Aluminum Paste by ECKART

Aluminum paste is a finely milled mixture of aluminum particles and a carrier solvent, used to impart metallic effects and enhance the properties of coatings, inks, and paints.

Aluminum Paste Overview

Metallic effect pigments are used in the paints and coatings industry for both their optical effects as well as for their functionality.

Metallic effect pigments are lamellar, or flake-like, with a particle size larger than that of color pigments. The particle size for color pigments falls within the visible light spectrum, whereas effect pigment flakes typically measure between 5 and 45 micrometers. These pigments are commonly supplied in a paste.

Close up of aluminum paste products.Close up of aluminum paste products.

What Aluminum Paste products does ECKART offer?

ECKART offers a variety of aluminum paste grades under the STAPA® name.

STAPA® METALLIC

Wide particle size distribution, good hiding power, and intensive color, especially the slightly greyish fine grades.

STAPA® MOBILUX

Narrow particle size distribution, high color purity, for clear color shades.

STAPA® METALLUX 200

Coarse sparkle grade, very narrow particle size distribution, for clear color shades with a sparkle effect.

STAPA® METALLUX 700

Aluminum pigment pastes for can and coil coatings.

STAPA® METALLUX 1000

Tinting strength/hiding power and sparkle effect in one pigment, high brilliance.

STAPA® METALLUX 1500 

Highly brilliant silverdollar, pronounced metallic effect, unique hiding power.

STAPA® METALLUX 2000

Standard silverdollar, narrow particle size distribution.

STAPA® METALLUX 3000

Silverdollar has a narrow particle size distribution, high brilliance, color purity, and dark flop.

STAPA® METALLUX 8000

Fine, narrow particle size distribution for clear color shades.

STAPA® METALLUX 9000

Silky gloss grades are very bright and fine, and they are low flop for silky gloss and bright color shades.

STAPA® SILVERSHINE 400

The latest Silverdollar technology.

STAPA® NDF

Non-degrading flakes, high shear stability, and fineness are needed for very clear color shades.

Aluminum Paste 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 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 later application by adding solvents or additives to the filter cake.

Diagram showing the process used to produce aluminum paste.Diagram showing the process used to produce aluminum paste.

Exploring the characteristics of Aluminum Paste?

Leafing Pigments

Wetting Behavior
Lubricant must be used in the various production processes to avoid cold welding. This typically affects 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, the binder does not wet leafing pigments. Therefore, they float on the wet film and orient themselves on top of the surface. This effect is achieved using stearic acid as a lubricant (Fig. 3).

Diagram showing aluminum paste leafing behavior.Diagram showing aluminum paste leafing behavior.

Table 1 shows typical properties and application fields.

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 taken into account when drawing up the paint formula. If necessary, special leafing-stabilized pigments should be used.

Table showing the typical properties of leafing aluminum pigments.Table showing the typical properties of leafing aluminum pigments.

Non-Leafing Pigments

Non-leafing pigments are created by adding strongly polar substances or wetting agents to leafing pigments or using special lubricants (e.g., oleic acid) directly during milling.

Non-leafing pigments are completely wetted and thus spread evenly throughout the paint film (fig. 4).

Diagram showing the behavior of non-leafing aluminum paste.Diagram showing the behavior of non-leafing aluminum paste.

Table 2 shows their typical properties and application fields.

Table listing the typical properties of wetting behavior of non-leafing aluminum paste.Table listing the typical properties of wetting behavior of non-leafing aluminum paste.

Particle Shape – Particle Size – Particle Size Distribution


Owing to its ductility, milling or crushing of the spattered aluminum in the ball mill creates flakes with shape factors of 1:50 to 1:500 (flake thickness: flake diameter).

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

Close up of aluminum paste product in a dish.Close up of aluminum paste product in a dish.

Optical Properties


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 the following:

  • 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; 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

For example, car finishes have a metallic effect achieved by the reflection and scattering of light on the flakes, which are fixed in a clear or transparent coating and more or less oriented.

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 (fig. 5).

Diagram showing the reflection and scattering properties of aluminum pigments found in aluminum paste products.Diagram showing the reflection and scattering properties of aluminum pigments found in aluminum paste products.

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 brightness alteration in dependency of the observation angle, also increases. The flop is displayed by the brightness contrast between vertical and horizontal automotive parts, thus looking at a car from different angles.

