Differences Between Polycrylic & Polyurethane

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If you're painting a wooden bench, you want to make sure you use the appropriate finish to prevent the wood from being damaged. Knowing the differences between chemical finishes, such as between polycrylic and polyurethane, can help you make better decisions when painting. These distinct uses come from the chemical compositions of the paint finishes themselves.

Polyurethane finish is basedon both oil and water while polycrylic is a water-based finish. That's just one example of a difference between polyurethane and polycrylic paint. But the answer of which one is better depends on the specific needs of the surface. Using one over the other can depend on factors like whether the material will be exposed to large amounts of heat or water that can affect the paint finish.

Polyurethane Finish

You might find a polyurethane as a viable polycrylic alternative for a shiny look on your wood. When comparing polycrylic vs. polyurethane keep in mind the benefits of polyurethane. Polyurethane is a liquid-based plastic that dries on both wood and oil surfaces. You can choose different polyurethane finishes like satin for a smooth look or glossy for a more shiny appearance.

When used on surfaces that may be exposed to water, polyurethane provides a crystal clear layer with a low odor and low toxicity. It can impart color tones without adding its own color to the painted wood. This water-based form of polyurethane isn't as heat-resistant as other finishes may be and, as a result, won't bode well in hot conditions.

Still, it's a suitable candidate for book cases, desks and picture frames in your house, for instance, although some oil-based polyurethanes are designed to withstand high temperatures.

Oil-based polyurethane is generally made to be more durable than the water-based option, and this lets it achieve greater resistances to heat. This version of polyurethane uses the natural richness of wood to add more feature to the color of the paint, and you can also use it to add a yellow color tint to the finish.

They're used in wood floors and kitchen tables, and this type of polyurethane takes more time to dry. It also gives off a strong odor so you'll need to use a respirator and work in a well-ventilated location when using it.

Polyurethane spray can also make it easier to apply on wooden surfaces. The polyurethane finish should make your material scratch-proof and hard. Be cautious as polyurethane can become flammable in its liquid form and it takes a long time to harden between layers. Make sure you use undiluted varnish between layers and let each one dry for 24 hours.

Polycrylic Spray

On the other hand, a finish like a polycrylic spray uses a bit of polyurethane and in some areas it's called "new polyurethane." It's water-based and can go over water and oil surfaces like polyurethane can. You can buy this finish in several sheens such as stain, semi-gloss and gloss to add subtle appearances with satin or more brightness with a glossy finish, and it's less toxic than polyurethane finish is.

Placing polycrylic over paint can make the top layer durable, making it a great candidate for desks and tables. When it dries, the paint color is much more clear than polyurethane and doesn't give off the tint of yellow color. Make sure you use even layers of it over your surfaces so each layer blends well with each other.This can be difficult because polycrylic spray or foam can flow easily, leading it to create runny surfaces.

You can follow a general method of applying polycrylic over paint by first clearing the surface from dust and debris, and then sanding it with sandpaper. Remove the dust after sanding and use a thin layer of polycrylic with brush or polycrylic spray paint. After it has dried and cured, sand it and apply two more layers.

Keep in mind that paint "drying" happens when the solvents evaporate from the coating so that the layer itself is dry, and paint "curing" is when the paint coating is as hard as it can be along with becoming dry.

You should also make sure you wait until each layer has dried and cured before adding more, but, because it dries relatively quickly, this shouldn't take long. Sometimes the dried layers can crack as well. Make sure you apply the paint as evenly as possible to prevent this.

Comparing Polycrylic vs. Polyurethane

Think about what you want and need from a specific paint project before choosing either a polycrylic or polyurethane finish. If you need to use the the wooden material in high temperatures and water (from sources such as rain or humidity), polyurethane based in oil will be more effective than polycrylic. If you want to apply sealer, you should use polycrylic paint because oil-based polyurethane can cause a yellow color tint. This can avoid the tint from seeping through the other layers you add onto the material.

Wooden floors should use oil-based polyurethane so that the surfaces are durable and smooth. Polycrylic dries much more quickly than polyurethane so it's more suitable for projects that you need finished in shorter time periods such as furniture you plan on using soon. If you're forced to work inside and don't have a safe way of ventilating the area, you should use polycrylic. It's much less toxic than polyurethane.

Large surfaces are better when coated with polyurethane. The polyurethane finish takes much longer to dry than polycrylic does so you can cover the entirety of a large surface with it while letting parts of it dry before coating them again. This makes the coating consistent across the surface.

Upright surfaces like walls or the sides of shelves should be coated with polyurethane because it's thicker and not as runny or drippy as polycrylic finish is. Polycrylic is also much more easy to clean up because it only takes soap and water to remove it, so it's better suited for projects where you don't want cleanup to be an issue. Finally, if money or cost-effectiveness is a concern, polycrylic is usually less expensive than polyurethane is.

Chemistry of Polycrylic vs. Polyurethane

The differences in chemistry between the two finishes depends on what you mean when you refer to "polycrylic" or "polyurethane." The name "polyurethane" is a certain type of polymer that has organic units, consisting of isoamines and alcohols, that are joined by urethane bonds. These urethane bonds have the molecular formula CH3CH2OC(O)NH2 of carbons C, oxygens O, nitrogens N and hydrogens H.

Chemists and engineers produce polyurethane links by reacting polyol and forms of poly-isocyanate using ultraviolet light or a catalyst, a biological enzyme that speeds up a reaction. Polyol takes the form R-OHn for the hydroxyl group OH alongside some R group, which is a group of atoms that connect a carbon or hydrogen to the rest of the atoms. The polyols and isocyanates that make up these molecules cause them to take the rigid form that they do when applied as finish.

The term "polycrylic," in contrast, refers to the paint finish itself, made from polyurethane and polyacrylates, liquid compounds made from molecules derived from acrylic and methacrylic acid. This chemical composition lets polycrylic protect wooden surfaces from solvents and water.

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About the Author

S. Hussain Ather is a Master's student in Science Communications the University of California, Santa Cruz. After studying physics and philosophy as an undergraduate at Indiana University-Bloomington, he worked as a scientist at the National Institutes of Health for two years. He primarily performs research in and write about neuroscience and philosophy, however, his interests span ethics, policy, and other areas relevant to science.