Study On The Properties Of Humic Acid And Its Compounds As Flame Retardants For Epoxy Resins

In recent years,biomass materials have many advantages such as non-toxicity,harmlessness,environmental protection and good biocompatibility,which are widely used in the field of flame retardant.Humic acid?HA?,as a biomass material with abundant raw materials and wide sources,has many oxygen-containing functional groups such as hydroxyl and carboxyl groups on its surface,which can be used as a new type of charring agent.In this paper,HA was introduced into the epoxy resin?EP?as a flame retardant for the first time.The effect of HA on the flame retardancy and mechanical properties of EP was systematically studied,and its flame retardant mechanism was revealed.In addition,for improving the thermal stability of HA,reducing humic acid?RHA?was prepared by hydrothermal reduction method to study the influence of RHA on the thermal stability of EP matrix.Secondly,four metal ions,Fe²⁺,Mn²⁺,Al³⁺and Cu²⁺,were used to chelate with HA to form humate.Metal ions were used to improve the dispersion,stability and flame retardancy of HA in EP.The effects of different humate on the performance of EP and their mechanism were studied in detail.Finally,according to the results of humate research and previous research work of our group,zinc ferrite/humic acid nanocomposites were prepared by hydrothermal method.The aim is to improve the dispersibility and flame retardancy of HA in EP using nano-zinc ferrite.Specific research contents are as follows:1.The influence of humic acid and reducing humic acid on the properties of epoxy resin First of all,5 wt.%,10 wt.%,20 wt.%and 30 wt.%HA were introduced into EP to form HA/EP composites,respectively.Then,thermogravimetric analysis?TGA?,dynamic mechanical analysis?DMA?,limiting oxygen index?LOI?and cone calorimetry were used to investigate the effects of HA on the thermal stability,mechanical properties and flame retardancy of EP composites.The results showed that,with the increase of HA content,the LOI of EP increased.When the content of HA was 30 wt.%,the LOI value of EP composite increased from 22%to24.7%,the peak heat release rate decreased by 37.7%,the peak smoke release rate decreased by39.9%,the amount of residual charring increased,and the time to reach the peak value was obviously delayed,showing good flame retardant effect.However,the glass transition temperature?T_g?of EP decreases significantly.This is because that the early decomposition of oxygen-containing functional groups on the surface of HA can generate water vapor to dilute the oxygen concentration.Moreover,the large size structure of HA and the formation of char residues can hinder the heat and mass transfer of gas-solid two phases,thus reducing the initial heat release rate and smoke release rate,and delaying the time of peak heat release.At last,to improve the initial thermal decomposition temperature of HA/EP,reducing humic acid?RHA?was prepared by hydrazine reduction.The results showed that RHA/EP had better thermal stability such as high initial thermal decomposition and T_g than HA/EP.2.Preparation and characterization of humate flame retardants and its effect on the properties of epoxy resinsIn order to improve the flame retardancy performance of HA,HA was complexed with four ions of Fe²⁺,Mn²⁺,Al³⁺,Cu²⁺to form four kinds of humate.The effects of four humates on the flame retardancy,thermal stability and mechanical properties of EP were studied.It was found that humate improved the initial thermal decomposition temperature,mechanical properties and flame retardancy of HA/EP composites.Among them,copper humate showed good mechanical properties and increased the char residue of HA/EP to 22.4%;Iron humate increased the initial thermal decomposition temperature of HA/EP about 23?;iron humate and manganese humate showed excellent flame retardancy properties.With the addition of 10 wt.%,iron humate and manganese humate increased the LOI of EP from 21.2%to 26.6%and 25.3%,respectively,and reduced the pHRR of EP by 36%and 35.5%,respectively.The morphological analysis of char residue showed that a more compact and continuous charring layer can be formed based on the charring ability of humic acid and the catalytic carbonization of iron and manganese,which plays a good physical barrier role,protecting the bottom polymer from decomposition,insulating the heat and oxygen permeation,and showing a better flame retardant effect.3.Preparation and characterization of zinc ferrate/humic acid nanocomposite flame retardant and its effect on the properties of epoxy resinZinc ferrite/humic acid?ZFO/HA?nanocomposite flame retardant was prepared by solvothermal method.Scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,X-ray powder diffraction?XRD?etc.revealed the zinc ferrite with about 60 nm size grew on the surface of HA and formed the nanocomposite.TGA results showed that ZFO/HA accelerated the early decomposition of EP and promoted the formation of carbon residue.The results of cross-section scanning and tensile test showed that the dispersion and the mechanical properties of HA in matrix can be improved after zinc ferrite modification.The results of LOI and cone calorimetry showed that the LOI value of EP increased from 22%to 26%with the addition of 5 wt.%ZFO/HA.The peak heat release rate and smoke release rate of EP decreased by 20%and 15.3%respectively,showing good flame retardancy.The morphological analysis of residual carbon showed that a compact carbon layer with high degree of graphitization was formed.The flame retardant mechanism is as follows:the catalytic carbonization,pin rolling of ZFO and charring ability of HA can form a dense carbon layer and play a physical barrier role;at the same time,the decomposition of HA produces non-combustible gases such as water vapor,diluting the oxygen concentration around the fire,thereby improving the flame retardant performance of EP composites.