Article
Torque rheological properties of polypropylene/cocoa pod husk composites
Journal of Thermoplastic Composite Materials 1–11 ª The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0892705715618743 jtc.sagepub.com
Koay Seong Chun, Salmah Husseinsyah and Chan Ming Yeng
Abstract The torque rheological properties of plastic wood composites are important to practical processing, but the research in this field is rare. In present, a Brabender plastrograph torque rheometer was used to analyse the rheological behavior of polypropylene (PP)/cocoa pod husk (CPH) composites. The effect of processing parameter, filler content, and addition of maleated polypropylene (MAPP) on torque rheological properties was investigated. The torque rheological data found that the processing torque increased with the increases of rotor speed, filler content, and addition of MAPP. The PP/CPH composites melt behavior as pseudoplastics and shear thinning occurred at higher shear rate. The decrease of power law index (n) evidenced the pseudoplasticity of PP/CPH composites increased at higher filler content and presence of MAPP. The increase of viscosity on PP/CPH was due to filler–filler interaction at higher filler content and strong filler–matrix adhesion after addition of MAPP. The activation energy of PP/CPH composites also increased with higher amount of CPH and addition of MAPP. Keywords Cocoa pod husk, polypropylene, composites, maleated polypropylene, torque rheology
Division of Polymer Engineering, School of Materials Engineering, Universiti Malaysia Perlis, Jejawi, Perlis, Malaysia Corresponding author: Koay Seong Chun, Division of Polymer Engineering, School of Materials Engineering, Universiti Malaysia Perlis, Jejawi, Perlis 02600, Malaysia. Email:
[email protected]
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Introduction In the past decade, wood plastic composites (WPC) made from agro-waste and thermoplastic have attracted industries and researchers interest due to their economic advantage, ecological awareness, and accumulation of agro-waste materials.1–5 Nowadays, the WPCs are enthusiastically used in building applications (e.g. deck and wooden fittings), consumer products (e.g. packaging tray and utensils), and automotive parts (e.g. door panel).2,4–7 Certainly, the filler–matrix adhesion is the main factor influencing the properties of thermoplastic composites containing agro-waste filler. This is because the agro-waste filler mainly contains cellulose, hemicellulose and lignin, which is hydrophilic in nature and it is incompatible with most thermoplastic materials.8–11 Therefore, various types of filler treatments and coupling agents have been reported in literature for improving the interfacial adhesion between agro-waste filler and thermoplastic matrix, such as used of maleated polymer,6,11,12–15 alkaline treatment,16,17 silane-based coupling agent,18–20 acid treatment,21,22,23 fatty acid, and its derivatives.4,5,8,19 Many of published information on WPC were related to how filler treatment and coupling agents enhancing the interfacial adhesion and improve the physical-mechanical properties (such as strength, thermal properties, and water absorption) of the composites.2,9,10,17,24 In addition, the rheological properties are also an important factor in processing quality of WPC.24 The processing of WPC is more difficult compared to virgin plastic resin, because of the WPC contains large amount of natural filler.25–27 The rheological properties of WPC are strongly influenced by type of filler, size of filler, quantity of filler, and the filler–matrix adhesion. Therefore, the understanding on how the filler and coupling agent influence the rheological properties of WPC is crucial in process optimization, trouble shooting, and equipment design.28,29 Unfortunately, the literature studies in the field on rheological properties of agro-waste based thermoplastic composites are scarce. Torque rheometer is one of the important equipment used to measure the rheological behavior of plastic material in an actual processing condition. Currently, there are only few companies made torque rheometer on the market, including Haake Buchler and Brabender.30 The torque rheometer provide a quantitative information on the rheological properties of plastics, changes of polymer chains during processing, and the effect of different additives on the processability of new formulations.31 In our previous work, an agro-waste-based thermoplastic composites have been developed from cocoa pod husk (CPH) and polypropylene (PP).12,19,22 However, research on torque rheological properties of PP/CPH composites has not found in any literature study. Thus, the investigation on rheological behavior of PP/CPH composites using torque rheometer has been underway. The previous study show the addition of maleated PP (MAPP) was remarkably improved the tensile and thermal properties of PP/CPH composites. This is because the presence of MAPP formed a covalent bonding between CPH and PP matrix, which led to strong filler–matrix adhesion.12 In present work, the research is focus on the effect of processing parameter and MAPP on torque rheological properties of PP/CPH biocomposites.
