Benoît Lecouvet

Elastic modulus of halloysite nanotubes

This work reports measurements of the elastic modulus of halloysite nanotubes. Nanoscale three-point bending tests were performed on individual nanotubes using an atomic force microscope. Nanotubes exhibit elastic behaviour at small deformations. The stiffness of the tubes, and hence their elastic modulus, was deduced from force curve measurements using an appropriate mechanical model. The boundary conditions were also identified by recording the stiffness profile of a tube along its suspended length. An average elastic modulus of 140 GPa is obtained for a set of tubes with outer diameters ranging between 50 and 160 nm. Moreover, the elastic modulus increases with decreasing outer diameter, with a steep jump below 50 nm. The size dependence of the elastic modulus may be attributed to: (i) surface tension effects for thinner tubes and (ii) a non-negligible contribution of shear deformations to the total deflection for larger tubes.

Highly loaded nanocomposite films as fire protective coating for polymeric substrates

The main fire retardancy mechanism of polymer–clay nanocomposites involves the progressive build-up of an inorganic-rich layer at the sample surface during combustion, by a combination of ablative reassembling and migration of the nanoparticles, reducing heat and mass transfer between gas and condensed phases. In this process, a non-negligible amount of polymer needs to be sacrificed before the entire surface of the residual material is shielded. This study aims to improve the fire behaviour of polymer–clay nanocomposites by accelerating the physical barrier formation through the development of new hybrid structures. For that purpose, a system based on polyamide 12 and halloysite nanotubes is chosen as reference and highly loaded nanocomposite films are coated on a virgin PA12 substrate. Both the influence of clay fraction in the nanocomposite film and its thickness on the flammability performance of the coated samples are evaluated by mass loss calorimetry and compared to that of homogeneous ‘bulk’ nanocomposites. Results show unambiguously that hybrid materials are more effective than conventional nanocomposites to flame retard polymers while substantially reducing the total filler loading, and so the cost of the final product. Moreover, a small amount of carbon nanotubes can be incorporated in the polymer substrate and/or in the nanocomposite film to suppress the vigorous bubbling of evolved degradation products as well as to reduce the flexibility of the thin protective skin, which can cause detrimental effects on its barrier properties.