Smart-Formulation-Journal-2012

6
Widening the Scope for Matting
Solvent-Free UV-Cured Coatings
It is relatively easy to produce a matt finish in conventional, solvent-based
coatings as opposed to UV-cured coatings. In extensive testing which has seen
it accomplish groundwork in this field, Evonik has come up with a more effective
way of matting these latter types of coating. The resulting product is a new
matting agent, ACEMATT® 3600, with a special surface treatment. This product
now broadens the options available for the formulation of matt UV coatings.
Model for insufficient matting
Earlier ”gel point”
= high shrinkage efficiency factor
= insufficient roughening
UV radiator
A Simple Model for a Complexity of Correlations
Application engineers at the Evonik have developed a
model for matting UV-cured coatings. Their model illustrates
how the non-materialization of film shrinkage
caused by the absence of solvents can be offset by other
mechanisms.
The first parameter which the developers examined
was particle size of the matting agent. They looked at
two conventional silicas used to create a matt finish. These
silicas had mean agglomerate particle sizes of 4.5 µm and
14.5 µm (in ACEMATT® OK 607and ACEMATT® HK 450,
respectively). Using this simple model which covers the
whole spectrum of particle sizes used in silica-based matting
agents, it is possible to qualify the impact various particle
sizes have. In this model, same-size spherical particles
in the highest possible packing density represent the
silica particles. The agglomerates are evenly distributed
throughout the liquid coating, creating a matting–agent
matrix which does not shrink as much as the binder–agent
matrix surrounding it does during the curing process. The
so-called volume shrinkage of coating during curing is
caused chiefly by the binding agent. The idea the coating
experts had was that it ought to be possible to control
matting to some extent by manipulating the particle size
in the matting agent.
Small Particles for More Efficient Matting in Thick-Layer
UV Coatings
According to the spherical-particles model, if a thick UVcured
coating (< 20 to 25 g / m 2 ) contains a matting agent
whose particles are relatively large (14.5 µm), the volume
shrinkage this produces during curing will result in the
surface of the coating exhibiting only weakly pronounced,
long-wave structures. The surface will be only-moderately
roughened and therefore exhibit only a minimal degree
of matting.
In contrast, finer-particle matting agents containing
particles of 4.5 µm in size will produce a coating-film
surface with a pronounced short-wave structure which
is ideal for the strong diffusion of incident light and thus
produces a high degree of matting (Fig. 1). The result is a
low level of shine at a 60 ° angle.
On the whole, the testing shows that fine silica particles
with mean agglomerate particle sizes of < 5.5 µm
are excellently suited for matting thick-layer UV coatings.
Coarse particle matting agent ACEMATT® HK 450/d50 = 14.5μm
Derived from model
Height difference appr.: 2.0μm
With low roughening and high gloss
at both measurement angels
Values from actual practice
60°-reflectometervalue: 46.1
85°-reflectometervalue: 86.0
Max. roughness profile height Rz: 0.77μm
Arithm. mean roughness value Ra: 0.08μm
Fine particle matting agent ACEMATT® OK 607/d50 = 4.5μm
Devided from model
Height difference appr.: 2.0μm
with higher roughening
Low gloss at 60° measurement angel
High gloss at 85° measurement angel
Figure 1
The surface structures of a thick coating film.
Werte aus der Praxis
60°-reflectometervalue: 22.6
85°-reflectometervalue: 77.8
Max. roughness profile height Rz: 1.20μm
Arithm. mean roughness value Ra: 0.14μm
Large Particles for Thin Layers
When it comes to the matting of thin layers, coating formulators
only gain limited benefit from polymerization
induced volume shrinkage since the layers are simply not
thick enough. For thin layers, therefore, coarser-particle
silica will always be a better choice than fine-particle acid
if the particle size approximately matches that of the layer
thickness.
For this, too, the devised model provides an explanation:
The large particles create a highly pronounced
structure (that is, a significant degree of coarseness) on
the film surface, while fine-particle matting agents create
minimally pronounced structures on thin layers, resulting
in barely any diffusion of incident light (Fig. 2). What this
means for thin coating layers is that the mean agglomerate
of the matting agent will ideally be 0.5 to 1 times the size
of the coating-film thickness.
