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Page 1: Intumescent flame retardant systems of modified rheology

Intumescent flame retardant systems of modified rheology

P. Annaa, Gy. Marosia,*, S. Bourbigotb, M. Le Brasc, R. Delobelc

aOrganic Chemical Technology Department, Budapest University of Technology and Economics, H-1111 Muegyetem rkp.3, Budapest, HungarybEcole Nationale Superieure des Arts et Industries Textiles, 9 rue de l’Ermitage, BP 30329, F-59056 Roubaix Cedex 01, France

cEcole Nationale Superieure de Chimie Lille, USTL, BP 108, F-59658-4-2 Villeneuve d’Ascq Cedex, France

Received 12 August 2001; accepted 5 November 2001

Abstract

A model system of intumescent flame retardants, consisting of ammonium polyphosphate and pentaerythritol was prepared andinvestigated in polypropylene and without the polymer matrix. Thermal scanning oscillation rheometric investigation in the temperaturerange of 170–500 �C was used to detect the rheological behaviour in the region of melting of the polymer and the plasticity of the

char formed at higher temperature. Addition of boroxo siloxane to the model system caused advantageous changes in both regions.Increased complex viscosity and viscoelsasticity of the melt and char respectively contributes to better flame retardancy. Accordingto DTG and FTIR studies the reactions of boroxo siloxane with pentaerythritol and ammonium polyphosphate are the reasons forthe rheological changes. # 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Polyolefins; Intumescence; Flame retardants; Boroxo siloxane; Thermal scanning; Oscillation rheometry

1. Introduction

A number of synergistic additives have been proposedrecently for improving the efficiency of intumescentflame retardant (IFR) systems but the rheologicalbackground of their effect was rarely studied [1–4]. Theboroxo siloxane elastomer (BSil) is also an effectivesynergist if combined with ammonium polyphosphate(APP) and pentaerythritol (PER) in polyolefins asdemonstrated earlier by LOI and cone calorimetricinvestigations [5–7]. Thermal scanning oscillation rheo-metric (OR) measurements, performed in a broad rangeof temperature, showed increased complex viscosity ofthe polymer melt, and improved plasticity of intumescentchar due to introduction of boroxo siloxane elastomer [8].The increased melt viscosity reduced the dripping, whilethe improved plasticity of the intumescent charincreased the barrier properties, and these resulted inimproved flame retardancy. The rheological details andchemical background of these phenomena are not yetfully understood.Increases of complex viscosity in OR studies of different

polymers were explained in different ways. The increaseof complex viscosity in N-isopropylacrylamide and

N,N0-methylenebisacryalmide copolymers could beexplained by formation and interaction of thermallysensitive micro-gel structure-blocks [9]. In the case ofpolyphenyleneoxide/liquid crystalline polymer blends,the increase of the apparent viscosity arose from thesemi-interpenetrating polymer network, formed from theblend components and a special type of liquid crystallinecompatibilizer agent in the interface of blend components[10].The behaviour of flame retardant compounds consisting

of zinc borate, metal hydroxide and ethylene vinyl acetatecopolymer were also characterised by the OR techniqueover a broad temperature range (300–400 �C) [11].According to these results zinc borate promotes theformation of a new material, having an elastic andsimultaneously plastic expanded foamed structure,which is an efficient flame retardant protective layer.In this work we tried to find analogies between the

explanations found in the literature and the effect of theBSil synergist.

2. Experimental

The materials used in the experiments are as follows:polypropylene (PP), Eltex P HV219 (Solvay); ammoniumpolyphosphate (APP), Exolit AP 422 (Clariant);

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

PI I : S0141-3910(02 )00040-X

Polymer Degradation and Stability 77 (2002) 243–247

www.elsevier.com/locate/polydegstab

* Corresponding author. Fax: +36-1-463-3648.

E-mail address: [email protected] (G. Marosi).

Page 2: Intumescent flame retardant systems of modified rheology

pentaerythritol (PER) R. P. grade (Aldrich) and boroxosiloxane elastomer (BSil) prepared in the laboratory asdescribed elsewhere (5) or in situ using reactive siliconeoligomer (Sil), Tegomer 2111 (Th. Goldsmit AG), andboron compound (B) e.g. boric acid. The IFR–PPcompounds contained 30% IFR additives. The ratio ofAPP/PER was 3/1, the 2% BSil elastomer was loaded atthe expense of the other IFR additives. The compoundswere prepared by homogenising the components in themixing chamber 350 of a Brabender Plasti-CorderPL2000 for 10 min at 200 �C. The rotor speed was 50 rpm.Sheets (100�100�3 mm) were prepared by compressionmoulding using a Dragon press at 190 �C and a pressureof 3 MPa. In the model experiments without the polymermatrix the IFR additives and BSil elastomer were usedin the same mass ratio as in the IFR PP compounds,and were homogenised in a mortar before analysis. Theirheat treatment was performed in thermal scanning ORequipment at 200 �C for 10 min. Thermogravimetry(TG) was carried out using Setaram Labsys equipmentwith the following parameters: sample weight: 10 mg,heating rate: 7.5 �C/min, flowing air atmosphere. ARheometric Scientific ARES-20A thermal scanning oscil-lation rheometer was used for rheological measurements inthe parallel plate configuration. Samples of 25�25�3mm size were positioned between the plates, with astarting gap of 1 mm. The normal force applied duringthe measurements was 0.98 N with frequency 10 rad/s.The initial static force was 30 g for compounds and 5 gfor powder like models. The heating rate was 7.5 �C/min.The FTIR measurements were performed using Impact400D type equipment (Nicolet Instrument Corp.).

