Stains which bake to a red-brown color, process for their...

Details Stains which bake to a red-brown color, process for their... Stains which bake to a red-brown color, process for their...

1999-02-02

2001-05-08

Marcheschi, Michael (Department: 1755)

Compositions: coating or plastic

Materials or ingredients

Pigment, filler, or aggregate compositions, e.g., stone,...

C106S457000, C106S460000, C106S482000

Reexamination Certificate

active

06228160

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to coloring bodies i.e. stains which fire to a red-brown color and comprise iron(III) oxide (Fe
2
O
3
) and/or one or more iron compounds which form Fe
2
O
3
on firing, as the coloring component in uniform distribution in a pulverulent matrix based on an oxide-like and/or silicate-like material. The invention furthermore relates to a process for the preparation of the stains and their use for coloring ceramic compositions.
DESCRIPTION OF THE PRIOR ART
Iron-containing stains are widely used in the production of so-called Grés Porcellanato, a fine stoneware composition for fast firing which is gaining ever-increasing importance in modern tile production, in order to give the composition an attractive red-brown color after firing.
Grés de Thiviers, a naturally occurring stain comprising about 90% quartz and about 10% goethite (FeOOH) is used to produce red-brown color shades. In this naturally occurring product, the iron oxide hydrate is protected from chemical reaction with the constituents of the composition during firing at about 1100 to 1250° C. by a covering of quartz. Disadvantages of this stain are the variations in the color shade, the comparatively high concentration of coloring material needed to achieve attractive color shades, and the dwindling natural resources of this pigment. The search for synthetic products with the same or better properties than those of the natural product is therefore of great importance.
According to JP 02-263870 A, violet colored pigments are accessible if FeOOH, SiO
2
and K
2
CO
3
are mixed, ground and calcined. The desired red-brown color shade is not obtained.
According to EP-A 0 563 688, red ceramic casting compositions can be produced by fast firing if clay is mixed, shaped and fired with customary additives and an iron oxide pigment. The pigment to be used must comprise less than 0.6% Si and less than 0.06% Mn, and comprises alpha-FeOOH, gamma-FeOOH, alpha-Fe
2
O
3
and/or Fe
3
O
4
.
As has been found in practice, commercially obtainable iron oxide pigments, including those with the above mentioned criteria in respect of the Si and Mn content, are not very suitable for coloring in a low concentration by themselves or as a mixture with quartz, since the iron oxide substantially dissolves in the compositions. Although a coloring effect results at a higher concentration of iron oxide, this is nevertheless limited to unattractive brown color shades. The reason for these properties is to be seen in the fact that iron oxide tends towards crystal growth at a high temperature, and the resulting coarse-particled iron oxide leads to colors which are not very brilliant in the ceramic composition.
It is known from Europa Chemie 31-32/91, page 4, that the wetting properties of the Bayer pigment Bayferrox® are improved by an after-treatment with aluminum oxide and such micronized Fe
2
O
3
red pigments are suitable for critical binder systems. The nature of the after-treatment, the amount of Al
2
O
3
and the structure thereof cannot be found from this document. From experiments by the inventors of the present application, The micronized red pigments mentioned are not suitable for use as stains in ceramic compositions. During firing, sintering of the Fe
2
O
3
evidently occurs, and with this a change in color from red to brown.
Ullmann's Enzykiopädie der technischen Chemie [Ullmann's 15 Encyclopaedia of Industrial Chemistry], 4th edition, volume 10, pages 437-438 describes compositions of enamels and glass fluxes for them. According to the definition for stains given on page 438, according to which the pigments contained therein do not change during stoving, a simple mixture of a glass frit and Fe
2
O
3
can not be regarded as stain, because the iron oxide would dissolve in the flux during firing. The document says nothing about the preparation of the stain. Although the glass frits employed in enamels are amorphous, the specific surface area measured by the BET method is usually about/less than 1 m
2
/g and their d
50
value mostly about 1 &mgr;m.
DE 32 11 327 A1 relates to iron oxide pigments with an iron(II) oxide content (page 10, line 11). Conventional pigments comprising FeG are black and have a BET surface area of 12 to 18 m
2
/g. Products which can exceed 40 m
2
/g are used only for magnetic purposes. Contrary to said document the present invention relates to stains which fire to a red-brown color and not a black color.
The inventors of the present application have attempted to coat iron oxide pigments with silica by a precipitation process in order to arrive at products with similarly good stain properties as Grés de Thiviers. In spite of a coating process, which was suitable per se, no products in which the thickness of the coating was sufficient to withstand attack by the constituents of the ceramic composition were accessible. Furthermore, a particle growth and therefore a reduction in the color brilliance occurred at a high temperature.
Another attempt was directed towards joint precipitation of silica and iron hydroxide from water-glass and iron salt solutions. Because of the fine division of the iron hydroxide formed and its high reactivity at a high temperature, these precipitated products were again exposed to considerable crystal growth; color shades, which were not very attractive, were the consequence.
A stain based on Fe
2
O
3
in a matrix of crystalline SiO
2
is commercially obtainable—CK 32055 from Ferro. In the coloring of ceramic compositions, this stain leads to colors of high blue content (high b* value) and inadequate depth of color (high L* value), determined in the CIELAB system in accordance with DIN 5033.
A coloristically improved product is the commercially obtainable CP RE-18 from Ithaka (JP). This stain comprises about 10 wt.-% Fe
2
O
3
in a matrix of silica (SiO
2
), of which about 15 wt.-% is crystalline and about 85 wt.-% is amorphous. According to trans-electron microscopy (TEM), the amorphous SiO
2
is in the form of substantially spherical particles with a particle diameter of 40 nm. The specific surface area of the stain, determined by the BET (N
2
) method in accordance with DIN 66131, is 36 m
2
/g.
SUMMARY OF THE INVENTION
The object of the invention is to provide a further stain for coloring ceramic compositions which is easily accessible in an economic manner and allows the production of brilliant red-brown ceramic tiles of intense color in a reliable manner.
DETAILED DESCRIPTION OF THE INVENTION
A stain, which fires to a red-brown color, comprising Fe
2
O
3
and/or one or more iron compounds, which form Fe
2
O
3
during firing, as coloring component in uniform distribution in a pulverulent matrix based on an oxide-like and/or silicate-like material, has been found, which is characterized in that the oxide-like and/or silicate-like material of the matrix has a specific surface area measured by the BET method (DIN 66131) equal to or greater than 40 m
2
/g and is amorphous to X-rays to the extent of at least 50 %, the stain comprises the coloring components in an amount of 1 to 25 wt. %, calculated as Fe
2
O
3
, and in that it is obtainable by bringing the coloring components and the oxide-like and/or silicate-like material into intensive contact with one another.
Stains according to the invention are obtainable by uniform distribution of the coloring component in a pulverulent matrix based on an oxide-like and/or silicate-like material, the coloring iron compound(s)and the oxide-like and/or silicate-like material of the matrix, including auxiliaries optionally present, being mixed and/or ground together intensively, that is to say with the introduction of an effective amount of mechanical energy, in a device for mixing and/or grinding.
Bringing the base material of the matrix and the coloring component into intensive contact with one another is of importance for the formation of the color of high red content, high brilliance and intensity which is red-brown when baked. Simple dry mixing of the constituents, for example treatment i

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