Cross Linkers for Superdurability

By Tony Pledger, EMS-Griltech

Since their humble beginnings in the early 1960s, thermosetting powder coatings, with near zero volatile organic compounds (VOCs), high levels of utilization and low hazardous waste, have been recognized as one of the world’s leading “green” industrial coating technologies.

Close to 50 percent of all powder coatings produced worldwide are based on weathering resistant binders, which can be broadly divided into polyurethane- and polyester-based systems. Although there are geographical variations, polyesterbased powder coatings have emerged as the dominant technology. Polyester coatings can be further broken down by the type of cross linker used and are often referred to as either TGIC powder coatings (from the chemical name Triglycidylisocyanurate TGIC) or by the brand name Primid® powder coatings (from the original range of beta-hydroxyalkylamide HAA cross linkers). Polyester Primid powder coatings were first introduced and commercialized more than 20 years ago. In the early 1990s, the acceptance of them for weathering resistant powder coatings grew steadily as powder coating manufacturers and end users gained experience and con- fidence in the new cross linking chemistry. A step-change in interest occurred in 1998, when the reclassification of TGIC as a category 2 mutagen was adopted into the law of all European Union member states. By the end of 1998, Primid was established as the main cross linker for exterior durable powder coatings in Europe, and it became a competitive alternative to TGIC worldwide.

Although its early success was driven by regulation in Europe, it is not an exclusively European coating solution. It is now used across all continents in developed and emerging powder coating markets and continues to gain market share from both polyester TGIC and polyurethane systems. In the United States, a fresh spotlight was put on TGIC’s long-term suitability for powder coatings when it was added to California Proposition 65, which lists chemicals known to the State of California to cause cancer or reproductive toxicity. Following the recession in 2009, the consumption of Primidbased powder coatings has bounced back and is again growing strongly. Figure 1 shows this growth since their commercial introduction in 1990. In 2011, annual powder coating production based on Primid cross linkers exceeded 490 million pounds (222 million kilos).

The main criterion for any exterior coating system is its resistance to degradation by environmental conditions commonly measured by natural weathering exposure. The suitability of Primid for weathering resistant powder coatings has been demonstrated through extensive natural weathering tests conducted in Florida. Figure 2 shows a typical Florida curve for Primid superdurable powder coatings over a period of five years exposure. In more practical terms, Primid powder coatings meet all the requirements of the international Qualicoat standard for architectural applications and many other standards.

Because exterior durability alone is not sufficient to meet the requirements of today’s highly specified powder coatings market, these cross linkers combine exterior durability with excellent mechanical performance and high levels of flow and levelling. A major factor in their success has been the unique combination of technical and economic benefits with the easy and safe handling associated with materials of very low toxicological impact.

Technical benefits include:

• Enhanced charging properties with both corona and tribo charging spray equipment.

• High first pass deposition rate.

• A high degree of chemical and physical storage stability.

• Consistent reactivity, which results in excellent batch to batch powder coating compatibility. This is particularly important when formulating high gloss, high definition coatings.

There are health and safety benefits as well. A distinguishing feature of the HAA chemistry is its clean toxicological profile. These cross linkers are:

• Non-irritating, non-sensitising and non-mutagenic.

• Require no hazard labels.

• Meet regulatory requirements in all of the key developed and emerging powder coating markets throughout the world.

• FDA-compliant for direct food contact (Primid XL-552, FDA 21 CFR 175.300).

Economic advantages include:

• The cross linkers are combined with polyesters in a ratio that is significantly lower than other weathering resistant powder coating cross linkers, significantly reducing binder costs.

• The charging characteristics and subsequent high transfer efficiency of these powder coatings helps reduce application costs and powder consumption.

• The clean toxicological profile of Primid and its ease of use greatly reduces administrative, handling and disposal costs for both powder coating manufacturers and applicators alike.

Viable, Successful Cross Linker

Polyester Primid powder coating is a proven technology. A great deal of time has been allocated in laboratories throughout the world developing and testing these powder coatings. It is work that has been extremely valuable in determining both the viability and success of Primid cross linkers. Nevertheless, no matter how extensive or how well such testing is conducted, the true test of any product is how it performs in practice. With two decades of commercial suc cess and more than 490 million pounds (222 million kilos) of powder coating consumed annually, Primid can boast an expansive range of applications. It is regularly used for architectural, automotive, agricultural and construction equipment (ACE), and industrial applications in all regions of the world.

High profile buildings coated with superdurable polyester Primid powder coatings can be found in many severe and challenging climates around the world. Automotive applications include surface primers, wheels, components and truck chassis. Industrial applications range from playground equipment to domestic and industrial fencing, to HVAC units and electrical housings, etc. Each application requires specific properties and performance unique to its end use and working environment.

Tony Pledger is market development manager, Primid Cross Linkers, at EMS-Griltech, headquartered in Switzerland. He can be reached via email at tony.pledger@emsgriltech.com.