Thermoplastic Polyurethane Blends

Thermoplastic Polyurethane Blends

Available online at www.sciencedirect.com ScienceDirect Materials Today: Proceedings 17 (2019) 778–786 www.materialstoday.com/proceedings RAMM 2018...

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Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 17 (2019) 778–786

www.materialstoday.com/proceedings

RAMM 2018

Miscibility, Morphology and Mechanical Properties of Compatibilized Polylactic Acid/Thermoplastic Polyurethane Blends M.S. Mahmuda, Y.F. Buysa*, H. Anuara, I. Sopyana a

Department of Manufacturing and Materials Engineering, International Islamic University Malaysia, Kuala Lumpur, Malaysia

Abstract

Polylactic acid (PLA) is a biodegradable polymer and has an excellent strength, however its inherent brittleness and low impact resistance has limited its application. One of the potential alternatives for enhancing the weakness of PLA is by blending it with thermoplastic polyurethane (TPU) which possesses several attractive properties such as high toughness, durability and flexibility. Nevertheless, few works have been reported on the effect of compatibilizer on physical properties of PLA/TPU blends. In this work, the effect of ethylene-methyl acrylateglycidyl methacrylate (EMA-GMA) compatibilizer addition in various blend compositions on miscibility, morphological development and mechanical properties of PLA/TPU blends was analysed. The blends were prepared through melt blending technique and analysis of miscibility and morphological development were conducted using dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM), while evaluation of mechanical properties were performed through tensile and impact tests. Inclusion of EMA-GMA improved miscibility of PLA/TPU blends by reducing the size of the droplets and uniformly dispersed the droplets throughout the matrix. Addition of EMA-GMA further improved mechanical properties of PLA which is showed by significant increment of elongation at break and impact strength of PLA/TPU blends with some composition exhibited non-break behaviour. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018. Keywords: PLA/TPU blends; miscibility; mechanical properties

* Corresponding author. Tel.: +60-3-6196-6549; fax: +0-3-6196-4477. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018.

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1. Introduction Polylactic acid (PLA) is an aliphatic thermoplastic polyester made from α-hydroxy acid, which can be extracted from renewable sources such as corn, wheat or sugarcane. In addition to biocompatibility which is suited to be used in biomedical field [1], the main attractive property of PLA is its eco-friendly behavior. Eco-friendly property is very crucial since polymer with this property is biodegradable, recyclable and compostable, while its production also consume carbon dioxide (CO2) [2]. PLA has been used in various applications such as packaging and biomedical fields. Even though the properties of PLA is as good as the conventional polymers, its major drawback, brittle property crippled its application as a multipurpose plastic. PLA is very brittle, which lead to poor toughness and low elongation at break [3]. On the other hand, thermoplastic polyurethane, TPU is a polymer that has been widely used to strengthen plastics like poly(butylene terephthalate), polyoxymethylene (POM), polypropylene, poly(methyl methacrylate), and especially PLA [4]. This type of polymer comes with several attractive properties such as high strength and toughness, durability, flexibility, biocompatibility, and biostability. Moreover, due to the ease of processing, most of the engineering division utilizes TPU in their application. Diverse efforts have been done to enhance crippled property of PLA such as plasticization, copolymerization and melt blending. Amongst of these efforts, melt blending is the simplest and less costly approach to acquire materials in need. Hence, blending PLA with TPU may improve the brittleness of PLA, which in turn may broaden the applications of the resulted blend, ranging from general purpose packaging to biomedical devices. Indeed, improvement in mechanical properties obtained by blending PLA with TPU have been reported by several researches [5,6]. Previous studies reported the toughening ability of TPU for PLA. Meanwhile, addition of compatibilizers in the polymer blends are expected to enhance the interfacial adhesion in the polymer blends. Besides, compatibilizer added in the polymer blends plays a crucial role in determining the distribution size, dispersion and adhesion between the polymers. Ethylene-methyl acrylate-glycidyl methacrylate (EMA-GMA) is a type of reactive compatibilizer that able to toughen certain type of plastics. This ability is contributed by the grafted copolymers, glycidyl methacrylate (GMA) and ethylene-methyl acrylate (EMA) which is ethylene-acrylic esters [4], is expected to show some degree of compatibility with polyolefin and acrylic polymers. Therefore, in this research, PLA, TPU and EMA-GMA were blended in an appropriate composition, and the effect of addition of TPU and compatibilizer on morphological development and mechanical properties of PLA is explored. 2. Experimental details 2.1. Materials PLA used in this research was Ingeo 3051D grade, supplied from Nature Works L.L.C with density of 1.25 g/cm3. While TPU is Desmopan 8785A with density of 1.20 g/cm3 supplied by Bayer Material Science. Lotader® AX8900 rubber, the compatibilizing agent was obtained from Arkema Inc, France. It is a terpolymer of ethylenemethyl acrylate-glycidyl methacrylate (EMA-GMA). 2.2. Sample Preparation Prior to mixing, PLA, TPU and EMA-GMA pellets were weighed according to appropriate composition. The selected composition of PLA/TPU blends with and without the addition of EMA-GMA is shown in Table 1. PLA and TPU pellets were then pre-dried in the oven at temperature of 65°C for 24 hours. Upon removal, the pellets were pre-mixed in the beaker and then, it was melt blended in a Haake Polylab Rheomix internal mixer machine at 190°C, 50 rpm speed, for approximately 10 minutes. Next, all the blended composition were compression molded to obtain samples with desired shape by using XH-406B Tablet Press machine. Each specimen was pre-heated first for 4 minutes and hot pressed for 3 minutes at 190°C under 17 MPa pressure. Then, it was left to cool down for another

