CA1236518A - Mineral filled seals for galvanic cells - Google Patents

Abstract

MINERAL FILLED SEALS FOR GALVANIC CELLS
ABSTRACT

Mineral filled polypropylene (as a preferred embodiment, but also filled thermoplastic material that is inert to electrolyte, such as poly-propylene, polyethylene, copolymers thereof, and nylon) is used for injection molded seals for galvanic cells. The mineral is generally talc, (anhydrous magnesium silicate) but may be calcium carbonate or mica, or other electrolyte-inert material. The use of such material provides a seal through which hydrogen generated as a result of internal corrosion in the cell will permeate; and particularly so as to provide better molding conditions and wider tolerances of mechanical design. At the same time, since the bursting or rupture strength of a membrane of filled thermo-plastic material is lower and can be more easily predicted and controlled, and since a filled thermoplastic membrane will stretch less before rupture, a lower membrane bursting headroom may be designed for, and a larger volume of active material may be installed in the cell. Due to the lower coefficient of linear thermal expansion and the higher compressive strength of filled thermoplastic material, the cells may be more effec-tively sealed.

Description

FIE~D 0~ T~E lNV~NTION:
This invention relate~ to sealed g~l~anic cells, BUCh aB primary alkaline cells and the like, and particularly relates to ~ealing aDd insul~tin~ members, hereafter referred to as seals, for such cells; which ~re produced or molded from filled thermopls~tic material such as polypro-p~lene, filled with a mineral, particularly talc, calcium carbonate or m~ca .
BACKGROUND OF T~E INV~NTION
The general construction of a sealed, cylindrisal gsl~anic cell iB
such ~hat i~B pri~cipal components, an anode and ~ cathode, are assembled into a can, together ~ith the appropriate separators, electrolyte, etc., and the cell i8 then closed by a seal placed in the open end of the can.
The seal precludes electrolye leakage from the cell snd insulates the electrode contacts of the cell from each other.
~ seal will desirably also permit hydrogen gas permeation from the cell 80 as to reduce pressure build-up within the cell, and yet ~l~o to inhibit moisture gain or 108E, and o~ygen or carbon dio~ide infiltration iDto the cell. Still further, the seal is usually ms~ufactured with a molded-in mffmbrane or thiD section 80 as to assure that the cell ~ill vent under certsiD couditions ~hen high ga~ pressure buildup within the cell msy occur, and to preclude rupture of the cell.

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D~TAILED DE8CRIPTION
ln keeping ~ith the preaent iu~ention, aeal~ ~re provided for use in cylindrical ~ealed cell~ and are molded, generally injection molded, fro~ a filled electolyte inert thermoplastic material, 0uch a~ poly-propylene, polgethylene, and nylon, (a~d particularly for h uh temperature spplication~, polysnlfone), and co-polymers thereof; usually polypropylene havin from 5% to 45% by ~eight of minersl filler, where the mineral m~y be chosen fro~ the group consistiug of talc ttheoretically, anhydrouc magne~iu~ silicate, ~g3SiO10(0~)2 ], calciu~ csrbo~ate, and mica. Depending on the purpo~e for which the sqal may be uaed, it may be annesled or not. ~hen polypropyleue i~ annealet, it may be annealed at temperstures of 7~ degsees C to 155 degree~ Celciu~, (other te~pere-tures bei~g appropriate for other thermoplastic matQrisls ~uch aa co-polymers of polypropyleue, polyethylene and nylon). Deairably the annealifig can be performet in an air-circulating oven or tunnel. ln ~ost instance~, mineral filled polypropyleue, and particularly talc filled polypropylene, has been fouud to be psrticulsrly u~eful.
A~ noted aboYe, auy aeal made accordiug to thi~ inveution, such a~
a filled may be provided having a rupturable membrane vent area that haa 8 reduoed cro0s-~ectional t~ickneos relative to the thickneae o~ the msterial ~urrounding the membraue. The precise confi$urstion of the ruptursble membrane veut area iB noe BpeCifiCally a aubject of the present inYention, ant the ~ealing and insulatiug member may take the kind of appearance a~ tho~e thst are described in the ~Canadian Patent No.
1,164,936 and co-pending Applicat~on No. 405,244 assigned -to a common Assignee herewith. The seal may otherwise be of any other sui-table design.

