Importance of Measurement of pH and
Conductivity of Fountain Solution used in Offset Printing Press
By Anuja Mahadik and
ArchanaJadhav , Hanna Equipments (India) Pvt Ltd, India
Introduction
Printing is a
means of graphic communications. It is the reproduction of quantities of
images, which can be seen or perceived visually. Regardless of the great number
and variety of printed products they all have one thing in common; each has the
same visible image produced in quantity.
Different Types of printing
· Offset
Lithography
· Engraving
· Thermography
raised printing
· Reprographics
· Digital Printing
· Letterpress
the original process founded by Gutenberg in 1440
· Screen a.k.a.
silk-screening
Modern printing technology
Across the
world, over 45 trillion pages (2005 figure) are printed annually. In 2006 there
were approximately 30,700 printing companies in the United States, accounting
for $112 billion.
Indian Printing
Industry has been considered as one of the largest industries, with the growth
of 12.5%.
Status of the Indian Print Industry Growth
· More than
160,000 Printing presses in active operation all over the country
· 1.6 million work
force
· 18 Printing
Engineering Colleges, Several diploma schools and many print training
institutions
· Over 2000
Printing Graduates every year
Offset printing
Offset printing
is a commonly used printing technique in which the inked image is transferred
(or "offset") from a plate to a rubber blanket, then to the printing
surface. When used in combination with the lithographic process, which is based
on the repulsion of oil and water, the offset technique employs a flat (Plano
graphic) image carrier on which the image to be printed obtains ink from ink
rollers, while the non-printing area attracts a water-based film (called
"fountain solution"), keeping the non-printing areas ink-free.
During Last 30
year’s offset printing developed into the most important printing process.
While Typographic, Gravure prints and screen printing based on the physical
principal, Senefelder defined lithography precursor of the offset printing as
the chemical printing process.
WORKING OF THE OFFSET PRINTING PRESS
- Formation of the image on the printing plate
Before the job
can be printed, the document must be converted to film and "plates."
In this case negatives are created from digital files. Images from the
negatives are transferred to printing plates in much the same way as
photographs are developed.
A measured
amount of light is allowed to pass through the film negatives to expose the
printing plate. When the plates are exposed to light, a chemical reaction
occurs that allows an ink-receptive coating to be activated. This results in
the transfer of the image from the negative to the plate.
Different
materials for plates are used including paper (which produces a lower-quality
product).
The best plate
material is aluminum, which is more costly.
Each of the
primary colors -- black, cyan (blue), magenta (red), and yellow -- has a
separate plate. Even though you see many, many colors in the finished product,
only these four colors are used (you'll also hear this called the four-color
printing process -- it's a little like the three-color process used in
television).
In web offset
lithography - The paper is fed through the press as one continuous stream
pulled from rolls of paper. Each roll can weigh as much as 2,000 pounds (1
ton). The paper is cut to size after printing. Offset lithography can also be
done with pre-cut paper in sheet fed presses.
Web presses
print at very high speeds and use very large sheets of paper. Press speeds can
reach up to 50,000 impressions per hour.
The press has to
maintain a constant balance between the force required to move the paper
forward and the amount of backpressure (resistance) that allows the paper to
remain tight and flat while traveling through the equipment.
Ink and water do
not mix -- this is the underlying principle of offset lithography. The ink is
distributed to the plates through a series of rollers. On the press, the plates
are dampened, first by water rollers, and then ink rollers. The rollers
distribute the ink from the ink fountain onto the plates.
The image area
of the plate picks up ink from the ink rollers. The water rollers keep the ink
off of the non-image areas of the plate. Each plate then transfers its image to
a rubber blanket that in turn transfers the image to the paper. The plate
itself does not actually touch the paper -- thus the term "offset" lithography.
All of this occurs at an extremely high speed.
The paper is run
through a long oven at about 375 degrees Fahrenheit (190 degrees Celsius). This
dries (sets) the ink so it won't smudge.
Immediately
after leaving the oven, the paper is run through a short series of large metal
rollers that have refrigerated water flowing through them. These chill rollers
cool the paper down instantly and set the ink into the paper. If this were not
done, the ink would rub off on your fingers.
Indian ink Market-
The estimated
size of the Indian ink market in terms of the volume is ~ 150k tonnes per
annum. The estimated size of this industry in terms of value stand for ~ Rs 15
bn.
Importance of Fountain solution in Offset Printing-
Fountain
solution is the water-based (or "aqueous") component in the
lithographic process that moistens the non-image area of the plate in order to
keep ink from depositing (and thus printing). Historically, fountain solutions
were acid-based and made with gum arabic, chromates and/or phosphates, and
magnesium nitrate. Alcohol is added to the water to make the water wetter (as
we say) and help cool the press a bit so the ink stays stable so it can set and
dry fast.
While the acid
fountain solution has come a long way in the last several decades, neutral and
alkaline fountain solutions have also been developed. Both of these chemistries
rely heavily on surfactants/emulsifiers and phosphates and/or silicates to
provide adequate cleaning and desensitizing, respectively. Since about 2000,
alkaline-based fountain solutions have started becoming less common due to the
inherent health hazards of high pH and the objectionable odor of the necessary
microbiological additives.
