Effect of epoxydized carbon nanotube master batch on polypropylene film properties

Cite this paper: Vietnam J. Chem., 2021, 59(2), 263-269 Article DOI: 10.1002/vjch.202000160 263 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Effect of epoxydized carbon nanotube master batch on polypropylene film properties Nguyen Van Khoi, Trang Vu Thang, Hoang Thi Phuong * Institute of Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam Submitted September 14, 2020; Accepted February 24, 2021 Abstract First, the surface of carbon nanotubes (CNTs) was modified by epoxidation reaction to create epoxidized CNTs (O<CNTs). Following step, master batches of O<CNTs (MB-O<CNTs) with different O<CNTs contents (5-12 wt%) were prepared by melt mixing in the presence of anhydride grafted polypropylene (PPgMAH). The CNTs content dispersed uniformly in master batch is 7 wt%. Finally, the effect of MB-O<CNTs contents on some properties of polypropylene film (PP film) was studied. The PP film samples with different MB-O<CNTs content (such that the O<CNTs content in the film varies from 0-1.0 wt%) were prepared on film blowing extruder. The effects of O<CNTs contents on the mechanical, thermal and electrical properties of PP film samples were investigated. The study found that tensile strength of PP films increased significantly with increasing O<CNTs content while elongation at break decreased. In addition, the presence of O<CNTs slightly increased the degree of crystallization and crystallization temperature of PP films while the melting behavior did not change much. From FE-SEM analysis, the good dispersion of O<CNTs in the PP films was obtained from PP films with O<CNTs content in the range of 0.2-0.8 wt%. The presence of O<CNTs also changed the electrical properties of PP. The PP films were achieved the antistatic effect at O<CNTs content in the range of 0.2-0.4 wt%. In addition, the results showed that the master batch processing helps O<CNTs to disperse more evenly in the PP film. Keywords. Polypropylene, carbon nanotubes, master batch. 1. INTRODUCTION Polypropylene (PP) is one of the most widely used plastic in packaging material, textile, electric appliances, and automobile parts because of good physical and chemical properties and low-cost. Though, to be suitable for applications requiring higher mechanical and antistatic properties, PP is generally modified by various additives to improve its properties. [1] Recently, nano-additives are widely used in the reinforcement of PP, especially carbon nanotubes (CNTs). The CNTs has unique structure, nano-size diameter, low volume resistivity advantage and its current price has been greatly reduced. Therefore, the CNTs are an excellent material to reinforce for PP. However, the biggest drawback when melt mixing PP and CNTs can exhibit agglomerate phenomenon and poor interfacial adhesion. CNTs have a propensity to agglomerates in polymer matrix. CNTs are difficult to be dispersed because of high surface energy and strong Van der Waals force. [2] Therefore, finding solution to improve the adhesion and compatibility between CNT and PP is an issue of research interest. To improve the interactions between CNTs and polymer matrix, different functional groups are attached directly to the CNTs’ sidewall [3-4]. At the nanotube surface, chemical functionalization helps functional groups (carboxylic acid, epoxy, hydroxyl,...) that may react with the functional groups of organic molecules, forming permanent bonds. In order to choose a suitable CNT functionalization method, the nature of the host polymer should be considered. In addition, the compatibility of PP and CNTs also could be improved by adding a compatibilizer. Some recent researches have reported that maleic anhydride - grafted polypropylene (PPgMAH) is able to use as a compatibilizer for improvement in the compatibility between PP and CNTs. [2,5] In addition, a kind of master batch technology was employed to support the process of dispersion of CNTs in PP. Usually, when using the melt blending method, CNT is directly mixed with the melted polymer. To uniformly disperse a large amount of CNT in the polymer matrix, the master Vietnam Journal of Chemistry Hoang Thi Phuong et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 264 batch process is an appropriate technique and necessary. [6] In this study, master batch MB-O<CNTs with different O<CNTs contents were prepared by melt mixing in the presence of PPgMAH. In this study, the effect of MB-O<CNTs contents on some properties of PP films was investigated. 