Technical Paper
Pigments
Lead is dead
Maintain opacity and durability and expand the colour range with new pigment chemistries
Contact:
Mark Ryan
The Shepherd Color Co.
T +1 513 874-0714
[email protected]
Mark Ryan
A new pigment chemistry, niobium tin pyrochlore
yellow, has been developed that expands the
envelope of durable colours available in paints and
coatings. It has the chromaticity and brightness of
organic pigments and the opacity and durability
of inorganic pigments. The new yellow is complemented by improvements in rutile tin zinc to
increase its red value. Together these pigments
provide an alternative to lead chromate pigments
and expand the envelope of durable colours in the
yellow and orange colour space.
Figure 1: A plot of hue angle and chromaticity
Figure 2: A hue v. chromaticity graph comparing the addition of RIO or RTZ
Orange to BiV
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M
arket demands and regulatory requirements
have placed pressure on coatings formulators
and chemists to find alternatives to lead chromate pigments. This middle-yellow colour space can be
addressed by a number of different pigmentations that
are a balance of performance and cost for a given colour.
Organic pigments offer high chromaticity, but less than
desired durability and opacity. They are often a compromise of performance that requires the use of special
coloured primers, and multiple coats of low opacity and
even clear coats to reach desired levels of weathering
performance. Inorganic pigments have opacity and durability, but alone currently lack the ability address all the
colours desired by the marketplace.
Niobium tin pyrochlore (NTP) yellow is a new, patented
pigment chemistry that addresses the need for bright
chromatic colours that are highly durable. It is an inorganic pigment that had been given the Colour Index (C.I.)
Pigment Yellow 227 designation (called PY 227 in this
article). It is a bright yellow similar in shade to middlechrome yellows (PY34) with excellent opacity, chromaticity, tint strength and inertness. To complement this
development, advances have been made to increase the
red tone of PY216 rutile tin zinc (RTZ) orange pigment
chemistry. The result is a pigment with a bright, chromatic-orange masstone and high tint strength. This tinting ability, especially the RTZ orange’s red value, makes
it very valuable in blends with the PY 227 to provide
chromatic, all-inorganic matches to a number of colours
from middle yellow to red shade yellow to orange. Both
pigments also complement other organic and inorganic
pigments to meet a wide range of colours and improve
coatings properties such as opacity and durability.
In order better to understand the significance of the two
new materials, it is instructive to position them in relation to other pigments. A plot of hue angle and chromaticity is a convenient way of showing how colours relate
to one other. In Figure 1, the horizontal axis represents
the hue angle of a pigment with redder pigments on the
right hand side and greener pigments on the left hand
side. The vertical axis represent the chromaticity of a pigment with less chromatic colours towards the bottom the
plot and higher chroma colours towards the top.
Figure 1 illustrates the relationship between PY53 (nickel
titanate) and PBr24 (chrome titanate). As can be seen,
PY53 (nickel titanate) is in the lower left corner and
PBr24 (chrome titanate) to the right. However, both are
at the bottom of the graph. These two important titanate pigments are standard components in high-durability systems, but are fairly low in chroma. PBr24 is redder
than PY53. Higher up near the upper left corner of the
graph, PY184 (bismuth vanadate) is to be found. It is
a very green shade, chromatic pigment. Arrayed in an
arc to the right are several of organic pigments ranging
from green shade yellows (PY54), oranges, to bright reds
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Technical Paper
Pigments
(PR254). Along this arc is PY34 (lead chromate). This
represents a middle-chrome pigment with a reddishyellow shade. Its chromaticity, opacity and economics
made it a standard pigment until regulations and market
pressure necessitated looking for alternatives.
Higher chroma
As can be seen, PY 227 is very close to the lead chromate
colour space and has much higher chroma than standard
high-durability inorganic pigments such as the titanates
mentioned earlier. Due to the need to move away from
lead chromates, the PY 227 provides a bridge between
inorganic and organic pigments in chromaticity, opacity
and durability.
PY 216 can be found in the lower right quadrant of the
graph. The increase in the redness of the latest product
can be seen by the offset to the right of PY 216. This
increase in redness produces a more an orange colour
which is more in demand than previous offerings seen to
its left. Besides this pleasing masstone, the RTZ Orange
makes an excellent adjunct to other pigments and makes
Organic
Formula percent
[wt.]
