Journal Pre-proof Effect of microcrystalline cellulose [MCC] fibres on the morphological and crystalline behaviour of high density polyethylene [HDPE]/polylactic acid [PLA] blends Siddharth Mohan Bhasney, Kona Mondal, Amit Kumar, Vimal Katiyar PII: S0266-3538(19)32932-X DOI: https://doi.org/10.1016/j.compscitech.2019.107941 Reference: CSTE 107941 To appear in: Composites Science and Technology Received Date: 18 October 2019 Accepted Date: 5 December 2019 Please cite this article as: Mohan Bhasney S, Mondal K, Kumar A, Katiyar V, Effect of microcrystalline cellulose [MCC] fibres on the morphological and crystalline behaviour of high density polyethylene [HDPE]/polylactic acid [PLA] blends, Composites Science and Technology (2020), doi: https:// doi.org/10.1016/j.compscitech.2019.107941. 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Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd. 1 Effect of microcrystalline cellulose [MCC] fibres on the morphological and 2 crystalline behaviour of high density polyethylene [HDPE]/polylactic acid 3 [PLA] blends 4 Siddharth Mohan Bhasney, Kona Mondal, Amit Kumar and Vimal Katiyar* 5 Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India *Corresponding author e-mail: vkatiyar@iitg.ac.in 6 7 Abstract 8 The present study describes the fabrication and characterization of eco-friendly composites 9 based on high density polyethylene (HDPE), poly (lactic acid) (PLA) with microcrystalline 10 cellulose (MCC) fibres via melt extrusion followed by injection moulding. The morphology of 11 PLA in both HDPE/PLA blends and HDPE/MCC fibres composites was irregular and 12 immiscible. The tensile strength at max load and % elongation at break of all polyblend 13 composites were reduced to ~17%, ~15%, and ~84%, ~89% than pure HDPE matrix, 14 respectively. The toughness of HDPE and its polyblend composites were decreased to ~ 92% 15 and ~ 91%, respectively. As higher number indicates greater resistance, the hardness of HDPE 16 was improved by ~7% and ~10% after adding PLA followed by MCC fibres. As revealed from 17 DSC analysis, the crystallinity of composites was considerably influenced by the MCC fibres 18 content due to the transcrystallization effect. The crystallinity of HDPE/PLA with MCC fibres 19 polyblend composites was decreased by ~ 41% as compared to the HDPE matrix because of 20 disorientation of fibres as seen in X-ray diffraction analysis. Eventually, the results revealed 21 that the presence or absence of MCC fibre loading had significant effects on the mechanical, 22 morphological, and thermal properties of HDPE/PLA blends. 23 24 Keywords: High density polyethylene (HDPE); Poly (lactic acid) (PLA); Microcrystalline 25 cellulose (MCC) fibres; Polyblend composites; Crystallinity. 1 26 1. Introduction 27 Plastic consumption has increased steadily since its manufacturing started on a 28 commercial scale in the 1930s. Conventional plastics are made from fossil fuels and are non- 29 degradable in nature. Plastic articles make our lifestyle smooth and consists of mainly 30 polypropylene (PP) and polyethylene (PE) products, which constitute more than 50% of all the 31 plastic waste found in the environment. Polyethylene has great commercial value due to its low 32 cost and good processing properties, and is classified as high-density polyethylene (HDPE), 33 low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE), based on its 34 melt flow index and density. PE has extensive applications in different industries such as 35 packaging, automobile, pharmaceutical, construction, agriculture, with HDPE constituting 36 almost half the share of disposable products from various industries [1]. HDPE has several 37 advantages over other plastic materials such as low cost of manufacture, better hydrophobicity, 38 and exceptionally good physical, chemical, mechanical and thermal properties like high 39 flexibility, great translucency, and considerable toughness at very low temperatures, while 40 some of its disadvantages include low stiffness, higher mold shrinkage, poor ultraviolet and 41 heat resistance [2]. 