Cellulose ether is a kind of cellulose derivative obtained by etherification modification of cellulose. It is widely used due to its excellent thickening, emulsification, suspension, film formation, protective colloid, moisture retention, and adhesion properties. It plays an important role in the development of the national economy in scientific research and industrial sectors such as food, medicine, papermaking, coatings, building materials, oil recovery, textiles and electronic components. In this paper, the research progress of etherification modification of cellulose is reviewed.
Cellulose ether is the most abundant organic polymer in nature. It is renewable, green and biocompatible. It is an important basic raw material for chemical engineering. According to the different substituents on the molecule obtained from the etherification reaction, it can be divided into single ethers and mixed cellulose ethers. Here we reviews the research progress on the synthesis of single ethers, including alkyl ethers, hydroxyalkyl ethers, carboxyalkyl ethers, and mixed ethers.
Key words: cellulose ether, etherification, single ether, mixed ether, research progress
The etherification reaction of cellulose ether is the most important cellulose derivatization reaction.Etherification of cellulose is a series of derivatives produced by the reaction of hydroxyl groups on cellulose molecular chains with alkylating agents under alkaline conditions. There are many kinds of cellulose ether products, which can be divided into single ethers and mixed ethers according to the different substituents on the molecules obtained from the etherification reaction. Single ethers can be divided into alkyl ethers, hydroxyalkyl ethers and carboxyalkyl ethers, and mixed ethers refer to ethers with two or more groups connected in the molecular structure. Among the cellulose ether products, carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC) are represented, among which Some products have been commercialized.
2.1 Synthesis of a single ether
Single ethers include alkyl ethers (such as ethyl cellulose, propyl cellulose, phenyl cellulose, cyanoethyl cellulose, etc.), hydroxyalkyl ethers (such as hydroxymethyl cellulose, hydroxyethyl cellulose, etc. ), carboxyalkyl ethers (such as carboxymethyl cellulose, carboxyethyl cellulose, etc.).
2.1.1 Synthesis of alkyl ethers
Berglund et al first treated cellulose with NaOH solution added with ethyl chloride, then added methyl chloride at a temperature of 65°C to 90°C and a pressure of 3bar to 15bar, and reacted to produce methyl cellulose ether. This method can be highly efficient To obtain water-soluble methyl cellulose ethers with different degrees of substitution.
Ethylcellulose is a white thermoplastic granule or powder. General commodities contain 44%~49% ethoxy. Soluble in most organic solvents, insoluble in water. the pulp or cotton linters with 40%~50% sodium hydroxide aqueous solution, and the alkalized cellulose was ethoxylated with ethyl chloride to produce ethyl cellulose. successfully synthesized ethyl cellulose (EC) with an ethoxy content of 43.98% by a one-step method by reacting cellulose with excess ethyl chloride and sodium hydroxide, using toluene as a diluent. Toluene was used as the diluent in the experiment. During the etherification reaction, it can not only promote the diffusion of ethyl chloride to the alkali cellulose, but also dissolve the highly substituted ethyl cellulose. During the reaction, the unreacted part can be continuously exposed, making the etherification agent It is easy to invade, so that the ethylation reaction changes from heterogeneous to homogeneous, and the distribution of substituents in the product is more uniform.
used ethyl bromide as etherification agent and tetrahydrofuran as diluent to synthesize ethyl cellulose (EC), and characterized the product structure by infrared spectroscopy, nuclear magnetic resonance and gel permeation chromatography. It is calculated that the degree of substitution of the synthesized ethyl cellulose is about 2.5, the molecular mass distribution is narrow, and it has good solubility in organic solvents.
cyanoethyl cellulose (CEC) through homogeneous and heterogeneous methods using cellulose with different degrees of polymerization as raw materials, and prepared dense CEC membrane materials by solution casting and hot pressing. Porous CEC membranes were prepared by solvent-induced phase separation (NIPS) technology, and barium titanate/cyanoethyl cellulose (BT/CEC) nanocomposite membrane materials were prepared by NIPS technology, and their structures and properties were studied.
used the self-developed cellulose solvent (alkali/urea solution) as the reaction medium to homogeneously synthesize cyanoethyl cellulose (CEC) with acrylonitrile as the etherification agent, and conducted research on the structure, properties and applications of the product. study in depth. And by controlling different reaction conditions, a series of CECs with DS values ranging from 0.26 to 1.81 can be obtained.