~365~

A number of ~dvantages are reslized by usiDg filled thermoplastic for molding sealing and insulating member6. The advantages that are achieved include molding and manufacturing econGmies with a better product, mechanical advantages a8 to the sssembly of the cell and design tolerances, chemical advantages as to the presence of the member in the cell, and phy~ical advantsges that give rise to enhanced cell operation and assist in the mechsnical and molding advantages.
For example, injection molded plastic parts that are made from t~lc loaded polypropylene can be produced substantially ~ithout any sinX mark~;
i.e., surface cavitation or irregularities, which may be particularly important when the ~eals are formed having a membrane. Moreover, because the parts can be molded without sink marks, the molding tolerences and conditions under which the parts are molded are less critical, A mineral filler such as talc, can permit lower molding temperatures. Al~o, a minersl filler such as talc may add lubricity to the surface of the thermoplastic material, 80 that it can act as an inter~al mold releaae a8ent. This iB particularly helpful because e~ternal ~old relea~e sgent0 such as ~ilicones may contaminate the molded parts, and tha~ contamination in turn can cause gassing with in the cell.
Z The mineral filler ttalc, cnlcium carbonate or mica) would not cau~e gassing within the cell. Because there iB less 6hrinkage after molding, especislly with higher concentrations of filler Quch a8 talc in ~he 20X to 40X range, there is grester a~surance thut the sesling and insulating members will be more precisely molded. Finally, the cost per piece of ~ineral filled plastics msy be less than that of unfilled material~.

12365~LB
We have discovered that there are subestantial mechanical advantages that are gained through thç use of filled thermoplastic Laterial. These include the fsct that there are better sealing snd venting charscteri~tic~ ~f the member. A filled thermoplastic has lo~er co-efficient of linear thermal espansion, ~hich relates more closely to the co-efficient of linear thermal e~pansion of the ~etal can that i6 u~ed for a~sembly of the cell. This results in a better ~e~l, particularly following thermal cycling. The filled thermoplas~ic has a higher com-pressive strength than an unfilled m~terial, resulting in a better seal at the cri~p. That feature iB even more greatly enh&nced when the sealing ~nd insulating members are annealed. The annealing stabilizes the dimensions of the sesling and insulatiDg members; that is, the physical dimensions of the part become~ ccnstant, and annealing al60 relieves any molded-in stresse~ that may have been c~used during the molding process.
The filler act~ as a stress concentrator to allow for tbe venting or membrane rupture to occur at minimal membrane deflection. When a filled ~eal i~ a~nealed, tbus relieving any mold0d-in stre~es that may h~e occurred, the membrane rupture pre~aure increases relntively le~s thun the corre~ponding increase imparted by annealing sn unfilled m~terinl. An sn~enled filled muterial provide~ greater predictability of the rupture strength of ~ membrane, At the same time, the provision of a filled thermoplastic reduces the headroom requirements by a~suring that the ruptursble vent will rupture ~ith less stretching than an unfilled thermoplastic material, and ~ithout e~ce~sive pressure build-up in the cell. Moreover, a thicker membrsne may be molded, thereby permitting wider molding tolerances, ~hile ~till remaining within the venting tolersnce design.

