Why Use Thermoplastic Coatings?

Posted on Tuesday, July 23, 2019

Those who have read this magazine before probably have at least a basic understanding of thermoset powder coatings. Parts are pretreated, powder is electrostatically sprayed onto them, and then the parts are baked until a chemical reaction crosslinks the polymers and creates an irreversible chemical bond. The result is the stronger, greener, better finish we all know and love. But what if an application requires even more durability and weatherability than what a thermoset powder can offer? Thermoplastic powders may hold the solution.

Thermoplastics can be applied using a fluidized bed in cases where part complexity and coating thickness are considerations.

Thermoplastic powder coatings had an early reputation for being expensive and difficult to apply. Significant advancements have been made over the years which have mitigated some of those costs and expanded the use of thermoplastics in many different applications. Let’s look at some of the differences between thermoset and thermoplastic powders and potential use cases for thermoplastics.

What Makes Thermoplastics So Different?
Thermoplastic powder coatings do not cure the same way as typical thermoset powder coatings; they must be applied post-heat per the manufacturer’s recommendation to fully melt the coating. Typically, they have a lower temperature requirement than thermoset coatings.

Thermoplastic powder coatings melt and encapsulate the part. They are typically applied at higher film builds and can be applied with a standard corona gun. It is wise to check with the thermoplastic coating supplier for gun setting suggestions, as they might be different than what you use for thermoset. For example, it may be recommended to reduce kV/uA or use the gun’s recoating preset function. Thermoplastics can also be applied using a fluidized bed in cases where part complexity and geometry are considerations, or when a thicker coating is desired.

Ambient temperature parts can build a film thickness up to seven to nine mils, but if a higher thickness is desired, the part will need to be preheated. For required applications, thermoplastic coating can be applied at 30+ mils. In such cases, they can be applied by flame or thermal spray which make field application or application to very large parts possible.

The thermoplastic coating in this coastal environment was applied 15 years prior. The coating is still in excellent shape in terms of corrosion performance and color/gloss retention.

A major advantage of thermoplastics is that they have no micro-porosity. Thermoset coatings leave tiny holes on the substrate and over time water and air can initiate the corrosion process. This can be mitigated by applying a primer, but that adds cost and time to applying a second coat. Thermoplastics can offer the same or better protection with a single-coat application. Thermoplastic coatings also have excellent weatherability. They are especially effective in harsh environments, such as coastal settings or industrial areas. Anything exposed to acid or caustics is also considered a harsh environment. Thermoplastics are resistant to acid rain and perform well on streets or walkways that are salted for snow or ice. With corrosion performance far superior to thermoset coatings, thermoplastics are a great option if you want to increase the lifetime of the item you’re coating.

Overall appearance is another difference between thermoplastic and thermoset powders. Typically, thermo-plastics have a “soft feel” and are easier to mar because they don’t have a hard shell, as do thermoset powders. In addition, the color and gloss retention are better than a standard durable polyester and they can even outperform superdurable polyesters.

What about parts with sharp edges? Parts that are stamped, laser cut, or produced on water jet or plasma machines have sharp edges. All coatings pull away from sharp edges. Therefore, to achieve the best corrosion performance, it is recommended to radius the edge.

Sand cast parts are especially tough to coat as they outgas and cause pin-holing in thermoset coatings. One way to reduce this phenomenon is to de-gas the part by preheating to the cure temperature of the thermoset coating. However, remember the micro-porosity mentioned earlier? Cast parts coated with thermoset coatings will tend to fail outdoors more quickly than a cold rolled steel part. While cast parts with thermoplastic coatings also require de-gassing, they will last longer in outdoor environments because micro-porosity is not a factor.

The most notable advantage of thermoplastic powder coatings is that they are field repairable. If the coating is damaged, it may be possible to heat it to the point where it can flow back in and fill itself. Simple scuffs that do not penetrate to the substrate simply melt back to a smooth surface with the application of heat from a hot air gun or similar tool. If this doesn’t happen, thermoplastic powder can be heated in a cup, applied to the damaged area with a spatula, then a heat gun can be used to smooth the coating.

Uses for Thermoplastics
It is quite easy to come up with a variety of everyday applications where thermoplastic powder coatings are advantageous. There are many places where coated items are constantly outside in the elements and heavily used or abused. Thermoplastics are used in the automotive sector on flexible steel or aluminum automotive parts that are exposed to stone chips. Railings, fences and picnic tabletops at public venues endure constant abuse from incidental hand contact with rings to children climbing on them and hitting them with objects. Besides these “normal” conditions, park benches are often also subjected to landscapers beating string trimmers against them and lawnmowers spitting stones that could easily scar the coating. (The plastic coating also helps keep benches and picnic tables cool on hot summer days.) Commercial trash containers along city streets are exposed to road and walkway salts. Playground equipment is constantly exposed to children climbing, swinging and jumping on it, and a thermoplastic coating provides anti-slip properties along with the other durability benefits. Agricultural equipment is in some of the harshest conditions one can imagine. All Thermoplastics are even suited for harsh conditions where agricultural equipment is used. these examples require the toughest coating possible, and thermoplastics are a great option to consider.

Some less obvious applications include thermoplastic coatings on glass, offering shatter protection for items such as light bulbs, tubes for medical facilities, or food containers. In the event a glass item is dropped or damaged, thermoplastic coatings will safely contain the glass and its contents.

Indoor pools with play structures are another application where thermoplastic coatings are an excellent option. Play structures coated with a thermoset powder tend to quickly experience corrosion in this application requiring extensive maintenance. However, if a thermoplastic coating is used very little maintenance is needed, and longevity is significantly improved. Even dishwasher baskets, freezer evaporation trays and refrigerator wire shelving are coated with thermoplastics for extreme corrosion protection.

Thermoplastic coatings can also be approved for potable water contact making them appropriate for water pipes, valves, and fittings. Thermoplastics feature excellent flexibility, weathering resistance, very good performance at elevated liquid temperatures, are resistant to cathodic disbondment and are BPA free.

The Bottom Line
Thermoplastic powder coatings characteristically have a low specific gravity. This is important in that specific gravity is used in determining applied cost. A simplified formula for calculating the applied cost is:

The specific gravity for thermoplastic powder can be below 1.0 resulting in an applied cost advantage over thermoset. However, one must consider more than specific gravity for an accurate cost calculation. Since thermoplastic requires a higher film thickness, the applied cost is often higher in the end. Yet, superior durability, improved longevity and minimal to no maintenance can very well make thermoplastic powder a cost-effective option for the right application. Therefore, thermoplastic powder coatings should be seriously considered when any of the following functions are priority:

• Corrosion protection.
• UV stability.
• Abrasion protection.
• Impact resistance.
• Chemical resistance.

The next time you have a coating application that requires durability be sure to include thermoplastics when assessing the best coating solution.

Michael Withers is segment leader, powder with Axalta Coating Systems.