Experimental characterization of the MCMK cable for fire ... · The cables compared were power installation cable MCMK 0.6/1 kV with PVC sheath and PVC insulation. ... 2 403 air 391

RESEARCH REPORT VTT-R-06873-12

Experimental characterization of the MCMK cable for fire safety assessment Authors: Johan Mangs, Simo Hostikka

Confidentiality: Public


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Preface

This study was carried out as a part of the “Risk Assessment of Large Fire Loads” project (LARGO) which is one of the projects in the National Nuclear Power Plant Safety Research 2011-2014 (SAFIR2014). The study has been financed by the State Nuclear Waste Management Fund (VYR). The Finnish participation in the OECD PRISME2 project takes place through the LARGO-project. The purpose of this study is to support the utilization of OECD PRISME2 cable flame spread test results both nationally and internationally. Dr. Tuula Leskelä (Aalto University School of Science and Technology) is acknowledged for carrying out the thermogravimetric and differential scanning calorimetry experiments. Mr. Risto Hiukka (VTT Expert Services) and his co-workers are acknowledged for exploring the chemical composition of the cable materials, Ms. Anna Matala (VTT) and Mr. Jyri Pekkanen (VTT Expert Services) for cone calorimeter experiments and Ms. Anna Matala also for microscale combustion calorimeter experiments. Espoo 14.5.2013 Authors


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Contents

Preface ........................................................................................................................ 3

1 Introduction ............................................................................................................. 5

2 Cable specimen ...................................................................................................... 6

3 Simultaneous thermal analysis ............................................................................... 8

4 Chemical composition of cable materials ............................................................. 13

4.1 FTIR -analysis ............................................................................................... 13 4.2 GC-MS-analysis ............................................................................................ 14 4.3 X-ray fluorescence analysis (XRF) ................................................................ 14 4.4 Calcination .................................................................................................... 15 4.5 X-ray diffraction analysis (XRD) .................................................................... 15 4.6 Conclusions from chemical composition analysis ......................................... 15

5 Cone calorimeter experiments .............................................................................. 17

6 Microscale combustion calorimeter (MCC) experiments....................................... 20

7 Flame spread experiments on 2 m samples ......................................................... 25

7.1 Methods ........................................................................................................ 25 7.2 Results .......................................................................................................... 26

8 Discussion ............................................................................................................ 29

9 Conclusions .......................................................................................................... 30

References ................................................................................................................ 31

Appendix A X-ray diffractograms for MCMK cable materials Appendix B Results from experiments with 2 m apparatus on PVC cable MCMK

3x2.5+2.5 mm2 “2012”


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1 Introduction

Cable flame spread is one of the most important research topics in the field of the nuclear power plant fire safety. Within the OECD PRISME2 project, full scale experiments of the cable flame spread on horizontal cable trays are performed. The cables used in the experiments are provided by the project partners. One of the cables is MCMK 3x2.5 mm2 PVC cable that is used at the TVO nuclear power plants in Finland. TVO sent the cable sample to IRSN, France, in early 2012 to be used in PRISME2 experiments. The cable sample was similar to but not identical with cables studied in previous research work at VTT in the year 2009 [1, 2]. Experimental characterization of the cable was therefore necessary to determine the applicability of the 2009 results concerning the fire performance characteristics and the pyrolysis model parameters. A piece of the same cable was therefore delivered to VTT. The characterization was carried out using Simultaneous Thermal Analysis (STA), flame spread experiments on 2 m long pre-heated samples, and cone calorimeter. The chemical composition of the cable materials was investigated using Fourier transformed infra-red spectroscopy (FTIR), gas chromatography combined with mass spectrometry (GC-MS), X-ray fluorescence analysis (XRF), X-ray diffraction (XRD) and calcination. The cable was also studied with a microscale combustion calorimeter (MCC). Specific heat release rate, pyrolysis residue, heat release capacity, heats of combustion and heat release temperature were determined for sheath, filler and insulation materials. The results are presented in this report and compared to the earlier cable results.


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2 Cable specimen

The cables compared were power installation cable MCMK 0.6/1 kV with PVC sheath and PVC insulation. The two cable types are named after the year of delivery, i.e. “2009” and “2012”. Both cable samples were delivered to VTT by TVO as “fresh” samples, new off the reel. Cable specifications given by the manufacturer are presented in Table 1. The structures of the cable specimen are presented in Figures 1 and 2. Structural parts and mass relations of the samples determined from 0.1 m long cable pieces are listed in Table 2. The PVC blend is composed of PVC resin, diisodecyl phthalate (DIDP) as plasticizer, CaCO3 and minor additional material according to information from the manufacturer1. The exact formulations of the cable materials were still unknown by the author for both “2009” and “2012” samples. Cable MCMK “2012” in this study corresponds to the cable used in the PRISME2 experiments. Table 1. Cable specifications given by the manufacturer.

