Views: 13 Author: Site Editor Publish Time: 2022-09-27 Origin: Site
The mechanism, way and changes of properties of cross-linking modification of different types of cross-linking agents and water-soluble cellulose ethers were introduced. Pass crosslinking modification can greatly improve the viscosity, rheological properties, solubility and mechanical properties of water-soluble cellulose ethers. Thereby improving its application performance. According to the chemical structure and properties of different cross-linking agents, the reaction types of cellulose ether cross-linking modification are summarized. The development direction of different cross-linking agents in various application fields of cellulose ether was concluded. In view of the excellent performance of the cross-linked modified water-soluble cellulose ether, and there are few internal and external studies, and the cross-linking modification of cellulose ethers has broad development prospects in the future. This is for the reference of relevant researchers and production enterprises.
Key words: cross-linking modification; cellulose ether; chemical structure; solubility property; application property.
Due to its excellent properties, cellulose ethers are used as thickeners, water-retaining agents, adhesives, binders and dispersants, protective colloids, stabilizers, suspending agent, emulsifier and film-forming agent, widely used in coatings, construction, petroleum, daily chemicals, food and medicine and other industries. Cellulose ethers mainly include Methyl Cellulose (MC), Hydroxyethyl Cellulose (HEC), Carboxymethyl Cellulose (CMC), Ethyl Cellulose (EC), Hydroxypropyl Methyl Cellulose (HPMC), Hydroxyethyl methylcellulose (HEMC) and various other mixed ethers. Cellulose ether is made of cotton fiber or wood fiber after alkalization, etherification, washing and separation. It is prepared by the process of core, drying and pulverization, and the etherifying agent used generally uses halogenated alkane or alkylene oxide.
However, in the application process of water-soluble cellulose ether, it is likely to encounter special environments, such as high and low temperature, acid-base environment, complex ionic environment, these environments will cause the thickening, solubility, water retention, adhesion, gum retention, stable suspension and emulsifying properties of water-soluble cellulose ethers to be affected. To a greater impact, or even lead to complete loss of its functionality.
In order to improve the application performance of cellulose ether, it needs to be cross-linked, and different cross-linking agents are used, resulting in different product properties. Based on the research on various types of cross-linking agents and their cross-linking methods, combined with the cross-linking process in the industrial production process, the research on the crosslinking of cellulose ethers is discussed, which provides a reference for the crosslinking modification of cellulose ethers.
Cellulose ether is one of cellulose derivatives, which is formed by three alcoholic hydroxyl groups on natural cellulose molecules and halogenated alkanes or alkylene oxides. It is synthesized by etherification substitution reaction. Due to the different substituents, the structure and properties of cellulose ethers are different. Crosslinking Reaction of Cellulose Ethers, it is mainly -OH on the cellulose ether molecular chain (-OH on the glucose unit ring or -OH on the substituent or carboxyl on the substituent) and has a divalent or the cross-linking agent with multifunctional groups undergoes etherification or esterification cross-linking reaction, so that two or more cellulose ether molecules are linked together to form a multi-functional cross-linking agent. The dimensional space network structure is cross-linked cellulose ether.
In general, HEC, HPMC, HEMC, MC, CMC and other aqueous solutions containing more -OH, cellulose ethers and cross-linking agents can generate ethers. Crosslinking or esterification. Since CMC contains carboxylate ions, the functional groups in the cross-linking agent can be esterified and cross-linked with carboxylate ions.
The cross-linking of cellulose ethers can be represented by the following three reaction equations (taking a cross-linking agent containing two functional groups as an example):
In the above formula, are cellulose ethers with different reactive groups are cross-linking agents, and is a functional group; are cross-linked cellulose ethers.
After the -OH or -COO- in the cellulose ether molecule reacts with the cross-linking agent, due to the reduction of the content of its water-soluble group, and the formation of the water in the solution. It forms a multi-dimensional network structure, and its solubility, rheology, mechanical properties, etc. will be changed. By using different cross-linking agents to react with cellulose ether, to improve the application properties of cellulose ethers and prepare cellulose ethers that meet the needs of industrial applications.
