Cellulose ether is a synthetic polymer made from natural cellulose through chemical modification. Cellulose ether is a derivative of natural cellulose. The production of cellulose ether is different from synthetic polymers. Its most basic material is cellulose, a natural polymer compound. Due to the particularity of the natural cellulose structure, the cellulose itself has no ability to react with etherification agents. However, after the treatment of the swelling agent, the strong hydrogen bonds between the molecular chains and the chains are destroyed, and the active release of the hydroxyl group becomes a reactive alkali cellulose. Obtain cellulose ether.
The properties of cellulose ethers depend on the type, number and distribution of substituents. The classification of cellulose ethers is also based on the type of substituents, degree of etherification, solubility and related application properties. According to the type of substituents on the molecular chain, it can be divided into monoether and mixed ether. We usually use mc as monoether, and HPmc as mixed ether. Methyl cellulose ether mc is the product after the hydroxyl group on the glucose unit of natural cellulose is substituted by methoxy group. It is a product obtained by substituting a part of the hydroxyl group on the unit with a methoxy group and another part with a hydroxypropyl group. The structural formula is [C6H7O2(OH)3-m-n(OCH3)m[OCH2CH(OH)CH3]n]x Hydroxyethyl methyl cellulose ether HEmc, these are the main varieties widely used and sold in the market.
In terms of solubility, it can be divided into ionic and non-ionic. Water-soluble non-ionic cellulose ethers are mainly composed of two series of alkyl ethers and hydroxyalkyl ethers. Ionic Cmc is mainly used in synthetic detergents, textile printing and dyeing, food and oil exploration. Non-ionic mc, HPmc, HEmc, etc. are mainly used in construction materials, latex coatings, medicine, daily chemicals, etc. Used as thickener, water retaining agent, stabilizer, dispersant and film forming agent.
Water retention of cellulose ether
In the production of building materials, especially dry-mixed mortar, cellulose ether plays an irreplaceable role, especially in the production of special mortar (modified mortar), it is an indispensable and important component.
The important role of water-soluble cellulose ether in mortar mainly has three aspects, one is excellent water retention capacity, the other is the influence on the consistency and thixotropy of mortar, and the third is the interaction with cement.
The water retention effect of cellulose ether depends on the water absorption of the base layer, the composition of the mortar, the thickness of the mortar layer, the water demand of the mortar, and the setting time of the setting material. The water retention of cellulose ether itself comes from the solubility and dehydration of cellulose ether itself. As we all know, although the cellulose molecular chain contains a large number of highly hydratable OH groups, it is not soluble in water, because the cellulose structure has a high degree of crystallinity. The hydration ability of hydroxyl groups alone is not enough to cover the strong hydrogen bonds and van der Waals forces between molecules. Therefore, it only swells but does not dissolve in water. When a substituent is introduced into the molecular chain, not only the substituent destroys the hydrogen chain, but also the interchain hydrogen bond is destroyed due to the wedging of the substituent between adjacent chains. The larger the substituent, the greater the distance between the molecules. The greater the distance. The greater the effect of destroying hydrogen bonds, the cellulose ether becomes water-soluble after the cellulose lattice expands and the solution enters, forming a high-viscosity solution. When the temperature rises, the hydration of the polymer weakens, and the water between the chains is driven out. When the dehydration effect is sufficient, the molecules begin to aggregate, forming a three-dimensional network structure gel and folded out. Factors affecting the water retention of mortar include the viscosity of cellulose ether, the amount added, the fineness of particles and the use temperature.
The higher the viscosity of the cellulose ether, the better the water retention performance, and the higher the viscosity of the polymer solution. Depending on the molecular weight (polymerization degree) of the polymer, it is also determined by the chain length of the molecular structure and the shape of the chain, and the distribution of the types and quantities of the substituents also directly affects its viscosity range. [η]=Kmα
[η] Intrinsic viscosity of polymer solution
m polymer molecular weight
α polymer characteristic constant
K viscosity solution coefficient
The viscosity of a polymer solution depends on the molecular weight of the polymer. The viscosity and concentration of cellulose ether solution are related to the application in various fields. Therefore, each cellulose ether has many different viscosity specifications, and the adjustment of viscosity is mainly realized by the degradation of alkali cellulose, that is, the breaking of cellulose molecular chains.
