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 of Recycled High-Density Polyethylene/(Natural Rubber)/(Kenaf Powder) Biocomposites Xuan Viet Cao,1 Hanafi Ismail,1 Azura A Rashid,1 Tsutomu Takeichi,2 Thao Vo-Huu3 School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300, Nibong Tebal, Malaysia School of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi, 4418580, Japan 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 powder and recycled high-density polyethylene/(natural rubber) blend by using an internal mixer at 165oC 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 confirmed changes in the chemical compositions and structural characteristics induced through the modification It was found that treated biocomposites offered higher tensile strength and tensile modulus, but lower elongation at break compared with untreated biocomposites Lower water absorption and higher thermal stability of the resultant biocomposites were also obtained when treated fillers were used J VINYL ADDIT C 2014 Society of Plastics TECHNOL., 00:000–000, 2014 V 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 environment [1–3] The advantages of natural fillers over inorganic 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 Correspondence to: Hanafi Ismail; e-mail: hanafi@eng.usm.my DOI 10.1002/vnl.21374 Published online in Wiley Online Library (wileyonlinelibrary.com) C 2014 Society of Plastics Engineers V JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2014 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 rubber (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 toughener to overcome the brittleness in thermoplastic composites 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 produce biocomposites Therefore, (kenaf core)-reinforced rHDPE/NR biocomposites could provide environmental advantages and cost reduction In previous studies, we have successfully prepared biocomposites 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 and cis-polyisoprene (NR) This drawback caused difficulties in achieving good dispersion and strong interfacial adhesion between the components, which led to composites with rather poor mechanical properties The use of (maleic anhydride)grafted polyethylene and (maleic 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 generally carried out with the use of reagents that contain functional groups that are able to react with the hydroxyl groups from fibers In this study, gaminopropyltriethoxysilane (APTES) was used as a silane-coupling agent for KP surface treatment In addition, 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 Supplies Sdn Bhd (Penang) with a melt flow index of 0.237 g/10 NR used was SMR L grade from the Rubber Research Institute of Malaysia (RRIM) APTES was supplied 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 grinding kenaf core in a table-type pulverizing 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 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 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 with acetic acid and stirred for h [12] Then, 10 g of KP was soaked in the solution and stirred continuously for h at room temperature 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 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 After min, NR was added After min, KP (or KP-APTES) was added The blend was mixed further for another min, at which time the stabilization torque was received, indicating the formation of a homogeneous sample The total mixing time was 12 for all samples The blend composites were then compression-molded in a JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2014 TABLE Formulation of rHDPE/NR/KP biocomposites.a Materials Composition (php) Recycled high-density polyethylene (rHDPE) Natural rubber (SMR L) Kenaf core powder (KP) 70 30 0, 10, 20, 30, 40 php, parts per hundred polymer a Similar biocomposites with KP-APTES were also prepared hydraulic hot press into 1-mm sheets for preparation of test samples The hot-press procedure involved preheating at 165oC for min, followed by compressing for at the same temperature, and subsequent cooling under pressure for 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 cm21 There were eight scans for each spectrum All FTIR spectra were obtained by using attenuated total reflectance About 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 Instron 3366 machine at a cross-head speed of 50 mm/min at 25 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 wt wo ị=wo Mt 5100C (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 DOI 10.1002/vnl loss temperature (T50%), and maximum degradation temperature (Td) RESULTS AND DISCUSSION Filler Characterization FIG FTIR spectra of untreated KP (a), KP-NaOH (b), and KPAPTES (c) [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] Polaron Division) for 45 to prevent electrostatic charging during evaluation Thermogravimetric Analysis (TGA) Analysis by TGA was carried out by using a PerkinElmer TG-6 Analyzer to determine the thermal stability of the composites About 10–20 mg of sample was heated at 10 C/min from 30 C to 600 C 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 Infrared spectra of untreated KP, KP-NaOH, and KPAPTES are presented in Fig 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 cm21 was assigned to unconjugated C5O groups in hemicellulose of the untreated KP This peak fully disappeared 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 1,0301,060 cm21 region and peak at 1,267 cm21 should also be noted These changes could be attributed to the presence of asymmetric stretching of Si O 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 cm21 range 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 shows the FIG SEM micrographs of untreated KP (a), KP chemically treated with NaOH (b) and APTES (c) DOI 10.1002/vnl JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2014 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 illustrates the effect of filler treatment on stabilization torque of rHDPE/NR/KP biocomposites with respect to filler