Carbon Nanotube Coatings to Reduce the Flammability of Polymeric Materials...
Within the scope of the study, in order to reduce the flammability of the foams, single-walled carbon nanotubes were first functionalized to be stabilized.
Preparation and Application of Carbon Nanotube Coatings to Reduce Flammability of Polymeric Materials – Part I
Generally, within the scope of the study, to reduce the flammability of foams, single-walled carbon nanotubes were first functionalized to achieve stability. Functionalized carbon nanotubes were deposited onto polyurethane foam. Additionally, a polyethylenimine layer enhanced interaction between the carbon nanotube and polymer layers, enabling achievement of a uniform, durable and thick coating. Single-walled carbon nanotubes (SWCNT) functionalized with polyethylenimine and polyethylenimine completely cover the internal and external surfaces of the foam. Microscopic images confirm strong polymer/nanotube interaction due to the polyethylenimine layer on the polyurethane foam surface and the well-distributed carbon nanotube network. The carbon nanotube network created through layer-by-layer (LBL) assembly significantly reduces the flammability of polymeric materials such as foams (for example, around 40%).1. Introduction
The layer-by-layer (LbL) assembly technique has been studied as a thin film production technique over the past twenty years [1-3]. LbL coatings/thin films are generally produced through alternating accumulation of positively charged and negatively charged layers (a pair of positive and negative layers is termed two layers, BL) [4]. Their properties and functional purpose are controlled by production parameters (solution pH, solution concentration and temperature) and the materials forming the coatings [5-11]. Recently, LbL coatings have been shown to significantly reduce the flammability of cotton fabrics, polymeric films and polyurethane foams (PUF) [11-18]. During combustion, polymer/nanoparticle coatings form a protective residue that prevents flame propagation and fire development. Research has shown that only polymer LbL coatings can reduce the flammability of cotton fabrics [16-18]. However, in coatings with added nanoparticles, the flammability of PUF can be greatly reduced. In previous studies, carbon nanofiber (CNF) and montmorillonite clay-based LbL coatings reduced the peak heat release rate (PHRR) of PUF by 40% [19,20]. CNF-based coating grew exponentially due to inter-diffusion of polyethylenimine (PEI) and poly(acrylic acid) (PAA). Clay-based coating, in contrast to CNF-based coating, exhibits slow and linear growth [10], but this is not practical for commercialization of flame retardant (FR) technology. However, the growth rate accelerates significantly when using a three-layer technique, and the thickness of clay-based coating reached 1 μm after this deposition [20]. Conventional clay-based LbL coatings use electrostatic attraction between clay platelets and polyelectrolytes, which is a very weak interaction. The three-layer deposition technique combines electrostatic attraction and hydrogen bonding by depositing an additional polyelectrolyte layer after the clay layer. This helps protect the clay in the coating and enables inter-diffusion between two polymer layers. Since the discovery of carbon nanotubes (CNT) in the early 1990s, carbon nanotube properties (for example, small size and high aspect ratio [21,22], high modulus [23] and high thermal conductivity [24]) have been attractive for enhancing the performance of polymeric materials. Compared with carbon nanofibers (CNF) having similar composition and much larger geometries, CNTs possess superior physical properties with higher surface area. Recently, CNTs have been deposited using LbL techniques and the resulting films have been observed to develop excellent properties for various applications [25-27]. However, this CNT-based coating cannot be practical as a flame retardant (FR) because the coatings are very thin (less than 100 nm even after 10 BL). Compared with earlier CNF [19] and clay-based [20] LbL coatings, MWCNT-based coating has significant challenges arising from MWCNT size and surface chemistry that makes dispersion in aqueous solutions and maintaining the dispersed state difficult. Researchers have improved MWCNT dispersion and stability by using non-covalent stabilizing agents (for example, surfactants [28-30], water-soluble polymers [31-33] and inorganic nanoparticles [34,35]) and by chemically modifying CNTs. Covalent functionalization is generally preferred to increase CNT solubility, but the production process is complex and requires strong acid treatment. Liao and colleagues investigated a single-step MWCNT functionalization method through direct amination without strong acid treatment [36]. Functionalized MWCNT shows excellent dispersion and stability in water without requiring any dispersing agent. In this study, SWCNT (single-walled carbon nanotube) LbL coating with high SWCNT concentration was focused on. The stability of SWCNT subjected to direct amination was supported, SWCNT deposition, advantages of LBL methodology and the importance of coatings including microstructure and fire performance were discussed.2. Material and Method
2.1. SWCNT Functionalization and LbL Methodology SWCNTs were initially functionalized with PEI to facilitate dispersion in deionized water and increase the persistence of SWCNTs in the coating. Amination of SWCNTs was prepared according to the procedure applied by Liao et al. [36]. A plastic bottle (500 mL) was filled with 50 g N,N-dimethylformamide (DMF), 5 g PEI and 0.5 g SWCNT. The suspension was subjected to sonication at 50 W for 1 hour and then stirred at 50°C for 2 days. Functionalized SWCNTs (SWCNT-PEI) were filtered using a 0.20 μm membrane and isolated from the suspension by washing four times with ethanol and water to remove excess PEI and DMF. SWCNT-PEI was dried over anhydrous calcium sulfate in a desiccator at least 3 days before use. SWCNT-PEI suspension in deionized water was prepared by filling a plastic bottle (250 mL) with 150 mL deionized water and 0.60 g SWCNT-PEI and applying sonication at 50 W for 1 hour. The suspension was diluted with deionized water (450 mL) and used immediately for the coating process.Associate Professor Fatih Şen / Department of Biochemistry - Faculty of Arts and Sciences - Dumlupınar University
Betül Şen / Master's Student - Department of Biochemistry - Faculty of Arts and Sciences - Dumlupınar University
Zeynep Daşdelen / Master's Student - Department of Biochemistry - Faculty of Arts and Sciences - Dumlupınar University
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