Regenerated-cellulosic fiber (RCF) is made from chemically dissolved natural cellulose (cotton linter, wood, bamboo, bagasse, etc.) following spinning treatment. It is a kind of artificial/man-made fibers without changing the chemical structure. Compared with the other man-made fiber, i.e., synthetic fiber which is produced from synthetic high molecular-weight polymers, the raw material of the RCF is natural polymers. Fig. 1 (Woodings, 2001) shows the classification of fibers and the difference of them.
Natural cellulose fiber can be dissolved by derivative and non-derivative dissolution systems. Cellulose is usually dissolved by grafting new groups onto the molecules to form a new intermediate in the derivative dissolution (Sixta, 2008). The typical derivative dissolution system is NaOH/carbon disulfide (CS2) for making viscose fibers and esterification dissolution in the production of cellulose acetate fiber. Natural cellulose suffers from mercerization, aging, and xanthation treated with CS2, the resultant cellulosic xanthate can be easily dissolved in dilute aqueous caustic soda (Treiber et al., 1962; Sayyed et al., 2019). Cellulosic derivatization will take place in the preparation of viscose spinning dope. However, a large amount of industrial discharge generated in this process will restrict the development of viscose fiber industry. About 25%–30% of CS2 would not be recovered. Approximately 20 kg of exhaust, 300–600 t of wastewater and some toxic waste residue were emitted with one-ton production of viscose fiber (Vanhoorne et al., 1991; Deo, 2001; Camper and Bott, 2006; Chen and Burns, 2006; Zhang, 2010). In the production of cellulose acetate fiber (LaNieve, 2006; Ertas and Uyar, 2017; Wang, 2018), purified cellulose fiber is treated with a mixture of acetic acid, acetic anhydride, and concentrated sulfuric acid for acetylation. After ripening, the flakes of cellulose acetate are precipitated, and then dissolved in acetone. Due to the use of acetone, the production process is environmentally unfriendly and even noxious. The merits and limitations of the viscose and cellulose acetate fibers processes are shown in Table 1 (Wang, 2010; Shaikh et al., 2012).
Merit Limitation Output (t) Viscose fiber 1) Low breaking strength, high elongation;
2) Moderate reaction; 3) Cheap
1) Heavy pollution loads;
2) Poor resiliency
4.5 million Cellulose acetate fiber 1) Excellent drapability, handle and comfort properties; 2) Good permeability and adsorbability 1) Poor physical strength;
2) Acetone pollution
800–900 thousand Cuprammonium fiber 1) Mechanical strength, abrasion resistance and softness are better than viscose; 2) Slender 1) High consumption in cuprammonium;
2) Poor acid resistant ability; 3) Expensive
Table 1. Comparison of several typical regenerated cellulosic fiber production processes
As some hagardous by-products generated in the derivative dissolution, the non-derivative one becomes the research hot-spot gradually. In the non-derivative dissolution system, cellulose can be dissolves directly in a solvent only accompanied by destroying the crystalline structure (Sayyed et al., 2019). These dissolution systems, including NaOH/H2O/dissolving promoter (for example, NaOH/H2O/urea, NaOH/H2O/ZnO, NaOH/H2O/zinc nitrate) (Luo et al., 2009; Fu et al., 2014; Wang et al., 2019), ionic liquid (Andre et al., 2010; Yuan et al., 2019), LiCl/N, N-dimethylacetamide (Ramos et al., 2011; Medronho and Lindman, 2014) and high concentration inorganic salt (for example, zinc chloride/H2O) (Thomas et al., 2000), have their own flaws, such as high cost, difficult recovery, heavy pollution, harsh dissolution condition, unstable and limited dissolution. Among them, the production of cuprammonium fiber has been industrialized. In this process, the raw materials are firstly purified by mechanical washing and chemical bleaching. After dissolved in cuprammonia, aging, deaerating, wet spinning, the cuprammonium filament are obtained (Woodings, 2001; Kotek, 2008). In addition, the characteristics of the cuprammonium manufacturing process can be found in Table 1 (Kang et al., 2011).
By comparing the characteristics of these typical and full-scale commercial processes, a new cellulose dissolution technology is desired. The dissolution mechanism of cellulose in N-methyl morpholine-N-oxide (NMMO) aqueous solution supports that the lyocell process is the most promising technology to replace the above processes, and it has been used industrially (Karimi and Taherzadeh, 2016; Sayyed et al., 2018).
