CoatingsPro Magazine

NOV 2015

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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62 NOVEMBER 2015 COATINGSPROMAG.COM Science Behind It Moisture-Cured Magic: Single-Component Urethane By Andrew Rogers, Marketing Coordinator for Tnemec Company, Inc. M ost people who have been in the coatings industry for a while know how a two-part epoxy cures. In the most basic terms, an epoxy resin is combined with a curing agent, or hardener, and the two of them chemically react with one another to create a cross-linked flm. Two-component urethanes work in a similar manner: An isocyanate reacts with an alcohol to yield a solid flm and, eventually, a high-performance coating. One common problem encountered when applying these coatings is the mix; the ratio must be correct, and the mixing quality needs to be a top priority. But what if you aren't able to use a two-part urethane? Or what if you need to use something else for a specifc solution, such as the Alpine Painting & Sandblasting crew needed on the Massapequa tank? Enter the single-component, moisture-cured urethane; the Alpine crew used Series 94-H2O Hydro-Zinc as its solution. Tese coatings feature a curing mechanism that chemically cross-links with moisture in the atmosphere to form a high-performing coating flm. Te convenience of these coatings is that they are "all in one," as the product comes in one package, making mixing and applying a much more efcient process. Chemistry of the Cure A ll moisture-cured polyurethanes cure in the same manner. It's a process that occurs in two stages: 1) Evaporation of solvent and leveling of the surface, accom- panied by reaction of the polyisocyanate with moisture for initial cure. This is normally called the "soft phase." 2) Further reaction with water available from the atmospheric moisture enables continued curing of the film. This is often referred to as the "hard phase." Te chemical reaction for this process is as shown in Figure 1. As the equation details, moisture in the atmosphere acts as the catalyst in the reaction. Te polyisocyanate and the water from the atmosphere combine to create the coating flm, and this reaction releases carbon dioxide, or CO 2 . Tis release typically occurs in the soft phase of the cure, and as more CO 2 releases, the coating becomes harder. Knowing the Cure Understanding how these products cure can help the appli- cation process, as there are a few practices the applicator can carry out to ensure this reaction goes as planned. As the name implies, one of the most important factors to consider when applying one of these urethanes is the moisture in the air — it can make or break the coating's cure. As the Alpine crew did on this project, the contractor must check the relative humidity before applying a moisture-cured product. In humid conditions, the moisture in the air can afect the coating's pot life and shorten the working time; in dry condi- tions, the lack of moisture can increase the cure and topcoat time. Generally, most moisture-cured urethanes are applied at a relatively low flm thickness — up to but not exceeding four mils (101.6 microns) per coat — to limit these humidity-re- lated issues and the entrapment of CO 2 in the coating's flm. Another consideration — again, having to do with moisture — is the surface preparation. Surface preparation is important for any coating project, especially in regard to the proper adhesion of a primer. But with moisture-cured products, ensuring that the surface is clean and dry will be a crucial step. Surface moisture, including condensation or rain water, can cause problems with the chemical reaction and afect the release of CO 2 . Same Cure, More Zinc W hen applied and cured, single-component, moisture-cured urethanes ofer excellent adhesion and corrosion protection, especially when they contain zinc. Tis is why they are speci- fed for so many steel tank projects. Additionally, many of these products are fast-cure and can cure at lower temperatures. Because the reaction's catalyst (water) is in the atmosphere at any temperature, the coating is more versatile in its curing capabilities in hot or cold environ- ments — a major beneft to the Alpine crew. In addition, many moisture-cured urethanes are compatible with optional accel- erators, or added catalysts, that can quicken cure time even in temperatures as low as 35° F (1.7° C). For the Alpine Painting & Sandblasting crew, the option of a single-component, moisture-cured urethane enabled them to complete portions of their project with greater ease than with another type of coating. Between the temperatures and the corrosion protection, it was the right ft for this client. CP R—NCO Polyisocyanate R—NCO R—NH—CO— NH—R Final Polyurethane Film H 2 O Water R-NH 2 Amine CO 2 Carbon Dioxide Figure 1. Chemical Reaction

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