Comprehensive analysis of cellulose ethers
1. Definition of cellulose ethers
Cellulose ethers are an important polymer compound. They are a general term for a class of derivatives obtained from natural cellulose through etherification reactions8. Cellulose is the main component of plant cell walls and the most widely distributed and abundant polysaccharide in nature. The cellulose content in cotton is close to 100%, while cellulose accounts for 40-50% in general wood, with 10-30% hemicellulose and 20-30% lignin.
Each glucose ring on the cellulose macromolecule contains three hydroxyl groups, including the primary hydroxyl group on the sixth carbon atom and the secondary hydroxyl groups on the second and third carbon atoms. In the etherification reaction, the hydrogen in the hydroxyl group is replaced by a hydrocarbon group to generate cellulose ether derivatives. Originally, cellulose macromolecules are difficult to dissolve, but after etherification, their hydrophilicity is improved, and their solubility in aqueous media is greatly improved, thus possessing a variety of unique properties.
2. Characteristics of cellulose ethers
(1) Solution properties
Thickening properties
Cellulose ethers have good thickening properties and can increase the viscosity of liquids. In many industrial applications, such as cement mortar, coatings, inks and other fields in building materials, it can change the rheological properties of the product, making the product more operable during application, printing and other operations. For example, in building exterior wall insulation mortar, cellulose ether increases the viscosity of the mortar, improves its fluidity and workability, prevents the mortar from flowing too quickly during application, and ensures the uniformity and quality of construction8.
In the food industry, it is used as a thickener in sauces, beverages and other products to increase its viscosity, make the sauce have better adhesion, and the ingredients in the beverage are not easy to precipitate, thereby improving the stability and taste of the product.
Good water solubility
Most cellulose ethers have good solubility in water and can form transparent solutions. This feature makes it used in many systems that need to be compatible with water, such as in the pharmaceutical field as an excipient for drugs, which can be dissolved and evenly dispersed in drug preparations to ensure the stability and effectiveness of the drugs.
In daily chemical products, such as shampoo, shower gel and other products, the water solubility of cellulose ether enables it to be evenly mixed with other ingredients to play a role in thickening and stabilization, without affecting the appearance and use experience of the product.
Suspension or latex stability
In latex paints and other systems, cellulose ethers can improve the stability of latex and prevent the aggregation and sedimentation of latex particles. It forms a protective film on the surface of latex particles to prevent the interaction between particles, thereby maintaining the stability of the latex system. This is very important for ensuring the quality and performance of the coating, for example, ensuring the uniformity and performance of the coating under long-term storage or different environmental conditions.
In some systems that require suspended solid particles, such as ceramic glaze slurry, cellulose ethers act as binders to suspend, deflocculate, and retain water, increase the strength of the raw glaze, reduce the drying shrinkage of the glaze, and make the embryo and glaze firmly bonded and not easy to fall off.
(2) Physical and chemical properties
Film-forming properties
Cellulose ethers have film-forming properties and can form a protective film in some applications. In the pharmaceutical field, for example, in the coating material of drugs, cellulose ethers can form a thin film on the surface of the drug to protect the drug and prevent the drug from being degraded by oxygen or moisture in the air. At the same time, it provides the desired drug release pattern after administration, masks the unpleasant smell or odor of the drug, or improves the appearance.
In food packaging, some cellulose ethers can form a thin protective film on the surface of food, which plays a role in preservation and moisture-proofing.
Water retention
In the field of building materials, water retention is an important characteristic of cellulose ethers. For example, in cement mortar, cellulose ether, as a water-retaining agent, can retain moisture in the mortar, so that the slurry will not crack due to drying too quickly after application, and enhance the strength after hardening. In gypsum mortar, it can improve thickening, water retention and slow setting, extend the operability time, and ensure the smooth progress of construction.
In agriculture, cellulose ether can be used for soil improvement. By retaining moisture in the soil and improving the soil’s moisture retention capacity, it is conducive to plant growth.
Adhesion
In building materials, cellulose ether can be used as an adhesive. In plaster mortar, gypsum, putty powder or other building materials, it can improve the spreadability and extend the operability time. It is used as an adhesive enhancer for pasting tiles, marble, and plastic decorations, and can also reduce the amount of cement. In the papermaking industry, cellulose ether can improve the strength of paper and make the paper stronger by bonding the fibers in the paper.
