What role does cellulose ether play in dry mixed mortar?

Cellulose ether is a synthetic polymer made of natural cellulose as raw material by chemical modification. Cellulose ether is a derivative of natural cellulose, cellulose ether production and synthetic polymer is different, its most basic material is cellulose, natural polymer compounds. Due to the particularity of natural cellulose structure, cellulose itself has no ability to react with etherifying agent. But after the treatment of swelling agent, the strong hydrogen bonds between molecular chains and chains were destroyed, and the activity of hydroxyl group was released into alkali cellulose with reaction ability, and cellulose ether was obtained through the reaction of etherifying agent — OH group into — OR group.

The properties of cellulose ethers depend on the type, number and distribution of substituents. The classification of cellulose ether is also based on the type of substituents, degree of etherification, solubility and related application can be classified. According to the type of substituents on the molecular chain, it can be divided into single ether and mixed ether. MC is usually used as a single ether, while HPmc is a mixed ether. Methyl cellulose ether MC is a natural cellulose glucose unit on the hydroxyl is methoxide replaced by the product structure formula [C O H7O2 (OH) 3-H (OCH3) H] X, hydroxypropyl methyl cellulose ether HPmc is a unit on the hydroxyl is part of the methoxide replaced, another part of the hydroxypropyl replaced product, The structural formula is [C6H7O2 (OH) 3-M-N (OCH3) M [OCH2CH (OH) CH3] N] X and hydroxyethyl methyl cellulose ether HEmc, which is widely used and sold on the market.

From the solubility can be divided into ionic type and non-ionic type. Water-soluble non-ionic cellulose ether is mainly composed of alkyl ether and hydroxyl alkyl ether two series of varieties. Ionic Cmc is mainly used in synthetic detergent, textile, printing, food and petroleum exploitation. Non-ionic MC, HPmc, HEmc and other mainly used in building materials, latex coatings, medicine, daily chemistry and other aspects. As thickening agent, water retention agent, stabilizer, dispersant, film forming agent.

Cellulose ether water retention

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), is an indispensable part.

The important role of water-soluble cellulose ether in mortar mainly has three aspects, one is excellent water retention ability, the second is the influence of mortar consistency and thixotropy, and the third is the interaction with cement.

Cellulose ether water retention, depends on the base of hydroscopicity, composition of mortar, mortar layer thickness, mortar water demand, condensation material condensation time. The water retention of cellulose ether comes from the solubility and dehydration of cellulose ether itself. It is well known that cellulose molecular chains, although they contain a large number of highly hydrated OH groups, are insoluble in water because of their highly crystalline structure. The hydration ability of hydroxyl groups alone is not enough to pay for the strong intermolecular hydrogen bonds and van der Waals forces. When substituents are introduced into the molecular chain, not only the substituents destroy the hydrogen chain, but also the interchain hydrogen bonds are broken due to the wedging of substituents between adjacent chains. The larger the substituents are, the greater the distance between molecules is. The greater the destruction of hydrogen bond effect, cellulose lattice expansion, the solution into the cellulose ether becomes water-soluble, the formation of high viscosity solution. As the temperature rises, the hydration of the polymer decreases and the water between the chains is driven out. When the dehydrating effect is sufficient, the molecules begin to aggregate and the gel folds out in a three-dimensional network. The factors affecting the water retention of mortar include cellulose ether viscosity, dosage, particle fineness and service temperature.

The greater the viscosity of cellulose ether, the better the water retention performance, the viscosity of polymer solution. The molecular weight (degree of polymerization) of polymer is also determined by the length and morphology of the molecular structure of the chain, and the distribution of the number of substituents directly affects the viscosity range. [eta] = Km alpha

Intrinsic viscosity of polymer solutions

M polymer molecular weight

α polymer characteristic constant

K viscosity solution coefficient

The viscosity of polymer solution depends on the molecular weight of the polymer. The viscosity and concentration of cellulose ether solutions are related to various applications. Therefore, each cellulose ether has many different viscosity specifications, viscosity regulation is also mainly through the degradation of alkali cellulose, namely the fracture of cellulose molecular chain to achieve.