Coarse particle matting agent ACEMATT® HK 450/d50 = 14.5μm
Derived from model
Height difference: > 2.0μm
with high roughening and low gloss
at both measurement angels
Fine particle matting agent ACEMATT® OK 607/d50 = 4.5μm
Derived from model
Height difference: < 1.0μm
with low roughening and high gloss
at both measurement angels
Figure 2
The surface structure of a thin coating film.
Values from actual practice
60°-reflectometervalue: 31.0
85°-reflectometervalue: 49.2
Max. roughness profile height Rz: 3.92 μm
Arithm. mean roughness value Ra: 0.21μm
Values from actual practice
60°-reflectometervalue: 46.8
85°-reflectometervalue: 83.8
Max. roughness profile height Rz: 0.86 μm
Arithm. mean roughness value Ra: 0.08 μm
New Matting Agent Widens Processing Window
Practice corroborates the test results, as Fig. 1 and 2 illustrates.
Matting can be better produced in thick coating
films using fine-particle silicas, and coarse-particle silica
is better used to create this effect in thin coating layers.
It also becomes clear that the new ACEMATT® 3600,
developed specifically for creating matt finishes for UVcured
coatings and post-treated with polydimethylsiloxan
(PDMS), has a significantly higher matting effect across
practically all layer densities. Given that it was also found
to have a positive influence on the matting agent’s rheological
properties and can therefore be added to the coatings
in a higher concentration, formulators now have a
much broader application window at their disposal.
Different Time Sequences in The Polymerization Process
Affect The Degree of Gloss
Another essential factor in matting is the time sequence
employed in the polymerization process to the point of
gelling (one of the defined time points in the processing
schedule). It is at this point in time that polymerization
of the binding agent has progressed to the stage that the
silica and binding-agent matrixes proceed to jointly shrink
further. From the point of gelling onward the agglomerates
of the matting agent are fixed in the emerging polymer
network and can thus no longer contribute to any
further coarsening. The later the gelling point is reached,
the more pronounced the matting effect will be (Fig. 3).
Model for effective matting
Later ”gel point”
= low shrinkage efficiency factor
= sufficiently high
roughening
Conveyor speed: 6m/min = 3 sec exposure
Reaction start
30 cm
”Gel point”
UV radiator
Reaction end
Figure 3
When the gelling point is reached prematurely, this results in an inadequate
poor degree of coarsening (model pictured at top). A later gelling point
allows time for sufficient coarsening to transpire (model pictured at bottom).
At which point in time the gelling point is reached
depends on a number of thermodynamic and reaction_
kinetic factors. The other components in the formulation
(acrylate oligomers; acrylate monomers and photoinitiators)
therefore also have a substantial impact on the mattability
of UV-cured coatings.
Number of Double Bonds Plays an Important Role
The existence of complex reciprocal dependencies means
it is difficult to make any definite assertions regarding the
mattability of oligomers. Evonik has, however, been able
to prove that the double-bonding density, which, in turn,
is contingent on functionality and molecular mass, is of
particular significance. Generally speaking, we can say
that the higher the double-bonding density of the oligomers,
the greater the mattability of the coating.
Formula for Calculation of the Double-Bonding Density
funtionalityt [DB/ Mol]
[DB]
DB-Density =
relative molar mass [g / Mol] x 1000 [g/ kg Oligomer] = [kg Oligomer]
Experiments with different monomers have shown that
their contribution to the matting effect depends primarily
on their molecular form and turnover rate during polymerization.
The photoinitiator also influences the mattability of
UV-cured coatings chiefly through its absorption maxima.
The actual curing process provides additional scope for
reducing the shine in a coating film, with parameters such
as the type of light source and its emissions spectrum, the
intensity and exposure time, the geometry of the lamps
used, the ambient temperature and type of substrate all
figuring into the equation. Evonik’s application engineers
have looked at these variables, too.
Coating formulation continues to be predominantly a
matter of employing experience and intuition. Evonik’s
analyses do, however, show that it is possible to expressly
fine-tune and coordinate the pertinent process and formulation
parameters and thus significantly improve the
degree of control employable in matting UV-cured coatings.
The new ACEMATT® 3600 now affords formulators
a greater array of options for the matting process.
Contact
Reinhard Behl
reinhard.behl@evonik.com
Rüdiger Mertsch
ruediger.mertsch@evonik.com
Smart Formulating Journal | Issue 9 | April 2012