DSpace at VNU: Effect of Filler Surface Treatment on the Properties of Recycled High-Density Polyethylene (Natural Rubber) (Kenaf Powder) Biocomposites

Effect of Filler Surface Treatment on the Properties ofRecycled High-Density Polyethylene/Natural Rubber/Kenaf Powder Biocomposites Xuan Viet Cao,1Hanafi Ismail,1 Azura A.. Figure 4 illu

Effect of Filler Surface Treatment on the Properties of

Recycled High-Density Polyethylene/(Natural Rubber)/(Kenaf Powder) Biocomposites

Xuan Viet Cao,1Hanafi Ismail,1 Azura A Rashid,1 Tsutomu Takeichi,2 Thao Vo-Huu3

1

School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia,

14300, Nibong Tebal, Malaysia

2

School of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi, 4418580, Japan 3

Department of Polymer Materials, Faculty of Materials Technology, Ho Chi Minh University of Technology,

Ho Chi Minh City, Vietnam

Biocomposites were prepared from a kenaf core

pow-der and recycled high-density polyethylene/(natural

rubber) blend by using an internal mixer at 165 o C and

50 rpm The effect of the filler content and the filler

surface treatment was studied Chemical modification

of kenaf filler was performed with alkali pretreatment

followed by treatment with silane Scanning electron

microscopy and infrared spectroscopy studies

con-firmed changes in the chemical compositions and

structural characteristics induced through the

modifi-cation It was found that treated biocomposites offered

higher tensile strength and tensile modulus, but lower

elongation at break compared with untreated

biocom-posites Lower water absorption and higher thermal

stability of the resultant biocomposites were also

obtained when treated fillers were used J VINYL ADDIT.

TECHNOL., 00:000–000, 2014 V C 2014 Society of Plastics

Engineers

INTRODUCTION

The development of biocomposites by use of recycled

or recyclable polymers and natural organic fillers is on

the rise because of the exhaustion of petroleum resources

and growing public concern of the effect on the

environ-ment [1–3] The advantages of natural fillers over

inor-ganic counterparts include availability in large quantities,

low cost, low density, reasonable strength, reduced energy

consumption, and biodegradability [4]

Kenaf (Hibiscus cannabinus L.) is recognized as green

lignocelluloses plants with both economic and ecological

advantages Unlike kenaf bast fibers, kenaf core is usually

used as a source material for paper products, fiberboard,

absorbents, and animal feeds It was reported that kenaf

core fibers are more homogeneous than hardwood fibers [5], and paper from kenaf core has a high tensile and burst strength compared with hardwood pulps [6] In addition, the availability of kenaf core, together with the ease of its processability, could make it a good substitute for inorganic fillers for biocomposites based on polymers (i.e., rubber, plastics, and thermoplastic elastomers) Thermoplastic elastomer (TPE), a blend of natural rub-ber (NR) and polyethylene, has received considerable attention recently Because of its unique microstructure, it demonstrates elastic properties at room temperature and flowability at high temperature In the TPE system, NR, a biodegradable polymer, plays a functional role as a tough-ener to overcome the brittleness in thermoplastic compo-sites A further benefit of TPEs is that TPEs provide high value-added products if the components are derived from waste sources (“upcycling”) [7] In this study, recycled HDPE (rHDPE)/NR blend was used as a matrix to pro-duce biocomposites Therefore, (kenaf core)-reinforced rHDPE/NR biocomposites could provide environmental advantages and cost reduction

In previous studies, we have successfully prepared bio-composites on the basis of a kenaf core powder (KP) and rHDPE/NR blend However, it was found that the kenaf has inherently low compatibility with nonpolar polymer matrices, such as polyethylene andcis-polyisoprene (NR) This drawback caused difficulties in achieving good dis-persion and strong interfacial adhesion between the com-ponents, which led to composites with rather poor mechanical properties The use of (maleic anhydride)-grafted polyethylene and (maleic anhydride)-anhydride)-grafted NR enhanced the properties of rHDPE/NR/KP biocomposites

to some extent [8, 9]

