1.Structure and preparation principle of cellulose ether

Figure 1 shows the typical structure of cellulose ethers. Each b-D-anhydroglucose unit (the repeating unit of cellulose) replaces one group at the C (2), C (3) and C (6) positions, that is, there can be up to three ether groups. Due to the intra-chain and inter-chain hydrogen bonds of cellulose macromolecules, it is difficult to dissolve in water and almost all organic solvents. The introduction of ether groups through etherification destroys intramolecular and intermolecular hydrogen bonds, improves its hydrophilicity, and greatly improves its solubility in water media.

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Typical etherified substituents are low molecular weight alkoxy groups (1 to 4 carbon atoms) or hydroxyalkyl groups, which may then be substituted by other functional groups such as carboxyl, hydroxyl or amino groups. Substituents may be of one, two or more different kinds. Along the cellulose macromolecular chain, the hydroxyl groups on the C(2), C(3) and C(6) positions of each glucose unit are substituted in different proportions. Strictly speaking, cellulose ether generally does not have a definite chemical structure, except for those products that are completely substituted by one type of group (all three hydroxyl groups are substituted). These products can only be used for laboratory analysis and research, and have no commercial value.

(a) The general structure of two anhydroglucose units of the cellulose ether molecular chain, R1~R6=H, or an organic substituent;

(b) A molecular chain fragment of carboxymethyl hydroxyethyl cellulose, the degree of substitution of carboxymethyl is 0.5, the degree of substitution of hydroxyethyl is 2.0, and the degree of substitution of molar is 3.0. This structure represents the average substitution level of etherified groups, but the substituents are actually random.

For each substituent, the total amount of etherification is expressed by the degree of substitution DS value. The range of DS is 0~3, which is equivalent to the average number of hydroxyl groups replaced by etherification groups on each anhydroglucose unit.

For hydroxyalkyl cellulose ethers, the substitution reaction will start etherification from new free hydroxyl groups, and the degree of substitution can be quantified by the MS value, that is, the molar degree of substitution. It represents the average number of moles of etherifying agent reactant added to each anhydroglucose unit. A typical reactant is ethylene oxide and the product has a hydroxyethyl substituent. In Figure 1, the MS value of the product is 3.0.

Theoretically, there is no upper limit for the MS value. If the DS value of the degree of substitution on each glucose ring group is known, the average chain length of the ether side chainSome manufacturers also often use the mass fraction (wt%) of different etherification groups (such as -OCH3 or -OC2H4OH) to represent the substitution level and degree instead of DS and MS values. The mass fraction of each group and its DS or MS value can be converted by simple calculation.

Most cellulose ethers are water-soluble polymers, and some are also partially soluble in organic solvents. Cellulose ether has the characteristics of high efficiency, low price, easy processing, low toxicity and wide variety, and the demand and application fields are still expanding. As an auxiliary agent, cellulose ether has great application potential in various fields of industry. can be obtained by MS/DS.

Cellulose ethers are classified according to the chemical structure of the substituents into anionic, cationic and nonionic ethers. Nonionic ethers can be divided into water-soluble and oil-soluble products.

Products that have been industrialized are listed in the upper part of Table 1. The lower part of Table 1 lists some known etherification groups, which have not yet become important commercial products.

The abbreviation order of the mixed ether substituents can be named according to the alphabetical order or the level of the respective DS (MS), for example, for 2-hydroxyethyl methylcellulose, the abbreviation is HEMC, and it can also be written as MHEC to highlight the methyl substituent.

The hydroxyl groups on cellulose are not easily accessible by etherification agents, and the etherification process is usually carried out under alkaline conditions, generally using a certain concentration of NaOH aqueous solution. The cellulose is first formed into swollen alkali cellulose with NaOH aqueous solution, and then undergoes etherification reaction with etherification agent. During the production and preparation of mixed ethers, different types of etherification agents should be used at the same time, or etherification should be carried out step by step by intermittent feeding (if necessary). There are four reaction types in the etherification of cellulose, which are summarized by the reaction formula (cellulosic is replaced by Cell-OH) as follows:

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Equation (1) describes the Williamson etherification reaction. R-X is an inorganic acid ester, and X is halogen Br, Cl or sulfuric acid ester. Chloride R-Cl is generally used in industry, for example, methyl chloride, ethyl chloride or chloroacetic acid. A stoichiometric amount of base is consumed in such reactions. The industrialized cellulose ether products methyl cellulose, ethyl cellulose and carboxymethyl cellulose are the products of Williamson etherification reaction.

