Application characteristics of cellulose ether in cement products
Cellulose ether is a multi – purpose additive that can be used in cement products.
Cellulose ether (abbreviated as CE) is formed by etherification of cellulose or one or more etherifying agents and dry grinding. Cellulose ethers can be divided into ionic and nonionic types. Among them, nonionic cellulose ethers have unique thermal gel properties and solubility properties, good salt and heat resistance, and suitable surface activity. As a water retention agent, suspending agent, emulsifier, film former, lubricant, binder and rheology modifier. The main areas of consumption are latex paints, building materials, and oil drilling. With the improvement of people’s health and environmental awareness, polymers such as water-soluble cellulose ether which are physiologically harmless and do not pollute the environment have been greatly developed and applied.
The cellulose ethers commonly used in the field of building materials are methyl cellulose (MC) and hydroxypropyl methyl cellulose (HPMC), which can be used as plasticizers, thickners for coatings, plasters, mortars and cement products. Water retaining agent, air entraining agent and retarder. Most of the building materials industry is used at room temperature. The conditions of use are that the powder is dry mixed with water, less involved in the solubility characteristics and thermogel properties of cellulose ether, but in the mechanized production of cement products and other special temperature conditions. These characteristics of CE will be more fully effective.
1 Chemical properties of cellulose ether
Cellulose ethers are obtained from a series of chemical and physical processes of cellulose. According to the chemical substitution structure, it can be generally divided into: MC, HPMC, hydroxyethyl cellulose (HEC) and the like. Each cellulose ether has the basic structure of cellulose – anhydroglucose. In the process of producing cellulose ether, the cellulose fiber is first heated in an alkaline solution, followed by treatment with an etherifying agent, and the fibrous reaction product is purified and ground to form a uniform powder of a certain fineness.
In the production of MC, only methyl chloride is used as the etherifying agent. In addition to methyl chloride, HPMC is used to obtain hydroxypropyl substituent groups. Various cellulose ethers have different methyl and hydroxypropyl substitution rates, thus affecting the organic compatibility of the cellulose ether solution, thermal gel temperature and the like.
The number of substituent groups on the anhydroglucose structural unit of cellulose can be expressed by mass percentage or the average number of substituent groups (i.e., degree of substitution D. S – Degree of Substitution). The number of substituent groups determines the nature of the cellulose ether product. The effect of average degree of substitution on the solubility of etherified products is as follows:
1.1 Low degree of substitution in lye;
1.2 A slightly higher degree of substitution in water;
1.3 High degree of substitution in a polar organic solvent;
1.4 Higher substitution in a non-polar organic solvent.
2 Method for dissolving cellulose ether
Cellulose ethers have unique solubility characteristics and are insoluble in water when heated to a certain temperature, and below this temperature their solubility increases with decreasing temperature. The cellulose ether is soluble in cold water (in some cases a specific organic solvent), and its dissolution process is swelling and hydration. Cellulose ether solutions do not have significant solubility limitations as seen in the dissolution of ionic salts. The concentration of the cellulose ether is generally limited by the viscosity that can be controlled by the production equipment, and also varies depending on the viscosity and chemical type required by the user. The solution concentration of the low viscosity cellulose ether is generally from 10% to 15%, and the high viscosity cellulose ether is generally limited to from 2% to 3%. Different types of CE (such as powdered or surface treated powdered or granulated) can affect the method of formulating the solution.
2.1 Untreated surface cellulose ether
Although cellulose ether is soluble in cold water, it must be completely dispersed in water to prevent agglomeration. In some cases, a high speed mixer or funnel can be used in cold water to disperse the cellulose ether powder. However, if the untreated powder is directly added to the cold water without being sufficiently stirred, a large amount of agglomerates are formed. The reason for the agglomeration is mainly that the cellulose ether powder particles are not completely wetted, and when only a part of the powder is dissolved, a gel film is formed, thereby preventing the remaining powder from continuing to be dissolved, so that the cellulose ether particles should be dispersed as much as possible before dissolving. . The following are the two dispersion methods generally used.
This method is the most common method used in cement products. Before adding water, the other powders are uniformly mixed with the cellulose ether powder to disperse the cellulose ether powder particles. The minimum mixing ratio is: other powder: cellulose ether powder = (3 ~ 7): 1.
