(54b) Modeling LLDPE Resins in Multilayer Film Heat-Sealing with Slip-Link Model

Optimization of the industrial processing of polymers requires accurate modeling of the rheological behavior. There are a number of molecular models for the rheology of polymeric materials. Their main advantages include a direct connection between molecular level details of polymer chains and macroscopic behavior of the material. However, such models are challenging to implement due to computational cost requirements for realistic molecular weights. Generally, significant additional effort is required to develop, optimize and apply these models to industrially relevant cases.

Heat sealing is a common industrial method of hermetically sealing flexible packaging made from polymer films. The typical heat-sealing operation involves hot bars that bring two polymer films together and apply pressure and heat to form the seal. The polymer films typically comprise multiple layers; different layers serve different purposes and contain different polymers. The simplest multilayer structure includes a toughness layer and a sealant layer. The sealant layer is designed to melt, flow and recrystallize during the operation cycle.

Here, we take the discrete slip-link model, an established computational rheological model for entangled polymers, and apply it to the simulation of heat sealing of multilayer LLDPE films. The model is attractive due to its applicability to broadly polydisperse polymers under non-linear flows. Additionally, we recently extended the model to semicrystalline and crystallizing polymers. We describe our efforts to adapt the model for heat-sealing simulations. The modifications include optimizing the computational cost and inverting the model's stress-strain relation. Next, we combine the rheological model with the heat transfer and crystallization kinetics models. The combined model of multilayer film sealing features toughness and sealant layers. A layer representation in the model is based on inputted molecular and rheological characterization information for each LLDPE resin. The model calculates temperature, crystallinity profiles, stresses, flow rates, film thickness evolution, and flow volume, from which conclusions can be drawn about hermeticity of the seal.