Weather stable pearlescent pigments

Abstract

Weather resistant pearlescent pigment comprising a basic pearlescent pigment which comprises a transparent substrate and at least one metal oxide layer with a refractive index > 1.8 and thereon a weather stabilizing top coating which comprises a first and a second metal oxide followed by an organofunctional surface modification layer, wherein in variant a) the weather stabilizing top coating comprises a first metal oxide, metal hydroxide or oxide hydrate of Zn followed by a second metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof or in variant b) the weather stabilizing top coating consists of a mixture of the first metal oxide, metal hydroxide or oxide hydrate of Zn and the second metal oxide, metal hydroxide or oxide hydrate of Si, Al and / or Zr and wherein in variant a) or b) the carbon content of the weather stabilizing top coating is less than 0.50 wt.%, based on the total weight of the weather resistant pearlescent pigment.

Description

[0001] Weather stable pearlescent pigments

[0002] The invention relates to weather stable pearlescent pigments having only small glosslosses in strengthened tests of weather stability and no substantial alteration of optical properties due to their top-coating.

[0003] Pearlescent pigments which comprise titanium dioxide in the top layer or which are constructed from particulate TiO2 possess a certain photocatalytic activity. In the presence of water and oxygen, the UV activity of the pearlescent pigment may trigger accelerated degradation of organic compounds such as a binder matrix. Even the UV fraction present in daylight may cause this reaction, i.e., for applications such as automotive finishes, which are exposed directly to weathering, it is necessary to use pearlescent pigments which have been specially stabilized. In order to counter this effect, which is deleterious for exterior application, pearlescent pigments can be furnished with a variety of protective coatings in order to reduce the photoactivity.

[0004] Such so called top coatings usually contain several metal oxides in small amounts and an organic surface modification. The metal oxide layers reduce the charge transfer which is induced by the photoactivity of the titania layer and therefore reduce the degradation of the binder matrix polymers in a coating. The organic surface modification enables a compatibility between the surface of the pearlescent pigment and the binder system which leads to stability in condensation water test which is part of weather stability testing.

[0005] Very often the top coating contains cerium oxide species which effectively reduce the photoactivity of titania layers due to the redox couple properties of cerium.

[0006] EP 0 141 174 describes pearlescent pigments of improved weathering stability which have a protective coating composed essentially of a rare earth metal compound, which is specified as cerium-oxide and a polysiloxane. Furthermore, application of the protective coating takes place in an aqueous suspension which may also include zinc salts and / or aluminum salts or silicate. The polysiloxane is coated together with the silicate or alumina coating.

[0007] EP 1682622 A1 discloses weather stable pearlescent pigments with a top-coating comprising a first layer of cerium-oxide followed by a SiO2-layer and an organic surface modification. EP 0 888 410 B1 discloses modified pearlescent pigments based on a platelet-shaped substrate coated with metal oxides. According to the teaching of EP 0 888 410 B1 the top layer is composed of at least two oxides, oxide mixtures or mixed oxides of silica, alumina, cerium oxide, titanium oxide or zirconium oxide, and a water-based oligomeric silane system.

[0008] EP 0 649 886 provides pearlescent pigments with a titanium dioxide or iron oxide coating which are after coated in aqueous phase with a combination of cerium and aluminum oxide hydrates and are subsequently dried. Such top coatings have the disadvantage that the amount of cerium oxide is quite limited as this metal oxide has a yellowish appearance which would change the optical properties of the pearlescent pigment too much. It is desirable that the top coating alters the optical properties such as interference color, gloss and chroma of the base pearlescent pigments as little as possible.

[0009] US 2013 / 213260 A1 discloses weather resistant pearlescent pigments having a top coating which comprises zinc oxide and other metal oxides like silica, alumina, cerium oxide or zirconium oxide. Clearly it is favored to have zinc oxide located as outer metal oxide coating of the top coating. Furthermore, it is mandatory that the total carbon content of the pearlescent pigment is > 0.5 wt.%, referred to the total amount of pearlescent pigment. The carbon content is caused by the organofunctional surface modification layer.

[0010] New customer demands involve a more rigorous testing method for weather stability which is a combination of two well-known methods. First the pearlescent pigments are coated onto KTL-panels in a suitable base coat according to automotive demands using a 1 K clear coat. These coatings are first treated for 1000 up to 4000 hours in a Xenon test and immediately afterwards in a condensation water test. This test procedure simulates relatively harsh conditions in a relatively sensitive coating application systems. In many cases it can cause irreversible UV reaction and swelling phenomena, finally causing either undesired color changes or significant degradation of gloss properties in the coating application.

[0011] Therefore, it is the object of this invention to provide weather stable pearlescent pigments which pass the above described rigorous test procedure of weather stability but are not altered too strongly in their optical properties compared to the basic pearlescent pigment without top-coating. The object was solved by providing a weather resistant pearlescent pigment comprising a basic pearlescent pigment which comprises a transparent substrate and at least one metal oxide layer with a refractive index > 1 .8 disposed on the transparent substrate and a weather stabilizing top coating disposed on the at least one metal oxide layer. The weather stabilizing top coating comprises a first and a second metal oxide followed by an organofunctional surface modification layer. The weather stabilizing top coating has two variants. Variant a) comprises a first layer of metal oxide, metal hydroxide or oxide hydrate of Zn followed by a second layer of metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof. Variant B consists of a mixture of a first metal oxide, metal hydroxide or oxide hydrate of Zn and a second metal oxide, metal hydroxide or oxide hydrate of Si, Al and / or Zr. In each variant (a or b) the carbon content of the weather resistant pearlescent pigment is less than 0.50 wt.%, based on the total weight of the weather resistant pearlescent pigment.

[0012] A second object is to provide a method of manufacture of such weather resistant pearlescent pigments which is easily practiced and cost effective.

[0013] This object is solved by providing a method of manufacturing the weather resistant pearlescent pigment comprises: i) suspending the basic pearlescent pigments in a solvent, iia) coating the basic pearlescent pigments from step i) in the solvent using a Zn oxide precursor at a predetermined pH1 , and obtaining a Zn oxide layer on the basic pearlescent pigments, and iib) contacting the pearlescent pigment from step iia) at a predetermined pH2 with a precursor of a second metal oxide forming a layer of metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof or iic) coating the basic pearlescent pigments from step i) at a predetermined pH3 by simultaneously and continuously adding a solution of a Zn oxide precursor and a solution of a precursor of the second metal oxide, wherein the feed rate of one of these precursor solutions is coupled with the control of the pH value and forming a mixed layer of metal oxide, metal hydroxide or oxide hydrate of Zn and of metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof iii) contacting the pearlescent pigments from step iib) or step iic) in the solvent with organofunctional silanes or water based pre-condensed organofunctional silanes, and iv) separating the coated pearlescent pigments, optionally washing with solvent, and drying at a temperature in a range of 80 to 160° C.

[0014] Basic Pearlescent pigments:

[0015] With the term “basic pearlescent pigments” the pearlescent pigments without the weather resistant top coating are meant throughout this invention. They comprise a transparent substrate and at least one metal oxide layer with a refractive index > 1 .8.

[0016] Natural mica is usually the thinnest transparent substrate and has a high surface area. The high surface area requires higher amounts of the refractive index metal oxide and particularly of TiO2 leading to higher photoactivity. Therefore, a more resistant top coating is needed here.

[0017] The optical properties of basic pearlescent pigments based on synthetic substrates have very high-quality optical properties. By optically very high-quality optical properties it is meant in particular an excellent gloss and color purity of the pearlescent pigments in an application medium. A perturbation of these optical properties by the top coating is a severe drawback.

[0018] The transparent platelet-shaped substrates are preferably selected from natural mica platelets, synthetic mica platelets, glass platelets, SiO2 platelets, AI2O3 platelets, synthetic boehmite platelets, BiOCI platelets and mixtures thereof and more preferably selected from synthetic mica platelets, glass platelets, AI2O3 platelets and mixtures thereof. The platelet-shaped synthetic substrates are particularly preferably selected from the group consisting of glass platelets, synthetic mica platelets and mixtures thereof.

[0019] The above-named synthetic substrates are known from a series of patent applications and patents. If, e.g., the platelet-shaped synthetic substrate consists of glass platelets, those produced according to the processes described in EP 0 289 240 A1 , WO 2004 / 056716 A1 and WO 2005 / 063637 A1 are preferably used. The glass platelets which can be used as substrate can, for example, have a composition corresponding to the teaching of EP 1 980 594 B1.

