Preparation And Properties Of The Efficient Flame Retardant Rigid Polyurethane Foams

Rigid polyurethane foams?RPUF?is applied in construction because of light weight and low thermal conductivity,sound-absorbing and shock-absorbing.However,a major limitation of RPUF which possessed a large number of hydrocarbon segment,porous structure,low density and large surface area is easily flammable.Thus,the improvement in flame-retardant properties of RPUF has become an urgent and attractive topic all over the world.Phosphorus-containing flame retardants have been widely used to impart RPUF with flame retardancy due to their high efficiency.The flame retardancy is limited in addition of a phosphorus-containing flame retardant,the simultaneously use of two or three phosphorus-containing flame retardants can exert the gaseous and condensed biphase flame retardant effects.TCPP is one of the most popular flame retardants used in RPUF because it is economical and efficient.However,the use of TCPP is currently restricted because of their potential toxicity and environmental problems.Recently attention has been paid to the synergetic flame-retardant between the inorganic phosphorus-containing FR additives aluminum hypophosphite?AHP?and melamine polyphosphate?MPP?with TCPP which improve the cell structure and mechanical property;the synergetic flame-retardant between aramid fiber and TCPP which not only reduce the heat release and the evolution of toxic smokes of RPUF,but also improve the cell structure and mechanical property based on heterogeneous nucleation.Recently,halogen-free IFR expandable graphate?EG?becomes the major trend in flame-retarding RPUF because of low smoke release and low toxicity.However,there are two problems about the flame retardant application of EG to be solved,including the so-called?popcorn effect?and the inorganic nature of incompatibility with the PU matrix.Moreover,with the increment of EG content,the mechanical property of the flame retardant RPUF will be reduced.On the one hand,EG was modified to improve the interfacial strength between MEG and PU.On the other hand,The simultaneously use of MEG and phosphorus-containing flame retardants system ammonium polyphosphate?APP?and DOPO derivatives?DTE-DOPO?not only can reduce the dosage of MEG,but also better flame retardancy was achieved than they were used alone.In order to prepare the RPUF with outstanding flame retardancy,mechanical property and cell structure,this article focuses on different FRs synergetic mechanism and the relationship between structure and property of flame retardant RPUF.The main research contents and conclusions are shown as below:We mixed AHP and MPP with TCPP and prepared flame-retardant RPUF.The flame retardancy and thermal decomposition of TCPP in combination with AHP and MPP was investigated in RPUF.The effect of TCPP,AHP and MPP on the flame retardancy and thermal decomposition in RPUF was characterized using vertical burning test?UL 94 VB test?,limited oxygen index?LOI?,cone calorimeter test?CCT?,thermogravimetric analysis/infrared spectrometry?TG–IR?and gas chromatography-mass spectrometry?GC–MS?.Scanning electron microscope?SEM?–energy dispersive X-ray?EDX?and in situ fourier transform infrared spectroscopy?in situ FTIR?analysis was used to analyze the char residue of the RPUF and flame-retardant RPUF.The gaseous flame-retardant mechanism of the PU/TCPP/AHP/MPP was investigated via GC–MS.When the content of TCPP was 10wt%,the ratio of MPP to AHP was 1:2,the common use of TCPP,AHP and MPP in the RPUF exhibited better flame retardancy and the regularity in cellular structure compared with adding AHP or MPP in PU/TCPP composites and adding TCPP,AHP and MPP standalone.The LOI of the PU/TCPP/AHP/MPP system was increased to 27.4%,and FR RPUF passed the V-1rating in the UL-94 test.The PHRR,THR,PSPR and TSR were decreased by 71.4,82.9,16.1and 39.9%respectively compared to that of pure RPUF,respectively.TCPP and AHP release PO and PO₂ free radicals with quench effect,while MPP releases inert gas,like CO₂,NH₃ and H₂O to dilute the oxygen and flammable gas during burning in the gaseous phase,thereby decreasing the smoke production and toxicity.Compared with the PU/TCPP₁₀/MPP₁₀ and PU/TCPP₁₀/AHP₁₀,PU/TCPP₁₀/MPP_3.3/AHP_6.7 had a higher carbon content,which could be explained by the catalyzing charring effect of polyphosphoric acid generated from TCPP and the oxidation reaction of phosphine.Simultaneously,the chemical cross-linking reaction between MPP and polyol from depolymerization of PU results in the formation of esterifiable structure with P–O–C groups to enhance the quality of the charred layers and form integrated,stable and tight charred layers.The combination of TCPP,AHP and MPP in the RPUF can greatly reduce the toxic gases–NCO containing products,HCN and hydrocarbons and increasearomaticcompoundsandwater.Theflameretardancyofthe PU/TCPP₁₀/MPP_3.3/AHP_6.7 increases with the increment of density.When the chemical structure,density,closed cell rate and wall thickness is invariable,in the cell size of0.17-0.3mm,at density of 93.1 kg/m³,the compressive strength and modulus of the PU/TCPP₁₀/MPP_3.3/AHP_6.7 increase with the increment of cell size.We mixed TCPP with MAF