Hydroxypropyl methyl cellulose ether plays an important role in dry mixed mortar

In dry mixed mortar, hydroxypropyl methyl cellulose ether is a major additive which can significantly improve the performance of wet mortar and affect the performance of mortar construction.The reasonable selection of cellulose ether with different varieties, different viscosity, different particle size, different viscosity degree and adding amount has a positive impact on the performance improvement of dry mortar.Currently, many masonry and plaster mortar water retention performance is not good, a few minutes of standing will appear water slurry separation.
Water retention is an important property of methyl cellulose ether, but also a lot of dry mortar manufacturers in China, especially in the south of the higher temperature area of the manufacturers concerned about the performance.The factors affecting the water-retaining effect of dry mortar include the addition amount of MC, the viscosity of MC, the fineness of the particles and the temperature of the operating environment.
Hydroxypropyl methyl cellulose ether is a synthetic polymer made from natural cellulose 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 special structure of natural cellulose, cellulose itself has no ability to react with etherification agent.However, after the treatment of swelling agent, the strong hydrogen bonds between and within the molecular chain are destroyed, the activity of the hydroxyl group is released into a reactive alkali cellulose, and cellulose ether is obtained after the reaction of etherifying agent -- OH group is converted into -- OR group.
The properties of hydroxypropyl methyl cellulose ether depend on the type, number and distribution of substituents.And the classification of hydroxypropyl methyl cellulose ether is also according to the type of substituent, etherification, solubility and related application type to classify.According to the type of substituents on the molecular chain, it can be divided into single ether and mixed ether. We usually use MC as single ether and HPMC as mixed ether.Methyl cellulose ether MC is the hydroxyl of natural cellulose glucose units will be replaced the product of methoxy structured for [C O H7O2 (OH) 3 - (OCH3) h] h x, hydroxypropyl methyl cellulose ether HPMC is part of the hydroxyl on the unit was replaced by the methoxyl group, the other part be replaced by hydroxypropyl get the product of formula for [C6H7O2 (OH) 3 - m - n (OCH3) m [OCH2CH (OH) CH3] n] x and hydroxyethyl methyl cellulose ether HEMC, these are widely used on the market at present the main items and sales.
In terms of solubility, it can be divided into ionic type and non-ionic type.Water-soluble nonionic cellulose ether is mainly composed of two series of alkyl ether and hydroxyl alkyl ether.Ionic CMC is mainly used in synthetic detergent textile printing and dyeing food and oil mining.Non-ionic MC, HPMC and HEMC are mainly used in building materials, latex coatings, medicine and daily chemistry.As thickening agent, water - retaining agent, stabilizer, dispersant, film - forming agent.
Hydroxypropyl methyl cellulose ether water retention: in the production of building materials, especially dry 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 the excellent water retention ability, two is the impact on the consistency and thixotropy of mortar, three is the interaction with cement.The water-retaining effect of cellulose ether depends on the water-absorbing capacity of the base, the composition of the mortar, the thickness of the mortar, the water requirement 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.It is well known that cellulose molecular chains, although containing a large number of highly hydrated OH groups, do not dissolve in water, because the cellulose structure is highly crystalline.The hydration of the hydroxyl groups alone is insufficient to pay for the strong intermolecular hydrogen bonds and van der Waals forces.Therefore, when substituents are introduced into the molecular chain, not only will the substituents break the hydrogen chain, but also the hydrogen bonds will be broken due to the wedging of substituents between adjacent chains. The bigger the substituents are, the greater the distance between molecules will be.The larger the hydrogen bond destruction effect is, after the expansion of cellulose lattice, the solution enters, cellulose ether becomes water-soluble, forming a high viscosity solution.As the temperature increases, the hydration of the polymer decreases and the water between the chains is expelled.When the dehydrating process is complete, the molecules begin to assemble, forming a three-dimensional mesh that folds out of the gel.
The factors affecting the water retention of mortar include the viscosity of cellulose ether, the amount of additives, particle fineness and the operating temperature.
The greater the viscosity of hydroxypropyl methyl cellulose ether, the better the water retention.Viscosity is an important parameter of MC performance. Currently, different MC manufacturers adopt different methods and instruments to measure the viscosity of MC. The main methods include Haake Rotovisko, Hoppler, Ubbelohde and Brookfield.For the same product, the viscosity results measured by different methods vary greatly, and some even vary exponentially.Therefore, the comparison of viscosity must be carried out between the same test methods, including temperature, rotor, etc.
