Agrowaste-based composites from cocoa pod husk and polypropylene: effect of filler content and...

Article

Agrowaste-basedcomposites fromcocoa pod husk andpolypropylene: Effectof filler content andchemical treatment

Koay Seong Chun and Salmah Husseinsyah

AbstractCocoa pod husk (CPH)-filled polypropylene (PP) composites were prepared via meltcompounding. The effect of filler content and chemical treatment using 3-mercaptopropyltrimethoxysilane (MPS) and sodium dodecyl sulfate (SDS) on propertiesof composites were investigated. The results indicated that the treated composites withMPS and SDS improved the tensile strength, tensile modulus, thermal stability, stabiliza-tion torque, water resistivity, and crystallinity of composites. The treated compositeswith SDS show better tensile properties and water resistivity than composites treatedwith MPS. Scanning electron microscopic results show that the interfacial bondingbetween CPH and PP matrix improved with the presence of MPS or SDS.

KeywordsCocoa pod husk, polypropylene, composites, sodium dodecyl sulfate, 3-mercaptopro-pyltrimethoxysilane

Introduction

Cocoa is an important agricultural export commodity and the most widely planted crop

in several tropical countries including Malaysia.1–3 Currently, cocoa pod husk (CPH) is a

major by-product of the cocoa industry. In the cocoa industry, CPH is a nonfood part of

Division of Polymer Engineering, School of Materials Engineering, Universiti Malaysia Perlis, Perlis, Malaysia

Corresponding author:

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

Perlis, 02600 Jejawi, Perlis, Malaysia.

Email: [email protected]

Journal of Thermoplastic Composite

Materials

1–20

ª The Author(s) 2014

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DOI: 10.1177/0892705714563125

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the cocoa pod, and it accounts for as much as 76% of cocoa pod by weight.2,3 For every

ton of dry cocoa bean produced, there are 10 tons of CPH generated as waste.4 The CPH

is readily abundant but does not have any marketable value; therefore, the utilization of

CPH as natural filler could provide a new and sustainable source of filler for plastic

industry. Meanwhile, the waste levels of cocoa industry could be reduced.

In recent years, composites from agrowaste and thermoplastic materials have gar-

nered interest among researchers and industries due to today’s environmental issues and

economic advantages as well as the accumulation of agricultural waste by-product.5–8

Nowadays, various combination of agricultural waste and thermoplastic material have

been successfully produced commercial products. IKEA injection-molded furniture is a

well-known example of commercial product, which is made from polypropylene (PP)/

wood flour composites.9 In Malaysia, a series of eco-tableware was made from rice

husk-filled thermoplastic eco-composites by Melsom Biodegradable Enterprise.6 Hence,

the present research was comprehensively to utilize CPH as filler for PP composites. The

development of such composites has a potential to replace current forest product, such as

wooden fittings, fixtures, deck, and furniture as well.

Generally, the compounding of natural filler in thermoplastic materials would not

produce a good composite due to the poor interfacial compatibility between the hydro-

philic natural filler and the hydrophobic matrix.10,11 Thus, filler treatment using silane

coupling agent is one of the effective methods used to modify the hydrophilic properties of

natural filler. Silane is able to form covalent linkage with the hydroxyl groups of natural

filler.The alkyl chains from silane provided hydrophobic properties to natural filler and

improved the interfacial compatibility.12,13 Thus, many researchers reported that the silane

coupling agent does remarkably to improve the water resistivity and mechanical and

thermal properties of composites.12–21 Moreover, some literatures reported that the use of

fatty acid for filler treatment will also give a coupling effect to the composites. Fatty acid is

made from sustainable resources, and it is inexpensive compared to commercial silane

coupling agent. Consequently, fatty acid is an alternative choose of coupling agent besides

silane. In our previous study, sodium dodecyl sulfate (SDS) was used as coupling agent to

enhance the filler–matrix adhesion between coconut shell–polylactic acid6 and chitosan–

polypropylene.22 The use of SDS shows a remarkable result in improving water resistance

and tensile and thermal properties of composites.

Currently, the studies on PP/CPH composites were reported in our previous research,1–3

and the use of 3-mercaptopropyltrimethoxysilane (MPS) in natural filler-based composites

is not found in the literature. Thus, the present work was undertaken to compare the effect

of filler treatment using MPS and SDS on torque development, tensile properties, water

absorption, thermal properties, and morphology of PP/CPH composites.

