Comparison of processing and mechanical properties of polypropylene/recycled acrylonitrile butadiene rubber/rice husk powder composites modified with silane and acetic anhydride compound

Abstract

Polypropylene (PP)/recycled acrylonitrile butadiene rubber (NBRr)/rice husk powder (RHP) composites were fabricated with silane and acetic anhydride (Ac) treatment agent. The in situ formed RHP-filled PP/NBRr composites were prepared by melt mixing technique. The mechanical properties of both the treatment methods were investigated with Instron mechanical analysis and Fourier transform infrared. The results indicated that 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 stronger interaction between anhydride moieties with PP/NBRr.

Keywords

Mechanical properties polypropylene acrylonitrile butadiene rubber rice husk powder acetic anhydride silane

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.^1

–4 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.^5

–8

The major disadvantage encountered during the incorporation of natural lignocellulosic 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,^{10

–18} silane treatment,^19,20 graft co-polymerization,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 lignocellulosic 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.^{10

–20} 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 γ-aminopropyltrimethoxysilane (γ-APS) for the purpose of manufacturing RHP-filled polypropylene (PP)/recycled acrylonitrile butadiene rubber (NBRr) composites.

The aim of the present work is to evaluate and compare the mechanical properties of RHP filler PP/NBRr composite utilizing Ac and γ-APS treatment agents in polymer waste 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,Taichung City, Taiwan) with speed of 2850 r min⁻¹, sieved at 300–500 μm in particle size and dried at 110°C for 24 h in a vacuum oven to produce RHP of homogeneous fractions.

Table 1.

Materials specification and description.

Material	Description	Source
Polypropylene (PP)	Code: 331; MFI: 14 g/10 min at 230°C; density: 0.9 g cm⁻³	Titan Pro Polymers (M) Sdn. Bhd., Johor, Malaysia
Recycled acrylonitrile butadiene rubber (NBRr)	Content: 33% of acrylonitrile; density: 1.015 g cm⁻³	Juara One Resources Sdn. Bhd., Penang, Malaysia
Rice husk powder (RHP)	Cellulose 35%; hemicellulose 25%; lignin 20%; ash 17%; density:1.4702 g cm⁻³; size: 300–500 µm	Thye Heng Chan Enterprise Sdn. Bhd. (City Parit Buntar, Perak, Malaysia)
Treatment agent	Acetic anhydride (Ac); γ-aminopropyltrimethoxysilane (γ-APS)	Alfa Aesar (M) Sdn. Bhd. (A Johnson Matthey Company, Lancashire, United Kingdom)

MFI: melt flow index.

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 ratio at 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 80°C for 24 h.

Silane treatment (γ-APS)

The γ-APS treatment reaction for RHP was carried out in a mixture of water and ethanol (40/60, vol, respectively). γ-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 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 80°C 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 110°C for 24 h in a vacuum oven prior to mixing. A constant PP and NBRr was used at 70 phr and 30 phr, respectively. Table 2 shows the formulation of PP/NBRr/RHP composites.

Table 2.

Formulation for PP/NBRr/RHP composites.

Composite materials	PP/NBRr/RHP composites (phr)
Composite materials	S1	S2	S3	S4	S5	S6	S7	S8	S9
PP	70	70	70	70	70	70	70	70	70
NBRr	30	30	30	30	30	30	30	30	30
RHP	–	5	10	15	30	–	–	–	–
Ac-treated RHP	–	–	–	–	–	5	10	15	30
γ-APS-treated RHP						5	10	15	30

PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder; γ-APS: aminopropyltrimethoxysilane; Ac: acetic anhydride.

The composites were prepared by melt mixing using a Haake Rheomix Polydrive R 600/610 mixer (International/Germany, Dieselstr. 4, 76227 Karlsruhe) at 180°C with the rotor speed of 50 r min⁻¹. PP was soaked in Ac and γ-APS for the RHP-treated composite to allow possible in situ grafting of PP and anhydride/carbon chain of γ-APS by heat during mixing. However, for control sample, PP was 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 80°C under vacuum prior to melt mixing in an internal mixer. The compounded samples were compression moulded in a Go-Tech compression moulding machine (Testing Machines Inc, Taichung, Taiwan). For test sample fabrication, the composites were pre-heated for 7 min at 180°C, compressed at 1000 psi for 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 (Instruments Inc., England). A according to ASTM D638.

