Torque Rheological Properties of Agro-Waste-Based Polypropylene Composites: Effect of Filler Content and Green Coupling Agent
Koay Seong Chun,1Salmah Husseinsyah,2Chan Ming Yeng1
1School of Engineering, Taylor’s University Lakeside Campus, No.1, Jalan Taylor’s, 47500 Subang Jaya, Selangor, Malaysia
2Division of Polymer Engineering, School of Materials Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia
This study investigates the effect of filler content and green coupling agent (GCA) on the torque rheological properties of polypropylene (PP)/cocoa pod husk (CPH) composites. The GCA was prepared from coconut oil and it was used to improve the properties of PP/CPH composites. The processing torque of PP/CPH compo- sites was increased with increasing of filler content and the presence of GCA. The power law index (n) of PP/CPH composites were less than 1.0. Thus, the com- posites melt showed pseudoplastic character and it exhibited shear thinning effect. The valuesn of PP/CPH composites increased with higher filler content and after filler modification using GCA. This indicated that the pseudoplasticity of composites were affected by filler–
matrix interaction. The viscosity of PP/CPH composites increased after increased filler content and the presence of GCA. The change of viscosity was related to filler–filler interaction and filler–matrix adhesion. The increase of fil- ler content and addition of GCA also increased the acti- vation energy (Ea) of PP/CPH composites. This indicated that the energy of fusion process of PP/CPH composites increased with higher filler content and the addition of GCA. J. VINYL ADDIT. TECHNOL., 00:000–000, 2016. VC 2016 Society of Plastics Engineers
INTRODUCTION
Nowadays, thermoplastic composite materials made from agro-waste and thermoplastic have gained great inter- est among the industries and researchers due to their eco- nomic advantage, ecological awareness, and accumulation of agro-waste materials [1–5]. To date, numerous combina- tions of agro-waste and thermoplastic materials have been made into composite materials and already enthusiastically used in building applications (e.g., deck and wooden fit- tings), consumer products (e.g, packaging tray and utensils),
and automotive parts (e.g., door panel) [2, 4–7]. In the pres- ent, this research was focused on producing thermoplastic composites by combining polypropylene (PP) and cocoa pod husk (CPH). The CPH was a major waste material in the cocoa industry which was readily abundant and widely available in Malaysia [8–10]. Generally, the CPH waste does not have any market potential and burning became a common disposal method for these CPH wastes [10]. For this reason, the CPH needs utilization as filler in producing thermoplastic composite materials and this will benefit the environment, economy and technology.
Certainly, the incompatible between thermoplastic mate- rial and agro-waste filler is a main factor influencing the properties of composites. This is because the agro-waste fil- ler mainly contains cellulose, hemicellulose, and lignin which contain plenty of polar hydroxyl groups and leads to weak adhesion and poor wettability with most thermoplas- tic materials [11–15]. Hence, different kinds of coupling agents, such as maleated polymer [6, 8, 16, 17], silane- based coupling agent [9, 18–21], fatty acid and its deriva- tives [4, 5, 9, 11] are usually used to impart better adhesion between agro-waste filler and thermoplastic material. In present work, a green coupling agent (GCA) was synthe- sized from coconut oil fatty acid. Generally, the GCA was a type of glycidyl fatty acid ester that was produced by react- ing sodium fatty acid from coconut oil and glycidyl chlo- ride. The GCA consists a reactive oxirane group as head and fatty acid group as tail in the structure. Ideally, the oxir- ane’s head can covalent bonded on natural filler surface leaving the fatty acid tail provide an organophilic character to natural filler (as shown in Fig. 1). This provides the natural filler better wetting to the thermoplastic matrix and further enhances the filler–matrix adhesion. The new GCA show several benefits compared to synthetic coupling agents, such as low cost and it is made from a sustainable resource. Currently, there are many published literatures related to effect of coupling agents on improving the filler–
matrix adhesion and enhancing the physical-mechanical
Correspondence to: K.S. Chun; e-mail: [email protected] DOI 10.1002/vnl.21561
Published online in Wiley Online Library (wileyonlinelibrary.com).
VC2016 Society of Plastics Engineers
properties of the composite materials (such as, strength, thermal properties, and water absorption) [2–5, 11–14, 22].
However, the information on how the coupling agent influ- encing the torque rheological properties of thermoplastic composites containing agro-waste filler are rare.
