A fire test device suitable for small-scale testing of fire retardant coatings

A Fire Test Device Suitable for Small-scale Testing of Fire Retardant Coatings

M. Kay BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN,

A. F. Price Department of Chemical Engineering, The University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET, UK

A fire test device suitable for small-scale, comparative testing as a first step in investigating the performance of fire retardant materials is described. The procedure developed is not intended to predict the performance of a given material in a real fire, since the local conditions can be extremely variable. However, the parameters in the device can be varied over a wide range to screen the performance of various materials in given situations. Typical results for an intumescent mastic consisting of epoxy resin, hardener and melamine phosphate are quoted, together with appropriate calibration curves. A method of estimating the thermal resistance of the developing char is also discussed.

INTRODUCTION

The assessment of a coating for fire protection requires that it be tested in a reproducible fire simulation situation. However, the development and behaviour of real fires vary widely, being dependent upon the nature of the fuel, the geometry of the site, the prevailing meteorological conditions and the pres- ence of different materials. Therefore a large number of tests have been specifically designed for certain situations.

Petroleum-fuelled fires develop with great rapidity, may reach 1100 "C in 3 min and engender high radiant heat fluxes. Mobil have developed a test of resistance to such fires' and the US Department of Transporta- tion have carried out spectacular tests using rail tank cars,2-6 while road tankers have been subjected to similar tests in the UK.7

In contrast, a wood-fuelled fire develops considerably more slowly, reaching a temperature of 500°C after about 5 min, and around 900°C after about 45 min. Such a temperature development is the basis of the British Standard test BS476: 1972, Part 8 for testing the fire resistance of structural elements.

Two parameters are of special importance in deter- mining the effect of fire in a particular situation: flame spread and fire resistance. The former indicates the rate at which a fire propagates itself while the latter is a measure of the destructive effects of fire to a structural element. This is a complex parameter incorporating measures of insulation (temperature rise of unex- posed areas), integrity (propagation of cracks allowing penetration of flames) and stability (resistance to phys- ical collapse) of the structural element. Fire resistance is therefore a property of a structure which may involve a coating; but it is not a property of the coating itself.

Due to the multitude of fire tests available and

the many variables involved in a real fire, the meaning of any fire test result will require careful interpreta- tion.'-'' A large-scale test is a better simulation of a real situation'* than small-scale laboratory tests, which produce lower surface temperatures and lower temperature gradients than exist in practice." However, despite the disadvantages of laboratory lire tests, it was considered necessary14 to devise a small-scale test facility which would enable a high throughput of samples to be examined at very low cost but under closely reproduced conditions. By this means, the effects of variations in formulations on intumescent performance can be determined to identify the more promising coatings which can then be subjected to a full-scale test.

The progress of samples under test was monitored by noting the substrate temperature, the thickness and the quality of the advancing char generated during intumescence. Low substrate temperatures for long periods of time coupled with a hard, durable char would indicate a good performance.

EQUIPMENT

The fire test device is shown in Fig. 1 and schematically outlined in Fig. 2, and was developed for the investigation of a melamine phosphate-epoxy resin intumescent mastic.lS It consists essentially of a tunnel burner of face measuring 152x 102 mm, burning a mixture of air and natural gas, mounted in a fume extraction hood. How rates of air and gas were con- trolled at the required values by the system shown in Fig. 3, which also indicates the safety devices.

Samples to be tested took the form of a 6-mm thick coating on a mild steel substrate measuring 95 mm square. This was placed in a heat resistant Sindanyo

OWiley Heyden Ltd, 1982

CCC-0308-0501 /82i0006-0111$04.00

FIRE AND MATERIALS, VOL. 6, NOS 3 AND 4, 1982 111

M. KAY AND A. F. PRICE

FACE TEMPERATURE

Figure 1. Fire test.

frame shown in Fig. 4, such that it was facing the burner and positioned at a suitable distance to achieve the desired face temperature conditions.

Temperatures were measured by chromel-alumel thermocouples connected to a manually compensated voltmeter accurate to +2"C in the range 0-1100°C. Two thermocouples of 3 m m diameter sheathed with Inconel were mounted on each side of the sample in such a way that conduction errors were minimized (Fig. 9). These were used during tests to ensure that experimental conditions were comparable: surface temperatures of the advancing face of an intumescing sample were measured by a separate technique. A thermocouple attached to the centre of the mild steel substrate was used to assess the performance of the sample with increasing time.

