Effect of polymer blend matrix compatibility and fibre

Thermochimica Acta 640 (2016) 52–61

Contents lists available at ScienceDirect

Thermochimica Acta journal homepage: www.elsevier.com/locate/tca

Effect of polymer blend matrix compatibility and fibre reinforcement content on thermal stability and flammability of ecocomposites made from waste materials Ruey Shan Chen a,∗ , Sahrim Ahmad a , Sinyee Gan a , Mohd Nazry Salleh a,b , Mohd Hafizuddin Ab Ghani a , Mou’ad A. Tarawneh c a

Material Science Programme, School of Applied Physics, Faculty of Science and Technology, The National University of Malaysia, 43600 Bangi, Malaysia School of Materials Engineering, Universiti Malaysia Perlis, 02600 Arau, Perlis, Malaysia c Department of Physics, Collage of Science, Al-Hussein Bin Talal University, P.o. Box 20, Ma’an, Jordan b

a r t i c l e

i n f o

Article history: Received 29 March 2016 Received in revised form 29 June 2016 Accepted 3 August 2016 Available online 4 August 2016 Keywords: Polymer-matrix composites (PMCs) Biocomposite Recycled thermoplastic Natural fibers Flame/fire retardancy Thermal analysis

a b s t r a c t This paper was aimed to evaluate the effects of matrix types (with or without ethylene-glycidyl methacrylate compatibilizer) and rice husk (RH) loadings (40–80 wt.%) on RH-reinforced recycled high-density polyethylene/recycled polyethylene terephthalate (rHDPE/rPET) ecocomposites. Results showed that the thermal stability and flammability resistance properties increased as the RH loadings increased. The addition of RH has effectively delayed the thermo-oxidation process of rHDPE/rPET matrix by 10 ◦ C, and evident flame retardant effect has also been observed (decreased burning rate up to 24% in comparison to neat polymer blend). It is interesting to note that compatibilization of polymer blend matrix has further increased the thermal stability of ecocomposites. SEM images confirmed the enhanced interfacial bonding of phases in the compatibilized matrix ecocomposites. It can be concluded from this study that the used agro-waste material (RH) is attractive reinforcements in recycled plastics from the standpoint of their thermal and flammability properties. © 2016 Elsevier B.V. All rights reserved.

1. Introduction During the period of 2010–2013, the average annual global rice production was 725 million metric tons and Asia region alone accounted for more than 90% of the total global rice production. Rice husk (RH) is an inexpensive by-product of rice processing where it is separated from rice grain during the rice milling process, and it is reported that, there is about 0.23 tons of RH is produced for every ton of rice produced [1]. The use of RH in the manufacture of composites panels provides advantageous characteristics, such as low density, biodegradability, toughness, resistance to weathering, and also making the final products more economically competitive. RH fibre-reinforced thermoplastic composites are now being commercialised in the green furniture, building construction for interior components, window and door frame, wall, partitioning and panelling industries as well as automotive industry [2]. The literature review reveals that PE, polypropylene (PP), polyvinyl chloride (PVC), polylactic acid (PLA) are among popular

∗ Corresponding author. E-mail address: [email protected] (R.S. Chen). http://dx.doi.org/10.1016/j.tca.2016.08.005 0040-6031/© 2016 Elsevier B.V. All rights reserved.

choices as matrices for RH filled polymer composites [1]. However, the application of RH in thermoplastic composites is restricted by its hydrophilic nature and low thermal stability [3]. Because of polymers are mostly nonpolar (hydrophobic), the weak compatibility (polarity) between RH and polymer matrix must be solved to avoid the fiber-fiber agglomeration and bad mixing of composite materials. Polymer modification with a coupling agent containing polar groups, such as, maleic anhydride is commonly used to improve the matrix-fiber interfacial adhesion [4]. In fact, most of the natural fibers including RH have a relatively low degradation temperature of about 200 ◦ C, which limits the processing temperature of composites and thus the melting point of polymer is an important consideration in choosing it as matrix materials for RH composites [5]. Thermal properties are crucial to understand the behaviour of the raw materials (either polymer or filler) and interfacial characteristics the end composite material [6]. Different thermal analysis techniques provide different essential information regarding the thermal stability of composites. Thermogravimetric analysis (TGA) can be used to determine the moisture content, thermal degradation temperature and thermal stability of composite materials. Differential scanning calorimetry (DSC) can be used for determining

