Transparent Ultrafine Mineral Filler Technologies
January 2017 www.coatingsworld.com Coatings World | 45
clarity in clear UV-cured resins is the
ideal match in Refractive Index (R.I.)
with that of the resins. The R.I. of neph-
eline syenite is compared with the R.I. for
several resin systems and other common
mineral fillers in Figure 2. The R.I. for
nepheline syenite is in the range of 1.50
to 1.53, matching several types of resin
systems and oligomers used for radiation
curing. Nepheline syenite’s R.I. matches
particularly well with acrylic, urea, and
urethane monomers and oligomers for
exceptional clarity when properly wet out
and dispersed in the host binder system.
Previous investigations showed that
ultrafine nepheline syenite has exceptionally high light transmittance in organic
binder systems commonly used for radiation curing in the critical UVA and UVB
wavelengths.1 Thus, unlike other mineral
fillers and pigments, ultrafine nepheline
syenite is not expected to interfere with
the UV curing process.
Additionally, it was discovered that
nepheline syenite, with sufficiently controlled particle top-size (i.e., D99 < 5
µm), not only maintains, but enhances
key properties in radiation-curable clear
coatings, such as optical clarity, image
clarity, haze, scratch resistance and
gloss, when compared to conventional
sizes (i.e., D99 < 30 µm) of nepheline and
other mineral fillers.1 An illustration of
the improved optics and relationship to
particle top-size is found in Figures 3a
and b. The quality, clarity, and warmth
of a UV-cured high-solids acrylic topcoat,
applied over stained and sealed maple veneer, filled at 10% with nepheline syenite
with a top-size of 5 microns ( 5 µm N.S.),
is shown in Figure 4.
Furthermore, initial testing by UV
coating producers and contract labs with
nepheline syenite fillers in UV-cured formulations indicated that ultrafine prototypes could have a positive impact on
hardness and cure rate. Further experiments measuring pendulum hardness
and double bond conversion of filled
films while regulating the cure energy
showed that ultrafine nepheline actually
Particle Shape Rectangular, angular, nodular
Specific Gravity, g/ml 2.56-2.61
Mohs Hardness 6.0
Brightness (TAPPI) 85-94
Oil Absorption, (ASTM 281) 22-35
Refractive Index 1.51-1.53
Moisture, (ASTM C-566) .05-. 15
pH (20% Slurry) 9. 5-10. 5
Figure 2. R.I. comparison of mineral fillers (top) with binders (bottom).
Table 1. Typical properties of nepheline syenite.
Figure 3. Haze and optical clarity as a function of N.S. top-size and concentration in an aqueous UV-cure
PUD with 2 mil DFT.
Figure 4. Acrylic high-solids UV-cure clear wood
finish filled with 5 µm N.S. at 10 by wt. based
on resin solids over stained maple veneer (DFT
= 3 mil).