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Al~o, bec~use there iB le~s headroom needed to permit vent rupturing, the ~eal may be placed blightly higher in the can, thus permit~ing installation of a greater amount of active ma~erial in the cell.
We have also discovered that lower molding temperatures may be possible. At the same time, when tbe filled thermsplastic i8 injection molded, its cooling characteristic in the mold is sufficiently better than tbat of an uhfilled material that the molding cycle may be reduced while at the same time broadening the ~window" of the inatant of time durLng the molding cycle when a pin muse be moved in the mold to accommodate the molding of the thin membrane section.
Becau~e the filled thermoplsstics of the invention permit hydrogen permeation, the cell in ~hich the member is instslled may tric~le vent.
Trickle ~enting permits sufficient hydrogen to escape from a cell, thus reducing the possibility of venting of the cell by membrane rupture. This iB also important in that the ~mount of mercury corrosion and gassing inhibitor constituent that may have to be in~talled in the cell, particularly in alkaline cells, may be reduced quite considerably, thereby reaulting not only in a more environmentally satisfactory product, but 2n al80 in better working conditiona for a~Dembly of the cell, and appreciable C08t s~vings.
An advantage that arises from the pre~ent invention ~hen compared to glass filled nylon, vhen used for cell seals, is that the filled thermopla~tic is much less subject to leaching when it iB expo~ed eO
electrolyte such aB 40~ potsssium hydro~ide solution. That is, talc, calcium carbonate or mica filled polypropylene will not decompoYe, even when exposed for long periods of time say four ~eeks at ~l~C to ~0~, ~uch 8B the glass filled nylon does.

The filled thermoplastic material~ that are considered herein, ~uch as tslc, calcium carbonate ~r ~ica filler, in quaDtities of from 5~ to 45 and usually 15~ to 40Z, the filler material i8 ~ery finely ground or iB a fine particulate material; and i~ such that when it iB within the molded thermoplastic ~ealing and insulating member, there iB ~ubBtantially no ~urface e~posure of the filler material in the molded part. In that regard~ particularly with respect to the use of talc a~ a filler, it i8 preferred tha~ approxima~ely 20~, but up to 40Z talc, may be u~ed as a filler, particularly in polypropylene.
AB noted, annealing provides certain advantage~ by Telieving any molded-in streases that mRy have occurred, and stabilizing the phy~ical dimenaions of the molded sealing and in~ula~ing member; but even if the ~eal iB unannealed, superior performsnce over an unfilled product of similar design can be achievedO Thi~ iB becau~e there i8 8 ~ider tolerance in the production of the membrane portion of the member, BO that it can be molded thicker, with better prediction an to the rupture pressure, and in any event BO as to accommodate somewhat higher rupture pre~sures without approaching the pre~ure at ~hich the cathode can would decrimp c~using release of the whole member and thus rupture of the cell.
A typical an~lysi~ of talc-filled polypropylene follows:
The polypropylene re~in, unfilled, may have a melt index of 1~ to 15. After filling, the ~elt iDde~ ~ill have been reduced to about 6 to ~;3fi~;~8 The talc iB a platy or platelet-like du~ty ~hite powder ~ub~t~nce, hAving no asbe~tos con~tit~ent, and hsving a chemical analysis that may be within the following ran~ea:
CO~STITU~NT ~ BY ~EIG~T
MgO 20-32%
SiO2 16-46X
CaO 9-30%
A1203 0.5-4Z
Fe203 0.2-4%
LOI 8-20%
[LOI ~LOB8 On Ignition) repre~ents the percentage of ca-rbon dioxiode released on heating of the talc.]
A typical mesh-~ize distribution of Particle size may be within the follo~ing ranges:
NESH SIZE PASSED % BY ~EIGHT
44 micronB 100%
30 microns 94-97 20 micron~ 75-90%
10 microns 40-60%
205 microns 18-32Z
4 micron~ 14-24%
3 miCronB 9-17Z

2 micron~ 5-lOZ
1 micron 4- 4Z

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Typical dry brightness is ~4-94; tapped density ia 60-~0 lbs/cu.ft., loose density iB 24-55 lbs./cu.ft.; specific grnvity iB
2.8-2.9; oil absorption g/100 g talc i6 20-35; p~ is 9-10~5; ~egman fi~eness i~ 1-3.5; 100% will pass 200 mesh and 98-99,9Z will pass 325 mesh.
A 6imilsr ground calcite (calcium csrbonate) would exhibit the following characteristics:
CONSTIT~ENTZ BY WEIG~T
C~Co3 95-g8Z
0 ~gC03 1- 2%
A1203 0.05-0.2 23 0.01-0.2%
SiO2 0.1- lZ
MnO ~OlX
Coppernot detected ~oi~ure 0.25 Organic Coating 0.5-2%
(typically ailane) Typical particle ~ize i8 100~ le~ than 15 micron0 (cpherical particles); with 95-99% less than 10 microns; 75-85% le~0 th~n 5 microns, 40-60% less than 2.5 microns; and 20-40% le~8 than 1 micron. The mesn particle ~ize may be in ~he order of 2.5 microns; with a ~pecific gravity of about 2.7 and a refractive inde~ of appro~imately 1.55.