R.S. Chen et al. / Thermochimica Acta 640 (2016) 52–61

glass transition temperature (Tg ), melting temperature (Tm ), melting enthalpy (Hm ) and crystallinity level (␹c) [7]. Flammability is an important criterion in material selection for consumer product, building and construction applications. Generally, synthetic polymers are highly flammable because of their petroleum source. However, Zhang et al. reported that addition of rice husk hold the potential to suppress the flammability of HDPE owing to the high content of silica present in RH [8]. In our previous research regarding on using recycled highdensity polyethylene (rHDPE) and recycled polyethylene terephthalate (rPET) and RH, the investigation of physical, mechanical and thermal properties of RH-filled rHDPE/rPET biocomposite extruded via single screw and twin screw extruder [9], the effect of RH filler loading and matrix types (uncompatibilized and compatibilized rHDPE/rPET blend) on RH-filled rHDPE/rPET biocomposites on water absorption and mechanical properties [10] and the effect interfacial modifications between blend matrix and RH fillers on water absorption and mechanical properties [2] have been discussed. In spite of these studies, the thermal analysis and flammability properties on RH-filled recycled HDPE/PET blend with respect to the presence of compatibilizer in immiscible polymer matrix has not been discussed deeply. The objectives of the study were to investigate the effect of matrix types (recycled polymer blend with and without compatibilizer) and RH content (40–80 wt.%) and on the thermal properties and flammability behaviour of the RH-incorporated HDPE/PET ecocomposites. The correlation between experimental and theoretical results for thermal properties are reported. 2. Experimental 2.1. Materials Recycled high-density polyethylene (rHDPE; density of 923 kg/m3 , melt flow index of 0.72 g/10 min at 190 ◦ C, 2.16 kg load) and recycled polyethylene terephthalate (rPET; Tg of 74.1 ◦ C, cold crystallization peak temperature of 119.9 ◦ C, melting peak temperature of 252.5 ◦ C and intrinsic viscosity of 0.68 dL/g) were obtained from a local plastic recycling plant. To improve the compatibility between immiscible polymer blend components of rHDPE and rPET, ethylene-glycidyl methacrylate copolymer (E-GMA) with a trade name of Lotader AX8840 (melt flow index of 5 g/10 min at 190 ◦ C, 2.16 kg load and a glycidyl methacrylate content of 8%) was used as compatibilizer. Rice husk (RH) with particle size of 100-mesh was used as agro-filler in the experiment. Maleic anhydride polyethylene (MAPE) with a melting peak temperature of 135.2 ◦ C was utilized as coupling agent. All the raw materials were supplied from a local factory namely BioComposites Extrusion Sdn. Bhd. 2.2. Preparation of RH filled ecocomposites The rHDPE and rPET were compounded using a laboratory scale co-rotating twin screw extruder (model Thermo Prism TSE 16PC, D = 16 mm, L/D = 25). The screw rotating speed was fixed at 30 rpm. The four barrel temperatures from the feeding to die zones were set as 250, 270, 240 and 190 ◦ C, respectively. The uncompatibilized polymer blend with the weight ratio of rHDPE/rPET at 75/25 (wt/wt) is labeled as rPB. Meanwhile, the compatibilized polymer blend with the same ratio of both plastics and 5% of E-GMA (based on the total weight of blend) is referred as rPB/E-GMA. The pre-extruded polymer blend (rPB and rPB/EGMA, respectively) pellets were melt-blended with RH and 3% of MAPE at temperatures profiles 170, 215, 210 and 195 ◦ C with the same screw speed as extrusion of recycled polymer blend. The loading level of RH was

53

varied at 40, 50, 60, 70 and 80 wt.%. In order to remove the trapped moisture inside RH flour, it was oven-dried at 90 ◦ C for 24 h before extrusion. After extrusion, the extrudates were cooled and granulated into pellets by a crusher. The fine granules were compression molded at 200 ◦ C under a pressure of 1000 psi by using a hot/cold press machine (LP50, LABTECH Engineering Company). The preheating, venting, full pressing and cold pressing times were set to 3, 2, 5 and 5 min, respectively. 2.3. Thermal analysis Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) was conducted using a Mettler Toledo TGA/SDTA851e and DSC 882e , respectively, on the samples of about 10–15 mg. Samples of TGA were tested at a heating rate of 10 ◦ C/min over the temperature range from 25 ◦ C to 600 ◦ C, the temperature of complete degradation. While the DSC samples were scanned from 25 ◦ C to 300 ◦ C at heating rate of 10 ◦ C/min, under atmospheric air flow condition. The melting temperature (Tm ) values were taken as the maximum of the endothermic melting peak. In order to confirm the moisture content of raw and dried RH determined by TGA, moisture analysis was carried on both RH using moisture analyzer (MS-70, A&D Company). 2.4. Flammabilty properties Burning test was performed in accordance to ASTM D 5048-90 (Procedure A – test of bar specimens) to determine the relative burning characteristics and flame resistance properties. The burning rates of specimens are calculated using the Eq. (1): V = 60

L t

(1)

where V is the burning rates (mm/min), L is the burned length (mm), t is the burning time (seconds). All the reported results of the burning tests are the average of five replicates for each formulation. 2.5. Scanning electron microscopy (SEM) The morphology of the fracture surface of broken sample from tensile testing was analysed using SEM (VPSEM Philips XL-30). The samples were sputter-coated with gold before examination of SEM at 2000×. 3. Results and discussion 3.1. TGA The thermo-oxidative degradation of rPB and rPB/EGMA ecocomposites with different RH loadings was studied using TGA under atmosphere air condition. The corresponding TGA curves are illustrated in Fig. 1(a) and (b). As can be seen in Fig. 1, the neat polymer blend matrix (rPB and rPB/EGMA samples without RH filler) experienced a dramatic weight loss through one-step degradation process from 400 ◦ C to 500 ◦ C. This result can be explained that neat polymer blend comprises a series of interchained small molecular species which easily undergo thermal decomposition at elevated temperature, which results in the formation of small-molecule radicals with a great activity, capable of initiating various reactions of macromolecules, first of all their thermo-oxidative degradation, depolimerization and destruction that preferably occur at the weak sites of the polymer chains. The maximum decomposition temperature of the rPB and rPB/EGMA samples was at 471 ◦ C and 469 ◦ C, respectively (Table 1). The lower decomposition temperature for rPB/EGMA than that of rPB was due to an increase of rHDPE-rPET interaction as a result of the incorporation of EGMA compatibilizer