The lyocell route to the RCFs is based on non-derivative dissolution of cellulose in an organic and aprotic solvent. Compared with the preparation of viscose fiber, lyocell has some advantages: 1) The NMMO/H2O solvent could directly dissolve the cellulose, without mercerization, aging, xanthation and other treatments. The whole process last for 3–4 h, and the production efficiency has been greatly improved (Liu, 1997; Mo, 2002); 2) No derivatization is involved, thus the original degree of polymerization (DP) of the cellulose could have the maximum conservation; and 3) Very few chemicals are applied, so the NMMO solvent recovery is higher than 98.5%–99.0%. In addition, the NMMO has the advantages of non-toxic, environmentally harmless, and biodegradable (Meiste and Wechsle, 1998).
The objective of this paper is aimed at reviewing the RCF with lyocell process, mainly including the raw materials, the keys and emphases of industrial preparation, the structure and properties of the lyocell.
The raw material of the RCF is defined as dissolving pulp. It is a kind of fiber pulp, with high content of cellulose and low content of other chemical components, obtained by chemical refinement/purification treatment. The raw material of dissolving pulp is mainly wood and cotton linter. The wood-based dissolving pulp mainly comes from the countries and regions rich in forest resources, e.g., Europe, U.S., Brazil and South Africa. Most dissolving pulps in China are produced from cotton linter. An undeniable fact is that the proportion of the wood-based dissolving pulp in China is increasing year by year.
The production process of regenerated cellulose fiber required certain properties and conditions of dissolving pulp, including high α-cellulose and DP, low metal ion and ash, and remarkable reactivity (or solubility) properties. According to these requirements, acid sulfite (AS) or pre-hydrolysis kraft (PHK) pulping are selected for producing wood-based dissolving pulp (Li et al., 2015). They account for 42% and 56% of the global production capacity of dissolving pulp, respectively. The PHK method, which has gradually become the mainstream technology, accounts for 78% of the process of dissolving pulp in China (Chen et al., 2015). It is a combination process of acid pre-hydrolysis with alkaline cooking. Compared with the AS, the PHK is widely adaptable to raw materials and controls the DP and viscosity easily. Besides softwood, the raw materials with high hemicellulose and resin content also can be used to produce dissolving pulp by the PHK. Fig. 2 shows a traditional industrial process of wood-based dissolving pulp produced by the PHK.
Figure 2. Traditional industrial process of wood-based dissolving pulp produced by prehydrolysis kraft pulping
The functions of each process of the PHK are as follows: 1) Pre-hydrolysis (Schild and Sixta, 2011). Removing part of hemicelluloses can destroy the primary wall of fiber, and lead to some voids opening on the cell wall. Thus, the accessibility of fiber cell wall can be improved. The subsequent reactions can be accelerated. Pre-hydrolysis involves acid pre-hydrolysis, hot water pre-hydrolysis and steam pre-hydrolysis. Steam pre-hydrolysis is generally used in the industrial production because of short heating time and rapid reaction. 2) Alkaline extraction (Ingruber et al., 1985; Liu et al., 2016). It is a refining section of the dissolving pulp production, which can dissolve the remaining hemicellulose. Cold caustic extraction and hot caustic extraction are available. The first one with the best protection for cellulose belongs to a physical purification process. It requires a high alkali concentration. As a comparison, the latter with a relatively low alkali concentration is the main method to purify dissolving pulp in the industrial production. 3) Bleaching (Vila et al., 2004; Viviana, 2010). The aim of this step is to decrease the content of the remaining lignin. The bleaching method involves oxygen delignification, hypochlorite and chlorine dioxide bleaching. The chlorine dioxide bleaching is generally used due to the selectively degradation of lignin and lightly damage of cellulose. 4) Acid treatment (Maréchal, 1993). It can remove alkali-resistant hemicellulose, ash and metal ion in the dissolving pulp, purify the cellulose to minimize the degradation of NMMO and improve the solubility of fiber.
The production processes and product features of varied RCFs have different demands on the performance of raw material. According to the researches and production experience, the requirements of the dissolving pulp for lyocell fibers are as follows:
(1) The DP and reactivity (or solubility). The DP of dissolving pulp directly affects the mechanical strength of lyocell fiber. In theory, the higher DP the better mechanical strength can be occurred. However, an excessively high DP may result in a poor solubility and an increase in the viscosity of spinning dope. How to balance the relationship between DP and the solubility of dissolving pulp has a great impact on the spinning process and the performance of corresponding fiber. The specific methods involve improving the pre-hydrolysis strength and moderating the cooking conditions. In the new process of lyocell production, a pretreatment process is used to improve solubility of lyocell fiber in the NMMO aqueous solution. It is found that the DP between 650 and 750 is better (Li and Zhuang, 2001).