Methodology

Materials

The discarded CPH was obtained from cocoa plantation, Perak, Malaysia. The CPH was

dried in an air circulatory oven at 80 C for 24 h. Then, the dried CPH was crushed into

2 Journal of Thermoplastic Composite Materials

2



small pieces and ground into fine powder. The average particle size of CPH produced

was 22 mm, which is analyzed by Malvem Particle Size Analyzer Instrument. PP, type

copolymer, grade SM 340, was supplied by Titan Petchem (M) Sdn. Bhd (Malaysia). The

melt flow index value of PP was 4.0 g/10 min at 230 C and density 0.9 g cm�3,

respectively. MPS (97%) and SDS (98%) were supplied by Sigma Aldrich (St Louis,

Missouri, USA). Ethanol (Fluka, Penang, 95%) was used as the solvent in filler

modification.

Filler treatment

Firstly, MPS (3% based on weight of filler) was dissolved into ethanol. The CPH

powder was added into MPS solution and stirred continuously for 1 h. The CPH was

soaked in MPS solution and left overnight. The soaked CPH was filtered and dried

in an oven at 80 C for 24 h. The similar procedure was carried out for filler

treatment using SDS.

Melt compounding and molding procedures

The formulation of PP/CPH composites are listed in Table 1. All composites were

compounded using a Brabender1 Plastrograph intermixer (model EC PLUS, Germany)

in counterrotating mode at 180 C and a rotor speed of 50 r min�1. 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

composites compound was collected from mixing chamber. All the compounds were

molded into 1-mm thick sheets using a compression molding machine (Gotech, Taiwan,

Model GT 7014A) at 180�C. The compression sequences involved were as follows: (i) pre-

heat the compound for 4 min; (ii) compress the compound under a pressure of 100 kgf cm�2

for 1 min; and (iii) cooling under the same pressure for 5 min. Then, the PP/CPH composite

sheets were cut into tensile specimens using a dumbbell cutter with dimensions according to

ASTM D638 type IV.23

Table 1. Formulation of PP/CPH biocomposites.

Materials Untreated PP/CPH Treated PP/CPH with MPS Treated PP/CPH with SDS

PP (phr) 100 100 100CPH (phr) 0, 10, 20, 30, 40 10, 20, 30, 40 10, 20, 30, 40MPS (wt%) – 3a –SDS (wt%) – 10, 20, 30, 40 3a

PP: polypropylene; CPH: cocoa pod husk; SDS: sodium dodecyl sulfate; MPS: 3-mercaptopropyltrimethox-

ysilane; phr: part per hundred.a3% from weight of CPH.

Chun and Husseinsyah 3

3



Processing torque measurement

The processing torque was measured during the compounding of composites using

Brabender Plastrograph internal mixer. The processing characteristics of composites

compound with the time were recorded and the torques versus time curves was plotted by

Brabender Mixer Program (WINMIX). The torque values at the end of processing time

were taken as stabilization torque.

Tensile testing

Tensile testing was performed by using an Instron Testing Machine (model 5569,

Norwood, Massachusetts, USA) according to ASTM D638 standard.23 The test was

carried out at 25 + 3 C. A crosshead speed of 30 mm min�1 was used and the load cell

selected was 50 kN. A minimum of five specimens were tested for each composite.

Morphological analysis

The fracture surface of tensile specimens were examined using scanning electron

microscope (SEM; model JEOL JSM-6460 LA, Japan). The samples were coated with a

thin layer of palladium for conductive purpose before they were analyzed at 5 keV.

Water absorption test

All composite samples with dimension 30� 25� 1 mm3 were prepared and dried before

immersed in water. The specimens were immersed in distilled water at room temperature

and the water absorption was determined by record sample weight at regular intervals. A

Mettler balance (model AJ150; Columbus, Ohio, USA) with precision of + 1 mg was

used to measure the weight of sample. The water absorption at time t (Wa) was calculated

by formulation below:

Wa ¼Wn �Wd

Wd

� 100; ð1Þ

where Wd and Wn are original dried weight and weight after exposure, respectively.

FTIR spectroscopy

Perkin Elmer Paragon 1000 Fourier transform infrared (FTIR) spectrometer (Waltham,

Massachusetts, USA) was used to characterize chemical functional groups in unmodified

and modified CPH. The attenuated total reflectance method was selected. The sample

was recorded with eight scans in the frequency range of 4000–600 cm�1 with a reso-

lution of 4 cm�1.