Aldehyde cross-linking agents refer to organic compounds containing aldehyde groups (-CHO), which are more active in chemical properties and can react with hydroxyl, ammonia, amide, etc. Compound to react. Aldehyde cross-linking agents for cellulose and its derivatives include formaldehyde, glyoxal, glutaraldehyde, glyceraldehyde, and the like. Aldehyde group in weak acid it is easy to react with two -OH under the conditions to generate acetal, and the reaction is reversible. Commonly used aldehyde cross-linking agent for cross-linking modification ，Cellulose ethers include HEC, HPMC, HEMC, MC, CMC and other aqueous cellulose ethers. The description of its cross-linking reaction process is shown in formula (4) .
It can be seen from the above formula that a single aldehyde group undergoes a cross-linking reaction with two hydroxyl groups on the molecular chain of cellulose ether, and the cellulose ether molecule is linked by the formation of acetal. Knot, forming a network-like space structure, thereby changing its solubility. Due to the reaction of the aldehyde cross-linking agent with the free -OH in the cellulose ether, leading to poor water solubility of the product, therefore, by controlling the amount of cross-linking agent, the cellulose ether is moderately cross-linked, which can delay the hydration time to prevent the product from dissolving too quickly in the aqueous solution causing localized clumping.
The effect of aldehyde-crosslinking cellulose ethers generally depends on the amount of aldehyde, pH value, uniformity of the cross-linking reaction, cross-linking time and temperature. Crosslinking temperature and pH too high or too low will cause irreversible cross-linking due to hemiacetal to acetal, resulting in complete insolubility of cellulose ether in water. The dosage and the uniformity of the cross-linking reaction directly affect the cross-linking degree of the cellulose ether.
Due to its high toxicity and strong volatility, formaldehyde is less used for cross-linking of cellulose ethers. In the past, formaldehyde was used more in the fields of coatings, adhesives and textiles. It is now gradually replaced by low-toxicity non-formaldehyde cross-linking agents. The cross-linking effect of glutaraldehyde is better than that of glyoxal, but it has a strong pungent odor. And the price of glutaraldehyde is relatively high. Considering it comprehensively, in industry, glyoxal is commonly used to cross-link water-soluble cellulose ethers to improve the dissolution of products. Performance. Generally, the cross-linking reaction can be carried out at room temperature and under weakly acidic conditions with pH of 5-7. The hydration and sticking time of the cross-linked cellulose ether and the total hydration time will be longer, the agglomeration phenomenon will be weakened, the solubility is better than the non-crosslinked product, and there will be no agglomerated and undissolved product in the solution. Beneficial for industrial applications. Zhang Shuangjian sprayed the cross-linking agent glyoxal before drying in the preparation of hydroxypropyl methylcellulose to obtain 100% dispersion of hydroxypropyl methylcellulose. The instant hydroxypropyl methylcellulose has no clumping when dissolving, disperses quickly, and dissolves quickly, which solves the constraints in practical application and expands the application field.
Under alkaline conditions, the reversible process of acetal formation of aldehyde-crosslinked cellulose ethers will be broken, the hydration time of the product will be shortened, and the fiber will be restored. Solubility properties of uncross-linked ethers. In the preparation and production of cellulose ether, the cross-linking reaction of aldehyde is usually after the etherification reaction process, which can be in the liquid phase of the washing process, it can also be carried out in the solid phase after centrifugation. Generally, the uniformity of the cross-linking reaction during the washing process is good, but the cross-linking effect is poor. However, due to the limitations of engineering equipment, crosslinking in the solid phase leads to poor crosslinking uniformity, but the crosslinking effect is relatively better, and the amount of crosslinking agent used is increased. Relatively small.
Aldehyde cross-linking agent modified water-soluble cellulose ether, in addition to improving its solubility, there are also research reports that it can be used to improve its mechanical properties, viscosity stability and other properties. For example, Zhang Peng  used glyoxal and HEC to cross-link, and explored the effect of cross-linking agent concentration, cross-linking pH and cross-linking temperature. Influence of HEC wet strength. The study found that under the optimal cross-linking conditions, the wet strength of the cross-linked HEC fibers increased by 41.5%, and its performance was to a significant improvement. Zhang Jin used water-soluble phenolic resin, glutaraldehyde, trichloroacetaldehyde, etc. to cross-link CMC. For CMC cross-linked by water-soluble phenolic resin, its solution has the smallest viscosity drop after high temperature treatment, that is, the best temperature resistance.