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4 minutes. The sample was labelled as xPxT, xPxT5E and xPxT10E which x refers to wt % composition, while P and T represent PLA and TPU; and 5E and 10E are for 5 phr and 10 phr EMA-GMA respectively. Table 1: Composition of investigated PLA/TPU/EMA-GMA blends. Sample

PLA (wt %)

TPU (wt %)

100P

100

0

75P25T

75

25

50P50T

50

50

25P75T

25

75

100T

0

100

75P25T5E

75

25

50P50T5E

50

50

25P75T5E

25

75

75P25T10E

75

25

50P50T10E

50

50

25P75T10E

25

75

EMA-GMA (phr)

0

5

10

2.3. Dynamic Mechanical Analysis (DMA) DMA measurement was conducted to investigate the miscibility of PLA/TPU blend. The samples were examined by Pyris Diamond DMA (Perkin Ekmer Instrument) machine and the liquid nitrogen was used to regulate cooling and heating processes. The samples were measured with single cantilever using 3-point bending mode over the temperature range of -100 to 160˚C at heating rate 5˚C/min and frequency of 1 Hz. The sample size was approximately 45 mm × 12 mm × 3 mm. 2.4. Scanning Electron Microscopy (SEM) SEM model InTouch Scope JSM-IT100 (JEOL) was used to analyze the morphological characteristic of PLA/TPU blends with and without EMA-GMA. The appropriate image of the samples were captured and analyzed in order to determine the miscibility of the sample blends. Before observation, the specimens were fractured in liquid nitrogen after about 20 minutes soaking in it. The samples were then sputtered with palladium by using Quorum SC7620 Sutter Coater machine in order to make the surface conductive before undergo SEM analysis. 2.5. Tensile Test Tensile test of different blends composition was conducted by using Shimadzu AGS-X Universal Testing Machine, according to ASTM D638 (Type V). Dumbbell shaped samples with 3 mm thickness were cut out from the molded sample and were tested at 10 mm/min crosshead speed. Minimum of five samples from each composition were tested and average values were reported. 2.6. Impact Test This test was conducted by using Dynisco Polymer Test, Advance Pendulum Impact (API) machine which operated with 7.5 J hammer. The impact resistance of notched samples were measured according to ASTM D6110, under Charpy-type test method. Notches were formed by Dynisco Polymer Test, Automatic Sample Notcher (ASN) machine. Minimum of five samples for each composition with dimension of 60 mm x 12 mm x 3 mm were prepared and average values were reported.