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DSC analysis

Differential scanning calorimetric (DSC) analysis was evaluated using DSC Q10 (TA

Instruments, USA). The specimen was cut into small pieces and placed into closed

aluminum pan with a weight in range of 7 + 2 mg. The specimen was heated from 30�Cto 200�C at 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 composite (Xc) can be cal-

culated from DSC data by following equation:

Xc ¼�Hf

�H0f

� 100; ð2Þ

where �Hf is the heat fusion of the PP composites, and �H0f is the heat of fusion for

100% crystalline PP (�H0f ¼ 209 J g�1).

The crystallinity of PP matrix (XPP) was calculated using following equation:

XPP ¼Xc

Wf PP

; ð3Þ

where Wf PP is the weight fraction of PP matrix.

Thermogravimetric analysis

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

Elmer apparatus. The samples were about 7 + 2 mg in weight and were placed into

platinum crucible. Then, the weight loss against temperature was measured at a heating

rate of 10�C min�1 and range of thermal scan from 30�C to 700�C.

Results and discussion

Processing torque

Figure 1 illustrated the processing torque versus times curves for the neat PP, untreated

PP/CPH composites with different CPH content. The first processing torque rose rapidly

due to the shearing action from the solid PP pellets. Then, the processing torque gra-

dually reduced indicating decreasing viscosity as PP pellets melt when subjected to high

temperature and continuous shearing. The second torque development can be found after

3 min. This is because the addition of the CPH which interrupted the flow of melted PP.

The processing torque decreased gradually and achieved stabilization torque after the

compound in homogenously mixed. Similar trends of processing torque have also been

reported by other researchers.1,16,24,25 The stabilization torque was increased with the

increasing of CPH content (as shown in Figure 2). This is due to the resistance from CPH

particles which increased the viscosity of PP.1,3 From Figure 2, the processing torque of

treated PP/CPH composites with MPS or SDS were higher as compared to untreated PP/

CPH composites. This might be because CPH treated with MPS or SDS have better filler

dispersion and adhesion with matrix that led to increase the viscosity of the composites.


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Tensile properties

Figure 3 shows the effect of filler content on tensile strength of untreated and treated PP/

CPH composites with MPS or SDS. The incorporation of CPH decreased the tensile

strength of PP matrix. This was a common trend for the particular natural filler

Figure 1. The torque–time curves of neat PP and PP/CPH composites with different filler content.PP: polypropylene; CPH: cocoa pod husk.

Figure 2. Stabilization torque of untreated and treated PP/CPH composites with MPS and SDS.PP: polypropylene; CPH: cocoa pod husk; MPS: 3-mercaptopropyltrimethoxysilane; SDS: sodiumdodecyl sulfate.


6



containing thermoplastic composites, whereas the similar trend was found in previous

work.5,6 Usually, the irregular shape CPH filler has low aspect ratio and poor efficiency

of transferring tensile stress from matrix to filler. Thus, the incorporation of CPH reduced

the tensile strength of PP matrix. The decreased tensile strength was also supported by the

weak interfacial adhesion and the presence of filler agglomeration. Alternatively, the

tensile strength of treated PP/CPH composites increased compared with the untreated PP/

CPH composites, but the tensile strength was still lower as compared to neat PP. The

treated CPH with MPS or SDS had long alkyl chains covalent bonded on its surface that

lead to increase the wettability with PP matrix and it enhanced the interfacial bonding

between CPH and PP matrix. Chun et al.6,12 reported that treated coconut shell with silane

coupling agent and SDS improved the tensile strength of the composites.

The elongation at break of untreated and treated PP/CPH composites is displayed in

Figure 4. The results indicated that the elongation at break of PP/CPH composites had

decreasing trend as the CPH content increased. The addition of rigid CPH particles

restricted the PP chains mobility, which increased brittleness of composites. This is a

common trend found by other researchers.13,14,16,21,24 Treated PP/CPH exhibited lower

elongation at break values compared to untreated PP/CPH. The filler modification with

MPS or SDS altered the CPH properties, making it less hydrophilic and increased the

filler–matrix adhesion. Thus, the flexibility of PP matrix was reduced by the stronger

filler–matrix adhesion. Other researchers reported a similar influence of silane coupling

agent20 and SDS22 on the elongation at break of PP/chitosan composites.

