Analysis and Modelling of Extrusion Foaming Behaviour of Low-Density Polyethylene using Isobutane and CO2

In this work, modelling of physical foaming extrusion of LDPE is carried out in order to achieve a better understanding of the mechanisms involved in foam manufacturing. Foaming by extrusion is a four-step process. First, the pellets are introduced and molten in an extruder. Gas is then injected under pressure and dissolved in the polymer matrix. The mixture is then significantly cooled to give more strength to the material while maintaining a certain level of pressure. Finally, foam expansion occurs at the die exit. At this location, the dissolved gas undergoes a strong decompression leading to the nucleation and growth of bubbles. The objective of this study is to better understand the origin of the limitation of foaming based on the combination of an experimental analysis of the foaming process and the prediction of a model.The modelling tools are focused on the expansion occurring at the die exit, in order to quantify all the important parameters for the control of the foaming structure. The model, labelled as “cell model”, is based on previous works [1,2] and considers the growth of a single bubble in a mixture of polymer matrix and dissolved gas (blowing agent) [3]. In order to take into consideration the viscoelastic character of the polymer, the rheological behaviour is represented by a multi-Maxwell model.The foaming behaviour of two LDPE commercial grades provided by Total Research & Technology Feluy (Belgium) is compared for two different foaming agents (isobutane and carbon dioxide). The extrudate expansion at the die exit is analysed experimentally for different conditions (mainly temperature and gas concentration). An analysis of the extrusion parameters is performed to determine the quantity of dissolved gas which is effectively used for the foaming process. In order to compare with the experimental results, the cell model considers the gas concentration and the relaxation spectrum of each LDPE grade. As a consequence, particular attention is devoted to the determination of the solubility and the diffusivity of the blowing agent in the molten polymer based on literature data. The main effort concerns the analysis of the influence of the rheological properties of the two LDPE grades and the properties of the blowing agent on the size and stability of the cells. The modelling predictions are compared with the foam expansion and the foam density, revealing that the use of the cell model provides an accurate estimation of the final properties of the foam in the case of isobutane. The difference in final foam density is used to make hypotheses on the physical phenomena which can limit the foam expansion. Indeed, gas loss or polymer crystallization can limit the foam expansion and this is related to the temperature at the die exit [4]. Nevertheless, there is an open question on the role of strain hardening behaviour on foam expansion [3,5]. These different hypotheses will be discussed. REFERENCES[1] Reglero Ruiz, J.A., Vincent, M., Agassant, J.‐F., Claverie, A. and Huck, S. (2015), Morphological analysis of microcellular PP produced in a core‐back injection process using chemical blowing agents and gas counter pressure. Polym Eng Sci, 55: 2465-2473.[2] Reglero Ruiz, J.A., Vincent, M. and Agassant, J.‐F. (2016). Numerical Modeling of Bubble Growth in Microcellular Polypropylene Produced in a Core-Back Injection Process Using Chemical Blowing Agents, Int. Polym. Proc., 31: 26-36.[3] Otsuki, Y. and Kanai, T. (2005), Numerical simulation of bubble growth in viscoelastic fluid with diffusion of dissolved foaming agent. Polym Eng Sci, 45: 1277-1287.[4] Naguib, H.E., Park, C.B. and Reichelt, N. (2004), Fundamental foaming mechanisms governing the volume expansion of extruded polypropylene foams. J. Appl. Polym. Sci., 91: 2661-2668.[5] Weingart, N., Raps, D., Lu, M., Endner, L. and Altstädt, V. (2020). Comparison of the Foamability of Linear and Long-Chain Branched Polypropylene—The Legend of Strain-Hardening as a Requirement for Good Foamability. Polymers, 12(3):725

Extrusion moussage de polypropylènes linéaires et branchés - apport de l'analyse thermomécanique de la pression dans la filère

International audienceThis paper aims at a better understanding of the polypropylene (PP) physical extrusion foaming process with the objective of obtaining the lowest possible foam density. Two branched PPs were compared to the corresponding linear ones. Their shear and elongation viscosities were measured as well as their crystalline properties. Trials were conducted in a single screw extruder equipped with a gear pump and a static mixer cooler to adjust the melt temperature at the final die. The effect of decreasing this temperature on the PP foamability and on the pressure drop in the die was analyzed. The foam density of branched PPs varies from high to low values while decreasing the foaming temperature. In the same processing conditions, the foam density of linear PPs does not decrease so much, as already evidenced in the literature. The foamability transition coincides with an increase of the pressure drop in the die. The originality of the work lies in the thermomechanical analysis of the polymer flow in the die which allows the identification of the relevant physical phenomena for a good foamability. The comparison of the experimental pressure drops in the die and the computed ones with the identified purely viscous behavior points out the influence of the foaming temperature and of the PP structure. At high foaming temperature the discrepancy between experimental measurements and the computed pressure drops remains limited. It increases when decreasing the foaming temperature, but the mismatch is much more important for branched PPs than for linear ones. This difference is analyzed as a combination of the activation energy of the viscosity, the elongational viscosity in the convergent geometry of the die which is much more important for branched PPs than for linear ones, and the onset of crystallization which occurs at higher temperature for branched PPs than for linear PPs

Analysis and Modelling of Extrusion Foaming Behaviour of Polyolefins using Isobutane and CO2