Generally speaking, the higher the viscosity, the better the water retention effect.However, the higher the viscosity, the higher the molecular weight of MC, and the lower its solubility, which has a negative impact on the strength and construction performance of mortar.The higher the viscosity, the more obvious the thickening effect on the mortar, but is not proportional to the relationship.The higher the viscosity, the wet mortar will be more viscous, not only during the construction, the performance of the adhesive scraper and high adhesion to the substrate.However, the increase in the structural strength of the wet mortar itself is of little help.When both construction, the performance is not obvious for anti - droop performance.On the contrary, some methyl-cellulose ether with medium and low viscosity but modified has excellent performance in improving the structural strength of wet mortar.
The greater the amount of cellulose ether added in the mortar, the better the water retention performance, the higher the viscosity, the better the water retention performance.
For particle size, the finer the particle, the better the water retention.After the large particles of cellulose ether contact with water, the surface immediately dissolved and formed a gel to wrap the material to prevent water molecules to continue to infiltrate, sometimes a long time stirring can not be evenly dispersed and dissolved, forming a cloudy flocculent solution or clumps.The water retention of hydroxypropyl methyl cellulose ether is greatly affected, and solubility is one of the factors to choose cellulose ether.Fineness is also an important property index of methyl cellulose ether.The MC used for dry mortar requires powder, low water content, and 20%-60% particle size less than 63um for fineness.Fineness affects the solubility of methyl cellulose ether.The thicker MC is usually granular and can be easily dissolved in water in fractions without caking, but the dissolution rate is slow and it is not suitable for use in dry mortar.In dry mortar, MC is dispersed between aggregates, fine fillers and cement and other cementing materials. Only a fine enough powder can avoid methyl cellulose ether caking when mixing with water.When the MC adds water to dissolve the caking, it is difficult to dissolve the caking in dispersion.The MC with thicker fineness not only wastes, but also reduces the local strength of mortar. When such dry mortar is applied in a large area, the curing speed of local dry mortar is obviously reduced, and cracks are caused by different curing time.For the spraying mortar with mechanical construction, the mixing time is shorter and the fineness is required to be higher.
The fineness of MC also has a certain influence on its water retention. Generally speaking, for methyl cellulose ether with the same viscosity but different fineness, the finer the fineness is, the better the water retention effect is under the same dosage.
The water retention of MC is also related to the temperature used. The water retention of methyl cellulose ether decreases with the increase of temperature.However, in the actual application of materials, dry mortar in many environments will often be in the high temperature (more than 40 degrees) under the conditions of the construction of hot substrate, such as the summer under the sun external wall putty plastering, which tends to accelerate the solidification of cement and hardening of dry mortar.The decrease of water retention rate leads to the obvious feeling that both the construction performance and the anti-cracking performance are affected.Although the methyl-hydroxyethyl cellulose ether additive is considered to be at the forefront of technological development, its dependence on temperature still leads to the weakening of the performance of dry mortar.Despite the increase in methyl-hydroxyethyl cellulose (summer formula), construction and cracking resistance are not enough to meet the use needs.Through some special treatment of MC, such as increasing the etherification degree, the water retention effect of MC can be kept better under the condition of high temperature, so that it can provide better performance under harsh conditions.
In addition, hydroxypropyl methyl cellulose ether thickening and thixotropy: cellulose ether second action - thickening action depends on: cellulose ether polymerization degree, solution concentration, shear rate, temperature and other conditions.The gelation properties of solution are 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, the gel properties are also related to the modification degree of hydroxyl alkyl.For MC and HPMC with low viscosity of solution concentration, 10%-15% solution can be prepared, MC and HPMC with medium viscosity can be prepared with 5%-10% solution, while MC and HPMC with high viscosity can only be prepared with 2%-3% solution, and the viscosity classification of cellulose ether is usually graded with 1%-2% solution.High molecular weight of hydroxypropyl methyl cellulose ether thickening efficiency is high, the same solution concentration, different molecular weight polymer with different viscosity, viscosity and molecular weight between available the following said, [eta] = 2.92 x 10-2 (DPn) 0.905, DPn is average polymerization degree is high.Low molecular weight cellulose ether to add more to achieve the target viscosity.Its viscosity has a small dependence on the shear rate, high viscosity to achieve the target viscosity, the need to add less, the size of the viscosity depends on the thickening efficiency.So to achieve a certain consistency must ensure that cellulose ether a certain amount of addition (solution concentration) and solution viscosity.The gel temperature of the solution 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 high at room temperature.
