When bonding lenses with Norland Optical Adhesive the minimum
full cure is estimated at 20 times the precure time using the
same intensity of ultraviolet light. This is a general statement
that works well in most applications.
The precure time is defined
as the time it takes to initially set or gel the adhesive so that
the two surfaces being bonded can no longer be moved. For example,
a 15 second precure requires 5 minutes (15 sec. x 20) for minimum
full cure. When different light sources are used in production
for the precure and the full cure, the precure time used in this
calculation must be measured using the full cure light source.
Factors to remember which affect
the precure time are:
1. Intensity and distance of
ultraviolet
light source from surface.
2. Thickness and transmission
of lens.
3. Thickness of adhesive in
bondline.
4. Cure rate of adhesive.
Varying one or more of these
can change the cure time. Further understanding of the
cure can be had from Fig.1 which illustrates the relation between
the cure rate or crosslinking and the exposure time. Note that the curve
is asymtotic and as the exposure time progresses the amount of crosslinking
approaches 100% but theoretically never reaches it. The time chosen
for minimum full cure is approximately 99.5% cross linked and
for efficient production is enough for most applications exposure
time. Further exposure will bring
no noticeable increase in adhesion but the additional crosslinking
will lower the moisture and solvent absorption. Since the absorption
of even a small amount of moisture can cause separations at the
edges we recommend 4 to 8 time the minimum full cure if lenses
are to be subjected to high humidity for extended periods of time.
