Note: Descriptions are shown in the official language in which they were submitted.
2162459
WO 94/26860 PCT/EP94/01387
Corrosion inhibitors for silver (II)
It is a generally known fact that silver "tarnishes"
even when it is not in use. It is only a matter of time
before it develops dark, brownish, bluish to blue-black
stains or completely discolors and, hence, is said in
common usage to have "tarnished".
In practice, the machine washing of table silver
also involves recurring problems in the form of tarnish-
ing and discoloration of the silver surfaces. In this
case, silver can react to sulfur-containing substances
which are dissolved or dispersed in the wash liquor
because, in domestic dishwashing machines (DDWM), food
residues, including mustard, peas, egg, and other sulfur-
containing compounds are introduced into the wash liquor.
The much higher temperatures prevailing in dishwashing
machines and the longer contact times with the sulfur-
containing food residues promote the tarnishing of silver
by comparison with manual dishwashing. In addition,
through the intensive cleaning process in dishwashing
machines, the silver surface is completely degreased and,
hence, becomes more sensitive to chemical influences.
Where detergents containing active chlorine are
used, tarnishing by sulfur-containing compounds can
largely be prevented because these compounds are reacted
to sulfones or sulfates by oxidation of the sulfidic
functions in a secondary reaction.
However, the problem of tarnishing in the case of
silver became topical again when active oxygen compounds,
such as sodium perborate or sodium percarbonate for
example, were used as an alternative to active chlorine
compounds to eliminate bleachable soils, for example tea
stains/tea coatings, coffee residues, dyes from vegetab-
les, lipstick residues and the like.
2162459
Wo 94/26860 2 PCT/EP94/01387
These active oxygen compounds are used in conjunc-
tion with bleach activators above all in modern low-
alkali machine dishwashing detergents of the new genera-
tion. These modern detergents generally consist of the
following functional components: builder component
(complexing agent/dispersant), alkali carrier, bleaching
system (bleaching agent + bleach activator), enzymes and
wetting agents (surfactants).
Basically, the silver surfaces react more sensitive-
ly to the modified formulation parameters of the new-
generation detergents free from active chlorine with
their reduced pH values and activated oxygen bleaching.
During the machine dishwashing process, these detergents
release the actual bleaching agent, hydrogen peroxide or
active oxygen, in the wash cycle. The bleaching effect
of detergents containing active oxygen is enhanced by
bleach activators so that a good bleaching effect is
obtained even at low temperatures. In the presence of
these bleach activators, peracetic acid is formed as a
reactive intermediate compound. Under the modified wash-
ing conditions, not only are sulfidic coatings formed in
the presence of silver, oxidic coatings are also formed
on the silver surfaces through the oxidizing effect of
the intermediately formed peroxides or the active oxygen.
Chloride coatings can also be formed in the presence of
high salt concentrations. In addition, tarnishing of the
silver is intensified by relatively high residual water
hardness values during the wash cycle.
Avoiding the corrosion of silver, i.e. the formation
of sulfidic, oxidic or chloridic coatings on silver, is
the subject of numerous publications. In these publica-
tions, the corrosion of silver is prevented above all by
so-called silver protectives.
GB 1,131,738 describes alkaline dishwashing deter-
gents containing benzotriazoles as corrosion inhibitors
2162~5~
Wo 94/26860 3 PCT/EP94/01387
for silver. US 3,549,539 describes highly alkalinemachine dishwashing detergents which may contain inter
alia perborate as oxidizing agent in conjunction with an
organic bleach activator. Additions of inter alia benzo-
triazole and iron(III) chloride are recommended to pre-
vent tarnishing. pH values of, preferably, 7 to 11.5 are
mentioned. EP 135 226 and EP 135 227 describe low-alkali
machine dishwashing detergents containing peroxy com-
pounds and activators in which inter alia benzotriazoles
and fatty acids may be present as silver protectives.
