Physical properties of dispersions and composites containing surface-grafted cellulose nanocrystals

Licentiate thesis, 2018

The needs and requests from the society for non-fossil-based materials as well as more lightweight products are constantly increasing. The increasing awareness of the sustainability of the resources available paralleled by a growing population, point to a need for changes, if the population also aim for constantly higher standard of living. Cellulose is a renewable material, produced by all plants on earth through photosynthesis using solar energy. It has promising properties, being the supportive and stiff structure in plants and trees, and has been widely used throughout history as a construction material and in everyday life.

The smallest entity of the cellulose fibre, the cellulose nanocrystals (CNC), exhibit even greater promising properties with their low weight, high aspect ratio and high specific stiffness in combination with biodegradability and renewability. Using CNC as reinforcements in a polymer matrix reduces in principle the need for fossil-based materials and reduces also the weight of the component. There are however some challenges, the hydrophilic nature of cellulose and the hydrophobic nature of most polymers causing low adhesion and poor dispersion. The temperatures needed for most thermoplastic processes also exceed the onset temperature for thermal degradation of cellulose, about 150-200 °C.

Here, azetidinium salts have been used to graft three different functional groups onto the CNC surface, aiming at improving the thermal stability of the CNC but also the compatibility between a polymer matrix and CNC in a composite material. The focus in the first part of the study was to evaluate the properties of and interactions between the grafted groups in aqueous dispersions of surface-treated CNC. This was followed by manufacturing of composites using dispersion mixing of CNC and an ethylene-acrylic acid copolymer and compression moulding of plaques. The composites were then evaluated in terms of mechanical properties and thermal stability.   

The surface-grafting resulted in a large increase in thermal stability of the CNC; the onset temperature for thermal degradation increased from 150 °C to 250 °C. The addition of CNC also had a strong influence on the mechanical properties of the composites, e. g. resulting in an up to three times higher elastic modulus when adding 10 weight-% CNC. Several experimental methods indicated enhanced interactions either between the grafted groups and/or between the polymer matrix and the grafted groups.