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used for acetic anhydride originally. For consistency,

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4 Please provide city name for the manufacturers Thye

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compression moulding machine and Wallace die cutter

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17

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Article

Comparison ofprocessing andmechanical properties ofpolypropylene/recycledacrylonitrile butadienerubber/rice husk powdercomposites modifiedwith silane and aceticanhydride compoundAQ 1

S. Ragunathan1, H. Ismail1 and K. Hussin2

AbstractPolypropylene (PP)/recycled acrylonitrile butadiene rubber (NBRr)/rice husk powder(RHP) composites were fabricated with silane and acetic anhydride (Ac) treatmentagent. The in situ formed RHP-filled PP/NBRr composites were prepared by melt mixingtechnique. The mechanical properties of both the treatment methods were investigatedwith Instron mechanical analysis and Fourier transform infrared. The results indicatedthat Ac treatment was found to exhibit better mechanical properties of RHP-filled PP/NBRr composites treated with silane. This was due to good compatibility and strongerinteraction between anhydride moieties with PP/NBRr.

KeywordsMechanical properties, polypropylene, acrylonitrile butadiene rubber, rice husk powder,acetic anhydride, silane

1 School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Nibong Tebal, Penang,

Malaysia2 School of Environmental Engineering, Universiti Malaysia Perlis, Arau, Perlis, Malaysia

Corresponding author:

H. Ismail, School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong

Tebal, Penang, Malaysia.

Email: hanafi@eng.usm.my

Journal of Thermoplastic Composite

Materials

00(0) 116

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DOI: 10.1177/0892705712475006

jtc.sagepub.com

1

Introduction

In recent years, the incorporation of lignocellulosic materials as reinforcing agents or as

fillers in polymer composites has received an increased attention. The addition of fillers

has a high impact upon economics for thermoplastics, while a general improvement in

certain properties is also achieved. Lignocellulosic materials exhibit a number of

attractive features including low density, low requirements on processing equipment,

less abrasion during processing, abundance and certainly biodegradability.14 The main

advantage of lignocellulosic materials upon mineral fillers is their environmental

friendliness. In general, polymer waste is disposed in large landfills causing serious

problems in the environment, while biodegradable materials are envisaged to be an

excellent alternative to tackle this problem by reducing the waste volume. Considerable

amount of studies has been carried out on utilizing natural fillers such as sago, sisal, short

silk fibre, oil palm empty fruit bunch, rice husk ash, jute fibre, rubber wood powder, jute,

hemp, sisal, cotton stalk, kenaf, sugarcane banana fibres and other cellulosic fibres as

reinforcement materials in various waste polymeric materials.4,5 Consequently, it has not

been surprising that the use of lignocellulosic materials in the production of composites

has gained significant importance in various manufacturing fields and industries.58

The major disadvantage encountered during the incorporation of natural lig-

nocellulosic materials into polymers is the lack of good interfacial adhesion between the

two components, which results in poor properties of the resulting material.9 The polar

hydroxyl groups on the surface of the lignocellulosic materials have difficulty in forming

a well-bonded interface with a non-polar matrix, as the hydrogen bonds tend to prevent

the wetting of the filler surfaces. Besides, the incorporation of lignocellulosic materials

in a synthetic polymer is often associated with agglomeration as a result of insufficient

dispersion, caused by the tendency of the fillers to form hydrogen bonds with each other.

This incompatibility leads to poor mechanical properties and high water absorption,

especially when the matrix is hydrophilic.

Thus, in order to develop composites with good properties, it is necessary to improve

the interface between the matrix and the lignocellulosic material. There are various

methods for promoting interfacial adhesion in systems where lignocellulosic materials

are used as fillers, such as esterification,1018 silane treatment,19,20 graft co-poly-

merization,21 use of compatibilizers,22 plasma treatment23 and treatment with other

chemicals.24 These methods are usually based on the use of reagents that contain

functional groups that are capable of bonding to the hydroxyl groups of the lig-

nocellulosic material, while maintaining good compatibility with the matrix. Interfacial

compatibilization improves the stress transfer between the two components and leads to

the improvement of mechanical and physical properties of the produced composites.

Esterification by means of acetylation and silane treatment is the common chemical

modification procedure that has been studied the most.1020 However, so far no work has

been reported on mechanical comparison of rice husk powder (RHP) acetylation using

acetic anhydride (Ac) and silane treatment using -aminopropyltrimethoxysilane (-APS) for the purpose of manufacturing RHP-filled polypropylene (PP)/recycled acrylo-

nitrile butadiene rubber (NBRr) composites.AQ 2

2 Journal of Thermoplastic Composite Materials 00(0)

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The aim of the present work is to evaluate and compare the mechanical properties of

RHP filler PP/NBRr composite utilizing Ac and -APS treatment agents in polymerwaste such as NBRr and PP. Processing stabilization torque, mechanical properties,

Fourier transform infrared (FTIR) and morphological properties of both the composites

were investigated and compared.