Finally, it is known from DE-OS 41 28 672 that peroxy
compounds activated by addition of known organic bleach
activators prevent the tarnishing of silver in highly
alkaline detergents.
It has now surprisingly been found that organic
redox-active substances, more particularly the primary
and/or secondary intermediates typically used in oxida-
tion dyes which have not hitherto been described as
corrosion inhibitors for silver, effectively prevent the
corrosion of silver in dishwashing machines.
The present invention relates to the use of organic
redox-active substances as corrosion inhibitors for
silver in dishwashing detergents.
The word "corrosion" is to be interpreted in its
broadest chemical sense. More particularly, "corrosion"
in the context of the present invention is intended to
stand for any visually just discernible change in a metal
surface, in the present case silver, whether for example
in the form of discolored spots or, for example, in the
form of stains covering a relatively large area.
"0rganic redox-active substances" are organic sub-
stances which are accessible to readily occurring,
reversible oxidation and/or reduction. For example,
typical complexing agents, for example EDTA or hydroxy-
ethane diphosphonic acid and related compounds, do not
2162~5~
WO 94/26860 4 PCT/EP94/01387
fall under this definition.
Typical "organic redox-active substances" are, for
example, ascorbic acid (vitamin C), indole, methionine
(~-amino-~-methylmercaptobutyric acid).
N-Mono-(C14-alkyl)-glycines, for example N-monomethyl
glycine, and N,N-di-(C14-alkyl)-glycines, for example N,N-
dimethyl glycine and 2-phenyl glycine, are also suitable.
Certain primary and/or secondary intermediates known
from oxidation dyeing are also particularly suitable.
The organic substances preferably used to prevent
the corrosion of silver are primary and/or secondary
intermediates selected from the group of diaminopyri-
dines, aminohydroxypyridines, dihydroxypyridines, hetero-
cyclic hydrazones, aminohydroxypyrimidines, dihydroxypyr-
imidines, tetraaminopyrimidines, triaminohydroxypyrimi-
dines, diaminodihydroxypyrimidines, dihydroxynaphtha-
lenes, naphthols, pyrazolones, hydroxyquinolines, amino-
quinolines, primary aromatic amines with another free or
C14-alkyl- or C24-hydroxyalkyl-substituted hydroxy or
amino group in the ortho, meta or para position and di-
or trihydroxybenzenes.
The primary and secondary intermediates used in
accordance with the invention to prevent the corrosion of
silver are the substances from the above-mentioned groups
normally used in oxidation dyes. Examples of such pri-
mary and secondary intermediates can be found, for exam-
ple, in Venkataraman, "The Chemistry of Synthetic Dyes",
Vol. V, Academic Press, New York/London, 1971, pages 478-
495 and in the literature cited therein. Primary and
secondary intermediates selected from the group consist-
ing of p-hydroxyphenylglycine, 2,4-diaminophenol, 5-
chloro-2,3-pyridinediol, l-(p-aminophenyl)-morpholine,
hydroquinone, pyrocatechol, hydroxyhydroquinone, gallic
acid, phloroglucinol, pyrogallol are particularly suit-
able for preventing the corrosion of silver.
2162~59
W0 94/26860 5 PCT/EP94/01387
The organic redox-active substances are preferably
coated, i.e. completely surrounded by a material which is
water-resistant, but readily soluble at the dishwashing
temperatures in order to prevent their premature decom-
position or oxidation during storage. Preferred coatingmaterials, which are applied by known methods, for
example by the Sandwik melt coating process used in the
food industry, are paraffins, microwaxes, waxes of
natural origin, such as carnauba wax, candellila wax,
beeswax, relatively high-melting alcohols, such as
hexadecanol for example, soaps or fatty acids. The
coating material, which is solid at room temperature, is
applied in molten form to the material to be coated, for
example by projecting fine-particle material to be coated
in a continuous stream through a continuously produced
spray mist zone of the molten coating material. The
melting point has to be selected so that the coating
material readily dissolves or rapidly melts during the
subsequent use of the silver corrosion inhibitor in a
dishwashing machine. For most applications, therefore,
the melting point should ideally be between 45C and 65C
and is preferably between 50C and 60C.