42 The petroleum-derived plastics have caused massive environmental problems due to its 43 non-degradable characteristic, which has resulted in increased carbon footprint and widespread 44 accumulation of plastic products in the ecosystem. Consequently, it is extremely necessary to 45 replace conventional plastics with biodegradable polymers. Among the biopolymers, an 46 aliphatic polyester, polylactic acid (PLA) is a biobased plastic and have excellent 47 characteristics like high modulus, biodegradability, biocompatibility, transparency and non- 48 toxicity, and thus, it can be used in various sectors of agriculture, medical, packaging and 49 automobile [3, 4]. However, PLA has few drawbacks such as high brittleness, relatively poor 50 thermal stability and high cost of production, which has turned out as the major drawbacks for 2 51 its large scale application in different industries [5]. On the other hand, natural fiber-based 52 polymer composites have gathered substantial importance in the recent years due to their 53 excellent eco-friendly characteristics such as hydrophilicity, non-toxicity, higher stiffness, low 54 density, biodegradability and cheap production cost and they are considered as good 55 replacement to the fossil fuel-based polymers [6, 7]. Microcrystalline cellulose (MCC), the 56 refined form of wood pulp, is a natural fiber, having the highest specific area among all the 57 cellulosic fibers. MCC fibers are being used as biofuel and also as reinforcing fillers in high- 58 performance packaging, due to its outstanding properties of fire resistance, anti-abrasiveness, 59 and biocompatibility [5, 8]. It is a well-known fact that the complete degradation of synthetic 60 plastics takes more than 500 years, wherein these plastic heaps create problems in the 61 environment. 62 Recycling of non-degradable plastics has been considered as a solution to this menace, 63 but this process is very expensive and the quality of recycled products is very inferior 64 compared to the new products. Moreover, replacement of synthetic plastics with biodegradable 65 polymers is not an effective solution as biodegradable polymers are expensive, have a short life 66 span and show reduced performance due to their characteristics. Consequently, a plausible 67 answer to this critical scenario of plastics waste management is using biodegradable polymers 68 as partial constituents of non-degradable plastics, e.g. blending of non-degradable polymers 69 with the biopolymers like polylactic acid (PLA), polyhydroxyalkanoate (PHA), poly-ε- 70 caprolactone (PCL), etc. [4, 9]. 71 The primary aim of this work is to develop bio-derived thermoplastic polyblend of 72 HDPE and PLA with MCC fibres and its optimization for improved mechanical and thermal 73 properties, in particular, optimizing the ratio of biodegradable polymers present in the polymer 74 bio-composites, so that if the microorganisms attack or degrade the composites, the non- 3 75 degradable constituents disintegrates in the waste dumping place. So far, no work has been 76 performed till now to characterize the properties of MCC fiber with HDPE/PLA composites. 77 78 2. Experimental 79 2.1. Materials 80 Commercial HDPE (Halene – H*, grade – F5400, density – 0.954 g/cm3 at 23° C, a 81 melt flow index – 0.09 g/10 min, Hardness – 65 shore D) and semi- crystalline PLA (grade 82 2003D, Mn – 88,500 Da and Mw/Mn – 1.8, density – 1.24 g/cm3) were procured from Haldia 83 Petrochemical Limited (Kolkata, West Bengal, India) and Nature Works LLC (Minnetonka, 84 Minnesota, USA), respectively. Highly crystalline and odourless white fine powder of MCC 85 (pH rang 5.0 – 7.0, size of particles – 10-15 µm, density - ~0.13 – 0.25 g/cm3) was bought from 86 Sigma Aldrich (Bangalore, Karnataka, India). 87 2.2. Fabrication of polyblend composites 88 All the materials were dried in a hot air oven at 60 °C for 24 hours to eliminate 89 impurities prior to melt compounding, and then mixed in varying proportions (Table 1). The 90 samples were obtained in molten form using co-rotating twin screw extruder (D76227, Haake, 91 Thermo Fisher Scientific, Germany) with L/D ratio 16:1 at 210 °C. The screw speed was 100 92 rpm and the recycle time was 1 minute. The melted sample was applied in the injection 93 moulding machine to make dumbbells at 215 °C cylinder temperature, 100 °C mould 94 temperature at 5 second holding time under the pressure range of 700 – 750 bar. 95 Table 1: Composition of HDPE, PLA and MCC and its polyblend composites. Sample designation (wt.