Catalyzed Coating Pot Life

Q. How long after a two-component paint is catalyzed can it be used? We are a custom coater and are applying two-component finishes in several colors where high-performance coatings are needed. My painter has been ending his day with several gallons of catalyzed paint left over. I'm old school and hate to throw anything away. 

A. As a Depression Era baby, my motto has always been “Waste not, want not.” However, use of catalyzed materials is a different situation. All catalyzed materials have a definite useful life, called “pot life.” They start to gel or cure from the time the components are mixed. In paints, the reaction rate is retarded by the presence of solvents in the formulation. After the coatings are applied, the solvents evaporate, the reaction rate accelerates and the paint film cures. The mixed paint left in the container is also gelling or curing but at a slower rate. This accounts for the fact that a two-component paint can have an eight-hour pot life and a half hour set time.

Since most chemical reaction rates are temperature dependant, it is possible, to save catalyzed coatings by refrigerating them to retard gellation. In other cases, catalyzed materials can be salvaged by adding freshly catalyzed paint on a percentage basis (I don’t advise this unless you really know what you are doing). The rule of thumb is that a batch of catalyzed coating is useful until its viscosity doubles. For example, a catalyzed coating with an initial viscosity of 40 sec on a Zahn cup can be used until its viscosity reaches 80 sec.
 

Use of Proportioner Equipment

Q. I have a follow up comment to your answer to the person who did not want to throw away the several gallons of catalyzed coating his painter didn’t use every day. Possibly what he needs is some proportioning equipment. In the spirit of full disclosure, my employer sells a full line of proportioning equipment, but there are others as well. A proportioner would allow him to mix only what he needs to spray. The money saved by not throwing away the paint could pay for the equipment. He should contact a finishing equipment dealer and discuss what options would work for him. Thanks and keep up the good work. 

A. I agree with you 100%, proportioning equipment cannot only limit the amount of material catalyzed, it provides precision to materials measurement. Taking it a step farther, the use of a two component spray unit, where the materials are proportioned and fed to the spray gun, where they are mixed, is the ultimate.

One of my projects in the early 1960s, when I had a real job, was to evaluate two-component spray units. Since we made durable electrical apparatus, two-component epoxy insulation was an important material. My article on the subject, “Applying two-component coating materials” was published in the March 1970, Products Finishing Magazine. Suppliers of the aforementioned equipment are listed under Painting Equipment.