However, the organic redox-active substances de-
scribed above are particularly suitable for preventing
the corrosion of silver when used in low-alkali machine
dishwashing detergents. This is all the more surprising
insofar as these silver corrosion inhibitors are not
affected in their performance by the presence of oxygen-
based bleaching agents typically present in low-alkali
detergents.
Accordingly, the present invention also relates to
low-alkali machine dishwashing detergents of which 1% by
weight solutions have a pH value of 8 to 11.5 and prefer-
ably 9 to 10.5 and which contain 15 to 60% by weight and
preferably 30 to 50% by weight of a water-soluble builder
2162~S~
WO 94/26860 6 PCT/EP94/01387
component, 5 to 25% by weight and preferably 10 to 15% by
weight of an oxygen-based bleaching agent, 1 to 10% by
weight and preferably 2 to 6% by weight of an organic
bleach activator, 0.1 to 5% by weight and preferably 0.5
to 2.5% by weight of an enzyme, based on the detergent as
a whole, and silver corrosion inhibitors, an organic
redox-active substance being present as the silver corro-
sion inhibitor. Particularly suitable silver corrosion
inhibitors are ascorbic acid, indole, methionine, N,N'-
di-(C14-alkyl)-glycine and 2-phenyl glycine, but above
all primary and/or secondary intermediates selected from
the group of diaminopyridines, aminohydroxypyridines,
dihydroxypyridines, heterocyclic hydrazones, aminohy-
droxypyrimidines, dihydroxypyrimidines, tetraaminopyri-
midines,triaminohydroxypyrimidines,diaminodihydroxypyr-
imidines, dihydroxynaphthalenes, naphthols, pyrazolones,
hydroxyquinolines, aminoquinolines, primary aromatic
amines with another free or C14-alkyl- or C24-hydroxy-
alkyl-substituted hydroxy or amino group in the ortho,
meta or para position and di- or trihydroxybenzenes.
Preferred dishwashing detergents contain primary
and/or secondary intermediates selected from the group
consisting of p-hydroxyphenylglycine, 2,4-diaminophenol,
5-chloro-2,3-pyridinediol, 1-(p-aminophenyl)-morpholine,
hydroquinone, pyrocatechol, hydroxyhydroquinone, gallic
acid, phloroglucinol, pyrogallol and mixtures thereof.
The organic redox-active substances are preferably
present in the detergents according to the invention in
a total quantity of 0.05 to 6% by weight and preferably
0.2 to 2.5% by weight, based on the detergent as a whole.
Basically, suitable water-soluble builder components
are any of the builders typically used in machine dish-
washing detergents, for example polymeric alkali metal
phosphates, which may be present in the form of their
alkaline, neutral or acidic sodium or potassium salts.
2162g~
WO 94/26860 7 PCT/EP94/01387
Examples include tetrasodium diphosphate, disodium
dihydrogen diphosphate, pentasodium triphosphate, so-
called sodium hexametaphosphate and the corresponding
potassium salts or mixtures of sodium hexametaphosphate
and the corresponding potassium salts or mixtures of
sodium and potassium salts. The quantities of phosphate
are up to about 30~6 by weight, based on the detergent as
a whole. However, the detergents according to the inven-
tion are preferably free from such phosphates. Other
possible water-soluble builder components are, for exam-
ple, organic polymers of native or synthetic origin,
above all polycarboxylates, which may act as cobuilders,
particularly in hard water systems. For example, poly-
acrylic acids and copolymers of maleic anhydride and
acrylic acid and also the sodium salts of these polymer
acids may be used. Commercial products are, for example,
Sokalan~ CP 5 and PA 30 (BASF), Alcosperse~ 175 or 177
(Alco), LMW~ 45 N and SPO2 N (Norsohaas). Native poly-
mers include, for example, oxidized starch (for example
German patent application P 42 28 786.3) and polyamino-
acids, such as polyglutamic acid or polyaspartic acid,
for example the products of Cygnus or SRCHEM.
