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Spanning the world with protective: bridge coatings.

Although frequently taken for granted, bridges play an important role in the daily life of millions of people. They span otherwise impassable geographic features, allowing individuals and societies to connect with one another in ways not possible before the advent of advanced civil engineering. Beyond this important function, bridges also represent creativity and ingenuity. As a result, almost everyone can name one or more famous bridges, even if they have never traversed them. Coatings help protect and preserve both the functional and aesthetic characteristics of these important features of the landscape. Bridges require high-performance coatings that can protect the structural components from constant exposure to sun, wind, rain, snow, ice, wind-blown sand, dirt, and other particles. Many bridges are also located in corrosive environments and need additional protection against this type of damage.

For many years, lead-based paint was often used to coat the structural components of steel bridges. Until the last decade, high-VOC solvent-borne formulations were preferred due to their performance characteristics. Advances in polysiloxane technology, though, have made it possible to protect bridges with more environmentally friendly systems.


Coating solutions from International Paint Protective Coatings have been chosen by transportation agencies to protect some of the most notable bridges around the world. Currently, Shanghai Zhenhua Heavy Industry Co., Ltd. (ZPMC) is using International Paint's Interzinc [R] 22 Inorganic Zinc Rich Silicate and Interfine [R] 979 Acrylic Polysiloxane paints to coat the 45,000-ton steel structures being used for the new east span of the San Francisco Oakland Bay Bridge.

"The Oakland Bay Bridge is an exciting project and the coating system is fairly unique," notes Chris McMillan, International Paint's heavy industry market manager for protective coatings. The bridge is being fabricated and coated in China, but there will be some touch-up work and ancillary steel pieces painted locally in the U.S. International Paint is providing the coatings from its manufacturing sites in China.

Specifications for the coatings were established by California's Department of Transportation (Caltrans). The agency was looking for a system comprised of a topcoat over an inorganic zinc primer that had extended weather-ability as well as excellent gloss and color retention compared to standard acrylic over inorganic zinc systems. More specifically, the topcoat needed to be an inorganic thermosetting hybrid coating based upon a polysiloxane resin co-reacted or blended with an epoxy, acrylic, or urethane resin--or some combination of these resins. In addition, both the topcoat and primer needed to be provided by the same paint manufacturer.

Interzinc 22 and Interfine 979 were extensively tested and found to meet all of the performance requirements for anticorrosion, abrasion, impact-resistance, and gloss and color-retention protection. ZPMC also chose International Paint materials because they are manufactured with the same quality standards in all of their global locations, according to a representative from ZPMC.

Coating systems from International Paint also protect other famous bridges in the U.S. (Whitestone Bridge Towers and Roosevelt Island Bridge in Queens, NY, and Throgs Neck Bridge in The Bronx, NY), the UK (QEII Bridge in Newcastle upon Tyne and Wearmouth Bridge in Sunderland), Australia (Sydney Harbour Bridge), and many other countries.


In each of these cases, systems were developed to address the specific protection needs for each bridge. Most systems involved three coats including a primer, basecoat, and topcoat.

The Sydney Harbour Bridge, at nearly 3,300 feet long, is a notable feature of the Sydney skyline. First opened in 1932, the bridge had been maintained using an alkyd/chlorinated rubber system. In 2006, the bridge was completely repainted for the first time. Officials wished to find a more cost effective, long-term solution with lower VOCs. They selected a combination of Interzinc 72, a fast recoatable organic zinc primer; Intergard 475HS, a high-build MIO-filled intermediate that provides an extended recoat interval; and Interthane 80HS, a high-gloss urethane with excellent brush and roll capabilities.

Wearmouth Bridge, a through arch bridge in Sunderland, UK, first opened in 1796, but has since been replaced twice. The current bridge was constructed in 1927. When officials wanted to repaint it in 2004, they required minimum disruption of traffic flow. They also were looking for a lower-VOC coating system that still provided excellent gloss and color retention. They turned to International Paint, which provided a three-coat system including Intercure 324, Intercure 384, and Interfine 979.

"It was imperative to the operator that the disruption on this key bridge was kept to an absolute minimum," observes McMillan. "An independent consultant had recommended a high-performance four-coat system. We were able to demonstrate that a three-coat system based on Intercure and polysiloxane products would provide the required protection--and assist in speeding up the project," he explains. The use of the system also reduced the VOC content by 22% compared to the four-coat system. And the polysiloxane system will provide the desired characteristics over a longer term at only a slightly higher initial investment, leading to overall long-term cost reduction.


The advances in primer and polysiloxane technologies make it possible today to protect important, high-value structures such as the Sydney Harbour Bridge with coating systems that provide long-lasting protection and desired appearance characteristics in a formulation that has a dramatically reduced environmental impact. "We of course continue to invest in R&D efforts to further improve our high-performance coating technologies. As we gain a better understanding of the chemical and physical properties of these specialized resins, we expect to develop even more highly functional coating systems for our customers," McMillan concludes.

Think about the coatings that are at work under your tires or feet the next time you cross a bridge.
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Publication:JCT CoatingsTech
Date:May 1, 2010
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