Green Coupling Agent for Agro-Waste Based Thermoplastic Composites

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Green Coupling Agent for Agro-Waste Based Thermoplastic Composites

Koay Seong Chun,¹Chan Ming Yeng,²Salmah Hussiensyah³

1School of Engineering, Taylor’s University Lakeside Campus, No. 1 Jalan Taylor’s, 47500 Subang Jaya, Selangor, Malaysia

2Centre for Engineering Programmes, HELP College of Arts and Technology, Kuala Lumpur 55200, Malaysia

3Division of Polymer Engineering, Universiti Malaysia Perlis, Perlis, Malaysia

The present work describes the use of agricultural by- product of the cocoa pod husk (CPH) as filler in the polypropylene composites. Besides, a green coupling agent (GCA) was developed from the coconut oil for filler modification. The primary objective of using GCA on CPH is to enhance the interfacial adhesion as improved stress transferability in the composites. The overall tensile properties, water absorption, thermal stability, and crystallinity of the composites are significantly improved by the occurrence of GCA. The fracture micrographs via the scanning electron microscopy shows homogeneous distribution of modified CPH particles and better interfacial bonding with matrix which leads to considerable improvement in the properties of the composites. The performance of GCA was comparable with the maleic anhydride grafted polypropylene and silane coupling agent.POLYM. COMPOS., 00:000–000, 2016.^V^C 2016 Society of Plastics Engineers

INTRODUCTION

Recently, the wood plastic composites (WPC) have received considerable attention in the industry due to the sustainable features and economic factors. Usually, the WPC refers to the composite that contains wood filler and thermoset or thermoplastic materials. The WPC is usually applicable in construction, furniture, packaging, household, and automotive products, for examples, deck- ing, window fitting, car panelling, rear shelves, packaging tray, and tableware [1–4]. The rapid growth of WPC mar- ket raised a serious concern on the limitation of wood filler resources to support the industrial needs [5]. This is because, the main resource of wood filler is from the forest and currently, the forest is declining at the rate of 13.0 million hectares per years in most developing

countries [6]. Thus, the agro-waste based composites are developed due to the high global demand for WPC, short- age of wood resource, and ecological concerns. In Malay- sia, many types of agro-waste materials are abundant and readily available, such as rice husk [6, 7], palm oil empty fruit bunch [8, 9], palm kernel shell [10, 11], coconut shell [3, 12, 13], and corn cob [2, 14]. At this moment, these agro-waste materials are being researched and made into composite materials for different applications. For example, Melsom Biodegradable Manufacturing Sdn Bhd has found success in making a series of tableware from rice husk filled thermoplastic composites and commercial- ly marketing those [3].

The cocoa pod husk (CPH) is a major waste product from the cocoa agricultural and industrial sectors as it is a non-food part of the cocoa pod after obtaining the cocoa beans [1, 2]. Generally, CPH accounts for as much as 76% of cocoa pod by weight and each ton of dry cocoa bean produced will generate ten tons of CPH as waste material [15, 16]. In the past, most of the CPH wastes were often recovered as raw material for anime feed [17], charcoal [18], compost, and fertilizer [19]. In most of the time, burning is one of the ways disposing the CPH wastes, because it has no commercial value. The present work emphasizes the use of CPH as alternative filler for thermoplastic composites, because they are in abundance and easily available in Malaysia. The utiliza- tion of CPH can be beneficial to the ecology, economy, and technology. To date, the development of polypropylene (PP)/CPH composites has been made into utensil produced in our laboratory (as illustrated in Fig. 1).

However, the drawback of the agro-waste is the nature of the hydrophilic properties because of plenty of hydroxyl groups [20–22]. This might lead to poor interfacial compatibility between the agro-waste filler and thermoplastic matrix and yield a composite with undesirable mechanical properties and water resistivity. The use of

Correspondence to: K.S. Chun; e-mail: [email protected] DOI 10.1002/pc.24228

Published online in Wiley Online Library (wileyonlinelibrary.com).

