A model of the heat sealing process for multilayer films with molecular detail

Heat sealing of multilayer polymer films is a critical step in the packaging of food, consumer products, and various other commercial items. Each layer comprises a resin that is specifically designed to impart a particular property to the final package. The sealant layer, in particular, must provide the package with a hermetic seal of sufficient strength to hold the product, and it must do so fast enough to accommodate high throughput. The toughness layer provides overall strength to the film. Recently, packaging manufacturers have sought to create all-polyethylene packaging. Polyethylene resins are tailored for different roles through adjustments in molecular weight and short-chain branching (SCB) distributions. To facilitate the design of resins for such multilayer packages, we develop a model for the heat sealing operation that accounts for the thermo-rheological behavior of the different crystallizable resins that compose the multilayer film, based on molecular-level characterization. The crystallizable discrete slip-link model, modified to account for partial crystallinity, is used to describe the rheology of each resin as it melts and recrystallizes. Melting and crystallization are modeled using the Toda and Nakamura models, which have been expanded to accommodate distributions of melting points associated with SCB in high-density polyethylene and linear low-density polyethylene resins. The heat sealing model is validated using results for hermeticity obtained from multilayer films processed in industrial packaging machines.