MCMK “2009” MCMK “2012”

Conductors x cross-section (no x mm2)

4x1.5 + 1.5 3x2.5 + 2.5

Nominal diameter (mm) 13 13

Nominal linear mass (kg/km)

220 250

Material

Sheath PVC PVC

Filler Not specified Not specified

Insulation PVC PVC

1 e-mail: Camilla Sandström, Prysmian Finland Oy, 1st February, 2013


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Figure 1. MCMK 4x1.5 mm2 ”2009”, nominal diameter 13 mm. Left cross-section, right structure of cable. Scale in mm.

Figure 2. MCMK 3x2.5 mm2 ”2012”,nominal diameter 13 mm. Left cross-section, right structure of cable. Scale in mm.

Table 2. Mass relations of cable samples.

Cable type Structural part Linear mass

g/m %

MCMK 4x1.5 mm2 ”2009” Total 217.5 100 Sheath 89.8 41.3 Plastic tape 0.9 0.4 Filler 32.1 14.8 Insulation 29.7 13.7

Metal 64.7 29.7

MCMK 3x2.5 mm2 “2012” Total 235.6 100

Sheath 86.9 36.9

Plastic tape 1.0 0.4

Filler 36.2 15.4

Insulation 27.2 11.5

Metal 84.2 35.7


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3 Simultaneous thermal analysis

Simultaneous thermal analysis2 (STA) including thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) was carried out on cable component materials in a range from room temperature up to 800 oC at Aalto University School of Science and Technology, Laboratory of Inorganic Chemistry. The materials and experimental conditions are presented in Table 3 and results in Figure 3 and Figure 4. Table 3. Materials for simultaneous thermal analysis and experimental conditions.

Exp. Cable Component Environment Heating rate (K/min)

T44 MCMK 4x1.5, “2009” Sheath air, N2 10

T45 -“- Filler air, N2 10

T46 -“- Insulation air, N2 10

T69 MCMK 3x2.5, “2012” Sheath air, N2 10

T70 -“- Filler air, N2 10

T71 -“- Insulation air, N2 10

Three main decomposition steps could be identified, occurring roughly in the temperature intervals 1. 200 – 320 oC 2. 430 – 530 oC, sheath and insulation

380 – 440 oC, filler in air 3. > 660 oC. The second step did not occur in the curves for filler in N2. The mass losses for the different steps were estimated according to EN ISO 11358 [3]. In cases of multistage decrease in mass where the TG curve does not indicate constant mass between the reaction stages, the standard states that the mass loss during the interval between the reactions is split and included in the mass loss of the nearby reactions. As the TG-curves were rippling at higher temperatures, this introduces some uncertainties in the mass losses of the second and third steps. The estimated mass losses are presented in Table 4.

2 Netzsch STA 449C instrument


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Figure 3. Thermogravimetric results for MCMK cable materials.


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Figure 4. DSC results for MCMK cable materials.


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Table 4. Cable material mass losses in the TG decomposition steps.

Layer Sample, environment

Mass loss (%)

Step 1 Step 2 Step 3 Total

Sheath “2009”, air 56 16 8 80

“2009”, N2 56 11 7 74

“2012”, air 55 16 10 81

“2012”, N2 55 9 12 76

Filler “2009”, air 23 7 23 53

“2009”, N2 28 - 18 46

“2012”, air 24 8 26 58

“2012”, N2 27 - 30 57

Insulation “2009”, air 51 19 10 80

“2009”, N2 51 12 8 71

“2012”, air 43 22 9 74

“2012”, N2 43 12 12 67

The onset temperatures for the two first mass loss steps were determined from the TGA curves and are presented in Table 5. Table 5. The extrapolated onset temperatures determined from the TGA curves.

Step Sheath Filler Insulation

“2009” “2012” “2009” “2012” “2009” “2012”

Tonset (oC) environment

1 278 air, N2

278 air, N2

280 air, N2

280 air, N2

281 air, N2

272 air, N2

Tonset (oC) environment

2 438 air, N2

438 air, N2

403 air

391 air

438 air, N2

441 air, N2

For the cable sheath, no differences between “2009” and “2012” curves can be seen up to about 460 oC, where small differences are present, i.e. 2…5 % mass loss differences in step 2 and step 3.


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The filler curves show a difference of 12 oC in the onset of the second step in air, while the initial parts of the curves are overlapping. The mass loss differs significantly only in step 3 in N2. The insulation curves show an onset temperature difference of 9 oC in the first step as the “2009” curve is somewhat steeper, and the mass loss differs with 8 % in step 1 between the “2009” and “2012” samples. The differences for steps 2 and 3 are less significant. The DSC curves show, that the first step in the sheath and insulation curves correspond to an endothermic reaction and the second step to an exothermic reaction. The filler DSC curves show two exothermic reactions. According to the cable manufacturer, both sheath and insulation materials are PVC. The STA curves for the filler material indicates that the filler material is different than the sheath and insulation material. The investigation of the chemical composition of the materials showed that the amount of PVC in the filler was ¼ of the PVC amount in the sheath and 1/3 of the amount of PVC in the insulation. The amount of CaCO3 was twice the amount in sheath and insulation. According to the STA results, the two cable samples are very near each other, except for a small difference for the insulation materials.