3~;~;18 Seversl e~ample~ follow:
EXAMPLE 1:
A number of A~ sized ~eals (tops) were molded from nylon, unfilled polypropylene, and polypropylene hav~ng 20X talc filler. ~alf of each of the polypropylene tops were annealed, and ths other hslf were not. All of the top~ were fully installed in ~ell~, except tbat the outer jacket was not placed on the cell~ and ob~ervations of lesksge were taken follo~ing Yarious storage and use te~ts.
Leakage ob~ervation~ revesled, for e~ample, that after storage for one week ~t high temperature, and a fur~her weeX of room t~mperature stabiliz~tion, the annealed top~ and the nylon tops showed substantially equal (one of thirty) and no failure, with higher failure of the unan-nealed tops. Other tops were stored at a somewhat lower, but ~till elevated temperature, for t~o weeks, followed by a further week of ro temperature ~tsbilization, with one nylon and one unfilled top failing, ~hile no filled top failed.
Still other top~ underwent storage nt Yery low temperature for one ~eek, followed by room temperature ~tabilization for snother week, with m~ch better performance for annealed filled top0 ~one of thirty) for 2n example, tban the nylon top~ of which 2V of 30 failed.
Likewi0e, tempersture cycling of ~no~her batch from low temperature to ~ relati~ely hiBb temperature followed by room temperAture stabili-zstion showed failure of only two of thirty annealed filled ~ops, vhile 15 of tbe thirtg nylon top~ failed.

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Other s~mples were stored at room temperaeure fo~ one dsy> after ~hich they were subjected to a 1.5 ampere charge, and all cells vented.
~oweYer, other top~ were stored at high temper~ture for several dsys and then given a 1.5 smpere charge; and three of three filled annealed poly-propylene tops YentedJ whereas one of three of the cell~ having a nylon top had a much more violent rupture.
EXAMPLE 2:
A number of AA, C snd D cell tops were molded, all having rupturable ~ent membranes molded in them having thicknesses of from 0.0035 to 0.005 inch; and of the molded ~amples, some were molded without talc filler, ~ome had 20g talc filler and some hat 40Z talc filler.
A number of top~ were cho~en a8 controls, and were not immersed in pota~sium hydroxide solution; whereas other groups of tops were immersed in potassium hydroxide solution at rooI temper~ture for two ~eeks, or a high temperature for two ~eeks, or a very high temperature for t~o weeks.
Thereafter, out-of-cell vent testing was performed, by which ~11 of the top~ that hsd been stored ~bile i3mersed in XOH ~howed Yenting result~
that were quite con~i0tent ~ith the coutrol group that hsd not been e~posed to or immersed in KO~. ln other words, all of the samples were chemically resistant to the ROa ~olution.
An import~t conclu~ion that can be dra~n from the~e venting te~ts i8 that the membrane or vent pre~sure relief portions of the filled poly-propylene eops are not affected by alkaline electrolyte at high tempersture; and that the membr~nes will rupture 80 that the cell~ ~ill vent at the Rame pressure both before and after long stor~ge at elevated temperatures. The came conclu~ion ca~ be dr~n with re~pect eo the other filled thermopla0tic mater;als di~cussed herein.

