Water-soluble cellulose ether derivatives

The crosslinking mechanism, pathway and properties of different kinds of crosslinking agents and water-soluble cellulose ether were introduced. By crosslinking modification, the viscosity, rheological properties, solubility and mechanical properties of water-soluble cellulose ether can be greatly improved, so as to enhance its application performance. According to the chemical structure and properties of different crosslinkers, the types of cellulose ether crosslinking modification reactions were summarized, and the development directions of different crosslinkers in various application fields of cellulose ether were summarized. In view of the excellent performance of water-soluble cellulose ether modified by crosslinking and the few studies at home and abroad, the future crosslinking modification of cellulose ether has broad prospects for development. This is for the reference of relevant researchers and production enterprises.
Key words: crosslinking modification; Cellulose ether; Chemical structure; Solubility; Application performance

Cellulose ether due to its excellent performance, as a thickening agent, water retention agent, adhesive, binder and dispersant, protective colloid, stabilizer, suspension agent, emulsifier and film forming agent, widely used in coating, construction, petroleum, daily chemical, food and medicine and other industries. Cellulose ether mainly includes methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, ethyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose and other kinds of mixed ether. Cellulose ether is made of cotton fiber or wood fiber by alkalization, etherification, washing centrifugation, drying, grinding process prepared, the use of etherification agents generally use halogenated alkane or epoxy alkane.
However, in the application process of water-soluble cellulose ether, the probability will encounter special environment, such as high and low temperature, acid-base environment, complex ionic environment, these environments will cause the thickening, solubility, water retention, adhesion, adhesive, stable suspension and emulsification of water-soluble cellulose ether are greatly affected, and even lead to the complete loss of its functionality.
In order to improve the application performance of cellulose ether, it is necessary to conduct crosslinking treatment, using different crosslinking agents, the product performance is different. Based on the study of various types of crosslinking agents and their crosslinking methods, combined with the crosslinking technology in the industrial production process, this paper discusses the crosslinking of cellulose ether with different types of crosslinking agents, providing reference for the crosslinking modification of cellulose ether.

1.Structure and crosslinking principle of cellulose ether

Cellulose ether is a kind of cellulose derivatives, which is synthesized by ether substitution reaction of three alcohol hydroxyl groups on natural cellulose molecules and halogenated alkane or epoxide alkane. Due to the difference of substituents, the structure and properties of cellulose ether are different. The crosslinking reaction of cellulose ether mainly involves etherification or esterification of the -OH (OH on the glucose unit ring or the -OH on the substituent or the carboxyl on the substituent) and the crosslinking agent with binary or multiple functional groups, so that two or more cellulose ether molecules are linked together to form a multidimensional spatial network structure. That’s crosslinked cellulose ether.
Generally speaking, cellulose ether and crosslinking agent of aqueous solution containing more -OH such as HEC, HPMC, HEMC, MC and CMC can be etherified or esterified crosslinked. Because CMC contains carboxylic acid ions, the functional groups in the crosslinking agent can be esterified crosslinked with carboxylic acid ions.
After the reaction of -OH or -COO- in cellulose ether molecule with crosslinking agent, due to the reduction of the content of water-soluble groups and the formation of a multi-dimensional network structure in solution, its solubility, rheology and mechanical properties will be changed. By using different crosslinking agents to react with cellulose ether, the application performance of cellulose ether will be improved. Cellulose ether suitable for industrial application was prepared.

2. Types of crosslinking agents

2.1 Aldehydes crosslinking agents
Aldehyde crosslinking agents refer to organic compounds containing aldehyde group (-CHO), which are chemically active and can react with hydroxyl, ammonia, amide and other compounds. Aldehyde crosslinking agents used for cellulose and its derivatives include formaldehyde, glyoxal, glutaraldehyde, glyceraldehyde, etc. Aldehyde group can easily react with two -OH to form acetals under weakly acidic conditions, and the reaction is reversible. The common cellulose ethers modified by aldehydes crosslinking agents are HEC, HPMC, HEMC, MC, CMC and other aqueous cellulose ethers.