[0020] The glass platelets, also called glass flakes, described in these documents are characterized by particularly homogeneous surfaces and thicknesses of the platelets. The thickness of the glass platelets in certain embodiments varies with a standard deviation of at most 20%, preferably of at most 15%, and most preferably of at most 10%. The average thicknesses of the glass platelets lie in a range of 20 to 2,000 nm, preferably in a range of 100 to less than 1 ,300 nm. These glass platelets are preferably those which are produced according to the following process (see EP 0 289 240 B1): feeding a stream of molten glass in a downwards direction into a rotating cup and allowing the glass melt to flow over the edge of the cup such that it is forced into a gap between two plates surrounding the cup, wherein the movement of the material takes place in a radial direction and is effected by a stream of air between the plates such that the radial stream is pushed in the radial direction such that it remains flat and the material, when it solidifies, is broken into flakes.

[0021] Particularly optically high-quality pearlescent pigments based on synthetic substrates are also known from EP 2 217 664 B1 . It can be learned here that substrates with a narrow size distribution surprisingly make it possible to provide pearlescent pigments with particularly high color purity and high gloss.

[0022] In an embodiment of the present invention, the weather stable pearlescent pigments according to the invention have a cumulative frequency distribution of a volume-averaged size distribution function with the values Dio, D50 and D90, wherein this cumulative frequency distribution has a span AD in a range of 0.7-1 .4. The span AD is calculated according to formula (I):

[0023] AD=(D9O-DIO) / D5O (I).

[0024] The span AD is used to characterize the particle size distribution. The smaller the span , the narrower is the particle size distribution. In particularly preferred embodiments, the weather stable pearlescent pigments according to the invention have a span AD in a range of 0.75 to 1 .5, further preferably in a range of 0.8 to 1 .4 and still further preferably in a range of 0.85 to 1.25. No sufficiently color-pure pearlescent pigments are obtained above a span AD of 1 .5. Within the framework of the customary methods, pearlescent pigments below a span of the size distribution of 0.7 can only be produced very laboriously and thus not economically.

[0025] Pearlescent pigments with the span AD values listed above as preferred have excellent color purities. This is particularly true for pearlescent pigments in which the substrate has an average thickness of 500 to 2,000 nm, further preferably of 500 to 1 ,300 nm. The aforementioned parameters of the size distribution function and average thickness of the pearlescent pigment substrate interact in a synergetic way with the weather stable coating according to the invention. In such optically high-quality pearlescent pigments even small optical losses as a result of insufficient stabilization on the one hand or a too thick or otherwise optically distorting weather stable layer (such as for example swelling layers) can have a very distorting effect.

[0026] The weather stable pearlescent pigments can have any average particle size D50. The D50 values of the pigments preferably lie in a range of 3 to 80 pm. The pigments preferably have a D50 value in a range of 5 to 63 pm, particularly preferably in a range of 7 to 56 pm and quite particularly preferably in a range of 9 to 40 pm.

[0027] The D10 values of the pigments preferably lie in a range of 1 to 25 pm. The pigments more preferably have a D10 value in a range of 2 to 21 pm, particularly preferably in a range of 3 to 18 pm and quite particularly preferably in a range of 4 to 14 pm.

[0028] The D90 values of the pigments preferably lie in a range of 6 to 250 pm. The pigments preferably have a D90 value in a range of 15 to 210 pm.

[0029] The D10, D50 or D90 values of the cumulative frequency distribution of the volume — averaged size distribution function, as obtained by laser diffraction methods, indicates that 10%, 50% or 90%, respectively, of the pigments according to the invention have a diameter which is equal to or smaller than the value indicated in each case. Here, the size distribution curve of the pigments is preferably determined with a device from Malvern (device: MALVERN Mastersizer 3000) according to the manufacturer’s instructions. The scattered light signals were evaluated according to the Fraunhofer approximation.

[0030] The average thickness of the platelet-shaped transparent substrates to be coated preferably lies in a range of 50 nm to 5000 nm, more preferably in a range of 60 nm to 3000 nm and particularly preferably in a range of 70 nm to 2000 nm.

[0031] In an embodiment of the invention, the average thickness for glass platelets as plateletshaped substrate to be coated preferably lies in a range of 750 nm to 1500 nm. Such glass platelets are widely available commercially. Thinner glass platelets offer further advantages. Thinner substrates lead to a smaller total layer thickness of the pigments according to the invention. For glass platelets, the average thickness is in a range of 100 nm to 700 nm, more preferably in a range of 150 nm to 600 nm, particularly preferably in a range of 170 nm to 500 nm and quite particularly preferably in a range of 200 nm to 400 nm, and they are thus also preferred substrates according to the invention.

[0032] In a further embodiment, the average thickness for synthetic mica as platelet-shaped substrate to be coated preferably lies in a range of 100 nm to 1200 nm, more preferably in a range of 120 nm to 900 nm, particularly preferably in a range of 140 nm to 600 nm and quite particularly preferably in a range of 150 nm to 450 nm.

[0033] If platelet-shaped transparent substrates below an average thickness of 50 nm are coated with, for example, highly refractive metal oxides, extremely fragile pigments are obtained which can break apart even when incorporated into an application medium which in turn results in a significant reduction in gloss. In addition, the times for coating these thin substrates with, for example, highly refractive metal oxides are very long because of the large specific surface areas, i.e. the surface area per weight unit of pigment, of these platelet-shaped transparent substrates, which gives rise to high production costs. Above an average substrate thickness of 5000 nm, the basic pearlescent pigments can be too thick overall. This can be associated with a poorer specific covering capacity, i.e. covered area per weight unit of pigment according to the invention, and a lower plane-parallel orientation in an application medium. A poorer orientation results in turn in a reduced gloss.

[0034] In a preferred embodiment the standard deviation of the thickness of the artificial substrate is 15% to 100%, particularly preferably 20 to 70% and quite particularly preferably 22 to 40%. Below a standard deviation of 15%, color-flop effect pigments are obtained. Above a standard deviation of 100%, much thicker pigments are contained in the entire pigment system such that poorer orientation and thus losses in gloss result.

[0035] The average thickness is determined using a cured coating film in which the effect pigments are orientated substantially plane-parallel to the base. For this, a cross-section of the cured coating film is examined under a scanning electron microscope (SEM), wherein the thickness of at least 100 pearlescent pigments is determined and statistically averaged.

[0036] The platelet-shaped transparent substrates used according to the invention can be coated or uncoated platelet-shaped substrates. For example, low refractive layers, for example in the form of AI2O3 and / or SiO2 can also be applied to the platelet-shaped transparent substrates. However, a highly refractive layer is preferably applied as outermost layer. An extremely thin layer of SnO2, which effects a rutilization of a subsequently applied TiO2 layer, can also be applied to the platelet-shaped transparent substrate. By a rutilization is meant that applied TiO2 is not present in the anatase structure but rather the formation of the rutile structure is induced. The rutile structure can, however, also be effected by applying SnO2 and TiO2 jointly with the result that a separate SnO2 layer is not absolutely necessary for the application of a TiO2 layer with rutile structure.

[0037] According to a preferred variant of the invention, uncoated platelet-shaped substrates are used.

[0038] The platelet-shaped substrates are coated with at least one highly refractive metal oxide layer in order to obtain the usual pearlescent effect based on interference. Within the framework of this invention, a highly refractive metal oxide layer is defined as a layer with a refractive index >1.8, preferably >2.0. Herein, the refractive index refers to literature values in the visible wavelength's region of the bulk materials.

[0039] The at least one highly refractive layer preferably contains or consists of metal oxides, metal hydroxides and / or hydrated metal oxides which are selected from the group consisting of TiO2, FegOs, FesCU, TiFegOs, FegTisOg, FeTiOs, ZnO, SnO2, CoO, CO3O4, ZrO2, CT2O3, VO2, V2O3, (Sn,Sb)O2 and mixtures thereof.

[0040] The at least one highly refractive metal oxide layer particularly preferably contains or consists of metal oxides, metal hydroxides and / or hydrated metal oxides which are selected from the group consisting of TiO2, FegOs, FesCU, TiFegOs, FegTisOg, FeTiOs and mixtures thereof.

[0041] In some embodiments, the transparent substrate is coated with only one (number: 1) highly refractive metal oxide layer which is preferably selected from the group consisting of TiO2, FegOs, TiFegOs, FegTisOg, FeTiOs and mixtures thereof.

[0042] In some embodiments the highly refractive metal oxide layer is composed of a mixture of TiO2 and Fe-oxide and contains Fe(ll) ions. Additionally a single layer of TiO2 may be present. Such pearlescent pigments are silvery, absorbing pearlescent pigments with optical properties reflecting a metallic look. These pearlescent pigments usually have optical properties such that the resulting color in reflection is essentially a neutral silver tone or a slightly colored tone and cover a color region from absorbing grey to anthracite shades. With respect to pearlescent pigments generally the color tone “anthracite” is also often referred to as “black”. In this invention the term “silvery, absorbing pearlescent pigments” is used for pearlescent pigments which have a combination of neutral silver or slightly colored reflection color and grey to anthracite absorption color tones providing a metallic-like characteristic. Examples of commercially available pearlescent pigments of this type are the Symic 604 series of Eckart GmbH, Germany.

