The invention relates to a method for dispersing and passivating particulate hard-
metal powders, ceramic powders and elemental powders of the 3rd and 4th main
group of the Periodic Table of the Elements in water and aqueous media.
The invention is particularly applicable in the powder-processing industry
(ceramics, hard-metal industry).
Today the production of molded articles from hard metals and ceramics is mainly
carried out via the powder technology process, in which the initial powders
together with additives are mixed in a liquid and optionally ground, and
subsequently the powder mixture is dried, pressed into molds and sintered. While
with silicate ceramics and oxide ceramics water is chiefly used thereby as a
grinding and mixing liquid (dispersing agent, suspending agent), non-oxidic
powders, such as hard-metal powders, nitride and carbide powders are
processed in nonaqueous dispersing agents in order to repress or suppress
oxidation and hydrolysis phenomena of the powder. Aliphatic and aromatic
hydrocarbons, alcohols and acetone are used as nonaqueous liquids for this.
Since for reasons of technical safety and industrial safety and environmental
protection the processing of particulate powders in these organic liquids is
relatively complex and cost-intensive (explosion-proof installations and buildings,
solvent recovery, recycling processes), attempts have been made to carry out a
dispersing of the powders in water or aqueous media with the addition of special
auxiliaries.
EP 1153652 describes a method for dispersing mixtures of tungsten carbide and
cobalt powders (hard-metal powder mixture) in part with the addition of further
hard materials, such as TiC, TaC, TiN, (W,Ti)C, in aqueous or ethanol media with
the addition of the cationic polyelectrolyte polyethylene imine. Through the
addition of 0.1 to 10 % by weight, preferably 0.1-1 % by weight, of polyethylene

imine with a molar weight of 5000 to 50,000, preferably 10,000 to 30,000 g/Mol a
good dispersion of the hard-metal powder mixture in water is thereby achieved
and a viscosity is regulated that is suitable for a subsequent spray-drying
process, e.g., 8-20 mPas with a shearing rate of 10-100 s-1. Aqueous-ethanol
hard-metal suspensions of WC-Co powder mixtures suitable for a spray drying
can also be realized according to WO 98/00256 by the addition of 0.01 to 10 %
by weight, preferably 0.1-5 % by weight dispersing agent such as polyacrylates,
hydroxyethylcellulose, styrene maleic acid copolymers and ethylene oxide
urethane copolymers. However, an increase in the oxygen content of the dried
powder ought to be associated therewith. However, this is not described in the
publication.
In WO 93/21127 surface-modified nanoscalar ceramic powders are produced,
such as, e.g., Si3N4, SiC, Al2O3 and ZrO2 by dispersing the respective unmodified
powder in water and/or organic solvents such as alcohols by means of a low-
molecular (molar weight up to 500 g/Mol) organic compound and subsequently
removing the suspending agent. Carboxylic acids, amines, p-dicarbonyl
compounds and organo-alkoxysilanes, for example, act as organic compounds
(auxiliaries). In DE 4336694 the auxiliaries for dispersing the ceramic powders
are expanded to low-molecular organic substances with a molar weight of up to
1000 g/Mol.
EP 0771316 describes the dispersion of nanoscalar non-oxidic powders such as
TiN, TiC, Si3N4 and SiC in organic suspending agents with the object of
producing sinterable green bodies, whereby polymolecular substances are used
as dispersing agent, which substances comprise one or more polar groups and
one or more long-chain aliphatic radicals, such as, e.g., alkyl-substituted imides
of dicarboxylic acids.
DE 19751355 describes a method for dispersing particulate inorganic powders in
preferably water or aqueous media, in which substances having a biological
genesis, such as, e.g., sugar, starch and/or chitin derivatives are used as a

dispersing agent. Dispersions with long-term stability are thereby produced in
particular in aqueous media.
Moreover, DE 19800310 describes a solution in which non-oxidic ceramics are
coated with amino acids of aqueous or organic solutions. Furthermore, DE
10130161 claims a method for conditioning non-oxidic very fine powders such as
titanium nitride or silicon carbon nitride, in which the coating of the powders with
a surfactant auxiliary that contains nitrogen in the polar head group, takes place
in an organic solvent in a first step and subsequently in a second step the coated
non-oxidic very fine powder is further processed in water or in air, whereby the
oxygen content of the very fine powders does not increase or increases only
slightly.
The disadvantage of the cited methods according to the prior art lies in the fact
that either the dispersion of fine and very fine powders does not take place in
water, but in organic or organic-aqueous media, or that no passivation of the
powders is explicitly achieved in a subsequent dispersion of the cited powders in
water. That means that for non-oxidic powders, although an adequate dispersion
through the listed auxiliaries in water is described, no protection against oxidation
and hydrolysis is described.
The object of the invention is now to propose a method for dispersing and
passivating particulate powders in water and aqueous media, with which all the
disadvantages of the solutions of the prior art are eliminated. In particular, the
object of the invention lies in finding a method for dispersing particulate powders
in water with which a passivation against the chemical attack of the water is
simultaneously achieved for non-oxidic powders and elemental powders, in
addition to the dispersion, and the oxygen content does not thereby increase or
increases only slightly compared to the treatment according to the prior art.
The object is attained through the invention disclosed in claim 1. Further
developments are the subject matter of claims 2 through 13. In the method
according to the invention water-soluble polyvinylamines and/or the initial
products thereof, such as, e.g., polyvinyl formamides, are used for dispersing