Chemical modification on natural fiber presents a promising approach for the establishment of covalent bonding between the filler and matrix [10, 11] It is

Correspondence to: Hanafi Ismail; e-mail: hanafi@eng.usm.my

DOI 10.1002/vnl.21374

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

V C 2014 Society of Plastics Engineers

generally carried out with the use of reagents that contain

functional groups that are able to react with the hydroxyl

g-aminopropyltriethoxysilane (APTES) was used as a

silane-coupling agent for KP surface treatment In

addi-tion, KP was pretreated with sodium hydroxide (NaOH)

to remove impurities and promote the possible reaction

between silane and filler The effect of filler content and

filler treatment on the performance of the biocomposites

was evaluated

MATERIALS AND METHODS

rHDPE was obtained from Zarm Scientific and

Sup-plies Sdn Bhd (Penang) with a melt flow index of 0.237

g/10 min NR used was SMR L grade from the Rubber

Research Institute of Malaysia (RRIM) APTES was

sup-plied by Sigma Aldrich Other chemicals, such as ethanol,

acid acetic, and NaOH, were used as received and were

provided by Bayer Chemicals (M) Sdn Bhd Kenaf core

fibers were obtained from Forest Research Institute

Malaysia (FRIM) Kenaf powder was produced by

grind-ing kenaf core in a table-type pulverizgrind-ing machine and

sieving to obtain the powder size in range of 32 to 150

mm

Filler Surface Treatment with APTES

First, KPs were pretreated with NaOH KP was

immersed in NaOH solution (5% w/v) for 2 h at room

temperature Then, the fillers were washed with distilled

water containing a few drops of acetic acid Subsequently,

fillers were washed thoroughly with distilled water After

washing, the fillers were kept in air and dried in an oven

at 80oC for 6 h

Second, silane treatment was carried out in a mixture

of water/ethanol (30/70 v/v) for the pretreated NaOH KP

(KP-NaOH) One gram of APTES was dissolved for

hydrolysis in 1,000 mL of the water/ethanol mixture The

pH of the solution was adjusted to 4 with acetic acid and

stirred for 1 h [12] Then, 10 g of KP was soaked in the

solution and stirred continuously for 3 h at room

tempera-ture The filler was filtered and dried in air Finally,

APTES-treated KP (KP-APTES) was dried in a vacuum

oven at 80oC for 24 h prior to compounding

Preparation of rHDPE/NR/KP Biocomposites

The formulation of rHDPE/NR/KP biocomposites is

given in Table 1 Prior to compounding, rHDPE and KP

were dried by using a vacuum oven at 80oC for 24 h The

rHDPE was first added to the mixer and melted for 3

min After 3 min, NR was added After 6 min, KP (or

KP-APTES) was added The blend was mixed further for

another 6 min, at which time the stabilization torque was

received, indicating the formation of a homogeneous

sam-ple The total mixing time was 12 min for all samples

The blend composites were then compression-molded in a

hydraulic hot press into 1-mm sheets for preparation of test samples The hot-press procedure involved preheating

at 165oC for 6 min, followed by compressing for 3 min at the same temperature, and subsequent cooling under pres-sure for 2 min

Fourier-Transform Infrared Spectroscopy (FTIR) FTIR testing was done by using a Perkin-Elmer 2000 testing instrument The FTIR spectrum was recorded in the transmittance range from 4,000 to 500 cm21 with a resolution of 4 cm21 There were eight scans for each spectrum All FTIR spectra were obtained by using atte-nuated total reflectance About 5 mg of KP was mixed with 95 mg of potassium bromide and pressed to form pellets FTIR was performed on the pellets to obtain the information on the chemical modification of KP

Tensile Properties The tensile properties were measured by using an Ins-tron 3366 machine at a cross-head speed of 50 mm/min

at 25 6 3oC according to ASTM D 412 Tensile strength, tensile modulus, and elongation at break (Eb) of the each sample were obtained from the average of five specimens