The finer the pigment and the more irregular its structure, the higher the proportion of scattered light. The more uniform and greyer the effect, the whiter the flop and the higher the hiding power and the DOI value (distinctiveness of image). The higher the DOI value, the clearer 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 that also provide good hiding power and DOI. However, these properties are contrary to the particle size distribution (see Fig. 6).

High-quality silver dollars fulfill these demands owing to their round shape and narrow particle size distribution, but they provide less hiding power. The STAPA® METALLUX 1500 series is innovative, offering Brilliance, a sparkling metallic effect, and good hiding power.

Diagram showing the influence of particle size within aluminum paste on coatings performance.Diagram showing the influence of particle size within aluminum paste on coatings performance.

Pigment Orientation

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 in obtaining an excellent metallic effect.

The 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 (see Fig. 7).

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 the alignment. This explains why a good pigment orientation and good optical properties are much easier achieved in low solid coatings than in high solid coatings.

Using high-solid coatings and choosing binding agents that release solvents quickly, such as cellulose acetate butyrate, or adding additives that stabilize the aluminum pigments can prevent a cloudy appearance. The function of wax dispersions or other surface active substances as "spacers" must be tested in the respective binder.

Diagram demonstrating the influence of particle orientation on optical effects.Diagram demonstrating the influence of particle orientation on optical effects.
Influence of coatings systems and application on the orientation on optical properties.Influence of coatings systems and application on the orientation on optical properties.

Fig. 9 / 10 compares different product series' Brightness and Flop Values.

Table charting the brightness of various ECKART aluminum paste products.Table charting the brightness of various ECKART aluminum paste products.
Table charting the flop value of various aluminum paste products by ECKART.Table charting the flop value of various aluminum paste products by ECKART.

Mechanical Properties

High mechanical shear forces can damage and deform the thin metallic effect pigments. The consequences might be the formation of spots, reduced optical effects, and, in the worst case, chemical reactions between the aluminum pigments and the coating system, which are caused by damages to the aluminum pigments and 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”).

Processing of Metallic Effect Pigments

Complete dispersion of the metallic pigments in the paint is essential for achieving full optical effects and a coating without spots. However, their mechanical properties require a different dispersion process from color pigments.

Generally, mixers are recommended for processing 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. Blade stirrers and flat or toothed dissolver discs are suitable shapes. The stirrer-to-dispersion vessel diameter ratio should ideally be between 0.5 and 0.7.

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

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

Aromatic hydrocarbons, such as xylol, and mineral spirits are suitable solvents for pre-dispersing leafing pigments. However, the use of polar solvents and dispersion-wetting agents should be avoided, as these substances will wet the pigments, causing a loss of 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, speeding up the wetting and separating of the pigments. Consequently, the paste is more stable, and the risk of pigment agglomeration is reduced.

Chlorinated hydrocarbons are not suitable, as they can split off hydrogen chloride under unfavorable conditions, which would react with the aluminum pigment. The solvent must not contain water to prevent a reaction of unstabilized pigments with water.

For easier processing, starting with the aluminum pigment paste and gradually adding the solvent while stirring is advisable. 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 appear thick.

It is recommended to produce the coating in the following order:

  • First, put the pigment paste in the dispersion vessel, then gradually add the solvent while stirring.
  • After the homogenization, the pigment slurry should be examined for undispersed paste residue.
  • After this check, the coating will be adjusted, and further components will be added, i.e., if necessary, more solvent will be added to achieve the paste consistency, and all other formulation ingredients, such as binder, additives, etc., will also be added.

In many cases, however, it is necessary to reverse the processing procedure, i.e., to add the pre-dispersed metallic effect pigment to the binder. The reason is a tendency to concentrate, which leads to a pigment or solvent shock, resulting in pigment agglomeration.

Therefore, laboratory tests are essential to determine the correct coating manufacturing order.

The paste must be covered as much as possible during the process to minimize the solvent's evaporation. Otherwise, agglomeration might occur.

Degradation Resistance

In the automotive industry, coatings are constantly pumped in circulation systems and led off if required. High shear forces arise in critical pump systems, especially at the pressure control valves between the circulation line and the spraying device, which can damage the metallic effect pigment. Subsequent dispersion processes (subsequent wetting through long dwelling times and turbulences) can change the optical effect.

Special aluminum pigments, "non-degrading flakes," have been developed for particularly aggressive circulation systems. The greater thickness of the flakes provides higher mechanical stability.