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4 Chemical composition of cable materials

The composition of the MCMK “2012” cable was investigated by VTT Expert Services [4] using the following analysis methods:

Fourier transformed infra-red spectroscopy (FTIR)

Gas chromatography combined with mass spectrometry (GC-MS),

X-ray fluorescence analysis (XRF)

X-ray diffraction

Calcination. From the cable, three part samples were separated for analyses: sheath (black), filler (whitish) and insulation (grey, blue and red).

4.1 FTIR -analysis

FTIR -spectra from the different layers of the cable were collected with Bruker Equinox 55 spectrometer using ATR -technique. The FTIR measurements were carried out at room temperature. Spectra from the layers of the cable sample are presented in Figure 5.

Figure 5. Spectra from the sheath layer (black curve), from the filler layer (green curve) and from the insulation of the three conductors: black (blue curve), brown (red curve) and grey coloured insulation (violet curve). According to the spectra all layers (sheath, filler and insulation) contained polyvinyl chloride (PVC) and carbonate. Between the sheath and filler layer there was a bright transparency layer which was polyester. Between the filler layer and insulation there was magnesium silicate (talc).


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The blue and red FTIR curves for insulation are very similar, but the violet insulation curve has peaks at 3676, 1008 and 669 cm-1, which are not present in the two other insulation curves. These come from the magnesium silicate (talc) present on the surface of this sample. The broad peak around 1390 cm-1 and the sharp peak 871 cm-1 come from the carbonate. The peak at 1725 cm-1, the two very small sharp ones on the right side of the 1725 peak and the 1275 cm-1 peak come from the phthalate.

4.2 GC-MS-analysis

Part samples were cut and extracted with petrol ether by using Accelerated Solvent Extraction (ASE) method. The extracts were evaporated to dryness and the residues were dissolved in dichloromethane. The samples were analysed by gas chromatography-mass spectrometry (GC-MS). The compounds detected were identified by using commercial mass spectrometry library search. The concentrations of the compounds were determined semiquantitatively with the external toluene standard response. With the GC-MS method used, nonpolar and semipolar organic compounds with boiling point < 450º C can be analysed. The results are presented in Table 6. The observed phthalate concentrations of 2…8 w-% are low compared to common PVC cable formulations, where plasticizer content is often in the range 20 – 30 w-%. This is probably due to the semiquantitative nature of the method used. Table 6. The concentrations (g/kg) of the detected compounds in the part samples separated from the cable MCMK 3 x 2.5 ”2012”.

Sheath black (g/kg)

Filler whitish (g/kg)

Insulation grey (g/kg)

Di-isodecyl-/didecylphthalate 20 80 40 Palmitic acid 0.02 0.06 0.12 Bisphenol A 0.02 # 0.06 Apidic acid esters, sum 0.27 # # Possibly C22-hydrocarbon # # 0.04

4.3 X-ray fluorescence analysis (XRF)

The samples were analysed by the X-ray fluorescence spectroscopy using the Philips PW2404 X-ray spectrometer and semiquantitative SemiQ program. The samples were analysed for fluorine (F) and heavier elements excluding noble gases, a total of 79 elements. The detection limit of the method is typically in the order of 0.01 %. The analysis results are shown in Table 7. The elements not listed in the table were below detection limits.


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Table 7. Results of the semi-quantitative X-ray analysis (w-%).

Element Sheath black

Filler whitish

Insulation grey

Sodium, Na 0.02 0.04 0.03 Magnesium, Mg 0.05 1.1 1.8 Aluminium, Al 0.05 0.07 0.18 Silicon, Si 0.09 0.94 1.4 Phosphorous, P 0.02 0.05 0.01 Sulphur, S 0.01 0.02 0.01 Chlorine, Cl 26 6.3 19 Potassium, K 0.01 0.03 0.01 Calcium, Ca 13 27 13 Titanium, Ti 0.03 0.01 0.37 Manganese, Mn 0.01 0.01 - Iron, Fe 0.03 0.09 0.05 Copper, Cu - 0.54 - Zinc, Zn 0.04 0.01 0.19 Strontium, Sr 0.03 0.06 0.03 Antimony, Sb 0.02 0.01 -

”-” Concentration is below the detection limit.

4.4 Calcination

Calcined residue of the samples at 950 °C were Sheath, black 19.4 % Filler, whitish 43.2 % Insulation, grey 25.2 %

4.5 X-ray diffraction analysis (XRD)

X-ray diffraction analysis was run on the samples using Philips X’Pert MPD diffractometer and the powder method. In this method crystalline compounds can be detected from the sample. Diffractograms of the samples are presented in Appendix A. According to X-ray diffraction analyses calcium carbonate (CaCO3) was detected from all the samples. In addition small amount of titanium dioxide (TiO2) was detected from the grey insulation sample.

