(394c) Investigation of Mass Transport Properties of Microfibrillated Cellulose (MFC) Films

Investigation of mass
transport properties of microfibrillated cellulose (MFC) films

M.
Minelli, M. Giacinti Baschetti, F. Doghieri, M. Ankerfors, T. Lindström, I.
Siró, D. Plackett

Cellulose-based nanocomposites are now emerging as promising new
materials suitable for a variety of applications. One source for such new
nanocomposites is microfibrillated cellulose (MFC), produced by delamination of
cellulosic fibers in high-pressure homogenizers. MFC is an organic and
biodegradable reinforcement in polymer nanocomposites because of its high
aspect ratio, good mechanical properties and ability to form networks: promising
properties that may make this material suitable for use in a number of
industrial fields and products.

The structure and transport properties of a four different films
based on two different generations of microfibrillated cellulose (MFC), alone
or in combination with glycerol as plasticizer, were investigated through
FE-SEM analysis and sorption or permeation experiments. FE-SEM revealed the existence
of complex structures in the different samples. A porous, closely packed fiber
network, more homogeneous in the samples containing glycerol, was
characteristic of the surface of MFC films; while film cross-sections presented
a dense layered structure with no evidence of porosity. Water vapor sorption
experiments confirmed the hydrophilic character of these cellulosic materials
and showed a dual effect of glycerol, which reduced the water uptake at low
water activity while enhancing it at high relative humidity, as showed in
Figure 1. The observed trends of water sorption can be described considering that
both physical adsorption on fiber surfaces and absorption in the cellulose
amorphous phase takes place and therefore a dual-model sorption model was
employed.

The water diffusivity (in the panel in Figure 1) in dry samples was
remarkably low for a porous material (10^-11-10^-12
cm²/s), confirming the existence of complex structures below the
film surface; however, when water is present in the system, D rapidly
increases with an exponential trend. Diffusivity is also definitely affected by
plasticization, being higher for glycerol-containing samples.

Similar behavior was observed in permeation experiments. Dry MFC
films showed excellent oxygen barrier properties, comparable with those of
polymers usually considered suitable for ultra-high barrier applications.
Indeed, when dry, the cellulose network presents a particularly compact and
stiff structure, causing the observed barrier effect. Furthermore, although MFC
films are characterized by a certain porosity, FE-SEM images of sample surfaces
suggests that the pores are not substantially interconnected

When the water content in the membrane is raised, however, a
dramatic decrease in these properties was observed (Figure 2).

Figure 1. Water vapor sorption isotherms
in MFC samples at 35°C, dotted lines are given by the best fitting of the data with
the dual-mode model equation. In the panel: water diffusivity at 35°C in the four different MFC samples as function of the average water concentration.

Figure 2. Oxygen permeability in the
four MFC samples at 35°C as a function of water

activity.