3. Results and discussion

Model experiments on the intumescent additive systemwere performed to give a chemical explanation for thecharacteristic behaviour of the melt and char of IFR–PP.The model measurements were performed in TG andOR equipment. The OR technique was used not onlyfor rheological characterisation of the tested substratebut also for indication of reactions occurring betweenthe components. The compositions of compoundsformed from the IFR additives at different stages ofmodelling processes were analysed by FTIR.As the IFR compounds can be considered as viscoelastic

materials both in the lower and higher temperaturerange, characterisation can be given by separating theelastic and viscous elements, or by giving the ratio ofthis elements. The complex viscosity is defined as

�� ¼ �0 þ i�00 ¼ G�=i!:

proportional to the complex Young modulus (G*),which is the ratio of complex stress to complex strain:

G� ¼ ��=��;with G� ¼ G0 þ iG00; G0 ¼ ðG�Þ cos�

and G00 ¼ ðG�Þsin�:

G0 is the storage modulus, and is a measure for thereversibly stored energy (this is the elastic share of thecomplex Young modulus), and the G00 is the loss mod-ulus, representing the irreversibly stored energy (this isthe viscous share of the complex Young modulus). Theratio of G00 and G0 gives the measure of damping calledthe dissipation factor or loss tangent

tan� ¼ G00=G0:

The values of �*,G0,G00 and tan � are directly measuredor calculated respectively using thermal scanning OR.In the discussion the term complex viscosity will beuniformly used for all tested substrates regardless of theconsistency (for more details see [12]).The tan � values of IFR–PP compounds without and

with BSil, are shown in Fig. 1. The curves show a char-acteristic run, and can be divided into three zones. Thefirst zone below 300 �C can be designated as the meltzone, the second between 300–420 �C as the reactionzone and the third, above 420 �C as the charring zone.The BSil-containing IFR–PP compound has lower tan �values than the compound without BSil in the wholerange of the first zone. Thus, in the presence of BSilchemical and/or physical processes take place, duringthe compounding, that result in decreased tan d (due toincreased G0). This manifested in earlier publishedobservations of increased complex viscosity [7,8]. In thereaction zone, due to intensive degradation reactionsand foaming of solid and liquid phases, the variation oftan � is too complex to interpret exactly. Above 420 �C,after consolidation of the foam, a rheologically stable

Fig. 1. The tan � of IFR–PP compounds, prepared with and withoutBSil, versus temperature as obtained by OR measurement.

244 P. Anna et al. / Polymer Degradation and Stability 77 (2002) 243–247

Page 3: Intumescent flame retardant systems of modified rheology

period can be observed, and interestingly the tan � of theBSil-containing IFR-PP compound appears to be higherthan without BSil. Consequently the BSil-containingcharred foam keeps its viscoelsastic character, which isfavourable for flame retardancy. This rheological behaviourmay be again a result of a chemical interaction betweenBSil and IFR additives.The interaction in the melting and charring zones

could be confirmed by TG measurements. The DTGcurves of IFR–PP compounds with and without BSil aregiven in Fig. 2. In the presence of BSil the degradationslows down slightly in the melting zone (170–250 �C)and in the charring zone (above 370 �C) (ringed inFig. 2). The influence of BSil is more pronounced inmeasurements carried out without the polymer matrixas shown in Fig. 3. Comparing Figs. 2 and 3 one canconclude that the same interactions appeared in bothcases but more clearly without the polymer matrix.Based on TG results better understanding of the

interactions of IFR additives could be expected fromthermal scanning ORmeasurements without the polymer.Fig. 4 shows the ‘complex viscosity’ values of the abovevariations of additives. The IFR compound without BSil

shows only slightly viscous behaviour in the temperaturerange below 300 �C, which may be the result of wettingof APP by the melted low molecular weight PER. Thusno formation of fluid macromolecular system can beobserved. Above 310 �C a rapid increase of viscositycan be seen, as a result of formation of carbonaceouspoly-phosphoric acid foam. In contrast with the previouslydiscussed behaviour, the IFR additives in the presence ofBSil already have a high viscosity at the very beginning(at 170 �C) of the measurement, and it decreases withincreasing temperature. This rheological behaviourindicates the formation of a viscous fluid from thecomponents as a consequence of reaction between theBSil and IFR additives. The formation of viscous reactionproduct in the charring zone starts at higher temperature(at 400 �C), than without BSil. The shift of the charringzone to higher temperature may again be a consequenceof the reaction between the components.Based on the previous results the question arises:

which component of IFR system is the dominant reactionpartner of BSil during the heat treatment process?Fig. 5 shows the complex viscosity values of the

separate component pairs APP/BSil and PER/BSil. This

Fig. 2. Influence of BSil synergist on the decomposition rate of IFR–

PP compound containing APP/PER flame retardant additives.

