Study On The Thermal Properties Of Silica Biomimetic Mineralized Poplar Composites

In nature,bio-silica mineralized wood has excellent mechanical strength,dimensional stability,flame-retardant and heat-resistant properties.This paper focused on natural bio-silica mineralization methods and non-toxic and harmless ethyl orthosilicate(TEOS)was used as a silicon source,a biomimetic mineralized micro-environment on the surface of wood cell walls was also builded to induce silica nucleation and growth.The physical and mechanical properties and flame-retardancy of silica mineralized wood were studied.Finally the degree of difficulty of the pyrolysis reaction was evaluated by the activation energy.Conclusions as below:(1)The modified wood was prepared by vacuum impregnation with SiO2 sol,and the mass ratio of ethyl orthosilicate(TEOS)and water with a mass ratio of 5:1 was determined by Nanoparticle size analyzer and Zeta potentiometer to obtain SiO2 particles that the size of about 200 nm and a lot of negative charges were observed.Scanning electron microscopy(SEM)results showed that SiO2 was mainly deposited in the cell walls and cell cavities of wood and the particles were easily aggregated into agglomerates.Furthermore,the water absorption was improved and flexural elastic modulus and mechanical strength were significantly enhanced of the as-prepared wood.X-ray diffractometer(XRD)indicated that the SiO2 formed in an amorphous;What is more,the cone calorimeter(CONE)showed that the heat release rate(HRR)and total heat release(THR)of the modified wood were reduced by 51.3%and 29.4%,respectively,compared with the control wood.In a summary,the flame retardant and thermal of composite materials has been significantly improved.(2)The layer-by-layer self-assembly method was adopted to make the poplar wood cell wall adsorb the nano-polyelectrolyte layers(diallyl dimethyl ammonium chloride(PDADMAC)and sodium polystyrene sulfonate(PSS)layers,which reduced the nucleation energy of SiO2 onto wood cell wall to prepare SiO2 mineralized wood.According to Infrared spectroscopy(FTIR)and X-ray diffractometer(XRD)images,it could be seen that amorphous SiO2 formed Si-O-C bond with the poplar cellulose hydroxyl group,and the SiO2 mineralized layer was uniformly distributed with different shapes as SEM shown.In addition,the experimental results of thermo-gravimetry(TG)and CONE suggested that the maximum heat release rate(pHRR)and total heat release(THR)of mineralized wood were reduced,and the polyelectrolyte layers could catalyze the heating of wood components into charcoal,cooperating with the SiO2 mineralized layer to slow down the heat transfer.It can be concluded that the thermal stability and flame retardancy of of the mineralized wood were improved.(3)The kinetic parameter activation energy was used to evaluate the difficulty of the pyrolysis reaction of SiO2 mineralized wood.Through the model-free method(Starink method,FWO method and modified-CR method),the average activation energy values of the control wood and SiO2 mineralized wood were 114～117 kJ/mol and 167～168 kJ/mol,respectively.Furthermore,during the preliminary thermal degradation of mineralized wood(≤242～278℃),the silica mineralized layer could better prevent the transfer of heat,leading the pyrolysis reaction more difficultly at low conversion rates,on the contrary,it was difficult for the silica mineralized layer to continue to prevent heat transfer into the poplar wood between the pyrolysis temperature range of 302℃ and 396℃.Not only that,it was found that the activation energy of modified poplar wood with inorganic SiO2 was larger than that of inorganic TiO2 and CaCO3,which led to more energy was needed to be absorbed for the pyrolysis reaction of the silica mineralized wood.In a word,studying the thermal properties of silica mineralized wood has certain theoretical significance and practical value.