In this method, the dispersion of the cellulose ether is completed in a dry state, and the other powder materials are used as a medium to disperse the cellulose ether particles, thereby avoiding the mutual adhesion of the cellulose ether particles when water is added, thereby affecting further dissolution. . Therefore, no hot water is required for dispersion, but the dissolution rate depends on the powder particles and the stirring.
First heat the required water volume from 1/5 to 1/3 to above 90 °C, add cellulose ether, stir until all the particles are dispersed and wetted, and then add the remaining water amount as cold water or ice water. The temperature of the solution is lowered, and once the dissolution temperature of the cellulose ether is reached, the powder begins to hydrate and the viscosity increases.
It is also possible to heat the entire amount of water, and then add the cellulose ether while stirring to cool until the hydration is complete. Sufficient cooling is important for the complete hydration and viscosity formation of the cellulose ether. To achieve the desired viscosity, the MC solution should be cooled to 0 to 5 °C, while the HPMC only needs to be cooled to 20 to 25 °C or below. Since full hydration requires adequate cooling, HPMC solutions are typically used where cold water cannot be used. At the same temperature at lower temperatures, HPMC has less temperature reduction than MC. It is worth noting that the hot water dispersion method only makes the cellulose ether particles uniformly dispersed at a relatively high temperature, but at this time, no solution is formed, and a solution having a certain viscosity must be cooled.
In many cases, cellulose ethers are required to have both dispersible and rapid hydration (viscosity formation) properties in cold water. The surface-treated cellulose ether is temporarily insoluble in cold water by special chemical treatment, which ensures that when the cellulose ether is added to water, it does not immediately form a significant viscosity, but can be in a relatively small shear condition. Disperse. The “delay” of hydration or viscosity formation is the result of a combination of the degree of surface treatment, temperature, pH value of the system, and concentration of the cellulose ether solution. The higher the concentration, the higher the temperature, and the greater the p H value, the slower the hydration delay. However, when the concentration of the cellulose ether is generally only 5% (mass ratio of water), the concentration value is taken as a factor to be considered.
For the best results, and to achieve the hydration, the surface treatment of the cellulose ether should be stirred under neutral conditions for a few minutes, while stirring to adjust the pH to 8.5 ~ 9. 0, until the maximum viscosity ( Usually takes 10 to 30 minutes). Once the pH is changed to alkaline (pH 8.5 to 9. 0), the surface treated cellulose ether can be completely and rapidly dissolved, and the solution can be stably present at a p H value of 3 to 11. However, it is worth noting that adjusting the pH of the high-concentration slurry will result in too high a viscosity to pump and pour, and the pH should be adjusted after the slurry has been diluted to the desired concentration.
In summary, the dissolution process of cellulose ether includes two processes of physical dispersion and chemical dissolution. The key is to disperse the particles of cellulose ether before dissolving to avoid agglomeration due to high viscosity at low temperature dissolution, affecting further Dissolved.
3 properties of cellulose ether solution
Different types of aqueous cellulose ether solutions will gel at their specific temperatures. The gel is completely reversible and forms a solution upon cooling again. The reversible thermogelability of the cellulose ether is unique. In many cement products, the viscosity of the cellulose ether and the corresponding water retention and lubricating properties are mainly utilized, and the viscosity is directly related to the gel temperature. Under the gel temperature, the lower the temperature, the more the viscosity of the cellulose ether High, the corresponding water retention performance is better.
The gelation phenomenon can now be explained in such a way that during the dissolution process, the thread-like polymer molecules are linked to the water molecule layer to cause swelling. The action of water molecules, such as lubricating oils, allows the distance between the long polymer molecular chains to be pulled apart, giving the solution a viscous fluid property that is easy to pour. When the temperature of the solution rises, the cellulose polymer gradually loses water, and the viscosity of the solution decreases. When the gel point is reached, the polymer
Complete dehydration, resulting in a link between the polymers, forming a gel. As the temperature remains above the gel point, the strength of the gel will continue to increase.
As the solution cools, the gel begins to reverse and the viscosity decreases. Finally, the viscosity change of the cooling solution returns to the initial temperature rise curve and increases with decreasing temperature. The solution can be cooled to its initial viscosity value. Therefore, the thermogel process of cellulose ether is reversible.
The main role of cellulose ether in cement products is as a thickner, plasticizer and water retention agent, so how to control the viscosity and gel temperature is an important factor.