%) HDPE (g) PLA (g) MCC (g) HDPE 7 0 0 PLA 0 7 0 HDPE90/10PLA 6.3 0.7 0 HDPE70/30PLA 4.9 2.1 0 4 HDPE50/50PLA 3.5 3.5 0 HDPE90/10PLA/3MCC 6.3 0.7 0.21 HDPE70/30PLA/3MCC 4.9 2.1 0.21 HDPE50/50PLA/3MCC 3.5 3.5 0.21 96 2.3. Characterization studies 97 2.3.1. Morphological studies - The morphological behaviour of the tensile fracture surface of 98 HDPE and its polyblend composites were observed using field emission scanning electron 99 microscopy (FESEM) (Zeiss Sigma, USA). Prior to the analysis, dumbbell of each sample was 100 kept in liquid nitrogen overnight, and then broken into two parts. These cryo-fractured samples 101 were placed on the aluminium stub fixed with carbon tape and were gold coated using sputter 102 coater (SC7620, Quorum). Additionally, thin film samples were also kept on a copper grid and 103 observed via transmission electron microscopy (TEM) (JEM-2100, JOEL, USA). Thin sections 104 of HDPE90/10PLA blend and HDPE90/10PLA/3MCC composite samples were first cut with 105 the help of a 1 mm2 trapezium shaped block. Later, an ultra-microtome (Leica EM UC7) was 106 used for the perfect sectioning of the mounted samples. Crystal density and crystal growth rate 107 of HDPE and its polyblend composites (in varying ratios) were studied using a polarized 108 optical microscope (POM) (Nikon Co., Japan) attached to a Linkam TST350 hot stage (Linkam 109 Scientific Instrument). ~ 0.3 mg sample was positioned between two micro cover glass slides 110 (18 x 18 mm2), kept on the hot platform, and gradually heated to 210 °C. The sample was then 111 soft-pressed until it converted into a thin fine layer. In the first heating cycle, the sample was 112 heated up to 210 °C at 40 °C/min and the kept steady for 3 min to erase any impurities. After 3 113 min, it was cooled to 120 °C at the same rate and kept steady for 30 min. The progress of the 114 spherulite was examined after every 5 min till 20 min. 115 2.3.2. Mechanical properties - Tensile strength at max load (ultimate tensile strength), 116 elongation at break (% EB), Young’s modulus (YM), tensile strength at yield (TS at yield), 117 toughness and hardness of HDPE and its polyblend composites were calculated at 5 mm/min 5 118 crosshead speed using an Instron - Dynamic Universal Testing Machine 8801J4051 according 119 to the ASTM D638 method. Five dog-bone shaped dumbbell samples were tested at a gauge 120 length section of 30×5×2 mm3. The hardness of the dumbbell samples was measured using a 121 Shore Durameter type D hardness tester. The average value of five readings were taken for 122 calculations. 123 2.3.3. Crystallinity studies - To measure the glass transition temperature (Tg), melting 124 temperature (Tm) and crystallization temperature (Tc) of HDPE and its polyblend composites 125 via differential scanning calorimetry (DSC) (Netzsch DSC204 F1 Phoenix, Germany), ~ 6 – 10 126 mg samples were taken in crucibles and heated to 200 °C, starting from 30 °C at 10 °C/min rate 127 to eliminate the thermal impurities, held for 3 min and then cooled from 200 °C to 30 °C at 10 128 °C/min rate, held for 3 min, and the second heating cycle was commenced again from 30 °C to 129 200 °C, under nitrogen gas atmosphere. % crystallinity of HDPE, PLA, MCC fibres and their 130 polyblend composites were measured by the X-ray diffractometer (XRD) (Rigaku, TTRAX III 131 18kW, Japan), operated at 100 mA, 50 kV voltage, in grazing incident way running on Cu-Kα 132 radiation (λ 1.5406Å), at 0.5 οC scan rate and 2θ ranging from 5 to 45ο. 133 134 3. Results and Discussion 135 3.1. Morphological properties 136 FESEM and TEM analyses - FESEM micrographs of HDPE and its polyblend composites are 137 shown in Fig. 1. Figs. 1(a) and 1(b) illustrates the stretching and irregularity in the HDPE and 138 PLA matrix along with the direction of tension. Since, PLA has brittle character, it showed no 139 void formation and unevenness on the smooth and coarse fractured surfaces [10]. Fig. 1(c) 140 demonstrated MCC in the form of small bunches of crystalline cellulose fibres showing partial 141 agglomeration due to the presence of hydroxyl groups. The average length of MCC fibres was 142 measured to be 23.7 ± 6.9 µm via image J software and it existed as isodiametric needle-shape 6 143 in the composites. The different L/D ratio of MCC fibres were calculated to be 1-100 microns. 144 The average particle size of PLA in HDPE90/10PLA, HDPE70/30PLA, and HDPE50/50PLA 145 blends was calculated to be 0.053 ± 0.02 µm, 1.07 ± 0.02 µm and 6.26 ± 4.17 µm, respectively. 146 The HDPE/PLA blends were incompatible with each other as a result of their dissimilar 147 polarity. The two- phase morphology of HDPE/ PLA in different ratio was clearly observed as 148 shown in Figs 1(d) to 1(f). PLA was found to be fragmented within the HDPE matrix due to 149 shearing during the melt extrusion process. The PLA particle existed in mostly spherical form 150 with an average diameter of 0.053 ± 0.02 µm. Small cracks were observed on the surface of 151 HDPE90/10PLA blend because of tensile testing [11]. Uneven dispersion of PLA on the HDPE 152 matrix surface was observed, resulting in the reduction of mechanical properties [12]. 