Effect of phosphorus-nitrogen additives on fire retardancy of rigid polyurethane foams

Effect of Phosphorus-Nitrogen Additives on FireRetardancy of Rigid Polyurethane Foams

Harpal Singh,1 A. K. Jain,2 T. P. Sharma1

1Fire Research Laboratory, Central Building Research Institute, Roorkee 247667, India2Department of Chemistry, Indian Institute of Technology, Roorkee 247667, India

Received 6 June 2007; accepted 14 February 2008DOI 10.1002/app.28324Published online 12 May 2008 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A flame retardant composition was pre-pared from phosphorus-melamine-urea-formaldehyde(PMUF) reaction. The flame retardancy of rigid polyur-ethane foam (RPUF) was studied by impregnating it withvarious concentrations of PMUF. Optimum impregnationtime, retention and density of RPUF samples with PMUFwere also studied. The morphology, elemental analysis,and thermal stability of RPUF and RPUF-PMUF sampleswere studied with scanning electron microscopy (SEM),CHNS analyzer, and energy dispersive X-rays (EDX) and,thermogravimetric analysis (TGA), derivative thermogravi-metric (DTG), differential thermal analysis (DTA), respec-tively. The deposition of PMUF on the cell walls and surfa-ces was exhibited by decreased cell size from 357 to 285(65) lm. CHNS and EDX studies show the introduction ofphosphorus and nitrogen into the RPUF-PMUF samples.The degradation of RPUF and RPUF-PMUF samples wasoccurred through two and three stages in nitrogen and air,respectively. RPUF-PMUF samples are decomposed com-

paratively at lower temperature than RPUF which isresulted into large amount of high temperature stable charresidue. This char residue acts as insulating blanket andprotects RPUF-PMUF from fire. Fire performance andsmoke density of RPUF and RPUF-PMUF samples wereinvestigated with BS: 4735, ASTM D2863 and ASTM E662,respectively. RPUF-PMUF samples demonstrate reducedextent burnt from 125 to 27 mm, burning rate 2.23–0.44mm/s and percent mass loss (PML) from 100 to 8.82%.Oxygen index was also increased from 17.8 to 23.8. Dmresults obtained during smoke density tests show thatRPUF-PMUF generates less and delayed smoke than RPUFunder flaming and nonflaming mode. � 2008 Wiley Periodi-cals, Inc. J Appl Polym Sci 109: 2718–2728, 2008

Key words: rigid polyurethane foams; impregnation; SEM;thermal stability; fire retardant; burning rate; percent massloss; oxygen index; smoke density; combustion; smoulder-ing

INTRODUCTION

Rigid polyurethane foams (RPUFs) are the reactionproducts of high functional 4,40-diphenylmethanediisocyanate (MDI) and low-molecular-weight poly-ether polyol. RPUFs have an extremely versatilerange of properties and applications. Some of theimportant engineering applications are building con-struction industry, domestic and commercial refrig-eration, transportation, automotive industry, packag-ing, furniture components, and decorative paneling,due to their superior heat insulation value resultedfrom closed cell structure.1,2 However, RPUFs havelow thermal stability and high flammability, primar-ily due to the presence of biuret, allophanate, ure-thane, and urea groups into their structure. Biuretand allophanate are the most thermally unstablegroups in the RPUF backbone and on exposure tofire immediately decompose to diisocyanate, urea,and urethane precursors. These precursors are againthermally unstable. During RPUF burning diisocya-

nate converts into yellow smoke containing HCN,and rest of the other groups convert to white smokecontaining CO2 and CO. HCN and CO are the prom-inent toxicant gases which on inhalation lead todeath.3–6

Various flame retardants such as phosphorus-halo-gen mixture, ammonium polyphosphate (APP), andorganophosphorus compounds either alone or incombination with nitrogen or silicone have beenused to impart flame retardancy to RPUFs.7–9 Thesecompounds may be incorporated into the foam for-mulation as reactive or nonreactive additives in theform of phosphorus, nitrogen, and halogen elements.Modesti et al. recently reported the successful incor-poration of various combinations of halogen freeflame retardants such as APP with melamine, ex-pandable graphite with triethyl phosphate and redphosphorus, and expanded graphite with melamineinto the RPUF formulations.10–13 As phosphorus andother halogen free flame retardants containingRPUFs give off less-toxic combustion products, thusthey are preferred over halogen containing flameretardants.14,15 Bromine and chlorine containingRPUFs on exposure to fire produce corrosive smokeof high toxicity. Organic phosphates do not cause

Correspondence to: H. Singh ([email protected]).

