Fabrication And Thermal Protection Performance Of Nano-micro Porous PTFE/SiO₂ Composite Aerogel Insulation Materials

Thermal protective clothing is an important protective equipment to protect firefighters from various thermal injuries,and its performance is directly related to the life safety and health of firefighters.At present,the general fire-fighting clothing on the market generally has problems such as heavy,stuffy,and limited flexibility of activities,which seriously affect the work efficiency of firefighters and endanger the safety of life.Therefore,it is extremely urgent to prepare a new type of thermal protective fabric that is lightweight and has excellent protective properties.However,research demonstrated that there is a contradiction between the thermal protection and lightweight comfort indicators of thermal protective fabrics,and it is crucial to improve both the comfort and the thermal protection requirements.Conventional fabrics and their structural design can no longer meet this requirement,and the research of new lightweight and low thermal conductivity fabrics is an important way to solve this problem,of which SiO₂aerogel is a typical representative.However,SiO₂ aerogel has defects such as low strength and poor continuity of internal gel framework,which leads to poor macro mechanical properties and brittleness of materials,and even structural collapse under external stress,limiting its large-scale application in thermal protective fabric.The purpose of this text is to replace the original insulation lining material with a new type of lightweight and excellent protective properties material.Firstly,focusing on the development of elastic SiO₂ composite aerogels,PTFE/SiO₂ composite aerogels that can be applied to the thermal liner were prepared with PTFE and SiO₂ aerogels as the basic building units;Secondly,combined with the structural design,PTFE/SiO₂ composite aerogel with excellent mechanical properties and thermal insulation properties was applied to the thermal liner of traditional thermal protective fabric,and the mechanism of the influence of moisture content and fabric system structure on the thermal protective performance of the new type of aerogel protective fabric was explored.Finally,based on the method of numerical simulation,the heat transfer model of multi-layer fabric containing aerogel material was established,and the heat transfer mechanism inside the material was systematically studied.The main achievements of the text are as follows:（1）Preparation and properties of bulk SiO₂ aerogel insulation materials.The SiO₂ aerogel prepared by the traditional method,which exists in the form of powder or particle and is difficult to form a complete shape,not only has poor mechanical properties but also needs to go through a tedious process of surface modification and solvent exchange,which consumes resources and harms the environment.To solve this problem,using water-glass and methyl trimethoxysilane（MTMS）as co-precursors,deionized water as the only solvent,we prepared the bulk SiO₂aerogel through the two-step sol-gel process and freeze-drying method.The controllable preparation of aerogel microstructure was realized by adjusting the molar ratio of co-precursors,and the relationship between the microstructure of aerogel and its mechanical properties and thermal insulation was studied.These results show that the prepared SiO₂ aerogel was obtained with good performance,such as high porosity（97.50%）,low bulk density（0.0549 g/cm³）,and low thermal conductivity（0.0154 W/m·k）.Besides,the aerogel can withstand 0.007 MPa compressive stress under 60%large compressive strain and completely recover to the original shape without any structural damage.However,with the further increase of the compressive strain,the aerogel appears to have obvious structural collapse and powder loss,which limits its application in thermal protective fabrics.（2）Preparation and properties of PTFE/SiO₂ composite aerogel thermal insulation materials by sol-gel method.Although the pure bulk SiO₂ aerogel material has good thermal insulation performance and certain compression recovery,it is still fragile and powdery,and cannot be directly used in thermal protection fabrics.The PHSiO₂ composite aerogel with a stable dual network structure was prepared by combining the basic building unit PTFE with SiO₂ sol,followed by the sol-gel process,freeze-drying technology,and a high-temperature curing method.A stable physical connection between the PTFE network and the SiO₂ aerogel network was established depending on the bonding effect of high-temperature PTFE,which ensures the good structural stability of aerogel materials.By adjusting the microstructure and optimizing the design of composite aerogels,a double network structure aerogel material with high flexibility and thermal insulation properties was obtained.The results show that the composite material can withstand compressive stress of 0.0492 MPa without structural collapse at a compressive strain of 60%,which is 7.03 times that of pure SiO₂ aerogel.Meanwhile,the density of aerogel is 0.0807 g/cm³,the porosity is 96.32%,and the thermal conductivity is0.0360 W/m·K.When applied to the thermal liner of thermal protective fabrics,its equilibrium temperature is 3.09%lower than that of traditional fabric combinations.However,the plastic deformation of this material after 10 fatigue tests is as high as 13.02%,and its long-term stable performance in thermal protective fabrics needs to be further improved.