Cellulose fibres, nanofibrils and microfibrils: The morphological sequence of MFC components from a plant physiology and fibre technology point of view

Abstract

During the last decade, major efforts have been made to develop adequate and commercially viable processes for disintegrating cellulose fibres into their structural components. Homogenisation of cellulose fibres has been one of the principal applied procedures. Homogenisation has produced materials which may be inhomogeneous, containing fibres, fibres fragments, fibrillar fines and nanofibrils. The material has been denominated microfibrillated cellulose (MFC). In addition, terms relating to the nano-scale have been given to the MFC material. Several modern and high-tech nano-applications have been envisaged for MFC. However, is MFC a nano-structure? It is concluded that MFC materials may be composed of (1) nanofibrils, (2) fibrillar fines, (3) fibre fragments and (4) fibres. This implies that MFC is not necessarily synonymous with nanofibrils, microfibrils or any other cellulose nano-structure. However, properly produced MFC materials contain nano-structures as a main component, i.e. nanofibrils.

Figure 1

Structure of wood pulp fibres. (a) Note the network of microfibrils covering the outer wall layer. (b) Microtomed cross section showing the S1, S2 and S3 layers. (c) Cross-sectional fracture area, showing the microfibrils in the S2 layer. Reproduced and modified from Chinga-Carrasco [11].

Figure 2

Microfibril of Pinus radiata. Image acquired with TEM. The black arrow indicates the boundaries of a microfibril, which is approximately 28 nm in diameter. The white arrows indicate a single elementary fibril, which is 3.5 nm in diameter. See also Chinga-Carrasco et al. [16].

Figure 3

Films made of cellulose materials with a grammage of 20 g/m². (A) Control film made of 100% P. radiata pulp fibres. (B) Film made of MFC, homogenised with three passes and 1,000 bar pressure. (C) Film made of MFC, homogenised with five passes and 1,000 bar pressure. (D) Film made of MFC produced with TEMPO-pre-treated fibres, three passes and 200 bar pressure. (E) Film made of MFC produced with TEMPO-pre-treated fibres, three passes and 600 bar pressure. (D) Film made of MFC produced with TEMPO-pre-treated fibres, five passes and 1,000 bar pressure. Dark threadlike structures indicate poorly fibrillated fibres or fibre fragments. The lighter the local areas, the higher the transparency levels. For details, see Syverud et al. [35].

Figure 4

Surfaces of films (20 g/m²) made of microfibrillated cellulose. (A) MFC obtained by mechanical homogenisation. The image corresponds to the film shown in Figure 3C. (B) MFC obtained with TEMPO-mediated oxidation as pre-treatment and mechanical homogenisation. The image corresponds to the film shown in Figure 3F. The insets in (A) and (B) represent the surface structure visualised at 50,000× magnification from areas without a metallic coating. Both MFC materials have been collected after passing five times through the homogeniser, at 1,000 bar. For details, see Chinga-Carrasco et al. [16].

Figure 5

Microfibrillated cellulose suspensions dried on glass slides. (A) MFC obtained by mechanical homogenisation. Note the relatively large structures remaining after a homogenisation process. The inset shows structures composed mainly of nanofibrils. (B) MFC obtained with TEMPO-mediated oxidation as pre-treatment and mechanical homogenisation. The inset shows the nanofibrils having relatively homogeneous sizes. Both MFC materials (A and B) have been collected after passing five times through the homogeniser, at 1,000 bar.