Abstract
Nanofibrillated cellulose (NFC) films have potential as oxygen barriers for, e.g., food packaging applications, but their use is limited by their hygroscopic characteristics. The three-dimensional microstructure of NFC films made of Pinus radiata (Radiata Pine) kraft pulp fibres has been assessed in this study, considering the structural development as a function of relative humidity (RH). The surface roughness, micro-porosity, thickness and their correlations were analyzed using X-ray microtomography (X–μCT) and computerized image analysis. The results are compared to those from scanning electron microscopy and laser profilometry. Based on a series of films having varying amounts of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-mediated oxidated nanofibrils, it was demonstrated that X–μCT is suitable for assessing the surface and bulk 3D microstructure of the cellulose films. Additionally, one of the series was assessed at varying humidity levels, using the non-destructive capabilities of X–μCT and a newly developed humidity chamber for in-situ characterization. The oxygen transmission rate (OTR) of the films (20 g/m2) was below 3.7mLm−2 day−1 at humidity levels below 60% RH. However, the OTR increased considerably to 12.4mLm−2 day−1 when the humidity level increased to 80% RH. The increase in OTR was attributed to a change of the film porosity, which was reflected as an increase in local thickness. Hence, the characterization techniques applied in this study shed more light on the structures of NFC films and how they are affected by varying humidity levels. It was demonstrated that in increasing relative humidity the films swelled and the oxygen barrier properties decreased.
Highlights
The fibrils are composed of 36 β-1,4-glucan chains, which are synthesised by rosette complexes, i.e., cellulose synthase proteins in the plasma membrane [3,4]
Nanofibrillated cellulose (NFC) refers to the material that is produced based on the disintegration of the fibre wall into a major fraction of individualized elementary fibrils and their aggregates
The thickness and roughness quantified by X–μCT are compared with the corresponding results from scanning electron microscopy (SEM) and laser profilometry (LP) respectively, which give similar estimates
Summary
Vascular plant cell walls are composed of elementary fibrils which have been reported to have a width of 3.5 nm [1,2]. Nanofibrillated cellulose (NFC) refers to the material that is produced based on the disintegration of the fibre wall into a major fraction of individualized elementary fibrils and their aggregates. Chemical pretreatments such as TEMPO mediated oxidation yield homogenous fibrillated materials with typical fibril diameters less than 20 nm (see [5,6,7]). For a good overview of oxygen permeability of NFC materials see [18], where values between 5 and 0.1 mL μm m−2 day−1 kPa−1 have been reported (measured at 23 °C and 50% relative humidity), and [13,19,20,21,22]. To our knowledge no direct assessment of the three-dimensional structural changes as a function of relative humidity has been reported
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