Acid-based
fountain solutions are still the most common variety and yield the best quality
results by means of superior protection of the printing plate, lower dot gains,
and longer plate life. Acids are also the most versatile, capable of running
with all types of offset lithe inks. However, because these products require
more active ingredients to run well than do neutrals and alkaline, they are
also the most expensive to produce. That said neutrals and, to a lesser degree,
alkaline are still an industry staple and will continue to be used for most
newspapers and many lower-quality inserts.
Components of the Fountain Solution-
Most Fountain
Solution is made of following five components,
1.
Water
2.
Acids
3.
Buffering
Agents
4.
Wetting
Agents
5.
Gum
Arabic
6.
Plate
Conditioners
· Water-It comprises of 98 % of the total
volume of fountain solution, it is important to be aware of water condition.
· Acids- Usually a mild acid, which helps to
reduce a pH and keeps the image area sensitive to ink and non-image area more
sensitive to water.
· Buffering Agents - A substance capable of
neutralizing acids and bases in solutions and thereby maintaining the acidity
or alkalinity level of the solution. "Keeps the pH constant or within a
certain range regardless of how much acid is added. “Neutralizes the effects of
paper, inks, gases, and humidity.
· Wetting Agents – Sometimes referred as
surfactants these are used to lower the surface tension of the water, allowing
it to maintain the wetting ability of the fountain solution in the non-image
areas, while allowing you to run less water. By reducing the surface tension of
the water the water will spread out to a thinner film.Common agents used are:
Isopropyl Alcohol, Glycerin, and various Glycols.
· Gum Arabic-Gum Arabic has already been added to
most fountain solution concentrates made today. Its function is to adhere to
the plate’s non-image area and protect it from accepting ink. Gum also serves
to protect the plate from humidity and chemical attack during press stops.
· Plate Conditioners (Chemical salts) -These are used
to minimize the corrosive action of the acid on the surface of an aluminum or
metal plate. This is designed to increase the print quality and to extend the
life of the plates. Most solutions also contain antifoaming and anti-fungus
ingredients.
1. WATER
The printing
process does not only require ink, but also water. These water must show
certain properties, which are reached by the water, treatment and special
fountain solution additives.
Properties of
water that influence the preparation of fountain solution-
Parameter in
water
|
Influence on
Printing
|
Required
Concentration
|
Ca/Mg(hardness)
|
Stripping, pH
value, Running blind of plate
|
|
Hydrogen
Carbonate
|
pH Value
|
100-200 ppm
|
Chloride
|
Corrosion
|
25 ppm
|
Nitrate
|
Corrosion
|
20 ppm
|
Sulphate
|
Corrosion
|
50 ppm
|
If the water
quality significantly differs from these nominal values, a water treatment is
recommended. Water treatment is carried out by following method.
· Water Softening
· Demineralization
· Reverse Osmosis
2. ACIDS
The acid
concentration in the fountain solution influences following properties during a
printing process.
• Running Clean Plate
• Water take up/ Emulsification of
printing ink
• Oxidative drying of printing ink
• Durability of the press material and
reaction of fountain solution with paper coating
Taking all the
influences into the consideration, a pH range of 4.8 to 5.3 has been
established for the offset printing in Europe.
3. BUFFERING AGENTS
Fountain
solution additives contain buffer systems, which consist of blending from acids
and bases. They only change the pH value minimally, if small quantities of acid
or alkaline impurities are added and thus guarantee a constant pH value even
with exterior influences like paper or ink components in water. The quality of
a buffer system is characterized by its buffer capacity. The buffer capacity
provides a measure for the quantity of other substances. E.g. calcium carbonate
from paper coating can be added, without considerably changing the pH value.
The pH buffer in
the fountain solution composition includes water-soluble organic acid and/or
inorganic acid or salts thereof.
These compounds
have effects on adjusting or buffering of the pH of the fountain solution and
appropriate etching or prevention of corrosion of the support in the
lithographic printing plate.
Examples of the
organic acid include citric acid, ascorbic acid, malic acid, etc.
Examples of the
inorganic acid include phosphoric acid, nitric acid, sulfuric acid, and
polyphosphoric acid. Furthermore, alkali
metal salts, alkaline earth metal salts, ammonium salts, and organic amine
salts of these organic acids and/or inorganic acids can be preferably used.
These organic
acids, inorganic acids and/or salts thereof may be used alone or as mixtures of
two or more.
The pH of the
fountain solution composition is preferably adjusted to a range of 3 to 7 by
these compounds. On the other hand, the fountain solution composition may be
used in an alkaline region of 7 to 11 by using an alkali metal hydroxide,
phosphoric acid, an alkali metal salt, an alkali metal carbonate, or a
silicate.
4. WETTING AGENTS
The wetting of
solid surfaces, e.g. printing plates and rollers, is determined by the surface
tension of the wetting liquids. Liquid with high surface tensions e.g. Mercury
shows very poor wetting properties, because their drops have spherical shape.
Therefore liquid films they form tend to break up. Liquids with low surface
tension, like alcohol in the contrary have excellent wetting properties,
because the liquid spreads across the surface.
