The flame retardant mechanism of polyolefins modified with chalk and silicone elastomer

FIRE AND MATERIALSFire Mater. 2003; 27:51–70 (DOI: 10.1002/fam.817)

The flame retardant mechanism of polyolefins modified withchalk and silicone elastomer

Anna Hermansson1,y, Thomas Hjertberg1,n and Bernt- (AAke Sultan2

1Department of Materials and Surface Chemistry, Chalmers University of Technology, SE-412 96 G .ooteborg, Sweden2Borealis AB, SE-444 86 Stenungsund, Sweden

SUMMARY

This paper presents the current understanding of the flame retardant mechanism of CasicoTM. The studyincludes the flame retardant effect of each individual component: ethylene–acrylate copolymer, chalk andsilicone elastomer, as well as the formation of an intumescent structure during heating. The flame retardantproperties were investigated by cone calorimetry and oxygen index tests. To obtain insight into the flameretardant mechanism, heat treatment under different conditions has also been performed. The resultsindicate that the flame retardant mechanism of Casico is complex and is related to a number of reactions,e.g. ester pyrolysis of acrylate groups, formation of carbon dioxide by reaction between carboxylic acid andchalk, ionomer formation and formation of an intumescent structure stabilized by a protecting char.Special emphasis is given to the formation of the intumescent structure and its molecular structure asevaluated from 13C MAS-NMR and 29Si MAS-NMR, ESCA and XRD analysis. After treatment at 5008Cthe intumescent structure consists mainly of silicon oxides and calcium carbonate and after treatment at10008C the intumescent structure consists of calcium silicate, calcium oxide and calcium hydroxide.Copyright # 2003 John Wiley & Sons, Ltd.

KEY WORDS: flame retardant; thermal degradation; NMR; ESCA; XRD; effervesce; intumescence;LDPE; EBA; calcium carbonate; silicone elastomer; silicon oxide; calcium silicate; larnite

1. INTRODUCTION

As a result of the concern with environmental and safety issues, a great deal of focus has latelybeen placed on flame retardants and in particular those based on halogens. For instance, somebrominated flame retardants have already been proposed for a ban while the use of others isstrictly regulated [1,2]. This has led to accelerating research in order to obtain safer andenvironmentally friendly halogen-free flame retardants and halogen-free flame retardant

Received November 2001Accepted 2 December 2002Copyright # 2003 John Wiley & Sons, Ltd.

nCorrespondence to: T. Hjertberg, Department of Materials and Surface Chemistry, Chalmers University ofTechnology, SE 412-96 Goteborg, Sweden.

yE-mail: [email protected]

Contract/grant sponsor: Borealis A/SContract/Grant sponsor: Knowledge Foundation via the Material Research School at Chalmers University ofTechnology

materials. There is also an increasing demand from the market for halogen-free flameretardants.

Traditionally, materials containing halogen, such as polyvinyl chloride (PVC), have been usedas primary insulation in low voltage power and telecommunication cables and therefore mostcables used today contain halogen. During a fire, materials containing halogen producecorrosive and toxic gases as well as dense smoke that makes escape difficult. Polyolefins havefavourable properties in terms of cable production such as excellent dielectrical properties,balance of physical and barrier performance, processing characteristics, low density and thermalstability. In addition, polyolefins do not contain plasticizers that may lead to migrationproblems. Recycling and incineration of polyolefins is also available. Furthermore, they emitnon-toxic and non-corrosive gases such as carbon dioxide and water vapour when burning andgood visibility is maintained throughout a fire.

However, polyolefins burn easily and have a high calorific value [3]. In order to slow down therate of combustion a variety of additives can be added to the polyolefin compounds. In the1980s, low smoke zero halogen (LSZH) materials, based on aluminium hydroxide or magnesiumhydroxide, were developed. These formulations are costly mainly due to their high loading (50–65 wt-%) and the production of cables based on aluminium hydroxide requires investments inextrusion equipment. Furthermore, the production speed of cables based on hydroxides isslower than the production of PVC cables. Therefore, these technologies are unfeasible asalternatives for standard cables. Instead, cables containing aluminium hydroxide are used incritical environments such as subways, airports, ships and nuclear power stations whereas cablescontaining magnesium hydroxide are generally reserved for automotive and special cables.

In the early 1990s, a new generation LSZH materials was developed. One was CasicoTM

consisting of a relatively small amount of chalk, and a small fraction of silicone elastomer mixedwith an ethylene–acrylate copolymer (EBA) [4]. The additives are quite inexpensive and sincethe addition is 35 wt-% the viscosity of Casico is similar to that of unfilled polyethylene. Hence,Casico can be extruded on normal polyethylene extruders providing the same production speedas the production of standard PVC cables. When exposed to fire, cables based on Casicomaterial form an intumescent structure protected by a stable charred layer. This structureprotects the material underneath from the heat of the fire hence lowering the amount ofcombustible gases transported to the flame front [5]. The overall properties of Casico make it apromising and cost-effective alternative for standard cables.

This paper evaluates the flame retardancy of Casico obtained at different temperatures alongwith the flame retardant effects of each individual component: ethylene–acrylate copolymer,chalk and silicone elastomer. The formation of the intumescent structure is shown and explainedand its molecular structure is analysed and discussed.

2. EXPERIMENTAL

2.1. Materials

Casico is short for calcium carbonate, silicone elastomer and an acrylate containing copolymer.In order to investigate the flame retardant mechanisms of Casico as well as the flame retardanteffect of each individual component, compounds were prepared according to the formulationsshown in Table I and subjected to various tests and analyses. Calcium carbonate is designated

Copyright # 2003 John Wiley & Sons, Ltd. Fire Mater. 2003; 27:51–70

A. HERMANSSON, T. HJERTBERG AND B.-A. SULTAN52

TableI.

Materials(compositionin

wt-%

).

