Effect of Sealant Structure and Sealing Condition on Heat Sealing Performance of Polyethylene Films

En dépit de l'importance significative du scellage à chaud dans l'industrie de l'emballage flexible, certains aspects de ce processus ont reçu une attention très limitée dans la littérature et n'ont pas encore été examinés en détail. Dans cette thèse, une étude approfondie a été menée sur les effets du procédé de scellage et des conditions de ce scellage sur sa performance pour des films à base de polyéthylène. Le procédé de scellage est basé sur le chauffage de films de matière plastique en les superposant et en les poussant ensemble entre des mâchoires chauffées. Cependant, il n'y a pas encore eu une étude approfondie sur le transfert de chaleur au cours du processus de scellage à chaud dans la littérature. Dans la première partie de ce travail, le transfert de chaleur se produisant lors du thermoscellage de films multicouches avec une couche de scellant en polyéthylène a été étudié en détail. Des films multicouches composés d'une couche externe en polyamide (PA), d'une couche intermédiaire de polyéthylène greffé avec de l'anhydride maléique (PE-g-MA) et d'une couche de scellant en polyéthylène ont été produits par co-extrusion en filière plate. La température à l'interface des deux côtés du scellage a été mesurée à l'aide d'un thermocouple très fin connecté à un système d'acquisition de données. Les effets de la température de la mâchoire chauffée, de l'épaisseur de la couche de polyéthylène, du type de polyéthylène et de la pression de scellage sur l'évolution de la température à l'interface entre les côtés du joint ont été examinés. Les résultats obtenus ont montré que la température de la mâchoire, l'épaisseur du scellant et la cristallinité du polyéthylène sont les paramètres les plus importants affectant le temps nécessaire à l'interface pour atteindre la température de scellage désirée.

Abstract

Despite the significant importance of heat sealing in flexible packaging industry, some aspects of this process have been poorly studied in the literature and few aspects have not been examined in detail yet. In this dissertation, attempts have been made to provide a comprehensive study on the effects of sealant structure and sealing condition on seal performance of polyethylene-based sealant films.Heat sealing process is based on heating of plastic films by sandwiching and pushing them together between heated jaws, but a comprehensive study on heat transfer in heat sealing process is lacking in the literature. In the first part of this work, heat transfer in heat sealing of multilayer films with polyethylene sealant layer was studied in detail. Multilayer films composed of a polyamide (PA) outer layer, a polyethylene grafted maleic anhydride (PE-g-MA) middle layer and a polyethylene sealant layer were produced by cast co-extrusion. The interface temperature between two seal sides was measured using a fine thermocouple connected to a data acquisition system. Effects of heated jaw temperature, polyethylene layer thickness, type of polyethylene, and sealing pressure on the temperature evolution at the interface between seal sides were examined. The obtained results showed that jaw temperature, sealant thickness and crystallinity of polyethylene are the most important parameters that can affect the time required for the interface to reach the set sealing temperature. In order to model heat transfer in heat sealing, all material properties needed for modeling of heat transfer including temperature variations of density, thermal conductivity, and specific heat capacity were measured experimentally. In addition, thermal conductivity was measured at different pressures and it was shown that, in the studied range, pressure did not have a considerable effect on thermal conductivity of polyethylene. In addition, the surface roughness of film samples was also determined by atomic force microscopy (AFM). Heat transfer during heat sealing of multilayer films was simulated using COMSOL Multiphysics software. Thermal contact resistance (TCR) was considered as the boundary condition between jaws and the outer PA layers because heat sealing was done at temperatures far below melting temperature of PA. By comparing model predictions with experimental results, it was shown that the Cooper−Mikic−Yovanovich (CMY) model predicts much better the boundary condition in this system. The simulation results were in very good agreement with experimental data both below and above melting temperature of sealant material. Moreover, the simulation could predict well observed effects of sealing conditions on the interface temperature. To our knowledge, this was the first simulation of heat transfer in heat sealing that did not require any fitting parameter.