Consistency can also be adjusted by selecting particle size and cellulose ether with different modification degree.The so-called modification is to introduce a certain degree of hydroxyalkyl substitution on the skeleton structure of MC.By changing the relative substitution value of the two substituents, that is, we often say the DS and ms relative substitution value of methoxy group and hydroxyl group.The various properties of cellulose ether can be obtained by changing the relative substitution values of two substituents.
Consistency and modification relationship: the addition of cellulose ether affects the water consumption of mortar and changes the water-binder ratio between water and cement, which is the thickening effect. The higher the dosage, the greater the water consumption.
Hydroxypropyl methyl cellulose ether, used in powdery building materials, must be rapidly dissolved in cold water and provide a suitable consistency for the system.If you give a certain shear rate and you still have a flocculent and a colloidal mass this is an unqualified or poor quality product.
The consistency of cement slurry and the amount of hydroxypropyl methyl cellulose ether also has a good linear relationship, cellulose ether can greatly increase the viscosity of mortar, the greater the amount of mixing, the more obvious the effect.The cellulose ether solution with high viscosity has high thixotropy, which is also a characteristic of hydroxypropyl methyl cellulose ether.Aqueous solutions of MC polymers usually have pseudo-plastic, non-thixotropic fluidity below their gel temperature, but Newtonian fluidity at low shear rates.Pseudoplasticity increases with the increase of molecular weight or concentration of cellulose ether, and is independent of the type and degree of substitution.Therefore, cellulose ether of the same viscosity grade, whether MC, HPMC, HEMC, as long as the concentration and temperature remain constant, always shows the same rheological properties.As the temperature increases, structural gels are formed, resulting in high thixotropic flows.Cellulose ether with high concentration and low viscosity exhibits thixotropy even below the gel temperature.This property is beneficial to the construction of building mortar to adjust its flow and hang.It is necessary to note 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 hydroxypropyl methyl cellulose ether, the lower 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 is not completely proportional.Some medium and low viscosity, but through the modification of hydroxypropyl methyl cellulose ether in wet mortar to improve the structural strength of a better performance, with the increase in viscosity, cellulose ether water retention increased.
Hydroxypropyl methyl cellulose ether retarded coagulation: cellulose ether the third effect is to delay the hydration process of cement.Hydroxypropyl methyl cellulose ether gives mortar a variety of beneficial properties, but also can reduce the early cement hydration heat, delay the cement hydration dynamic process.This is detrimental to the use of mortar in cold areas.This retarding effect is caused by the adsorption of cellulose ether molecules on hydration products such as c-s-h and ca (OH) 2. As the pore solution viscosity increases, cellulose ether reduces the activity of ions in solution, thus delaying the hydration process.The higher the concentration of cellulose ether in the mineral gel material, the more obvious the hydration delay.Cellulose ether not only delays the setting, but also delays the hardening process of cement mortar system.The retarding effect of cellulose ether depends not only on its concentration in the mineral gel system, but also on its chemical structure. The higher the degree of HEMC methylation, the better the retarding effect of cellulose ether, and the stronger the retarding effect of hydrophilic substitution than that of hydrophilic substitution.However, the viscosity of cellulose ether has little effect on the hydration kinetics of cement.
With the increase of hydroxypropyl methyl cellulose ether content, the setting time of mortar increased significantly.The initial setting time of mortar has a good linear correlation with the content of cellulose ether, and the final setting time has a good linear correlation with the content of cellulose ether.We can control the operational time of mortar by changing the content of cellulose ether.
In summary, in ready-mixed mortar, hydroxypropyl methyl cellulose plays a role in water retention, thickening, delaying cement hydration power, and improving construction performance.Good water retention ability makes cement hydration more complete, can improve the wet mortar wet viscosity, improve the mortar bond strength, can adjust the time.Mechanical spraying mortar adding hydroxypropyl methyl cellulose ether can improve the mortar spraying or pumping performance, as well as structural strength.Therefore, hydroxypropyl methyl cellulose ether is widely used as an important additive in premixed mortar.