Polyurethanes vs. Epoxies

Question:

I would like to know what the differences are between epoxy and polyurethane coatings. I have been an applied coatings technician for 15 years and had never given it a second thought until my boss somehow ended an unrelated argument with that question. I was instantly baffled. Apparently no painter has ever been able to give him an answer. I am aware of the different applications of each coating but do not know why or what determines it. What are their characteristics alone and used in conjunction with each other? My boss would be as enlightened as I would if you could shed some light on this topic. R.M.

Answer:

Your question can be answered simply or in detail. Because of the space constraints of "Painting Clinic," I will give the simple answer without going into the chemistry. There are several types of polyurethane coatings. Types IV and V are two-component. Types I, II, III and VI are single-component and do not need the addition of a catalyst. The most versatile are the two-component types. One of the components is based on aromatic or aliphatic prepolymers or adducts containing isocyanates. The second component is generally a polyol or polyamide or a resin containing active hydrogen groups. When the components react, they form films that can be weatherable, flexible, wear resistant and chemical resistant. Hence, they are excellent as topcoats for exterior exposures.

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On the other hand, one component of the two-component epoxies is generally based on a resin made from epichlorohydrin and bisphenol A, although polyols are also used. The other component can be amines, polyamides and other cross-linking agents. When the components react they form films that have excellent adhesion and provide corrosion resistance. This accounts for their use as primers. Epoxies generally do not have good color and gloss retention in exterior exposures.

Two-component epoxies and polyurethanes are higher priced than single-component conventional coatings. Indeed, cost is always an important consideration when choosing finishing systems for products. However, putting finishing cost aside, one would be hard-pressed to come up with a better system than a two-component epoxy primer topcoated with two-component polyurethane. These finish systems are used in extreme outdoor exposure conditions.