Rigid polyurethanes foaming with CO2 as physical blowing agent

There is significant industrial interest in the development of innovative and efficient materials for thermal insulation applications. Indeed, the energy issues are becoming more and more important because of possible energy shortage in the future compounded by global warming. Moreover, regulations on thermal insulation in the household sector, building trade, aeronautics and gas transport are becoming ever stricter. One of the solutions to these issues is to fabricate materials with very low thermal conductivity. Many cellular materials are used in thermal insulation to take advantage of the good insulation capacity of some gases, in particular rigid polyurethane foams. The thermal conductivity of these polymers can become lower than the relevant gas, because of the Knudsen effect that limits the heat conduction via a confined gaseous phase. Fabrication of low density material within which the gas mobility is restricted is a challenge in terms of obtaining a very low thermal conductivity material. In this dissertation, the foaming of rigid polyurethanes by using high pressure carbon dioxide (CO2) as physical blowing agent was investigated starting from the knowledge of the behavior of the whole system in the presence of CO2. The study of CO2 sorption in the polymeric precursors of rigid polyurethane foam (polyol and isocyanate), by using a coupled gravimetry-Axisymmetric Drop Shape Analysis, was conducted to design the process and the equipment and to optimize the foaming. In particular, to address the recent interest in combining the gas (physical) foaming with the classical (chemical) polyurethane foaming, a novel instrumented pressure vessel was designed for studying: i) gas sorption under high pressure on the different reactants, kept separate and ii) synthesis under high gas pressure, upon mixing, by spectroscopic investigation and iii) foaming upon release of the pressure. In the literature, no papers addressed the use of CO2 as a physical blowing agent in polyurethane foams (as well as in other thermosetting polymers), where CO2 solubilization is conducted in both the reactants before mixing in lab-scale. Furthermore, in industrial processes, typically liquid CO2 is mechanically mixed (not solubilized) under pressure in polyurethane foam reactants to froth the mixture by release pressure. The two novelties, shown in this thesis, are the possibility to solubilize the gas (1) as physical blowing agent (2) in both the reactants of a polyurethane foam before the mixing. As results, from sorption measurements the maximum value of CO2 pressure usable for foaming experiments was defined, in order to avoid extraction of low molecular weight fractions of the polymeric precursors in CO2. Rigid polyurethane foams obtained during this Ph.D. are described in terms of their morphology. In conclusion, the developed lab-scale apparatus allows to obtain rigid polyurethane foams by solubilizing CO2, as physical blowing agent, both in polyol and isocyanate. By optimizing some chemical and processing parameters is possible to control the morphology and so the thermal conductivity of the final foam. In the first part of this thesis, an overview of the current chemistry and foaming processes used in the production of rigid polyurethane foam are reported. The main part will be occupied by the description of CO2 solubility measurements, of the new equipment to study sorption, synthesis and foaming of rigid polyurethane foams and of experimental results.