Other possible builder components are naturally
occurring hydroxycarboxylic acids such as, for example,
monohydroxysuccinic acid, dihydroxysuccinic acid, ~-
hydroxypropionic acid and gluconic acid. Preferred
builder components are the salts of citric acid, more
particularly sodium citrate. The sodium citrate used may
be anhydrous trisodium citrate and is preferably dihydra-
ted trisodium citrate. Dihydrated trisodium citrate may
be used in the form of a fine or coarse crystalline
powder. Depending on the pH value ultimately established
in the detergents according to the invention, the acids
corresponding to citrate may also be present.
Suitable oxygen-based bleaching agents are, above
2162459
Wo 94/26860 8 PCT/EP94/01387
all, sodium perborate monohydrate and tetrahydrate or
sodium percarbonate. The use of sodium percarbonate has
advantages because sodium percarbonate has a particularly
favorable effect on the corrosion behavior of glasses.
Accordingly, the oxygen-based bleaching agent is prefer-
ably a percarbonate salt, more particularly sodium per-
carbonate. Since active oxygen only develops its full
effect at elevated temperatures, so-called bleach activa-
tors are used to activate it in the dishwashing machine.
Suitable bleach activators are organic bleach activators,
for example PAG (pentaacetyl glucose), DADHT (1,5-diace-
tyl-2,4-dioxohexahydro-1,3,5-triazine) and ISA (isatoic
anhydride), N,N,N',N'-tetraacetyl ethylenediamine (TAED)
being preferred. In addition, it can also be useful to
add small quantities of known bleach stabilizers, for
example phosphonates, borates or metaborates and metasil-
icates and also magnesium salts, such as magnesium
sulfate.
To improve the removal of protein-, fat- or starch-
containing food remains, the dishwashing detergents
according to the invention contain enzymes, such as
proteases, amylases, lipases and cellulases, for example
proteases, such as BLAP~ 140 (Henkel); Optimase~ -M-440,
Optimase~ -M-330, Opticlean~ -M-375, Opticlean~ -M-250
(Solvay Enzymes); Maxacal~ CX 450.000, Maxapem~ (Ibis);
Savinase~ 4.0 T, 6.0 T, 8.0 T (Novo); Esperase~ T (Ibis),
and amylases, such as Termamyl~ 60 T, 90 T (Novo);
Amylase-LT~ (Solvay Enzymes) or Maxamyl~ P 5000, CXT 5000
or CXT 2900 (Ibis); lipases such as Lipolase~ 30 T
(Novo); cellulases, such as Celluzym~ 0.7 T (Novo
Nordisk). The dishwashing detergents preferably contain
proteases and/or amylases.
In a preferred embodiment, the detergents according
to the invention additionally contain the alkali carriers
present in typical low-alkali machine dishwashing deter-
2162~59
WO 94/26860 9 PCT/EP94/01387
gents, for example alkali metal silicates, alkali metalcarbonates and/or alkali metal hydrogen carbonates. The
alkali carriers normally used include carbonates, hydro-
gen carbonates and alkali metal silicates with a molar
ratio of SiOz to M2O (M = alkali metal atom) of 1.5:1 to
2.5:1. Alkali metal silicates may be present in quan-
tities of up to 30% by weight, based on the detergent as
a whole. The highly alkaline metasilicates are preferab-
ly not used as the alkali carrier. The alkali carrier
system preferably used in the detergents according to the
invention is a mixture of - essentially - carbonate and
hydrogen carbonate, preferably sodium carbonate and
hydrogen carbonate, which is present in a quantity of up
to 60% by weight and preferably 10 to 40% by weight,
based on the detergent as a whole. The ratio of car-
bonate used to hydrogen carbonate used varies according
to the pH value ultimately required or established.