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3. Results and Discussion 3.1. Miscibility of Compatibilized PLA/TPU Blends DMA provides the analysis of dynamic mechanical properties of polymer blends such as storage modulus, loss modulus and damping factor (tan δ). Through this testing, the effect of addition of TPU and compatibilizer in PLA/TPU blends on the glass transition temperature was examined. Determination of glass transition temperature of polymer blends may explain the miscibility of the respective polymer blend. In this study, glass transition temperature, Tg is defined as the temperature located at the peak of tan δ curves obtained. Fig. 1 shows tan δ graph of neat PLA, neat TPU and PLA/TPU blends with and without addition of EMA-GMA obtained from DMA. Based on the curves, Tg of PLA and TPU were obtained at about 67°C and -23° C respectively. Curve of the blends showed two detectable peaks which implies the existence of two Tg values correspond to PLA and TPU. In the other words, indicates the presence of significant phase separation in the blends at a microscopic level. [7]. According to several literatures, Tg for PLA is in the range of 61°C to 67°C while Tg value of TPU is in the range of -18°C to -47°C which show that PLA and TPU were not thermodynamically miscible [5,8,9].

Fig. 1: Tan δ graph of neat PLA, neat TPU and PLA/TPU blends with and without addition of EMA-GMA.

Besides, different ratios of PLA and TPU also influenced the miscibility of the blends. For the sake of clarity, Table 2 summarizes the Tg values of PLA and TPU in PLA/TPU blends, with and without addition of EMA-GMA. Theoretically, improvement in miscibility is displayed by the shifting of two Tg values towards each other [10], and the shifting occurred in between the values of Tg of neat polymers. As shown in Table 2, Tg values of PLA and TPU in polymer blends shifted towards each other when 5 phr of compatiblizer was added. Further shifting towards each other between the two Tg values was observed with addition of 10 phr EMA-GMA in PLA/TPU blends, except for 50P50T10E composition. Addition of 10 phr EMA-GMA in 50P50T10E composition has slightly shifted the Tg value of TPU towards lower temperature, compared to the Tg value of TPU in 50P50T5E composition. The excessive amount of compatibilizer present in 50P50T10E composition might had disrupted the interaction between PLA and TPU in molecular level and thus increased the chain mobility of TPU. This shifting phenomena indicates improvement in miscibility between PLA and TPU, which proved the positive compatibilization effect of EMAGMA in PLA/TPU blends. Thus, addition of EMA-GMA enhanced the miscibility of PLA and TPU.

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SAMPLE 75P25T 75P25T5E 75P25T10E 50P50T 50P50T5E 50P50T10E 25P75T 25P75T5E 25P75T10E

Table 2: Glass transition temperature, Tg of PLA/TPU blends with and without addition of EMA-GMA. Tg TPU (°C) Tg PLA (°C) -47.87 70.62 -47.42 71.85 -46.85 74.25 -48.73 75.08 -38.24 73.62 -42.65 75.94 -41.96 76.21 -29.96 75.08 -26.34 71.54

3.2. Morphology of Compatibilized PLA/TPU Blends Development of phase morphology significantly affect the properties of polymer blends, and this morphological development was influenced by the different blend compositions. Fig. 2 shows the SEM images of the cryofractured surface of neat polymers, non-compatibilized and compatibilized PLA/TPU blends with various weight compositions. According to Fig. 2, neat PLA and TPU show very smooth fractured surfaces. When immiscible polymers were melt blended, two major morphologies can be expected which are the sea-island or matrix-droplet morphology and a cocontinuous morphology [11,12].

