Abstract
For the successful commercialisation of microfibrillated cellulose (MFC) it is of utmost importance to carefully characterise the constituent cellulose particles. This could for instance lead to the development of MFC grades with size distributions tailored for specific applications. Characterization of MFC is challenging due to the heterogeneous chemical and structural nature of MFC. This study describes a fractionation approach that combines two steps of physical sieving of larger particles and a final centrifugation step to separate out the smallest, colloidally stable particles, resulting in four distinctly different size fractions. The properties, such as size and charge, of each fraction were studied, as well as MFC filtration time, film formation, and film properties (mechanical and optical). It was found that virtually all surface charges, determined by polyelectrolyte adsorption, are located in the colloidally stable fraction of the MFC. In addition, the amount of available surface charges can be used as an estimate of the degree of fibrillation of the MFC. The partly fibrillated particles frequently displayed a branching, fringed morphology. Mechanical testing of films from the different fractions revealed that the removal of large particles may be more important for strength than achieving full fibrillation. Overall, this study demonstrates that by controlling the size distribution in MFC grades, property profiles including dewatering time to make films by filtration, rheology, film strength and optical transmittance could be optimised.Graphical abstract
Highlights
Today, strong research efforts are focused on finding sustainable materials as alternatives to non-biodegradable petroleum-based materials
The results show that tailored size distributions is a feasible route towards new microfibrillated cellulose (MFC) grades where property profiles are optimised, for instance in terms of dewatering time for making films and the strength of these films
It should be noted that the losses probably contain dissolved matter or small low-aspect-ratio colloids, a type of matter that (Tanaka et al 2012) reported to constitute 5–10% of the MFC grades they studied
Summary
Strong research efforts are focused on finding sustainable materials as alternatives to non-biodegradable petroleum-based materials. Due to the heterogeneous chemical and structural nature of wood and of the pulp produced from it, this type of process naturally produces a chemically and structurally heterogeneous material This heterogeneity is often decreased by further ultrasonication to break down aggregates remaining after homogenisation and/or ultracentrifugation to exclude particles that do not fulfil some defined size quality criterion (Saito et al 2006; Ahola et al 2008; Wagberg et al 2008; Aulin and Lindstrom 2011). This is hardly a feasible alternative for large-scale production and it is crucial to characterise the material produced and to facilitate its proper use in suitable products. To make things even more complex, microfibrillated cellulose (MFC) can be produced from basically any cellulose fibre raw material, and this affects the properties of the MFC produced even if the process itself is standardised
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