V^C2016 Society of Plastics Engineers

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organic acid [8, 11, 13, 23], silane [12, 24], maleated co- polymer [25, 26], fatty acid and its derivatives [1–3, 26, 27] has been reported as the coupling agent in order to overcome the interfacial incompatible hydrophilic natural filler and hydrophobic matrix. At present, a green coupling agent (GCA) is being made from the virgin coconut oil. This GCA has few advantages compared to the commercial coupling agents (e.g., silane and maleated polymer) as the coupling agent is made from sustainable resources, reactive to natural filler, and inexpensive. The GCA is a glycidyl fatty acid ester produced by reacting sodium fatty acid derived from the virgin coconut oil and glycidyl chloride (Fig. 2). The CGA consists of a reactive oxirane group and long alkyl tail. During the filler modification, the GCA will react with the hydroxyl group of filler via oxirane group and the long alkyl tails are chemically attached to filler surface (as illustrated in Fig. 3) which provided the organophilic character to the filler and improved the wettability, dispersion as well as filler–

matrix adhesion. Our previous research found that the

tensile strength and modulus of the PP/CPH composites were remarkably improved with 3% of GCA [1].

The aim of this work was to evaluate the effects of the GCA on the tensile, water absorption, thermal, and morphological properties of the PP/CPH composites. At similar amount of coupling agent, the comparison on the properties of PP/CPH composites with GCA, 3- mercaptopropyltrimethoxysilane (MPS) [1], and maleated polypropylene (MAPP) [2] were studied.

MATERIALS AND METHODS

Agro-waste explored in this study was the discarded CPH from a cocoa plantation (Perak, Malaysia). The collected CPH was dried in an air circulatory oven at 808C for 24 hr. Then, all the dried CPH was crushed into small pieces and ground into fine powder by using a miniature grinder (RT-34, Taiwan). The average particle size of CPH produced was 22mm (analyzed by the Malvem Par- ticle Size Analyzer Instrument). PP (type co-polymer, grade SM 340) was supplied by Titan Petchem (M) Sdn.

Bhd. The melted flow index was 4.0 g/10 min at 2308C and density 0.9 g/cm³. Glycidyl chloride was a product of Sigma Aldrich and ethanol was obtained from Fluka, Penang. In our laboratory, the virgin coconut oil used in this experiment was made from fresh coconut milk via the fermentation method. The GCA was prepared by reacting sodium fatty acid (made from virgin coconut oil through saponification method) and glycidyl chloride (as shown in Fig. 2).

Filler Treatment

The CPH was treated by immersing in the GCA- ethanol solution (3% vol/vol) under constant stirring for 1 hr. Then, the CPH was soaked in the GCA-ethanol

FIG. 1. Utensil made from CPH filled polypropylene composites. [Col- or figure can be viewed at wileyonlinelibrary.com]

FIG. 2. Schematic reaction of (i) saponification of coconut oil and (ii) glycidyl chloride and sodium fatty acid to produce gylcidyl fatty acid ester.

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solution for 12 hr. The treated CPH was filtered and dried in an oven at 808C for 24 hr to completely remove the solvent.

Melt Compounding and Molding

The formulation of PP/CPH composites is listed in Table 1. All the composites were compounded using a BrabenderV^R Plastrograph intermixer, Model EC PLUS in counter-rotating mode at 1808C and a rotor speed of 50 rpm. The compounding procedures involved were as follows: (i) the PP pellets were added into the mixing chamber for 3 min until it melted homogeneously; (ii) the unmodified or modified CPH was incorporated to the melted PP and continuously mixed for 5 min. Finally, the composite compound was collected from the mixing chamber. All the compounds were molded into 1-mm thick sheets using a compression molding machine, model GT 7014A at 1808C. 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; and (iii) cooling under the same pressure for 5 min. Then, the PP/CPH composite sheets were cut into the tensile specimens using a dumbbell cutter with dimensions according to ASTM D638 type IV [28].

Tensile Test

The composite specimens were tested for their tensile properties by using an Instron Testing Machine model 5569 in accordance with ASTM D638 [28]. A cross head speed of 30 mm/min was used and the load cell selected was 50 kN. The test was performed at 25638C. A mini- mum of five replicate specimens was tested for each composite formulation.

The data of tensile test for each unmodified and modified PP/CPH composites with GCA were statistically analyzed by using Microsoft Excel 2013. The single factor for analysis of variance (ANOVA) was used to determine the significance of the difference between means. The difference between means is considered significant when P0.05.

Water Absorption Test

The water absorption test on composites was performed according to ASTM D 570 [29]. All composite samples with dimension 30 mm3 25 mm 3 1 mm were immersed in the distilled waste at room temperature (258C). The water absorption was determined by the recorded sample weight at regular intervals. A Mettler Balance Model AJ150 (USA) with the precision of 61 mg was used to measure the sample weight. The water absorption at time t,Wa, was calculated by the formulation below:

Wa5Wn2Wd

Wd

3100% (1) Where Wd and Wn are original dried weight and weight after exposure, respectively.