Pure

polymers

Binary

compounds

Territory

compounds

Abbreviation

EBA

LDPE

SiEBA

SiLDPE

CaEBA

CaLDPE

CaSiEBA

CaSiLDPE

Material

Ethylene-butylacrylate,EBA

n99.8

87.3

69.8

57.3

OE6408,1.8

mole-%

BA

MFR

2.16:0.45,density:0.925g/cm

3

Low

density

polyethylene,

LDPEy

99.8

87.3

69.8

57.3

MFR

2.16:0.16,density:0.920g/cm

3

Calcium

carbonate

z30.0

30.0

30.0

30.0

OmyaEXH1SP,coateddensity:2.7g/cm

3

meanparticle

size:1.4

microns

Siliconeelastomer

masterbatchz

12.5

12.5

12.5

12.5

40wt-%

highmolecularPDMS} /60wt-%

LDPE1

Antioxidant}

0.2

0.2

0.2

0.2

0.2

0.2

0.2

0.2

Irganox1010

nprovided

byBorealisA/S.

yprovided

byOMYA.

zprovided

byBorealisA/S.

}methylterm

inatedpolysiloxanewithpendantvinylgroupsmanufacturedbyDow

Corning(Silastic

Q4-2735).

}provided

byCibaspeciality

chem

icals.


FLAME RETARDANT MECHANISM OF MODIFIED POLYOLEFINS 53

chalk, or Ca as an abbreviation, and in all compounds containing chalk the content is 30wt-%.Similarly, the silicone elastomer masterbatch is called Si as an abbreviation and is referred to assilicone elastomer in the body text. In all compounds containing silicone elastomer masterbatchthe actual content of the silicone elastomer is 5wt%. The silicone elastomer is apolydimethylsiloxane (PDMS) and is referred to as PDMS. The polymers used are low densitypolyethylene and ethylene–acrylate copolymer are referred to as the abbreviations LDPE andEBA, respectively. All compounds also contain 0.2 wt-% Irganox 1010. A phenolic antioxidantmanufactured by Ciba Specialty Chemicals.

2.2. Sample preparation

Compounds were produced on a laboratory two-roll mill at 1808C at 5 rpm for 20min. Thefeeding order and the feeding time was the following: polymer and antioxidant were mixed for5min, then chalk was added and mixed for 10min. Finally, the silicone elastomer was addedand mixed for an additional 5min. The foils were pelletized using a granulator providing coarsegranules.

2.3. Flame retardant tests

The oxygen index (OI) is a method for evaluation of the flammability of materials. It determinesthe minimum oxygen concentration of an O2/N2 mixture required to sustain combustion [6].The OI was determined using a Stanton Redcroft FTA Flammability Unit according to ISO4589 part 2 (using a step size of 0.5% for successive changes in the oxygen concentration) [7].The results are based on ten test specimens of dimensions 150� 6� 3mm3. These were stampedout of plates pressed in a Collin 300 press by applying low pressure (20 bar) at 1508C for 1minfollowed by high pressure (200 bar) for 5min at the same temperature. The cooling rate was108C/min under high pressure.

Cone calorimetry is based on the principle of oxygen consumption calorimetry. Generally theheat of combustion of any organic material is directly related to the amount of oxygen requiredfor combustion and 13.1 MJ of heat is released per kg oxygen consumed. A Stanton Redcroftcone calorimeter was used to obtain results such as weight loss, rate of heat release, total heatrelease, ignition time and burning time according to ISO 5660 part 1 [8]. A test specimen, ofdimension 100� 100� 3mm3, was punched from a compression moulded plate and exposed toa heat flux of 35 kW/m2 and an airflow of 24 l/s. The cone calorimeter data represent an averageof triplicate cone results where deviations of up to 10% might occur within the results. Thesedeviations most probably result from the formation of the intumescent structure duringburning, which may be inhomogeneous. The cone test is terminated when the heat release rate istoo low to be reliably determined. For some compounds this led to a residue of organic materialafter the test was terminated.

2.4. Thermal treatments

Three different thermal treatments were used to simulate different stages of the fire process. Heattreatment at 3008C for 1 h in a flowing stream (18ml/min) of technical air or N2 was performedin a tubular furnace in order to study the initial effervescence process both visually and bychemical changes, respectively. Coarse granules were pressed at 1508C and test specimens ofdimensions 10� 30� 1mm3 were placed on a microscope glass. Duplicate tests were performed.



The tubular furnace was originally constructed for thermal degradation of polyethylene andfurther details can be found in reference [9].

A Heraeus Instruments M110 (220V, 2.9 kW, 11008C end temperature) was used for ashing inair at 5008C and 10008C for 1 h. Ashing was done in order to determine mass loss and forobserving the effervescence process. Coarse granules, 3 g, were placed in a ceramic cup and theresult is an average of at least two tests. There is a risk of material loss when the samplevigorously self-ignites in the high temperatures. In order to study the thermal stability anddegradation of the chalk used in this study, the chalk was heat treated for 1 h at 5008, 6008,7008, 8008, 9008 and 10008C, respectively. The result is obtained from one test at eachtemperature.

In order to obtain mass loss rates and degradation temperatures of the polymer components,a TGA850 Mettler Toledo with a TS801RO sample robot was used to perform thermogravi-metric analysis (TGA) on the compounds. Samples, 7–8mg, were contained in ceramic pans andheated at 108C/min from ambient temperature to 6008C in a flowing stream (50ml/min) oftechnical air. A 1:2.3 mixture of chalk and PDMS, produced on a Brabender Plasti-CorderPL2000 (1208C for 10min at 50 rpm) was also analysed. The results obtained from the TGA arebased on one test per sample.

2.5. Analytical techniques

In order to follow changes in the chemical structures of samples treated at different temperaturesFTIR, ESCA, MAS-NMR and XRD were used. Infrared spectroscopy (FTIR) measurementswere performed using a Perkin Elmer 2000 (resolution: 4 cm�1, number of scans: 30). Non-thermally treated samples were analysed using a beam condenser. Samples treated thermally inthe tubular furnace were analysed with diffuse reflectance after being ground in liquid nitrogenand blended with KBr. There are probably differences between bulk and surface, but this wasnot taken into consideration and the spectra represent an average of the burned samples. Singletests were performed.

