Tammaro, Daniele (2016) Fundamentals of cell opening in polymer foaming. [Tesi di dottorato]

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Polymeric foams are ubiquitous in foods and industrial manufacturing. Since, they are used in a number of applications as thermal and acoustic insulators, in some cases it is desirable to create foams with cells not interconnected (i.e. closed cells), while in others cases an efficient interconnections between cells (i.e. opened cells) is required, as instance for culture substrates for living cells. In both cases, a fundamental understanding of the physics governing the cell opening process is needed to improve the final product and reduce the polymeric manufacturing cost. In this dissertation, the physical mechanisms leading to cell opening in foams is investigated from a fundamental point of view. As such, the complex foaming process (i.e. involving different physical mechanisms) was studied with a bottom-up process, dividing it in four elementary steps namely: 1) cells growth, 2) cells interaction, 3) rupture and 4) retraction of the cells walls. Different experimental techniques are employed in this thesis; most of them were designed during the Ph.D. to reproduce particular experimental conditions, which are difficult to be obtained with typical foaming equipment. In fact, different new experimental apparatus were developed (i.e. Mini-batch, Interfacial bubble, Breaking bubble) and specifically designed to make unique measurements. The new apparata are particularly useful for testing theoretical predictions on some types of simplified systems useful for the study of the foaming process. The main and novel result of this thesis is the fundamentals understanding of the entire foaming process that leads to a fundamental comprehension of how to produce a particular foam morphology, called fully opened cell. In the literature, there was not fundamentals understanding of the mechanisms behind the cell opening in thermoplastic foaming, since the reported foaming models stop the modeling at the rupture event in the cell walls, without considering the retraction event of the produced hole. The introduction of the retraction as the fundamental step to produce a fully open cell morphology is the novelty of this thesis. Moreover, the comprehension of the retraction step, leads to us to identify the importance of the role of the viscoelasticity for making a fully opened cell foam, that is a new concept that is unique and it was not covered earlier in the previous literature. Moreover, a model of the entire foaming process was developed and it was identified a criterion that employs the computed stresses, the elongational rate and the film thickness among the bubbles to predict the rupture of the polymeric layer between the bubbles and its retraction. As a result, the foaming process model is able to make predictions on the final foam morphology, starting from any polymer/gas solution. Independent experiments to assess the validity of each step of the proposed approach were performed. In conclusion, the developed methodology allows to design the materials and processing conditions to control foam morphology. In the first part of this thesis, a general overview of the foaming process is supplied, focusing the attention on the crucial points of each foaming steps pointed out by the existent literature. The main part will be occupied by the contributions published during the years of this Ph.D. because they represent the steps ahead achieved with respect to the literature.

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