2. EXPERIMENTAL 2.1. Materials PP resin 1126NK was supplied by IRPC (Thailand). The MFI and density of PP is 11 g/10 min and 0,93 g/cm 3 , respectively. PPgMAH with a percentage of MAH 1.0 % was supplied by Addivant, USA. The CNTs was provided by Kumho petrochemical Co., Ltd, Korea (diameter 5-15 nm, length 10-20 µm). Methyltrioxorhenium (MTO) grade M1296 of Tokyo Chemical Industry Co., LTD and Triphenylphosphine grade T84409 of Sigma were used. 2.2. Methods Epoxidation of CNTs [7] : 2 g CNTs were placed in a 500 mL Schlenk flask attached with addition funnel. After evacuation for 30 min to remove any adsorbed O2, a toluene (150 mL) solution of MTO (0.4 g) was added. Triphenylphosphine (0.456 g dissolved in 150 ml of toluene) was added dropwise to the suspension over 15 min. The reaction was stirred at 55 °C for 24 hours. The sample was centrifuged at 4000 rpm for 15 min, washed with fresh toluene (3×280 mL). After drying overnight under vacuum at 60 o C, a solid was obtained O<CNTs. - Preparation of master batch content O<CNTs: Master batch of PP, O<CNTs, PPgMAH were prepared with various O<CNTs contents, as list in Table 1. The master batch samples were prepared by melt blending technique using a internal mixer (Brabender PlastographEC, Germany) at 190 o C, 70 rpm for 7 min, followed by cooling to room temperature. The dispersion of O<CNTs in master batch was determined by melt flow index and torque. Table 1: Composition of PP/O<CNTs/PPgMAH master batch samples Sample Content (wt%) O-CNTs PPgMAH PP MB- O<CNTs5 5 3 92 MB- O<CNTs7 7 3 90 MB- O<CNTs9 9 3 88 MB- O<CNTs12 12 3 85 - PP film preparation: PP films (30±3 µm) were prepared in a blowing machine using single screw extruder SJ-35 (35 mm screw, L/D:28/1). PP and O<CNTs were prepared via master batch. The O<CNTs content was incorporated into the PP film at a concentration of 0-1.0 wt%, the samples were designated PP-0, PP/O<CNTs0.2, PP/O<CNTs0.4, PP/O<CNTs0.6, PP/O<CNTs0.8 and PP/O<CNTs1.0. - Determination of melt flow index (MFI): Melt flow index of samples were measured by using BP- 8164-A instrument (China), according to ASTM D 1238 standard. - Torque: the torques of O<CNTs masterbatch samples in melt processing were determined and recorded by CANfig software of Brabender PlastographEC connected with computer. - Surface resistivity tests:Surface resistivity of samples were measured using a SL-030 surface resistance meter (China) at 25 o C, relative humidity 65% (65% RH), 9V voltage, according to ASTM D257. - Mechanical measurements: The mechanical measurements, including tensile and elongation at break properties of samples were performed using a tensile tester (INSTRON 5980, USA) in Polymer center, Hanoi University of Science and Technology, according to ASTM D882. - Field emission scanning electron microscopy (FESEM): The fracture surface of samples were obtained using a field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) in institute of materials science. The samples surfaces were coated with a thin layer of platinum in vacuum chamber for conductivity before examination. The accelerator voltage is 2 and 5 kV. - Thermal characteristic: Differential scanning calorimetry (DSC) was performed on DSC 204F1 Phoenix Instrument (Germany) performing the following thermal cycle under nitrogen atmosphere: at 10 o C/min from -50 to 200 o C, a dwell at 200 o C for 10 min, a cooling down to -50 o C. The degree of crystallinity (Xc%) values of nanocomposites were calculated by using the following Equation (1). (1) Where ∆Ho is the heat of fusion for 100% crystalline PP which is 209 J/g, ∆Hm is crystallinity enthalpy. 3. RESULTS AND DISCUSSION 3.1. Effect of O<CNTs contents on the dispersion of O<CNTs in master batch 3.1.1. Torque of samples in melt processing Vietnam Journal of Chemistry Effect of epoxydized carbon nanotube © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 265 Torque is defined as the load required by the mixer to perform the mixing process and is a quantity that characterizes the process ability of a material. The process ability of the material can be represented by a torque diagram. The mixing torque vs time is shown on the figure 1. Figure 1: Torque curves of neat PP and master batch samples vs. time The study results showed that the torque values of all samples increased sharply in initial stages of the mixing process. In this stage, the components were still in a solid state, the mixture has a high viscosity so the force exerted on the rotor is higher. When the mixture is melted and subjected to the mechanical shear forces, inside the mixing chamber, the temperature starts to rise leading to a transition into the