Electron spectroscopy for chemical analysis (ESCA) provided quantitative data of theelemental composition and binding energy of the outermost layers (measuring depth approx.50 nm) of CaSiEBA treated at 5008C and 10008C. Spectra and atomic concentrations wereobtained using a Quantum 2000 scanning ESCA microprobe from Physical Electronics with amonochromatic Al Ka X-ray source. The beam size was 100 mm and the power 20W/15 kV.Duplicate tests were performed.

Magnetic angle spinning- nuclear magnetic resonance (MAS-NMR) was used to analyse theintumescent structure obtained from CaSiEBA and CaSiLDPE after thermal treatment at5008C and 10008C for 1 h. A 1:2.3 mixture of chalk and PDMS, treated at 10008C for 1 h wasalso analysed. The 29Si and 13C MAS-NMR spectra were obtained using a Chemagnetics CMXInfinity NMR spectrometer operating at 270MHz for 1 h. 29Si MAS-NMR was run using a 408excitation pulse and 100 s recycle delay and 13C MAS-NMR was run using a 408 excitation pulseand 20 s or 200 s recycle delay. All samples were run with a 10mm probe. The abbreviationNMR will be used, instead of MAS-NMR, throughout the text. Single tests were performed.

X-ray diffraction (XRD) was used to analyse the ash residue obtained from CaSiEBA afterthermal treatment at 5008C and 10008C. Difractograms were obtained with a Siemensdifractometer D5000 using a copper characteristic radiation of wavelength 154 (AA. Single testswere performed.



3. RESULTS

3.1. Burning characteristics obtained from oxygen index and cone calorimetry

Pure polyolefins, such as EBA, have OI values between 17.0 and 18.0 [10,11]. Addition ofsilicone elastomer or chalk to EBA, SiEBA and CaEBA, respectively, has almost no effect onthe OI. The highest flame retardancy was obtained for compounds containing the ternaryformulation chalk/silicone elastomer/polymer. When combining silicone elastomer, chalk andEBA the OI was very much affected (Table II). Replacing the LDPE with the EBA copolymerincreased the OI by 6 units, from 24.5 to 30.5. Due to the large effect of the copolymer in thetertiary system, compounds based on EBA were mainly concentrated on. In some casescomparisons were made with the LDPE systems to demonstrate the effect of the acrylate group.

Data from the cone calorimeter, presented visually in Figure 1, show that EBA had a shortignition time and a very short and intensive heat release. Addition of silicone elastomer to EBAhad a minor effect on the combustion properties (not shown in Figure 1). By adding chalk toEBA the ignition time was prolonged and the maximal rate and total heat release was reduced.When combining chalk, silicone elastomer and EBA, CaSiEBA, the burning intensity wasreduced substantially. The ignition time was lengthened by almost 1.5min compared with pure

Table II. Oxygen index results.

Material OI

LDPE 18EBA 18SiEBA 19CaEBA 20CaSiLDPE 24.5CaSiEBA 30.5

Figure 1. Heat release rate of EBA, CaEBA and CaSiEBA at a heat flux of 35 kW/m2.



EBA and the heat release rate was reduced dramatically. Table III provides further data fromthe cone calorimetry such as heat of combustion and mass loss.

In general, polyolefins have a high heat release rate. The low heat release of CaSiEBA andCaSiLDPE can be explained by their low mass loss, as an effect of chalk providing lesscombustible material. The maximal heat release rate of CaSiLDPE was similar to that ofCaSiEBA. The higher flame retardancy of CaSiEBA was manifested by an almost 1minlengthening of the ignition time. Hence the acrylate group was important for obtaining the highflame retardant behaviour discussed.

3.2. Intumescence

During treatment at 3008C it was observed that most samples effervesced to some extent,generating an intumescent structure. The intumescent structure of SiEBA and CaEBA collapsedduring treatment and the sample spread over a large area. The intumescent behaviour was mostpronounced for CaSiEBA, which maintained its structure after treatment. CaSiLDPE was theonly sample that shrank during treatment. All samples became very sooty.

The intumescent behaviour of samples placed in a hot furnace differed somewhat from theintumescent behaviour obtained in the tubular furnace. At 5008C no SiEBA was left. The othersamples produced an effervesced residue that differed in the degree of intumescence, andconsequently differed in char structure. CaEBA formed a stable convex char. Of all theformulations, CaSiEBA effervesced the most. Unfortunately, material stuck to the walls of thecup so there was not enough material left to provide a stable char. The char of CaSiLDPEcollapsed.

At 10008C the char of CaEBA collapsed during burning while CaSiEBA formed anintumescent structure with a compact and flat char (see Figure 2). CaSiLDPE formed a moreporous intumescent structure, compared with CaSiEBA, with many cracks in the charred layer.

3.3. Volatilisation

Isothermal heat treatment for 1 h was used to study the thermal stability and degradation ofchalk (see Figure 3). The weight loss started at 5008C and at 9008C the weight loss was constant,providing a residue of 56%. The residue correlates with the theoretical value when all calciumcarbonate has been transformed into calcium oxide. IR analysis confirmed the transformation

Table III. Data from the cone calorimeter.

Material

HRRa

(kW/m2)(max)

Ignitiontime (s)

Burningtime (s)

HCb

(MJ/kg)Mass

loss (%)

LDPE 1420 76 229 41.0 99.4EBA 1304 77 236 40.9 98.1SiEBA 1044 84 224 33.4 98.7CaEBA 658 102 232 26.3 71.7CaSiLDPE 320 95 374 26.0 69.4CaSiEBA 326 148 414 24.1 65.9

aHRR=heat release rate. b HC=heat of combustion. max=maximum.



(not shown). The small weight loss occurring between 5008C and 6008C was due to theevaporation of the surface coating of the chalk particles.

The course of volatilization of EBA, SiEBA, CaEBA, CaSiEBA and CaSiLDPE was studiedwith TGA and the results are seen in Figure 4. Significant degradation leading to the observedweight losses appeared to start at about 3008C. A big difference concerning the temperaturerange when the major weight loss occurred was seen amongst the samples. Compared with EBAthe addition of silicone elastomer, SiEBA, shifted the degradation process about 20–308Ctowards higher temperatures. At 6008C nothing was left of either system. The addition of chalkalone or in combination with silicone elastomer caused a dramatic shift to higher temperatures.CaEBA gave the expected 30 wt-% residue while CaSiEBA gave a higher residue, 31.5 wt-%.

