Construction And Performance Study Of Surface Coated Flame Retardant Polyurethane Rigid Foam

Polyurethane rigid foam is a closed-cell structure with the advantages of good thermal insulation,light weight,high specific strength and easy construction,etc.It also has the characteristics of sound insulation,shockproof,electrical insulation,heat resistance,cold resistance and solvent resistance,etc.However,when polyurethane rigid foam is in the burning process,the flame spreads very quickly,causing many fire safety accidents,so enhancing the flame retardant ability of polyurethane rigid foam has attracted the high attention of the whole society.The currently used flame retardant strategy can be defined as"overall flame retardant",which is to disperse the flame retardant in the whole foam matrix,because the polyurethane rigid foam is easy to become carbon and hot thick material during the combustion process.During this period,the flame retardant is mainly distributed in the surface layer to take on the flame retardant effect,because the combustion mainly takes place in the surface layer of the matrix,the internal flame retardant is still unburned,and the flame retardant in the solid matrix is still intact,which makes it impossible for the high proportion of the flame retardant in the matrix to play its flame retarding role effectively,thus,the“overall flame retardant”strategy is not efficient.Some research shows that when the burning polyurethane rigid foam is cut open,it can be observed that the surface layer is a black charred layer,and there is still unburned foam inside,which indicates that the burning of polyurethane rigid foam is mainly carried out in the surface layer.Therefore,the combination of matrix flame retardant and surface layer flame retardant is considered to construct a composite flame retardant system and improve its flame retardant efficiency.The ignition barrier is constructed on the surface of polyurethane foam to effectively improve the fire safety performance of polyurethane hard foam,while a certain amount of ammonium polyphosphate(APP)is added to the matrix to study the synergistic effect of matrix flame retardant and surface flame retardant to strengthen its fire safety performance.A polyurethane rigid foam with high fire safety performance is studied to expand the application scope of polyurethane rigid foam.It not only has a broad market prospect,but also can crack the urgent problem in the field of building energy saving in China.In this paper,by using hollow glass microspheres(HGM)combined with gypsum,common exterior coating and phenol formaldehyde resin,the ignition barriers of three systems,gypsum/HGM,common exterior coating/HGM and phenol formaldehyde resin/HGM were constructed on the surface of polyurethane hard foam.A simple painting method was used to coat 500g/m~2on the surface of polyurethane hard foam with different amounts of APP additions to study the combustion performance and influencing factors of various fire retardant polyurethane hard foam systems.The main research works are as follows:(1)By using building gypsum as the cementing material for the surface coating of polyurethane rigid foam,the coating with different hollow glass microspheres contents(0,10 wt%,20 wt%)was formulationted by adding hollow glass microsphere with low thermal conductivity of hollow structure as the filling material.This method mainly utilizes the non-combustible thermal insulation property of the coating to block out most of the heat,which enhances the flame retardancy of the polyurethane rigid foam.The thermal stability of the coatings with HGM content of(0,10 wt%,20 wt%)was tested by thermogravimetric analyzer,and the structure showed that the addition of HGM could effectively improve the thermal stability of the coatings,and the residual amount at 700°C was 2.7%and 3.4%higher than that when it was not mixed,respectively.The flame retardant performance of polyurethane rigid foam was tested by limiting oxygen index(LOI),and the results showed that pure polyurethane rigid foam was only 20%,indicating that it was extremely easy to burn,and the addition of 10 wt%and 20 wt%ammonium polyphosphate(APP)to the rigid foam matrix could only raise the LOI to 23%and 24.5%,respectively,which could not reach the building material burning class B2.In contrast,when the LOI test was conducted on the polyurethane rigid foam specimens coated with gypsum coating,it was found that the gypsum-coated polyurethane rigid foam samples appeared to be unable to sustain combustion after contacting the flame and could not sustain combustion even after increasing the oxygen concentration,indicating that the noncombustible gypsum coating could effectively inhibit flame propagation along the surface of the foam samples.The combustion performance of the samples was analyzed by conical calorimeter,and the results showed that the combined effect of gypsum/HGM coating and APP flame retardant added to the matrix could effectively delay the ignition time of polyurethane rigid foam(from 3s to29s for pure samples),reduce the peak heat release rate of polyurethane rigid foam,and significantly reduce the total heat release.It indicates that the gypsum/HGM coating can significantly improve the fire safety of polyurethane hard foam.(2)By using silicone propylene emulsion for ordinary exterior wall coatings as the base material,the exterior wall coatings/HGM flame retardant coatings were constructed on the surfaces of polyurethane rigid foam and polyurethane rigid foam with different proportions of APP flame retardant by adding different proportions(10wt%/20 wt%)of hollow glass microspheres,and the flame retardant and combustion properties of the samples were tested by limiting oxygen index and cone calorimeter.The LOI test showed that compared with pure polyurethane rigid foam,the ultimate oxygen index of polyurethane rigid foam coated with exterior coating/HGM coating(0HGM,10%HGM,20%HGM)increased from 20%to 22%,23.5%and 26%respectively,but none of them reached the requirement of oxygen index(30%)for building material combustion class B1,indicating that to a certain extent,the exterior coating/HGM system can hinder the spread of flame,but the outer coating still burns under higher oxygen concentration,which shows that it is not as effective as gypsum.Coating cone calorimeter test results show that this exterior coating/HGM coating is less effective than gypsum/HGM coating system in delaying ignition time and advancing the peak time of heat release rate,failing to effectively reduce the peak heat release rate and peak smoke release rate.This is due to the fact that the coating contains organic group components,which will also release heat when decomposing in the process of heat,and cannot effectively form a heat insulation protection layer,thus making the flame retardant effect of polyurethane hard foam poor.(3)By using phenol formaldehyde resin with better carbon formation performance as the film-forming substance,phenol formaldehyde resin/HGM flame retardant coatings(0HGM,10%HGM,20%HGM)were constructed on the surface of polyurethane hard foam by adding different proportions of hollow glass microspheres.The flame retardancy and combustion properties of the samples were also tested by limiting oxygen index and cone calorimetry.The LOI results showed that polyurethane rigid foam coated with phenol formaldehyde resin coating(0HGM,10wt%HGM,20wt%HGM)increased from 20%to 32%,35%and 36%,respectively,indicating that the flame retardant effect of phenol formaldehyde resin/HGM system is good.In the comparison of the coated samples,the doping of HGM can enhance the flame retardant effect.This is because of the excellent char formation property of phenol formaldehyde resin,which forms a dense char layer after combustion and can separate the flame from the foam matrix,effectively playing the role of physical thermal mass barrier shielding.Thermal radiation can still induce a small amount of pyrolysis of polyurethane rigid foam through the carbon layer,but the combustible gas generated by the pyrolysis of polyurethane rigid foam cannot be released due to the existence of dense carbon layer,so the phenol formaldehyde resin/HGM flame retardant coating shows good self-extinguishing performance for polyurethane rigid foam.