Aluminum Pigment Pastes for Metallic Effects

In many cases the properties of metallic effect pigments can be deduced directly from the production process. For safety reasons aluminum pigments are produced almost exclusively in a wet milling process adding white spirit (Hall process).

The raw material - atomized, spattered aluminum with a minimum purity of 99.5 % according to EN 576 is milled respectively shaped to flake-like particles in ball mills, filled with grinding aids (lubricant).

The milling parameters and the lubricant are determined by the application for which the pigment is intended. The pigment slurry is screened, pressed on filter presses, and adjusted in mixers to a ratio of 65% solids and 35% solvent.

The final product can be tailor-made for its later application by the addition of particular solvents or additives to the filter cake.

Wetting Behavior
A lubricant must be used in the various production processes to avoid coldwelding. This has a typical effect on the wetting behavior of the metal effect pigments. A difference is made between leafing and non-leafing pigments.

Leafing Pigments
Due to their high surface tension leafing pigments are not wetted by the binder, therefore, they float on the wet film and orient themselves on top of the surface. This effect is achieved using, stearic acid as lubricant.

In paint systems with strongly polar solvents or binders there is always a danger that the leafing pigments will “drown” through wetting and turn into non-leafing pigments. This possibility must be considered when drawing up the formula of paints. If necessary special leafing-stabilized pigments should be used.

Non-Leafing Pigments
Non-leafing pigments are created either by the addition of strongly polar substances or wetting agents to leafing pigments or using special lubricants (e.g., oleic acid) directly during the milling process.

Non-leafing pigments are completely wetted and thus spread evenly throughout the entire paint film.

In paint systems with strongly polar solvents or binders there is always a danger that the leafing pigments will “drown” through wetting and turn into non-leafing pigments.

This possibility must be considered when drawing up the formula of paints. If necessary special leafing-stabilized pigments should be used.

The flake form varies from irregular cornflakes to almost completely round silverdollars. Silverdollars are aluminum pigments with extraordinary optical properties. They are produced in an elaborate process from very fine granules.

The particle sizes of the aluminum pigments vary according to the product and milling procedure. They show an (advanced) Gaussian distribution which decisively influences the optical properties of a coating. Therefore, the measuring of the particle size distribution is essential to classify the pigments and check their quality.

Metallic effect pigments create their characteristic effects through reflection and scattering of light falling on the surface of the pigment. The metallic effect depends mainly on:
 • Particle size distribution.
 • Particle shape.
 • The smoothness of the surface.
 • Pigment orientation in relation to the surface of the substrate.
 • Wetting behavior (leafing – non-leafing).

It is somewhat difficult to describe or measure the visual impression of the metallic effect as it is made up of a number of characteristic individual effects:
 • Color shade.
 • Brilliance (sparkle and metallic gloss).
 • Brightness.
 • Flop.
 • Color saturation.
 • Tinting strength (hiding power).
 • DOI (DOI = distinctiveness of image).

Color Shade
Aluminum pigments have no color of their own, they are achromatic, thus distinguishing from black and white pigments by their brilliance or metallic gloss. Colored aluminum pigments whose surfaces are coated with coloring agents are available too. The best-known examples are gold-colored aluminum pigments coated with iron oxides (Paliocrom®, a trademark of BASF, Ludwigshafen).

Color Shade, Brilliance, Brightness, Color Saturation, Flop, Hiding Power and DOI
The metallic effect of e.g., car finishes is achieved by the reflection and scattering of light on the flakes, which are fixed in a oriented way in a clear or transparent coating. The visual impression created depends on the ratio between reflected and scattered light. The proportion of reflected light increases with the size of the pigment areas; the proportion of scattered light increases with the number of edges scattering the light.

The coarser the pigments and the rounder their shape, the higher the proportion of reflected light and, thus, the brilliance, brightness and color saturation in colored metallic coatings. The flop, i.e., the alteration of the brightness in dependency of the observation angle, increases as well.

The flop is clearly displayed by the contrast of brightness between vertical and horizontal automotive parts, thus, looking at a car in different angles.

The finer the pigment and the more irregular its structure, the higher is the proportion of scattered light. The more uniform and greyer the effect, the whiter is the flop and the higher are the hiding power and the DOI value (distinctiveness of image).

The higher the DOI value, the clearer is the reflection of objects such as buildings, clouds, or trees on the coated surface (paint gloss).

There is a high demand for bright, brilliant metallic effect pigments with a strong flop which also provide good hiding power and DOI. But these properties are contrary to the particle size distribution.

High-quality silverdollars fulfill these demands owing to their round shape and narrow particle size distribution, but they provide. less hiding power.

In addition to color shade, brilliance, brightness, color saturation, hiding power, flop and DOI, the orientation of the aluminum pigments in the paint film is a crucial factor to obtain a good metallic effect.

Best metallic effects are achieved when the pigments are aligned parallel to the film surface. Poor orientation results in a cloudy appearance or a turbulent “salt-and-pepper” effect.

The orientation of the pigment in the coating is determined by the formulation and the conditions of the application: By the evaporation of the solvent the wet film shrinks, and the aluminum pigments orient themselves parallel to the surface. The higher the proportion of solvent in the coating, the more significant is the alignment. In turn, this explains why a good pigment orientation and good optical properties are much easier achieved in low solid coatings than in high solid coatings.

If high solid coatings are used, the cloudy appearance can be avoided by using binding agents which release the solvents quickly (e.g., cellulose acetate butyrate) or by the addition of additives which fix the aluminum pigments. The function of wax dispersions or other surface active substances as “spacer” needs to be tested in the respective binder.

