Flame Retardancy And Smoke Suppression Of Rigid Polyurethane Foam With Expandable Graphite

In recent years, under the pulling of application in the field of building energy saving,flame retardant rigid polyurethane foam (RPUF) attracts more and more attention ofindustrial and academic fields home and abroad. The expansion temperature ofhalogen-free flame retardant expandable graphite (EG) could match with the thermaldegradation temperature of RPUF. The resulting “worm-like”char residue formed when EGburns could cover the porous surface of RPUF as a barrier of heat and mass transfer. ThusEG could be employed for improving flame retardancy of RPUF. However, there are twoproblems about the flame retardant application of EG to be solved, including (1) theso-called “popcorn effect” leads the incompact structure of the expanded “worm-like” charlayers which could be easily blown off by the turbulence of flame impingement, reducingthe efficiency of flame retardancy and smoke suppression; and (2) the inorganic nature ofEG is incompatible with RPUF, leading to a deteriorative mechanical performances of thefinal materials. The combination of phosphorus-containing flame retardant compounds withEG and surface modification of EG are the main technical ways to solve the problems.Nevertheless, it is very important for solving above problems to study the interactionsbetween phosphorus-containing flame retardants and EG, the interactions between EG andsurface modification groups in RPUF flame retardant system, and the mechanism of flameretardancy and smoke suppression in RPUF filled with EG. This dissertation consists of thefollowing three parts:(1) The combustion and thermal degradation behaviors of RPUF flame retarded withEG and ammonium polyphosphate (APP) composite were investigated, and a flameretardant mechanism model of the above RPUF was established. Based on the optimumratio of physical intumescent flame retardant EG and chemical intumescent flame retardantAPP, the interactions among EG, APP and RPUF were researched by thermal gravimetricanalysis (TG), smoke density test, combustion performance test and scanning electronmicroscope (SEM). The factors effecting on the flame retardancy and smoke productionwere studied. The thermal decomposition behaviors and mechanism of the flame retardantRPUF were revealed by thermogravimeter coupled with infrared spectrophotometer(TG-FTIR) and X ray photoelectron spectroscopy (XPS). The results showed that the synergistic effects between EG and APP were superior in RPUF at the optimum ratio (7:3)of EG and APP, the RPUF’s LOI reached to36%. Moreover, EG/APP composite enhancedthe compactness and thermal stability of the char layer of flame retardant RPUF at hightemperature. The mount of residual char by the chemical action of APP with RPUF washigher than by the physical action of EG with RPUF. But the thermal oxidation resistantability of RPUF/APP system was the poor. Therefore, for RPUF/EG/APP system, theimprovement of thermal stability might be attributed to the combination of physical andchemical intumescent flame retardant.(2) EG was modified by two methods, and the use of the modified EG in RPUF wasexamined. The first modification method is the surface modification of EG by silanecoupling agent and boric acid, and MEG was obtained. The second one is the coatingmodification of EG by polyvinyl alcohol and boric acid, and CMEG was obtained. MEGand CMEG were characterized by SEM, FTIR, XPS, X ray diffraction (XRD) andelemental analysis. Base on their results, the morphology, composition and structure wereanalyzed, respectively. And the respective modification mechanisms were confirmed.Meanwhile, the conditions for preparing MEG and CMEG were optimized. Theperformances of MEG and CMEG were studied by TG, expansion experiment, combustionand mechanical performance tests, respectively. Furthermore, the effects of themodification methods on the pore structure of flame retardant RPUF and morphology ofchar layer were analyzed. MEG could be prepared by reacting silane coupling agent(KH550) with the boric acid on the surface of EG, and the percentages of actual reaction ofKH550and boric acid are about80.6%and27.0%respectively. The CMEG was preparedby coating EG with polyvinyl alcohol and then loading boric acid on the surface of EG, andthe percentage of actual reaction of boric acid is about22.5%. For both two modified EGs,the high expansion ratio was maintained, and the compactness and thermal stability ofexpansion char layer was improved. Compared with EG, the modified EGs can improve theflame retardancy properties of RPUF, but has limited effects on improvement of mechanicalproperties. The flame retardant performances of RPUF/MEG are better than RPUF/CMEG,while the compression performance of RPUF/CMEG is better than RPUF/MEG. Thisindicates that silicon boron compounds can effectively restrain the "popcorn" effects and enhance the flame retardancy due to the formation of a borosilicate ceramic precursor. Theinterface compatibility between CMEG and RPUF is efficiently improved, hence, themechanical properties of RPUF/CMEG is enhanced.(3) The smoke suppression mechanism and the combustion behaviors ofRPUF/EG/APP were investigated under the presence of the smoke suppressants Cu2O andMoO3. The smoke suppression mechanism models of Cu2O and MoO3in RPUF wereestablished. The effects of Cu2O and MoO3on the flame retardancy, smoke production andthermal stability of RPUF/EG/APP was characterized by the limiting oxygen index,horizontal-vertical burning test, smoke density test and thermo gravimetric analysis,respectively. The smoke suppression mechanism of Cu2O and MoO3was analyzed byCONE calorimeter, XPS and FTIR. The results indicates that Cu2O and MoO3have littleeffects on the limiting oxygen index, horizontal-vertical burning of RPUF/EG/APP system,but significantly reduce the total heat release, the smoke production and the CO generationof RPUF/EG/APP. Moreover, Cu2O and MoO3enable to enhance the thermal stability ofRPUF/EG/APP, and increase the amount of char residue at high temperature. The effects ofCu2O on reducing heat release are better than that of MoO3, because Cu2O can promote therelease of oxygen containing molecules from flame retardant RPUF and leads to a loweffective heat of combustion. MoO3has better effects on smoke suppression and charformation than Cu2O. This is attributed to three reasons: firstly, MoO3can promote thefragment from soft segment in polyurethane to form large molecules, which was helpful forincreasing the amount of char producing by acid catalysis and dehydration cross linking ofpolyurethane derivatives. Secondly, MoO3can retain more phosphorus containingcompounds in condensation phase. This is helpful to inhibit the thermal decomposition andvolatilization of the aromatic compound in the hard segment of polyurethane and to formmore char. Thirdly, MoO3can catalyze the combustion of volatiles more sufficiently andpromote the oxidation of CO to generate CO2. This is also beneficial for reducing thesmoke production and the toxicity of volatiles.