Morphological Analysis

The fracture surface of tensile specimens was observed with a scanning electron microscope (SEM), model JEOL JSM-6460 LA, under an acceleration voltage of 5 keV.

The specimen was coated with a thin layer of palladium for the conductive purpose before the examination. The SEM micrograph on the filler dispersion and filler–matrix adhesion of the composites was investigated.

Differential Scanning Calorimetry Analysis

Differential scanning calorimetry (DSC) measurements were carried out with a DSC Q10 analyzer (TA

FIG. 3. Schematic reaction between CPH and GCA.

TABLE 1. Formulation of PP/CPH composites.

Materials

PP

(phr) CPH (phr)

GCA (wt %)

Unmodified PP/CPH 100 0, 10, 20, 30, 40 –

Modified PP/CPH 100 10, 20, 30, 40 3^a

phr5part per hundred resin.

a3% from weight of CPH.

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Instruments). The specimens (762 mg) were placed in a closed aluminum pan and analyzed by heating from 308C to 2008C with a heating rate of 108C/min under nitrogen atmosphere. The nitrogen gas flow rate was 50 mL/min.

The degree of crystallinity of the composite (Xc) can be calculated from the DSC data by using the following equation:

Xc5 DHf=DH_f^o

3100% (2) whereDHfis the heat fusion of the PP composites, and D H^o_f is the heat fusion for 100% crystalline PP (DH_f^o5209 J/g)

Thermogravimetric Analysis

The thermogravimetric analysis (TGA) was carried out using the TGA Pyris Diamond Perkin Elmer apparatus.

The specimens (762 mg) were placed in a platinum cru- cible. Then, the weight loss against the temperature was measured at a heating rate of 108C/min and range of the thermal scan from 308C to 7008C under nitrogen atmosphere. The nitrogen gas flow rate was 50 mL/min.

RESULTS AND DISCUSSION

Tensile Properties

The stress–strain traces of the neat PP, unmodified and modified PP at 20 phr CPH are depicted in Fig. 4. The entire composites exhibited a similar stress–strain traces but different in the tensile strength and elongation. The addition of CPH dramatically reduced the strength and elongation at the break of composites as compared to the neat PP. However, the filler modification using GCA has significantly increased the strength of PP/CPH composites along a slight drop of the elongation at break.

Figure 5 shows the tensile strength of the unmodified and modified PP/CPH composites as a function of the CPH content. The tensile strength of the PP matrix

decreased with the presence of CPH as an expected result for the particulate filler filled composites. Usually, the filler in particulate, irregular dimension, and low aspect ratio would offer a low stress transfer to the composites [2, 3, 12, 13]. In this study, the use of CPH as a filler was in the form of particulate. Therefore, the tensile strength of the PP/CPH composites was reduced once subjected to the tensile stress. Besides, the natural hydrophilic behavior of CPH leads to the poor wettability and poor interfacial compatibility with hydrophobic PP matrix. Upon increasing of CPH content, CPH also tends to agglomerate due to the formation of hydrogen bonding within each CPH particles. Hence, the presence of weak interfacial adhesion of CPH and PP matrix and presence of filler agglomeration caused poor stress transfer which causes the decrease of the tensile strength. However, the modified PP/CPH composites with GCA had significantly higher tensile strength (P0.05) than the unmodified PP/

CPH composites. The modified CPH with GCA was pre- dicted to enhance the properties of composites. The presence of GCA reacted to CPH through the formation of ester bonding between the oxirane group of GCA and hydroxyl group of CPH. Thereby, the hydrocarbon chains of GCA chemically attached to CPH are likely a hydrophobic layer on the surface. This might avoid CPH from

FIG. 4. Stress–strain curves of neat PP, unmodified and modified PP/

CPH composites.

FIG. 5. Effect of GCA on tensile strength of PP/CPH composites.

FIG. 6. Effect of GCA on tensile modulus of PP/CPH composites.

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the agglomeration and allow CPH to have better wetting and dispersion in matrix. As a result, the modified CPH had homogeneous dispersion and stronger interfacial adhesion with PP matrix that leads to increase in the tensile strength.

