However, as previously shown, the correlation between surface energy and contact
angle does not extend to stain repellency
even upon considering the chemical nature of the staining agents.
While not perfect or absolute, the data
suggests that correlation coefficient is not
an adequate tool to describe whether a
relationship exists between contact angle
or surface energy and stain repellency.
The fact that some subjectivity exists in
all of the test methods could explain the
less than perfect relationships. For example, while the contact angle is determined
using precision equipment, the choice of
where to place the interpretative lines is
decided by an operator. Additionally,
there does exhibit some variation in the
stain removal. An average of multiple
values recorded over a range of the area
of the cleaned stains and the choice of
where to take the readings with a colorimeter are determined by an operator.
This does not mean that the testing in this
study is flawed, rather that it is imperfect
due to inherent human error.
Summary and Conclusions
This study evaluated 171 samples for stain
repellency, contact angle with at least two
different solvents, and calculated surface
energy to determine if a correlation exists
between stain repellency, contact angle,
and surface energy. This was done in an
attempt to determine if a staining agent is
more repelled (shows less staining), or easier to clean if the staining agent exhibited
a higher contact angle (beading behavior).
Had this been the case, stain repellency
development work might be done more
expeditiously since no stain testing would
have to be done. However, the opposite
was found to be true, at least with respect
to contact angles with water and dodecane.
With the exception of a weakly positive correlation between ASTM black stain media
and water contact angle, no other relationship was shown to exist within the context
of the staining agents used and the chemical
agents used to measure contact angle.
No correlation was noted between the
chemical nature of the staining agents and
the contact angles of any test liquid or
subsequently calculated surface energies.
Hansen Solubility Parameters (HSP) may
Table 10: Water contact angle correlation coefficient data
Correlation Calculation Property (L*)
(n = 171 samples)
Correlation Coefficient
Ketchup 0.34
Red wine 0.52
Mustard 0.20
Soya oil 0.34
Soy sauce 0.37
Coffee 0.43
Cola 0.39
Red lipstick 0.54
ASTM black stain media 0.61
Total stain L* Value 0.49
Table 11: Dodecane contact angle correlation coefficient data
Correlation Calculation Property (L*)
(n = 158 samples)
Correlation Coefficient
Ketchup 0.10
Red wine 0.14
Mustard 0.04
Soya oil 0.03
Soy sauce 0.10
Coffee 0.06
Cola 0.07
Red lipstick 0.04
ASTM black stain media 0.13
Total stain L* Value 0.09
Table 12: Diiodomethane contact angle correlation coefficient data
Correlation Calculation Property (L*)
(n = 13 samples)
Correlation Coefficient
Ketchup 0.44
Red wine 0.47
Mustard 0.39
Soya oil 0.64
Soy sauce 0.67
Coffee 0.57
Cola 0.55
Red lipstick 0.63
ASTM black stain media 0.34
Total stain L* Value 0.49