Figure 2. The intumescent behaviour of CaSiEBA (a) and CaEBA (b) after thermal treatment in air(10008C for 1 h in ceramic cups).

Temperature [°C]

Ash

con

tent

[%]

100

80

60

40

20

0500 1000600 700 800 900

Figure 3. Isothermal degradation of chalk in technical air (1 h/temperature).



This extra residue most probably originated from PDMS. The influence of the polymer in theternary formulation was also investigated by thermogravimetry. LDPE was more thermallystable than EBA (not shown) but no major differences in mass loss were seen between theternary systems using this method.

Ashing was then used to study the thermal stability, by mass loss, of binary and tertiarycompounds as well as for pure polymers. The temperatures, 5008C and 10008C, were used basedon the fact that chalk was unaffected at 5008C and fully decomposed to calcium oxide at 10008Cas shown previously. As seen in Table IV, CaEBA provided a 29.2 wt-% residue at 5008C. Thepolymer was volatilized at this temperature hence the residue consisted of calcium carbonate. At10008C one would expect a 16.4 wt-% residue of calcium oxide (originating from 29.2 wt%chalk) compared with the theoretical value of 16.8 wt-% expected from 30 wt% chalk. The samereasoning applies to CaLDPE treated at 5008C and 10008C.

The residues of CaEBA and CaLDPE, treated at 5008C, were higher than expected due tosoot formation on the walls of the cups. The mass residues of CaSiEBA and CaSiLDPE, treatedat 10008C, were higher compared with the mass residues of CaEBA and CaLDPE, respectively.The additional experimental residue, besides calcium oxide, was 2.3 wt-% for CaSiEBA and 2.2

Figure 4. Weight losses of EBA, SiEBA, CaEBA and CaSiEBA as a functionof temperature (108C/min, technical air).

Table IV. Experimental residues after thermal treatment at 5008C and 10008C, respectively (1 h, air).

MaterialTotal residue5008C [wt-%]

Total residue10008C [wt-%]

LDPE 0 0EBA 0 0CaEBA 29.2 16.7SiEBA 0 0CaLDPE 29.8 17.2SiLDPE 0 0CaSiEBA 30.6 19.0CaSiLDPE 30.5 19.4



wt% for CaSiLDPE. This indicates that almost half of the silicon added from the start (5wt%)was retained in the samples.

3.4. Structural changes

Thermal treatment, 3008C, was performed in a nitrogen atmosphere in order to avoid oxidativereactions and soot formation. Figure 5 shows that the characteristic ester peak of EBA at1735 cm�1 diminished and a new peak representing carboxylic acid arose at 1705 cm�1. ForSiEBA only the carboxylic acid formation was observed and almost no silicon residue was left(Figure 6).

Figure 5. IR spectra of thermally treated (3008C, 1 h, N2) EBA, SiEBA, CaEBA and CaSiEBA incomparison with untreated samples.

1000 1000 1000 (cm-1)CaSiEBA

(untreated)CaSiEBA SiEBA

Figure 6. IR spectra of CaSiEBA and SiEBA treated for 1 h at 3008C in N2 compared with an untreatedCaSiEBA. Marked peaks are typical silicon absorption bands originating from PDMS.



For EBA mixed with chalk, CaEBA, only a minor amount of carboxylic acid was visible aftertreatment at 3008C whereas the ester peak was no longer visible. Instead, at 1560 cm�1, ashoulder at the CaCO3 peak was observed as seen in Figure 5. The spectrum of CaSiEBA, alsoshown in Figure 5, was similar to CaEBA but with an even more pronounced shoulder at1560 cm�1 due to the carboxylate ions.

3.5. Char structure

PDMS degraded around 3008C, as seen in Figures 6 and 7, but when mixed with chalk andEBA, silicon was still present in samples treated at 5008C and 10008C, respectively. This ispresented in Table V showing the atomic concentrations obtained from ESCA analysis of thesamples. Due to the stabilizing effect of the chalk (Figure 7), the degradation of PDMS occurredat higher temperatures. As a result, PDMS contributed to the intumescent structure and thechar formation at higher temperatures.

ESCA was used to analyse the char after thermal treatment of CaSiEBA at 5008C and 10008C(Table V). With respect to the presence of calcium and silicon two general distinctions could bedrawn (not shown). First, due to the vigorously effervescent behaviour of CaSiEBA no realchar, with clearly defined top- and bottom surfaces, was formed. Therefore, no significant

Figure 7. TGA curve (108C/min, technical air) of PDMS, chalk and a modelmixture (30/70) of chalk and PDMS.

Table V. Atomic concentrations [atomic-%] of the top surface of CaSiEBA after treatment inair at 5008C and 10008C, respectively. Atomic concentration after subtraction of CaCO3 from

CaSiEBA treated at 5008C is also shown (*).

Element 5008C 5008C* 10008C

C1s 10 0a 23O1s 68 38a 60Si2p 12 12a 11Ca2p 10 0a 6



differences were observed between the top surface and the bottom of the top surface ofCaSiEBA treated at 5008C. Secondly, for CaSiEBA treated at 10008C the amount of silicon washigher at the top surface whilst the amount of calcium was much higher at the bottom of the toplayer.

Besides providing quantitative data on the elemental composition, ESCA also provided theamount of carbon in different functional groups. In the C1 s region, CaSiEBA treated at 5008Cand 10008C, respectively, had two distinctive peaks. One peak at 284 eV and the other peak at289.3 eV and 288.5 eV, respectively. Peaks originating from CaSiEBA treated at 10008C areshown in Figure 8. The peak at 284 eV originated from C-Si and/or C-C. The peak at 289.3 eV(50 area-%), from CaSiEBA treated at 5008C, originated from carbonate, i.e. carbon with threeoxygen bonds. The peak at 288.5 eV (20 area-%), from CaSiEBA treated at 10008C, representedthe formation of carbon with two oxygen bonds where one is a double bond, or carbon withthree oxygen bonds. The peak at 288.5 eV indicated that calcium carbonate can be present inCaSiEBA after treatment in 10008C.

