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ESMA (European
Screenprinting
Manufacturers’
Association) is an
Association of
European
Manufacturers of
machinery and
consumables for the
specialist printing
industry, including
screen, digital and
pad printing
processes.
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ESMA is a non-profit organisation, funded by manufacturers, for manufacturers and to the ultimate benefit of their customers.
ESMA has close links to FESPA, Europe’s leading trade federation for this industry. For further details visit www.esma.com
I. Basic Information regarding UV inks and their
curing
The most commonly used ink types for graphic screen
printing still are 1-component solvent based inks.
These inks dry physically by evaporation of solvents.
UV inks on the other hand “dry” by a chemical
curing reaction initiated by UV radiation. Basically
there are two types of reaction, radical and cationic
reaction. Apart from a few rare exceptions the radical
version is mainly used for screen applications.
Components of UV inks (prior to curing):
Oligomer binder, main component, highly viscous,
contains active groups for cross linkage.
Monomer Used as “thinner” (substitute), low viscosity,
reacts with the oligomer during the curing process.
Photoinitiator UV-sensitive component; under UV
radiation polymerisation is initiated by its
decomposition into radicals.
Pigments
Additives
Radical Curing Reaction:
Under UV-radiation so-called free radicals form by
degradation of the photo initiator. These initiate
polymerisation between the double bonds of the
oligomers and monomers. In that respect it is
important that an adequate amount of UV energy will
initiate a sufficient amount of photoinitiators to form
radicals. For optimal polymerisation the UV-radiation
has to penetrate the complete ink film to the substrate.
The curing reaction depends on many influencing
factors:
1. Binder base of the inks:
Oligomers, and above all monomers show quite
different reactions in reference to their cross-Iinking
speed depending on their raw material base. There
are quite reactive products (e.g. inks for paper,
thermoplastics, plastics), however there are also
systems with a much slower reaction by factor 5-10
(inks for duroplastics, coated surfaces metals).
2. Pigmentation of inks:
UV light can penetrate non-pigmented varnishes
without any major loss. Thus practically the whole
amount of energy emitted by the UV radiator is
available for curing.
Pigmented inks exhibit a certain absorption ability
towards UV-radiation, which depends on the type of
pigment and the amount contained in the formulation. So pigments inhibit the polymerisation to some extent.
Therefore pigmented inks require more UV energy
than non-pigmented inks in order to cure. Under most
unfavourable conditions, e.g. very high ink deposit,
polymerisation may therefore be insufficient.
3. UV-radiators:
Depending on the composition of the spectrum of the
UV lamp there are different effects on the curing
process. The usual “conventional” high pressure
mercury radiators used for graphic screen printing
show a more or less even energy distribution covering
the whole UV-spectrum.
The flash discharge lamps used for the so-called
flash cure on the other hand have more UV-A
radiation. Inks for these lamps have to contain special
photo initiators which react to this UV spectrum.
4. Efficiency of the UV-radiators
The efficiency of a UV-radiator is given in W/cm e.g.
80 W/cm, 120 W/cm. This value refers to the
efficiency supplied and serves as an indication
regarding the efficiency of the UV-drier. However
these values do not refer to the actual energy amount
emitted to the substrate or the ink film.
Discharge of the UV-energy of the radiator
decreases with life time to a large extent. Thus
uncontrolled working methods will result in adhesion
problems sooner or later.
5. Construction of the UV-Iamp reflector:
For optimal use of the energy of UV-Iight discharged
by the UV-radiators the UV-tunnel is surrounded halfway
by a reflector in order to reflect the light emitted
towards the top down to the print. The quality of the
reflector (mirrorplated, chromium-plated, Aluminium
polished, mat Aluminium) and construction (focussed,
semi-focussed, defocussed) has a significant influence
on the quality of curing.
For reasons of heat emission distribution of the
UV-radiator (prevention of the burning glass effect),
defocussing or semi-focussing reflectors are often to
cure temperature sensitive substrates. However highly
pigmented dark colours, especially those with a low
reactivity may then show iriferior curing quality. In
that case focussing reflectors are required.
6. Substrate colour:
White substrates re-reflect a big portion of the UVradiation
emitted through the ink film. Therefore the
ink reacts and thus cures faster. Transparent substrates
do not reflect the UV-Iight. lt will be absorbed by the
dark color of the drier. The missing reflected share of
light has to be compensated by a higher radiation emitted
by the lamp. Dark substrates show similar properties.
II. Evaluation of ink curing and indicators for
insufficient curing:
Drying of air drying solvent based inks is usually
evaluated as follows:
1. Stacking/blocking resistance:
Printed sheets do
not block under significant load.
2. Finger nail test:
Ink cannot be scratched off the substrate (under the precondition that
the ink is basically suitable for the substrate).
3. Cross-hatch testing:
(under the pre-condition that the ink is basically suitable for the substrate).
However residues of solvents contained in the ink cannot be determined by the above
testing methods. These have to be found by analytical methods. The most simple method
is a weighing process of the printed specimen by oven curing.
