The pulp and paper industry is going through a transition to developing new products and markets to improve its competitiveness. The main strength of this industry is its capacity to produce high-value products along with conventional products from lignocellulosic biomass.
Cellulose and its derivatives are potential alternatives to petroleum-based polymers for flexible and rigid packaging, composites, and films, among others. The most critical segment in cellulose processing corresponds to regenerated cellulose products, primarily fibers (rayon), films, membranes, and sponges. These derivatives are obtained from the dissolving grade pulps (Gindl and Keckes 2005; Yousefi et al. 2011; Ghaderi et al. 2014). Yield, residual lignin content, brightness, and strength properties are the main characteristics of papermaking chemical pulp grade (unbleached and bleached). The most relevant requirements when it comes to dissolving pulp are high content of α-cellulose (90–99 %), low content of non-cellulosic compounds (lignin, hemicelluloses, extractives, and ash), good reactivity to derivatizing chemicals, and specified cellulose molar mass and its distribution (Gavrilescu 2013). The use of raw material with relatively low hemicellulose content, such as eucalyptus, is ideal as it produces dissolving pulp with a significantly low xylans content and high yield. Currently, the primary efforts in dissolving pulp production are focused on selective removal of hemicelluloses and increasing reactivity (Kumar and Christopher 2017).
Conventional dissolving pulp is obtained from cotton linter or wood by soda, kraft, or sulfite pulping. The main advantage of lignocellulosic waste (i.e., bagasse, straw, sawdust, others) as raw material instead of cotton linter or commercial wood pulp is the decrease in water and (or) land use.
However, industrial products from lignocellulosic wastes can only compete with those obtained from conventional raw materials, such as petroleum, if they are optimally exploited. This trend catalyzed the development of novel and integrated processes based on biorefinery concepts.
Dissolving pulps require low hemicelluloses, lignin, and extractives contents, so their production in biorefineries has considerable potential (Kumar and Christopher 2017). Traditional pulp mills can be biorefinery platforms due to the raw materials characteristics, infrastructure, processes know-how, and sustainability (renewable feedstock, integrated production, as well as heat and power generation) (Popa 2013).
Dissolving pulp is mainly used to produce viscose, but it is also a raw material for other cellulose derivatives, such as cellulose acetate, cellophane, carboxymethyl cellulose, and other high-value-added cellulose products (Chen et al. 2019). In the last years, regenerated cellulose production caused a significant growth of the dissolving pulps market (Sixta et al. 2013). Between 2016 and 2017, the global production of dissolving pulp was 8 million tons, approximately (Kumar and Christopher 2017; Chen et al. 2019). Its price is affected by the price of bleached chemical pulp, which usually is 150–300 USD more expensive than bleached chemical pulp. The United States, Canada, Brazil, and Europe produce around 60 % of global dissolving pulp production from wood (Chen et al. 2019). Currently, Asia–Pacific regions are the principal producers of viscose (80 % of the global production) due to lower labor costs and softer environmental regulations compared to Europe and the United States. Within the region, China is the largest global producer of viscose (62 % of the total), and it also imports other regenerated cellulosic fibers such as Lyocell and modal. Overcoming the cost and environmental burden of the transportation of dissolving pulps from wood sources could enhance the development of the local textile industry in various regions (Rana et al. 2014).
The challenges of dissolving cellulose demand improved processes and technology to overcome the limitations for larger-scale use and application of cellulose-based green products. The supramolecular structure of cellulose causes it to be insoluble in most common organic solvents. The dissolution of cellulose involves the disruption of a strong hydrogen bonding network, both inter-and intramolecular and different dissolution alternatives exist. While direct cellulose dissolution using non-derivatizing solvents breaks the hydrogen bonds and avoids their re-conformation, derivatizing solvents modify the –OH groups, thus avoiding the hydrogen bonding formation.
The viscose process is the most used to dissolve and mold cellulose. In this process, the cellulosic pulp is treated with CS2 to obtain cellulose xanthate - a metastable intermediate - solubilized in dilute aqueous alkali to form the viscose solution. For the molded products, the substituents are removed from the viscose solution, and then regenerated into highly pure cellulose. This process is highly efficient and fully known but it poses great environmental concerns. The use of CS2 causes sulfur products to be released during the cellulose regeneration, creating contamination at both atmospheric and aqueous levels, which are very expensive and difficult to remove (Vo et al. 2010; Chen et al. 2016). These economic and environmental reasons have driven the manufacturers of regenerated cellulose products to continuously search for alternative manufacturing practices more environmentally friendly processes (Liu et al. 2016). NaOH and urea mixtures at low temperatures have been heavily investigated in the last years, showing improved performance than in the case of dissolving cellulose using pure NaOH. This process causes the fast dissolution of native cellulose with the additional advantage of alkali recovery, avoiding contaminants generation (Chen et al. 2016; Liu et al. 2016).
The conventional cellulose dissolution method involves the following stages: (i) cellulose activation, (ii) formation of intermediates, (iii) processing and shaping, and (iv) regeneration (Gericke et al. 2013). The most commonly used solvents for the direct dissolution of cellulose are LiCl/DMAc, NMMO, NaOH, and BmimCl (Gindl and Keckes 2005; Shibata et al. 2013). However, not all of these apply to industrial scales due to their low dissolution rates, toxicity, and inability to be recycled.
Cellulose carbamate is a cellulose derivative produced by the reaction of cellulose with urea (CO(NH2)2). The cellulose carbamate process is an environmentally friendly alternative to the viscose processes with great potential. However, its industrial application is at an early stage and it requires more research and technological developments (Pinnow et al. 2008; Zhang et al. 2019). Obtaining regenerated cellulose through the carbamate process involves several steps: (i) cellulose and urea react at high temperatures to form cellulose carbamate, (ii) cellulose carbamate is dissolved in NaOH solution, and (iii) the products are formed in a regenerating bath using diluted sulfuric acid and sodium sulfate as co-solvent media. The preparation of cellulose carbamate solution is the more critical stage of this process (Fu et al. 2014c) due to required conditions in the synthesis step (catalyst, solvents, reaction time, and temperature).
In this study, we assess the possibility of obtaining regenerated cellulose products (beads and films) from eucalyptus sawdust dissolving pulps (EPs) produced by non-conventional processes. The performance of the products was compared with a commercially available dissolving pulp. EPs were obtained by soda pulping followed by a sequential oxidative treatment (oxygen and ozone), followed by alkaline extraction. Beads and films were produced prepared by coagulating and regenerating processes. Beads were characterized by average diameter and porosity, and films by tensile strength.