However, an excess of sodium hydrogen carbonate is
normally used, so that the ratio by weight of hydrogen
carbonate to carbonate is generally from 1:1 to 15:1.
Surfactants, more particularly low-foaming nonionic
surfactants, may optionally be added to the detergents
according to the invention to improve the removal of fat-
containing food remains. They also serve as wetting
agents, as granulation aids or as dispersion aids to
improve and homogenize the distribution of the silver
corrosion inhibitors in the wash liquor and on the silver
surfaces. The surfactants are used in quantities of up
to 5% by weight and preferably in quantities of up to 2%
by weight. Extremely low-foaming compounds are normally
used and preferably include C1218 alkyl polyethylene
glycol polypropylene glycol ethers with up to 8 moles of
ethylene oxide and propylene oxide units in the molecule.
However, it is also possible to use other nonionic
surfactants known as low foamers, including for example
2I 62~ 5~
WO 94/26860 10 PCT/EP94/01387
C12l8 alkyl polyethylene glycol polybutylene glycol ethers
containing up to 8 moles of ethylene oxide and butylene
oxide units in the molecule, end-capped alkyl polyalky-
lene glycol mixed ethers and the foaming, but ecological-
ly attractive C814 alkyl polyglucosides with a degree ofpolymerization of about 1 to 4 (for example APG~ 225 and
APG~ 600, Henkel KGaA) and/or C12l4 alkyl polyethylene
glycols containing 3 to 8 ethylene oxide units in the
molecule. A bleached quality should be used because
otherwise brown granules are formed. Surfactants from
the family of glucamides such as, for example, alkyl-N-
methyl glucamides (alkyl = C6l4 fatty alcohol) are also
suitable. In some cases, it is of advantage to use the
described surfactants in the form of mixtures, for
example a mixture of alkyl polyglycoside with fatty
alcohol ethoxylates or a mixture of glucamides with alkyl
polyglycosides, etc.
If the detergents foam excessively in use, a foam-
suppressing compound, preferably from the group of
silicone oils, mixtures of silicone oil and hydrophobi-
cized silica, paraffin oil/Guerbet alcohols, paraffins,
hydrophobicized silica, bis-stearic acid amides and other
known commercially available defoamers, may be added to
them in quantities of up to 6% by weight and preferably
in quantities of about 0.5 to 4% by weight. Other
optional additives are, for example, perfume oils.
The dishwashing detergents according to the inven-
tion are preferably present as powders, granules or
tablets which may be produced in known manner, for
example by mixing, granulation, roll compacting and/or by
spray drying.
To produce detergents according to the invention in
tablet form, all the constituents are preferably mixed
together in a mixer and the mixture obtained is tabletted
in a conventional tabletting press, for example an
21 62459
WO 94/26860 11 PCT/EP94/01387
eccentric or rotary press, under pressures of 200-105 Pa
to 1500-105 Pa. Breaking-resistant tablets with a
flexural strength normally in excess of 150 N, which
still dissolve sufficiently rapidly under in-use condi-
tions, are readily obtained in this way. A correspond-
ingly produced tablet weighs 15 g to 40 g and, more
particularly, 20 g to 30 g for a diameter of 35 mm to 40
mm.
The production of machine dishwashing detergents in
the form of non-dust-emitting, storable free-flowing
powders and/or granules with high apparent densities of
750 to 1000 g/l is characterized in that, in a first
process step, the builder components are mixed with at
least part of the liquid components with an increase in
the apparent density of this premix, after which the
other components of the machine dishwashing detergent,
including the organic redox-active substances, are
combined with the premix obtained, if desired after
drying.
Since the possible presence of alkali metal car-
bonate can have a considerable effect on the alkalinity
of the product, the intermediate drying step must be
carried out in such a way that the decomposition of
sodium bicarbonate to sodium carbonate is minimal (or at
least constant). Any additional sodium carbonate formed
as a result of drying would of course have to be taken
into consideration in the formulation for the granules.