Journal of Applied Polymer Science, Vol. 109, 2718–2728 (2008)VVC 2008 Wiley Periodicals, Inc.

any of these problems. The flame retardant activityof phosphorus compound starts in condensed phase.On exposure to heat, phosphorus compound decom-poses at lower temperature than polyurethane foamsto produce phosphoric or polyphosphoric acids.These acids catalyze the char formation in the con-densed phase. This phosphorus rich char (carbona-ceous layer) prevents heat transfer by acting as insu-lating blanket.16,17 Grassie et al. reported that phos-phoric acid reacts very efficiently with carbodiimideformed by the condensation of the isocyante releasedon the thermolysis of polyurethane foams. Phos-phoric acid-carbodiimide reaction leads to crosslink-ing which resulted into carbonaceous char formation.This carbonaceous char prevents the heat penetrationinto the foam surface.18

The main objective of this study is the preparationof phosphorus-nitrogen additives based compositionfrom phosphoric acid-melamine-urea-formaldehyde(PMUF) reaction and to investigate the effect on thefire retardancy of rigid polyurethane foam impreg-nated with PMUF. In the PMUF composition, phos-phoric acid is the source of phosphorus while mela-mine and urea are the sources of nitrogen. Earliersuch flame retardant compositions have been stud-ied in cellulosic paper and wood,19 but never inRPUFs to the best of our knowledge. Rigid polyur-ethane foam RPUF impregnated with PMUF compo-sition was prepared in our own laboratory and theresults are reported.20 RPUF samples of similar den-sity were impregnated with different concentrationsof PMUF. The density of RPUF samples impregnatedwith PMUF was measured as per ASTM D1622.Morphological characteristics, elemental analysis,and thermal behavior properties in nitrogen and airatmosphere of conventional and PMUF impregnatedRPUF samples were studied with scanning electronmicroscopy (SEM), CHNS analyzer, and energydispersive X-rays (EDX) and, thermogravimetricanalysis (TGA), derivative thermogravimetry (DTG)differential thermal analysis (DTA), respectively. The

flammability characteristics and smoke densityunder flaming and nonflaming mode were evaluatedwith BS: 4735 and ASTM D2863, and ASTM E662,respectively.

EXPERIMENTAL

Materials

Phosphoric acid (88% ortho, GR grade; Merck, Mum-bai, India), formalin (37% formaldehyde solution, GRgrade; BDH, Mumbai, India), melamine and urea(crystallized, LR grade; Loba, Mumbai, India) andsodium hydroxide (pallets, Merck, Mumbai, India)were used as they received. Distilled water used wasgenerated in the laboratory. Rigid polyurethanefoam (49.19 kg/m3) samples used for impregnationwere also prepared in the laboratory. Some of thephysical characteristics of the chemicals used in thestudy are listed in Table I.

Preparation of PMUF composition

41% formaldehyde solution (183.0 g, 2.5 mol) mixedwith distilled water (126.0 g, 7.0 mol) was chargedin a 1000-mL three-neck round-bottom flaskequipped with thermometer, stirrer and reflux con-denser. The medium of the solution was adjusted toalkaline by adding a few drops of 3N sodium hy-droxide solution. The solution was then heated overa water bath until it attains the temperature of 80–858C. A mixture of urea (60.06 g, 1.0 mol) and mela-mine (31.53 g, 0.25 mol) was then added incremen-tally with constant stirring over 20 min. Upon thecompletion of the addition, the reaction mixture wasallowed to reflux for 10 min. The resulting solutionwas cooled to ambient temperature by putting theflask under cold-water stream. 88% ortho-phosphoricacid (111.4 g, 1.0 mol) was then added slowly withconstant stirring. During the addition of ortho-phos-phoric acid, the whole assembly was continuously

TABLE IPhysical Characteristics of the Chemicals Used in the Study

Chemical Phosphoric acid Formaldehyde Melamine Urea Water

Molecular formula

Molecular weight 98 30 126.12 60.06 18Melting point (8C) – – 354 132.7 –Miscibility Miscible with water Miscible with water Sparingly miscible Miscible with water –Phosphorus (%) � 15 – – – –Nitrogen (%) – – � 67 � 47 –

EFFECT OF NITROGEN ADDITIVES ON FIRE RETARDANCY 2719

Journal of Applied Polymer Science DOI 10.1002/app

kept under cold-water stream to avoid the heat upof flask due to the heat generated by the reaction ofortho-phosphoric acid and mixture solution. The finalproduct was a colorless viscous liquid mixture ofphosphorus-melamine-urea-formaldehyde. This vis-cous mixture was designated as PMUF. The basicformulation used for the PMUF preparation is pre-sented in Table II.

