O R I G I N A L P A P E R
Effect of green coupling agent from waste oil fatty acid on the properties of polypropylene/cocoa pod husk composites
Koay Seong Chun1•Salmah Husseinsyah2• Chan Ming Yeng1
Received: 4 August 2015 / Revised: 2 April 2016 / Accepted: 18 April 2016 ÓSpringer-Verlag Berlin Heidelberg 2016
Abstract Green coupling agent (GCA) made from waste oil fatty acid was used as an effective coupling agent for polypropylene (PP)/cocoa pod husk (CPH) com- posites. The results indicated that the incorporation of 0.5 phr of GCA shows a remarkable improvement on tensile strength, elongation at break and tensile mod- ulus of PP/CPH composites. The composites with GCA also exhibited a higher crystallinity and thermal stability as well as water resistivity. The addition of GCA improved the filler dispersion and interfacial adhesion between PP and CPH.
Moreover, some properties of PP/CPH composites with GCA are better compared to PP/CPH composites with MAPP or MAA. This outcome implied that GCA could be a potential coupling agent for thermoplastic composite materials.
Keywords Cocoa pod huskPolypropyleneCompositesGreen coupling agent Waste oilFatty acid amine
Introduction
Cocoa is an important and widely planted crop among tropical countries including Malaysia [1–3]. Cocoa pod husk (CPH) is outer shell of the cocoa pod covered the cocoa pulp and beans. It usually accounts for up to 76 % of the whole weight of the cocoa pod and each ton of cocoa bean produced 10 tonnes of CPH as waste material [2,3]. In cocoa producing countries, an approximately 40 million ton of CPH was generated annually [4]. The CPH shows good features to be used as filler in the
& Koay Seong Chun
1 School of Engineering, Taylor’s University Lakeside Campus, No. 1 Jalan Taylor’s, 47500 Subang Jaya, Selangor, Malaysia
2 Division of Polymer Engineering, School of Materials Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia
DOI 10.1007/s00289-016-1682-7
production of composite materials as it consists of high cellulosic, lignocellulosic and hemicellulosic content [5]. The used of this CPH in producing thermoplastic composite material may offer an economic advantage and reduce the current environmental issue cause by disposal of CPH. Currently, most of the wood-plastic composites (WPC) are made from agricultural waste filler and thermoplastic materials due to the limitation of forest resources, sustainability, and economic factor as well as the accumulation of agro-waste [6]. Polli-BerTMis an example of composite material made from rice husk and post-consumer thermoplastic materials and it was developed by Miniwiz Sustainable Energy.
The major drawback of producing agricultural waste-based composite materials is the interfacial incompatibility between hydrophilic filler and hydrophobic matrix [7–9]. Natural filler is a well-known hydrophilic material that consists of plenty of hydroxyl groups in chemical structure which leads to poor wetting by hydrophobic thermoplastic material and caused weak filler-matrix adhesion. Currently, many kinds of method and chemical compound were used to pre-treat the natural filler or added during the compounding process for the purpose of enhancing filler dispersion and adhesion with the polymer matrix. However, chemical pre-treatment of natural filler by using alkaline [10, 11], organic acid [1, 12–14] and silane coupling agent [15–17] is seldom used by industry because it involves higher capital costs. The most common methods used by the industry are adding maleated polymer [2, 3, 18–20], fatty acid and its derivative [21–26] during the compounding of composites. This method is rather simple and effective.
The chemical compound based on fatty acid is widely used in the processing of natural filler-based composites because it is inexpensive and sustainable. The Ultra- PlastTMTP01 is an example of commercial plastic additive containing a fatty acid amide that acts as a coupling agent between polymer and natural filler. The amide is a reactive site to form hydrogen bonding with hydroxyl groups of the natural filler.
