Cellulose ether in cement based products

Cellulose ether is a kind of multipurpose additive which can be used in cement products. This paper introduces the chemical properties of methyl cellulose (MC) and hydroxypropyl methyl cellulose (HPMC /) commonly used in cement products, the method and principle of the net solution and the main characteristics of the solution. The decrease of thermal gel temperature and viscosity in cement products was discussed based on practical production experience.

Key words: cellulose ether; Methyl cellulose; Hydroxypropyl methyl cellulose; Hot gel temperature; viscosity

1. Overview

Cellulose ether (CE for short) is made of cellulose through etherification reaction of one or several etherifying agents and dry grinding. CE can be divided into ionic and non-ionic types, among which non-ionic type CE because of its unique thermal gel characteristics and solubility, salt resistance, heat resistance, and has appropriate surface activity. It can be used as water retaining agent, suspension agent, emulsifier, film forming agent, lubricant, adhesive and rheological improver. The main foreign consumption areas are latex coatings, building materials, oil drilling and so on. Compared with foreign countries, the production and application of water-soluble CE is still in its infancy. With the improvement of people’s health and environmental awareness. Water-soluble CE, which is harmless to physiology and does not pollute the environment, will have great development.

In the field of building materials usually selected CE is methyl cellulose (MC) and hydroxypropyl methyl cellulose (HPMC), can be used as paint, plaster, mortar and cement products plasticizer, viscosifier, water retention agent, air entraining agent and retarding agent. Most of the building materials industry is used at normal temperature, using conditions are dry mix powder and water, less involving the dissolution characteristics and hot gel characteristics of CE, but in the mechanized production of cement products and other special temperature conditions, these characteristics of CE will play a more full role.

2. Chemical properties of CE

CE is obtained by treating cellulose through a series of chemical and physical methods. According to the different chemical substitution structure, usually can be divided into: MC, HPMC, hydroxyethyl cellulose (HEC), etc. : Each CE has the basic structure of cellulose — dehydrated glucose. In the process of producing CE, cellulose fibers are first heated in an alkaline solution and then treated with etherifying agents. The fibrous reaction products are purified and pulverized to form a uniform powder of a certain fineness.

The production process of MC only uses methane chloride as etherifying agent. In addition to the use of methane chloride, the production of HPMC also uses propylene oxide to obtain hydroxypropyl substituent groups. Various CE have different methyl and hydroxypropyl substitution rates, which affects the organic compatibility and thermal gel temperature of CE solution.

The number of Substitution groups on the dehydrated glucose structural units of cellulose can be expressed by the percentage of mass or the average number of substitution groups (i.e., D S — Degree of Substitution). The number of substituent groups determines the properties of CE products. The effect of average degree of substitution on solubility of etherification products is as follows:

(1) low substitution degree soluble in lye;

(2) slightly high degree of substitution soluble in water;

(3) high degree of substitution dissolved in polar organic solvents;

(4) Higher degree of substitution dissolved in non-polar organic solvents.

3. Dissolution method of CE

CE has a unique solubility property, when the temperature rises to a certain temperature, it is insoluble in water, but below this temperature, its solubility will increase with the decrease of temperature. CE is soluble in cold water (and in some cases in specific organic solvents) through the process of swelling and hydration. CE solutions do not have the obvious solubility limitations that appear in the dissolution of ionic salts. The concentration of CE is generally limited to the viscosity that can be controlled by the production equipment, and also varies according to the viscosity and chemical variety required by the user. The solution concentration of low viscosity CE is generally 10% ~ 15%, and high viscosity CE is generally limited to 2% ~ 3%. Different types of CE(such as powder or surface treated powder or granular) can affect how the solution is prepared.

3.1 CE without surface treatment

Although CE is soluble in cold water, it must be completely dispersed in water to avoid clumping. In some cases, a high speed mixer or funnel may be used in cold water to disperse CE powder. However, if the untreated powder is added directly to cold water without sufficiently stirring, substantial lumps will form. The main reason for caking is that the CE powder particles are not completely wet. When only part of the powder is dissolved, a gel film will be formed, which prevents the remaining powder from continuing to dissolve. Therefore, before dissolution, the CE particles should be fully dispersed as far as possible. The following two dispersion methods are commonly used.