4.6 Conclusions from chemical composition analysis

According to the analysis, all layers (sheath, filler and insulation) contained polyvinyl chloride (PVC) and calcium carbonate. Phthalates were used as softeners. Between the black (sheath) and whitish (insulation) layer there was a bright transparency layer which was polyester. Between the whitish layer (filler) and insulation there was magnesium silicate (talc).


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Combining results from XRF with the identification of PVC, CaCO3 and TiO2, one can estimate the amounts of these compounds from atomic weights. Adding results from GC-MS one obtains the total mass of identified components as presented in Table 8. The calcined residues are in very good agreement with the mass loss determined with TG for “2012” cable in air (Table 4). Table 8. Mass proportions of identified components in the different cable materials.

Compound Sheath, black (weight-%)

Filler, whitish (weight-%)

Insulation, grey (weight-%)

Sodium, Na 0.02 0.04 0.03 Magnesium, Mg 0.05 1.1 1.8 Aluminium, Al 0.05 0.07 0.18 Silicon, Si 0.09 0.94 1.4 Phosphorous, P 0.02 0.05 0.01 Sulphur, S 0.01 0.02 0.01 Vinyl chloride C2H3Cl 45.83 11.11 33.49 Potassium, K 0.01 0.03 0.01 CaCO3 32.46 67.42 32.46 TiO2 0.05 0.02 0.62 Manganese, Mn 0.01 0.01 Iron, Fe 0.03 0.09 0.05 Copper, Cu 0.54

Zinc, Zn 0.04 0.01 0.19 Strontium, Sr 0.03 0.06 0.03 Antimony, Sb 0.02 0.01

Total from above FTIR+XRF+XRD

78.73 81.52 70.29

Di-isodecyl-/didecylphthalate 2 8 4 Palmitic acid 0.002 0.006 0.012 Bisphenol A 0.002 0.006 Apidic acid esters, sum 0.027 Possibly C22-hydrocarbon 0.004 Total from GC-MS 2.03 8.01 4.02

Sum 80.76 89.52 74.31


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5 Cone calorimeter experiments

Standard cone calorimeter tests [5] were carried out on MCMK 3x2.5 mm2 “2012” samples. The experiment was carried out twice at the radiation exposure level 50 kW/m2 and the experiments were continued until no flames were visible. The samples were tested as such without previous conditioning. The main results are presented in Table 9. The heat release rate (HRR) curves are shown in Figure 6, the mass loss rate (MLR) curves in Figure 7 and the effective heats of combustion (EHC) in Figure 8. The EHC curves were determined by dividing the HRR curve by the MLR curve. In the light of the all three quantities, the three experimental results are very close to each other. The relative mass losses of the “2009” and “2012” cables are close to each other. Total heat release (THR) per unit area and the effective heat of combustion (THR/mass loss) for “2009” sample is within the scatter of the “2012” samples. The only significant difference in these figures is the ignition time, which is shorter for “2009” than for “2012” samples. This is also seen from repeatability for results in Table 9 which according to ISO 5660-1 [5] are Ignition time repeatability: 6…7 s THR repeatability: 15.9…16.5 MJ/m2 THR/mass loss repeatability 2.1 MJ/kg The THR and THR/mass loss values are all within the repeatability limits but ignition time for “2009” is on the limit of repeatability compared to”2012” experiment 2 and outside the limit compared to experiment 3. When evaluating the importance of the observed difference in the ignition times, it is important to know that in a recent (unpublished) inter-laboratory round robin of cone calorimeter experiments, where PMMA samples were burned at 35 kW/m2 heat flux, the ignition times observed with VTT’s cone calorimeter were clearly longer then the values observed by the other laboratories. Table 9. Main results from standard cone calorimeter experiments.

Exp. Sample Dia. (mm)

Initial mass (g)

Mass loss (g (%))

Ignition time (s)

THR (MJ/m2)

THR/mass loss (MJ/kg)

1 MCMK 4x1,5 mm2 ”2009”

11 174.2 70.0 (40 %) 17 131.2 17.1

2 MCMK 3x2.5 mm2 “2012”

12 190.4 67.2 (35 %) 23 133.4 17.5

3 MCMK 3x2.5 mm2 “2012”

12 190.3 66.8 (35 %) 26 124.9 16.5


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Figure 6. Heat release rates from standard cone calorimeter experiments at 50 kW/m2 heat flux.

Figure 7. Mass loss rates from standard cone calorimeter experiments at 50 kW/m2 heat flux.


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Figure 8. Effective heat of combustion from standard cone calorimeter experiments at 50 kW/m2 heat flux.