In cement products, the temperature is usually controlled below its initial gel temperature point. The lower the temperature, the higher the viscosity, and the more obvious the viscosity-increasing water retention effect. The test results carried out in the production line of the extrusion cement board also show that the lower the temperature of the cellulose ether under the same dosage, the better the viscosity-increasing and water-retaining effect. Since the cement system is a system with extremely complex physicochemical properties, there are many factors that affect the temperature and viscosity changes of the cellulose ether gel, and the influence trends and degrees of various factors vary. Therefore, in practical applications, it has also been found that the actual gel temperature point of various cellulose ethers (i.e., the viscosity-increasing and water-retaining effects are significantly reduced at this temperature) after being incorporated into the cement system are lower than the gel temperature indicated by the product. Therefore, in the selection of cellulose ether products, the factors causing the gel temperature drop should be considered. The main factors affecting the viscosity and gel temperature of the cellulose ether solution in cement products are as follows:
3.1 The effect of pH on viscosity
Since MC and HPMC are non-ionized, the viscosity of their solutions generally has a broader range of p H stability than the viscosity of natural ionic rubber, but if the pH exceeds the range of 3 to 11, they are at higher temperatures or at After a long period of storage, the viscosity will gradually decrease, especially in high viscosity solutions. The viscosity of the cellulose ether product solution in a strong acid or alkali solution is reduced, mainly because the alkali and acid promote the dehydration of the cellulose ether. Therefore, the viscosity of the cellulose ether generally has a certain degree of decline in the alkaline environment of the cement product.
3.2 Effect of heating rate and stirring on the gel process
The temperature of the gel point is a combination of the heating rate and the agitation shear rate. High speed agitation and rapid temperature rise generally result in a significant increase in gel temperature. This is advantageous for mechanized agitated cement products.
3.3 Effect of concentration on thermal gel
Increasing the concentration of the solution generally lowers the gel temperature, and the low viscosity cellulose ether has a higher gel point than the high viscosity cellulose ether.
3.4 Effect of additives on thermal gel properties
Many materials in the field of building materials are inorganic salts, which can have a significant impact on the gel temperature of the cellulose ether solution. Some of the additives may increase the thermal gel temperature of the cellulose ether, while others may lower the thermogel temperature of the cellulose ether depending on whether the additive acts to promote coagulation or solubilization. For example, solubilizer ethanol, PEG 400 (polyethylene glycol), propylene glycol, etc. can increase the gel point. Salts, glycerin and sorbitol reduce the gel point. Nonionic cellulose ethers are generally not precipitated due to polyvalent metal ions, but when the concentration of electrolyte or other dissolved substances exceeds a certain limit, the cellulose ether product can be salted out in solution, which is due to The competition of the electrolyte for water leads to a decrease in the hydration of the cellulose ether and salting out the cellulose ether product. The amount of salt in the HPMC product solution is generally slightly higher than that of the MC product, and the salt content is slightly different in different HPMC.
In summary, (1) cellulose ether is obtained by etherification of natural cellulose.
The basic structural unit having anhydroglucose has different properties depending on the kind and amount of the substituent group at the position of substitution. Among them, nonionic ethers such as MC and HPMC can be widely used as building materials in tackifiers, water retaining agents, and air entraining agents. (2) Cellulose ethers have a unique solubility, forming a solution at a certain temperature (thermogel temperature) and forming a solid gel or a mixture of solid particles at the thermogel temperature. Dissolution methods mainly include dry mixing and dispersing methods, hot water dispersion methods, etc., and commonly used in cement products are dry mixed dispersion methods. The key is to disperse the cellulose ether evenly before it dissolves and form a solution at low temperatures. (3) The concentration of the solution, the temperature, the pH of the solution, the chemical nature of the additive and the stirring rate all affect the gel temperature and viscosity of the cellulose ether solution, especially the inorganic salt solution in which the cement product is alkaline, which usually decreases. The gel temperature and viscosity of the cellulose ether solution have an adverse effect. Therefore, depending on the characteristics of the cellulose ether, it is first necessary to use it at a lower temperature (below the gel temperature), and secondly, the influence of additives and the like.
Landercoll® cellulose ethers are unique in the industry and have been developed to impart a range of properties in dry mix mortars. Depending on the end-user requirements, Landercoll® products provide,
• Increased water retention
• Improved consistency to make thin layer products workable
• Controlled rheology to provide sag resistance
• Reduced segregation of different formulation ingredients
• Improved adhesion on porous substrates
• Optimized air pore stability for improved workability
• Improved adhesion to polystyrene boards