding and Moulding Procedures
The unmodified and modified PP/CPH composites listed in Table 1 were compounded using a Brabender® Plastrograph intermixer, Model EC PLUS in counter-rotating mode at 180 oC and a rotor speed of 50 rpm. The mixing procedures involved were as follows: i) the PP was transferred into the mixing chamber for 3 min until it melted homogeneously; ii) the unmodified or modified CPH was added to the melted PP and continuously mixed for 5 min. The total time for compounding was 8 min. All the compounds were moulded into 1 mm-thick sheets using a hotpress, model GT 7014A at 180 oC. The compression sequences involved were as follows: i) preheat the compound for 4 min; ii) compress the compound under a pressure of 100 kgf/cm2 for 1 min; iii) cooling under the same pressure for 5 min. The PP/CPH composite sheets were cut into tensile bars using a dumbbell cutter with dimensions according to ASTM D638 type IV.
Processing Torque Measurement
The processing torque was measured during the compounding of the PP/CPH composites using a Brabender® Plastrograph internal mixer. The torque changes of the composites with time were recorded and the torque versus time curves were plotted. The torque values at the end of processing time were taken as stabilization torque.
Tensile Testing
Tensile testing was carried out using an Instron Universal Testing Machine, model 5569. The load cell selected was 50 kN and the cross-head speed was 30 mm/min. The test was performed at 25 ± 3 oC.
Morphology Analysis
The tensile fracture surfaces of the PP/CPH composites were analysed using a scanning electron microscope (SEM), model JEOL JSM-6460 LA. The samples were coated with a thin layer of palladium for conductive purposes and analysed at 5 keV.
Fourier Transmission Infra-Red (FTIR) Spectroscopy
PerkinElmer Paragon 1000 FTIR spectrometer was used to characterize chemical groups in pure MAA, virgin PP, unmodified and modified CPH, and PP/CPH composites. The Attenuated Total Reflectance (ATR) method was selected. The sample was recorded with 4 scans in the frequency range 4000-600 cm-1 with a resolution of 4 cm-1.
Differential Scanning Calorimetry (DSC) Analysis
DSC analysis was evaluated using a DSC Q10, Research Instrument. The samples were cut into small pieces and placed into a closed aluminum pan with sample weights in the range of 7 ± 2 mg. The specimens were heated from 30 oC to 200 oC with a heating rate of 10 oC/min under a nitrogen atmosphere. The nitrogen gas flow rate was 50 mL/min. The degree of crystallinity of the composites (Xc) can be evaluated from DSC data using following equation,
Xc = (ΔHf / ΔHf0) x 100 (1)
where ΔHf is the heat of fusion of the PP/CPH composites, and ΔHf0 is the heat of fusion for 100% crystalline PP (ΔH100 = 209 J/g).
Thermogravimetric Analysis (TGA)
TGA analysis was carried out using a TGA Pyris Diamond PerkinElmer apparatus. The samples were about 7 ± 2 mg in weight and were placed into a platinum crucible. The samples were then heated from 30 oC to 700 oC at a heating rate of 10 oC/min under a nitrogen atmosphere with a nitrogen flow rate of 50 mL/min.
RESULTS AND DISCUSSION
Torque Development
The torque-time curves of the neat PP, unmodified, and modified PP/CPH composites with different CPH contents are shown in Fig. 1. Generally, once the PP pellets were transferred into the mixing chamber, the torque rose rapidly due to the resistance exerted on the rotors by unmelt PP pellets. The torque decreased gradually with time as the PP pellets melted, which subjected them to the shearing reaction at high temperature. The torque in the PP/CPH composites increased again just after 3 min. This was due to the addition of CPH into the melted PP. The torque gradually decreased and achieved stabilization torque as the PP and CPH were compounded homogenously. Similar trends of torque development have also been reported by many researchers (Shaari Balakrishana et al. 2012; Osman et al. 2012).
Fig. 1. The torque-time curves of neat PP and unmodified and modified PP/CPH composites
Figure 2 illustrates the stabilization torque vs. filler content for the unmodified and modified PP/CPH composites. As shown, both PP/CPH composites’ stabilization torque increased with increasing CPH content. This result indicated that the dispersed CPH particles in the melted PP hindered the polymer chain mobility. The disperse resistance from the CPH led to an increase in stabilization torque at a higher CPH content. Meanwhile, the modified PP/CPH biocomposites had a higher stabilization torque than the unmodified PP/CPH biocomposites. This is attributed to the modified CPH with MAA having better dispersion and filler-matrix interactions compared to unmodified CPH in the PP matrix; therefore, modified CPH yields a higher viscosity of PP/CPH biocomposites.