International audienceThis study aims to better understand the polypropylene (PP) foamability by physical extrusion foaming comparing branched chains with strain hardening versus linear ones. Trials were conducted in a single screw extrusion equipped with a gear pump for the gas dissolution step (same extrusion parameters, 1wt% CO2) and a static mixer cooler allowing to decrease the melt temperature before the final die (referred as foaming temperature). The effect of decreasing the foaming temperature on the PP foamability was analyzed.The foam density of branched PP varies from high to low values while decreasing the foaming temperature. This foamability transition coincides with an increase of the pressure drop in the die. As reported, branched PPs depict a better foamability than linear grades. As the pressure drop in the die is responsible of the polymer foaming, a thermomechanical analysis of the polymer flow was conducted to better understand the foamability transition.The pressure drop was calculated in the die using dedicated analytical expressions for the converging and capillary parts and a power law for the viscosity curve. Calculated pressures are lower than the measured values. The discrepancy is interpreted as an additional contribution due to the elongational flow in the converging channel, which can be estimated. The pressure drop variation with the foaming temperature follows an Arrhenius dependence in the case of linear grades. In the case of branched grades, the Arrhenius dependence is valid for large foaming temperatures but a large discrepancy is reported for low foaming temperatures. Two phenomena (presence of strain hardening for branched PP and/or of crystallization) can be at the origin of this discrepancy. These hypotheses will be examined and discussed for the different polymer grades in order to clarify the physical scenario for the foaming process

Analysis of extrusion foaming of polyolefins using isobutane and CO2

International audienc

Foamability of linear and branched polypropylenes by physical extrusion foaming - Input of the thermomechanical analysis of pressure drop in the die

This study aims to better understand the polypropylene (PP) foamability by physical extrusion foaming comparing branched chains with strain hardening versus linear ones. Trials were conducted in a single screw extrusion equipped with a gear pump for the gas dissolution step (same extrusion parameters, 1wt% CO2) and a static mixer cooler allowing to decrease the melt temperature before the final die (referred as foaming temperature). The effect of decreasing the foaming temperature on the PP foamability was analyzed.The foam density of branched PP varies from high to low values while decreasing the foaming temperature. This foamability transition coincides with an increase of the pressure drop in the die. As reported, branched PPs depict a better foamability than linear grades. As the pressure drop in the die is responsible of the polymer foaming, a thermomechanical analysis of the polymer flow was conducted to better understand the foamability transition.The pressure drop was calculated in the die using dedicated analytical expressions for the converging and capillary parts and a power law for the viscosity curve. Calculated pressures are lower than the measured values. The discrepancy is interpreted as an additional contribution due to the elongational flow in the converging channel, which can be estimated. The pressure drop variation with the foaming temperature follows an Arrhenius dependence in the case of linear grades. In the case of branched grades, the Arrhenius dependence is valid for large foaming temperatures but a large discrepancy is reported for low foaming temperatures. Two phenomena (presence of strain hardening for branched PP and/or of crystallization) can be at the origin of this discrepancy. These hypotheses will be examined and discussed for the different polymer grades in order to clarify the physical scenario for the foaming process

Extrusion foaming of linear and branched polypropylenes - Input of the thermomechanical analysis of pressure drop in the die

International audienceThis study aims at a better understanding of the polypropylene (PP) foamability by physical extrusion foaming. Trials were conducted in a single screw extruder equipped with a gear pump for the gas dissolution step (same extrusion parameters, 1wt% CO2) and a static mixer cooler allowing to decrease the melt temperature before the final die (referred as foaming temperature). The effect of decreasing this temperature on the PP foamability and on the pressure drop in the die is analyzed. The behavior of branched and linear PP grades is compared.The foam density of branched PPs varies from high to low values while decreasing the foaming temperature. In the same processing conditions, the foam density of linear PPs does not decrease so much as already evidenced in the literature. The foamability transition coincides with an increase of the pressure drop in the die. The thermomechanical analysis of the polymer flow in the die is used to identify the relevant physical phenomena for a good foamability. The comparison of experimental pressure drops in the die and computed ones (analytical expressions for converging and capillary dies with identified purely viscous behaviors) points out the influence of the foaming temperature and of the polypropylene structure. At high foaming temperature the discrepancy between experimental and computed pressure drops remains limited. It increases when decreasing the foaming temperature, but the mismatch is much more important for branched PPs than for linear ones. This difference is analyzed as a combination of the activation energy of the viscosity, extensional flow in the converging part of the die, and onset of crystallization in the die. These hypotheses will be examined and discussed for the different polymer grades in order to clarify the physical scenario for the foaming process [1]. Reference :1. Sandino C., Peuvrel-Disdier E., Agassant J.F., Laure P., Boyer S.A.E., Hibert G., Y. Trolez Y., Extrusion foaming of linear and branched polypropylenes-Input of the thermomechanical analysis of pressure drop in the die, Intern. Polym. Proc., 2022 accepte

Near-Infrared Laser Direct Writing of Conductive Patterns on the Surface of Carbon Nanotube Polymer Nanocomposites

International audienceNear-infrared (NIR) laser annealing is used to write conductive patterns at the surface of Polypropylene/Multi-walled carbon nanotube nanocomposite (PP/MWCNT) plates. Before irradiation, the surface of the nancomposite is not conductive due to the partial alignment of the MWCNT which occurs during injection molding. We observe a significant increase of the surface sheet resistance using NIR laser irradiation, which we explain by a randomization of the orientation of MWCNTs in the PP matrix melt by NIR laser irradiation. After only 5s of irradiation, the sheet resistance of PP/MWCNT, annealed with a laser at a power density of 7 W/cm 2 , decreases by more than 4 decades from ~ 100 MΩ/sq to ~ 1 kΩ/sq. Polarized Raman, TEM, and SEM are used to investigate the changes in the sheet resistance and confirm the physico-chemical processes involved. This allows directwriting of conductive patterns using NIR laser at the surface of nanocomposite polymer substrates, with the advantages of a fast, easy, and low-energy consumption process