CA1195728A

CA1195728A - Preforms for reinforcement of battery grids

Info

Publication number: CA1195728A
Application number: CA000430810A
Authority: CA
Inventors: Joseph J. Duffy
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1982-06-21
Filing date: 1983-06-21
Publication date: 1985-10-22
Also published as: JPS598271A; US4395449A

Abstract

ABSTRACT OF THE DISCLOSURE
Network preforms for battery grids are prepared from cross-lapped assemblages of spaced bundles of inorganic filaments which are bonded at crossover points by ice.

Description

TITLE
Preforms For Reinforcement of Battery Grids _ACRGROUND OF THE INVENTION_ ~his invention relates to improved techniques for battery grid manufacture. Lead-acid batteries and their operation are well-k~own. The grids are the support for the active material of the plates and conduct the current to and ~rom the active materials. The grid must be corrosion resistant and m~st possess sufficient stren~th and stiffness to pre~ent damage or distortion during manufacture of the grids, preparation of the plate from the grid and construction of the battery itself. Pure lead is the most corrosion resistant material for lead-acid battery gridsl but it is weak and tends to creep. In use, the grids are subject to stress due to their own weight and the weight of active material supported by the grids and the stresses imposed through expansion and contraction of the active material in the 2Q charge-discharge cycle. If the grids are broken during use, the active material is isolated or shed and lost for the electrochemical reaction. From the standpoint of economies of weight, cost and cell capacity, grid thickness is also a significant factor ~5 Battery grids are generally made by a casting process. The grid-casting machine consists of a book or center-parting grid mold, trimming mechanism and melting pots. The grid mold consists of two cast iron parts, each with a grid design for a face. In order to avoid premature solidification the mold is heated before introduction of the molten metal. The lead or lead alloy is first mel~ed in the ~elting pots and transported to the grid mold. After the mold is Eilled with molten metal, it is allowed QP-2916 35 to cool. It is then opened, the cast grid is removed '7~

and the operation begins anew for fabrication of ~he next grid. This is done on a mass production schedule and it is important that no undue delay occurs in the cycle.
Alloys are generally used as the preferred grid material because pure lead is easily deformed.
The proper selection of the lead alloy depends on the intended use and economics of the lead-acid battery application because the alloys often introduce other problems such as cost, corrosion, difficulties in manufacture or reduced battery life.
Reinforcement of grids is a relatively new concept and shows promise of resolving many of the problems mentioned above. Efforts to use fiber to 15 reinforce the grid structure have been found to present new problems in manufacture. Hand lay-up o filaments in the open casting mold is a time-consuming operation that is inconsistent with mass production techniques. Also, conditions around 20 the grid casting area are not conducive to careful hand lay-up of the filaments. In some cases handling of the filaments causes discontinuities and breaks.
When this occurs in the casting mold, it results in poor quality grids. The novel preform of the instant 5 invention offers a solution to these problems.
SUMM~RY OF THE INV~NTION
This invention provides a network sheet preform for reinforcement of a lead or lead alloy battery grid comprising an array of spaced bundles of 30 inorganic filaments superimposed upon and directionally displaced from another array of spaced bundles of inorganic filaments, said arrays being bonded to each other at the filament bundle crossover points with a ugitive binder having a melting point 35 of at least about 0C and a boiling point below about 300C. The spaced bundles in each array are preferably essentially parallel to each other. Also provided is a method for making preforms comprising depositing bundles of inorganic filaments in the grcoves of a fixture having a group of 9roGves intersecting another group of grooves, wetting the filament bundles with the fugitive binder at a temperature above its melting point, free~ing the fugitive binder thereby bonding the filamentary bundles at crossover points and removing the ~rozen preform from the fixture.
DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic of a preform.
F~. 2 is a schematic of a section of a preform fixture.
DETAII.ED DESCRIPTION OF THE INVENTION
The preform of the invention is a grid or skeletal framework preferably for reinforcement of a lead or lead alloy battery grid. It may correspond in configuration to a battery grid and comprises a preformed sheet structure wherein an array or assemblage of spaced bundles of inorganic filaments lying in a single plane is bonded with certain fugitive binders at crossover points to a 25 cross lapped array of spaced bundles of inorganic filaments. FIG. 1 is a schematic of such a preform in which assembla~e 1 of parallel bundles of inorganic Eilaments coated with ice overlaps assemblage 2 of parallel bundles of inorganic 30 fil~ments also coated wi~h ice and is a~tached thereto at crossovers 3 through bonds of ice. The spaci ng o~ f ilament bundles and the size and shape of the preform are selected so that the prefsrm can be readily inserted in the grid casting mold with the 35 filamentary bundles resting in the mold depressions formlny -the X and Y direction bars of the grid to be cast. Normally the X and Y bar directions are perpendicular.
The filament bundles of the preform should be made up of inorganic high temperature resistant fibers. In this class are glass, graphite and alumina fiber. The bundles are preferably untwisted or only slightly t~isted and should bear little or no finish oils. It is preferred to use alumina fiber an~ still more preferred to use the lead coated fiber described in Hartmann ~.SO Patent No.
~,282,922 because of the fact that it is more easily wettable by lead or lead alloys. A useful range of reinforcement for the grid is from 1 to 25 volume percent, preferably from 2 to 8 volume percent.
As taught in Canadian Patent Application Serial No. 430 811 of H.~. Hartmann, filed 1983 June 21, it is preferred that little or no residual foreign matter rernain in the battery grid, that is, the grid should consist essentially of lead or lead alloy and the inorganic fiber. Organic residues are particularly undesirable because they interfere with the wetting of the inorganic fiber by the lead or lead alloy. In accordance with the present invention, a fugitive binder having a melting point of at least about 0C and a boiling point below about 3~0C is used to bond the filamen-t bundles at crossovers. Ice is preferred since it is inexpensive and has the advantage that it is easily and 3~ completely removed after it has served its purpose.
Thus, where the frozen preform is placed in the grid casting mold and the mold closed, -the ice turns to water. Evacuation of the mo~d by vacuum to remove air prior to introduction of molten lead or lead alloy, results also in removal of water leaving only 7~