Meanwhile, the fatty acid chains provided a hydrophobic character to natural filler [27]. Suryadiansyah et al. [26] also reported that the use of ethylene diamine dilaurate from palm oil fatty acid improved the properties of waste paper filled polypropylene composites and its performance is comparable to maleated polypropylene. Previously, we had developed an ethylene diamine dilaurate based on coconut oil fatty acid which is called coconut oil coupling agent (COCA). The incorporation of COCA shows a remarkable improvement on tensile properties, water resistivity and thermal stability of thermoplastic composites containing corn cob [22] and palm kernel shell [23]. Currently, a fatty acid amide from waste oil fatty acid was initially developed in our laboratory and it was used as a green coupling agent (GCA) in cocoa pod husk filled polypropylene composites.
This study is focused on the influence of cocoa pod husk (CPH) content and GCA on tensile properties, water absorption, thermal properties and morphological properties of PP/CPH composites.
Experimental Materials
A discarded coco pod husk (CPH) was obtained from cocoa plantation (Perak).
Firstly, the collected CPH was dried in a circulated air oven at 80°C for a day to remove moisture and then ground into a fine powder. The average particle size of the CPH obtained was 22lm and measured using Malvern Particle Size Analyzer Instrument. Table1 shows the chemical composition of CPH. Polypropylene (PP, type co-polymer, SM 340) used in this experiment was supplied by Titan Petchem (M) Sdn. Bhd, in pellet form with melt flow index of 4.0 g/10 min (230°C) and density 0.9 g/cm3, respectively. The GCA was prepared from waste cooking oil fatty acid and ethylene diamine in our laboratory according to the method developed by Chun et al. [22]. The fatty acid content of waste cooking oil is listed in Table2.
Figure1shows the schematic reaction of preparation of GCA.
Preparation of PP/CPH composites
The CPH and PP were compounded into composites using a BrabenderÒ Plastrograph intermixer, Model EC PLUS in counter-rotating mode at 180°C and a rotor speed of 50 rpm. The PP/CPH composites with different GCA content were compounded according to Table3. The PP pellets were charged into the mixing chamber for 3 min until it completely melted. Then, the CPH and GCA were added and mixed continuously for another 5 min. The total mixing time was conducted for 8 min. The composite compounds were compressed into 1 mm thickness sheet by hot-press machine (model GT 7014A, GoTech, Taiwan). The procedures included:
(1) pre-heated compound at 180°C for 8 min; (2) compressed compound into uniform flat surface for 1 min at the same temperature with pressure 9.81 MPa; (3) cool the samples for 5 min. All composite sheets were cut into dumbbell shapes using a Wallace Dumbbell cutter according to ASTM D 638 type IV [28]. First, the PP/CPH composites were prepared with different GCA content to determine the optimum content of GCA. According to the experimental results, the 0.5 phr of GCA content was an optimum content for this composite system. Then, the PP/CPH
Table 1 Chemical composition
of cocoa pod husk [5] Composition Content (%)
Cellulose 26.38
Lignin 24.24
Hemicellulose 8.72
Carbohydrate 17.52
Moisture content 10.50
Ash content 9.02
Crude protein 2.09
Fat content 1.53
Total 100
composites with and without 0.5 phr of GCA at different filler content were prepared.
Testing and characterization
The specimens were tested for tensile properties according to ASTM D 638 [28]
using an Instron universal testing machine, model 5569. A cross-head speed of 30 mm min-1was selected and the load cell was 50 kN. An average of minimum 5 replicate samples for each formulation was tested and the tensile properties including tensile strength, elongation at break and tensile modulus were recorded and calculated by the instrument software.
The data of tensile test for each PP/CPH composite with and without GCA were statistically analyzed using Microsoft Excel 2013 statistical analysis. The single
Fig. 1 Schematic reaction between fatty acid from waste oil and ethylene diamine to produce green coupling agent
Table 3 Water absorption constants and diffusion coefficient of PP/CPH composites with and without GCA or other coupling agents
Materials Ms(%) n k910-4(g/g s2) D910-13(m2s-1)
PP/CPH:100/20 without GCA 4.07 0.53 5.30 1.40
PP/CPH:100/40 without GCA 11.22 0.53 6.48 1.80
PP/CPH:100/20 with GCA 3.19 0.54 4.37 1.27
PP/CPH:100/40 with GCA 9.05 0.54 5.29 1.60
PP/CPH:100/40 with MAPPa 8.98 0.54 5.32 1.65
PP/CPH:100/40 with MAAb 9.05 0.54 5.35 1.66
Table 2 Fatty acid content of
waste cooking oil Fatty acid Name Content
C14:0 Myristic acid 0.8
C16:0 Palmitic acid 51.8
C18:0 Stearic acid 7.4
C18:1 Oleic acid 30.5
C18:2 Linoleic acid 9.5
factor for analysis of variance (ANOVA) was used to determine the significance in the difference between means. The difference between means was considered significant whenP B0.05.
