1. Introduction
Hydroxypropyl methylcellulose (HPMC) is a cellulose ether widely used in building materials. In the application of putty powder, HPMC plays a vital role in its adhesion and water retention, which directly affects the construction performance and use effect of putty powder.

2. Basic characteristics of HPMC
HPMC is a non-ionic cellulose ether obtained by chemical modification of natural cellulose. Methoxy and hydroxypropoxy groups are introduced into its molecular structure, which makes HPMC have good water solubility and film-forming properties. In addition, HPMC also has high chemical stability and resistance to enzymatic hydrolysis, which makes it widely used in building materials.

3. Mechanism of enhancing the adhesion of putty powder
3.1 Surface activity and wettability
HPMC has good surface activity, which can reduce the interfacial tension between putty powder and the substrate surface and increase the wettability of the material. When putty powder contacts the substrate, HPMC can promote the uniform distribution of fine particles in the putty powder and closely contact the substrate surface to form a dense coating, thereby improving the adhesion of the putty powder.

3.2 Film-forming properties
HPMC can form a stable colloidal solution in an aqueous solution. As the water evaporates, HPMC will form a tough and elastic film on the surface of the substrate. This film can not only physically enhance the bonding between the putty powder and the substrate, but also buffer the stress caused by temperature changes or slight deformation of the substrate, thereby effectively preventing the cracking and shedding of the putty powder layer.

3.3 Bonding bridge effect
HPMC can act as a binder in putty powder to form a bonding bridge. This bonding bridge not only enhances the adhesion of the components in the putty powder, but also improves the mechanical interlocking effect between the putty powder and the substrate. The long-chain molecules of HPMC can penetrate into the pores or rough surface of the substrate, thereby further enhancing the adhesion of the putty powder.

4. Mechanisms for improving water retention of putty powder
4.1 Water retention and delayed drying
HPMC has good water retention properties and can delay the volatilization of water in putty powder. This property is particularly important during the construction process, because putty powder needs sufficient water for hydration reaction and gelation during the drying process. HPMC can effectively retain water, so that putty powder can maintain a suitable construction consistency for a long time, thereby improving construction performance and preventing cracking caused by drying too quickly.

4.2 Increase the uniformity of water distribution
The mesh structure formed by HPMC in putty powder can evenly distribute water and avoid the problem of excessive or insufficient local water. This uniform water distribution not only improves the operability of putty powder, but also ensures uniform drying of the entire coating, reducing the uneven shrinkage and stress concentration problems that may occur during the drying process.

4.3 Improve moisture retention
HPMC adjusts the humidity of putty powder by absorbing and releasing water, so that it can maintain a suitable degree of wetness under various construction conditions. This moisture retention not only prolongs the open time of putty powder, but also increases the working time of putty powder, allowing construction workers to complete construction operations more calmly and reduce the need for rework and repair.

5. Application Examples
In actual applications, the concentration of HPMC in putty powder is usually between 0.1% and 0.5%, and the specific concentration depends on the formula of putty powder and construction requirements. For example, when constructing in high temperature or dry environment, the amount of HPMC can be appropriately increased to improve the water retention and anti-drying ability of putty powder. On the other hand, in occasions where higher adhesion is required, the bonding performance of putty powder can also be enhanced by increasing the content of HPMC.

The application of HPMC in putty powder significantly improves its adhesion and water retention. These two aspects of enhancement are achieved through the surface activity, film-forming properties, bonding bridge effect of HPMC, and its water retention, delayed drying and moisture retention ability. The introduction of HPMC not only improves the construction performance of putty powder, but also improves the overall quality of the coating, reduces problems during construction, and provides reliable technical support for the development of building decoration materials.