PC Summitry:Polyester Vs. Polyurethane

By Kevin Biller

One of the neatest parts of being in this industry is the fact that you sometimes get to meet really cool people. Cool people are pretty easy to identify, but for me they always engender an inherent curiosity, a thirst for knowledge and quest to continuously innovate and improve their operations. One of these “cool” people posed a great question to me recently. Here’s what she asked:

Polyester vs. polyurethane: I have a question concerning these two chemistries. Which one is a better choice for outdoor furniture? I hear great things about polyurethane being great for outdoor applications, and yet I hear the same for polyester. I am not sure what the difference in the two would be. I thought maybe it had something to do with the gloss level. Maybe the polyurethanes need high gloss to fulfill their corrosion protection quality? Or maybe polyester has better impact and mar resistance. Anything that you could offer on this would be helpful.

From a fundamental standpoint, polyester and polyurethane powder coatings have many similarities. The base polymer used in both chemistries is very similar. The reason for this pertains to the limited amount of monomers available to the polymer scientist to create a solid resin. Hence, the major portion of the polymers used in polyester and polyurethane powders is comprised of the same building blocks. Consequently, the same building blocks yield the same performance that is to a point.

The main differences in these two chemistries emanate more from the crosslinkers than the base polymers. Polyester powder coatings are formulated with a polyester resin cured with triglycidyl isocyanurate (TGIC) or alternately hydroxyl-alkyl-amide (HAA). Primid® is a common trade name for HAA. Polyurethanes, on the other hand, are cured with a multifunctional isocyanate and derive much of its performance from this crosslinker.

The binder or resinous portion of polyester powders is comprised of a carboxyl polyester cured with a relatively low level of crosslinker (typically 5 to 10 percent). Polyurethane powder binders contain a hydroxyl functional polyester cured with an isocyanate crosslinker. The isocyanate crosslinker level can range from 15 to 60 percent and therefore strongly influences the ultimate coating properties.

Here is a rundown of the top properties and how they compare:

Exterior Durability

Excellent exterior durability is possible with either chemistry. Standard grade and “Superdurable” polymers are available in both polyester and polyurethane varieties. The superdurable products have the potential to maintain appearance and film performance for up to 6 or 7 years in south Florida environment.

Hardness and Abrasion Resistance

Polyesters are typically somewhat softer and less abrasion resistant than their polyurethane counterparts. The additional crosslinker inherent in a polyurethane powder formulation is responsible for the added performance. Using the somewhat archaic pencil hardness test, polyesters clock in at H hardness whereas polyurethanes can achieve H to 5H depending upon the specific formulation. The higher hardness products typically sacrifice mechanical flexibility.

Low Gloss and Matte Finishes

Polyurethanes can be formulated from matte to high gloss with very good to excellent coating performance. Polyesters are more difficult to formulate in lower gloss and film performance can be reduced with some of these formulation schemes.

Chemical Resistance

Both chemistries provide fairly good chemical resistance however polyurethanes significantly outshine polyesters if formulated appropriately. In fact a highly crosslinked polyurethane powder coating provides chemical resistance that rivals epoxy powder performance.

Cure Response

Low temperature cure is much easier to develop in a polyester than a polyurethane powder coating. A couple caveats need to be noted. HAA polyesters are more difficult to catalyze than TGIC based polyesters. HAA polyesters can cure as low as 325°F whereas TGIC polyesters can be formulated to cure at temperatures as low as 250°F. Polyurethanes bottom-out around 300°F.

Film Thickness

One peculiarity of polyurethanes is the need for a chemical blocking agent in the crosslinker. These blocking agents volatilize out of the coating during the bake cycle. Blocking agents make the chemistry work best (balance of reactivity and cure) but also can create pinhole defects in thick film coatings. Hence polyurethanes are difficult to apply successfully at film builds greater than 5.0 mils, whereas TGIC based polyester powders can be applied at upwards of 10 mils. On the other hand polyurethanes inherently provide very good film smoothness at thin films (>1.5 mils).

Cost

In general, you will pay more for a polyurethane powder coating as compared to a polyester based powder. That said, the powder formulator can cheapen a polyurethane powder product by taking a substantial amount of quality out of the coating.

In the end, both chemistries have their place in the powder coating industry. Polyesters and polyurethanes can provide good to excellent outdoor durability. More often than not, polyurethanes have polyesters beat on chemical resistance, hardness and thin film appearance; however, this added performance almost always comes at a premium. Thanks for the cool question, Bonnie.

Kevin Biller is technical editor for Powder Coated Tough magazine. He can be reached at kevinbiller@yahoo.com.