particulate powders in water and/or aqueous media and furthermore for
passivating non-oxidic particulate powders in water. Polyvinylamines have the
general summation formula - [CH2-CH-NH2]n - and in each structural unit have an
NH2-(amine-) group that can be protonated in water. Polyvinyl formamides have
the general summation formula -[CH2-CH-NH-CHO]n - and have an -NH-CHO-
(formamide) group that can be protonated in water.
The water-soluble polyvinylamines and/or the initial products thereof used have a
molar weight of 5000 to 350,000, preferably 5000 to 100,000. They are used in a
concentration of 0.01 to 10 % by weight, based on the solids content of the
suspension, preferably 0.1 to 0.5 % by weight. Primarily hard-metal powders,
non-oxidic ceramic powders and/or elemental powders are used as particulate
powders. The hard-metal powders used thereby comprise carbides, nitrides
and/or carbon nitrides of the elements of the auxiliary groups IV, V and/or VI of
the Periodic Table of the Elements (PTE) and Co, Ni and/or Fe. These hard-
metal powders can thereby comprise a mixture of WC and/or in part other hard
materials , such as TiC, TaC, NbC, Cr3C2, VC and Co and/or in part Ni and/or Fe
as binder metal and/or a mixture of TiC or TiCN and Mo2C and Co and/or Ni
and/or Fe as binder metal. Nitrides, carbides, borides and/or silicides, such as,
e.g., Si3N4, SiC, AIN, BN, B4C, TiN, TiC, ZrC and/or ZrN are used as non-oxidic
ceramic powders. Crystalline and/or amorphous elements of the 3rd and 4th main
group of the Periodic Table of the Elements (PTE) are used as elemental
powders. These include, e.g., elementary boron and silicon and carbon in the
form of graphite, diamond powder, carbon blacks and other amorphous and
partially crystalline C-modifications.
According to the invention the auxiliary polyvinylamine and/or the initial products
thereof are dissolved in the given concentration in water while being stirred and
subsequently the particulate powder is added in portions with further stirring and
optionally with ultrasonic treatment. Subsequently, a mixed grinding can follow in
a ball grinder, e.g., in an agitator ball mill. The solids content of the aqueous
suspensions is 40 - 90 % by weight, preferably 60 - 85 % by weight. A
suspension that is stable for the time of the further processing is obtained with

the polyvinylamine and/or the initial products thereof as auxiliaries and viscosities
that are very suitable for spray-drying processes are established. With solids
contents of 70 % the viscosities are, e.g., in the range of 12-100 mPa*s or with
solids contents of 85 %, e.g., in the range of 20 - 300 mPa*s.
With an addition concentration of, e.g., 0.2 % by weight polyvinylamine and/or
the initial products thereof based on the solids contents, the dynamic mobility,
which as a measured variable for the electrostatic repelling powers or for the
calculation of the zeta potential describes the stability and dispersibility of the
particulate powder particles in the suspension, reaches values of 0.8 to 2
m2(V*s)-1, which represents a multiple of the values that can be achieved with the
addition of auxiliaries that suffice in the prior art.
It was found that the described method for dispersing particulate powders with
polyvinylamine and the initial products thereof, such as, e.g., polyvinyl foramide
as auxiliary with non-oxidic particulate powders and with elemental powders
furthermore leads to a passivation of these powders, i.e., to the protection thereof
from oxidation and hydrolysis and thus does not lead to an increase or only to a
slight increase in the oxygen content compared to treatment according to the
prior art. It is assumed that protonated NH2 groups that are on each structural
unit of the polyvinylamine and that do not break the continuous carbon chain as,
e.g., with polyethylene imine, are particularly suitable for bringing about an
effective protection against the water molecules due to electrostatic and steric
interactions with the charge and structural conditions of the powder surface.
Through the method according to the invention it is possible to process
particulate powders that cannot otherwise be processed in water, such as hard-
metal powders, non-oxidic ceramic powders and elemental powders in water in a
cost-effective and environmentally compatible manner without their oxygen
content rising perceptibly compared to processing according to the prior art. All
the disadvantages of the prior art can be eliminated and the objects attained with
the solution according to the invention.

