Technology Interchange: Polyester Powder Coatings: TGIC vs. HAA

By: Kevin Biller

Polyester powders are ubiquitous regardless of what part of the world you live. These are the mainstay of the outdoor durable powder market and represent anywhere from 32 to 45 percent of powders sold, depending on global region and who you’re talking to.

This technology has been around since the mid-1970s and continues to dominate the outdoor durable finishing world. Here is a review of the prevailing technologies and their performance.

Polyester powder coatings fall into two broad categories based on what material is used to crosslink these polymers. Common vernacular describes polyesters as “TGIC” and “TGICfree.” In addition, some folks call the non-TGIC containing polyesters HAA-polyester or even “Primid® polyesters.” TGIC refers to the curing agent triglycidyl isocyanurate whereas HAA describes β-hydroxyl-alkyl amide, an alternate curing agent. Primid® is the registered tradename of EMS-Griltech, the original supplier of HAA curing agent. In this Technology Interchange, I present a comparison of properties of these closely related powder coating types. For the sake of this discussion, I will use the terms TGIC-polyester and HAA-polyester.

One important issue I will not be covering is toxicity. Volumes of information are available that discuss the handling issues associated with TGIC. Regulations for handling TGIC vary by jurisdiction and global regions. The European Union and Australian regulatory agency require strict labeling for TGIC-containing products. Consultation with the appropriate regulating agencies will provide more insight into whether TGIC-based powders can be used in your location.

Base Polymers.

Both types of polyester powders use strikingly similar polymers. The backbones are comprised mainly of terephthalic and isophthalic acids with neo-pentyl glycol. Other monomers are used to alter melt viscosity, flexibility and coating physical properties. And both polyester types have reactive carboxyl groups for crosslinking.

Curing Agents and Cure Mechanism.

TGIC is a compact molecule based on a heterocyclic ring with three pendant epoxide groups. These epoxide (aka glycidyl) groups react with the carboxyl groups on the polyester resin. HAA is another rather compact molecule that contains four fairly reactive hydroxyl groups that crosslink with the carboxyl functionality on the polyester resin.

The reactive glycidyl groups on the TGIC molecule chemically bond with the carboxyls on the polyester resin in an additive fashion. That is, there is no condensate or volatile emitted during cure. Alternately the HAA-carboxyl reaction produces water as a condensate which evolves from the coating film during cure.

Outdoor Durability.

The durability of a coating is predicated on a number of formulation variables. The UV resistance of the primary polymer, in this case a polyester resin, has the most significant influence on durability. Hence, the choice of specific polyester resin will dictate the expected weathering resistance in a polyester irrespective of whether it is cured with TGIC or HAA. The use of TGIC or HAA is weathering neutral as both materials inherently provide good UV resistance.

Both standard durable and “superdurable” versions of TGIC and HAA polyester powders are available to the coating user. Standard durable polyesters withstand about 18 months south Florida exposure before exhibiting signs of degradation. Superdurable polyesters typically endure up to 5 years exposure in south Florida before they degrade.

Other formulation variables affect outdoor durability. Non-durable pigments and additives can reduce weathering resistance, whereas ultraviolet (UV) absorbers and light stabilizers can enhance UV durability—especially in clear coats. These variables are independent of the crosslinker used in the formula (e.g., HAA vs. TGIC).

Physical Properties.

Physical film properties such as impact resistance, flexibility, hardness and abrasion resistance are generally equal between TGIC and HAA polyester powder coatings. Formulation approaches can influence these properties, but neither polyester chemistry is inherently better than the other for basic physical film performance.

Chemical Resistance.

TGIC polyesters typically have slightly better chemical resistance than their HAA polyester counterparts. The HAA curing mechanism emits water during the reaction which makes the resultant coating slightly more water soluble than a TGIC cured polyester. Consequently, the alkali and acid resistance of HAA polyesters is slightly less than TGIC polyesters.

Corrosion Resistance

Because of the potential for residual H2O from the cure mechanism, HAA polyesters are somewhat less corrosion resistant than TGIC polyesters. Of course formulation conventions affect the ultimate corrosion resistance but a straight-up comparison demonstrates a slight advantage to the TGIC coating variants.

Processing Issues: Manufacture, Application, Cure.

TGIC polyester formulas are easier to manufacture because TGIC melts in the extrusion process whereas HAA, having a higher melt point, is more difficult to process. This requires a more closely managed powder coating manufacturing process with particular attention to extrusion conditions.

Conversely, HAA polyester powders typically apply better than TGIC powders. HAA based powders tend to possess higher first pass transfer efficiency than TGIC types and therefore create less reclaim and better overall electrostatic deposition. Most HAA polyester users claim this technology penetrates cavities and tight corners better than TGIC powders.

Interestingly TGIC has the advantage in low temperature cure capability. Powder coatings cured with TGIC can be cured at temperatures as low as 266°F (130°C) but more practically around 293°F (145°C). HAA-based polyesters can be cured no lower than 311°F (155°C) but in practice are cured about 20°F higher than this.

TGIC polyester powders can be applied at a wide range of film thickness. Typically, the low coating threshold is around 1.5 mils (38 microns) but consistent films of up to 10 mils (250 microns) are possible. HAA powders emit water during cure which can create pinhole defects at thick films. Consequently, the high thickness threshold is around 3.5 to 4.0 mils (90 to 100 microns) for these products.

Color stability of TGIC based powders is considered the best for main stream powder coating technology. Very little color drift is observed over a large cure temperature (and time) range. Alternately HAA based powders tend to yellow with overbake conditions. More stable versions of HAA are available but more expensive than standard grade HAA. In addition, antioxidants can be used to reduce oven yellowing of HAA polyesters.

Cost/Performance Balance.

Both of these coating types are relatively economical and demonstrate high performance. Outdoor durability is very good to excellent (1 to 5 yrs. Florida). Film performance exhibits a very good balance of flexibility, hardness and abrasion resistance. TGIC polyesters are more forgiving to process variables in powder manufacture and curing. HAA powders have an advantage in electrostatic behavior and transfer efficiency. Neither product is cost prohibitive and both provide good overall value to the coating user.

One final note, handling issues, specifically toxicity, tilt the decision making choice to HAA polyester powders, however in most markets outside of Europe and Australia, TGIC polyester powders can be easily handled with the proper personal protective equipment (PPE) and containment engineering.

Kevin Biller is technical editor of Powder Coated Tough magazine and president of The Powder Coating Research Group. He can be reached via email at kevinbiller@yahoo.com.