PCT Engineered Comparison of UV and EB Technology for Printing and Packaging Applications - Manual (Page 2)

28 RADTECH REPORT SEPTEMBER/OCTOBER 2008
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The energy for photons is determined
by the wavelength. The range of
wavelengths for UV curing applications
is typically about 250 to 450 nm. The
shorter the wavelength, the higher the
energy. Wavelength units may be
converted to other energy units for
comparison. For example, a 350 nm
photon is equivalent to 3.5 electron
volts (eV). UVcuring processes are
often characterized by the total
amount of applied UV energy
impinging per unit surface area (also
known as the irradiance). The UV
energy needed for a curing process
depends on the material and the
application. For an ink, coating or
adhesive for a packaging application,
the UV energy typically ranges from
about 0.1 to 0.5 J/cm. 2
The smallest “bit” of EB energy is
the electron. The energy of the
electrons is determined by the
accelerating potential of the EB
equipment. The range of accelerating
potential used for typical packaging
applications is about 80 to 180 kV. The
electrons lose some energy when
passing through the foil window and
the air space between the window and
the substrate. For example, the
electrons from an EB unit operating at
100 kV have an average energy of
about 70 keV when they reach the
substrate. EB curing processes are
often characterized by the total
amount of energy absorbed per unit
mass of the substrate (also known as
the cure dose). The dose for EB curing
depends on the material and the
application. For an ink, coating or
adhesive for a packaging application,
the cure dose typically ranges from
about 20 to 40 kGy (2 to 4 Mrads).
It is interesting to compare the
energy of a typical UV photon (3.5 eV)
to an EB electron (70,000 eV). Clearly,
EB electrons are much more energetic
than UV photons. This has a significant
impact on how this energy interacts
with the media to be cured. The typical
chemical bond energy in an organic
material that is the basis of an ink,
coating or adhesive is on the order of
5 eV. Curing reactions are initiated with
the breaking of a chemical bond. Since
UV photons have less energy than the
bond energy, they cannot initiate
curing on their own. A photoinitiator is
needed which can be activated by the
lower energy photons. The energy of
the EB electrons easily exceeds the
bond energy of the curable materials;
thus they will initiate curing without an
added photoinitiator. EB is also known
as ionizing radiation because of its
ability to break chemical bonds. UV is
non-ionizing radiation.
In addition to considering the
energy of the individual photons and
electrons, it is useful to compare the
total energy applied in the curing
process. As can be seen from the
discussion above, UV curing is
characterized by the energy absorbed
per unit area (irradiance), while EB
curing is characterized by the energy
per unit mass (dose). If one considers
a given thickness and density of the
substrate, it is possible to make a
direct comparison of the total applied
energy in UV- and EB-curing
processes. A typical modern low-
voltage EB unit operating at 125 kV
will penetrate into a 50 g/m 2 layer.
Thus, given 1 kGy = 1 J/gram, and
assuming a 50 gram/m 2 substrate,
then; 20 to 40 kGy = 0.1 to 0.2 J/cm 2
for typical EB curing compared to:
0.1 to 0.5 J/cm 2 for typical UV curing.
The lesson from this exercise in
energy unit conversions is that
although EB electrons are much more
energetic than UV photons, the total
amount of energy applied in a typical
curing process is not all that different.
UV and EB Penetration
The nature of the energy determines
how it penetrates into a material.
Curing can only occur in areas that
are effectively exposed. Figure 1
provides a cross-sectional illustration
of the differences between UV and
EB penetration.
Penetration of UV energy depends
on the optical density (OD) of the
Figure 1
Penetration of UV and EB energy