Recent Advancements in Bio-based Powder Coating Technology

In the spring of 2014, I reported on the current state of bio-based powder coating technology with an article aptly named It’s Not Easy Being Green. At that juncture, there had been no commercial success with introducing bio-based technology into powder coatings, although a number of attempts had been made.

Here is how I characterized the state-of-art of bio-based powders back then:

“Regardless of where the monomers come from and how the resins are produced, the ultimate product has to perform to become a commercial reality. Nearly all bio-based products synthesized to date for powder coatings have underperformed the state-of-the-art technology. In most cases the bio-renewal product has been inferior in color stability and UV durability. These shortcomings truly are show-stoppers for the powder coating formulator and her customers. The rubric ‘half the performance at twice the price’ comes to mind.”

The good news is that significant effort has been made since then and results are promising enough to warrant an update.

Bio-Based Powder Technology

Approaches to powder coatings based on renewable resources typically involve reengineering the components of a thermosetting binder system. Scientists isolate monomers from plant-based materials and use them as building blocks for powder coating resins or crosslinkers. Monomers can be derived from sources such as soybeans, corn, pine trees (rosin), cellulose (e.g., cotton, wood and hemp), sugar cane, palm trees and linseed oil (flax). These monomers are then used to design resins or crosslinkers that have chemically reactive functional groups such as carboxyl, hydroxyl or glycidyl moieties. Two or more reactive sites are requisite to function in a thermoset binder system.

Below are some of the monomers that have been derived from plant materials and used to synthesize powder resins and crosslinkers.

Isosorbide (from soybean oil).

Epoxidized soybean oil (from vegetable oil).

Rosin based glycidyl crosslinker (from pine trees).

C18 Diacid (from palm or soybean oil).

Why Bio-Based?

The main driver for bio-based powder coatings is to reduce reliance on fossil-based feedstocks, namely petroleum. The reasons for this are two-fold: 1. Fossil based feedstocks are limited in supply and will eventually be depleted, and 2. Many sources of petroleum involve unstable nations such as Saudi Arabia, Iran, Iraq, Venezuela, Nigeria and Russia. While these two points are debatable, it is nevertheless wise to explore alternatives to petroleum-based sources for powder coating binder components. Although the price of a barrel of oil is currently at a relatively low price, relying on unstable governments and fluid world political events can affect raw material price stability and continuity of supply.

Previous Attempts

Not long ago, a major resin supplier developed carboxyl polyester resins on a monomer derived from soybean oil. Isosorbide was the monomer selected and the United Soybean Board funded the initial work. Two polyesters were developed, one for general industrial use and the other an outdoor durable type. In 2009, the resins were introduced to the European powder coating industry with the less-than- ideal timing just after the Great Recession. This marketing strategy relied on Europe’s penchant for green technology and the presence of the headquarters of a number of global powder coating suppliers.

Major powder producers evaluated these resins and concluded that the resultant coating performance was not impressive enough to encourage any of them to introduce them to their product lines. My lab performed a thorough evaluation and observed the same results. Another drawback was that these resins carried a cost premium because of higher feedstock prices.

In 2014, Washington State University developed a glycidyl functional curing agent designed for powders with a grant provided by the Center for Biopolymers and Biocomposites (CB 2 ). This oligomer possessed two functional groups and was evaluated with various carboxyl functional polyester resins. Poor cure and inadequate film properties were observed, undoubtedly due to low crosslink density from the only two chemically reactive groups.

Promising Recent Work

Over the last 15 years, resin producers have investigated a number of schemes to develop bio-based powder coating resins. A prevailing approach is based on using recycled PET (polyethylene terephthalate) as a building block for powder polyester resins. Recently, Allnex developed a line of bio- based resins based on recycled PET and renewable monomers derived from C5 and C6 sugars. The details are proprietary; however, these carboxyl-based polyesters can be used in a variety of powder coating systems, including epoxy-polyester hybrids, polyester-HAA (hydroxyl-alkyl amide) and TGIC cured formulas.

Resin E-04342 has a 30 acid number and can be cured with a 70/30 ratio of polyester to epoxy resin. Powder coatings based on this resin exhibit good impact and solvent resistance. In addition, this system provides an improvement in blooming resistance observed versus conventional hybrid polyesters.

Resin E-04367 is designed to cure with HAA at a 95/5 ratio of polyester to crosslinker. This approach exhibits mechanical film performance similar to conventional polyester/HAA powder coatings. However, a reduction in outdoor durability is observed in accelerated and natural sunlight testing. Pilot size samples of these resins are available for testing.

Another promising development has been achieved by a Battelle Memorial Institute project funded by the United Soybean Board. The project’s scope of work was similar to previous ones – isolate a monomer from soybean oil and use it to synthesize a solid thermosetting powder coating resin. Jeffrey Cafmeyer, the project’s principal investigator at Battelle, used high oleic soybean oil to synthesize long-chain aliphatic diacids. These diacids were then reacted with di-ethanol amine to create highly aliphatic polyester amide resins possessing carboxyl functionality.

Resins based on this synthetic approach were evaluated in powder coating formulations using various glycidyl functional and hydroxyl-alkyl amide curing agents. Excellent film performance was attained, especially with TGIC cured formulations.

Battelle Bio-based Resin Characteristics:

Figure 1. Generalized resin synthesis scheme of C18 carboxyl functional diacid polyester-amide resin. (Branched functionality due to diethanolamine omitted for clarity.) Courtesy of Battelle Memorial Institute.

Figure 2. a) C18 polyester-amide resin from the bulk condensation reaction, and b) Mechanically powdered resin. Courtesy of Battelle Memorial Institute.

Most remarkable about this polyester-amide resin is its excellent film performance when formulated in a powder coating. When cured with TGIC, this polymer produces very smooth films with 160 inch pounds impact resistance and excellent solvent resistance. Because of its relatively low melt point temperature this chemistry can be formulated to cure at temperatures as low as 135 degrees Celsius. This makes it a candidate for temperature sensitive substrates such as MDF (medium density fiberboard), glass filled composites, pultrusions and many plastics. In addition, the UV durability is exceptional, eclipsing 4000 hours QUV-B exposure with less than 50 percent gloss loss. Equally important is that this resin possesses a uniquely stable melt viscosity profile. It is solid until about 105-110 degrees Celsius then exhibits a fairly low melt viscosity at about 125 degrees Celsius, enabling it to form a smooth, continuous film.

Dynamic melt viscosity (poise) of bio-based powder coating resin.

ASTM D-4587 accelerated UV durability testing of bio-based powder coating vs. industry standard products.

Powder coatings based on the Battelle resin technology have been scaled up to pilot size. Application trials have been conducted at infrared curing test facilities and MDF powder coating operations with good results. Plans for 2019 include further resin and powder coating scale-up and efforts to explore commercialization of this interesting technology.

Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.