Study On Structural Retulation Of Layered Double Hydroxides And Their Catalytic Oxidation And Smoke Suppression Properties For Polypropylene

The combustion process of polypropylene（PP）produces a large amount of smoke and toxic gases,which pose a serious threat to people’s lives,health,and property.Layered double hydroxides（LDHs）,as two-dimensional layered materials,have attracted much attention and are characterized by a variety of controllability.By adjusting their composition and structure,they are widely used in many fields as catalytic materials,rubber and plastic additives,photoelectric materials,etc.Therefore,conducting research on the catalytic pyrolysis and smoke suppression functions of LDHs on polypropylene is of great significance.This paper uses LDHs as the design unit,utilizing the controllability of the composition and defect structure of the main layer of LDHs.By introducing transition metals such as Fe,Co,Ni with catalytic oxidation performance into the layer,the structure of LDHs with different layer elements and defect contents was designed and synthesized.By adding it to PP,the influence of it on the catalytic oxidation process and product composition of PP was investigated,and the action rules were obtained.The mechanism of transition metal elements as nucleophilic reagents for catalytic oxidation and CO₂generation was discovered.Further research on the smoke suppression performance of PP was conducted,and the results showed that under the catalysis of LDHs,the oxidation of PP decomposition products to form small molecule CO₂ gas was promoted,reducing the production of large molecule substances,the main component of smoke released by PP during heating,and improving its smoke suppression performance.The specific research results are as follows:1.By regulating the elemental composition of LDHs laminates,four catalytic oxidation materials with different metal laminates,Ni₂Al-LDH,Co₂Al-LDH,Ni₂Fe-LDH,and Co₂Fe-LDH,were prepared.LDHs/PP composite materials were prepared by adding them to PP at a dosage of 5 phr.In situ detection and analysis of the thermal decomposition process of PP were carried out using TG-IR,TG-MS,and other methods.The results showed that Ni₂Al-LDH,Co₂Al-LDH,Ni₂Fe-LDH,and Co₂Fe-LDH all promoted the thermal decomposition of the PP main chain.Transition elements such as Co,Fe,and Ni provided unpaired electrons to act as nucleophilic reagents to attack the tertiary carbon in the PP carbon chain,thereby achieving carbon end oxidation of PP,Promote the catalytic oxidation of decomposition products to form CO₂,and the effect is Co₂Fe-LDH>Ni₂Fe-LDH>Co₂Al-LDH>Ni₂Al-LDH.Among them,the addition of Co₂Fe-LDH advanced the maximum decomposition rate temperature of PP by 15℃,and the maximum intensity of carbon dioxide infrared absorption peak increased by 447.6%.Composite smoke suppression studies have shown that Co₂Fe-LDH exhibits excellent smoke suppression performance,with a maximum smoke density reduced by 28.45%compared to pure PP samples.A new smoke suppression mechanism based on the catalytic oxidation of PP to promote the formation of CO₂ small molecule substances from decomposition products has been proposed.2.By adjusting the element ratio of LDHs laminates,different Co/Fe ratios of Co_mFe-CO₃-LDHs（m=2,3,4）were prepared and used as catalytic oxidation and smoke suppression materials.The prepared LDHs and PP materials were mixed with a 5 phr addition.The analysis results of TG-IR,TG-MS,NBS smoke density,etc.showed that the oxidation effect of hydrocarbon substances generated by the thermal decomposition of the PP main chain increased with the increase of Fe content in the laminates,Therefore,it is proposed that Fe³⁺,due to its presence of more unpaired electrons than Co²⁺,can provide more lone pair electrons and act as nucleophilic reagents,which is more advantageous for the catalytic oxidation and smoke suppression of PP in an air atmosphere.In addition,the catalytic oxidation and smoke suppression effects of LDHs on PP increase with the increase of Co₂Fe-LDH addition,but the change is small after the addition reaches 5 phr.3.Explored the effect of surface defects in LDHs on the catalytic oxidation and smoke suppression performance of PP.Different amounts of metal element Al were introduced into Co₂Fe-LDH laminates to synthesize Co₂²⁺Fe_n³⁺Al_m³⁺-CO₃-LDHs（n=0.9,0.75,0.5,m=0.1,0.25,0.5）.The Al in the laminates was etched/dissolved with Na OH to change the surface electronic state of the material,regulate the surface defect structure,and prepare N-LDHs with different metal defect structures.The prepared N-LDHs and PP materials were composite with a 5 phr addition.TG-IR,TG-MS,NBS smoke density tests were conducted,and the results showed that the catalytic oxidation and smoke suppression effect of hydrocarbon substances generated by the thermal decomposition of the PP main chain increased with the increase of defects in N-LDHs.The maximum smoke density of N-Co₂²⁺Fe_0.5³⁺Al_0.5³⁺-CO₃-LDH/PP composite material was 179.58,which decreased by 35.61%compared to the blank PP sample（278.92）.Therefore,it is proposed that when the content of LDHs surface defects increases,its charge state changes,and the electron binding energy of Fe decreases under the interaction of Co and Fe,which strengthens its role as a nucleophilic agent to attack the tertiary carbon on the PP carbon chain,thus improving the catalytic oxidation and smoke suppression effects.