By Dr. Michael R. Van De Mark and Ameya M. Natu, Missouri S&T Coatings Institute
VOC or volatile organic content has been one of the main driving forces behind coatings technological innovation and research efforts for the past 40 years since the advent
of the EPA and the clean air act.1 The approaches to the problem of an ever decreasing VOC has taken two directions. First,
the application of the “art” of formulation, where experience and
trial and error are used to reduce the level of VOC. At first, this
approach worked but as the levels of allowed VOC dropped it
found limits. The second approach is to use “science” to define
what will work and the true rules. This second approach is the
one we use at the Missouri S&T Coatings Institute.
The importance of the use of solvents in coatings has been
over blown. All solvents cost money and are only there to deliver the coating to the substrate, then, they leave. All solvents
cost to ship including water and we should look at solvent as
something we would want to eliminate if we can.
The resin and the pigment, including the extender pigments
which help lower the coating cost, are the two major paint components influencing the viscosity or flow of paints. The pigment
increases the viscosity as its concentration increases. As the aspect
ratio of primarily the extender increases from a sphere, ratio of 1,
to platy talc with its ratio of 40, the viscosity will increase significantly at the same volume fraction. Thus low PVC and low aspect
ratio will yield the lower viscosity and aid with VOC reduction.
The most influential is the resin which is the focus of the paper.2
Resin and Solvent Interaction Issues
The resin is the major contributor to the viscosity of a coating. The
choice of solvent can play a major role in how the viscosity responds
to the added solvent which is demonstrated by the Mark-Houwink
equation3, Equation 1. In a good solvent, the resin usually is in a
random coil configuration with an ‘a’ of 0.4-0.7. If the polymer has
charges on the chain, they repel one another and the chain becomes
a rigid rod with an exponent of 2. If the polymer chain is in a ball
shape with no solvent inside (also known as hard sphere), the exponent is 0 and thus the viscosity becomes low and independent of
molecular weight. 4 Examples of hard sphere resins are latex, dispersions and water reducible resins including CUPs.
The viscosity of a resin in a good solvent can be controlled by
adding a poorer solvent which reduces the value of exponent ‘a’ and
gives viscosity which allows spray. This technology is simply a re-
duction with solvent which reduces the viscosity by more than sim-
ply adding more of the good solvent since the exponent is lowered.
Equation 1
The Mark Houwink equation also indicates that if we want to lower
the viscosity we can also reduce the molecular weight. However, as
the molecular weight is lowered below approximately 100,000, the
tensile strength drops. Thus, although the viscosity is now sprayable
at let’s say 40,000 MW, the mechanical properties of the coating are
inferior and cannot be used. 5 The answer to this is reaction cure
“cross-link” the resin after application. This is how 2K urethanes,
urea, and epoxy coatings work by using building blocks of resins
with 200-1500 molecular weight which combine many times to
reach a molecular weight giving good mechanical performance. 6 In
addition, reactive diluents can be used which are actually monomers,
which act as both a solvent and are incorporated into the resin.
WR vs CUP: a new approach
The term water reducible (WR) has been around since about the
mid-1950s. It refers to a process by which a water insoluble resin
is dissolved in a high boiling, water soluble, organic solvent and
base is added to form a salt. Most of the chain is hydrophobic. When water is added, the chains collapse into a hard sphere
mainly containing only one polymer chain per particle depending
upon the polymer concentration when water is added. 7
The term CUP was coined a few years ago in our research
group. It stands for a colloidal unimolecular polymer particle
dispersed in water without solvent. The process to form CUP
particles is very similar to that of water reduction. The major
differences are the use of a low boiling organic solvent, lower
than water, and after reduction, the solvent is stripped off to
yield a VOC free resin suspended in water, Figure 1.8 This process is the true pinnacle of what we referred to as reduction in
lowering the exponent of the Mark Houwink equation to zero.
The measured and calculated (from GPC data) diameters and distribution of CUP particles are virtually identical indicating a true unimolecular polymer particle. 9 These particles are thermodynamically
The following article is a summarized version of the full research paper recently delivered as a presentation on this topic of
solvent reduction technology at the Roof Coatings Manufacturers Association’s 2014 International Roof Coatings Conference
(IRCC).
Solvent Reduction Technology –
What are the Rules?
