Utilization of Cocoa Pod Husk as Filler in Polypropylene Biocomposites: Effect of Maleated Polypropylene

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published online 26 November 2013Journal of Thermoplastic Composite MaterialsKoay Seong Chun, Salmah Husseinsyah and Hakimah Osman

Effect of maleated polypropyleneUtilization of cocoa pod husk as filler in polypropylene biocomposites:

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Article

Utilization of cocoapod husk as filler inpolypropylenebiocomposites: Effectof maleatedpolypropylene

Koay Seong Chun1, Salmah Husseinsyah1 andHakimah Osman1

AbstractThe aim of the research was to utilize cocoa pod husk (CPH) in polypropylene (PP)biocomposites. Maleated polypropylene (MAPP) was used as coupling agent to improvethe properties of PP/CPH biocomposites. The addition of MAPP had increased thestabilization torque of PP/CPH biocomposites. The tensile strength and modulus ofPP/CPH with MAPP were higher compared to PP/CPH biocomposites without MAPP,except the elongation at break decreased. The crystallinity and thermal stability of PP/CPH biocomposites with MAPP increased. These improvements were due to theenhanced interfacial bonding between CPH and PP matrix, which were proved by SEManalysis.

KeywordsCocoa pod husk, polypropylene, biocomposites, maleated polypropylene

Introduction

Cocoa (Theobroma Cacao) is an important agricultural crop in several tropical countries.1–3

Cocoa pod husk (CPH) is a by-product of the process of obtaining cocoa bean from cocoa

1 Division of Polymer Engineering, School of Materials Enginering, Universiti Malaysia Perlis, Jejawi, Perlis,

Malaysia

Corresponding author:

Salmah Husseinsyah, Division of Polymer Engineering, School of Materials Engineering, Universiti Malaysia

Perlis, Jejawi 02600, Perlis, Malaysia.

Email: [email protected]

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Materials

1–15

ª The Author(s) 2013

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pod, and it usually accounts for 52–76% of the cocoa pod wet weight. In the cocoa industry,

every ton of dry cocoa bean produced will generate 10 ton of CPH as waste.2,3 In general, the

CPH is readily abundant but does not have any market value. Therefore, the utilization of

CPH as natural filler in thermoplastic materials will provide a new application route for CPH

as the useful resource for thermoplastic industry. Moreover, the utilization of CPH can bring

economic benefit and reduce the environmental impact. Recently, the use of natural filler in

thermoplastic materials also gained great interest among researchers and industries due to

some advantage of natural filler compared to mineral filler (e.g., calcium carbonate, kaolin,

mica, and talc), for example, low cost, renewable, minimal health hazard, low density, less

abrasion to machine, biodegradable, and ecofriendly4–7

Currently, there are numerous combinations of agricultural by-products (such as

coconut shell,4–7 palm kernel shell,8 corn cob,9 durian seed,10 rice husk,11 banana fiber,12

rapeseed,13 sunflower stalk,14 and sunflower seed cake15) and thermoplastic materials

had been developed by researchers to produce biocomposites. Moreover, some of the

biocomposites already marketed in Malaysia, like Melsom Biodegradable Enterprise

made eco-friendly tableware from rice husk-based thermoplastic biocomposites mate-

rial.7 In our previous studies, coconut shell and corn cob was introduced to polylactic

acid thermoplastic to produce ecopackaging and ecotableware materials.6,9 Currently,

the developments of polypropylene (PP)/CPH biocomposites have the potential to

replace forest product, such as wooden fittings, fixtures, deck, and furniture. This will

reduce the forest consumption in cutting trees and give benefit to environment.