XRD, isothermal treatment, and IR clearly showed that calcium carbonate was unaffected at5008C. Therefore, the atomic concentration obtained with ESCA originating from calciumcarbonate can be subtracted from CaSiEBA hence obtaining the atomic concentration of theremaining elements. The atomic concentration of CaSiEBA, before and after subtraction ofcalcium carbonate, is shown in Table V. After corrections only oxygen and silicon were leftin the sample indicating formation of SiO3. The slight convex baseline obtained by XRD(Figure 9), also indicated the formation of amorphous silica, SiOx, supporting the resultsobtained from ESCA.

Although ESCA analysis showed the presence of carbonate in CaSiEBA treated at 10008C(Figure 8), the calcium carbonate could not be subtracted from the atomic concentration, as forCaSiEBA treated at 5008C. At 10008C the calcium carbonate decomposed into CaO as wasclearly seen from isothermal treatments, IR and XRD. Therefore, all four elements; carbon,oxygen, silicon and calcium were left in CaSiEBA after treatment at 10008C. XRD showed thatCa(OH)2 and Ca2SiO4 were formed as seen in Figure 9. As for CaSiEBA treated at 5008C theslightly convex baseline, also seen in Figure 9, indicated the presence of amorphous silica.Ca2SiO4 was also obtained for CaSiLDPE, treated at 10008C, according to XRD analysis (notshown).

292 290 288 286 284 282 280

1300

1200

1100

1000

900

800

700

600

C1

C4

C2C3

Binding Energy [eV]

c/se

c

Figure 8. ESCA spectrum of the C1s region originating from CaSiEBA treated at 10008C for1 h in air. The lines represent different carbons as obtained from spectra deconvolution: C1 isdue to C–C and/or C-Si; C2 is due to C–O–C; C3 is due to O–C–O and/or C=O; C4 is due to

O-C=O and/or carbon bound to three oxygen atoms.



The 29Si NMR spectra of CaSiEBA, treated at 5008C and 10008C, are shown in Figure 10.PDMS should have a peak at �21.3 ppm due to the O2Si(CH3)2 groups [12] but this peak wasabsent and thus the silicone elastomer had decomposed confirming the results obtained fromTGA and IR. Instead, broad peaks between �80 to �110�ppm were seen for CaSiEBA treated

Figure 9. XRD difractogram of CaSiEBA treated at 5008C and 10008C respectively (1 h, air) and expectedpeaks for (&) CaCO3; (*) Ca2SiO4; ($) Ca(OH)2; (&) CaO. CaCO3 might be present in CaSiEBA treatedat 10008C in the form of vaterite and aragonite. The peaks coincide with some of the peaks of (*) Ca2SiO4

and are therefore not marked in the spectrum.



at 5008C. These were most likely due to a mixture of silica [13]. The spectrum of CaSiEBAtreated at 10008C was very different compared with the spectrum of CaSiEBA treated at 5008C.Only one distinct and very narrow peak was observed at �71.1 ppm (Figure 10). This peakcorresponded to the crystalline structure of Ca2SiO4 [14].

The 13C NMR spectrum showed that calcium carbonate was present in CaSiEBA treated at5008C while carbon was absent in CaSiEBA treated at 10008C (not shown). However, anindication of calcium carbonate was seen when analysing CaSiEBA, treated at 10008C, using alonger recycle delay (200 s). No other peaks were seen in the 13C NMR spectra.

A 29Si NMR spectrum of the model mixture of PDMS and chalk, treated at 10008C for 1 h,was obtained as well (not shown). One distinct peak was seen at �71.1 ppm, as for CaSiEBAtreated at 10008C, originating from Ca2SiO4. Therefore the copolymer, EBA, did not affect theformation of Ca2SiO4.

4. DISCUSSION

The results presented above clearly demonstrate that the Casico system has significant flameretardant properties. Compared with pure LDPE and EBA, the OI increased from 18 to 30.5.(Note that data discussed in this paper originate from experiments performed in a smalllaboratory scale. CaSiEBA manufactured in full scale production, using more effective mixingequipments, receives an OI of 38 [15]). In the cone calorimetry experiments, the maximum heatrelease rate decreased dramatically and the time to ignition increased significantly. Theseparameters are highly relevant to a real fire situation with respect to ease of ignition of materials,fire spreading and escape time, respectively.

Although the general trends are the same with the two methods, the results do not alwayscorrelate quantitatively. One explanation is the difference in design between the two tests.Results from OI are affected by the dripping tendencies of the materials. During OI testing thetest specimen behaves like a candle where drops run along the specimen. The drops causeignition further down on the test specimen hence contributing to the fire propagation. Bycontrast, in the cone calorimeter the test specimen is a horizontally placed plaque and therefore

Figure 10. 29Si NMR spectrum of the top surface of CaSiEBA treated at 5008C and10008C respectively, for 1 h in air.



no dripping can occur. In addition, the results from the cone calorimeter reveal information thatis relevant to the flame retardant mechanism. Together with the different components of thesystem, the principal parameters discussed are maximum heat release rate, heat of combustion,ignition time and burning time.

A fire involving building wires based on Casico reached its peak at around 8008C [16].Therefore, the thermal treatments, at 5008C and 10008C, respectively, used in this study do notcorrespond to a real fire scenario. The temperatures were chosen based on the decompositiontemperature of the chalk.

4.1. Effect of chalk in EBA

Compared with the two pure polymers, the addition of 30% chalk to EBA led to a dramaticdecrease in the heat of combustion. This was to be expected since the polymer, which burns in afire, was diluted with an inert material that did not contribute to the fire. This is an effect that isseen with any inorganic filler. In addition, chalk could have a more active contribution as CO2

can be released in the endothermic reaction:

CaCO3 ! CaOþ CO2 " ð1Þ

Heat treatment showed that this reaction started at 6008C and was completed at 9008C. In thecone calorimeter test the temperature for the sample reached approx. 6708C and CO2 wasreleased to a certain extent. However, in this experiment the time scale must be considered. After5.5min the heat release had decreased to a level where the experiment was stopped and the massloss indicated that only 6% of the chalk had reacted. In a real fire a larger amount of CO2 iscertainly released but it should be remembered that this reaction takes place at too high atemperature to contribute to the initial fire retardation.

