HYDROXYETHYL CELLULOSE (HEC) - Ataman Kimya

yılında Cengiz Tuncel, Şelale Kimya' yı devralmış ve yılında hammadde ithalatı ve ihracatı amacıyla ATAMAN Kimya Ltd' yi kurmuştur. Ataman Kimya yılında Cengiz Tuncel tarafından bir sonraki nesile devredilmiştir ve yılında ATAMAN Kimya A.Ş adını almıştır.

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For the first time, we succeeded in manufacturing a 2-hydroxyethyl cellulose (HEC)-based composite membrane with improved thermal stability, for use as a battery separator, coating a HEC polymer solution to a polypropylene (PP) support and using a vacuum-assisted process. A HEC polymer solution was prepared by utilizing HEC and lactic acid (LA) as a plasticizer. A vacuum-assisted process was used to move ethanol, which a mobile phase to permeate a plasticized region in the HEC polymer side for pore formation. The pores formed with uniform nano sizes, and areas in which some large pores formed were observed. The thermal stability of the composites was measured using TGA. The thermal decomposition temperatures were measured at about 250 °C for the neat HEC, about 210 °C for the HEC/LA film, and about 335 °C for the HEC/LA/PP membrane before the process. After the vacuum-assisted process, the first and second thermal decomposition were observed at about 360 °C and 450 °C, respectively. The HEC/LA/PP membrane after the process showed greater thermal stability than before the process. This means that the adhesion between the HEC polymer and the PP support was created through the rearrangement of the HEC chain, as LA escaped after the process, and it was seen indirectly that the mechanical strength was enhanced. In particular, the surface of the membrane was observed by SEM to investigate whether the HEC penetrated into the PP to block its pores, and whether the HEC region collapsed. Furthermore, the interaction of the HEC chain with the additives and the rearrangement of the HEC was confirmed using FT-IR. As a result, we demonstrated that the mechanical strength and thermal stability of the manufactured HEC/LA/PP membrane were enhanced.

1. Introduction

Recently, the battery industry has become very important through the rapid development of electric vehicles and portable electronic devices [1,2]. In particular, lithium-ion batteries (LIBs) are among the most widely used power sources for energy-storage systems due to their long lifespan, low self-discharge, and high energy density [3]. However, LIBs are less thermally stable than other battery systems. Therefore, many fire accidents caused by electric vehicles and mobile phones have recently occurred [4]. The major reason for this is presumed to be the separator, which porous the polymer membrane is used to prevent direct contact between the cathode and the anode [5]. The separators used in LIBs are polyolefin materials, such as polyethylene and polypropylene. These materials offer many advantages, such as chemical stability and uniform pore distribution, but they also have low electrolyte absorption and ionic conductivity, and their low thermal stability causes internal short circuits and safety issues at high temperatures [6,7,8,9]. Therefore, it is important to develop separators with good thermal stability and mechanical properties [10].

There are two ways to solve the problems described above. The first involves manufacturing a composite separator by coating an organic/inorganic material on the surface of a polyolefin separator [11]. In the case of a separator coated with Al2O3, it is more hydrophilic than PE-separator surfaces, the wettability is improved by polar liquid electrolytes. Improved wettability affects both liquid-electrolyte absorption and ionic conductivity. In addition, the interfacial resistance of the battery is reduced, and the battery has the ability to retain more electrolytes in the separator than when PE separators are used. Therefore, ion shortage and leakage can be prevented [12]. However, these methods do not solve all problems. Another option is to develop a new type of membrane. The polyimide (PI)/SiO2 composite separator not only combines the excellent thermal stability of polymers, but also exhibits the high wettability and excellent mechanical properties of silica [13]. The silica layer imparts hydrophilicity to the outside of the composite separator and greatly improves the wettability of liquid-electrolyte diffusion. However, this is an expensive method due to the electrospinning and the various other processes involved. Therefore, studies on the manufacturing of separators have been actively conducted, with a particular focus on cellulose series, which are cheap and eco-friendly biodegradable natural polymers with high wettability, and good thermal stability [14,15,16,17].

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On the other hand, porous polymer membranes that uses a battery separator are produced in various ways, and various manufacturing methods are being developed. In particular, the phase-inversion method is mainly used in the field of membrane manufacturing. Non-solvent-induced phase separation (NIPS) and thermally induced phase separation (TIPS) are generated through interactions between polymers and solvents. In NIPS, an asymmetrically shaped surface is created, while in TIPS, a porous and symmetrical structure is created. The use of NIPS is effective in reducing the ion-migration resistance inside the battery due to changes in the electrolyte concentration inside the separator. The TIPS method is relatively simple and easy to manufacture, since a separation membrane is formed in the process of mixing and cooling the polymer solution. In addition, it has the advantages of good reproducibility and the low occurrence of defects. However, both methods have the disadvantage that pores are randomly formed and the curve is very large, meaning that the movement distance of the lithium ions becomes long [18]. This affects the charging speed of the battery. In order to increase the charging speed of the battery, there is a method for reducing the moving distance by making the pore shape of the separator straight. The track-etch method [19] previously used for manufacturing straight-shaped pores, is a very expensive process using radiation.

Therefore, in our laboratory, a new approach was found, in which a linear separator is manufactured cellulose acetate (CA)/specific additives composite membrane with a hydraulic method and specific additives. In this process, metal salts are used as additives. However, it has a disadvantage, in that the cost is high. Additionally, many salts, such as Zn and Ni, are utilized, and the required energy is relatively high [20,21]. Furthermore, there is an additional disadvantage in that it is difficult to apply uniform pressure, and the associated costs are relatively high. In addition, CA with a molecular weight of 30,000 g/mol was used, and its mechanical strength is confirmed to be weak. To solve these problems, a new cellulose-based material should be developed and it was necessary to introduce new methods, and eco-friendly additives and processes are required.

Therefore, in this study, the aim was to form pores in a separator using 2-hydroxyethyl cellulose (HEC), a cellulose-based material, and lactic acid (LA), an eco-friendly plasticizer. In addition, a vacuum-assisted process was used to support for pore formation using instead of the hydraulic method to reduce costs. HEC with a molecular weight of 90,000 g/mol was used to increase the mechanical strength, which is a disadvantage of CA, 2-hydroxyethyl cellulose (HEC) with a molecular weight of 90,000 g/mol was used. Due to its strong viscosity, HEC is a good candidate for ultra-thin coating due to its strong viscosity [22]. Lactic acid (LA), a plasticizing agent, causes a hydration effect through its strong interaction with water molecules used as a solvent [23]. Therefore, LA was dispersed between the HEC chains to form hydrated parts of various sizes, and it was intended to be converted into pores through the vacuum method. It was confirmed through SEM images that the LA was well dispersed, and uniform pores are formed. In addition, lactic acid is an eco-friendly material produced through fermentation [24,25], and has the advantage of reducing process costs, and is environmentally friendly. Thus, the aim of this study was to analyze the characteristics of porous membranes manufactured using the vacuum-assisted process, which has more advantages than the hydraulic process, and to confirm that it offers improved physicochemical properties.

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