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Essentially
a printing is no more than a means of transferring a
coloured pigment onto a substrate. Having got it there
quickly and efficiently –not always the easiest of tasks
–the ink system must ensure that it remains there under
whatever conditions the end user requirements of the print
demands.
Sometimes
these demands can be very stringent, and have necessitated a
range of tests for resistance properties to be established
by ink makers, many of which have applications that are
independent of the printing process.
Adhesion
A
basic requirement of any ink is that it adheres to the
substrate upon which it is printed. With paper and board,
this is really a problem. With non-absorbent substrates such
as plastics and foils, it can be. Three very simple test
methods have been established which give a very good
assessment of ink adhesion- fingernail scratching, scotch
tape test and wrinkling. The fingernail test is a very
useful technique for foils, films and plastic, including
polythene bottles and sheet plastics. The ink film is
scratched with the nail and the ease or difficulty with
which the ink is removed is a measure of the adhesion or the
hardness of the ink film.
The
scotch tape is probably the most widely used, but in many
ways, is the least reliable.
A length of tape is pressed down onto the ink film
and quickly pulled off. The print is then examined for ink
removal. Unfortunately the test is very dependent on how
well the tape is pressed onto the ink, what type is used,
how long it remains and at what angle and how rapidly it is
pulled off. The wrinkle test is commonly used on film. A
piece of print is grasped between the thumb about one inch
apart. The hands are then put together and moved in a rotary
motion so that the face of the print is rubbed against
itself for a specified number of rubs. This is a quite a
severe test, but more realistically matches the conditions
that a printed film will come under during its lifetime.
Rub
resistance
The
need for good resistance to rubbing, or abrasion, is most
significant for carton inks as they are subjected to
considerable scuffing. Rub resistance of ink is a difficult
property to quantity and should always be assessed by
comparison with standard ink of known performance on the
board under test. Several mechanical rub tests are available
of which the Patra is the most familiar and the most likely
to give reproducible results. A large disc of plain board is
placed face upwards on the circular platforms and a smaller
a printed disc is placed face downward on top of this. A
weight is dropped onto the test disc and the machine is run
for a specified number of revolutions (usually 50). The
large and small discs revolve at the same rate but in such a
way that their centers do not coincide, thus causing
planetary motion. The print is removed and the degree of ink
transfer to the larger plain disc assessed in comparison to
a standard. The severity of the test can be varied in two
ways; firstly by means of different weights of loading and
secondly, by altering the number of revolutions. This test
cannot be readily modified to suit lightweight substrates
but others have been devised for such cases, for instance
heavy duty sacks and dry rub of coloured inks against white
on packaging lines.
Grease /
rub resistance
Grease
/ rub resistance is important for inks designed for use on
snack food packs. A satra rub tester assesses the ink by
rotating a weighted 1" diameter felt soaked in a known
quantity of a particular grease or oil on a print sample for
a given number of revolutions, normally 500. One such test
specifies that after 100 rubs, no removal should be noted
with a maximum of 50 percent removal after the full 500 if
the ink is to be considered satisfactory.
Blocking
and set off
Blocking
is the term used to describe the condition when sheets or
reels of print stick together. Set-off describes the
transfers of ink from one print onto the back of the print
above. Both can be related to the drying of ink in some way.
One of the major causes if blocking is failure to remove all
solvent from the ink film. It is most common in flexo and
gravure printing of plastic films can be a problem in screen
printing of sheet plastic, particularly PVC. Set-off is less
severe manifestation of slow drying and although the problem
can arise in any of the print processes, the term is
normally used to describe the poor settings of a litho ink.
Laboratory
equipment used to evaluate these two properties may simply
be a stack of paper or board left on a bench or a
sophisticated hydraulic press needed to simulate the
extremely high pressures (5-20 tons per square inch) that
may be found at the center of a reel of film whatever the
test method, any transfer of ink is noted and, as the result
can't be assessed quantitatively, compared against control
sample.
Other
physical resistance properties
The
action of light on an ink film can induce and catalyze chemical changes in the pigment molecules which lead to
marked colour changes or fading. Stability to light is an
important property when the in is to be used for outdoor
work or for display in a shop window. Testing for light
fastness can be carried out by the traditional method of
exposing the print outdoors for several months. This has the
advantage of assessing the inks weather ability at the same
time and is therefore particularly suitable for testing the
inks for road signs for example. A much faster method but
which gives no clue to weather ability is to usr an
accelerated exposure machine such as an Xenotester where
exposure of the print to a xenon discharge lamp can simulate
3 months sunlight in less than one day. On many occasions,
it is desirable to be able to heat a pack over an ink film
without the ink smearing. The ink film must thus be
resistance to heat. The apparatus used consists of a pair of
electrically heated jaws with provision for accurate control
of temperature, pressure and dwell time. A cruder method
uses to domestic iron with base drilled to take the stem of
a small dial thermometer.
More
and more print is now being subjected to deep-freeze
conditions which can place severe demands on ink and varnish
films. A satisfactory ink will maintain adhesion and/ or
flexibility at temperatures of -30°c
to -40°c
and can be tested by subjecting the print to a series of
deep-freeze/thawing cycles plus wrinkle testing in the
presence of water and checking the appearance of the print.
Resistance
to chemical influences.
Many
inks need to be formulated to resist the product that they
are designed to pack and may also be required to withstand
other chemical influences. Sometimes a chemically resistance
varnish may be used to seal off ink films from their
surroundings and, in so doing, ink formulation and colour
availability are made that much easier.
The
resistance of inks to chemical influences is dependent on
both the pigments used and the resin system. Some resins
have poor resistance to specific chemicals (e.g. the poor
grease resistance of polyamides) and their use is avoided at
the initial formulating stage. Resistant pigments are often
more expensive that alternative grades and are only to be
used if it is absolutely essential. Thus it is important
that any requirement for chemical resistance is specified
when the ink is ordered. Chemical resistance tests can, if
carefully chosen, indicate the likely product resistance of
an ink or varnish but often the only way to obtain
absolutely reliable results is to test with the exact
product being packed. No two detergents for example are
exactly alike and may give different results. The packaging
buyer often specifies test methods. The technique varies
depend on the product, whether be solid or liquid for
example, but essentially all such tests are similar and
involve bringing the print into contact with the product
under specified conditions. Typical of them all is the soap
gel test in which a 25% soap solution is prepared and allow
to cool to form to a gel. A portion of print is damped and
pressed, face down, on the gel to ensure close contact.
After storage, the print is removed and compared for shade
and strength with the original print, and the soap gel is
examined for any sign of bleed.
In
such a limited space, it has only been possible to outline
the test methods used. Most are far more detailed in their
application; often modified to test specific properties
identified by printers and end user. The ink maker may even
design new tests to simulate specific production conditions.
It is however, difficult to simulate packaging and transit
conditions in the laboratory and therefore, close
cooperation between ink maker, printer and end user is
essential, ideally, this should be followed from laboratory
testing through print and production trials.
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