Investigation on Thermal Degradation and Combustion Behaviors of Typical Outwall Insulation RPU

Abstract

As energy crisis deepens, application of exterior wall insulation materials highlights to reduce energy consuming in the building sector. Rigid polyurethane foam (RPU) has been used as insulation slabs for its excellent insulation performance related to super low thermal conductivity. However, several disastrous fires involved ignition and combustion of RPU have attracted attention on its thermal degradation and combustion behaviors, which are fundamental and necessary to flame spread and fire related studies. Thus to gain a comprehensive knowledge of fire occurrence, development, and acceleration, to build related chemical kinetic models, also to tailor design flame retardant RPU, we are motivated to deeply investigate thermal degradation mechanism, thermal degradation kinetics, ignition property, combustion characteristics as well as relationship between thermal degradation and combustion via experimental, theoretical, and numerical simulation methodologies.

Firstly, thermal degradation kinetics of RPU is analyzed with compensation use of model free and model-fitting methods at four oxidative atmospheres. Effective activation energies (Eα) depending on conversion (α) are determined simultaneously by traditional graphic methods and advanced isoconversional method at four linear heating rates. Results show that the increasing of oxygen concentration has a slight influence on competing of pyrolysis and oxidation at low conversion, while during high conversion stage activation energies at each conversion show non-monotonic change. When α> 0.6, degradation process with constant Eα is discriminated as kinetically dominated by various n-order models at different heating rates in air; while in low oxygen-containing atmospheres, n-dimensional diffusion models compete with n-order models. Meanwhile, Friedman and model reconstruction methods are verified unsuitable for analyzing complex RPU thermal degradation due to large imprecision. Evolutions of thermodynamic functions based on Eα further show the critical oxygen concentration for primary mechanism transformation existing between 10% and 15%. In addition, kinetic prediction forecasts good thermal stability of RPU applied in isothermal conditions and reactive flame retardants show significantly influence on kinetic parameters.

Subsequently, fundamental thermal degradation behaviors of RPU in non-oxidative and oxidative atmospheres are studied, including effect of oxygen on thermal stability, changes in solid phase, identification of volatile products, and hypothesis of degradation mechanisms. With a model building insulation RPU based on polymeric 4,4-diphenylmethane diisocyanate and polyether polyol, Thermogravimetric analysis (TGA) and in-situ Fourier transform infrared spectroscopy (FTIR) results for condensed phase show that presence of oxygen starts to accelerate the entire process at 120 oC while the communal partial reactions occur at least 50 oC delay in nitrogen. In gas phase, more than 20 characteristic products are identified by thermogravimetry-Fourier transform infrared spectroscopy coupled with gas chromatography and mass spectrometry (TG-FITR-GC/MS). With fruitful information of evolved gaseous products, mechanism of RPU thermal degradation can be adequately deduced.

To further seek accurate kinetic inputs for thermal degradation as well as following combustion modeling of RPU, TG experiments and modeling simulations are conducted with different heating rates under variant oxygen concentration surroundings. With increase of heating rate, TG curves show slight high-temperature reversion phenomena and shoulder peak of main mass loss stage splits due to secondary reactions between regenerated isocyanates and polyols at high temperature. Increasing oxygen concentration greatly enhances thermal degradation by respectively catalyzing the pyrolysis and oxidation at onset and high-temperature stages.Generally, a comprehensive five-step model is proposed and implemented with an optimization genetic algorithm (GA) to estimate kinetic parameters of RPU pyrolysis and thermal oxidation simultaneously. Accuracy and validation are identified by the ability to reproduce and predict the experimental TG results, and also ignition time can be calculated precisely. In addition, the calculated kinetic parameters show that variation of oxygen concentration mainly impacts on the oxidation of aliphatic ether alcolhols and aromatic isocyanates among five sub-reactions.

Finally, combustion characteristics of RPU at various external heat fluxes under piloted ignition, and radiative ignition modes are investigated using Cone calorimeter with different heat fluxes gradient (12, 15, 20, 25, 30, 35, and 50 kW/m2). The ignition mode puts a significant impact on critical heat flux, yet minimum effects on ignition time and heat release. At piloted ignition mode, ignition time decreases proportionally with increase of external heat flux, but discrepancy of evolution trends are found in low, medium and high regions. Also increasing the external heat flux raises the peak heat release rate and impresses on the vertical temperature distribution greatly. Based on the experienced ignition formula, a methodology to predict ignition times of RPU under piloted ignition mode has been developed theoretically. By analyzing in-depth radiation absorption along sample thickness and thermal conductivity evolution accompanied heating, both radiations from cone calorimeter and heat loss from convection flow are considered and calculated theoretically. The experimental ignition time and temperature distribution agree well with the theoretical ones in studied conditions; however, the observed critical ignition heat flux is higher than the calculated one under piloted-ignition mode because heat loss process with outgoing gases, especially in lower heat flux radiation, is not considered in this developed methodology.

Relationship between thermal degradation and combustion of RPU is explored with calculation of kinetic parameters and ignition time with GA as well as investigation of thermal degradation and char forming mechanism. The results of this study have implications concerning kinetic triplet determination method and escaped gas analysis during polymer waste pyrolysis process.

Date of Award	31 Aug 2018
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Siu Ming LO (Supervisor) & Jinhua Sun (External Supervisor)