The tensile modulus values versus the CPH content of PP/CPH composites are plotted in Fig. 6. The tensile modulus of PP/CPH composites increased continually with the increasing of CPH content. The deformability of PP matrix was reduced with the addition of neat CPH as the friction that occurs between the CPH and PP matrix restricted the chain mobility of the PP matrix. Therefore, the higher filler content increased the stiffness and rigidi- ty of the PP/CPH composites. In addition, the modified PP/CPH composites showed higher tensile modulus than the unmodified PP/CPH composites with considerable significance of P0.05. This is due to the coupling effect from GCA that increased the filler–matrix adhesion and lowered the deformability of matrix.

Figure 7 presents the elongation at break of the PP/

CPH composites. The elongation at break was sharply decreased on addition of CPH. This can be explained by the presence of CPH that leads to the lower deformability of matrix and the filler agglomeration also contributed to the premature failure. Therefore, the larger filler content highly reduced the ductility of the PP/CPH composites.

Consequently, the elongation at the break of PP/CPH composite was significantly reduced (P0.05) after the filler modification with GCA. This is the main drawback of using the coupling agent, which improved the

interfacial adhesion and increased the strength and stiffness of the composites but loss of ductility.

Table 2 shows the average percentage change in the tensile properties of the modified PP/CPH composites with GCA, MAPP, and MPS. The modified PP/CPH composites with MAPP [2] demonstrated the highest improvement in the tensile strength compared to the modified PP/

CPH composites with GCA and MPS [1]. This indicated that the MAPP is the most effective coupling agent in enhancing the interfacial adhesion between the CPH and PP matrix. The modified CPH with GCA and MPS showed better wettability with the PP matrix and resulted in better interlocking at the interfacial region. However, the MAPP achieved a strong interfacial bonding by forming a molecular entanglement between the filler and matrix. For this reason, the MAPP exhibited a better performance in improving the tensile strength compared to GCA and MPS. Furthermore, all the modified PP/CPH composites were improved in the tensile strength and tensile modulus but the ductility of composites was dramatically reduced. Particularly, the modified PP/CPH composites with MPS [1] and MAPP [2] showed more than 49% reduction in the elongation at break. The modified PP/CPH composites with GCA lowered the loss in ductility as compared to the modified PP/CPH composites with MPS [1] and MAPP [2].

The relationship between the filler–matrix adhesion and filler modification can be expressed quantitatively by a simple model developed by Pukansky [30]. The model considers the major factors influencing the tensile strength including (i) n, which is the change of specimen dimensions during the deformation and raise of tensile strength due to strain hardening; (ii) (1 –/)/(112.5/)—

effect of the reduced load bearing cross-section of matrix due to filling; and (iii) exp (B/)—interfacial adhesion [30, 31].

rT5rT0kⁿ 12/

112:5/expðB/Þ (3)

where r_T and r_T0 are the true strength of the composite and polymer matrix, respectively (rT5rk, whereris the measured engineering tensile strength), kis relative elongation (k5L/L0, where L0is the original length and L is the length at the failure point), n is related to the strain hardening exponent of polymer matrix, / is the volume friction of filler, and B is a parameter that expresses the

FIG. 7. Effect of GCA on elongation at break of PP/CPH composites.

TABLE 2. Tensile properties of unmodified and modified PP/CPH composites with different coupling agents at 40 phr of filler content.

Composites Tensile strength (MPa) Elongation at break (%) Tensile modulus (MPa)

Unmodified PP/CPH composites 15.6760.6 13.7160.6 1169.72639

Modified PP/CPH composites with CGA 19.6560.8 8.3860.3 1512.40647

Modified PP/CPH composites with MPS^a 16.9760.6 5.6760.2 1346.77658

Modified PP/CPH composites with MAPP^b 21.8160.9 4.2360.2 1357.76643

a,bResults were obtained from Refs. 1, 2, respectively.

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load bearing capacity of filler corresponding to the effects of interfacial adhesion. The model can be expressed in a linear form such as in Eq.4.

lnrT_red5lnrT

kⁿ

112:5/

12/ 5lnrT₀1B/ (4) Figure 8 illustrates the plot of ln reduced tensile strength of unmodified and modified PP/CPH composites with GCA, MAA, and MAPP as a function of the filler content. A straight linear correlation was observed. Thus, the parameterB related to the stress transfer and filler–matrix adhesion can be determined with more accuracy. The

slope of the line was changed after the addition of GCA, MPS, and MAPP. The parameter B of the modified PP/

CPH composites with GCA was 2.75 and it was higher than the composites without GCA (1.55). This evidenced that the interfacial adhesion between CPH and PP matrix was enhanced by the presence of GCA. The modified PP/

CPH composites with MAPP showed the highest value of B (2.88) compared to the modified PP/CPH composites with GCA (2.75) or MPS (2.11). This proved that the MAPP had better performance in improving the interfacial adhesion compared to MPS and GCA.

