Effect of heat transfer coefficient, draw ratio, and die exit temperature on the production of flat polypropylene membranes

Abstract

In this work, a stable numerical scheme has been developed for the 1.5-dimensional film casting model of Silagy et al. [Polym. Eng. Sci. 36, “Study of the stability of the film casting process,” 2614–2625 (1996)] utilizing the viscoelastic modified Leonov model as the constitutive equation and energy equation coupled with the crystallization kinetics of semicrystalline polymers taking into account actual temperature as well as cooling rate. The model has been successfully validated on the experimental data for linear isotactic polypropylene taken from the open literature. Drawing distance, draw ratio, heat transfer coefficient, and die exit melt temperature were systematically varied in the utilized model in order to understand the role of process conditions in the neck-in phenomenon (unwanted film width shrinkage during stretching in the post die area) and crystalline phase development during flat film production. It is believed that the utilized numerical model together with the suggested stable numerical scheme as well as obtained research results can help to understand a processing window for the production of flat porous membranes from linear polypropylene considerably.

Steady-state modeling of extrusion cast film process, neck-in phenomenon, and related experimental research: A review

“…It is important to mention that a different definition of relaxation time can be found in the reviewed literature. In the studies based on single-mode constitutive equations, the utilized Maxwell relaxation time 27 and the shortest 35 or characteristic relaxation time (determined by the reciprocal frequency at the intersection of the storage modulus G ′ and the loss modulus G ′′ curves 36 or by fitting the strain rate dependent steady uniaxial extensional viscosity data 37,38 ) are typically used to calculate De. In the case of multi-mode constitutive equations, the relaxation time for each mode [39][40][41][42][43][44] or an average relaxation time, λ, is calculated to determine De by using the following expression: [45][46][47]…”

“…Individual research groups focused on experimental works 30,48,52,55,[65][66][67][68][69][70][71][72][73][74] dealing with flow visualization, effects of temperature, crystallization, molecular weight distribution, or long chain branching on kinetics of the film casting process (see Table I). Theoretical research has not disappeared, and attention has been drawn to the development and use of numerical models (primarily considering steady-state conditions) of different dimensionalities such as 1D, 22,25,26,45,[75][76][77][78][79][80][81][82][83][84][85] 1.5D, 18,27,29,35,37,38,[41][42][43][44]47,53,[86][87][88][89][90]…”

“…The model demonstrated the ability to describe the film casting experimental data for linear (PP) and branched (LDPE) polymers even by using a single mode (i.e., utilizing a single pair of λ and G) only 35,37,38 (see Fig. 18).…”

“…The parameter nL was kept equal to zero.) The modified Leonov model was used in the following studies: 1D (0), 1.5D (3 works), 35,37,38 2D (0), and 3D (0). The original Leonov model was used only in the following study: 1.5D (1 work).…”

A fundamental rule: Determining the importance of flow prior to polymer crystallization

“…On the other hand, in typical polymer processing methods such as polymer extrusion 13,14 , injection molding 15,16 , film blowing 17,18 , fibre production 19 ,and extrusion-based three-dimensional (3D) printing 20 , processing temperatures are usually above the melting point of the polymer, and crystallization follows in the absence of flow under non-isothermal conditions. Thus, the nucleation rate is typically non-constant, depending on both the decaying temperature and the flow-induced polymer deformation, which will begin to relax at the cessation of flow.…”