Fountain
solution with high surface tension forms the big drops and hence results in the
uneven wetting of the printing plate. The fountain solution with low surface
tension does not have these problems because they form evenly thin, stable
films. The required reduction of the surface tension in alcohol dampening units
is reached by using isopropyl alcohol (IPA).
In addition to
reduction of the surface tension, alcohol has other important tasks, like
increase of viscosity to improve the transport as well as the control of
emulsification.
5. GUM ARABIC
Gum Arabic is
added to the fountain concentrate today. The gum function is to adhere to the
plates, non-image area and protect it
from accepting ink. Gum also serves to protect the plate from humidity and
chemical attack during press stops. Today, some gum substitutes are being used
such as CMC (carboxymethyl cellulose), etc. Gum Arabic, however, is still the
most effective material for keeping the non-image areas of the plate clean.
6. PLATE CONDITIONERS / ADDITIVES
They minimize
the corrosive action of the acid on the aluminum plate. This will extend plate
life and improve the overall print quality. Most solutions also contain rust
preventives, Preservatives (anti-fungus agents), defoaming agent, perfume.
Preservatives
are used to exhibit effects to bacteria, fungus and yeast. the amount depends
on the kinds of bacteria, fungus and yeast present .Two or more of the
preservatives are preferably used in combination so as to be effective to
various kinds of fungus, bacteria and yeast.
Examples of rust
preventives for use in the present invention include benzotriazole,
5-methylbenzotriazole, thiosalicylic acid, benzimidazole, and derivatives
thereof.
A trace amount
of (g) perfume is preferably used for improving the working environment of
printing. Examples thereof for masking odors of solvents include perfumes of
fragrant olive, lemon, and vanillin. Thus, the fountain solution becomes
further easy to use.
Defoaming agents
includes siliconedefoaming agent. The
silicone defoaming agent may be an emulsified dispersion type or a soluble
type.
In general, the
gum, conditioners, additives, acids are combined into what is known as
"fountain concentrate" or "fountains etch". Concentrated
composition of the fountain solution can be used by diluting with tap water or
well water. At this time, calcium ions or the like contained in city water or
well water sometimes adversely affect printing and causes for printed matters
to be liable to be scummed. However, such a defect can be prevented by adding a
chelating compound. Examples of preferred chelating compounds include organic
phosphoric acids
EFFECT OF THE pH VALUE IN FOUNTAIN SOLUTION
pH is the
negative logarithm of the hydrogen ion concentration. The Fountain solution has
pH value between 4.8 to 5.3.
Increase in pH
value results in the following problem
1. Piling on blanket- By the reaction of
the acid with coating particles, the paper coating starts to dissolve, which
causes a partial loss of the inner coherence of the paper coating. Loose
coating particles may pile on the blanket. This error mostly becomes more
evident in the last printing units.
2. Stripping of ink rollers- The calcium
ions dissolved in the fountain solution react with citrates frequently
contained in the buffers and form insoluble salts. These are stored in the pores
of the ink rollers which consequently become hydrophilic. The water infiltrates
the ink film, the ink is displaced and the ink rollers partly do no longer
transport ink.
3. Running blind of printing press- In
addition to the mechanic wear the running blind of printing plates can also be
caused by insoluble calcium salts. These hydrophilic salts deposit on the
printing plate. The ink film on the copy layer is infiltrated by water , the
ink is expelled and the ink is no longer transferred to the printing area
(Running blind of printing plate).
ROLE OF CONDUCTIVITY IN FOUNTAIN SOLUTION
pH of the
solution cannot be used to determine the added quantity of the fountain
solution additive. Such measure is possible by measuring the conductivity.
Conductivity is
a measure for the ability to conduct electric current, in solution it is caused
by breaking down salts in electrically loaded particles, so called ions.
The conductivity
in fountain solution is determined by water quality and used fountain solution
additives.
Conductivity is
not relevant for the printability i.e unlike for the pH value there is no
favorable range for printing above or below which printability problem can
occur. Despite this fact it is recommended not to exceed a conductivity of 1500
micro Siemens for freshly mixed solution.
The conductivity
is strongly influenced by the alcohol concentration as well as by the
impurities from ink and paper components. Therefore a determination of the
concentration is only possible with freshly prepared fountain solutions and a
constant alcohol concentration.
On the other
hand a regular measurement of the conductivity allows conclusions regarding the
pollution level of the fountain solution, because the conductivity increases
with increasing pollution.
Hanna Instruments
Hanna
Instruments is the largest independent manufacturer of electrochemistry
products in the world.
Hanna
Instruments has vast range of the pH conductivity combination meter for the
measurement of this parameter. These multi parameter meters are available in
the portable type also. The Hanna Instruments Cal Check and GLP give the user
additional information about the electrode condition and information about
calibration status of the meter. Hanna Instruments also offer handheld type
combo tester HI 98129, HI98130 excellent for the quick, easy and accurate
measurement of this pH and Conductivity parameters.
http://www.hannainst.com/usa/prods2.cfm?id=002002&ProdCode=HI%2099171