Hydroxypropyl methylcellulose (HPMC) is a polymer compound widely used in construction, medicine, food and other fields. Due to its excellent water retention, it plays an important role in cement mortar, putty powder, coatings and pharmaceutical preparations. The water retention of HPMC is affected by multiple factors, including its molecular structure, degree of substitution, viscosity, addition amount, ambient temperature, water absorption of the substrate and formulation system.

1. Influence of molecular structure and degree of substitution
HPMC consists of a cellulose skeleton structure and methoxy (–OCH?) and hydroxypropoxy (–OCH?CHOHCH?) substituents, and its degree of substitution plays a key role in water retention. The presence of substituents increases the hydrophilicity of HPMC, while also affecting its solubility and film-forming properties. Generally speaking, the higher the degree of hydroxypropyl substitution, the stronger the hydrophilicity and water retention of HPMC. A higher methoxy content helps to improve solubility, making it easier to lock in water and slowing down the rate of water evaporation.

2. Effect of viscosity
The viscosity of HPMC is an important parameter to measure the rheological properties of its solution, usually expressed as the viscosity of a 2% aqueous solution (mPa·s). The solution formed by high-viscosity HPMC is denser and can form a more stable water film on the surface of the material, delaying the evaporation and penetration of water and improving the water retention capacity. The low-viscosity HPMC solution has strong fluidity and is suitable for application environments that require rapid water release. Therefore, in fields such as building mortar, high-viscosity HPMC is more conducive to improving water retention, while low-viscosity HPMC is suitable for application scenarios that require faster drying.

3. Effect of addition amount
The water retention performance of HPMC increases with the increase of addition amount, but the more the better. The appropriate amount of HPMC can form a stable hydration film in the mortar or coating system, reduce the rapid loss of water, and improve the construction operability. However, excessive use may lead to excessive viscosity and affect the construction performance, such as reducing the fluidity of the mortar and prolonging the setting time. Therefore, in practical applications, the amount of HPMC needs to be comprehensively considered to achieve the best water retention effect.

4. Effect of ambient temperature
Temperature has a significant effect on the water retention of HPMC. Under high temperature, water evaporates faster, and the water in mortar or paint is easily lost, resulting in a decrease in construction performance. HPMC has certain thermal gel properties. When it exceeds its gel temperature, it will precipitate water, affecting the water retention effect. Therefore, in hot or dry environments, it is necessary to select suitable HPMC varieties and appropriately increase its addition amount to ensure water retention. In addition, measures such as covering and wet curing can be taken to reduce water loss.

5. Water absorption rate of substrate
Different substrates have different water absorption capacities, which will also affect the water retention performance of HPMC. Substrates with high water absorption, such as bricks, gypsum boards, etc., will quickly absorb water, reduce the water in the mortar or putty layer, and affect the adhesion and construction performance. In this case, the use of high-viscosity, high-substitution HPMC can form a more durable water-retaining film on the surface to reduce water loss. In addition, appropriate adjustments to the formula, such as adding water-retaining agents or reducing the water absorption rate of the substrate, can also improve the overall water retention performance.

6. Influence of the formulation system
HPMC usually works together with other components in mortar, putty or coating systems, and its water retention will be affected by the overall formulation. For example, the proportion of cement, lime, hydrated gypsum and other cementitious materials in cement mortar directly affects the hydration reaction rate and water retention capacity. The use of admixtures such as air entraining agents, thickeners, and fibers will also affect the distribution state of HPMC, thereby changing its water retention effect. Therefore, when designing the formulation, it is necessary to comprehensively consider the interaction between HPMC and other ingredients to optimize the final water retention performance.

The water retention of HPMC is affected by many factors, including molecular structure, degree of substitution, viscosity, addition amount, ambient temperature, water absorption rate of the substrate, and formulation system. In specific applications, it is necessary to select the appropriate HPMC variety and addition amount according to different use environments and material properties to achieve the best water retention effect. In addition, adjusting the formula and process in combination with construction conditions can also further optimize the water retention performance and improve the quality and use effect of the final product.