However, the increase of CPH content has increased the tensile modulus of treated

and untreated PP/CPH composites (as illustrated in Figure 5). This is due to the fact that

Figure 3. Effect of filler content on tensile strength of untreated and treated PP/CPH compositeswith MPS and SDS.PP: polypropylene; CPH: cocoa pod husk; MPS: 3-mercaptopropyltrimethoxysilane; SDS: sodiumdodecyl sulfate.


7



the friction at interface region between CPH particles and PP matrix led to a rigid

interface which restricted the polymer chain mobility. This increased the rigidity and

stiffness of composites. A similar observation also reported by other researchers.26,27

Furthermore, the tensile modulus of PP/CPH composites increased by filler treatment

using MPS and SDS. It can be seen that the treated CPH with MPS or SDS had better

interfacial adhesion with PP matrix, which led to increase tensile modulus. A similar

result was also reported in the literature for treated composites with silane coupling and

fatty acid.5,19,24–27

The effect of filler treatment on interfacial interaction can be expressed quantitatively

by a simple model developed by Pukansky.28 This model (Equation (4)) takes into

account the most important factors influencing the tensile strength, such as (i) �n—the

change of specimen dimensions during the deformation and the rise of tensile strength

due to strain hardening; (ii) 1� �=1þ 2:5�—effect of reducing load bearing cross

section of matrix due to filling; and (iii) exp B�ð Þ—interfacial adhesion.28,29

�T ¼ �T0n

1� �1þ 2:5�

exp B�ð Þ� �

; ð4Þ

where �T and �T0are the true strength of the composite and the polymer matrix,

respectively (�T ¼ ��, where � is the measured engineering tensile strength), � is

relative elongation (�¼ L/L0, where L0 is the original length and L is length at the failure

point), n is related to strain hardening exponent of polymer matrix, � is the volume

fraction of filler, and B is a parameter expresses the load-bearing capacity of filler, which

Figure 4. Effect of filler content on elongation at break of untreated and treated PP/CPH compo-sites with MPS and SDS.PP: polypropylene; CPH: cocoa pod husk; MPS: 3-mercaptopropyltrimethoxysilane; SDS: sodiumdodecyl sulfate.


8



is related to the effect of interfacial adhesion. Equation (5) can be rewritten and

expressed in linear form:

ln �Tred ¼ln�T

n 1þ2:5�1��

� � ¼ ln�T0þ B�: ð5Þ

The plot of ln-reduced tensile strength (�Tred) as a function of filler content should

give a straight line with a slope of B. Figure 6 displayed the plot of reduced tensile

strength of untreated and treated PP/CPH composites versus filler content. A good linear

correlation is observed; therefore, parameter B expressing stress transfer and interfacial

adhesion can be determined with more accuracy. The slope is changed after filler treated

with SDS or MPS. The parameter B of treated PP/CPH composites with SDS or MPS

were 2.44 and 2.11, respectively, which is higher than untreated PP/CPH composites

(1.55). This confirms the interfacial adhesion was enhanced by filler treatment with SDS

or MPS. Meanwhile, the parameter B of treated PP/CPH composites with SDS was larger

than treated PP/CPH composites with MPS. The results indicated that the treated CPH

with SDS was better in enhancing interfacial adhesion and improved stress transfer. The

efficiency between SDS and MPS might be related to the molecular orientation on the

filler surface. The SDS molecules are possibly perpendicular attached on filler surface,

which result in better wetting of matrix to filler surface as shown in Figure 7(a). From

literature, it is found that the coupling agent like stearic acid would form a perpendicular

oriented structure on filler surface as illustrated in Figure 7(a). However, the molecules

of silane coupling agent might resulting in flat or bridge-like structure rather than per-

pendicular on filler surface as illustrated in Figure 7(b)- and (c).30 This probably

Figure 5. Effect of filler content on tensile modulus of untreated and treated PP/CPH compositeswith MPS and SDS.PP: polypropylene; CPH: cocoa pod husk; MPS: 3-mercaptopropyltrimethoxysilane; SDS: sodiumdodecyl sulfate.