Morphological study of composites was carried out using a scanning electron microscopy (SEM, model JOEL, JSM-6460LA). The fracture ends of specimens were sputter coated with a thin layer of palladium for conductive purpose. The specimens were examined under an acceleration voltage of 5 keV.
Water absorption test on PP/CPH composites was performed according to ASTM D570 [29]. The specimens (30 mm925 mm91 mm) were immersed in distilled water at room temperature (25°C). The water absorption was measured by weighing the specimens at regular intervals with a Mettler balance; model AX 200, Shimadzu (Japan, precision of±1 mg). The percentage of water absorption,Mawas calculated by formula below:
Ma¼WnWd
Wd 100% ð1Þ
whereWdandWnare original dried weight and weight after exposure, respectively.
Differential Scanning Calorimetry (DSC) analysis was carried out using DSC Q10, Research Instrument. Specimen with a weight of 7±2 mg was placed in close aluminum pan and heated from 30 to 200°C with a heating rate of 10°C/min under nitrogen atmosphere. The nitrogen gas flow rate was 50 ml/min. The degree of crystallinity of composites (Xc) can be calculated from DSC thermogram using the following equation:
Xc ¼ ðDHf=DHf0Þ 100 ð2Þ where DHf is the heat fusion of the composites, and DHf0
is the heat fusion for 100 % crystalline PP (DH100=285 J/g) [1,2].
The crystallinity of PP Matrix (Xpp) was calculated using Eq. (3), where theWfpp is the weight fraction of PP matrix.
Xpp¼Xc=Wfpp ð3Þ Thermogravimetric analysis (TGA) was carried out using TGA Pyris Diamond Perkin Elmer apparatus. The weight of the specimens was 7±2 mg. The weight loss of the specimen against the temperature was measured at a temperature range from 30 to 700°C with a heating rate of 10°C min-1, under nitrogen atmosphere.
The nitrogen gas flow rate was 50 ml min-1.
Results and discussion Tensile properties
The effect of various GCA content on tensile strength and tensile modulus of PP/
CPH composites with 40 phr filler content is shown in Fig.2a. The addition of 0.5 phr of GCA shows optimum improvement (PB0.05) of tensile strength and
tensile modulus of PP/CPH composites. However, the tensile strength and tensile modulus of PP/CPH composites decreased (PB0.05) after 1 phr of GCA content.
This is probably related to the relationship between surface coverage of the GCA and wettability of matrix on the filler surface as illustrated in Fig.3. The 0.5 phr of GCA was attached and widely covered on CPH filler surface and there are some spaces that allow PP matrix to fully wet the filler surface as illustrated in Fig.3a.
Therefore, a good interfacial adhesion between filler and the matrix was achieved.
This also contributed to the increase of tensile strength and tensile modulus of composites. Once the GCA content more than 0.5 phr, the CPH surface was fully covered by GCA and the GCA formed a tightly packed fatty acid layers on filler surface. This might restrict the PP matrix from wetting the filler surface (as shown in Fig.3b). As result, a decrease of tensile strength and tensile modulus was observed due to the poor filler-matrix adhesion. This statement was agreed by Danyadi et al. [24]. Demjen and Pukanszky [30] also claimed that the coupling agent molecules form a perpendicularly oriented structure on the filler surface at low concentration, which resulting in improve of tensile strength and tensile modulus. However, the tensile strength and modulus of the composite decreased while high concentration of coupling agent was used. This is because a high
Fig. 2 Effect of GCA content onatensile strength and tensile modulus,belongation at break of PP/CPH composites at 40 phr of CPH content
concentration of coupling agent forms physisorbed layers on filler surface, which is easily debonded from the matrix under the effect of external load. Thus, the 0.5 phr of GCA was the optimum content for this composite system. Besides, the elongation at break of PP/CPH composites was significantly increased (PB0.05) with the increases of GCA content (as displayed in Fig.2b). This is due to the lubricating effect of GCA. Suryadiansyah et al. [26] also reported that the increase of ethylene diamine dilaurate content increased the elongation at break of waste paper filled polypropylene composites due to the lubricating effect.
