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August 10, 2018 10:47AM EST

Open Time

Open time (or working time) in the chemistry of two-component coatings is the elapsed time between when the two coating components are mixed together and when they have sufficiently reacted that they are no longer workable.  Gel time in high performance coatings is the elapsed time from when a coating is placed onto a surface until that coating can no longer be moved around.

Typically, pure polyurea coatings react very quickly.  Historically, they’ve been applied using high heat and high pressure spray equipment, with gel times typically in the three- to seven-second range.  The application equipment has a spray gun, the direct impingement gun, bringing the two components of the coating system together under very high pressure, forcing them to react with each other.  Frequently referred to as hot spray, it’s widely used in polyurethane spray foams and two-component coating systems like polyurea and polyurethane coatings.

However, the latest formulation techniques available today allow specific raw materials to be blended together, providing a much greater open time.  The longer open time enables the polyurea or polyurethane coating, or polyurethane spray foam, to be sprayed through a relatively inexpensive gun that contains a static mixer spray nozzle rather than a high-cost impingement spray gun.  This equipment is frequently referred to as cold spray.

The crucial point about open time is that the faster the coating gels, or sets up, the less able it is to flow into, and conform to, the landscape of the surface being coated (known as surface profile or anchor profile).

In discussions of coatings, you may hear references to the wet out properties of a coating product.  The property of wet out, or wetting out, of a coating is its ability to conform to all the surface profile—that is, fill in all the little valleys and crevices so that a good mechanical bond is obtained.  The graphic below demonstrates this point.

Everyone wants a fast cure product; but, if it cures too fast to conform to the surface profile, the risk is catastrophic adhesion failure.

WB Sealer Systems, LLC uses formations which have open times up to 20 minutes for successful application and installation. This provides efficient installation with superior properties in the finished coating.

 


 

August 8, 2018 20:23 EST  —Over the next couple of months we will be re-posting a series  about the issues a customer needs to understand when purchasing or installing High performance materials. These apply to Primer, Coatings, and Stains. When you have better knowledge and facts we believe the customer will be empowered to make better choices.

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Thank you   —Clifford