54


Fig. 1. Effect of RH loadings on TGA of (a) rPB and (b) rPB/EGMA-based composites, (c) raw and oven-dried RH at 90 ◦ C.

that has a lower decomposition temperature [11]. At temperature beyond 600 ◦ C, the amount of residue was relatively small because the thermal degradation materials of neat polymer blend will further breakdown into gaseous products at higher heating temperature [12]. For ecocomposites filled with RH, the thermal degradation process can be divided into three zones, namely initial, active and passive zones. In initial zone, temperature peak (1) was located at 138–143 ◦ C which indicating the evaporation of moisture trapped in the RH fibers as reported in the literature [13]. As for comparison, the raw (without oven-dried) and oven-dried pure (100%) RH at 90 ◦ C for 24 h (used RH for extrusion) showed 6 and 3% of weight loss in initial zone (Fig. 1(c)). This indication of moisture presented in RH has proved by the moisture analysis in which the raw and oven-dried RH possessed 7.3% and 1.2% moisture, respectively. This result showed that the oven-drying for RH removed

most of the moisture in RH but not fully removal. As shown in Table 1, the weight losses at this zone were recorded for 3.2–4.3%, which were slightly higher than that of pure oven-dried RH (weight loss at 3%). For composites containing RH, although the RH filler was oven-dried at the 90 ◦ C for 24 h before extrusion, the exposure of composites to the air after the extrusion and hot pressing process caused the moisture to be trapped into composites. Some of the small RH flour could be blown out from the open-design sample holder [14], and this was another factor that contributed to this weight loss. The active zone started from 230 ◦ C and the maximum decomposition temperature is recorded at 338–345 ◦ C which corresponds to the decomposition of hemicellulose and cellulose components [15]. Meanwhile, passive zone was measured at the temperature peak (3) which was at about 469–481 ◦ C. This peak is associated with the degradation of non-cellulose such as lignin component and polymer matrix in the ecocomposites. Compared to


55

Table 1 Thermogravimetric data for RH-filled rPB and rPB/EGMA composites. Temperature Peak (◦ C)

Samples

Residues after 600 ◦ C (%)

Weight Loss (%)

1

2

3

1

2

3

– 138 139 140 141 142

– 345 345 342 342 343

471 481 480 480 479 479

– 3.29 3.42 3.56 3.90 3.91

– 16.33 21.83 31.19 34.40 34.41

97.24 67.10 58.49 42.66 37.09 36.56

3.02 16.06 17.56 23.10 24.89 26.29

Composites rPB/EGMA-RH – 0 wt.%RH 40 wt.%RH 138 50 wt.%RH 139 142 60 wt.%RH 143 70 wt.%RH 80 wt.%RH 143

– 343 341 339 340 338

469 480 480 481 478 479

– 3.35 3.49 3.57 4.05 4.25

– 24.23 28.19 34.98 38.67 39.01

97.54 56.23 46.67 36.18 28.45 27.74

2.74 20.70 22.03 25.48 28.50 29.63

Composites rPB-RH 0 wt.%RH 40 wt.%RH 50 wt.%RH 60 wt.%RH 70 wt.%RH 80 wt.%RH

the neat polymer blend (rPB and rPB/EGMA), the temperature peak (3) of ecocomposites containing RH has shifted to higher temperatures, from 469–471 ◦ C to 479–481 ◦ C (Table 1). This behaviour reflects the enhanced thermal stability polymer blend matrix by adding the RH into the ecocomposites. The presence of RH filler limited the mobility of polymer chains and thereby delayed the thermal degradation. When the biocomposite samples were heated to nearly 250 ◦ C, the RH component started to decompose and produced by-products such as silica residues. The silica ash was then gradually accumulated to form a protective network and absorbed the heat generated during degradation, and consequently slow down the thermo-oxidation process of polymer blend matrix [15]. From Fig. 1(a) and (b), it is evident that the residue weight of samples after heating at 600 ◦ C showed an upward trend with RH content in the ecocomposites. The control samples (rPB and rPB/EGMA without RH filler) had the smallest amount of residues, which were about 3.02% and 2.74%, respectively. The amount of residues increased continuously until 26.29% and 29.63% for ecocomposites containing 80 wt.% RH based on rPB and rPB/EGMA matrix, respectively. This result was supported by the high silica content in the RH and the formation of silica ash upon heating.

Interestingly, it is evident that the effect of RH incorporation was more pronounced than the effect of matrix types (rPB, rPB/EGMA). This can be proven that there were no significant changes in all temperature peaks by using different matrix types; those were rPB and rPB/EGMA for RH-filled ecocomposites. The possible reason for this phenomenon is the investigated ecocomposites were reinforced with a very high content of RH filler (40–80 wt.%). However, the residue weight after 600 ◦ C for rPB/EGMA-based ecocomposites was higher (10–29%) than that of rPB-based ecocomposites at all RH contents. This was probably due to the increased interfacial interaction as a result of the reaction of the acid groups from MAPE coupling agent and hydroxyl groups on the fiber surface which promoted the interaction between the degradation process of matrix component and filler, indicating the polymer blend might contribute together with RH fiber in the ash generation process [16]. For rPB (without EGMA compatibilizer), the presence of MAPE (same amount with rPB-based ecocomposites) in the ecocomposites containing RH reacted not only with RH but also hydrophilic rPET. Thus, the interaction between MAPE and RH filler in rPB ecocomposites was lower than that of rPB/EGMA ecocomposites. This has resulted in a lower amount of residue weight for rPB-based ecocomposites.

