The Importance of Water Retention of HPMC
April 24th 2024
Building gypsum powder is the main material of gypsum putty and is the basic material to ensure the bond strength and impact strength, therefore there are strict requirements on its quality.
Physical properties
Fineness: all passed through 120 mesh sieve.
Initial setting time: not less than 6min
Final setting time: less than 30min
2h flexural strength: greater than 2.1MPa
2h compressive strength: more than 4.9MPa
Whiteness: greater than 85 when used directly in the production of decorative layer plaster putty; and higher than that used in the production of paint or wallpaper base.
In the ingredients of gypsum putty, although CMC has a certain viscosity, it can have damaging effects of varying degrees on the strength of gypsum, especially the surface strength. So a small amount of binder should be added, making it migrate to the surface during the drying process of gypsum putty and increase the surface strength of gypsum putty.
Otherwise, there will appear surface dusting phenomena for the gypsum putty scraped onto the walls, because of not spraying coatings for a long time. But using MC, HPMC or HEC instead, we will not need to add binders, because they are different from CMC, can be used as powdery binders and will cause no or little decrease on the gypsum strength. The commonly used binders of gypsum putty are gelatinized starch, starch, oxidized starch, water-soluble polyvinyl alcohol at room temperature and redispersible polymer powder, etc.
A very small amount of penetrant is added to make the gypsum putty bond better to the substrate. Commonly used penetrants include anionic and non-ionic types.
Talcum powder in the gypsum putty is mainly to improve the applicability of the paste, to facilitate scraping and to increase the surface finish. Fineness: all pass through 325 mesh sieve; Na2O content: less than 0.10%; K2O content: less than 0.30%.
For Wall Putty/Plaster
The blade coating performance of gypsum putty slurry is mainly ensured by water retention agent, which can ensure the compatibility of plaster putty slurry, so that the water in the gypsum putty layer will not be absorbed by the wall too quickly, and avoid the dusting and peeling phenomena caused by insufficient water required for hydration of plaster. Water retention agents are mainly derivatives of cellulose ethers, such as methyl cellulose (MC), hydroxyethyl cellulose (HEC), hydroxypropyl methyl cellulose (HPMC) and carboxymethyl cellulose (CMC).
The hardened gypsum is inherently soft and brittle. Once the gypsum putty layer is too thick, the interfacial layer can easily flake off, so adding a certain amount of flexibiliser and penetrant can increase the degree of flexibility of the gypsum putty and can further improve the operational performance of the gypsum putty slurry. Commonly used softening agents include a variety of sulphonates and lignin fibres.
Although some cellulose ethers and binders have a retarding effect on gypsum, the retarding effect does not meet the time requirements of gypsum putty, so a certain amount of retarder is also required.
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