Furthermore, the rheological properties are also an impor- tant factor in producing a quality thermoplastic composite materials [23]. The processing of thermoplastic composites containing large amounts of agro-waste filler is more diffi- cult compared to neat plastic resin [24–26]. In addition, the rheological behavior of thermoplastic composites highly depends on type of filler, amount of filler, and the filler–
matrix interaction. Therefore, the information on relationship of natural filler and coupling agent on rheological properties of thermoplastic composites is crucial in process optimiza- tion, trouble shooting, and equipment design [27, 28]. The torque rheometer is an important equipment used to measure the rheological behavior of thermoplastic material in actual processing conditions. Nowadays, there are only several companies that made torque rheometer on the market, including Haake Buchler and Brabender [29, 30]. The quan- titative information on the rheological properties of thermo- plastic materials can obtained from torque rheometer, such as changes of polymer chains during processing, and the effect of different additives on the processability of new formulations [30].
In our previous studies, thermoplastic composites have been produced from CPH and polypropylene (PP) [8–10, 31]. Unfortunately, information on torque rheological properties of PP/CPH composites was not found in any literature yet. For this reason, the research on torque rheo- logical properties of PP/CPH composites using torque rheometer has been underway. This work described the effect of filler content and GCA on torque rheological properties of PP/CPH composites.
METHODOLOGY
Materials
The discarded CPH waste was obtained from cocoa plan- tations at Sungai Hilir, Perak. Then, the CPH waste was dried at 808C using circulatory air oven for 24 h. The dried CPH was crushed into small pieces and further ground into fine powder using a miniature grinder (model RT-34, manu- factured by Mill Powder Tech, Taiwan). The CPH powder was sieved to obtained an average particle size of 22 lm measured by Malvern Particle Size Analyzer Instrument.
The polypropylene type co-polymer (grade SM 340) used in this experiment was supplied by Titan Petchem (M) Sdn.
Bhd, Malaysia. The green coupling agent from coconut oil (GCA) was prepared by reacting glycidyl chloride and
FIG. 1. Schematic reaction between natural filler and GCA.
FIG. 2. (i) Preparation of sodium fatty acid from coconut oil and (ii) reaction of sodium fatty acid and gylcidyl chloride to produce GCA. [Color fig- ure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
sodium fatty acid produced from coconut oil via saponifica- tion (Fig. 2). The preparation of GCA was used the method developed by Chun & Husseinsyah [31] and Chun et al.
[32]. All the chemicals used to produce GCA were supplied by Fluka Malaysia.
Filler Modification
First, 3% of GCA was dissolved in ethanol. The CPH was modified by immersing them in GCA-ethanol solu- tion under a constant stirring for 1 h. After that, the CPH was soaked in GCA-ethanol solution for 12 h. The modi- fied CPH was filtered and dried in an oven at 808C for 24 h to completely remove the ethanol.
Torque Rheology Analysis
The PP/CPH composites with and without GCA were prepared by using Brabender Plastrograph torque rheome- ter intermixer, Model EC PLUS with counter-rotating mode. All composites were compounded according to the formulation in Table 1. The processing parameter of com- pounding process was listed in Table 2. The compounding procedures were the following: (i) the PP resin transferred into mixing chamber for 3 min until it fully melted; (ii) the modified or unmodified CPH powder was added into mixing chamber and compounded for 5 min. The process- ing characteristics (plot of torque versus time) were recorded by the Brabender Mixer Program (WINMIX).
The processing torque detected by the Brabender Plastrograph torque rheometer intermixer can be con- verted to rheological data by following the method sug- gested by Goodrish & Porter [33]. Referring to that method, the torque M versus speed (rpm) S at constant temperate was plotted on log-log scale, as the expression proposed by Babbar & Mathur [27]:
M5CSb (1)
whereC is a constant that depends on machine geometry andb is a constant characteristic of the polymer melt. In all cases, the plots of logM versus logS were a straight lines and the slopebcan be taken as equivalent to a melt flow index (n) related to any fluid that following the power law given by:
s5Kcn (2)
wheresis shear stress,Kis a constant, andcis shear rate.
The shear stress (s) and shear rate (c) and viscosity (g) of compound can be determined from torque and speed data obtained from instrument using following equation used for coaxial cylinder and modified for non-Newtonian materials [32, 33]:
s15 M1
2pR2mh (3)
c15 2S1
nR2=nm ðR22=ni 2R22=ne Þ (4) g5s1
c1 (5)
Where the effective instrument dimension considered for BrabenderVR Plastrograph torque rheometer, Model EC PLUS were: radius of inner cylinder (Ri) 5 1.65 cm;
radius of outer cylinder (Re) 5 1.85 cm; average radius of cylinder (Rm) 51.75 cm; and length of cylinder (h)5 4.6 cm.