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6 Microscale combustion calorimeter (MCC) experiments

Microscale combustion calorimetry, or pyrolysis combustion flow calorimetry, is a method for measuring the combustibility of milligram samples [6, 7]. The MCC experiment is carried out using controlled heating of small, milligram specimens and complete thermal oxidation of the decomposition gases from the specimen. The specimens are weighed before they are inserted in a pyrolysis chamber and thermally decomposed in either an oxygen-free (anaerobic) or oxidizing (aerobic) environment at a constant heating rate of 0.2…2 K/s. The gases released are swept from the pyrolysis chamber, mixed with additional oxygen, and completely oxidized in a high temperature combustion chamber. The oxygen consumed in the combustion process is measured to calculate by oxygen consumption calorimetry the amount and rate of heat released by combustion of the specimen during controlled heating. Dividing the heat release rate by the initial sample mass gives the specific heat release rate q (W/g). The specific heat release rate is determined as a function of both temperature and time. MCC experiments were carried out on MCMK “2012” sheath, filler and insulation samples using an oxygen-free (anaerobic) pyrolysis environment. Maximum temperature was 900 °C in all experiments except filler experiment 1 and insulation experiments 2-4 where maximum temperature was 750 °C. The constant heating rate was 1 K/s in all experiments. Series of three experiments were carried out for sheath and filler, and five experiments for insulation, of which the first one failed. Specific heat release rate versus temperature are presented in Figure 9. There are very small differences between individual specific heat release curves in the series. Moreover, the heat release curves of the three cable components are quite similar. Two major reactions producing combustible gases are seen for all components, one starting at 200 C and the other starting at about 400 C. A third but much weaker reaction is observed at 700 C.


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Figure 9. Specific heat release rate for MCMK ”2012” sheath, filler and insulation materials in MCC tests.


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Numerical results from the MCC tests are given in Table 10. The notations in the table are

m0, initial sample mass Y (-), pyrolysis residue, residual sample mass divided by initial sample

mass, hc (J/g), heat of combustion of sample, total heat released divided by initial

mass hc, gas (J/g), heat of combustion of sample gases, total heat released divided

by mass loss during test Tmax (K), heat release temperature, the temperature at the maximum value

of specific heat release (kJ/kgK), heat release capacity, maximum value of the specific heat

release rate divided by the average heating rate.

Table 10. MCC results for cable MCMK "2012" sheath, filler and insulation. Notations are explained in the text above.

Sheath m0 (mg) Y (-) hc (MJ/kg) hc, gas (MJ/kg)

Tmax ( C)

(kJ/kg•K)

Exp 1 6.24 0.26 14.7 19.9 297 262.2

Exp 2 8.83 0.26 14.8 20.0 301 258.6

Exp 3 7.81 0.26 14.6 19.7 302 287.9

Mean 7.63 0.26 14.7 19.9 300 269.6

Filler

Exp 1 14.15 0.50 6.8 13.6 324 138.5

Exp 2 7.14 0.46 7.0 12.8 321 151.0

Exp 3 10.19 0.46 7.3 13.5 323 145.6

Mean 10.49 0.47 7.0 13.3 322 148.3

Insulation

Exp 2 6.95 0.30 12.3 17.6 303 209.4

Exp 3 5.2 0.32 11.8 17.4 297 239.5

Exp 4 7.41 0.29 12.7 17.9 302 206.8

Exp 5 6.11 0.28 12.2 17.0 301 197.4

Mean 6.68 0.30 12.3 17.5 301 213.3


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Combining the heat of combustion of sample gases hc,gas for the different cable parts with the combustible mass fractions

=,

(1)

where mi is the mass of component i (i = sheath, filler, insulation) and mtot, comb is the total mass of combustible parts of the cable (Table 2) one gets the effective heat of combustion for the whole MCMK cable

, = , , = 17.7 (2)

This is in good agreement with the average of the two cone calorimeter results 17.0 MJ/kg (Table 9). Comparing the pyrolysis residue figures Y in Table 10 with the calcined residue (Section 4.4) shows relative deviations 34 % for sheath, 9 % for filler and 19 % for insulation.

Integrating each peak in the specific heat release rate - time curves gives the heat (energy) released by the reaction(s) contributing to the peak normalised by the initial mass. Dividing this heat released by the corresponding relative mass loss measured by thermogravimetry (Table 4) gives the heat of combustion relating to the considered peak. Table 11 presents these results together with the corresponding temperature ranges for the two first peaks. The analysis was not carried out for the third weak peak, as the third step in the TG-curves in Section 3 is mainly due to the decomposition of CaCO3 producing CO2 but no combustible gas (cf. Section 8).


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Table 11. Heat released and heat of combustion of main peaks for cable MCMK "2012" sheath, filler and insulation.