PY154
Benzimidazolone
55.20
PW6
Titanium dioxide
33.50
PY181
Monoazo/ Benzimidazolone
11.20
PG7
Phthalocyanine
Inorganic
0.10
Table 1:
Organic pigment
formulation
Formula percent [wt.]
PW6
Titanium dioxide
0.5
PY227
NTP yellow
77.8
PBr24
Chrome titanate
12.7
PY53
Nickel titanate
9.0
Table 2:
Inorganic pigment
formulation
Results at a glance
PY 227 and RTZ orange pigments have durable
colour and chromaticity which widens the colour
envelope
Their physical properties are very positive
They exhibit excellent weathering properties and
high stability in difficult conditions
The pigments are also inherently safe; however,
the NTP PY 227 and RTZ orange have obtained regulatory approvals
Figure 3: A comparison of the effect of blending RIO rather than RTZ Orange with BiV
They demonstrate that a colourant that cannot
only have excellent properties, but can also be used
in sensitive applications.
The two new pigment chemistries provide
high-performance, high-durability colour envelope
expanding tools for the coatings chemist and paint
formulator
They can be used in high durability systems such
as coil and extrusion, powder, silicate-based paints
and coatings for the architectural, ACE, automotive,
signage and corporate colour marketplace.
They provide viable alternatives to lead chromate
pigments in high-value systems where the highest
performance is expected.
Figure 4: hue v. chromaticity for RAL colours inside the envelope
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European Coatings J OURNAL
75
Technical Paper
Pigments
Figure 5: A comparison of contrast ratio v. film thickness for inorganic and organic pigment matches
it possible to offer redder tones than have been available
to date with organic pigments.
Until the redder shade of RTZ pigments were developed, a
chromatic red-shade yellow was made from purely inorganic highly durable pigments. A common option would
have been to start with a green-shade bismuth vanadate
(PY 184) and add red iron oxide (PR 101) to make the
colour redder. Figure 2 is a more selective chroma and
hue angle plot which illustrates the colours of this combination with the line labelled ‘blends of BiV-RIO’. The
iron oxide shifts the yellow from its green-shade to redder shades, but there is a loss of chroma. If the bismuth
vanadate is instead blended with the RTZ orange to shift
to colour redder, the resulting colours are represented by
the line marked ‘blends of BiV-RTZ’. These colours shift
Figure 6: A comparison of the opacity of PVDF/acrylic coatings as the pigment-to
binder ratio of different
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redder as with the red iron oxide blends, but they are
more chromatic. The area between the two lines represents the increase in colour space available to colour
formulators if the desire to stay with all inorganic colourants for durable colours.
As can be seen in Figure 3, PY 227 can also be blended
with the bismuth vanadate and the RTZ orange pigments
to make a wide range of colours. They are represented
by the lines labelled ‘blends of BiV-NTP’ and “blends of
NTP-RTZ. The difference between these two lines and
the ‘blends of BiV-RTZ’ in Figure 2 represents the increase
in colour space available to colour formulators for highly
durable colours through the use of the PY 227.
This increase in the colour envelope available with all
inorganic pigments represents a very popular and useful
colour space. As is apparent from Figure 4, it includes a
number of the highly popular RAL 1000 series of colours.
RAL 1003 is a very common colour and it can be matched
in a number of ways.
The two formulations are based on either inorganic or
organic pigments. While they give the same colour, they
have different costs and performance characteristics.
One of the most apparent properties in bright yellow
colours is opacity. The two pigment formulations were
used with an acrylic binder at a pigment volume concentration (PVC) of about 22 %. Due to the large difference
in the specific gravity of the two pigment formulations,
the pigment-to-binder ratio was 1.2 for the inorganic and
0.4 for the organic. Both represent fairly high loadings of
pigment as illustrated in Tables 1 and 2.
The acrylic paints were then drawn down at various
thicknesses and the opacity of the films recorded and
plotted. Figure 5 clearly shows that the inorganic based
match to the RAL 1003 made mainly of PY 227 has much
higher opacity at a lower film thickness than the organic
pigment based match. This relationship holds for other
bright yellow colours.
Expanding colour envelope
PY 227 was incorporated into a polyester resin in a
23-micron film over a chromate primer on a steel substrate. When standard organic pigments are used to try
to match the PY 227 panel, they do not keep enough of
their chromaticity due to lack of opacity to hide the substrate and reach the colour. However, the combination of
PY 227 and RTZ orange dramatically pushes the edge of
the durable colour envelope when it comes to thin film
applications.