Table 2 Summary of Tm, melting enthalpy (Hm) and ␹c (%) for rHDPE, rPET, neat polymer blend (rPB and rPB/EGMA) and RH-filled composites. Composites

HDPE component Tm (◦ C)

PET component

Hm (J/g)

Tm (◦ C)

c (%)

Hexp

Hcal a

Obtained

% I/Db

134.7 –

145.3 –

–

49.6 –

– –

Composites rPB-RH 135.6 0 wt.%RH 130.4 40 wt.%RH 50 wt.%RH 130.6 60 wt.%RH 130.5 70 wt.%RH 131.6 80 wt.%RH 134.3

125.2 82.8 60.7 31.7 21.9 10.6

109.0 (+15%) 63.5 (+30%) 52.9 (+15%) 42.3 (−25%) 31.7 (−31%) 21.2 (−50%)

57.0 64.7 56.9 37.1 34.3 24.9

Composites rPB/EGMA-RH 133.6 0 wt.%RH 132.3 40 wt.%RH 132.1 50 wt.%RH 60 wt.%RH 131.2 70 wt.%RH 130.5 80 wt.%RH 132.1

114.8 94.1 63.5 37.5 26.9 16.5

103.7 (+11%) 60.4 (+56%) 50.4 (+26%) 40.2 (−7%) 30.2 (−11%) 20.2 (−18%)

54.9 77.2 62.5 46.1 44.1 40.5

rHDPE rPET

a b

Hm (J/g)

c (%)

Hexp

Hcal a

Obtained

% I/Db

– 252.5

– 36.0

–

– 30.1

– –

+15% +30% +15% −25% −31% −50%

252.8 250.4 249.4 249.2 249.6 249.5

7.7 6.1 4.3 2.9 1.9 1.1

9.0 (−15%) 5.3 (+16%) 4.4 (−2%) 3.5 (−17%) 2.6 (−27%) 1.8 (−37%)

25.7 34.8 29.7 25.0 22.1 19.5

−15% +16% −2% −17% −27% −37%

+11% +56% +26% −7% −11% −18%

251.2 249.3 250.0 249.2 249.9 249.6

5.8 6.4 4.4 3.1 2.0 1.2

8.6 (−32%) 5.0 (+28%) 4.2 (+5%) 3.3 (−6%) 2.5 (−20%) 1.7 (−29%)

20.6 38.2 31.6 28.4 24.1 21.3

−32% +28% +5% −6% −20% −29%

The values in parentheses are the deviation between the Hexp and Hcal . The percentage of increase or decrease of c with respect to that of neat rHDPE or rPET.

56


3.2. Comparison of experimental TGA curves with the modelling curves By employing a rule of mixture (ROM) model, the weight loss for composites at the heating temperature (T), wC (T), can be predicted from the following equation: for rPB-based ecocomposites, wC1 (T ) = wrPB (T ) × W rPB + wRH (T ) × W RH

(2)

for rPB/EGMA-based ecocomposites, wC2 (T ) = wrPB/EGMA (T ) × W rPB/EGMA + wRH (T ) × W RH

(3)

where wC1 (T) and wC2 (T) are the weight los of the rPB-based and rPB/EGMA-based ecocomposite at temperature T; wrPB (T), wrPB/EGMA (T) and wRH (T) are the weight loss of rPB, rPB/EGMA and RH at temperature T; WrPB , WrPB/EGMA and WRH are the corresponding weight fractions of the three components in the composite. The comparison of the prediction results obtained from Eqs. (4) and (5) for both rPB and rPB-based ecocomposites with the TGA experimental results is made in Fig. 2. The experimental curves fitted the ROM model curves reasonably well for weight loss determined TGA, especially for rPB-based ecocomposites. It can be observed that the rPB/EGMA-based ecocomposites with comparatively lower RH content (at 40–60 wt.%RH, as in Fig. 2(a–c)) show an obvious discrepancy between the measured and modeled curves which appeared in the range of 345–440 ◦ C. In fact, within this temperature range the polymer blend matrix kept in molten state which exhibited a higher thermal transfer behaviour than that of RH filler. As a result of the better dispersion and interfacial bonding between rPB/EGMA-RH phases, the thermal transferring process in the rPB/EGMA-based ecocomposites has been expedited which could further accelerate their decomposition kinetics. This suggests that the compatibilized rPB with EGMA led to the positive effect in improving the adherance and interaction between the rPB/EGMA matrix and RH filler. Somehow, the discrepancy became less significant with increasing RH content. The similar observation is reported by Zhao et al. [8]. At temperature over 440 ◦ C, as the increase of RH in the ecocomposites irrespective to matrix types (rPB and rPB/EGMA), the experimental curves shifted toward the right, higher temperature, as compared to the predicted ones. This observation indicates the delay of the decomposition of polymer blend matrix, as supported by the weight loss induced by neat polymer blend matrices within the temperature range of 440–500 ◦ C (Table 1). These results confirmed the enhancement in thermal stability of polymer blend matrices by incorporating high loading RH. 3.3. DSC Fig. 3 depicts the DSC thermograms of the rHDPE, rPET and their ecocomposites with various RH loadings. Table 2 describes the results of changes in melting temperature (Tm ) peak and for pure polymer, polymer blend matrix and RH-filled ecocomposites. As seen in Fig. 3(a), the neat rHDPE showed a melting temperature of 134.7 ◦ C. For neat rPET, a glass transition temperature (Tg ) of 74.1 ◦ C, cold crystallization peak of 119.9 ◦ C and the melting temperature (Tm ) of 252.5 ◦ C were found. Referring to Table 3, two endothermic peaks were observed for all rPB and rPB/EGMA-based ecocomposites which corresponded to the Tm of rHDPE and rPET components, respectively. Based on the observation of Fig. 3(b) and (c), there is no indication for Tg peak of the neat rPB and rPB/EGMA matrix as well as their ecocomposites. This could be ascribed to the less portion of rPET which was only 5–24 wt.% in the ecocomposites and thus, resulted in the failure of showing the Tg peak. Both Fig. 3(b) and (c) also shows the effect of RH loadings on Tm for ecocomposites, however, no remarkable changes is visible (Table 2). These

