LEE- Polimer Bilim ve Teknolojisi Lisansüstü Programı

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http://hdl.handle.net/11527/19428

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Yazar "Doğu, Mustafa" ile LEE- Polimer Bilim ve Teknolojisi Lisansüstü Programı'a göz atma
  • Öge
    Industrial scale sustainable nanocomposite production by melt mixing technique
    (Graduate School, 2022-08-12) Avcıoğlu, Nesrin ; Nofar, Reza M. ; Doğu, Mustafa ; 515191011 ; Polymer Science and Technolog
    Scientists are in search of new biopolymers as alternatives for petroleum-based polymers due to environmental concerns. Polylactic acid (PLA) is a biopolymer that is produced from renewable feedstock and has high performance with biodegradable and sustainable nature. This is while PLA suffers from brittleness, poor processability, and impact strength. The development of PLA composites and blends are possible ways to overcome such drawbacks. Cellulose nanocrystals (CNCs) are good alternative nanoparticles to improve some of these drawbacks compared to other reinforcements for not only providing good mechanical and physical properties but also being low density, biobased and biodegradable filler. Unfortunately, the production of nanocomposites on a large scale with good CNCs dispersion is still a challenge. This study is aimed to develop sustainable polymer nanocomposites by direct melt-mixing on an industrial scale through two approaches that are with and without masterbatch preparation. For this purpose, a corotating twin-screw extruder (Gülnar 1101402) was used and its specifications are 25 mm screw diameter and L/D ratio of 42. This extruder has 8 barrels with volumetric feeders. PLA was reinforced with CNC (1, 2, and 4% (w/w)) in this industrial-scale co-rotating twin-screw extruder. Polyethylene glycol (PEG) was used as an agent that could enhance the interfacial interaction between PLA and CNC to promote dispersion and acts as a plasticizer to overcome the brittleness of PLA. PEG concentration was kept constant at 10%(w/w) throughout all formulations. In this process, PLA was fed in the main feeder and solvent casted (distilled water was used as a solvent) PEG/ PEG-CNC after shredding was fed into the side feeder. For all trials, the vacuum pump that is located after the side feeder was set as on and the screw speed was set as 130 rpm. The temperature profile for each heating zones was 40°C, 170 °C, 175°C, 180°C, 185°C, 180°C, 185°C, 190°C and 185°C. Obtained granules from this process were dried at 50°C in a vacuum oven overnight both for injection molding and compression molding. In the compression molding process, the mold was set at 190°C for 5 minutes and pressure was gradually enhanced to 1.5 tons. At the end of the heating process under pressure, the mold was cooled quickly with water circulation. Samples for rheological characterization, SEM analysis, XRD analysis were prepared with this method. For the injection molding process temperature of the cylinder and temperature of the mold were set as 190°C and 70°C, respectively and residence time was optimized as 15 seconds. Dog-bone samples for the tensile test were obtained with this processing technique. Fabricated nanocomposites were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), rheological analysis, scanning electron microscopy (SEM) and tensile test. It was seen that from the DSC cooling thermogram, the addition of CNC resulted in non-isothermal crystallization behavior and improvement of crystallization capacity and crystallization temperature. When the consideration of the second heating cycle of normally compounded particles, the addition of CNC resulted in decreasing melting temperature. This means that crystal nucleation is improved with the addition of CNC and small crystals tend to reduce of melting point. The highest change of degree of crystallinity and melting temperature in comparison to the neat PLA were observed for PLA/PEG/CNC2 from 39% to 59% and from 177 oC to 172 oC, respectively. Whereas, for the masterbatch preparation method, the crystal growth observed instead of nucleation. This might be related to the agglomeration of the nanoparticles in the MB preparation technique, but the agglomeration could not be understandable from the SEM results. Besides, low melting shoulders are observed for the only normally compounded particles due to the existence of small crystals in the structure. DSC first heating thermograms were only considered for comparison of the results of XRD because of the thermal history of the materials. The neat PLA shows two distinct peaks in 2θ= 16.8 and 19.2. The different amounts of CNC in the nanocomposites result in some changes in diffraction patterns. This shift in the diffraction patterns means some structural changes in the crystallite. Moreover, loading more amount CNC in the nanocomposites leads to an enhancement of the intensity of crystalline peaks. The addition of CNC to the PLA matrix making a positive effect on the crystallite size and the ordering of the material is better in the nanocomposites that are produced from the MB preparation technique. Additionally, the thermogravimetric test of the neat PLA after extrusion, PEG, PLA/PEG blend, and its nanocomposites were performed to understand the thermal degradation behavior of materials. It was observed that neat PLA, PEG, and nanocomposites have shown one step degradation process. However, the PLA-PEG blend showed a two-step degradation process. The initial thermal degradation temperature of each nanocomposite ranges from about 260°C to 290°C. Maximum degradation temperature observed in neat PEG as 412 °C. These TGA curves demonstrate that the thermal stability of PLA decreases with the addition of PEG. The reduction of the thermal degradation temperature of PLA with the addition of the PEG can be related to some impurities after the chemical reaction between the OH- groups of PEG and ester groups of PLA. With the addition of the 1, 2, and 4 % (wt) CNC on the PLA/PEG, the decomposition temperature shifted to the higher temperatures of 261, 268, and 287°C, respectively. Additionally, the materials which were obtained by the MB preparation technique resulted in a thermal degradation temperature of 284 °C for both materials. Whereas PLA has better thermal degradation temperature in comparison to nanocomposites. Moreover, complex viscosity and storage modulus were considered for the rheological behavior of the materials. The results evidenced that neat PLA and its blend and nanocomposites show a Newtonian plateau at low frequencies, and it was observed that shear thinning behavior at high frequencies. Existence of PEG can be triggered transesterification reactions with PLA in the extrusion process, and these reactions were resulted in the degradation of the PLA matrix meanwhile reduction of polymer molecular weight. This reduction resulted in the reduction of PLA's complex viscosity and even addition of CNC could not enhance the complex viscosity. Considering the storage modulus, the addition of PEG made a negative effect on the moduli of PLA in the low and high-frequency range whereas some enhancements were observed in the storage modulus at the low frequencies with the addition of CNC. This improvement could not be obtained for the MB preparation technique due to the agglomerates. Changing of rigid and brittle nature of PLA with the addition of additives was analyzed with the tensile test. Tensile strength and Young's Modulus of neat PLA before extrusion are 50 MPa and 3500 MPa, respectively. However, after extrusion, it has been seen that a significant decrease in these values as about 42 MPa and 1720 MPa, respectively. PEG was selected as not only a processing aid to improve PLA-CNC interaction but also as a plasticizer. Hence, the addition of PEG into the PLA matrix results in an increment for elongation at the break. Besides, Young's modulus as well as the tensile strength of the blend slightly decreased. With the CNC loading, tensile strength and modulus are slightly enhanced. Whereas elongation at break is reduced due to the addition of CNC which is a stiffer phase. The masterbatch preparation aggregates might result in stress concentration points, so tensile properties cannot enhance. Consequently, it needs to be noted that in the twin screw extrusion process, materials were exposed to high shear. Thus, materials can be easily degraded. Even though PEG used as a well dispersing agent, these results were demonstrated that melt mixing is an inappropriate route to produce PLA-CNC nanocomposites. There is no surface treatment of CNC and that can promote the agglomeration of nanoparticles dramatically.