Low drying temperatures not only counteract the decompos-
ition of sodium bicarbonate, they also increase the solu-
bility of the granulated detergent in use. Accordingly,
drying is advantageously carried out at an inflowing air
temperature which, on the one hand, should be as low as
possible to avoid the decomposition of bicarbonate and
which, on the other hand, must be as high as necessary to
obtain a product having good storage properties. An
2162~59
Wo 94/26860 12 PCT/EP94/01387
inflowing air temperature of around 80C is preferable
for drying. The granules themselves should not be heated
to temperatures above about 60C. In the first stage of
the mixing process, the liquid components are applied to
the builder generally after it has been mixed with at
least one other component of the dishwashing detergent.
For example, the liquid nonionic surfactants and/or the
solution of perfumes may be applied to and thoroughly
mixed with the builder component in the form of a mixture
with perborate. The remaining components are then added
and the mixture as a whole is compounded and homogenized
in the mixer. There is generally no need to use addi-
tional quantities of liquid, i.e. additional water. The
mixture obtained is present in the form of a free-flow-
ing, dust-free powder with the required high apparent
density of around 750 to 1000 g/l.
The granules are then mixed with the missing com-
ponents of the dishwashing detergent, including organic
redox-active substances, to form the end product. In all
the cases illustrated here, the mixing time both in the
preliminary stage of compacting mixing in the presence of
liquid components and in the following final mixing stage
where the other components are incorporated is a few
minutes, for example from 1 to 5 minutes.
In one particular embodiment, it can be useful in
the production of fine granules to ensure further stabi-
lization and equalization by dusting the surface of the
granules formed with powder. Small amounts of waterglass
powder or powder-form alkali metal carbonate are par-
ticularly suitable for this purpose.
The detergents to be used may be used both in
domestic dishwashing machines and in institutional dish-
washing machines. They are added either by hand or by
means of suitable dispensers. The in-use concentration
in the dishwashing liquor is about 2 to 8 g/l and prefer-
2162~9
WO 94/26860 13 PCT/EP94/01387
ably 2 to 5 g/l.
The dishwashing program is generally extended andterminated by a few intermediate rinses with clear water
after the main wash cycle and by a final wash cycle using
a commercial rinse aid. Not only completely clean and
hygienically satisfactory crockery but also and above all
shining silverware is obtained after drying.
E x a m p 1 e s
Silver spoons (type WMF, hotel cutlery, style
Berlin) were cleaned with a silver cleaner, degreased
with naphtha and dried. Three spoons were then placed in
the cutlery basket of a domestic dishwashing machine
(DDWM) of the Bosch S 712 type. The wash program (65C,
16dH) was then started and 50 g of a soil~l) and 30 g of
the detergent were directly introduced into the machine
during the main wash cycle. After rinsing and drying,
the DDWM was opened for 10 minutes, then closed again and
operated in the same way. After the tenth wash cycle,
the spoons were removed and evaluated. Tarnishing was
evaluated on a scale of 0 to 4 where 0 = no tarnishing,
1 = very slight yellowing, 2 = stronger yellowing, 3 =
spoons completely gold to brown in color, 4 = spoons
violet to black in color; values in the upper left-hand
part of Tables 1 to 3).
Composition of the soil:
Ketchup: 25 g
Mustard (extra sharp)25 g
Gravy: 25 g
Potato starch: 5 g
Benzoic acid: 1 g
Egg yolk: 3 eggs
Milk: 1/2 l
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WO 94/26860 14 PCT/EP94/01387
Margarine: 92 g
Local water: 608 ml
At the same time, china was evaluated for the
removal of tea stains. Evaluation was based on a scale
of o to 10 where 0 = no removal of tea stains and 10 =
complete removal of tea stains; values in the lower
right-hand part of Tables 1 to 3.