Fig. 2. The stabilization torque of unmodified and modified PP/CPH composites
Tensile Properties
Figure 3 illustrates the effect of CPH content and MAA modification on the tensile strength of thePP/CPH composites. The increase in CPH content reduced the tensile strength of the unmodified and modified PP/CPH composites. This was a common observation for the particular natural filler-containing thermoplastic composites; similar results were also found in previous studies (Chun et al. 2013a; Salmah et al. 2012c; Chun and Husseinsyah 2013). This particular form of natural filler usually has a low aspect ratio, and its ability to transfer stress from the matrix was poor; therefore, the addition of CPH decreased the tensile strength of the PP matrix.
Another reason for the decrease in tensile strength was poor wettability between the hydrophilic CPH and the hydrophobic PP matrix. The poor wettability contributed to poor filler dispersion and poor interfacial bonding. The poor interfacial adhesion between the filler and the matrix caused poor stress transfer and also allowed initial crack propagation. This finding is also supported by the presence of filler agglomeration. The modified PP/CPH composites, however, showed higher tensile strength than the unmodified PP/CPH composites.
The MAA modification increased the filler-matrix interaction by reacting with hydroxyl groups on the CPH surface via esterification. As a result, the modified CPH had better wettability in thePP matrix, which improved the filler dispersion and filler-matrix adhesion. Chun et al. (2013b) and Salmah et al. (2012a) reported that modifying coconut shell powder with maleic acid and acrylic acid improved the tensile strength of the resulting composites.
Fig. 3. Effect of filler content on tensile strength of unmodified and modified PP/CPH composites
The elongation at break of the unmodified and modified PP/CPH composites is shown in Fig. 4. The results indicate that the elongation at break of both composites had decreasing trends as the CPH content increased. The decrease in elongation at break was probably caused by the presence of rigid CPH particles, which inhibit the PP chain mobility, resulting in more brittle composites. This is a general trend that has also been reported by other researchers (Shaari Balakrishana et al.2012; Osman et al. 2012; Salmah and Ismail 2008). Nevertheless, modified PP/CPH with MAA showed lower elongation at break values compared to unmodified PP/CPH composites. The addition of MAA chemically altered the CPH surface, making it more hydrophobic, leading to enhanced filler-matrix interfacial bonding; therefore, the ductility of the PP/CPH compo-sites was reduced by the enhanced interfacial bonding. Salmah et al. (2011a) also found a similar effect of modified chitosan with acrylic acid on the elongation at break of PP/chitosan composites.
Contrary to the negative effect on the elongation at break, the incorporation of unmodified and modified CPH yielded PP/CPH composites with an increase in tensile modulus (as shown in Fig. 5). It is possible that the friction between the CPH particles and the PP matrix generated a rigid interface, which induced the flexibility of the PP matrix. This led to more rigid and stiffer composites. A similar observation was also reported by Faisal et al. (2013). Consequently, modification of PP/CPH composites with MAA increased the tensile modulus. It can be seen that modified CPH had better filler dispersion and interfacial interaction with the PP matrix. The improvement in filler dispersion increased the surface area of the filler-matrix interaction and it, along with the enhanced interfacial bonding, yielded a stiffening effect on the PP/CPH composites. Additionally, the MAA modification also enhanced the nucleating effect of CPH on the PP matrix and it increased the stiffness of the composites as the crystallinity increased. According to Farsi (2010), the tensile modulus of PP/wood flour composites was improved by acrylic acid treatment; the presence of acrylic acid increased the bonding strength between the wood flour and the PP matrix.
Fig. 4. Effect of filler content on elongation at break of unmodified and modified PP/CPH composites
Fig. 5. Effect of filler content on tensile modulus of unmodified and modified PP/CPH composites
Morphology Study
SEM micrographs of the tensile fracture surface of the unmodified PP/CPH composites at 20 and 40 php of CPH content are displayed in Figs. 6 (a) and (b). The SEM micrographs show that the CPH particles were poorly dispersed in the PP matrix and the poor interfacial adhesion between the CPH particle and the PP matrix. This observation was demonstrated by the presence of holes due to the filler pull-out and detached CPH particles. In contrast, modified PP/CPH composites exhibit a more brittle fracture surface compared to unmodified PP/CPH composites (as shown in Figs. 6 (c) and (d)). Other than that, the modified CPH particles were well dispersed and embedded in the PP matrix. This indicates that modified CPH has better wettability with the hydrophobic PP matrix. The less filler pull-out evidenced the better filler-matrix adhesion between the modified CPH and the PP matrix.