wang 2013

Fire Safety Journal 55 (2013) 168–181

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Fire Safety Journal

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Experimental study of hydrothermal aging effects on insulative propertiesof intumescent coating for steel elements

L.L. Wang a, Y.C. Wang a,b,n, G.Q. Li a

a College of Civil Engineering, Tongji University, Chinab School of Mechanical, Aerospace and Civil Engineering, University of Manchester, UK

a r t i c l e i n f o

Article history:

Received 29 September 2010

Received in revised form

9 January 2012

Accepted 17 October 2012Available online 9 December 2012

Keywords:

Intumescent coating

Hydrothermal aging test

Fire test

Effective thermal conductivity

Fire resistance

12/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.firesaf.2012.10.004

esponding author at: University of Manche

ce and Civil Engineering, PO Box 88, Manche

ail addresses: [email protected]

[email protected] (Y.C. Wang).

a b s t r a c t

This paper reports the results of an experimental study of degradation in fire protection performance of

two types of intumescent coating after different cycles of accelerated hydrothermal aging tests.

Intumescent coating (without top coating) was applied to steel plate to make a test specimen. After

subjecting the specimen to the aging test, fire test was carried out to obtain the steel plate temperature.

In order to help understand the aging mechanism of intumescent coating, TGA tests, XPS tests and FTIR

tests were also conducted on the intumescent coating after the accelerated aging test. In total, tests

were performed on 56 intumescent coating protected steel specimens, of which 16 specimens were

applied with type-U intumescent coating and the other 40 with type-A intumescent coating. Results of

the degradation mechanism study reveal that the hydrophilic components of intumescent coating

move to the surface of the coating and can be dissolved by moisture in the air, which can destroy the

intended chemical reactions of these components with others and deter formation of the desired

effective intumescent char. The consequence of this is reduced expansion of the intumescent coating

and increased effective thermal conductivity. Compared to specimens without hydrothermal aging,

after 42 cycles of hydrothermal aging (to simulate 20 years of exposure to an assumed exposure

environment), the effective thermal conductivity of type-U intumescent coating was 50% higher and

that of type-A intumescent coating 100% higher than that of the fresh coating. These increases in

effective thermal conductivities resulted in increases in steel temperatures of up to 150 1C and 220 1C

higher than the steel temperatures of the specimens without hydrothermal aging for the type-U

intumescent coating and type-A intumescent coating specimens, respectively.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Among different forms of fire protection to steel structures,intumescent coating is particularly favored by architects because itallows the attractive steel structural form to be exposed. Intumescentcoatings are now widely used as passive fire protection to steelstructures and in countries such as the UK, the use of intumescentcoating dominates the passive fire protection market [3]. The coat-ings, which usually are composed of organic components containedin a polymer matrix, are designed to decompose and expand whensubjected to high temperatures so as to provide an insulating, foamedchar to protect the underlying substrate.

When specifying intumescent coating fire protection for steelstructures, the following assumptions are made:

(1)
the type and thickness of the intumescent are correctlyspecified;
ll rights reserved.

ster, School of Mechanical,

ster, M60 1QD, UK

,

(2)
the intumescent coating is correctly applied; (3) the fire protection performance of intumescent coating does
not degrade in time.

Assumptions (1) and (2) may not be fulfilled in practice, butthe problem is not a technical one. Assumption (3) deals withdurability of intumescent coating. Since most of the chemicalcomponents in intumescent coating are organic, it would not beunreasonable to expect that they react with the exposed environ-ment and that the fire protection function of intumescent coatingsdeteriorates over time.

There are very few reported research studies in open literatureon durability of intumescent coatings. Sakumoto et al.[8] carriedout some accelerated aging tests according to the standards of[7,2] to investigate the principal environmental factors that affectthe durability of intumescent coatings; [9,10] carried out accel-erated aging tests in a SH60CA weatherometer according to [1]standard. However, despite progresses made in these studies,there was no quantification of how the fire resistance perfor-mance of intumescent coatings reduces over time.

This paper reports the results of a comprehensive experimentalstudy to provide some quantitative information on reduced fire

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dx.doi.org/10.1016/j.firesaf.2012.10.004



mailto:[email protected]

mailto:[email protected]


Table 1Main test parameters.

Coating

type

Coating

DFT (mm)

No. of cycles of

accelerated aging

Simulating time

in service (years)

Specimen

ID

U 1 0 0 UI-1–00-i

(i¼1–4)

11 5 UI-1–11-i

(i¼1–4)

21 10 UI-1–21-i

(i¼1–4)

42 20 UI-1–42-i

(i¼1–4)

A 1 0 0 AZ-1–00-i

(i¼1–4)

4 2 AZ-1-04-i

L.L. Wang et al. / Fire Safety Journal 55 (2013) 168–181 169

protection performance of degraded intumescent coatings tosteel structures. Two series of tests have been conducted. In seriesone (to be referred to as fire test), intumescent coating specimenswere subjected to different cycles of accelerated aging and thentested in fire. The measured data include final expanded thick-nesses and the substrate steel temperatures. From these tests, theeffects of hydrothermal aging on effective thermal conductivities(related to the original intumescent coating thickness) of intumes-cent coatings were obtained. In the accompanying series of tests(to be referred to as chemical analysis tests), the aged intumescentcoatings were subjected to TGA, XPS and FTIR tests to measuretheir mass loss, change of element contents and migration ofcomponents in the intumescent coating system. The tests help toexplain the degradation processes.

(i¼1–4)

11 5 AZ-1-11-i

(i¼1–4)

21 10 AZ-1-21-i

(i¼1–4)

42 20 AZ-1-42-i

(i¼1–4)

2 0 0 AZ-2–00-i

(i¼1–4)

4 2 AZ-2-04-i

(i¼1–4)

11 5 AZ-2-11-i

(i¼1–4)

21 10 AZ-2-21-i

(i¼1–4)

42 20 AZ-2-42-i

(i¼1–4)

Fig. 1. Specimen dimensions.

2. Fire tests

2.1. Specimen preparations

A total of 56 specimens were tested. Each specimen was madeof 16 mm thick steel plate coated with 1 mm or 2 mm Dry FilmThickness (DFT) intumescent coatings on all sides. Of the testspecimens, 16 were protected by type-U intumescent coating (tobe referred to as type-U specimens) and 40 were applied withtype-A intumescent coating (to be referred to type-A specimens).The two different types of intumescent coating were supplied bytwo different manufacturers operating in the Chinese market. Theprincipal components of type-U and type-A intumescent coatingare APP-MEL-DPER (aided with zinc borate) and APP-MEL-PER,respectively; the acid resins of type-U and type-A intumescentcoatings are ethylene benzene–acrylic and single componentacrylic, respectively.

For the 16 type-U specimens, four replicate tests were per-formed for each of the following 4 cycles of accelerated aging: 0(no aging), 11 cycles (simulating 5 years in service), 21 cycles(simulating 10 years in service) and 42 cycles (simulating 20years in service). All specimens were coated with 1 mm DFT. Forthe 40 type-A specimens, 20 were coated with DFT 1 mm coatingand the other 20 with DFT 2 mm coating. For each coatingthickness, four replicate tests were performed for each of thefollowing 5 cycles of accelerated aging: 0 (no aging), 4 cycles(simulating 2 years in service), 11 cycles (simulating 5 years inservice), 21 cycles (simulating 10 years in service) and 42 cycles(simulating 20 years in service). In all cases, the substrate steelplate measured 200 mm by 270 mm by 16 mm thick. A primerwas applied to the steel surface first to act as an aid to adhesion ofthe intumescent coating; this was then followed by differentlayers of intumescent coating to achieve the desired DFT.However, no top coating was applied. For each specimen, DFTwas measured and recorded before the accelerated aging test.Three thermocouples (2.0 mm diameter, type K) were embeddedin each steel plate. Table 1 lists the main specimen parametersand Fig. 1 shows the specimen dimensions, where d is theintumescent DFT.

2.2. Hydrothermal aging test

Intumescent coating aging is an extremely complicated pro-cess of physical and chemical interactions between the chemicalcomponents of intumescent coatings and the external environ-ment. Whilst it would be ideal to carry out real time aging test,this process would be extremely long, running into many tens ofyears. An alternative is to conduct accelerated aging test, in whicha real environmental condition over a long period of time isrepresented by a short cyclic exposure of the intumescent coating

to a concentrated dosage of the environment. During any accel-erated aging test, it is necessary to determine the environmentalconditions that the product (intumescent coating) will be exposedto, the length of time of the exposure and the performancecriterion based on which the effect of aging is assessed.

The accelerated aging test was performed according to theEuropean guideline ([5]). In this guidance, four types of environ-mental exposure are simulated: (a) type X for all conditions;(b) type Y for internal and semi-exposed conditions; (c) type Z1for internal conditions which have above zero temperatures andhigh humidity; and (d) type Z2 for internal conditions that haveabove zero temperatures but humidity conditions that are not inclass Z1. The accelerated aging test reported in this paper adoptedexposure condition Z1, simulating the more severe exposurecondition of application around the coastal provinces in China.

L.L. Wang et al. / Fire Safety Journal 55 (2013) 168–181170

For exposure condition type Z1, each cycle of exposure is asfollows:

8 h at (4073) 1C and (9872)%RH;16 h at (2373) 1C and (7572)%RH.

According to ETAG018, 21 cycles of accelerated aging isequivalent to 10 years in service. Based on this correlation, 0 cycle,4 cycles, 11 cycles and 42 cycles correspond to fresh coating,2 years, 5 years and 20 years in service.

Fig. 4. Furnace door with observation holes.

2.3. Surface appearance

After the accelerated aging test but before the fire test, thespecimens were checked for their coating surface appearance.Figs. 2 and 3 show typical appearance of type-U and type-Aspecimens after having gone different cycles of accelerated agingtest. Type-U specimens did not appear to suffer any change in

Fig. 2. Type-U coating appearance after different cycles of hydrotherm

Fig. 3. Type-A coating appearance after different cycles of hydrothermal aging

appearance after 11 and 21 cycles of hydrothermal aging tests(Fig. 2(b) and (c)). After 42 cycles, wrinkles can be clearly seen(Fig. 2(c)). In contrast, type-A specimens experienced noticeablechanges in appearance after every accelerated aging test. After

al aging test. (a) UI-1-00, (b) UI-1-11, (c) UI-1-21 and (d) UI-1-42.

test. (a) AZ-1-00, (b) AZ-1-04, (c) AZ-1-11, (d) AZ-1-21 and (e) AZ-1-42.

Fig. 5. Specimens in furnace. (a) Specimens hung on steel beams and (b) Specimens laid flat on steel beams.

Fig. 6. Bubble appearance on the surface of type-A specimens. (a) AZ-1-00, (b) AZ-1-21 and (c) AZ-1-42.


only 4 cycles, the surface of type-A specimens appeared uneven(Fig. 3(b)). After 11 cycles of accelerated aging test, bumpsappeared on the surface (Fig. 3(c)). After 21 and 42 cycles, thespecimen surface was uneven, with very large bumps(Fig. 3(d) and (e)). As will be shown later in this paper, there isstrong link between the surface appearance and fire protectionperformance of an intumescent coating. Coating surface appear-ance may be explored when determining a replacement strategyin real applications.

2.4. Fire test

After the specimens were subjected to hydrothermal aging asdescribed in the previous section, they were placed in a furnace(Fig. 4) and exposed to fire. The furnace temperature wasmeasured by four thermocouples and the average furnace tem-perature was regulated according to the ISO 834 ([6]) standardtemperature–time relationship. The ISO 834 standardtemperature–time curve, or a very similar one, is followed world-wide in assessment of fire resistance of construction elements,including intumescent coating protected steel structures.

Fig. 5 shows four specimens tested together in the furnace. Thesteel temperature was measured by three thermocouplesembedded in the steel plate and recording was made everyminute continuously. Four observation holes were placed on thefurnace door to enable the fire tests to be observed and pictures ofthe surface of the specimens to be taken. Each test was continueduntil the steel temperature reached 700 1C.

3. Test results

3.1. Experimental phenomena

When exposed to flame, intumescent coatings for all speci-mens underwent the following main steps of chemical reaction:

(1)
melting of the acid base; (2) expansion due to release of gas by the blowing agent; (3) char formation; (4) char degradation due to oxidation.
Depending on the composition of the chemical componentsand the fire exposure condition, these reactions may happen insequence or together. Type-A intumescent coatings began toexpand earlier than type-U intumescent coatings. In the intumes-cence (expansion) stage, bubbles that appeared on the surface oftype-A intumescent coatings were much larger than that those onthe surface of type-U intumescent coatings.

Intumescent coatings for both types are highly ‘‘engineered’’ topass the standard fire resistance test when freshly applied.Hydrothermal aging causes some chemical components in theintumescent coatings to migrate to the surface, altering thechemical reactions. In the intumescence stage, the blowing agentin the intumescent coating decomposes to produce gas, a fractionof which is trapped within the molten matrix to cause the coatingto expand. From the pictures taken of the specimens through theobservation holes on the furnace door, many bubbles appeared


during the intumescence stage on both types of intumescentcoating after 0 or 4 cycles of hydrothermal aging tests, as shownin Figs. 6 and 7(a). This means a large amount of gas wasproduced due to decomposition of the blowing agent. After 11and 21 cycles of hydrothermal aging tests, the number of bubblesdecreased drastically and the bubble distribution was much lessuniform, shown as Figs. 6 and 7(b). After 42 cycles, bubbles werealmost non-existent, see Figs. 6 and 7(c).

Fig. 7. Bubble appearance on the surface of type-U sp

Fig. 8. Cross sectional view of expanded intumescent char after different cycles of acc

and (f) UI-1-42.

The observed phenomena for type-A specimens with both1 mm and 2 mm DFTs were generally similar.

The most important parameters that directly reflect the fireprotection performance of intumescent coatings are the finalexpanded thickness and internal structure of the char [11].Fig. 8 shows the expanded heights of both types of coatings afterdifferent cycles of aging test. These figures also give someindication of the consistence of the char.

ecimens. (a) UI-1-00, (b) UI-1-21 and (c) UI-1-42.

elerated aging test. (a) AZ-2-00, (b) AZ-2-42, (c) AZ-1-04, (d) AZ-1-42, (e) UI-1-11


It can be seen from Fig. 9 that the expanded thicknessdecreased greatly for both types of intumescent coating after 42cycles of aging test. In addition, the integrity and consistency ofthe char for the specimens after 42 cycles of aging test are poorFig. 8.

Table 2 lists the measured DFTs, the measured final thick-nesses and the expansion ratios of the different specimens. Itmust be pointed out that due to unevenness of the final char, themeasured final thickness in some cases is around the averagevalue. Fig. 10 plots the expansion ratio as a function of thenumber of cycles of accelerated testing. It can be seen that theexpansion ratio decreases considerably after only a few cycles ofaging test. After 21 cycles (simulating 10 years in service), theexpansion ratios of all three groups of intumescent coatings wereabout 60% of those without aging. The expansion ratio may beused to give a measure of the effective thermal conductivity ofintumescent coating. This means that after 21 cycles, the effectivethermal conductivity of the intumescent coating is about 1.7 times(1/0.66) the effective thermal conductivity without aging. After 42cycles, the expansion ratio was about 1/3rd of that without aging.

3.2. Temperature results

The average furnace temperature followed the ISO 834 stan-dard temperature–time relationship.

Fig. 10 presents the measured steel substrate temperature–time curves for the replicate tests of each specimen.

0

12

24

36

48

0

number/cycles of hydrothermal aging

expa

nsio

n ra

tio

U group

AZ-1 group

AZ-2 group

11 22 33 44

Fig. 9. Reduction of expansion ratio with number of cycles of hydrothermal aging.

Table 2Expansion ratios for specimens.

Specimen Initial thickness(mm)

Type-U specimens (U-group)

UI-1–00 0.95

UI-1–11 1.01

UI-1–21 1.04

UI-1–42 0.94

Type-A specimens with 1 mm coating (AZ-1 group)

AZ-1–00 1.02

AZ-1–04 1.05

AZ-1–11 1.10

AZ-1–21 1.08

AZ-1–42 1.06

Type-A specimens with 2 mm coating (AZ-2 group)

AZ-2–00 2.20

AZ-2–04 2.16

AZ-2–11 2.09

AZ-2–21 2.18

AZ-2–42 2.22

It can be seen from Fig. 10 that the replicate tests givegenerally consistent results even though some discrepanciesexist. The average values of temperatures of the steel substrateswill be used. Figs. 11 and 12 compare the average steeltemperature–time relationships to show the effect of aging onsteel substrate temperature.

It can be seen from Figs. 11 and 12 that compared to speci-mens without aging, there is sharp increase in steel substratetemperature after a certain number of cycles of aging test. Fortype-A coating (Fig. 11), 11 cycles (representing 5 years in service)appear to mark the beginning of sharp increase in the steelsubstrate temperature. For type-U coating (Fig. 12), the increasein steel temperature appears to be more even over the entirerange of aging cycles.

Figs. 13 and 14 present the steel substrate temperatures afterdifferent cycles of aging at the same time when the specimenswithout aging reached 400 1C, 500 1C and 600 1C. Furthermore,Tables 3 and 4 present fire resistance times that may be achievedby the different specimens if the steel limiting temperature is400 1C, 500 1C, 600 1C and 700 1C.

The increase in steel temperature or the reduction in fireresistance time, due to degradation in intumescent coatingperformance is very high. A question may arise on when intu-mescent coating should be replaced. On the assumption ofreplacing degraded intumescent coatings after suffering a loss of20% in its fire resistance rating, then type-A coating (Table 3)would need replacing after 11 cycles (representing 5 years inservice) and type-U coating (Table 4) would need replacing after21 cycles (representing 10 yes in service). If no loss of fireresistance rating is allowed, the only alternative solution wouldbe to specify intumescent coating DFT based on the degradedintumescent coating performance. For example, look at the resultsin Fig. 14 for type-U intumescent coating. Suppose the steellimiting temperature is 604 1C. If it is desired to use the intumes-cent coating after 42 cycles (corresponding to 20 years in service),then when specifying the fresh intumescent coating DFT, a steellimiting temperature of 500 1C should be used.

3.3. Thermal conductivity

The simple quantity of effective thermal conductivity (refer-ring to the initial, not expanded, intumescent coating thickness)may be used to indicate the overall effects of hydrothermalaging on fire performance of intumescent coatings. The effec-tive thermal conductivity may be obtained from the following

Final thickness(mm) Expansion ratio

28.00 29.47

22.00 21.78

19.00 18.27

10.00 10.64

47.00 46.08

42.00 40.00

35.00 31.82

28.00 25.93

10.00 9.43

85.00 38.64

76.00 35.19

69.00 33.01

52.00 23.85

35.00 15.76


equation ([4]):

lp,t tð Þ ¼ dp �V

Ap� cara � 1þ f=3

� ��

1

yt�ya,t

� �Dt

" #

�½Dya,tþ ef=10�1� �

Dyt� ð1Þ

where

0

200

400

600

800

0 20 40 60 80

time (minute)

AZ-1-00-1

AZ-1-00-2

AZ-1-00-3

0

200

400

600

800

0 14 28 42 56 70

time (minute)

AZ-2-00-1

AZ-2-00-2

AZ-2-00-3

0

200

400

600

800

0 20 40 60 80

time (minute)

tem

pera

ture

of

stee

lsu

bstr

ate

(°C

)te

mpe

ratu

re o

f st

eel

subs

trat

e (°

C)

tem

pera

ture

of

stee

lsu

bstr

ate

(°C

)

UI-1-00-1

UI-1-00-2

UI-1-00-3

Fig. 10. Replicate temperature–time

0

200

400

600

800

0

time (minute)

AZ-1-00

AZ-1-04

AZ-1-11

AZ-1-21

AZ-1-42

tem

pera

ture

of

stee

lsu

bstr

ates

(°C

)

20 40 60 80

Fig. 11. Effect of aging on steel substrate tempera

Dya,t is the increase in steel temperature during the timeinterval Dt

lp,t is the effective thermal conductivity of intumescentcoating during the time interval Dt

dp is the initial DFT of intumescent coatingca is the specific heat of steelra is the density of steelAp/V is the section factor of the protected steel sectionyt is the furnace temperature at time t

0

200

400

600

800

0 12 24 36 48 60

time (minute)

AZ-1-42-1

AZ-1-42-2

AZ-1-42-3

0

200

400

600

800

0 12 24 36 48 60

time (minute)

AZ-2-42-1

AZ-2-42-2

AZ-2-42-3

tem

pera

ture

of

stee

lsu

bstr

ate

(°C

)te

mpe

ratu

re o

f st

eel

subs

trat

e (°

C)

tem

pera

ture

of

stee

lsu

bstr

ate

(°C

)

0

200

400

600

800

0 12 24 36 48 60

time (minute)

UI-1-42-1

UI-1-42-2

UI-1-42-3

relationships of steel substrate.

0

150

300

450

600

750

0

time (minute)

tem

pera

ture

of

stee

lsu

bstr

ates

(°C

)

AZ-2-00

AZ-2-04

AZ-2-11

AZ-2-21

AZ-2-42

14 28 42 56 70

ture–time relationship for type-A specimens.


ya,t is the steel temperature at time t

Dya,t is the increase of furnace temperature during the timeinterval Dt

f¼ cprpdpAp

caraV

Dtr30 s

Since the specimen was exposed to fire on all sides, the sectionfactor was calculated as Ap/V¼142 m�1.

In theory, the temperature dependent specific heat and densityof intumescent coating should be used when calculating the

0

200

400

600

800

0

time (minute)

tem

pera

ture

of

stee

lsu

bstr

ates

(°C

)

UI-1-00

UI-1-11

UI-1-21

UI-1-42

14 28 42 56 70

Fig. 12. Effect of aging on steel substrate temperature–time relationship for type-

U specimens.

350

400

450

500

550

600

650

700

750

10 20 30 40

time

tem

pera

ture

of

stee

lsu

bstr

ates

(°C

)

350

400

450

500

550

600

650

700

750

10 20 30 40

time

tem

pera

ture

of

stee

lsu

bstr

ates

(°C

)

592°C

506°C

400°C

5

443°C

621°C

530°C

400°C

450°C

420°C

424°C

Fig. 13. Temperatures reached when the specimens without aging reached 400

effective thermal conductivity of intumescent coating usingEq. (1). However, since the amount of heat stored inside theintumescent coating is very small and may be considered to benegligible compared to that in the steel substrate, FE0 so Eq. (1)may be simplified to Eq. (2) below:

lp,t tð Þ ¼ dp �V

Ap� cara �

1

yt�ya,t

� �Dt

" #� Dya,t ð2Þ

For each specimen and for each time interval, the intumescentcoating temperature yp may be taken as the mean of the steel andfire temperature so that

yp ¼ytþya,t

2ð3Þ

Figs. 15 and 16 present some of the results of coating thermalconductivity–temperature curve.

It can be seen from Figs. 15 and 16 that the effective thermalconductivity of both types intumescent coating starts to fallsharply after the temperature of intumescent coating reachedabout 100 1C, indicating chemical reactions starting at about100 1C. The effective thermal conductivity of the coating becamestable after reaching temperatures over 400 1C, a clear indicationthat the coating had reached full expansion. Afterwards, theeffective thermal conductivities increase with temperature, whichmay be explained by increased radiation inside the bubbles atincreasing temperatures [11]. It is the stable, fully expanded stageof intumescent coating that is providing the fire protectionfunction, and the discussions below will focus on this stage.

It can be seen from Figs. 15 and 16 that there are some smalldiscrepancies between the results for the same three nominallyidentical specimens. Nevertheless, the three replicate tests gavegenerally consistent results. In the discussions to follow, the

50 60 70 80

(minute)

AZ-1-00

AZ-1-04

AZ-1-11

AZ-1-21

AZ-1-42

50 60 70 80

(minute)

AZ-2-00

AZ-2-04

AZ-2-11

AZ-2-21

AZ-2-42

530°C

67°C

643°C

500°C

711°C

600°C

675°C

556°C

641°C

500°C

600°C

658°C

630°C

532°C

630°C

1C/500 1C/600 1C. (a) Type AZ-1 specimens and (b) Type AZ-2 specimens.

350

400

450

500

550

600

650

700

750

10 20 30 40 50 60

time (minute)

tem

pera

ture

of

stee

lsu

bstr

ates

(°C

)

UI-1-00

UI-1-11

UI-1-21

UI-1-42

483°C 438°C

400°C

525°C

547°C

500°C

600°C

703°C

423°C

604°C 624°C

649°C

Fig. 14. Temperatures reached when the specimens without aging reached 400 1C/500 1C/600 1C.

Table 3

Specimen Limiting steel temperature(1C)

400 500 600 700

(a) Fire resistance times for type AZ-1 specimens (in minutes)

AZ-1–00 30 41 51 63

AZ-1–04 27 38 48 59

AZ-1–11 25 35 44 53

AZ-1–21 22 30 37 47

AZ-1–42 17 23 30 40

(b) Fire resistance times for type AZ-2 specimens (in minutes)

AZ-2–00 33 43 54 66

AZ-2–04 30 40 50 62

AZ-2–11 28 37 48 59

AZ-2–21 23 31 39 50

AZ-2–42 19 25 31 40

Table 4Fire resistance times for type U specimens (in minutes).

Specimen Limiting steel temperature(1C)

400 500 600 700

UI-1–00 23 32 41 52

UI-1–11 21 30 39 49

UI-1–21 20 28 37 47

UI-1–42 17 24 32 41


average effective thermal conductivities for intumescent coatingtemperatures between 250 1C and 750 1C at 50 1C interval will beused. In order to help clarify discussions, the raw results inFigs. 15 and 16 are smoothed and the average results are shownin Fig. 17.

The changes in effective thermal conductivity of all coatingsfollow a consistent pattern as a function of the number of cyclesof hydrothermal aging when the temperature increases. There-fore, the change in average thermal conductivity at the intumes-cent coating temperature range of 650–750 1C, which correspondto the practically interesting steel temperature range of 500–6001C, may be used. The results are presented in Fig. 18.

The effect of hydrothermal aging is to cause the hydrophiliccomponents of intumescent coating to move to the surface of thecoating and then be dissolved by moisture. Hence, although type-A coating of the same DFT (1 mm) performed better than type-Ucoating if neither suffers any aging effect (as can be seen in Fig. 18by the lower thermal conductivity of type-A coating at 0 cycle),type-A coating suffers from hydrothermal aging much morequickly than type-U coating due to it having a lower waterresistance as explained in Section 2.3. Whilst the thermal con-ductivity of type-U coating increases very gradually as thenumber of cycles increases, type-A coating suffers significantlymore loss in performance after only 4 cycles of hydrothermalaging.

Comparing the performance of 1 mm DFT type-A coating withthat of 2 mm DFT type-A coating, the same degradation processoccurs, but the thicker DFT 2 mm coating delays the processslightly so that the large change occurs between 11 and 21 cycles

of hydrothermal aging for the thicker DFT instead of after 4–11cycles of hydrothermal aging for the thinner DFT.

4. Chemical analysis tests

A number of chemical analysis tests were carried out to furtherexamine the degradation process in more detail. The chemicalanalysis tests included TGA test, XPS test, FTIR test and SEM test.

4.1. TGA test results

TGA test gives mass loss as a function of temperature. The TGAtests were conducted using a Pyris diamond TG/DTA instrumentunder nitrogen at a heating rate of 20 1C min� i within a tempera-ture range of 25–800 1C. Fig. 19 presents the measured mass lossresults.

It can be seen from Fig. 19 that there is little difference in theTGA test results after different cycles of hydrothermal aging. Thisindicates that the hydrothermal aging tests did not cause thechemical components to be any different. However, the optimummatching of chemical components in the intumescent coatingchanged due to migration of the hydrophilic components to thesurface. Hence, the expansion ratios of the fire test specimenswere different. This also suggests that when detecting changes inintumescent coating performance over time, the TGA test wouldnot be suitable.

4.2. FTIR test results

FTIR (Fourier transform infrared spectroscopy) test was usedto investigate the migration of chemical components for bothtypes of specimens with different cycles of hydrothermal aging.The FTIR test was conducted on samples extracted from thesurface layer of intumescent coating, using the EQUINOXSS/HYPERION2000 device. The experimental results are presentedin Fig. 20.

0

0.05

0.1

0.15

0.2

0 200 400 600 800

temperatrure of coating (°C)

effe

ctiv

e th

erm

al c

ondu

ctiv

ity

W/ (

m•°

C)

UI-1-00-1

UI-1-00-2

UI-1-00-3

0

0.05

0.1

0.15

0.2

0.25

0 200 400 600 800


effe

ctiv

e th

erm

al c

ondu

ctiv

ity

W/ (

m•°

C)

UI-1-42-1

UI-1-42-2

UI-1-42-3

Fig. 15. Effective thermal conductivity (lp)–coating temperature (yp) relationships for type-U specimens. (a) Type U specimens with 0 cycles of aging and (b) Type U

specimens with 42 cycles of aging.

0

0.05

0.1

0.15

0.2

0 200 400 600 800


effe

ctiv

e th

erm

al c

ondu

ctiv

ity

W/ (

m•°

C)

AZ-1-00-1

AZ-1-00-2

AZ-1-00-3

0

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0.09

0.12

0.15

0 200 400 600 800


effe

ctiv

e th

erm

al c

ondu

ctiv

ity

W/ (

m•°

C)

AZ-1-42-1

AZ-1-42-2

AZ-1-42-3

0

0.06

0.12

0.18

0.24

0.3

0 200 400 600 800


effe

ctiv

e th

erm

al c

ondu

ctiv

ity

W/ (

m•°

C)

AZ-2-00-1

AZ-2-00-2

AZ-2-00-3

0

0.05

0.1

0.15

0.2

0.25

0 200 400 600 800


effe

ctiv

e th

erm

al c

ondu

ctiv

ity

W/ (

m•°

C)

AZ-2-42-1

AZ-2-42-2

AZ-2-42-3

Fig. 16. Effective thermal conductivity (lp)–coating temperature (yp) relationships for type-A specimens. (a) Type AZ-1 specimens with 0 cycles of aging, (b) Type AZ-1

specimens with 42 cycles of aging, (c) Type AZ-2 specimens with 0 cycles of aging and (d) Type AZ-2 specimens with 42 cycles of aging.


Although the FTIR test results can only be used to gain aqualitative understanding of the effects of aging, they can stillgive some indication of the extent of aging in service.

For type-A coatings (Fig. 20a), the troughs at 3321 cm�1,3176 cm�1, 2957 cm�1 and 1668 cm�1 indicate N–H bondscontained in MEL and APP, O–H bonds in PER, C–H bond in acrylicacid resin and PER, and C¼O bonds in acrylic acid resin,respectively. The trough at 1428 cm�1 is the overlapping peakof the absorption peak of CH2 group contained in acrylic acid resinand PER and the absorption peak of triazing rings which are themain structure of MEL. The troughs at 1253 cm�1, 1078 cm�1,1013 cm�1 and 889 cm�1 indicate P¼O bonds in APP, C–O–Hbonds in PER, C–O–C bonds in acrylic acid resin and triazing ringsin MEL, respectively. Similar wave numbers of these bonds can beobserved in Fig. 20(b) for type-U coating.

It is clear from Fig. 20 that the absorption peak of the abovementioned different chemical bonds contained in PER and APP are

enhanced with increasing number of cycles of aging. This indi-cates that PER and APP migrated from within the coating to thesurface of the coating after different cycles of aging.

Compared to AZ-1–00, the absorption peak at 1428 cm�1 ofAZ-1–11 was weakened whereas the width of the peak increased.This indicates that the polymer binder degraded under the effectof water and oxygen and some of the CH2 groups were oxidatedinto C¼O groups. This enhanced the absorption peak of C¼Obonds at 1668 cm�1 with increasing number of cycles of aging.Degradation of the polymer binder (acrylic acid resin) waspresent during the whole process of aging. The absorption peaksat 1428 cm�1and 889 cm�1 were also enhanced, indicatingmigration of MEL from within the coating to the surface of thecoating after different cycles of aging.

It is observed from the above analysis that the degradation ofpolymer binder (acrylic acid resin) and migration of flameretardant system (APP-MEL-DPER) happened at the same time

0

0.03

0.06

0.09

0.12

0.15

200

temperature of coating θp(°C)

effe

ctiv

e th

erm

al c

ondu

ctiv

ity λ

p W

/(m

•°C

) AZ-1-00 AZ-1-04

AZ-1-11 AZ-1-21

AZ-1-42

0

0.03

0.06

0.09

0.12

0.15

effe

ctiv

e th

erm

al c

ondu

ctiv

ityλp

W/ (

m•°

C)

AZ-2-00 AZ-2-04

AZ-2-11 AZ-2-21

AZ-2-42

0

0.02

0.04

0.06

0.08

0.1

effe

ctiv

e th

erm

al c

ondu

ctiv

ityλp

W/(

m•°

C)

UI-1-00

UI-1-11

UI-1-21

UI-1-42

300 400 500 600 700 800 900

200


300 400 500 600 700 800 900

200


300 400 500 600 700 800 900

Fig. 17. Effects of aging on effective thermal conductivity of intumescent coatings. (a) Effect of aging on effective thermal conductivity of Type AZ-1 coating, (b) Effect

of aging on effective thermal conductivity of Type AZ-2 coating and (c) Effect of aging on effective thermal conductivity of Type U coating.

0

0.017

0.034

0.051

0.068

443322110


effe

ctiv

e th

erm

al c

ondu

ctiv

ity

λp W

/(m

•°C

)65

0°C

~75

0°C

Type-U Type-A (1mm)

Type-A (2mm)

Fig. 18. Effect of aging on effective thermal conductivity of intumescent coating.

0 100 200 300 400 500 600 700 800

40

50

60

70

80

90

100

wei

ght l

oss

(%)

temperature of coating (°C) temperature of coating (°C)

AZ-1-00 AZ-1-11 AZ-1-21 AZ-1-42

0 100 200 300 400 500 600 700 800

40

50

60

70

80

90

100

wei

ght l

oss

(%)

UI-1-00 UI-1-11 UI-1-21 UI-1-42

Fig. 19. TGA curves of intumescent coating after different cycles of aging.



during the process of aging, which resulted in the reduced fireprotective properties of intumescent coatings.

In practical application, when examining the effects of agingon intumescent coatings, if on-site FTIR test shows little change inthe absorption peaks, then there is high confidence that theeffects of aging are minimal.

4.3. XPS test results

XPS (X-ray photoelectron spectroscopy) test gives informationon the amount of chemical elements being examined. For example,Fig. 21 presents the amounts of Carbon and Nitrogen existent onthe surface layer of both types of intumescent coatings after

3500 3000 2500 2000 1500 1000 500

1428

88910781253

166829573321 3176

AZ-1-42

AZ-1-21

AZ-1-11

AZ-1-00

Wavenumber (cm-1)

Fig. 20. FTIR test results. (a) FTIR test results for Type-A

10000

1000

2000

3000

4000

5000

6000

7000

8000

O 1s

N1s

C1s

N (

E)

Binding Energy (eV)

AZ-1-00

0

1000

2000

3000

4000

5000

6000

7000

8000

N (

E)

AZ

0

1000

2000

3000

4000

5000

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7000

8000

O1s

N1s

C1s

N (

E)

UI-1-00

0

1000

2000

3000

4000

5000

6000

7000

N (

E)

UI

800 600 400 200 0 1000

Binding

800 6

1000

Binding Energy (eV)

800 600 400 200 0 1000

Binding

800 60

Fig. 21. XPS te

different cycles of aging. The XPS test was conducted using anelemental analyzer VARIOEL 3.

C element is contained in MEL(C3H6N6) which acts as theblowing agent and in DPER/PER and (C(CH2OH)4) acting as thecharring agent; N element is contained in MEL andAPP(NH4)nþ2PnO3nþ1) which act as the catalytic agent. Table 5lists the percentage of Carbon and Nitrogen elements obtainedfrom the XPS tests for representative samples of both types ofintumescent coatings.

It can be seen from Table 5 that compared to specimenswithout aging, the contents of C and N elements on the surfacelayer of type-A and type-U specimens increased with increasingnumber of cycles of aging. The change in N element is much more

3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1)

1435

1078 8901251

1661

294031153325

UI-1-42

UI-1-21

UI-1-11

UI-1-00

coating and (b) FTIR test results for Type-U coating.

O1s

N1s

C1s

-1-21 AZ-1-42

0

1000

2000

3000

4000

5000

6000

7000

8000

O1s

N1s

C1s

N (

E)

O1s

N1s

C1s

-1-21

0

1000

2000

3000

4000

5000

6000

7000

8000

O1s

N1s

C1s

N (

E)

UI-1-42

Energy (eV)

00 400 200 0 1000

Binding Energy (eV)

800 600 400 200 0

Energy (eV)

0 400 200 0 1000

Binding Energy (eV)

800 600 400 200 0

st results.


sensitive to the change in C element. The current research is notsufficiently comprehensive, but further extensive testing shouldbe done to ascertain whether it would be possible to link thechanges in C and N elements to the changes in fire protectionperformance of different intumescent coatings.

4.4. SEM test results

SEM test gives some information on the change in internalstructure of chars after different cycles of aging. The SEM micro-graphs of chars obtained from type-A coating after 11, 21 and 42cycles of aging are presented in Fig. 22.

Both Fig. 22(a) and (b) show the expected honeycomb struc-ture, but the pore size in Fig. 22(b) is much larger than that inFig. 22(a) and the number of pores decreases. For Fig. 22(c),although the coating can still expand to form a char structureafter 42 cycles of aging, the aging process has damaged itsexpanding effect and the ‘‘honeycomb’’ structure of intumescentchar does not exist.

5. Conclusions

This paper has presented the results of a series of fire tests onintumescent coating protected steel plates after the intumescentcoatings have been exposed to different cycles of hydrothermal

Table 5Contents of C/N.

Element

contents(%)

Specimen

Element AZ-1–00 AZ-1–21 AZ-1–42 UI-1-00 UI-1–21 UI-1–42

C 61.6 64.1 65.5 63.7 63.8 64.9

N 7.9 8.7 10.5 9.9 10.1 13.4

Fig. 22. SEM micrographs of type-A intumescent chars after differ

aging according to exposure condition Z1 in European guide ETAG018 Part 2. The numbers of cycles were 0, 4, 11, 21 and 42,corresponding to 0, 2, 5, 10 and 20 years of nominal service. Theresults have been presented in terms of the expansion ratio, thesteel temperature and effective thermal conductivity. