"Primary" batteries are those that, once discharged, are not readily capable of being recharged, and as a result, are discarded. Early primary batteries used liquid electrolytes for power and were not very portable. As technology advanced, a "dry" process for combining energy-generating compounds was developed which allowed for greater battery portability. These primary batteries became known as "dry cell" batteries.
A cross-sectional diagram of a typical ready-to-use consumer dry cell battery of the prior art is provided in FIG. 8. The energy generating components of that battery are encased within a metal canister, referred to as the battery wall, battery can, or battery cell. The battery can itself is typically encased in a battery cover. Such battery covers can serve three functions. First, they may insulate or otherwise protect the materials contained in the battery cell from the outer environment. Second, they may protect the consumer and the product into which the battery is placed from battery leakage. Third, they provide a surface for affixing a labelling image or other decoration identifying the product.
Until recently, most battery makers typically enclosed battery cans in "metal jacket" battery covers, with a protective fiberboard layer of insulation between the metal jacket and the battery can to prevent shorting. The labelling image in a metal jacket cover is imprinted on the jacket. Battery manufacturers that still use metal jacket covers typically contract with independent sources for the supply of such covers. These battery manufacturers typically apply the metal jacket covers and insulating fiberboard to the battery cans in the final stage of the battery manufacturing process.
Metal jacket battery covers suffer from two principal drawbacks. First, the metal casing of the cover is relatively thick (up to 3/64th of an inch) especially when combined with the insulating fiberboard layer. Within the limits of each battery size, the cover thickness limits the volume available to store chemical compounds within the battery can. Since battery life is a function of both the type and amount of compounds used, use of these relatively thick metal jacket covers leads to reduced battery life. Second, metal jacket covers allow a relatively high incidence of battery can leakage wherein inner substances leak through both the battery can and the metal jacket cover. Such leakage may result in battery short circuits, thereby terminating this battery's life. Additionally, this leakage may cause outwardly visible discoloration and corrosion, which, though relatively harmless to a battery from a functional standpoint, can seriously erode consumer confidence in a battery manufacturer's product.
Alternative battery cover systems were developed and made possible by technological breakthroughs in applications for polyvinyl chloride, or "PVC". PVC is a clear, plastic material which can meet or exceed the durability, flexibility, and strength characteristics of a metal jacket with much less thickness of material. In PVC-based battery covers, one or more very thin layers of PVC film replace both the metal jacket and fiberboard insulating layer of the traditional metal jacket cover. Typically, a hard grade of PVC film is employed in such constructions, and this material may be referred to as "HPVC". Since PVC-based covers are considerably thinner than their metal jacket counterparts, they allow for much more dry cell material to be included in an enlarged battery cell, and thus allow for relatively longer-lived batteries. Other positive characteristics of PVC, including its strength, flexibility, and durability, allow PVC-based battery covers to perform the leak-prevention and insulation functions of battery covers better than metal jacket battery covers. Finally, PVC is a superior medium over the metal jacket for the printing of battery label information and decoration. For alkaline batteries, the combination of alkaline cell material and the larger cell size (achieved through use of the thinner PVC battery covers) leads to a dramatic increase in battery life. As a result, alkaline battery manufacturers have rapidly adopted the new PVC-based battery cover technology.
Pressure-sensitive PVC battery covers are flat covers, sometimes referred to as "labels", for ready-to-use consumer dry-cell batteries. Typically, pressure sensitive PVC battery labels have the following characteristics: (1) one or more layers of PVC film designed to allow for shrinkage in only the hoop (circumferential) direction when applied to the battery body, (2) printed label information and other decoration, (3) a layer of metal, typically vapor-deposited aluminum, (4) a layer of adhesive material which allows the label to adhere to the battery body, and (5) a backer or liner which is removed just before the label is applied by the battery manufacturer. Pressure sensitive battery labels are normally sized so that the label is fractionally longer than the battery can at both ends. In applying these labels to batteries, the labels are initially removed from their backer and wrapped around the battery can. Next, the wrapped battery passes through a small heating unit where the ends of the label are heated, causing the label to shrink circumferentially (in the hoop direction) at its ends where it extends beyond the battery body. This shrinkage causes the label to wrap tightly around the ends of the battery can, forming a tight protective seal.
Pressure sensitive PVC labels can be arranged in a number of different configurations depending on, among other factors, the number of PVC layers used. Labels composed of a single layer of PVC are known as monofilm labels, those formed of two layers of PVC are known as duplex labels, and labels composed of three layers of PVC are known as triplex labels. As mentioned above, these labels typically include a thin metal layer, which gives the label a bright background and an overall appearance superior and brighter than that achievable through printing of metallic inks alone. Depending on the position of the metal layer relative to the PVC, the labels are further classified. For example, a metal-down duplex label has two layers of PVC with a metal layer disposed between the bottom (or innermost) layer of PVC and the battery body. Similarly, in a metal-up duplex label the metal layer lies between the two PVC layers. The position of the metal layer and the number of PVC layers used in a given application depend on the battery cell involved as well as battery-manufacturer preference.
In order to facilitate the application of the metal layer to the PVC and adherence of the metal to the plastic film, metalization primers have been employed between the metal layer and the PVC film. Primer layers have also been used in other locations within the label to enhance printing. For example, one such triplex label having a primer layer between the metal layer and PVC is shown in Ast, Adhesive Labeling--Suitable Especially For Plastics, Packung & Transport, Vol. 1, 1984. That triplex label has the following layers (from bottom to top): (1) backing, (2) silicone, (3) adhesive, (4) HPVC, (5) primer, (6) metal, (7) adhesive, (8) HPVC, (9) imprint, (10) adhesive, and (11) HPVC. The primer layer of this label (layer 5) and an adhesive layer (layer 7) are sandwiched between two layers of HPVC layers 4 and 8).
The prior art recognizes the undesirability of having layers of primer and adhesive oriented between layers of film. The specific drawbacks of this arrangement include optical as well as functional defects. The optical defects often appear in the imprint on the labels and in the form of folds after the labels are applied. Noted functional defects include non-uniform shrinkage of the labels after application to the battery bodies, peeling off of the label at the point of overlap, and delamination of the various layers of the composite label.