The coating process for single-part wet-cured polyurethane coatings

Single-component wet-cured polyurethane coating process
room force, Li Junxia (Hebei Shijiazhuang Zhengyuan Chemical Company, Hebei Shijiazhuang, 050051)
Li Minfeng (China Coatings Industry Association Expert Committee, Beijing, Beijing) 100079)
0 Introduction
Cold exhaust device is the key equipment of chemical plant, consisting of carbon steel pipes in side by side, pipe wall thickness 4 to 8 mm, pipe diameter 8 to 12 mm. In the steel pipe through H2S, SO2, NO2 and other mixed gases, the outside temperature of 150 to 170 degrees C, 365 d a year need to be sprayed non-stop circulating water for cooling, water added anti-fouling, anti-algae and other reagents, corrosion environment is harsh. If the outside of the cold exhaust device is not sprayed with anti-corrosion paint, equipment worth 350,000 yuan less than 1 a will be scrapped due to pipe wall corrosion perforation, economic losses are greater
China
. Since 2008, Zhengyuan Chemical Company's Technology Development Department, after many consultations, has adopted three kinds of supporting coatings: the first, inorganic zinc silicate primer 80 m and high temperature silicone paint resistant to 200 degrees C 50 m, Total film thickness of 130 m, within 1 year from soybean-like coating shedding; m, the coating condition is good within 2 years, but the temperature is below 0 degrees C construction coating shedding seriously; Wet-cured polyurethane finishes of 55 m and a total film thickness of 150 m, after 3 years, the coating is complete, free of peeling, rusting, foaming, etc., after cleaning as shiny as new. According to the above results: In 2010, the Company selected the United States "SWD" company's single-part wet cured polyurethane coating coated with more than a dozen cold exhaust devices. The single-part wet-cured polyurethane coating (MCPu) not only has excellent corrosion resistance, but also has high solid content and low VOC (volatile organic compounds); This article describes the coating process of MCPu.
1 MCPu's coating process
The main component of MCPu is the high-performance polyurethane-polyurethane resin, which has a unique film-forming process:
R-NCO-H2 → R-NH2-CO2
R-NCO-R-NH2→ polyurethane
has a process of absorbing moisture and releasing CO2 in the above reactions, which shows that the coating construction of MCPu is the key to determine the quality of the coating. To this end, we use the following coating processes for the corrosion characteristics of cold-emission units and the excellent performance of MCPu.
1.1 Surface Treatment
Step 1, De-oiling: Cold exhaust device in the production process, steel surface is inevitably contaminated with oil, need to be high safety, low toxicity of 200 s solvent gasoline (pine perfume) wipe removal.
Step 2, Blast: MCPu is a low-surface coating, but for newly manufactured cold-emission units, due to the harsh environment, sand blasting is required to ensure adhesion. After sand blasting, the surface status reaches 2 mass levels: Rust removal grade: ISO 8501-1 Sa 2.5, average roughness Rz: ISO 8503 Rz=45 to 65 m (set to 1/3 of the specified film thickness).
conditions for sand blasting:
blaster type: pressure blaster; compressed air: installation of oil and water separator, no oil, no water; blast pressure: 0.55 to 0.65 MPa; grinding Material: mixed copper ore (0.8 to 3.0 mm), recycled 2 times; nozzle: material, tungsten carbide; structure, "Venturi"; aperture, 0.8 to 1.0 cm.
step 3, blowing ash: Dust and other dirt is blown away with dry, clean compressed air.
1.2 Spray
McPu's performance metrics are the technical basis for spraying, as in Table 1.
pre-painting: welds and cutting edges in cooling devices, etc., due to thermal stress, especially prone to corrosion, pre-painting these parts is the key to the coating process. Pre-painting can only be applied by brush, not roller coating. Special thinners may be added to the MCPu at a rate of no more than 5%.
coating method: Although MCPu is suitable for brushing, roller coating, air spraying, airless spraying, but due to the complex structure of the cooling device, the pipe diameter is small, in order to save paint, brush coating should be used, the flatness of the coating is better. Also under the premise of full stirring, the dilution rate is less than 5%. Brushing is also used to strictly control the thickness of the film. Because mcPu primers have a low viscosity, good permeability, MCPu film thickness is too high, CO2 is difficult to volatile, easy to foam the coating. During the construction of MCPu, moisture should be prevented from entering. Paint after opening the can can can not be shelved overnight, so as not to affect the quality of the paint film. In addition, under different temperature and humidity conditions, the coating interval is strictly controlled (see Table 1) to ensure the adhesion of the coating.
drying and curing: MCPu coatings can be self-drying without heating and baking.
requirements for environmental conditions: MCPu can be constructed at low temperatures below 0C and relative humidity greater than 95%, but dew points must be effectively managed. During construction, the surface temperature of the steel (base) must be greater than the dew point temperature of 3 degrees C.
the cure speed of a single-part wet-cured polyurethane coating depends on temperature and relative humidity. If the temperature is too high, curing speed is too fast, the coating is easy to cortify, affecting inter-layer adhesion and coating quality.of the coating of the 2 cold
mechanical damage may occur during the use of the cold exhaust unit. In order to ensure safety, after the suspension of maintenance, the surface dirt is washed away by high-pressure water, after drying, the damage is removed loose old paint film and rust spots, exposed steel needs to be polished to ISO 8501 St 2 level, allowing the retention of solid old coating and some minor rust spots.
the brush, brush 1 MCPu primer at the polishing site. After the rest of the old MCPu coating has been hair-coated, 1 MCPu finish is sprayed until the specified film thickness is reached.
MCPu coating is a good low surface treatment coating, with the old coating adhesion excellent, energy saving, consumption reduction effect is very obvious.