Method for the production of electrodes for lead storage batteries

An improvement in the production of plate type electrodes for use in lead storage batteries from a dry lead powder mixture is disclosed. The lead powder is mixed with a small amount (0.1 to 3 weight percent) of polytetrafluoroethylene (PTFE) powder, the mixture is heated and treated to convert substantially all of the PTFE into fibrous form. The mixture is applied to an electrode grid under conditions whereby a coherent, unitary structure is formed. A hot liquid (e.g., water) may be added to the mixture to form a paste.

BACKGROUND OF THE PRESENT INVENTION 
The present invention concerns a method for the production of electrodes 
for lead storage batteries and the resulting electrodes. More 
specifically, the invention concerns electrodes and a method for forming 
electrodes where the active material in the form of lead powder is made to 
form a connected or continuous structure before or while it is applied to 
a supporting grid. 
The starting material for the production of active material to form lead 
electrodes is called "lead powder", which term is somewhat inaccurate as 
the lead powder consists of lead powder of varying degrees of oxidation, 
and generally between 70 and up to 100%. By degrees of oxidation is meant 
the percentage of the total weight of the mixture which consists of lead 
oxides. As used herein, "lead powder" is intended to mean this highly 
oxidized lead powder mixture as known in the art. The lead powder is 
applied to a grid which usually consists of a diamond pattern or a number 
of connected ribs made of lead or lead alloys. There occur great 
variations, however, in the grid constructions, both with regard to their 
formation and the choice of material. A grid may, for example, consist of 
a number of suitably formed lead wires, or it may contain parts made of 
plastic or some other metal besides lead. Lead grids are usually produced 
by means of casting, but they can also be formed from expanded metal or 
stamped sheeting. 
With certain electrode types such as the so-called tube electrodes and 
similar electrode types where the active material is surrounded by an 
outer sheath, the active material is applied on the grid in the form of 
dry powder. With such electrodes as well as with other electrode types the 
active material may also be applied as a so-called lubricating mass. This 
is accomplished, as is known in the art, by mixing lead powder with water 
and some sulfuric acid, and perhaps smaller amounts of other additions. In 
this way, a lubricant paste is obtained. In this paste, the lead powder 
forms a coherent structure. Its porosity depends on the addition used, and 
in order to obtain a high porosity, which is desirable in certain cases, 
special pore-makers may be added. 
A special problem in connection with lead storage battery electrodes is 
caused by the volume variations that occur with both the positive and the 
negative electrodes. This is due to the fact that the active material in 
the case of the positive electrode in charged condition is composed of 
lead dioxide, and in the case of the negative electrode, of pure lead, 
while in the discharged condition the material in both electrodes is 
present in the form of lead sulphate. These different compounds have 
different volumes per unit of mass which causes a tendency to increase the 
volume of active material in electrodes with a subsequent separation of 
the active material, which forms a precipitate and sinks down to the 
bottom of the storage battery cell. This separation of active material in 
turn leads to a reduced capacity of the electrodes and shortens their life 
span. In order to avoid this problem, the electrodes, while being built 
into their respective cells, have been provided with supports of different 
kinds, e.g., with separators provided with glass fiber mats or sheets that 
are in contact with the electrodes. Such devices, however, can be 
relatively expensive, and even though these have led to the desired 
result, at least to a certain extent, additional improvements in this 
respect remain desirable. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of this invention to reduce or alleviate the problems of 
the prior art. 
It is also an object of this invention to provide a method for producing 
plate-type electrodes utilizing a lead powder mixture and the resulting 
electrode. 
In one aspect, the present invention provides a method for the production 
of a plate-type electrode for a lead storage battery comprising forming a 
mixture of lead powder which normally is at least 70% oxidized and from 
0.1 to 3 weight percent polytetrafluoroethylene, heating the mixture, 
applying shear stresses to said powder mixture sufficient to convert 
substantially all of the polytetrafluoroethylene into fibrous form and 
applying the heated mixture to an electrode grid under conditions whereby 
a unitary, coherent structure is formed. 
In another aspect, the present invention provides the electrode produced by 
this process. 