Figure4a, b shows the effect of filler content on tensile strength and tensile modulus of PP/CPH with and without GCA. The addition of CPH significantly reduced the tensile strength of composites. This is caused by the following reasons:
(1) particular filler has a low aspect ratio and poor in stress transfer and (2) interfacial incompatibility between hydrophilic filler and hydrophobic matrix, and (3) poor filler dispersion [1, 2, 12, 21]. Alternatively, the incorporation of CPH increased the tensile modulus of composites. This is also supported by the decrease of elongation at break as exhibited in Fig.4c. The PP matrix became less ductile as the chains mobility of PP molecules reduced by the present of friction between filler and matrix [13,16]. Thus, the composites became stiffer and more brittle at higher filler content. In addition, the presence of filler agglomeration in composites also acts as a stress concentration point that causes composites fracture at lower elongation [22]. The introduced GCA had increased (PB0.05) the tensile strength and tensile modulus of PP/CPH composites. The presence of GCA is able to react with the hydroxyl group of CPH. The GCA is attached on the CPH filler surface via hydrogen bonding. Meanwhile, the fatty acid group of GCA provides a hydrophobic character to CPH filler (as shown in Fig.5). Thus, this improved the wetting of the PP matrix on CPH filler and enhanced the interfacial interaction. As result, a better stress transfer was achieved by strong interfacial interaction and it increased the tensile strength and tensile modulus. However, the elongation at break of PP/CPH composites was increased (P B0.05) with the addition of GCA. This is probably due to the presence of GCA reduced filler agglomeration by enhancing the filler dispersion. Besides, the lubricating effect of GCA also contributed to improve the
Fig. 3 Different idealized surface coverage of GCA and wettability of matrix on CPH surface
Fig. 4 Effect of GCA (0.5 phr) onatensile strength,btensile modulus, andcelongation at break of PP/
CPH composites
ductility of composites. As a result, the PP/CPH composites with GCA show a higher elongation at break. The similar observation was also reported by Chun et al.
[22] and Jandas et al. [31].
Table5 shows the comparison of the tensile properties of PP/CPH composites with GCA, MAPP, and MAA at similar filler content. The PP/CPH composites with GCA exhibited 2.3 and 1.4 % higher tensile strength and tensile modulus than PP/
CPH composites with MAA [1]. However, PP/CPH composites with GCA have 22.1 and 10.4 % lower tensile strength and tensile modulus compared to PP/CPH composites with MAPP [2]. The MAPP coupling agent shows great performance in improving tensile strength and tensile modulus of PP/CPH composites. This is because the MAPP covalent bonded with CPH and PP backbone from MAPP forms a strong molecular chain entanglement with PP matrix. The interfacial bonding was enhanced due to the formation of molecular chains entanglement at interface region and it is stronger compared to usual interfacial bonding that established via mechanical interlocking mechanism [32]. Hence, the PP/CPH composites with MAPP exhibited highest tensile strength and tensile modulus. In contrast, the PP/
CPH composites with GCA show highest elongation at break compared to PP/CPH composites with MAA [1] and MAPP [2]. The main drawback of using MAPP and MAA is reducing the ductility of PP/CPH composites. However, the main benefit of using GCA is improving the tensile strength and tensile modulus, while improving the ductility of PP/CPH composites.
