Surface-modified conductive pigment

Details Surface-modified conductive pigment Surface-modified conductive pigment

1995-04-05

2002-06-25

Wood, Elizabeth D. (Department: 1755)

Compositions: coating or plastic

Materials or ingredients

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

C106S403000, C106S404000, C106S418000, C106S425000, C106S429000, C106S430000, C106S431000, C106S441000, C106S442000, C106S445000, C106S446000, C106S447000, C106S448000, C106S455000, C106S460000, C106S475000, C106S490000, C106S491000, C428S403000, C252S500000, C252S512000, C252S521600

Reexamination Certificate

active

06409815

ABSTRACT:

The invention relates to conductive pigments having a partly modified surface.
BACKGROUND OF THE INVENTION
Conductive pigments are usually metal powders, non-metallic powders, such as carbon black and doped metal oxides, or pulverulent carrier materials coated with conductive layers. The first group includes, for example, silver powder.
Examples of doped metal oxides are antimony-doped tin oxide, halogen-doped tin oxide or aluminum-doped zinc oxide, which are described in U.S. Pat. No. 4,655,966, EP 0 441 427 and U.S. Pat. No. 5,171,364.
Carrier materials which are used for the third group of conductive pigments are, inter alia, phyllosilicates, in particular mica, silicon dioxide, titanium dioxide and barium sulfate. These carrier materials are in general coated with a conductive layer of doped metal oxides. DE 40 17 044 describes an electrically conductive barium sulfate, the conductive layer of which comprises antimony-doped tin oxide. A conductive pigment comprising a mica and antimony-doped tin oxide is described in EP 0 139 557. Hollow beads of silicon dioxide coated with antimony-doped tin oxide are known from EP 0 359 569.
If the conductive pigments mentioned are incorporated into a non-conductive matrix, conductive composite materials are only obtained if the concentration in the total matrix lies above a critical concentration range at which the first conductivity pathways can form. This concentration range depends on the pigment and matrix. This range is also called the percolation threshold. It depends chiefly on the geometric shape and the density of the particles.
Conductive pigments are useful for addition to non-conductive materials, such as plastics, in order to make them antistatic or to make them conductive. When made antistatic, the materials do not conduct electric current; however, they prevent the accumulation of electrostatic charges because such charges can flow off the materials. The antistatic region lies between 10
6
and 10
9
ohms. At conductance above this antistatic region, the material is an insulator and, at conductance below this region, the material is electrically conductive.
Concentrations in the range of or above the percolation threshold are necessary for both the antistatic and electrically conductive regions.
In order to be able to establish a surface resistance of about 1 M&OHgr; for a pigmented polyester material, about 30 to 50% by weight of conductive pigment, based on the resin employed, must be added. Thus, if Minatec® 30 CM (manufacturer: E. Merck, Darmstadt) is used, a surface resistance of 2 M&OHgr; is achieved at a pigment content of 30% by weight. Minatec® 30 CM comprises mica coated with titanium oxide and antimony-doped tin oxide as the conductive layer.
If Sacon® P 401 (manufacturer: Sachtleben Chemie GmbH, Duisburg), a barium sulfate coated with antimony-doped tin oxide, is used, 50% by weight, based on the resin employed, is necessary to achieve a surface resistance of 20 M&OHgr;.
However, such high pigment contents adversely affect the mechanical properties of the system to be used.
SUMMARY OF THE INVENTION
An object of the invention is to provide a conductive pigment which imparts an adequate conductivity to the pigmented system even at significantly below the normal percolation threshold.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