During the course of a test, the advancing face of the expanding char achieved a higher temperature than the thermocouples mounted in the Sindanyo frame due to the smaller distance from the burner. There- fore, two 1 mm rapid response thermocouples were mounted along the central axis of the test region, one in the plane of the sample frame, in front of a mild steel plate, and the other 45 mm nearer the burner. These were then calibrated against the two thermocouples mounted in the frame, in the absence of an intumescent sample. All thermocouples were blac- kened to compare favourably with the black char obtained during intumescence. It was then possible to relate the temperature indicated by the thermocouples on each side of the sample together with the char thickness to the temperature of the front face of the char. According to the char thickness, either of the calibration curves or a suitable derived curve was used to model the front face temperature of a char. These curves are shown in Fig. 5 : samples were tested at 875 f 25 "C and 775 +25 "C.

RADIOMETRY

Incident radiation levels at various points within the fire test apparatus were measured using a gold disc radiometer, of the type described by McGuire and Wraight," loaned by the Fire Research Station. This

n

Figure 2. Schematic outline of fire test rig. A: burner; B: Sindanyo sample holder; C: sliding glass doors; D: door frames; E: working surface.

112 FIRE AND MATERIALS, VOL. 6, NOS 3 AND 4, 1982

DEVICE SUITABLE FOR TESTING FIRE RETARDANT COATINGS

-1 I I I I I

Figure 3. Flow control system. 1 : burner; 2: air blast injector; 3: air flow measurement (rotameter, size 24); 4: gas flow measurement (rotameter, size 18); 5: valve with butterfly positioning; 6: air pressure measurement (mercury manometer); 7: pressure reduction valves (Spirax); 8: Shut-off valve; 9: pressure gauge (0-100 p.s.i.); 10: mixture pressure measurement (water manometer); 11 : gas pressure (reduced) measurement (water manometer); 12: shut-off valve; 13: zero governor; 14: gas pressure measurement (water manometer); 15: pressure control governor; 16: solenoid-operated shut-off valve; 17: non-return valve (otan); 18: shut-off valve; 19: photocell; 20: electrical control.

enabled a correlation to be determined between test conditions of heat input and the position of a test sample within the apparatus, taking account also of the effects of convective heat transfer as obtained from the air flow patterns within the apparatus. Such air flow patterns were determined using smoke tubes. They are shown schematically in Fig. 6.

The fire test apparatus was operated using air and gas flow rates of 3701min-’ and 261min-’, respectively, and radiometer measurements made at a height of 175 mm above the working surface. The results are shown in Fig. 7. Rapid fluctuations in recorded output were noted at all positions at which the radiometer was held, which varied in magnitude according to the distance from the burner. The magnitude of these fluctuations is shown by the bars in Fig. 7. A height of 175 mm is in the upper part of a test sample; a smaller number of observations were made at lesser heights, and although these showed slightly lower heat levels (except very close to the burner, where the highest levels were near the centre of the burner), the same general trends as described below were apparent.

The most noticeable observation was an increase in the rate of decay of incident heat with distance when the range of the radiometer was around 360 mm. This coincides with the near edge of the exhaust flue where

A I

1 I I ‘I 6’10 A ,’

4.3

57

Figure 4. Schematic diagram of Sindanyo sample holder. A: sample well; B: holes for thermocouple measuring face temperature; C: mild steel plate.

the major updraught occurs. The decay in radiation with range may be satisfactorily modelled using either an exponential decay model

w = a . exp ( b R ) (la) .. In w =In a + b R (Ib)

w = pR4 (24 .. In w =In p + q In R (2b)

or a power law decay model

which, both show a change of slope in their logarithmic plots (Fig. 8). Both models approximate the actual results well; the advantage of the exponential decay model is that it gives a finite value of source radiation output at R = 0.

The linear nature of the plots using logarithmic axes allowed linear regression analysis to be used for computational determination of the parameters of Eqns (1) and (2). The power law model probably has a better basis in physics, and showed less variation in the

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1 Central situation,ot the front of the frame 2 Central situation,45 mm in front of frame 3 Right-handsideof samplewell 4 Left-hond side of sample well

Viewed from burner

I I I I I I I 100 200 300

Range ( rnm)

Figure 5. Thermocouple measurements in the fire test device.



