The Research On The Mechanism And Application Of Blowing-out Flame Retarded Epoxy Resins

In China, the flame retardant industry experienced rapid development over thepast30years. Research about flame retardant, flame retardant materials and flameretardant mechanism is more and more in-depth and comprehensive, and plays a moreimportant role in the related fields of international. At present, as the human attentionon environmental protection and the human health problems, the research andapplication about mature halogen-contained flame retardant seems to have problemswhich are difficult to solve. Therefore, development and application of halogen freeflame retardant has became the focus of the flame retardant field.The topic of my paper is using9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and polyhedraloligomeric silsesquioxanes (POSS) to flame retard epoxy resins (EP), the phosphorusand silicon are expected to play a synergy effect. Polyhedral oligomericsilsesquioxanes of the caged structures with the DOPO groups (DOPO-POSS) weresynthesized. The blowing-out effect was detected in epoxy resins flame retarded byDOPO-POSS, and the flame retardant mechanism of blowing-out of epoxy resin havebeen studied in detail. The “blowing-out effect” is that:“after the sample was ignited,it showed an unstable flame for several seconds; with the pyrolytic gaseous productsjetting outward from the condensed-phase surface, the flame was extinguished, itlooks like that the gas blew out the flame”. This study has opened up a new directionfor the research of halogen-free flame retardant epoxy resin.1. The DOPO-POSS were synthesized by two-step method using DOPO andvinyl triethoxy silane (VTES). The DOPO-POSS and its synthesis method hasobtained patent license. The DOPO-POSS was used as novel flame retardant to flameretard epoxy resins and the blowing-out effect was detected in this system. Based onthe blowing-out, the flame retardant efficiency of phosphorus and silicon wasimproved obviously. In order to understand the flame retardant mechanism ofblowing-out effect, octaphenyl POSS (OPS), octaaminophenyl POSS (OAPS) andpolyphenylsilsesquioxane (PPSQ), which are synthesized in our lab, are used alone orcompanied with DOPO to flame retard EPs. The studies show that although OAPS can react with epoxy monomer, it hassimilar flame retardant action compared with unreactive OPS. The OPS and OAPScompanied with DOPO could make epoxy resins shows remarkable blowing-outeffect. Comparing OPS and OAPS, the OPS is more effective and helpful to theformation of blowing-out effect. However, the epoxy resins with ladder-type PPSQcannot occur the blowing-out effect. This result is caused by that the cross-linking andcharring in the condensed phase caused by PPSQ cannot match the intumescent andcharring process of the EPs during combustion. Therefore, such a solid char may bevery brittle, and may crack very easily. This char layer cannot accumulate thepyrolytic gases to form the blowing-out effect. On the contrary, cross-linking in thecondensed phase caused by OPS seems to have the desired retarding effect. Theseresults indicate that only when the release speed of pyrolytic gases and the condensedphase structures match each other, the blowing-out out effect can show the bestperformance.2. The epoxy monomers in this paper are diglycidyl ether of bisphenol A(DGEBA) and tetraglycidyl-4,4’-methylene dianiline (TGDDM), the curing agents arethe aromatic4,4’-diaminodiphenylsulfone (DDS), m-phenylenediamine (m-PDA) andthe aliphatic oligomeric polyamide650(PA650). In this research, the blowing-outeffect can be detected in the DGEBA/m-PDA resins, DGEBA/DDS resins andTGDDM/DDS resins, however, the blowing-out effect did not show up in theDGEBA/PA650resins. This result indicates that the structures of epoxy resinsmonomer and curing agents are very important for the blowing-out effect. Throughanalysis the relationship between curing structures and blowing-out effect in theDGEBA/DDS resins and DGEBA/PA650resins, we find that the more aromaticstructures in the curing net is helpful to the blowing-out effect. This is because themore aromatic structures are easy to form crosslinked char layer, which couldaccumulate the pyrolytic gases easily and enable the formation of blowing-out effect.In the TGDDM/DDS resins, the typical blowing-out effect can be detected. TheDOPO-POSS and OPS/DOPO are used to flame retard the TGDDM/DDS resins. Theblowing-out effect can be detected during the UL-94tests of them. The UL-94resultsindicate that DOPO-POSS or OPS/DOPO could make the UL-94test of TGDDM/DDS resins reach V-0rating. The morphology of the char layer ofTGDDM/DDS flame retarded by OPS/DOPO is like a honeycomb, and severalindividual cavities can be detected under this char layer. This kind of char layer makesthe OPS/DOPO system showed more intensive blowing-out effect than theDOPO-POSS system.3. Thermal gravimetric analyzer was coupled with Fourier transform infraredspectrometry (TGA-FTIR), thermal gravimetric analyzer was coupled with massspectrometry (TGA-MS), x-ray photoelectron spectroscopy (XPS), x-raydiffractometer (XRD), scanning electron microscopy (SEM), thermal gravimetricanalyzer (TGA), differential scanning calorimetry (DSC), fourier transform infraredspectrometry (FTIR), and stress rheometer are used to qualitative or quantitativeanalysis of the flame retardant materials and its combustion products (gas phase andthe condensed phase). The cone