3.1.1 Dry mix dispersion method

This method is most commonly used in cement products. Before adding water, mix other powder with CE powder evenly, so that CE powder particles are dispersed. Minimum mixing ratio: Other powder: CE powder =(3 ~ 7) : 1.

In this method, CE dispersion is completed in the dry state, using other powder as the medium to disperse CE particles with each other, so as to avoid the mutual bonding of CE particles when adding water and affecting further dissolution. Therefore, hot water is not needed for dispersion, but the dissolution rate depends on the powder particles and stirring conditions.

3.1.2 Hot water dispersion method

(1) The first 1/5~1/3 of the required water heating to 90C above, add CE, and then stir until all particles dispersed wet, and then the remaining water in cold or ice water added to reduce the temperature of the solution, once reached the CE dissolution temperature, the powder began to hydrate, viscosity increased.

(2) You can also heat all the water, and then add CE to stir while cooling until hydration is complete. Sufficient cooling is very important for complete hydration of CE and the formation of viscosity. For ideal viscosity, MC solution should be cooled to 0~5℃, while HPMC only needs to be cooled to 20~ 25℃ or below. Since full hydration requires sufficient cooling,HPMC solutions are commonly used where cold water cannot be used: according to the information,HPMC has less temperature reduction than MC at lower temperatures to achieve the same viscosity. It is worth noting that hot water dispersion method only makes CE particles disperse evenly at a higher temperature, but no solution is formed at this time. To obtain a solution with a certain viscosity, it must be cooled again.

3.2 Surface treated dispersible CE powder

In many cases, CE is required to have both dispersible and rapid hydration (forming viscosity) characteristics in cold water. Surface treated CE is temporarily insoluble in cold water after special chemical treatment, which ensures that when CE is added to water, it will not immediately form obvious viscosity and can be dispersed under relatively small shear force conditions. The “delay time” of hydration or viscosity formation is the result of the combination of the degree of surface treatment, temperature, pH of the system, and CE solution concentration. The delay of hydration is generally reduced at higher concentrations, temperatures, and pH levels. In general, however, the concentration of CE is not considered until it reaches 5% (the mass ratio of water).

For best results and complete hydration, the surface treated CE should be stirred for a few minutes under neutral conditions, with the pH range from 8.5 to 9.0, until the maximum viscosity is reached (usually 10-30 minutes). Once the pH changes to basic (pH 8.5 to 9.0), the surface treated CE dissolves completely and rapidly, and the solution can be stable at pH 3 to 11. However, it is important to note that adjusting the pH of a high concentration slurry will cause the viscosity to be too high for pumping and pouring. The pH should be adjusted after the slurry has been diluted to the desired concentration.

To sum up, the dissolution process of CE includes two processes: physical dispersion and chemical dissolution. The key is to disperse CE particles with each other before dissolution, so as to avoid agglomeration due to high viscosity during low temperature dissolution, which will affect further dissolution.

4. Properties of CE solution

Different kinds of CE aqueous solutions will gelate at their specific temperatures. The gel is completely reversible and forms a solution when cooled again. The reversible thermal gelation of CE is unique. In many cement products, the main use of the viscosity of CE and the corresponding water retention and lubrication properties, and the viscosity and gel temperature has a direct relationship, under the gel temperature, the lower the temperature, the higher the viscosity of CE, the better the corresponding water retention performance.

The current explanation for the gel phenomenon is this: in the process of dissolution, this is similar

The polymer molecules of the thread connect with the water molecular layer, resulting in swelling. Water molecules act like lubricating oil, which can pull apart long chains of polymer molecules, so that the solution has the properties of a viscous fluid that is easy to dump. When the temperature of the solution increases, the cellulose polymer gradually loses water and the viscosity of the solution decreases. When the gel point is reached, the polymer becomes completely dehydrated, resulting in the linkage between the polymers and the formation of the gel: the strength of the gel continues to increase as the temperature remains above the gel point.

As the solution cools, the gel begins to reverse and the viscosity decreases. Finally, the viscosity of the cooling solution returns to the initial temperature rise curve and increases with the decrease of temperature. The solution may be cooled to its initial viscosity value. Therefore, the thermal gel process of CE is reversible.

