Study On Features And Mechanism Of Production Exothermic And Thermal Decomposition Process About Flexible Polyurethane Foam

Flexible polyurethane flexible foam（FPUF）is a polyurethane material with an open-cell structure.Due to its low thermal conductivity,it takes a long time for heat released from production reaction to be completely transferred from the inside of the foam each time.Once the proportion of FPUF formula components changes,the reaction heat release increases and the continuous high temperature inside the foam,which may lead to the decomposition and spontaneous combustion of the FPUF.Therefore,it is necessary to understand the production exothermic feature and thermal decomposition feature of FPUF to prevent fire accidents in FPUF production.In this thesis,it takes a common production formula of flexible polyether polyurethane foam as the research object.Firstly,a manual foaming method that can obtain small-volume FPUF was proposed.Then,the experiments were conducted from the perspective of raw material component dosage（polyisocyanate,polyol,blowing agent and catalyst）to study the production exothermic features and thermal decomposition process features of FPUF,and the dangerous formula and optimized formula of FPUF were further discussed.Finally,the production exothermic mechanism and thermal decomposition process mechanism of FPUF were deeply analyzed from the molecular group level by the Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy（ATR-FTIR）and the absorbance ratio analysis method（derived from the Beer-Lambert law）,and the initial process of FPUF thermal decomposition was deduced.The main conclusions of this study are as follows:（1）The effect of component dosage change on the production temperature and morphology quality of FPUF was studied by the component dosage change experiments.It was found that the insufficient polyol dosage and the simultaneous increase in polyisocyanate and water dosage（with constant TDI index）were the two production situations most likely to lead to autothermal decomposition of FPUF,which not only result in a significant increase in FPUF production temperature,but also make it difficult to detect abnormalities from morphological changes.In addition,by adjusting the proportion of component dosage,the dangerous formula and optimized formula were further discussed.It was found that when the low-density FPUF is produced with only water for foaming and meets the situation of insufficient polyether polyol dosage,the FPUF production temperature can reach more than 200℃,which is very likely to cause the decomposition and spontaneous combustion of FPUF.If the production temperature of the FPUF formula is to be lowered,it can be considered to appropriately increase the ratio of polymer polyol and physical blowing agent in the formulation.（2）By conducting heat treatment experiments at 210/220/230℃for the FPUFs made by component dosage change experiments,and comparing the morphology changes of FPUFs before and after heat treatment,it was found that under the same heat treatment conditions,if there is any one of insufficient polyisocyanate,insufficient physical blowing agent,insufficient or excessive water,excessive polyol,and excessive catalyst in the formula,the FPUF generated by it will be more prone to coking than the FPUF made with the qualified formula.In addition,compared with a uniform network skeleton,the parts inside the foam that become dense due to shrinkage are more prone to coking,and the thermal stability of high-density FPUF is poorer than that of low-density FPUF,and the use of polymer polyol instead of partial polyether polyol will also make the thermal stability of FPUF weaker.（3）The ATR-FTIR test results of the FPUF were analyzed by the absorbance ratio analysis method to reveal the influence of the change of the component dosage on the molecular group of the FPUF.Then,combined with the above findings and the influence of the component dosage change on the FPUF production temperature,it was found that the main factors for the variation of FPUF production temperature are the number of groups,the weight of foam,the vaporization endotherm of blowing agent and the activity of catalyst.The more the number of groups synthesized such as urethane group and urea bond,the greater the heat release of the reaction and the higher the production temperature.The increase of foam weight,the enhancement of heat absorption effect by blowing agent（water and methylene chloride）vaporization,and the increase of catalyst（stannous octanoate and triethylenediamine）activity will all lead to the decrease of FPUF production temperature.（4）The ATR-FTIR test and the absorbance ratio analysis method were used to reveal the molecular group changes of the FPUF made by different component dosages before and after the heat treatment.Then,combined with above findings and the morphology changes of FPUF before and after the heat treatment,it was found that the main factors causing the differences in the thermal decomposition process of FPUFs were the number of hydrogen bonds,the number of urethane groups and the number of（unreacted）alcoholic hydroxyl groups.The more hydrogen bonds formed by amino groups and carbonyl groups between the molecules of FPUF,the FPUF needs to absorb more heat to achieve rapid pyrolysis.The more urethane groups in the molecular structure of the FPUF,the higher the decomposition degree of the FPUF under the same environmental conditions（above 200°C）.The more alcohol hydroxyl groups contained in the FPUF,the easier it is for the polyol in FPUF to undergo the thermal oxidation reaction which can accelerate the thermal decomposition process of FPUF because it can occur continuously through autocatalysis and can release a large amount of heat.In addition,combined with the pyrolysis conditions of groups in FPUF moleculars,it was found that there were a variety of groups decomposed during the process of FPUF from room temperature to 230°C,which are sorted from easy to difficult:allophanate2,carbonyl compounds and so on.In summary,this thesis discussed the production situations that may lead to high temperature and autothermal decomposition of FPUF from the abnormal proportion of raw materials,and analyzed the root causes of the increase in FPUF production temperature and the reduction of FPUF thermal stability from the perspective of molecular groups.These studies provide practical guidance and theoretical basis for subsequent production workers to design safer FPUF formulations and solve the defects of FPUF formulations.