9



influences the wettability between filler and matrix. Furthermore, the better wettability

yields a stronger interfacial bonding between filler and matrix. As a result, the treated

CPH with SDS shows a better adhesion with PP matrix compared to the treated CPH with

MPS. Demjen and Pukanszky31 also reported that the interfacial bonding was enhanced

when the molecules of coupling agent form a perpendicularly orientation on the filler

surface which improves the tensile strength and tensile modulus.

Morphology study

Figure 8(a) and (b) show SEM micrograph of tensile-fractured surface of untreated PP/

CPH composites with 20 and 40 wt% of CPH content. The SEM micrograph show that

the untreated CPH had poor dispersion and easily to form agglomeration, especially at

40 phr of filler content. The presence of holes due to filler pull out and detached CPH

particles can be observed. This indicated the weak interfacial bonding between untreated

CPH and PP matrix. In contrary, SEM micrographs of treated PP/CPH composites with

MPS or SDS illustrated a brittle fracture surface as shown in Figures 9(a) and (b) and

10(a) and (b). CPH treated with both MPS and SDS was embedded and well covered

by PP matrix. This evidenced the treated CPH had better adhesion with PP matrix.

Water absorption

The water absorption of untreated and treated PP/CPH composites with MPS or SDS is

illustrated in Figure 11. The water absorption of all composites increased with the

increasing of the filler content. Clearly, the untreated PP/CPH composites exhibited

Figure 6. Reduced tensile strength of untreated and treated PP/CPH composites plotted againstfiller content in the linear form of equation (4). Symbols: (a) untreated, (b) treated with MPS, and(c) treated with SDS.PP: polypropylene; CPH: cocoa pod husk; MPS: 3-mercaptopropyltrimethoxysilane; SDS: sodiumdodecyl sulfate.


10



higher water absorption than treated PP/CPH composites with MPS and SDS. The

increment of water absorption of composites is correlated to the ability of hydrophilic

CPH to form hydrogen bonding toward water molecules. The water absorption of

Figure 9. SEM micrograph of treated PP/CPH composites with MPS at (a) 20 phr and (b) 40 phr offiller content.SEM: scanning electron microscopy; PP: polypropylene; CPH: cocoa pod husk.

Figure 8. SEM micrograph of untreated PP/CPH composites at (a) 20 phr and (b) 40 phr of fillercontent.SEM: scanning electron microscopy; PP: polypropylene; CPH: cocoa pod husk.

Figure 7. Different idealized molecular orientation of coupling agent on filler surface: (a) perpen-dicular oriented, (b) flat laying, and (c) bridge-like structure.30


11



composites is higher at more filler content. In contrast, the water absorption of com-

posites is reduced with the presence of MPS and SDS. This reason is that the CPH treated

with MPS or SDS has hydrophobic aliphatic chains attached on filler surface, which

decreases the hydrophilicity of CPH. As a result, the tendency of treated CPH to bond

with water molecules was lower. Besides, the treated PP/CPH composites with SDS had

lower water absorption than treated PP/CPH composites with MPS. The treated CPH

Figure 11. Water absorption of neat PP (a); untreated PP/CPH at 20 phr (b) and 40 phr (c) fillercontent; treated PP/CPH with MPS at 20 phr (d) and 40 phr (e) filler content; treated PP/CPH SDSat 20 phr (f) and 40 phr (g) filler content.PP: polypropylene; CPH: cocoa pod husk; MPS: 3-mercaptopropyltrimethoxysilane.

Figure 10. SEM micrograph of treated PP/CPH composites with SDS at (a) 20 phr and (b) 40 phrof filler content.SEM: scanning electron microscopy; PP: polypropylene; CPH: cocoa pod husk; SDS: sodium dode-cyl sulfate.


12



with SDS probably had more efficiency in preventing diffusion of water molecules into

CPH due to the presence of longer hydrocarbon chains.