The relationship between GCA and interfacial adhesion can be expressed quantitatively by a simple model developed by Puka´nsky [33]. The model considers the major factors influencing the tensile strength, including: (i)nis the change of specimen dimensions during the deformation and the raise of tensile strength due to strain hardening; (2) (1 - /)/(1?2.5 /)-effect of reducing load bearing cross section of the matrix due to filling; and (iii) exp (B/)-interfacial adhesion [33,34].
rT¼rnT0 1/
1þ2:5/expðB/Þ ð4Þ
whererTand rT0 are the true strength of the composite and the polymer matrix, respectively (rT=rk, whereris the measured engineering tensile strength),k is
Fig. 5 Schematic reaction between GCA and CPH
the relative elongation (k=L/L0, whereL0is the original length andLis length of the failure point), n is related to strain hardening exponent of polymer matrix,/is the volume friction of filler, and B is a parameter expressing the load bearing capacity of filler that corresponds to the effect of interfacial adhesion. The model can be expressed in the linear form, such as Eq. (5).
lnrTred¼lnrT kn
1þ2:5/
1/ ¼lnrT0þB/ ð5Þ
Figure6illustrates the plot of ln reduced tensile strength of PP/CPH composites without and with GCA as a function of filler content. A straight linear correlation is observed. Thus, the parameterB related to stress transfer and interfacial adhesion can be determined with more accuracy. The slope of the line is changed after the addition of GCA. The parameterB of composites with GCA was 2.16 and it was higher than composites without GCA (1.55). This evidenced that the interfacial adhesion between CPH and PP matrix was enhanced by the presence of GCA.
Water absorption
The comparison of water absorption between PP/CPH composites with and without 0.5 phr of GCA is shown in Fig.7. The results indicated that the water absorption of composites rose with increasing filler content and immersion time. The water absorption of composites was due to the capillary transport of water molecules into the micro gap or flaw that is present in the interface between filler and matrix as poor interfacial interaction [35]. Furthermore, the CPH filler contains plenty of hydroxyl groups in the chemical structure and this given the natural hydrophilic behave to CPH. The CPH filler is easily bonded to water molecules through hydrogen bonding and the present of CPH filler also supported to the water uptake of composites [18, 21]. Therefore, the increase in the filler content increased the
Fig. 6 Reduced tensile strength of PP/CPH composites (a) with and (b) without GCA plotted against filler content
water uptake ability of composites. The incorporation of GCA was significantly reduced the water absorption of PP/CPH composites. The presence of GCA was attached and formed a fatty acid layer on the CPH filler surface, which can provide a hydrophobic character to CPH filler. Thus, the tendency of CPH filler to bond with water molecules was reduced. This might result in decreasing water absorption of PP/CPH composites.
Moreover, the water absorption behavior of PP/CPH composites can be determined using Fick’s law. The studies of diffusion mechanism and kinetic of composites were performed based on Eq.6.
Mt=Ms¼ktn ð6Þ
where Mt is the moisture content at time t, and Ms is the moisture content at saturated point. Thekandn are constants.
The moisture diffusion behavior of composites can be divided as: (1) case I (n=0.5): Fickian diffusion; (2) case II (0.5\n\1): non-Fickian or anomalous diffusion; and (3) super case II (n[1) [35]. The constantkandncan be determined from the fitting curve of plot log Mt/Ms versus log t (as showed in Fig.8a). The diffusion coefficient (D) is the most important parameter in Fick’s model and it shows the ability of water molecules to penetrate inside the composites structure [35,36]. The parameterDis calculated from the slope of the linear part of the plot ofMt/Ms versus time (t0.5) (as illustrated in Fig.8b) using Eq.7.
Mt=Mt¼ ð4=hÞðD=pÞ0:5t0:5 ð7Þ whereh is the thickness of the sample.
The k, n, and D values obtained from curves fitting are listed in Table3. The n values of all composites are close to the valuen =0.5. Therefore, all PP/CPH composites approached toward the Fickian diffusion behavior. Most of the studies on the water absorption behavior of natural filler filled polymer composites were following Fickian behavior [35–38]. The valuekof PP/CPH composites was found
Fig. 7 Water absorption of (a) Neat PP, PP/CPH composites without GCA at (b) 20 phr and (c) 40 phr CPH content, PP/CPH composites with GCA at (d) 20 phr and (e) 40 phr CPH content
to increase with increasing filler content, but it reduced with the presence of GCA.
