Technical Paper

HEUBACH GmbH • P.O. Box 1161 • 38679 Langelsheim • Germany • Phone +49-5326-52-0 • Fax +49-5326-52-213 • sales@heubachcolor.de

IR-Ref lect ing Pigments

Page 1 / 4August 2011

“A Phi losophy is gaining Momentum”By Dr. Thomas Sowade

It is well known that modern IR-reflecting Inorganic Complex Pigments do not only impart super durable color shades to a surface but enable coating formulators to create products showing less interaction with the solar radiation compared to using normal pigments and therefore showing lower heat build-up. The most known application is found in the area of coil coatings for facades and roofs (key word: cool roofs). Aim is to lower heat transfer into buildings and as a result reduce energy consumption of air conditioners to cool down the room interior. As positive side effect an enhanced lifetime of the coating due to reduced t h e r m o m e c h a n i c a l impact is observed. There are several programs available defining criteria and approving products for cool roof applications. Out of these the Energy Star and Cool Roof rating Council of the United States can be named as the most known programs, further programs are Green Globes (Canada & US), LEED (US) and one from the Singapore Environment Council. In Europe a similar program called Cool Roofs Europe has been started and first products have been approved.

While the application of IR-reflecting pigments in coil coatings is accepted and subject to constant improvement, further applications are under initial development.

This paper will describe recent findings in car refinish paints, plasters for architectural thermal insulation systems, and will last but not least present a new dark IR-reflecting pigment combining jetness with a high solar reflection.

Inf luence of Solar radiation on Pigments

Pigments interact with the solar terrestrial radiation described in Figure 01. Possible interactions are absorption (transmission), reflection and scattering.

The interaction of color pigments with the visible part of the electromagnetic spectrum (380 – 780 nm) is obvious in form of our color impression. Pigments can additionally interact with UV-radiation (250 – 380 nm) and/or NIR

(near infrared; 780 – 2500 nm) radiation. While the (detrimental) influence of the UV-radiation is well known in form of e.g. pigment fading, the interaction with the NIR-radiation is its poor invisible cousin, although the integral energy of the NIR-radiation accounts for appr. 50 % of the total integral energy of solar radiation.

Heat build-up effect of dif ferent Pigments in a Topcoat

The amount of total energy absorbed and emitted by a topcoat determines the heat build-

up of a coated surface and results in a final surface temperature after reaching an equilibrium state. The pigmentation of the coating material has the main influence on heat build-up performance. To achieve cool surfaces the pigments need to reflect as much energy as possible. This reflection ability can be expressed as the Total Solar Reflectance value (100% = total reflection; 0 % = total absorption). Therefore pigments with high TSR-values show a high reflection combined with low heat build-up and viceversa.

The pigment with the highest

reflection ability is based on TiO2 while the pigment with lowest reflection is based on Carbon Black. If from aesthetical point of view one likes to stay with a dark color instead of white, there is the choice to use dark Inorganic Complex Pigments like P.G. 17 (Chromium Green Black) or P.Bk. 30 (Chrome Iron Nickel Black Spinel) as IR-reflecting option. These pigments show higher TSR values compared to Carbon Black and are an excellent choice for e.g. grey color shades while in mono pigmented systems the TSR-value is higher but the jetness is lower compared to Carbon Black.

The following graphs demonstrate TSR values of different pigmented topcoats (Figure 02) with the corresponding heat build-up measurements Figure 03).

Heat build-up effect of different Pigments in a Multi-Layer-System like Car Refinish

Although the usage of black Inorganic Complex Pigments with higher TSR-values compared to Carbon Black is unlikely in full shade Car Refinish- or OEM-TopCoat due to missing jetness there is still some room for it in the total Multi-Layer-Setup: Conventionally pigmented topcoats (high performance organic pigments) are transparent or at least partially transparent to NIR-radiation. As consequence the NIR-radiation passes through the TopCoat and the IR-reflectance ability of this Multi-Layer-System is then determined by the behavior of the next layer, in this case the primer. Primers are available in different colors, including light grey and dark grey, to support the color of the Topcoat e.g. in case of scratches.We modified a conventional white primer to a light grey and a dark grey primer, one

Figure 02: TSR spectra of different inorganic “black” pigments (20% pigmentation in alkyd/melamine)

Figure 03: Heat-build up of different inorganic “black” pigments and equilibrium heat level [˚C] compared to Carbon Black (20% pigmentation in alkyd/melamine) according to ASTM D4803

