Preparation Of Titanate Nanotube-based Composite Flame Retardants And Their Flame Retardant Properties

Titanate nanotube（TNTs）is a new one-dimensional material with flame retardancy properties.Firstly,TNTs can be well dispersed in polymer matrix to form a net-work structure,thereby enhancing the stability of polymeric materials and changing their degradation pathways.Secondly,TNTs have high specific surface,which can absorb the flammable volatile organic compounds,and suppress the smoke during combustion of polymer;they also can act as radical trappers to interrupt combustion reaction,which is better to inhibit the thermal-degration of polymer.Thirdly,TNTs,with a mount of-OH on the surface,can dehydrate and release water vapor to the combustion zone during the thermal-degration,thus cooling down the polymer matrix and diluting the concentration of oxygen.At last,TNTs exhibit high char catalytic activity,which can catalyze the formation of chars and enhance the thermal stability of chars,thereby acting as effective physical barriers to resist the diffusion of oxygen,the release of flammable gases,and the heat transmission.However,TNTs are easy agglomerated in the polymer matrix ascribe to the poor comparability,Van der Waals’force and hydrogen-bond interaction;simultaneously,TNTs are also easy coated by the catalyzed chars that would lead to reduce the catalytic effective during the char formation process;furthermore,the catalyzed chars are so weak that the flammable organic volatiles are easily breakthrough the physical barrier.To prepare nanocomposites flame retardants is one of the effective strategies to solve these drawbacks.In this work,a series of TNTs-based nanocomposite flame retardants,zinc hydroxystannate/titanate nanotube（ZHS-TNT）,titanate nanotubes/graphene oxide（TNTs/GO）and antimony doped titanate nanotubes/graphene oxide（Sb-TNTs/GO）,are prepared via nano engineering technology in view of the strategy of composites material and the design concept of flame retardants to optimize the structure and flame retardancy performance of TNTs.In order to evaluate the flame retardancy performance,to explore application value of TNTs in the field of flame retardants,and further reveal the flame retardancy mechanism of the nanocomposites,the TNTs-based nanocomposite flame retardants are incorporated to flexible polyvinyl chloride（PVC）.The main research of this work are displayed as following.1.Preparation of ZHS-TNT nanocomposite-flame retardant and its flame retardancy propertiesZHS-TNT nanocomposite-flame retardant with sugar-coated haw-like was prepared successfully via a solution synthesis method.The properties of ZHS-TNT were detected by evaluating the mechanical property,thermal stability and flame retardancy performance of the ZHS-TNT/PVC composites.The dispersion and thermal stability of the composites were enhanced by incorporated ZHS-TNT nanocomposite flame retardant in compared with ZHS/PVC or TNTs/PVC composites.The limited oxygen index（LOI）was improved to 29.6,which is higher than the values that was calculated by rule of mixture,indicating that ZHS and TNTs have a significant synergistic effect when they are combined together.The results of cone test show that the peak heat release rate（pHRR）,total heat release（THR）,total smoke release（TSR）and peak specific extinction area（pSEA）were decreased obviously comparing with the pure PVC.Especially,the TSR and pSEA were reduced remarkably by 40%and 60%with only 2.5 wt.%content of ZHS-TNT,respectively.The results indicate that ZHS-TNT nanocomposite flame retardant has a good flame retardancy and smoke suppression effective.The reason could be ascribed to the synergistic effect of the ZHS and TNTs,namely,a gaseous phase and condensed phase synergistic effect.First,the one-dimensional ZHS-TNT can promote the crosslinking of PVC chain during polymerization process and accelerate its dehydrochlorination process at the initial stage of degradation,thereby increasing the thermal stability of PVC-matrix composites;the non-flammable gas produced by dehydrochlorination reaction play a gas barrier in the gaseous phase,which is helpful to the flame retardancy performance.In the meantime,the metal compound residues formed during the thermal decomposition of ZHS-TNT nanocomposite flame retardant can dwell in the chars to enhance the char stability,and the chars formed at different stages of combustion can exhibit a so-called’labyrinth effect’to prolong the diffusion pathway of the pyrolysis flammable volatiles.As a result,the flame-retardancy of PVC is highly increased,and its smoke is effectively suppressed.2.Preparation of TNTs/GO nanocomposite flame retardant and its flame retardancy propertiesIn this part,the TNTs/GO nanocomposite flame retardant was prepared by a simple solution reaction method.The characterization of the structure and morphology is shown that TNTs were dispersed on the GO nano layer.TNTs inhibited the folding and aggregation of GO,while GO also can promoted the dispersion of TNTs in the polymer matrix.Furthermore the tensile strength of composites was maintained owing to the well dispersion of TNTs/GO in PVC matrix.Thermogravimetric analysis（TG）showed that the stability of PVC composites was enhanced by 30 ^oC at high-temperature area after adding TNTs/GO.When incorporating 2.5 wt.%to PVC matrix,the LOI increased from 25.8 of pure PVC to 27.4,pHRR,THR,TSR and the average mass loss rate（AMLR）are decreased by 21%,30%,16%and 30%respectively.And more,compared with TNTs alone,the second heat release of composites was controlled successfully.Further study of the char revealed that the chars of composites with TNTs/GO are more compact and continuous.The reason could be ascribed to the synergistic effect of the TNTs and GO in condensed phase.This is because TNTs can suppress the re-stack of GO,and the GO skeleton can act as a template for the carbonaceous char to promote the formation of multiple char;titanium dioxide transformed from TNTs during combustion can be anchored in the char layers enhancing the thermal stability of the char layers.Therefore,a compact,continuous and stable physical barrier covered on the polymer matrix that can inhabit the heat and mass transmission effectively.Finally,the flame retardancy properties of composites was enhanced.3.Preparation of Sb-TNTs/GO nanocomposite flame retardant and its flame retardancy propertiesSb-TNTs/GO nanocomposite flame retardant was prepared by introducing antimony to TNT then doped GO.The results showed that LOI were further increased to 28.1 comparing with that of TNTs/GO nanocomposite flame retardant;the pHRR and THR were decreased by 23%and 53%,the AMLR was further reduced to 13.6 g s^-1,and the residual were increased to 8.2%when the adding content is 2.5 wt.%;the time to the pHRR also was delayed to 110 s,which is helpful for evacuation.These enhancement can be explained by the gaseous phase and co-condensed phase synergistic effect mechanism.In the early stage of combustion,Sb promoted the dehydrochlorination reaction and the carbonization,and act as free radical trapper in the gaseous phase to interrupt combustion reaction.In the solid phase,GO is as the carbon skeleton template for the catalyzed char by TNTs,therefore a continuous,dense carbon layers were formed.And more,titanium dioxide transformed from TNTs during combustion can be anchored in the char layers to enhance the thermal stability of the char layers,thereby preventing the heat and mass transmission effectively.