Rheological Properties of Microfibrillated Cellulose
Cellulose has for a long time been used by man. It is renewable and an easily recyclable resource. Mostly it has been used in either its native form as wood or in the form of fibres. But in the early 1980s it was shown that it was possible to extract even smaller elements out of the fibres when wood pulp was subjected to intense mechanical shearing. The material that was obtained was called microfibrillated cellulose or abbreviated MFC. It is believed (and to some extent shown) that these microfibrils potentially have better mechanical properties than the cellulose fibres. They can also exhibit other interesting features. It is thus of interest to use microfibrils in certain applications, but, in the form of suspensions, these microfibrils present very different rheological properties than the cellulose fibres. Thus, based on the interest in microfibrillated cellulose for different applications, such as transparent and high toughness papers, there is a need to extend the understanding of the rheological properties of the MFC-suspensions.
The rheological properties of two different types of MFC-suspensions with a concentration of 2 weight-% (in water) were determined both in shear and in extension. In shear, the shear storage and shear loss moduli of the suspensions were assessed in terms of their stress and frequency dependence. It was shown that the suspensions exhibited a linear region where the above material parameters were independent of the stress level (linear viscoelasticity). Above a critical stress level, this is no longer valid anymore and the network structure of the suspension starts to deteriorate. Creep measurements confirmed that the suspensions exhibited a linear region in which the creep compliance was independent of the stress applied. This was the case up to ca 10-15 Pa for one of the studied suspensions. The creep measurements was fitted to simple models used to describe viscoelastic behaviour and it was, somewhat unexpectedly, noted that quite simple models could describe the behaviour quite well
The extensional viscosity of some MFC-suspensions was determined with a contracting flow technique. The material was found to exhibit a strain-thinning behaviour, and the viscosity decreased when the concentration of cellulose decreased (in the concentration range below 1 weight-%), but it increased when the amount of added salt increased. This is believed to be due to screening of the surface charges on the fibrils caused by the salt added to the suspension, increasing the fibril-fibril interactions. The extensional viscosity was much higher than the shear viscosity of the suspensions.