[0043] In further embodiments, the substrate is coated with only one (number: 1) highly refractive metal oxide layer which consists of TiO2 in the rutile modification. By rutile modification (or rutile TiO2) is meant in the sense of this invention that at least 99 wt.-% of the TiO2 is present as rutile, wherein the indication in wt.-% refers to the total TiO2 content in the respective layer.

[0044] In a further, particularly preferred embodiment, the highly refractive coating comprises rutile TiO2, i.e. TiO2 in the rutile modification, in a quantity from a range of 30 to 80 wt.%, relative to the total weight of the weather resistant pearlescent pigment. The particularly intensive interference color shades of the third order typically lie in this range. The proportion of the rutile TiO2 coating further preferably lies in a range of 40 to 70 wt.%, still further preferably in a range of 45 to 60 wt.%, in each case relative to the total weight of the weather resistant pearlescent pigment. Such amounts of rutile TiO2 correspond, individually depending on the fineness and thus on the specific surface area of the platelet-shaped substrate, to a preferred average thickness of the rutile TiO2 layer in a range of 80-280 nm. The average thickness of the rutile TiO2 layer particularly preferably lies in a range of 100 to 270 nm.

[0045] At these layer thicknesses, as a rule an interference color of the third order may be obtained. Due to the large layer thickness of the rutile TiO2 layer the pigments have a particularly high photocatalytic activity. Accordingly, an undesired discoloration of a color or coating layer containing TiO2-containing pigments occurs very readily with the action of UV radiation.

[0046] In further preferred embodiments the basic pearlescent pigments have a so-called multilayer structure wherein at least one alternating sequence of high - low - high refractive index layers are employed. Preferred high refractive index layers herein are TiO2, Fe2O3 or mixtures thereof and preferred low refractive index layers are SiO2, AI2O3 or MgO.

[0047] Such multilayer pearlescent pigments have high chroma and depending on the optical thicknesses of the layers may also exhibit color flopping properties. Examples of commercially available products are the Lumina® Royal-series (Sun Chemicals), Automotive Multiorora® series and Multiorora® Axion series (both of CQV )or Setalic® series (Kuncai).

[0048] An alternative multilayer technology denotes to basic pearlescent pigments having a so- called “spacer layer” in between two high-refractive index layers. This spacer layer is rather porous having large cavities and certain so-called connections and has a very low effective refractive index. Such pearlescent pigments are described in EP 3034562 B1 , EP 3034563 B1 , EP 3034564 B1 and WO 2016 / 097421 A1 .

[0049] In this invention basic pearlescent pigments having a “silvery” or a “colored” interference color may be distinguished. For the purposes of this invention basic pearlescent pigments with a “silvery” interference color are basic pearlescent pigments whose chroma values C*15° are 20, preferably 18, more preferably 15, and very preferably 10. The chroma values C*15° of the basic pearlescent pigments with silvery interference color of the invention are preferably in a range from 1 to 20, more preferably in a range from 2 to 19, more preferably in a range from 3 to 18, and with particular preference in a range from 4 to 17.

[0050] In the case of multilayer pearlescent pigments these effect pigments may, in addition to their silver interference color, have a colored appearance when viewed from outside of the specular angle. This colored appearance may be induced, depending on the nature of the coating of the platelet-shaped transparent substrate, by the inherent color of the coating material and by its layer thickness. On account of the high luster, any absorption color possibly present will be outshone at the specular angle, and the overall appearance of the multilayer pearlescent pigment of the invention will be presented to the viewer as silver- colored. In other embodiments, the silver interference color may also have a slightly pastel hue at the specular angle, but the observer perceives a predominantly silver appearance.

[0051] The chroma values herein are determined from the following applications: a nitrocellulose varnish (Dr. Renger Erco Bronzemischlack 2615e; Morton) containing 6.0% by weight of basic pearlescent pigments, the % by weight figure being based on the total weight of the varnish, is applied, depending on the D50 value and pigment opaqueness, in a wet film thickness chosen to provide sufficient hiding power, to BYK-Gardner black / white drawdown charts (Byko-Chart 2851), and subsequently dried at room temperature. Then, using a BYK-MAC (BYK Gardner), colorimetric evaluations may be performed on these drawdown charts, with measurement taking place on the black background of the drawdown chart. The incident angle is 45° and the chroma value employed is the chroma value at an observation angle of 15°.

[0052] Likewise, basic pearlescent pigments with a “colored interference color” are defined under these conditions to have a C*15° > 20, preferably > 18, more preferably > 15, and particularly preferably >10. These lower limits of the C*i5°-values are to be interpreted complementary to the values defining a silver basic pearlescent pigment as disclosed above.

[0053] The color alterations of the basic pearlescent pigment by the weather stable top coating may affect the interference color or the mass tone. The mass tone is mainly affected when colored metal oxides like Ce-oxides are used. As this is not the case within this invention there is still a demand that the interference tone does not change too much. Such interference tone changes can be best measured on black underground and the details of which are specified in the experimental section.

[0054] Weather resistant top coating:

[0055] The weather stabilizing top coating comprises a first and a second metal oxide followed by an organofunctional surface modification layer. The weather stabilizing top coating has two variants. Variant a) comprises a first metal oxide, metal hydroxide or oxide hydrate of Zn followed by a second metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof. Preferably in variant a) the first and a second metal oxide of the weather stabilizing top coating consist of a first metal oxide, metal hydroxide or oxide hydrate of Zn followed by a second metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof. Variant b) consists of a mixture of the first metal oxide, metal hydroxide or oxide hydrate of Zn and the second metal oxide, metal hydroxide or oxide hydrate of Si, Al and / or Zr. In each variant (a or b) the carbon content of the pearlescent pigment is less than 0.50 wt.%, based on the total weight of the weather resistant pearlescent pigment. In variant a) the metal oxides are precipitated in a consecutive manner with the Zn oxide layer being the inner and the second metal oxide being the outer layer. In further embodiments of variant a) the second metal oxide is precipitated directly onto the first metal oxide layer of the top coating. In further embodiments of variant a) the top coating consists of the first metal oxide followed by the second metal oxide and the organofunctional surface modification layer.

[0056] The pearlescent pigments are usually not calcined after the weather stable top coating is applied. Therefore, the metal oxides may be a mixture of metal oxide, metal hydroxide or oxide hydrate, respectively. However, for simplification and in relation to analytical detection the first metal oxide, metal hydroxide or oxide hydrate is abbreviated as “ZnO” in this invention. Likewise, the second metal oxide, metal hydroxide or oxide hydrate is abbreviated as “SiO2, AI2O3 or ZrCh”, respectively in this invention. Generally the amount of ZnO is preferably in a range of 0.1 O to 1.50 wt.%, and more preferably in a range of 0.20 to 1 .30 wt.%, each based on the total amount of the pearlescent pigment. Below an amount of 0.10 wt.% the weather stability might be adversely affected. Above 1 .5 wt.% zinc oxide is precipitated in rather irregular manner which leads to roughened surface affecting the optical properties adversely.

[0057] The amount of zinc oxide can be preferably determined with XRF (X-ray fluorescence analysis) and may be expressed as wt.% of the weather resistant pearlescent pigment.

[0058] Generally the amount of the second metal oxide is preferably in a range of 0.80 to 4.00 wt.%, more preferably in a range of 0.90 to 3.50 and most preferably in a range of 1 .00 to 3.00 wt.%, each based on the total amount of the pearlescent pigment.

[0059] Particularly for variant a) the content of the second metal oxide is preferably in a range of 1 .3 to 4.0 wt.% and more preferably in a range of 1 .50 to 3.00 wt.%, each based on the total amount of the pearlescent pigment. The second metal oxide is selected from the group of SiO2, AI2O3 and ZrO2 and more preferably of SiO2 and AI2O3 and most preferably SiO2.

[0060] In variant b) it is preferred that the molar ratio of Zn in the first metal oxide layer to the metal of the second metal oxide in the weather stabilizing top coating is in a range of 0.20 to 0.40, more preferred in a range of 0.22 to 0.38 and most preferred in a range of 0.23 to 0.36. It turned out that for such ratios the pearlescent pigments are weather stable and have good optical properties. Below or above such ratios the pearlescent pigments may become agglomerated.

