Estimation of cellulose nanofibre aspect ratio from measurements of fibre suspension gel point
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Cellulose nanofibre aspect ratio controls the properties of sheets made from nanofibres and processing conditions, but aspect ratio is very difficult to measure. In this paper, aspect ratio was estimated from the gel point of a cellulose nanofibre suspension, the solids concentration at which the transition from a dilute to a semi-dilute suspension occurs. Four batches of cellulose nanofibres were tested. Two were produced from softwood fibres using ball milling. Commercially produced microfibrillated cellulose material was also used, both in as supplied form and after removal of the larger fibres by filtering. The average diameter measured from SEM images of fibres ranged from 33 to 73 nm. One sample was too heavily treated and an average dimension could not be measured. The gel-point was measured both from the height of a layer of cellulose nanofibres sedimented from a dilute suspension or from the lowest solids concentration at which a yield stress could be measured using a vane rheometer. The two methods were closely in agreement for all samples. Aspect ratio was then calculated using either the effective medium (EMT) or crowding number (CN) theories. Aspect ratio calculated with an assumed fibre density of 1,500 kg/m3, using the CN theory ranged from 155 to 60. Use of the EMT theory reduced the calculated aspect ratio by between 11 and 23 %. Reducing the assumed density in suspension from 1,500 to 1,166 kg/m3 reduced the calculated aspect ratio by 12–14 %. The heavily treated sample had by far the lowest aspect ratio.
KeywordsCellulose nanofibres Gel point Aspect ratio Yield stress Sedimentation
The authors would like to acknowledge the facilities used with the Monash Center for Electron Microscopy. The authors would like to thank Wade Mosse and Mae-Gene Yew for assisting us with rheological measurements and Liyuan Zhang and A.Prof. Takuya Tsuzuki for helping us to develop the method for separating nanofibres from MFC sample. We acknowledge the financial support of the Australian Research Council, Australian Paper, Nopco Australasia, Norske Skog, SCA Hygiene Australasia and Visy through Linkage Project Grants LP0989823 and LP0990526. Swambabu Varanasi thanks Monash University for a MGS and FEIPRS Scholarship.
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