Will Bio-based Materials Find a Foothold in Powder Coatings?

How are powder coating formulas made bio-based? Typically, scientists look for ways to isolate monomers from plant based materials that can be used as building blocks for powder coating resins. Monomers can be derived from sources such as soybeans, corn, cellulose (e.g., cotton, wood and hemp), sugar cane, palm trees and linseed oil (flax). For powder coatings they explore means to synthesize monomers that can be used to make polyester resins. These can be carboxyl or hydroxyl functional and must have two or more reactive sites to become part of a polymeric chain.

But, why use bio-based materials for powder coatings? The answer is not so obvious. It’s easy to recognize that biobased products could reduce the reliance on petroleum-based feedstocks for our resins, curing agents and additives. There are other reasons as well.

One long term aspect of using biobased materials is control over the raw material supply chain for the resins and hence price stability. Petroleum-based feedstocks are influenced by world and political events in sometimes unstable oil producing nations such as Saudi Arabia, Iran, Iraq, Russia, Venezuela and Nigeria. With careful management and planning, bio-renewable sources can be limited to U.S. farm production, ensuring stability in supply and prices.

Developing and using bio materials for industrial products elicit a host of issues beyond simple supply, demand and raw material costs however. There are many more matters to consider before committing resources to the development of bio materials for coatings. The following is a discussion on some of these issues.

Cost to Produce, Environmental Impact

One of the biggest challenges is developing a process that is not energy intensive or cost prohibitive. The process to convert the raw plant material to a usable monomer must not require onerous logistics and complex or lowyield processes, otherwise these costs may outweigh the benefit of going bio. From a practical basis, the process of synthesizing the polymer competes with the conversion cost of prevailing polymerization techniques. The cost/performance balance may skew negatively if the bio-based process costs more than using petroleum-based monomers.

If the process to convert plant-based materials to monomers and the subsequent polymerization require more logistics, infrastructure and energy, then the environmental impact may conflict with the benefit of using a renewable feedstock. This has been a bone of contention with producing ethanol for incorporation into gasoline formulas. Some analysts claim the energy costs to distill corn byproducts into ethanol outweigh its energy contribution as a fuel. With monomers for resins the process costs are still being developed, however it doesn’t appear that they will be lower than those associated with petroleum-based polymers.

How the plant-based materials are produced undoubtedly influences environmental impact. Will the new demand for coating monomers affect farming density, arable land use and watershed runoff of fertilizers? Growing more crops to supply industrial products such as powder coatings could potentially have negative environmental effects. Ideally, organic feed stocks would emanate from existing farm production by using byproducts such as soy oil, which is produced when the more sought after soy proteins are separated from soy beans. While soy oil is commercially used in a variety of consumer products (mainly salad and cooking oils), the soy protein is in far bigger demand mainly for feed materials for livestock. These situations create a “win-win” scenario by consuming an already available material from an existing product line.

Quality, Performance Are Key

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 industry just cannot accept a product simply because it is synthesized on plant-based materials. Quality and performance must justify the switch in technology. Equal performance is the bar and improved performance is a more worthy goal. Anything short of parity in performance is unacceptable.

Impact on the Food Chain

Another key issue is how using a plantbased material for industrial products will affect the food supply chain. The use of corn to produce starch, which subsequently is converted into sugars (mainly glucose) then diacids or glycols, will have an impact on food supply. Interestingly, the vast majority of corn production is used for livestock feed. If the worldwide consumption of beef products lessened then the ability to use corn for industrial purposes could very well open up. In the meantime, it is difficult to justify interfering with corn production to make a powder coating resin.

Historic Attempts

So, what has been developed for powder coatings thus far? A number of programs have been endeavored by the universities, non-profit organizations and the private sector (e.g., Technische Universiteit Eindhoven, United Soybean Board, Battelle and Momentive Specialty Chemicals). These research projects have investigated the use of a variety of monomers that were synthesized from plant-based materials. Most prominent are isosorbide, succinic acid and citric acid.

This extensive research has produced promising results in synthesizing polyester resins based on one or more of these monomers. In some cases the polyester resin was hydroxyl functional and cured with a blocked isocyanate. In other experiments, polymer scientists synthesized carboxyl functional polyesters and cured them with hydroxyl-alkyl-amide or TGIC. In all cases, however, problems were experienced with low resin Tg (melting point temperature), yellowing and poor weathering performance. To a lesser extent, impact resistance and chemical resistance were deficient.

The Future

In spite of the shortcomings observed with bio-based powder coatings developed to date, research and development is being renewed. The Powder Coating Research Group has become a founding member of the Center for Bioplastics and Biocomposites (dubbed CB2) member with the hope of reinvigorating the quest to find renewable materials to incorporate into powder coatings. In addition researchers are retracing their steps to determine how earlier versions of bio-based polyesters can be improved upon.

Indeed, as the song, “Bein’ Green” indicates: it may not be easy to be green; however, with careful analysis and diligent development efforts, we may someday find the key to synthesizing affordable, high-performance powder coating resins based on bio-renewable materials. Jim Henson and his Muppets—especially Kermit—would be proud.