PH-value
Acid
or alkaline reactions of watery solutions are
caused by their content of hydrogen ions.
This
reaction is described as pH value, which is
expressed on a logarithmic scale in figures
between 0 and 14.
Neutral
water has a pH value of 7, figures below 7
indicate increasing acidity of the liquid,
higher figures indicate increasing alkalinity. Contrary to the exact scientific
explanations of the pH value, the offset printer
should rather imagine that the pH value is
comparable to a “ruler” with a division from
0 to 14.
It
should also be kept in mind, that an increase or
decrease of the pH value by one numerical value
means a multiplication of the acidity resp,alkalinity by the factor ten. In
practical work a pH value of the damping medium
of 5.5 to 5.8 has proven advantageous for offset
printing. A
slightly acid influence on the water-carrying
parts of the plate and increased surface tension
are achieved thereby.
The
pH value is usually measured by means of
specially treated paper, so-called indicator
paper which changes its color upon contact with
the liquid; this decoloration is compared with a
given color scale and thus permits to determine
the pH value. Unsuitable
pH values of the dampening medium can be very
detrimental to the printing result.
By adding a buffer solution to the
dampening medium, the pH value can be kept in
the pH range of 5.5-5.8, which is favorable for
offset printing.
Possible
consequences when using an acid dampening medium
with a pH value below 5:
1.
Drying difficulties of the printing ink
2.
Oxydation of metallic inks
3.
Shorter operating life of the printing
plate.
And
when using alkaline dampening medium with a pH
value above 7:
1.
Reduction of surface tension between
printing ink and dampening
medium; the ink
emulsifies.
2. The plate tends to scum.
Water
hardness
The
quality of the water used is of extraordinary
importance for trouble free printing.Tap water is not water in its chemically
pure form. Its suitability for use as dampening
medium is above all determined by the type and
by the quantity of the salts dissolved in it. The
water hardness is a measure for the content of
dissolved salts of alkaline earth metals; it is
expressed in “degrees German hardness” (0dH). 10 dH is the equivalent of 10 mg calcium oxide in one
litre of
water.
The
following guide provides the classification of
the water hardness:
|
0–40 dH very soft |
12–180
dH rather hard |
|
4–80 dH soft |
18-300
dH hard |
|
8–120 dHmedium soft |
> 300
dH very hard |
|
10dH
= 1,25 eH (English Standard)
= 1,79 fH
(French standard)
|
The
water hardness varies geographically very much,
it can even fluctuate temporarily.
A water analysis, which can be done e.g.
by the manufacturers of damping water additives,
will give the information on the quality of the
available water. The printer himself can also
determine the hardness by means of small
indicator rods or test solutions.
For
offset printing the water hardness should not be
higher than 100 dH. If
the harder water is used, calcium and magnesium
salts may settle on printing plates, blankets
and rollers and disturb the ink respectively.
Moreover, the chemical reaction of
calcium sales with fatty acids in the printing
ink may cause lime soaps to develop.
These then act as wetting agent and can
also disturb the printing process by making
ink-receptive areas water-receptive and vice
versa.These
difficulties can be avoided by installing a
water softener.
If
such deposits are already present on the
surfaces, they can be removed by treating the
surfaces with a solution of 50-g tartaric acid
in 1 litre water.This method, however, is very time
consuming and can by no means replace water
softening.
|