Looking at Table III it can be seen that the ignition time increased from about 75 to 100 s dueto the addition of chalk to EBA. Beside the diluting effect, the results in Figure 4 indicate thatchalk might have had an active influence as well. The addition of chalk to EBA postponed majormass loss of the polymer to substantially higher temperatures. This means that release ofvolatile, and flammable, low molecular weight compounds was delayed to higher temperatures.As the effect was similar when nitrogen is used in the TGA experiment (not shown) it was notlikely that chalk could delay the degradation reactions. One possible reason could be that chalkabsorbed the small molecules formed at around 3508C and that they were released at highertemperatures. However, this needs to be confirmed by further experiments before it can beestablished.

4.2. Effect of silicone elastomer in EBA and LDPE

Addition of silicone elastomer to EBA did not have any significant effect on the fire properties asshown by the results from both the cone calorimeter and the OI test. This is indeed what is to beexpected as the silicone elastomer is volatilized at a relatively low temperature. In the TGA test(Figure 7), PDMS was volatilized between 2908C and 3608C. The IR spectrum of SiEBAthermally treated at 3008C for 1 h (Figure 6), confirmed that this also took place when siliconeelastomer was mixed with a copolymer without chalk.

At low temperatures, the degradation of PDMS occured by splitting off cyclic oligomers fromchain ends or after internal cyclization [17]. The cyclic trimer (D3) has been reported as the mostabundant product, with decreasing amounts of tetramer (D4), pentamer (D5), hexamer (D6)



and higher cyclic oligomers (D74) [17–20]. The degradation can be seen as a depolymerizationsince the polymerization can be done by ring opening of cyclic oligomers. The relativelylow degradation temperature is explained by the fact that the acidic polymerizationcatalyst, which remains in the PDMS, acts as a depolymerization catalyst at higher temperatures[21–23].

Addition of chalk had a significant influence on the PDMS. The IR spectra of CaSiEBA orCaSiLDPE heat treated at 3008C clearly show that silicon remained in the samples. Thestabilizing effect of chalk was confirmed by the TGA test on a model mixture between chalk andPDMS (Figure 7), with a 1008C shift for the major weight loss of the PDMS. A plausibleexplanation is that the chalk neutralized the acidic catalyst left in the PDMS from itspolymerization process, thus shifting the degradation to higher temperatures.

The stabilization of PDMS had a very significant effect on the flame retardancy of the tertiarycompounds. The residues from both the cone calorimeter and the ashing experimentsdemonstrate this. From the cone calorimeter one would expect a residue of 30% comparedwith the 34% obtained for CaSiEBA, and from ashing at 10008C the residue of CaSiEBA andCaSiLDPE are more than 2% higher than expected. The most likely explanation is that theexcess residue consisted of silicon oxides formed from the PDMS [5], see below.

Considering the rapid increase in temperature in the ashing experiments, both depolymeriza-tion and degradation occur simultaneously. Depolymerization results in extra fuel anddegradation results in the formation of a surface char. In order to obtain the best flameretardant results depolymerization must be suppressed and degradation must lead to a stablechar.

4.3. Systems with both chalk and silicone elastomer

To obtain a significant fire retardant effect one must add both chalk and silicone elastomer tothe polymer. An increase in OI was observed only for CaSiLDPE and CaSiEBA fromapproximately 18 to 24.5 and 30.5, respectively. In the cone calorimeter experiments (Table III),these materials both showed a dramatic decrease in the maximum heat release rate to less than25% of that of the pure polymers, and a corresponding increase in the burning time. In fact, theheat release profile of the two materials was very similar with one very important exception, i.e.the ignition time of CaSiEBA was 50% longer than that of CaSiLDPE.

Of course, the complete systems benefit from the isolated effect of the chalk, dilution with aninert filler, and delayed release of volatile substances. The additional decrease in heat release rateis explained by the formation of an intumescent structure providing a heat-insulating layer forthe underlying material. The intumescent structure was formed by foaming, due to gases andvolatile substances formed at increased temperatures due to the degradation reactions. Theconditions in the ashing experiments are not the same as in a fire but the effervescence canclearly be seen. All materials effervesce to some extent forming an intumescent structure but thebehaviour of this structure differed significantly between the materials. A reasonably stableintumescent structure was formed only in the case of the two complete systems, CaSiLDPE andCaSiEBA.

To obtain the most promising flame retardant effect it is necessary to add silicone elastomerand chalk to an acrylate copolymer, i.e. CaSiEBA. The reason can be found in the reactionsinduced by the specific degradation products generated in the acrylate moiety. It is well knownthat copolymers between ethylene and acrylates with b-hydrogen in the alkyl group undergo



ester pyrolysis at temperatures close to 3008C, e.g. for EBA [24,25]:

Polymer� COOC4H9 ! Polymer� COOHþ CH2 ¼ CH� CH2 � CH3 " ð2Þ

The ester pyrolysis was confirmed by the change of the carbonyl absorption from ester tocarboxylic acid in the IR spectrum of EBA heat treated at 3008C (Figure 4). For EBAcontaining chalk the carboxylic acid groups are instead converted to carboxylate. This is theexpected result of a reaction between COOH groups and chalk:

2Polymer� COOHþ CaCO3 ! ðPolymer� COOÞ2Ca2þ þ CO2 " þH2O " ð3Þ

This reaction contributes to the flame retardant mechanism in two ways. First, it leads to theformation of foaming gases at a lower temperature than the normal degradation of the polymerbackbone. This means that a heat-insulating layer is formed at an earlier stage, which couldaccount for an increased ignition time, provided that the intumescent structure is stable enough.In addition, the gases formed are inert and may have a suffocating effect on the fire. Theformation of ionomers between carboxylate groups and calcium ions is a second importantcontribution. This introduces crosslinks in the melt increasing its viscosity thus stabilizing theinitially formed intumescent structure.