A single aldehyde group is crosslinked with two hydroxyl groups on the cellulose ether molecular chain, and the cellulose ether molecules are connected through the formation of acetals, forming a network space structure, so as to change its solubility. Due to the free -OH reaction between aldehyde crosslinking agent and cellulose ether, the amount of molecular hydrophilic groups is reduced, resulting in poor water solubility of the product. Therefore, by controlling the amount of crosslinking agent, moderate crosslinking of cellulose ether can delay the hydration time and prevent the product from dissolving too quickly in aqueous solution, resulting in local agglomeration.
The effect of aldehyde crosslinking cellulose ether generally depends on the amount of aldehyde, pH, the uniformity of crosslinking reaction, crosslinking time, and temperature. Too high or too low crosslinking temperature and pH will cause irreversible crosslinking due to the hemiacetal into acetal, which will lead to cellulose ether completely insoluble in water. The amount of aldehyde and the uniformity of crosslinking reaction directly affect the crosslinking degree of cellulose ether.
Formaldehyde is less used for crosslinking cellulose ether because of its high toxicity and high volatility. In the past, formaldehyde was used more in the field of coatings, adhesives, textiles, and now it is gradually replaced by low-toxicity non-formaldehyde crosslinking agents. The crosslinking effect of glutaraldehyde is better than that of glyoxal, but it has a strong pungent odor, and the price of glutaraldehyde is relatively high. In general consideration, in industry, glyoxal is commonly used to cross-link water-soluble cellulose ether to improve the solubility of products. Generally at room temperature, pH 5 ~ 7 weak acidic conditions can be carried out crosslinking reaction. After crosslinking, the hydration time and complete hydration time of cellulose ether will become longer, and the agglomeration phenomenon will be weakened. Compared with non-crosslinking products, the solubility of cellulose ether is better, and there will be no undissolved products in the solution, which is conducive to industrial application. When Zhang Shuangjian prepared hydroxypropyl methyl cellulose, the crosslinking agent glyoxal was sprayed before drying to obtain the instant hydroxypropyl methyl cellulose with a dispersion of 100%, which did not stick together when dissolving and had fast dispersion and dissolution, which solved the bundling in practical application and expanded the application field.
In the alkaline condition, the reversible process of forming acetal will be broken, the hydration time of the product will be shortened, and the dissolution characteristics of cellulose ether without crosslinking will be restored. During the preparation and production of cellulose ether, the crosslinking reaction of aldehydes is usually carried out after the etheration reaction process, either in the liquid phase of the washing process or in the solid phase after centrifugation. Generally, in the washing process, the crosslinking reaction uniformity is good, but the crosslinking effect is poor. However, due to the limitations of engineering equipment, the cross-linking uniformity in solid phase is poor, but the cross-linking effect is relatively better and the amount of crosslinking agent used is relatively small.
Aldehydes crosslinking agents modified water-soluble cellulose ether, in addition to improving its solubility, there are also reports that can be used to improve its mechanical properties, viscosity stability and other properties. For example, Peng Zhang used glyoxal to crosslink with HEC, and explored the influence of crosslinking agent concentration, crosslinking pH and crosslinking temperature on the wet strength of HEC. The results show that under the optimal crosslinking condition, the wet strength of HEC fiber after crosslinking is increased by 41.5%, and its performance is significantly improved. Zhang Jin used water-soluble phenolic resin, glutaraldehyde and trichloroacetaldehyde to crosslink CMC. By comparing the properties, the solution of water-soluble phenolic resin crosslinked CMC had the least viscosity reduction after high temperature treatment, that is, the best temperature resistance.