Preparation of the tea stain
16 Liters of cold local water (16dH) are heated
briefly to boiling point in a tank. 96 g of black tea
are allowed to draw for 5 minutes in a nylon net with the
cover on, after which the tea is transferred to an
immersion apparatus with a heating system and stirrer.
60 Tea cups were immersed in the tea thus prepared 25
times at 1-minute intervals at a temperature of 70C.
The cups are then removed and placed on a metal plate to
dry with the opening facing downwards.
Detergent composition
The following low-alkali basic product was first
prepared (a 1% by weight solution in distilled water
having a pH value of 9.5):
56.0% trisodium citrate dihydrate
36.1% sodium hydrogen carbonate
6.1% sodium carbonate, anhydrous
1.8% mixture of nonionic surfactants of APG 225 (C810
alkyl oligoglucoside) and Dehydol~ LS2 (C1zl4 fatty
alcohol 2E0 ethoxylate) (1:1)
Test variations corresponding to the following
formulation were then carried out with this basic prod-
uct. The results are set out in Tables 1 to 3.
2162~59
Wo 94/26860 15 PCT/EP94/01387
81 - 86% by weight basic product
12 % by weight sodium percarbonate
0 - 10% by weight TAED
0 - 3% by weight pyrocatechol, gallic acid or hydro-
5quinone
1 % by weight protease
1 % by weight amylase
2I62 159
WO 94/26860 16 PCT/EP94/01387
Table 1
Removal of tea stains/protection of silver aqainst
corrosion
Machine: Bosch S 712 Tea: 1 = no removal
Dosage: 30 g 10 = optimal removal
Program: 65C universal Silver: 0 = no tarnishing
Water 4 = heavy tarnishing
hardness: 16H
Redox-active substance Scores: tarnishing/tea
Pyrocatechol
3 0% ~ / ~ /
l 0% / 3 / / / ~ /
//////
0.0% ~ /5
0.0 % 1.0 % 2.0% 3.0 % 4.0 % 10.0 %
TAED
2162~S9
WO 94/26860 17 PCT/EP94/01387
Table 2
Removal of tea stains/protection of silver against
corrosion
Machine: Bosch S 712 Tea: 1 = no removal
Dosage: 30 g 10 = optimal removal
Program: 65C universal Silver: 0 = no tarnishing
Water 4 = heavy tarnishing
hardness: 16H
Redox-active substance Scores: tarnishing/tea
Gallic acid
5~ ~ / / /
0.0 % 1.0 % 2.0% 3.0 % 4.0 % 10.0 %
TAED
2162459
WO 94/26860 18 PCT/EP94/01387
Table 3
Removal of tea stains/protection of silver against
corroslon
Machine: Bosch S 712 Tea: 1 = no removal
Dosage: 30 g 10 = optimal removal
Program: 65C universal Silver: 0 = no tarnishing
Water 4 = heavy tarnishing
hardness: 16H
Redox-active substance Scores: tarnishing/tea
Hydroquinone
/////
2-0% ~2
1.5% 0 / 1 / 1 / 2 / 3
1.0% ~ ~ ~ ~ ~ /
~ ~ ~ ~3 ~ ~ 10 /
0.2% ~ ~ ~ ~ ~ /
0.0% 4 / 3 / 4 / 4 / 4 / 4
/ 2 / 5 / 7 / 9 / 10 / 10
0.0 % 1.0 % 2.0% 3.0 % 4.0 % 10.0 %
TAED
2162~59
WO 94/26860 19 PCT/EP94/01387
In addition, machine dishwashing detergents with the
following compositions were prepared (see Table 4). Com-
pounds A to M were used as silver corrosion inhibitors:
A: p-hydroxyphenylglycine
B: 2,4-diaminophenol
C: 5-chloro-2,3-pyridinediol
D: l-(p-aminophenyl)-morpholine
E: ascorbic acid
F: monomethylglycine
G: N,N-dimethylglycine
H: 2-hydroxy-4-aminopyrimidine
I: 2,4-dihydroxy-5-methylpyrimidine
K: indole
L: methionine
M: 2-phenylglycine
2162~9
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WO 94/26860 22 PCT/EP94/01387
The silver spoons were all awarded a score of 0 to
1, i.e. "very slight tarnishing, if any". Identical
compositions, but without silver corrosion inhibitors A
to D, turned silver spoons yellow to violet in color
(score: 2 to 4).
