Strength and barrier properties of MFC films

The preparation of microfibrillar cellulose (MFC) films by filtration on a polyamide filter cloth, in a dynamic sheet former and as a surface layer on base paper is described. Experimental evidence of the high tensile strength, density and elongation of films formed by MFC is given. Typically, a MFC film with basis weight 35 g/m² had tensile index 146 ± 18 Nm/g and elongation 8.6 ± 1.6%. The E modulus (17.5 ± 1.0 GPa) of a film composed of randomly oriented fibrils was comparable to values for cellulose fibres with a fibril angle of 50°. The strength of the films formed in the dynamic sheet former was comparable to the strength of the MFC films prepared by filtration. The use of MFC as surface layer (0–8% of total basis weight) on base paper increased the strength of the paper sheets significantly and reduced their air permeability dramatically. FEG-SEM images indicated that the MFC layer reduced sheet porosity, i.e. the dense structure formed by the fibrils resulted in superior barrier properties. Oxygen transmission rates (OTR) as low as 17 ml m⁻² day⁻¹ were obtained for films prepared from pure MFC. This result fulfils the requirements for oxygen transmission rate in modified atmosphere packaging.