Basically UV-inks can be tested for sufficient curing by the same methods.
However, the following has to be taken into consideration:
1. Stacking/Blocking resistance:
Printed sheets do not block, even under significant
load. Instead of insufficient curing the reason for blocking of UV ink in the rack may also
be a thermoplastic effect. The ink is sufficiently cured, however, when stacking it is still
too “soft’ due to too much heat absorption and will therefore stick. This especially happens
with very elastic inks, above all if there is no cooling period for the prints (mere UV-drier
with low cooling efficiency, UV-unit is installed at the end of a circulation drier).
“Thumb test’:A quick testing method is the “thumb test’. The printed, cured ink is
pressed with the thumb with high pressure. Also with high pressure the thumb tip is
tumed several times (approx. 90 - 1200) on the spot. lf the pressure point will then look
mat, or if there was any abrasion curing has been insufficient.
2. Finger nail test:
(under the pre-condition that the ink is basically suitable for the
substrate).
The polymerisation of the UV ink in the ink film takes place from the top (facing the
lamp) to the bottom. Insufficiently cured UV-inks show a rigid surface,
however they are still soft inside. Thus the finger nail test will result in a “smearing”
scratching off after the finger nail has penetrated the film.
lf, however the scratch test results in rigid, dry splitting off from the substrate there is
probably an adhesion problem and rarely over-curing. Flaking due to over-curing can be
excluded with thin thermoplastics as they would deform due to strong heat influence
before the over-curing reaction of the ink.
3. Adhesion on substrate by cross-hatch testing:
(under the pre-condition that the ink
is basically suitable for the substrate).
This test is a common method to determine basic suitability of the ink for the substrate.
If the ink is insufficiently cured there is a “smeary” scratch off of ink at the bottom of the
ink layer pulled away with the tape. Ink residues remaining on the printed matter by
splitting of the ink layer with the tape may show the same property.
4. Chemical tests:
Evaluation methods used for solvent based inks or other chemical
substances cannot be used. Depending on their type UV-inks show different reactions in
reference to chemical resistances. Even if an adequate testing solution is available there
may be significant falsifications. Insufficiently cured UV-inks may show rigid surfaces,
however their underlayer is still soft and un-cured. The surface will be resistant to the
testing solution and thus show a positive result, which is wrong.
5. Other Methods:
There are no other testing methods the printer can carry out.
III. Control of degree of curing with a UV-dosage meter.
This measuring device, commonly known as a UV-integrator and mostly formed as flat
disc or plate contains a sensor, adapted to measuring of UV-radiation. When going through
the UV-radiators the device will measure the whole energy hitting the surface in millijoule
per cm2.. This is in direct proportion to the belt speed of the drier (J=W x s) and the
number of UV-radiators.
The best possible curing parameter of an ink, differentiated according to ink type,
colour- shade, possibly additives and fabric used are determined empirically. Then
measurement will be carried out with the UV-integrator in the curing unit to be used. The
value(s) determined (mJ/cm²) is (are) the essential values for the future productions. When
starting an order the UV-drier is adjusted to the corresponding measurement value and test
prints are made. As described adhesion, scratch, crosshatch and thumb tests are carried out.
Usually the results should be positive, production can start.
Attention: Measurement devices from different manufacturers can show deviations up to a
factor of 2, sometimes even more. Measurement values under the same conditions can be
quite different. Even devices of the same make and manufacturer may show deviations of
up to 50%. These devices can only be used
individually for safe quality control with the
measurement values determined as described
above.
Measurement values cannot be used for other
devices.
In that respect there is a requirement for
standardisation !
IV. Determination of content of residue
monomers (non polymerised) in UV-inks
As already mentioned, when curing an ink the
splitting of the double bonds of molecule structures
of the monomers and oligomers will result in a
polymerisation/cross linkage between both
substances. Net work complexes are the result of
this film formation. Depending on the properties of
the UV-ink these net works show a coarse or fine
mesh and various properties, e.g. very flexible/soft:,
very rigid/brittle or extremely chemical resistant.
1. Degree of cross linkage: A UV-ink is considered
to be cured, e.g. optimally cross linked, if it fulfils
the properties described in the requirement profile.
Basically these are the criteria regarding adhesion,
scratch resistance, tape resistance, stackability, in
individual cases especially high mechanical or
chemical resistance. This optimal cross linkage
however, does not present a real 100% cross linkage
of the double bonds. UV-inks reach their best
possible cross linkage at approx. 70-90% average.
The remaining 10-30% of non-split double bonds
cannot form any further bonds as due to the very high
density of the 70-90% network there is no further
room left. These non-cross linked molecules are
closed in the network and cannot evaporate.
Safety: An optimally cross-Iinked ink can be
considered as completely cured and does not present
any risk in reference to irritants.
Special cases in reference to consumer protection:
Food and partially toys and health articles are
problematic applications of UV-inks.