153 Consequently, particle size and percentage of PLA increased uniformly in the HDPE matrix in 154 the blends. In HDPE70/30PLA and HDPE50/50PLA blends, voids (holes) were detected on the 155 fractured surfaces of HDPE as PLA particles were embedded in the matrix. In addition, some 156 PLA particles were pulled out and voids were made on the opposite side of the other tensile- 157 fractured surface and vice-versa. It was noticed that the HDPE/PLA blends were highly 158 incompatible polymer blends and separate particle interfaces were observed, suggesting weak 159 interfacial interactions [5]. On the other hand, FESEM micrographs depicted irregular 160 distribution of MCC fibres on the surface of the HDPE/PLA blends in Figs. 1(g) to 1(i). With 161 the addition of 3 wt.% MCC fibres to the HDPE/PLA blends, a better dispersion occurred 162 between the phases. The average particles size of PLA in HDPE90/10PLA/3MCC, 163 HDPE70/30PLA/3MCC and HDPE50/50PLA/3MCC composites were calculated as 0.57 ± 164 0.27 µm, 2.72 ± 0.45 µm, and 11.42 ± 12.57 µm, respectively. Crazing (fine cracks on the 165 material surface) appeared on the surface of the samples of HDPE and its polyblend composites 166 due to mechanical testing. Crazes were observed initially, followed by microscopic voids over 167 the sample surface, as a result of external stretching in the composites. Moreover, the 7 168 occurrence of one void resulted in the formation of other voids in the region due to the 169 application of maximum external forces in vertical directions. Also, less quantity of fibres was 170 attached over the fractured surface of the polymer samples. But, finally, good adhesion was 171 seen between the hydrophobic HDPE, PLA and hydrophilic MCC fibres quite unpredictably. It 172 might be due to the fact that broken fibres and the absence of fibre pull-out resulted in better 173 interaction between the matrix and the fibres. This happened because HDPE, PLA and MCC 174 fires were extruded at higher processing temperature and viscosity difference [14]. 175 176 Fig. 1: FESEM micrographs of (a) HDPE, (b) PLA, (c) MCC, (d) HDPE90/10PLA, (e) 177 HDPE70/30PLA, 178 HDPE70/30PLA/3MCC and (i) HDPE50/50PLA/3MCC polyblend composites. (f) HDPE50/50PLA, 179 8 (g) HDPE90/10PLA/3MCC, (h) 180 Fig. 2: TEM images of (a) HDPE90/10PLA and (b) HDPE90/10PLA/3MCC polyblend 181 composites. 182 TEM analysis of HDPE90/10PLA blend and HDPE90/10PLA/3MCC composite are shown in 183 Fig 2(a) and 2(b). In addition, Fig. S1 also depicts the distribution of PLA in the HDPE matrix. 184 It can be clearly observed from Fig. S1 that with the blending of PLA into HDPE, the 185 percentage of oxygen had increased (as shown by FESEM-EDX mapping for the oxygen 186 element). MCC fibres were more attracted towards PLA rather than HDPE matrix due to their 187 polarity and wettability characteristic [15]. This kind of morphology was the reason of 188 reduction in the elongation at break of the blends and composites. In Fig. 3(b), arrows show the 189 MCC fibres and PLA particles in the HDPE90/10PLA/3MCC composite. The partly exfoliated 190 or intercalated MCC fibres were observed due to the shearing force and the lower aspect ratio 191 of fibres [5]. 192 POM analysis - The distinction of the crystal morphologies of HDPE and its polyblend 193 composites are illustrated in Fig. 3. It shows that the HDPE matrix had a complex morphology. 194 The capability of crystallization and the size of HDPE spherulites were enhanced at 120 ºC, 195 and the crystallization was completed within ten seconds at the constant crystallization 196 temperature. The spherulites formed a Maltese cross pattern and resembled the sea-island 197 structure [15]. The spherulites overlapped above one another and formed concentric rings and 198 thus, it was difficult to calculate the crystal density and spherulite growth rate with time. 199 Another observation was that one spherulite was always near and attached to another spherulite 200 which prevented the growth of other spherulites and so the spherulites could not attain 201 complete shape [20]. The PLA spherulite existed as a radial fibril structure in the Maltese cross 202 shape. Its radius was calculated at four different time intervals and then linear-fitted. The PLA 203 spherulite size enhanced gradually till 16 min and was calculated to be 57.2 µm, 123.4 µm, 204 200.4 µm, and 263.1 µm at every 4 min interval, and its growth rate was found to be 16.7 9 205 µm/min. The density of crystal (no. of spherulites in the picture/ area of the picture, in µm2) 206 was calculated as 0.11 after 8 min interval from the micrograph (Fig. 