This is in line with Brauman [26] who suggests two factors that might have an influence on theformation of bubbles and closed cells in intumescent systems namely melt viscosity andcrosslinking ability. Viscous melt is formed deep into the bulk of the material during thermaltreatment and fumes trapped within this viscous melt could have more difficulty escaping andtherefore obtain a longer residence time in the crosslinking environment of the melt.Crosslinking of the membranous wall of the bubbles could increase their strength and allowthese bubbles to retain their shape during further pyrolysis.

The behaviour during heat treatment at 3008C clearly demonstrates the stabilization of theintumescent structure. Among the samples CaSiEBA effervesced the most and the intumescentstructure was retained, while the corresponding sample without copolymer, CaSiLDPE, shrank.The stabilization of the melt, and thus decreased dripping tendencies, is also a major reason forthe increased OI of CaSiEBA compared to CaSiLDPE. The possibility of forming a stableintumescent structure, preferably covered by a char, is very important since it protects thematerial underneath from the heat, thus preventing combustible gases to maintain the flamewith fuel. CaSiEBA treated at 5008C effervesced to such an extent that most material splashedonto the inside walls of the cup. Therefore no stable and protective char was formed. Whentreating CaSiEBA at 10008C a dense and mechanically stable char was formed thus protectingthe intumescent structure underneath.

The reduction of heat release rate, seen in the cone calorimeter test, is most likely a result ofisolation obtained by the intumescent structure. The formation of an intumescent structure alsoexplains the increased mass residue, seen in TGA and thermal treatment at 10008C, forCaSiEBA and CaSiLDPE. The intumescent structure and the char of CaSiLDPE are somewhatless stable than the intumescent structure and the char of CaSiEBA. Most likely the presence ofionomers in CaSiEBA is limited due to the short time they can be formed but still ionomers seemto have an effect not only on the initial fire properties but on the formation of the intumescentstructure as well.

Thermal treatment, 13C NMR, and XRD analysis clearly showed that chalk was present inCaSiEBA at 5008C and indicate that SiOx was formed from the PDMS. After corrections forcalcium carbonate, regarding the atomic concentration obtained by ESCA analysis, the presence



of silicon and oxygen indicate formation of silicon oxides. The silicon oxides were also seen in29Si NMR. According to the literature, SiO2 has a peak at �112 ppm, while protonated silicaobtain peaks at �92 ppm and �102 ppm [27,28]. The broad series of overlapping peaksbetween �80 to �110 ppm seen in CaSiEBA treated at 5008C originate from silica (SiO4)-likematerials [12].

XRD and 29Si NMR showed that mainly CaO, Ca(OH)2 and Ca2SiO4, were formed aftertreating CaSiEBA at 10008C for 1 h. CaO was formed through the transformation of chalk asshown in Equation (5). Ca2SiO4 was formed, at high temperatures, through a reaction betweenCaO and SiOx. The calcium silicate, Ca2SiO4, is also known as the shorthand notation b-C2Swhere C=CaO and S=SiO2 [14]. Ca2SiO4 is also formed when treating CaSiLDPE and thePDMS/chalk mixture at 10008C. Therefore, the polymers, EBA and LDPE, do not affect theformation of the calcium silicate. The calcium silicate may be formed according to the reactionsequence Equations (4) to (6) presented below:

PDMS ! SiOx þ ðVOC "Þð> 5008CÞ ð4Þ

CaCO3 ! CaOþ CO2 " ð> 6008CÞ ð5Þ

CaOþ SiOx ! Ca2SiO4 ð> 6008CÞ ð6Þ

Both Ca(OH)2 and CaCO3 were seen in CaSiEBA after being treated at 10008C and they wereboth due to reactions occurring after the thermal treatment as shown below. CaO formsCa(OH)2 by absorbing water from the atmosphere Equation (7). This is a fast reaction.Ca(OH)2 then reacts with carbon dioxide in the atmosphere and forms calcium carbonateEquation (8). This is a much slower process than formation of Ca(OH)2, as shown below:

CaOþH2O ! CaðOHÞ2 ð7Þ

CaðOHÞ2 þ CO2 ! CaCO3 þH2O ð8Þ

The formation of CaO, Ca(OH)2 and Ca2SiO4 explains the presence of calcium, oxygen andsilicon in CaSiEBA treated at 10008C as shown by ESCA. However, according to ESCAanalysis a high amount of carbon was also present in the sample. A small amount of carbonoriginating from carbonate could be seen with XRD and 13C NMR. Two things can explain thepresence of calcium carbonate after treatment at 10008C. First, all natural calcium carbonatescontain some high-temperature calcium carbonate phases such as aragonite and vaterite. Thesephases were detected with XRD analysis (not shown). Secondly, the formed calcium hydroxidereacts with carbon dioxide hence calcium carbonate reforms on the surface as shown inEquation (8).

However, all the carbon seen in ESCA cannot be explained by the presence of calciumcarbonate. Both XRD and NMR measure the bulk of the analysed samples, whereas ESCA is avery surface sensitive method. Since carbon was only detected with ESCA the carbon must bedue to contaminants collecting at the surface of the sample. Hence, the composition of the charof the intumescent structure and the bulk of the intumescent structure differed.

Internal reports (performed in 2000 by Lomakin and Zaikov, at the Institute of BiochemicalPhysics of Russian Academy of Sciences) showed that silicon oxycarbide, SiOxCy, was formedafter treating polypropylene and PDMS formulations at 7008C in N2 atmosphere. They basedtheir findings on IR, GCMS, 13C NMR and 29Si NMR. A 29Si NMR spectrum originating fromsilicon oxycarbide showed a broad peak at �71.3 ppm originating from an amorphous



structure. One can assume that silicon oxycarbide was formed also from Casico formulationstreated at 10008C. Hence the presence of silicon oxycarbide might represent a small amount ofthe carbon seen with ESCA after treating CaSiEBA at 10008C.

5. CONCLUSIONS

The combination of silicone elastomer, chalk and acrylate containing copolymer, Casico, isnecessary to reach optimal flame retardancy measured by OI and cone calorimetry. Thermaltreatments, IR and visual observations support this. A formulation of silicone elastomer, chalkand low density polyethylene was used as a comparison.