the inorganic filament bundles in the grid casting mold. The fugitive binder should preferably be one that is relatively inert at temperatures below its boiling point. Illustrative of such substances are 5 naphthalene, glycerine, ice and dodecanol.
FIG. 2 represents a section of a fixture which may be used to prepare the preform. It is not the same as the mold which is used for cast;ng the battery grid. It may be formed from plastic, metal lO or other convenient material. As shown, it has two groups of groove~, X and Y, the grooves in each gro~p being essentially parallel and ~he groups intersecting at essentially right angles in bonding areas T. Holes extend through the fixtures in the 15 bonding areas. In practice, the filament bundles or yarns are laid up by hand or mechanically in the X-direction grooves of the fixture and then in the Y-direction grooves. An assemblage of parallel filament bundles may be prepared in a dispensing cartridge and fed to the fixture directly from the cartridge as a group of bundles. Conveniently the overall configuration of the preform fixture corresponds to that of the battery grid casting mold.
After the filamentary bundles are deposited in the preform fixture~ water ox other suitable ugitive substance may be introduced to wet the filament bundles. Alternatively, wet filament bundles are laid in the grooves of the preform fixture. In either case, the water on the fiber bundle array is frozen and the ice serves to bond filamentary b~ndles at crossover points. The preforms are removed and kept frozen until used~
The preforms are next taken to the battery grid casting area and placed in the open molds which are then closed. The casting molds are heated as 7~

mentioned previously to prevent premature solidification of the molten metal which is subseq~ently introduced. A vacuum is then applied to exhaust gases from the mold cavity. Application of vacuum facilitates infiltration of the fiber bundles by the molten metal by removal of air or gases from the mold prior to introduction of the molten metal.
Any water or steam flashing off from the ice can escape in a similar manner. Molten lead or lead alloy is then introduced into the mold and allowed to solidify by cooling. The mold is then opened and the battery grid removed making the mold ready to receive the next preform.
The following example is illustrative of the 5 inven~ion and is not intended to limit it in any way.

Lead coated alumina yarn (800 filament, 20 microns per filament) such as described in U~S.
Patent No. 4,282,922 was moved from a bobbin through a water bath thoroughly wetting the fiber bundle.
The yarn was cut to the length of each cavity of a freezing fixture shown in FIG. 2, ~hen placed first in one direction and then in the other~ The wet fiber in its f ixture was then frozen by passing a cold gas over the fiber array or by fogginy the fiber array with a stream of solid carbon dioxide particles from a high pressure cylinder. The preform was removed from the fixture by moving ejector pins thro~gh the holes in the fixture at the crossover points, f reeing the unitary preform. It was ~hen kept in a freezing chamber for storage. This preform geometry matched that of the lead casting mold~ This pxeform was later placed into the casting mold, the mold halves closed, clamped and then preheated in an oven. During the preheat cycle, a vacuum was d.awn on the mold cavity to evacuate air and moisture. The mold was then submerged into a pot of lead. Lead was admitted to the mold cavity by removing a plug in a tube entering the bottom of the vertically positioned mold. A freeze plug positioned in the vacuum line stopped lead flow when the mold was f ull 4 The mold was then removed from the lead pot and transferred to a cooling station where a water spray was used ~o expedite cooling. The cast grid was then removed from the mold. The gate and riser were then trimmed, completing the fabrication process.

2~

Claims

8

1. A network sheet preform comprising an array of spaced bundles of inorganic filaments superimposed upon and directionally displaced from another array of spaced bundles of inorganic filaments, said arrays being bonded to each other at filament bundle crossover points through bonds of a fugitive binder having a melting point of at least about 0°C and a boiling point below about 300°C.

2. The preform of Claim 1 wherein the inorganic filaments are alumina fibers.

3. The preform of Claim 1 wherein the fugitive binder is ice.

4. The preform of Claim 1 wherein the bundles of filaments in each array are essentially parallel.

5. The preform of Claim 4 wherein the arrays are perpendicular to each other.

6. A method for making a preform comprising depositing bundles of inorganic filaments in the grooves of a fixture having a group of grooves intersecting another group of grooves, wetting the bundles by introducing a fugitive binder at a temperature above its melting point into the grooves, the fugitive binder having a melting point of at least 0°C and a boiling point below about 300°C, freezing the fugitive binder thereby bonding the filamentary bundles at crossover points and removing the frozen preform from the mold.

7. The method of Claim 6 wherein the bundles of filaments are wet with water before being deposited in the grooves of the mold.