（3）Structural optimization and application of PTFE/SiO₂ composite aerogel thermal insulation materials.To improve the durability of PTFE/SiO₂ composite aerogel and improve the lightweight comfort of thermal protective fabrics,the multi-level cellular PTFE/SiO₂composite aerogel（PMSiO₂）with good mechanical properties was obtained by using PTFE and MTMS as precursor materials and combining the methods of freeze-drying technology and high-temperature calcination.Stable cross-linking points were formed through the two-way bonding of high-temperature PTFE and polysiloxane（PMSQ）.The results show that the composite aerogel can withstand the stress of 0.0487 MPa under 60%compression strain while maintaining its thermal insulation performance.After 100 compression cycles,the maximum compression stress remains above 82%of the first cycle;Replace the original insulation layer with the composite aerogel,and a new double-layer structure of aerogel thermal protective fabric was obtained.The obtained double-layer structure aerogel thermal protection fabric has a balanced temperature of 1.07%lower than that of the multi-layer fabric composed of PHSiO₂after undergoing high-temperature insulation performance testing,which indicates that it has better high-temperature heat insulation performance.Meanwhile,after undergoing thermal protection performance testing,it can increase the TPP value and secondary burn time by 20.37%and 20.33%,reduce the middle layer quality by 69.89%,and improve the lightweight comfort of thermal protection fabric,when compared with traditional fabrics.Besides,adjusting the wetting state of the outer layer,as the water content increases to 50%,the TPP value and secondary burn time increase by 84.71%and 89.14%,respectively,but when it increases to100%,the TPP value and secondary burn time decrease by 12.08%and 11.97%,respectively,compared to 50%.（4）Numerical simulation and heat transfer mechanism of aerogel thermal protective fabric.Based on the microstructure characteristics of PTFE/SiO₂ composite aerogel and the face-centered cube（FCC）foam model,the random geometric modeling of the material was established by introducing the random pore diameter d^*and the pore diameter variation coefficient Cv value,and the thermal parameters of the aerogel dual network composite skeleton structure were calculated by using the numerical simulation method.Exploring the effect of the distribution of pore structure on their thermal conductivity for composite materials through numerical simulation methods.Moreover,a multi-layer fabric model containing an aerogel layer was established,and the mechanism of the influence of the thickness of the air layer,the thermal conductivity of the aerogel material,and the external environment on the heat transfer of the fabric system was explored through numerical simulation.It was found that the relative error of the aerogel homogeneous geometric model was kept within 3.12%,and the random geometric model could reduce the error to 1.60%.Both models could effectively simulate the heat transfer process inside the material;For a multi-layer fabric model,when the air layer thickness is less than 6 mm,the maximum relative error of temperature during the thermal exposure stage is 4.48%,and the second degree burn time is 5.53%.This model can effectively simulate the heat transfer in the fabric system with an air layer thickness in a range of 3 mm to 6 mm.The thickness of the air layer has an important impact on the heat transfer process in the new double-layer structure S-3,which can slow down the rate of heat transfer during the heat exposure stage,absorb the heat storage from the release of the fabric system during the cooling process,and improve the fabric system’thermal protection performance.With the thickness of the air increased from 3 mm to 6 mm,the secondary burn time of S-3increased by 21.38%;however,as the thickness increases to 9 mm,the occurrence of natural convection increases the heat transfer efficiency and slows the growth trend of thermal protection performance,which can lay a theoretical foundation for the structural design and thermal protection performance prediction of the subsequent development of new thermal protection fabrics.