Fig. 2. Comparison of thermogravimetric curves from experimental measurement with the modelling curves for rPB and rPB/EGMA-based composites with (a) 40 wt.%RH, (b) 50 wt.%RH, (c) 60 wt.%RH, (d) 70 wt.%RH and (e) 80 wt.%RH.


57

Table 3 Flammability behaviours and burning rate of the investigated composites. Composite sample

Flame characteristics

Char

Burning rate (mm/min)

Composites rPB-RH 0 wt.%RH 40 wt.%RH 50 wt.%RH 60 wt.%RH 70 wt.%RH 80 wt.%RH

Pronounced dripping, little black smoke No dripping, huge black smoke No dripping, huge black smoke No dripping, huge black smoke No dripping, huge black smoke No dripping, huge black smoke

Little Higher Higher Higher Higher Higher

43.8 ± 1.2 41.2 ± 0.7 40.2 ± 0.3 38.8 ± 0.3 37.1 ± 0.2 35.3 ± 0.3

Composites rPB/EGMA-RH 0 wt.%RH 40 wt.%RH 50 wt.%RH 60 wt.%RH 70 wt.%RH 80 wt.%RH

Pronounced dripping, little black smoke No dripping, huge black smoke No dripping, huge black smoke No dripping, huge black smoke No dripping, huge black smoke No dripping, huge black smoke

Little Higher Higher Higher Higher Higher

42.4 ± 1.0 40.0 ± 0.4 39.0 ± 0.3 38.1 ± 0.4 36.6 ± 0.2 34.5 ± 0.4

results suggest that Tg and Tm of ecocomposites were strongly influenced by the polymer matrix rather than the presence of RH and its concentrations [7]. According to Kiziltas et al., the melting temperature of the ecocomposites plays an important role in determining the processing temperature and thermal properties [7]. The rPB had Tm at 135.6 ◦ C and 252.8 ◦ C for rHDPE and rPET phase, respectively. Meanwhile, rPB/EGMA sample exhibited lower Tm values, which were 133.6 ◦ C (rHDPE phase) and 251.2 ◦ C (rPET phase), this is because of the improved compatibility between both rHDPE and rPET polymers with the use of EGMA compatibilizer. The enhanced interaction between the PB/EGMA matrix and MAPE-coupled RH can be further explained by the chemical reaction occurred between the epoxy functionality of EGMA with the hydroxyl and carbonyl terminal groups of PET via formation of covalent bonding (Fig. 4(a)). Meanwhile, the hydroxyl end group of PET from rPB matrix interacted with the carbonyl group of MAPE-coupled RH, and wetting of hydrophobic polymer chains (Fig. 4(b)). In overall, it can be noted that the incorporation of RH filler into polymer blend matrix reduced the Tm by 1.2–5.2 ◦ C. This result was agreed by Yao et al. [17] who studied on rice straw fiber reinforced HDPE composite and stated that this phenomenon imply an enhancement on the processing temperature of recycled HDPE after filling rice straw components. 3.4. Correlation of melting enthalpy with crystallinity percentage from DSC The melting enthalpy and crystallinity percentage are listed in Table 2. Considering the rHDPE/rPET composition and RH fibre proportions, the prediction of melting enthalpy of HDPE and PET in the formulated ecocomposites (Hcal ) can be calculated from the Eq. (4). H cal = H exp(p) × Wf

(4)

where Hcal is the calculated melting enthalpy, Hexp(p) is the experimental melting enthalpy for neat rHDPE or rPET, and Wf is the weight fraction of HDPE or PET in the ecocomposites. The values of experimental melting enthalpy (Hexp ) provide important information about the crystallinity of the polymer based ecocomposites. The crystallinity level (␹c ) of HDPE and PET was obtained using the following equation: c (%) =