Sample preparation

Conventional RPUF samples of 49.19 kg/m3 den-sities were used for impregnation with PMUF.PMUF concentration, impregnation time, retention,and density are considered to be the most importantparameters during the preparation of RPUF samples.PMUF is water based composition, thus its variousconcentrations ranging from 10 to 100% were pre-pared by mixing it with water. Optimum impregna-tion is the time at which RPUF samples retain themaximum quantity of PMUF with optimum time pe-riod. Optimum impregnation time was investigatedby impregnating the RPUF samples with 100%PMUF solution concentration from 10 to 60 min.with an increment of 10 min at the ambient condi-tions. Retention is the quantity of PMUF solutionconcentration absorbed by the RPUF sample andsubsequently retained in it. Retention of RPUF sam-ples was investigated by impregnating the 30 num-ber of samples for optimum time with 10–100%PMUF concentrations with an increment of 10%. Theeffects of PMUF concentration and impregnationtime on retention were studied by impregnating the180 numbers of RPUF samples into the PMUF solu-tion concentration ranging from 10 to 100% and ev-ery three samples were removed from the solutionat an interval of 10 min. The effect on the density ofRPUF samples was observed by impregnating thesamples for optimum time with 10–100% PMUF con-centrations with an increment of 10%. After com-plete impregnation, RPUF samples were removedfrom their respective PMUF solution concentrationand allowed to dry for 24 h at room temperature.PMUF solution concentration and optimum impreg-nation time of RPUF samples (RPUF-PMUF) areshown in Table III. In the sample code, PMUFdenotes the phosphorus-melamine-urea-formalde-

hyde composition concentrations used for impregna-tion. RPUF samples impregnated with PMUF areshown in Figure 1(a).

Conventional RPUF and RPUF-PMUF sampleswith maximum retention were prepared for scanningelectron microscopy (SEM), CHNS analyzer andEDX and, thermal analysis (TGA/DTG/DTA). Theflammability characteristics of conventional RPUFand RPUF-PMUF samples of dimensions 150 3 50 313 and 150 3 12.5 3 12.5 mm3 was investigated byimpregnating the three samples with each PMUFconcentrations ranging from 0 to 100% with an incre-ment of 10%. After impregnation the RPUF sampleswere removed from the solution and dried for 24 hat room temperature. Dried RPUF samples weremarked across their width by a line (gauge mark)25 mm from one end. Smoke density of conventionalRPUF and RPUF-PMUF samples of size 75 3 75 325 mm3 under flaming and nonflaming mode wasinvestigated by impregnating the samples with max-imum PMUF concentration.

Measurements

PMUF impregnation extent to the RPUF samplesmainly depends upon the concentration, impregna-tion time, and retention. These three parameters arelinked to each other in which impregnation timedepends upon the maximum retention, and retentiondepends on both concentration and impregnationtime. Optimum impregnation time was measured onthe basis of maximum PMUF retention attained bythe RPUF samples with respect to time. Further, op-timum impregnation time on the basis of maximumretention of every three RPUF-PMUF samples wascalculated and averaged. Retention on each PMUFconcentrations were also calculated and averaged.

TABLE IIBasic Chemical Formulation of PMUF

Chemical Quantity per mole

Ortho-Phosphoric acid 1.0Melamine 0.25Urea 1.0Formaldehyde solution 3.0Water 8.0

TABLE IIIRigid Polyurethane Foam samples (RPUF-PMUF)a

Impregnatedb with Phosphorus-Nitrogen AdditivesBased Composition

Sample codes(RPUF-PMUF)

PMUFConcentration (%) Water (%)

RPUF-0.0 0.0 0.0RPUF-PMUF 10 90RPUF-PMUF 20 80RPUF-PMUF 30 70RPUF-PMUF 40 60RPUF-PMUF 50 50RPUF-PMUF 60 40RPUF-PMUF 70 30RPUF-PMUF 80 20RPUF-PMUF 90 10RPUF-PMUF 100 0.0

a PMUF denotes phosphorus-melamine-urea-formalde-hyde composition.

b 20 min optimum impregnation time.

2720 SINGH, JAIN, AND SHARMA


The density of conventional RPUF and RPUF-PMUFsamples was measured according to ASTM D1622.The size (length3width3thickness) of the specimenwas 30 3 30 3 30 mm3, respectively. ConventionalRPUF and RPUF-PMUF specimens were conditionedat 258C and 55% relative humidity for 48 h prior totheir density measurement. The density of five speci-mens per sample were measured and averaged.

The morphology of conventional RPUF and RPUF-PMUF samples was observed with Leo (Cambridge,UK) 438 VP SEM. The samples were cryogenicallyfractured and gold coated to render them conductiveprior putting under scanning observation. Duringscanning 15 kV accelerating voltage was used. TheSEM was used to observe the PMUF deposition onthe cell walls, surfaces and the difference in the shapeand size of the cells between conventional RPUF andRPUF-PMUF samples. To define the cell size, meas-ured cell sizes were averaged except the sizes for thelargest and smallest cells. Elemental analysis of con-ventional RPUF and RPUF-PMUF samples was car-ried out with CHNS analyzer (Elementar Andysensys-teme GmbH VarioEL III, Germany) and EDX. RPUFand RPUF-PMUF samples powder were mixed withoxidizing agent and dropped into the CHNS chamberat about 7008C. The same samples which were usedfor morphological observation were also used for theEDX study. The main purpose was to detect the qual-itative and quantitative presence of elements intro-duced into the structure of RPUF samples by thePMUF impregnation. Thermal properties of conven-tional RPUF and RPUF-PMUF samples were meas-ured on a TGA/DTG/DTA apparatus (Pyris Dia-mond, PerkinElmer, Shelton, CT). The samples werecrushed into power. Platinum pan was used to placethe samples powder inside the heating chamber. Thesamples were heated up to 10008C at a rate of 108C/min in nitrogen and air atmosphere at a flow rate of200 mL/min. After complete heating of the samplesTGA, DTG and DTA curves obtained were taken foranalysis.