Generally, the poor adhesion between natural filler and thermoplastic matrix due to

the polarity difference are common issue among researchers and industries. Therefore,

without coupling agents, biocomposites with good filler dispersion, filler–matrix

adhesion, and mechanical strength are hard to achieve. Usually, maleated polymers

(such as maleated polypropylene (MAPP) and maleated polyethylene (MAPE)) are used

as coupling agents in the production of natural filler-based biocomposites. During com-

pounding process at temperature above 170�C, maleated polymer is active and then the

maleic anhydride group is chemically reacting with natural filler via esterification.16 The

long polymer chains covalently bond to the filler surface provide more efficient interfa-

cial bonding with matrix. Thus, many researchers reported the biocomposites properties

were remarkably improved by adding maleated polymer.10,14–21

Nowadays, there is no research on CPH-filled PP biocomposites. Against this back-

ground, the present research has been undertaken, with the aim to utilize CPH as filler in

PP biocomposites. The present research is to investigate the effect of filler content and

MAPP on processing torque, mechanical properties, thermal properties, and morphology

of PP/CPH biocomposites.

Methodology

Materials

CPH was collected from cocoa plantations, Perak, Malaysia. First, the CPH was dried in

oven at 80�C for 24 h. The dried CPH was crush into small pieces and further ground into

2 Journal of Thermoplastic Composite Materials

2




fine powder. The CPH powder was sieved and average particle size of CPH powder was

22 mm, which was measured by Malvern Particle Size Analyzer Instrument (Italy). PP

type co-polymer, grade SM 340 used as matrix, was supplied by Titan Petchem (M) Sdn.

Bhd (Malaysia). MAPP used was obtained from Sigma Aldrich (Penang, Malaysia).

Melt compounding and molding procedures

The PP/CPH biocomposites with and without MAPP were compounded using Bra-

bender1 plastograph intermixer, Model EC PLUS (Germany) at a counterrotating mode

of 180�C and rotor speed of 50 r min�1 (Table 1). The mixing procedures involved (i) PP

transferred into mixing chamber for 3 min until it melted homogeneously and (ii) CPH was

added to molten PP and mixed continuously for 5 min. The total time for compounding

was 8 min. All the compounds were molded into 1 mm thickness sheet using hot press,

model GT 7014A (Taiwan) at 180�C. The compression sequences involved (i) preheat

compound for 4 min; (ii) compress under pressure of 100 kgf cm�2 for 1 min; and (iii) cool

under the same pressure for 5 min. The PP/CPH biocomposites sheet was cut into tensile

bar using dumbbell cutter with the dimension referring to ASTM D638 type IV.

Processing torque measurement

The processing torque was measured during the compounding of PP/CPH biocomposites

by using Brabender plastrograph internal mixer. The torque changes with time in

biocomposites were recorded, and the torques versus time curves were plotted by com-

puter. The torque values at the end of processing time were taken as stabilization torque.

Tensile testing

The tensile tests were carried out by Instron Universal Testing Machine, model 5569

(Massachusetts, USA). The load cell selected was 50 kN and the cross-head speed used

was 30 mm min�1. The test was performed at 25 + 2�C condition.

Morphological analysis

The tensile fracture surface of PP/CPH biocomposites were analyzed using SEM, model

JEOL JSM-6460 LA (Japan). The samples were coated with a thin layer of palladium for

conductive purpose and analyzed at 5 keV.

Table 1. Formulation of PP/CPH biocomposites.a

Materials PP (phr) CPH (phr) MAPP (phr)

PP/CPH without MAPP 100 0, 10, 20, 30, 40 –PP/CPH with MAPP 100 10, 20, 30, 40 5*

aCPH: cocoa pod husk; PP: polypropylene; MAPP: maleated polypropylene; phr: part per hundred resin.*5 phr from weight of PP.

Chun et al. 3

3




Fourier transform infrared (FTIR) spectroscopy

FTIR analysis of neat PP, PP/CPH biocomposites with and without MAPP was carried

out by PerkinElmer Spectrum FTIR, Model Paragon 1000 (Germany) and the attenuated

total reflectance (ATR) method was applied. The sample was recorded with 4 scans in

the frequency range of 4000–650 cm�1 with a resolution of 4 cm�1.

Differential scanning calorimetry analysis

Differential scanning calorimetry (DSC) analysis was evaluated using DSC Q10,

Research Instrument (California, USA). The sample was cut into small pieces and placed

into closed aluminum pan with sample weight in range of 7 + 2 mg. The specimen was

heated from 30 to 200�C with a heating rate of 10�C min�1 under nitrogen atmosphere.