For particle size, the finer the particle, the better the water retention. Large particles of cellulose ether contact with water, the surface immediately dissolve and form a gel to wrap up the material to prevent water molecules from continuing to penetrate, sometimes long time stirring can not be evenly dispersed dissolved, the formation of a muddy flocculent solution or agglomerate. The solubility of cellulose ether is one of the factors to choose cellulose ether.

Thickening and thixotropy of cellulose ether

The second effect of cellulose ether – thickening depends on: cellulose ether polymerization degree, solution concentration, shear rate, temperature and other conditions. The gelation property of solution is unique to alkyl cellulose and its modified derivatives. Gelation characteristics are related to degree of substitution, solution concentration and additives. For hydroxyl alkyl modified derivatives, gel properties are also related to the degree of hydroxyl alkyl modification. For the solution concentration of low viscosity MC and HPmc can be prepared 10%-15% concentration solution, medium viscosity MC and HPmc can be prepared 5%-10% solution and high viscosity MC and HPmc can only be prepared 2%-3% solution, and usually the viscosity of cellulose ether is also graded by 1%-2% solution. High molecular weight cellulose ether thickener efficiency, the same concentration of solution, different molecular weight polymers have different viscosity, viscosity and molecular weight can be expressed as follows, [η]=2.92×10-2 (DPn) 0.905, DPn is the average polymerization degree of high. Low molecular weight cellulose ether to add more to achieve the target viscosity. Its viscosity is less dependent on shear rate, high viscosity to achieve the target viscosity, the amount needed to add less, viscosity depends on the thickening efficiency. Therefore, to achieve a certain consistency, a certain amount of cellulose ether (concentration of solution) and solution viscosity must be guaranteed. The gelation temperature of the solution decreased linearly with the increase of the concentration of the solution, and gelation occurred at room temperature after reaching a certain concentration. HPmc has a high gelation concentration at room temperature.

The consistency can also be adjusted by selecting particle size and cellulose ethers with different degrees of modification. The so-called modification is the introduction of hydroxyl alkyl group in a certain degree of substitution 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 methoxy and hydroxyl groups. Various properties of cellulose ether are required by changing the relative substitution values of two kinds of substituents.

the relationship between consistency and modification. In Figure 5, the addition of cellulose ether affects the water consumption of mortar and changes the water-binder ratio of water and cement, which is the thickening effect. The higher the dosage, the more water consumption.

Cellulose ethers used in powdery building materials must dissolve quickly in cold water and provide the system with the right consistency. If a given shear rate is still flocculent and colloidal it is a substandard or poor quality product.

There is also a good linear relationship between cement slurry consistency and the dosage of cellulose ether, cellulose ether can greatly increase the viscosity of mortar, the greater the dosage, the more obvious the effect.

Cellulose ether aqueous solution with high viscosity has high thixotropy, which is one of the characteristics of cellulose ether. Aqueous solutions of Mc type polymers usually have pseudoplastic, non-thixotropic fluidity below their gel temperature, but Newtonian flow properties at low shear rates. Pseudoplasticity increases with the increase of molecular weight or concentration of cellulose ether and is independent of substituent type and degree. Therefore, cellulose ethers of the same viscosity grade, whether MC, HPmc or HEmc, always show the same rheological properties as long as the concentration and temperature remain constant. When the temperature increases, structural gel is formed and high thixotropic flow occurs. Cellulose ethers with high concentration and low viscosity exhibit thixotropy even below the gel temperature. This property is of great benefit to the construction of building mortar to adjust its flow and flow hanging property. 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, the corresponding reduction of its solubility, which has a negative impact on the mortar concentration and construction performance. The higher the viscosity, the more obvious the thickening effect of mortar, but it is not a complete proportional relationship. Some low viscosity, but modified cellulose ether in improving the structural strength of wet mortar has a more excellent performance, with the increase of viscosity, cellulose ether water retention improved.

Post time: Mar-30-2022