Carboxylic acid crosslinking agents refer to poly-carboxylic acid compounds, mainly including dibasic or polybasic carboxylic acids such as succinic acid, malic acid, tartaric acid and citric acids. Carboxylic acid cross-linking agents were first used to cross-link fabric fibers to improve their flatness . The cross-linking mechanism is: carboxyl groups and hydroxyl groups on cellulose molecules,the group undergoes an esterification reaction to generate an esterified cross-linked cellulose ether. Welch and Yang et al.[7-8] were the first to study the crosslinking mechanism of carboxylic acid crosslinking agents. According to research reports, the cross-linking process is: under certain conditions, two adjacent carboxyl groups in the carboxylic acid cross-linking agent are first dehydrated to form a cyclic acid anhydride.
The OH in cellulose molecules reacted to form cross-linked cellulose ethers with a network-like space structure.
Carboxylic acid cross-linking agents generally carry out cross-linking reaction with cellulose ethers containing hydroxyl substituents, and the reaction process is described as shown in formula (5).
Due to their water-soluble and non-toxic characteristics, carboxylic acid cross-linking agents have been widely used in the study of wood, starch, chitosan, cellulose, etc. in recent years. Esterification and cross-linking modification of various natural polymers such as derivatives, thereby improving the performance of their application fields.
Hu Hanchang et al.  used sodium hypophosphite catalyst, using four kinds of polycarboxylic acids with different molecular structures: propane tricarboxylic acid (PCA), 1,2,3,4-Butanetetracarboxylic acid (BTCA), cis-cyclopentanetetra-carboxylic acid (cis-CPTA), cyclohexanehexa-carboxylic acid (cis-CHHA) for easy-press finishing of cotton fabrics, The results show that the cotton fabrics treated with cyclic poly-carboxylic acids have better wrinkle recovery properties. Therefore, the cyclic poly-carboxylic acids because of its high rigidity, the crosslinking modification effect is better than that of chain carboxylic acid molecules, and it is a potential effective crosslinking agent.
Wang Jiwei et al.  used citric acid and acetic anhydride mixed acid to double-modify starch by esterification and cross-linking, and tested the water separation rate and paste transparency. The results show that the esterified cross-linked starch has better freeze-thaw stability, lower paste transparency, and better viscosity and thermal stability than starch. After the carboxylic acid group reacts with the active -OH in various polymers, it can improve its solubility, resistance to biodegradation, mechanical properties, etc., and the carboxylic acid compound has non-toxic or low-toxic properties, which is suitable for cross-linking modification of water-soluble cellulose ethers in food-grade, pharmaceutical-grade and coating fields. There are broad prospects.
The epoxy compound crosslinking agent contains two or more epoxy groups, or is an epoxy compound containing active functional groups, under the action of a catalyst
Epoxy and functional groups undergo substitution reaction with -OH in organic compounds to generate macromolecules with a network structure [11-12], so they can be used in fiber
Cross-linking of vitamin ethers. The description formula of the crosslinking reaction process of epoxy compound and cellulose ether is shown in formula (6).
The cross-linking of cellulose ether by epoxide can increase its viscosity and improve mechanical properties. Epoxy compounds were first used to treat fabric fibers, which it has a good finishing effect, and there are few reports on the cross-linking modification of cellulose ether by epoxide. Hu Cheng et al.  developed a novel multifunctional epoxy compound crosslinking agent: EPTA, the wet elastic recovery angle of silk fabrics after finishing is increased from 200º before treatment to 280º, and this cross the positive charge of the linking agent can significantly improve the dyeing rate and exhaustion rate of acid dyes on silk fabrics. The epoxy compound crosslinking agent used by Chen Xiaohui et al. :Polyethylene glycol diglycidyl ether (PGDE) is cross-linked with gelatin, and the gelatin hydrogel after cross-linking has excellent elastic recovery performance, elasticity the response rate is as high as 98.03%. Referring to the research on the cross-linking modification of natural polymers such as fabrics and gelatin by epoxides in the literature, the properties of cellulose ethers were studied. Epoxide crosslinking modification also has development prospects.