Fig. 3. Influence of BSil synergist on the rate of degradation of flame

retardant additive mixture APP/PER.

Fig. 4. Influence of BSil synergist on the rheological behaviour of

APP/PER flame retardant additive mixture.

Fig. 5. Influence of BSil synergist on the rheological behaviour of

APP and PER flame retardant additives.

P. Anna et al. / Polymer Degradation and Stability 77 (2002) 243–247 245

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figure helps to determine which pair of componentsproduces the strong increase of viscosity in the melt zone ofthe IFR–PP compound. The APP/BSil component pair hasrelatively high complex viscosity in the temperature rangeof 170–300 �C, but does not show any variation as thetemperature increases. It behaves like a cohered powder.Above this temperature the increasing value, due to thestart of formation of carbonaceous poly-phosphoricacid, turns to a decreasing slope at 350 �C, which can beattributed to the reaction of APP and BSil resulting in amore deformable material structure.The PER/BSil component pair has an extremely high

complex viscosity at the starting temperature of the test(170 �C), which decreases characteristically in two steps asthe temperature increases. The high viscosity and strongdependence of viscosity on temperature are typical ofmacromolecular fluid systems, while the step-wisetemperature dependence of viscosity suggests crosslinkingof the macromolecules. At 390 �C BSil starts to transformto an inorganic material.The thermal scanning OR measurements confirmed

the probability of the following earlier assumptions:

the increased melt viscosity of BSil synergist-containing IFR–PP compounds can be attrib-uted to a crosslinked macromolecular structureformed from PER/BSil components, and

the plastic char formed in the charring zone isthe result of the reaction between APP and BSilcomponents.

The occurrence of the PER–BSil reaction could bedemonstrated by comparing the FTIR spectra of

mixture of the components with their reactionproduct at 200 �C. This result is published elsewhere[13].To confirm the APP and BSil reaction, the FTIR

spectra of APP and BSil mixtures prepared at ambienttemperature and heat treated for 10 min at 420 �Crespectively were compared. The spectra of mixtureprepared at ambient temperature can be seen in Fig. 6.The most characteristic peaks, the nNH4+ valencyvibration at 3300 cm1, the dNH4+ deformation vibrationat 1480 cm1 and the valency vibration of PO4

3+ at1100 cm1, show the presence of ammonium polypho-sphate, while the d Si (CH3)2 deformation vibration at1250 cm1 and at 800 and 760 cm1 show the presenceof BSil. The FTIR of the same material after heattreatment at 420 �C can be seen in Fig. 7. This spectrumis dominated by the vibrations characteristic of B–O, Si–Oand P–O. The remaining dimethyl siloxane appears as ashoulder between 700 and 800 cm1. The formation oftraces of aryl-oxy-silane can be estimated on the basis ofa slightly deformed peak at 940 cm1, as n SiO(Ar)vibration, which coincides with the n vibrations ofSi(–O–)4 groups. These spectral data are characteristicof low melting glasses like phosphor-borosilicate com-pounds also containing some alkyl siloxane and aryl oxysilane segments.The FTIR analysis of model systems made the char-

acteristic changes in rheological and thermal analyticalinvestigations more clear, confirming the reactionbetween PER and BSil at lower temperature andbetween APP and BSil at higher temperature, but fur-ther investigations are needed for optimisation of theestimated positive changes.

Fig. 6. FTIR spectra of BSil–APP mixtures prepared at ambient temperature.

246 P. Anna et al. / Polymer Degradation and Stability 77 (2002) 243–247

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4. Conclusion

The boroxo silane elastomer is an effective synergist withammonium polyphosphate and polyol-containing intumes-cent flame retardant systems and advantageously usablein polyolefins. Thermal scanning oscillation rheometricmeasurements suggested the formation of a crosslinkedPER–BSil macromolecular system and improved plasticityof the intumescent carbonaceous foam formed at hightemperature due to APP–BSil interaction. The consequ-ences of the interactions appeared in the rate of weight lossin the low and high temperature range determined byDTGmeasurements. Occurrence of chemical reactions betweenthe flame retardant additives and BSi elastomer has beendemonstrated by FTIR measurements on model systems.

Acknowledgements

This work was financially supported by the Commissionof the European Communities through contract No.G5RD-CT-1999–00120 and by the Hungarian ResearchFund through projects OTKA T32941 and T026182.

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P. Anna et al. / Polymer Degradation and Stability 77 (2002) 243–247 247