Chemical Modification of Microfibrillated Cellulose: Effects on Film Barrier Properties

Abstract

A global demand for environmental sustainability is a strong driving force towards the development of enhanced barrier concepts and the use of new materials, especially for packaging applications. Abundant and renewable, cellulosic fibers have been widely used as one of the main constituents in the fiber-based packaging. However, the porous and hydrophilic structure of cellulose network requires the use of barrier polymer coatings to create an additional resistance against water, water vapors and gases. The ability of forming strong translucent films, the low oxygen permeability and the potential of chemical modification for hydrophobicity improvement open up the possibilities of using fibrillated celluloses as coating layers in barrier packaging.

The research focus of this thesis was therefore on the surface chemical modification for altering hydrophobicity while preserving the oxygen barrier properties and the morphology of cellulose fibrils. The work contains results achieved within the scope of the competence developing project initiated by the Paper and Fiber Research Institute (PFI): “Fiber based packaging materials: Development of innovative and sustainable barrier concepts (SustainBarrier)”.

Microfibrillated cellulose (MFC) was surface functionalized via esterification treatments with several chemical compounds. Acetylated with acetic anhydride MFC was used for the preparation of water repellent cellulose films. Pure MFC films were subjected to gas-phase reaction with mixtures of trifluoroacetic anhydride, acetic acid and acetic anhydride. Efficient esterification and purification procedures were developed. TEMPO-oxidized celluloses with variable carboxylate contents were prepared from hardwood and softwood pulps by oxidation with the TEMPO/NaClO/NaBr system. TEMPO-oxidized cellulose nanofibers (TOCN) were obtained as a result of a slight mechanical treatment and used for preparation of nanofiber dispersions and high oxygen barrier films.

The physical and chemical properties of native and modified MFC as well as TOCN (in dispersions and films) have been investigated with advanced characterization methods. Infrared spectroscopy and X-ray photoelectron spectroscopy was applied for the characterization of the chemical modifications. Field emission electron microscopy was used to examine the morphological changes of the MFC films after a reaction treatment and surface morphology/cross-sectional analysis of the TOCN films. Atomic force microscopy was applied for measuring the average nanofiber width in TOCN dispersions. In addition, generic laboratory characterization methods, like titration, contact angle analyses, mechanical strength tests, oxygen permeability measurements, laser profilometry, light microscopy, water vapor transmission rates, birefringence and sedimentation tests, viscosity determination, fibrillation yields, UV-Vis transmittance, have been employed to obtain sufficient knowledge about effect of the modifications applied to MFC and oxidation degrees to TOCN.