Note: Descriptions are shown in the official language in which they were submitted.
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"Cooling liquid for use in units made of magnesium"
This invention relates to an alcohol-based cooling or heating medium. It may
be used in
cooling or heating systems, such as air-conditioning systems, heat exchangers
and, in
particular, cooling systems for internal combustion engines. The cooling
liquid is especially
designed to cool units, especially internal combustion engines, made of
magnesium and/or
magnesium alloys.
Cooling systems in general and, in particular, cooling systems for internal
combustion
engines, such as motor vehicle engines, consist of a variety of different
metals, such as
copper, brass, steel, cast iron, aluminum, magnesium and alloys thereof.
Furthermore
solders, such as soldering tin, are generally present. This material
composition brings with
it particular problems of corrosion, especially in automobile cooling systems
where high
temperatures, pressures and flow speeds are present in the cooling system.
Corrosion
shortens the service life of the cooling system and leads to a reduction in
efficiency through
the build-up of undesirable deposits. Cooling liquids which are suitable for
automobile
cooling systems, for example, have therefore not only to have freezing points
which are
markedly below 0°C, for example between -20 and -30 °C, but must
also be effectively
protected against corrosion.
At present, grey cast iron or aluminum engines are the most commonly used in
the
automobile industry. The prior art, as referred to below, discloses
serviceable corrosion-
inhibiting cooling liquids for these materials.
A water-soluble liquid alcohol component, in particular ethylene glycol, is
conventionally
used for the purpose of lowering the freezing point. In addition to these
alcohol components,
anti-corrosive agents are absolutely essential. Nowadays, these are required
to be effective
in particularly low concentrations and as far as possible not to contain
substances which
might be questionable from the point of view of toxicology and/or damaging to
the
environment.
Antifreeze agents for cooling systems for grey cast iron and aluminum engines
are known
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from the prior art which contain, in addition to the alcohol components, an
anti-corrosive
system based on a combination of certain carboxylic acids with triazoles, the
efficiency of
which may be improved by further anti-corrosive additives, such as borates,
phosphates or
silicates.
For example, EP-A-251 480 discloses a corrosion-inhibited concentrated
antifreeze which
contains, in addition to from 90 to 99 wt. % alcohol, 0.1 to 5 wt. %
alkylbenzoic acid or the
salts thereof, 0.1 to 5 wt. % of an aliphatic monocarboxylic acid having from
8 to 12 carbon
atoms and 0.1 to 0.5 wt. % of a triazole.
EP-B-308 037 discloses an antifreeze composition having corrosion-inhibiting
properties,
which consists essentially of: 90 to 99 wt. % of a water-soluble liquid
alcohol medium for
depressing freezing point, 0.1 to 5 wt. % of an aliphatic monobasic acid
having from 6 to 12
carbon atoms, 0.1 to 5 wt. % of an alkali metal/borate compound and 0.1 to 0.5
wt. % of a
triazole.
An alcohol-based concentrated antifreeze is known from EP-B-229 440 which
contains 0.1
to 15 wt. % of an aliphatic monocarboxylic acid having from 5 to 16 carbon
atoms, 0.1 to
15 wt. % of a dicarboxylic acid having from 5 to 16 carbon atoms and 0.1 to
0.5 wt. % of
a triazole, wherein the weight percentages are based on the amount of liquid
alcohol present.
According to the relatively narrow teaching of these three documents, ethylene
glycol is
preferably used as the alcohol component and benzotriazole or tolyltriazole
are preferably
used as the triazole. DD-A-218 635 proposes the use of a mixture containing 2-
ethylhexanoic
acid, mercaptobenzothiazole and carboxymethylcellulose or reaction products of
these three
components as the anti-corrosive system for cooling or heating media.
DE-A-195 46 472 proposes a concentrated antifreeze in which the anti-corrosive
system
consists of from 0.005 to 5 wt. % branched aliphatic carboxylic acids having
from 6 to 11
carbon atoms and a synergistic combination of from 0.005 to 0.04 wt. % of each
of
tolyltriazole and benzotriazole.
When used to cool engines of grey cast iron or of aluminum alloys, these
antifreeze agents
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adequately fulfil the technical requirements. However, attempts are currently
being made in
the automobile construction industry to reduce the weight of internal
combustion engines by
making them partially or completely of magnesium and/or magnesium alloys.
Tests have
shown that, owing to the increased chemical reactivity of these materials, the
conventional
antifreeze agents do not fulfil the anti-corrosive requirements when diluted
with considerable
amounts of water, as is conventional. An object of the present invention is
therefore to
provide a cooling liquid with which units, such as internal combustion engines
of magnesium
and/or magnesium alloys in particular, may be cooled without unacceptable
corrosion
damage.
