Note: Descriptions are shown in the official language in which they were submitted.
9395
B~C~GRO~D O~ T~i_ IMV~NTION:
The shelf lifc of electrolyte compositions ~or capa-
citors has been a problem for over 40 years. Most of ~he re-
search has been directed to the formation of an adequate seal
for the oxide film on the anode electrode which is covered by
a skin or film of an oxide produced by anodic oxidation which
functions as a dielectric. The purpose of the seal is to make
the anode less susceptible to degradation by hydration of the
oxide.
B. Chesnot, in U.S. Patents 3,546,119 and 3,638,077,
dlscloses an electrolyte for single and multiple aluminum elec-
trode capacitors comprising about 0.5 to 10 moles of boric acid
per 11 moles of a solvent such as gamma-butyrolactone, gamma-
valerolactone and N-methylpyrrolidone. Chesnot's compositions
can contain between 0.1 to 2 moles of a trialkylamine per mole
of the solvent. The anion generating agent may be such acids
as acetic, acrylic, butyric and citric acids. ThesP compositions
are substantially non-aqueous but are dependent on the presence
of a small amount of water necessary to reform the anode (oxide)
layer produced in situ by the dehydration of boric acid into
metaboric acid and water. Without this water balance the elec-
trolyte is subject to pre~ature destruction through self-sustained
~. .
- ~1939S
elev~tion of the leakage current or an unwanted change in the
electrical characteristics of the capacitor. Such composi~ions
also require the presence of the afore-mentioned anode-generat-
ing acids in order to attain lowered resistivity. Ethylene
glycol is incorporated in the compositions of the '077 p~tent
as a solubilizer.
U.S. Patent 3,83~,055, by B. Chesnot, discloses an
aqueous electrolyte for aluminum-anode capacitors which is com-
posed of maleic acid, a maleate of N-methylbutylamine, triethyl-
amine or tributylamine and phosphoric acid dissolved in dimethyl-
for~amide (DMF) wherein the maleic acid and the maleate salt
are present in an amount between 10-3 moles up to their up limits
of solubility per mole of solvent ~DMF) and the phosphoric acid
is present in an amount between 10-3 to 10 1 moles per mole of
solvent (D~).
These compositions are described as being very slightly
aqueous and function where low temperatures, high frequencies
and possibly high temperatures in the order of at least 85C are
encountered. Stability over a temperature range of -55C to
~125C is disclosed. High faradic efficiency of the anodic
oxidation and a limited service voltage, i.e. with maximum build-
up voltage,are disclosed by Chesnot. Tests show that the com-
bin~tion of the maleic acid and its salt produces an electrical
conductivity (mmho/cm at 35C)which is more than additive and
that the addition of phosphoric acid raised the build-up voltage
without affecting the electrical conductivity by more than 10%.
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~119395
Also, a small amount, i.e, 0.01% to 3% by weight of water and
various lactones and D~-ethylene glycol solvent combinations
can be used with the phosphoric acid being an essential ingre-
dient.
SUMMARY OF THE INVENTIO~:
The instant invention provides electrolytes for capa-
citors, using ethylene glycol as the solvent, that are charac-
terlzed by exhibiting only moderate changes in capacitance and
diss~pation factor and leakage currents that are not excessive
when maintained at elevated temperatures, either with or without
an applied voltage. Also, the compositions of this invention
exhibit a leakage current factor K defined by the formula:
K ~ IL (~ a)
~ MF x V
which i9 below that of compositions using other solvents and
well below a standard ammonium pentaborate electrolyte at 7.5
VDC, 300,000 ~, and well below this value at 100 hours and
up t~ 500 hours or more shelf time (85C), The compositions
of this invention have been tested at 60 VDC and lO,OO0 MF
and found to exhibit long shelf life with a low K value. In
the above formula, IL ~ leakage current in microamps; MF ~
capacitance in microfarads; and V ~ applied DC voltage in volts,
These and other advantages of using the combination of
ethylene glycol and a our carbon atom unsaturated dicarboxylic
acid in the form of an ammonium or Cl to C4 primary, secondary
~L119395
or tertiary aliphatic amine salt will be described. Particular advantages
are obtained using an electrolyte containing between about 0.001 to 0.1
mole of the acid, i.e. fumaric acid, per mole of ethylene glycol and between
about 0.001 to 0.2 mole of the amine, i.e. triethylamine, per mole of
ethylene glycol. From this it is seen that the molar concentration of the
salts herein disclosed is maintained at no more than about 0.3 per mole of
ethylene glycol. The amount of the unsaturated aliphatic acid is determined
by the desired conductivity, and the amount of base required is sufficient
to attain a final pH of about 4.9 to 7.2. Also, the electrolytes of this
invention exhibit low voltage shelf lives that exceed the industry imposed
standards of 300,000 MFD and 7.5 VDC, and exhibit very little change in
2000 hours.
