Transparent Ultrafine Mineral Filler Technologies
January 2017 www.coatingsworld.com Coatings World | 47
to air dry for 10 minutes, followed by
10 minutes at 50 ºC. A small section
of the coating was scored with a razor
blade, then peeled off the release chart
using scotch tape. The coating was then
attached to a metal holder with tape for
measurement using FTIR.
The curing reaction was measured
with a Bruker Vertex 70 FTIR in rapid
scan mode. A typical 3-D plot of the
spectrum for the area of interest can be
found in Figure 6. The resolution for
testing was set at 8 cm-1 and measured
from 1800 to 600 cm-1 with the liquid N2
MCT detector and a 0.5 mm aperture.
Approximate scan time was about one
minute in which a total of 750 measurements were performed.
Curing was followed by taking the
FTIR band area of the reacting resin car-bon-carbon double bond (C=C, 800-820
cm-1) and normalizing with a band of the
resin that does not change during curing
(825-845 cm-1). The cure rate was determined by measuring the time required
to react half of the available C=C (t1/2),
where complete curing is taken as the intensity of the C=C band after exposure to
UV radiation for the full one minute.
5 µm N.S. Loading and
Photo-initiator Interaction
via DOE
A design of experiments (DOE) was employed to determine the effects of loading levels of 5 µm N.S. and photoinitiator
on the cure rate of the UV-cure PUD test
formulation. The DOE was set up using
statistical software as a multifactorial design with two factors and one response.
The two factors studied in the DOE were
loading levels of photoinitiator (g) and of
5 µm N.S. (% solids in cured film). The
response was t1/2, which is the time required to react half of the available C=C
in seconds, and is expressed as the desirability level. A desirability of 1 is ideal, as
it represents the lowest cure time for t1/2.
The fastest t1/2 was typically around 0.5
seconds. A desirability of 0 represented
the slowest t1/2 and was typically around
1.5 seconds. A diagram illustrating this
DOE can be found in Figure 7. This DOE
was run in triplicate, producing 27 separate measurements.
The cure rate was determined by
measuring the time required to react
half of the available C=C (t1/2), where
completed curing is taken as the intensity of the C=C band after exposure to
UV radiation for one minute. The total degree to which the coatings were
cured could be determined by analyzing
the amount of this band remaining at
the end of curing. The degree of curing,
or crosslinking, is expected to have a
substantial effect on coating properties
such as hardness and flexibility.
Assignment of the C=C bands in the
FTIR spectra of the urethane acrylate
resin was based on comparison with other spectra. Good agreement was found
with spectra of various methacrylate
resins published on the NIST Chemistry
Webbook.2 The vibrational modes of
the vinyl moiety can be assigned by comparison with the well-known assignments
of the vibrational spectrum of ethylene.
FTIR bands at 800-820 and 1400-1410
cm-1 are both due to C-H bending modes
of the vinyl moiety of the acrylate. 3 Two
bands in the FTIR spectra at approximately 800 and 1400 cm-1 were used
to determine the cure rates of the testing
coatings. The 1400 cm-1 band was relatively stronger than the band at 800 cm-1
and could potentially be used to consider
other filler types that might otherwise interfere with the 800 cm-1 band.
Results and Discussion
The first series of experiments studied
the kinetics of the UV curing reaction as
affected by factors of applied film thickness, 5 µm N.S. filler loadings levels, and
photoinitiator levels (100% and 50%
levels). Of most interest was the effect of
ultrafine nepheline syenite curing behavior in filled, UV-cure PUD clear versus a
typical unmodified formulation.
As previously noted, the time required
to react half of the available C=C is presented as a measure of cure rate, which occurs in the first couple of seconds after the
UV source is turned on. Additional curing
up to 60 seconds drives the reaction forward and increases crosslinking density,
resulting in a thoroughly cured coating.
Filler Loading Level
The cure rate results for 5 µm N.S. filled
systems versus the unmodified control
are presented in Figures 8a and 8b at 1.5
and 3.0 wet film thickness, respectively.
When looking at the 3.0 mil films, the 5
µm N.S. filler loading had a faster cure
rate at 6% than at 12% loading. Both
Figure 6. Three-dimensional spectra of a sample obtained by FTIR with real-time curing. Peak is at approximately 820 cm-1.
Figure 7. Diagram of DOE showing levels of the
two factors: 5 µm N.S. and photoinitiator level.
