Recent Advancements in Bio-based Powder Coating Technology
Posted on Friday, March 22, 2019
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
Below are some of the monomers that have been derived
from plant materials and used to synthesize powder resins
Isosorbide (from soybean oil).
Epoxidized soybean oil (from vegetable oil).
Rosin based glycidyl crosslinker (from
C18 Diacid (from palm or soybean oil).
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.
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
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
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
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:
- 85% Bio-based.
- 2.1 Functionality.
- Low color.
- Sharp melting point - 105°C.
- Acid Value 44-49.
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.