In the textile industry, cellulose ether is used as an adhesive for fabric finishing to improve the strength and wear resistance of textiles.
Colloid protection
Cellulose ether can protect the colloid system and prevent the colloid from settling. In some food and pharmaceutical colloid systems, it increases the repulsive force between particles by adsorbing on the surface of colloid particles and maintains the stability of the colloid. For example, in some emulsion-type pharmaceutical preparations, cellulose ether protects the stability of the emulsion system and ensures the uniform dispersion and stability of the drug.
(3) Other characteristics
Non-toxic, odorless, biocompatible
Cellulose ether is non-toxic and odorless, which makes it widely used in food, medicine, daily chemicals and other fields that come into direct contact with the human body. In the food industry, it is used as a thickener, stabilizer and moisturizer without causing harm to human health.
In the pharmaceutical field, good biocompatibility is its important feature. It can be used as a pharmaceutical excipient for drug delivery and encapsulation, such as in the production of plant capsules, to make it gel and film-forming, and avoid cross-linking and curing reactions. In the fields of cell culture and tissue engineering, the biocompatibility of cellulose ethers is also favored. It will not cause the body’s immune response and is conducive to cell growth and tissue repair.
Stability
Cellulose ethers have good stability and can remain stable at different temperatures and pH values. In industrial applications, such as in some chemical production processes, it can exist stably under different reaction conditions and play its due role. In pharmaceutical preparations, it can remain stable under different physiological environments to ensure the quality and efficacy of the drug.
3. Uses of cellulose ethers
(1) Construction field
Mortar additives
In cement mortar, cellulose ethers are used as water retainers and retarders to make the mortar pumpable. It can retain moisture in the mortar and prevent the mortar from drying out due to excessive evaporation of moisture, thereby improving the initial strength of the mortar and avoiding cracking. It is used as an adhesive in building materials such as plaster, gypsum, and putty powder to improve spreadability and extend the operability time. For example, in water-resistant putty, it can replace traditional industrial glue, improve the water retention, viscosity, scrub resistance and adhesion of putty, and eliminate formaldehyde hazards.
In tile bonding mortar, cellulose ether can improve the anti-sagging ability of the bonding mortar, improve the early bonding strength of the mortar, and resist strong shear forces to prevent tiles from slipping. In gypsum mortar, it can improve thickening, water retention and slow setting, improve the performance of the mortar, and facilitate construction operations.
PVC production
In the production of polyvinyl chloride (PVC), cellulose ether is used as a dispersant and is the main auxiliary agent for the preparation of PVC by suspension polymerization. It can adjust the density of PVC resin, improve the thermal stability of the resin and control the particle size distribution, and improve the apparent physical properties, particle characteristics and melt rheology of PVC resin products. By uniformly dispersing PVC resin particles, the quality and performance of the product can be improved.
(2) Pharmaceutical field
Drug excipients
Cellulose ether is widely used in the pharmaceutical industry and can be used as coating materials, film materials, rate-controlling polymer materials for sustained-release preparations, stabilizers, suspending agents, tablet adhesives, thickeners, etc. When used as a coating material, it can protect the drug from the influence of the external environment, such as preventing the drug from being degraded by oxygen or moisture in the air, while controlling the release rate of the drug to achieve the purpose of sustained or controlled release.
As a suspending agent, it can make the drug disperse evenly in the liquid medicine and prevent the drug from settling. In tablet production, as a tablet binder, it can be used to cause powder particles to bond during the granulation process and improve the strength of the tablet.
Drug delivery and encapsulation
In recent years, the application of some cellulose ethers such as hydroxypropyl methylcellulose (HPMC) in the pharmaceutical industry has received great attention. HPMC has good biocompatibility, strong load capacity and mechanical properties, making it stand out among many cellulose derivatives and being made into various forms for drug delivery and encapsulation. For example, in some new drug delivery systems, HPMC can encapsulate drugs, deliver drugs to specific parts, and then slowly release drugs to improve the efficacy of drugs and reduce the side effects of drugs.
(3) Food field
Thickening and stabilization
Cellulose ethers can be used as food thickeners, stabilizers and humectants. In seasoning additives, it can thicken, increase the viscosity of sauces, increase the taste durability, and help thicken and shape. In beverage additives, non-ionic cellulose ethers are generally used. They are compatible with beverages, have suspending properties, and thicken without covering up the taste of beverages.
