An Experimental and Numerical Investigation on Bubble Growth in Polymeric Foams.

Daniele Tammaro,Pier Luca Maffettone,Massimiliano M Villone,Gaetano D’Avino

doi:10.3390/e24020183

Abstract

The cellular morphology of thermoplastic polymeric foams is a key factor for their performances. Three possible foam morphologies exist, namely, with closed cells, interconnected cellular structure, and open cells. In the gas foaming technology, a physical blowing agent, e.g., or , is used to form bubbles at high pressure in softened/melted polymers. As a consequence of a pressure quench, the bubbles grow in the liquid matrix until they impinge and possibly break the thin liquid films among them. If film breakage happens, the broken film may retract due to the elastic energy accumulated by the polymeric liquid during the bubble growth. This, in turn, determines the final morphology of the foam. In this work, we experimentally study the growth of bubbles in a poly(e-caprolactone) (PCL) matrix under different pressure conditions. In addition, we perform three-dimensional direct numerical simulations to support the experimental findings and rationalize the effects of the process parameters on the elastic energy accumulated in the liquid at the end of the bubble growth, and thus on the expected morphology of the foam. To do that, we also extend the analytic model available in the literature for the growth of a single bubble in a liquid to the case of a liquid with a multi-mode viscoelastic constitutive equation.

Highlights

Due to their mechanical, transport, acoustic, and impact absorption properties, polymeric foams are used in a multitude of different applications, e.g., transportation, construction, packaging, food, extraction and separation, tissue engineering, leisure, and sport [1,2]
It is worth remarking that the computational curves stop earlier than the experimental data because the adopted numerical technique is not capable to deal with topological changes, namely, when the thickness of the liquid film separating the bubbles becomes too little and the bubbles coalesce
The broken liquid wall might retract depending on the amount of elastic energy that it has stored during bubble growth

Summary

Introduction

Transport, acoustic, and impact absorption properties, polymeric foams are used in a multitude of different applications, e.g., transportation, construction, packaging, food, extraction and separation, tissue engineering, leisure, and sport [1,2]. The physical gas foaming technology, which makes use of a physical blowing agent (such as carbon dioxide or nitrogen) to form bubbles in softened/melted polymers, is the most widely employed process for the making of polymeric foams, mainly because of its high productivity [1,3]. The sequence of operations involved in the gas foaming technology is the following [1]: (1) blowing gas solubilization (yielding a polymer–gas solution); (2) bubble nucleation induced by an instantaneous pressure quench or temperature rise; (3) bubble growth; (4) foam setting These operations are interconnected and their interplay strongly affects the cell morphology and, the properties of the final product. In principle, it is possible to make measurements to quantify such properties, it is very challenging to predict their combined influence on a foam, especially under transient conditions

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