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Yana Bohodyst
National Technical University of Ukraine
“Igor
Sikorsky Kyiv Polytechnic Institute”, Ukraine
Structure formation of metallized colloid systems for
printing purposes
The problem of stability
of dispersive systems is one of
the most important in the
printing industry. Maintenance of stability of metallized paints
is a necessary basis for getting a quality printing impession [2].
Coagulation structure is a thixotropic structured liquid if two
conditions are fulfilled for the structure:
1. fluctuation begins at any arbitrary low
strain shift;
2. steady, stationary,
laminar fluidity can be realized without rupture of continuity
with the exit to Newtonian
fluidity with viscosity which corresponds to the extremely destroyed
structure [5].
The presence of metallized
particles of aluminum or copper
powders in ordinary printing inks somewhat changes their physical and chemical properties.
The main factor determining their kinetic stability
is the degree
of sedimentation of colloidal solutions.
Until recently, the size of
the powder particles for offset
printing was 5-7 microns and for flexographic
– 11-13 microns. The method of vacuum
metallization significantly
expanded the granulometric range of the resulting
powders in the direction of
a significant reduction in their size
up to 0.5-1 microns. With the increase
of the specific
surface of metal particles, the stability of
the entire dispersion system increases. As a result of the
difference in the specific gravity,
the components of the ink
exhibit agglomerate instability. The particles of the
dispersed phase are gradually deposited
if the density
of the particles
is greater than the density
of the medium dô>dc, or they expire if dô<dc . The stable condition of such
colloidal systems is the nature
of sedimentation-diffusion equilibrium.
It is proved
that the equilibrium by sedimentation is established rather slowly: the time
of installation is proportional to 1/r2,
where r is the radius
of the particle
[3].
One of the most important
factors in the stabilization of a dispersive system is the
hydrodynamic resistance to displacement of the liquid dispersion medium from the
interlayer between converging particles. Given its heterogeneity,
it is difficult
to predict the effect of
viscous properties on such a hydrodynamic
characteristic.
The thinning-down of the layer
of the dispersive medium
in the process
of coagulation is associated with
the outflow of the fluid from a narrow gap
between the particles. With high viscosity of the dispersion
medium, the resistance of the
layer to the thinning-down can provide virtually
unlimited stability of the system
to coagulation (Fig. 1) [4].
Figure 1. The scheme
of displacement of volume of
liquid between two flat-parallel particles.
The value ∆p characterizes the viscous resistance of the medium
to the thinning-down of the film when
the particles converge and can
be considered as an analogue
of the positive
disjoining pressure
of its binding
component. If the film surrounding
the metal particle is ≥ 1 micron, the outflow of the binding
component in the ink is caused by capillary pressure.
At critical
concentrations of metallic pigments in colloidal solutions,
as well as
high viscosity and storage temperature,
conditions for the appearance of xerogels are
possible [1]. In case of
complete displacement of the medium
from the gap (at breakage
of the adsorption-solvent shell), a direct local collision of particles is
achieved. In this case, together
with the van der Waals
forces, the short-range (valence)
forces involved in the area
of direct contact can also
participate in the coupling of
the particles. Their contribution to the strength
of the contact
can be estimated
in order of magnitude:
where
Summing up, it can be
noted that the stabilization of metallized colloids
for printing purposes is not
systemically technological,
but rather relates to the
selective management of physico-chemical processes that determine the behavior of
metallic paints, especially before their use.
References:
1. Morozov. A. S. Homogenization
of metallized colloids. Seventeenth International Scientific and Practical Conference
on Publishing and Printing Industry.
Kyiv, December 19, 2013. - Oct. 2010. – 104 p.
2. Morozov, A. S Metallized paints for advertising products / A. S. Morozov, E. Yu. Gavrishchak, V. K. Kicha // Technology and technique of
printing. - 2011. - No. 3.
- December 12. – 128 p.
3. Morozov, A. S. Stabilization
of metallized colloidal solutions for printing purposes
/ A. S. Morozov // Technology
and technique of printing. - ¹1. - 2008 -
p.110-117.
4. Kirchok, P.O. Metals and composite materials
/ P. O. Kirichok, T. A. Roik, A. S. Morozov // The textbook. - K .: NTUU
"KPI", 2011. – 216 p.
5. Ovchinnikov,
P.F. Vibroeology / P.F.Ovchinnikov
// K .: Naukova dumka. –
1983. – 272 p.