H exp × 100 H.Wf

(5)

where Hexp is the experimental melting enthalpy, H is the assumed melting enthalpy of fully crystalline HDPE or PET, and Wf is the weight fraction of HDPE or PET in blends of ecocompos-

ites. H fully crystalline HDPE is 293.0 J/g, whereas H fully crystalline PET is 119.8 J/g [18]. The ␹c of rHDPE and rPET in RH-filled ecocomposites decreased with the increasing content of RH fillers, probably due to the restriction of movement in PE chain during the crystallization process [19]. The rPB/EGMA-RH ecocomposites exhibited a higher crystallinity compared to those made from rPB matrix owing to the improved compatibility which promotes the diffusion of PE chains to form transcrystals around the surface of RH. Based on the analysis of DSC data in Table 2, three possible interpretations can be made for the relationship between Hm and ␹c (%) as the following (shown by positive or negative sign in parentheses and% I/D): (i) If Hexp of the ecocomposites is lower than the Hcal , it implies that the RH fibres interact with the polymer matrix by decreasing its crystallinity percentage with respect to that of neat polymer compenent (rHDPE or rPET); (ii) If Hexp of the ecocomposites is higher than the Hcal , it implies that the RH fibres interact with the polymer matrix by increasing its crystallinity percentage with respect to that of neat polymer compenent (rHDPE or rPET); (iii) If Hexp of the ecocomposites is the same as Hcal , it implies that the RH fibres do not interact with the polymer matrix. Another striking observation can be seen in which the values shown in parentheses and% I/D are the same. This phenomenon indicates the relationship between Hexp , Hcal and ␹c can be induced by given the Eq. (5): c (%) =

H exp × c(p) Hcal

(5)

where ␹c(p) is the crystallinity level of neat polymer component. 3.4. Flammability properties The flammability characteristics and burning rate of rPB and rPB/EGMA-based ecocomposites containing different RH filler contents are demonstrated in Table 3. It will be noted that the ecocomposites with the presence of RH filler released a lot black smokes during burning but no dripping of burned materials is visible. This behaviour was different from the polymer blends, neither rPB nor rPB/EGMA, which badly dripped and released a little black smoke during the burning process. As expected, the burning of all RH-containing ecocomposites produced a high amount of ash. This phenomenon can be related to the presence of high lignin content (about 26–31%) from the used raw RH as filler in ecocomposites [11].

58


Fig. 4. Proposed schemes of (a) chemical reaction for rPB/EGMA based biocomposites and (b) rPB matrix/RH filler/MAPE interaction (rPB based biocomposites).

rPB, indicates that the rPB/EGMA sample possessed slightly higher flammability resistance property than rPB. This might be due to the improved compatibility of rHDPE/rPET blend with the aids of EGMA compatibilizer. For ecocomposites containing RH, the flammability resistance increased by 6–24% with the increase of RH filler content from 40 to 80 wt.%. This improvement is attributed to the nature of silica (15–17% in the raw RH) which slowed down the combustion [20]. Based on Table 3 again, it can be noted that the use of rPB/EGMA as a matrix for RH-reinforced ecocomposites provided a lower flammability as compared to rPB matrix-based ecocomposites. From the observation, the compatibilized rHDPE/rPET with EGMA (rPB/EGMA matrix) seems to be more effective in enhancing the interaction between the RH filler and matrix as compared to an immiscible polymer blend (rPB matrix). 3.5. Mechanism for the flame retardancy

Fig. 3. DSC heating curves for (a) rHDPE and rPET, (b) rPB- and (c) rPB/EGMA-based biocomposites at different RH loadings.

According to the calculation of burning rate in Eq. (3), the burning rate is inversely proportional to the times taken to burn a sample. This implies that as the burning rate is lower, the burning or flammability resistance is higher because of the longer burning time is taken during the combustion. From Table 2, neat polymer blend exhibited the highest burning rate that was about 43.8 mm/min for rPB and 42.4 mm/min for rPB/EGMA. A lower burning rate for rPB/EGMA, which was approximately 3.3% less than

Rice husk consists of 35% cellulose, 33% hemicelulose, 23% lignin, and 23% silica [21]. It is known that cellulose and hemicellulose contribute to the flammability of RH, whereas lignin promotes the fire resistance of RH by charring [22]. Comparing to other lignocellulosic fillers, RH contains a high content of inorganic silica which is speculated to play an important role for the flame retardant effect demonstrated in the RH-incorporated HDPE/PET ecocomposites. In which they can be considered as silica-filled polymer ecocomposites [8]. The flame retardant effect of silica-filled polymer ecocomposites could be explained by the barrier mechanism. When RH is exposed to heat, the organic components from RH were gradually decomposed to generate volatile components, which in turn


59

(Fig. 6b). In which the white surface (top) layer is an intumescent crust with a white amorphous silica powder which indicating the almost complete burning of RH on the surface layer, whereas the black bottom layer composed of black powders that meant there was a huge amount of carbon which was not calcinated at this mass scale [23]. These observations have been similarly reported in the previous study on flame retardancy of HDPE/RH ecocomposite [8]. The phenomenon in Fig. 6b confirmed the barrier action of RH in the ecocomposite as explained above, in which the surface layer can serve as a heat insulator that delays the heat transfer to the bottom layer material, and so the heat contact of surface layer material to the flame is prolonged that causes the complete combustion to achieve (proved by white surface layer). 3.6. Morphology of blend and RH/blend ecocomposites Fig. 5. The charred residues and burning rate of composites as a function of RH content.

react with oxygen energetically, and thus leaving silica as the main constituents of the residues. Progressive accumulation of these silica residues finally led to the formation of a carbonaceous-silicate charred layer. This layer can serve as a heat insulator and diffusion barrier which could delay the gasification of degradation products and prevent the access of oxygen as well as retard the heat transfer to the underlaying material [8,22]. Therefore, the effectiveness of a flame retardancy which represented by the burning rate is correlated with the amount of silicate charred layer. Fig. 5 depicts the charred residues generated by TGA testing and burning rate of ecocomposites as a function of RH content. As the increasing RH content, the amount of charred residues produced increased and so to decrease the burning rate of composites. In comparison to rPB-based ecocomposites, the RH ecocomposites based on rPB/EGMA exhibited lower burning rate with more charred residues. This reflects that rPB/EGMA-RH ecocomposites provide more prominent flame retardant effect. The mechanism action of RH during combustion of polymer composites could be confirmed with the evidence of appearances for combusted residues. As shown in Fig. 6, the residues of the combusted RH reinforced rPB/EGMA composite showed different appearances with respect to the different content of RH in comparison to the neat polymer blend. For 40 wt.% RH, the combustion yielded almost entirely black powders of residues (Fig. 6a). The combustion residue of composite with 80 wt.% RH showed a twolayer structure which appeared with absolutely different colours