Morphological Properties

Figure 9a and b presents the SEM micrographs of the tensile fracture surface of unmodified PP/CPH composites with 20 phr and 40 phr filler content, respectively. The SEM micrographs (Fig. 9a and b) show the appearance of holes due to the filler pull-out and the detached filler indicates poor filler–matrix adhesion. The poor adhesion was usually caused by the poor wetting of the filler by polymer matrix. On the other hand, the modified PP/CPH with GCA exhibited a brittle fracture surface as illustrated in Fig. 9c and d. The modified CPH particles found were well dispersed and covered by the polymer as well as embedded in the PP matrix. This evidenced that the modified CPH has better adhesion with PP matrix. Thus, the

FIG. 8. Reduced tensile strength of unmodified and modified PP/CPH composites plotted against filler content.

FIG. 9. SEM micrographs on tensile fracture of unmodified PP/CPH at (a) 20 phr and (b) 40 phr filler content; modified PP/CPH with GCA at (c) 20 phr and (d) 40 phr filler content.

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modified PP/CPH composites showed improvement in the tensile properties due to the better interfacial adhesion.

Water Absorption

The water absorption of the unmodified and modified PP/CPH composites with 20 phr and 40 phr filler content is presented in Fig. 10. It can be observed that the water absorption PP/CPH composites increased with the increasing of CPH content. This is because the CPH is naturally hydrophilic and can absorb large amount of moisture.

This is mainly due to the plenty of hydroxyl groups on the CPH that might be easily bonded with water molecules via hydrogen bonding [3, 32]. Even CPH compounded with polymer can absorb a considerable amount of water. The presence of micro gap or flaw at interface of filler and matrix due to poor adhesion also contributed to the water uptake of the composites. The water molecules also can penetrate into composites via capillary action and get trapped in the interface region [32, 33]. By contrast, the modified filler with GCA had significant effect on the water absorption of the PP/CPH composites.

The presence of GCA in modified CPH alternated the hydrophilic character by providing a hydrophobic layer on the CPH surface. Thus, the water uptake capacity of the entire modified PP/CPH composites was lower compared to the unmodified PP/CPH composites.

Furthermore, the studies of water absorption behavior of the unmodified and modified PP/CPH composites were carried out by using the Fick’s law. The diffusion mecha- nism and kinetic of the PP/CPH composites were studied according to Eq. 5 where Mt is the moisture content at time t, and Ms is the moisture content at saturated point.

Thekandnare a constant.

Mt=Ms5ktⁿ (5)

The moisture diffusion characteristic of composites can be divided into three cases, including: (i) Case I (n50.5) refers to the Fickian diffusion behavior; (ii) Case II

(0.5<n<1) relates to the non-Fickian or anomalous diffusion; and (iii) Super Case II (n>1) [33, 34]. The con- stantkandncan be calculated from the plot of logMt/Ms

against the logt of the unmodified and modified PP/CPH composites (Fig. 11). The diffusion coefficient (D) refers to the ability of water molecules to penetrate inside the composites structure. The value D can be determined from the slope of linear part of the Mt/Ms versus time (t^0.5) according to Eq.6(as shown in Fig. 12).

Mt=Ms5ð4=hÞðD=pÞ^0:5t^0:5 (6) whereh5thickness of the sample.

Table 3 shows the k, n, and D values of the unmodified and modified PP/CPH composites. The n values of all the PP/CPH composites are close to the valuen50.5, which indicated that the composites approached toward the Fickian diffusion behavior. Many literatures also reported that the water absorption behavior of natural filler that filled the thermoplastic composites was following the Fickian behavior [34–36]. According to Table 3, thek and D values of the PP/CPH composites increased with the increasing of filler content. This indicates that the composites show high water uptake ability at the higher

FIG. 10. Water absorption of neat PP (a), unmodified PP/CPH composites at 20 phr (b) and 40 phr (c) filler content and modified PP/CPH composites with GCA at 20 phr (d) and 40 phr (e) filler content.

FIG. 11. Plot of logMt/Msversus log timetof unmodified and modified PP/CPH composites with GCA.

FIG. 12. Plot ofMt/Msversus logt^0.5of unmodified and modified PP/

CPH composites with GCA.