The flammability characteristics of conventionalRPUF and RPUF-PMUF samples were evaluatedaccording to BS: 4735 and ASTM D2863. The speci-mens were weighed before placing horizontally onsupport gauge inside the noncombustible chamber.The farthest end away from gauge mark of the speci-men was exposed for 60 s to 10 mm diameter wingtop fitted LPG burner of 38 mm nonluminous flameheight. Exposed RPUF-PMUF samples are shown inFigure 1(b). After complete fire exposure extentburnt, burning rate, percent mass loss (PML) andextinction time of three specimens per sample weremeasured and averaged for analysis. Oxygen index(OI) of conventional RPUF and RPUF-PMUF sam-ples was carried out with Stanton Redcroft FTA,flammability apparatus, (UK). OI was determined byigniting the specimen in the minimum level of oxy-gen in an oxygen-nitrogen mixture which supportsthe candle like burning of the test specimen. Thespecimens were clamped vertically exactly in thecenter of the column and 100 mm below the top ofthe open column. The limiting oxygen concentrationis always approached from both sides to establishthe OI. The tip of the specimen is ignited in givenoxygen concentration atmosphere and maintainedfor at least 30 s. If the sample burns for 3 min orpass the mark, the oxygen concentration is graduallyreduced. Conversely, if the specimen is extinguishedin less than 3 min or covers a smaller burning area,the oxygen concentration is increased. During thisprocedure an oxygen concentration is reached atwhich sample extinguished at specified length andtime and this is taken as the OI. After complete flam-mability exposure OI of five specimens per samplewas measured and averaged. Smoke density of con-ventional RPUF and RPUF-PMUF samples wasmeasured under flaming and nonflaming modeaccording to ASTM E662. Specific optical density(smoke concentration) measurement is made of theattenuation of a light beam (transmittance) by smokeaccumulating within a closed chamber due to non-

Figure 1 Photographs of PMUF impregnated RPUF samples (RPUF-PMUF): (a) before flammability test and (b) afterflammability test.



flaming decomposition and flaming combustion ofconventional RPUF and RPUF-PMUF samples. Thespecimens were mounted vertically inside the non-combustible chamber. The specimens were exposedto electrically heated radiant heat source in the pres-ence of six tube LPG burner and, electrically heatedradiant heat source for 20 min under flaming andnonflaming mode respectively. After complete expo-sure percent transmittance (pT) and specific opticaldensity (Ds) of three specimens per sample undereach mode were measured and averaged.

RESULTS AND DISCUSSION

PMUF composition

Chemical reactions involved in the preparation ofPMUF composition were already studied. It wasfound that 1 mol of urea in alkaline medium reactswith 1 mol of formaldehyde to form monomethylolurea. Similarly to urea, melamine in alkaline me-dium reacts with formaldehyde to yield monomethy-lolated product. When the amount of formaldehydeis increased to 2 mol, the dimethylolated product isformed. The rate of dimethylolated product forma-tion is very slow at room temperature. When thetemperature is raised in the range of 80–858C, theend product was dimethylol and no monomethylolproduct was formed. Thus the major intermediatesin the PUMF composition are N,N0-dimethylolatedurea and N,N0-dimethylolated melamine. IR spec-trum of dimethylolated products exhibited that sub-stitution occurred on the carbon double bond. NMRspectrum of dimethylolated products suggests thatwhen phosphoric acid was added, the hydroxyl pro-ton was no longer coupling with the methylene pro-tons, and most probably 1 mol of water was elimi-nated to give a 6-membered ring compound(Schemes 1 and 2). Polymerization of this ring com-pound occurred subsequently which probably actsas fire retardant.19,21