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365~3 ~8AMPLE 3:
Otber venting te~ts uere carried out, wherein tops having vent membrane~ of a thickness of 0.0055 inch molded therein ~ere tested in the presence of various height cle~rance~ above the membrane. Unfilled poly-propylene tDps failed to vent or vented at very high pres~ure~ uith lo~
clearances, ~hereas 20% talc filled tsps vented at rea~onable pressures.
At 0.091 inch clearance, an unfilled snd unannealed polypropylene top tid not vent st a pressure of 850 p8i; whereas a similsr top having 0.145 inch clearance vented at 280 psi. ~nannealed 20% taIc filled polypropylene top~ vented at 480 psi with the lowest clearance noted above, and at 280 psi with the higheRt clearance; whereas annealed 20% talc filled polypro-pylene tops vented at 420 psi ~ith the lowest clearsnce and 400 psi with the highest clearance.
EXAMPL~ 4-Glass filled nylon, unfilled polypropylene, 20Z talc filled polypro-pylene ~nd 40Z talc filled polypropylene topa ~ere molded) after which they ~ere stored at 71C for one month in RO~ electrolyte. The solutiDn following the stor~ge wss acidified and snulyzed for traces of met~llic elements. ~xcept for a slightly higher resding of calcium lesched from the ~0~ talc filled polypropylene tops, none of the polypropylene tops showed any significant leaching of nickel, cadmium, strontium, cobalt, tit~nium, molybdenum, le~d, copper, iron, vanadium, chromium, sluminum, ~ilicon or calcium. The glass filled nylon, on the other band, ~ho~ed that significant amounts of titanium, copper, iron, ~anadium, aluminum, ~ilicon and calcium had been leached.

~65~8 The use of filled thermoplastic materials, ~otably but not exclu~ively mineral filled polypropylene, and particulnrly t~lc filled polypropylene, a~ ~ msterial for injectio~ molding of seals or tops for cell~ ha~ been discussed above. ln ~o~e re~pe ts, the action of filled and unfilled polypropylene have been ob~erved to be ~ubstantially the Bame; whereaB in other re~pects, the action of material~ ~uch a8 filled polypropylene (~ith 20~ to 40~ of talc in the tests ~pecifically referred to~ have sho~n marked improvement over unfilled polypropylene, and over ~uch other material~ as glass filled nylon. That improvement bas been lo psrticularly noted in re6pect of predictable venting pre~sure and venting operation by rupture of the membrane, and an ability to withstand temperature cyclingO
Moreover, considersble other advantages are obtained by filled polypropylene when compared to unfilled polypropylene, including better molding characteri~tics st lo~er molding temperatures~ better seali~g chsracteristics; and permitting wider de~ign and molding tolerances.
In all even~, the filled polypropylene top~ that ~ere specifically tested also Dhowed better re~ults tban commercial nylon or 8l8~8 filled nylon tops.
The configuration of filled thermoplsstic ~eals according to the present invention i8 dependent upond esign considerations that are beyond the scope hereof; but the de6igner may permit ~ider tolerance~ in molding, and he may design for thicker rupturable ~ent me~branes with the assurance that cell performance ~ill be as expected when the cells are in the field.
The appended claim~, however, define the ambit of the present invention, a6 herein ~et forth.

Claims (9)

WHAT IS CLAIMED IS:

1. For use in crimp sealed galvanic cells, a seal comprised of a thermoplastic material having from 5% to 45% by weight of filler, where said thermoplastic material and said filler are each chemically inert to the electrolyte material that will be used in the sealed cell, and having a rupturable member vent area with reduced cross-sectional thickness relative to the thickness of the material surrounding said vent area.

2. The seal of claim 1, when said thermoplastic material is chosen from the group consisting of polypropylene, polyethylene, and co-polymers thereof; and said filler is chosen from the group consisting of talc, calcium carbonate; and mica.

3. The seal of claim 2, when made from polypropylene having talc filler.

4. The seal of claim 3, when talc is present in the amount of 15% to 40%.

5. The seal of claim 1, when annealed.

6. The seal of claim 4, when annealed at a temperature of from 70°C to 155°C.

7. The seal of claim 4, when annealed for at least 1 hour at a temperature of from 70°C to 155°C.

8. The seal of claim 4, when said talc has 8 particle size less than 44 microns maximum dimension, with at least 2% by weight greater than 1 micron, at least 18% greater than 5 microns and at least 40%
greater than 10 microns.

9. A galvanic cell comprising a container crimp sealed with the seal of claims 1, 3 or 7.