Blister packs are usually sealed with an adhesive coating using heat and pressure, but the need to maintain the pressure while waiting for the adhesive coating to be heated to form a bond requires the equipment to be operated at slower speeds than would be feasible if the bond were formed more quickly. The heated platens must be aligned with precision, thus requiring the equipment to be expensive both to fabricate and to operate. Each style of blister pack requires a unique platen or set of platens.
Pressure-sensitive adhesives have been used for sealing blister packs, but involve problems that have limited their use. For example, their tacky nature makes it desirable that they be applied only in areas of contact between the flanges of the blisters and the cardboard or other material on which the blisters are mounted. This is difficult to control, and slight misalignment might allow the contents to contact the tacky adhesive or allow the listers to open accidentally.
Heat applied in the sealing of a blister pack, often at temperatures of 175.degree. C., can damage items being packaged, even though the damage may not be evident. For example, medical pills, i.e., capsules or tablets, are often individually packaged by a pharmacist in medication cards having one punch-open compartment per pill, and the small size of each compartment brings the heat close to each pill. The pharmacist might not be aware of the deleterious effect that such heat may have on specific medications, and it is believed that for this reason such equipment is banned in the United Kingdom. Where such equipment is used, a preheating delay is required and there are safety hazards. During periods of hot weather, pharmacists complain about heat discharged into the working area.
A variety of other types of containers also are sealed by adhesive coatings using heat and pressure, and the speed at which the sealing equipment can be operated is likewise reduced by the delay in forming bonds.
Labels often are applied to containers and other objects using adhesive coatings that are sealed by heat and pressure. In addition to involving the aforementioned time delays, the labels, if misaligned, cannot be economically repositioned. Instead, solvent-activated adhesives are often used even though they involve similar time delays and set so quickly that repositioning is impractical. While pressure-sensitive adhesives eliminate such delays, those that that are sufficiently aggressive to make the labels tamper-resistant do not permit repositioning of misaligned labels.
Neither solvent-activated nor heat-activated adhesives are practical for many important applications such as adhering body-side molding to an automobile, thus requiring the use of pressure-sensitive adhesives, but for applications that require strong, permanent bonding, misalignment often cannot be economically corrected.
Some containers are sealed using heat-activated adhesives, e.g., peel-open tabs that close openings in the lids of drink-containing cans. The speed of sealing such closures also is reduced by the delay in forming the bond. The operating speed of equipment for heat-fusing two articles is likewise limited by the time delay to fuse material. For example, documents such as drivers' licenses are often protected between two heat-sealable plastic sheets. Such equipment is expensive and its high temperatures involve a safety hazard.