The Role of Redispersible Latex Powder in Tile Adhesives
Release Time:
2016-09-12
Source:
The Role of Redispersible Latex Powder in Tile Adhesives
Redispersible Polymer Powder or emulsions, differ substantially from hydraulic mineral cementitious binders. Remarkably, even the addition of only a few weight percent of a white polymer powder can produce a new material that is entirely distinct from conventional cement mortars. It is precisely this modifying effect on mortars that ultimately gave rise to today’s thin-set tile adhesives and their associated application methods. Compared with earlier construction practices, this approach is highly economical in both time and material consumption. In the early 1930s, the first patents were recorded for the modification of cementitious systems using synthetic polymer emulsions. Less than three decades later, such products began to be available in various powdered forms. These formulations simplified on-site handling and ultimately opened up the market.
The various direct and indirect effects of redispersible polymer powder on the bond strength of tile adhesives are as follows: on the one hand, the latex film forms a flexible interface between the mortar and the tile; on the other hand, in fresh mortar, redispersible polymer powder increases the air content and influences surface formation and wettability. Subsequently, during the setting process, it also affects shrinkage behavior and the hydration of cement. All these parameters influenced by redispersible polymer powder ultimately impact bond strength.
During mixing, the redispersible polymer powder in fresh mortar spontaneously redisperses throughout the mix without agglomerating with cement particles. The “lubricating effect” of the redispersible polymer powder particles imparts excellent workability to the mortar mix, resulting in superior construction performance. In addition, the air-entraining properties of the redispersible polymer powder make the mortar more compressible, thereby facilitating troweling operations. Depending on the specific type of redispersible polymer powder used, the modified mortar can exhibit either improved plasticity or increased viscosity.
Cellulose ethers and redispersible polymer powders help retain water within the mortar. Some of this water is lost through evaporation during drying, leading to the formation of capillary pores. The remaining water is consumed by cement hydration, resulting in gel pores. Consequently, the number of gel pores serves as an indicator of the degree of hydration. Adding cellulose ether at a dosage of 0.4% by weight of the dry mortar reduces the degree of hydration; compared with dense concrete, tile adhesives exhibit an astonishingly high total porosity—exceeding 40% by volume. However, since tile adhesives are not subjected to high compressive stresses, such a high porosity does not pose any problems. To further enhance workability, tile adhesives typically contain very fine fillers; yet the finer the filler, the greater the water demand.
The incorporation of redispersible polymer powder has a positive effect on the development of bond strength. The tensile strength of the polymer film is one order of magnitude higher than that of plain mortar. Therefore, the addition of redispersible polymer powder can enhance the tensile (cohesive) strength of the mortar. The soft, high-tensile-strength polymer film can adhere effectively to the smooth, vitrified tile surface, thereby improving the overall performance of the mortar, including achieving exceptionally high bond strength. Polymers and cement coexist in a novel composite material with outstanding properties, which exhibits superior quality and new functional characteristics.