This evidenced that the composites had high water absorption at higher filler content, but the water absorption tends to be reduced with the addition of GCA. The PP/CPH composites with GCA also showed lower diffusion coefficient (D) com- pared to PP/CPH composites without GCA. This is because modified CPH with GCA has better wetting and interfacial bonding with PP matrix. There are less gaps and flaws at interfacial region and also GCA provides a hydrophobic surface to CPH. As a result, the ability of water molecules to penetrate the composite is reduced. According to Table3, the value ofMs,kandDof PP/CPH composites and GCA was not much different compared to PP/CPH with MAPP and MAA. This
Fig. 8 Plot ofalogMt/Msversus log timetandbMt/Msversus logt0.5of PP/CPH composites without and with GCA
indicated that the GCA has similar performance as MAPP and MAA in improving the water resistivity of PP/CPH composites.
Morphological study
The CPH was irregular oriented shape particulate filler. For this reason, the addition of CPH reduced the tensile strength of PP/CPH composites. Figure9a, b shows the SEM micrographs of the tensile fracture surface of PP/CPH composites without GCA. The filler agglomeration was found in SEM micrograph of PP/CPH composites without GCA. This indicated that the CPH particles were poor dispersion in the PP matrix. In addition, numerous voids due to filler puller out can be observed as highlighted using an arrow. A detached CPH particle is also found in Fig.9b. This evidenced poor interfacial adhesion between PP matrix and CPH filler.
Fig. 9 SEM micrographs of PP/CPH composites without GCA ata20 phr andb40 phr of CPH content
The observations supported the decrease of tensile strength of PP/CPH composites.
The SEM micrographs of PP/CPH composites with 0.5 phr of GCA are illustrated in Fig.10a, b. The filler agglomeration was absent in SEM micrographs and the CPH particles found in the SEM micrographs were embedded in a PP matrix.
Furthermore, there are less filler pull out can be observed in the SEM micrographs of PP/CPH composites with GCA (Fig.10a, b). This indicated that the presence of GCA improved the dispersion and adhesion between CPH and PP matrix. This is also proven that the improvement of tensile properties of PP/CPH composites was due to the addition of GCA.
Differential scanning calorimetry (DSC)
Figure11shows the DSC curves of PP/CPH composites with and without 0.5 phr of GCA. Table4shows the data extracted from the DSC analysis. The heat fusion of
Fig. 10 SEM micrographs of PP/CPH composites with GCA ata20 phr andb40 phr of CPH content
composites (DH) of composites and crystallinity of PP matrix increased with the increasing filler content. This is because the incorporation of CPH provided a site for nucleation [1, 2]. Furthermore, the PP/CPH composites with GCA exhibited higher DH and crystallinity than PP/CPH composites without GCA. This is contributed to the improvement of filler dispersion and interfacial adhesion between CPH and PP matrix, which further enhanced the nucleating effect on composites.
Many researchers also reported that the crystallinity of composites is generally influenced by filler content, filler dispersion, and the filler-matrix adhesion [21,23, 39,40]. The melting temperature (Tm) of composites was not influenced by change of filler content and presence of GCA. Regarding Table4, Th Xpp of PP/CPH
Fig. 11 DSC thermograph of neat PP, PP/CPH composites with and without GCA
Table 4 DSC data of neat PP, PP/CPH with and without GCA or other coupling agent
Materials Tm(°C) DH(J/g) Xc(%) Xpp(%)
Neat PP 165 27 27 27
PP/CPH:100/20 without GCA 165 35 28 34
PP/CPH:100/40 without GCA 165 52 31 43
PP/CPH:100/20 with GCA 165 68 32 37
PP/CPH:100/40 with GCA 164 71 34 48
PP/CPH:100/40 with MAPPa 163 83 40 56
PP/CPH3:100/40 with MAAb 164 91 43 60
Tm=Melting temperature Xc=Crystallinity 8 of composites DH=Heat fusion of composites XPP=Crystallinity of PP matrix
a, b DSC data of composites were taken from Ref. [1,2], respectively
composites with GCA shows 17 and 25 % lower compared to PP/CPH composites with MAPP and MAA. In this case, the presence of MAPP and MAA exhibited a stronger nucleating effect on PP/CPH composites.