content 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 Mixing torque of rHDPE/NR/KP biocomposites at and 20 KP (parts per hundred polymer, php) is shown in Fig Unlike the blend, the composite torque curve has three peaks corresponding to loading peaks of rHDPE, NR, and KP Generally, torque decreased with mixing Generally, the incorporation of KP into the rHDPE/NR blend reduced the tensile strength and Eb while increasing the tensile modulus of the composites More detailed discussion 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 biocomposites Figure depicts the effect of silane treatments on the tensile strength of the composites APTES pretreatment 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 impurities and waxy substances from the fiber surface and create a rougher topography Thus, the mechanical interlocking would be promoted, and the interface quality was enhanced further by silane treatment [13] The interaction between phases was expected to improve because the KP surface became less hydrophilic because of chemical bonding between APTES and the OH groups at the filler surface Nevertheless, the low silane concentration and difficulty of grafting reaction might render the effectiveness of APTES [[19]] Figure illustrates the effect of filler treatment on the Eb of the rHDPE/NR/KP biocomposites It was clearly FIG Effect of filler treatment on stabilization torque of rHDPE/NR/ KP biocomposites FIG Effect of filler treatment on tensile strength of rHDPE/NR/KP biocomposites FIG Effect of filler treatment on the torque-time curves of rHDPE/ NR/KP biocomposites SEM micrographs of filler surface before and after chemical 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, lignin, 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 KP-APTES compared with KPNaOH However, after silane treatment, the surface of KP seemed to be covered with an additional layer, which corresponded to the deposition of siloxane Mixing Torque JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2014 DOI 10.1002/vnl FIG Effect of filler treatment on elongation at break 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, catastrophic failure occurs after small strain deformations Figure 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, FIG Effect of filler treatment on tensile modulus of rHDPE/NR/KP biocomposites 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 fracture surface of the composites The SEM micrographs of untreated and (KP-APTES)-treated composite samples at FIG SEM micrographs of rHDPE/NR/KP biocomposites at 40 phr of KP (a) untreated (32003), (b) KP-APTES (3200), and (c) KP-APTES (3400) DOI 10.1002/vnl JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2014 KP was pretreated with NaOH to remove lignin, hemicelluloses, 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 FIG Effect of filler treatment on water absorption of rHDPE/NR/ KP biocomposites 40 phr of KP are shown in Fig 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 visible 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 mechanical 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) Figure 11a and b shows the TGA and DTG curves of untreated and treated rHDPE/NR/KP biocomposites at different 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 330 C was caused by the thermal degradation of cellulose [23] TGA parameters for rHDPE/NR/KP biocomposites can be seen in Table Generally, silane treatment improved the thermal stability of rHDPE/NR/KP biocomposites to some extent, 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 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 FIG 10 Water uptake at 63 days of rHDPE/NR/KP biocomposites JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2014 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.] DOI 10.1002/vnl REFERENCES TABLE TGA data for rHDPE/NR/KP biocomposites Samples rHDPE/NR 10KP 10KP-APTES 40KP 40KP-APTES Temperature at 5% weight loss T5% ( C) Temperature at 50% weight loss T50% ( C) Maximum degradation temperature Td ( C) Weight residue (%) 301.3 304.7 331.0 277.4 289.1 480.5 464.3 470.0 463.0 461.6 487.0 487.3 488.6 488.0 488.2 0.0 0.17 0.98 7.3 5.92 particularly at low filler content All the degradation temperatures in the first degradation region were shifted to a higher temperature, which suggested that at low temperatures 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 hemicelluloses and lignin to a considerable extent [25] and thus led to a better thermal stability over this temperature range (disappearance of hemicelluloses peak in DTG curve) However, lignin seems to be more stable than celluloses and hemicelluloses 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 roughness 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 molecular 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 financial support through a Collaborative Research (CR) grant DOI 10.1002/vnl K.L Yam, B.K Gogoi, C.C Lai, and S.E Selke, Polym Eng Sci., 30, 693 (1990) Y Lei, Q Wu, F Yao, and Y Xu, Compos Part A: Appl Sci Manuf., 38, 1664 (2007) F.P La Mantia and M Morreale, Compos Part A: Appl Sci Manuf., 42, 579 (2011) A.K Bledzki and J Gassan, Prog Polym Sci., 24, 221 (1999) J.L Bowyer, Economic and Environmental Comparisons of Kenaf Growth versus Plantation Grown Softwood and Hardwood for Pulp and Paper, in Kenaf Properties: Processing and Products, Mississippi State University, Mississippi, USA, 323 (1999) H.S Sabharwal, M Akhtar, R.A Blanchette, and R.A Young, Tappi J., 77 105 (1994) O Grigoryeva, A Fainleib, O Starostenko, A Tolstov, and W Brostow, Polym Int., 53, 1693 (2004) X.V Cao, H Ismail, A.A Rashid, T Takeichi, and T VoHuu, BioResources, 6, 3260 (2011) X.V Cao, H Ismail, A.A 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Figure depicts the effect of silane treatments on the tensile strength of the composites APTES pretreatment... DOI 10.1002/vnl FIG Effect of filler treatment on elongation at break of rHDPE/NR/ KP biocomposites observed that KP-APTES treatment had an adverse effect on Eb The lower Eb of composites with