Sheath Peak 1 Peak 2

Temperature range ( oC)

Energy/initial mass (kJ/g)

Heat of combustion

(kJ/g)

Temperature range ( oC)

Energy/initial mass (kJ/g)

Heat of combustion

(kJ/g)

Exp 1 169 - 359 10.5 19.1 361 - 573 4.0 25.3

Exp 2 168 - 357 10.5 19.1 357 - 627 4.2 26.2

Exp 3 166 - 358 10.5 19.1 358 - 588 4.1 25.4

Mean 10.5 19.1 4.1 25.6

Filler

Exp 1 172 - 371 5.4 22.5 370 - 543 1.4 18.0

Exp 2 174 - 353 5.2 21.7 354 - 529 1.3 15.9

Exp 3 175 - 368 5.3 22.3 369 - 535 1.4 16.9

Mean 5.3 22.2 1.4 16.9

Insulation

Exp 2 182 - 340 8.1 18.9 360 - 567 3.9 17.6

Exp 3 187 - 330 7.9 18.3 367 - 545 3.7 16.7

Exp 4 198 - 352 7.8 18.2 353 - 565 4.2 19.1

Exp 5 183 - 354 8.1 18.8 355 - 573 3.9 17.5

Mean 8.0 18.5 3.9 17.8


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7 Flame spread experiments on 2 m samples

Flame spread experiments were carried out on MCMK 3x2.5 mm2 “2012”. The dependence of vertical flame spread on ambient temperature was determined with the VTT 2 m apparatus [8].

7.1 Methods

The 2 m long sample is heated to desired temperature with hot air circulating inside the test rig. Air is circulated at highest possible speed in the heating phase, and the air speed is lowered to 0.3 m/s before starting the flame spread experiment. Maximum gas temperatures (Tmax) in the test channel are measured just before lowering the flow rate to the test conditions. After this the temperatures decrease somewhat before the ignition of the burner. The ‘nominal’ temperature of the flame spread experiment is denoted Tave and calculated as the average temperature in the test channel just before turning the burner on. The flame spread experiment starts when the propane burner is ignited below the lower end of the sample, putting the lower part of the sample in flame contact. Time of the ignition and the origin of the time scale in the figures refer to ignition of the propane burner. The heating phase is not shown in the figures. After ignition, fresh air is drawn into the test rig through the heating resistors at sufficiently high flow rate (0.3 m/s) to provide oxygen for burning during the experiment. Combustion products exit through an outlet in the upper part of the system. Flame spread is monitored with 20 thermocouples close to the sample surface at 100 mm vertical intervals. Data acquisition rate is 1 Hz. Rate of flame spread is deduced from the temperature measurements determining the moment of temperature rise above 300 oC at the specific thermocouple heights. Plotting this flame spread as a function of time one notes that after some time (length of burning), the flame spreads at constant velocity. A straight line is then fitted to this part of the curve, giving the flame spread rate as the slope of the line. The temperature range studied was from room temperature 20 oC to 176 oC. Higher temperatures were not attempted as thermogravimetric results (Section 3) showed that the cable sheath starts to decompose at approximately 200 oC indicating an upper temperature limit for experiments. The propane burner was on until the thermocouples indicated that the sample had ignited and the flame spread was established. The burner output power was 510...600 W and the burning duration varied between 1 min 53 s and 5 min 46 s. The burner flame height without a sample is approximately 200 mm at output power 600 W.


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7.2 Results

A summary of the flame spread results is here presented. Actual temperature-time curves at different heights, and flame spread as a function of time with straight line fitting are presented in Appendix B. Table 12 lists the flame spread experiments presenting the average maximum gas temperature, average gas temperature immediately before the ignition of the propane burner (Tave), the duration of the propane burner flame, the pre-heating time to burner ignition, and the rate of flame spread. The cable ignited and burned to full length in all experiments. The results from the previous flame spread experiments on “2009” cable [1] are included in the table for comparison. Observed flame spread rates and the corresponding temperature ranges were MCMK 4x1.5 mm2 “2009”: 4.2 ... 22.9 mm/s 23 ... 176 oC MCMK 3x2.5 mm2 “2012”: 3.4 ...14.4 mm/s 20 ... 170 oC The observed flame spread rates as a function of ambient temperature are presented in Figure 10. Horizontal “error bars” on the right of the markers indicate Tmax showing the degree of cooling before ignition. Vertical error bars are ± one standard deviation (10 %) of the flame spread rate measurement, as described in [9]. The measured flame spread rates Vf were plotted against Tdec - Tave, where Tdec is the minimum temperature at which decomposition starts, and a power law dependence

nf TTV avedec (3)

was fitted for each sample. Here Tdec = 252 oC was used, which is the extrapolated onset temperature determined for ”2009” cable sheath material with TGA at heating rate 2K/min [10]. The results of the power law fit are presented in Figure 10 and Figure 11. The results for the “2012” sample are slightly lower than for the “2009” samples.


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Table 12. Experimental features in flame spread experiments with MCMK “2009” and “2012” cable samples.