Figure 6. Operational air flow patterns.

computational parameters derived as an increasing number of points were included in the analysis up to the boundary at which the change in slope begins to take place. The exponential model showed a progres- sive decrease in the absolute values of these parame-

5 0

4 0

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5 3 0 3 - - 0 01 L

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I I I I I I 200 400 600

Range ( rnrn I Figure 7. Radiometer readings.

ters as the number of points was increased. The inclu- sion of points at ranges in excess of around 300mm led in both cases to an increase in the absolute values of all the parameters with the increased rate of decay of incident heat, and the first occurrence of this sys- tematic increase was used to determine the boundary for computational purposes. Separate linear regression analysis was then carried out for mid- and far-range points.

The computational analyses gave the following equ- ations for the results shown in Fig. 7 using, respectively, exponential and power law decay models:

w = 6.27 exp (-0.00428 R ) R < 300 mm (3a) w = 14.76 exp (-0.00685 R ) R > 300 mm (3b) w = 268 Rp0.877 R < 300 mm ( 4 4 w = 7.85 x lo6 Rp2."" R > 300 mm (4b)

w: incident heat (W cmp2) R : range, distance of radiometer

from burner (mm)

Examination of Fig. 7 shows that a linear decay model is a reasonable approximation within the limits 100mm<R<500rnm, although it has no basis in fact:

( 5 ) w = 4.37 -0.00835 R w(W cm-2), R (mm)

FIRE TESTING PROCEDURE

The process of fire testing was designed to give infor- mation about the reaction to heat and fire of a variety


DEVICE SUITABLE FOR TESTING FIRE RETARDANT COATINGS

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substrate on the upper face before application of the coating, and more qualitatively by noting the thickness of the char present throughout the test and monitoring the progress of tests photographically.

Fire testing was normally carried out under three sets of conditions, i.e. constant ranges of 125mm or 190 mm giving heat fluxes of 4.5 x lo4 W m-2 or 2.8 x lo4 W mp2, respectively, or to a gradually increasing heat flux to 4.5 x lo4 W m -2 effected by continually moving the sample in its holder towards the burner by means of Tufnol rods attached to the back of the holder. The somewhat higher incident heat levels at the top of any sample compared with the base of the sample due to convective effects resulted in faster reaction and char degradation at the top; however, central monitoring of substrate temperatures gave a reasonable indication of sample performance.

EXPERIMENTAL RESULTS

I I I I I I 100 200 300 400 500 600

Range ( m m )

Figures 9 and 10 show typical char formations for two different formulations tested under identical conditions. The continual generation of foamed char is clearly shown in Fig. 9, where the initial layers of char

Figure 9. Typical voluminous char; about 100 m m obtained after about 10 min in a test at 750-800°C.


intumescence would be less extensive and the temperature would remain considerably lower. Rapid intumescence always led to char ablation and early failure of a sample. For example, tests with samples incorporating large needles of stoichiometry (C3H6N6)I.2. H,PO, produced 95 mm and 45 mm of intumescence after 7imin and only 10mm and 20 mm, respectively, after 30 min. The first sample followed the upper response curve while the second followed the lower (better) curve. It is not clear at this stage what factors determine whether a particular sample will follow the better or poorer performance path.

10 20 3 0 40 50 Time (min)

Figure 11. Typical time-temperature responses a t 750-800 "C for mastics containing 12% melamine phosphate. 1: medium needles (C3H6N&,,, . H,PO,. large needles (C3H6N6)l.2. H,PO,; 2: very large needles (C3H6N6)l,o. H,PO,; 3: small needles (C3H6N6)l,z. H,PO,; 4: small block plates (C3H6N6)1.z. H,PO,; 5: thin reflective plates (C3H6N6)l.3 . H,PO,, plates (c3H6N6)z.o ' H,PO,.

have collapsed downwards as the newer char pushed forwards towards the burner.

Figures 11 and 12 show typical temperature responses of the substrate for various formulations of intumescent material^.'^ The end of a test was taken as the time that the substrate temperature reached about 450 "C, or when the top third of the sample had ablated completely. Distinctly different responses are apparent for the samples tested, showing that the method can be used to compare the behaviour of different materials under similar conditions. This could be helpful in reducing substantially the amount of expensive investigation that would be necessary using standard methods.