Concentration, impregnation, retention,and density

The add-on of phosphorus-melamine-urea-formalde-hyde (PMUF) composition to the RPUF samples wasmainly measured from PMUF concentration, impreg-nation time, and retention. Results presented in Fig-ure 2 showed that the retention of RPUF sampleswas ranged from 1.98 to 64.29% when they wereimpregnated with 10–100% PMUF concentration for10–60 min, respectively. After 20 min impregnationthe retention of RPUF samples were ranged from1.98–62.98% with 10–100% PMUF concentrationrespectively, and further up to 60 min impregnationonly 1.09% average increase in the retention wasobserved. This may be possible due to the fact thatthe vacant cell space was fully occupied by PMUFsolution after 20 min of impregnation, and noenough space was left for further impregnation atthe same rate. Thus after 20 min and up to 60 min,no appreciable increase in the retention wasobserved. Therefore, 20 min is considered as the op-timum impregnation time of RPUF samples. Theretention and density of conventional and RPUFsamples impregnated for optimum time are shownin Figure 3. As shown in Figure 3, the densityincreases as the retention of RPUF-PMUF samplesincreases. When retention of RPUF samples increasesfrom 2.49 to 62.98%, the density increases from 50.42to 80.17 kg/m3, respectively. Thus the impregnationof RPUF samples with PMUF solution increases theirdensities up to 62.98%.

Morphology

The cross-sectional surfaces of RPUF and RPUF-PMUF samples were observed at the similar magnifi-

Scheme 1 Methylolation reaction of formaldehyde andurea.

Scheme 2 Dehydration reaction of dimethylol urea.

Figure 2 Effect of PMUF concentration and impregnationtime on retention of RPUF samples (RPUF-PMUF).



cation under SEM. Micrographs of the RPUF andRPUF-PMUF samples are shown in Figure 4(a,b). Asshown in Figure 4(a), RPUF sample has the polyhe-dral and spherical cell structure of 357 (65) lm aver-age cell size. Figure 4(b) shows the deposition ofPMUF on the cell surfaces in addition to the polyhe-dral and spherical cell structure of RPUF. The aver-age cell size of RPUF-PMUF sample was reduced to285 (65) lm. This suggests that the decrease in thecell size may be due to the deposition of PMUF onthe cell walls also. The PMUF deposition on theRPUF skeleton is further confirmed by the fact thatpolyurethane phase is optically transparent.22 ThusPMUF was deposited on the cell surfaces and wallsof impregnated RPUF sample.

Elemental analysis

The elemental analysis results of the RPUF andRPUF-PMUF samples obtained with CHNS analyserand EDX are compiled in Table IV which shows the

presence of various elements. The elements whichare present in the minor quantities are not shown inthe table. Conventional RPUF shows the presence ofCarbon, oxygen, hydrogen, and nitrogen which arepart of the backbone and thus, appear in the higherratio. Chlorine and silicone are present due to themethylene chloride and polysiloxane ether as physi-cal blowing agent and surfactant respectively, intothe RPUF formulation. Minute quantities of bromineand magnesium were also found to be present whichappear due to the impurities in the raw materials.RPUF-PMUF exhibits the presence of phosphorusand increased percentage of nitrogen from 5.05 to12.51 in addition to the elements which are part ofthe RPUF backbone. Thus the introduction of phos-phorus and enhanced percentage of nitrogen showthat RPUF is properly impregnated with PMUF com-position.

Thermal analysis

TGA/DTG/DTA profiles for conventional RPUF andRPUF-PMUF samples in nitrogen and air atmos-phere are shown in Figures 5(a,b) and 6(a,b), respec-tively. The data of the TGA/DTG/DTA investiga-tions are summarized in Table V. DTG curves ofRPUF and RPUF-PMUF decomposition in nitrogenshow only two weight loss stages, while the DTGcurves in air indicate that three main weight lossstages occur during degradation. It suggests thatRPUF and RPUF-PMUF samples degrade throughsame process and degradation mechanism in air ismuch more complex than that in nitrogen.4,23 Innitrogen atmosphere, RPUF sample does not showdistinct weight loss until the temperature rises to2258C, whereas, the degradation of RPUF-PMUFbegins at 1808C. During first stage, the rate of weightloss of RPUF begins to increase gradually to a maxi-mum between 285 and 3108C through two successivedecomposition processes. RPUF-PMUF begins to de-

Figure 3 Effect of PMUF concentration on the retentionand density of RPUF samples (RPUF-PMUF).

Figure 4 Scanning electron micrographs of RPUF samples: (a) conventional RPUF and (b) RPUF-PMUF impregnated.



grade between 180 and 3408C with a maximumweight loss at 2708C in step first. RPUF shows mini-mum weight loss during pyrolysis (second stage) af-ter the sample is heated above 5008C which leads to12% char residue at 9008C. The degradation ofRPUF-PMUF starts between 400 and 5208C at maxi-mum rate of weight loss and leads to 38% char resi-due at 5208C during second step. This char residuereduces to 23% as temperature rises to 9008C. Itshows that RPUF-PMUF produces more quantity ofchar residue than conventional RPUF. The pyrolysislimits the development of volatile compounds whichallows more quantity of black char residue at highertemperature. This black residue may contain carbon,and condensed aromatic contents, because biuret, al-lophanate, urea, urethane, and disubstituted urea arethe thermally weakest links in the polyurethane net-work.4,5 On the basis of the TGA/DTG/DTA traces,it can be seen that decomposition of RPUF-PMUFbegins at lower temperature than conventionalRPUF. This suggests that RPUF and PMUF reactfirst. The lower initial thermal decomposition tem-perature and correspondingly higher amount of sta-ble char yield may indicate that PMUF is an effectivefire retardant composition. Wang et al. reported thatall the phosphorus containing polyurethanes de-grade at low temperatures due to the cleavage of theP��O��C bonds and hydroxyl groups whichresulted into considerable amount of high tempera-

ture stable char residue.24 The stable char residueprovides the barrier effect to the polymer surface byreducing the heat, oxygen flexes and rate of weightloss at higher temperature and consequentlyincreases the flame retardancy of the polymer.11

