Investigation Of POSS-based Multi-element Flame-retardants In The Modification Of Epoxy Resins

In order to meet the high requirements of the market for the flame retardants,reserchers are committed to develop the novel green flame retardants with low toxicity,low smoke,no pollution,high efficiency.Studies have shown that there is a good synergistic effect among a variety of flame retardant elements,so the introduction of two or more flame retardant elements can play a better flame retardant effect.The key to the study is that how to combine multiple flame retardant elements,especially the metal,Si,P and N,to construct the molecules,give full play to their flame retardant effect and synergistic effect and obtain the materials with high performance.In this paper,from the point of molecules design,the multiple flame-retardant elements were constructed into one molecule,which exhibited more efficient synergistic effect.POSS containing silicon,DOPO containing phosphorus,triazine group containing nitrogen,isopropyl titanate containing titanium and n-butyl zirconate were used as the sources of flame retardant elements and three kinds of POSS-based flame retardants were constructed,including PDT,Ti-POSS-bisDOPO and Zr-POSS-bisDOPO.Furthermore.the above three flame retardants were adopted to modify the epoxy resins(EP)obtaining the corresponding composites,and the flame retardancy,thermal stability,mechanical property and the flame retardant mechanism were discussed in depth.The highlight of this paper was that the microscopic change at the molecular level by the design of novel molecules could bring the macroscopic performance improvement of the composites.The main achievements of this work were displayed as follows(1)A multi-element P/N/Si synergistic flame retardant PDT was synthesized by nucleophilic substitution reaction with the triazine unit as the center and NH2-POSS and DOPO as raw materials.The PDT had a special chemical structure,which combined the nano-cage POSS,phosphaphenanthrene group and triazine unit simultaneously.It gave full play to the functions of each group at the molecular level and exhibited more excellent synergistic effect than the physical mixing of different materials simply.With the addition of PDT,the limiting oxygen index(LOI)values,UL-94 rating,glass transition temperature and the residual char were improved,and the total heat release,the peak of heat release rate were declined obviously,which proved that PDT enhanced the flame retardancy and thermal stability of EP.Furthermore,based on the analysis of scanning electron microscope(SEM)and X-ray spectroscopy(EDX)for the fractures,PDT could self-assemble into nanoparticles,palyed a role of nano-reinforcing filler and elevated the mechanical property.In the exploration of flame retardant mechanism,SEM,Raman spectra and X-ray photoelectron spectroscopy(XPS)results showed that the complex compounds generated by PDT could promote the cross-linking producing dense and continuous char,which worked in the protection of the matrix during combustion.The results of TG-FTIR and Py-GC/MS illustrated that the phosphorus-containing and nitrogen-containing free radicals from the decomposition of PDT could terminated the chain reactions in the gas phase.It can be concluded that the flame retardant PDT can work effectively in both gas and condensed phase exhibiting excellent flame retardancy.(2)The flame retardant Zr-POSS-bisDOPO,a kind of zirconium hybrid POSS flame retardant modified by double DOPO units,was obtained through "open cage reaction","cap reaction" and "Kabachnik-Fields reaction" with POSS,DOPO and n-butyl zirconate as the raw materials.Zr-POSS-bisDOPO was a kind of metal-inorganic-organic ternary hybrid compound,which was based on the cage structure of POSS with the zirconium atom being introduced into the cage and modified by DOPO group.It was adopted to endow the EP with good performances.The improved LOI values,UL-94 rating and residul char demonstrated that Zr-POSS-bisDOPO had good flame retardancy and heat resistance.The residual char was dense,continuous and demonstrated the folded lamellar morphorlogy,which was promoted by the complicated compounds with aromatic rings,Si-O-Si and Zr-O.The key to the molecule design of Zr-POSS-bisDOPO was to incorporate metallic titanium into the inorganic-organic POSS,realizing the metal-inorganic-organic ternary hybrid and performing the intramolecular synergy of metal and organic compounds truly.(3)The DOPO-modified titanium hybrid POSS flame retardant Ti-POSS-bisDOPO was successfully synthesized by "cage opening reaction","cap reaction" and"Kabachnik-Fields reaction" with POSS,DOPO and isopropyl titanate as the reagents.Ti-POSS-bisDOPO dispersed well in the epoxy matrix and self-assembled into nanoparticles with sizes ranging from 150 nm to 1.5 ?m.Compared with pure EP,both LOI values and residual char were raised dramatically and the corresponding heat release and smoke release were significantly reduced.Ti-POSS-bisDOPO was endowed with biphase flame retardant mechanism and the titanium,organophosphorus and silicon exhibited efficient intramolecular synergistic effect.XRD results revealed the important catalytic role of titanium,i.e.,the organic titanium was transformed into thermodynamically stable TiO2 at high temperature,which greatly promoted the residual char formation during the combustion.Furthermore,Metallic titanium displayed much better flame-retardant catalytic effct than zirconium via lateral comparison and EP/Ti-POSS-bisDOPO demonstrated dramatically excellent comprehensive performance.