New class of treated aluminium pigments.

A new class of aluminium pigments with a champagne colour metallic appearance is going to open the way to new styling effects, according to the manufacturers, especially when they are used in combination with transparent colour pigments to create novel colour `flops'.

LAMELLA FLAKES

of aluminium are well known in the paint and coatings industry as silver bronze pigments. They are efficient reflectors of radiation over the entire range of the spectrum because of, first, the spectral response of the metallic aluminium; secondly, the high area-to-weight ratio of the flake particle; and thirdly, the highly preferred flat, parallel to the surface orientation of the flakes in a paint film.

In general aluminium pigments can be divided in two groups: the leafing grades are those which are distributed only at the surface and non-leafing grades are those which orientate throughout the depth of the coating. Metallic non-leafing aluminium pigments, in combination with transparent or opaque colour pigments, have given colour stylists a significant styling tool for product and automotive finishes. The primary contribution of the metallic pigments is their ability to change lightness/colour and reflectance with a change in angle of viewing and/or illumination. When the surface coating is viewed face-on, the colour appears bright and when viewed sideways, the colour appears dark or in a different colour shade. This optical property is known as `flop effect', `two-tone' or `travel' and provides optical highlights to curved surfaces.

A wide range of styling possibilities is opened by the particle size distribution and particle shape of the aluminium pigments -- from a glittering sparkle-effect to a silky gloss-like surface; everything is possible.

With the second generation of aluminium pigment, the Silverdollars, the request for brighter and more brilliant metallics is fulfilled. With these nearly round-shaped aluminium flakes coatings with an excellent brilliancy and a very strong (dark) flop can be prepared.

Nowadays the majority of automotive basecoats contain aluminium pigments. The particle size, particle size distribution, concentration, reflectivity, the type and amount of colour pigment, as well as the dispersion and orientation in the paint films, are factors that control the optical quality of a finish[1].

Stabilised aluminium pigments[2] meet the requirements set by waterborne coatings in the automotive industry[3]. They provide good gassing stability and show similar optical performance to that of conventional aluminium pigments in solvent-borne coating systems.

Deposition of transparent highly refractive iron oxide on aluminium pigments by CVD (chemical vapour deposition) techniques has led to a new class of pigments. Their optical properties can be described as a combination of mirror reflection at the aluminium surface, absorption caused by the iron oxide and interference of the light reflected from the upper surface of the iron oxide and the light reflected from the iron oxide/aluminium interface[4]. These pigments combine the excellent hiding power, high lightness flop and fastness properties of aluminium with the colouring properties of iron oxide. It is not possible to match the bright colour shades of these pigments in coatings by mixing pure aluminium flakes with transparent iron oxide pigments.

Thin transparent highly refractive metal oxide coatings on mica create the nacreous effect of the well known pearl lustre pigments by adjusting the thickness of the metal oxide coating to the point where interference takes place[5]. Selectively absorbing coloured layers of metal or metal oxides can alter the visual properties of these pigments[6].

In this article tinted oxidised aluminium flakes are presented. Upon oxidation of conventional aluminium pigments, thin layers of aluminium oxide on the surface of the flakes are formed. The yellowish colour shade of the pigments is dependant on the reaction conditions of the oxidising process. Especially in combination with transparent colour pigments, the champagne coloured Aloxal opens up a new field of coloration.

Synthesis

Heating aluminium pigments in water-miscible solvents like alcohols in the presence of defined amounts of water and a basic catalyst leads to a strong exothermic reaction. The aluminium flakes are oxidised under formation of hydrogen: 2A1 + (3+n) [H.sub.2]O [right arrow] [Al.sub.2][O.sub.3] x n[H.sub.2]O + 3[H.sub.2]

The process can be operated in a batchwise manner (Figure 1). When the reaction has been completed the work up of the reaction mixture is similar to the manufacturing process for conventional aluminium flakes. After sieving, most of the solvent is removed in the filter press and can be recycled. The exhaust gas is cleaned to remove any volatile organic solvent and the catalyst. In the final drying step the formed aluminum oxide layer is dehydrated and densified.

[Figure 1 ILLUSTRATION OMITTED]

All available leafing and non-leafing aluminium pigments can be oxidised to form tinted products without any degreasing or cleaning pre-treatment of the starting material[7]. Leafing stearine coated aluminium flakes need a longer incubation time before the reaction starts. Non-leafing oleic acid coated aluminium flakes yield products with better optical properties. The alloy composition of the unoxidised aluminium flake does not influence the resulting colour shade of the oxidised product very much[8]. Depending on the oxidation process, the resulting colour shades range from light yellow to brown (Figure 2). The increase in chroma of the oxidised flakes compared to the unoxidised starting material is correlated with a reduction in lightness by the following equation: L* = 116 - 1.7C*. The hue and chroma are linked together by the following equation: H* = 113 - 1.9C*.

[Figure 2 ILLUSTRATION OMITTED]

Electron microscopy scans of the oxidised flakes show that a thin oxide layer is formed around the metallic core (Figure 3). The thickness of the coating (in general around 20-30nm) is strongly dependent on the reaction conditions. The oxide layer features a porous structure consisting of needle-like crystalline material. BET measurements reveal an increase in surface area of around 500% compared to the unoxidised starting material.

[Figure 3 ILLUSTRATION OMITTED]

For every aluminium pigment grade, a defined degree of oxidation has to be obtained to get the maximum colour strength. Finer grades with higher surface areas have to be reacted to a higher degree of oxidation than the coarser grades to reach the same colour strength (Figure 4). The metal content of the available Aloxal grades is in a range from 58%, for the fine types up to 71% for the sparkling type.