Fig. 7 shows the SEM micrograph of tensile fractures surfaces for (a) rPB, (b) rPB/EGMA, (c) rPB composite with 40 wt.% RH, (d) rPB/EGMA composite with 40 wt.% RH, (e) rPB composite with 60 wt.% RH, (f) rPB composite with 60 wt.% RH, (g) rPB composite with 80 wt.% RH and (h) rPB/EGMA composite with 80 wt.% RH. As can be seen in Fig. 7(a), rPB showed an obvious phase separation structure between the rHDPE and rPET components which suggesting rPB is an immiscible blend. The surface morphology of rPB was relatively rough and it is evident that no interaction between the spherical rPET particles with the rHDPE matrix as the holes and gaps were presented at the polymer-polymer interphase. The lack of adhesion between rHDPE-rPET confirmed the incompatibility of the two polymer components and this can be attributed to the great difference in their solubility parameters [24], where rHDPE is nonpolar but rPET is polar. On the contrary, rPB/EGMA in Fig. 7(b) displayed homogenous morphology structure with a fine dispersion of rPET domain within the rHDPE phase and smaller size matrix domains. The presence of EGMA as compatibilizer in rPB/EGMA played a role in increasing the interfacial adhesion between the two phases (rHDPE and rPET) and thereby resulted in an increase of mechanical properties [24]. These similar morphologies [Fig. 7(a) and (b)] were noticed for the matrix part of the rPB and rPB/EGMAbased ecocomposites containing RH, as shown in Fig. 7(c, e, g) and Fig. 7(d, f, h) respectively. By comparing Fig. 7(c/d, e/f, and g/h), a striking observation can be seen in the surface morphology changes of ecocomposites with different RH loadings. The RH filler at the concentration of 40 wt.% and 60 wt.% were perfectly attached and strongly embedded in the polymer blend matrix, indicating the efficiency of the composite material mixing, which is attributed to a good fibers-matrix

Fig. 6. Digital optical photographs for the combusted residues of rPB/EGMA-based composites with (a) 40 wt.%RH and (b) 80 wt.%RH.

60


Fig. 7. SEM micrograph of (a) rPB, (b) rPB/EGMA, (c) rPB composite with 40 wt.% RH, (d) rPB/EGMA composite with 40 wt.% RH, (e) rPB composite with 60 wt.% RH, (f) rPB composite with 60 wt.% RH, (g) rPB composite with 80 wt.% RH and (h) rPB/EGMA composite with 80 wt.% RH.


interfacial interaction. Another interesting observation is found to confirm the good interface bonding, that was the presence of fiber fracture (predominant than fiber pullout) in the fractured surface of rPB/EGMA with 60 wt.% RH [Fig. 7(f)]. This proposed the better matrix-filler interfacial bonding which resulted in the well stress propagation between the filler and matrix polymer. Meanwhile, a lesser amount of fiber dispersion and more clear gaps or cavities resulted by insufficient adhesion and fiber pullout were visible in Fig. 7(g) and (h). When the RH filler content increased, there was insufficient filler-matrix interfacial bonding causing the fiber-fiber contact to dominate in the matrix so-called fiber agglomeration. This might be ascribed to the insufficient amount of coupling agent (MAPE) added at high RH loading (80 wt.%) where the amount of MAPE used was fixed for all RH loadings. 4. Conclusions High RH loading ecocomposites based on recycled HDPE/PET (75/25 wt/wt) with and without EGMA compatibilizer copolymer were successfully fabricated through a two-step melt-blending followed by hot pressing. This study evaluated effects of matrix types (rPB and rPB/EGMA) and RH loadings (40–80 wt.%) on thermal, flammability and morphological properties of ecocomposites. Due to the high silica content in raw RH, the incorporation of RH and its concentrations have effectively improved thermal stability and flammability resistance. It is interesting to note that these properties of ecocomposites based on rPB/EGMA (matrix with EGMA) were better than rPB-based ecocomposites (matrix without EGMA), irrespective of fiber loadings. This is due to the compatibilization of rHDPE/rPET blend which is inherently immiscible. The improved interfacial bonding and adhesion of rHDPE-rPET and polymer blend-filler matrix in the rPB/EGMA-based ecocomposites was confirmed by SEM images. Based on the results in this work, it can be concluded that the ecocomposites containing recycled plastics with agro-waste fillers offered comparable properties and lower costs as well as more environmental friendly with respect to competitive materials, especially those based on virgin plastics and synthetic fibers. Acknowledgements The authors gratefully thank UKM Research Grant DPP-2015035, BioComposites Extrusion Sdn Bhd, and MyPHD Scholarship Programme. References [1] R. Arjmandi, A. Hassan, K. Majeed, Z. Zakaria, Rice husk filled polymer composites, Int. J. Polym. Sci. (2015). [2] R.S. Chen, M.N. Salleh, M.H. Ab Ghani, S. Ahmad, S. Gan, Biocomposites based on rice husk flour and recycled polymer blend: effects of interfacial modification and high fibre loading, BioResources 10 (2015) 6872–6885.

