Summary:
As the most important additive in tile adhesives, cellulose ether has a strong influence on the drawing strength and open time of tile adhesives, and these two items are also important indicators of high-performance tile adhesives. The effects of ethers on the properties of tile adhesives are summarized and reviewed.
Cellulose ether; Pulled knot strength; Open time
1 Introduction
Cement-based tile adhesive is currently the largest application of special dry-mixed mortar, which is composed of cement as the main cementitious material and supplemented by graded aggregates, water-retaining agents, early strength agents, latex powder and other organic or inorganic additives mixture. Generally, it only needs to be mixed with water when used. Compared with ordinary cement mortar, it can greatly improve the bonding strength between the facing material and the substrate, and has good slip resistance and excellent water and water resistance. It is mainly used to paste decorative materials such as building interior and exterior wall tiles, floor tiles, etc. It is widely used in interior and exterior walls, floors, bathrooms, kitchens and other building decoration places. It is currently the most widely used Tile bonding material.
Usually when we judge the performance of a tile adhesive, we not only pay attention to its operational performance and anti-sliding ability, but also pay attention to its mechanical strength and opening time. Cellulose ether in tile adhesive not only affects the rheological properties of porcelain adhesive, such as smooth operation, sticking knife, etc., but also has a strong influence on the mechanical properties of tile adhesive.
2. Influence on the open time of tile adhesive
When rubber powder and cellulose ether co-exist in wet mortar, some data models show that rubber powder has stronger kinetic energy to attach to cement hydration products, and cellulose ether exists more in the interstitial fluid, which affects more Mortar viscosity and setting time. The surface tension of cellulose ether is higher than that of rubber powder, and more cellulose ether enrichment on the mortar interface will be beneficial to the formation of hydrogen bonds between the base surface and cellulose ether.
In the wet mortar, the water in the mortar evaporates, and the cellulose ether is enriched on the surface, and a film will be formed on the surface of the mortar within 5 minutes, which will reduce the subsequent evaporation rate, as more water is removed from the thicker mortar Part of it migrates to the thinner mortar layer, and the film formed at the beginning is partially dissolved, and the migration of water will bring more cellulose ether enrichment on the mortar surface. As shown in Figure 1
Therefore, the film formation of cellulose ether on the surface of the mortar has a great influence on the performance of the mortar. 1) The formed film is too thin and will be dissolved twice, which cannot limit the evaporation of water and reduce the strength. 2) The formed film is too thick, the concentration of cellulose ether in the mortar interstitial liquid is high, the viscosity is high, and it is not easy to break the surface film when the tiles are pasted. It can be seen that the film-forming properties of cellulose ether have a greater impact on the open time.
The type of cellulose ether (HPMC, HEMC, MC, etc.) and the degree of etherification (substitution degree) directly affect the film-forming properties of cellulose ether, as well as the hardness and toughness of the film.
Migration state of cellulose ether in wet mortar (upper part is dense ceramic tile, lower part is porous concrete base)
3 Influence on the pull-out strength
In addition to imparting the above-mentioned beneficial properties to mortar, cellulose ether also delays the hydration kinetics of cement. This retarding effect is mainly due to the adsorption of cellulose ether molecules on various mineral phases in the cement system being hydrated, but generally speaking, the consensus is that cellulose ether molecules are mainly adsorbed on water such as C-S-H and calcium hydroxide. On the chemical products, it is rarely adsorbed on the original mineral phase of clinker. In addition, cellulose ether reduces the mobility of ions (Ca2+, SO42-, …) in the pore solution due to the increased viscosity of the pore solution, thereby further delaying the hydration process.
Viscosity is another important parameter, which represents the chemical characteristics of cellulose ether. As mentioned above, the viscosity mainly affects the water retention capacity and also has a significant effect on the workability of the fresh mortar. However, experimental studies have found that the viscosity of cellulose ether has almost no effect on cement hydration kinetics. Molecular weight has little effect on hydration, and the maximum difference between different molecular weights is only 10min. Therefore, molecular weight is not a key parameter to control cement hydration.
The general trend is that for MHEC, the higher the degree of methylation, the less retarding effect of cellulose ether. In addition, the retarding effect of hydrophilic substitution (such as substitution to HEC) is stronger than that of hydrophobic substitution (such as substitution to MH, MHEC, MHPC). The retarding effect of cellulose ether is mainly affected by two parameters, the type and quantity of substituent groups.
Our systematic experiments also found that the content of substituents plays an important role in the mechanical strength of tile adhesives. We evaluated the performance of HPMC with different degrees of substitution in tile adhesives, and tested the effect of cellulose ethers containing different groups under different curing conditions on The influence of the mechanical properties of tile adhesives, Figure 2 and Figure 3 are the effects of changes in methoxyl (DS) content and hydroxypropoxyl (MS) content on the pull-out properties of tile adhesives at room temperature.
In the test, we consider HPMC, which is a compound ether, so we have to put the two pictures together. For HPMC, it needs a certain degree of absorption to ensure its water solubility and light transmittance. We know the content of substituents It also determines the gel temperature of HPMC, which also determines the use environment of HPMC. In this way, the group content of HPMC that is usually applicable is also framed within a range. In this range, how to combine methoxy and hydroxypropoxy In order to achieve the best effect is the content of our research. Within a certain range, an increase in the content of methoxyl groups will lead to a downward trend in the pull-out strength, while an increase in the content of hydroxypropoxyl groups will lead to an increase in the pull-out strength. There is a similar effect for opening hours. The effect of HPMC with different substituent content on the mechanical properties under the condition of open time of 20 minutes.
The change trend of the mechanical strength under the open time condition is consistent with the normal temperature condition, which is consistent with the toughness of the cellulose ether film we talked about in Section 2. The content of methoxyl (DS) is high and the content of hydroxypropoxyl HPMC with low (MS) content has good toughness of the film, but it will affect the wettability of the wet mortar to the surface material.
4 Summary
Cellulose ethers, especially methyl cellulose ethers such as HEMC and HPMC, are essential additives in many dry-mix mortar applications. The most important property of cellulose ethers is their water retention in mineral building materials. If cellulose ether is not added, the thin layer of fresh mortar will dry out very quickly, so that the cement cannot be hydrated in a normal way, so that the mortar cannot harden and cannot obtain good bonding properties to the base layer. There are many factors that affect the water retention of cellulose ether, such as dosage and viscosity, and its internal composition: the degree of substitution has a greater impact on the final performance of mortar. For a long time, we have believed that the viscosity of cellulose ether is important for cement-based materials. The setting time of cement has a great influence. Recent studies have found that the change of viscosity has little effect on the setting time of cement. On the contrary, the type and combination of substituent groups are the most important factors affecting the function of cellulose ether. When we expect a high-performance tile adhesive product, we not only have to consider the rheological property changes brought about by cellulose ether, which makes the mortar easier to handle, but also consider the mechanical effects of cellulose ether products with a suitable degree of substitution. contribute.
Post time: Feb-09-2023