Accordingly, the invention provides a surface-modified conductive pigment which is obtainable by partial coating of a conductive pigment with an organic modifying agent. The partially coated conductive pigment is prepared by dispersing a conductive pigment, subsequently adding the organic modifying agent, if appropriate as a mixture with a solvent, to the pigment dispersion, and in particular in an amount such that the powder electrical conductance measured in “siemens” (S)=1/&OHgr;, of the partly coated conductive pigment is about 80% or less, preferably 50% or less, of the starting value, but does not fall below a lower limit value of 10
−7
S, preferably 10
−5
S, removing the volatile reaction products and the solvent present, where appropriate, with further agitation and heating of the pigment to 100° C., and finally drying the partly coated pigment.
The invention also relates to coatings, thermoplastics, casting resins, printing inks and powder coatings pigmented with the pigment according to the invention.
All types of conductive pigments can be employed for partial coating with the modifying agents mentioned below, in particular metallic powders, such as, for example, silver powder and aluminum powder, and furthermore non-metallic powders, such as, for example, carbon black, carbon fibers, antimony-doped tin oxide, aluminum-doped zinc oxide or fluoride-doped tin oxide, and pulverulent substrates which are coated with conductive layers, such as, for example, mica, kaolin or barium sulfate coated with doped metal oxides.
Compounds with organic radicals which can be fixed onto the surface of conductive pigments can be employed as modifying agents. Suitable modifying agents include, for example, organometallic compounds of the elements tin, zirconium, titanium, aluminum and silicon; silanes being preferred. These agents can be used separately or as mixtures.
Particularly suitable compounds are those of the formula I
XR
1
R
2
R
3
R
4
  I
wherein the radicals R
1-4
have the following meanings:
R
1
and/or R
2
is alkyl having 1-30 C atoms, and in particular 10-30 C atoms, including cycloalkyl-containing alkyl groups, wherein one or two non-adjacent CH
2
groups can also be replaced by —CH═CH— and/or —O—, with decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosanyl, heneicosanyl, docasanyl, tricosanyl, tetracosanyl, pentacosanyl, hexacosanyl, heptacosanyl, octacosanyl, nonacosanyl, tricontanyl, 12,12-dimethyltetradecyl, 11-propyl-12-butylpentadecyl and 8,8-dimethyl-12-propyl-13-propylhexadecyl radicals being particularly preferred;
the remaining radicals R
1-4
(i.e., R
2
-R
4
when R
1
is as defined above, R
1
and R
3
-R
4
when R
2
is as defined above and R
3
-R
4
when both R
1
and R
2
are as defined above) are halogen or alkoxy having 1-20 C atoms, in particular 1-10 C atoms, wherein one or two non-adjacent CH
2
groups can also be replaced by —CH═CH—, —O—, —CO—, —COO— or —OCO—; fluorine, chlorine, methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, octoxy, nonoxy or decoxy radicals are particularly preferred; and
x is Si, Sn, Ti, Zr or Al, the radical R
4
being omitted in the case where X=Al.
Particularly preferred compounds are those of the formula
(C
n
H
2n+1
)X(OC
m
H
2m+1
)p
wherein
X has the above-mentioned meaning,
n is 1-30,
m is 1-10, and
p is 3 when X is Si, Sn, Ti or Zr, and 2 when X is Al.
Among these compounds, n-hexyldecyl-tri-ethoxysilane and methyl-tri-ethoxysilane are preferably employed as modifying agents in the context of the invention.
Silanes are known from EP 0 492 223 for coating pigments. The pigments described therein are used for inhibiting yellowing of plastics.
Partial coating of the conductive pigment is carried out by the process described in DE-OS 40 41 663. For this, the pigment and the modifying agent are mixed intensively in a high-performance mixer at temperatures of preferably about 50-100° C., if appropriate with addition of preferably about 1-50% by weight, especially 5-10% by weight, of solvent, based on the pigment. The mixing time is preferably about 1 hour.
If a solvent is used for the coating, then it serves merely for better distribution of the modifying agent on the pigment surface.
If silanes are used, it may be necessary to subject them to preliminary hydrolysis, because the silane does not react directly with the hydroxyl groups on the surface of the pigment. In this case, water and a solvent are added to the silane and the mixture is subjected to preliminary hydrolysis by stirring at room tempe

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