[0061] Such molar ratios may be determined by XRF, if the second metal, which can be Si, Al or Zr is not contained in the platelet base substrate of the pearlescent pigment. Otherwise the relative molar ratios may be determined by first preparing a sample of pearlescent pigments particles which are selectively deposited with a micromanipulator on a laser-cut Ge bar having a thickness of a few tens of pm, fixed and thinned to electron transparency using a focused Ga ion beam (FIB). Afterwards, the structure of the weather stable layer may be determined by TEM (transmission electron microscopy) and the molar ratio of the first and the second metal therein may be determined by EDX spectroscopy (energy- dispersive X-ray spectroscopy).

[0062] The organofunctional surface modification layer is necessary to compatibilize the surface of the effect pigment with the organic binder or polymer medium of the final application. Such surface modification is well known to improve adhesion and mechanical stability of coatings containing weather stable pearlescent pigments.

[0063] The term “organofunctional surface modification layer” should not be understood as a separate layer of organic material is analytically detectable as individual layer. It simply refers to the fact that this surface modification layer is deposited on top of the first and second metal oxide of the weather stable top coating.

[0064] In this invention it surprisingly turned out that the carbon content which is mainly determined by this surface modification layer is rather low. The carbon content is less than 0.50 wt.% based on the total weight of the weather resistant pearlescent pigment. In preferred embodiments the carbon content is less than 0.48 wt.% and more preferred less than 0.44 wt%, based on the total weight of the weather resistant pearlescent pigment. In a preferred embodiment the carbon content is in a range of 0.04 to 0.044 wt.% and more preferred in a range of 0.05 to 0.40 wt.%, each based on the total weight of the weather resistant pearlescent pigment.

[0065] To reach such low carbon contents most preferably water based pre-condensed organofunctional silanes are used as precursor for the organofunctional surface modification layer. Water-based, oligomeric silanes are generally known from EP 0 675 128 A2, EP 0 716 127 A2 and EP 0 716 128 A2. Using these particulate organofunctional silanes as organofunctional modification of weather resistant pearlescent pigments have been described in US 6,176,916 B1 , for example.

[0066] These water-based, oligomeric silanes contain at least one kind of functional binding group. By a functional binding group is meant, within the framework of this invention, a functional group which can interact chemically with the binder. The chemical interaction can consist of a covalent bond, a hydrogen bond or an ionic interaction. The choice of a suitable functional group depends on the chemical nature of the binder. A functional group that is chemically compatible with the functionalities of the binder is preferably chosen in order to allow good binding. This property is very important with regard to weatherresistant and UV-resistant pearlescent pigments, as a sufficiently strong adhesion between pigment and cured binder is achieved in this way. This can for example be verified in adhesion tests such as the cross cutting test with condensation water test loads according to DIN 50 017. Passing such a test represents a necessary precondition for the use of weather-resistant pearlescent pigments in an automotive coating.

[0067] These water-based, oligomeric silanes must contain amino groups as functional groups. The amino function is a functional group which can enter into one or more chemical interactions with most groups present in binders. This can comprise a covalent bond, such as e.g. with isocyanate or carboxylate functions of the binder, or hydrogen bridge bonds such as with OH or COOR functions or also ionic interactions. An amino function is therefore very suitable for the purpose of the chemical binding of the pearlescent pigment to different types of binder.

[0068] In further preferred embodiments the water-based, oligomeric silanes have alkyl groups of from 1 to 18 C atoms. The pigment surface is partially hydrophobized by the alkyl groups, which allows a repulsion of water and a better plane-parallel orientation in the application medium. The water-based, oligomeric silanes further preferably contain alkyl groups of from 2 to 10 C atoms and particularly preferably from 3 to 6 C atoms. The alkyl groups can be linear, branched and optionally cyclic.

[0069] The water-based, oligomeric silanes are preferably produced by mixing water-soluble aminoalkylalkoxysilanes of general formula II:

[0070] R-Si (R1)y(OR1*)3-y, (II) preferably aminopropyltriethoxysilane, aminopropylmethyldiethoxysilane, aminopropyltrimethoxysilane or aminopropylmethyldimethoxysilane, with water-insoluble alkyltrialkoxysilanes of general formula Illa:

[0071] R2— Si(OR1”>3 (Illa) preferably propyltrimethoxysilane, propyltriethoxysilane, methyltriethoxysilane, methyltrimethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, isobutyltrimethoxysilane or isobutyltriethoxysilane, and / or water-insoluble dialkyldialkoxysilanes of general formula IV:

[0072] AA'-Si(OR1***)2(IV) and being preferably dimethyldimethoxysilane, dimethyldiethoxysilane, methylpropyldimethoxysilane or methylpropyldiethoxysilane, and / or mixtures of waterinsoluble alkyltrialkoxysilanes and dialkyldialkoxysilanes of general formulae III and IV, wherein R is an aminofunctional organic group, R1, R1*, R1” and R1” independently represent a methyl or ethyl radical, R2represents a linear or cyclic or branched alkyl radical with 1 to 8 C atoms, A represents an unbranched or branched alkyl radical with 1 to 3 C atoms and A' represents an unbranched or branched alkyl radical with 1 to 3 C atoms and 0<y^1 . Water is added to the mixture and the pH of the reaction mixture is adjusted to a value between 1 and 8 and the alcohol already present and / or formed during the reaction is removed., preferably by distillation. Preferably the amount of water that is added during the distillative separation of the alcohol is equal to the amount of alcohol or alcohol / water mixture that is removed from the reaction medium. Monobasic acids are particularly suitable for adjusting the pH. Products prepared in this way also release no further alcohols through hydrolysis upon dilution with water and have a flashpoint well above 70° C.

[0073] The obtained water-based oligomeric silanes (water-based organopolysiloxane-containing compositions) are substantially free of organic solvents and have a flashpoint above 70° C. As the alkoxy groups have already been substantially hydrolyzed by mixing with water, less than 5 wt.-% alcohols (methanol, ethanol) are released by hydrolysis upon dilution with water. The oligomeric silane can also be produced by mixing Q moles of water-soluble aminoalkylalkoxysilanes of general formula II as described above with M moles of waterinsoluble alkylalkoxysilanes of general formula lllb:

[0074] R3— Si(OR1”)3(lllb)

[0075] Herein R1** represents a methyl or ethyl radical and R3represents a linear or cyclic or branched alkyl radical with 1 to 6 C atoms or a ureidoalkyl group of general formula V:

[0076] NH2— CO— NH— (CH2)b— , with 1 <b<6 (V) wherein preferably b=3, and 0<y^1 . The molar ratio is 0<M / Q^2, and water is added to the mixture, the pH of the reaction mixture is adjusted to a value between 1 and 8 and optionally the alcohol already present and / or formed during the reaction is removed.

[0077] The oligomeric silane can also be obtained by mixing water-soluble organosilanes of general formula VI:

[0078] H2N(CH2)f(NH)g(CH2)i— Si(CH3)h(OR°)(3-h) (VI) wherein 0^f^6, g=0 if f =0, g=1 if f>1 ,0^i^6.0^h^1 and R° is a methyl, ethyl, propyl or isopropyl group, with organosilanes of general formula VII: wherein 0<h^1 and R° represents a methyl, ethyl, propyl or isopropyl radical, preferably glycidyloxypropyltrimethoxysilane, and / or of general formula VIII :

[0079] H2C=CR'— COO(CH2)3— Si(CH3)h(ORO)(3-h) (VIII) wherein 0^h^1 , R° represents a methyl, ethyl, propyl or isopropyl radical and R' a methyl or hydrogen radical, preferably methacryloxypropyltrimethoxysilane, and a water-insoluble organosilane of general formula IX: R"— Si(CH3)h(OR°)(3-h) (IX) wherein 0^h^1 , R° represents a methyl, ethyl, propyl or isopropyl radical and R" a linear, branched or cyclic hydrocarbon radical with 1 to 8 C atoms, preferably propyltrimethoxysilane, in the molar ratio M=a / (b+c+d), wherein a is the total number of moles of organosilanes according to Formula VI, b the total number of moles of organosilanes according to Formula VII and c the total number of moles of organosilanes according to formula VIII and d the total number of moles of organosilanes according to formula IX, with 0^M^3, in particular for M equal to 0 with a equal to 0 and / or c equal to d equal to 0 and b^1 and preferably 0.5^M^3, adding a water / acid mixture to the mixture, adjusting the pH of the reaction mixture to a value between 1 and 8 and optionally removing the alcohol already present and / or formed during the reaction.

[0080] Preferably the amount of water that is added during the distillative separation of the alcohol is equal to the amount of alcohol or alcohol / water mixture that is removed from the reaction medium. Monobasic acids are particularly suitable for adjusting the pH. Products prepared in this way also release no further alcohols through hydrolysis upon dilution with water and have a flashpoint well above 70° C.