Fig. 2: Cryogenic fractured surface morphology of, a) neat PLA, b) neat TPU, c) 75P25T, d) 75P25T5E, e) 75P25T10E, f) 50P50T, g) 50P50T5E, h) 50P50T10E, i) 25P75T, j) 25P75T5E and k) 25P75T10E.

Morphology of the blends indicated that formation of isolated particles phase throughout the matrix phase. The inclusion of TPU in PLA lead to the development of two-phase structure, in which PLA was the continuous phase matrix and TPU was the separated phases forming round-shaped domain, as shown in Fig. 1 (c) and (f). This phenomenon is vice-versa for 25P75T blend compositions, as indicated in Fig. 1 (i), (j) and (k). While addition of

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50% TPU, some of the TPU particles phase size increased in PLA matrix. Non-compatibilized PLA/TPU blends indicate inhomogeneous distribution and irregular shape of separated phase domains with the presence of some debonded holes indicates weak interfacial interaction between matrix and dispersed phase [13]. This confirmed that, PLA and TPU were immiscible with each other. Besides, the immiscible and incompatible blends contain high interfacial tension which lead to coarser morphology compared to the corresponding blends with the addition of compatibilizer [14]. Addition of EMA-GMA in the PLA/TPU blends exhibit a finer and uniformly dispersed droplet phase in the matrix phase. As shown in Fig. 1 (i), (j) and (k), addition of EMA-GMA caused uniformly distribution of PLA droplets throughout the TPU matrix with significant reduction in size, due to good compatibility between EMAGMA and PLA [15]. For 75P25T and 50P50T blends composition, in the presence of EMA-GMA, the size of TPU droplets appeared to be about the same size and had been well distributed in the PLA matrix. This indicate the effective compatibilization effect in PLA/TPU blends. Aside from reducing the size of the droplet phase, inclusion of EMA-GMA promotes uniform distribution of droplet phase throughout the matrix phase, which is presumed as an indication of good compatibility between the two neat polymers. Improvement in compatibility is shown by the reduction of the size of the dispersed domain caused by the increased attraction between phases [13]. Thus, improvement in mechanical properties of polymer blends can be expected. 3.3. Mechanical Properties of PLA/TPU Blends Compatibilized with EMA-GMA The representative stress-strain curves of PLA/TPU blends, with and without the addition of EMA-GMA are shown in Fig. 3. Neat PLA showed common brittle behavior with high tensile modulus but with no obvious yield occurred before fracture. PLA was deformed with a steep linear increase in stress, followed by a yield point and a very short necking. Finally, it fractured catastrophically at very low elongation due to lack of crack deviation and cavitation mechanism [16,17]. While TPU exhibited excellent flexibility which is shown by a significant yield before fracture occurred. Meanwhile, the addition of TPU and EMA-GMA in PLA lead to obvious yield and neck formation before fracture. This indicates transition from brittle behavior of PLA to ductile behavior of PLA/TPU blends, with and without addition of EMA-GMA. This transition of behavior is due to the plasticizing effects of the soft segments of thermoplastic elastomer added [18].

Fig. 3: a) Young's modulus, b) Tensile strength and c) Elongation at break of neat PLA, neat TPU and PLA/TPU blends with and without addition of EMA-GMA.

Fig. 4 shows tensile properties of neat PLA, neat TPU and PLA/TPU blends, with and without addition of EMAGMA. Based on the result obtained, neat PLA possessed highest Young’s modulus and tensile strength which are 2657.86 MPa and 63.54 MPa respectively. However, neat PLA exhibit extremely low elongation at break, 4.35% which implies its inherent brittle property. PLA is a hard and brittle material which would not elongate more than 10% [19]. Meanwhile, TPU shows the lowest Young’s modulus which is 25.51 MPa, tensile strength of 57.13 MPA,

