Daniel J. Mania, Wacker Chemical Corp., Adrian, MI
Abstract
Architectural coatings are becoming increasingly robust as we move into the
21st century. Consumers demand that
coatings be more stain repellent or easy-to-clean. To meet this demand, paint
companies advertise highly cleanable,
stain repellent paints. Many people believe that if a material beads on a surface,
it will either not stain or the stain will be
easy to remove. In some instances, stain
repellency or ease of cleaning may occur; however, there are times when beading will concentrate the stain in a small
area, making the stain more noticeable or
harder to remove.
Three basic starting point interior flat
paint formulations were used in this study.
Various binder chemistries were used
including all-acrylic, vinyl-acrylic, and
ethylene-vinyl acetate. Additionally, 18
commercial paints were also evaluated.
Four different stain repellent additives
were included (either included in the
formulated paints or post-added to the
commercial paints). The stain repellent
materials were added to approximately
80% of these samples to increase stain
repellency performance variability. Two
main output variables were used to de-
termine if a correlation exists: (1) contact
angle as measured by goniometer, and
(2) stain repellency as determined by a
standard coating stain repellency test
method, measured using a colorimeter
for absolute change in the L* value in the
L*a*b* color space.
No correlation was found to exist between the contact angle of water or dodecane and stain repellency to nine different
household chemicals. Within a very small
set of samples, slight correlations were
found to exist between diiodomethane
and several of the household chemicals.
However, this set was less than 10% of
the total sample set of 171 samples, thus
it may not be statistically relevant. This
suggests that contact angle should not be
used as a developmental predictive tool
for stain repellency testing.
Introduction
Adhesion is a very important concept
for many industries and applications. A
simple Merriam-Webster definition of adhesion is “the act of sticking or attaching
something”. 1
More precisely, according to Kinloch2,
there are four main components or mechanisms of adhesion: mechanical interlocking, diffusion theory, electronic theory,
and adsorption theory. Depending upon
the type of system involved, one or more
of these mechanisms can apply.
Since this paper involves correlating
contact angle with stain repellency (or
lack of adhesion), we will focus on adsorption theory starting with the Dupre
equation, also known as the Work of
Adhesion equation3:
WA=γs + γlv - γsl (1)
where
WA - sum of free energies
γs - surface free energy of the solid phase
γlv - surface free energy of the liquid phase
γsl - interfacial free energy
The issue with the Dupre equation is
that it contains components that are diffi-
cult if not impossible to measure, specifi-
cally, the interfacial free energy. Young2, 3
developed an equation to describe the re-
lationship between interfacial energy and
contact angle:
γs - γsl = γlvcosθ (2)
where θ is the contact angle of the liquid on surface s.
Combining equations 1 and 2 yields
the Young-Dupre equation:
WA = γlv (1 + cosθ) ( 3)
The Young-Dupre equation allows for
predictability of adhesion based upon the
surface free energy of the liquid in question and the contact angle of the liquid
upon the surface in question. In this case,
the smaller the contact angle (θ), the better are the chances of good adhesion.
When θ ≥ 90°, cosθ ≤ 0, which should
indicate lack of adhesion.
Is There A Correlation
Between Contact Angle
And Stain Repellency?