The slope of log viscosity versus log reciprocal of abso- lute temperature can be calculated as activation energy (Ea) of compound that referring to Arrhenius equation [32, 33]:
g5Kexp Ea
RT
(6) where K is the pre-factor, R is gas constant (8.314 J mol21K21), andTis processing temperature.
RESULTS AND DISCUSSION
Torque Rheological Properties
Figure 3 shows the plot of torque versus rotor speed of PP/CPH composites with and without GCA. The increase of rotor speed and filler content increased the torque of PP/CPH compounds. The friction and constraint between molecular chains increased with increased rotor speed.
Therefore, the torque of the compound was increased lin- early with increased rotor speed. Furthermore, the addi- tion of solid CPH particles was restricted the melt flow of PP matrix. With increased filler content, the friction between the surface of mixing chamber and the CPH par- ticles, and also between CPH particles and melted PP increased. In addition, the CPH particles were more easily agglomerated at higher filler content. The formation of CPH agglomeration also hinder the melt flow of PP matrix. Thus, the increase amount of CPH particles will hindered the flowability of PP/CPH compounds. Rahman et al. [34] also reported the formation of filler
TABLE 1. Formulation of PP/CPH composites with and without GCA.
Materials PP (phr) CPH (phr) GCA (%)
PP/CPH composites without GCA
100 0, 10, 20, 30, 40 —
PP/CPH composites with GCA
100 10, 20, 30, 40 3*
phr5part per hundred resin * 3% based on weight of CPH.
TABLE 2. Processing parameter for torque rheological analysis.
Parameter (i) (ii)
Rotor Speed (rpm) 40, 50, 60, 70 50
Processing Temperature (8C) 180 180, 190, 200, 210
Time (min) 8 8
agglomeration obstructed the flowability of high density polyethylene/rice husk compounds. However, the process- ing torque of PP/CPH composites with GCA was higher compared to PP/CPH composites without GCA. As men- tion before, the addition of GCA was able to covalently bond on CPH surface, and promote better adhesion at the interfacial region between CPH and PP matrix. This strong interfacial adhesion restrained the flowability of PP/CPH compound and caused the increase of viscosity.
For this reason, the viscosity of melted PP/CPH compos- ite increased with the presence of GCA. Chun et al. [11]
also discovered that the filler modification using sodium dodecyl sulfate as coupling agent increased the processing torque of recycled PP/kapok husk due to the presence of better filler–matrix adhesion which increased the viscosity of compound.
The effect of shear rate on shear stress of PP/CPH composites with and without GCA at different CPH con- tent were demonstrated in Fig. 4. Regarding to power law, the change of the shear stress was proportional to the change of shear rate. At a similar shear rate, the increased amount of CPH content increased the shear stress of PP/CPH compounds. As discuss earlier, the vis- cosity of PP/CPH compounds increased with more filler content. Hence, a higher shear stress was required to gen- erate more shearing action in PP/CPH compounds, to overcome the friction from filler–matrix interaction and filler–filler interaction [35]. Alternatively, the shear stress of PP/CPH compounds increased after filler modification using GCA. This is because the presence of GCA enhanced the filler–matrix interaction and it further
increased the melt viscosity of PP/CPH compounds.
Therefore, shear stress was increased to generate more viscous flow on PP/CPH composites with GCA. The power law index (n) can calculated from the plot of log torque versus log speed (as shown in Fig. 3) using the curve fitting method. Figure 5 displays the valuenof PP/
CPH composites with and without GCA as function of filler content. The values n of neat PP, PP/CPH compo- sites with and without GCA were less than 1.0. This indi- cates the neat PP and both PP/CPH composites exhibited a pseudoplastic characteristic. The results show that the values nwere reduced at higher filler content. This show the pseudoplasticity of all PP/CPH composites increased with the increasing of filler content. The PP/CPH compo- sites with GCA also show higher values n compared to PP/CPH composites without GCA. This evidenced the PP/CPH composites exhibited a shear thinning effect at higher filler content, but this effect was reduced with the presence of GCA. A similar observation also found by other researchers [23, 36, 37].
The viscosity of PP/CPH composites with and without GCA as function of shear rate were illustrated in Fig. 6.