Tensile test

The tensile properties were measured using an Instron 3366 machine (Instron Worldwide Headquarters, Massachusetts, USA) with a cross head speed of 5 mm min⁻¹ at 25 ± 3°C, according to ASTM D638. Tensile strength, tensile modulus and elongation at break (E _B) of the each sample were obtained from the average of five specimens with their corresponding SDs.

FTIR spectroscopic analysis

FTIR spectroscopic analysis of the composites was carried out using Perkin Elmer Spectrometer 2000 FTIR (California, USA). Scanned range was predetermined at 400–4000 cm⁻¹. All the samples including control, γ-APS and Ac-treated RHP filler were characterized individually 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 micrographs 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 torque–time curves of untreated RHP with γ-APS-treated RHP and Ac-treated RHP-filled PP/NBRr composites at 15 phr of RHP. The peak A for γ-APS-treated RHP-filled PP/NBRr composite is lower than the peak B for Ac-treated RHP-filled PP/NBRr composite due to lubricant action of γ-APS. However, all curves become completely homogenous and stabilized at the end of 6 min mixing time.

Figure 1.

Effect of Ac and γ-APS treatment on the torque–time curves of the RHP-filled PP/NBRr composites. PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder; γ-APS: aminopropyltrimethoxysilane.

The stabilization torque of both γ-APS-treated RHP and Ac-treated RHP-filled PP/NBRr composites increased with increasing filler content, with Ac showing higher torque than γ-APS (Figure 1). The results explained that both Ac and γ-APS provided high viscosity to the composites due to the interaction between functional group of the compatibilizer 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).

Figure 2.

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

Tensile properties

Figures 3 to 5 show the tensile properties of RHP-filled PP/NBRr composites as a function of filler content together with Ac and γ-APS contribution. The tensile properties can be 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 matrix^4

–8 and filler moisture uptake.

Figure 3.

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

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 interfacial 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 γ-APS. At a similar content, the tensile strength for both the treated PP/NBRr/RHP composites increased and also showed higher strength than control composites with 42–50% improvement in Ac and 25–35% in γ-APS. At similar filler loading, incorporation of 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 fibre–matrix bonding.

The general mechanism of reaction between filler’s surface with functional group of the treatment agent is shown in Figure 4. Since the filler–matrix 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 adhesion mechanism is inter-diffusion. With the strengthening of interfacial interaction between RHP and PP/NBRr matrix by anhydride, the resultant efficient stress transfer from the PP/NBRr matrix to the fibre leads to enhance in tensile strength.

Figure 4.

General mechanism of the reaction between fibre’s surface with functional group of the treatment agent (Ac).

Compared with untreated PP/NBRr/RHP composites, application of γ-APS did resulted 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 γ-APS as coupling agent was proven to be effective in enhancing dispersion, adhesion and compatibility of systems consisting of hydrophilic filler and hydrophobic matrix through modification of polymer–filler 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, Si–O cross links are formed between the filler surface and the adjacent functional groups in a condensation reaction with the elimination of water.

Figure 5.

General bond mechanism of silane coupling agent to fibre’s surface.

E _B of untreated composite and Ac-/γ-APS-treated RHP-filled PP/NBRr composites is shown in Figure 4. E _B is maximum for untreated composites up to 5 phr of filler content, but then decreased steadily at higher filler content. The presence of γ-APS and Ac further decreased even though at the lowest filler content. For both the cases, once the composites become harder and stiffer, the E _B is certainly lowered. 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.^19

–23 They found that the decrease in E _B at lower filler content may be due to low E _B of the filler and this restricts the polymer molecules flowing past between each other. Incorporation of Ac yield composites with the E _B of compatibilized composites lower than that of γ-APS-treated composites and untreated composites. The lower E _B of the compatibilized composites was associated with its higher stiffness as well as dramatic increase in Young’s modulus due to the rigidity of the composites. The increase in stiffness upon addition of Ac made the composites more brittle (Figure 6).

Figure 6.

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

In both cases, untreated composite and Ac-treated/γ-APS-treated RHP-filled PP/NBRr composites (Figure 7), increasing filler content resulted in enhanced Young’s 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 20–33% was observed at all filler content when Ac was added when compared 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 (1–2%) increases the nucleation capacity of fillers for PP and alters the crystal morphology of PP around the fillers. Consequently, surface crystallization 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 γ-APS treatment on Young’s modulus of RHP-filled PP/NBRr composites. PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder; γ-APS: aminopropyltrimethoxysilane.