Experimental

Materials

The materials used for the preparation of RHP-filled PP/NBRr composites are shown in

Table 1. The rice husk powder (RHP) were ground in a table-type pulverizing machine

(Rong Tsong Precision Technology Co., Product id: RT-34) with speed of 2850 r min1,sieved at 300500 mm in particle size and dried at 110C for 24 h in a vacuum oven toproduce RHP of homogeneous fractions.AQ 3

Ac treatment

The RHP fibre were dipped in glacial acetic acid for 30 min. The acid was drained and

the fibres were dipped in 50% Ac solution and stirred for 1 h, with filler to solution ratioat 1:25. A few drops of concentrated sulphuric acid were also added as catalyst. The RHP

fibre is finally washed in distilled water for few times and then dried in the vacuum oven

at 80C for 24 h.

Silane treatment (-APS)

The -APS treatment reaction for RHP was carried out in a mixture of water and ethanol(40/60, vol, respectively). -APS of 3 g was first introduced into 1000 mL of water/ethanol mixture and was allowed to stand for 1 h. The pH of the solution was maintained

Table 1. Materials specification and description.AQ 4

Material Description Source

Polypropylene (PP) Code: 331; MFI: 14 g/10 min at230C; density: 0.9 g cm3

Titan Pro Polymers (M) Sdn.Bhd., Johor, Malaysia

Recycled acrylonitrilebutadiene rubber (NBRr)

Content: 33% of acrylonitrile;density: 1.015 g cm3

Juara One Resources Sdn. Bhd.,Penang, Malaysia

Rice husk powder (RHP) Cellulose 35%; hemicellulose25%; lignin 20%; ash 17%;density:1.4702 g cm3; size:300500 mm

Thye Heng Chan Enterprise Sdn.Bhd.

Treatment agent Acetic anhydride (Ac); -aminopropyltrimethoxysilane(-APS)

Alfa Aesar (M) Sdn. Bhd.

MFI: melt flow index.

Ragunathan et al. 3

3

at 4 by the addition of acetic acid. Then, 10 g of RHP was added into the solutions and

continuously stirred for 1.5 h. The treated RHP was filtered, dried by air and then by

vacuum oven at 80C for 24 h.

Processing and sample preparation

PP was mixed with NBRr and RHP at various loading (0, 10, 15, 20 and 30 phr). RHP

was dried at 110C for 24 h in a vacuum oven prior to mixing. A constant PP and NBRrwas used at 70 phr and 30 phr, respectively. Table 2 shows the formulation of PP/NBRr/

RHP composites.

The composites were prepared by melt mixing using a Haake Rheomix Polydrive R

600/610 mixer at 180C with the rotor speed of 50 r min1. PP was soaked in Ac and -APS for the RHP-treated composite to allow possible in situ grafting of PP and anhy-

dride/carbon chain of -APS by heat during mixing.AQ 5 However, for control sample, PPwas first charged directly into the mixer and melted for 4 min, NBRr was added at the

4th minute, and the RHP was added at 6th minute. The mixture was allowed to further

mix for another 3 min to obtain the stabilization torque. The total mixing time was

9 min for all samples. The recycled NBRr powder was dried for 24 h at 80C undervacuum prior to melt mixing in an internal mixer. The compounded samples were com-

pression moulded in a Go-Tech compression moulding machine. For test sample fab-

rication, the composites were pre-heated for 7 min at 180C, compressed at 1000 psifor 2 min and then cooled for 2 min into 1 mm thickness sheets. Moulded samples were

then cut into dumbbell shapes with a Wallace die cutter S6/1/6.A according to ASTM

D638.AQ 6

Tensile test

The tensile properties were measured using an Instron 3366 machine with a cross head

speed of 5 mm min1 at 25+ 3C, according to ASTM D638.AQ 7 Tensile strength, tensilemodulus and elongation at break (EB) of the each sample were obtained from the average

of five specimens with their corresponding SDs.

Table 2. Formulation for PP/NBRr/RHP composites.