calorimeter analysis, limited oxygen index andUL-94vertical burning test are used to investigate the flame retardant property andcombustion process of epoxy resin materials. In addition to the above routineexperimental method, in order to study the flame retardant mechanism of blowing-out,two kinds of effective experimental methods were built by myself to study theblowing-out effect in epoxy resin. The first method is that during the combustionprocess of cone calorimeter test of epoxy resins, the condensed phase samples ofdifferent time and the condensed phase samples of different position are investigated.Based on this method, we find the changes of chemical structures and element contentin the external, internal and bottom residues. The second method is that athermocouple which was used to identify the temperature of condensed phase wasembedded in the EPs. This method enabled us to grasp the temperature data in thecondensed phase during the UL-94vertical burning test of pure epoxy resin and theepoxy resin with blowing-out effect. These two kinds of experimental methodsprovide essential data and materials for the research of flame retardant mechanism ofblowing-out effect. The detail of all experimental methods can be found in everychapter.4. In this paper, the research about flame retardant mechanism of blowing-outeffect includes that: (1) An experimental method was set up to investigate the condensed phasesamples of different time and the condensed phase samples of different positionduring the combustion process of cone calorimeter test of epoxy resins. These resultsindicate that the ignition of EP is caused by flammable volatile fragments due to thescission of the EP chain in surface. Then, the internal degradation and combustiblegas release would outward supply the fuel to support the flame, and the C elementbegins to increase due to fast crosslinking reaction. At this point, if the char layer cangather pyrolysis gas, blowing-out effect can happen. And if sample can’t formeffective char layer, with the acceleration of thermal degradation reaction of EPmatrix, more combustible volatiles will migrate to the surface, the samples will reachflashover state. The char residues were investigated in detail by FTIR and XPS. Theinteractions between DOPO and silsesquioxane in the condensed phase are caused bythe formation of the-P(=O)-O-Si-structure. The-P(=O)-O-Si-structure is helpful toaccelerate the formation of effective char layer. During the vertical burning tests, thiskind of char layer could accumulate the pyrolytic gases easily and enable theformation of blowing-out effect.(2) The SEM, stress rheometer, video analysis were used to investigate thecondensed phase product created during the combustion process, we found that thequick formation of effective char layer can be observed in every blowing-out flameretardant system. Furthermore, the big cavities can be detected under this char layer,which is helpful to accumulate the pyrolytic gases. This kind of cavities with pyrolyticgases (gasbags) will become a thermal insulation layer, which is effective to inhibitthe heat transfer from the fire to the unburned polymer matrix. With the increase ofgas pressure in the cavities, the char layer would be broken, then, the blowing-outeffect would present.(3) In addition, to measure the internal temperature profiles in the ignited end ofsamples in UL-94tests, a thermocouple was embedded in the epoxy resins. Thisexperiment indicates that the blowing-out effect can slow the heat transfer from thefire to the unburned matrix, and also take away part of the heat in the surface zone bythe spurting gases. This heat insulation effect prolongs the decomposition of theunburned EP matrix at low temperature. This is the key factor for the formation of blowing-out effect.(4) The TGA-FTIR, TGA, XPS were used to analyse the condensed phaseproducts and gas phase products of different temperature. These results indicate thatthe low temperature decomposition results in more char formation during combustionwhich is helpful to the accumulation of pyrolytic gases. The gas phase analysisindicates that the low temperature decomposition results in a high proportion of CO2in the pyrolytic gases, which means the jetting gas are lower flammable gaseousproducts. This kind of jetting gases can extinguish the remnant fire easily.(5) Depending on the summarization of the impact factors and experimentalanalysis of blowing-out effect, a physical model of blowing-out effect was establishedin this paper. According to this physical model, the flame retardant mechanism ofblowing-out effect is quite different with the traditional mechanism that gas phaseflame retardant mechanism, condensed phase flame retardant mechanism, andintumescent flame retardant mechanism et al.5. This paper, the blowing-out effect improve the flame retardant efficiency ofsilicon and phosphorus elements for epoxy resins, and reduce the content of flameretardants obviously. When content of silicon and phosphorus element is about0.5wt%, four EP materials with UL-94V-0rate (1.6mm and3.2mm) and four EPmaterials with UL-94V-1rate (3.2mm) were obtained. When content of silicon andphosphorus element is about0.9wt%, one EP material with UL-94V-0rate (3.2mm)was obtained. The lower content of flame retardants means lower damage of theoriginal properties of epoxy resin materials. So epoxy resin materials flame retardedby blowing-out effect possesses huge potential application value.