The nitrogen gas flow rate was 50 ml min�1. The degree of crystallinity of biocomposite

(Xc) can be evaluated from DSC data using the following equation:

Xc ¼ �Hf =�H0f

� �� 100; ð1Þ

where �Hf is the heat fusion of the PP/CPH biocomposites and �H0f the heat fusion for

100% crystalline PP (�H100 ¼ 209 J g�1).

Thermogravimetric analysis

Thermogravimetric analysis (TGA) was carried out using TGA Pyris Diamond Perkin

Elmer apparatus (California, USA). The sample was about 7 + 2 mg in weight and was

placed into platinum crucible. Then, the sample was heated from 30 to 700�C at a heating

rate of 10�C min�1 under nitrogen atmospheric condition with the nitrogen flow rate of

50 ml min�1.

Results & discussion

Processing torque

The processing torque verses time curves for PP/CPH biocomposites with and without

MAPP are shown in Figure 1. The first processing torque increased rapidly, while the PP

pellets transferred into the mixing chamber. This was due to the shearing action from the

solid PP pellets. The processing torque was reduced gradually with the change in

viscosity as the PP pellets was melted at high temperature and under shearing. For

PP/CPH biocomposites, a second processing torque was increased at time after 3 min.

This is due to the fact that dispersive resistance from CPH particles increased the

viscosity of PP matrix. Furthermore, the processing torque gradually decreased and

achieved the stabilization torque after PP and CPH were homogenously mixed. This was

a common trend that was also followed in our previous research and by other

researchers.22,23 Figure 2 shows the stabilization torque of PP/CPH biocomposites with

and without MAPP. The stabilization torque of both biocomposites increased with

increasing of CPH content. This was because the dispersed CPH particles in molten PP


4




restricted the polymer chain mobility. Thus, the disperse resistance tend to rise the

stabilization torque at higher filler content. Moreover, the presence of MAPP increased

the stabilization torque of PP/CPH biocomposites. This might be the MAPP enhanced

the dispersion and filler–matrix adhesion of PP/CPH biocomposites, which contributed

to higher viscosity and resulting higher stabilization torque compared to PP/CPH

biocomposites without MAPP.

Figure 1. The torque versus time curves of PP/CPH biocomposites with and without MAPP atdifferent filler contents. CPH: cocoa pod husk; PP: polypropylene; MAPP: maleated polypropylene.

Figure 2. Stabilization torque of PP/CPH biocomposites with and without MAPP at different fillercontent. CPH: cocoa pod husk; PP: polypropylene; MAPP: maleated polypropylene.

Chun et al. 5

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Tensile properties

Figure 3(a) shows the effect of CPH content on tensile strength of PP/CPH biocompo-

sites with and without MAPP. The tensile strength of both PP/CPH biocomposites

decreased with increasing CPH content. The decrement in tensile strength might be

attributed the low ability of stress transfer by CPH particle with low aspect ratio. Poor

wet ability, filler dispersion, and interfacial bonding between hydrophilic CPH and

hydrophobic PP matrix also contributed to a weak interfacial bonding. Therefore, the

efficiency of stress transfer was reduced by the weak interfacial bonding between filler

and matrix. The tensile strength of PP/CPH biocomposites with MAPP was higher than

PP/CPH biocomposites without MAPP. However, the presence of MAPP in the bio-

composites does not bring much change compared to neat PP. This was because the

maleic anhydride group from MAPP reacted with hydroxyl group on the CPH filler

surface via esterification. The PP chain from MAPP covalently bond on the CPH

particles provided a better wettability and enhanced the filler–matrix interaction. This

statement was agreed by many other researchers10,14–24

The elongation at break of PP/CPH biocomposites with and without MAPP is shown

in Figure 3(b). The result indicated the elongation at break of PP matrix was abruptly

Figure 3. Effect of filler content on (a) tensile strength, (b) elongation at break, and (c) tensilemodulus of PP/CPH biocomposites with and without MAPP. CPH: cocoa pod husk; PP: polypro-pylene; MAPP: maleated polypropylene.