Chemical Properties

The chemical properties of aluminum pigments limit the choice of binders and solvents (Table 3). However, stabilized aluminum pigments have been developed for water-based coatings, which allow the formulation of stable water-based paints for storage.

Table listing many of the aluminum paste behaviors with various chemicals.Table listing many of the aluminum paste behaviors with various chemicals.

Water-Based Coating Systems

The escalation of pollution and the depletion of raw materials have become increasingly severe in recent years. The paint industry, especially the paint processing sector, is being urged to contribute to the reduction of environmental pollution through relevant legislation and regulations.

This resulted in enhancements to application processes, such as more efficient methods achieved through electrostatic coatings, and to mechanical and thermal air purification processes, including filtration and thermal post-combustion. Most notably, it spurred the development of environmentally friendly coating systems, namely those with low or no solvent content.

Water-Based Coating Systems

The chemical properties of aluminum pigments described on page 17 mean that their use in water-based paint systems (dispersions, soluble systems, anodic and cathodic electro-dip coating, etc.) is highly critical.

The main challenge is not making the basically hydrophobic metallic effect pigments water-dispersible by using suitable tensides or adding water-miscible solvents; rather, it is ensuring the storage stability and processing properties of water-based metallic effect pigment paints by using appropriate protection mechanisms.

Aluminum pigments for water-based coating systems must be stable in contact with water and with various binder systems. Gassing stability, therefore, depends to a large extent on the composition of the coating and is thus only a relative parameter. The pH value, the nucleophilia of the alkalines used to neutralize the binder, and the presence of chemical groups in the binder with an affinity to the pigment, as well as the organic solvent contained as a co-solvent, play a vital role.

Therefore, the gassing stability of an aluminum pigment should be tested in the given water-based coating. Measuring hydrogen development in the coating during storage at 40 °C in a gas bubble meter has proved valuable for quantitative assessment.

Two main processes have become standard for the stabilization of Aluminum pigments:

  • Absorption of suitable corrosion inhibitors on the surfaces of the pigments
  • Encapsulation of the pigments with a protective coating

Suitable products for use in water-based systems are, amongst others:

Additive-Stabilized Pigments

  • STAPA® HYDROXAL

  • STAPA® HYDROMIC

Encapsulated Pigments

  • STAPA® HYDROLAN

PVD-Pigments

  • HYDROSHINE

For further information about water-based coating systems, please request the brochure "STAPA®  Aluminum pastes for aqueous coating systems."

Quality Control and Testing Methods

Colorimetry

The visual appearance of metallic coatings strongly depends on the viewing angle and the light conditions. A spectrophotometer for metallic colors (e.g., BYK-Gardner BYK-mac) helps to measure a metallic coating colorimetrically. The characterization requires simultaneous consideration of different measurements, such as brightness L* vs. color strength (chroma C*), red and green value a* vs. yellow-blue value b*. The angles of 25°, 45°, and 75° for standard products 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. Figures 11 and 12 compare aluminum pigments in a blue metallic two-coat finish.

Table charting the color field of various aluminum paste products.Table charting the color field of various aluminum paste products.
Table charting the color field of various aluminum paste products.Table charting the color field of various aluminum paste products.

Guarantee of Quality Standard

The batches' quality control uses differential measurement against the defined corresponding standard. A precondition is that the samples are simultaneously applied in the same coating system under constant environmental parameters. This minimizes processing-related variations of the sample application.

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

Image of a member of the ECKART aluminum paste production team.Image of a member of the ECKART aluminum paste production team.

Quality Control

The quality control of metallic effect pigments comprises tests of optical properties and the quality criteria mentioned in the technical data sheets. A distinction is drawn between the tests of the pigment and the application.

Tests of 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

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)

Aluminum pigment pastes for water-based coating systems are tested on gassing stability (not standardized).

Determination of the Particle Size Distribution by Laser Granulometer

The measuring of the particle size distribution (typical value) follows the method of laser granulometry according to ISO 13320-1.

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 pre-disperse the sample in an external ultrasonic bath.

The higher the ultrasonic frequency, respectively, 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, the finest particles will be generated by mechanical breaking off from the original pigment.

The longer the dispersion time, the smaller the particle size distribution value (D 50). The sample appears finer again.

The dispersion medium has little effect on the measurement results. Isopropanol is usually used for quality control. The device's material properties should be checked before using other solvents.

A detailed description of ECKART's testing method (test instruction) can be obtained upon request.

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