Polymerization mechanism, micro-macro properties, and carbonization of polyurethane foams

Abstract

Polyurethane is one of the most diversified macropolymers with versatile properties for many applications including construction, transportation, personal wear, household appliance, etc. The research of this PhD study covers many aspects of polyurethane, including modelling on urethanisation and foaming mechanism, cell microstructure and packing polyhedrons, macroscopic properties and performance, and functional carbon materials developed from carbonisation of polyisocyanurate (PIR) foams. The work contains both theoretical modelling and experimental measurements. Urethanisation Kinetics The catalysed polyisocyanurate reaction kinetic model was developed based on generalized copolymerization scheme. PIR/PUR ratio was derived from mathematical manipulation on rate equations. The structural unit effects of isocyanurate, urethane and urea were evaluated based on Mayo-Lewise tercopolymerization scheme. Two reaction scenarios – bifunctional and macropolyol – were taken into consideration. Two ratios of isocyanate/polyol and urethane/urea rather than isocyanurate concentration were found to have impact on isocyanate conversion. Cell Growth and Foaming Process The cell microstructural configuration model was developed based on FOAMAT reactivity profiling (FOAMAT is a foam qualification system to measure the formation by curing and foaming parameters). The cell constructions were well understood by characterization of interstitial border area between cells. The cell anisotropic degree was calculated based on 2D cell shapes deformation comparison between free rising and stress stretching. The foaming process of continuous line panel production was further modelled based on cell anisotropic stretching. Plateau Borders The geometric Plateau border model for closed cell polyurethane foam was developed based on volume integrations of approximated 3D four-cusp hypocycloid structure. The tetrahedral structure of convex struts was orthogonally projected into 2D three-cusp deltoid with three central cylinders. The idealized single unit strut was modeled by superposition. The volume of each component was calculated by geometric analyses. The strut solid fraction f_s and foam porosity coefficient δ were calculated from developed strut model based on representative elementary volume (REV) of Kelvin and Weaire-Phelan structures. The specific surface area Sv derived from packing polyhedra model and deltoid approximation model respectively were put into contrast against strut dimensional ratio ε. The characteristic parameters modeled from this semi-empirical method were further employed to predict foam thermal conductivity. The correlation results show good agreement with actual measurement. The deviation gap can be caused by disorderedness and irregularity of actual cells. The periodical numerical method still has limit in predicting foam mechanics. Foam Defect Microstructure Streak and blister cell defects pose extensive surface problems for rigid polyurethane foams. In this study, these morphological anomalies were visually inspected using 2D optical techniques, and the cell microstructural coefficients including degree of anisotropy, cell circumdiameter, and the volumetric isoperimetric quotient were calculated from the observations. A geometric regular polyhedron approximation method was developed based on relative density equations, in order to characterize the packing structures of both normal and anomalous cells. The calculated cell volume constant, C_c, from polyhedron geometric voxels was compared with the empirical polyhedron cell volume value, C_h. The geometric relationship between actual cells and approximated polyhedrons was characterized by the defined volumetric isoperimetric quotient. Binary packing structures were derived from deviation comparisons between the two cell volume constants and the assumed partial relative density ratios of the two individual packing polyhedrons. The modelling results show that normal cells have a similar packing to the Weaire-Phelan model, while anomalous cells have a dodecahedron/icosidodecahedron binary packing. Insulation Performance Polyurethane (PU) is a commonly used insulation material for cold storage warehouses. The insulation performance of PU sandwich panels made from blended blowing agents were re-assessed by k-factor measurements and the insulation thickness was calculated based on cold warehouse design standard. The purposes of this study is test the impact of thermal conductivity value from experimental measurements on insulation barrier thickness calculation, and try to identity the gap between experimental data and empirical data in real practice and its impact on insulation design. The building design standard of cold warehouse can be a good benchmark to showcase this difference in aggressive cooling environment. The results have confirmed significant positive impact of blowing agents for energy saving. Post-curing Stability Problem The foam post-curing stability was evaluated by mathematic manipulation. The developed 3D paraboloid model based on gridding measurements has provided a scientific solution to foam panel shrinkage problem. Cell microstructure characterisation and post-growth angle coefficients calculation were further performed in this study. The results show the cell microstructure undergoes severe contraction during cooling and some cell destruction has happened on foam defects. Meanwhile, the cell anisotropic degree is getting more uniformed and this phenomenon is considerably prominent in central position. Thermal Degradation The thermal degradation of polyisocyanurate foam samples were studied by TG/DTA, FTIR, and SEM. All samples with different isocyanate index (NCO/OH = 100, 200, 300) were pre-treated by H2SO4, K2CO3, and NaOH before heating. The measurements of DTG and DTA presented corresponding variability for different acidic and alkaline treatments. The activation energy of thermal decomposition was calculated based on kinetic reaction evaluation. The pronounced polyol and isocyanate regenerations were observed over degradation. Further FTIR measurements at elevated temperatures suggested the possibility of acidic hydrogen bonding catalyzation and alkaline reversible amide regeneration during degradation by chemical treatments. The morphology study by SEM show localized corrosion is severe for high temperature carbonisation by acidic treatment (H2SO4) and microcrystallization occurs for alkalic treatments (K2CO3 and NaOH). The microcrystals vary by geometric shape. Carbonization The isocyanate index (NCO/OH) of diisocyanate and macropolyol can dictate the carbonisation of polyisocyanurate (PIR) foams. The carbon amorphousness was characterized by DSC which suggests the disorderedness can be aggravated by acidic pre-treatment. XRD investigation on crystallography suggests intercalation layered structure was created during carbonisation. Synchrotron-based X-ray photoelectron spectroscopy (XPS) analyses of all carbonaceous residues reveal that N-doping carbonisation has been realized by isocyanate dipolar cycloaddition. The N-doping structures with pyridinic (N-6) and pyrrolic (N-5) nitrogen atoms were found in carboncyclic rings, but no graphitic (N-Q) structure was identified. Higher isocyanate index (NCO/OH) can increase the opportunity for N-doping by creating more pyrrolic (N-5) nitrogen structure. The acidic treatment by H2SO4 can promote pyridinic (N-6) structure formation by cyanic acid trimerization. The derived carbons from higher isocyanate index (NCO/OH) were further found from electrochemical tests to possess improved capacitance but with negative resistivity which is attributed to more capacitive but amorphous N-doping carbon structure.

Date of Award	8 Nov 2017
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	Tao Wu (Supervisor), Peng Chuang (Supervisor) & Wei Sun (Supervisor)