The invention relates to a metal/air cell having an anode comprising zinc and an air cathode and having an adhesive sealant between a portion of the cathode and the cathode casing. The invention also relates to a pad transfer process of applying the adhesive sealant to a portion of the inside surface of the cathode casing.
Zinc/air depolarized cells are typically in the form of a miniature button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 12 mm and a height between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO2 alkaline cells and even larger sizes.
The miniature zinc/air button cell typically comprises an anode casing (anode cup), and a cathode casing (cathode cup). The anode casing and cathode casing each can have a closed end an open end. After the necessary materials are inserted into the anode and cathode casings, the open end of the anode casing is typically inserted into the open end of the cathode casing and the cell sealed by crimping. The anode casing can be filled with a mixture comprising particulate zinc. Typically, the zinc mixture contains mercury and a gelling agent and becomes gelled when electrolyte is added to the mixture. The electrolyte is usually an aqueous solution of potassium hydroxide, however, other aqueous alkaline electrolytes can be used. The closed end of the cathode casing (when the casing is held in vertical position with the closed end on top) typically has a raised portion near its center. This raised portion forms the positive terminal and typically contains a plurality of air holes therethrough. The cathode casing closed end also typically has an annular recessed step which surrounds the positive terminal.
The cathode casing contains an air diffuser (air filter) which lines the inside surface of the raised portion (positive terminal contact area) at the casing""s closed end. The air diffuser can be selected from a variety of air permeable materials including paper and porous polymeric material. The air diffuser is placed adjacent air holes in the raised portion of the casing closed end. Catalytic material typically comprising a mixture of particulate manganese dioxide, carbon and hydrophobic binder can be inserted into the cathode casing over the air diffuser on the side of the air diffuser not contacting the air holes. A cathode catalytic assembly can be formed by laminating a layer of electrolyte barrier material (hydrophobic air permeable film), preferably Teflon (tetrafluoroethylene), to one side of the catalytic material and an electrolyte permeable (ion permeable) separator material to the opposite side. The cathode catalytic assembly is then typically inserted into the cathode casing so that its central portion covers the air filter and a portion of the electrolyte barrier layer rests against the inside surface of the step.
In high drain or other demanding service electrolyte can migrate to the edge of the catalytic cathode assembly and leakage of electrolyte from the cathode casing can occur. The leakage, if occurring, tends to occur along the peripheral edge of the cathode catalytic assembly and the cathode casing and then gradually seep from the cell through the air holes at the cathode casing closed end. The potential for leakage is also greater when the cathode casing is made very thin, for example, having a wall thickness of about 0.01 inches (0.25 mm) and lower, for example to 0.05 mm. Such low wall thickness is desirable, since it results in greater internal cell volume. However, there is a greater tendency for the thin walled cathode casing to relax after the cell is closed by crimping. Such casing relaxation can result in the development or enlargement of microscopic pathways between the cathode catalytic assembly and the inside surface of cathode casing step, in turn providing a pathway for electrolyte leakage.
The cathode casing can typically be of nickel plated stainless steel, for example, with the nickel plate forming the cathode casing""s outside surface and stainless steel forming the casing""s inside surface. The anode casing can also be of nickel plated stainless steel, typically with the nickel plate forming the casing""s outside surface. The anode casing can be of a triclad material composed of stainless steel having an outer layer of nickel and an inner layer of copper. In such embodiment the nickel layer typically forms the anode casing""s outside surface and the copper layer forms the anode casing""s inside surface. The copper inside layer is desirable in that it provides a highly conductive pathway between the zinc particles and the cell""s negative terminal at the closed end of the anode casing. An insulator ring of a durable, polymeric material can be inserted over the outside surface of the body of the anode casing. The insulator ring is typically of high density polyethylene, polypropylene or nylon which resists flow (cold flow) when squeezed.
After the anode casing is filled with the zinc/electroyte mixture and after the air diffuser, catalyst, and electrolyte permeable (ion permeable) separator is placed into the cathode casing, the open end of the anode casing can be inserted into the open end of the cathode casing. The peripheral edge of the cathode casing can then be crimped over the peripheral edge of the anode casing to form a tightly sealed cell. The insulator ring around the anode casing prevents electrical contact between the anode and cathode cups. A removable tab is placed over the air holes on the surface of the cathode casing. Before use, the tab is removed to expose the air holes allowing air to ingress and activate the cell. A portion of the closed end of the anode casing can function as the cell""s negative terminal.