FTIR spectroscopy analysis

Figure 12 shows the FTIR spectra of untreated and treated CPH. The broad peak at 3291

cm�1 was assigned to the hydroxyl groups (–OH) of CPH. The peak at 2927 cm�1 related

to C–H stretching and a small peak at 1732 cm–1 was attributed to C¼O stretching of

carboxyl groups from hemicellulose. The peak of C¼C stretching from hemicellulose

and –OH group from embed moisture were detected at 1604 cm–1. The small peak

presence at 1518 cm–1 was indicated as conjugated C–O group from aromatic skeletal in

lignin. The peak at 1434 cm–1 was corresponding to CH2 deformation vibration of

cellulose. Moreover, the peak at 1372 cm–1 was referring to C–H group deformation in

cellulose and hemicellulose. A peak found at 1247 cm–1 was assigned as C–O groups

from acetyl group in lignin. Another broad peak detected at 1038 cm–1 was C–H group

vibration in cellulose. There are small peaks in the 700–900 cm�1 range related to C–H

vibration of lignin. The treated CPH with MPS or SDS show less hydrophilic properties

then untreated CPH as the peak’s intensity at 3291 and 1605 cm�1 were reduced. The

peak intensity at 1736 cm�1 increased after CPH treated with MPS. This is due to the

Figure 12. FTIR spectra of (a) neat CPH and treated CPH with (b) MPS and (c) SDS.FTIR: Fourier transform infrared; CPH: cocoa pod husk; MPS: 3-mercaptopropyltrimethoxysilane;SDS: sodium dodecyl sulfate.


13



presence of covalent linkage between MPS and CPH. The peak’s intensity at 775 cm�1

significantly increased due to presence of Si–C group from MPS on CPH. Alternatively,

the other characteristic peaks of MPS could not be found in modified CPH. This might be

due to the overlapping of characteristic peaks between MPS and CPH. Figure 13 shows

the schematic diagram of chemical reaction between MPS and CPH. The reaction steps

of MPS involved are as follows: (i) MPS undergoes hydrolysis to become silanols and

some will be self-condensation to form silanol oligomer; (ii) silanol oligomer is physi-

cally absorbed to hydroxyl group of CPH; (iii) condensation of silanol oligomer and

form Si–O–C bond between MPS and CPH. Besides, the treated CPH with SDS also

shows an increasing of peak intensity at 1737 cm�1 that attributed to the formation of

ester linkage between SDS and CPH. The peak intensity at 2923 and 2853 cm�1 increased

due to the presence of long alkyl chains from SDS attached on CPH surface via covalent

bonding. The schematic reaction between SDS and CPH was shown in Figure 14.

Differential scanning calorimetry

The DSC curves of neat PP and untreated and treated PP/CPH composites are shown in

Figure 15. The DSC data of neat PP and all composites are listed in Table 2. The melting

temperature (Tm) of neat PP can be observed at 165 C and the crystallinity of neat PP is

27%. According to Table 2, the increase in CPH content increased the crystallinity of PP/

CPH composites and PP matrix as well. The presence of CPH provided a site for

nucleation as it promotes the migration and diffusion of PP chain nucleation sites for

initiation of spherulites growth. Furthermore, the treated PP/CPH composites with MPS

or SDS show a higher crystallinity than untreated PP/CPH composites. This result

indicated treated CPH with MPS or SDS that might had better adhesion with PP matrix

Figure 13. Schematic reaction between MPS and CPH.CPH: cocoa pod husk; MPS: 3-mercaptopropyltrimethoxysilane.


14



and resulting a strong nucleating effect to composites. The addition of natural filler gives

nucleating effect to composites and the nucleating effect is increased with the presence

of silane coupling agent or fatty acid were reported by other researchers.5,12,15,20 The Tm

of PP/CPH composites was not influenced by changes of CPH content and SDS or MPS

Figure 14. Schematic reaction between SDS and CPH.CPH: cocoa pod husk; SDS: sodium dodecyl sulfate.

Figure 15. DSC curves of neat PP, untreated and treated PP/CPH with composites MPS and SDS.DSC: differential scanning calorimetry; PP: polypropylene; CPH: cocoa pod husk; SDS: sodiumdodecyl sulfate; MPS: 3-mercaptopropyltrimethoxysilane.


15



modification. Some literatures also reported that the increase of filler content or filler

modification did not tend to change the Tm of composites.6,12,22,32 Generally, the Tm of

the semicrystalline polymer is increased with the increasing of spherulites dimensions.33

The presence of particulate filler usually causes a heterogeneous nucleating effect to

semicrystalline polymer.34,35 The number of smaller spherulites are probably growth due

to the increase of filler content or filler modification, which increased the crystallinity of

composites, but it does not affect the Tm.36,37

Thermogravimetric analysis

The derivative thermogravimetric (DTG) and TGA thermograms of neat PP, CPH and

untreated and treated PP/CPH composites are illustrated in Figures 16 and 17, respec-

tively. The DTG and TGA data of samples are summarized in Table 3. As shown in

Figure 16, the CPH exhibited three decomposition steps. First, the dehydration of

moisture and other volatile compound in CPH at temperature about 60 C. Then, the

weight loss is attributed to the decomposition of hemicellulose at temperature about 250

C, followed by further decomposition of lignin and cellulose at temperature above 350 C.