In baked food additives, it can improve texture, reduce oil absorption, inhibit food moisture loss, make baked food crispier, and make its surface texture and color more uniform. The superior adhesiveness of cellulose ethers can improve the strength, elasticity and taste of flour products.
Other applications
In extruded food additives, it can reduce the generation of powder and improve texture and taste. In some processed meat products, cellulose ethers can retain moisture and improve the taste and juiciness of the product. In gel foods such as jelly and pudding, cellulose ethers can form gels, providing unique taste and stability.
(4) Daily chemical field
Thickening and stabilization
In daily chemical products such as shampoo, shower gel, lotion, etc., cellulose ethers can be used as thickeners, emulsifiers and stabilizers to improve product quality and stability. It can increase the viscosity of the product, making it easier for the product to adhere to the skin or hair during use, while preventing stratification and demulsification of systems such as emulsions.
Other applications
In some cosmetic products such as foundation and blush, cellulose ether can improve the texture of the product, making it easier to apply and evenly distribute on the skin, while improving the stability of the product and extending the shelf life of the product.
(5) Other industrial fields
Oil extraction
In oil extraction, cellulose ether can be used in the preparation of drilling fluid. It can increase the viscosity of the drilling fluid, improve the sand carrying capacity of the drilling fluid, and prevent the well wall from collapsing. At the same time, in the later maintenance of the oil well, cellulose ether can be used to seal the cracks in the oil layer and improve the recovery rate of the oil well.
Textile industry
In the textile industry, cellulose ether can be used as an adhesive and thickener in the textile industry, which can improve the quality and processing performance of textiles. As an adhesive, it can be used in the processing of fabric printing, coating, etc. to improve the fastness of printing and coating. As a thickener, in textile printing and dyeing slurry, it increases the viscosity of the slurry and ensures the uniformity of printing and dyeing.
Papermaking industry
In the papermaking industry, cellulose ether can improve the strength, water resistance and flexibility of paper. It can improve the physical properties of paper by interacting with the fibers in the paper and increasing the bonding force between the fibers. At the same time, in the process of surface sizing of paper, cellulose ether can be used as a sizing agent to improve the water resistance of paper.
Leather industry
In the leather industry, cellulose ether can be used in leather tanning, finishing and other processes. In the tanning process, it can be used as an auxiliary agent of tanning agent to improve the tanning effect. In the finishing process, it can be used as a thickener and stabilizer to improve the performance of leather finishing agent, make the leather surface smoother and flatter, and improve the quality and appearance of leather.
4. Classification of cellulose ethers
(1) Classification by ionization
Ionic cellulose ethers
Ionic cellulose ethers refer to molecules with positive charge, which can form hydrogen bonds with anions or protons and have strong hydrophilicity. For example, carboxymethyl cellulose (CMC) and polyanionic cellulose (PAC) are ionic cellulose ethers.
Ionic cellulose ethers perform well in some applications that require high hydrophilicity, such as in detergents, where they can improve the detergency of detergents due to their good water solubility and thickening properties. In terms of food additives, some ionic cellulose ethers can be used as thickeners and stabilizers. However, it also has some limitations. For example, compared with non-ionic cellulose ethers, its temperature resistance, salt resistance and stability are poor, its performance is greatly affected by the outside world, and it is easy to react with Ca2+ contained in some coatings and building materials to produce precipitation, so it is less used in the field of building materials and coatings2.
Non-ionic cellulose ethers
Non-ionic cellulose ethers refer to molecules that do not contain positive charges. They cannot form hydrogen bonds with anions or protons and have strong hydrophobicity. Therefore, non-ionic cellulose cannot form gels in water, but they have excellent heat preservation, tolerance, transparency and acid resistance.
Non-ionic cellulose ethers include hydroxypropyl methylcellulose (HPMC), methyl cellulose (MC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC) and hydroxyethyl methyl cellulose. Among them, ethyl cellulose is non-water-soluble. Non-ionic cellulose ethers are widely used in industry, such as emulsifiers, antioxidants, preservatives, antibacterial agents, anti-aging agents, gelling agents, suspending agents, antifreeze agents, baking aids, foaming agents, flavor enhancers, preservatives, colorants, anticoagulants and anticoagulants. Because they are superior to ionic cellulose ethers in terms of temperature resistance and salt resistance, and have high product added value, although ionic cellulose ethers are currently the mainstream product in the market (accounting for about 60%), the market development prospects of non-ionic cellulose ethers are good.