The greater the amount of cellulose ether added to the mortar, the better the water retention performance, and the higher the viscosity, the better the water retention performance.
For the particle size, the finer the particle, the better the water retention (see Figure 3). After the large particles of cellulose ether come into contact with water, the surface immediately dissolves and forms a gel to wrap the material to prevent water molecules from continuing to infiltrate. Sometimes it cannot be uniformly dispersed and dissolved even after long-term stirring, forming a cloudy flocculent solution or agglomeration . It greatly affects the water retention of cellulose ether, and solubility is one of the factors for choosing cellulose ether.
Thickening and Thixotropy of Cellulose Ether
The second function of cellulose ether – thickening depends on: the degree of polymerization of cellulose ether, solution concentration, shear rate, temperature and other conditions. The gelling property of the solution is unique to alkyl cellulose and its modified derivatives. The gelation properties are related to the degree of substitution, solution concentration and additives. For hydroxyalkyl modified derivatives, the gel properties are also related to the modification degree of hydroxyalkyl. For mc and HPmc with low viscosity, 10%-15% concentration solution can be prepared, 5%-10% solution can be prepared for medium viscosity mc and HPmc, and 2%-3% solution can be prepared for high viscosity mc and HPmc, and usually The viscosity classification of cellulose ether is also graded with 1%-2% solution. High molecular weight cellulose ether has high thickening efficiency. In the same concentration solution, polymers with different molecular weights have different viscosities. High degree. The target viscosity can only be achieved by adding a large amount of low molecular weight cellulose ether. Its viscosity has little dependence on the shear rate, and the high viscosity reaches the target viscosity, and the required addition amount is small, and the viscosity depends on the thickening efficiency. Therefore, to achieve a certain consistency, a certain amount of cellulose ether (concentration of the solution) and solution viscosity must be guaranteed. The gel temperature of the solution also decreases linearly with the increase of the concentration of the solution, and gels at room temperature after reaching a certain concentration. The gelation concentration of HPmc is higher at room temperature.
Consistency can also be adjusted by choosing particle size and choosing cellulose ethers with different degrees of modification. The so-called modification is to introduce a certain degree of substitution of hydroxyalkyl groups on the skeleton structure of mc. By changing the relative substitution values of the two substituents, that is, the DS and ms relative substitution values of the methoxy and hydroxyalkyl groups that we often say. Various performance requirements of cellulose ether can be obtained by changing the relative substitution values of the two substituents.
Cellulose ethers used in powdered building materials must dissolve quickly in cold water and provide a suitable consistency for the system. If given a certain shear rate, it still becomes flocculent and colloidal block, which is a substandard or poor quality product.
There is also a good linear relationship between the consistency of cement paste and the dosage of cellulose ether. Cellulose ether can greatly increase the viscosity of mortar. The larger the dosage, the more obvious the effect, see Figure 6
High-viscosity cellulose ether aqueous solution has high thixotropy, which is also a major characteristic of cellulose ether. Aqueous solutions of Mc-type polymers usually have pseudoplastic and non-thixotropic fluidity below their gel temperature, but Newtonian flow properties at low shear rates. Pseudoplasticity increases with the molecular weight or concentration of cellulose ether, regardless of the type of substituent and the degree of substitution. Therefore, cellulose ethers of the same viscosity grade, no matter mc, HPmc, HEmc, will always show the same rheological properties as long as the concentration and temperature are kept constant. Structural gels are formed when the temperature is raised, and highly thixotropic flows occur. High concentration and low viscosity cellulose ethers show thixotropy even below the gel temperature. This property is of great benefit to the adjustment of leveling and sagging in the construction of building mortar. It needs to be explained here that the higher the viscosity of cellulose ether, the better the water retention, but the higher the viscosity, the higher the relative molecular weight of cellulose ether, and the corresponding decrease in its solubility, which has a negative impact on the mortar concentration and construction performance. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not completely proportional. Some medium and low viscosity, but the modified cellulose ether has better performance in improving the structural strength of wet mortar. With the increase of viscosity, the water retention of cellulose ether improves.
Post time: Nov-22-2022