In epoxy compounds, epichlorohydrin (also known as epichlorohydrin) is a commonly used cross-linking agent, which is often used for cross-linking treatments containing -OH, -NH2 and other activities. The group of natural polymer materials. The viscosity, acid and alkali resistance, temperature resistance, salt resistance, shear resistance, strength of the material cross-linked by epichlorohydrin, academic performance, etc. will be improved [15-18]. Therefore, the use of epichlorohydrin in the crosslinking of cellulose ethers has great research significance. For example, Su Maoyao  adopted epichlorohydrin cross-links CMC to prepare high adsorption materials. The influence of material structure, degree of substitution and degree of cross-linking on adsorption performance is discussed. It is concluded that,
Products made with about 3% crosslinker showed a 26-fold and 17-fold increase in water retention value (WRV) and brine retention value (SRV), respectively. When Chang guang et al. prepared ultra-high viscosity carboxymethyl cellulose, epichlorohydrin was added after etherification for cross-linking. By comparison, the viscosity of the cross-linked product was
Up to 51% higher than the uncrosslinked product.
Boron-based cross-linking agents mainly include boric acid, borax, borates and organoboron and other borate-containing cross-linking agents. The cross-linking mechanism is generally considered to be boric acid (H3BO3) or borate (B4O72-) Formation of tetrahydroxyborate ion (B(OH)4 in solution),and then decomposed with -OH in the compound water combines to form cross-linked compounds with a network structure . The reaction process is described as formula (7).
Boric acid cross-linking agents are widely used and can be used as additives in medicine, glass, ceramics, petroleum and other fields. After the material treated with boric acid cross-linking agent, its mechanical strength will be improved, which can be used for cross-linking of cellulose ether, thereby improving its performance.
In the 1960s, the cross-linking agent of water-based fracturing fluid in oil and gas field development was mainly inorganic boron (borax, boric acid and sodium tetraborate, etc. ).Mainly, borax was the first cross-linking agent used. Due to the short time of inorganic boron cross-linking and poor temperature resistance, the development of organic boron cross-linking agent has become a research hotspots, the research of organic boron began in the 1990s, due to its high temperature resistance, easy to break glue, controllable delayed cross-linking and other characteristics. It has achieved good application results in oil and gas field fracturing[23-24]. Liu Ji et al.  developed a polymer crosslinking agent containing phenylboronic acid groups. The cross-linking agent reacts with acrylic acid and polyol polymers with succinimidyl ester groups, and the obtained bioadhesive has excellent comprehensiveness. It can show good adhesion and mechanical properties in a humid environment, and can be easily debonded. Yang Yang et al.  prepared high temperature resistant Zirconium-boron cross-linking agent can greatly improve the temperature resistance and shear resistance of fracturing fluid after cross-linking treatment of guar gum base fluid. It has been reported that Boric acid the cross-linking modification of carboxymethyl cellulose ether by carboxymethyl cellulose has been used in petroleum drilling fluids .Research on cross-linking of cellulose ethers in various fields such as construction and coatings.
Phosphide crosslinking agents mainly include phosphorus oxychloride (phosphorus oxychloride), sodium trimetaphosphate, sodium tripolyphosphate, etc. The crosslinking mechanism is P-O bond or P-Cl and -OH in the molecule undergo an esterification reaction in an aqueous solution to produce a phosphoric acid diester and form a network structure. The cross-linking reaction process is described as formula (8)～(10) shown.
Phosphide cross-linking agents are widely used in the cross-linking modification of food and pharmaceutical polymer materials, such as starch, chitosan, etc., due to their non-toxicity or low toxicity. Natural polymer cross-linking treatment . Studies have shown that adding a small amount of phosphide cross-linking agent to starch can significantly change the gelatinization and swelling properties of starch. After the starch is cross-linked, the gelatinization temperature increases, the paste stability improves, the acid resistance is better than that of native starch, and the film strength increases.