Figure 01: Typical solar terrestrial radiation spectrumFigure 04: Dark grey and light grey primers tinted with carbon black (non-NIR) and inorganic black (NIR)

Following these lab findings tests in outdoor exposure have been performed: During an extraordinary warm summer day (38˚C maximum temperature) samples colored with P.Bk. 30 and P.Bk. 7 at 1/3 STD and P.G. 17 and P.Bk. 7 at 1/9 STD were placed in the sun in Germany/Langelsheim (51.9˚N, 10.3˚E) facing south. While the non-NIR reflecting samples reached surface temperatures of around 100 ˚C the NIR-reflecting modified samples stayed appr. 10 ˚C cooler (see setup of experiment in Figure 09 and summary of results in Table 02).

Considering the thermal form stability of EPS (depending on compressive stress by load) to be 75-85 ˚C in long term and 100 ˚C in short term many manufacturers of EPS recommend to keep temperatures < 75 ˚C. The above findings of temperatures up to 102 ˚C implement the question, if a detrimental ageing effect can be observed. That was tested in our lab with non-NIR-reflecting and NIR-reflecting modified samples (Figure 10).

While the non-IR-reflecting samples show already after a short period of time (i.e. 5h) a softening and irreversible dimensional alteration of the EPS the NIR-reflecting samples show the expected better performance. The dimensional alteration, if ongoing, may cause cracks in the plaster and surface delamination.

modification with carbon black, the other with inorganic IR-reflecting black. Both modifications were tinted to the same coloristic values.

Due to the better tinting strength of carbon black a higher amount of inorganic black was necessary to achieve the similar coloristic result (Figure 04).

The light grey and the dark grey non-NIR primer show an appr. 8˚C higher end temperature compared to the white or the NIR primers. Remarkable is especially the very small difference in between the white and the two NIR primers which proof the efficiency of using IR reflecting inorganic blacks (Figure 05).

To check the influence of the primer pigmentation we applied a blue and a red basecoat including a clearcoat onto the different primers to complete the full automotive refinish set-up (Figure 06).

In practice a red basecoat is more likely to be combined with a white or light grey primer while a blue basecoat is more likely to be combined with a light or dark grey primer, but not a white one.

The TSR values including the maximum temperatures of the heat build-up measurements of different multilayer combinations are listed in the following table (Table 01): In both cases (red & blue basecoat) the modification of the standard grey primers (containing Carbon Black) with Inorganic IR-reflecting Black Pigments shows a significant effect for the Multi-Layer-Setup in total performance.

Heat build-up effect of different Pigments in a composite Thermal Insulation System

Modern composite thermal insulation systems are used to keep the interior of buildings warm in winter and cold in summer to reduce energy consumption for heating respectively cooling. The insulation layer (commonly expanded Polystyrene, EPS) largely reduces heat transport in any direction.

The darkness of the applied plaster on the EPS is limited as it may suffer from thermal / thermomechanical stress especially at the interface between Plaster and EPS layer. Therefore thermal insulation Systems are only offered to a certain point of luminosity values, indicating the darkness of the plaster.

Using IR reflecting pigments, the heat build-up on the composite thermal insulation is reduced, with expected positive

impact on prevention of premature failure (Figure 07). As a consequence using (dark) grey products with lower luminosity

values is becoming reality.

In a lab experiment grey plasters with a 1/3, 1/9 and 1/25 standard depth were formulated using for each sample a different pigment chemistry (P.Bk. 7, P.Bk. 11, P.Bk. 30 and P.G. 17) and applied as thermal insulation system.

In heat build-up tests for all standard depths the systems with P.Bk.7 and P.Bk.11 show a higher equilibrium

temperature compared to the systems using P.Bk. 30 or P.G. 17 (Figure 08 showing the example for 1/3 standard depth).

Technical Paper

HEUBACH GmbH • P.O. Box 1161 • 38679 Langelsheim • Germany • Phone +49-5326-52-0 • Fax +49-5326-52-213 • sales@heubachcolor.de

IR-Ref lect ing Pigments

Page 2 / 4August 2011

Figure 05: Heat build-up measurements on white, light grey and dark grey primer

Figure 06: Multilayer set up of refinish coating with different primers and a red respectively a blue basecoat

Primer Red Basecoat Blue Basecoat

White

Non-NIR Light Grey

Non-NIR Dark Grey

NIR Light Grey

NIR Dark Grey

TSR [%] 35

Max. Temperature [˚C] 50.0

Table 01: Matrix of test results (TSR and maximum temperature) for multilayer Car Refinish systems

20

56.6

30

51.3

12

59.6

10

61.6

18

52.3

15

52.8

Figure 07: Temperature profile of composite thermal insulation with and without IR-reflecting pigments

Figure 08: Comparison of surface temperatures on the surface of composite thermal insulation for four different pigments at 1/3 STD

interior, it can be expected that the attention will turn back to the coating once the research for glazing is finished and the contribution of the car body/reflective coating gets higher again in relation.

