Flame retardancy of polyisocyanurate–polyurethane foams: use ofdifferent charring agents

M. Modesti*, A. Lorenzetti

Department of Chemical Process Engineering, Padova University, v. Marzolo 9, 35131 Padova, Italy

Received 5 March 2002; received in revised form 21 May 2002; accepted 25 May 2002

Abstract

The influence of different charring agents on physical-mechanical properties and fire behaviour of polyisocyanurate–polyurethane

(PIR–PUR) foams has been investigated. In particular the use of varying amounts of ammonium polyphosphate, melamine cya-nurate and expandable graphite has been analysed; all, when involved in fire, lead to the formation of a char layer on the polymersurface, but their ways of flame retardancy are different. The results obtained show that the higher the filler content the lower thecompression strength; in particular the worst results have been obtained in the presence of melamine cyanurate. Moreover, the

presence of ammonium polyphosphate or melamine cyanurate causes any significant worsening on thermal conductivity, whileexpandable graphite leads to a quite marked increase of the thermal conductivity. The fire behaviour has been studied by means ofcone calorimeter apparatus and oxygen index test; it has been observed that the best results, i.e. the lowest rate of heat release and

the highest oxygen index, are achieved with expandable graphite. Also the ammonium polyphosphate brings slight improvement infire behaviour, whereas the effect of melamine cyanurate is negligible.# 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Polyisocyanurate–polyurethane foams; Ammonium polyphosphate; Melamine cyanurate; Expandable graphite; Char

1. Introduction

The increasing awareness of public opinion towardthe problem of fire safety of materials has led to theapproval of new regulations [1] where toxicity and den-sity of the smokes are very important factors thatshould be considered in evaluating fire safety. There-fore, there is today a need to find halogen-free flameretardants, as effective as the phospho-halogen ones.Such compounds, in fact, allow considerable improve-ment in the fire behaviour of the foams, but, on theother hand, cause the development of very dense andtoxic smokes [2].Therefore, in this work we have studied the influence

of different halogen-free flame retardants on fire beha-viour of PIR–PUR foams blown with n-pentane. Ourattention has been put on charring agents: in particularwe study the effect on fire behaviour of PIR–PURfoams due to the presence of various amounts ofammonium polyphosphate, melamine cyanurate and

expandable graphite. All those compounds lead to theformation of a superficial char layer that prevents fur-ther decomposition, but they act in three differentways:

� Ammonium polyphosphate (APP) leads to theformation of a char layer through the linking ofphosphates to the ester group; the latter arereadily eliminated forming conjugated doublebonds, which finally cyclize to give char [3];

� Melamine cyanurate (MC) acts through endo-thermic decomposition that leads to evolution ofammonia and formation of condensation poly-mers such as melam, melem, melom, which con-stitute the superficial char layer [4]. Two schemesof condensation have been proposed [5]: the firststates that condensation leads to the fused-ringstructure of cyameluric triamide which reacts asa trifunctional monomer to give the final con-densate; the second states that the melamine unitis the trifunctional monomer which progressivelycondenses to give a product in which triazinerings are linked by –NH– bridges.

0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(02 )00184-2

Polymer Degradation and Stability 78 (2002) 341–347

www.elsevier.com/locate/polydegstab

* Corresponding author. Fax: +39-049-8275555.

E-mail address: michele.modesti@unipd.it (M. Modesti).

� Expandable graphite (EG) leads to the formationof a char layer characterized by the presence of‘‘worms’’, deriving from its expansion. Accord-ing to some authors [6], the expansion of EG isdue to a redox process between H2SO4, inter-calated between graphite layers, and the graphiteitself that originates the blowing gases accordingto the reaction:

C þ 2H2SO4 ! CO2 þ 2 H2O þ 2 SO2

Therefore, the aim of this work is to analyse the effec-tiveness of such char layers, i.e. char formed by differentinteractions between filler and polymer.Moreover, the influence of the presence of flame

retardants on the physical-mechanical properties of thefoams has also been analysed.

2. Experimental

2.1. Raw materials

The raw materials used were:

� Polymeric MDI (methane diphenyl diisocya-nate): Tedimon 385 (Enichem): NCO%=30.5;average functionality=2.8.

� Polyester polyols: Glendion 9801 (Enichem):n�OH=351 mgKOH/g, viscosity to 25 �C=5600mPa.s; Kitane (Enichem) n� OH=435 mgKOH/g, viscosity to 25 �C=1100 mPas.

� Catalyst: pentamethyldiethylentriamine(PMDETA) (Abbot) and potassium octoate(K15) (Air Products).

� Surface-active agent: polysiloxane-polyethercopolymer Tegostab B8469 (Goldschmidt-Italy).

� Blowing agent: n-pentane technical grade (CarloErba).

� Flame retardant: expandable graphite: GrafGuard 160–80 N, medium particle size 150 mm(Ucar Graph. Tech); melamine cyanurate: Budit314, N=48%, particle size=5–10 mm (Buden-heim R. A. Oetker); ammonium polyphosphate:FR Cross 484, P2O5=72%; N=14%, mediumparticle size =10 mm (Budenheim R. A. Oetker).