This object is achieved by using a cooling liquid which, based on the total
composition,
comprises:
(a) 0.005 to 0.5 wt. % tolyltriazole
(b) 0.005 to 0.5 wt. % benzotriazole
(c) 0.005 to 10 wt. % of one or more corrosion inhibitors selected from
branched
aliphatic carboxylic acids having 6 to 11 carbon atoms and alkanolamine
phosphates
(d) 70 to 99.985 wt. % of a water-soluble liquid alcohol having a boiling
point at normal
pressure of over 100°C
this being made up to 100 wt. % with water, alkalis and/or other active
substances, for
cooling units made of magnesium and/or magnesium alloys.
Pure magnesium is not generally used for the construction of internal
combustion engines,
but rather magnesium alloys. Examples of such alloys are: AS 21 and AZ 91.
The use according to the present invention differs from the use of similar
cooling liquids
known in the prior art for materials other than magnesium and magnesium alloys
in that the
cooling liquid having the above-mentioned composition is used as it is. Thus,
it is not further
diluted with water. The water content of the cooling liquid used according to
the present
invention consequently amounts to a maximum of 30 wt. % and is preferably
less. The
cooling liquid preferably contains no more than approximately 15 wt. % water.
By way of
example, cooling liquids of this type may be used in accordance with the
present invention
having a water content of less than 5 wt. % .
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When carboxylic acids are mentioned in the present context, what is generally
meant is the
acids in protolysed or non-protolysed form, i.e. the acids may be present as
such or as
anions. The protolysis equilibrium of the acids is adjusted according to the
acidity constants
depending on the pH of the cooling liquid.
The branched aliphatic carboxylic acids (c) are present in the cooling liquid
within the
preferred concentration range of from 0.5 to 4 wt. % and the tolyltriazole and
benzotriazole
are each present within the preferred concentration range of from 0.005 to
0.05 wt. % . If an
alkanolamine phosphate is selected as the corrosion inhibitor (c), its
concentration is
preferably set at from about 0.1 to about 10 wt. %. In this case,
trialkanolamine phosphate,
and especially triethanolamine phosphate is selected as the alkanolamine
phosphate.
The water-soluble liquid alcohol should have a boiling point at normal
pressure of over
100°C and especially over 120°C. The "liquid" criterion is to be
understood to mean that
the selected alcohol is liquid at the temperatures which may occur when the
unit to be cooled
is either at a standstill or in operation. The temperature range which is in
practice relevant
extends from about -35 to about 110°C. The cooling liquid may be used
in a closed cooling
system, in which an operating pressure markedly above normal pressure may
build up during
operation of the unit to be cooled. The decisive factor is whether the alcohol
used remains
liquid under the prevailing pressure and temperature conditions.
The water-soluble liquid alcohol is preferably selected from alkene glycols,
especially from
monoethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol
and/or from
water-miscible monoethers thereof, for example monomethyl, monoethyl,
monopropyl and
monobutyl ethers of the above-mentioned glycols. Monoethylene glycol and/or
propylene
glycol are particularly preferable.
If branched aliphatic carboxylic acids are selected as the anti-corrosive
component (c), the
latter are preferably selected from 2-ethylhexanoic acid, 2,2-dimethyloctanoic
acid and 3,5,5-
trimethylhexanoic acid or mixtures thereof. Owing to its easy availability and
good anti-
corrosive action, 2-ethylhexanoic acid is particularly preferable.
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It is also preferable for the cooling liquid additionally to contain as a
further active substance
from 0.5 to 1$ wt. % of one or more linear saturated or unsaturated aliphatic,
araliphatic or
aromatic mono- or poly-basic carboxylic acids having from 4 to 20 carbon
atoms. The anti-
corrosive effect is markedly improved thereby. The lower limit for the carbon
number, i.e.
4 carbon atoms, relates to aliphatic carboxylic acids. Aromatic carboxylic
acids must contain
at least 7 carbon atoms. The carboxylic acids which may be optionally
introduced are
preferably selected from sebacic acid, caprylic acid, nonanoic acid, decanoic
acid,
undecanoic acid, benzoic acid, cinnamic acid, gluconic acid or mixtures
thereof. Acids which
are particularly preferred are sebacic acid, caprylic acid and cinnamic acid.
In addition to
these acids, the cooling liquids may contain polybasic carboxylic acids having
a particularly
marked complexing action, for example tartaric acid and especially citric
acid.