The inherent advantages in opacity over even other inorganic pigments can be seen in Figure 6 that shows
the opacity of PVDF/acrylic coatings as the pigment-to
binder ratio of different pigments is increased. PY 227
has much higher opacity than the organic alternatives. A
plot of the opacity of a coating made with bismuth vanadate is used as reference. While PY 227 does not compete
directly with bismuth vanadate, because of the relative
prices, performance and hue it has higher opacity than
the bismuth vanadate.
Besides the appearance properties of colour and opacity,
the NTP and RTZ pigments have other desirable properties. They have excellent UV stability, weathering resistance in accelerated tests and acid/alkali resistance. In
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Pigments
Table 3: QUV masstone weathering performance with PBr 24 and PY 216
Description
Yellow 29 (PBr 24)
Orange 10C341 (PY 216)
Masstone PVDF/acrylic QUV testing
1000 hrs
2000 hrs
3000 hrs
D60°
D60°
DE*
gloss
DE*
gloss
DE*
0.7
0.5
0.8
-1.0
0.9
1.0
1.0
1.0
0.0
0.8
D60°
gloss
-2.0
-4.0
addition they have a range of advantageous physical
properties.
The pigments are dense because of their inorganic metal-oxide structure and a particle size of around 1 micron.
Like many inorganic pigments, they have low surface
areas and oil absorption for easy incorporation as well
as the potential for high pigment loads for increased
opacity or higher loading levels in dispersion bases. The
pigments have listed heat stabilities of 320 °C, however
this is system dependant. The pigments themselves do
Figures 7: Tint
colour stability of
various pigments
not react with molten resins in the way that pigments
like BiV do. In fact, if the pigment can be protected from
oxygen, it can survive much higher temperatures than
those listed. An example of this would be in a silicate
system based system cured at 500 °C for 10 minutes. PY
154 and a PY 184 undergo a significant colour change due
to the temperature and high-pH of the system, but the PY
227 retains its colour.
Aside from the performance in a basic system, the periodic Kesternich acid stability test was run on cured masstone and tint PVDF/acrylic panels. After seven cycles, the
PY 227 had similar colour and gloss retention properties
when compared to a PY 53 (nickel titanate), the standard
CICP green-shade pigment.
Accelerated weathering tests available
When exposed to UV light, the pigment’s high opacity
protects sensitive substrates and primers. At the same
time, the UV light that it does absorb is not released as a
free radical, rather it is dissipated by a vibration mechanism (heat) that is negligible in a paint film exposed to
sunlight.
The NTP yellow and improved RTZ Orange are still being tested in real-world situations. This will be reported
in the future, but accelerated weathering results are already available.
When NTP yellow is incorporated into a PVDF/acrylic film
over a chromate based primer on a steel substrate, excellent EMMAQUA weathering results are obtained. Most
notable feature of the masstone colour change and gloss
loss graphs in Figures 7 and 8 is that the PY 227 weathers
very similarly to the green-shade standard pigment for
highly durable systems, PY 53 (nickel titanate). PY 227
also performs better than PY 184 (bismuth vanadate)
and PY 139 (isoindoline).
Using the above resin system and tinting with TiO2,
the PY 227 performs very well in the same accelerated
weathering testing. It has excellent colour retention,
especially compared to the organic pigment. It loses
gloss, but as can be seen in comparing the masstone
and tint gloss change data, the major driver in the gloss
change is the TiO2, even though a highly durable grade
was used.
The new RTZ orange show excellent accelerated QUV
masstone weathering comparable to the red-shade
standard, high-durability pigment PBr 24(chromium titanate) –see Table 3.
Conclusion
Together the new NTP yellow and improved RTZ orange
pigments provide high-performance, high durability colour envelope expanding tools for the coatings chemist
and paint formulator. Their high opacity and chromaticity
increase the colours that can be obtained in thin film
coatings while pushing the edge of the colour envelope in high durability systems like coil and extrusion,
pwoder, silicate-based paints and coatings for the architectural, ACE, automotive, signage and corporate colour
marketplace. They also make alternatives to lead chromate pigments in high-value systems where the highest
performance is expected.
Figure 8: Tint gloss
change of various
pigments
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