BARRIER PROPERTIES OF FILMS BASED ON MICROFIBRILLATED CELLULOSE AND POLYLACTIC ACID

Microfibrillated cellulose (MFC) produced by delamination of cellulosic fibers in high-pressure homogenizers is emerging as a new, sustainable and biodegradable material with interesting properties for many applications, including packaging. MFC consists of cellulose microfibrils with very large aspect ratios, and pure MFC films show excellent gas barrier properties, due to a high crystallinity and cohesive energy density. Unfortunately, the strong hydrophilicity prevents the use of MFC in the packaging industry, because the oxygen barrier effect is lost in the presence of humidity. To the aim of creating a fully degradable and renewable material with optimal oxygen and moisture barrier properties, multilayer films were developed by using a hydrophobic substrate formed by a biopolymer (polylactic acid, PLA) coated by a thin MFC layer. In this work we present experimental oxygen and moisture transport data in pure MFC films and multilayer PLA-MFC materials. The MFC studied are obtained from aqueous dispersions obtained via different pretreatments prior the delamination stage, and were kindly provided by Innventia. Pure MFC films are obtained by evaporation on Petri dishes. Multilayer films are obtained with an innovative, solvent-free protocol which, despite the intrinsic incompatibility of the two moieties, allows a good adhesion between the two layers, relying only on a physical pretreatment of the substrate. The dry oxygen barrier properties of pure MFC films are excellent and comparable with those of polymers for high barrier applications, but at higher R.H. values, a dramatic worsening of the oxygen barrier effect is observed. The moisture sorption tests allow to quantify the vapour absorption of neat MFC films in a wide range of activities and temperatures, confirming their marked hydrophilic character. On the other hand, the dry oxygen permeability of the multilayer PLA-MFC material is almost two orders of magnitude lower than neat PLA and similar to the value evaluated on self-standing MFC films. Those tests allow to conclude that the novel multilayer material couples the intrinsic properties of the two layers, namely the hydrophobicity and the oxygen barrier ones, paving the way for a fully renewable material with properties comparable to the petrochemical ones.