The invention is described in more detail below on the basis of exemplary
embodiments.
Example 1
An aqueous suspension containing 72 % by weight WC/Co powder (thereof 90 %
by weight WC and 10 % by weight Co), is produced by dissolving in water 0.15 %
by weight polyvinylamine with a molar weight of < 10,000 g/mol, based on the
solids content of the suspension and subsequently stirring in the hard-metal
powder mixture. The WC grain size was 0.5 µm. After 6 h of mixed grinding in
an attrition mill, the viscosity was 28 mPa*s, with a shearing rate of 240 min-1.
The oxygen content after grinding was 0.38%. The dynamic mobility was 0.9
m2(V*s)-1. Without polyvinylamine as an additive, the oxygen content rose to
0.7%.
Example 2
An aqueous suspension containing 72 % by weight hard-metal powder (thereof
59 % by weight WC, 16 % by weight TiC, 11.2 % by weight TaC, 4.8 % by weight
NbC and 9 % by weight Co) is produced by dissolving in water 0.15 % by weight
polyvinylamine with a molar weight of 45,000 g/mol, based on the solids content
of the suspension, and subsequently stirring in the hard-metal powder mixture.
After 6 h mixed grinding in an attrition mill, the viscosity was 15 mPa*s, with a
shearing rate of 240 min-1. The oxygen content after grinding was 0.54%. The
dynamic mobility was 1.2 m2(V*s)-1.
Example 3
An aqueous suspension containing 40 % by weight ceramic silicon nitride powder
mixture (thereof 90 % by weight Si3N4, 6 % by weight Y2O3 and 4 % by weight
Al2O3), is produced by dissolving in water 0.6 % by weight polyvinylamine with a
molar weight of 45,000 g/mol, based on the solids content of the suspension, and
subsequently stirring in the ceramic powder mixture. After 3 h mixed grinding in a
laboratory attrition mill, the viscosity was 32 mPa*s, with a shearing rate of 240
min-1. The dynamic mobility was 1.2 m2(V*s)-1. Without the addition of

polyvinylamine, the viscosity was 299 mPa*s and the dynamic mobility was -0.9
m2(V*s)-1.

We Claim:
1. Method for dispersing and passivating particulate powders selected
from hard metal powders, non-oxidic ceramic powders and/or
elemental powders in water and aqueous media, whereby additives
are added determined by the process, consisting of water soluble
polyvinylamines and/or the initial products thereof.
2. Method as claimed in claim 1, wherein the polyvinylamines and/or
the intial products thereof have a molar weight of 5000 to 350,000.
3. Method as claimed in claim 2, wherein polyvinylamines and/or the
intial products thereof with a molar mass between 5000 and
100,000 are used.
4. Method as claimed in one or more of claims 1-3, wherein the
polyvinylamines and/or the initial products thereof are used in a
concentration of 0.01 to 10% by weight, based on the solids
content of the suspension.
5. Method as claimed in claim 4, wherein polyvinylamines and/or the
initial products thereof are used in a concentration of 0.1 to 0.5%
by weight, based on the solids content of the suspension.

6. Method as claimed in one or more of claims 1-5, wherein the
polyvinylamines and/or the initial products thereof are dissolved in
water while being stirred before the addition of the particulate
powders.
7. Method as claimed in one or more of claims 1-6, wherein initial
products of polyvinylamine are used as auxiliaries.
8. Method as claimed in claim 1-6, wherein the polyvinylamines used
is polyvinyl formamides.
9. Method as claimed in claim 1, wherein carbides, nitrides and/or
carbon nitrides of the elements of the auxiliary groups IV, V and
VI of the PTE (Periodic Table of the Elements) and Fe, Co, Ni
individually, mixed or alloyed as powders containing binder metal
phase, are used as hard-metal powders.
10. Method as claimed in claim 1, wherein nitrides, carbides, borides
and/or silicides are used as non-oxidic ceramic powders.

11. Method as claimed in claim 1, wherein crystalline and/or
amorphous elements of the 3rd and 4th main group of the PTE are
used as elemental powders.
12. Method as claimed in one or more of claims 1-11, wherein the
oxygen content of non-oxidic particulate powders such as hard-
metal, ceramic and elemental powders used is not increased or is
increased only slightly.

Method for dispersing and passivating particulate powders such as herein
described in water and aqueous media, whereby additives are added
determined by the process, characterized in that water-soluble
polyvinylamines and/or the initial products thereof are used as auxiliaries.