films cured faster than the unfilled film.
This shows that 5 µm N.S. accelerates
the initial cure rate relative to the unfilled
formulation. Moreover, there appears to
be an optimal loading level of 5 µm N.S.
to accelerate the curing time.
At 1.5 mil wet thickness, the effect of
the 5 µm N.S. loading level on cure rate
is smaller, but still observable. It should
be noted that the cure rate at 6% did
not change much with film thickness.
However, the cure rate was much faster
than the thinner film loaded at 12% and
the unfilled control. The data suggests
that 5 µm N.S. could be even more beneficial for curing at greater film thickness.
Photoinitiator Level
The second series of FTIR experiments
studied the effect of 5 µm N.S. on cure
rate of formulations with a 50% reduced
concentration of the photoinitiator. The
results shown above suggested that nepheline syenite acts as a UV cure catalyst by
an unknown mechanism. The results for
testing at standard and at 50% reduced
photoinitiator levels were measured as
percentage cured versus time. The plot-ted functions are found in Figure 9. Time
for initial cure (50% C=C conversion) is
found in Figure 10. Reducing the photo-initiator level by 50% in the unmodified
control has a significant negative effect
on the initial cure rate and cure profile.
However, when using 12% 5 µm N.S. in
the coating with 50% lower photoinitiator, the cure decreased only slightly.
Again, the initial cure rate with 5 µm N.S.
provides a superior and accelerated curing profile versus the unmodified control,
and more thorough cure after one minute.
DOE: 5 µm N.S. and
Photoinitiator Impact
on Cure
To further study the effect of 5 µm N.S.
on radiation curing and interactions with
the photoinitiator, a design of experi-
ments (DOE) was conducted. The design
parameters and results of these measure-
ments are listed in Tables 5 and 6. The
standard deviation in the curing rate, as
expressed by the half–time, is approxi-
mately 10%. The error analysis for each
FTIR measurement band showed the R2
or correlation values both exceeded 90%,
which is excellent for this model and
shows a high level of correlation and con-
fidence in the predictions.
With the aid of statistical graphing
software, the contributions of 5 µm N.S.
and photoinitiator to the curing rate were
compared with response surface plots,
Figure 8. Time (sec) for 50 of the C=C in PUD-Acrylate to be converted.
Figure 9. Percent cure rate of the unmodified UV-cure PUD and 5 µm N.S. filled (12% by wt.) with standard
photoinitiator and 50% reduction of the photoinitiator level.
Figure 10. Time (sec) for 50 of the C=C in the unmodified UV-cure PUD and 5 µm N.S. filled (12% by
wt.) with standard photoinitiator and 50% reduction of the photoinitiator level.