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filler content. However, the modified PP/CPH composites show lower k and D values compared to the unmodified PP/CPH composites. This evidences that the presence of GCA is able to reduce the hydrophilicity of CPH and also improve the interfacial bonding which prevents the water uptake of the composites. Table 3 shows theMs,k, andD values of the modified PP/CPH composites with GCA were the lowest compared to the modified PP/CPH composites with MPS and MAPP. This indicates that the GCA had more efficiency in preventing water molecule diffuse and form bonding to the hydroxyl group of CPH as compared to other coupling agents.

Thermal Properties

The DSC thermograms of neat PP, the unmodified and modified PP/CPH composites are presented in Fig. 13.

The melting temperature (Tm), fusion enthalpy (DHf), and degree of crystallinity (X_c) of the composites are listed in Table 4. The X_c of the PP/CPH composites increased as the CPH content in PP composites increased. This observation indicates that the addition of CPH gave a nucleating effect to the PP composites as the presence of CPH provides sites for the initiation of nuclei growth. Thus, the crystallinity composites increased at the higher CPH content. In addition, the modified PP/CPH exhibited higher crystallinity compared to the unmodified PP/CPH composites. From the paper published in the literature, it was found that the crystallinity of the composites is usually influenced by the filler dispersion and content, and

the surface chemistry of the filler [5]. In this case, the modified CPH with GCA had better filler dispersion and interfacial interaction which promoted the nucleating effect on the PP composites. Therefore, the modified CPH might have more nuclei growth sites that attribute to the higher crystallinity. The modified PP/CPH composites with GCA show similar manner with the modified PP/

CPH composites by other coupling agent. Alternatively, the Tmof PP/CPH composites was not affected by either the CPH content or filler modification by GCA.

The derivative thermogravimetry (DTG) and TGA thermograms of the neat CPH and PP, and unmodified and modified PP/CPH composites are illustrated in Figs. 14 and 15. The thermogravimetric analysis results are summarized in Table 5. The neat PP demonstrated the single step thermal degradation above 3008C in which the depolymerization of carbon–carbon bond in PP chain occurred [37, 38]. The neat CPH had three steps of the thermal degradation reaction as follows: (i) evaporation of the volatile compound and moisture (30–1008C); (ii) thermal degradation of hemicelluloses (200–3508C); and (iii) thermal degradation of lignin and cellulose (above 3508C). Based on the degradation temperature at 5%

weight loss (Td5%), the PP/CPH composites showed an early thermal degradation as the Td5% was lower compared to the neat PP. Moreover, the Td5% of PP/CPH composites decreased with the increasing of the CPH content. The early thermal degradation was related to the weight loss of volatile compound/moisture and hemicellu- lose in CPH. On the other hand, the thermal stability of the PP/CPH composites increased at higher temperature, which can be observed via the shift of degradation temperature at maximum rate (Tdmax) to the higher

TABLE 3. Water absorption data of unmodified and modified PP/CPH composites with GCA and other coupling agents.

Materials Ms(%) n k310²⁴(g/g s²) D310²¹³(m²s²¹)

Unmodified PP/CPH:100/20 4.07 0.53 5.30 1.40

Unmodified PP/CPH:100/40 11.22 0.53 6.48 1.80

Modified PP/CPH:100/20 (GCA) 2.43 0.54 4.24 1.22

Modified PP/CPH:100/40 (GCA) 9.05 0.54 5.29 1.60

Modified PP/CPH:100/40 (MPS) 9.23 0.54 5.27 1.68

Modifed PP/CPH:100/40 (MAPP) 8.98 0.54 5.15 1.65

FIG. 13. DSC thermograms of neat PP, unmodified and modified PP/

CPH composites with GCA at selected filler content.

TABLE 4. DSC data of unmodified and modified PP/CPH composites with GCA and other coupling agents.

Materials Tm(8C) Xc(%) DH(J/g)

Neat PP 165 27 57

Unmodified PP/CPH:100/20 165 28 59

Unmodified PP/CPH:100/40 165 31 65

Modified PP/CPH:100/20 (GCA) 164 34 72

Modified PP/CPH:100/40 (GCA) 164 40 83

Modified PP/CPH:100/40 (MPS)^a 165 35 73

Modified PP/CPH:100/40 (MAPP)^b 163 40 83

Tm5melting temperature.

Xc5degree of crystallinity.

a,bDSC data of composites was taken from Refs. 1, 2 respectively.

