Preparation And Performance Of Transition Metal Based-layered Double Hydroxide Flame-retardant Silicone Foam

Silicone foam（SiF）is a kind of high-performance elastomer material with porous structure,which is prepared by physical foaming or chemical foaming of silicone rubber in the crosslinking process.Due to its excellent comprehensive performance,it is widely used in many fields.However,its side chains contain a large number of flammable organic groups and its porous structure,making it still necessary for flame retardant modification.At present,SiF mainly improves its flame retardancy by adding flame retardant fillers.In recent years,layered double hydroxide（LDH）,with low toxicity,low cost,and highly adjustable structure and function,has been widely used as a green halogen-free flame retardant and smoke suppressant in the flame retardant modification of polymer materials.In this work,LDH was designed by changing the types of metal cations on the main layer and interlayer anions,and discussed the effect of LDH on the performance of SiF.This thesis mainly works as follows:（1）The following six LDHs were prepared by chemical coprecipitation method:CoAl-LDH-CO₃^2-,CoAl-LDH-SO₄^2-,NiAl-LDH-CO₃^2-,NiAl-LDH-SO₄^2-,NiCo-LDH-SO₄^2-and NiTi-LDH-SO₄^2-.The crystal structure,chemical composition,microscopic morphology,specific surface properties and thermal stability of LDHs were characterized by X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,inductively coupled plasma optical emission spectrometry（ICP-OES）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）,Brunauer-Emmett-Teller test（BET）and thermogravimetric analysis（TGA）.（2）CoAl-LDH-CO₃^2-,CoAl-LDH-SO₄^2-,NiAl-LDH-CO₃^2-and NiAl-LDH-SO₄^2-were added to SiF as flame retardant additives to obtain SiF/M（Co,Ni）Al-LDHs nanocomposites.The effects of metal elements Co,Niand interlayer anions CO₃^2-and SO₄^2-on the properties of SiF were discussed respectively.The micromorphology,flame retardancy,smoke suppression performance,thermal stability and thermal insulation performance of the sample were analyzed by ultra-depth microscope,vertical flame testing（VFT）,limiting oxygen index（LOI）,cone calorimetric test（CCT）,smoke density test,TGA and thermal conductivity test,the conclusions can be drawn as follows:（1）The presence of Co can improve the porosity and thermal insulation performance of SiF.The presence of Nican improve the flame retardancy and smoke suppression performance of SiF;（2）LDHs-SO₄^2-can improve the porosity and thermal insulation performance of SiF,and have a good smoke suppression effect,meanwhile,it can also delay the decomposition of SiF at high temperatures.In combination with the above performance tests,SiF/CoAl-LDH-SO₄^2-and SiF/NiAl-LDH-SO₄^2-nanocomposites with better comprehensive properties were selected,and their pyrolysis gas products and char residues were studied by Thermogravimetry-fourier transform infrared spectrometry（TG-IR）and Scanning electron microscopy coupled with an energy dispersive X-ray spectroscopy（SEM-EDS）,to further analyze the flame retardant mechanism of these two LDHs on silicone foam.Finally,the compression test results show that the addition of LDHs would reduce the compressive strength of SiF.（3）Based on Nielement,metal elements（Co,Ti）with catalytic synergism and thermal stability were introduced into the main layer by chemical coprecipitation method,and sulfate ions were used as interlayer anions to prepare NiCo-LDH-SO₄^2-and NiTi-LDH-SO₄^2-,which were added to SiF,and the effect of Ni/Co and Ni/Ti on the performance of SiF was compared.The microscopic morphology,flame retardancy,smoke suppression performance,thermal stability and thermal insulation performance of the sample were analyzed by ultra-depth microscope,VFT,LOI,CCT,smoke density test,TGA and thermal conductivity test.The results are as follows:（1）The synergistic effect of Ni/Ti can significantly improve the porosity and thermal insulation performance of SiF;（2）The synergistic effect of Ni/Co can significantly extend the TPHRR of SiF,reduce the THR,and have a good smoke suppression effect;（3）LDHs containing two transition metals（Ni/Co and Ni/Ti）,compared with LDHs containing a single transition metal（Co/Al and Ni/Al）,have a high flame retardant efficiency.The addition of only 1 phr can increase the LOI of SiF from 28.7%to 30.5%(SiF/NiCo-LDH-SO₄^2-)and 30.3%(SiF/NiTi-LDH-SO₄^2-),respectively,and reach the V-0 level.In addition,TG-IR and SEM-EDS were used to study their pyrolysis gas phase products and char residues,respectively.Finally,the compression test results show that NiCo-LDH-SO₄^2-and NiTi-LDH-SO₄^2-have less influence on the mechanical properties of SiF than CoAl-LDH-SO₄^2-and NiAl-LDH-SO₄^2-.（4）Take NiCo-LDH-SO₄^2-as example to further explore the flame retardant and smoke suppression mechanism of LDH.The specific mechanism is as follows:during combustion,endothermic reaction occurs during thermal decomposition of NiCo-LDH-SO₄^2-,which can reduce the temperature of combustion system.At the same time,non-combustible gases such as H₂O are released to dilute combustible gases.The layered double oxides（LDO）formed after pyrolysis can adsorb small molecular gas or smoke particles,which has a good smoke suppression effect.The presence of transition metals in NiCo-LDH-SO₄^2-can promote the formation of cyclic oligomers,further oxidize them to CO₂ and SiO₂,and thus play a role in gas phase flame retardancy.NiCo-LDH-SO₄^2-is conducive to promoting the formation of stable and dense char layer for playing a good physical barrier role.