An unusual phenomenon is shown in Fig. 12 as the 'dual response pattern'. During the examination of a large number of sample^'^ it was noted that apparently identical samples tested in the same fashion would behave in one of two ways. Either the samples in- tumesced voluminously in the early part of a test, allowing the substrate temperature to rise slowly, or

6oo I

I I m- 10 ;o ' 40 50 Time ( min)

Figure 12. Typical time-temperature responses at 750-800 "C for mastics containing 30% melamine phosphate. 1 : needles (C3H6N6)l.o. H,PO,, plates (C3H6N6)l,l . H,PO,- dual response pattern; 2: large needles (C3H6N6)q,2. H,PO, - dual response pattern; 3: small needles, small block plates (C3H6N6)l.2. H,PO,-dual response pattern; 4: thin reflective plates (C3H6N6)l,3 . H,PO,, plates (C3H6N6)2,0 . H,PO,.

HEAT TRANSFER ACROSS THE TEST SAMPLE

Complete insulation of the rear face of the substrate to avoid heat losses was found to be impossible; therefore, rather than use any form of partially effective insulation, heat losses were measured in a series of specially designed experiments, allowing the assessment of the thermal resistance of chars after compen- sation for such losses. All substrate temperatures measured during experiments designed as comparative tests of different coatings are therefore the resultant of heat transfer across the sample and heat loss from the back face of the substrate, i.e.

heat flu conducted rate of increase heat flu loss through char of internal from back of

energy of sub- substrate strate

where h = film coefficient of heat transfer from substrate to

air rn =mass of substrate

t = time x = char thickness A =thermal conductivity of char T = chosen time

A =surface area of substrate C, = specific heat capacity of substrate T, = air temperature behind substrate T, =temperature at char face surface facing burner T, =temperature of substrate

( x / A ) ) , represents the resistance to heat transfer across the char at time T, assuming a purely conductive process of heat transfer. This equation may be applied to the results of tests on various coatings in the form

where

1 = thickness of substrate = 1.65 mm, a mean value

p = density of mild steel substrate = 7860 kg m-3 for most tests

C, =specific heat capacity of mild steel= 420 J kg-' K-'


DEVICE SUITABLE FOR TESTING FIFE RETARDANT COATINGS

E

-

which is independent of the cross-sectional area of the substrate. Thus differences in temperature across the substrate at any point in a test do not affect the calculation. Substituting the above values, the equation simplifies to

Determination of (A/x), therefore requires the determination of h, the film coefficient of heat transfer to the environment from the substrate, and the values of (dT,/dt), T,, T, and T, throughout a test. The determination of these parameters is described below,

(1) h: the film coefficient for heat losses was determined from cooling curves obtained from a thermocouple placed in an aluminium alloy insert substrate of diameter 12.5 mm replacing a disc cut out of the centre of a standard substrate and insulated from the remaining standard substrate by a layer of fire clay. This special substrate, shown in Fig. 13, was then coated with a typical coating in the normal way, cured and the sample heated in the fire test apparatus until the substrate temperature was around 270°C, at which point the burner was switched off and the sample allowed to cool. Cool- ing curves were also obtained for the air temperature with thermocouples placed at distances of 2 mm, 4.5 mm and 9 mm behind the substrate. The following procedure was then followed: (a) Eqn (6) was applied and integrated17 assuming no heat transfer in either direction through the char during the cooling period (after an initial period of around 5 min) compared with the heat loss from the back face. (b) Given one experimental value of temperature

Pvruma

piote i l I ' I ' ( I

Wires to meter measuring substrate temperature

Figure 13. Modified substrate for measurement of thermal resistance of char. Thermocouples: 0: substrate temperature; 1 : air temperature 2 mm behind substrate; 2: air temperature 4.5 mm behind substrate; 3: air temperature 9 m m behind substrate; A: aluminium substrate; B: Pyruma insulating an- nulus; C: mild steel substrate plate; D: coating; E: sample holder; F: wires from thermocouple Q to meter.

at a particular time, calculated temperatures at later times were obtained substituting an estimated value of the film coefficient of heat transfer and compared with the experimental temperatures obtained. Calculated temperatures were obtained from the integrated form

(hA:irn)) A Tcalc = A Tn . exp

where AT = (T, - Ta), the superscript 0 refers to an experimental standard result and T to the time

(c) The sum of the squares of the difference in calculated and experimental temperatures over the period of the test

of ATcalc.

1 (AT,,,, - AT,,,)^ 7

was minimized for different values of estimated film coefficients of heat transfer using a computer. This method was used due to difficulties in accu- rately assessing (dT,/dt), from the cooling curves.

A value of h = 3 . 9 ~ k 0 . 3 W m - ~ K - ' was obtained from several tests using values of T, obtained at different distances from the substrate.

(2) T,: modelled as described earlier in the section on face temperature according to char thickness.