According to a broad aspect of the invention there is provided
an electrolyte for electrolytic capacitors having aluminium anodes, the
electrolyte consisting essentially of the salt formed by the reaction of:
a dicarboxylic acid having the formula
R' R
HOOC - C = C - COOH
wherein R and R' are hydrogen or a Cl to C4 alkyl group; and a base con-
sisting of ammonia and/or an amine of the formula
R-NH2, RR'NH or RR'R"N,
wherein R, R' and R" are hydrogen or a Cl to C4 alkyl or alkylene group;
said salt being dissolved in ethylene glycol, the total molar
concentration of said acid and base being not more than about 0.3 per
mole of said ethylene glycol;
and wherein the pH of said electrolyte is substantially between
4.9 and 7.2.
DESCRIPTION OF THE DRAWINGS:
The accompanying drawing is a graph of low voltage shelf life
wherein the abscissa is the number of hours of storage at 85C and the
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~9395
ordinate is the K factor or leakage current factor.
DESCRIPTION OF PREFERRED EMBODrMENTS:
In order to demonstrate the invention, a number of electrolytes
were prepared and tested for low voltage shelf life as measured by the K
factor (leakage current factor). The compositions are set forth in the
following TABLE 1.
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, ~ . .
,X!~ A.
.
~1~9395
., ~
T A B L E
ELECT~OLYTE COMPOSITIONS
NO. IDENTIFICATION INGREDIENTS AMOUNTS
.
1. (Standard glycol) ethylene glycol 16,13 mols,
300, 000 M}~, 7 . 5 VDC boric acid 4, 71 mols .
ammonia O. 95 mols .
~, (M-50) ethylene glycol ~ mols~
12 300, 000 MF, 7 . 5 Vl)C fumaric acid ~.~Z; mols .
lJ triethylamine 0.11 mols.
Composition No. l was prepared by mixing ~6.13 mols. of
ethylene glycol with 4.71 moL~ of boric acid and slowly adding
0.95 mols. of ammonia. The mixture is then heated until the
boiling point reaches 125C, the final pH is sbout 5..1 and the
composition exhibits 2 resistivity o~ 940 okm-centimeter~ at
30C.
Composition No. 2 was prepared by mixing 1.32 mol~ . of
~OSS'
ethylene glycol with 0,54 mols. of f~maric acid and slowly
0.1~
adding 0.4 mols. of triethylamine, while keeping the tempera-
ture below about 35C until the final p~ is about 6.5 and the
resistivity is 420 ohm-centimeters at 30 C.
A number of capac~tors with service voltages of 7.5 V.
with etched aluminum anodes were impregnated with each of the
fore~oing electrolytes after aging for four hours, and subjected
to low voltage shelf life te~s. The re~ults are sho~ in the
~ 395 48.261
accompanying graph wherein the capacitors bear the same
numhers as the aforesaid composition numbers.
Each point on the graph represents an average of
ten (lO) capacitance determinations at the indicated hours
exposure to a temperature of 85C. Capacitors Nos. l and
2 were rated at 300,000 MFD.
