| Rigid polyurethane foams(RPUF)have been widely used in the construction industry,refrigeration equipment and insulation pipes because of their low density,porosity,low thermal conductivity,high specific strength and excellent mechanical properties.However,RPUF without flame retardant treatment are extremely easy to ignite,which have a high speed of combustion and release toxic gases such as hydrogen cyanide(HCN)and carbon monoxide during the combustion process.This will greatly limit the use of RPUF.Flame retardancy research has always been a key research topic in the polyurethane industry.In view of the fact that halogen-based flame retardants will release a large amount of toxic gases during the combustion process,the halogen-free flame retardants have become the inevitable trend for RPUF.Thus,the following contents were studied systematically in this paper.(1)Inorganic flame retardants:aluminium hydroxide(ATH),ammonium polyphosphate(APP)and expanded graphite(EG)have a certain effect on the flame retardancy of RPUF,while the excessive addition of the fillers will lead to the increase of the viscosity,which is not conducive to the foams’processing.Besides,it will destroy the physical properties of foams.In this study,polyisocyanurate modified polyurethane foams(PIR-PUR foams)and dimethyl methylphosphonate(DMMP)were used as the matrix.And ATH,APP and EG were added respectively as flame retardants.Furthermore,their flame retardant mechanism and synergistic effect were explored.The results showed that ATH could be dehydrated during the combustion process to generate alumina,which would absorb the heat of the matrix,thus delaying the combustion reaction.In addition,the alumina that covered the surface of the substrate could also play a role in strengthening the carbon layer.The polyphosphoric acid produced by the decomposition of APP could catalyze the dehydration of the hydroxyl compounds produced by the degradation of the PIR-PUR foams,and the stable carbon layer was covered on the surface of the substrate.EG would degrade to form a worm-like carbon layer,thereby preventing heat and flammable gases from spreading.The good flame retardant formulations are:10 phr DMMP,5 phr ATH,15 phr APP and 20phr EG,which could significantly improve the flame retardant properties of PIR-PUR foams without destroying the mechanical properties,thermal insulation properties and preparation process.(2)The synergistic effects between ATH and APP were proposed and the synergistic mechanism was studied in detail.First,the mixed solid particles of ATH and APP could be uniformly dispersed in the matrix to avoid their respective agglomeration,thereby avoiding the destruction of the cells.Therefore,the pressure applied on the substrate was uniform.The matrix was not prone to large deformation.Secondly,when the mass ratio of ATH and APP was 1:3,part of polyphosphoric acid produced by APP decomposition reacted with aluminum produced by decomposition of ATH to form aluminum metaphosphate,and the other part of polyphosphoric acid catalyzed the dehydration of hydroxy compound formed by PIR-PUR foams degradation.Under the action of aluminum metaphosphate and polyphosphate,PIR-PUR foams could be degraded to produce the most effective carbon layer to prevent the substrate from burning.Further studies showed that ATH and APP also had synergistic effects in polyurethane elastomers.On the one hand,the compatibility of the mixed solid particles with the substrate could be significantly improved.On the other hand,the combination of ATH and APP could promote the polymer matrix to produce a more continuous and dense carbon layer,thereby preventing the combustion from proceeding.(3)Two kinds of halogen-free organic flame retardants based on 9,10-dihydro-9-oxa-10-phosphorinphilippines-10-oxide(DOPO):10-(2’,5’-bis(hydroxyl)phenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (ODOPB)and 1,1’-bis(4-hydroxyphenyl)-metheylene-bispheny-1-oxophosphine oxide(DOPO-Ph OH)were synthesized and added to PIR-PUR foams,and their effects on the flame retardant properties of PIR-PUR foams were studied.The mechanical properties and flame retardancy of the PIR-PUR/DOPO-Ph OH foams were superior to PIR-PUR/ODOPB in the same P addition amount,which was because that DOPO-Ph OH didn’t destroy the integrity of the cells.In addition,the polyaromatic ring structure of DOPO-Ph OH resulted in higher thermal stability and high temperature charring of PIR-PUR foams.The residues of PIR-PUR/ODOPB and PIR-PUR/DOPO-Ph OH covered a continuous dense carbon layer.The residues of PIR-PUR/ODOPB were thin two-dimensional membrane morphology,but the residues of PIR-PUR/DOPO-Ph OH were a thick three-dimensional fabric.The three-dimensional fabric-like covering has a more effective barrier effect,so that the flame retardant properties of PIR-PUR/DOPO-Ph OH were better than that of PIR-PUR/ODOPB.(4)A new metal-organic compound 9,10-dihydro-9-oxa-10-phosphofluoro-10-oxide-Mg(DOPO-Mg)was designed and prepared by introducting the magnesium element to the DOPO structure,and was added to the PIR-PUR foams as flame retardants.The results show that DOPO-Mg could play a flame retardant role in both gas and condensed phases.DOPO-Mg could improve the thermal stability and carbon formation of PIR-PUR foams.DOPO-Mg could also change the degradation path of PIR-PUR foams and inhibit the release of combustible gases and toxic gases.And the products of degradation preferentially were kept in the agglomerated phase,facilitating the formation of stable carbon residues.(5)In the further study,we explored the structural flame retardant of RPUF.The melamine based polyether polyol(HMMM-PG)was prepared by alcoholizing the melamine ether.HMMM-PG was an oligomeric mixture containing five different ether exchange products with an average hydroxyl value of 447 mg KOH/g.HMMM-PG had higher thermal stability and carbon formation compared with the widely used 380A.HMMM-PG showed shear thinning characteristics,which helped to reduce the difficulty of foams’processing.During the decomposition process,the C element and N element in HMMM-PG formed stable compounds in the condensed phase.The RPUF prepared with HMMM-PG as a polyol had a lower thermal conductivity and a higher compressive strength.RPUF-HMMM-PG had continuous and dense residues that could prevent heat and mass transfer between the gas phase and the condensed phase,thereby preventing the internal polymer matrix from being further attacked by heat flow.Therefore,the flame retardant properties of foams using HMMM-PG were significantly higher than those of 380A or castor oil polyol.And the improvement of the flame retardancy was mainly manifested in the condensed phase.(6)Polyurethane foams with high performance were prepared by introducing flame retardants such as DMMP,ATH,APP and EG into HMMM-PG based polyurethane foams system.The results showed that the physical properties and the flame retardancy of RPUF were greatly improved under the flame retardant effect of 10 phr DMMP,5 phr ATH,15 phr APP and 20 EG.The compressive strength of RPUF was increased to 0.22 MPa,while the LOI value was increased to 30.2%and the peak heat release rate was reduced to 91.7k W/m~2. |
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