The increased CPH content reduced the thermal stability of PP/CPH composites com-

pared to neat PP as evidenced from the temperature at 5% weight loss (Td5%). This

indicated that the PP/CPH composites had undergone an early thermal decomposition

and the weight loss at Td5% was attributed to the loss of moisture, volatile material, and

hemicellulose in the CPH. However, the temperature at maximum weight loss (Tdmax) of

the PP/CPH composites shifted to higher temperature and the residue content increased

as CPH content increased. This is because the addition of more CPH increased the

thermal stability of PP/CPH composites to higher decomposition temperature. This

phenomenon occurred due to the formation of char residue from thermal decomposition

of hemicellulose, which acts as thermal protecting layer on the composites and delaying

the process of thermal decomposition.38,39 Araujo et al.40 also reported that increasing

the curaua fiber content raised the formation of char residue and inhibited the thermal

decomposition of the biocomposites. The thermal stability of treated PP/CPH composites

with SDS or MPS was higher than untreated PP/CPH composites, as can be seen from the

Table 2. DSC data of neat PP, untreated and treated PP/CPH composites with MPS and SDS.

Materials Tm (�C) �H (J g�1) Xc (%) Xpp (%)

Neat PP 165 57 27 27Untreated PP/CPH:100/20 165 59 28 34Untreated PP/CPH:100/40 165 65 31 43Treated PP/CPH:100/20 (MPS) 165 69 33 40Treated PP/CPH:100/40 (MPS) 165 73 35 49Treated PP/CPH:100/20 (SDS) 164 78 37 44Treated PP/CPH:100/40 (SDS) 164 86 41 57

DSC: differential scanning calorimetry; PP: polypropylene; CPH: cocoa pod husk; SDS: sodium dodecyl sulfate;

MPS: 3-mercaptopropyltrimethoxysilane; Tm: melting temperature; Xc: crystallinity of composites; �H: heat

fusion of composites; XPP: crystallinity of PP matrix; PP: polypropylene


16



Figure 16. DTG curves of CPH, neat PP, untreated and treated PP/CPH composites with MPSand SDS.DTG: derivative thermogravimetry; PP: polypropylene; CPH: cocoa pod husk; SDS: sodium dode-cyl sulfate; MPS: 3-mercaptopropyltrimethoxysilane.

Figure 17. TGA curves of CPH, neat PP, untreated and treated PP/CPH composites with MPS andSDS.TGA: thermogravimetric analysis; PP: polypropylene; CPH: cocoa pod husk; SDS: sodium dodecylsulfate; MPS: 3-mercaptopropyltrimethoxysilane.


17



increase in Td5%, Tdmax, and residue content. This was because the filler modification with

MPS and SDS improved the filler dispersion and filler–matrix adhesion that further

enhanced the thermal stability of composites against thermal decomposition. Arbelaiz

et al.41 found that the PP/flax fiber composites with silane modification had improved the

thermal stability of the composites. Arrakhiz et al.42 reported the PP composites with alfa

fiber treated with palmitic acid show a better thermal stability.

Conclusions

The tensile strength and elongation at break of PP/CPH composites decreased with

increasing CPH content. However, the processing torque, tensile modulus, water

absorption, and crystallinity of PP/CPH increased with increasing CPH content. The

incorporation of CPH causes early thermal decomposition to PP/CPH composites. The

processing torque, tensile strength, tensile modulus, water resistivity, crystallinity, and

thermal stability of PP/CPH composites increased with the presence of MPS or SDS. The

properties of PP/CPH composites improved due to the enhanced filler–matrix adhesion

by filler treatment with MPS or SDS. The SEM results evidenced that the CPH treated

with MPS or SDS has better filler dispersion and interfacial adhesion with PP matrix.

Overall, PP/CPH composites treated with SDS show better performance in enhancing

tensile properties and water absorption than PP/CPH composites treated with MPS, but

PP/CPH composites treated with both MPS and SDS show similar improvement in the

thermal properties.

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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