Extreme Thermal Shielding Effects in Nanopaper Based on Multilayers of Aligned Clay Nanoplatelets in Cellulose Nanofiber Matrix

摘要

Currently used flame retardants for polymers, such as some halogen compounds, are often recognized as being environmentally hazardous due to the risk of bioaccumulation in humans and the production of toxic byproducts during fires. Furthermore, in specific applications such as protective garment for fire fighters or interior components in aircrafts, it is desirable to have thin, lightweight, flexible/moldable, and more efficient flame shielding solutions. A few commercially available inherently flame retardant polymers exist, including polyphenylenesulfide and oxidized polyacrylonitrile. However, these high temperature polymers, especially in the form of fibers, are expensive and have substantial environmental impact in terms of high energy expenditure and carbon footprint. Although biobased cellulosic fibers are used in flame retardant applications, the required chemical modification routes utilize toxic chemicals, as is apparent from reviews on commercial materials. The potential of clay as a “green” nanocomponent in fire retardant textiles has been demonstrated using thin clay-polymer coatings applied in a layer-bylayer (LbL) nanotechnology approach with closely controlled nanostructure and high clay content. The LbL technique is versatile in that different types of particles can be used in the layers, but it is also slow since each deposited layer tends to be very thin thus requiring a high number of layers (20–40) to reach the desired performance. Recently, somewhat thicker (4–6 μm) fire retardant coatings containing 82–87 wt% clay were produced by LbL and showed good performances already with less than ten bilayers when deposited on a polystyrene sheet of 3.2 mm thickness. Still, for the purpose of thicker films or coatings in the 50–100 μm range, the LbL technique is in many cases impractical.