There are also many studies on the crosslinking of chitosan by phosphide crosslinking agents. After crosslinking, its mechanical strength and chemical stability can be improved [33-34].At present, there is no research report on the use of phosphide cross-linking agent to cross-link cellulose ether, because cellulose ether and starch, chitosan and other natural polymers contain more active -OH, and phosphide cross-linking agents have non-toxic or low-toxic physiological properties, which are used for the cross-linking of cellulose ethers，research is also potentially promising. For example, when CMC is used in food and toothpaste grades, it can be modified with a phosphide cross-linking agent, which can improve its thickening and rheology performance. MC, HPMC, HEC used in the pharmaceutical field can be improved with phosphide cross-linking agent.
The above-mentioned crosslinking of aldehydes, epoxides and cellulose ethers belongs to etherification crosslinking, and carboxylic acid, boric acid and phosphide crosslinking agents belong to esterification crosslinking. In addition, the cross-linking agents used for cellulose ether cross-linking include isocyanate compounds, nitrogen methyl compounds, mercapto compounds, metal cross-linking, the common characteristic of its molecular structure is that the molecule contains multifunctional groups that are easy to react with -OH, and can form after cross-linking. Multidimensional network structure. The performance of the cross-linked product has a certain relationship with the type of cross-linking agent, the degree of cross-linking and the conditions of cross-linking.
Badit Paban Kondu and others used toluene diisocyanate (TDI) to crosslink methylcellulose, and the glass transition temperature (Tg) after crosslinking. It increases with increasing percentage of TDI, and its aqueous solution stability improves. TDI is also commonly used for cross-linking modification in adhesives, coatings and other fields. After modification, the bonding performance, temperature resistance and water resistance of the film will be improved. Therefore, TDI is very important for construction, coatings, adhesives after the cross-linking modification of cellulose ether used in the field, its application performance can be improved.
Disulfide bond cross-linking technology is mostly used in the modification of medical materials, and has certain research value for the cross-linking of cellulose ether products in the field of medicine. Shu Jun et al.  coupled β-cyclodextrin with silica microspheres and cross-linked thiolated chitosan and dextran through a gradient shell to remove silica microspheres. After the spheres, disulfide-crosslinked nanocapsules were obtained, which exhibited good stability in simulated physiological PH.
Metal cross-linking agents mainly include inorganic and organic compounds of high-valent metal ions such as Zr(IV), Al(III), Ti(IV), Cr(III) and Fe(III).The valence metal ions are polymerized to form polynuclear hydroxyl bridge complex ions through hydration, hydrolysis, hydroxyl bridge, etc. Studies generally believe that the cross-linking of high-valent metal ions. Mainly through the polynuclear hydroxyl bridge ion, which is easy to combine with the carboxylic acid group and cross-link to form a body-shaped multi-dimensional space structure polymer [37-38]. Xu Kai et al [39-40]. The flow of Zr(IV), Al(III), Ti(IV), Cr(III) and Fe(III) series of high-valent metals after cross-linked carboxymethyl hydroxypropyl cellulose (CMHPC) was investigated. After denaturation and application compatibility, the thermal stability, filtration loss, sand suspension capacity, glue-breaking hydration residue and salt compatibility, etc., the research results show that the gold cross-linking agent has the properties required by the oil well fracturing fluid gelling agent.
Among the cellulose ethers, HEC, HPMC, HEMC and MC are mainly used in the fields of construction and coatings. Such cellulose ethers must have good properties.
Good water retention, thickening, salt and temperature resistance, shear resistance, etc., commonly used in cement mortar, latex paint, tile adhesives, exterior wall coatings, real
Shi Lacquer et al . Due to the requirements of materials with good mechanical strength and stability in the fields of construction and coatings, etherified crosslinking agents are generally used to Cross-linking modification of cellulose ether, such as using epoxy halogenated alkanes, boric acid cross-linking agents, etc., can improve product viscosity, salt resistance, etc. Temperature resistance, shear resistance and mechanical properties.
MC, HPMC and CMC in water-soluble cellulose ethers are commonly used in drug coating materials, pharmaceutical sustained-release additives and thickeners for liquid pharmaceuticals and emulsion stabilizers and other pharmaceutical fields . CMC is also used as an emulsifier and thickener in yogurt, dairy products and toothpaste. For HEC and MC in the field of daily chemicals, it has the functions of thickening, dispersing and homogenizing. Since the fields of medicine, food and daily chemicals require safe and non-toxic materials, for Such cellulose ethers can use phosphoric acid, carboxylic acid cross-linking agents, mercapto cross-linking agents, etc. After cross-linking modification, the viscosity and biological stability of the product can be improved and qualitative performance.