One cannot exclude with absolute certainty that
non-crosslinked or nonlinked components of the
cured UV-ink migrate from the UV-ink film to the
surface or penetrate into foods, e.g. butter, grease
etc. and thus present a potential health risk to human
beings.
For that reason UV inks are not used for print
likely to come in direct contact with food.
In border cases, such as medicine and toys, the
possible interactions of Uvprints with certain
substances is tested in time-consuming analysis
processes.
2. Measurement of degree of cross linkage
Actual measurement of degree of cross linkage of a
cured UV-ink can only be carried out by scientific analytical methods. Literature available mentions the
following processes:
IR spectroscopy: Measurement of concentration of
double bonds.
Temperature sequence measurement: Temperature
emission during polymerisation.
Discoloration reaction: Coloration of non-cross
linkable components with potassium permanganate
(tetraoxomanganate).
IR Spectroscopy: With this method, especially the
FTIR-spectroscopy (real time Fourier transform-IR
spectroscopy) the reaction amount of the printing ink
is measured after UV-radiation. This is a comparing
process, i.e. printing ink is measured before exposure
and after UV-curing. Based on the different peak
curves (decrease of the C=C double bonds) the degree
of cross linkage can be determined.
Temperature sequence measurement: This
measurement method has been described in
connection with the topic ink curing with flash
discharge lamps. Measurement of conventional UV
radiators may not be possible, as there is a significant
temperature increase of UV-radiators during the
radiation process.
During the curing process heat is emitted from
the polymerising ink (exothermic reaction). First this
process measures the low temperature of the ink layer
from a blind specimen (non cross linkable ink without
photoinitiator). Then temperature development of a
cross linkable ink is measured. The heat emitted
during the polymerisation will rise above the level
of mere heat emission of the flash lights. Following
a number of flashes the temperature curve will not
increase any further. This indicates optimal cross
linkage and further reaction heat is not emitted.
The process described in relevant literature is
based on measurements carried out with high layer
thicknesses (2mm) and non-pigmented varnishes :
Suitability for usual screen ink layer thicknesses is
questionable.
Discoloration: With this process non cross-linked
ink components become visible by discoloration
reaction using potassium permanganate
(tetraoxomanganate). The reduction of the potassium
permanganate by reaction of chemicals with the noncross
linked ink components results in manganese
dioxide. The stronger its coloration the lower the
cross linkage and vice versa.
This method is only suitable for non-pigmented
varnishes. Evaluation factors regarding colour scale
of the brown coloration have to be determined
empirically for each varnish type.
3. Residue monomers: Residue monomers are noncross
linked monomers of an insufficiently cured UVink,
which can exit from the ink layer due to insufficient cross-Iinkage/film formation and present
an active irritation potential or quality problem.
4. Measurement of residue monomers: Insufficient
cross linkage (just like the degree of cross linkage)
can be recognised indirectly by adhesion, scratch,
cross-hatch and strain tests. Direct measurement of
residual monomers on the other hand can only be
carried out by scientific analytical methods.
This is possible e.g. by substance analysis with
the gas chromatograph. Using this device first the
non cross-Iinked reactive component of an ink are
dissolved individually using a suitable solvent. Each
solution will then be analysed with the gas
chromatograph. A characteristic peak line appears for
each reactive raw material (on a time axis). Then a
potentially insufficiently cross-Iinked ink film will
be placed in a solvent. No linked reactive components
from the insufficiently cross-Iinked film will then
elute into the solution, which will now be analysed.
A comparison of this peak curve with the curves of
non-cross linked raw materials will result in
determination of residual monomers.
5. Summary: Various evaluation methods used to
determine sufficient or insufficient curing of UV-inks
have been described. In addition to suitable, manual,
albeit, insecure methods, there are quite some
demanding scientific, analytical methods. There is
no intermediate solution for screen printers, i.e. todate
there is no simple, cost efficient and exact
evaluation method. According to many discussion
with chemists and members of the technical
committee of FOGRA there is no solution at present.
In our opinion the only way is the two-way-concept:
a) The method for daily work: manual - quick - easy -
cost-efficient - however, too inaccurate for some
applications
b) The method to exclude (if required) any safety inks:
scientific - time-consuming - complex - expensive -
accurate.
Literature:
• Georg Bolte: “Den Nachteilen geht die Luft aus”,
UV-Trocknung mit Sauerstoffreduzierung Druck &
Medien-Magazin 5/ 2001 (Disadvantages disappear
- UV drying with oxygen reduction)
• Georg Meichsner / Klaus Vogg “Mehr
Aufmerksamkeit für einen Strahlertyp: Die UVBlitzlampe”
• Untersuchungen zur Lackhärtung (more attention
for a radiator type: the UV flash radiator,
examinations about ink curing)
• Andrea Prager-Duschke / Reiner Mehnert “UVhärtende,
lösemittelfreie Tiefdruckfarben” (UVcuring,
solvent based gravure inks)
• Fa. Fusion UV-Systems Inc., Maryland-USA
“Fascination UV”