It is desired to produce a zinc/air cell which has a tight seal between the cathode assembly and the cathode casing.
It is desired to produce a zinc/air cell which prevents leakage of electrolyte around the edge of the cathode assembly and escaping through air holes in the cathode casing.
An aspect of the invention is directed to a process for applying an adhesive sealant to a portion of the inside surface of a cathode casing for a zinc/air cell. The zinc/air cell is typically in the form of a miniature button cell. Such miniature button cells have particular application as a power source for electronic hearing aids. The adhesive is preferably applied to the inside surface of a recessed step which surrounds a central positive terminal on the cell""s cathode casing at the closed end of the casing. The adhesive sealant prevents leakage of electrolyte from the cell. If the closed end of the cathode casing is flat, that is, does not have a recessed step, the adhesive sealant can be applied to the inside surface of the closed end adjacent the outer peripheral edge thereof.
The process of the invention involves the steps of preparing an etched or grooved surface on a plate, filling the etching or grooves on the plate with an adhesive mixture to form an adhesive pattern, applying a silicon pad to the adhesive pattern on the plate whereby the adhesive pattern transfers to the pad, then applying the pad to a portion of the inside surface of the cathode casing, whereby the adhesive pattern transfers to the inside surface of the cathode casing as the pad is lifted from the surface. The cathode casing preferably has an annular recessed step surrounding the positive terminal on the cathode casing. The adhesive pattern is preferably transferred from the pad to the inside surface of said annular recessed step surrounding the positive terminal on the cathode casing. The transferred adhesive pattern is preferably in the form of a continuous ring conforming to the shape and size of the recessed step, typically a circular ring. Thus, the adhesive pattern can desirably be applied to the inside surface of the annular step as a continuous ring contacting and tracing the inside surface of the step. The adhesive pattern has a width which can be the same as the width of said recessed step desirably between about one third to two thirds of the width of the step. Thus the adhesive preferably covers a large portion of-the inside surface of the step preferably most of the inside surface of the step. The adhesive is applied to the step so that it has a small uniform thickness. The thickness of the adhesive pattern (wet) transferred to the cathode casing step is about the same as the depth of the etching, preferably between about 20 and 40 micron (0.020 and 0.040 millimeter). If the etching is too shallow the pad will not pick up enough of the adhesive; if the etching is too deep the pad will only pick up a portion of the adhesive material. A desirable depth range for the etching is between about 20 and 40 micron (0.020 and 0.040 mm). Preferably the thickness of the transferred adhesive is about 30 micron (0.030 mm) (wet) and about 7 micron (0.007 mm) (dry).
In one aspect an electrolyte barrier sheet is applied to the adhesive ring on the inside surface of said recessed step, preferably so that the edge of the sheet adheres to the adhesive. The barrier sheet has the property that it is hydrophobic, that is, not permeable to water and alkaline electrolyte, yet it is permeable to air. A preferred barrier sheet is a sheet of Teflon (tetrafluoroethylene). The adhesive ring forms a permanent bond between the barrier sheet and the recessed step. A catalytic cathode assembly comprising a catalytic cathode layer typically comprising a mixture of manganese dioxide, carbon and binder is applied over the electrolyte barrier sheet so that the barrier sheet is between the cathode assembly and the positive terminal. The cathode assembly can include an electrolyte permeable separator placed over the catalytic cathode layer. The separator separates the cathode assembly from the anode mixture comprising zinc and alkaline electrolyte within the cell""s interior. The adhesive bond between the edge of the electrolyte barrier sheet and the inside surface of the recessed step around the positive terminal provides a tight seal preventing electrolyte from seeping around the cathode assembly and leaking from the cell.
The adhesive pattern which is transferred to the inside surface of the cathode casing step by the process of the invention is preferably a solvent based polyamide adhesive mixture. A preferred adhesive mixture is formed of a low molecular weight polyamide adhesive gel or solid dissolved in solvent. A desirable molecular weight of the polyamide is between about 195 and 390 atomic mass units. The ratio of polyamide material to solvent can be adjusted so that the solution viscosity is optimized for pad transfer by the process of the invention. A preferred viscosity for the adhesive mixture employing a low molecular weight polyamide is desirably between about 1000 and 1300 centipoise, preferably about 1100 centipoise.