The heat-seal coating influence the appearance and physical integrity of the package. Heat-seal coatings provide a bond between the plastic blister and the printed lidding material. These solvent or water based coatings can be applied to rolls or sheets of printed paperboard using roll coaters, gravure or flexographic methods, knives, silk-screening, or sprays. It is essential that the proper coating weight be applied to the lidding material for optimum heat-sealing results.

A good heat-seal coating for blister packages must exhibit good gloss, clarity, abrasion resistance, and hot tack and must seal to various blister films. Hot tack is particularly important because the product usually is loaded into the blister and the lidding material heat-sealed in place onto the blister. When the package is ejected from the heat-seal jig, the still-warm bond line must support its entire weight. A relatively low heat-seal temperature is desirable for rapid sealing and to prevent heat distortion of the blister film.

Although heat-seal coatings used for blister packaging still are predominantly solvent-based vinyls (because of their superior gloss), water-based products are making some inroads. However, they must be evaluated carefully for hot-tack properties, gloss retention, adhesion to specific inks, and sealability to selected blister films.

The heat-seal coating must precisely match the lidding material and the plastic material of the forming films. It means that with pre-determined sealing parameters, a permanent sealing effect between the lidding material and the forming film must be guaranteed under any climatic conditions. The heat-seal coating must also ensure constant sealing for any given sealing parameter. With the proper heat-seal coating, strong fiber-tearing bonds can be obtained. Specifically, the sealing strength must fall within predetermined tolerance limits and must be suitable for push-through or peel-off opening. The heat-seal coating also protects the printed area and provides a glossy finish.

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