Protection for the plant1 January 2002
Using polyurea coatings can help protect hydro power plants from corrosion. John Henningsen offers a guide on choosing the best polyurea for your application
IN the past decade, the use of spray-applied polyurea coatings for protecting concrete, steel and other industrial substrates from the degrading effects of corrosion and abrasion has increased dramatically. This new polymer coating technology is 100% solids, contains zero volatile content and generates a fast gel time which is ideal for moist environments such as those found in the hydroelectric industry. However, with so many coatings available, how can you be convinced polyurea is the best choice? It's no longer as simple as the best price. You must now consider substrate, particulates, application equipment and climatic conditions to ensure long term performance.
Know your substrate
It is important to determine the general condition of all the substrates in your facility. This includes all the concrete, steel and wooden surfaces. Take note whether these surfaces have ever been painted, sealed or coated in the past, and pay close attention to any cracks (dynamic or static), corrosion or water stains in the substrates. You should also look closely at whether any of the concrete has been patched or repaired in the past.
This may seem like concentrating on insignificant details but it is critical to ensure the best adhesion for the coating and, ultimately, the longevity of the concrete. While polyurea is a versatile coating, its final performance is dependent on the quality of substrate and its preparation.
As a guide, concrete will take a minimum of 28 days at 70?F (21?C) to fully cure. Over this time period, concrete will dry out and achieve its maximum strength. If the concrete is rushed to cure or coated too soon, excessive moisture in the concrete may cause blistering and delamination of coatings like polyurethanes or epoxies. On the other hand, a pure polyurea is hydrophobic and not affected by moisture in the concrete. The polyurea will therefore achieve 90% of its final physical properties within 30 seconds, 'sealing' the moisture in the concrete and preventing pinholes and delamination.
Even with the moisture insensitivity of polyurea it is always important to thoroughly dry the concrete surface before priming and coating. If you are unsure of the moisture content of the concrete, one way to be certain is to utilise the American Standards for Testing and Measurement (ASTM ) D-4263 test. This test involves using a 4-mil clear plastic sheet to cover a 1m2 area of the concrete. The process involves sealing the edges with tape and directing a heat lamp over the covered area for 4-8 hours. If the covered area is darker than the exposed area, the concrete is too wet to coat. Also, if there is an oily residue on the underside of the plastic there is still oil contamination present.
Preparing the surface
Obviously, in a hydro power plant, moisture is going to be present. If a substrate is moist or has condensation on it, polyurea will perform much better than polyurethanes or epoxies. For example, Texaco Chemical Company (now Chevron Texaco Corporation) produced a video of polyurea being sprayed over ice and water without affecting the reaction of the components. This demonstration, as impressive as it is, is however generally not encountered in real life applications. Applying any coating over an excessively wet or unprimed/unprepared substrate can have detrimental effects on the adhesion and is not
Organisations such as SSPC (Steel Structures Painting Council) issue guidelines for properly preparing and priming concrete surfaces. A properly prepared surface will improve adhesion, especially for critical applications such as moist concrete for containment lining and flooring, as well as adhesion protection for geotextile, wood and steel. In these applications, a polyurea system is claimed to provide very efficient results.
Before any polyurea coating is applied it is crucial to thoroughly clean and grit blast the concrete and steel protrusions. All grease, dirt, old coatings, oil and laitance must be removed, and any damaged areas or small voids should be filled with an epoxy repair gel. Make certain to ground any rough protrusions, ridges, spurs and rough edges as all concrete should be prepared to the texture of medium-grit sandpaper. All non-ferrous metal in an immersion application should be free of surface contamination, steam cleaned and prepared with an abrasive grit blast to
SSPC-5P 'near white finish'. Once all the dust is removed the surface is ready for coating and a profile of 2-3 mils should be achieved.
The next step is to steam clean at the highest available pressure (1500psi or greater), or with 10% TSP (tri-sodium phosphate) in a hot water solution. A thorough rinsing to remove any trace of the TSP should immediately follow this wash.
On concrete that has never been coated, use an acid etching. A 10% to 15% muriatic acid (hydrochloric acid), phosphoric acid or other suitable acid solution should be used to achieve the required surface texture. The acid should remain on the surface until the frothing and bubbling action stops. The acid must then be drained, thoroughly rinsed away and scrubbed to remove any residue.