3) [5]. Fig. 3 exhibits the 207 POM micrographs of different HDPE/PLA blends, both with and without reinforcing MCC 208 fibres. In HDPE90/10PLA blend, less effect of PLA was found on the HDPE matrix. The shape 209 of the HDPE matrix was constant in the blend, but, incompatibility between the polymers was 210 clearly visible. Both the polymers expanded at their own pace with the HDPE matrix taking 211 very less time in comparison to PLA spherulites which took more time for growth in the 212 HDPE70/30PLA and HDPE50/50PLA blends. Both HDPE and PLA did not influence each 213 other significantly during the crystallization process, resulting in the immiscible behaviour of 214 these blends. When 3 wt.% MCC fibres were incorporated into the various HDPE/PLA blends, 215 a transcrystallization trend appeared with the spherulites adhering around the wall of the fibre. 216 This helped in the increase of adhesion between the polymers and the fibre. This observation 217 was confirmed in XRD analysis where an increase in crystallinity was seen in the 218 HDPE90/10PLA/3MCC composite, signifying that the polymeric materials and the fibres were 219 miscible. So, it was concluded that HDPE and PLA blends with MCC fibres affected one 220 another synergistically during the crystallization process [15]. 221 10 222 Fig. 3: POM micrographs of the crystallized HDPE, PLA and its polyblend composites. Images 223 were taken at 4, 8, 12, and 16 min intervals (from left to right) at 120 °C. 224 3.2. Mechanical properties 225 The tensile properties of HDPE, PLA and their blends at varying ratios, with or without MCC 226 fibres, was calculated after they were subjected to various studies such as transfer of stress to 227 reinforcement from the matrix and vice-versa, optimization of the quantity of additive, matrix 228 crystallinity, compatibility, interaction among all the constituents and aspect ratio of the 229 additive (L/D). All features were controlled by the addition of different fractions of additive 230 into the matrix and the disperse phase, which changed their physical and chemical properties 231 [1]. The complete tensile properties and stress vs strain curve of HDPE and its composites are 232 presented in Figs. 4(a) -4(d) and Table 2. The UTS, EB, and YM of independent HDPE and 233 virgin PLA were calculated as ~ 30 MPa, 60%, and 0.25 GPa, and ~ 62 MPa, 18%, and 1.47 234 GPa, repectively. 235 Table 2: Mechanical properties of HDPE and its polyblend composites. Sample designation (wt.%) HDPE PLA HDPE90/10PLA HDPE70/30PLA HDPE50/50PLA HDPE90/10PLA Tensile strength at Elongation Young’s Tensile Hardness strength Toughness at Yield 3 (Shore-D) at Break Modulus (%) (GPa) 29.9±1.9 59.9±8.5 0.25±0.02 14±0.3 12.9±1 64.4±2.7 61.8±3.9 17.8±2.1 1.47±0.14 43.6±1.9 9±0.2 81.4±2.1 (107.1) (-70.3) (485.9) (211.9) (-30.2) (26.4) 25.7±1.5 60.5±7.5 0.25±0.04 14.2±0.7 10±1.4 67±1.7 (-14) (1.0) (0.6) (1.8) (-22.5) (4) 22.6±2.8 21.4±10 0.36±0.03 14.9±0.2 3.6±1.8 67.6±1.5 (-24.3) (-64.3) (42.9) (6.6) (-72.1) (5) 24.8±1.5 9.5±5.5 0.55±0.05 18.8±0.9 1.1±0.3 69±1 (-16.8) (-84.1) (118.9) (34.9) (-91.5) (7.1) 25.1±3.7 48.5±14.1 0.7±0.06 14.1±0.5 8.5±3 68.2±1.3 max load (MPa) 11 (J/m ) (MPa) /3MCC (-15.7) (-19) (8.6) (0.5) (-34.1) (5.9) HDPE70/30PLA/ 23.4±1.3 11.5±3.7 0.46±0.01 16.8±0.8 2±0.6 68.6±0.9 3MCC (-21.6) (-80.8) (81) (20.3) (-84.5) (6.5) HDPE50/50PLA 25.2±0.8 6.7±1.4 0.66±0.06 18.9±0.7 1.2±0.3 70.8±0.4 /3MCC (-15.3) (-88.8) (162.1) (35.3) (-90.7) (9.9) 236 Note: % improvement are shown in brackets 237 In the HDPE90/10PLA blend, UTS was reduced to ~26 MPa from ~30 MPa (pure HDPE) 238 while no significant change in EB was observed. It might be due to the fact that HDPE is a 239 flexible material that undergoes large deformation before breakdown whereas PLA behaves as 240 a rigid material. UTS and EB of HDPE70/30PLA blend were decreased to ~22 MPa and ~21% 241 as compared to HDPE whereas EB was increased than PLA (~ 17%). The reason for this 242 behaviour could be the incompatibility and poor interfacial adhesion between the polymers 243 [16]. In the HDPE50/50PLA blend, UTS and EB decreased to ~25 MPa and ~84% as compared 244 to virgin HDPE. As the percentage of PLA gradually increased in the HDPE matrix, UTS 245 became nearly constant but EB diminished continuously because of the stiffness of PLA in the 246 disperse phase, due to which YM of the blends enhanced suggesting the presence of strong 247 intermolecular force in the PLA chains which hindered the movement of HDPE polymeric 248 chains and resulted in poor interfacial adhesion, voids and immiscibility between the phases 249 [17]. Since, the mechanical properties of the composites were significantly modified at the 250 interface of the matrices and the fibres, maximum stress developed at or near the interface and 251 optimal stress transfer from the matrix to the fibres happened at the interface, and resulted in an 252 early breakdown of the composites [18]. When 3 wt.