Abstract:
” This PhD thesis have been made in the frame of a European project, where the objective is to produce a 100% cellulosic food packaging material that would be biobased, biodegradable and recyclable. Microfibrillated cellulose films, have shown interesting barrier properties to grease and oxygen. However, the barrier properties to water and water vapor of such film are very poor because of the hydrophilic nature of cellulose. A process of chemical modification named chromatogeny and used at pilot scale, consists to the deposition, diffusion and grafting of fatty acids at the surface of cellulosic materials to bring them hydrophobic properties. This technology has shown interesting results in the case of porous cellulosic substrates but present some grafting limitations when applied to dense structures such as microfibrillated cellulose film. In this PhD work, the objective was to bring a better understanding about the diffusion and grafting phenome in a dense microfibrillated cellulose film as well as the structure and water barrier property relationship. To do so, a gas phase process established at lab scale and already used for cellulose aerogel grafting have been investigated. In a first step, a new gas phase reactor has been designed in order to have temperature profiles to study the vaporization and condensation phenomena. Then, this process has been used to modify cellulose aerogel, using reagent with different chain length. One goal was to find the process parameters that allow to have a modification restricted to the surface or that modify the core of cellulose aerogel. In a second step, this gas phase process was used to modify microfibrillated cellulose films. Firstly, a structure modification protocol has been determined and has shown the increase of the specific surface area and the porosity of the microfibrillated cellulose film. Then, those structure modified films have been grafted using the gas phase process and different level of grafting have been reached. Different level of hydrophobic properties has also been reached and correlated with the substitution degree. Finally, a theoretical study has investigated the penetration depth of the grafting reagent in the microfibrillated cellulose film thickness. Molecular modeling and a calculation based on the volume expansion of the film are the tools that have been used. Those calculations have shown a very high increase of the thickness linked to the chemical modification with a fatty acid which have been confirmed by the measurements of the thickness of gas phase modified films. Also, a coloration technic was used to show the location of the grafting reagent in the thickness of a modified film.
