Under normal circumstances, we
would be evaluating adhesion by attempting to find a polymeric material that
has a low contact angle on a substrate so
as to increase chances of successful adhesion. However, here we are looking to
do the opposite. We are attempting to
establish stain repellency, by either ease
of cleaning or a lack of adhesion, which
in this case would be equal to a lack of
adhesion, which in turn would be equal
to a high contact angle.
To determine if a correlation exists, we
need to measure the contact angle and calculate the surface free energy, in addition
to determining stain repellency. Multiple
methods exist to determine stain repellency4-8, some of which utilize a colorimeter
to measure the color difference in a given
color space between an un-stained/soiled
area, and a stained/soiled and cleaned
area. We used the light-dark (L*) value in
the L*a*b* color space for this work.
Surface energy analysis can be accomplished in multiple ways9-14, e.g., Fowkes,
Owens-Wendt, Wu, Schultz, Oss-Good,
extended Fowkes, Zisman, and the
Neumann Equation of State to name a few.
All these methods involve some type of
measurement and a calculation. We chose
the method of Wu because it requires only
two liquids of known and disparate polar
and dispersive components. Additionally,
the Wu method is suitable to determine
the surface energy of polymeric materials
with values up to 40 mN/m. 9 We chose
water as a mostly polar liquid and either
diiodomethane or dodecane as our mostly
dispersive liquid. The values for the surface free energy of each liquid used in this
study are listed in Table 1.
We used the sessile drop technique16, 17
with a goniometer to measure the contact
angle, which was then used to calculate
the surface free energy. Surface energy
can be calculated from contact angles using various formulae as mentioned previously. We focused on the Wu method
since it is useful for calculating surface
energies in the expected range of our
samples (< 40 mN/m). Water and dodecane were tried with every sample, but
in about 10% of the samples, dodecane
completely wet the surface too quickly
for us to measure the contact angles.
In those cases, we used diiodomethane.
Using these solvents’ known free energies, the contact angles, and the following
equation (calculated using Kruss Advance
Drop Shape software), we were able to
calculate the dispersive, polar, and total
surface free energy of each substrate.
( 4)
where superscripts D and P represent
the Dispersive and Polar component,
respectively. A correlation coefficient
analysis data tool pack within Microsoft
Excel® was used to determine correlation
coefficients between the variables listed in
Table 2.
Methods and Materials
A total of 171 paint samples, including
formulated and commercial coatings,
were used in this evaluation. Eighteen dif-
ferent commercial paints from six differ-
ent paint manufacturers were used as part
of this study. These paints were used as
is, and four additional samples per paint
were prepared using four different stain
repellent post-add additives. Commercial
paints accounted for 89 of the test samples.
The remaining 82 samples were produced
from six different internally developed
paint formulations using the same post-
add stain repellent additives. The testing
formulation variables were:
Polymer:
• Rhoplex TM 585 (all-acrylic)
• RovaceTM 9900 (vinyl-acrylic)
• VINNAPAS® EF 8001 (EVA)
PVC (%):
• 47
• 57
Stain repellent additive:
• None
• Variant 1
• Variant 2
• Variant 3
• SILRES® BS 6500 A
WACKER Developmental Stain re-
pellent additive concentration (%):
• 0.75
• 1.50
Test panels were prepared according
to ASTM D3450 - Standard Test Method
for Washability Properties of Interior
Architectural Coatings5, using a 10-mil
Table 1: Surface free energy of solvents2
Solvent Dispersive
Component
γDlv (mN/m)
Polar Component
γPlv (mN/m)
Total Surface
Energy
γlv (mN/m)
Water 22.0 50.2 72.2
Diiodomethane 48. 5 2.3 50. 8
Dodecane 25. 4 0.0 25. 4
Table 2: Variables used in calculating correlation coefficients
Variables
Dispersion surface free energy
Polar surface free energy
Total surface free energy
Water contact angle
Dodecane contact angle
Diiodomethane contact angle
Absolute L* value difference from unstained to stained area
