Study of the thermal conductivity of bulk and micronized nanocellular polymers

Resumen

The lack of knowledge regarding the thermal conductivity of nanocellular polymers has motivated this thesis. The main objective is the study of the thermal conductivity of nanocellular polymers. To achieve this goal, the following secondary objectives are fixed: Measuring the thermal conductivity of nanocellular polymers with steady-state methods. To achieve this goal, two routes will be followed; understanding the heat transfer mechanisms taking place in nanocellular polymers, evaluating their potential and limitations; and enhancing the thermal insulation behavior of nanocellular polymers by means of a new route based on micronization. To achieve these goals, the thermal conductivity of microcellular and nanocellular poly (methyl methacrylate) (PMMA) has been investigated. PMMA has been chosen as the polymer matrix due to its high ability to produce low-density nanocellular materials. First, the method for measuring the thermal conductivity in steady-state conditions is established. Second, the thermal conductivity is studied for bulk nanocellular polymers based on PMMA, both experimentally and theoretically, to understand the heat transfer mechanisms. Then, the material will be micronized to generate a new material: a micronized nanocelullar powder, which is studied in detail. The analysis of this novel material will also include a detailed characterization of the thermal conductivity of the powder and the possibility of compacting the material to generate vacuum insulation panel’s (VIP) core prototypes, as well as the optimization of the thermal conductivity by adding infrared blockers. Taking into account the general objectives and the methodology of this work, the following activities have been carried out: 1-Preparing low-density nanocellular polymers with large dimensions suitable to be measured in conventional steady-state machines. 2-Characterizing the thermal conductivity of insulating samples with small dimensions. 3-Analyzing the thermal conductivity of bulk microcellular and nanocellular PMMA- 4-Producing and characterizing micronized microcellular and nanocellular PMMA. 5-Fabricating and analyzing self-standing compacted panels based on micronized microcellular and nanocellular PMMA. 6-Evaluating the performance of the compacted panels as VIP core materials. 7-Optimizing the performance of the panels by adding IR blockers. Thus, it was possible to measure the thermal conductivity of nanocellular polymers using reliable techniques and use the results to understand in detail the heat transfer mechanisms in the bulk materials and model their thermal conductivity. Then, the thermal insulation behavior was enhanced using the main novelty of this thesis: the micronization of nanocellular polymers to give rise to a new material, that can be compacted in self-standing panels too. The micronized and compacted materials were studied in detail to understand the heat transfer mechanisms and develop models to predict their behavior. Quite promising results were obtained in the compacted panels, with minimum conductivities of 10.7 mW/(m·K) under vacuum and as low as 9.6 mW/(m·K) when infrared blockers were added.