[0081] This results, for example, in compounds with the following approximate structure:

[0082] [HO[Si(A)(OH)Z(CH3)(i-z)O]a[Si(B)(OH)Y(CH3)(i-Y)O]b[Si(C)(OH)W(CH3)(i-w)O]c[Si(D)(OH)v (CH3)(i-v)O]d[H (HX)]e (X) wherein:

[0083] A=aminoalkyl radical derived from general formula VI, B=glycidetheralkyl radical derived from general formula VII, C=acryloxyalkyl or methacryloxyalkyl radical of general formula VIII, D=alkyl radical of general formula IX, HX=monobasic acid, wherein X=inorganic or organic acid radical, such as e.g. chloride, nitrate, formate, acetate, v is equal to 0 or 1 and w is equal to 0 or 1 and y is equal to 0 or 1 and z is equal to 0 or 1 and a+b+c+d^4 and a^e^2a, with 0^a / (b+c+d)^3, in particular for a / (b+c+d) is equal to 0 with a=0 and / or c is equal to d is equal to 0 and b^1 and for 0.5^a / (b+c+d)^3. The task of the silicon-functional hydroxyl groups is to form chemical bonds to the hydroxyl groups of the metal oxide layer on the pearlescent pigment surface. A stable bond is thereby formed between silane and pigment surface.

[0084] The organofunctional groups of the oligomeric silane have the task of producing bonds to the polymer of the waterborne coating. As the oligomers can be provided with several functional groups which are different from each other, the pigment can be used in different waterborne coating systems. Being provided with methacryl and amino groups means, for example, that the pigment can be used for waterborne coating systems with polyester as polymer and for waterborne coating systems with polyurethane as polymer.

[0085] The obtained water-based oligomeric silanes (water-based organopolysiloxane-containing compositions) are advantageously substantially free of organic solvents and have a flashpoint above 70° C. As the alkoxy groups have already been substantially hydrolyzed by mixing with water, less than 5 wt-% alcohols, such as e.g. methanol or ethanol, are released through hydrolysis upon dilution with water.

[0086] Examples of water-based oligomeric silanes are aqueous, alcohol-free aminosilane hydrolyzate (Dynasylan Hydrosil 1151), aqueous, alcohol-free amino / alkylfunctional siloxane co-oligomer (Dynasylan Hydrosil 2627), aqueous, alcohol-free diamino / alkylfunctional siloxane co-oligomer (Dynasylan Hydrosil 2776), aqueous, alcohol- free amino / vinylfunctional siloxane co-oligomer (Dynasylan Hydrosil 2907), aqueous, alcohol-free amino / alkylfunctional siloxane co-oligomer (Dynasylan Hydrosil 2909), aqueous, alcohol-free amino / alkylfunctional siloxane co-oligomer (Dynasylan Hydrosil 2909, Hydrosil 2926) or aqueous, alcohol-free amino / methacrylatefunctional siloxane co- oligomer (Dynasylan Hydrosil 2929), oligomeric diaminosilane system (Dynasylan 1146).

[0087] In further embodiments the ratio of the C-content in wt.% and the ZnO content in wt.% is preferably in a range of 0.10 to 0.90, more preferably in a range of 0.11 to 0.88, and most preferably in a range of 0.12 to 0.85. If this ratio is below of 0.10 the organofunctional silanes do not provide enough organic functions in order to interact with the organic binder. If the ratio is above 0.90 the organofunctional silanes usually have a too large amount of hydrophobic alkyl groups leading to a bad intermediate adhesion in the final application.

[0088] Method of manufacturing weather stable pearlescent pigments: Disclosed herein is a method of manufacturing the weather resistant pearlescent pigment comprising the following steps: i) suspending the basic pearlescent pigments in a solvent, iia) coating the basic pearlescent pigments from step i) in the solvent using a Zn oxide precursor at a predetermined pH1 , and obtain a Zn oxide layer on the basic pearlescent pigments, and iib) contacting the basic pearlescent pigment having a Zn oxide layer from step iia) at a predetermined pH2 with a precursor of the second metal oxide to form a layer of metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof or iic) coating the basic pearlescent pigments from step i) at a predetermined pH3 by simultaneously and continuously adding a solution of a Zn oxide precursor and a solution of a precursor of the second metal oxide, wherein the feed rate of one of these precursor solutions is coupled with the control of the pH value and forming a mixed layer of metal oxide, metal hydroxide or oxide hydrate of Zn and of metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof, iii) contacting the coated pearlescent pigments from step iib) or step iic) in the solvent with organofunctional silanes or water based pre-condensed organofunctional silanes, and iv) separating the coated pearlescent pigments, optionally washing with solvent, and drying at a temperature in a range of 80 to 160°C.

[0089] Steps iia) and iib) describe the manufacture of variant a) of the weather resistant pearlescent pigment and step iic) describes the manufacture of variant b) of the weather resistant pearlescent pigment. The other steps are the same for both variants.

[0090] The solvent can be water or an organic solvent. Preferred organic solvents are alcohols like methanol, ethanol, n-propanol, isopropanol, n-butanol, 2-methylpropanol, 2- methoxypropanol, butyl glycol, etc. Also possible are mixtures of these alcohols in any desired proportions. Preferred alcoholic solvents are ethanol, isopropanol or mixtures thereof. Water is preferred as solvent and in this case the precursor materials for the coatings of metal oxides should be well water soluble. For the first metal oxide chlorides or nitrates of zinc are preferably used. For the second metal oxide coating preferably waterglass for SiO2 and chlorides or nitrates of Al or Zr are used.

[0091] For preferred embodiments of variant a) the pH1 value is in a range of 5.0 to 8.5 and more preferred in a range of 5.5 to 8.0. If pH1 is above 8.5 zinc oxide is precipitated too fast leading to an irregular layer with adverse effect to the optical properties. If pH1 is below 5.0 the rate of precipitation is too low.

[0092] In other preferred embodiments the precursor of the second metal oxide is a water glass solution and pH2 is then in a range of 6.0 to 10.8 and more preferably in a range of 6.5 to 10.6 for producing a pearlescent pigment according to variant a).

[0093] For variant b) the pH3 is preferably in a range of 6.5 to 10.8 and more preferably in a range of 6.7 to 10.6.

[0094] For variant a) pH1 may also be essentially the same as pH2 and for this case pH1 and pH2 are preferably in a range of 6.5 to 8.0 and more preferably in a range of 6.7 to 7.8.

[0095] A preferred precursor for manufacturing the zinc oxide layer is based on ZnCh. Aqueous solutions of this salt are acidic while water glass solutions are typically basic. Therefore, the different nature of these precursor solutions can be used to affect the pH value of the whole reaction dispersion. Particularly for manufacture of variant b) the water glass solution can be used to control pH3. Depending on the concentrations of the initial solutions of zinc chloride and water glass and the envisaged amounts of ZnO and SiO2 in the top coating the feed rate of the water glass solution may be coupled with a pH measurement and thus be adjusted to keep the pH3 value essentially constant at a predetermined value. In other embodiments the pH can be controlled by co-addition of either acidic solutions (for example HCI solution) or basic solutions (for example NaOH solution) while adding precursor solutions of the metal oxides, as well known in the art.

[0096] The temperature for the coating reactions is preferably in a range of 50 °C to the boiling point of the solvent used, more preferably in a range of 55 °C to 85 °C. The drying step vi) can be done at ambient pressure or under vacuum.

[0097] A further object of this invention is solved by the use of the weather resistant pearlescent pigments in coatings, printing inks, powder coatings, architectural coatings or plastics, preferably in automotive coatings.

[0098] A further embodiment of this invention are formulations of coatings, preferably automotive base coat formulations, powder coatings, architectural coatings, printing inks or plastics containing the weather resistant pearlescent pigments. EXAMPLES

[0099] A Manufacture of Examples and Comparative Examples:

[0100] Example 1 (Co precipitation):

[0101] 100 g of commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 10-50 pm (Symic C604, Eckart GmbH) was suspended in 900 g of water. Subsequently, the dispersion was heated to 70 °C and pH was adjusted to 7.0. 12.0 g solution of zinc chloride w(ZnCl2)= 7.0 wt.% was added continuously and in parallel a water glass solution (8.5 wt.-% SiO2) was added continuously to keep pH at 7.0. The feed rate of the water-glass solution was coupled with a pH meter in order to keep the pH constant.

[0102] Next the suspension was stirred for 1 h before adding a solution of 5.7 g of Hydrosil 2627 diluted with 24.3 g of demineralized water. After additional stirring for 180 minutes, suspension was filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under 100 mbar vacuum.