Thermogravimetric analysis (TGA)
The derivative thermogravimetric (DTG) and TGA thermograms of neat PP, PP/
CPH composites without and with 0.5 phr of GCA are shown in Fig.12a, b, respectively. The data from TGA analysis are summarized in Table5. The incorporation of CPH assigned to an early thermal decomposition on composites as the increase in the filler content decreased the decomposition temperature at 5 % weight loss (Td5 %). The weight loss at early thermal decomposition was assigned to weight loss from moisture and hemicellulose from CPH. Alterna- tively, increase in the filler content increased the decomposition temperature at the maximum rate (Tdmax) of composites. This indicated that the thermal
Fig. 12 aDTG andbTGA thermographs of neat PP, PP/CPH composites with and without GCA
stability of composites increased with more filler content. This is because of the generation of char residue from early thermal decomposition delaying the thermal decomposition process of composites [1, 2]. Araujo et al. [41] also found that the thermal stability of composites increased along with the charged of curaua fiber content. The authors claimed that the increases of curaua fiber content also increased the char residue content. Thus, they suggested the increase of thermal stability of composites was related to the effect of char formation. The Td5 % and Tdmax of PP/CPH composites shifted to higher temperature with the presence of GCA. The char residue of composites significantly increased after addition of GCA. This might be due to the addition of GCA promoted the process of char formation. Moreover, the increase of char content further increased the thermal stability of composites. Hence, the char residue of PP/CPH composites also increased after the addition of GCA. Chun et al. [21] and Faisal et al. [42] also reported that the use of fatty acid-based coupling agent (sodium dodecyl sulfate) improved the thermal stability of thermoplastic composites consisting of coconut shell and chitosan. However, the reason on how the present coupling agent influences the thermal stability of composites was not in depth study. As a comparison, the presence of GCA, MAPP, or MAA was increased theTd5 %andTdmaxof PP/CPH composites. This suggested that the GCA has a similar effect as MAPP and MAA in improving the thermal stability of PP/CPH composites.
Conclusions
The increase of CPH has significantly reduced the tensile strength and elongation at break, but increased the tensile modulus of composites. The incorporation of CPH also increased the crystallinity and water absorption of composites. The PP/CPH composites show an early thermal degradation compared to neat PP. The addition of 0.5 phr of GCA was the optimum content for PP/CPH composites. The presence of 0.5 phr of GCA was improved the tensile strength and tensile modulus of PP/CPH
Table 5 TGA data of neat PP, PP/CPH composites without and with GCA or other coupling agent
Sample Td5%(°C) Tdmax(°C) Residue at 700°C (%)
Neat PP 336 418 1.22
PP/CPH:100/20 without GCA 272 422 2.69
PP/CPH:100/40 without GCA 246 432 4.22
PP/CPH:100/20 with GCA 278 428 4.62
PP/CPH:100/40 with GCA 252 445 6.22
PP/CPH:100/40 with MAPPa 251 449 6.49
PP/CPH:100/40 with MAAb 249 455 6.27
Td5 %=Decomposition temperature at 5 % weight loss Tdmax=Decomposition temperature at maximum rate
a, b TGA data of composites were obtained from Ref. [1,2], respectively
biocomposites, but the improvement was slightly lower compared to PP/CPH composites with MAPP and MAA. Moreover, the presence of GCA increased the elongation at break of PP/CPH composites which show the opposite trend compared to PP/CPH composites with MAPP and MAA. The GCA also improved the water resistivity, crystallinity and thermal stability of PP/CPH composites and the performance is similar to MAPP and MAA. The improvement of properties was due to better interfacial adhesion between CPH and PP matrix. The SEM micrographs evidenced that the PP/CPH composites with GCA had better filler dispersion and interfacial adhesion compared to PP/CPH composites without GCA. The GCA made from waste oil fatty acid was a new potential coupling agent for thermoplastic composite materials.
Acknowledgments The authors are thankful to Dr. Alias from Cocoa Research and Development Centre (Hilir Perak), Malaysian Cocoa Board for providing the cocoa pod husk waste for this research.
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