# Production of Microfibrilated Cellulose from Fast-Growing
## 1 Introduction  
The social and environmental challenges resulting from the widespread consumption of fossil fuels are driving research efforts to solve the current societal problems. The general strategy is to make the bioeconomy concept a reality through the efficient usage of renewable resources in the sustainable production of chemicals, materials and energy [1]. This scenario has led to a growing interest in the design and development of different products from cellulose [2]. Cellulose is the most abundant natural polymer on Earth, where it exists as a structural component in the cell walls of plants and some marine animals, algae and fungi, as well as biofilms produced by bacteria [3]. It is a very promising polymer due to its specific properties, i.e., renewably sourced, biodegradable, biocompatible and easily modified or functionalized, which make cellulose a material with great potential for applications in different technologies and in a wide range of products. Although this biopolymer is currently used in different industrial sectors such as pulp and Paper, textiles and pharmaceuticals, among others, its structural and chemical versatility opens the door to applications in different industrial sectors (packaging, construction, separation membranes, bioplastics, bioengineering and optoelectronics) [2]. Nevertheless, in order to meet the specific requirements for these new applications, it is necessary to use cellulose at the micro/nano-scale [4],resulting in unique mechanical, optical, barrier, thermal and rheological properties that far exceed those of raw cellulose. Nanocellulose refers to long and entangled cellulose nanofibers (CNFs), as well as crystalline rod-like cellulose nanocrystals (CNCs). Regarding CNFs, their production generally encompasses two different steps, using chemical pulp fibers as the raw material [2]. Firstly, a pretreatment step is necessary to reduce fiber length, increase external fibrillation and separate fiber bundles, as well as to introduce highly charged functional groups to the fibers or to modify their chemical composition and degree of crystallinity [5]. Secondly, a nanofibrillation step using equipment such as high-pressure homogenizers or microfluidizers is necessary to further reduce the fiber length and longitudinally fibrillate the fibers to nanometer-sized diameters [4]. Unfortunately, this process consumes a lot of energy. Therefore, to achieve energy savings, it might be sufficient for some applications to reach only microscale diameters in the fibers during fibrillation [6]. The pretreatment step also contributes to avoid not only excessive energy consumption but also process-ability restrictions [4]. Different pretreatment technologies have been reported [4; 5; 7; 8], including physical (mechanical refining by PFI mills, disc refiners, Valley beaters), chemical (2,2,6,6-tetramethyl)piperidine 1-oxyl radical (TEMPO)-mediated oxidation, cationization, carboxymethyl) and biological (enzymatic hydrolysis) pretreatments. Regarding mechanical refining, a dilute pulp fiber suspension is forced through a gap between the rotor and stator discs. These discs have surfaces fitted with bars and grooves against which the fibers are subjected to repeated cyclic stresses, producing profound changes in morphology and size of the pulp fibers and, consequently, being more exposed to fibrillation during the microfluidization step [9]. As commented above, nanocellulose is typically obtained from chemical pulp fibers, using traditional woody species either from hardwoods (e.g., eucalypt, aspen and birch) or softwoods (e.g., pine, spruce and fir) [7; 10; 11]. In addition to these woody species, dedicated crops and agricultural residues may be interesting and attractive raw materials for nanocellulose production [4].