% MCC fibres was blended to the 253 HDPE/PLA blend, UTS and EB were decreased to ~ 25 MPa and 48.5% than HDPE but EB 254 was increased than PLA. The HDPE70/30PLA/3MCC and HDPE50/50PLA/3MCC composites 255 showed no significant improvement in their UTS but their EB was decreased to ~ 11.5% and ~ 256 7%. MCC fibres and PLA as minor phase restricted the mobility, resulted in poor stress transfer 12 257 and decreased the free volume between HDPE polymeric chains due to which all the 258 biocomposites became stiffer. Fig. 4 (b) illustrates the TS at yield, which describes the 259 maximum stress that occurs under the elastic limit. YM and TS at yield of HDPE90/10PLA 260 blend were measured as 0.25 GPa and 14.2 MPa, respectively, which was almost similar to 261 HDPE matrix (0.25 GPa and ~ 14 MPa) but less than PLA (1.47 GPa and ~ 43 MPa). It was 262 also observed that YM of HDPE70/30PLA and HDPE50/50PLA blends changed to 43% and 263 119%, whereas TS at yield of both the blends improved by ~7% and ~35%, respectively, as 264 compared to HDPE matrix. With an increase in the PLA content of the blend, YM and TS at 265 yield values increased, the sample became stiffer and required more stress to break. When 3 266 wt.% MCC fibres was added to the HDPE/PLA blends, YM of all the composites varied from 267 0.7 to 0.66 GPa and TS at yield improved slightly from ~14 to ~19 MPa. It was thus concluded 268 that the MCC fibres were less effective in improving the YM and TS at yield of the bio- 269 composites due to the stiffness of PLA and disorientation of fibres which shortened the aspect 270 ratio (L/D ratio) of the MCC fibres, due to which the fibres could not make a network for better 271 stress transfer between the constituents of the composites [19]. Fig. 4(c) and 4(d) demonstrates 272 the stress vs strain curve and the toughness of the HDPE and its polyblend composites. The 273 toughness describes the ability of a material to absorb energy per unit volume before breaking 274 and is determined by the area underneath the stress vs strain graph. It represents the equilibrium 275 between strength and ductility. HDPE matrix had the highest toughness value (~13 J/m3) and 276 covered large area in the curve as compared to the blends, composites and PLA. It is known 277 that PLA has brittle characteristic, and so requires more energy to break, and therefore, had a 278 low value of toughness (~ 9 J/m3), covering lesser area in the graph. The toughness value of 279 HDPE90/10PLA blend was less than HDPE but a little more than PLA. Hence, the blend was 280 strong and tough as PLA. Similarly, the toughness of HDPE 70/30PLA and HDPE50/50PLA 281 blends was further reduced to ~ 4 J/m3 and ~ 1 J/m3. Lesser area was detected underneath the 13 282 graph gradually and less energy was required to break the blend samples. This could be the 283 reason of the hindrance of mobility of HDPE polymeric chains from lower to higher PLA 284 percentages. 285 HDPE90/10PLA/3MCC composite to ~ 9 J/m3 (~ 34% from HDPE). Also, the toughness value 286 of HDPE70/30PLA/3MCC and HDPE50/50PLA/3MCC composites decreased by ~ 84% and ~ 287 91%, respectively in comparison to the matrix phase. As a result, the bio-composites became 288 stiffer with less area under the curve, and thus, less strength was needed to break the dumbbell- 289 shaped samples. It was observed that the use of MCC as reinforcing filler had no significant 290 effect on the composites [5]. Fig. 4(c) illustrates the hardness of HDPE and its composites. The 291 hardness of polymer describes its resistance to deformation by mechanical indentation with 292 Shore D method and has no unit. A higher number on shore D scale (0 to 100) indicates higher 293 resistance. HDPE was softer than PLA because it had longer chains and so, presented lower 294 resistance to deformation, giving a low hardness value (~ 64) while PLA showed higher 295 hardness value (~ 81) [20]. Addition of MCC fibres (3 296 14 wt.%) decreased the toughness of 297 Fig. 4: Mechanical properties of HDPE and its polyblend composites (a) Tensile strength at 298 max load and Elongation at break, (b) Young's modulus and Tensile strength at yield, (c) 299 Toughness, and Hardness, and (d) Stress vs Strain curve. 