The Study On Synthesis Of Different Hydrotalcites And Their Synergistic Flame Retardant In Epoxy Resin

Hydrotalcite(LDH),as a green and new type nano-material,containing a large number of hydroxyl,acid radical and metal ions in the structure,so it was often used as a flame retardant synergist and catalyst.In this paper,on the basis of magnesium-aluminum hydrotalcite,three transition metal elements of nickel,cobalt and iron were introduced to prepare seven kinds of LDH,and two kinds of hydrotalcite-polyaniline hybrids(LDH-PANI)were synthesized at the same time.And the crystallization behavior and microscopic morphology of different LDH and LDH/PANI were studied systematically,the effects of several LDH and LDH/PANI as synergistic flame retardants and APP on the thermal stability,flame retardancy and mechanical properties of epoxy resin(EP)were investigated.The specific research contents were as follows:Firstly,the Mg-Al LDH(LDH₁),Mg-Fe LDH(LDH₂),Co-Fe LDH(LDH₃),Ni-Fe LDH(LDH₄),Mg-Ni-Fe LDH(LDH₅),Mg-Co-Fe LDH(LDH₆)and Ni-Co-Fe LDH(LDH₇)were synthesized by co-precipitation method,and two hydrotalcite-polyaniline hybrids(LDH₂/PANI and LDH₇/PANI)were prepared by in-situ oxidative polymerization.The results of infrared analysis(FT-IR),X-ray diffractometer(XRD)and scanning electron microscope-X-ray energy spectroscopy(SEM-EDS)showed that the seven LDH all have the infrared characteristic absorption peaks of traditional hydrotalcite,and the high crystallinity and irregular regular hexagonal lamella structure were discovered in seven kands of samples.At the same time,the analysis results of FT-IR and SEM-EDS showed that LDH/PANI were successfully synthesized,their morphology was rod-like or strip-like structure,the particle size was smaller than that of polyaniline(PANI),and appearing a certain degree of entanglement and agglomeration phenomenon.In addition,the thermogravimetric(TG)results confirmed that the thermal decomposition of the seven LDH can be roughly divided into three stages,and with the introduction of the three metals of nickel,cobalt and iron,the initial thermal decomposition temperature shifted to the low temperature direction,but its residual carbon content was higher than that of Mg-Al LDH at high temperature.This may be attributed to the interaction force of main laminate and the interlayer anions was reduced with the introduction of nickel,cobalt and iron,so the hydroxyl and interlayer anions can be decomposed earlier during the heating process.The TG curve of LDH/PANI showed that the initial thermal decomposition temperature of the two LDH/PANI was higher than the corresponding LDH and PANI,but the carbon residue was much lower than the corresponding LDH and slightly higher than PANI,this is mainly because there is a small amount of LDH in LDH/PANI to increase the initial decomposition temperature,and LDH will also catalyzed the rapid cross-linking of PANI into carbon.Therfore,the amount of char was higher than that of pure PANI.Secondly,when the total additive amount of flame retardant was same,different EP composites were prepared by adding APP,LDH and LDH/PANI to EP materials in a certain proportion.TG results proved that the addition of several flame retardant systems can reduce the initial thermal decomposition temperature of and increase the amount of carbon residue EP composites,while the compound addition of LDH and APP can farther promote the generation of more carbon residue.The results of oxygen index(LOI)and vertical combustion(UL-94)demonstrated that the alone addition of LDH can only prevent the dripping of EP composites.The LOI value of EP composites increased to 34.5%with the addition of APP and UL-94 reached to V-0 level.Moreover,when LDH or LDH/PANI was used in combination with APP,the LOI value of EP composites was further improved,which showed that these two flame retardants and APP have a good synergistic effect.The results of cone calorimetry(CONE)and SEM proved that the synergistic use of LDH and APP can effectively reduce the emission of smoke and heat,and promote the generation of more continuous and dense carbon residues,improving the flame retardant performance and reducing the risk of fire of EP composites.Among them,LDH₇ has the best synergistic flame retardant effect.When LDH₇ and APP were added together,the LOI value of EP composites increased to 44.6%,and the EP composite material self-extinguishes in 300s.Meanwhile,its heat release rate(THR),total smoke emission(TSP)and average carbon monoxide(Av-COY)were reduced to 23 MJ/m²,9.1 m² and 0.094 kg/kg,respectively,which decreased by 63%,91%and 41%than pure EP,and the residual carbon content was increased to 63.5%,which increased by 36.5%compare with pure EP.In addition,the Pk-HRR was reduced with addition of LDH/PANI and APP,and the burning intensity of EP materials was slowed down,but the emission amount of flue gas and heat was not decreased when the addition of LDH/PANI.It might result from a large number of benzene ring structures in PANI,which makes the EP material difficult to burn violently.But the amount of smoke and heat released was not reduced effectively because of the smolder phenomenon of EP composites.Thirdly,the tensile performance test results indicated that the tensile strength of pure EP was 65.3±0.5 MPa,and the tensile strength of the EP composite reduced to 46.5±0.2 MPa with addition of APP,the addition of LDH and APP can maintain or even improve the tensile properties of EP materials,which proved that the dispersion of LDH was more uniform in EP materials,and the addition of LDH is beneficial to promote the dispersion of APP.Furthermore,due to the high concentration of PANI in LDH/PANI,it is prone to agglomeration when it was added to EP materials with APP,which the tensile strength of EP composites was decreased.In summary,APP was an effective flame retardant for EP materials,and when LDH or LDH/PANI was used in conjunction with APP,it can decompose to produce a large amount of flame-retardant gas,and accelerate the formation of continuous and dense carbon residue.Thereby,the intensity of fire was decreased,and the flame retardant property was further improved of EP materials.Moreover,since LDH was a nanoparticle,it has better dispersibility in EP materials,which effectively improves the mechanical properties of EP materials.