Composite materials

PP/NBRr/RHP composites (phr)

S1 S2 S3 S4 S5 S6 S7 S8 S9

PP 70 70 70 70 70 70 70 70 70NBRr 30 30 30 30 30 30 30 30 30RHP 5 10 15 30 Ac-treated RHP 5 10 15 30-APS-treated RHP 5 10 15 30

PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder; -APS:

aminopropyltrimethoxysilane; Ac: acetic anhydride.

4 Journal of Thermoplastic Composite Materials 00(0)

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FTIR spectroscopic analysis

FTIR spectroscopic analysis of the composites was carried out using Perkin Elmer

Spectrometer 2000 FTIR.AQ8 Scanned range was predetermined at 4004000 cm1. All the

samples including control, -APS and Ac-treated RHP filler were characterized individu-ally by FTIR to confirm the chemical reaction between RHP filler and PP/NBRr matrices.

Fractography studies

The failure mode of the fractured tensile specimens was examined using field emission

scanning electron microscope (Zeiss Supra 36VP-24-58). Scanning electron micro-

graphs were taken at various magnifications. Prior to the scanning electron microscopic

observation, the fractured ends of the specimens were mounted on aluminium stubs and

were sputter coated with a thin layer of gold to avoid electrical charging during

examinations.

Results and discussion

Torque development

Comparisons were made in Figure 1 between torquetime curves of untreated RHP with

-APS-treated RHP and Ac-treated RHP-filled PP/NBRr composites at 15 phr of RHP.

Figure 1. Effect of Ac and -APS treatment on the torquetime curves of the RHP-filled PP/NBRrcomposites. PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice huskpowder; -APS: aminopropyltrimethoxysilane.

Ragunathan et al. 5

5

The peak A for -APS-treated RHP-filled PP/NBRr composite is lower than the peak Bfor Ac-treated RHP-filled PP/NBRr composite due to lubricant action of -APS. How-ever, all curves become completely homogenous and stabilized at the end of 6 min mix-

ing time.

The stabilization torque of both -APS-treated RHP and Ac-treated RHP-filled PP/NBRr composites increased with increasing filler content, with Ac showing higher torque

than -APS (Figure 1). The results explained that both Ac and -APS provided highviscosity to the composites due to the interaction between functional group of the compa-

tibilizer and coupling agent with RHP or PP/NBRr matrix. Meanwhile, higher stabilization

torque of the Ac treated with RHP might be due to better interaction between RHP and PP/

NBRr matrix, thus increasing the total viscosity of the composites (Figure 2).

Tensile properties

Figures 3 to 5 show the tensile properties of RHP-filled PP/NBRr composites as a func-

tion of filler content together with Ac and -APS contribution. The tensile properties canbe translated to a degree of reinforcement provided by the filler to the composites.2,3

Tensile strength of control sample (PP/NBRr/RHP) is shown in Figure 3. It can be seen

that the tensile strength continuously decreased with increasing RHP filler content. The

decrease in the tensile strength may be due to poor dispersion of the filler in the matrix,

increase in the interfacial defects or debonding between the filler and the matrix48 and

filler moisture uptake.

Figure 2. Effect of Ac and -APS on the stabilization torque of RHP-filled PP/NBRr composites.PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder; -APS:aminopropyltrimethoxysilane.

6 Journal of Thermoplastic Composite Materials 00(0)

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Since the RHP is highly hydrophilic in nature, its strong inter-filler hydrogen bonding

allows them to cling together, thus resisting dispersion of the filler, leading to weak inter-

facial bonding with consequent problems such as poor stress transfer, small void spaces,

debonding in the resulting composites and picked up moisture during storage, processing

and testing.8

In order to reduce the surface hydrophilicity, the filler surface is treated with Ac and

-APS. At a similar content, the tensile strength for both the treated PP/NBRr/RHPcomposites increased and also showed higher strength than control composites with 42

50% improvement in Ac and 2535% in -APS. At similar filler loading, incorporationof Ac into RHP-filled composites has the highest tensile strength. This improvement was

due to the interaction through chemical bonds between anhydride moiety of the Ac and

the hydroxyl groups in the filler, which would form covalent bonds and ester linkages,

thus improving fibrematrix bonding.

The general mechanism of reaction between fillers surface with functional group of

the treatment agent is shown in Figure 4. Since the fillermatrix bonding is improved, the

PP long chains become compatible with the NBRr. The presence of anhydride groups

lowered the surface tension of the filler and increases its wettability with the PP/NBRr

matrix. Furthermore, the sufficient numbers of anhydride moiety allow better diffusion

into the matrix polymer, which indicates easier entanglement with the polymer matrix.