(2) α-cellulose. Results of some studies have shown that the dissolving pulp which is suitable for lyocell fiber does not demand a high α-cellulose (Tong et al., 2005; Zhang, 2007). It is because that in the production of spinning dope, hemicelluloses in the dissolving pulp also can be dissolved in the NMMO aqueous solution. However, an exorbitantly high hemicellulose may reduce mechanical strength of lyocell fiber.
(3) Others. Lyocell fiber demands that dissolving pulp should be relatively pure, i.e., with low metal ions and ash. The metal ions, especially iron ions, are easy to degrade the NMMO, thus affecting the dissolution of fiber and the recovery of the NMMO. The viscosity, the filtration of spinning dope, and the stability of spinning section also will be influenced by other impurities, such as blocking the nozzle.
Project Index Project Index α-cellulose (%) ≥92 Resin (%) < 0.3 Ash (%) < 0.1 Moisture (%) ≤10 Degree of polymerization 650–750 Carboxyl group (%) < 0.2 Pentosan (%) < 2.5 Dust (mm2/m2) ≤0.2 Intrinsic viscosity (mL/g) 280–350 Iron (mg/L) < 5 Whiteness (%) ≥90 Copper content (mg/L) < 1
Table 2. Index demand of dissolving pulp for lyocell fiber
2.1. Raw materials for regenerated cellulosic fiber
2.2. Dissolving pulp for lyocell fiber
Lyocell fiber is quite different from viscose fiber and native cotton fiber in structure. The cross-sections are lace shape, roughly circular and flat for viscose, lyocell and cotton, respectively, as shown in Fig. 7 (Woodings, 2001; Yang and Yang, 2002; Gao et al., 2019b).
Figure 7. Cross sections of viscose, lyocell and cotton fibers: (a) viscose fiber; (b) lyocell fiber; (c) cotton fiber
The lyocell fiber is easy to form a skin-core structure (Zhang, 1999; Kongdee et al., 2004; Abu-Rous et al., 2007; Biganska and Navard, 2009). It seems that a thin skin wraps the fibrils together. The skin is very thin with low degree of crystallinity, and the percentage of crystallinity is about 3% in area. The core is mainly composed of numerous fibrils arranged along the fiber axis with high orientation, regular structure, low contact area and lateral cohesion between fibril bundles. Table 3 shows structural parameters for different cellulosic fibers (Schuster et al., 2004). It can be seen that the DP of lyocell is higher than that of viscose fiber.
Parameter Lyocell Ordinary viscose High-modulus viscose Cotton DP 550–650 290–320 450–500 2000 Crystallinity 0.62 0.39 0.39 0.74 Note: DP is degree of polymerization.
Table 3. Comparison of structural parameters of cellulosic fibers
Fibrillation is defined as a phenomenon that some filaments split along the fiber axis under the action of swelling, mechanics and friction, always in the wet state (Wan and Wang, 1999). Fibrillation is commonly seen in fiber, especially in lyocell owing to the weakened intermolecular binding force by swelling (Rohrer et al., 2001). Furthermore, the high ordering of the structure causes less entanglement between fibrils, which is beneficial to the separation of fibrils (Zhang, 1999). The fibrillation effect provides the imagination for the application of lyocell. Although the fibrillation has been regarded as a disadvantage for some applications, the filaments create excellent touch which is so-called "peach skin" (Periyasamy and Khanum, 2012). Some approaches are employed to improve the fibrillation, such as enzyme, easy care resins and crosslinking chemicals treatments (Reddy and Yang, 2014).
The property comparison of the lyocell and other cellulosic fibers are listed in Table 4 (Hohberg and Thumm, 1998; Perepelkin, 2007). The mechanical strength of lyocell is higher than those of viscose and cotton. These can be attributed to the difference in the structure (Schuster et al., 2004; Eva, 2008): 1) Almost no degradation of cellulose in the spinning of the lyocell type. The cellulose chain is long and the intermolecular hydrogen bonds are strong; 2) high crystallinity and a tightly contact of cellulose molecule; and 3) high orientation in both crystalline and amorphous regions of lyocell, the highly ordered cellulosic chains arrangement and the strong valence bonds.
Elongation, dry (%) Elongation, wet (%) Strength (cN/tex) Deformation modulus (GPa) Deformation modulus, wet (GPa) Lyocell 11–16 17–19 35–47 8–10 3–4.5 Viscose 18–25 21–23 20–26 3–5 0.6–1 High-modulus viscose 12–15 13–15 32–36 5–6.5 1.5–2 Cotton 8–10 12 25–40 5–9 –
Table 4. Property comparison of different kinds of cellulosic fibers