Mixed cellulose ethers
Mixed cellulose ethers include hydroxyethyl carboxymethyl cellulose and hydroxypropyl carboxymethyl cellulose. This type of cellulose ether combines some of the characteristics of ionic and non-ionic cellulose ethers and has advantages in some specific application fields. For example, in some industrial applications that require certain hydrophilicity and special properties at the same time, mixed cellulose ethers can be selected according to specific needs.
(2) Classification by substituents
Single ethers
Single ethers refer to cellulose ethers that are substituted by only one ether group on the cellulose macromolecule. For example, methyl cellulose (MC) is a single ether obtained by replacing the hydroxyl group in the cellulose molecule with a methyl group. Single ethers have relatively simple chemical properties and application performance, and their performance mainly depends on the properties of the ether groups replaced.
Mixed ethers
Mixed ethers refer to cellulose ethers substituted by two or more different ether groups on the cellulose macromolecule. For example, hydroxypropyl methylcellulose (HPMC) is a mixed ether obtained by replacing the hydroxyl group in the cellulose molecule with two ether groups, hydroxypropyl and methyl. Because mixed ethers have multiple ether groups, their performance is often more complex and diverse than that of single ethers. They can combine the characteristics of multiple ether groups and have a wider range of applicability in different application fields.
(3) Classification by solubility
Water-soluble cellulose ethers
Water-soluble cellulose ethers have good solubility in water and can form transparent solutions. Most common cellulose ethers such as hydroxypropyl methylcellulose (HPMC) and carboxymethyl cellulose (CMC) are water-soluble cellulose ethers. Water-soluble cellulose ethers are widely used in systems that need to be compatible with water, such as as thickeners and stabilizers in food, medicine, daily chemicals and other fields.
Water-insoluble cellulose ethers
Water-insoluble cellulose ethers have poor solubility in water, but have certain solubility in some organic solvents. For example, ethyl cellulose (EC) is a water-insoluble cellulose ether, which has good solubility in some organic solvents such as ethanol and toluene. Water-insoluble cellulose ethers are used in some special industrial applications, such as in some coatings. When organic solvents are needed in systems such as printing inks, etc., they can play a unique role.
5. Production process of cellulose ether
(1) Traditional production process
Alkalinization reaction
The traditional production process of cellulose ether first involves alkalizing refined cotton with sodium hydroxide under certain conditions to produce sodium cellulose. During the alkalization process, sodium hydroxide reacts with the hydroxyl groups in the cellulose molecules to activate the cellulose molecules and prepare for the subsequent etherification reaction. For example, refined cotton is put into a reactor, an appropriate amount of sodium hydroxide solution is added, and an alkalization reaction is carried out at a certain temperature and reaction time. The conditions of the alkalization reaction, such as temperature, alkali concentration, reaction time, etc., have an important influence on the efficiency of the subsequent etherification reaction and the quality of the product. If the alkalization reaction is incomplete, the etherification reaction will be uneven, affecting the performance of the product.
Etherification reaction
The alkalized sodium cellulose is then etherified by an etherifying agent such as propylene oxide, ethylene oxide, methyl chloride or chloroacetic acid, and reacts under certain conditions to produce different types of cellulose ethers. The etherification reaction is a key step in the production of cellulose ethers, and different etherifying agents will produce different types of cellulose ethers. For example, hydroxypropyl methylcellulose (HPMC) can be produced by using propylene oxide and methyl chloride as etherifying agents. The conditions of the etherification reaction include temperature, pressure, reaction time, and the amount of etherifying agent. In the etherification reaction, these reaction conditions need to be precisely controlled to ensure the quality and performance of the product. For example, when producing HPMC, too high a reaction temperature may lead to an increase in side reactions, affecting the purity and performance of the product.
Post-processing steps
After the etherification reaction is completed, post-processing steps such as neutralization, solvent recovery, washing, drying, and crushing are required to finally obtain a powdered finished product. The neutralization step is to adjust the pH value of the reaction system and remove excess alkali. The solvent recovery step can recover the organic solvent used in the reaction process, reduce production costs and reduce the impact on the environment. The washing step can remove impurities in the product and improve the purity of the product. The drying step removes moisture from the product to obtain a dry cellulose ether product, and the crushing step crushes the dried product into the required particle size for easy packaging and use.