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 γ-APS with the PP/NBRr matrices. Figure 8 presents FTIR spectra corresponding to the RHP-filled PP/NBRr composites before and after incorporation of Ac and γ-APS. All spectra show different bands at around 3200–3500, 1740 and 1635 cm⁻¹, which are associated with the stretching vibrations of the OH, C=O and C=C groups, respectively. Rice husk is mostly composed of cellulose, hemicellulose, lignins and some pectins. The C–OH of the cellulose backbone (C–O secondary and C–O primary alcohols) corresponded to the 1056 and 1030 cm⁻¹ peaks, respectively.

Figure 8.

FTIR spectra corresponding to the RHP-filled PP/NBRr composites with or without Ac and γ-APS. PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder; γ-APS: aminopropyltrimethoxysilane; FTIR: Fourier transform infrared.

As for Ac, an increase in the band at 1260 cm⁻¹ corresponding to the ester (CO–O) group formation and a vibration band at 1740 cm⁻¹ corresponding to the carbonyl groups (C=O) related to the ester functions for Ac-treated RHP is observed. Similar finding was observed by Bessadok et al.²⁶ on Alfa fibres modified by chemical treatments with Ac.

As for γ-APS, the broad intense bands around 1162 and 1105 cm⁻¹ were assigned to the stretching of the –Si–O–cellulose and –Si–O–Si– bonds, respectively. The large band around 1047 cm⁻¹, present in the spectrum of the untreated composite, was attributed to –Si–OH groups. This band disappeared after the surface modification and was replaced by a wide band around 1020 cm⁻¹, which is a characteristic of –Si–O–Si– moiety.²⁷

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 γ-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 γ-APS-treated RHP at same filler content were compared, it can be seen that there are more ductile composites morphology with less filler detachment sites for the Ac-treated composites indicating better adhesion between filler and matrix compared with the γ-APS-treated composites. It is also possible to observe a crack running through the filler for the compatibilized composites.

Figure 9.

(a) Micrograph of tensile fracture surfaces of control untreated RHP-filled PP/NBRr composites at a magnification of ×100 and at different filler contents (5 phr). (b) Micrograph of tensile fracture surfaces of control untreated RHP-filled PP/NBRr composites at a magnification of ×100 and at different filler contents (15 phr). (c) Micrograph of tensile fracture surfaces of control untreated RHP-filled PP/NBRr composites at a magnification of ×100 and at different filler contents (30 phr). PP: polypropylene; NBRr: recycled acrylonitrile butadiene rubber; RHP: rice husk powder.

Figure 10.

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

Figure 11.

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

Conclusion

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

The processing torque and tensile modulus increased with increasing RHP filler content for all composites, which were attributed to the brittle nature of RHP filler.

Both RHP-treated composites exhibit higher processing stabilization torque, tensile strength, tensile modulus and E _B compared with control (untreated RHP) composites due to enhance interfacial bonding between RHP filler and PP/NBRr matrices.

Ac treatment in comparison with silane (γ-APS) treatment was found to exhibit better 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.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

References

Albuquerque

Joseph

Carvalho

. Effect of wettability and ageing conditions on the physical and mechanical properties of uniaxially oriented jute-roving-reinforced polyester composites. Compos Sci Technol 2000; 60: 833–844.

Bledzki

Gassan

. Composites reinforced with cellulose based fibres. Progr Polym Sci 1999; 24: 221–274.

Joseph

Thomas

. Effect of processing variables on the mechanical properties of sisal-fiber-reinforced polypropylene composites. Compos Sci Technol 1999; 59: 1625–1640.

Nabi

Jog

. Natural fiber polymer composites: a review. Adv Polym Technol 1999; 18(4): 351–363.

Satyanarayana

Arizaga

GGC

Wypych

. Biodegradable composites based on lignocellulosic fibers: an overview. Progr Polym Sci 2009; 34: 982–1021.

Tajvidi

Falk

Hermanson

. Effect of natural fibers on thermal and mechanical properties of natural fiber polypropylene composites studied by dynamic mechanical analysis. J Appl Polym Sci 2006; 101: 4341–4349.