6




reduced by the addition of CPH. It is possible that the presence of rigid interface between

CPH particle and PP matrix decreased the deformability of PP matrix. This led to more

rigid and stiffer biocomposites. The PP biocomposites with coconut shell,7 rattan,22 rice

husk,23 wood flour,24,25 and chitosan26,27 also showed a similar trend on elongation at

break. Furthermore, the PP/CPH biocomposites with MAPP exhibit lower elongation at

break than PP/CPH biocomposites without MAPP. This was because the MAPP

enhanced the interfacial interaction between CPH and PP matrix, and it generated a

stronger interfacial bonding due to the reduction of molecular chain flexibility. This was

a general observation that was also found by other researchers.24,25

Figure 3(c) illustrated the tensile modulus of PP/CPH biocomposites with and without

MAPP increased with the CPH content. Both biocomposites showed increased trend on

tensile modulus because the CPH particles were rigid than the PP matrix. Therefore, the

stiffness of biocomposites increased with the additional of CPH as expected. The increase

in tensile modulus was also supported by the increased surface crystallization over bulk

crystallization of PP matrix. The presence of CPH promoted transcrystalline formed

around the filler surface. As a result, the crystalline region in PP matrix increased, and it

might increase the tensile modulus of biocomposites. The tensile modulus of PP/CPH

biocomposites was significantly higher with the addition of MAPP. The increase in tensile

modulus of biocomposites was due to the improvement in interfacial bonding between the

CPH and the PP matrix. A strong interfacial bonding also enhanced the nucleating effect of

CPH on PP matrix, and it yields stiffer biocomposites. Some other researchers also found

that the addition of natural filler increased the tensile modulus of PP biocomposites and the

tensile modulus was further increased by MAPP.10,14–24

Morphological study

Figure 4(a) and (b) displays the SEM micrograph of tensile fracture surface of PP/CPH

biocomposites without MAPP at 20 and 40 phr CPH content. The SEM micrographs

Figure 4. SEM micrograph of PP/CPH biocomposites without MAPP at (a) 20 phr and (b) 40 phr ofCPH content. CPH: cocoa pod husk; PP: polypropylene; MAPP: maleated polypropylene.

Chun et al. 7

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show poor filler dispersion and agglomeration of CPH particles. This indicated the

incompatibility between hydrophilic CPH and hydrophobic PP matrix. Figure 4(a) and

(b) also shows fibrils, indicating plastic deformation of PP matrix. This was because of

the poor filler–matrix interaction and the PP matrix can deform independently until CPH

particles inherent in the deformation. The poor filler–matrix interaction also assigned to

the holes present and detached CPH particle. In contrast, SEM micrographs of PP/CPH

biocomposites with MAPP exhibit brittle fracture surface, and the CPH particles were

embedded as well as coated by the PP matrix (as shown in Figure 5(a) and (b)). It can be

explained as due to the incorporation of MAPP, which enhanced the interfacial

interaction between the CPH particles and the PP matrix, increasing the stiffness of

biocomposites and inducing brittle fracture behavior. The SEM micrographs also show

the CPH particles were well dispersed and the agglomeration was not observed, because

the MAPP improved the compatibility between CPH and PP matrix.

FTIR analysis

The FTIR spectrums of neat PP, PP/CPH biocomposites with and without MAPP are

shown in Figure 6. The main characteristic peak of neat PP and PP/CPH biocomposites is

listed in Table 2. The peaks at 3000–2800 cm�1 were contributed by C–H stretching

vibrations in PP chains. The peaks found at 1457 and 1376 cm�1 were assigned to –CH2

and –CH3 bending vibration in PP. Moreover, 3 small peaks found at 1167, 998, and 973

cm–1 were due to –CH3 symmetric deformation vibration and –CH3 rocking vibration of

PP. The broad peak of around 3300 cm�1 on PP/CPH biocomposites was attributed by –

OH group from CPH. Furthermore, the absorption peak at 1601 cm�1 assigned to C¼C

stretching from hemicelluloses and the broad absorption peak at 1043 cm�1 was

exhibited at the C–O–C and C–O groups from the main carbohydrates of cellulose and

lignin. Regarding the FTIR spectrum, a new peak at 1737 cm�1 on PP/CPH biocomposites

Figure 5. SEM micrograph of PP/CPH biocomposites with MAPP at (a) 20 phr and (b) 40 phr ofCPH content. CPH: cocoa pod husk; PP: polypropylene; MAPP: maleated polypropylene.