The flame retardant effect of Casico starts at low temperature. Ester pyrolysis of EBA, at3008C, results in a reaction between the chalk and the carboxylic acid leading to formation ofgases and ionomers. The gases dilute and reduce the combustible gases transported to the flamefront and cause the melt to effervesce hence generating an intumescent structure. Theintumescent structure acts as a heat insulating layer protecting the material underneath fromburning thus preventing new combustible gases to reach the flame front. The intumescentstructure is reinforced by the ionomers creating crosslinks in the melt.

The silicone elastomer is stabilized by the chalk thus surviving to higher temperatures where ittakes an active part in the formation of the intumescent structure. After treatment at 5008C theintumescent structure consists of calcium carbonate and silicon oxides. At higher temperatures,the silicon oxides react with CaO, formed from the degradation of chalk, thus forming Ca2SiO4.Also CaO and Ca(OH)2, are present in the intumescent structure obtained after thermaltreatment at 10008C.

ACKNOWLEDGEMENTS

Borealis A/S and the Knowledge Foundation via the Material Research School at Chalmers University ofTechnology (MARCHAL) are gratefully acknowledged for financial support.The authors also thank Mrs Anne Wendel, Department of Polymer Technology, Chalmers University of

Technology, for ESCA measurements, Associate Professor Vratislav Langer at Department of InorganicEnvironmental Chemistry, Chalmers University of Technology, for XRD measurements, and AndrewRoot, Fortum Oil and Gas OY, for the NMR analysis.

REFERENCES

1. http://www.environmentdaily.com/articles/, 23 January 2001.2. http://www.environmentdaily.com/articles/, 6 September 2001.3. Schwarz RJ. In Flame Retardancy of Polymeric Materials, Kuryla WC, Papa AJ (eds). Marcel Dekker: New York,

1973; 83–133.4. Davidson NS, Wilkinson K. Flame Retardant Polymer Composition. EP0393959 B1, 24.10.90.5. Sultan B- (AA, Ericson K, Hirvensalo M et al. Novel Halogen Free Flame Retardant Polyolefins Intended for Internal

Wiring}Properties and Flame Retardant Mechanism. Proceedings of the 47th International Wire and CableSymposium, Philadelphia, 1998.

6. Troitzsch J. International Plastics Flammability Handbook. Hanser Publishers: Munich, 1990; 218.7. ISO 4589:1984. Plastics- Determination of Flammability by Oxygen Index. International Organization for

Standardization: Geneva, Switzerland, 1984.



8. ISO 5660-1:1993. Fire Tests- Reaction to Fire- Part 1: Rate of Heat Release from Building Products (ConeCalorimeter Method). International Organization for Standardization: Geneva, 1993.

9. Holmstr .oom A, S .oorvik E. Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content.Journal of Chromatography 1970; 53:95–108.

10. Nelson GL. In Fire and Polymers II—Materials and Tests for Hazardous Prevention, Nelson GL (ed.). ACS:Washington, 1995; 1–26.

11. Coad EC, Rasmussen PG. In Fire and Polymers II: Materials and Tests for Hazardous Prevention, Nelson GL. (ed.).ACS: Washington, 1995; 256–266.

12. Marshall GL. The use of 29Si fourier transform nuclear magnetic resonance spectroscopy for the characterisation ofpolymers and paints. British Polymer Journal 1982; 14:19–22.

13. Brandolini AJ, Hills DD. NMR Spectra of Polymers and Polymer Additives. Marcel Dekker: New York, 2000; 415.14. Sun G, Brough AR, Young JF. 29Si NMR study of the hydration of Ca3SiO5 and b-Ca2SiO4 in the presence of silica

fume. Journal of American Ceramics Society 1999; 82:3225–3230.15. Andreasson A. The Influence of Mixing Conditions on the Properties of Casico. Master Thesis, Department of

Chemical Engineering, Chalmers Lindholmen University College: Gothenburg, 1999.16. Fagrell O, Robinson JE. Low cost building wire in a large scale fire. Plastics in Telecommunications VIII, IOM:

London, UK, 1998.17. Grassie N, Scott G. Polymer Degradation and Stabilisation. Cambridge University Press: Cambridge, 1985; 35–39.18. Grassie N, Macfarlane IG. The thermal degradation of polysiloxanes—I. Poly(dimethylsiloxane). European Polymer

Journal 1978; 14:875–884.19. Hunter MJ, Hyde JF, Warrick EL, et al. Organosilicon polymers. Cyclic dimethylsiloxanes. Journal of American

Chemical Society 1946; 68:667–672.20. Thomas TH, Kendrick TC. Journal of Polymer Science: Part A-2. 1969; 7:537–549.21. Austin PJ, Buch RR, Kashiwagi T. Gasification of silicone fluids under external thermal radiation Part 1.

Gasification rate and global heat of gasification. Fire and Materials 1998; 22:221–237.22. Lewis CW. The pyrolysis of dimethylpolysiloxanes. II. Journal of Polymer Science 1959; 37:425–429.23. Kucera M, Lanikova J, Jelinek M. Neutralization of residual catalyst in polydimethylsiloxane. Effect of

neutralization on the thermal stability of the polymer. Journal of Polymer Science 1961; 53:301–310.24. Sultan B- (AA, S .oorvik E. Thermal degradation of EVA and EBA—A comparison. I. Volatile decomposition products.

Journal of Applied Polymer Science 1991; 43:1737–1745.25. Sultan B- (AA, S .oorvik E. Thermal degradation of EVA and EBA—A comparison. II. Changes in unsaturation and side

group structure. Journal of Applied Polymer Science 1991; 43:1747–1759.26. Brauman S. Char-forming synthetic polymers. 2. Char characterization. Journal of Fire Retardant Chemistry 1979;

6:266.27. Maciel GE, Sindorf DW. Silicon-29 NMR study of surface of silica gel by cross-polarisation and magic-angle

spinning. Journal of American Chemical Society 1980; 102:7606–7607.28. Sindorf DW, Maciel GE. 29Si NMR study of dehydrated/rehydrated silica gel using cross-polarisation and magic-

angle spinning. Journal of American Chemical Society 1983; 105:1487–1493.



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The flame retardant mechanism of polyolefins modified with chalk and silicone elastomer