2.2. Foam preparation

The foams formulations are reported in Table 1. Wehave prepared PIR–PUR foams, blown with n-pentane,characterised by a constant NCO index (250) and filledwith several amounts (0, 15, 25 wt.%) of APP, MC orEG. The amount of pentane was calculated in order toobtain foams with a constant density (35�4 kg/m3).

The foams were prepared by hand mixing technique,that is the isocyanate was added to the formulatedpolyol (i.e. the mixture of polyols, catalysts, surfactant,filler, blowing agent); then the mixture mixed for 15 sand poured into an aluminium cup. During the expan-sion several kinetic parameters (cream time, gel timeand tack free time) were recorded. After the preparationthe foams were put in a oven at 70 �C for 24 h, in orderto complete the polymerisation reaction, before carryingout physical-mechanical and fire behaviour character-isation.

2.3. Test methods

For all the foams produced, we analysed both physi-cal-mechanical properties and fire behaviour. The phy-sical and mechanical properties were measured usingstandard test methods. In particular, the apparent den-sity was measured according to ISO 845, the compres-sion strength according to ISO 844 and the thermalconductivity according to ISO 8301. Fire behaviour wasinvestigated by means of DIN 4102-B2 and oxygenindex test according to ISO 3216. Moreover, the firebehaviour was analysed by using a cone calorimeter.This apparatus allows measurement of a wide variety ofparameters, rate of heat release (RHR), effective heat ofcombustion (EHC), mass loss, smoke extinction area(SEA), development of CO and CO2. Among them, wetake into consideration the RHR and CO/CO2 weightratio, which are the more important parameters in orderto understand the fire performance of a material: in fact,RHR peak value is believed by many fire scientists asthe responsible for the ‘‘flashover’’ phenomena in a realfire situation [7] while CO/CO2 weight ratio, being anindex of combustion completeness, can be considered asan index of smoke toxicity.At the moment, standard regulation exists only for

RHR (ISO 5660), while for the other parameters themeasurements are not standardized. In order to over-come problem of poor repeatability of data, five speci-mens for each sample were submitted to each kind oftest. Therefore in the graphs representing experimentalresults the error bars are also reported.

3. Results and discussion

3.1. Physical and mechanical properties

The data for foam characterisation are reported inTable 1. The results show that, for foams filled with MCor EG, the higher the filler content the lower the com-pression strength, as it is expected. This could be due, incase of MC, to the fact that the melamine cyanurateleads to an increase on polymer friability, as alreadyobserved in previous work [8], and therefore brings a

342 M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347

worsening of compression strength. In the case of EG,although the effect is less significant than in case of MC,the worsening could be due to the fact that the fillerdoes not locate in the struts but between the cell walls(Fig. 1) because of its great particle dimensions (150mm). This, causing an inhomogeneous cellular structure,could be responsible for the lower values of compres-sion strength. The thermal conductivity results showthat the presence of EG, causing an increase on themean cell size [9], leads to an increase of thermal con-ductivity. Moreover, the presence of either APP or MCdoes not significantly affect the thermal conductivity: infact, as both APP and MC are characterized by meanparticle size (ffi 10 mm) smaller than the mean cell size,these fillers locate on the struts (Figs. 2 and 3) thusavoiding increase of the mean cell size.

Table 1

Foams formulations, kinetic parameters and physical-mechanical properties

Formulation Ref. AP15 AP25 M15 M25 EG15 EG25

Glendion 9801 70 70 70 70 70 70 70

Kitane 30 30 30 30 30 30 30

PMDETA 0.2 0.2 0.3 0.2 0.3 0.2 0.2

K15 1.8 2.2 2.4 2.2 2.4 2.5 2.2

Tegostab B 8469 2.5 2.5 2.5 2.5 2.5 2.5 2.5

H2O 1.5 1.5 1.5 1.5 1.5 1.5 1.5

n-Pentane 10 28 35 28 35 14.5 24

FR Cross 484 0 74.9 143.9 0 0 0 0

Budit 314 0 0 0 74.9 143.9 0 0

GRAF Guard 160–80N 0 0 0 0 0 72.5 140.1

Tedimon 385 290 290 290 290 290 290 290

Cream time (s) 38 32 34 35 23 24 30

Gel time (s) 75 120 105 110 88 47 75

Tack free time (s) 94 155 125 130 102 54 100

Density (kg/m3) 35 35 35 32 32 39 38

Thermal conductivity (mW/mK) 26.2 26.4 26.6 27.0 26.9 28.6 30.6

Parallel comp. strength (kPa) 210 177 184 146 126 186 164

Perpend. comp. strength (kPa) 90 131 107 68 78 66 48

Fig. 1. Particle of expandable graphite (marked by black circle)

between cell walls.