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temperature and the increase of residue content at 7008C.

It can be explained that the pyrolysis compound generated during the early thermal degradation provided a thermal protective layer on the composites, resulting in delaying the thermal degradation process [12, 13]. The similar thermal behavior in the composites was also observed by other researchers [39, 40]. The Td5% and Tdmax of the modified PP/CPH composites shifted to higher temperature and the residue content at 7008C was higher compared to the unmodified PP/CPH composites. This result indicates the modified PP/CPH composites had better thermal stability than the unmodified PP/CPH composites.

The similar observation was also found by other researchers with modified filler with fatty acid [3, 37]. However, the reason why the addition of coupling agent enhanced the thermal stability of the composites was not found in any literature study. The probable reason for the increase of Td5% of the modified PP/CPH composites was related to the dispersion of CPH filler. Thus, the well dispersed CPH particles have high tendency to be covered by the PP matrix and protected from early thermal degradation.

Furthermore, the increase of Tdmax of the modified PP/

CPH composites might be correlated with the effect of char formation. The presence of coupling agent (GCA,

MPS, and MAPP) possibly promoted char forming during the thermal degradation of the CPH filler and it further improved the thermal stability of the composites. At similar filler content, the modified PP/CPH composites with GCA showed similar thermal behavior as compared to the modified PP/CPH composites with MPS [1] and MAPP [2].

CONCLUSIONS

The modified PP/CPH composites with GCA showed a remarkable improvement of the tensile strength and modulus. The filler modification by GCA alternated the hydrophilic properties of CPH and reduced the water uptake ability of its composites. The crystallinity and thermal stability of the PP/CPH composites were also improved by the filler modification using GCA. The properties of composites improved due to the modification of CPH with GCA, which enhanced the filler dispersion and interfacial interaction between CPH and PP matrix. The SEM micrographs also confirmed the better adhesion between the modified CPH with GCA and PP matrix.

Eventually, modified PP/CPH with GCA had comparable properties with the modified PP/CPH composites with MPS and MAPP. Thus, GCA was potential as a new coupling agent for the agro-waste based thermoplastic composites.

REFERENCES

1. K.S. Chun and S. Husseinsyah, J. Thermoplast. Compos.

Mater.,29, 1332 (2016).

2. K.S. Chun, S. Husseinsyah, and H. Osman, J. Thermoplast.

Compos. Mater.,28, 1507 (2015).

3. K.S. Chun, S. Husseinsyah, and F.N. Azizi, Polym. Plast.

Technol. Eng.,52, 287 (2013).

4. S. Panthapulakkal and M. Sain, J. Polym. Environ., 14, 265 (2006).

5. N. Ayrilmis and A. Kaymakci, Ind. Crops Prod., 43, 457 (2013).

6. G.B. Hattotuwa, H. Ismail, and A. Baharin, Polym. Test., 21, 833 (2002).

FIG. 14. DTG thermograms of CPH, neat PP, unmodified and modified PP/CPH composites with GCA at selected filler content.

FIG. 15. TGA thermograms of CPH, neat PP, unmodified and modified PP/CPH composites with GCA at selected filler content.

TABLE 5. TGA data of unmodified and modified PP/CPH composites with GCA and other coupling agents.

Sample

Td5%

(8C)

Tdmax

(8C)

Residue at 7008C (%)

Neat PP 336 418 1.22

Unmodified PP/CPH:100/20 272 422 2.69

Unmodified PP/CPH:100/40 246 432 4.22

Modified PP/CPH:100/20 (GCA) 285 439 4.39

Modified PP/CPH:100/40 (GCA) 258 450 6.54

Modified PP/CPH:100/20 (MPS)^a 251 449 6.12 Modified PP/CPH:100/20 (MAPP)^b 251 449 6.49

Td5%5degradation temperature at 5% weight loss.

Tdmax5degradation temperature at maximum rate of weight loss.

a,bTGA data of composites was obtained from Refs. 1, 2, respectively.

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7. H.D. Rozman, M.H. Lee, R.N. Kumar, A. Abusamah, and Z.A. Mohd Ishak,J. Wood Chem. Technol.,20, 93 (2000).

8. A. Hassan, A.A. Salema, F.N. Ani, and A.A. Bakar, Polym.

Compos.,31, 2097 (2010).