8




with MAPP was found. This evidenced the presence of ester linkage (C–O) between CPH

and MAPP. The schematic reaction of MAPP and CPH is illustrated in Figure 7.

DSC properties

The DSC curves of neat PP, PP/CPH biocomposites with and without MAPP are illustrated

in Figure 8. The DSC data of neat PP and both biocomposites were summarized in Table 3.

The neat PP exhibited a melting temperature (Tm) at 165�C and the crystallinity (Xc) was

Figure 6. FTIR spectrums of neat PP, PP/CPH biocomposites with and without MAPP. CPH: cocoapod husk; PP: polypropylene; MAPP: maleated polypropylene; FTIR: Fourier transform infrared.

Table 2. Functional groups of neat PP and PP/CPH biocomposites.a

Wave number (cm�1) Functional group

3300 Hydroxyl group (–OH) of CPH2950, 2918, 2868, 2839 C–H stretching vibration of PP1737 Ester linkage (C–O) between MAPP and CPH1601 C¼C stretching from hemicellulose1457 –CH2 bending vibration of PP1376 –CH3 bending vibration of PP1167 –CH3 symmetric deformation vibration of PP1043 C–O–C and C–O groups from main carbohydrates of cellulose and

lignin998, 973 –CH3 rocking vibration of PP

aCPH: cocoa pod husk; PP: polypropylene.

Chun et al. 9

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27%. In Table 3, the Tm of PP/CPH biocomposites showed no significant change with the

addition of CPH. However, the increase in the CPH content had increased the crystallinity

of PP/CPH biocomposites. This could be explained by the nucleating effect from the CPH

particles. This result was consistent with a previous study.4–7,26,27 The Tm of PP/CPH

biocomposites with MAPP was slightly dropped to lower temperature. Similar observation

Figure 7. Schematic reaction between MAPP, CPH, and PP matrix. CPH: cocoa pod husk; PP:polypropylene; MAPP: maleated polypropylene.

Figure 8. DSC curves of PP/CPH biocomposites with and without MAPP. CPH: cocoa pod husk;PP: polypropylene; MAPP: maleated polypropylene; DSC: differential scanning calorimetry.


10




was also found by Nadir et al.15 Alternately, the crystallinity of PP/CPH biocomposites

increased with MAPP. This is due to the presence of MAPP, promoting the migration and

diffusion of PP chain to growth crystalline structure on the CPH filler surface.

Properties of TGA

The derivative thermogravimetry (DTG) and TGA curves of neat PP, PP/CPH

biocomposites with and without MAPP are illustrated in Figure 9 and 10. The TGA data

are summarized in Table 4. Figure 10 shows, the neat PP decomposed in single step at

temperatures above 300�C. The CPH decomposed in 3 steps, included (i) evaporation

of moisture in CPH at a temperature of 30–100�C; (ii) decomposition of hemicelluloses

at a temperature of 200–350�C; and (iii) decomposition of lignin and cellulose at a

Table 3. DSC data of PP/CPH biocomposites with and without MAPP.a

Materials Tm (�C) �H (J/g) Xc (%)

Neat PP 165 57 27PP/CPH:100/20 (without MAPP) 165 59 28PP/CPH:100/40 (without MAPP) 165 65 31PP/CPH:100/20 (with MAPP) 163 68 33PP/CPH:100/40 (with MAPP) 163 83 40

aCPH: cocoa pod husk; PP: polypropylene; MAPP: maleated polypropylene; Tm: melting temperature; Xc:

degree of crystallinity of biocomposite; �H: heat fusion of the PP/CPH biocomposites; DSC: differential

scanning calorimetry.

