Why Is My EPS Foam Bending After Applying a Hard Coat?

If you’re working with long, thin EPS boards and urethane hard coats for your theming project, you should be aware of issues that can occur during the application and cure process. Foam boards of this size and thickness or at lower densities have a limited ability to resist bending or warping after they have been coated. This typically occurs when the material is applied too thick in a single pass.

Why Hard Coat Thickness Matters

Thickness directly impacts the coating’s appearance, performance, and durability. All urethane hard coat products come with a recommended application thickness rate for proper coverage and to extend the life of the EPS piece. Due to the different ways hard coatings are applied, the achievable thickness and uniformity may be different for a brushable hard coat vs a high-pressure or Qwik Spray application.

The viscosity of the material can also play a role in the final thickness of the coating. Higher-viscosity materials, like brushable coatings, will generate a thicker film build, while lower-viscosity materials, like sprayable coatings, will generate thinner builds. Brushable coatings can make applying an even amount of coating more difficult, which can cause more tension on one end of the foam piece and result in uneven curling.

It’s important to spray enough of the coating to protect your piece, but there is a certain point where too much coating can affect the surface below. Some applications require the coating to be applied very thick (up to 250 mils) for increased protection. If you apply it that thick in a single pass, it will cause issues for your piece.

As polyurethane hardens, the chemical reaction generates heat, with temperatures getting as high as 175°F. While curing, the coating experiences some degree of shrinkage, which creates tension on the surface of the foam. The more material you apply, the hotter the coating will get as it dries, and the more it will shrink. The coating will pull the foam out of shape and permanently bend or curl it. Thinner sheets and lower densities of EPS bend because they have less rigidity and will respond to this stress.

Note: Applying too much hard coat at once can also lead to sagging or wrinkling of the coating and even difficulty drying or curing properly. Excessively thick layers will be even more prone to cracking or chipping.

Solutions

Always refer to the technical data sheet of the product for thickness requirements and application techniques. To prevent your foam pieces from bending, you’re going to want to spray or brush on the coating in multiple thinner passes. Several even layers are always better than fewer heavy ones. This will prevent the heat generated in the curing process from adding stress and shrinking the foam.

VFI’s recommended thickness ranges per product are:

• VFI-2519 75 D Brushable Hard Coat and VFI-2626 65 D Brushable Hard Coat: Apply a minimum of 60 mils to create a uniform film on the piece. For the best performance, they should be applied at 100 mils or more. You can build up to this thickness in 2-3 coats.
• VFI-6170 70 D Spray Hard Coat and VFI-6171 Qwik Spray Hard Coat: Apply between 40-80 mils to create a uniform film on the piece. For increased impact resistance and outdoor applications, they should be applied between 60-120 mils. You can build up to this thickness in 2-3 coats.

You can add as many coats as you want as long as the previous coat has set up and the recoat window has not ended. If you miss the recoat window, lightly sand the surface before applying the next coat or use a compatible primer. Doing so will help ensure intercoat adhesion.

Alternatively, you could encapsulate the piece to balance the stress the coating application creates. After spraying the top of the piece, allow it to cure before flipping it over to the other side and spraying so there is a similar opposing force to straighten it out.

If you don’t need to work with flat, thin pieces, we recommend spraying 3D objects or thicker boards that are harder to bend. If you must work with large, thinner sheets of foam, consider reinforcing the foam with a rigid backing material first or increasing the density of your foam.

Contact VFI if you are still experiencing issues with your EPS hard coat, including bubbling, curing, or failure problems.