Rutile-based pigment and a method for the production thereof

Abstract

The invention covers a fine-particle, brilliant and strongly hiding rutile-based pigment that is devoid of any metal or reactive metal compounds relevant to mill abrasion detectable by application technology but whose particle-size in terms of particle diameters ranges from 50 to 1000 nm, for mono-, bi-, tri- or oligo-modal size distribution and a primary maximum ranges from 230 to 400 nm, wherein optionally for a bi- or poly-modal frequency distribution, a secondary maximum is less than 25% of the primary maximum between 400 and 1000 nm. The process for producing said pigment comprises treating an inorganic mixed-phase rutile structured oxide pigment by high-speed grinding in suspension in a ball grinding mill provided with a mechanically and chemically resistant coating until said particle-size and a substantially isometric rounded particle shape are attained. The invention optionally provides for viscosity adjustment and surface conditioning of the pigment. The inventive pigment differs from prior art by improved gloss, low whiteness or reduced whitening, relatively high hue saturation, by extremely high hiding power which had been unattainable in said class of products up to now and exhibits a low photoactivity with respect to known fine-particle rutiles. In practice neither abrasivity nor interaction of possible grinding residues with an application matrix are observed.

Description

RELATED APPLICATION [0001] This is a U.S. national phase continuation application of PCT application PCT / EP2005 / 008892 filed Aug. 16, 2005, published Mar. 2, 2006 under WO 2006 / 021354, and which claims the priority of German Application No. 10 2004 040 384.8, filed Aug. 20, 2004. The prior application is hereby incorporated by reference, in its entirety.FIELD OF THE INVENTION [0002] The invention relates to a fine-particle, bright, and highly opaque rutile-based pigment without contamination by reactive metal compounds and to a method for the production of such pigments. BACKGROUND OF THE INVENTION [0003] Nickel antimony titanium yellow pigments are by nature pale yellow pigments with high opacity. Overdyeing, i.e., over-coloring, with high quality organic pigments facilitates to obtain highly saturated full-tone colors covering the entire color spectrum, with the exception of blue and violet hues. This overcoloring results in a synergy between the relatively high opacity of the cos...

AI Technical Summary

Benefits of technology

[0021] As is consequently evident, the particle size for the type described should be considered an essential feature of the present invention. This shall be explained in greater detail in terms of the technology. Surprisingly, it has been demonstrated that the sharp particle size maximum must sit on a “base” in the distribution curve, which may also feature secondary maxima of up to 15%, preferably 5%. Without this explanation being limiting or exhaustive, this special embodiment of a sharply asymmetrical, bi- or oligomodal particle distribution or a “deep drag” for attaining optimum brightness and saturation may be tracked down to the need to provide optimum space occupation in the matrix by approaching a certain portion of the pigment to the “Fuller curve”. As long as they remain a minority, the finest fraction (diameters 150 nm<D<250 nm) may certainly cause the absorption edge of the pigment to be steeper due to stronger absorption, which likewise causes intensification of Raleigh scattering, resulting in improved brilliance.

[0022] The texture must also receive the required “polishing” in order to permit smooth, optimized interaction with a matrix, improve gloss, and protect usual application tools. This can be achieved efficiently by adding auxiliary agents in the same work step of the inventive method, which will be described subsequently. The particle surface is ground to a rounded shape using the selected method and appropriate additives and coatings, which helps moderating the abrasiveness of the particles and improves the flowability of highly-pigmented preparations in later applications. The tendency of fine rutile pigments to agglomerate is thus effectively addressed during creation, i.e., in the grinding batch, to be described further below.

[0024] The inventive fine-particle, brilliant, and highly opaque rutile-based pigments also result in excellent gloss values. A pigment in accordance with the invention is distinguished in that it has a 200 reflectometer gloss value of at least 42 according to DIN 67 530 and a 60° reflectometer gloss value of at least 80, in particular a 20° reflectometer gloss value of at least 50, in particular 55 to 70, and a 60° reflectometer gloss value of at least 83, in particular 83 to 93. The excellent hiding power of the claimed pigments is particularly valuable. These are distinguished in that the hiding power in accordance with DIN 55 987 is greater than 100% relative to a standard rutile pigment comparison substance, in particular is greater than 110%, and in particular is between 115 and 130%. Neither dry grinding used today as the state of the art nor conventional sand / bead mills are suitable for producing the inventive pigment of the type characterized; these known methods and processes merely yield particle diameters of the 600 through 1200 nm range at best.

[0025] The present invention also proves valuable because the accordingly obtained products do not feature an evenly distributed increase of remission over the entire wavelength spectrum, but feature a maximum in the yellow range of the visible spectrum (approximately 570 to 600 nm) instead. This also prevents the b* values from decreasing excessively with the unwanted inherent increase in brightness (i.e., the non-wavelength-specific remission).

Problems solved by technology

This overcoloring results in a synergy between the relatively high opacity of the cost-efficient nickel antimony titanium yellow and the high color intensity of the organic overcoloring pigments, which are generally quite expensive.

Another disadvantage of titanium white overcoloring is the photocatalytic effect of titanium white pigments, resulting in a sharp decrease of light and weather-fastness of the expensive organic colorants.

In the past, this decisive use of nickel antimony titanium yellow was rarely used because the nickel titanium pigments currently available on the market are abrasive (grain-hard and sharp-edged), have poor gloss, and are inferior in terms of hiding power compared to titanium white.

As colored pigments, nickel antimony titanium yellows themselves do not provide for a satisfactory option to replace 100,000 annual tons of lead sulfochromate yellow and molybdate red pigments pursuant to the hazardous materials laws and environmental protection laws that have become increasingly stringent since 1980.

The reason for this is the deficiencies, considered unchangeable, that are manifested in particular by the insufficient opacity, compared to titanium white and to chromium and cadmium yellows and by inadequate gloss and high abrasiveness.

In addition due, to their high grinding costs expensive, highly doped, highly-fired products are unknown because they are inconsistent in terms of coloristic assessment.

Surface enlargement below a mean particle diameter of 300 nm should be avoided for color pigments because they become transparent when their size drops below a particle diameter of about half the wavelength of the light reflected by them, which is undesirable for applications of nickel antimony titanium yellow pigments.

As the hardness of the particles is high, abrasiveness increases for spiky and sharp-edged particles.

Nevertheless, as a further object set herein for avoiding color effects under different types of illumination (metamerism), a relatively smooth surface and also approximate uniformity of the (projected) edge lengths should be obtained; a spherical shape is unattainable under any conditions, however.

However, there are limits for hue control depending on how the reactive iron content will assume uncontrollable amounts if a non-metallic mill with resistant lining is not used as it is inventively in this case.

If iron abrasion is permitted in the milling process, the material will gray and results in interferences in PVC-based matrices.

However, this quoted application does not provide any information on particle size distribution and does not specify the type of milling precisely.

Although in this method small quantities of aluminum chloride are metered to the titanium tetrachloride for “rutilization”, it being unresolved how many lattice places in the rutile are really occupied by aluminum ions, this method is not promising e.g. for application of antimony and nickel chlorides to titanium tetrachloride upstream of the burner.

The hydroxyl groups on the surface of the freshly precipitated oxides and hydroxides represent a good promoter for the diffusive penetration of the rutile lattice with its numerous vacancies with foreign metal ions after evaporating the water above 150° C. However, the finer grain sizes possible due to the lower firing temperature are not yielding reproducible color intensity.

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