In air atmosphere, RPUF shows three weight lossstages at the temperature regions 216–337, 350–478,and 480–6508C which are consistent with the previ-ous studies conducted and reported in the litera-ture.3–4,9,25 RPUF-PMUF also degrades through threedistinct weight loss stages, however, DTG analysisindicates that its initial thermal decomposition startat 1608C. RPUF exhibits minimum weight loss infirst and second stages which appears to increasegradually between 480 and 6508C with a maximumat about 5268C during third stage. The third stage,which has 0% char residue at 9008C, appears to bethermo-oxidative in nature since it does not exist innitrogen atmosphere.4,6,23 Thus, maximum weightloss occurs in third stage is due to combustion. Dur-ing combustion oxygen reacts with carbon of charredresidue to converts it into volatile carbon dioxideresulting into no residue was left behind. RPUF-PMUF sample degrades through single process withmaximum rate of weight loss (3.22%/min) at 3018Cvisible on DTG peak in first stage. During secondstage RPUF-PMUF sample degrades exothermicallyat 5218C. Third steps are from the combustion whichleads to 18% high temperature char residues at

TABLE IVElemental Analysis of Conventional and PMUF Impregnated RPUF Samples

RPUF sample codes Carbona (%) Hydrogena (%) Oxygena (%) Nitogena (%) Phosphorusa (%)

RPUF-Cb 55.15 5.67 34.14 5.05 -RPUF-PMUFc 43.74 5.62 21.48 12.51 16.65

a (60.05%).b Conventional RPUF.c Phosphorus-melamine-urea-formaldehyde impregnated RPUF.

Figure 5 Thermograms of RPUF samples in nitrogen atmosphere: (a) conventional RPUF and (b) RPUF-PMUF impreg-nated.



9008C. The increased char residue quantity may bedue to the phosphorus-nitrogen additives. The pres-ence of phosphorus-nitrogen additives into theRPUF-PMUF affects its thermal decomposition inseveral ways such as decreasing initial thermaldecomposition temperature, attaining the maximumdecomposition temperature in the early stage of deg-radation and degradation through single exothermicdecomposition process which leads to the formationof large quantity of char residue stable at high tem-perature. The decomposition of RPUF-PMUF atlower temperature increases the rate of maximumweight loss which resulted into maximum formationand stabilization of char residue. Lowered initialthermal decomposition temperature, maximumdecomposition temperature during first step andlarge quantity of stable char residue in RPUF-PMUFsample is very interesting in terms of fire retardancy.Phosphorus additive reacts with RPUF in the con-densed phase and lowers its initial thermal decom-position temperature. Consequently, due to thedecomposition at lower temperature the large quan-tity of stable char residue is formed in the early

stage of degradation which increases the fire retard-ancy of foam.9

Fire behavior

Fire performance of RPUF and RPUF-PMUF samplesand their comparison is mainly measured fromextent burnt, burning rate, PML, and extinction timeobtained during the fire test. All these parametersare expressed in terms of average values. The varia-tions of extent burnt and burning rate, PML andextinction time of RPUF and RPUF-PMUF sampleswith respect to PMUF concentrations are shown inFigures 7 and 8, respectively. Figure 7 presents theresults as expected with an increasing concentrationof PMUF from 0 to 100%, the extent burnt and burn-ing rate were decreased from 125 to 27 mm and2.23–0.44 mm/s, respectively. However, exponentialdifference was observed between RPUF and RPUFsamples impregnated with minimum (10%) PMUFconcentration in their extent burnt and burningrate which were 125 mm, 2.23 mm/s, and 45 mm,1.07 mm/s respectively. The rates of decrease of

Figure 6 Thermograms of RPUF samples in air atmosphere: (a) conventional RPUF and (b) RPUF-PMUF impregnated.

