Polyurethane coatings are most commonly two-package systems consisting of one isocyanate component and one polyol componenet. Because of the reactivity of the isocyanate, polyurethanes are moisture-sensitive, and the gloss may drop when the wet film is exposed to high humidity. The toxicity of the isocyanate component is of great concern, and personal protection, including respirators, must be used when applying these coatings. they require skilled applicators.
Types of Polyurethanes
Polyurethane polymers are reaction products of isocyanates and polyols. By judicioususe of reactants, polyurethane coatings are available in diffrent variations to provide a variety of services.
The isocyanates may be aromatic, such as toluene diisocyanate (TDI) or diphenylmethane (MDI), or they may be aliphatic such as hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI). The aliphatic reaction products have better resistance to ultraviolet light, while the aromatic reaction products have better chemical resistance.
Selection of the polyol (organic compounds with two or more reactive hydrogen atoms) is important in determining the properties of the cured coating. Greater cross-linking will result in a harder, more chemically resistant product. Polyurethanes can also be reduced chemical resistance. Two-package coatings based on acrylic polyols have the best chemical resistance.
Moisture-curing polyurethanes are one-package coatings in which isocyanate groups react with moisture from the air to polymerize while releasing carbon dioxide gas as a by- product. Because of this reaction, this products are more tolerant of surface moisture than other polyurethane types. However, a relative humidity of %30-80 is normally required for their complete curing. They function well in corrosion control (e.g. primers for steel), extrior topcoats (aliphatic types),on roofs, on secondary containments, and wood and concrete floors.
*Pipe Lining and Repair
*Pulp and Paper Plants
*Concrete/Steel Water Storage Tanks
*Water and Wastewater Treatment Plants
*Can be low in VOCs
*Good water resistance
*Good hardness or flexibility
*Aliphatics have good gloss and color retention
*Aromatics have good chemical resistance
*Good abrasion resistance
*Low-temperature curing achievable
*Highly toxic; need personal protection
*Moisture-sensitive; gloss may drop
*Skilled applicator needed
*Limited pot life
*Blast cleaned surface required
*More expensive than epoxies
FACTORY APPLICATION OF POLYURETHANE COATING
STORAGE AND MIXING OF MATERIALS
- Manufacturer’s Recommendations: Unless otherwise indicated, the coating manufacturer’s printed recommendations and instructions for thinning, mixing, handling, applying, and protecting its coating materials, for preparation of surfaces for coating, and for all other procedures relative to coating shall be strictly observed.
- Materials shall be used within the manufacturer’s recommended shelf life.
- Storage and Mixing: Materials shall be stored and mixed per the manufacturers recommendations.
- Pipe shall be clean and free of contaminants. If not the pipe surfaces shall be cleaned in accordance with SSPC-SP1, to remove oil, grease, and other soluble contaminants. No residue shall remain on the pipe. Remove burrs, weld splatter and gouges.
- Prepare the metal surface to achieve a metal finish and cleanliness in accordance with SSPC-SP10 to an angular profile of 3.00-mil minimum measured and recorded using surface profilometer or surface replica tape.
- Pipe temperatures shall be at least 5 degrees F warmer than the dew point temperature and within the coating manufacturers recommended temperature range per the technical data sheet. Pipe shall be warmed if necessary.
- Pipe shall not be allowed to flash rust before coating is applied.
- Equipment: Two-component, 1:1 mix ratio, heated airless spray unit.
- Temperature: Minimum 5 degrees F above dew point temperature. The temperature of the surface shall not be less than 60 degrees F during application.
- Humidity: Heating of pipe surfaces may be required to meet requirements of 2.01E if relative humidity exceeds 80 percent.
- Do not thin or mix resins; use as received. Store resins at a temperature above 55 degrees F at all times.
- Application: Conform to coating manufacturer’s recommendations. Apply directly to substrate to achieve specified thickness. Multiple-pass, one-coat application process is permitted provided maximum allowable recoat time specified by coating manufacturer is not exceeded.
- Recoating: Recoat only when coating has cured less than maximum time specified by coating manufacturer. When coating has cured for more than recoat time, brush-blast or thoroughly sand coating surface. Blow-off cleaning using clean, dry, high-pressure compressed air.
- Curing: At ambient temperature above 0 degrees F. Do not handle pipe until coating has been allowed to cure as follows:
Ambient Temperature Minimum Full Cure Time Over
70˚F 7 days
50 to 70˚F 9 days
0 to 50˚F 12 days
- Galveston, Polyurethane Coatings on Steel or Ductile Iron Pipe, Date of access: 08 November 2016, http://www.galvestontx.gov/DocumentCenter/Home/View/1602
- LifeLast, Inc., Polyurethane Coating on Steel Pipe, Date of access: 08 November 2016, http://lifelast.com/main/wp-content/uploads/2012/03/Polyurethane-Coating-Specification-Steel-Pipe-20120327.pdf
- International Journal of Science and Engineering Investigations, Application Polyurethane as Coating in Oil and Gas Pipelines, Date of access: 08 November 2016, https://www.researchgate.net/profile/Amir_Samimi3/publication/267801691_Application_Polyurethane_as_Coating_in_Oil_and_Gas_Pipelines/links/5547332b0cf23ff7168707a7.pdf