Note: Descriptions are shown in the official language in which they were submitted.
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Surface-modified manganese sulfide, method for producing the
same and use thereof
The invention relates to surface-modified manganese sulfide,
a method foxy its preparation as well as the use of the
surface-modified manganese sulfide as an auxiliary press
material and as an additive in the sintering of molded
articles. Furthermore r_he present invention relates to a
sinter powder containing an amount of the surface-modified
manganese sulfide, a method for the preparation of a molded
article using the inventive sinter powder as well as a molded
article obtainable by l.he method.
The machined processing, especially by cutting, of sintered
1!~ molded articles is often quite difficult due to tool wear and
is alleviated by the addition of sulfides. Manganese sulfide
(MnS) has proved to be an especially suitable additive for
sintering, since its thermal stability allows it to
participate in the sintering process without thermal
degradation. For this, 1_he manganese sulfide must be
incorporated into the :~:i.nter powder in as finely dispersed a
form as possible. In the=_ U.S. patent 3,705,020, a manganese
sulfide particle size o.f 10 to 100 ~,m, preferably 30 to 40 ~m
is cited and., according to the European patent application EP
0 183 666, the particle size is 10 ~m or less. A small
particle size enables homogeneous incorporation of the
sulfide and the product:_ion of mechanically stable molded
articles. In. particular,, the crack formation is reduced.
However, the small particle size and the large surface of the
manganese sulfide associated therewith promotes its reaction
with, in particular, moist air to MnO, Mn02 and MnS04 and,
with this, a clumping together to larger particles or
agglomerates. This behavior causes not only storage problems;
in the use cf the manganese sulfide as a sinter additive, the
clumping then leads to a structural weakening of the finished
molded articles. Especially under mechanical and thermal
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stress, the formation of. cracks and the destruction of the
molded articles follow.
In the U.S. patent 5,768,678, the use of a special
composition containing 'manganese sulfide as a sinter additive
is proposed to avoid the oxidation of the manganese sulfide.
This composition is made of a mixture of 50-65 wt.o
manganese, 30-38 wt.% :sulfur and 5-25 wt.% iron and is added
to the metal powder' to be sintered in an amount 0.2-2.0 wt. o.
lc) According to this patent, the oxidation protection is
afforded by the iron, which should be present in non-oxidized
form. It is, however, ~~ disadvantage of the sinter additive
of U.S. 5,768,678 that it contains additional useless iron
sulfide and, furthermorE=, a surface reaction can be merely
reduced but not avoided.. A further disadvantage of this
sinter additive is its costly production.
There therefore exists a need for a storable and easily
obtainable manganese st~7_fide. It is therefore the goal of the
present invention to provide such a manganese sulfide without
the features known in the prior a:rt.
The inventors have surprisingly found that such a manganese
sulfide can :be obtained in that powdered manganese sulfide is
provided, a ~~oating a~_Yent selected from a wax, an ester of an
inorganic or organic acid or a polymer with a low melting
point or mixtures thereof is selected, is added in an amount
of 0.01 to 10 wt.% re=Lative to the weight of the manganese
sulfide used, and the mixture is mixed over a period of time
which is sufficient te:~ ensure a homogeneous mixture.
The studies of the inventors have indicated that the
oxidation protection and the reduction of the moisture uptake
by the otherwise hygrr~scopic material and the agglomeration
resulting therefrom az:~e surprisingly prevented by even small
amounts of the coating agent.
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According to the invention coating agent is used in the
production of the coated manganese sulfide in an amount of
0.01 wt.o to 10 wt.%, preferably 0.01 to 5 wt. o, especially
preferred 1.0 to 3.0 wt~~ relative to the weight of the
manganese sulfide.
Depending on the melt:i:2g point of the coating agent, the
coating is generally carried out at a temperature of 20°C to
150°C. Depending on t.hf=_ coating agent used, one can however
also operate at a ternpe:rature around the freezing point
(0°C). Preferably, ore operates around room temperature,
since then, the mixer does not have to be heated. The mixing
is normally performed ~=or a period of time of 1 minute to 60
minutes, preferably for 5 minutes to 20 minutes.
Surprisingly, the manganese sulfide obtained in this way can
be used directly, in other words it does not have to be
submitted to further treatment steps such as drying for
storage or for its use.
In addition to the use as a sinter additive for improving the
processability o.f molded sinter articles, it was determined
within of the scope of t:he present invention that the
inventive surface-modified manganese sulfide also proves to
be a lubricating press auxiliary and it can therefore be
generally used as an additive for improving compression
characteristics. A further advantageous characteristic of the
inventive manganese sulfide, which proves to be very
favorable in connection. with .its storage and use, is that it
3C is dustless.
