Energy requirements for the disintegration of cellulose fibers into cellulose nanofibers.docx
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1、Cellulose (2012) 19:831842 1 3 DOI 10.1007/s10570-012-9694-4 OR IGINAL PAPER Energy requirements for the disintegration of cellulose fibers into cellulose nanofibers Alvaro Tejado Md. Nur Alam Miro Antal Han Yang Theo G. M. van de Ven Received: 9 December 2011 / Accepted: 15 March 2012 / Published o
2、nline: 27 March 2012 Springer Science+Business Media B.V. 2012 Abstract Cellulose nanofibers have a bright future ahead as components of nano-engineered materials, as they are an abundant, renewable and sustainable resource with outstanding mechanical properties. However, before considering real-wor
3、ld applications, an efficient and energetically friendly production process needs to be developed that overcomes the extensive energy consumption of shear-based existing processes. This paper analyses how the charge content influences the mechanical energy that is needed to disintegrate a cellulose
4、fiber. The introduction of charge groups (carboxylate) is achieved through periodate oxidation followed by chlorite oxidation reactions, carried out to different extents. Modified samples are then subjected to different levels of controlled mechanical energy and the yields of three different fractio
5、ns, separated by size, are obtained. The process produces highly functionalized cellulose nanofibers based almost exclusively on chemical reactions, thus avoiding the use of intensive A. Tejado (&) Md. N. Alam M. Antal H. Yang T. G. M. van de Ven Department of Chemistry, Pulp & Paper Research Centre
6、, McGill University, 3420 University St., Montreal, QC H3A 2A7, Canada e-mail: Present Address: A. Tejado Tecnalia Research & Innovation, Area Anardi 5, 20730 Azpeitia, Spain mechanical energy in the process and consequently reducing drastically the energy consumption. Keywords Cellulose nanofibers
7、 Mechanical energy Disintegration Pulp Periodate Introduction From the most basic to the most advanced use, cellulose seems always to be one step ahead of any other material, be it natural or synthetic. Besides being the most abundant biopolymer on earth, as well as being renewable, biodegradable an
8、d carbon-neutral, cellulose has unique properties that have been crucial for the existence of life on earth. It has served mankind as the primary source of heat, clothes and building material, to cite the most relevant ones. Because of its proven record of applications, it is not surprising that the
9、 use of cellulose nanostructures, especially cellu- lose nanofibers (CNF) or nanofibrils, promises to play an essential role in the development of the next generation high-tech nanostructured materials. The cellulose fiber wall, with a typical diameter (d) ranging 1535 lm, is a compounded material m
10、ainly composed of cellulose microfibers (d * 40100 nm), arranged in different orientations, embedded in a poly- meric network of hemicelluloses, pectins and lignins (Somerville et al. 2004), with the percentage of each 2 Cellulose (2012) 19:831842 1 3 constituent varying in the radial direction thro
11、ugh well defined layers. The microfibers themselves are composed of several nanofibrils (d * 210 nm) made of crystalline and amorphous domains. Whether these domains are arranged in an alternating configuration or a coreshell distribution (Ding and Himmel 2006) is still an open question, although tr
12、aditionally the first possibility has been the most widely accepted (Habibi et al. 2010). Finally, the number of cellulose polymeric chains that builds up one nanofibril is also a matter of discussion, but lately a molecular model consisting of a 36-glucan-chain elementary fibril forming both crysta
13、lline and subcrys- talline structures is being preferentially considered (Ding and Himmel 2006; Gross and Chu 2010). However, cellulose nanofibrils from different sources are known to have different diameters and thus a different number of elementary chains associated with them. CNF are then the pri
14、mary complete building entities in the hierarchy of plants. From the point of view of materials science, their fibrillar shape of small diameter and very high aspect ratio makes them ideal to be used as reinforcing elements, but by themselves they are also ideal to form strong and transparent films
15、(Henriksson et al. 2008; Siro and Plackett 2010; Saito et al. 2009) that can compete with polymeric ones. However, there are still two major problems that require solution before considering real-world applications for the CNF (Hubbe et al. 2008; Siro and Plackett 2010): first, finding an efficient
16、and energetically favourable way to isolate them. Because neighbouring nanofibrils are either chemically cross-linked (Somerville et al. 2004) or physically entangled by single-chain polysaccharides (Keckes et al. 2003), it seems that their isolation always requires a considerable amount of shear, i
17、.e. mechanical action, regardless of the type of pretreatment. So far, existing methods (Henriksson et al. 2007; Herrick et al. 1983; Hubbe et al. 2008; Isogai et al. 2011; Siro and Plackett 2010; Turbak et al. 1983) make use of a considerable amount of mechanical energy to disrupt the fiber wall, a
18、 process that, in addition to other environmental implications, requires a high energy input and high cost. The second step to be mastered has to do with the problem of dispersing hydrophilic CNF into hydrophobic media, e.g. polymeric matrices. Despite several strategies that have been developed to
19、minimize this effect, such as grafting hydrophobes onto them (Siro and Plackett 2010) or coating them with surfactans (Heux et al. 2000), the high crystallinity is often an issue since it limits reactivity. It is very likely that without fully addressing these two features, CNF will have a hard time
20、 to find their way out of the laboratories and into the factories. In recent years, the use of enzymatic or chemical pretreatments on cellulose fibers has become popular with the aim of reducing the amount of mechanical energy required to liberate the nanostructures. The enzymatic route typically in
21、volves mixtures of various cellulases which are able to partially digest both the crystalline and amorphous regions (Paakko et al. 2007; Henriksson et al. 2007) facilitating the subsequent mechanical disintegration of the fibers. Alternatively, the introduction of carboxylate groups (COO) onto the s
22、urface of the nanofibrils leads under mild alkaline conditions to the appearance of repulsive forces that also weaken the structure. In this direction the preferred pathway is the 2,2,6,6-tetramethylpiperi- dine-1-oxyl (TEMPO) radical-mediated oxidation with hypochlorite and chlorite salts as the mo
23、st common oxidizing agents (Iwamoto et al. 2010; Saito et al. 2009, 2010; Fukuzumi et al. 2009; Siro and Plackett 2010; Isogai et al. 2011), by which one of the three hydroxyl groups in the accessible glucose units of cellulose is converted to a carboxylic group. The use of such nitroxyl radicals an
24、d nitrosonium salts as an oxidative route to transform hydroxyl functions into carboxyl and/or aldehyde groups is disclosed elsewhere (Bobbitt and Flores 1988; Chang and Robyt 1996). Both enzymatic and chemical modifications allow reducing the disintegration energy of cellulose fibers from somewhere
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