61

[3] H. Fang, Y. Zhang, J. Deng, D. Rodrigue, Effect of fiber treatment on the water absorption and mechanical properties of hemp fiber/polyethylene composites, J. Appl. Polym. Sci. 127 (2013) 942–949. [4] A. Choudhury, S. Kumar, B. Adhikari, Recycled milk pouch and virgin low-density polyethylene/linear low-density polyethylene based coir fiber composites, J. Appl. Polym. Sci. 106 (2007) 775–785. [5] M.J. John, S. Thomas, Biofibres and biocomposites, Carbohydr. Polym. 71 (2008) 343–364. [6] Y.A. El-Shekeil, S.M. Sapuan, K. Abdan, E.S. Zainudin, Influence of fiber content on the mechanical and thermal properties of kenaf fiber reinforced thermoplastic polyurethane composites, Mater. Des. 40 (2012) 299–303. [7] A. Kiziltas, D. Gardner, Y. Han, H.-S. Yang, Thermal properties of microcrystalline cellulose-filled PET?PTT blend polymer composites, J. Therm. Anal. Calorim. 103 (2011) 163–170. [8] Q. Zhao, B. Zhang, H. Quan, R.C.M. Yam, R.K.K. Yuen, R.K.Y. Li, Flame retardancy of rice husk-filled high-density polyethylene ecocomposites, Compos. Sci. Technol. 69 (2009) 2675–2681. [9] R.S. Chen, M.H. Ab Ghani, S. Ahmad, M.N. Salleh, M.a. A. Tarawneh, Rice husk flour biocomposites based on recycled high-density polyethylene/polyethylene terephthalate blend: effect of high filler loading on physical, mechanical and thermal properties, J. Compos. Mater. 49 (2014) 1241–1253. [10] R.S. Chen, M.H. Ab Ghani, M.N. Salleh, S. Ahmad, M. a. A. Tarawneh, Mechanical, water absorption, and morphology of recycled polymer blend rice husk flour biocomposites, J. Appl. Polym. Sci. 132 (2015) 2461–2472. [11] B.K. Deka, T.K. Maji, Study on the properties of nanocomposite based on high density polyethylene polypropylene, polyvinyl chloride and wood, Compos. Part A Appl. Sci. 42 (2011) 686–693. [12] H.-S. Kim, S. Kim, H.-J. Kim, H.-S. Yang, Thermal properties of bio-flour-filled polyolefin composites with different compatibilizing agent type and content, Thermochim. Acta 451 (2006) 181–188. [13] S.M.L. Rosa, N. Rehman, M.I.G. De Miranda, S.M.B. Nachtigall, C.I.D. Bica, Chlorine-free extraction of cellulose from rice husk and whisker isolation, Carbohydr. Polym. 87 (2012) 1131–1138. [14] Q. Zhao, B. Zhang, H. Quan, R.C.M. Yam, R.K.K. Yuen, R.K.Y. Li, Flame retardancy of rice husk-filled high-density polyethylene ecocomposites, Compos. Sci. Technol. 69 (2009) 2675–2681. [15] N. Chand, P. Sharma, M. Fahim, Tribology of maleic anhydride modified rice-husk filled polyvinylchloride, Wear 269 (2010) 847–853. [16] J.R. Araújo, W.R. Waldman, M.A. De Paoli, Thermal properties of high density polyethylene composites with natural fibres: coupling agent effect, Polym. Degrad. Stab. 93 (2008) 1770–1775. [17] F. Yao, Q. Wu, Y. Lei, Y. Xu, Rice straw fiber-reinforced high-density polyethylene composite: effect of fiber type and loading, Ind. Crops Prod. 28 (2008) 63–72. [18] B. Wunderlich, M. Dole, Specific heat of synthetic high polymers. VIII. Low pressure polyethylene, J. Polym. Sci. 24 (1957) 201–213. [19] K.S. Chun, S. Husseinsyah, N.F. Syazwani, Properties of kapok husk-filled linear low-density polyethylene ecocomposites: effect of polyethylene-grafted acrylic acid, J. Thermoplast. Compos. Mater. (2015). [20] S. Arora, M. Kumar, M. Kumar, Flammability and thermal degradation studies of PVA/rice husk composites, J. Reinf. Plast. Compos. 31 (2012) 85–93. [21] M. Jonoobi, R. Oladi, Y. Davoudpour, K. Oksman, A. Dufresne, Y. Hamzeh, R. Davoodi, Different preparation methods and properties of nanostructured cellulose from various natural resources and residues: a review, Cellulose 22 (2015) 935–969. [22] M. Nikolaeva, T. Kärki, A review of fire retardant processes and chemistry, with discussion of the case of wood-plastic composites, Balt. For. 17 (2011) 314–326. [23] T.D. Hung, Affects of mass burning scales on properties of rice husk ash as raw material to produce geopolymer, Int. J. Emerg. Technol. Adv. Eng. 3 (2013) 422–428. [24] R.S. Chen, M.H. Ab Ghani, M.N. Salleh, S. Ahmad, S. Gan, Influence of blend composition and compatibilizer on mechanical and morphological properties of recycled HDPE/PET blends, Mater. Sci. Appl. 5 (2014) 943–952.