Cable type Experiment

Average maximum gas temperature (oC)

Average gas temperature at ignition of propane burner (o C)

Duration of propane burner flame (min:s)

Pre-heating time to burner ignition (min:s)

Rate of flame spread (mm/s)

MCMK 4x1.5 mm2 , “2009” 1 23 23 4:27 - 4.2

2 142 134 2:46 15:26 12.7

3 142 135 2:40 15:38 10.4 4 144 137 2:38 15:33 11.0

5 188 176 1:54 23:50 22.9

6 162 155 1:41 18:14 13.2 7 87 82 3:54 8:00 5.7

MCMK 3x2.5 mm2 , “2012” 1 20 20 4:19 - 3.4

2 143 113 2:40 17:45 5.0

3 182 170 2:16 23:50 14.4


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Figure 10. Rate of flame spread as a function of ambient temperature for MCMK cable samples.

Figure 11. Power law fit to rate of flame spread as a function of Tdec – Tave.


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8 Discussion

The results from MCMK cable material can be compared to the results of Beneš et al. [11], which the studied lifetime simulation and thermal characterization of commercial PVC cable insulation. The PVC cable insulation sample contained the following additives: dioctyl phthalate (DOP) as plasticizer in the amount of 22 % mass/mass, CaCO3 as filler, a Pb based compound as thermal stabilizer, Pb and Ca stearate as lubricants and carbon black. Combined TG/FTIR (5K/min in air) analysis showed the results presented in Table 13 during the observed three degradation steps. Table 13. Mass loss determined by TG for commercial PVC cable insulation materials on heating in air in the range 20…800 oC [11].

Degradation step

1 2 3 Total

Temperature range (oC)

200…340 360…540 540…800 20…800

Mass loss (%) 45.9 17.2 8.5 71.6

Released compound

HCl, phthalate, CO21) CO2 2) CO2 3)

1) CO2 release assumed to be due to the reaction between HCl and CaCO3. 2) CO2 release assumed to be due to the burning of the polymer backbone. 3) Intense CO2 release assumed to be due to the thermal degradation of the carbonate filler. The main components of this insulation material are similar to those of MCMK, although the phthalate is not the same. The temperature ranges in Table 13 includes parts of the non-steady intervals between the reactions. Taking this into account, the observed temperature ranges and the mass losses of the Beneš study and for MCMK “2012” insulation in can be considered similar. The assumed reactions for the different degradation steps in Table 13 are probably the same for MCMK. The “2009” and “2012” cable types differ in structure by the number of conductors and cross-sections, 4x1.5 mm2 for “2009” sample and 3x2.5 mm2 for “2012” sample. The linear mass for Cu metal is about 20 g/m greater for “2012” sample. Linear mas for combustibles are quite similar with differences 3…4 g/m. It is possible that the difference in conductor masses contributes to the different flame spread rates observed in the 2-m apparatus, but this effect cannot be confirmed.


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9 Conclusions

Thermal, chemical and fire performance characterization has been carried out for the PVC-cable (MCMK “2012”) to be used in the cable flame spread experiments of the OECD PRISME2-project. Results from our previous studies on a similar cable (MCMK “2009”) have been included in the report for comparison and to enable the evaluation the applicability of the previously determined pyrolysis model parameters. According to the thermal analysis, the two cable samples are very close to each other in terms of thermal degradation performance, except for a small difference for the insulation materials, where the mass loss for the first step is 8 % greater for “2009” sample. According to the chemical composition analysis, all the layers (sheath, filler and insulation) contained polyvinyl chloride (PVC) and calcium carbonate. Phthalates were used as softeners in all layers. Between the black (sheath) and whitish (insulation) layer there was a bright transparency layer which was polyester. Between the whitish layer (filler) and insulation there was magnesium silicate (talc). The chemical composition was not determined for the “2009” sample. Cone calorimeter tests showed differences in ignition times, which were 6…9 s (26…35 %) shorter for the “2009” sample. However, the experimental uncertainty of the ignition time measurement was high. Other quantities were close to each other for “2009” and “2012” samples. The “2012” cable was studied with the new microscale combustion calorimeter (MCC) introduced at VTT in the beginning of the year 2013. Specific heat release rate, pyrolysis residue, heat release capacity, heats of combustion and heat release temperature were determined for sheath, filler and insulation materials. Vertical flame spread rates along a single cable were slightly higher for the “2009” sample. At room temperature and at 100 oC, the flame spread rate on the “2009” cable was 1.3 times the rate for “2012” and 1.6 times at 200 oC. The present comparison of the MCMK “2009” and “2012” samples showed that the cable performance in the tests carried out was rather similar, with minor differences in the ignition time (cone calorimeter) and flame spread rate (2-m apparatus).


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References

1. Mangs, J. & Matala A. Flame spread and pyrolysis of PVC cables. Research Report VTT-R-00626-10. 18 p. + App. 26 p.

2. Matala, A. & Hostikka, S. Pyrolysis Modelling of PVC Cable Materials. Fire Safety Science 10, 10th International IAFSS Symposium, University of Maryland, USA, 19-24 June, 2011. The International Association for Fire Safety Science, IAFSS (2011), 917-930. doi: 10.3801/IAFSS.FSS.10-917

3. EN ISO 11358:1997. Plastics - Thermogravimetry (TG) of polymers – General principles. Brussels: European Committee for Standardization. 11 p.