Water Absorption

A water absorption test was carried out by immersing samples in distilled water at room temperature (25oC) After immersion in water, samples were removed, patted dry with a soft cloth, and weighed at regular intervals on

an electronic balance The percentage of water absorption,

Mt, was calculated by

Mt5100 ¸C wð t–woÞ=wo (1) where wo and wt are the original dry weight and weight after exposure, respectively

Scanning Electron Microscopy (SEM) The topology of filler and tensile fractured specimens was analyzed with a Supra-35VP field emission scanning electron microscope All samples were coated with gold/ palladium by a sputter-coating instrument (Bio-Rad

TABLE 1 Formulation of rHDPE/NR/KP biocomposites.

Recycled high-density polyethylene (rHDPE) 70

Kenaf core powder (KP) 0, 10, 20, 30, 40 php, parts per hundred polymer.

a Similar biocomposites with KP-APTES were also prepared.

Polaron Division) for 45 min to prevent electrostatic

charging during evaluation

Thermogravimetric Analysis (TGA)

Analysis by TGA was carried out by using a

Perkin-Elmer TG-6 Analyzer to determine the thermal stability

of the composites About 10–20 mg of sample was heated

at 10C/min from 30C to 600C with a nitrogen flow

rate of 20 mL/min The weight loss curve (TGA) and

derivative weight loss curve (DTG) were analyzed to

obtain the 5% weight loss temperature (T5%), 50% weight

loss temperature (T50%), and maximum degradation tem-perature (Td)

RESULTS AND DISCUSSION Filler Characterization

Infrared spectra of untreated KP, NaOH, and KP-APTES are presented in Fig 1 Chemical modification of

KP led to a change of molecular interactions that showed wave number shifts in the FTIR spectra A peak at 1,732

cm21was assigned to unconjugated C5O groups in hem-icellulose of the untreated KP This peak fully disap-peared after pretreatment with NaOH Treatment of filler with amino silane also showed some peaks shifts at 710 and 460 cm21, corresponding to the Si O Si asymmetric stretching and Si O C asymmetric bending, respectively [13, 14] Slight changes in the peaks found in the 1,030-1,060 cm21 region and peak at 1,267 cm21 should also

be noted These changes could be attributed to the pres-ence of asymmetric stretching of Si and/or

Si O C bonds [13, 15] The appearance of siloxane bonds was indicative of a polysiloxane depositing on the filler, whereas the alkoxysilane bonds seemed to confirm the occurrence of a condensation reaction between APTES and KP In addition, bands in the 3,200-3,600 cm21range became broader, which might be because of the NH2 stretch vibration from APTES [16]

The change in surface morphology of the treated KP was examined by analysis by SEM Figure 2 shows the

FIG 1 FTIR spectra of untreated KP (a), NaOH (b), and

KP-APTES (c) [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

FIG 2 SEM micrographs of untreated KP (a), KP chemically treated with NaOH (b) and APTES (c).

SEM micrographs of filler surface before and after

chemi-cal treatment It can be seen that the KP-NaOH surface

(Fig 2a) appeared rougher and cleaner than the untreated

KP (Fig 2b) because external impurities were mostly

removed from the surface of KP The pretreatment of KP

with NaOH was expected to remove hemicelluloses,

lig-nin, and waxes present on the surface of the fiber This

observation was in good agreement with FTIR results,

which confirmed the disappearance of the peak at 1,732

cm21 Figure 2c shows the topography of KP-APTES It

can be observed that there was no significant change in

surface morphology of APTES compared with

KP-NaOH However, after silane treatment, the surface of KP

seemed to be covered with an additional layer, which

cor-responded to the deposition of siloxane

Mixing Torque

Mixing torque of rHDPE/NR/KP biocomposites at 0

and 20 KP (parts per hundred polymer, php) is shown in

Fig 3 Unlike the blend, the composite torque curve has

three peaks corresponding to loading peaks of rHDPE,

NR, and KP Generally, torque decreased with mixing

time and reached stabilization torque at the end of mixing time as the composites became homogenous However, higher torque values in the case of KP-APTES were observed after the sixth minute Figure 4 illustrates the effect of filler treatment on stabilization torque of rHDPE/NR/KP biocomposites with respect to filler con-tent It can be observed that the stabilization torque of treated composites was higher than that of the untreated composites Silane modification of KP might result in increased melt viscosity of the composites owing to enhanced interaction between filler and polymer