TABLE VThermal Behavior and Thermal Stability of Conventional and PMUF Impregnated RPUF Samples in

Nitrogen and Air Atmosphere

Sample codes A/N Ton (8C) T1max (8C) R1max (%/min) T2max (8C) R2max (%/min) T3max (8C) R3max (%/min) Yc (%)

RPUFa N2 221 285 8.00 393 2.00 – – 12Air 216 275 4.86 308 3.92 526 6.74 00

RPUF-PMUFb N2 180 270 1.97 520 1.52 – – 23Air 160 301 3.22 521 1.93 693 1.46 18

A/N denotes the air and nitrogen atmosphere.Ton denotes on-set degradation temperature.T1max, T2max and T3max denotes maximum degradation temperatures in first, second and third stage respectively.R1max, R2max and R3max denotes maximum rate of degradation in stage first, second and third stage respectively.Yc denotes the char yield at 9008C.a Conventional RPUF.b Phosphorus-melamine-urea-formaldehyde (PMUF) impregnated RPUF.



extent burnt and burning rate are almost linear withthe increase of PMUF concentration. However, whenPMUF concentration increases over 90% up to 100%,there is an increase of 16.39% in the retention ofRPUF-PMUF, whereas, extent burnt and burningrate are decreased only by 4 mm and 0.05 mm/s,respectively. Thus there is no appreciable decreasein the rate of extent burnt and burning rate, at theincreasing rate of PMUF concentration. As shown inFigure 8, the increasing concentration of PMUF from0 to 100% reduces the PML from 100 to 8.82%, butextinction time increases slightly from 56 to 60 s,respectively. RPUF and RPUF samples impregnatedwith minimum (10%) PUMF concentration showgreat differences in their PML and extinction timewhich are 100%, 56 s and 16.92%, 40 s, respectively.The rate of PML reduction is almost linear to 90%PMUF concentration and above this PML reduces

slightly as the concentration increases to 100%.RPUF samples in flammability test are consumed upto gauge mark (125 mm) in 56 s. For comparison,56 s is considered as extinction time of RPUF. Theextinction time of RPUF sample reduces to 42 s withminimum PMUF concentration, however, increasesto 61 s with maximum concentration. This slightlydifferent flame extinction behavior of RPUF-PMUFsamples can be explained by considering the fire re-tardant action of phosphorus-nitrogen additive. Dur-ing flammability test the presence of phosphorus-nitrogen additive accelerates the decomposition offoam at lower temperature which leads to an increasein the amount of high temperature stable char resi-due.7,9 Flame extinction time is increased slightlywith the increase of PMUF concentration and thecombined effect of these two resulted into the forma-tion of large amount of stable char residue. The stabi-lized char residue acts as protective thermal barrierwhich does not allow further flame spread. This mayleads to reduced burning rate which resulted intoenhanced flame retardancy. Thus depending uponthe decrease 24.8% in extent burnt, 21.9% in burningrate and 89.7% in PML and, 46.59% increase in thedensity of RPUF-PMUF samples, PMUF concentrationof 90% can be considered as optimum.

Oxygen index

Oxygen index (OI) is a fundamental tool in basicresearch on polymer combustion and mechanism offire retardancy. OI also provides an evaluation ofintrinsic flammability of the polymeric materials.26

Thus, flammability of RPUF and RPUF-PMUF sam-ples is also evaluated by determining their oxygenindexes with respect to various PMUF concentra-tions and values are presented in Figure 9. OI of

Figure 7 Effect of PMUF concentration on extent burntand burning rate of conventional and PMUF impregnatedRPUF samples.

Figure 8 Effect of PMUF concentration on PML andextinction time of conventional and PMUF impregnatedRPUF samples.

Figure 9 Effect of PMUF concentration on oxygen index(OI) of conventional and PMUF impregnated RPUF samples.



RPUF is 17.8 which increase from 19.9 to 24.1 as thePMUF concentrations are increased from 10 to 100%,respectively. An OI of RPUF-PMUF is probablyincreased due to the presence of phosphorus andnitrogen into the PMUF composition. Phosphorusand nitrogen have synergistic effect between themwhich enhances the OI of RPUF-PMUF (Wang,2005). The trend of these data shows that OIincreases with increasing concentrations of PMUF.

Smoke density

Figures 10 and 11 demonstrate the smoke evolutionfrom the RPUF and RPUF-PMUF under flaming andnonflaming mode, respectively. Smoke density andevolution pattern are expressed in terms of specificoptical density (Ds) and percent transmittance (pT)from RPUF and RPUF-PMUF under flaming andnonflaming mode. As shown in Figure 10, the pT ofRPUF decreases from 287 to 157 and Ds increasesfrom 0 to 34.82 after 8 min of exposure. After 8 min,pT again increases to 212 and Ds decreases to 17.34at the end of exposure. These results indicate thatRPUF generates maximum smoke after 8 min expo-sure and the quantity of smoke decreases slightlytowards the end of exposure. Under similar expo-sure conditions, the pT of RPUF-PMUF decreasesfrom 274 to 170 and Ds increases from 0 to 28.28 af-ter complete 20 min combustion. The pT and Ds val-ues exhibit that RPUF-PMUF generates maximumsmoke towards the end of exposure. The aboveresults show that maximum specific optical density(Dm) of RPUF and RPUF-PMUF are 34.82 and 28.28,respectively, as maximum value of Ds is referred asDm. The values of Dm indicate that RPUF-PMUF gen-erates less smoke than that of RPUF. As shown inthe Figure 11, during RPUF smouldering under non-flaming mode the pT decreases and Ds increases to

