(1) Building Gypsum Powder
Building gypsum powder is the main material of gypsum putty and the basic material to ensure the bond strength and impact strength, so there are stricter requirements for its quality.
Fineness: all through 120-mesh sieve;
Initial Setting Time: no less than 6min
Final Setting Time: Less than 30min
2h Flexural Strength: Greater than 2.1MPa
2h Compressive Strength: Greater than 4.9MPa
Whiteness: Higher than 85 when directly used for making the gypsum putty of decorative layer; and higher than if used for making that of paint or wallpaper base course.
(2) Talcum Powder
Talcum powder in the gypsum putty is mainly to improve the application property of slurry, make it easy for blade coating and to increase the surface smoothness. Fineness: all through 325-mesh sieve; Na2O Content: less than 0.10%; K2O Content: less than 0.30%.
(3) Water-retaining Agents
The blade coating performance of gypsum putty slurry is mainly guaranteed by water-retaining agents, which can ensure the workability of gypsum putty slurry, make the moisture in the gypsum putty layer not absorbed by walls too quickly, and avoid dusting and shedding phenomena arising from shortage of water required for gypsum hydration. Water-retaining agents are mainly derivatives of cellulose ethers, such as methyl cellulose (MC), hydroxyethyl cellulose (HEC), hydroxypropyl methyl cellulose (HPMC) and carboxymethyl cellulose (CMC), etc.
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.
Although some cellulose ethers and binder have a retarding effect on the gypsum, the retarding effect cannot meet the requirements for the using time of gypsum putty, so a certain amount of retarder is also need to be added.
In order to make the gypsum putty better bonded with the base, a very small amount of penetrant is added into it. The commonly used penetrants include anionic and nonionic types.
(7) Flexibility agents
The hardened gypsum is soft and brittle itself. Once the gypsum putty layer is too thick, it will be very easy to peel off the interface layer, so adding a certain amount of flexibility and penetrating agents can improve the flexibility degree of gypsum putty, and can further improve the operation properties of sypsum putty slurry. The commonly used flexibility agents include a variety of sulfonates and lignin fibers, etc.
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