2.2 Carboxylic acid crosslinking agents
Carboxylic acid crosslinking agents refer to polycarboxylic acid compounds, mainly including succinic acid, malic acid, tartaric acid, citric acid and other binary or polycarboxylic acids. Carboxylic acid crosslinkers were first used in crosslinking fabric fibers to improve their smoothness. The crosslinking mechanism is as follows: the carboxyl group reacts with the hydroxyl group of cellulose molecule to produce esterified crosslinked cellulose ether. Welch and Yang et al. were the first to study the crosslinking mechanism of carboxylic acid crosslinkers. The crosslinking process was as follows: under certain conditions, the two adjacent carboxylic acid groups in carboxylic acid crosslinkers first dehydrated to form cyclic anhydride, and the anhydride reacted with OH in cellulose molecules to form crosslinked cellulose ether with a network spatial structure.
Carboxylic acid crosslinking agents generally react with cellulose ether containing hydroxyl substituents. Because carboxylic acid crosslinking agents are water-soluble and non-toxic, they have been widely used in the study of wood, starch, chitosan and cellulose in recent years
Derivatives and other natural polymer esterification crosslinking modification, so as to improve the performance of its application field.
Hu Hanchang et al. used sodium hypophosphite catalyst to adopt four polycarboxylic acids with different molecular structures: Propane tricarboxylic acid (PCA), 1,2,3, 4-butane tetracarboxylic acid (BTCA), cis-CPTA, cis-CHHA (Cis-ChHA) were used to finish cotton fabrics. The results showed that the circular structure of polycarboxylic acid finishing cotton fabric has better crease recovery performance. Cyclic polycarboxylic acid molecules are potentially effective crosslinking agents because of their greater rigidity and better crosslinking effect than chain carboxylic acid molecules.
Wang Jiwei et al. used the mixed acid of citric acid and acetic anhydride to make esterification and crosslinking modification of starch. By testing the properties of water resolution and paste transparency, they concluded that esterified crosslinked starch had better freeze-thaw stability, lower paste transparency and better viscosity thermal stability than starch.
Carboxylic acid groups can improve their solubility, biodegradability and mechanical properties after esterification crosslinking reaction with the active -OH in various polymers, and carboxylic acid compounds have non-toxic or low-toxic properties, which has broad prospects for the crosslinking modification of water-soluble cellulose ether in food grade, pharmaceutical grade and coating fields.
2.3 Epoxy compound crosslinking agent
Epoxy crosslinking agent contains two or more epoxy groups, or epoxy compounds containing active functional groups. Under the action of catalysts, epoxy groups and functional groups react with the -OH in organic compounds to generate macromolecules with network structure. Therefore, it can be used for the crosslinking of cellulose ether.
The viscosity and mechanical properties of cellulose ether can be improved by epoxy crosslinking. Epoxides were first used to treat fabric fibers and showed good finishing effect. However, there are few reports on the cross-linking modification of cellulose ether by epoxides. Hu Cheng et al developed a new multifunctional epoxy compound crosslinker: EPTA, which improved the wet elastic recovery Angle of real silk fabrics from 200º before treatment to 280º. Moreover, the positive charge of the crosslinker significantly increased the dyeing rate and absorption rate of real silk fabrics to acid dyes. The epoxy compound crosslinking agent used by Chen Xiaohui et al. : polyethylene glycol diglycidyl ether (PGDE) is crosslinked with gelatin. After crosslinking, gelatin hydrogel has excellent elastic recovery performance, with the highest elastic recovery rate up to 98.03%. Based on the studies on the cross-linking modification of natural polymers such as fabric and gelatin by central oxides in literature, the cross-linking modification of cellulose ether with epoxides also has a promising prospect.