2.1.2 Synthesis of hydroxyalkyl ethers
Fan Junlin et al prepared hydroxyethyl cellulose (HEC) in a 500 L reactor using refined cotton as raw material and 87.7% isopropanol-water as solvent by one-step alkalization, step-by-step neutralization and step-by-step etherification. . The results showed that the prepared hydroxyethyl cellulose (HEC) had a molar substitution MS of 2.2-2.9, reaching the same quality standard as the commercial grade Dows 250 HEC product with a molar substitution of 2.2-2.4. Using the HEC in the production of latex paint can improve the film-forming and leveling properties of the latex paint.
Liu Dan and others discussed the preparation of quaternary ammonium salt cationic hydroxyethyl cellulose by the semi-dry method of hydroxyethyl cellulose (HEC) and 2,3-epoxypropyltrimethylammonium chloride (GTA) under the action of alkali catalysis. ether conditions. The effect of adding cationic hydroxyethyl cellulose ether on paper was investigated. The experimental results show that: in bleached hardwood pulp, when the substitution degree of cationic hydroxyethyl cellulose ether is 0.26, the total retention rate increases by 9%, and the water filtration rate increases by 14%; in bleached hardwood pulp, when When the amount of cationic hydroxyethyl cellulose ether is 0.08% of the pulp fiber, it has a significant reinforcing effect on paper; the greater the degree of substitution of cationic cellulose ether, the greater the cationic charge density, and the better the reinforcing effect.
Zhanhong uses the liquid-phase synthesis method to prepare hydroxyethyl cellulose with a viscosity value of 5×104mPa·s or more and an ash value of less than 0.3% through the two-step process of alkalization and etherification. Two alkalization methods were used. The first method is to use acetone as a diluent. The cellulose raw material is directly basified in a certain concentration of sodium hydroxide aqueous solution. After the basification reaction is carried out, an etherification agent is added to directly carry out the etherification reaction. The second method is that the cellulose raw material is alkalized in an aqueous solution of sodium hydroxide and urea, and the alkali cellulose prepared by this method must be squeezed to remove excess lye before the etherification reaction. The experimental results show that factors such as the selected diluent amount, the amount of ethylene oxide added, the alkalization time, the temperature and time of the first reaction, and the temperature and time of the second reaction all have a great influence on the performance of the product.
Xu Qin et al. carried out etherification reaction of alkali cellulose and propylene oxide, and synthesized hydroxypropyl cellulose (HPC) with low substitution degree by gas-solid phase method. The effects of mass fraction of propylene oxide, squeeze ratio and etherification temperature on the degree of etherification of HPC and effective utilization of propylene oxide were studied. The results showed that the optimum synthesis conditions of HPC were propylene oxide mass fraction 20% (mass ratio to cellulose), alkali cellulose extrusion ratio 3.0, and etherification temperature 60°C. The structure test of HPC by nuclear magnetic resonance shows that the degree of etherification of HPC is 0.23, the effective utilization rate of propylene oxide is 41.51%, and the cellulose molecular chain is successfully connected with hydroxypropyl groups.
Kong Xingjie et al. prepared hydroxypropyl cellulose with ionic liquid as a solvent to realize the homogeneous reaction of cellulose so as to realize the regulation of the reaction process and products. During the experiment, the synthetic imidazole phosphate ionic liquid 1, 3-diethylimidazole diethyl phosphate was used to dissolve microcrystalline cellulose, and hydroxypropyl cellulose was obtained through alkalization, etherification, acidification, and washing.
2.1.3 Synthesis of carboxyalkyl ethers
The most typical carboxymethyl cellulose is carboxymethyl cellulose (CMC). The aqueous solution of carboxymethyl cellulose has the functions of thickening, film forming, bonding, water retention, colloid protection, emulsification and suspension, and is widely used in washing. Pharmaceuticals, food, toothpaste, textiles, printing and dyeing, papermaking, petroleum, mining, medicine, ceramics, electronic components, rubber, paint, pesticides, cosmetics, leather, plastics and oil drilling, etc.
In 1918, German E. Jansen invented the synthesis method of carboxymethyl cellulose. In 1940, the Kalle factory of the German I.G. Farbeninaustrie Company realized industrial production. In 1947, the Wyandotle Chemical Company of the United States successfully developed a continuous production process. my country first put into CMC industrial production in Shanghai Celluloid Factory in 1958. Carboxymethyl cellulose is a cellulose ether produced from refined cotton under the action of sodium hydroxide and chloroacetic acid. Its industrial production methods can be divided into two categories: water-based method and solvent-based method according to different etherification media. The process using water as the reaction medium is called the water medium method, and the process containing an organic solvent in the reaction medium is called the solvent method.