Sgriccia

Hawley

Misra

Characterization of natural fiber surfaces and natural fiber composites. Compos A: Appl Sci Manuf 2008; 39: 1632–1637.

Mohanty

Khan

Hinrichsen

. Surface modification of jute and its influence on performance of biodegradable jute-fabric/Biopol composites. Compos Sci Technol 2000; 60: 1115–1124.

Frisoni

Baiardo

Scandola

. Natural cellulose fibers: heterogeneous acetylation kinetics and biodegradation behavior. Biomacromolecules 2001; 2(2): 476–482.

10.

Rowell

Young

Rowell

. Paper and composites from agrobased resources. Boca Raton, FL: CRC Press, 1997.

11.

Khalil

Ismail

Ahmad

. The effect of various anhydride modifications on mechanical properties and water absorption of oil palm empty fruit bunches reinforced polyester composites. Polym Int 2001; 50: 395–402.

12.

Hill

CAS

Khalil

HPSA

Hale

. A study of the potential of acetylation to improve the properties of plant fibres. Ind Crop Prod 1998; 8: 53–63.

13.

Sreekala

Kumaran

Joseph

. Stress relaxation behaviors of composites based on short oil palm fibres and phenol formaldehyde resins. Compos Sci Technol 2001; 61: 1175–1188.

14.

Rana

Basak

Mitra

. Studies of acetylation of jute using simplified procedure and its characterization. J Appl Polym Sci 1997; 64: 1517–1523.

15.

Sun

. Structural and thermal characterization of acetylated rice, wheat, rye, and barley straws and poplar wood fiber. Ind Crop Prod 2002; 16: 225–235.

16.

Sreekala

Thomas

. Effect of fibre surface modification on water-sorption characteristics of oil palm fibres. Compos Sci Technol 2003; 63: 861–869.

17.

Pothan

Thomas

. Polarity parameters and dynamic mechanical behaviour of chemically modified banana fiber reinforced polyester composites. Compos Sci Technol 2003; 63: 1231–1240.

18.

Sun

. Acetylation of sugarcane bagasse using NBS as a catalyst under mild reaction conditions for the production of oil sorption-active materials. Biores Technol 2004; 95: 343–350.

19.

Ismail

Mega

. The effects of a compatibilizer and a silane coupling agent on the mechanical properties of white rice husk ash filled polypropylene/natural rubber blend. J Polym Plast Technol Eng 2001; 40: 463–478.

20.

Ismail

Ragunathan

Hussin

. The effects of recycled acrylonitrile butadiene rubber content and maleic anhydride modified polypropylene (PPMAH) on the mixing, tensile properties, swelling percentage and morphology of polypropylene/recycled acrylonitrile butadiene rubber/rice husk powder (PP/NBRr/RHP) composites. J Polym Plast Technol Eng 2010; 49: 1323–1328.

21.

Bledzki

Reihmane

Gassan

. Properties and modification methods for vegetable fibers for natural fiber composites. J Appl Polym Sci 1996; 59: 1329–1336.

22.

Razavi

Jafarzadeh

Oromiehie

. Mechanical properties and water absorption behaviour of chopped rice husk filled polypropylene composites. Iran Polym J 2006; 15: 757–766.

23.

Ismail

Ragunathan

Hussin

. Tensile properties, swelling, and water absorption behavior of rice-husk-powder-filled polypropylene/(recycled acrylonitrile-butadiene rubber) composites. J Vinyl Addit Technol 2011; 17(3): 190–197.

24.

Ismail

Mega

Abdul Khalil

HPS

. Effect of a silane coupling agent on the properties of white rice ash-polypropylene/natural rubber composites. Polym Int 2001; 50: 606–611.

25.

Zhang

Huang

Mai

. Crystallization and dynamic mechanical properties of polypropylene/polystyrene blends modified with maleic anhydride and styrene. J Appl Polym Sci 2005; 96: 2038–2045.

26.

Bessadok

Roudesli

Marais

. Alfa fibres for unsaturated polyester composites reinforcement: effects of chemical treatments on mechanical and permeation properties. Composites A 2009; 40: 184–195.

27.

Abdelmouleh

Boufi

Belgacem

. Short natural-fibre reinforced polyethylene and natural rubber composites: effect of silane coupling agents and fibres loading. Compos Sci Technol 2007; 67: 1627–1639.