The main role of CE in cement products is as a viscosifier, plasticizer and water retention agent, so how to control the viscosity and gel temperature has become an important factor in cement products usually use its initial gel temperature point below a section of the curve, so the lower the temperature, the higher the viscosity, the more obvious the effect of viscosifier water retention. The test results of extrusion cement board production line also show that the lower the material temperature is under the same content of CE, the better the viscosification and water retention effect is. As cement system is an extremely complex physical and chemical property system, there are many factors affecting the change of CE gel temperature and viscosity. And the influence of various Taianin trend and degree are not the same, so the practical application also found that after mixing cement system, the actual gel temperature point of CE (that is, the glue and water retention effect decline is very obvious at this temperature) are lower than the gel temperature indicated by the product, therefore, in the selection of CE products to take into account the factors causing gel temperature decline. The following are the main factors that we believe affect the viscosity and gel temperature of CE solution in cement products.

4.1 Influence of pH value on viscosity

MC and HPMC are non-ionic, so the viscosity of the solution than the viscosity of natural ionic glue has a wider range of DH stability, but if the pH value exceeds the range of 3 ~ 11, they will gradually reduce the viscosity at a higher temperature or in storage for a long period of time, especially high viscosity solution. The viscosity of CE product solution decreases in strong acid or strong base solution, which is mainly due to the dehydration of CE caused by base and acid. Therefore, the viscosity of CE usually decreases to a certain extent in the alkaline environment of cement products.

4.2 Influence of heating rate and stirring on gel process

The temperature of gel point will be affected by the combined effect of heating rate and stirring shear rate. High speed stirring and rapid heating will generally increase the gel temperature significantly, which is favorable for cement products formed by mechanical mixing.

4.3 Influence of concentration on hot gel

Increasing the concentration of the solution usually lowers the gel temperature, and the gel points of low viscosity CE are higher than those of high viscosity CE. Such as DOW’s METHOCEL A

The gel temperature will be reduced by 10℃ for every 2% increase in the concentration of the product. A 2% increase in the concentration of F-type products will reduce the gel temperature by 4℃.

4.4 Influence of additives on thermal gelation

In the field of building materials, many materials are inorganic salts, which will have a significant impact on the gel temperature of CE solution. Depending on whether the additive is acting as coagulant or solubilizing agent, some additives can increase the thermal gel temperature of CE, while others can decrease the thermal gel temperature of CE: for example, solvent-enhancing ethanol, PEG-400(polyethylene glycol), anediol, etc., can increase the gel point. Salts, glycerin, sorbitol and other substances will reduce the gel point, non-ionic CE generally will not be precipitated due to polyvalent metal ions, but when the electrolyte concentration or other dissolved substances exceed a certain limit, CE products can be salted out in solution, this is due to the competition of electrolytes to water, resulting in the reduction of hydration of CE, The salt content of the solution of the CE product is generally slightly higher than that of the Mc product, and the salt content is slightly different in different HPMC.

Many ingredients in cement products will make the gel point of CE drop, so the selection of additives should take into account that this may cause the gel point and viscosity of CE changes.

5.Conclusion

(1) cellulose ether is natural cellulose through etherification reaction, has the basic structural unit of dehydrated glucose, according to the type and number of substituent groups on its replacement position and has different properties. The non-ionic ether such as MC and HPMC can be used as viscosifier, water retention agent, air entrainment agent and other widely used in building materials products.

(2) CE has unique solubility, forming solution at a certain temperature (such as gel temperature), and forming solid gel or solid particle mixture at gel temperature. The main dissolution methods are dry mixing dispersion method, hot water dispersion method, etc., in cement products commonly used is dry mixing dispersion method. The key is to disperse CE evenly before it dissolves, forming a solution at low temperatures.

(3) Solution concentration, temperature, pH value, chemical properties of additives and stirring rate will affect the gel temperature and viscosity of CE solution, especially cement products are inorganic salt solutions in alkaline environment, usually reduce the gel temperature and viscosity of CE solution, bringing adverse effects. Therefore, according to the characteristics of CE, firstly, it should be used at a low temperature (below the gel temperature), and secondly, the influence of additives should be taken into account.