300 The hardness values of HDPE90/10PLA, HDPE70/30PLA, and HDPE50/50PLA were slightly 301 enhanced as 67, ~ 68 and 69, respectively. This indicated good interfacial interaction between 302 the phases [3]. Moreover, on the incorporation of MCC fibres in the blends, the hardness value 303 of HDPE90/10PLA/3MCC, HDPE70/30PLA/3MCC and HDPE50/50PLA/3MCC composites 304 increased to ~ 68, ~ 69, and ~71, respectively, which was greater than that of HDPE matrix but 305 lesser than PLA. Addition of MCC fibres might have improved the interfacial interaction 306 among the constituents of composites [21]. This could be the reason of the lower aspect ratio 307 and disorientation of MCC fibres along the matrix axis due to which the composites became 308 stiffer and more resistant to deformation. 309 3.3. Crystallinity studies 310 DSC and TGA analyses – The second heating curve data of HDPE and its composites are 311 depicted in Fig. 5 and Table S1. The thermal degradation phenomena, namely, Tg, Tcc, Tm, 312 ∆Hcc and ∆Hm were observed during the phase transition of the polymeric samples. Tg and Tcc 313 were absent which indicated that the HDPE matrix behaved like a highly crystalline polymer 314 (%Xc 69.2). However, only a single melting peak was noticed in the HDPE thermograms (Tm 315 =133.7 ºC) [16, 22]. The Tg, Tcc, and Tm of PLA were determined as 63.8 ºC, 126.4 ºC, and 316 153.4 ºC, respectively. After blending of PLA to HDPE in various ratios, two melting peaks 317 were observed in the DSC heating arch. The first high temperature peak was related to PLA 318 while the low temperature peak was related to HDPE. The Tm of HDPE was not significantly 319 changed but Tm of PLA was enhanced to 155.3 ºC from 150.2 ºC. The Tcc disappeared as PLA 320 was added since HDPE improved the crystallization behaviour of PLA and formed crystals 321 during the cooling cycle. In the HDPE90/10PLA and HDPE50/50PLA blends, the crystallinity 15 322 and melting enthalpies decreased to 65.9%, 50.3%, and 180.4 J/g and 97.4 J/g, respectively 323 than HDPE (69.2%, 203.1 J/g). It was due to the addition of stiffer PLA into the HDPE matrix 324 which decreased the chain length of HDPE, therefore, the mobility of HDPE polymeric chains 325 was hindered and they could not rearrange themselves into ordered form. Furthermore, these 326 polymer blends required less heat to be melted after the transformation of HDPE chains into 327 shorter chains [23]. Consequently, melting temperature of the blends were reduced to 97.4 ºC 328 (lesser than HDPE matrix) and Tm of HDPE was found to be lower than PLA, which meant that 329 some interaction occurred at the interface of HDPE and PLA. Similar phenomena was observed 330 during the FESEM study reported in the above section [24]. On addition of 3 wt.% MCC fibres 331 from HDPE90/10PLA to HDPE50/50PLA blend, Tm was increased from 149.7 ºC to 154.4 ºC, 332 but the crystallinity and melting enthalpy were decreased to 54.1 % and 105.8 J/g as compared 333 to 334 HDPE50/50PLA/3MCC composites was decreased by 61.1%, and 54.7%, respectively. pure HDPE. Also, the crystallinity of the HDPE70/30PLA/3MCC and 335 336 Fig. 5: DSC thermograms of (a) HDPE, (b) PLA, (c) HDPE90/10PLA, (d) HDPE70/30PLA, 337 (e) HDPE50/50PLA, (f) HDPE90/10PLA/3MCC, (g) HDPE70/30PLA/3MCC and (h) 338 HDPE50/50PLA/3MCC polyblend composites. 16 339 HDPE is a moderately flexible polymer whereas PLA and MCC fibres are stiffer materials. 340 Initially, HDPE polymeric chains became inflexible because of PLA macromolecule and then, 341 MCC fibres broke its chains to shorter chains. PLA and MCC also limits the molecular 342 movement and reduce the available place which were present in the longer polymer chains. 343 Consequently, disorientation and irregularity was detected in the HDPE matrix. Thus, the 344 polyblend composite required less heat to melt. Additionally, no significant change was 345 observed in the crystallinity of polyblend composites because very fast crystallization occurred 346 in HDPE at crystallization temperature. So, PLA chain could not affect the crystallization of 347 HDPE [15, 25]. Finally, a combined effect was observed on their temperatures after addition of 348 PLA and MCC fibres in the HDPE matrix, which could be explained with the help of 349 Information S1, Table S2 and Figure S2. 