Without anhydride, the only adhesionmechanism is inter-diffusion.With the strengthening

Figure 3. Effect of Ac, -APS and filler content on tensile strength of RHP-filled PP/NBRr com-posites. PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder;-APS: aminopropyltrimethoxysilane.

Ragunathan et al. 7

7

of interfacial interaction betweenRHPand PP/NBRrmatrix by anhydride, the resultant effi-

cient stress transfer from thePP/NBRrmatrix to the fibre leads to enhance in tensile strength.

Compared with untreated PP/NBRr/RHP composites, application of -APS didresulted in some increment in the tensile strength for all filler content but decreased with

increasing RHP filler content (Figure 3). The result indicates that the use of -APS as

Figure 4. General mechanism of the reaction between fibres surface with functional group of thetreatment agent (Ac).

Figure 5. General bond mechanism of silane coupling agent to fibres surface.

8 Journal of Thermoplastic Composite Materials 00(0)

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coupling agent was proven to be effective in enhancing dispersion, adhesion and com-

patibility of systems consisting of hydrophilic filler and hydrophobic matrix through

modification of polymerfiller interface.24 Figure 5 illustrates the reaction mechanism

that occurs in three steps. First, the alkoxy group in the coupling agent undergoes a

hydrolysis process. Water for the hydrolysis may come from the surface humidity of the

filler (in the case of the silane treatment). Next, the group reacts with the hydroxyl of the

filler surface by hydrogen bond formation. Then, SiO cross links are formed between

the filler surface and the adjacent functional groups in a condensation reaction with the

elimination of water.

EB of untreated composite and Ac-/-APS-treated RHP-filled PP/NBRr compo-sites is shown in Figure 4. EB is maximum for untreated composites up to 5 phr

of filler content, but then decreased steadily at higher filler content. The presence

of -APS and Ac further decreased even though at the lowest filler content. For boththe cases, once the composites become harder and stiffer, the EB is certainly low-

ered. The improved adhesion in the presence of bonding agent restricts the mobility

of polymer segments, which finally results in a reduction in elongation.18 Similar

behaviour has also been reported by many researchers.1923 They found that the

decrease in EB at lower filler content may be due to low EB of the filler and this

restricts the polymer molecules flowing past between each other. Incorporation of

Ac yield composites with the EB of compatibilized composites lower than that of

Figure 6. Effect of Ac and -APS filler content on EB of RHP-filled PP/NBRr composites. PP:polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder; -APS:aminopropyltrimethoxysilane; EB: elongation at break.

Ragunathan et al. 9

9

-APS-treated composites and untreated composites. The lower EB of the compati-bilized composites was associated with its higher stiffness as well as dramatic

increase in Youngs modulus due to the rigidity of the composites. The increase

in stiffness upon addition of Ac made the composites more brittle (Figure 6).AQ 9In both cases, untreated composite and Ac-treated/-APS-treated RHP-filled PP/

NBRr composites (Figure 7), increasing filler content resulted in enhanced Youngs

modulus since it represents the stiffness of the composites. The enhanced modulus is

easily understood because filler in fibrous form may carry more tensile load with

increasing filler content. Besides, filler is much stiffer than polymer matrix and

as a result, it adds stiffness to the composites. For overall trend, a better modulus

of about 2033% was observed at all filler content when Ac was added when com-pared with their counter parts. First, this improvement could be related to better

adhesion between the fibre and the matrix by chemical interactions. Better adhesion

yields more restriction to deformation capacity of the matrix in the elastic zone and

increasing modulus. According to Zhang et al.,25 the addition of anhydride groups

even at low levels (12%) increases the nucleation capacity of fillers for PP andalters the crystal morphology of PP around the fillers. Consequently, surface crystal-

lization dominates over bulk crystallization and a transcrystalline can be formed

around the fillers. From that it can be seen that the effects of crystallites have much

higher modulus compared with the amorphous regions and resulted in increase in the

modulus.

Figure 7. Effect of Ac and -APS treatment on Youngs modulus of RHP-filled PP/NBRr compo-sites. PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder; -APS: aminopropyltrimethoxysilane.

10 Journal of Thermoplastic Composite Materials 00(0)

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Result from FTIR analysis can confirm that the improvement in tensile results was

due to the irreversible chemical bonding of the silane onto the cellulose surface through

surface modification with Ac and -APS with the PP/NBRr matrices. Figure 8 presentsFTIR spectra corresponding to the RHP-filled PP/NBRr composites before and after

incorporation of Ac and -APS. All spectra show different bands at around 32003500, 1740 and 1635 cm1, which are associated with the stretching vibrations of theOH, CO and CC groups, respectively. Rice husk is mostly composed of cellulose,hemicellulose, lignins and some pectins. The COH of the cellulose backbone (CO sec-

ondary and CO primary alcohols) corresponded to the 1056 and 1030 cm1 peaks,respectively.

