Note: Descriptions are shown in the official language in which they were submitted.
CA 02662238 2009-03-02
GT64732PC D'usseldorf, September 11, 2006
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Goldschmidt TIB GmbH
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A novel additive for chromium electrolytes
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The present invention relates to the field of additives for chromium
electrolytes,
especially to the field of the additive surfactants for chromium electrolytes,
and to
the field of the additives for polymer metallizations.
For electrochemical chromium-plating, which typically takes place in chromium
electrolytes at high concentration of aggressive chromic acid, a wide variety
of
different additives are proposed in order to prevent the formation of
aggressive
spray mists. However, it has been found to be difficult in practice to find
compounds which, on the one hand, have favourable properties but, on the other
hand, survive the aggressive conditions in the chromium-plating.
Particularly suitable for the reduction of spray mist are foam-forming wetting
agents which, by lowering the surface tension, not only reduce the spray
losses but
also greatly reduce the entrainment of the chromium electrolyte. For this
purpose, a
wide variety of different products have been proposed, for example
perfluoroalkylsulphonic acids (PFOAs). These products are also stable to the
highly oxidative properties of chromic acid. However, their use is problematic
and
already banned in many applications. These fluorine surfactants are not
biodegradable, since they do not undergo any photolytic, hydrolytic, oxidative
or
reductive transformation whatsoever. They are biodegraded neither aerobically
nor
anaerobically. Owing to their physicochemical properties, perfluoro
alkylsulphonic acids remain as end metabolites and are not degraded any
further.
DE 1034945 has proposed alkylmethylsulphonates as additives, which are said to
have the properties of a surfactant and simultaneously bring about process-
improving influences with regard to the smoothing of the chromium layer.
However, these additives are unsuitable in practice, since they decompose in
the
course of chromium-plating within a very short time.
It is therefore an object of the present invention to find an additive for
chromium
electrolytes which reduces the disadvantages detailed above.
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This object is achieved by an additive according to Claim 1. Accordingly, an
additive, especially a smoothing additive for chromium electrolytes, is
provided,
characterized in that a chromic acid solution which contains 0.1 g/1 of
additive and
250 g/l of chromic acid has a surface tension of <_ 30 mN/m, and 0.1 g/1 of
additive,
at 45 C and 6000 Ah of charge passage, in a chromic acid solution which
contains
270 g/I of chromic acid, has a stability of _ 4 h.
It should be noted that the term "additive" within the present invention can
relate
either to an individual substance or to a substance mixture; for reasons of
readability and clarity, however, "additive" is only referred to in the
singular
within the present invention. If the additive used is a substance mixture,
what is
meant in each case is that the substance mixture has the properties described,
but
the individual components of the mixture may also have the properties
described.
In the context of the present invention, "stability" means especially the
lasting
efficacy of the additive with regard to the surface tension under the
chemically
demanding conditions of a chromium electrolyte.
In particular, in the context of the present invention, "stability" over a
certain
period means that the surface tension increases by not more than 5 mN/m over
this
time.
In particular, in the context of the present invention, "chromium
electrolytes"
and/or "chromic acid solution operations" are understood to mean chromium
electrolytes or chromic acid solutions which comprises catalysts and/or
further
acids.
It has been found that, surprisingly, when such an additive is added to
chromic acid
solutions in chromium-plating operations, in most applications of the present
invention, at least one, usually more than one, of the following advantages
can be
achieved:
- The use of the inventive additive improves the operation of the chromium
electrolytes in a lasting manner.
- The use of the additive leads to the formation of significantly smaller gas
bubbles, which is associated with a drastic reduction in the emission
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nuisance.
- It is likewise possible to considerably reduce entrainment losses.
- Depending on the process, when the additive is used, the dispersibility of
the electrolytes in many applications is improved.
- The additive does not adversely affect the properties of the layer, not even
with regard to layer properties such as hardness, crack network, structure,
etc.
A chromic acid solution which contains 0.1 g/l of additive and 250 g/l of
chromic
acid preferably has a surface tension of <_ 28 mN/m, even more preferably
<_ 25 mN.
A chromic acid solution which contains 0.1 g/l of additive and 400 g/1 of
chromic
acid preferably has a surface tension of <_ 35 mN/m, even more preferably <_
30
mN/m.
0.1 g/l of additive at 45 C in a chromium electrolyte which contains 270 g/l
of
chromic acid preferably has a stability of _ 8 h, even more preferably of _ 12
h.
In a preferred embodiment of the present invention, the additive is free of
fluorine
surfactants. This is understood to mean especially that the additive does not
contain
any organofluorine compound, or that the proportion of organofluorine
compounds
in the additive is below the detection limit.
In a preferred embodiment of the present invention, the additive is
biodegradable.
This is understood to mean especially that, according to OECD criteria, _
99.5%,
preferably _ 99.8%, of the additive has degraded in the screening test after 8
days.
In many applications, such an additive contributes to minimizing expenditure
with
regard to preventing the contamination of the environment, or even to making
it
entirely superfluous.
A chromium electrolyte which contains 0.1 g/l of additive and 250 g/1 of
chromic
acid preferably has a current density of _ 30 A/dm2 to < 60 A/dm2, even more
preferably _ 40 A/dm2 to_ 50 A/dm2.