350 XRD analysis - X-ray beams diffract in various directions when they are incident over the 351 crystallographic plane for measuring the angle, orientation of crystalline phase, percentage 352 crystallinity and intensities of diffracted rays via the XRD technique. Fig. 6 illustrates the % 353 crystallinity, crystallographic plane and 2θ angle of HDPE and its blend and composites in 354 different ratios. The degree of crystallinity of HDPE. PLA and MCC fibres were calculated as 355 85.5%, 80.9%, and 19.2%, respectively. The HDPE matrix exhibited two sharp peaks at the 356 angular position of 2θ at ~ 22.1°, 24.6° which were related to [110], [200] lattice planes with 357 the interlayer d-spacing of 4.02 Å, 3.62 Å, respectively. Independently, the crystalline structure 358 of HDPE was nearly the same throughout the process due to the existence of the crystalline 359 characteristic peaks of the HDPE matrix in the blend and composites [26]. XRD pattern for the 360 semi-crystalline PLA revealed a broader amorphous hump at 2θ at ~ 16.4° without a noticeable 361 crystalline peak that was associated with [211]/[110] crystallographic plane with d-spacing of 362 5.40 Å [10]. MCC fibres displayed slightly broad and visible peaks at 15.1°, 22.6° and 34.6° of 363 2θ associated to [101], [002] and [004] crystallographic planes, respectively. Also, the 17 364 interlayer d-spacing of 5.71, 3.93, and 2.95 Å was calculated using Bragg’s law that is 365 analogous to the cellulose I polymorph [5]. The strength of the fibres depends on the crystalline 366 behaviour of the natural fibre [27]. The crystallinity and intensity of HDPE gradually decreased 367 to 30.7% with an increase in PLA content because of the variation in the crystalline size. It 368 happened because PLA occupied more amorphous region in HDPE matrix, and the transfer of 369 amorphous phase to the crystalline region hampered the ordered arrangement of HDPE, 370 thereby, decreasing the crystallinity of the matrix phase. The addition of 3 wt.% MCC fibres to 371 the HDPE/PLA blend resulted in continual decrease in the crystallinity of the composites. 372 HDPE matrix and PLA as the disperse phase were incompatible with each other, and so, when 373 the MCC fibres were added, a network in similar direction could not be formed among all of 374 them, leading to the disorientation of cellulosic material on the surface of the HDPE/PLA 375 blend. No substantial shift was detected in the diffraction position of the three polymeric 376 constituents when polymer composites were designed. Therefore, the crystal lattice nature of 377 all the polymeric materials was unaffected [9, 28]. 378 18 379 Fig. 6: X-ray diffractograms of (a) MCC and HDPE, (b) PLA, (c) HDPE90/10PLA, (d) 380 HDPE70/30PLA, 381 HDPE70/30PLA/3MCC and (h) HDPE50/50PLA/3MCC polyblend composites. (e) HDPE50/50PLA, (f) HDPE90/10PLA/3MCC, (g) 382 383 4. Conclusions 384 In the present work, MCC fibres were used as reinforcing agents in the HDPE and PLA 385 composites at various loadings and their properties was compared. With an increase of PLA 386 and MCC fibres in the HDPE matrix, there was a decline in both the tensile strength at 387 maximum load and elongation at break of the polymer blend. The Young’s modulus and tensile 388 strength at yield of HDPE/PLA blends and its composites were enhanced in comparison to the 389 HDPE matrix. The toughness and hardness value of HDPE/PLA and its polyblend composites 390 with integrated MCC fibres were less and more than the HDPE matrix, respectively. After 391 optimization, HDPE50/50PLA blend and HDPE50/50PLA/3MCC composite would be able to 392 be disintegrated and fragmented by microbial attack or degradation behaviour at the waste- 393 dumping sites. It was concluded that the MCC fibres and PLA had a synergic reinforcing effect 394 on the HDPE matrix and further fulfilled the objective of the present work for developing a 395 bio-derived thermoplastics polyblend of HDPE and PLA with MCC fibres and its optimization 396 for better mechanical properties and thermal stability. Henceforth, the melt-extruded polyblend 397 composites could be commercially utilized in sports, marine, construction, automobile and 398 aviation fields. 399 400 Acknowledgments 401 The authors sincerely acknowledge CoE-Suspol, IIT Guwahati, funded by the Department of 402 Chemicals and Petrochemicals, Ministry of Chemicals and Fertilizers, Government of India for 19 403 the financial support. Departments of Chemical, Mechanical Engineering and Physics and CIF, 404 IIT Guwahati are also acknowledged for providing facilities to execute this research work. 405 406 References 407 1. M.V.S. Murty, E.A. Grulke, D. Bhattacharyya, Influence of metallic additives on thermal 408 degradation and liquefaction of high density polyethylene (HDPE), Polym. Degrad. 409 Stab., 61(3) (1998) 421-430. 410 2. S. Deng, J. Ma, Y. Guo, F. Chen, Q. 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