The electrolytes formed in accordance with this
invention employ ethylene glycol as the solvent contain-
ing the ammonium or amine salt of an unsaturated aliphatic
dicarboxylic acid represented by the cis and trans isomers,
maleic and fumaric acid, maleic anhydride, and the methyl-
substituted members of the series including citraconic acid,
mesaconic acid, dimethyl maleic acid and dimethyl fumaric
acid. In one embodiment the acids used contain two central,
double-bonded carbon atoms irrespective of the length of
the side chains. Also, mixtures of the dicarboxylic acids
can be used, as well as their anhydrides.
The nitrogen-containing base having up to 12 carbon
atoms, used to obtain a final pH of between about 4.9 to
7.2 can be ammonia or a Cl to C4 alkyl or alkylene-substi-
tuted primary, secondary or tertiary amine. Examples of
such amines may be represented by the formulas:
RNH2; RR'NH and RR'R"N
whereln R, R' and R" are hydrogen or Cl to C4 alkyl or
alkylene groups and mixed alkyl or alkylene groups.
~19395
Specific example~ o~ amine~ ~re methyl am~ne, dimcthyl-
a~lne, ethylamine, diethylamine, trimethylamine, n-propylamine,
di-n-propylamine, triethylamine and n-butylamine. Other examples
are the di- and tri-butylamines and the mixed ~lkyl amines
N-methyl-n-butylamine, N-methylethyla~ine, the preferred amine
being triethylamine. Mlxtures of amines or an amine and ammonia
can be used.
The following TABLE II lists additional examples of
electrolyte compositions coming within the invention wherein
tbe molar concentration of each ingredient is shown in paren-
theses:
T A B L E II
1. Ethylene glycol ( 1.61 M)
Maleic acid ( 0.03 M)
Ammonia ( O.03 M)
2. Ethylene glycol ( 1.61 M)
M~leic acid ( 0.02 ~)
Ammonia (0.064 M)
Boric acid (0.042 M)
3. Ethylene glycol ( 1.6L M)
Maleic acid ( 0.06 ~)
TriethyLamine ( 0.06 M)
19395
` 4, E.hylene slycol (1.61 ~
Maleic a cid (0 . 06 M )
Triethylamine (0.16 M)
Phosphoric acid (0.01 M~
~. Ethylene glycol (~
,~ Maleic acid (; ~
Trlethylamine ~ ~ M)
Ammoni~n dihydrogen phosphate (0, 0 087M )
6, Ethylene glycol (1.32 M)
Fumaric a cid (0 . 0 54 M )
Triethylamine (0,11 M)
7. Ethylene glycol (4,08 M)
Fumaric acid (0.052 M)
Triethylamine (0.13 M)
Ammonium p~ntaborate (0,008 M)
It is noted that such additives as boric acid, phosphoric
acid, ammonium dihydrogen phosphate and ammonium pentaborate
have been included in some of the cornpositions of Table II.
Such additives are included in small amounts fo~ stability of
the equivalent series resistance ~ESR) as required for certain
capàcitors without substantially affecting the shelf life.
Contrary to expecta-ion, it W25 found that the additives
listed above had to be limited to less than 0. 045 molar concen-
tration to achieve the desired capaci.or properties,
The pH of the compositions is adjusted to the indicated
levels so that the corrosivity of the compositions to alurninum
$s eliminated.
48.261
~19395
In one embodiment of this invention an electrolyte
for electrolytic capacitors having aluminum anodes is
provided consisting essentially of the salt formed by
the reaction of a dicarboxylic acid having the formula
R' R
HOOC C = C - COOH
wherein R and R' are substituents of group consisting
of hydrogen and Cl to C4 alkyl; and a base which may be
ammonia or an amine of the formulas
R-NH2; RR'NH and RR'R"N
wherein R, R' and R" are hydrogen and Cl to C4 alkyl and
alkylene groups dissolved in ethylene glycol, with the
total molar concentration of the salt being about 0.3
per mole of ethylene glycol.
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