Brondi, Cosimo (2020) Polyurethane foams: novel processing and novel additives for improved thermal insulation properties. [Tesi di dottorato]

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Global energy consumption is expected to increase by more than 50% within the next ten years, as a result of the energy efficiency technologies that still do not keep up with the rapid growth of new buildings and the refrigeration sectors. As a consequence of the rising energy demand, environmental issues are becoming more apparent. Moreover, regulations on thermal insulation are becoming ever stricter. For these reasons, there is high industrial interest to design innovative and efficient materials that are capable of lowering the thermal conductivity and then reducing the heat loss in buildings. In this field, polyurethane foams already proved to be one of the most efficient materials in reducing the thermal conductivity. In this dissertation, focus was given to develop new knowledge and gain fundamental understanding about the PU foaming process and the various influencing factors. New strategies were developed in order to obtain rigid polyurethane as well as polyisocyanurate foams with low foam density and reduced cell size that could lead to improved thermal insulation properties. In particular, two main strategies have been followed and defined that can be resumed as follows. The first approach consisted in the investigation of foaming process of rigid polyurethanes obtained by high-pressure CO2. High-pressure CO2 foaming technology proved very effective with thermoplastics. In fact, with the pressure quench method, utmost performances in terms of cell number densities have been reached with numerous thermoplastic polymers: microcellular (cell size in the range 1–10 µm) and, more recently, nanocellular (cell size in the range 1–100 nm) foams have been produced, characterized by improved thermal insulating and mechanical properties as compared to standard cell-sized foams. With regard to thermosetting foams, the same approach used in thermoplastics has been implemented here in this work. To this aim, firstly, (i) the governing factors that affect the curing process of the rigid PU under high-pressure CO2 were monitored and studied by FT-NIR spectroscopy. (ii) Once that the effect of the CO2 on the curing process have been clarified, it is necessary to develop a new strategy to cope with the very different timescales of the processes under consideration, namely the rapid depressurization of the sorbed CO2 and the slow curing reaction. In this way, the polymer does not undergo to excessive stresses due to the depressurization and suitable polyurethane foams can be obtained. In this approach, by pushing on CO2 bubble nucleation, (iii) microcellular foams by high-pressure CO2 have been produced. The second approach consisted in the investigation and monitoring of the foaming process of rigid polyurethane and polyisocyanurate foams obtained by liquid-type organofluorine additives. Liquid-type additives have been extensively used in the last years to reduce the average cell size. Several works have shown the improvement of polyurethane and polyisocyanurate foam morphologies (and, correspondingly with improved mechanical and thermal properties) as a consequence of the introduction of these additives. However, besides the achievements based on characterization results only, the mechanisms induced by addition of these liquids have not been deeply studied. In our study, it has been found that organofluorine additives are suitable to this purpose, by inhibiting the Ostwald Ripening that affect the late foaming process. With the aim of investigating the effect of these additives, firstly, (i) the competing bubble formation mechanisms and how these are affected by the cell degenerations mechanisms were studied and monitored by in-situ optical observation. (ii) Once that the mechanisms affecting the final morphology are elucidated, it is necessary to develop a new strategy that allows to regulate the aforementioned governing factors. In this approach, by pushing on the depletion of the cell degeneration mechanisms, (iii) polyurethane as well as polyisocyanurate foams by liquid organofluorine additives have been produced. In conclusion, the developed methodologies allow to control the processing conditions in order to obtain the desired foam morphology.

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