The coating of the manganese sulfide can be performed in any
way known to one of ord.i.nary skill in the art. The coating
agent can be added to a conventional mixing apparatus such as
a ploughsharf=_ mixer or_ a. tumbler-mixer all at once or in
intervals. It is for example also possible to spray the
coating agenl~ into the rriixing container. According to the
invention, any normal manganese sulfide can be used,
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irrespective of its purity or particle size. If it is
necessary, f:or exampi.e for the use as a sinter additive, the
manganese sulfide is finely ground prior to coating, so that
in general i.t has an a,~~?rage particle size, in other words a
particle diameter, of :L to 200 urn, preferably of 1 to 10 Vim,
and especially preferred of about 5 to 10 Vim. It is however
also possible in this c=ase to use larger particles, as long
as these confer sufficic=_nt mechanical stability in later
sintering of the molded articles.
According th.e invention, compounds preferred as coating
materials are either liquid at room temperature or have a
relatively low melting point of under 150°C. In this way, and
as indicated above, it is possible to operate without or only
with very weak heating of the mixer in the production of the
inventive manganese su1_f=ide .
For the use of the inventive manganese sulfide as a sinter
additive, the inventive coating agent should furthermore have
an evaporation temperature at normal pressure of under 500°C,
preferably in the range of about 200 to 300°C and should
decompose or evaporat:~ without residue during the heating of
the sinter mixture, s:in.c:e in this way the formation of
possible contaminations in the sintered molded article is
reduced.
Coating mate=rials used according to the invention and
possessing the previol.zsly cited physical characteristics are
preferably low melon<~ polymers, oils including paraffinic
oils and sil=icon oils, waxes, a mono- or mufti-functional
aliphatic alcohol with 2 t=o 12 carbon atoms, organic and
inorganic esi:ers, irl the latter case especially phosphoric
acid esters.
One example of what is to be understood by waxes in the sense
of the invent=ion are Easters of higher straight-chained fatty
acids such a:~ palmitic:~ acid, hexaicosanoic acid with higher
straight-cha:_ned, monofunctional alcohols such as palmityl
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alcohol, stearyl alcohol or octac~ec-9-en-1-ol. Another
example of what is tc:~:be understood as waxes in the sense of
the invention are materials which are mostly of natural
origin, which contain. the fatty acid esters named above
5 partially or predomir:~antly, and which transition without
degradation to a melting liquid, low viscous condition above
approximately 30°C to <~0°C. An example of such waxes are
Japan wax, lanolin or beeswax.
le) Furthermore, synthetic polymers, with waxy characteristics
such as the so-called polyolefin waxes also fall under the
definition of waxes here. Examples of such synthetic polymers
which find application in the present invention are low-
molecular polyethylene glycols and polypropylene glycols.
One example of what is too be 'understood as oils in the
present invention are ~ili:phatic oils based on mineral oil
such as the paraffinic oils. The aliphatic oils preferably
have a chain length of 6 to 20 carbon atoms. The term oil
also further encompasses synthetic oils such as the silicon
oils such as known to ore of ordinary skill in the art as a
general term inter al.iG~ for clear, colorless, hydrophobic
liquids with congealing points of about -80 to -40°C from
linear polydimethylsi:loxanes and polymethylphenylsiloxanes.
Compounds to be understood here as a low-melting polymer are
those which consist of macromolecules and which have a
melting poin~~ of underr 150°C and an evaporation temperature
of under about 500°C at normal pressure. Preferred
representatives of this class of compounds are polyesters,
polyamides and polyal_i_phatic compounds.
According to the invention, esters of inorganic or organic
acids can al:~o be usee~ a~~ the coating agent . Preferred
inorganic acids are phosphoric acid and sulfuric acid however
other acids can also be used according to the invention such
as sulfur ac:~_ds, carbonic acid, ... . Formic acid and lower
carbonic acids with 2 to 6 carbon atoms such as for example
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acetic acid are preferably used as organic acids. Aliphatic
as well as aromatic alcohols are used as ester alcohols. The
aliphatic a=~cohols preferably have a chain length of 1 to 20
C-atoms. Preferred az-omatic alcohols are the phenols. These
can have mu7_tiple hydroxyl groups or multiple aliphatic
substituent~~ with a chain length of preferably one to six C-
atoms.
According to the invention, an alcohol which is to be
understood as a mono- or multi-functional aliphatic alcohol
with 2 to la: carbon at«ms is one which, due to its
evaporation and flammability characteristics, is suitable for
use in the inventive mE=_thod.