At constant shear rate, the viscosity of all PP/CPH com- posites increased at higher filler content. As mention before, the increases of viscosity of PP/CPH composites was related to filler–matrix adhesion and filler–filler interaction. From the results show that the viscosity of PP/CPH composites was significantly reduced with the increasing shear rate. The change in viscosity as function of shear rate or filler content was due to the following reasons: (i) the reduction of chain entanglement of
FIG. 3. Plot of log torque versus log speed of PP/CPH composites: (a) without GCA and (b) with GCA.
FIG. 4. Plot of log shear stress versus log shear rate of PP/CPH composites: (a) without GCA and (b) with GCA.
polymer matrix at high shear rate which reduced the shear thinning effect on composites; and (ii) the increase of shear rate generated higher shear stress to overcome the filler–filler interaction. At higher shear rate, the polymer chains mobility was increased and filler agglomeration was reduced. Hence, the viscosity of PP/CPH composites was reduced. Feng et al. [38] and Yang et al. [39] agreed that the increase of shear rate reduced the viscosity of thermoplastic composite containing natural filler. This was because the present of shear thinning effect on com- posite materials. Moreover, the viscosity of PP/CPH increased after addition of GCA. This confirms that the presence of GCA increased viscosity of PP/CPH compo- sites. After filler modification, the interfacial adhesion between CPH and PP matrix was improved and this
strong filler–matrix adhesion further hindered the flow- ability of melted PP/CPH compound. Thus, the viscosity of PP/CPH composites increased after filler modification.
Figure 7 exhibits the plot of log viscosity against log reciprocal of absolute temperature (1/T) of PP/CPH composites with and without GCA. The viscosity of both PP/CPH composites was reduced with the increasing processing temperature. This indicated that the raise of temperature increased the molecular chain mobility, resulting in decrease of entanglement density and inter- molecular interaction. Therefore, the viscosity of PP/CPH composites decreased at higher temperature. This observa- tion was also found by other researchers [1, 35, 36]. The activation energy (Ea) is defined as the minimum quantity of energy that the polymer chains required to overcome molecular entanglement and to move to an adjacent posi- tion. Generally, flowability of the polymer melt will increase with the increasing of Ea [23]. The Ea of PP/
CPH composites can determined from the slope of log viscosity versus log reciprocal of absolute temperature.
Figure 8 displays theEa of PP/CPH composites with and without GCA at different filler content. The value Ea of all PP/CPH composites increased with the increased filler content. The results showed the viscosity of PP/CPH composites were less sensitive toward the change of proc- essing temperature compared to neat PP. This is because CPH particles only undergo elastic deformation and do not contribute to the viscous behavior of composites melt [40]. At higher filler content, the amount of viscous PP melt was reduced. Hence, the viscosity of PP/CPH com- posites with more filler content become less sensitive to
FIG. 5. Power law index of PP/CPH with and without GCA at differ- ent filler content.
FIG. 6. Plot of log viscocity versus log shear rate of PP/CPH composites: (a) without GCA and (b) with GCA.
FIG. 7. Plot of log viscocity versus temperature (1/T) of PP/CPH composites: (a) without GCA and (b) with GCA.
temperature change. This indicates higher energy might be required for the fusion process of PP/CPH composites as the filler content increased. In addition, theEaof PP/CPH com- posites increased with the addition of GCA. The present of GCA enhanced the adhesion between CPH and PP matrix.
For this reason, the viscosity of PP/CPH composite melts became less sensitive to the temperature change. This also indicates the energy for the fusion process of PP/CPH com- posites with GCA was higher compared to PP/CPH compo- sites without GCA. Xu et al. [23] also applied a coupling agent in polyvinyl chloride/wood flour compound. The author claimed that the Ea of fusion process of polyvinyl achohol (PVA)/wood flour compound with coupling agent increased due to the improvement of filler–matrix adhesion.
CONCLUSIONS
The processing torque of PP/CPH composites increased with increased filler content and the presence of GCA. The PP/CPH composites melt show pseudoplastic character and it also exhibited shear thinning at higher shear rate. The values of n show the increase of filler content and additional of GCA increased the pseudoplasticity of PP/CPH composites.
The change of viscosity in PP/CPH composites was attributed by the amount of filler content and the strong filler–matrix interaction after filler modification. TheEaof PP/CPH com- posite compounds also increased with the increased filler content and addition of GCA. This showed the fusion process of PP/CPH composites required higher energy when the filler content was increased and with the presence of GCA.
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FIG. 8. Activation energy of PP/CPH composites with and without MAPP at different filler content.
