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Multifunctional thermoset composites were made from polyester resin, glass fiber mats and carbon nanofiber sheets (CNS). Their flaming behavior was investigated with cone calorimeter under well-controlled combustion conditions. The heat release rate was lowered by pre-planting carbon nanofiber sheets on the sample surface with the total fiber content of only 0.38 wt.%. Electron microscopy showed that carbon nanofiber sheet was partly burned and charred materials were formed on the combusting surface. Both the nanofibers and charred materials acted as an excellent insulator and/or mass transport barrier, improving the fire retardancy of the composite. This behavior agrees well with the general mechanism of fire retardancy in various nanoparticle-thermoplastic composites.

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Carbon nanotubes and carbon nanofibers have been used as nanofillers for high performance damping composite materials in recent years. The large specific area (1000 m2/g) and aspect ratio (>1000) of carbon nanotubes and nanofibers promote significant interfacial friction between carbon nanotubes/nanofibers and the polymer matrix. The high stiffness and strength of carbon nanotubes and nanofibers enlarge the differences in the strains of individual constituents of the composites, which causes much higher energy dissipation in the polymer matrix. However, adding small amount of carbon nanotubes and nanofibers will significant increase the viscosity of polymer resin, which makes the dispersion and resin flow through the porous fiber mats extremely difficult. In addition, the fiber mats will filter carbon nanotubes and nanofibers during liquid molding process such as Resin Transfer Molding (RTM) and Vacuum-Assisted Resin Transfer Molding (VARTM). A unique concept of manufacturing nanocomposites with carbon nanotube/nanofiber based nanopaper sheets for structural damping applications has recently been explored. This approach involves making carbon nanopaper sheet by the filtration of well-dispersed carbon nanotubes and carbon nanofibers under controlled processing conditions. Subsequently, carbon nanopaper sheets are integrated into composite laminates using Vacuum Assisted Resin Transfer Molding (VARTM) process. In this study, several nanocomposite plates were fabricated with carbon nanopaper sheet as surface layer. For the comparative study, the regular composite plates without carbon nanopaper sheet were also fabricated. To identify the damping characteristics of each specimen, the Frequency Response Function (FRF) was estimated by a pair of piezoceramic patches: one as an actuator to excite the specimen and the other as a sensor to detect the induced vibrations. From the FRF, the damping ratio of the specimen at each modal frequency of interests was calculated. The experimental results clearly show a significant improvement of damping properties of nanocomposites plates. This research demonstrates structural damping enhancement via carbon nanopaper sheets and provided basic understanding of the damping characteristics for the optimal design and fabrication of high performance damping composites, which have the potential to be used as structural components for many applications.

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