Home' FLEXO Magazine : September 2015 Contents 56 FLEXO | SEPTEMBER 2015
roll sleeve, OPS, polyole n, and OPP require slightly di erent printing
techniques compared with PVC roll sleeve and, sometimes, bene t
from specially made primers.
Typical fountain ink formulations are comprised of 65 percent water
or solvent in a mix of pigments, resins and modifying additives. Water
based inks require alteration of the surface energy of the lm to pro-
mote bonding. As a rule of thumb, the di erence in surface energy of
the ink and the lm should be 10-12 dynes per centimeter. Since most
of these inks have surface energies in the 40 dyne range, most lms
would need to be treated to enhance the dyne level to greater than 50.
Care must be taken when treating low dyne level materials, such as OPS
and OPP, to higher surface energies (see Ta b l e 5 ). Overtreatment can
result in oxidation of the lm surface, resulting in poor ink adhesion.
Solvent based inks generally do not require surface enhancement.
Most are comprised of an 80 percent/20 percent ratio of alcohol to
acetate. is blend will change, based on the polymer substrate, as,
for example, the acetate content in OPS inks would be reduced to 10
percent or lower as acetates, hydrocarbons, and ketones could attack
the polymer bonds and so en or swell the lm. All substrates are sus-
ceptible to plasticization, depending on the solvent used, which could
result in an anti plasticization e ect, causing brittleness and potential
web breaks in the printing or label application processes. Drying con-
ditions and the e ectiveness of drying are major factors in minimizing
solvent retention and/or blocking.
Radiation cured inks are 100 percent solids and utilize cationic or free
radical mechanisms in the presence of UV light to complete curing.
Curing is completed in cationic mechanisms a er an initial exposure
to UV light, where the reaction of the photoinitiator---as in an epoxy
based adhesive---and UV creates a catalyst that actually completes
the cure. Free radicals, as in acrylic based chemistry, require constant
UV exposure. Transmission of UV
light through the lm is an important
consideration with this type.
Long time exposure to short wave UV
(UVB, UVC) light can break bonds in
some polymers, such as PVC roll sleeve,
and cause chromophores---color de-
velopment due to the resultant carbon/
carbon double bond formation. PETG
roll sleeve absorbs UVB and UVC, so
minimal curing would occur. Either
can be successfully cured with UVA
and a corresponding adhesive system
designed to cure in this wavelength.
Polyole n and OPP will transmit
UVB at 70 percent and 60 percent,
respectively. OPS will transmit about
Selection of the proper ink for printing
these polymers also depends on the
method of printing application. Typical printing types are exo, gra-
vure and rotary o set, as well as a growing volume of digital printing
equipment. All of these polymer substrates require special coatings
to augment digital ink receptivity. Given the variety of commercially
available equipment and ink systems, it is recommended the printer
work together with the ink and substrate suppliers to determine and
select the optimal ink package suited for the given substrate. ese
vendors may suggest a solution that requires no surface treatment, or
may suggest a surface treatment or o er a primer that is well suited
for ink adherence to the substrate. Tab l e 2 summarizes basic printing
methods by polymer. e red dots illustrate that polymer surface
modi cation is required to improve printability or that special care
is needed to print the material. As discussed, PETG roll sleeve can
be successfully printed with UV inks, but the curing wavelength is
limited to UVA or catalyst based ink systems.
Chart 1 shows the shrink curves for the polymer substrates plotted by
shrink percentage at a given temperature. e curves in this chart are
representative of the movement or shrinkage that the material would
exhibit in a steam tunnel. Not only can shrink initiation and ultimate
shrinkage be obtained from a shrink curve, predictive shrink perfor-
mance and the quality of shrinkage with respect to possible defects in
the nished label can be understood.
For example, rapid shrinkage (greater than 40 percent over a 10
degree Celsius temperature range, within 10 degrees Celsius to 20 de-
grees Celsius of shrink initiation) can lead to wrinkles. Fast shrinking
lms with low shrink force can also cause wrinkles to develop. ese
lms will stop shrinking at the moment of contact with the container
and could show localized areas with poor shrinkage commonly re-
ferred to as " sheyes." Low shrink force lms can cause problems with
incomplete shrinkage at high shrink positions on a container, such as
narrow necks. Films with insu cient shrinkage can show wrinkles or
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