5.98 after 7 min which remains stable up to 12 min.A little increase in pT and decrease in Ds from 5.98to 5.56 are observed towards the end. The trend ofthese data shows that RPUF generates maximumsmoke between 7 and 12 min which reduces slightlyin the end. The RPUF-PMUF exhibits that pTdecreases and Ds increases up to 11 min and remainstable up to 17 min. A very little decrease in pT andincrease in Ds are observed at the end of smoulder-ing. Thus RPUF-PMUF generates maximum smokebetween 11 and 17 min which further increasesslightly toward the end. The Dm values of RPUF andRPUF-PMUF shown are 5.98 and 5.42, respectively.These results indicate that RPUF generates rapid andmore smoke in the beginning whereas, RPUF-PMUFgenerates slow and less smoke toward the end ofsmouldering.

It is quite clear that RPUF and RPUF-PMUF gener-ate more smoke under flaming than nonflamingmode. However, RPUF-PMUF generates less anddelayed smoke than RPUF under both flaming andnonflaming mode. The generation of less anddelayed smoke from RPUF-PMUF may be due to thesynergistic effect between phosphorus and nitrogen.7

The suppression of smoke in RPUF by the synergis-tic effect between phosphorus-nitrogen is furtherconfirmed by the earlier research work whichdescribes that the presence of phosphorus alone inburning foam increases the production of smoke.27

CONCLUSIONS

To understand the effect of PMUF composition onthe structure, thermal stability, flammability charac-teristics and smoke density of RPUF, large numberof RPUF samples impregnated with various concen-trations of PMUF were investigated using SEM,CHNS analyzer and energy dispersive X-rays (EDX),

Figure 10 Smoke density of conventional and PMUFimpregnated RPUF samples under flaming mode.

Figure 11 Smoke density of conventional and PMUFimpregnated RPUF samples under nonflaming mode.



thermogravimeric analysis (TGA), derivative ther-mogravimetric (DTG), differential thermal analysis(DTA), horizontal flammability as per BS: 4735, oxy-gen index as per ASTM D2863 and smoke densityaccording to ASTM E662. Flame retardant chemicalcomposition was prepared from ortho-phosphoricacid, melamine, urea and formaldehyde solution(PMUF). PMUF was a colorless viscous liquid misci-ble with water. PMUF concentrations were decreasedwith the increasing water content. Retention anddensity of RPUF-PMUF samples were increased withthe increased concentration of PMUF for optimumimpregnation time of 20 min. Retention and densityof RPUF samples with optimum impregnation timewere increased up to 62.98% and 80.17 kg/m3

respectively. The results of morphology showed thatthe cell size of impregnated RPUF samples wasdecreased from 357 to 285 lm. The cell size wasdecreased by the increased thickness of cell wallwhich may be due to the deposition of PMUF on thecell walls and surfaces. Elemental analysis of RPUF-PMUF shows the introduction of phosphorus andnitrogen into the RPUF backbone. Thermal behaviorshows that RPUF and RPUF-PMUF samples weredegraded through two and three stage process undernitrogen and air atmosphere respectively. Thirdstage is attributed to combustion under air. Thus,degradation process is more complex in air thannitrogen. RPUF-PMUF samples are decomposedcomparatively at lower temperature than RPUFwhich is resulted into large amount of high tempera-ture stable char residue. This char residue acts asinsulating blanket and renders RPUF-PMUF fire re-tardant. The results of extent burnt, burning rate,PML and oxygen index under flammability testsindicate that an optimum 90% PMUF concentrationis adequate to render RPUF fire retardant. RPUFsamples containing 46.59% PMUF retention exhibit46.6% increase in the density, however, remarkabledecrease in extent burnt (66.25%), burning rate(90.19%) and PML (91.18%) were observed underflammability test. Oxygen index was also increasedfrom 17.8 to 23.8. Dm results obtained during smokedensity tests show that RPUF-PMUF generates lessand delayed smoke than conventional RPUF underflaming and nonflaming mode. However, the genera-tion of smoke is more under flaming than nonflam-ing mode. Thus, it can be concluded that PMUFimpregnation introduces the phosphorus-nitrogenadditives into the RPUF structure which is con-firmed by the enhanced thermal stability, fire retard-

ancy and reduced level of smoke generation underflaming and nonflaming mode.

The authors are grateful to the Director, Central BuildingResearch Institute for his encouragement and kind sup-port. They also acknowledge the services rendered by ShriRakesh Kumar.

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Effect of phosphorus-nitrogen additives on fire retardancy of rigid polyurethane foams

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