Why Is My Hard Coat Project Failing?

In the past, we’ve had customers think they could use a hard coat in a casting capacity for theming projects. They spray the hard coat into a mold and demold it once cured to create a lightweight part without adding support beneath it. The most likely explanation for why the coating is failing is that it was left in high temperatures without support.

Reasons Unsupported Hard Coats Fail

By definition, coatings are materials designed to create a layer over or cover the surface of an object or substrate. Hard coats are best at creating durable, protective films over EPS foam. They are not made to be a casting material or to create hollow shapes to save money.

You might think that since hard coats are hard and durable, they can create three-dimensional shapes without a supportive surface underneath. However, they lack the structural rigidity and strength to support themselves. Without a support structure, they are thin, slightly flexible, and won’t last long due to various conditions.

An unsupported hard coat will eventually weaken and deform when exposed to high temperatures and UV rays. This can lead to a ruined project or noticeable imperfections in the piece. A support structure can help the coating maintain its dimensional stability, preventing warping, cracking, and distortion.

Outdoor temperatures can surpass the coating’s heat deflection temperature (HDT). HDT is a property that measures a material’s resistance to distortion under a given load at an elevated temperature. In other words, it will tell you how well your coating can handle a high-heat environment before it will deform.

Hard coats with a lower HDT than the environment they are being installed in will need to be fully supported. An example of this would be if you were to use an EPS hard coat in an outside environment. The hard coat gets structure from the EPS at high temperatures. If you are installing your project in a location where temperatures get above the HDT and you are not supporting it, that hard coat is not recommended.

Depending on what it is used for, a hard coat can also deform from impacts. They’re sprayed thin (up to 120 mils or a 1/4 of an inch), so they can be susceptible to cracking under impact. A support layer will absorb some energy from the impact, reducing the risk of damage to the hard coat.

Solutions

Polyurethane is a versatile material that can accommodate a variety of applications. Its properties can be adjusted to make it hard or soft. Depending on the raw materials used, it can be made into a foam, coating, rubber, or plastic. Hard coats have a similar feeling to plastic, which is why they may be useful in some casting scenarios.

• The first solution to prevent your hard coat from deforming is to spray over a surface that will support the coating. These coatings are typically sprayed over EPS foam to harden it. They create a durable, weather-resistant finish without diminishing the details of the underlying structure. EPS foam has good heat and thermal stability, so the hard coat shouldn’t fail unless the foam fails. Using foam also doesn’t add too much weight to the piece. Plus, it is cost-effective, so it won’t increase the price too much, either.
• Another solution is to spray the urethane coating into a mold, treating it like a fast-setting plastic. It must be backfilled with expanding foam once it has cured. A material like urethane foam is lightweight like EPS and will provide the support the hard coat needs. There is a range of urethane foam options with varying densities to accommodate your specific project.
• Rather than use a hard coat, you could use a urethane plastic with a high heat deflection. These high-performance plastics are good for high-impact applications and can be exposed to environmental temperatures up to 205°F (96.1°C) without deforming. The only downside is that you’d need more material to make these castings, so they will cost more.
• One last option would be to use a urethane plastic capable of being rotocast and then backfill it with expanding foam for support. Using this material will depend on your intended application. Not all plastics have high heat deflection temperatures, so you’ll want to be aware of that, too.

Contact VFI for more information on our urethane sprayable plastic coatings and plastics.