9. M.F. Arif, P.S.M. Megat-Yusoff, and F. Ahmad, J. Reinf.

Plast. Compos.,29, 2009 (2015). (

10. H. Salmah, A. Romisuhani, and H. Akmal,J. Thermoplast.

Compos. Mater.,26, 3 (2013).

11. H. Salmah, B.Y. Lim, and P.L. Teh, J. Thermoplast. Com- pos. Mater.,26, 1155 (2013).

12. K.S. Chun, S. Husseinsyah, and H. Osman,J. Polym. Res., 19, 1 (2012).

13. K.S. Chun, S. Husseinsyah, and H. Osman, Polym. Eng.

Sci.,53, 1109 (2013).

14. C.M. Yeng, S. Husseinsyah, S.S. Ting, BioResource, 2, 2910 (2013).

15. C.M. Lucia, D.M.C.A. Renata Dias, and D.O.P. Carmen Lucia,Ind. Crop Prod.,34, 1173 (2001).

16. C.M. Lucia, D.M.C.A. Renata Dias, and D.O.P. Carmen Lucia,LWT- Food Sci. Technol.,49, 108 (2012).

17. A. Donkoh, C.C. Atuahene, B.N. Wilson, and D. Adomako, Anim. Feed Sci. Technol.,35, 161 (1991).

18. M. Syamsiro, H. Saptoadi, B.H. Tambunan, and N.A.

Pambudi,Energy Sustain. Dev.,16, 74 (2012).

19. K. Thiagalingam and N. Sriskandarajah, Resour. Conserv., 13, 135 (1987).

20. H.P.S. Abdul Khalil, A.M. Issam, M.T. Ahmad Shakri, R.

Suriani, and A.Y. Awang,Ind. Crops. Prod.,26, 315 (2007).

21. M.M. Kabir, H. Wang, L.T. Lau, and F. Cardona,Compos.

Part B,43, 2883 (2012).

22. A.K. Bledzki, S. Reihmane, and J. Gassan, Polym. Plast.

Technol. Eng.,37, 451 (1998).

23. R. Rahman, M. Hasan, M. Huque, and N. Islam,J. Thermo- plast. Compos. Mater.,22, 2945 (2009).

24. Y. Xie, C.A.S. Hill, Z. Xiao, H. Militz, and C. Mai, Com- pos. Part A,41, 806 (2010).

25. L. Danyadi, T. Janecska, Z. Szabo, G. Nagy, J. Moczo, and B. Pukanszky,Compos. Sci. Technol.,67, 2838 (2007).

26. L. Danyadi, J. Moczo, and B. Pukanszky, Compos. Part A, 41, 199 (2010).

27. R.G. Raj and B.V. Kokta, J. Appl. Polym. Sci., 38, 1987 (1989).

28. ASTM Standard D638, Standard Test Method for Tensile Properties of Plastics, ASTM International, West Consho- hocken, PA (2010). doi: 10.1520/D0638-10.

29. ASTM Standard D570, Standard Test Method for Water Absorption of Plastics, ASTM International, West Consho- hocken, PA (2010).doi: 10.1520/D0570-98R10E01.

30. B. Pukanszky,Composites,21, 255 (1990).

31. L. Danyadi, K. Renner, J. Moczo, and B. Pukanszky,Polym.

Eng. Sci.,47, 1246 (2007).

32. S. Hussiensyah, K.S. Chun, A. Hadi, and R. Ahmad, J.

Vinyl. Addit. Technol.,22, 200 (2016).

33. P. Chindaprasirt, S. Hiziroglu, C. Waisurasingha, and P.

Kasemsiri,Polym. Compos.,36, 604 (2015).

34. Y. Zhao, J. Qiu, H. Feng, and M. Zhang, J. Appl. Polym.

Sci.,125, 3211 (2012).

35. J. George, S.S. Bhagawan, and S. Thomas, Compos. Sci.

Technol.,58, 1471 (1998).

36. P.K. Kushwaha and R. Kumar,Polym. Plast. Technol. Eng, 49, 45 (2010).

37. F. Amri, S. Husseinsyah, and K. Hussin, Compos. Part A, 46, 89 (2013).

38. H.S. Kim, S.W. Choi, B.H. Lee, S. Kim, H.J. Kim, and C.W. Cho,J. Therm. Anal. Calorim.,89, 821 (2007).

39. A. Arbelaiz, B. Fernandez, J.A. Ramos, and I. Mandragon, Thermochim. Acta,440, 111 (2006).

40. Y.F. Shih and C.C. Huang,J. Polym. Res.,18, 2335 (2011).