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Table 1. Formulation of PP/CPH composites with and without MAPP. Materials PP/CPH composites (without MAPP) PP/CPH composites (with MAPP)
PP (phr)
CPH (phr)
MAPP (%)
100 100
0, 10, 20, 30, 40 10, 20, 30, 40
– 3a
PP: polypropylene; CPH: cocoa pod husk; MAPP: maleated polypropylene; phr: part per hundred resin a Based on weight of PP.
Table 2. Processing parameter for torque rheological analysis. Parameter Rotor speed (r/min) Processing temperature ( C) Time (min)
(i)
(ii)
40, 50, 60, 70 180 8
50 180, 190, 200, 210 8
Methodology Materials The PP type copolymer (grade SM 340) used in this experiment was obtained from Titan Petchem (M) Sdn. Bhd (Malaysia). A discarded CPH was collected from cocoa plantations at Sungai Hilir, Perak. The collected CPH was dried at 80 C using circulatory air oven for 24 h. Then, the dried CPH was crushed into small pieces and further ground into fine powder using miniature grinder (model RT-34, manufactured by Mill Powder Tech, Taiwan). The CPH powder was sieved and average particle size of CPH powder was 22 mm, which was measured by Malvern Particle Size Analyzer Instrument (UK). The MAPP used as coupling agent was supplied by Sigma Aldrich (St Louis, Missouri, USA).
Torque rheology analysis The PP/CPH composites were compounded using Brabender Plastrograph torque rheometer intermixer, Model EC PLUS (Germany) with counter-rotating mode. All compounds were prepared according to formulation in Table 1. Table 2 shows the different processing parameter for compounding of PP/CPH composites. Firstly, the PP and MAPP resin were pre-mixed and transferred into mixing chamber for 3 min until it fully melted. Then, the CPH powder was added into mixing chamber and continually mixed for 5 min. The processing characteristics (plot of torque vs. time) were recorded by the Brabender Mixer Program (WINMIX). The processing torque detected by the Brabender Plastrograph torque rheometer intermixer can be converted to rheological data by following the method suggested by Goodish & Proter.32 Referring to that method, the torque M versus speed (rpm) S at
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constant temperature was plotted on log-log scale, as the expression proposed by Babbar and Mathur28: M ¼ CS b ;
ð1Þ
where, C is a constant that depends on machine geometry and b is a constant characterizing of the polymer melt. In all cases, the plots of log M versus log S were straight lines and the slope b can be taken as equivalent to a melt flow index (n) related to any fluid that follows the power law given by: ¼ K n
ð2Þ
where, is shear stress, K is a constant, and is shear rate. The shear stress (), shear rate (), and viscosity () of compound can be determined from torque and speed data obtained from instrument by using following equation used for coaxial cylinder and modified for non-Newtonion materials: 1 ¼ 1 ¼
2=n nRm
M1 ; 2R2m h
2S1 ; 2=n 2=n Re Ri
¼
1 ; 1
ð3Þ ð4Þ ð5Þ
where, the effective instrument dimension considered for Brabender1 Plastrograph torque rheometer, Model EC PLUS were radius of inner cylinder (Ri) ¼ 1.65 cm; radius of outer cylinder (Re) ¼ 1.85 cm; average radius of cylinder (Rm) ¼ 1.75 cm; and length of cylinder (h) ¼ 4.6 cm.
Results and discussion Torque rheological properties The effect of filler content on torque as function of rotor speed of PP/CPH composites with and without MAPP was presented in Figure 1. The torque of compound was increased with the increasing of rotor speed and filler content. As the rotor speed increased, the amount of the friction and constraints on the compound also increased. Thus, the torque of the compound was increased linearly as change of rotor speed. Otherwise, the addition of solid CPH particles was hindering the flow of melted PP. The increases in amount of CPH particles increased the frictional forces between the mixing chamber surface and the CPH particles and also between CPH particles and melted PP. As the filler content increased, the CPH particles were tend to agglomerate. The presence of CPH agglomeration might also obstructed the flow of melted PP. Hence, the flowability of melted PP/CPH biocomposites decreased at higher filler content. Rahman et al.33 also discovered that the flowability of melted high-density polyethylene (HDPE)/ rice husk compound was reduced due to the present of filler agglomeration at higher
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Figure 1. Plot of log torque versus log speed of PP/CPH composites: (a) without MAPP and (b) with MAPP. PP: polypropylene; CPH: cocoa pod husk; MAPP: maleated polypropylene.