molting state, which significantly reduces the torque value at second minute at all samples. The torque value decreased and remained constant after seven minutes of mixing. At this stage, almost complete well-mixed compound, homogeneously mixed and the torque values start to constant. Overall, the torque value of master batch samples at all times increased with increasing O<CNTs content. This is because when the O<CNTs content increases, O<CNTs tends to aggregate that restricted the mobility of molecular chain of PP. Thus, the mobility of PP chain decreases with increasing O<CNTs content, which contributes to increase torque value. [8] 3.1.2. Melt flow index The melt flow property of thermoplastics was assessed through MFI, which is a very important parameter and is commonly used to determine the properties of resins. The dispersion of O<CNTs was measured through the MFI. In addition, the uniform dispersion of O<CNTs in PP matrix was determined through the repeatability of the MFI at five different measurements. The MFI of master batch samples with various O<CNTs contents are described in figure 2. Figure 2: Effect of O<CNTs content on melt flow index of master batch samples Figure 1 indicated that the MFI of master batch samples decreased with increasing O<CNTs content. This indicated that the structure of O<CNTs and PP was interconnected through interaction between epoxy of CNTs and anhydride maleic groups to interfere with the molecular movement of the polymer chain. [9] The MFI value of neat PP was 11 g/10 min and was decreased to 7.2 g/10 min (with 12 wt% O<CNTs content, decreased 28 %). The uniform dispersion of O<CNTs in the master batch was determined through the repeatability of the MFI measurement. Master batch samples with low O<CNTs content (5-7 wt%) showed higher repeatability than samples with high O<CNTs (9-12 wt%). This shows that, O<CNTs dispersed in master batch more uniform at the content of 5-7 wt% compared with content of 9-12 wt%. In this master batch processing, the presence of both O<CNTs and PPgMAH could effectively improve the surface adhesion between O<CNTs and PP surfaces. Considering the chemical structures of PPgMA and O<CNTs, the anhydride from PPMA may react with the epoxy group. The mechanism of interaction between epoxy and anhydride maleic groups is shown in the Fig. 3. [10] Therefore, master batch with 7 wt% O<CNTs content was selected for preparation of PP films. 0 5 10 15 20 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 T o rq u e (N .m ) Mixing time (min.) MB/O<CNT12 MB/O<CNT9 MB/O<CNT 7 MB/O<CNT 5 PP-0 4 6 8 10 12 14 3 5 7 9 11 13 M el t fl o w i n d ex ( g /1 0 m in ) O<CNT loading (WT%) 1st measurement 2 nd measurement 3 rd measurement 4 th measurement 5 th measurement Vietnam Journal of Chemistry Hoang Thi Phuong et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 266 Scheme 1: Reaction of maleic anhydride and epoxide group 3.2. Influence of O<CNTs contents on characteristics of PP films 3.2.1. Mechanical properties The PP film’s mechanical properties in machine direction (MD), including of tensile strength and elongation at break are reported in table 2. Table 2: Result of mechanical properties of PP and PP/O- CNTs films with different contents of O<CNTs Samples O<CNTs contents (%) Tensile strength -MD (MPa) Elongation at break- MD (%) PP-0 0 30.24 268.2 PP/O<CNT s0.2 0.2 32.31 134.4 PP/O<CNT s0.4 0.4 34.51 98.2 PP/O<CNT s0.6 0.6 35.27 74.6 PP/O<CNT s0.8 0.8 36.54 52.1 PP/O<CNT s1.0 1.0 34.11 21.6 Tensile strength is significantly affected when adding O<CNTs to the PP. Tensile strength of PP increased from 30.24 MPa in PP film to 36.51 MPa for PP/O<CNTs0.8. The rate of increase in tensile strength is 20.8 % compared to sample without O<CNTs. This showed good dispersion and adhesion between CNTs and PP phase. This may be due to the O<CNTs was distributed in the PP matrix which master batch was used. [6] However, the tensile strength of PP has a slight decrease when the content of O<CNTs is increased 1.0 wt%. With increasing of O<CNTs content, the tensile strength of PP/O<CNTs nanocomposites decreases due to the formed aggregates act as mechanical failure concentrators. [11] In contrast to the tensile strength, the elongation at break is significantly decreased when increasing the O<CNTs content in PP. The elongation at break is related to the flexibility of the polymer chains. The presence of O<CNTs in PP phase restricted the mobility of polymer chains. Therefore, the percentage elongation of PP decreased with increasing O<CNTs content from 0-1.0 wt%. [12] Prashantra et al. reported