Tensile Properties Generally, the incorporation of KP into the rHDPE/NR blend reduced the tensile strength andEbwhile increasing the tensile modulus of the composites More detailed dis-cussion of filler content has been presented elsewhere [8, 9] This study mainly focused on the influence of filler treatment on the properties of rHDPE/NR/KP biocompo-sites Figure 5 depicts the effect of silane treatments on the tensile strength of the composites APTES pretreat-ment of filler showed a positive effect on the tensile strength but the increment was only between 3.7% and 6.6% The reason for improvement in tensile strength might be because of better filler dispersion in the matrix and a fair degree of adhesion at the interface [17, 18] Indeed, previous NaOH treatment could remove impur-ities and waxy substances from the fiber surface and cre-ate a rougher topography Thus, the mechanical interlocking would be promoted, and the interface quality was enhanced further by silane treatment [13] The inter-action between phases was expected to improve because the KP surface became less hydrophilic because of chemi-cal bonding between APTES and the OH groups at the filler surface Nevertheless, the low silane concentration and difficulty of grafting reaction might render the effec-tiveness of APTES [[19]]

Figure 6 illustrates the effect of filler treatment on the

Eb of the rHDPE/NR/KP biocomposites It was clearly

FIG 3 Effect of filler treatment on the torque-time curves of rHDPE/

NR/KP biocomposites.

FIG 4 Effect of filler treatment on stabilization torque of rHDPE/NR/

KP biocomposites.

FIG 5 Effect of filler treatment on tensile strength of rHDPE/NR/KP biocomposites.

observed that KP-APTES treatment had an adverse effect

on Eb The lower Eb of composites with KP-APTES was

associated with enhanced adhesion between the filler and

matrix Better adhesion gives way to more restriction of

the deformation capacity of the composites; thus,

cata-strophic failure occurs after small strain deformations

Figure 7 shows that with increasing KP content, there

was an increase in tensile modulus for untreated and treated

biocomposites Significant improvement in the modulus of

the treated composites could be related to better adhesion

between the fiber and the matrix through a grafting reaction,

because the silane coupling agent reduced incompatibility between the fibers and the rHDPE/NR matrix Therefore, it increased their interfacial adhesion Better adhesion led to more restriction of the deformation capacity of the matrix in the elastic zone and increased modulus [20, 21]

Morphological Study Analysis by SEM was used to evaluate the effect of the filler treatment on the morphology of the tensile frac-ture surface of the composites The SEM micrographs of untreated and (KP-APTES)-treated composite samples at

FIG 6 Effect of filler treatment on elongation at break of rHDPE/NR/

KP biocomposites.

FIG 7 Effect of filler treatment on tensile modulus of rHDPE/NR/KP biocomposites.

FIG 8 SEM micrographs of rHDPE/NR/KP biocomposites at 40 phr of KP (a) untreated (32003), (b) KP-APTES (3200), and (c) KP-APTES (3400).

40 phr of KP are shown in Fig 8 In the case of the

untreated composites, some fiber detachment and voids

can be seen (Fig 8a) In contrast, in Fig 8b, the presence

of a number of fibers sticking out of the matrix was

visi-ble and less fiber pullout was observed on the fractured

surface of the treated composites This detachment and

voids occurred because the rough surface of treated filler,

in addition to the chemical bond, facilitated the

mechani-cal locking between the KP filler and the matrix A closer

examination of the fracture surface revealed that the level

of adhesion between filler and matrix was greatly

improved because the filler was embedded in the matrix

and broken under the tensile load (Fig 8c)

