Cellulose aerogel-based fiber reinforcement – towards thermoformable composite materials

Abstract

Cellulose is renewable and characterized by a strong polymeric backbone. Thus, it is increasingly gaining attention as functional additive in polymers. Melt blending thermoplastic polymers with nanocellulose yields cellulose nanocomposites. However, nanocellulose particles tend to aggregate making a homogenious nanofiber distribution challenging.[1] Cellulose aerogels (CA) consist of a 3D randomnly arranged cellulose nanofibers and therefore isotropic by preparation. These open porous aerogels can be prepared by various routes that mostly involve the dissolution of the native cellulose (e.g. in aqueous salt hydrate melts such as ZnCl2) followed by regeneration and drying in supercritical CO2.[2] In order to overcome aggregation, cellulose aerogels can be used as a preformed nanofiber felt. By means of capillary force-assisted infusion molding of the pore structure with a thermoset matrix, light and faultless nanofiber-matrix composites are achieved. Such composites are called cellulose aerogel reinforced polymers (CARPs). CARPs having fiber fractions ranging from 6 to 22 vol.-% considerably affect the resulting mechanical properties and formability. Fiber fractions as low as 14 vol.-% yield composites with multiplied Youngs’ moduli observed with epoxy, phenolic and unsaturated polyester resins. Although, the composites were cured in a final processing step, CARPs show significant formability at elevated temperatures. The interface of CA and matrix combines friction, mechanical interlocking, and chemical fiber-matrix bonding yielding complete stress transmission and thus, joint deformation of both composite phases, the fiber and the matrix. In conclusion, CARPs appear as a single component material making it impossible to attribute the resulting properties to either the fiber or the matrix phases.