Robert Ruckle, Ph.D., Siltech Corporation, Toronto, ON
Why do we call them additives? They are added to a formulation and they add additional properties: what else could we call them? “How do they
work” and “How do we design them”? These are more difficult questions.
Perhaps this is being overly simplistic, but a coating, at its
heart, is a carrier or solvent and a film forming resin or binder.
Actually egg tempera paint, which was used in pre-1400 medieval frescoes, was composed of egg yolk, pigment and water. Some artists today still use egg tempera. One could also say
that chili is meat, beans and ketchup. While partially true, there
are countless chili recipes with a multitude of spices, different
meats, and many more sometimes secret ingredients (ask anyone
from Texas!). By analogy, additives are the flavors and spices of
a coating.
Modern coatings are complex formulas with thickeners,
emulsifiers, dispersants, pigments, defoamers, leveling agents,
photo-stabilizers, curing catalysts and other additives. These
additives enhance the inherent properties of the resin.
Flow and Leveling
Flow and leveling agents are probably the most important
class of additives. A myriad of defects can occur in the forming film with colorful names like fisheyes and orange peel,
and the very technical sounding Bénard cells.1 A critical parameter in the formation of these defects is localized surface
tension gradients which physically form the defect and hold
it while the film dries. Surfactant-based flow and leveling
agents work by reducing the surface tension of the carrier liquid, which allows the liquid to flow more easily and reduces
the strength of the gradients.
Additive designers, a self-imposed and admittedly lofty term,
have several base chemistries to choose from when building
surface tension reducing agents. In general, amphiphilic molecules, derived from hydrocarbon-based hydrophobic groups,
can lower aqueous surface tension from pure water’s 72 mN/m
into the 30 mN/m range. Polydimethylsiloxane (aka silicone)
hydrophobe-based materials lower surface tension into the
20 mN/m range and perfluoroalkyl surfactants into the 15-20
mN/m range. There are some notable exceptions in the organic
materials such as acetylenic diols and small linear alcohols that
can reach into the high 20 mN/m range.2, 3
The use level needed for a surfactant also changes for the different basic chemistries. In water, surfactants have a measurable
property called the critical micelle concentration. 4 This is the
concentration at which micelles are formed and where surface
tension reduction is greatest. For our purposes this is essentially
the minimum effective use level. Organic surfactants typically
have a CMC in the 10-2 range, silicone surfactants in the 10-3
range and fluoroalkyls can be even lower. Hence the typical use
levels of 0.1%, which is well above these CMCs, are effective.
Gloss
Gloss improvement from additives is generally a result of improved flow and leveling. An even, defect-free coating has a
high distinctness of image (DOI) 5 allowing one to see the gloss
of the resin to its maximum effect. Additive designers can also
use flow and leveling agents with higher refractive indices which
will give a higher gloss.
The lower the surface tension, the more powerful the flow
and leveling agent is. However, with great power sometimes
comes great problems – to borrow a phrase from Spiderman.
Surface active agents are driven to all of the interfaces including
gas/liquid, liquid/solid, etc. This can cause many problems for
formulators.
Interfacial Problems/Perspective
From the perspective of a coating being applied, surface active
agents at the liquid/solid interface can cause poor adhesion to a
substrate. The same phenomenon at the cured film/air interface,
can provide a surface that is difficult to wet from the perspective
of the next coat.
In order for a liquid to be able to wet a surface the surface
tension of the liquid must be lower than the surface energy of
the surface. Otherwise the liquid beads up as pure water beads
on most surfaces due to its inherently high surface tension. If
the surface which needs to be wetted is a previous coating, the
surfactants on what is now the cured solid/air interface of the
Understanding the
Function of Additives
