I. Definitions of Several Terms
In construction and decoration, putty and paint are frequently used. Due to regional differences between north and south, variations in material sources, and differences in living standards between urban and rural areas, different decoration materials are often used in different places.
In rural areas of northern China, "scraping white" is still commonly used. The main raw material for scraping white is "double-flying powder," also called "white powder." Its main component is heavy calcium carbonate, also simply called "heavy calcium," which is typically used to fill, cover, level, and whiten walls and ceilings.
In urban home decoration, putty is commonly used as a wall base and for leveling. The main components of putty are gypsum powder, talcum powder, white cement, and adhesive, used together. Gypsum powder accelerates drying, white cement acts as a binder, and talcum powder makes the wall base smooth and delicate. In construction and decoration, putty is classified by form into paste putty, dry powder putty, and two-component putty.
Our ideal unfinished apartment should look like this
But in reality, the unfinished apartments we receive look like this
According to the JG/T3049-1998 standard "Building Interior Putty," interior wall putty is classified into Y-type and N-type. Y-type is the general type, and N-type is the water-resistant type, which has a certain degree of water resistance. Y-type putty is commonly known as ordinary putty, 821 putty, etc. Currently, this type of putty on the market is basically a low-priced, low-quality building decoration material produced using pregelatinized starch, carboxymethyl cellulose mixed with double-flying powder, talc powder, and other raw materials. Because the binder used itself has biodegradable properties, the coating is prone to chalking, and it quickly molds and degrades when exposed to moisture, easily causing the coating to chalk, crack, blister, and peel off. Currently, interior wall putty produced in most parts of my country still commonly uses polyvinyl alcohol 107 glue, carboxymethyl cellulose mixed with double-flying powder, talc powder, and other powder materials. Although the cost is lower than imported glue powder and emulsion-mixed powder materials, it still has the problems of complex processes and higher costs. In particular, it contains large amounts of harmful substances such as formaldehyde, which can damage the nervous system and, in severe cases, cause poisoning and cancer, especially posing a greater risk to infants, the elderly, and those with weakened immune systems.
Water-resistant putty is typically composed of lime powder, talc powder, and other raw materials, along with hydroxypropyl methylcellulose ether (HPMC), polyvinyl alcohol powder, and redispersible latex powder. Lime powder is a white powdery substance mainly composed of CaO and Ca(OH)₂. CaO reacts with water to form Ca(OH)₂, which in turn reacts with CO₂ in the air to form hard CaCO₃, thus giving the coating excellent water resistance. Water-resistant putty is white, fine-textured, and has a good feel; it has a porcelain-like effect (requiring an additional polishing process); and it has excellent water resistance, remaining intact and not peeling even after soaking in water for 7 days.
In the Central Plains region, it is called porcelain-like coating, but it is actually a type of water-resistant putty. After polishing, the putty surface achieves a remarkably smooth finish.
II. The Role and Mechanism of Polyvinyl Alcohol Powder in Putty
Polyvinyl alcohol powder, when dry-mixed with other powders, forms a paste-like slurry upon the addition of water. After the water evaporates, a framework structure composed of inorganic and organic binders is formed. This consists of a brittle, rigid skeleton formed by hydraulic materials, and flexible connections formed by the polyvinyl alcohol powder in the gaps and on the solid surface. This connection can be imagined as many tiny springs connected to a rigid skeleton. Because the tensile strength of the polymer resin film formed by polyvinyl alcohol is orders of magnitude higher than that of hydraulic materials, the strength of the putty itself is enhanced, i.e., its cohesion is increased. Due to the flexibility and deformation capacity of the polymer, which is far greater than that of the rigid structure formed by hydraulic materials, the deformation capacity of the putty is improved, and the stress dispersion effect is greatly enhanced, thereby improving the crack resistance of the putty. Polyvinyl alcohol (PVA) powder has slightly weaker water resistance than redispersible latex powder. However, in alkaline systems of cement-containing water-resistant putty, PVA is saponified by the alkali generated from cement hydration and absorbed by quartz-like materials. This significantly weakens the hydrophilicity of PVA, thus not affecting the water resistance of the putty.
III. Causes and Solutions for Putty Powdering and Small Particles
The strength of exterior wall putty partly depends on the strength of the cement itself. Cement is a hydraulic material and requires adequate water curing to achieve its strength. Because the putty coating is relatively thin, insufficient moisture curing can lead to powdering and flaking. In such cases, increasing the amount of cellulose ether or adjusting the amount of PVA powder can help.
The strength of thin-layer putty partly comes from the organic binder. Generally, adding 3-5 kg of PVA powder should prevent powdering.
Sometimes, finished putty products exhibit powdering and a rough texture, primarily due to the common problem of impure fillers. It's necessary to check the quality of the lime and cement in the fillers. Spot checks in Beijing, Hebei, Shanxi, Shandong, Henan, and Shenyang markets revealed that lime contained over 60-70% whitish powder, and cement contained over 60%. Alternatively, improper finishing time during application (80% dryness is optimal for finishing) can easily lead to powdering. A rough texture is mainly caused by the heavy calcium carbonate and lime not reaching a fineness of 400 mesh or higher. A simple method to check the quality of lime and cement is to separately mix them with water to form pastes, apply them to the wall, and rub them after drying. If they harden and don't powder, they are genuine and of acceptable quality; otherwise, they are counterfeit or substandard products.
The appearance of small particles in imitation porcelain putty after polishing and finishing is closely related to the quality of the polyvinyl alcohol powder. The fineness of the polyvinyl alcohol powder and the quality of the polyvinyl alcohol raw materials both affect the quality of the imitation porcelain putty. Many domestic vendors use 180-mesh or 200-mesh as a selling point, which is actually misleading customers. Internationally, powder fineness is described by sieve passing rate. For example, polyvinyl alcohol powder products produced in Japan and Taiwan, which we usually call Grade S, have a standard of 95% passing through an 80-mesh sieve. It's virtually impossible for all products to reach 180 mesh or higher in large-scale production. The finer the powder, the stronger the static electricity, making it impossible to pass through a vibrating sieve, thus hindering industrial production. The powder particle size should not exceed 180 micrometers. Excessively coarse polyvinyl alcohol powder does not dissolve quickly in water; polyvinyl alcohol powder with a high degree of polymerization also affects its solubility in water. It is generally recommended to use polyvinyl alcohol powder with a degree of polymerization below 1700. Based on our experience with putty manufacturers, polyvinyl alcohol powder produced using the calcium carbide method is prone to producing small particles in the putty powder during use. We recommend using polyvinyl alcohol powder produced using the ethylene method (or oil and gas method) for formulation and production.
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