Figure 2. Plot of log shear stress versus log shear rate of PP/CPH composites: (a) without MAPP and (b) with MAPP. PP: polypropylene; CPH: cocoa pod husk; MAPP: maleated polypropylene.
filler content. Alternatively, the processing torque of PP/CPH composites increased with the addition of MAPP. As discussion in our previous study, the presence of MAPP was covalent bonded on CPH particles, and it provided a molecular chain entanglement at the interfacial region between CPH and PP matrix. The presence of strong entanglement between filler and matrix obstructed melt flow of PP matrix. Therefore, the viscosity of melted PP/CPH biocomposites increased with addition of MAPP. Li and Wolcott25 also reported that the addition of maleated polyethylene increased the viscosity of HDPE/ maple wood composites due to the presence of strong interfacial bonding between HDPE and maple wood after addition of MAPE. Figure 2 shows the effect shear rate on shear stress of PP/CPH composites with and without MAPP at various CPH content. According to power law, a higher amount of force required to produce the higher shearing action. It was obvious that shear stress increased linearly with an increase in shear rate. At constant shear rate, the shear stress of PP/CPH composites increased as function of filler content. As mention before, the flowability of melted PP/CPH composites reduced at higher filler content. The shear
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Figure 3. Power law index of PP/CPH with and without MAPP at different filler contents. PP: polypropylene; CPH: cocoa pod husk; MAPP: maleated polypropylene.
stress is increased to generate the shearing action in melted PP/CPH composites as a higher shear stress need to overcome the friction cause by filler–matrix interaction and filler–filler interaction.34 Nevertheless, the shear stress of PP/CPH composites with MAPP was higher compared to PP/CPH composites without MAPP. This due to the addition of MAPP improved the interfacial adhesion and increased the melt viscosity of composites. For this reason, a higher shear stress was required to generate a viscous flow on melted PP/CPH composites with MAPP. The power law index (n) can be determined from log shear stress versus log shear rate graph (Figure 2) using curve-fitting method. The n of PP/CPH composites with and without MAPP at different filler content were displayed in Figure 3. The values n of neat PP, PP/CPH composites with and without MAPP were less than 1. This indicates the neat PP and all PP/CPH composites melt following the non-Newtonian or pseudoplastic behavior. From the results, the n was found to decrease with the increasing of filler content, which demonstrated the pseudoplasticity increased at higher filler content. This evidenced the composites melt show more shear thinning effect at higher filler content. A similar finding also reported other researchers.24,35,36 The values n of PP/CPH composites were reduced with presence of MAPP. This indicates the pseudoplasticity of PP/CPH composites increased with presence of MAPP. Figure 4 displays the log viscosity versus log shear rate graph of PP/CPH composites with and without MAPP at different filler content. At low shear rate, the viscosity of PP/CPH composites melt increased with increasing of filler content. As discuss before, the increases of viscosity of composites was due to the friction between filler and matrix, and also the presence of filler agglomeration that restriction the flow of melted polymer. The result exhibited that the viscosity was significantly reduced as shear rate increased for neat PP and PP/CPH composites. The decrement in viscosity of PP/CPH composites at higher shear rate or filler content due to the following reasons: (i) shear thinning effect
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Figure 4. Plot of log viscocity versus log shear rate of PP/CPH composites: (a) without MAPP and (b) with MAPP. PP: polypropylene; CPH: cocoa pod husk; MAPP: maleated polypropylene.
Figure 5. Plot of log viscocity versus temperature (1/T) of PP/CPH composites: (a) without MAPP and (b) with MAPP. PP: polypropylene; CPH: cocoa pod husk; MAPP: maleated polypropylene.
on PP/CPH composites. This is due to the reduction of chain entanglement of polymer matrix at high shear rate; and (ii) the shear stress increased at higher shear rate, a higher shear stress is able overcome the filler–filler interaction. The filler agglomeration is reduced, thus the viscosity of composites is reduced. Feng et al.37 and Yang et al.38 agreed that the reduction of viscosity of composite melt with increasing of shear rate. This is due to the fact of shear thinning action occurs on composites. At constant shear rate, the viscosity of PP/CPH with MAPP was higher than PP/CPH composites without MAPP. This confirms that the increase of viscosity was related to presence of MAPP. After addition of MAPP, a strong interfacial bonding exists between CPH particles and PP matrix and it restricted the flowability of melted PP/CPH composites. The plot of log viscosity against log reciprocal of absolute temperature (1/T) of PP/ CPH composites with and without MAPP was showed in Figure 5. A linear increment of log viscosity versus 1/T lines were found on both composites. The viscosity of both PP/ CPH biocomposites decreased with the increases of temperature. This is attributed by the increasing of polymer chain mobility at high temperature, which increased the free
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Figure 6. Activation energy of PP/CPH composites with and without MAPP at different filler contents. PP: polypropylene; CPH: cocoa pod husk; MAPP: maleated polypropylene.