Epichlorohydrin (also known as epichlorohydrin) is a commonly used crosslinking agent for the treatment of natural polymer materials containing -OH, -NH2 and other active groups. After epichlorohydrin crosslinking, the viscosity, acid and alkali resistance, temperature resistance, salt resistance, shear resistance and mechanical properties of the material will be improved. Therefore, the application of epichlorohydrin in cellulose ether crosslinking has great research significance. For example, Su Maoyao made a highly adsorbent material by using epiclorohydrin crosslinked CMC. He discussed the influence of material structure, degree of substitution and degree of crosslinking on the adsorption properties, and found that the water retention value (WRV) and brine retention value (SRV) of the product made with about 3% crosslinking agent increased by 26 times and 17 times, respectively. When Ding Changguang et al. prepared extremely viscous carboxymethyl cellulose, epichlorohydrin was added after etherification for crosslinking. By comparison, the viscosity of the crosslinked product was up to 51% higher than that of the uncrosslinked product.
2.4 Boric acid crosslinking agents
Boric crosslinking agents mainly include boric acid, borax, borate, organoborate and other borate-containing crosslinking agents. The crosslinking mechanism is generally believed to be that boric acid (H3BO3) or borate (B4O72-) forms tetrahydroxy borate ion (B(OH)4-) in the solution, and then dehydrates with the -Oh in the compound. Form a crosslinked compound with a network structure.
Boric acid crosslinkers are widely used as auxiliaries in medicine, glass, ceramics, petroleum and other fields. The mechanical strength of the material treated with boric acid crosslinking agent will be improved, and it can be used for the crosslinking of cellulose ether, so as to improve its performance.
In the 1960s, inorganic boron (borax, boric acid and sodium tetraborate, etc.) was the main crosslinking agent used in water-based fracturing fluid development of oil and gas fields. Borax was the earliest crosslinking agent used. Due to the shortcomings of inorganic boron, such as short crosslinking time and poor temperature resistance, the development of organoboron crosslinking agent has become a research hotspot. The research of organoboron began in the 1990s. Due to its characteristics of high temperature resistance, easy to break glue, controllable delayed crosslinking, etc., organoboron has achieved good application effect in oil and gas field fracturing. Liu Ji et al. developed a polymer crosslinking agent containing phenylboric acid group, the crosslinking agent mixed with acrylic acid and polyol polymer with succinimide ester group reaction, the resulting biological adhesive has excellent comprehensive performance, can show good adhesion and mechanical properties in a humid environment, and can be more simple adhesion. Yang Yang et al. produced a high temperature resistant zirconium boron crosslinking agent, which was used to cross-link the guanidine gel base fluid of fracturing fluid, and greatly improved the temperature and shear resistance of the fracturing fluid after cross-linking treatment. The modification of carboxymethyl cellulose ether by boric acid crosslinking agent in petroleum drilling fluid has been reported. Because of its special structure, it can be used in medicine and construction
Crosslinking of cellulose ether in construction, coating and other fields.
2.5 Phosphide crosslinking agent
Phosphates crosslinking agents mainly include phosphorus trichloroxy (phosphoacyl chloride), sodium trimetaphosphate, sodium tripolyphosphate, etc. The crosslinking mechanism is that P-O bond or P-Cl bond is esterified with the molecular -OH in aqueous solution to produce diphosphate, forming a network structure.
Phosphide crosslinking agent due to non-toxic or low toxicity, widely used in food, medicine polymer material crosslinking modification, such as starch, chitosan and other natural polymer crosslinking treatment. The results show that the gelatinization and swelling properties of starch can be significantly changed by adding a small amount of phosphide crosslinking agent. After starch crosslinking, the gelatinization temperature increases, the paste stability improves, the acid resistance is better than the original starch, and the film strength increases.
There are also many studies on chitosan crosslinking with phosphide crosslinking agent, which can improve its mechanical strength, chemical stability and other properties. At present, there are no reports on the use of phosphide crosslinking agent for cellulose ether crosslinking treatment. Because cellulose ether and starch, chitosan and other natural polymers contain more active -OH, and phosphide crosslinking agent has non-toxic or low toxicity physiological properties, its application in cellulose ether crosslinking research also has potential prospects. Such as CMC used in food, toothpaste grade field with phosphide crosslinking agent modification, can improve its thickening, rheological properties. MC, HPMC and HEC used in the field of medicine can be improved by phosphide crosslinking agent.
