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    Wrong pH - an invitation to troubles in offset printing

Wrong pH of the dampening solution is the main cause of lot many printing problems. Attention to pH is  very much decisive in the quality of any offset printing product. Described here are printing problems that may be caused by wrong pH – value in the dampening solution. The focus is on alkaline printing papers which are increasingly offered in the market and they could cause unwelcome chemical reactions with the acidic dampening solutions.

What is pH?
Water (H2O) not only consists of neutral hydrogen-oxyn molecules. The electric charge of hydrogen-oxyn particles determines whether a liquid is acid or alkaline.

In letters this is
H = one hydrogen atom (neutral)
H+ = one hydrogen ion (positive)
O = one oxygen ion (negative)

and the chemical formula is
H2O = H+ + OH-

The arrows of this equation mean that one normal water molecule H2O may split into 2 oppositely charged ions plus reserve.
The quantity of the (dissociated) water molecules determines whether a hydrous solution is acidic or alkaline. If a hydrous solution contains more than 10-7 mol (the quantity unit for small particles) of H+ ions per litre, it is acidic. If, However, it contains less than 10-7 mol of H+ ions per litre, it is alkaline. The pH value is calculated on the basis of these figures.

In order to simplify the way by which the acidity or alkalinity of hydrous solutions is determined in figures, the clumsy expression 10
-7 was dropped. Now only the exponent is used. Thus the pH value is a logarithmic value:
  pH=-log [H
+]          [H+]=10-pH

It will do to determine the H
+ ions since the ratio of hydroxide ions is constant.  
+] x [OH] = constant
1    2    3     4    5    6    7     8     9     10    11   12   13   14
                             strongest      acidic      neutral       alkaline         strongest 
acid                                                               base

How to measure pH?

In most cases, so-called indicators are used for measuring pH. They are, e.g.

Colour Range

Indicator Acidic Alkaline pH
methyl orange red orange/yellow 4.0
litmus red blue 6.8
phenolphthalein without care red 8.4

Mostly the indicators are integrated in paper strips, which only have to be put into a test solution in order to determine the pH fairly accurately. They are called indicator strips. This method can only be used for solutions without colour or that are faintly coloured. The electrical resistance is measured by means of the conductivity of ions. Ions conduct current depending on how they are charged, in neutral solutions (e.g. pure distilled water), however, no current will be conducted due to a lack of ions. In print shops electrical measurements are seldom made.

The pH in offset printing
For the printing process, a pH-value of approx. 5.5 has proved advisable.
Dampening solution that is too acidic has the following effects:

The printing layer of the plate is fretted resulting in sharp pointed halftone dots. The useful life of the plate is impaired.
Ink drying is delayed. In extreme cases, ink does not dry at all.

The effects of alkaline solutions are:

High dot increase
Tendency towards scumming and emulgating
Inks with metallic pigments will oxydate resulting in blunt quality in printing

The adjustment of pH-value with phosphoric or citric acid requires constant supervision. Meanwhile, dampening solution additives with buffer effect are being offered which help maintain a pH of 5.5. These buffers stabilize the pH-value.

Attention: The pH-value has nothing to do with alcoholic dampening. Alcoholic additives facilitate the guidance of water. They do not change the pH-value.

Why are papers alkaline?
Alkaline papers contain calcium carbonate. Calcium carbonate prevents a decrease of acidity and in turn the “ageing” of paper. In paper manufacturing, calcium carbonate is an inexpensive raw material with good flowing properties, and it gives the paper a good opacity. Due to these positive properties, more and more papers are being made alkaline.

How do alkaline papers and dampening solutions behave and which reaction results?
While dampening solutions have a pH of 4.8 to 5.5, alkaline papers have a pH of 8 - 11. A reaction between these two cannot be avoided. The calcium carbonate in the paper is dissolved by the acidic dampening solution. The calcium salts can enrich in the circuit of the dampening solutions, and this enrichment can result in a series of troubles in printing for e.g.

  • Increase of the pH-value

  • Accumulations on the blanket  
    The dissolved components of the coating mostly accumulate in the last printing unit. 

  • Inking rollers running blank  
    Salts that do not dissolve easily will build up in the pores of the inking rollers and render them hydrophilic. The water infiltrates the ink film, the ink will be displaced and the inking rollers will partly run without ink. The regeneration of blind inking rollers can only be achieved with specific washing agents.  

  • Printing plates running blind

Recommendations for the prevention of problems  
PH has a decisive effect on the dissolving speed of calcium carbonate. The higher the pH value, the slower the reaction of calcium carbonate and dampening solution. It is recommended therefore to stabilize the pH at the upper level of the tolerance range at approx. 5.1-5.5.

Attention: higher pH values may result in printing plates not running clean and well (scumming) and they may result in an increased water absorption by the printing ink.
The alcohol concentration must also be reduced to the lowest possible degree.

At present dampening additives are being offered that keep the pH value constant (buffers). A harmonization of the pH of the dampening solution with the pH of the paper should be achieved. On principle, the use of suitable dampening additives is recommended. It would be a good idea to contact once ink supplier for further clarification.


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