According to the present invention, the lead powder is mixed with a smaller 
amount of polytetrafluoroethylene (PTFE) and is then processed in a 
particular manner. In this way a material is obtained which with further 
treatment during the transference on electrode grids forms a coherent, 
unitary structure. Despite the changes and transformations which the 
electrode material undergoes in connection with charging and discharging, 
the stability of this structure is maintained over a longer period of 
time, and the electrodes according to the invention in no way have poorer 
qualities than conventional electrodes. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The lead powder utilized in the powder mixture of this invention can be any 
of the conventional lead powders normally used in the dry powder 
production of tube electrodes for lead storage batteries having varying 
degrees of oxidation, generally from about 70 to about 100%. 
Polytetrafluoroethylene powder (PTFE) is mixed with the lead powder in an 
amount of from about 0.1 to 3, preferably from about 0.2 to 0.5, percent 
by weight. 
The mixture is then carefully heated to an elevated temperature and the 
heated mixture is then treated by applying shear stresses thereto so that 
a substantial portion of the PTFE is converted into fibers in the mixture. 
The mixture is generally heated to a temperature of about 80.degree. C. 
The shear stresses may be applied by any suitable means such as, for 
example, by disposing the heated powder mixture between the walls of a 
rotating drum and a rotating cylinder for a time sufficient to form the 
PTFE into fibrous form by application of shear stresses. The powder 
changes its character and acquires somewhat reduced flow qualities. 
Then the powder mixture is applied to an electrode grid, and in connection 
with this application, the mixture is further exposed to stresses whereby 
a continuous or coherent, unitary structure is formed. 
The method according to the present invention has proven especially 
suitable with the production of electrodes of the so-called Faure type, as 
the treatment leading to the formation of a coherent, unitary structure 
takes place at the same time as the active material is applied to the 
electrode grid. 
There are also other possibilities for the production of electrodes 
according to the invention; for example, plates or staves of the powder 
mixture can be pressed under conditions whereby substantially all of the 
PTFE is formed into fibers and converted into a coherent structure and 
then be applied to the electrode grid. 
Pore-formers as Na.sub.2 SO.sub.4 or Al.sub.2 (S0.sub.4).sub.3, known in 
the art, can also be mixed into the powder mixture in minor amounts as 
conventionally utilized.

The invention is additionally illustrated in connection with the following 
Examples which are to be considered as illustrative of the present 
invention. It should be understood, however, that the invention is not 
limited to the specific details of the Examples. 
EXAMPLE 1 
Dry lead powder is mixed with 0.2 percent in weight of dry PTFE powder in a 
rotating drum mixer which consists of a rotating drum with a similar 
rotating cylinder in contact with the inside of the cylindrical wall. The 
drum's rotation speed is about 50 rpm and the mixing time is 5 minutes. 
The mixture is then heated to about 80.degree. C and treated again in the 
mixer for about 10 minutes, during which time the rotation speed is about 
40 rpm. Because of this treatment, fibers of PTFE are formed in the 
mixture and the qualities of the mixture are changed so that the dust 
binding inclination is greatly reduced and the flow qualities are somewhat 
impaired. This mixture is rubbed into an electrode grid consisting of a 
diamond pattern made of a cast lead alloy using conventional equipment. In 
the application of the mixture to the grid, further stresses are applied 
to the powder material as it is pressed into the grid. In the process, a 
coherent, unitary structure is formed from the active material. 
EXAMPLE 2 
1.5 kg lead oxide (71 weight percent PbO, 29 weight percent Pb) with a mean 
particle size of about 15 .mu.m, 7.5 grams of Teflon.RTM. PTFE powder 
"KlON" and 45 grams of MgSO.sub.4 were premixed during 10 minutes and 
heated to 110.degree. C over night. A sigma blade mixer was heated by 
IR-lamps to 100.degree. C. The premixed and hot powder was charged into 
the mixer of Example 1 and the mixing started immediately. In less than 2 
minutes a substantial amount of the PTFE powders were formed into fibers 
and the mixing was stopped. 