2.6 Other crosslinking agents
The above aldehydes, epoxides and cellulose ether crosslinking belong to etherification crosslinking, carboxylic acid, boric acid and phosphide crosslinking agent belong to esterification crosslinking. In addition, the crosslinking agents used for cellulose ether crosslinking also include isocyanate compounds, nitrogen hydroxymethyl compounds, sulfhydryl compounds, metal crosslinking agents, organosilicon crosslinking agents, etc. The common characteristics of its molecular structure is that the molecule contains multiple functional groups that are easy to react with -OH, and can form a multi-dimensional network structure after crosslinking. The properties of crosslinking products are related to the type of crosslinking agent, crosslinking degree and crosslinking conditions.
Badit · Pabin · Condu et al. used toluene diisocyanate (TDI) to crosslink methyl cellulose. After crosslinking, the glass transition temperature (Tg) increased with the increase of the percentage of TDI, and the stability of its aqueous solution improved. TDI is also commonly used for crosslinking modification in adhesives, coatings and other fields. After modification, the adhesive property, temperature resistance and water resistance of the film will be improved. Therefore, TDI can improve the performance of cellulose ether used in construction, coatings and adhesives by crosslinking modification.
Disulfide crosslinking technology is widely used in the modification of medical materials and has certain research value for the crosslinking of cellulose ether products in the field of medicine. Shu Shujun et al. coupled β-cyclodextrin with silica microspheres, crosslinked mercaptoylated chitosan and glucan through gradient shell layer, and removed silica microspheres to obtain disulfide crosslinked nanocapses, which showed good stability in simulated physiological pH.
Metal crosslinking agents are mainly inorganic and organic compounds of high metal ions such as Zr(IV), Al(III), Ti(IV), Cr(III) and Fe(III). High metal ions are polymerized to form multi-nuclear hydroxyl bridge ions through hydration, hydrolysis and hydroxyl bridge. It is generally believed that the cross-linking of high-valence metal ions is mainly through multi-nucleated hydroxyl bridging ions, which are easy to combine with carboxylic acid groups to form multi-dimensional spatial structure polymers. Xu Kai et al. studied the rheological properties of Zr(IV), Al(III), Ti(IV), Cr(III) and Fe(III) series high-priced metal cross-linked carboxymethyl hydroxypropyl cellulose (CMHPC) and the thermal stability, filtration loss, suspended sand capacity, glue-breaking residue and salt compatibility after application. The results showed that, The metal crosslinker has the properties required for the cementing agent of oil well fracturing fluid.

3. Performance improvement and technical development of cellulose ether by crosslinking modification

3.1 Paint and construction
Cellulose ether mainly HEC, HPMC, HEMC and MC are more used in the field of construction, coating, this kind of cellulose ether must have good water resistance, thickening, salt and temperature resistance, shear resistance, often used in cement mortar, latex paint, ceramic tile adhesive, exterior wall paint, lacquer and so on. Due to the building, coating field requirements of materials must have good mechanical strength and stability, generally choose etherification type crosslinking agent to cellulose ether crosslinking modification, such as the use of epoxy halogenated alkane, boric acid crosslinking agent for its crosslinking, can improve product viscosity, salt and temperature resistance, shear resistance and mechanical properties.
3.2 Fields of medicine, food and daily chemicals
MC, HPMC and CMC in water-soluble cellulose ether are often used in pharmaceutical coating materials, pharmaceutical slow-release additives and liquid pharmaceutical thickener and emulsion stabilizer. CMC can also be used as emulsifier and thickener in yogurt, dairy products and toothpaste. HEC and MC are used in daily chemical field to thicken, disperse and homogenize. Because the field of medicine, food and daily chemical grade needs materials safe and non-toxic, therefore, for this kind of cellulose ether can be used phosphoric acid, carboxylic acid crosslinking agent, sulfhydryl crosslinking agent, etc., after crosslinking modification, can improve the viscosity of the product, biological stability and other properties.