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and exhibit highest elongation at break, 699.25%, which indicates excellent flexibility or ductility property of TPU. Tensile properties of neat PLA was changed significantly with the addition of TPU and EMA-GMA. Compared to the neat PLA, Young’s modulus and tensile strength values started to gradually decrease as the inclusion of TPU content increase. This could be related with elastomeric properties of TPU with low modulus value. Decrease in Young’s modulus value indicates the reduction of stiffness in a material and vice versa. In the other words, that material become less brittle and possess ductile property. Moreover, tensile strength values were significantly reduced as higher amount of TPU added in PLA. This might be related to the poor miscibility between PLA and TPU which lead to the obvious reduction in tensile strength. From the data obtained in Fig. 4 (c), as the TPU content in the blend increase, the elongation at break of PLA/TPU blends increased. This increment in elongation at break implies improvement in elastic properties of the blends. It can be concluded that, plastic deformation of PLA is improved as the inclusion of TPU in the PLA/TPU blends. Besides, addition of EMA-GMA exhibited further improvement in modulus, strength and elongation at break of PLA/TPU blends. Compared to the binary blends, the addition of 5 phr EMA-GMA in the blends had slightly increased modulus value. This increment indicates the stiffening effect that might have restrict the chain mobility of polymer matrix. However, addition of 10 phr EMA-GMA lead to slight reduction of Young’s modulus value of all blends composition indicating there is an optimum amount of compatibilizer in stiffen the blends. While addition of 5 phr EMA-GMA has increased tensile strength of 75P25T5E and 25P75T5E samples. However, this increment is not significant in 50P50T5E. Meanwhile, the inclusion of 10 phr EMA-GMA has reduced tensile value of 75P25T10E and 50P50T10E samples, but further increased the tensile value of 25P75T10E sample. It has been reported that, rubber toughening of polymers leads to reduction in strength and stiffness and enhanced toughness provided that a strong interface exists between the phases [17]. In addition, the inclusion of EMA-GMA has further increased the elongation at break values of all blends composition. This significant increment in elongation at break with addition of EMA-GMA in PLA/TPU blends implies the improvement in flexibility and thus, brittle property of PLA has been successfully encountered. Addition of compatibilizer enhanced the interfacial adhesion in the polymer blends, hence lead to the improvement in mechanical performance. Impact test was done to determine the ability of a material to absorb impact energy at very high speed. Fig. 5 shows the result of impact test for neat polymers, non-compatibilized and compatibilized PLA/TPU blends. Based on the result obtained, neat PLA showed very low impact strength of 5.5 kJ/mm2. Due to the inherent brittleness of neat PLA, when it was subjected to an impact load, the crack easily propagated from the notched and lead to fracture. This implies that PLA only absorb small amount of energy when it is subjected to sudden impact load, or in the other words, PLA has low resistance to crack propagation when sudden load is applied. However, dissimilar observation was obtained with neat TPU. Since thermoplastic elastomer possessed excellent flexibility, it exhibited non-break behavior when sudden impact load was applied.

Fig. 4: Representative stress-strain curve of neat PLA, neat TPU and PLA/TPU blends with and without addition of EMA-GMA.

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While blends composition exhibit higher impact strength which significant enhancement is observed with increasing amount of TPU added. However, for 75P25T sample, the impact strength only increased slightly with addition of 25% TPU. Based on the result obtained, when 25% TPU is added, only slight increment in impact strength is observed compared to the addition of 50% TPU, in which from 5.46 kJ/mm2 (neat PLA), the impact strength increased to 6.54 kJ/mm2 with addition of 25% TPU. Meanwhile this impact strength was significantly increased to 55.81 kJ/mm2 with addition of 50% TPU. In the binary blends, the sudden impact load was passed from PLA matrix to TPU particles via interface. Besides, PLA blends with greater amount of TPU able to absorb more impact energy since the energy could be transmitted between the dispersed TPU particles in the PLA matrix. This indicates, the inclusion of TPU in PLA enhances the impact strength of PLA and the increment in impact strength is greater with increasing TPU content. Moreover, the impact strength of the blends was improved by the elongation transformation mechanism experienced by the particulates in the polymer matrix while absorbing the impact load [4].