Figure 9. DTG curves of PP/CPH biocomposites with and without MAPP. CPH: cocoa pod husk;PP: polypropylene; MAPP: maleated polypropylene; DTG: derivative thermogravimetry.

Chun et al. 11

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temperature above 350�C. All PP/CPH biocomposites show an early thermal decom-

position compared to neat PP as it can be observed through the temperature at 5% weight

loss (Td5%). The Td5% of PP/CPH biocomposites dropped with increasing content of the

CPH. The weight loss of early decomposition was contributed by the decomposition of

low thermal stability hemicellulose from CPH. Therefore, PP/CPH biocomposites with

higher filler content would undergo early decomposition at lower temperature. These

phenomena also found in coconut shell powder and corn cob filled thermoplastic

biocomposites, in our previous research.5–9 On the other hand, the decomposition

temperature at maximum rate (Tdmax) of PP/CPH biocomposites was shifted to higher

temperature as compared with neat PP. The Tdmax of PP/CPH biocomposites also rep-

resent the thermal stability of PP matrix. The increased thermal stability was also

reflected to higher residue content at a temperature of 700�C. The residue content of PP/

CPH biocomposites increased with increase in CPH content. This phenomenon can be

Figure 10. TGA curves of PP/CPH biocomposites with and without MAPP. CPH: cocoa pod husk;PP: polypropylene; MAPP: maleated polypropylene; TGA: thermogravimetric analysis.

Table 4. TGA data of PP/CPH biocomposites with and without MAPP.a

Sample Td5% (�C) Tdmax (�C) Residue at 700�C (%)

Neat PP 336 418 1.22PP/CPH:100/20 (without MAPP) 272 422 2.69PP/CPH:100/40 (without MAPP) 246 432 4.22PP/CPH:100/20 (with MAPP) 283 443 3.70PP/CPH:100/40 (with MAPP) 251 449 6.49

aCPH: cocoa pod husk; PP: polypropylene; MAPP: maleated polypropylene; Td5%: temperature at 5% weight

loss; Tdmax: decomposition temperature at maximum rate; TGA; thermogravimetric analysis.


12




explained by the generation of high thermal stability pyrolysis product from early

decomposition of hemicelluloses provided thermal insulative behavior to matrix, thus

the process of thermal decomposition of biocomposites was delayed.4–7 As a result, the

Tdmax and residue content of PP/CPH biocomposites increased at higher CPH content.

Furthermore, the thermal stability of PP/CPH biocomposites with MAPP was better than

PP/CPH biocomposites without MAPP, because the PP/CPH biocomposites with MAPP

showed higher Td5%, Tdmax, and residue content. The existence of MAPP will enable a

better thermal protecting layer chemically bonded on CPH surface and providing

stronger filler–matrix interaction, thus assigned to better thermal stability.

Conclusions

The following conclusions were drawn from the above studies:

1. The processing torque of PP/CPH biocomposites increased with increasing CPH

content. The torque of PP/CPH biocomposites with MAPP were higher compared

to PP/CPH biocomposites without MAPP.

2. The increase in CPH content decreased the tensile strength and the elongation at break

of PP/CPH biocomposites but increased the tensile modulus. The addition of MAPP

improved the tensile strength and tensile modulus of PP/CPH biocomposites.

3. The PP/CPH biocomposites without MAPP exhibit poor filler dispersion, incompat-

ibility and interfacial interaction between CPH and PP matrix. The SEM micrograph

had proved the filler–matrix interaction of PP/CPH biocomposites was improved by

MAPP.

4. The crystallinity of PP matrix increased with the presence of CPH content, and it

increased with a change in CPH content. However, the Tm of PP matrix was not

influenced by the CPH content. The crystallinity of PP/CPH biocomposites

increased with the addition of MAPP.

5. The PP/CPH biocomposites exhibit an early thermal decomposition, but they showed

higher thermal stability at higher temperature. The thermal stability of PP/CPH bio-

composites with MAPP improved as the Td5%, Tdmax, and residue content at 700�Cincreased.

Funding

This research received no specific grant from any funding agency in the public, commer-

cial, or not-for-profit sectors.

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