Aromatic compounds such as biphenyl etc. are also suitable as
further coating agents, as long as these compounds, as with
the other coating agenl:s usable according to the invention,
are characterized by a low-soot and residue-free evaporation.
A preferred class coat=ing materials used according to the
invention are (low-viscosity) esters of the phosphoric acids,
in particular with lcnc~er chain aliphatic alcohols with chain
lengths between 3 and =L5 C-atoms, preferably 6 to 13 C-atoms
as well as with phencl;~ or other aromatic alcohols with side
chains of 1 to 12 C-atoms, preferably 1 to 3 C-atoms such as
cresol. These compound; are known to one skilled in the art
as softeners. Examples of such compounds are tris-(2-
ethylhexyl)-phosphate, t=ris(2-butoxyethyl)-phosphate,
triphenylphosphate or ciiphenylcresylphosphate.
Diphenylcresylphosphate is an especially suitable
representative of this class of compounds, since it is
nontoxic, liquid at room temperature and has low viscosity,
it evaporates at approx_Lmately 230°C and burns soot-free. A
further advantage of diphenylcresylphosphate is its relative
temperature-independent: viscosity.
For the use as a sinter additive, the studies of the
inventors have shown that a thusly surface-modified manganese
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sulfide improves the machined processing, especially by
cutting, of sintered molded articles if the manganese sulfide
is incorporated into a sinter powder in the amount of 0.1
wt.% to 1.0 wt.%, pref~=rably from 0.2 to 0.6 wt. o, relative
to the weight of the sinter powder.
Accordingly, a further embodiment of the present invention is
directed to a sinter powder characterized by an amount of the
inventive manganese su:L:fide.
For use as an additive :in sintering, the manganese sulfide is
preferably treated with the inventive coating agents in the
manner explained above prior to the addition to the selected
metal powder. However __t= is also possible within the scope of
1> the invention to first rnix unmodified manganese sulfide with
the metal powder to be ;sintered and finally to add the
coating agent to the sinter mixture.
A further embodiment of: the present invention therefore also
relates to a method foxy producing a molded article using the
inventive sinter powder, wherein the method comprises the
following steps:
a) compression of the' sinter powder :in a sinter mold having
an inner shape corresponding to the end contour of the
2~~ finished molded article;
b) heating the green compact to a temperature above the
evaporation temperature of the coating agent used in the
production of the manganese ;sulfide and, if necessary,
maintaining the green compact at this temperature for a
period of time s~.zfficient to ensure complete evaporation
of the ~~oating agent;
c) sintering the gr~-yen compact from step b) ;
d) removing the cooled molded product. from the sinter mold.
A further embodiment therefore encompasses a molded article
obtainable b~~ the inventive production/sinter method
elaborated above.
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The present invention. is to be described by way of one non-
limiting example.
Example 1: Preparation of surface-modified manganese sulfide
20 g of liquid dipher~y:Lcresylphosphate were added to 2000 g
of manganese sulfide with a particle diameter of 4.96 ~m
(D50) in a L~oedige-mixer. The batch was subsequently mixed at
25°C for 20 minutes.
Determinaticn of the resistance to oxidation
The manganese sulfide, which had been treated with to
diphenylcresylphosphate,, obtained in Example 1 was stored
with uncoated manganese. sulfide of the same particle
diameter, that is according to EP 0 183 666 (Example 2) as
well as with a mangane~>e sulfide composition to U.S.
5,768,678 (Example 3) i.n an atmosphere of 75o humidity at
room temperature for 6 days. Subsequently the extent of
oxidation of the manganese sulfide on the surface to MnO,
MnOz and MnSO4 (according to the schematic reactions MnS + H20
=> MnO; Mn02; MnS04) wa~~ determined for every sample by
measurement of the weight increase (Table 1).
Table 1 Weiq:ht increase_after 6 days
Particle size D50 Weight increase
Example Manganese Sulfide
(gym) (%)
1 Inventive MnS 4.69 0.2
2 Uncoated MnS 4.49 25.8
3 MnS with :LO°s FeS 7.98 11.1
As can be SeE=_n in Tab_l.e 1, the inventive coated manganese
sulfide absorbs significantly less moisture than the uncoated
material or t=he manganese sulfide composition of U.S.
5,768,678. In standing with this, the inventive material is
still a fine, free-flowing powder lacking agglomerates after
10 weeks, while during this same time the uncoated manganese
sulfide changed to a c:~oarse-grained, dark product due to
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absorption of moistux-e and the formation of manganese
dioxide.
