Nanomaterials 2023 , 13 , 2536
2of 17
minimize these limitations, new approaches are being developed based on the in situ production and use of fit-for-use CNFs. Although CNFs can be obtained from a wide variety of cellulose sources such as wood (hardwood and softwood), seed fibers (cotton, coir, etc.), bast fibers (flax, hemp, jute, kenaf, ramie, etc.), grasses (bagasse, bamboo, etc.), marine animals (tunicate), algae, fungi, invertebrates, and bacteria [8], the trend for in situ production is focused on the utilization of virgin and secondary pulps, depending on the plant location [6]. Although virgin pulps have been widely studied for CNF production at industrial scale, further knowledge related to the use of recycled pulp is still necessary. The methods used to obtain CNFs include a variety of fabrication techniques, each of which is categorized using different labels like top-down and bottom-up approaches, or ac- cording to their nature, which includes physical, chemical, and biological methods, or even depending on whether they involve spinning or non-spinning techniques [9]. Spinning methods can be further categorized into electrospinning techniques, which utilize electric voltage for fiber morphology control; and alternative spinning approaches which employ forces like pressurized air (bubble electrospinning) or centrifugal forces (centrifugal spin- ning) [10]. Among all of them, physical methods (top-down approaches) based on intensive mechanical pressure (high-pressure homogenization, microfluidization, grinding, refining, or milling) are commonly used to obtain CNFs from virgin and secondary cellulose fibers. Prior to this mechanical treatment, various pretreatments, including chemical, enzymatic, mechanical, or combined approaches, can be employed to enhance nanofibrillation and the separation of the individual nanofibers. Furthermore, the pretreatments contribute to reduce the energy demand of the subsequent mechanical treatment [11]. On the other hand, the final properties of the CNFs are influenced by several factors, including the cellulose raw material used, the type and intensity of the pretreatment, and the type and severity of the mechanical defibrillation process. These variables collectively determine the final characteristics of the produced CNFs [12]. Different properties are required based on the final application. For example, the medical and electronic fields require CNFs with high purity, fibrillation yield, and homogeneity [13,14] and transparent nanopaper production requires high transmittance [15], but these properties are not always key for reinforcing some composites or paper products [6]. There are several studies related to the use of recovered paper to obtain CNFs at lab scale. Some researchers have employed different types of recycled cellulosic materials, such as recycled newspapers [16]; recycled pulp [17]; waste paper [18,19]; old corrugated boards [20,21] or recycled milk-container board [22]. In these studies, CNFs were obtained through ultrafine grinding or sonication as mechanical treatments, without the application of any pretreatment. Only Ukkola et al. (2021) pretreated the pulps in a deep eutectic solvent solution based on choline chloride and urea to obtain nanofoams from crosslinked CNFs [22]. Recent studies have focused on utilizing deinking pulp (DIP) as raw material to obtain CNFs. Le Van et al. (2018) successfully obtained CNFs by employing TEMPO- mediated oxidation followed by homogenization in a digital homogenizer mixer [23]. Ang et al. (2020 and 2021) produced CNFs through a combination of mechanical pretreat- ment (PFI refining) and high-pressure homogenization (HPH) [24,25]. Zambrano et al. (2021) obtained CNFs solely through ultrafine grinding using DIP as raw material [26]. Balea et al. (2019) produced CNFs from two different types of recycled pulps, old newsprint (ONP) and old corrugated container (OCC), at different TEMPO-mediated oxidation lev- els prior to the homogenization process [6]. The state of the art shows a fragmentated knowledge from which is not possible to produce fit-for-use CNFs. The effect of different pretreatments and homogenization conditions have been stud- ied for virgin bleached softwood and hardwood chemical pulps [27] and thermomechan- ical pulp [28] which also determined the energy applied for the nanofibrillation process. However, to the best of our knowledge, the impact of treatment intensity, involving var- ious pretreatment methods and homogenization conditions applied to recovered paper-
Made with FlippingBook Digital Proposal Creator