4. Hiukka, R. Identification of materials. Research Report VTT-M-04829-12. 6 p.

5. ISO 5660-1:2002 Reaction-to-fire tests -- Heat release, smoke production and mass loss rate -- Part 1: Heat release rate (cone calorimeter method). Geneva: International Organization for Standardization. 39 p.

6. Lyon, R.E. & Walters, R.N. Pyrolysis combustion flow calorimetry . J. Anal. Appl. Pyrolysis Vol. 71 (2004), p. 27–46.

7. ASTM D7309 – 11. Standard Test Method for Determining Flammability Characteristics of Plastics and Other Solid Materials Using Microscale Combustion Calorimetry. ASTM International, West Conshohocken, PA, 2013. 10 p.

8. Mangs, J. A new apparatus for flame spread experiments. VTT Working Papers 112. Espoo 2009. 51 p. + App. 28 p. http://www.vtt.fi/inf/pdf/workingpapers/2009/W112.pdf

9. Mangs, J. & Hostikka, S. Experiments and numerical simulations of vertical flame spread on charring materials at different ambient temperatures. Proceedings of the 10th IAFSS Symposium, College Park, Md, 19-24 June 2011. Fire Safety Science 9 (2011), p. 499 – 512.

10. Mangs, J. & Hostikka, S. Vertical flame spread on charring materials at different ambient temperatures. Fire and Materials. Published online in Wiley Online Library (2012). doi: 10.1002/fam.2127

11. Beneš, M., Pla ek, V., Matuschek, G., Kettrup, A. A., Györyová, K., Emmerich, W. D. and Balek, V. Lifetime simulation and thermal characterization of PVC cable insulation materials. Journal of Thermal Analysis and Calorimetry, Vol. 82 (2005), p. 761–768.

http://dx.doi.org/10.3801/IAFSS.FSS.10-917

http://www.vtt.fi/inf/pdf/workingpapers/2009/W112.pdf

http://dx.doi.org/10.1002/fam.2127


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Appendix A. X-ray diffractograms for MCMK cable materials

Figure A1. Diffractogram of sample sheath black.

Figure A2. Diffractogram of sample filler whitish.


A2 (2)

Figure A3. Diffractogram of sample insulation grey.

Figure A4. Collection of the three diffractograms.


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Appendix B. Results from experiments with 2 m apparatus on PVC cable MCMK 3x2.5+2.5 mm2 “2012”

Figure B1. Vertical temperatures in MCMK 3x2.5+2.5, “2012”, flame spread experiment 1.

Figure B2. Flame and extinction fronts as a function of time in MCMK 3x2.5+2.5 “2012”, flame spread experiment 1.

y = 3.41x - 602.10R² = 0.95

y = 3.63x - 1 363.03R² = 0.95

y = 3.62x - 1 432.60R² = 1.00

0

200

400

600

800

1000

1200

1400

1600

1800

2000

100 200 300 400 500 600 700 800 900

Hei

ght (

mm

)

Time (s)

25.4.2012 MCMK 3x2.5 mm2 "2012", Exp. 1, Tave = 20 C

ignext 450 Cext 400 CLinear (ign)Linear (ext 450 C)Linear (ext 400 C)


B2 (3)



y = 5.04x - 1 574.49R² = 0.90

y = 4.53x - 2 232.30R² = 0.97

0

200

400

600

800

1000

1200

1400

1600

1800

2000

100 200 300 400 500 600 700 800 900

Hei

ght (

mm

)

Time (s)

14.5.2012 MCMK 3x2.5 mm2 "2012", Exp. 2 ,Tave = 113 C

ign 300 C

ext 400 C

Linear (ign 300 C)

Linear (ext 400 C)


B3 (3)



0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700 800

Tem

pera

ture

(C)

Time (s)

16.5.2012 MCMK 3x2.5 mm2 "2012", Exp. 3, Tave = 170 C T1 [C]

T2 [C]

T3 [C]

T4 [C]

T5 [C]

T6 [C]

T7 [C]

T8 [C]

T9 [C]

T10 [C]

T11 [C]

T12 [C]

T13 [C]

T14 [C]

T15 [C]

T16 [C]

T16 [C]

T17 [C]

T18 [C]

T19 [C]

y = 14.39x - 3 826.45R² = 0.95

y = 6.89x - 2 998.40R² = 0.66

y = 6.9946x - 2850.8R² = 0.6339

0

200

400

600

800

1000

1200

1400

1600

1800

2000

100 200 300 400 500 600 700 800 900

Hei

ght (

mm

)

Time (s)

16.5.2012 MCMK 3x2.5 mm2 "2012", Exp. 3 ,Tave = 170 C

ign 300 C

ext 400 C

ext 450 C

Linear (ign 300 C)

Linear (ext 400 C)

Linear (ext 450 C)

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