≡ Positive findings for lower heat build-up of thermal insulation systems using IR-reflecting black pigments instead of carbon black or black iron oxide have been reported for lab and outdoor tests. Detrimental effects on dimensional integrity of the expanded Polystyrene insulation layer can be seen under real outdoor conditions which can be minimized using IR-reflecting Pigments. All in all one can expect a lower energy consumption for cooling the buildings interior as well as an enhanced lifetime of the

thermal insulation system reducing thermomechanical stress at the interface EPS/Plaster.

≡ A new inorganic black pigment has been developed combining jetness and a neutral colour shade in reductions with an extraordinary IR-reflecting ability bridging the gap between existing products.

Note: All in this paper cited TSR-values have been determined in a 20% pigmentation in Alkyd/melamine. The powder pigments itself show even higher TSR-values.

The main difference compared to products which typically show such high TSR-values can be seen in the visible part of the spectrum, especially in between 650nm – 800nm. Here the reflection is significantly lower resulting in a more neutral color shade (Figure 12) and a higher jetness.

While in full shade the new pigment is more neutral compared to PG 17 or PBr 35 (figure 12) this behaviour can be found also in the 1:1 reduction (see figure 13). To achieve this color shift to the more neutral side while staying with the high TSR-value a lower tinting strength needs to be accepted.

Conclus ion

Besides the well known application of IR-reflecting Pigments in coil coatings possible benefits in two other areas have been found and a new inorganic black pigment combining jetness and high TSR-value was developed:

≡ In Multilayer systems it is possible to formulate brilliant surfaces combining the brilliance of an NIR-transparent topcoat (organic pigments) with the NIR-reflection ability of the below primer layer (NIR-reflecting pigments instead of carbon black) to reduce heat build-up of the whole system. Such Multilayer systems can be found e.g. in automotive OEM or Refinish as well as in

other industrial coatings applications. Although in car industry research is focussed at the moment to optimize the window glazing due to its higher influence on heat build-up in the car

Optimized IR-ref lect ing Inorganic Black P igment

Although there are different options available when choosing Inorganic Black Pigments with NIR-reflecting ability it is a common rule that Pigments with higher jetness show lower TSR values. Therefore the market demands for products with higher jetness without reducing the TSR-value.

The Heubach GmbH has now added a new product to its product line of IR-reflecting Pigments combining jetness and a neutral color shade in reductions with an extraordinary NIR-reflecting ability.

Figure 11 compares the reflectance spectra of different Inorganic Black options . The new product has an extraordinary high TSR-value of appr. 23% (see integral spectrum from 250nm – 2500 nm).

Technical Paper

HEUBACH GmbH • P.O. Box 1161 • 38679 Langelsheim • Germany • Phone +49-5326-52-0 • Fax +49-5326-52-213 • sales@heubachcolor.de

IR-Ref lect ing Pigments

Page 3 / 4August 2011

Figure 09: Outdoor exposure of thermal insulation systems on a hot summer day

Pigment

P.Bk. 30

P.Bk. 7

P.G. 17

P.Bk. 7

Table 02: Equilibrium surface temperature of samples placed in the sun on a hot summer day

STD

1/3 STD

1/3 STD

1/9 STD

1/9 STD

Insulation Layer

20 mm

20 mm

110 mm

110 mm

Slope

~ 45 ˚

~ 45 ˚

~ 15 ˚

~ 15 ˚

Max. Temp.

86.0 ˚C

97.5 ˚C

88.2 ˚C

102.0 ˚C

Figure 10: Illustration of macroscopic effects of excessive heat build-up on EPS in composite thermal insulation

Figure 11: TSR-spectra of different Inorganic Black Pigments (20% pigmentation in alkyd/melamine)

Figure 12: Color space of different Inorganic Black Pigments (Full shade)

Figure 13: Color space of different Inorganic Black Pigments (1:1 Reduction)