Electrochemical measurements
Sample preparation:
Instead of silver cutlery, silver wire (D = 2 mm,
99.99%) was used as sample material for the tests. The
silver wire was cut into approximately 10 cm long pieces,
that part of the sample dipping into the measuring
solution being rubbed with SiC abrasive paper (600
grain). The samples were then thoroughly rinsed with
twice-distilled water and any abrasion residues adhering
to the samples were wiped off with a fluff-free cloth.
If desired, this procedure was repeated several times
until the sample left a visually satisfactory impression.
After rubbing with the abrasive paper, the samples were
immediately used for the measurement to forestall any
reaction of the metallic silver with the laboratory air.
The effective surface area of the sample immersed in the
solution amounted to 0.70 cm2.
Electrolytes and electrodes:
The experiments were conducted in a Duran glass
cell. The above-mentioned silver wires (A = 0.70 cm2)
were used as the measuring electrodes. The counterelec-
trode consisted of a gold foil (99.99%) with a surface
area of 1 cm2. In view of the alkaline electrolyte
solutions, the reference electrode was an Hg/HgO/0.1 M
NaOH electrode which was connected to the electrolyte by
a Haber-Luggin capillary. The measurements were carried
out with 5 g/l of detergent in tap water having a hard-
ness of 16dH and a salt concentration of around 600 mg
2I62i5~
W0 94/26860 23 PCT/EP94/01387
(dry residue).
To prepare the detergent solutions, the low-alkali
basic product (see above) was first dissolved and the
resulting solution was heated to 65C. The bleaching
agent and the bleach activator and/or the silver corro-
sion inhibitor were added immediately before the measure-
ment. The electrochemical measurement was then carried
out. During the electrochemical experiments, the elec-
trolyte solutions were kept at 65C and purged with air.
Apparatus and recording of the measuring curves:
To record the current/voltage curves, the electrode
potential was increased at a constant rate from -0.62 V,
based on a standard hydrogen electrode (SHE). After a
total increase of 1.1 V, the potential was reduced at the
same rate. A standard potentiostat consisting of a
regenerative amplifier, differential amplifier, adder and
impedance transformer and a function generator (Prodis 16
of Intelligent Controls CLZ GmbH) were used for this
purpose.
Results:
The corrosion behavior was characterized on the
basis of current/voltage curves. Essential information
comes from the zero-axis crossing of the current/voltage
curve (quiescent potential which is spontaneously estab-
lished even without any external influencing of the
potential) and the slope of the curve at the zero-axis
crossing (reciprocal polarization resistance), E. Heitz,
R. Henkhaus, A. Rahmel, ~Rorrosionskunde im Experiment~
Verlag Chemie (1983), pages 31 et seq.; H. Kaesche, "Die
Rorrosion der Metalle", 2nd Edition, Springer Verlag
(1979), pages 117 et seg.. The addition of the silver
corrosion inhibitor produces a shift in the potential of
the zero-axis crossing to lower values and a reduction in
2I 624 ~9
WO 94/26860 24 PCT/EP94/01387
the slope. Accordingly, electrochemical measurements
also show that the corrosion of silver is considerably
reduced by addition of the silver corrosion inhibitors.
Composition of Position of zero- Slope at zero
Detergent axis crossing axis crossing
E (mV) (SHE) di/dE (mA/V)
Basic product (86%) 319 0.5
+ 12% Percarbonate
+ 2% Hydroquinone