With the deepening of research and the advancement of technology, new reaction conditions have been applied to the synthesis of carboxymethyl cellulose, and the new solvent system has a significant impact on the reaction process or product quality. Olaru et al. found that the carboxymethylation reaction of cellulose using ethanol-acetone mixed system is better than that of ethanol or acetone alone. Nicholson et al. In the system, CMC with low degree of substitution was prepared. Philipp et al prepared highly substituted CMC with N-methylmorpholine-N oxide and N, N dimethylacetamide/lithium chloride solvent systems respectively. Cai et al. developed a method for preparing CMC in NaOH/urea solvent system. Ramos et al. used the DMSO/tetrabutylammonium fluoride ionic liquid system as a solvent to carboxymethylate the cellulose raw material refined from cotton and sisal, and obtained a CMC product with a substitution degree as high as 2.17. Chen Jinghuan et al. used cellulose with high pulp concentration (20%) as raw material, sodium hydroxide and acrylamide as modification reagents, carried out carboxyethylation modification reaction at set time and temperature, and finally obtained carboxyethyl base cellulose. The carboxyethyl content of the modified product can be regulated by changing the amount of sodium hydroxide and acrylamide.
2.2 Synthesis of mixed ethers
Hydroxypropyl methyl cellulose ether is a kind of non-polar cellulose ether soluble in cold water obtained from natural cellulose through alkalization and etherification modification. It is alkalized with sodium hydroxide solution and added a certain amount of Amount of isopropanol and toluene solvent, the etherification agent that adopts is methyl chloride and propylene oxide.
Dai Mingyun et al. used hydroxyethyl cellulose (HEC) as the backbone of the hydrophilic polymer, and grafted the hydrophobizing agent butyl glycidyl ether (BGE) onto the backbone by etherification reaction to adjust the hydrophobic group butyl group. The degree of substitution of the group, so that it has a suitable hydrophilic-lipophilic balance value, and a temperature-responsive 2-hydroxy-3-butoxypropyl hydroxyethyl cellulose (HBPEC) is prepared; a temperature-responsive property is prepared The cellulose-based functional materials provide a new way for the application of functional materials in the fields of drug sustained release and biology.
Chen Yangming and others used hydroxyethyl cellulose as a raw material, and in the isopropanol solution system, added a small amount of Na2B4O7 to the reactant for homogeneous reaction to prepare mixed ether hydroxyethyl carboxymethyl cellulose. The product is instant in water, and The viscosity is stable.
Wang Peng uses natural cellulose refined cotton as the basic raw material, and uses a one-step etherification process to produce carboxymethyl hydroxypropyl cellulose with uniform reaction, high viscosity, good acid resistance and salt resistance through alkalization and etherification reactions Compound ether. Using one-step etherification process, the produced carboxymethyl hydroxypropyl cellulose has good salt resistance, acid resistance and solubility. By changing the relative amounts of propylene oxide and chloroacetic acid, products with different carboxymethyl and hydroxypropyl contents can be prepared. The test results show that the carboxymethyl hydroxypropyl cellulose produced by one-step method has a short production cycle, low solvent consumption, and the product has excellent resistance to monovalent and divalent salts and good acid resistance. Compared with other cellulose ether products, it has stronger competitiveness in the fields of food and oil exploration.
Hydroxypropylmethylcellulose (HPMC) is the most versatile and best-performing variety among all kinds of cellulose, and it is also a typical representative of commercialization among mixed ethers. In 1927, hydroxypropylmethylcellulose (HPMC) was successfully synthesized and isolated. In 1938, Dow Chemical Co. of the United States realized the industrial production of methyl cellulose and created the well-known trademark “Methocel”. Large-scale industrial production of hydroxypropyl methylcellulose began in the United States in 1948. The production process of HPMC can be divided into two categories: gas phase method and liquid phase method. At present, developed countries such as Europe, America and Japan are more adopting the gas phase process, and the domestic production of HPMC is mainly based on the liquid phase process.
Zhang Shuangjian and others refined cotton powder as raw material, alkalized it with sodium hydroxide in reaction solvent medium toluene and isopropanol, etherified it with etherifying agent propylene oxide and methyl chloride, reacted and prepared a kind of instant hydroxypropyl methyl alcohol base cellulose ether.
Cellulose is an important chemical and chemical raw material that is rich in resources, green and environmentally friendly, and renewable. The derivatives of cellulose etherification modification have excellent performance, wide range of uses and excellent use effects, and meet the needs of the national economy to a large extent. And the needs of social development, with the continuous technological progress and the realization of commercialization in the future, if the synthetic raw materials and synthetic methods of cellulose derivatives can be more industrialized, they will be more fully utilized and realize a wider range of applications Value.