The consequences might be a formation of spots, reduced optical effects and in the worst-case chemical reactions between the aluminum pigments and the coating system – caused by damages of the aluminum pigments which result in unprotected surfaces.

Attention should be paid to the mechanical shear forces regarding their influence on aluminum pigments during dispersing and processing in critical pump systems and circulation lines (see “Processing of metallic effect pigments”).

Generally, mixers are recommended for the processing of metallic effect pigments that do not place a high mechanical shear force on the individual flake.

Stirrers (dissolvers) should be operated at low speeds of 500 – 800 rpm. Suitable shapes of dissolver discs are blade stirrers, flat or toothed dissolver discs. The ratio of the stirrer to dispersion vessel diameter should ideally be between 0.5 and 0.7.

During dissolving the mixing blade should be close to the bottom of the mixing vessel in order to ensure complete dispersion of the aluminum pigment paste.

A pre-dispersing of the metallic effect pigment in the solvent supports the homogenization of the paste. The ratio of aluminum pigment paste to solvent should be about 1:1 to 1:2.

Solvents suitable to pre-disperse leafing pigments are aromatic hydrocarbons (e.g., xylol) and mineral spirit. Polar solvents and dispersion wetting agents must not be used, otherwise the pigments are wetted, making them lose their leafing characteristics.

Solvents suitable to pre-disperse non-leafing pigments are polar solvents (e.g., alcohols, esters, ether ester) and mixed solvents.

Adding suitable wetting agents may improve the processing, i. e. the wetting and separating of the pigments is speeded up. Consequently, the paste is more stable, and the risk of a pigment agglomeration is being reduced.

Chlorinated hydrocarbons are not suitable, as due to unfavorable conditions they can split off hydrogen chloride which in turn would react with the aluminum pigment.

It is important that the solvent does not contain any water to prevent a reaction of unstabilized pigments with water.

For easier processing, it is advisable to start with the aluminum pigment paste and after this add gradually the solvent while stirring. This premix can either be homogenized after a while (“soaking of the pigment paste”) or immediately with a stirrer (dissolver). When all pigments are fully dispersed, the premix will show a thick appearance.

The following production steps are suggested:
 • First, put the pigment paste in the dispersion vessel, and gradually add the solvent while stirring.
 • After homogenization, examine the pigment slurry for undispersed paste residue.
 • The coating will be adjusted and further components added. Add solvent and other ingredients if necessary (binders, additives etc.).

In many cases it is necessary to reverse the processing procedure, i.e., to add the predispersed metallic effect pigment to the binder. The reason is a tendency to concentrate, which leads to pigment agglomeration.

Therefore, it is essential to carry out laboratory tests to determine the correct order of manufacturing a coating. During the whole processing it is important, that the paste is covered as much as possible to minimize the evaporation of the solvent. Otherwise, agglomeration might occur.

Degradation Resistance
Coatings are pumped into circulation systems and removed in the automotive industry if needed. High shear forces can damage metallic effect pigment at pressure control valves. Unique aluminum pigments, "non-degrading flakes," are developed for aggressive circulation systems with higher mechanical stability due to greater thickness.

Chemical Properties
The chemical properties of aluminum pigments limit the choice of binders and solvents (table 3). For water-based coatings stabilized aluminum pigments (STAPA® Hydro…) have been developed which allow the formulation of storage stable water-based paints.

Colorimetry
The aesthetics of metallic coatings depends on viewing angles and light conditions. A spectrophotometer for metallic colors helps to colorimetrically measure a metallic coating. The characterization requires a simultaneous consideration of different measurements, such as brightness L* vs. color strength (chroma C*), red green value a* vs. yellow blue value b*. For standard products the angles of 25°, 45° and 75° are measured. Subsequently, the results are presented in a colorimetric system, such as CIEL*a*b* or L*C*h°. Colorimetric systems, such as CIEL*a*b* and L*C*h°, visualize the results graphically.

Guarantee of Quality Standard
The quality control of the batches is done by means of differential measurement against the defined corresponding standard.

It is a precondition that the samples are simultaneously applied in the same coating system and under constant environmental parameters. In doing so, processing related variations of the sample application are minimized.

The values of brightness and tinting strength are very important assessment criteria. The color strength value characterizes the hiding power of a metallic effect pigment.

Quality Tests on the pigment:
 • Screen analysis (near-mesh sieving) according to DIN 53196 respectively ASTM 11.
 • Particle size distribution by laser granulometry according to ISO 13320-1.
 • Volatile and non-volatile content based on DIN 55923.

Quality Tests on the application:
 • Metallic effect (flop)
 • Brightness
 • Distinctiveness of image (DOI).
 • Color saturation.
 • Tinting strength.
 • Hiding power.
 • Measuring of gloss.
 • Measuring of effect (sparkle).

In addition to the hardware (equipment manufacturer and type) and software the results of the laser granulometer are highly dependent on the following parameters:
 • Way of dispersion.
 • Dispersing device.
 • Dispersing medium.
 • Dispersion energy.
 • Dispersion time.

Usually, the sample is dispersed by ultrasound. It is possible to use the integrated ultrasonic bath or, preferably, to predisperse the sample in an external ultrasonic bath.

The higher the ultrasonic frequency, the greater the energy concentration in the dispersing vessel, the finer the sample will appear, because more of the finest particles have been dispersed. In the case of a very high energy concentration, finest particles will be generated by mechanical breaking off from the original pigment.

The longer the dispersion time, the smaller the value of the particle size distribution (D 50). The sample appears finer again. The dispersion medium has little effect on the measurement results. For quality control, isopropanol is used. The material properties of the device should be checked before using other solvents.