HEC is less used in the fields of medicine and food, but because HEC is a highly soluble nonionic cellulose ether, compared with MC, HPMC and CMC have their unique advantages. In the future, safe and non-toxic cross-linking agents will be used to cross-link and modify them, and they will be used in the fields of medicine and food have great development potential.
CMC and carboxylated cellulose ethers are often used as industrial drilling mud treatment agent, fluid loss control agent and thickener. HEC as a non-isolated
Sub-type cellulose ether, because of its good thickening effect, strong and stable sand suspension ability, heat resistance, high salt capacity, small pipeline resistance, less liquid loss, and broken. It has the characteristics of fast glue and low residue, and is also widely used in the field of oil drilling. At present, more research is to use boric acid cross-linking agent and metal cross-linking agent, there are few reports on the cross-linking modification of non-ionic cellulose ethers, but the hydrophobically modified CMC is used in the field of cross-linking modification. Non-ionic cellulose ether, showing remarkable viscosity increase, temperature and salt resistance and shear stability, good dispersibility and resistance to biological enzymatic hydrolysis. Since the cellulose ethers used in the field of oil drilling and production are cross-linked and modified by cross-linking agents such as boric acids, metals, epoxides, epoxy halogenated alkanes, etc. Thickening, salt and temperature resistance, stability, etc. have been improved, and it has great application prospects in the future.
Cellulose ethers are widely used due to their excellent properties in thickening, emulsification, film formation, protective colloid, moisture retention, adhesion, and anti-allergy. In addition to the above fields, it is also used in various fields such as papermaking, ceramics, textile printing and dyeing, and polymerization. According to the requirements of material properties in various fields different cross-linking agents can be used for cross-linking modification to meet application requirements. In general, cross-linked modified cellulose ethers can be divided into two categories: ethers cross-linked cellulose ethers and esterified cross-linked cellulose ethers. Aldehydes, epoxides and other cross-linking agents react with -OH on cellulose ethers to form ether-oxygen bonds (-O-) is an etherified crosslinking agent. Carboxylic acid, phosphide, boric acid and other cross-linking agents react with -OH on cellulose ether to form ester bonds. Chemical crosslinking agent. The carboxyl group in CMC reacts with -OH in the cross-linking agent to produce esterified cross-linked cellulose ether. Less, there is room for development in the future. Since ether bonds are more stable than ester bonds, etherified cross-linked cellulose ethers have stronger stability and mechanical properties. According to different application fields, suitable cross-linking agents can be selected to cross-link and modify cellulose ethers to obtain products that meet application needs.
At present, glyoxal is mostly used in industry to cross-link cellulose ether to delay its dissolution time and solve the problem of agglomeration when the product is dissolved. Glyoxal cross-linked cellulose ether can only change its solubility and has no obvious improvement on other properties. At present, the use of other cross-linking agents other than glyoxal for the cross-linking of cellulose ethers is less researched. Because cellulose ethers are widely used in petroleum drilling, construction, coatings, food, medicine and other industries,
Its solubility, rheology, mechanical properties, etc. play a crucial role in its application, and its application in various fields can be improved through cross-linking modification to meet application requirements. For example, carboxylic acid, phosphoric acid, and boric acid cross-linking agents can improve the performance of cellulose ether after esterification and cross-linking. Application performance in the field of medicine. Aldehydes cannot be used in the food and pharmaceutical industries due to their physiological toxicity. Boric acid, metal cross-linking of cellulose ethers for petroleum drilling is of great help in improving the performance of fracturing fluids in oil and gas fields. Other alkyl crosslinkers such as epichlorohydrin
After the cellulose ether is cross-linked by propane, the viscosity, rheology and mechanical properties of the cellulose ether can be improved. With the continuous development of science and technology, the requirements for material properties in various industries continue to increase. In order to meet the performance requirements of cellulose ethers in various application fields, the cross-linking research of cellulose ethers in the future has
Broad development prospects.