and prepared flame-retardant RPUF.Phosphoric acid was used to modify AF to improve the compatibility with PU matrix and flame retardant property.Effect of tris?1-chloro-2-propyl?phosphate and modified aramid fiber on cellular structure,thermal stability and flammability of rigid polyurethane foams has been investigated.Based on the in suit FTIR of the char residue and TG–FTIR characterization of volatile products,the thermal degradation mechanism of PU/TCPP/MAF under non oxidizing gaseous environment can be evaluated.The mechanism of synergetic flame-retardant effect of TCPP/MAF in PU foams was analyzed.Moreover,when the content of TCPP and MAF was 20wt%and 5wt%respectively,the common use of TCPP and MAF effectively ensure both the regularity in cellular structure and enhancement in fire resistance of RPUF compared with TCPP and MAF standalone.Compared with foams without flame retardant,the LOI was increased by 39.2%,the MSD,PHRR,PSPR,THR and TSR was decreased by 25,59.6,10.7,78.2 and 26.7%,respectively.TCPP and MAF is not only active in the gaseous phase for releasing gaseous PO,Cl free radicals with quench effect and water with dilution effect,but also has a strong activity in the condensed phase.The char residue of PU/TCPP/MAF at high temperature of 700°C was increased to 15.6%,because the combination of the partial phosphorous of TCPP and P-OH group from MAF under pyrolysis generated more phosphoric acid which catalyzed poyol from depolymerization of PU to promote the formation of phosphorus-rich residual char.PU/TCPP/MAF exhibited lower smoke and toxicant production and increased char residue compared to RPUF containing TCPP.Production of toxic gases such as HCN was significantly decreased,nonflammable gases CO₂ and water were increased.At the same density,the LOI of PU/TCPP/MAF increases with the increment of fiber content.When the content of MAF is above 3 wt%,the trend of the LOI of PU/TCPP/MAF increases with the increment of fiber content is more obvious.When the chemical structure,density,closed cell rate and wall thickness is invariable,in the cell size of 0.17-0.3mm,at density of 93.1 kg/m³,the compressive strength and modulus of the PU/TCPP/MAF decrease with the increment of cell size.When the chemical structure,density,and cell size is invariable,the compressive strength and modulus of the PU/TCPP/MAF decrease with the decrement of closed cell rate.We mixed APP and DTE-DOPO with MEG and prepared flame-retardant RPUF.Boric acid and sulfuric acid were used to modify EG to improve the compatibility with PU matrix and mechanical property.Effect of APP,DTE-DOPO and MEG on cellular structure,thermal stability and flammability of RPUF has been investigated.The condensed flame-retardant mechanism was analyzed by SEM-EDX and FTIR.The gaseous flame-retardant mechanism was investigated via GC–MS.The mechanism of synergetic flame-retardant effect of APP/DTE-DOPO/MEG in PU foams was analyzed.Compared with neat PU,PU/APP/DTE-DOPO/MEG foams showed excellent flame-retardant and mechanical properties.When the total amount of flame retardants was 20wt%,the content of MEG was7wt%,the ratio of APP to DTE-DOPO was 4:1,the common use of APP,DTE-DOPO and MEG effectively ensure both the regularity in cellular structure and enhancement in compressive strength and flame-retardant property of RPUF compared with APP,DTE-DOPO andMEGstandalone.TheLOIvalueandcompressivestrengthof PU/APP_10.4/DTE-DOPO_2.6/MEG₇ foams was increased by 84.7%and 27.7%respectively.The PHRR,PSPR,THR,TSR and PEHC were decreased by 39.6,47.8,58,48.3 and 26.5%,respectively.During combustion,DTE-DOPO released PO and PO₂ free radicals,which quenched the flammable free radicals?e.g.,·H and·OH?produced by the substrate,thereby inhibiting the chain reaction of combustion and decreasing the burning intensity of the PU foams.APP act as acid source and blowing agent to release inert gas,like CO₂,NH₃ and H₂O to dilute the oxygen and flammable gas generated from pyrolysis of PU,thereby decreasing the smoke production and toxicity.The char residue of PU/APP_10.4/DTE-DOPO_2.6/MEG₇ at high temperature of 700°C was increased to 23.2%,polyphosphoric acid from APP and DTE-DOPO under pyrolysis reacted with the decomposed products from the PU matrix and boron phosphate from MEG to form a strong and compact carbonaceous layer,thereby enhancing the barrier effect of the residual char.The combination of APP,DTE-DOPO and MEG in the RPUF can greatly reduce the toxic gases–NCO containing products,HCN and hydrocarbons and increase water and aromatic compounds.PU/APP_10.4/DTE-DOPO_2.6/MEG₇of high density possesses more compact char,less plastic deformation,the char of PU/APP_10.4/DTE-DOPO_2.6/MEG₇ at low density has many holes.When the chemical structure,density,closed cell rate and wall thickness is invariable,in the cell size of 0.25-0.61mm,at densityof90.3kg/m³,the compressivestrength andmodulusofthe PU/APP_10.4/DTE-DOPO_2.6/MEG₇ increase with the increment of cell size.When the chemical structure,density,closed cell rate and cell size is invariable,in the wall thickness of17-19.5?m,at density of 90.3 kg/m³,the compressive strength and modulus of the PU/APP_10.4/DTE-DOPO_2.6/MEG₇ decrease with the increment of wall thickness.