Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1998-05-29
2001-01-30
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S457000, C106S460000
Reexamination Certificate
active
06179907
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to black iron oxide pigments that are stable at high temperatures and to a method of making such pigments.
BACKGROUND OF THE INVENTION
There are few black colorants that may be used in high temperature applications, such as concrete or fiber cement. Black mixed metal oxides like manganese ferrite have been the pigment of choice in the past for such applications. Although this type of pigment enjoys high heat stability (up to 800-1000° C.), it is expensive to produce and shows inferior tinting strength. Moreover, there are rising environmental concerns regarding the use of heavy metal ions in the manufacturing process for such pigments. In reality, the temperature requirement of the majority of these high temperature applications does not demand the high degree of temperature tolerance provided by such pigments.
Thus there exists a need for an alternative, simple, low cost black pigment for such applications.
Black iron oxide has always been considered as a suitable alternative to mixed metal oxides. It is relatively inexpensive to produce and exhibits much higher tinting strength when compared to mixed metal oxides. However, black iron oxide is not currently used in applications where high temperature is involved because of its instability towards heat and oxidation. When exposed to heat, iron (II) oxide, which is responsible for the black color in black iron oxide, is known to be oxidized to iron (III) oxide, which is red. This oxidation process is caused by oxygen transfer to the surface, and the process is facilitated by heat. To increase the heat stability of black iron oxide, oxygen supply to the oxide surface must to be diminished. This can be achieved by placing a protective coating onto the surface of the oxide.
There are some examples in prior art of such coatings. Unfortunately these coatings have their limitations. U.S. patent application Ser. Nos. 08/684,417 and 08/653,357 reveal the use of alkylphenols as surface coatings on iron oxide. These surface-bound organic molecules are effective in lowering the supply of oxygen onto the iron oxide surface and hence enhances the resistance of the iron oxide to heat. However, the alkylphenol molecules render the pigment so hydrophobic that they may not be suitable for use in applications that employ very polar media like alkaline cement.
U.S. Pat. No. 4,975,214 describes the use of silica in the range of 0.2-0.5 weight percent in the synthesis of magnetic iron oxide. U.S. Pat. No. 5,718,755 also discusses the use of an iron oxide coating with 0.5-2% weight percent of silica. However, it is shown that even a surface coating of 4 percent (by weight) of silica on the pigment alone, will not significantly raise the onset temperature of oxidation for the iron oxide.
European patent EP 305,819 discloses the preparation of a ferromagnetic metal powder comprising a ferromagnetic metal particle composed mainly of iron, a silicon compound layer formed on the surface of the ferromagnetic metal particle in such an amount that the amount of silicon is 0.1 percent to 1 percent by weight based on iron in the metal particle, and a layer containing a non-ferrous transitional metal element compound, which is formed on the silicon compound layer. In accordance with this patent, the final product is a metal powder, and not an iron oxide. Moreover, as shown in latter part of the patent, a dosage of 1 percent is not enough to raise the onset temperature significantly. Furthermore, the preparation procedure for this powder is lengthy, inefficient and involves heavy metal ion like cobalt.
United Kingdom Patent No. 1,494,746 discloses the use of an inner coating of silica and an outer coating of an aromatic carboxylic acid or salt thereof onto the surface of lead chrome pigment to increase its heat stability. However, such a method demands high dosage (at least 14 weight percent silica and 17-34 weight percent salt of an aromatic carboxylic acid) of the individual coatings. Moreover, the procedure involves the complicated sequence of pH adjustment and lengthy mixing time (up to 3 hours in one single step). Furthermore, the process is only applicable to a washed slurry. In other words, washed pigment must be re-dispersed before such a procedure can be applied. All of these requirements cause disruption in the manufacturing process and add tremendous cost to the final pigment.
Accordingly, a goal of the present invention is to increase the heat stability of black iron oxide; preferably, using a modifying agent that is inexpensive. Further, the modification process should be simple, fast and non-invasive towards the manufacturing process.
SUMMARY OF THE INVENTION
A method of improving the heat stability of black iron oxide pigment is provided that comprises contacting an aqueous slurry of black iron oxide pigment with a water-soluble silicate, in an amount from about 2 to about 50 percent, preferably from about 2 to about 20 percent, based on the weight of the pigment, at a temperature from about 40° C. to about 90° C., and subsequently contacting the slurry with an alkylphenol in an amount from about 0.1 to about 5 percent, preferably from about 0.1 to about 2 percent, based on the weight of the pigment, at a temperature from about 40° C. to about 90° C.
The present invention also provides a black iron oxide pigment having a coating comprising a silicate and an alkylphenol. Further, the present invention provides black iron oxide pigment having improved heat stability. Onset temperature, as indicated in latter part of the patent, is actually the degradation temperature.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
General Method
An aqueous slurry of black iron pigment is first heated to about 60° C. and contacted with a sufficient quantity of a solution of silicate to realize about a 4 wt % coating of silica on the pigment surface. The silicate solution may contain, for example, about 19 percent sodium silicate, by weight. Stirring is continued for about 10 minutes. The alkylphenol, such as dinonylphenol, is then added to the mixture and the mixing process is continued for about 10 to about 30 minutes. The pigment is then filtered, as by using a Buchner funnel with a Whatman #1 filter paper. The pigment is then washed with tap water or distilled water until the conductivity of the filtrate falls down to background value. The pigment is then dried as in a convection oven, as at 90° C. overnight. The dried pigment is pulverized, as by using a coffee mill before subjecting it to thermal evaluation. Recovery and washing are required unless a slurry obtained from re-dispersion of dry pigment or washed press-cake is used.
Specification of Black Iron Oxide
The black iron oxide may be either natural black iron oxide (i.e. natural magnetite) or synthetic black iron oxide from known synthetic processes (e.g. precipitation or Aniline Process or calcination of yellow iron oxide under a reducing gas medium). The black iron oxide can be cubical, spherical or acicular. Usual average particle size can extend from about 0.5 to about 2 microns when dispersed. However, finely ground natural black magnetite can have sizes below 0.1 micron when dispersed. The black iron oxide is inherently magnetic, but the magnetic strength can stretch from 60 to 300 Oe. The usual ratio of FeO/Fe
2
O
3
is from 2/8 to 3/7 (w/w), depending on the blackness of the pigment.
Specification of Slurry
A suitable slurry will be one obtained immediately after the synthesis of the black iron oxide pigment has been completed, or, less desirably, as obtained via re-dispersion of washed presscake or dried powder. The slurry should have a pigment content of about 10 to about 50 percent by weight, preferable between about 10 and about 30 percent. The conductivity of the slurry may be as high as about 18,000 micro S.
A native slurry, as preferably used in the present invention, is the slurry obtained in the production line after completion of the synthetic process and before the filtration step. This slurry is usually contaminated with salts (e.g. s
Kwan Wing Sum Vincent
Tai Lily W.
Leydig , Voit & Mayer, Ltd.
Marcantoni Paul
Marconi Data Systems Inc.
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