[0103] Example 2:

[0104] 100 g of commercially available interference red pearlescent pigment based on glass flakes of fineness 10-60 pm (Luxan C241 , Eckart GmbH) was suspended in 900 g of water. Subsequently, the dispersion was heated to 70 °C before 21.6 g solution of zinc chloride w(ZnCl2)= 7.0 wt.% was added. By continuously adding 10% KOH solution, pH was adjusted to 8.5. Thereafter, 61 .7 g water glass solution (2.5 wt.-% SiO2) was slowly introduced into the suspension where the pH value was kept constant at pH 8.5 by adding 10 wt.-% HCI solution.

[0105] After the solution was completely added, suspension was stirred for 1 h before adding a solution of 7.5 g of Hydrosil 2776 diluted with 22.5 g of demineralized water. After additional stirring for 180 minutes, suspension was filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under 100 mbar vacuum. The weather stable pearlescent pigment had a C*15° of 21 .21 compared to 20.42 for the base pearlescent pigment when applied in a draw down as described in section B and measured with a Byk-Mac apparatus.

[0106] Example 3:

[0107] 100g of commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 10-50 pm (Symic C604, Eckart GmbH) was suspended in 900 g of water. Subsequently, the dispersion was heated to 70 °C before 21 .6 g solution of zinc chloride w(ZnCl2)= 7.0 wt.% was added. By continuously adding 10% KOH solution, pH was adjusted to 8.5. Thereafter, 102.8 g water glass solution, 2.5 wt.-% SiO2, was then slowly introduced into the suspension where the pH value was kept constant at pH 8.5 by adding for example 10 wt.-% HCI solution.

[0108] After the solution was completely added, the suspension was stirred for 1 h before adding a solution of 7.5 g of Hydrosil 2627 diluted with 24.3 g of demineralized water. After additional stirring for 180 minutes, suspension was filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under 100 mbar vacuum.

[0109] Example 4 (Co precipitation):

[0110] 100 g of commercially available silvery pearlescent pigment based on titanium dioxide coated synthetic mica of fineness 30-120 pm (Symic E001 , Eckart GmbH) was suspended in 900 g of water. Subsequently, the dispersion was heated to 70 °C and pH was adjusted to 7.0. 12.0 g solution of zinc chloride w(ZnCl2)= 7.0 wt.% was added continuously and in parallel a water glass solution (8.5 wt.% SiO2) was added continuously to keep pH at 7.0. The feed rate of the water-glass solution was coupled with a pH meter in order to keep the pH constant. After the solution was completely added, the suspension was stirred for 1 h before adding 2.0 g Dynasilan 1189. After additional stirring for 180 minutes, suspension was cooled, filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under 100 mbar vacuum.

[0111] Example 5 (Co precipitation):

[0112] 100g of commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 1 -15 pm (Symic A604, Eckart GmbH) was suspended in 900 g of water. Subsequently, the dispersion was heated to 70 °C and pH was adjusted to 7.0. 12.0 g solution of zinc chloride w(ZnCl2)= 7.0 wt.% was added continuously and in parallel a water glass solution (8.5 wt.-% SiO2) was added continuously to keep pH at 7.0. The feed rate of the water-glass solution was coupled with a pH meter in order to keep the pH constant.

[0113] After the solution was completely added, the suspension was stirred for 1 h before adding 2.0 g Dynasilan 1189. After additional stirring for 180 minutes, suspension was cooled, filtered off and the filter cake was washed with demineralized water before it was dried at

[0114] 95 °C under 100 mbar vacuum.

[0115] Example 6 (Co precipitation):

[0116] 100 g of commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 10-50 pm (Symic C604, Eckart GmbH) was suspended in 900 g of water. Subsequently, the dispersion was heated to 70 °C before 21 .6 g solution of zinc chloride w(ZnCl2)= 7.0 wt.% was added continuously. In parallel a water glass solution (8.5 wt.-% SiO2) was added continuously until a final pH of 10.5 was reached.

[0117] After the solution was completely added, the suspension was stirred for 1 h before adding a solution of 5.7 g of Hydrosil 2627 diluted with 24.3 g of demineralized water. After additional stirring for 180 minutes, suspension was filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under 100 mbar vacuum.

[0118] Example 7a:

[0119] 100 g of commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 10-50 pm (Symic C604, Eckart GmbH) was suspended in 900 g of water. Subsequently, the dispersion was heated to 70 °C before

[0120] 21 .5 g solution of zinc chloride w(ZnCh) = 7.0% was added. At the same time, the pH value was kept constant at 5.0 by adding a 10% KOH solution. Afterwards the pH was adjusted to 8.5 by adding of 10% KOH solution. Thereafter, 102.8 g water glass solution,

[0121] 2.5 wt.-% SiO2, was then slowly introduced into the suspension while the pH value was kept constant at pH 8.5 by adding 10 wt.% HCI solution.

[0122] After the solution was completely added, the suspension was stirred for 1 h before adding a solution of 7.5 g of Hydrosil 2909 diluted with 24.3g of demineralized water. After additional stirring for 180 minutes, suspension was filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under vacuum.

[0123] Example 7b:

[0124] Preparation of Example 7b used the same procedure as Example 7a but only 11 .9 g solution of zinc chloride w(ZnCh) = 7.0 wt.% were added.

[0125] Example 7c:

[0126] Preparation of Example 7c used the same procedure as Example 7a but only 11.9 g solution of zinc chloride w(ZnCh) = 7.0wt.% and 45.2g of water glass solution were added. Example 7d:

[0127] Preparation of Example 7d used the same procedure as 7a but only 45.2g of water glass solution were added.

[0128] Example 8a: Co - precipitation

[0129] 100g of commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 10-50 pm (Symic C604, Eckart GmbH) was suspended in 900 g of water. Subsequently, the dispersion was heated to 70 °C before

[0130] 21 .5 g solution of zinc chloride w(ZnCl2) = 7.0 wt.% was added. 93.3 g water glass solution (8.5 wt.-% SiO2) were added continuously and a final pH of 7.0 was reached.

[0131] After the solution was completely added, the suspension was stirred for 1 h before adding

[0132] 7.5 g of Hydrosil 2909 diluted with 24.3 g of demineralized water. After additional stirring for 180 minutes, suspension was filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under vacuum.

[0133] Example 8b :

[0134] Like Example 8a, but only 11 .9 g solution of zinc chloride w(ZnCh) = 7.0 wt.% and only 49.0 g of water glass solution were added.

[0135] Example 9a:

[0136] 100g of commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 10-50 pm (Symic C604, Eckart GmbH) was suspended in 900 g of water. Subsequently, the dispersion was heated to 70 °C before

[0137] 21.5 g solution of zinc chloride w(ZnCL) = 7.0 wt.% was added. By continuously adding 10% KOH solution, pH was adjusted to 7.5. Thereafter, 102.8 g water glass solution (2.5 wt.-% SiO2) was slowly introduced into the suspension while the pH value was kept constant at 7.5 by adding 10 wt.% HCI solution.

[0138] After the solution was completely added, the suspension was stirred for 1 h before adding a solution of 7.5 g of Hydrosil 2909 diluted with 24.3 g of demineralized water. After additional stirring for 180 minutes, suspension was filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under vacuum.

[0139] Example 9b:

[0140] Like Example 9a, but only 11 .9 g solution of zinc chloride w(ZnCh) = 7.0 wt.% were added. Example 9c:

[0141] Like Example 9b, but only 45.2 g water glass solution (2.5 wt.% SiO2) were added.

[0142] Example 9d (AM-20-608 adjusted:

[0143] Like Example 9a, but only 45.2 g water glass solution (2.5 wt.% SiO2) were added.

[0144] Example 10:

[0145] Like Example 9c, but the amount of Hydrosil 2909 was 30.0 g diluted with 100.0 g of demineralized water.

[0146] Comparative Example 1 : According to Example 1 of US 2013 / 0213260 A 100 g of commercially available blue interference pearlescent pigment based on titanium dioxide coated synthetic mica of fineness 10-50 pm (Symic C261 , Eckart GmbH) was suspended in 1000 g of water. Subsequently, the dispersion was heated to 55 °C, while stirring and the pH of the slurry was adjusted to 2.1 with HCI solution. 32.6g Cerium chloride solution 8.09 wt.% was added into the slurry.

[0147] Next, a mixture of 1 .33 g zirconium oxychloride *8H2<D, 0.43g sodium hypophosphite and 15 g HCI solution 8 wt.% was simultaneously added dropwise for 60 minutes. In the meantime, the pH is kept at 2.1 with NaOH solution. After 30 minutes, the pH was adjusted to 5.5 with NaOH solution, 31.6 g aluminum chloride *6H2O solution 15 wt.% and sodium hydrate solution were simultaneously added dropwise at pH 5.5.