[Figure 4 ILLUSTRATION OMITTED]

Stability

The formation of a `sandwich' type structured aluminium pigment (see Figure 3) yields very shear stress-stable pigments. To simulate the mechanical stress for flakeshaped pigments in the circulation lines and pumps far the automotive industry, diverse laboratory equipment, for example, gear-feed pumps, dissolvers or Waring Blendors, are used. Lightness in different angles is measured before and after the test; the difference (Cielab dL* value) allows a statement about the mechanical stability of the pigment.

Running the Waring-Blendor test with the shear stress-susceptible starting material for the manufacturing of oxidised aluminium pigments under the same conditions, an unacceptable variation of the standardised lightness can be observed while the oxidised product does not show any significant change (Figure 5).

[Figure 5 ILLUSTRATION OMITTED]

Although the oxidised aluminium pigments are obtained by a controlled reaction with water, they are not storage-stable in all aqueous coating systems. They have to be protected by hydrophobic inhibitors, for example, a phosphorus-organic compound, to repel the water from the pigment surface, thus preventing gassing.

Among the several different methods looking at the gassing stability of aluminium pigments in waterborne coating systems in the automotive industry, we choose the one that stores the sample in an aqueous base coat at elevated temperature (40 [degrees] C). The criteria for passing this test are dependant on the base coat system and the specific components of the paint suppliers. Stabilised, oxidised aluminium flakes (Aloxal Hydro) pass these tests and are stable in nearly all established waterborne coating systems.

The oxidised aluminium pigments show the known advantages of conventional aluminium pigments, including UV stability and weather fastness. The positive results of the accelerated weathering tests (QUV and Xenontest) are fully proved by the Florida test. After 24 months' natural weathering, only a very slight lightening can be observed (Figures 6 and 7).

[Figure 6-7 ILLUSTRATION OMITTED]

Optical properties

The most eminent property of the Aloxal pigments in this context is their `champagne-coloured' shade. The optical properties of combinations of lustre and transparent colour pigments can in general be described as a superposition of the absorption of colour pigments and the reflection of the aluminium lustre pigments and, in case of Aloxal, the yellowish interference-reflectance. Aluminium pigments exhibit their reflected light mainly at the specular reflection angle. At larger angles, the contribution of the lustre pigments to the reflection is low and the main contribution comes from the light absorption and scattering of the colour pigments. As a result, the anisotropic light reflection of the metallic lustre pigments provides the possibility to create two-tone colour coatings.

In Figure 8 the Cieldb a*, b* co-ordinates of two different finishes depending on the observing angle are shown. The finishes are combinations of conventional aluminium pigments and oxidised aluminium pigments with two colour pigments (PG 7 and PB 15). The use of the tinted oxidised aluminium pigments instead of the conventional aluminium pigments gives rise to a yellow colour shift, which in general generates warmer colour shades. Especially in the green, blue and red colour ranges, this opens very attractive styling possibilities closer to `natural' colours.

[Figure 8 ILLUSTRATION OMITTED]

The angle-dependent colour hues cannot be matched by incorporating colour pigments to aluminium-containing pigment finishes. To prove this in practice, a master panel based on Aloxal was prepared and tried to match using conventional aluminium pigments and colour pigments. Figure 9 shows clearly what also visually was observed: a real matching can be done only in one angle (45 [degrees]). The finish with Aloxal displays not only an increased chromaticity but also a higher flop in hue.

[Figure 9 ILLUSTRATION OMITTED]

Conclusion and outlook

With Aloxal a new pigment generation was developed opening new coloristic directions without loosing the well known and appreciated advantages of classic metal pigments, like hiding power and weather resistance, for indoor and outdoor applications.

While the first cars with Aloxal-shades are to see on the roads since beginning of this year further developments to a colourful future are running. The coating of flakeshaped aluminium pigments with colour pigments will yield a high variety of coloured flakes. Combination of those with differently coloured transparent pigments will open the way to a nearly unlimited amount of new effects.

Type                     D50          metal       Description
                         (appr)       content
                                       (appr)
Stapa Aloxal
PM2010                  19[micro]m      58%        Fine grade
Stapa Aloxal
PM3010                  20[micro]m      60%        Silverdollar
Stapa Aloxal
PM4010                  32[micro]m      71%        Sparkling grade

Table 1: Aloxal pigments

References

[1.] R Rolles; KE Luyk; Treatise on Coatings, Volume 3, Pigments, Part I (Ed: RR Myers and JS Long) Marcel Dekker, New York 1975.

[2.] a) W Reifier; A Fetz; E Roth, Polymers Paint Colour Journal, February 1995, S 10. b) Besold R; W Reifier; E Roth, farbe+lack, 97, 311, 1991.

[3.] J Niemann, Prog Org Coatings, 2 I, 189, 1992.

[4.] N Mronga; V Radtke; B Baumann, New Technologies for Coatings and Inks, Congress Papers, 3rd Nurnberg Congress, paper 18, 1995.

[5.] KD Franz; R Emmert; H Hartner; K Nitta, Ullmanns Encyclopadie der Technischen Chemie, 5th Edition, Vol 20 A, VCH Weinheim, 1992.

[6.] W Ostertag, Nachr Chem Techn Lab 42, 854, 1994.

[7.] Lj Virin, Zurnal prikladnoy chimii 32, No 5, 1050, 1959.

[8.] DE Dt Patentanmeldung 195 20 312.