HEC is rarely used in the fields of medicine and food, but because HEC is a non-ionic cellulose ether with strong solubility, it has its unique advantages over MC, HPMC and CMC. In the future, it will be crosslinked by safe and non-toxic crosslinking agents, which will have great development potential in the fields of medicine and food.
3.3 Oil drilling and production areas
CMC and carboxylated cellulose ether are commonly used as industrial drilling mud treatment agent, fluid loss agent, thickening agent to use. As a non-ionic cellulose ether, HEC is also widely used in the field of oil drilling due to its good thickening effect, strong sand suspension capacity and stability, heat resistance, high salt content, low pipeline resistance, less liquid loss, fast rubber breaking and low residue. At present, more research is the use of boric acid crosslinking agents and metal crosslinking agents to modify CMC used in oil drilling field, non-ionic cellulose ether crosslinking modification research reports less, but the hydrophobic modification of non-ionic cellulose ether, showing significant viscosity, temperature and salt resistance and shear stability, good dispersion and resistance to biological hydrolysis. After being crosslinked by boric acid, metal, epoxide, epoxy halogenated alkanes and other crosslinking agents, cellulose ether used in oil drilling and production has improved its thickening, salt and temperature resistance, stability and so on, which has a great application prospect in the future.
3.4 Other Fields
Cellulose ether due to thickening, emulsification, film forming, colloidal protection, moisture retention, adhesion, anti-sensitivity and other excellent properties, more widely used, in addition to the above fields, also used in papermaking, ceramics, textile printing and dyeing, polymerization reaction and other fields. According to the requirements of material properties in various fields, different crosslinking agents can be used for crosslinking modification to meet the application requirements. In general, crosslinked cellulose ether can be divided into two categories: etherified crosslinked cellulose ether and esterified crosslinked cellulose ether. Aldehydes, epoxides and other crosslinkers react with the -Oh on cellulose ether to form ether-oxygen bond (-O-), which belongs to etherification crosslinkers. Carboxylic acid, phosphide, boric acid and other crosslinking agents react with the -OH on cellulose ether to form ester bonds, belonging to esterification crosslinking agents. The carboxyl group in CMC reacts with the -OH in the crosslinking agent to produce esterified crosslinked cellulose ether. Currently, there are few researches on this kind of crosslinking modification, and there is still room for development in the future. Because the stability of ether bond is better than that of ester bond, ether type crosslinked cellulose ether has stronger stability and mechanical properties. According to the different application fields, appropriate crosslinking agent can be selected for cellulose ether crosslinking modification, in order to obtain products that meet the needs of application.

4. Conclusion

At present, the industry uses glyoxal to crosslink cellulose ether, in order to delay the dissolution time, to solve the problem of product caking during dissolution. Glyoxal crosslinked cellulose ether can only change its solubility, but has no obvious improvement on other properties. At present, the use of other crosslinking agents other than glyoxal for cellulose ether crosslinking is rarely studied. Because cellulose ether is widely used in oil drilling, construction, coating, food, medicine and other industries, its solubility, rheology, mechanical properties play a crucial role in its application. Through crosslinking modification, it can improve its application performance in various fields, so as to meet the application needs. For example, carboxylic acid, phosphoric acid, boric acid crosslinking agent for cellulose ether esterification can improve its application performance in the field of food and medicine. However, aldehydes cannot be used in food and medicine industry because of their physiological toxicity. Boric acid and metal crosslinking agents are helpful to improve the performance of oil and gas fracturing fluid after crosslinking cellulose ether used in oil drilling. Other alkyl crosslinking agents, such as epichlorohydrin, can improve the viscosity, rheological properties and mechanical properties of cellulose ether. With the continuous development of science and technology, the requirements of various industries for material properties are constantly improving. In order to meet the performance requirements of cellulose ether in various application fields, the future research on cellulose ether crosslinking has broad prospects for development.