[0148] Next, the pH was adjusted to 6.2 and zinc chloride solution 4.85 wt.% and sodium hydroxide solution were simultaneously added dropwise at pH 6.2. Then, a mixture of 1 .0 g Dynasilan MEMO, 1 .0 g Dynasilan GLYMO, 2.0 g Dynasilan 9116 was added into the solution and was kept for 15 minutes. Afterwards, the pH was raised to 7.0 with NaOH solution and kept for 1 hour. Afterwards the slurry was cooled, filtered off and washed with ion exchanged water. The cake was dried at 130°C. for 2.5 hrs. The dried sample was sieved by 32pm sieve.

[0149] Comparative Example 2:

[0150] Commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 10-50 pm, Symic C604, Eckart GmbH (D50 of 22 pm) without any weather resistant top coat. Comparative Example 3 in accordance with EP 1682622 A1 :

[0151] 100 g of commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 10-50 pm (Symic C604, Eckart GmbH) was suspended in 800 g of water. Subsequently, the dispersion was heated to 70 °C and at a pH of 6.5 a solution consisting of 0.95 g CeNO3*6H2O dissolved in 40mL deionized water was added. At the same time, the pH value was kept constant by adding a 10 wt.% KOH solution. Thereafter, the pH value was raised with 5 wt.% KOH solution to 7.5 and stirred for 15 min.

[0152] A water glass solution (17.1 g water glass solution, 6.0 wt.% SiO2, mixed with 7.9 g of demineralized water) was then slowly introduced into the suspension where the pH value kept constant at pH 7.5.

[0153] After the solution was completely added, the suspension was stirred for 1 h before adding 1.5 g of Dynasilan 6490. After additional stirring for 180 minutes, suspension was filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under 100 mbar vacuum. The pigment had a Ce2O3 content of 0.3 wt.%, based on the total weight of the pigment.

[0154] Comparative Example 4:

[0155] 100 g of commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 10-50 pm (Symic C604, Eckart GmbH) was suspended in 800 g of water. Subsequently, the dispersion was heated to 70 °C before 61 .7 g water glass solution, 2.5 wt.% SiO2, was then slowly introduced into the suspension where the pH value was kept constant at pH 7.5 by adding 10 wt.% HCI solution. After solution was completely added, suspension was stirred for 1 h before adding 21.6 g of a solution of zinc chloride w(ZnCl2)= 7.0wt.%. By continuously adding 10% KOH solution, pH was adjusted to 7.5. Thereafter the suspension was stirred for 1 h before adding a solution of 7.5 g of Hydrosil 2909 diluted with 22.5g of demineralized water. After additional stirring for 180 minutes, suspension was filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under 100 mbar vacuum.

[0156] Comparative Example 5a: single precipitation vice versa

[0157] 100 g of commercially available silvery pearlescent pigment based on titanium dioxide and iron oxide coated synthetic mica of fineness 10-50 pm (Symic C604, Eckart GmbH) was suspended in 900 g of water. Subsequently, the dispersion was heated to 70 °C and pH adjusted before 102.8 g water glass solution, 2.5 wt.% SiO2, was then slowly introduced into the suspension. pH value was kept constant at pH 7.5 by adding 10 wt.% HCI solution. After 30 min additional stirring pH was adjusted to 5.0 before 21 .5 g solution of zinc chloride w(ZnCl2) = 7.0 wt.% was added continuously (corresponding to theoretically 0.9 wt.% of ZnO). By adding a 10 wt.% KOH solution the pH value was kept constant. After the solution was completely added, the suspension was stirred for 1 h before adding a solution of 7.5 g of Hydrosil 2909 diluted with 24.3 g of demineralized water. After additional stirring for 180 minutes, suspension was filtered off and the filter cake was washed with demineralized water before it was dried at 95 °C under vacuum.

[0158] Comparative Example 5b:

[0159] Analogous to Comparative Example 5a, but only 11 .9 g of zinc chloride solution were added (corresponding to theoretically 0.5 wt.% of ZnO).

[0160] Comparative Example 5c:

[0161] Analogous to Comparative Example 5b, but only 45.2 g of the water glass solution were added.

[0162] Comparative Example 5d:

[0163] Analogous to Comparative Example 5a, but only 45.2 g of the water glass solution were added.

[0164] Comparative Example 6:

[0165] Like Example 10c, but the amount of Hydrosil 2909 was 57.0 g diluted with 190.0 g of demineralized water.

[0166] In table 1 all experimental parameters (especially amounts of SiO2 added by water glass solution) and / or analytical results regarding the composition of the top-layer are summarized. Table 1 : Experimental parameters and top coating composition and structure of the samples

[0167] * Measured with XRF and calculated in wt.% as ZnO, if not indicated otherwise

[0168] ** Theoretical amount in wt.% related to amount of basic pearlescent pigment

[0169] **** Measured with XRF and calculated in wt.% as respective metal oxide, if not indicated otherwise DS = Dynasylan

[0170] B Testing methods:

[0171] B1 Photocatalytic Activity: In order to determine the photocatalytic activity of the pigment powders a special gas phase photoreactor was used, presented schematically in Figure 1 .

[0172] 150 mg of pigment powder was weighed in the center of a petri dish which afterwards was been placed at the bottom of the photoreactor. The temperature of the photoreactor was adjusted and kept constant at 50°C.

[0173] The photoactivities of the samples were measured in ambient air which circulates by using a gas pump through a FTIR measurements system and back to the photoreactor. The initial water and CO2 contents of the air were about 0.2% and 400 ppm, respectively.

[0174] As model pollutant 1 pL of acetone was injected over a septum inside the system. After the pollutant was homogeneously distributed the UV-lamp (90 mW / cm2at 405 nm) was switched on and acetone started to degrade forming CO2 quantitatively.

[0175] The degradation of acetone and formation of CO2 was monitored with a FTIR spectrometer as a function of time and fits to a 1 st order reaction kinetics and 1st order rate constants were obtained. The initial formation rates of acetone and carbon dioxide are approximated linearly and the formation rates are obtained in ppm / h units.

[0176] The higher the CO2 formation in ppm per hour, the higher the photocatalytic activity of the pigment powder is. The test was passed if the rate of CO2 formation was 1 ppm / h or lower.

[0177] For each test series a zero spectra needed to be imposed beforehand, by using same procedure without pigment and acetone injection. The values of this blind test were subtracted from the values of the samples with pigments.

[0178] B2: Color-Shade Constancy and Optical Properties of sample compared with the Starting Material without top coating:

[0179] The samples were applied as doctor-blade drawdowns on black-white opacity charts (Byko-Chart 2853, Byk Gardner) using a 40 pm spiral bar for samples based on synthetic mica and a 50 pm spiral bar for samples based on glass flakes. The effect pigments were dispersed in a conventional nitrocellulose coating (Dr. Renger Erco bronzing mixed varnish 2615e; from Morton, pigmentation of 6.0 wt.-% relative to the total weight of the wet varnish). In comparison to each sample without the weather stable top coating the change in color was determined using a MINOLTA CM-700d apparatus in diffuse reflection. The color values on the black background of the opacity charts were measured and the differences were expressed with the well-known Hunter formula:

[0180] AE* = (AL*2+ Aa*2+ Ab*2)1 / 2

[0181] Such differences are characteristic for changes of the interference color tone of the pearlescent pigment by applying the weather stable top coating. An overall color change AE* of < 2.00 was acceptable. Results are depicted in table 2, fifth column.

[0182] Gloss and chroma of samples compared to the initial pearlescent pigments without top coating were further evaluated on a qualitative basis. In case of pearlescent pigments having Fe-oxides in the interference layers additionally the hiding power was evaluated. This qualitative evaluation was denoted the following note system:

[0183] 1 : optical properties like basic pearlescent pigment, very good

[0184] 2: optical properties almost like basic pearlescent pigment with a little gritty texture, good 3: optical properties almost like basic pearlescent pigment but gritty texture, not acceptable

[0185] 4: agglomerated

[0186] 5: strongly agglomerated

[0187] Only notes 1 and 2 are acceptable. The results are depicted in table 2 (last column).

[0188] B3 2 -coat pneumatic gun application:

[0189] A white grounded aluminum test panel was painted with a 1 K aqueous-based test base coat varnish comprising acrylate-polyurethane binder and pearlescent pigments of the examples and comparative examples in an amount of 1 .5 wt.% using an Oerter APL1 .2 spray gun application. The wet-thickness of the coatings were 13 -17 pm and the coated panels were dried at 80 °C for 12 min.

[0190] A 1 K conventional acrylate-based lacquer was used as clearcoat at a thickness of 37 - 43 pm and a drying temperature of 140 °C for 30 min. The use of a 1 K clearcoat instead of the 2K clearcoat used conventionally enhances the testing conditions.