As for Ac, an increase in the band at 1260 cm1 corresponding to the ester (COO)group formation and a vibration band at 1740 cm1 corresponding to the carbonyl groups(CO) related to the ester functions for Ac-treated RHP is observed. Similar finding wasobserved by Bessadok et al.26 on Alfa fibres modified by chemical treatments with Ac.

As for -APS, the broad intense bands around 1162 and 1105 cm1 were assigned tothe stretching of the SiOcellulose and SiOSi bonds, respectively. The large band

around 1047 cm1, present in the spectrum of the untreated composite, was attributed toSiOH groups. This band disappeared after the surface modification and was replaced

by a wide band around 1020 cm1, which is a characteristic of SiOSi moiety.27

Figure 8. FTIR spectra corresponding to the RHP-filled PP/NBRr composites with or without Acand -APS. PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk pow-der; -APS: aminopropyltrimethoxysilane; FTIR: Fourier transform infrared.

Ragunathan et al. 11

11

Morphological properties

Scanning electron microscopy was used to examine the fracture surface of the

composites after tensile testing of the samples. In Figures 9 to 11, the fractured

surfaces of untreated RHP, Ac-treated RHP and -APS-treated RHP-filled PP/NBRr composites are shown, respectively. Observations indicate poor adhesion

between filler and matrix for all samples especially at higher filler content. In

Figures 10(b) and (c) and 11(b) and (c), it is also possible to observe same patterns

from RHP fillers, which were so weakly bonded to the matrix because of higher

filler content. They have been detached from the matrix during fracture, which

shows decrease in the strength of the composites. If the micrographs of the Ac-

treated RHP with -APS-treated RHP at same filler content were compared, it can

Figure 9. (a) Micrograph of tensile fracture surfaces of control untreated RHP-filled PP/NBRrcomposites at a magnification of 100 and at different filler contents (5 phr). (b) Micrograph oftensile fracture surfaces of control untreated RHP-filled PP/NBRr composites at a magnificationof 100 and at different filler contents (15 phr). (c) Micrograph of tensile fracture surfaces of con-trol untreated RHP-filled PP/NBRr composites at a magnification of 100 and at different fillercontents (30 phr). PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: ricehusk powder.

12 Journal of Thermoplastic Composite Materials 00(0)

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be seen that there are more ductile composites morphology with less filler detach-

ment sites for the Ac-treated composites indicating better adhesion between filler

and matrix compared with the -APS-treated composites. It is also possible toobserve a crack running through the filler for the compatibilized composites.

Conclusion

The following conclusions can be drawn based on the results presented in this work.

1. The processing torque and tensile modulus increased with increasing RHP filler con-

tent for all composites, which were attributed to the brittle nature of RHP filler.

2. Both RHP-treated composites exhibit higher processing stabilization torque,

tensile strength, tensile modulus and EB compared with control (untreated RHP)

composites due to enhance interfacial bonding between RHP filler and PP/NBRr

matrices.

Figure 10. (a) Micrograph of tensile fracture surfaces of Ac-treated RHP-filled PP/NBRr compo-sites at a magnification of 100 and at different filler contents (5 phr). (b) Micrograph of tensilefracture surfaces of Ac-treated RHP-filled PP/NBRr composites at a magnification of 100 andat different filler contents (15 phr). (c) Micrograph of tensile fracture surfaces of Ac-treatedRHP-filled PP/NBRr composites at a magnification of100 and at different filler contents (30 phr).PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder.

Ragunathan et al. 13

13

3. Ac treatment in comparison with silane (-APS) treatment was found to exhibit bet-ter mechanical properties for RHP-filled PP/NBRr composites. This may be due to

the enhanced adhesion between RHP filler and PP/NBRr matrix as shown in the

scanning electron micrographs.

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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Figure 11. (a) Micrograph of tensile fracture surfaces of -APS-treated RHP-filled PP/NBRr com-posites at a magnification of 100 and at different filler contents (5 phr). (b) Micrograph of tensilefracture surfaces of -APS-treated RHP-filled PP/NBRr composites at a magnification of 100 andat different filler contents (15 phr). (c) Micrograph of tensile fracture surfaces of -APS-treatedRHP-filled PP/NBRr composites at a magnification of100 and at different filler contents (30 phr).PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder; -APS:aminopropyltrimethoxysilane.

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