The amount of hydroxypropyl methylcellulose (HPMC) in putty powder is one of the key factors that determine the performance of putty powder. Reasonable HPMC addition can improve the workability, water retention, adhesion and durability of putty powder, while excessive or insufficient addition will affect the final effect of putty powder.

1. The role of HPMC in putty powder
HPMC is a commonly used water-soluble polymer with the following main functions:

(1) Enhance water retention
The main function of HPMC is to improve the water retention capacity of putty powder, making it difficult for water to be lost, prolong the open time of putty powder, and reduce cracking and powdering caused by rapid evaporation of water.

(2) Improve workability
HPMC can improve the lubricity of putty powder, make the scraping smoother, reduce construction resistance, improve construction efficiency, and reduce the labor intensity of construction personnel.

(3) Improving adhesion
HPMC can enhance the adhesion between putty powder and wall base, prevent the putty layer from falling off, and improve durability.

(4) Preventing sliding
During facade construction, HPMC can effectively prevent putty powder from sliding due to gravity, improving construction quality, especially when thick layers are constructed.

2. Factors affecting HPMC
The amount of HPMC is affected by multiple factors, including the formula of putty powder, construction environment, and the quality of HPMC.

(1) Formula of putty powder
Putty powder is usually composed of heavy calcium (calcium carbonate), double fly ash, cement, lime powder, glue powder, etc. Different formulas have different requirements for HPMC. For example, cement-based putty requires more water for its hydration reaction, so the amount of HPMC used will be relatively high.

(2) Construction environment
Temperature, humidity, and water absorption rate of the base layer also affect the amount of HPMC used. In a high temperature and dry environment, in order to avoid excessive evaporation of water, it is usually necessary to increase the amount of HPMC.

(3) HPMC quality
HPMC of different brands and models has different properties such as viscosity, substitution degree, and fineness, and has different effects on putty powder. High-viscosity HPMC has better water retention, but it may affect workability, so it is necessary to select the appropriate model according to the specific situation.

3. Recommended dosage of HPMC
The recommended dosage of HPMC generally varies according to the type of putty powder:

(1) Interior wall putty powder
The recommended dosage of HPMC is usually 0.2%~0.5% (relative to the total mass of putty powder). If the viscosity of HPMC is high, the recommended dosage is close to the lower value; if the viscosity is low, it can be appropriately increased.

(2) Exterior wall putty powder
Exterior wall putty requires better weather resistance and crack resistance, so the amount of HPMC added is generally between 0.3%~0.6% to enhance water retention and adhesion.

(3) Thick layer putty
For thick layer putty, in order to prevent rapid water loss and cracking, the amount of HPMC added can be appropriately increased, generally between 0.4% and 0.7%.

4. Precautions
(1) Avoid excessive addition
Adding too much HPMC may cause the viscosity of the putty powder to be too high, making construction difficult, scraping not smooth, and even affecting the strength after curing, causing cracking or powdering.

(2) Choose the right model
HPMC with different viscosities is suitable for different types of putty powder. For example, HPMC with a lower viscosity (400-20,000mPa·s) is suitable for general interior wall putty, while HPMC with a higher viscosity (75,000-100,000mPa·s) is more suitable for exterior wall putty or thick layer construction putty.

(3) Reasonable dispersion and dissolution
HPMC should be evenly dispersed during the production process to avoid agglomeration caused by direct addition into water. It is recommended to add gradually under low-speed stirring, or use the premixing method to mix with other powders and then add water to stir.

(4) Use with other additives
HPMC is often used in combination with other additives (such as starch ether, redispersible latex powder, etc.) to optimize the performance of putty powder.

The amount of HPMC in putty powder is an important factor affecting the workability and quality of the finished product. Generally speaking, its addition amount is between 0.2% and 0.6%, which is adjusted according to the specific formula and construction requirements. When selecting HPMC, its viscosity, degree of substitution and other characteristics should be combined to ensure that the putty powder has good water retention, adhesion and construction performance. At the same time, reasonable combination with other additives and mastering the correct dispersion method can make the best use of the role of HPMC, thereby improving the overall quality of putty powder.