(3) T,: obtained at intervals of 1 min from graphed results of experimental observations of fire tests.

(4) T,: determined from cooling curves described for the determination of h. (T, - T,) is constant for a given T, in any test:

(T,-Ta)=0.49T,-20 (9) The determination of T, led to the greatest errors in the analysis due to the width of the film across which cooling of the air was taking place. The above equation is based on observations at a distance of 4.5 mm from the substrate, and gives a larger heat loss term than use of observations at a distance of 2 mm (the use of which can give nega- tive (Alx) values).

(5) (dT,ldt),: determined from adjacent values of T' using

where A t is the standard time interval (60s). By this means profiles of thermal resistance ( x / h ) were obtained for several fire tests. A value of (x/A) = 1.5 m2 K W-' for an intumescent char of good structure and thickness around 120mm is typical. A poorer char of thickness 40 mm is likely to give a maximum thermal resistance of only 0.5 m2 K W-'. A typical value of thermal conductivity of a fully developed char of good structure (obtained by dividing by char thickness) is 0.035 W m-' K-', which is comparable with that of fibrous insulations such as Kapok (0.03 W m-' K-') and somewhat lower than that of concrete (0.1 W m-' K-'). Intumescent chars, however, ablate with time, and thus thermal resis-



tance falls. A well-formulated intumescent coating is, to give effective temporary protection to structures until active fire fighting measures can be taken.

Acknowledgements The authors would like to express their appreciation for the assis- tance and encouragement offered by Dr W. D. Woolley and his colleagues at FRS, Borehamwood.

REFERENCES

1. J. H. Warren and A. A. Corona, Hydrocarbon Process. 54(1), 121 (1975).

2. C. Anderson et a/., Railroad Tank Car Fire Test: Test No. 6, US Department of Transportation, Federal Railroad Ad- ministration, Office of Research, Development and De- monstrations; Report FRA-OR 81 D 75-36, Washington, DC (August 1973).

3. C. Anderson et al., Railroad Tank Car Fire Test: Test No. 7, ibid; Report FRA-OR 81 D 75-37, Washington, DC (December 1973).

4. C. Anderson and E. B. Norris, Fragmentation and Metallur- gical Analysis of Tank Car RAX201, ibid; Report FRA-OR & D 75-30, Washington (April 1974).

5. C. Anderson et at., The Effects of a Fire Environment on a Rail Tank Car Filled with LPG, ibid; Report FRA-OR 81 D 75-31, Washington, DC (September 1974).

6. W. Townsend et al., Comparison of Thermally Coated and Uninsulated Rail Tank Cars Filled with LPG Subjected to a Fire Environment, ibid., Report FRA-OR & D 75-32, Washington, DC (December 1974).

7. A. J. Taylor and D. J. Hands, Engulfment Fire Tests on Road Tanker Sections, Technical Report, Ministry of Defence - Royal Armaments Research and Development Establishment (July 1975).

8. H. L. Malhotra, The meaning of fire resistance tests. Confer- ence of the Institution of Structural Engineers: Structural

Design for Fire Resistance, Aston University, Birrningham (9-11 September 1975). pp. 92-114.

9. H. L. Malhotra, The philosophy and design of fire tests. Symposium: Fire Safety of Combustible Materials, Edin- burgh 15-17 October 1975), pp. 149-155.

10. W. Becker, Systematics of fires and fire test methods. Ibid.,

11. G. R. Nice, Fire tests and the assessment of fire hazard. 25th Annual Conference of Fidor Ltd (7-10 June 19781, London.

12. 1. A. Benjamin, Problems in correlation of small and large scale tests. Symposium: Fire Safety of Combustible Materi- als, Edinburgh (15-17 October 19751, pp. 141-148.

13. D. P. Crowley et at., Fire Technol. 8(3). 228 {August 1972). 14. M. Kay, Melamine phosphate as a component of intumes-

cent coatings. PhD thesis, Chemical Engineering Depart- ment. University of Aston in Birmingham (1980).

15. G. M. Phillips, UK Patent 1373908 (November 1974) as- signed to National Research Development Corporation.

16. J. H. MacGuire and H. Wraight, J. Sci. Instrum. 37. 128 (April 1960).

17. J. P. Fletcher and A. P. H. Jordan, Private communication, Chemical Engineering Department, University of Aston in Birmingham.

pp. 162-168.

Received 12 February 1982


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A fire test device suitable for small-scale testing of fire retardant coatings