volume of composites and it reduced the entanglement of polymer chain as well as intermolecular interaction. A similar observation was also reported by other researchers.1,34–35 The activation energy (Ea) is defined as the minimum quantity of energy that the polymer chains required in order to overcome molecular entanglement and to an adjacent position. Typically, the Ea increases as the molecular chains mobility increase.24 The Ea of compound can calculated from the slope of log viscosity versus log reciprocal of absolute temperature (Figure 5). The Ea of PP/CPH composites with and without MAPP at various filler content were displayed in Figure 6. The value Ea of both composites increased inappreciably with increase of filler content. The observation indicates the viscosity of PP/CPH composites are less sensitive on change of temperature compared to neat PP. This is because the solid filler only undergoes elastic deformation and it does not contribute to the viscous behavior of composites melt.39 For this reason, the viscosity of PP/CPH composites becomes less sensitive to temperature. This also implied that the PP/CPH compound required more energy to undergo fusion at higher filler content. Figure 6 showed that the Ea of PP/CPH composites with MAPP was higher than the PP/CPH composites without MAPP. The viscosity of melted PP/CPH composites is less sensitive to change of temperature after addition of MAPP. This indicates fusion of PP/CPH compounds with MAPP required higher energy. Xu et al.24 also reported that the higher Ea is required for fusion of polyvinyl chloride/wood flour compound after improving the filler–matrix adhesion using coupling agent.
Conclusions The results from torque rheological analysis reveal that processing torque of PP/CPH composites increased with increase of filler content and addition of MAPP. The PP/CPH
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composites melt behavior as pseudoplastics and undergo shear thinning at higher shear rate. The pseudoplasticity of PP/CPH composites were increased with change of filler content and presence of MAPP, which can be observed from the decrease of value n. The increases of viscosity in PP/CPH composites melt was related to amount of filler content and the change of filler–matrix adhesion after addition of MAPP. The increase of filler content and addition of MAPP increased the Ea of PP/CPH composites melt. This indicates the higher energy was required for fusion of PP/CPH composites at higher filler content and presence of MAPP. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding The author(s) received no financial support for the research, authorship, and/or publication of this article. References 1. Li H, Law S and Sain M. Process rheology and mechanical property correlationship of wood flour-polypropylene composites. J Reinf Plast Compos 2004; 23: 1153–1158. 2. Bledzki AK, Reihmane S and Gassan J. Thermoplastics reinforced with wood fillers: a literature review. Polym Plast Technol Eng 1998; 37: 451–468. 3. Yeng CM, Husseinsyah S and Sam ST. Chitosan/corn cob biocomposites film by cross-linking with glutaraldehyde. BioResour 2013; 8: 2910–2923. 4. Chun KS and Husseinsyah S. Polylactic acid/corn cob eco-composites: effect of new coupling agent. J Thermoplast Compos Mater 2014; 12: 1667–1678. 5. Chun KS, Husseinsyah S and Azizi FN. Characterization and properties of recycled polypropylene/coconut shell powder composites: effect of sodium dodecyl sulfate modification. Polym Plast Technol Eng 2013; 52: 287–294. 6. Matuana LM and Kim JW. Fusion characteristics of rigid PVC/wood-flour composites by torque rheometry. J Vinyl Add Technol 2007; 13: 7–13. 7. Chun KS, Husseinsyah S and Syazwani NF. Properties of kapok husk-filled linear low-density polyethylene ecocomposites: effect of polyethylene-grafted acrylic acid. J Thermoplast Compos Mater. Epub ahead of print 22 April 2015. DOI: 10.1177/0892705715583175. 8. Chun KS, Husseinsyah S and Yeng CM. Green composites from kapok husk and recycled polypropylene: processing torque, tensile, thermal, and morphological properties. J Thermoplast Compos Mater. Epub ahead of print 1 Febuary 2015. DOI: 10.1177/0892705715569822. 9. Nabi Saheb D and Jog JP. Natural fiber polymer composites: a review. Adv Polym Technol 1999; 18: 351–363. 10. Faruk O, Bledzki AK, Fink HP, et al. Biocomposites reinfored with natural fibers: 2000-2010. Prog Polym Sci 2012; 37: 1552–1596. 11. Yang HS, Wolcott MP, Kim HS, et al. Effect of different compatibilizing agents on the mechanical properties of lignocellulosic material filled polyethylene bio-composites. Compos Struct 2007; 79: 369–375.
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