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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
25、 in the order of 100 kWh/kg for unmodified cellulose preparations to as little as 12 kWh/kg (Isogai 2009; Siro and Plackett 2010), depending on the extent of the treatment. These new limits are comparable with those required to produce so called mechanical pulps out of wood, which means that are ind
26、ustrially viable. The main drawback of these two approaches, however, is that either they require the input of a substantial amount of energy or they fail to provide reasonable production yields. Another issue, concerning the chemical pretreatments, is that the maximum carboxylic content that can be
27、 introduced by means of TEMPO oxidation is limited (in case of TEMPO/NaBr/NaClO around 1.7 mmol/g, i.e. millimoles of COO per gram of dried fibers, and, if TEMPO/NaClO2/NaClO is used, below 1 mmol/g (Okita et al. 2010; Isogai et al. 2011). In principle, if one considers that the interior of the crys
28、talline 1 3 Cellulose (2012) 19:831842 833 domains is not accessible, the combination of the amorphous regions and the exterior surfaces of the crystals would still account for more than 3 mmol/g, taking an average degree of crystallinity of 60 % (corresponding to a ECF-bleached pine kraft pulp (Lii
29、tia et al. 2003), assuming that the nanofibrils are composed of 36 cellulose chains in a hexagonal conformation (Ding and Himmel 2006) and consider- ing that only every second glucose unit has the primary hydroxyl pointing out of the crystal. Apparently, diffusion problems prevent taking the modific
30、ation further toward the theoretical maximum. Other esti- mations, however, claim that 1.7 mmol/g correspond to the entire surface oxidation of cellulose nanofibrils of wood origin (Okita et al. 2010). In order to surpass TEMPO moderate oxidation limits, this work uses a different and well studied o
31、xidation route, namely periodate oxidation (Potthast et al. 2009; Potthast et al. 2007; Kim et al. 2000), to produce dialdehyde cellulose, followed by chlorite oxidation to convert aldehydes into carboxylic groups. It has been recently established that periodate oxida- tion attacks the crystalline d
32、omains of CNF already in the early stages of the treatment (Potthast et al. 2009). Such chemical treatment allows reaching carboxylic contents in the order of 6.5 mmol/g (Yang 2011), although in this work only up to 3.5 mmol/g is reported. The paper analyses the relation between the carboxylic conte
33、nt of cellulose fibers and the disinte- gration energy required to convert them into nanofi- brils. It shows that the oxidative treatment ultimately results in the spontaneous liberation of the CNF from the cell wall without the necessity of applying any mechanical energy other than that required to
34、 stir fiber suspensions during the chemical treatments. However, the length and especially the crystallinity of the nanostructures are severely affected, which in turn could become beneficial for certain applications such as biofuel production. The study brings some new light in understanding the me
35、chanisms that hold the nano- fibrils together inside the fiber cell wall and anticipates the production of CNF exclusively by chemical means by defining a charge threshold beyond which cellulose fibers need no mechanical energy to be disintegrated. Finally, since the process involves the introductio
36、n of a large amount of functional groups onto the CNF surfaces, the final product obtained is expected to show a higher reactivity and thus to be more prone to further derivatization than any previous preparation. Materials and methods Materials Unbeaten bleached softwood kraft pulp (SKP), sup- plie
37、d by Domtar Inc. Canada as never-dried pulp, was used as raw material for the chemical treatments. Sodium meta-periodate (NaIO4; Sigma-Aldrich), sodium chloride (NaCl; ACP Chemicals Inc.), hydrox- ylamine hydrochloride (NH2OH HCl; Sigma- Aldrich), hydrochloric acid (HCl; ACP Chemicals Inc.), sodium
38、hydroxide (NaOH; ACP Chemicals Inc.), sodium chlorite (NaClO2), hydrogen peroxide (H2O2; Sigma-Aldrich), 2,2,6,6-tetramethylpiperi- dine-1-oxyl (TEMPO; Sigma-Aldrich), sodium phos- phate buffer, sodium hypochlorite (NaClO), ethanol (ACP Chemicals Inc.) and a mix-bed ion exchange resin (Sigma-Aldrich
39、) were used as received. Double deionized water was used throughout the experimen- tation, except for the dialysis purification where Milli- Q ultrapure water (Millipore Corp.) was used. Chemical treatments Cellulose fibers in the form of pulp suspensions were subjected to two successive chemical tr
40、eatments, carried out to various extents in order to achieve various degrees of oxidation. Initially, periodate oxidation was carried out in aqueous media using a glass beaker with overhead stirrer, with the following reaction conditions: bleached softwood kraft pulp (3 g), NaIO4 (1.98 g; 10.75 mmol
41、; 50 mol % based on moles of anhydroglucose in pulp) and NaCl (11.7 g; 1 M based on overall solution) were added to 200 mL water. The beaker was totally covered with aluminium foil before starting the reaction, in order to prevent light from activating side reactions, and the mixture was gently stir
42、red at room temperature. After the desired reaction time, the modified pulp (dialdehyde cellulose) was filtered out and thoroughly washed with deionized water repeatedly. In order to convert aldehyde moieties into carboxylic groups, periodate- oxidized pulp (3.5 g), NaClO2 (80 % pure; 2.76 g; 24.5 m
43、mol) and H2O2 (30 wt.% solution; 2.76 g; 24.5 mmol) were added to 150 mL water. This mixture was stirred at room temperature for 20 h, during which the pH was kept at 5 by drop wise addition of NaOH solution (especially necessary during the first 3 h). 1 3 4 Cellulose (2012) 19:831842 Determination
44、of aldehyde and carboxylate content The aldehyde content of the dialdehyde cellulose produced by the periodate oxidation reaction was determined using the hydroxylamine-hydrochloride (NH2OH HCl) titration method, by which the HCl released from its reaction with aldehydes is back- titrated with a NaO
45、H solution of known concentration. More specifically, a water suspension of periodate- oxidized cellulose fibers (20 mL; 0.65 g dry basis) was mixed with 40 mL of isopropanol, making a final proportion of isopropanol/water of 2/1 v/v, and the mixture was sufficiently stirred to prepare a well- dispe
46、rsed slurry. The pH of the mixture was then adjusted to 23 by adding a few drops of concentrated HCl and then carefully adjusted to 3.5 with NaOH 1.1 N. 10 mL of 10 wt.% NH2OH HCl solution was added to this mixture, allowing it to react for 10 min. Finally, the HCl released from the reaction was tit
47、rated with 0.5 N NaOH solution until pH 3.5 was reached again. The aldehyde content was then calculated using the following equation: Aald VNaOH N=wcell Here Aald is the aldehyde content (mmol/g cellulose), VNaOH the volume of NaOH (mL) consumed in the titration, N is the normality of the NaOH (eq/L
48、) and wcell the weight of dry cellulose (g) initially suspended. The carboxylate content of the samples was deter- mined by conductometric titration. To a 120 mL of 1.2 wt.% water suspension of the cellulosic product (20.4 mg dry basis) 2.5 mL of a 0.02 M NaCl solution was added and the mixture was
49、gently stirred. Then 0.1 M HCl was slowly added to the mixture to set the pH value in the range of 2.53.0. Using an 836 Titrando titrator (Metrohm, Switzerland) a 0.005 M NaOH solution was added at a rate of 0.05 mL/min until the mixture had reached pH 11. The carboxylate content of the sample was determined from the conductivity curves using the following equation: COO VNaOH MNaOH=wcell Here COO is the carboxylate content in mmol per gram cellu