Fig. 2. Particle of ammonium polyphosphate (marked by black circle)

in cell struts.Fig. 3. Particle of melamine cyanurate (marked by black circle) in cell

struts.

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3.2. Fire behaviour

The fire reaction of filled PIR–PUR foams has beenanalysed by use of DIN 4102-B2 and oxygen index tests.The results of the DIN 4102-B2 test are reported inTable 2. Only the foams filled with EG, containing atleast 15 wt.% of filler, can be classified as B2 materials.This is a very successful result as pentane blown foamscan very seldom be rated as B2 materials; in fact, thefoams filled with APP and MC, even if containing veryhigh flame retardant amount (25 wt.%), can not berated as B2.

Fig. 4. Oxygen index results.

Table 2

DIN 4102-B2 test results

Sample Ignited on

Corner Surface

Ref. Not B2 Not B2

AP15 Not B2 Not B2

AP25 Not B2 Not B2

M15 Not B2 Not B2

M25 Not B2 Not B2

EG15 B2 B2

EG25 B2 B2

Fig. 5. Maximum RHR values as a function of the filler amount.

344 M. Modesti, A. Lorenzetti / Polymer Degradation and Stability 78 (2002) 341–347

The oxygen index (OI) test (Fig. 4) showed that theOI increases with increasing filler content. In particular,while the presence of MC does not significantly changethe OI, the presence of APP or EG leads to an increaseof about 25 and 35% respectively, using 25 wt.% offiller.The most important result from the RHR (Figs. 5 and

6) is the considerable decrease that is achieved in pre-sence of 25 wt.% of EG: the maximum value of RHRlowers by about 60% and the mean value by about80%; the results are also satisfactory in the presence of15 wt.% of EG. Moreover, it has been observed that

both APP and MC are more effective at lower amountas the maximum RHR value is lower with 15 wt.% thanwith 25 wt.% of filler. It seems therefore that the flameretardancy of the foams does not increase continuouslywith the filler content but rather shows an optimum, asalready observed by Piechota [10] for APP. No sig-nificant influence of either APP or MC on the meanRHR value is observed.The CO/CO2 weight ratio results are reported in

Fig. 7. In the presence of EG the values of the ratio arefairly high, while in the presence of APP or MC theratio becomes lower.

Fig. 6. Mean RHR values as a function of the filler amount.

Fig. 7. CO/CO2 average values as a function of the filler amount.

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Besides fire behaviour characterisation, in order tofurther test the effectiveness of the char layer formed bythe different charring agent, we analysed foams after theoxygen index test by means of SEM. In particular asection at 1.5 mm below the superficial char layer hasbeen taken into account. As can be seen in Fig. 8, thechar formed by expansion of EG is very effective inpreventing decomposition of the material: beside thepresence of worm like structure deriving from expansionof EG (that begins at 160 �C), the polymer below thechar layer is almost not decomposed. Otherwise, belowthe char formed by APP or MC the polymer is softened:the ‘‘drops’’ are clear evidence of polymer degradation(Figs. 9 and 10). This means that in the presence of EGthe temperature reached underneath the char is almostlower than in presence of APP or MC, i.e. the charformed by EG is much more insulating and/or compactand therefore limits the heat and/or oxygen transfer tothe polymer; therefore the effectiveness of the EG charlayer is greater than that of APP or MC.As the EG allows a good fire protection of the polymer,

it is clear that the results obtained for EG filled foams arebetter, that is the material can be classified as B2, the OI isthe highest, the RHR, both maximum and mean value,

are the lowest. Otherwise, the fire performances of APPor MC filled foams are less satisfactory, particularly forMC, as the char layers formed are less effective in pre-venting further decomposition of the polymer. In pre-sence of MC, the only positive effect is the lowering ofCO/CO2 weight ratio, that is probably due to a decreasein flame temperature due to the endothermic reactionsaccompanying the decomposition of MC, which favoursthe left-hand side of the equilibrium reaction:

CO2 þ C *) 2 CO �H ¼ þ172 kJ=mol

i.e. favouring the CO2 development.

4. Conclusions

The results obtained have clearly shown that theeffectiveness of char layers formed by different charringagents is not the same. The best results, that is the bestfire performances have been obtained in the presence of25 wt.% of EG, although the fire behaviour is alsosatisfactory in the presence of 15 wt.% of EG. The onlyundesired effect observed is a slight increase on theincomplete combustion in the presence of a highamount (25 wt.%) of EG. The fire performances of APPor MC filled foams are, in general, worst, except for theCO/CO2 weight ratio.The physical-mechanical characterisation showed that

the EG affects the physical-mechanical properties, andparticularly the thermal conductivity, much more thanAPP or MC. Otherwise, the effect is more marked athigher filler content (25 wt.%). However the foamsproduced show suitable physical-mechanical properties.

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Fig. 8. Polymer below the char layer formed by EG.

Fig. 9. Polymer below the char layer formed by APP.

Fig. 10. Polymer below the char layer formed by MC.

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