Rheology of printing inks (springer出版图书章节)

清泥

Rheology is the study of the flow and deformation of matter. The aim of
the science is to define and evaluate such words as consistency, tack,
length of flow, stiffness and body, in terms of the physical properties of
the materials. These words are used in everyday language and with
special significance in many sections of industry. At first sight such
terms may appear to be self-explanatory and simple to define; in fact
the converse is true and the greater the need for precision the greater is
the complexity of separating the factors involved.
In the printing industry there are two main areas of interest for the
rheologist.
. Lithographic and letterpress inks where the ink is required to be ready
for the press without addition or modification.
. Gravure and flexographic inks where the inkmaker supplies ink at a
greater viscosity than that required for use on the press and the printer
accepts the responsibility for adjustments in viscosity to satisfy his
printing conditions.
In lithographic and letterpress inks the need for precise evaluation on
numerical scales arises because of two factors which are diametrically
opposed. For good printing a lithographic ink should be as stiff as possible
but when a printing machine is run at high speed the stiffness of the ink
must be sufficiently low to avoid the paper surface being torn away from
the body of the paper (i.e. it will not pluck) and to avoid the generation of
too much heat on the machine.
With flexographic and gravure inks there are two reasons for the inkmaker
to supply ink at a viscosity much higher than required by the press
conditions. First, it overcomes the tendency of the pigment to settle to the
bottom of the container used for transport. Secondly, it allows the printer
to thin the inks to a viscosity suitable for the press and running speed and
enables him to adjust the balance of solvents to achieve the desired
solvent release and drying rate.
Most laboratory workers concerned with printing ink do not require a
specialist knowledge of rheology. They will be better served if they
acquire a more general appreciation of this subject and refer problems to
the specialist for explanations when the need arises. The depth of knowledge
required may be stated as follows.
. In branch operations where the laboratory worker is called upon to mix
inks to pattern, an understanding of ‘tack’ and an elementary appreciation
of flow should suffice.
. The chemist responsible for ink development should understand tack,
Newtonian and non-Newtonian aspects of flow and gain experience of
how these change in relation to the choice of pigment, pigment concentration,
the ink media and their composition, particularly with
regard to molecular size and pigment media interaction. A consideration
of these parameters will lead to a greater appreciation of the
visco-elastic nature of most ink systems under the high shear stress
and shear rates encountered on a printing press.
. The specialist rheologist will have an appreciation of the flow of liquids
and pastes which will extend far beyond the information recorded
here. Such knowledge may be gained by a study of the extensive
literature. Membership of the British Society for Rheology would be
useful for their literature surveys and conferences on a wide range
rheological topics.