Preparation and Applications of Carbon Nanotube Coatings to Reduce the Flammability of Polymeric Materials - I

Generally, within the scope of the study, to reduce the flammability of foams, single-walled carbon nanotubes were first functionalized to stabilize them. The functionalized carbon nanotubes were deposited onto polyurethane foam. Additionally, a polyethylenimine layer supported the interaction between carbon nanotube and polymer layers, enabling the achievement of a uniform, durable and thick coating. Single-walled carbon nanotubes functionalized with polyethylenimine (SWCNT) and polyethylenimine completely cover the inner and outer surfaces of the foam. Microscopic images confirm strong polymer/nanotube interaction due to the polyethylenimine layer on the polyurethane foam surface and the well-dispersed carbon nanotube network. The carbon nanotube network created through layer-by-layer (LBL) deposition significantly reduces the flammability of polymeric materials such as foams (e.g., around 40%).

1. Introduction

Layer-by-layer (LbL) assembly technique has been studied as a thin film production technique for the past twenty years [1-3]. LbL coatings/thin films are generally produced through alternating deposition of positively charged and negatively charged layers (a pair of positive and negative layers is termed two-layer, BL) [4]. Their properties and functional purpose are controlled by production parameters (solution pH, solution concentration and temperature) and the materials constituting 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 create a protective residue, preventing flame propagation and fire development. Research has shown that only polymer LbL coatings can reduce the flammability of cotton fabrics [16-18]. However, with nanoparticle-added coatings, 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 has grown exponentially due to inter-diffusion of polyethylenimine (PEI) and poly(acrylic acid) (PAA). Clay-based coating, unlike CNF-based coating, has slow and linear growth [10], but this is not practical for commercialization of flame retardant (FR) technology. However, the growth rate accelerates significantly when a three-layer technique is used, and the clay-based coating thickness has 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, leaving an additional polyelectrolyte layer after the clay layer. This helps protect clay in the coating and enables inter-diffusion between the two polymer layers.

Since the discovery of carbon nanotubes (CNT) in the early 1990s, the properties of carbon nanotubes (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 CNFs having similar composition and much larger geometries, CNTs possess superior physical properties with higher surface area. Recently, CNTs have been deposited using LbL technique 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 too thin (less than 100 nm even after 10 BLs). Compared with previous CNF [19] and clay-based [20] LbL coatings, MWCNT-based coating has significant challenges due to MWCNT size and surface chemistry that complicate its dispersion in aqueous solutions and maintenance of the dispersed state. Researchers have improved MWCNT dispersion and stability 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 one-step MWCNT functionalization method through direct amination without strong acid treatment [36]. Functionalized MWCNT exhibits excellent dispersion and stability in water without need for any dispersing agent. In this study, focus was placed on SWCNT LbL coating with high SWCNT (single-walled carbon nanotube) concentration. The stability of SWCNT with direct amination applied was supported, and the deposition of SWCNT, the advantages of LBL methodology and the importance of coatings including microstructure and fire performance are discussed.

2. Material and Method 2.1. SWCNT Functionalization and LbL Methodology

SWCNTs were first functionalized with PEI to facilitate dispersion in deionized water and increase the permanence 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 with 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 samples were dried in a desiccator with anhydrous calcium sulfate 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 Zeynep Daşdelen / Master's Student / 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

Figure 1. SWCNT/polymer coating process

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