Fig. 5: Impact strength of neat PLA, neat TPU and PLA/TPU blends with and without addition of EMA-GMA.

On the other hand, the addition of EMA-GMA had further enhanced impact strength of PLA/TPU blends. Tremendous increment in impact strength was achieved with the combination of high TPU content and inclusion of compatibilizer in the blends. Ternary blend of 50P50T with addition of 5 phr EMA-GMA possessed the maximum increment of impact strength, which is from 55.8 kJ/mm2 (non-compatiblized blend) to 165.8 kJ/mm2. This remarkable improvement in impact strength indicates that higher impact energy can be absorbed when the material is subjected with sudden load. In the other words, this ternary blends has very high resistance to crack propagation. Besides, this improvement could be attributed to the morphological development of the blends. However, when 10 phr EMA-GMA added in 50P50T composition, the impact strength was slightly reduced to 142.6 kJ/mm2. The presence of too many EMA-GMA particles in 50P50T composition might have interrupt the toughening effect of EMA-GMA in that respective blend composition. Some of the EMA-GMA particles are presumed might not have reacted with PLA and thus, not contributed as a toughening agent in the blends. When 75% TPU content was added, the blend exhibited non-break behavior. The same result obtained with the addition of 5 phr and 10 phr EMA-GMA in 25P75T composition. This non-break behavior is due to addition of high amount softer or rubbery component which allow PLA/TPU blends to absorb more impact energy during testing. Besides, EMA-GMA terpolymer have imparted relatively higher impact strength by showing excellent toughening effect with a non-break behavior due to the addition of higher amount of softer second component which enable more stresses to be transmitted between the dispersed second particles, thus more impact energy is absorbed and lead to the remarkable increment in impact strength [12]. Excellent compatibility between PLA and EMA-GMA lead to significant improvement in elongation and impact strength [15]. This proves that, addition of EMA-GMA in

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PLA/TPU blends has further improved the impact strength of PLA. Previous studies has proved that, rubber containing glycidyl moieties were generally used as impact modifiers and/or as compatibilizers with variation results [17,20,21]. Moreover, the inclusion of rubber has changed the properties of PLA from brittle to ductile through transformation of deformation behavior which including crazing, cavitation, shear bending, crack bridging and shear yielding that are popular in toughened polymer blends [12]. 4. Conclusion DMA curve of the all PLA/TPU blends composition yield two detectable Tg values. Inclusion of TPU indicates shifting of the Tg peak of TPU towards lower temperature proved the immiscibility between PLA and TPU. Meanwhile, addition of EMA-GMA resulted in further shifting of the two Tg peaks towards each other, showing that there are some further improvement in miscibility in the PLA/TPU blends. Addition of TPU in PLA indicated formation of separated phase, which confirmed the immiscibility of PLA and TPU. Inclusion of EMAGMA improved miscibility of PLA/TPU blends by reducing the size of the droplets and uniformly dispersed the droplet throughout the matrix. Addition of TPU and EMA-GMA improved the brittleness of PLA. This showed by improvement of elongation at break of PLA blends with addition of TPU, while addition of EMA-GMA further improves the elongation at break of PLA/TPU blends. Impact testing reveals that blending PLA with thermoplastic elastomer results in very significant increases in impact strength, and when these blends were added with EMAGMA, the increment in impact strength is much more significant. For 25P75T blends, with and without addition of EMA-GMA exhibit non-break behavior in impact test due to high content of elastomeric particle in the blends that able to absorb high amount of impact. Addition of EMA-GMA affect the dispersion, interfacial adhesion and decreased the average size of the droplet phase that resulting in improvement of mechanical strength. Acknowledgements This work was supported by Research Initiative Grant Scheme RIGS 16-085-0249, International Islamic University Malaysia. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

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