Water Absorption

Water absorption is one of the key parameters in the

evaluation of quality of lignocellulosic fiber composites

Water absorption of rHDPE/NR/KP biocomposites at 0

and 20 phr of KP as a function of the immersion time is

shown in Fig 9, whereas Fig 10 shows the water uptake

of the rHDPE/NR/KP biocomposites at 63 days It was

obvious that silane treatment resulted in lowering of the

water uptake when compared with the untreated samples

KP was pretreated with NaOH to remove lignin, hemicel-luloses, and other impurities This pretreatment is an effective means of advocating better property retention of composites when exposed to moisture [22] In addition, the improved interaction between the matrix and filler after APTES treatment through hydrogen bonding led to the reduction of water absorption of the composites

Thermogravimetric Analysis Figure 11a and b shows the TGA and DTG curves of untreated and treated rHDPE/NR/KP biocomposites at dif-ferent filler contents All composites were less thermally stable than the rHDPE/NR blend because of the lower thermal stability of the kenaf fiber As expected, two peaks of DTG curve were observed for all samples, which corresponded to two main degradation stages that occurred from the matrix materials In the rHDPE/NR/KP composites, two other peaks were also obtained A peak was observed at 140oC corresponding to the dehydration

of KP fiber and a major peak at 330C was caused by the thermal degradation of cellulose [23] TGA parameters for rHDPE/NR/KP biocomposites can be seen in Table 2 Generally, silane treatment improved the thermal stabil-ity of rHDPE/NR/KP biocomposites to some extent,

FIG 9 Effect of filler treatment on water absorption of rHDPE/NR/

KP biocomposites.

FIG 10 Water uptake at 63 days of rHDPE/NR/KP biocomposites.

FIG 11 (a) Typical TGA curves of rHDPE/NR/KP biocomposites (b) Typical DTG curves of rHDPE/NR/KP biocomposites [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

particularly at low filler content All the degradation

tem-peratures in the first degradation region were shifted to a

higher temperature, which suggested that at low

tempera-tures fiber with high lignin and hemicelluloses content

exhibited low thermal stability, whereas fiber with higher

cellulose content showed better thermal stability [24] The

alkaline-silane filler treatment did reduce the

hemicellulo-ses and lignin to a considerable extent [25] and thus led to a

better thermal stability over this temperature range

(disap-pearance of hemicelluloses peak in DTG curve) However,

lignin seems to be more stable than celluloses and

hemicel-luloses at high temperatures; hence, lower lignin content

resulted in a lower thermal resistance at a high temperature

range Alkaline pretreatment also caused a decrease in the

char yield because it removed a portion of the cell structure

(hemicelluloses or lignin) and eliminated some inorganic

matter [26]

CONCLUSIONS

Spectroscopic analysis (FTIR) and analysis by SEM

revealed that alkali pretreatment increased the surface

rough-ness by removing hemicelluloses and lignin, which could

promote a better mechanical interlocking with the matrix

Silane treatment further enhanced the compatibility of kenaf

with the polymer matrix by introducing a compatible

molec-ular structure on the filler surface Surface modification of

kenaf filler showed a positive effect on tensile strength of

the composites Tensile modulus of the treated biocomposites

was increased but Eb was reduced compared with the

untreated biocomposites This improvement was a result of

the enhanced interfacial adhesion between the rHDPE/NR

matrix and KP via physical and chemical bonding between

the components Water absorption was found to reduce with

fiber treatment In addition, the filler treatment also improved

the thermal stability of rHDPE/NR/KP biocomposites to

some extent, particularly at low filler content

ACKNOWLEDGMENT

The authors thank AUN/SEED–Net/JICA for the

finan-cial support through a Collaborative Research (CR) grant

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TABLE 2 TGA data for rHDPE/NR/KP biocomposites.

Samples

Temperature

at 5% weight

loss T 5%

(  C)

Temperature

at 50%

weight loss T 50%

(  C)

Maximum degradation temperature

T d (  C)

Weight residue (%)