Another active substance which reinforces the anti-corrosive action,
especially in the case of
non-ferrous heavy metal components, is mercaptobenzothiazole, which is
introduced into the
cooling liquid in amounts of between 0.0001 and 0.$ wt. %, preferably between
0.002 and
0.0$ wt. % . The action of the mercaptobenzothiazole is further increased by
the additional
use of carboxymethyl-cellulose. It is therefore preferable for the cooling
liquid additionally
to contain, as a further active substance, from 0.0001 to 0.$ wt. %,
especially 0.002 to 0.0$
wt. %, of carboxymethylcellulose. Mercaptobenzo-thiazole and
carboxymethylcellulose may
be added to the cooling liquid independently of each other. However, according
to the
teaching of DD 218 635 it is preferable to prepare at an elevated temperature
($0 to 6$°C)
a preliminary mixture of branched aliphatic carboxylic acid (c),
mercaptobenzothiazole and
carboxymethyl-cellulose, in which preliminary mixture partial reactions of
these reactants
may occur. This preliminary product is preferably produced by starting with a
concentrated
aqueous-alkaline carboxymethylcellulose solution, to which the
mercaptobenzothiazole is
added with stirring in the temperature range of from $0 to 6$°C, the
branched carboxylic
acid being slowly added to this mixture after many hours of reaction time. The
weight ratio
of the three active substances carboxymethylcellulose, mercaptobenzo-thiazole
and branched
aliphatic carboxylic acid preferably ranges from 1 : 1 : 1 to 1 : $ : $0. If
it is desired to
increase the amount of branched aliphatic carboxylic acid in the cooling
liquid to above the
upper limit given in this quantitative ratio, the desired amount of acid is
additionally added.
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The cooling liquid may contain additional inhibiting components known from the
prior art.
For example, zinc salts and alkali metal or ammonium molybdates may be
introduced in
amounts of from 0.01 to 2 wt. %. A prerequisite of this, however, is that
sufficient water
must be present in the cooling liquid for these components to dissolve.
Furthermore, a cooling liquid is preferably used which is free from nitrite,
borate and
silicate.
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Examples
Table 1 contains Examples of anhydrous cooling liquids for use according to
the present
invention. The composition is given in wt. %. "Anhydrous" in this context
should be
understood to mean that no water has been added to the cooling liquid.
However, it is not
impossible that the cooling liquid may contain small amounts of water owing to
the
hygroscopic properties of monoethylene glycol.
Table 1: Anhydrous cooling liquids
Example (in wt. %)
(made up to 100
wt. % with
monoethylene glycol)
Ex. 1 Ex.2 Ex.3
Benzotriazole 0.02 0.01 0.015
Tolyltriazole 0.02 0.01 0.01
2-ethylhexanoic acid2.5 3.8 -
Sebacic acid 1 - -
Caprylic acid - 1 -
Mercaptobenzo-thiazole- - 0.03
Carboxymethyl- 0.0001 0.0002 -
cellulose
Triethanolamine - - 2
phosphate
The anti-corrosive effect was tested in accordance with ASTM testing
specification D 1384-
70. Samples of metals typically present in motor vehicle cooling systems were
immersed
completely in the antifreeze solution for 336 hours with simultaneous
aeration. The
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temperature was 88°C. The corrosion-inhibiting properties of the test
solutions were
evaluated on the basis of the changes in weight of the samples. Each test was
carried out 3
times and the average weight change was determined for each metal. Before the
test started,
the samples were bright-polished with a moist scrubbing brush and ground
pounce, washed
with water and then acetone, dried and weighed. When the test was complete,
the corrosion
products on the samples were removed by brushing and by immersion in acid
solutions. The
samples were then washed, dried and weighed again. Table 2 contains the weight
losses (in
g/mz) for various metals using the exemplary or comparative solutions.
The test solutions were the solutions according to Examples 1 to 3 having
different amounts
of added water. The percentages in Table 2 are wt. % based on the resultant
total mixture.
The following Examples were examined: the anhydrous cooling liquids according
to Example
1 additionally had 10 wt. %, 20 wt. % and 30 wt. % water added, the cooling
liquid according
to Example 2 had 10 wt. % water added and the cooling liquid according to
Example 3 had
S wt. % water added. As a Comparative Example, the cooling liquid according to
Example
1 had 50 wt. % water added.
The corrosion inspections carried out on the tested metals show that all the
test liquids
including the comparative solution provided acceptable corrosion results apart
from in the
case of magnesium. In the case of magnesium, however, acceptable corrosion
behavior was
noted only when the test liquid contained up to 30 wt. % water. As the
Comparative Example
shows, when the water content is 50 wt. % , severe corrosion with pitting
occurs.
CA 02307621 2000-04-17
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