A chromium electrolyte which contains 0.2 g/1 of additive and 350 to 400 g/l
of
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chromic acid preferably has a current density of _ 5 A/dmz to <_ 25 A/dmz,
even
more preferably _ 8 A/dm2 to S 20 A/dm2.
In a preferred embodiment of the present invention, the additive comprises a
material selected from the group of long-chain alkylmonosulphonic acids, long-
chain alkyldisulphonic acids, long-chain alkylpolysulphonic acids, salts of
the
long-chain alkylmonosulphonic acids, salts of the long-chain alkyldisulphonic
acids, salts of the long-chain alkylpolysulphonic acids and mixtures thereof.
In this context, "long-chain" is understood to mean C4 and greater. The long-
chain
alkyl radicals are preferably unbranched, but it is also possible to use
branched
alkylmono-, -di- and -polysulphonic acids and salts thereof.
The salts used in a preferred embodiment of the present invention are alkali
metal
salts, alkaline earth metal salts, NH4+ salts, NR4+ salts (where R = alkyl)
and
mixtures thereof.
In a preferred embodiment of the present invention, the additive comprises, as
at
least one component, the compound CH3(CH2)nSO3H or salts thereof, where n _ 10
and n<_ 18. In practice, these compounds often have a particularly elevated
stability and are preferred in this respect.
More preferably, the additive comprises, as at least one component, the
compound
CH3(CH2)nSO3H or salts thereof, where n _ 12 and n<_ 17; even more preferably,
the additive comprises, as at least one component, the compound CH3(CH2)nSO3H
or salts thereof, where n> 14 and n< 16.
The present invention also relates to use of the inventive additive as a
smoothing
agent in chromium electrolytes. In a preferred embodiment of the present
invention, the concentration of additive is between 0.05 g/1 and <_ 20 g/l,
more
preferably _ 0.1 g/I and <_ 10 g/l, and most preferably ? 1 g/I and <_ 3 g/l.
The present invention also relates to use of the inventive additive as an
additive in
polymer mordants. The inventors have found that, surprisingly, the inventive
additive can be used not only in chromium electrolytes but also in the
pretreatment
of polymer metallizations. In these mordants, the additive has a wetting
effect and
lowers the surface tension of the chromic acid-containing mordants. The
positive
influence on chromic acid mist formation and entrainment is comparable to the
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effects described above for chromium electrolytes.
The present invention also relates to use of an additive which comprises, as
at least
one component, the compound CH3(CH2)nSO3H or salts thereof, where n _ 10 and
n< 18, more preferably n> 12 and n 5 17, even more preferably n _ 14 and n<_
16,
or mixtures thereof, as an additive in polymer mordants.
The aforementioned components to be used in accordance with the invention, and
those claimed and described in the working examples, are not subject to any
particular exceptional conditions in their size, three-dimensional
configuration,
material selection and technical design, such that the selection criteria
known in the
field of use can be used without restriction.
Further details, features and advantages of the subject-matter of the
invention are
evident from the subclaims and from the description of an inventive example
which follows, in which - in a purely illustrative and non-restrictive manner -
one
use of an inventive additive is detailed.
Example I:
In a bath containing 400 g/l of chromic acid, 5 g/l of phosphoric acid, 3 g/1
of
potassium nitrate, 3 g/l of rare earth fluorides (e.g. cerium, lanthanum) and,
as the
inventive additive, 2 g/l of sodium pentadecanesulphonate, at a temperature of
20-25 C and a current density of 20 A/dm2, a black chromium coating was
performed.
It was possible to lower the surface tension to a value of 29.8 mN/m by the
addition of the inventive additive.
In the subsequent studies, it was found that the chromium layer deposited had
a
very uniform appearance. In particular, the dispersibility of the electrolyte
was
improved. The testing of several sheets gave a dispersion of the chromium
layer
which was improved by an average of 1.0-1.5 cm according to corresponding
tests
in a Hull cell.
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MATERIALS AND METHODS
The surface tension was measured with a K8 tensiometer from Kruss Hamburg.
The unit works by the Du Nouy ring method. The force of a liquid lamella drawn
up by the ring is measured. The liquid is raised until there is contact of the
ring
with the surface. With the aid of a torsion balance, the force required to
raise the
platinum ring is measured. The further the ring is pulled out of the liquid,
the
greater this is. At the point of the highest force applied, when the liquid
lamella
breaks off, there is a force equilibrium from which the surface tension of the
liquid
can be calculated. The ring geometry is taken into account by means of an
instrument-specific calibration by the manufacturer.
The current density is determined by means of a measurement of the currents
with
an amperemeter and reference to the known surface geometry of the components
to
be chromium-plated.
The dispersibility of an electrolyte is determined by the evaluation of sheets
after
tests with the electrolyte in a Hull cell. The dispersion of the chromium
layer is
determined by measuring the expansion of the coated surface on the sheet after
it
has passed through the test run. The measurement is effected with a ruler. In
general, several sheets are coated and measured under the same conditions in
order
to obtain reliable averages.
