Tammaro Daniele, Villone Massimiliano M, D'Avino Gaetano, Maffettone Pier Luca
Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale V. Tecchio, 80, 80125 Napoli, Italy.
Entropy (Basel). 2022 Jan 26;24(2):183. doi: 10.3390/e24020183.
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., CO2 or N2, 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 CO2 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.
热塑性聚合物泡沫的泡孔形态是影响其性能的关键因素。存在三种可能的泡沫形态，即闭孔、相互连通的泡孔结构和开孔。在气体发泡技术中，使用物理发泡剂，如二氧化碳或氮气，在软化/熔融的聚合物中在高压下形成气泡。由于压力骤降，气泡在液体基质中生长，直到它们相互碰撞并可能打破其间的薄液膜。如果发生液膜破裂，破裂的液膜可能会因聚合物液体在气泡生长过程中积累的弹性能而收缩。这反过来又决定了泡沫的最终形态。在这项工作中，我们通过实验研究了在不同压力条件下二氧化碳气泡在聚己内酯（PCL）基质中的生长情况。此外，我们进行了三维直接数值模拟，以支持实验结果，并阐明工艺参数对气泡生长结束时液体中积累的弹性能的影响，进而对预期的泡沫形态产生影响。为此，我们还将文献中用于单个气泡在液体中生长的解析模型扩展到具有多模式粘弹性本构方程的液体情况。