For grit blasting, a light grit blast (wet or dry) of silica or equal (16-30 mesh size particles) should be used. For best results, direct grit into the concrete surface at an angle of approximately 45?. As a final step, remove all the blast from the surface with a vacuum. For previously coated concrete, any old coatings may compromise the service life and performance of polyurea. If this is not possible, Methyl Ethyl Ketone (MEK) can be used to provide proper adhesion to the aged coating. The appropriate primer must then be applied within ten minutes of the MEK application.
Polyurea is known to be highly effective for the protection of steel substrates. However, there is a common misconception, including in the hydro power industry, that polyurea is a 'universal' product capable of meeting the challenges of any application. For example, many believe that recoating water turbines which have become cavitated from the constant abrasion of use can save money. Sadly, not even the effective physical properties of polyurea can work in this application.
Damage to water turbines occurs over time as the water is sliced by the rapidly spinning turbine. Ripples in the water cause deflection. The point just beyond the deflection area is where pitting will occur in the substrate. The result is called generated cavitation erosion.
According to an industry expert, several years ago the University of Pennsylvania undertook an extensive study on solving the problem. They found that no coating produces a surface smooth enough to avoid the problem. The only solution might lie in molding a turbine from polyurethane resins. Unfortunately, an exceptional mold with perfectly smooth surfaces must be used. This is unlikely to happen, as it would be cost prohibitive due to the fact that you will only get 1-2 parts per mold. Molds can be more expensive than milling a solid turbine from steel.
Concrete versus steel
When water is pumped through a buried concrete pipe, the resulting pressure can actually force water through the uncoated concrete into the surrounding earth. This saturation can cause landslides. However, there are solutions to this problem. Using a polyurethane or polyurea elastomeric coating to line the interior of the pipe will help reduce the likelihood of water penetration through cracks in the concrete pipe. Alternatively, using something as simple as steel pipe coated with an elastomeric polyurea can also help eliminate the problems.
Another area for consideration is the existence of particulates in the water. As water flows under pressure from lakes and rivers, it carries small rocks and other debris which might not be screened out during intake. These particulates can cause severe abrasions on unprotected steel and concrete. Once again, a coating of highly abrasion resistant polyurethane or polyurea (84-98 Shore A Hardness) will help rectify the problem.
Power of polyurea
Polyureas are growing in popularity among contractors and specifiers building and maintaining hydro power and industrial manufacturing facilities. Polyurea materials have two components: the isocyanates quasi-prepolymer and a resin blend. Unlike polyurethanes or epoxies, there are no polyols used in the construction of polyurea resins. When the materials are mixed together in the application equipment, the isocyanates and the resins react almost instantly to form polyurea.
Fast reaction time is a particular advantage of polyurea. In the case of facility maintenance or rejuvenation, owners want to regain usage of the facility as soon as possible. A fast reacting polyurea will fully cure within a few hours, whereas most polyurethane or epoxy coatings require 24-48 hours before the coated areas can be used to their full potential.
The application characteristics of polyurea are considered equally advantageous. In waterproofing applications a low modulus and a high elongation elastomer are required. Polyurethanes have traditionally been regarded as having higher elongation and a lower modulus than a polyurea with a similar hardness. However, the new polyureas are designed to stretch with much less force.
The increase in the use of polyurea is often attributed to its 100% solids characteristics. Not only does this feature allow compliance with volatile organic compound (VOC) regulations, it also permits a safe environment for contracting crews, and greater film thickness per coat.
Many polyureas are based on aliphatic isocyanate prepolymers that are highly weather resistant and colour stable. On the other hand, products based on aromatic isocyanate prepolymers are not colour stable and will tend to chalk or darken in colour with extended exposure. In the past, aromatic polyurea was the primary version of polyurea promoted to the industry. In the mid-1990s, aliphatic and aliphatic-modified polyureas were developed. Now, end-users can safely choose a polyurea for applications that would be constantly exposed to sunlight, without fear of it discolouring and chalking.
Nowadays, polyureas can be applied to concrete floors, walls, containment areas or decks. They can be used on steel, iron and many other metals. The coating is also widely used over polyurethane foam for wall and roofing insulation systems in building, residential housing and temperature controlled containment tanks.
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