The above-formed mixture was heated to 130.degree. C and about 200 grams 
were now placed on a heated plate which had a surface temperature of about 
80.degree. C. A regular staggered grid of lead-antimony alloy, with the 
dimension of 148 .times. 148 .times. 4.2 millimeters heated to 130.degree. 
C in an oven, was placed above the teflon-lead powder-MgSO.sub.4 -mixture 
and pressed firmly against it. Another 200 grams of the mixture formed as 
described above were now placed on the upper side by a roller about 2 
centimeters in diameter, which was rolled over the paste under a firm 
pressure. Both sides of the sandwich were treated in this way and a plate 
with a well adherent paste was obtained. The plate was dropped 
(vertically) on the table from 1 centimeter several times and no loss of 
material was noted. 
The invention described above comprises a mixture of PTFE powder and lead 
oxide, which by applying stresses to the mixture is worked to a dough 
including fibers of PTFE formed from substantially all of the PTFE powders 
originally present, which is well suited to paste into grids intended for 
use in lead acid storage batteries. The mechanical effort necessary to 
apply to this powder mixture in order to form PTFE fibers is not 
extensively large in the beginning of the mixing period, but increases as 
the fibers increase in length. In some cases, the mixture forms into a 
dough in a rather short period, which dough is very difficult to treat 
further in the mixer. 
This problem may be avoided by the addition of a suitable liquid, 
preferably hot water or another hot liquid electrolyte containing anions 
able to form insoluble or slight soluble lead compounds acting as pore 
fillers. The liquid may be added in an amount sufficient to form a paste, 
which amount may be readily determined by one skilled in the art. 
By adding water or a suitable electrolyte, the internal friction will 
decrease and the mixing can continue, which, for example, is necessary for 
most mixers in order to empty the mixer. The shear stresses on the mixture 
are reduced but not eliminated and further the substantial amount of 
fibers are already formed. The water or suitable electrolyte, should 
preferably be hot (above 60.degree. C), thus allowing the fiber formation 
to continue or at least to prevent the already formed fibers to break with 
further mixing. 
Suitable electrolytes include water solutions of Na.sub.2 SO.sub.4 or 
MgSO.sub.4 or diluted sulfuric acid, but are not restricted to only these 
electrolytes. 
EXAMPLE 3 
1.5 kg of lead oxide (71 weight percent PbO, 29 weight percent Pb) with a 
mean particle size of about 15.mu.m was premixed in a rotating drum of 
ambient temperature without application of shear stresses with 7.5 grams 
of "Teflon.RTM. " PTFE powder "KlON" for 10 minutes and then heated over 
night to 110.degree. C. A sigma-blade mixer was heated by IR-lamps to 
100.degree. C. The premixed hot powder was charged into the sigmamixer and 
the mixing started immediately with shear stresses being applied to the 
mixture. After 2 minutes, a substantial amount of fibers formed and the 
powder was about to form a dough. 270 ml of boiling water was poured into 
the mixture with continuous mixing. The internal friction of the paste 
noticed by the forces the motor had to apply to the mixture, were 
decreased. The product formed was well suited to apply by pasting in 
conventional pasting machines into grids intended for lead storage 
batteries. 
EXAMPLE 4 
The procedure of Example 3 was followed except that 400 ml of boiling water 
was added and after cooling the mixture to 35.degree. C. 170 ml of H.sub.2 
SO.sub.4 (density 1.40) was added dropwise during 20 minutes with 
continuous mixing. This product showed the same amount and length of 
fibers as in Example 3 and was well suited to apply into grids by pasting. 
The wet paste weight was 316 grams/5 in.sup.3 determined by measuring the 
volume of a weighed lump of the paste in a graduated glass with water. 
The handling of the active material right on to its transference on 
electrode grids is simplified according to the method of the present 
invention, in comparison with the usual production and application of a 
lubricating mass. 
The principles, preferred embodiments and modes of operation of the present 
invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as limited to the particular forms disclosed, since these are to 
be regarded as illustrative rather than restrictive. Variations and 
changes may be made by those skilled in the art without departing from the 
spirit of the invention.