[0191] B4 Strengthened weathering test (combined Xenon and condensation water test): The panels obtained by B3 were subjected to Xenon accelerated weathering test according to SAE J 2527 for 1000h. Directly afterwards the panels were subjected to a 72 h condensation water test according to DIN EN ISO 6270-2 at a temperate of 40°C.

[0192] The gloss was measured using a 20° gloss meter (micro-TRI-gloss of BYK Gardener GmbH) according to DIN EN ISO 2813 before and after the testing and expressed as difference in % relative to the gloss before testing. A decrease of the relative gloss Agloss 20° of less than -40% was acceptable. Results are depicted in table 2, sixth column.

[0193] B5 Condense water cross cut test:

[0194] The pearlescent pigment samples were incorporated into a waterborne coating system and the test applications produced by spray painting. The base coat was overcoated with a 1 K clear coat customary in the trade and then stoved. These applications were tested according to DIN 50 017 (condensation water — constant climates). The adhesion was tested by means of cross cutting according to DIN EN ISO 2409 immediately after completion of the test in comparison with the unloaded sample. Here, Gt 0 means no change and Gt 5 a very significant change. Gt 0 and Gt 1 are acceptable. Results are depicted in table 2, seventh column.

[0195] B6 Carbon content determination:

[0196] The carbon-content of all samples was determined by a combustion of the sample in an oxygen stream and detection of evolving CO2 by IR spectroscopy using an analyzer of LECO instrument GmbH, Germany.

[0197] B7 XRF Analysis:

[0198] The Zn-content of the samples and the metal oxide of the rare earth metals of Comparative Example 1 were determined by means of x-ray fluorescence analysis (XRF). For this purpose, the respective pigments were incorporated into a lithium tetraborate glass tablet, fixed in solid sample measuring cups and analyzed therefrom. The measuring instrument used was the Advantix ARL system from Thermo Scientific. The measurements were first calibrated with appropriate standard methods. The results are listed in table 1 . The Si- contents of the top coating of the samples could not be determined with this method, because the transparent synthetic mica or glass substrates already have a high amount of silica. Instead, the theoretical values assuming 100% reaction rates are depicted in table 1 . C Results and Discussion:

[0199] The results of the testing are expressed in table 2.

[0200] Table 2: Test results of samples

[0201] All tests were passed by the inventive Examples. The test of color change constancy compared to pearlescent pigments without the weather stable top coating (AE* of < 2.00) was passed by all samples, but not by Comparative Example 1 where Ce2O3 was used in the top coating (according to Example 1 of US 2013 / 0213260 A). In Comparative Example 3 a top-coating was used which was made according to the teaching of EP 1682622 A1 and here the photoactivity was too high and the strengthened weathering test was not passed.

[0202] In Examples 7a to 7d the top layer had consecutive layers of first ZnO followed by SiO2 at two different amounts of ZnO and of SiO2. Examples 9a to 9d were manufactured analogously, but the pH was held constant here. Comparative Examples 5a to 5d were conducted the similar like the Example 7 series with respect to the variations of ZnO and SiO2 amounts, but the first layer here was SiO2 followed by ZnO which is most preferred variant in US 2013 / 0213260 A. Here Com. Examples 5c and 5d didn't pass the strengthened weathering test but had good optical properties. Comparative Examples 5a and 5b passed the strengthened weathering test, but the optical properties were not good. Most of the Comparative Examples didn't pass the cross cut test. Clearly and surprisingly a layer order ZnO / SiO2 is better than the order SiO2 / ZnO in the top layer.

[0203] Regarding a mixed layer of ZnO and SiO2 Examples 8a and 8b revealed good results. In all inventive example the carbon content was rather low (below of 0.50 wt.%). Here, water based pre-condensed organofunctional silanes are used as precursor for the organofunctional surface modification layer throughout. When using such kind of oligomeric silanes only a small amount seems to be adsorbed on the surface of the pearlescent pigments. In contrast, in Comparative Example 6 a very large amount of such organofunctional surface modification layer was employed leading to a carbon content of 0.48 wt.%. In this Comparative Example the cross cut test was not passed and the sample was agglomerated. The strengthened weathering test was passed, but this could be also a consequence of the agglomeration which leads to an overall small pigment surface.

Claims

Claims:1 . Weather resistant pearlescent pigment comprising a basic pearlescent pigment which comprises a transparent substrate and at least one metal oxide layer with a refractive index > 1 .8 and thereon a weather stabilizing top coating which comprises a first and a second metal oxide followed by an organofunctional surface modification layer, wherein the weather stabilizing top coating can comprise variant a) or variant b), variant a) comprising a first metal oxide, metal hydroxide or oxide hydrate of Zn followed by a second metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof, variant b) consisting of a mixture of the first metal oxide, metal hydroxide or oxide hydrate of Zn and the second metal oxide, metal hydroxide or oxide hydrate of Si, Al and / or Zr, and wherein in each variant a) or b) the carbon content of the pearlescent pigment is less than 0.50 wt.%, based on the total weight of the weather resistant pearlescent pigment.

2. Weather resistant pearlescent pigment according to claim 1 , wherein the amount of ZnO is in a range of 0.30 to 1.50 wt.%, based on the total amount of the pearlescent pigment.

3. Weather resistant pearlescent pigment according to any of the preceding claims, wherein for variant a) the content of the second metal oxide is in a range of 1 .3 to 4.0 wt.%, based on the weather resistant pearlescent pigment.

4. Weather resistant pearlescent pigment according to claim 1 , wherein for variant b) the molar ratio of Zn in the first metal oxide layer to the metal of the second metal oxide in the weather stabilizing top coating is in a range of 0.20 to 0.40.

5. Weather resistant pearlescent pigment according to any of the preceding claims, wherein the carbon content is less than 0.48 wt.%, based on the total weight of the weather resistant pearlescent pigment.

6. Weather resistant pearlescent pigment according to any of the preceding claims, wherein water based pre-condensed organofunctional silanes are used as precursor for the organofunctional surface modification layer.

7. Weather resistant pearlescent pigment according to any of the preceding claims, wherein the ratio of the carbon content in wt.% and the ZnO content in wt.% is in a range of 0.10 to 0.90.

8. Weather resistant pearlescent pigment according to any of the preceding claims, wherein the transparent platelet-shaped substrate from the basic pearlescent pigment is selected from the group consisting of natural mica platelets, synthetic mica platelets, glass platelets, SiO2 platelets, AI2O3 platelets, synthetic boehmite platelets, BiOCI platelets and mixtures thereof.

9. Weather resistant pearlescent pigment according to any of the preceding claims, wherein in variant a) the second metal oxide is precipitated directly onto the first metal oxide layer in the top coating.

10. Weather resistant pearlescent pigment according to any of the preceding claims, wherein in variant a) the weather stabilizing top coating consists of the first metal oxide followed by the second metal oxide and the organofunctional surface modification layer.

11. A method of manufacturing the weather resistant pearlescent pigment of any of claims 1 to 10 comprising the following steps: i) suspending the basic pearlescent pigments in a solvent, iia) coating the basic pearlescent pigments from step i) in the solvent using a Zn oxide precursor at a predetermined pH1 and obtaining a Zn oxide layer on the basic pearlescent pigments, and iib) contacting the basic pearlescent pigment having a Zn oxide layer from step iia) at a predetermined pH2 with a precursor of a second metal oxide and forming a layer of metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof, or iic) coating the basic pearlescent pigments from step i) at a predetermined pH3 by simultaneously and continuously adding a solution of a Zn oxide precursor and a solution of a precursor of the second metal oxide, wherein the feed rate of one of these precursor solutions is coupled with control of the pH value and forming a mixed layer of metal oxide, metal hydroxide or oxide hydrate of Zn and of metal oxide, metal hydroxide or oxide hydrate of Si, Zr or Al or mixtures thereof iii) contacting the coated pearlescent pigments from step iib) or step iic) in the solvent withorganofunctional silanes or water based pre-condensed organofunctional silanes, and iv) separating the coated pearlescent pigments, optionally washing with solvent, and drying at a temperature in a range of 80 to 160°C.

12. A method of manufacturing according to claim 11 , wherein pH1 is in a range of 5.0 to 8.5.

13. A method of manufacturing according to claim 10 or 11 , wherein the precursor of the second metal oxide is a water glass solution and pH2 is in a range of 6.0 to 10.8 or pH3 is in a range of 6.5 to 10.8.

14. Use of the weather resistant pearlescent pigments from any of claims 1 to 10 in coatings, printing inks, powder coatings, architectural coatings or plastics, preferably in automotive coatings.

15. Formulations of coatings, preferably automotive base coat formulations, powder coatings, architectural coatings, printing inks or plastics containing the weather resistant pearlescent pigments according to any of claims 1 to 10.

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