Reaction formula (2) is the addition reaction of base-catalyzed epoxides (such as R=H, CH3, or C2H5) and hydroxyl groups on cellulose molecules without consuming base. This reaction is likely to continue as new hydroxyl groups are generated during the reaction, leading to the formation of oligoalkylethylene oxide side chains: A similar reaction with 1-aziridine (aziridine) will form aminoethyl ether: Cell-O-CH2-CH2-NH2. Products such as hydroxyethyl cellulose, hydroxypropyl cellulose and hydroxybutyl cellulose are all products of base-catalyzed epoxidation.

Reaction formula (3) is the reaction between Cell-OH and organic compounds containing active double bonds in alkaline medium, Y is an electron-withdrawing group, such as CN, CONH2, or SO3-Na+. Today this type of reaction is rarely used industrially.

Reaction formula (4), etherification with diazoalkane has not been industrialized yet.

1. Types of cellulose ethers

Cellulose ether can be monoether or mixed ether, and its properties are different. There are low-substituted hydrophilic groups on the cellulose macromolecule, such as hydroxyethyl groups, which can endow the product with a certain degree of water solubility, while for hydrophobic groups, such as methyl, ethyl, etc., only moderate substitution High degree can give the product a certain water solubility, and the low-substituted product only swells in water or can be dissolved in dilute alkali solution. With the in-depth research on the properties of cellulose ethers, new cellulose ethers and their application fields will be continuously developed and produced, and the biggest driving force is the broad and continuously refined application market.

The general law of the influence of groups in mixed ethers on solubility properties is:

1)?Increase the content of hydrophobic groups in the product to increase the hydrophobicity of ether and lower the gel point;

2)?Increase the content of hydrophilic groups (such as hydroxyethyl groups) to increase its gel point;

3)?The hydroxypropyl group is special, and proper hydroxypropylation can lower the gel temperature of the product, and the gel temperature of the medium hydroxypropylated product will rise again, but a high level of substitution will reduce its gel point ; The reason is due to the special carbon chain length structure of the hydroxypropyl group, low-level hydroxypropylation, weakened hydrogen bonds in and between molecules in the cellulose macromolecule, and hydrophilic hydroxyl groups on the branch chains. Water is dominant. On the other hand, if the substitution is high, there will be polymerization on the side group, the relative content of the hydroxyl group will decrease, the hydrophobicity will increase, and the solubility will be reduced instead.

The production and research of cellulose ether has a long history. In 1905, Suida first reported the etherification of cellulose, which was methylated with dimethyl sulfate. Nonionic alkyl ethers were patented by Lilienfeld (1912), Dreyfus (1914) and Leuchs (1920) for water-soluble or oil-soluble cellulose ethers, respectively. Buchler and Gomberg produced benzyl cellulose in 1921, carboxymethyl cellulose was first produced by Jansen in 1918, and Hubert produced hydroxyethyl cellulose in 1920. In the early 1920s, carboxymethylcellulose was commercialized in Germany. From 1937 to 1938, the industrial production of MC and HEC was realized in the United States. Sweden started the production of water-soluble EHEC in 1945. After 1945, the production of cellulose ether expanded rapidly in Western Europe, the United States and Japan. At the end of 1957, China CMC was first put into production in Shanghai Celluloid Factory. By 2004, my country’s production capacity will be 30,000 tons of ionic ether and 10,000 tons of non-ionic ether. By 2007, it will reach 100,000 tons of ionic ether and 40,000 tons of Nonionic ether. Joint technology companies at home and abroad are also constantly emerging, and China’s cellulose ether production capacity and technical level are constantly improving.

In recent years, many cellulose monoethers and mixed ethers with different DS values, viscosities, purity and rheological properties have been continuously developed. At present, the focus of development in the field of cellulose ethers is to adopt advanced production technology, new preparation technology, new equipment, New products, high-quality products, and systematic products should be technically researched.