Hydroxypropyl methylcellulose (HPMC) is a non-ionic cellulose ether that is widely used in building materials, especially in dry-mixed mortar systems. It has become a key additive due to its excellent film-forming, water-retaining, thickening and other properties.
1. Film-forming mechanism of HPMC
HPMC has good water solubility and exists as a polymer dispersed in the water phase in the mortar. When the mortar begins to hydrate and harden after mixing, the HPMC molecules gradually gather and interweave with the evaporation of water, and finally form a continuous polymer film on the surface or inside of the mortar. This film layer can wrap cement particles and fill the mortar pores, playing a bonding and protective role on the overall structure.
The process of HPMC film formation is affected by temperature, humidity, water-cement ratio and HPMC dosage. Generally speaking, at suitable temperature (20℃～40℃) and relative humidity, HPMC is more likely to form a flexible and continuous film to enhance the overall performance of the mortar.
2. Effect of HPMC film formation on mortar performance
Enhance water retention
After film formation, HPMC can form a closed microenvironment in the mortar, slowing down the speed of water migration outward, thereby effectively prolonging the cement hydration time and improving the degree of hydration. This plays an important role in preventing early cracking, improving bonding strength and durability.
Improve construction performance
The film formed by HPMC gives the mortar good lubricity and smoothness, reduces the phenomenon of mortar sticking to tools during construction, and improves construction efficiency. Especially in tile adhesives, plaster mortars, and self-leveling mortars, the film-forming properties have a significant improvement on the operating feel.
Improve bonding performance
After film formation, HPMC forms a bonding bridge between the mortar and the base or veneer, enhancing the interface adhesion ability. At the same time, due to the presence of its elastic film, it can relieve stress concentration, improve the crack resistance of the bonding layer, and reduce the risk of falling off.
Improve crack resistance and flexibility
The HPMC film has a certain flexibility and elasticity, which can effectively absorb the stress caused by shrinkage, temperature change or base deformation, prevent mortar cracking, and thus improve durability and aesthetics.
Improve surface appearance
The film-forming HPMC can make the mortar surface denser and smoother, reduce water seepage, help subsequent finishing construction, and improve the decorative effect.
3. Factors affecting film-forming performance
Molecular weight and degree of substitution
The higher the molecular weight of HPMC, the better the strength and toughness after film formation, but the dissolution rate becomes slower. The degree of substitution (the content of methoxy and hydroxypropyl) determines its hydrophilicity and film-forming temperature. A moderate degree of substitution is conducive to the dual effects of film formation and water retention.
Addition amount
The amount of HPMC in mortar is generally controlled at 0.1%~0.5%. Excessive amount will lead to prolonged setting time and reduced strength. Too little will result in incomplete film formation, affecting water retention and construction performance.
Construction environment
A high temperature and dry environment will accelerate the evaporation of water, which may cause HPMC to be "burned dry" before it can fully form a film, affecting its performance. During construction, attention should be paid to maintenance and humidity control to facilitate the smooth completion of the film-forming process.
The film-forming performance of HPMC in mortar is the basis for the realization of its various functions. By forming a continuous polymer film before hardening, HPMC significantly improves the water retention, workability, adhesion and crack resistance of the mortar. Reasonable selection of the model and dosage of HPMC, and optimization of the formula in combination with the actual construction environment, are the key to exerting its film-forming advantages. With the development of green building materials and the improvement of construction quality requirements, HPMC, as a high-performance modified material, will play a more important role in the field of building mortar.
Post time: Apr-07-2025