that when adding PPgMAH, CNT dispersed better in the PP matrix but elongation at break decreased when the O<CNTs content increased. [13] 3.2.2. Thermal properties The thermal properties (melting temperature (Tm), melting enthalpy (∆Hm) and degree of crystallinity (Xc), crystallization temperature (Tc) crystallization enthalpy (∆Hc)) of samples were investigated using DSC and result are reported in table 3. As shown in table 3, when O<CNTs is added to the samples, it can be seen that melting temperature (Tm) and the melting behavior have not change much. The melting temperature of PP/O<CNTs and PP is in range of 175-177 °C. This shows that the presence of O<CNTs with concentrations up to 1 % did not affect the melting behavior and melting temperature of PP. The results of this research are consistent with those reported by other authors. [14] On the contrary, the crystallization temperature (Tc) of PP films contents O<CNTs increased from roughly 3 to 7 o C compared neat PP film. In addition, the degree of crystallization slightly increased with increasing O<CNTs content. It indicated that CNTs acted as nucleating agents, which induce easier and faster crystallization under isothermal and non-isothermal condition. When the O<CNTs continued to increase to 1.0 wt%, the Xc value reduced to 41.1 %. This can be explained by the existence of CNTs aggregations in PP that acted as barriers to the crystal growth. Salid Hakan Yetgin [15] has obtained similar results in his study of the thermal properties of PP/CNTs composites. 3.2.3. Surface resistivity Table 4 shows the effect of O<CNTs content on the surface resisvity of PP films. Vietnam Journal of Chemistry Effect of epoxydized carbon nanotube © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 267 Table 3: Melting and crystallization parameters of PP and PP/O<CNTs films with different contents of O<CNTs Sample Melting crystallization Tm ( o C) ∆Hm (J/g) Tc ( o C) ∆Hc (J/g) X (%) PP-0 175 80.1 113 83.01 40.1 PP/O<CNTs0.2 176 82.6 116 84.25 40.7 PP/O<CNTs0.4 176 83.08 117 85.35 41.2 PP/O<CNTs0.6 177 84.2 118 86.73 42.01 PP/O<CNTs0.8 176 84.6 120 89.91 43.02 PP/O<CNTs1.0 177 84.1 118 85.9 41.1 Table 4: The surface resistivity of PP films with various O<CNTs contents Sample Content of O<CNTs (wt%) Surface resistivity (Ω/sq) PP-0 0.0 >10 12 PP/O<CNTs0.2 0.2 10 9 PP/O<CNTs0.4 0.4 10 6 PP/O<CNTs0.6 0.6 10 4 PP/O<CNTs0.8 0.8 10 3 PP/O<CNTs1.0 1.0 <10 3 The industry standards ANSI/EIA-541-1988 (USA) and ANSI/ESD S541-200 (USA) defined and classified based on the surface resistivity. Accordingly, conductive, static dissipative, antistatic and insulator materials have surface resistivity of 10 1 -10 5 , 10 6 -10 8 , 10 9 -10 11 and 10 12 -10 16 Ω/sq, respectively. Therefore, it can be seen that surface resistivity of neat PP film is higher than 10 12 Ω/sq, it was insulator material. However, the surface resistivity of PP/O<CNTs0.2 and PP/O<CNTs0.4 films was 10 9 and 10 6 Ω/sq, respectively. This indicates that both of the PP/O<CNTs0.2 and PP/O<CNTs0.4 films could be as antistatic materials. The antistatic mechanism of CNTs is explained by the CNTs as conductive particles, in PP matrix lies in that static is dissipated by a conductive network of CNTs. Thus the dissipation of static charges can be promoted. However, Chensha Li et al. [16] stated that if CNTs were blended directly in PP, to have antistatic effect, the content of CNT must be greater than 15 wt%. This may be due to that unmodified CNTs has very poor interaction with PP resin, the process of mixing is not uniform, to achieve antistatic effect, a large amount of CNTs is required. But PP films can not be produced in such high CNTs content. While, in this study, the PP films were achieved the antistatic effect at O<CNTs content in range of 0.2-0.4 wt%. This shows that the epoxy functionalized CNTs and through the master batch process containing O<CNTs has significantly improved the dispersion of the O<CNTs phase in the PP matrix, helped the O<CNTs phase to be evenly distributed in the PP matrix. The uniform distribution of O<CNTs formed conductive channels of O<CNTs in PP phase that static is dissipated. Consequently, to obtain a mixture that O<CNTs can evenly disperse in the polymer, the master batch process is a very efficient process. However, with an increase in the O<CNTs content from 0.6 wt% up to 1.0 wt%, there was a sharp decrease in surface resistivity, the PP films became electrically conductive. At high O<CNTs content, with the uniform dispersion of O<CNTs in the PP matrix, forming a percolated network of O<CNTs, which can be regarded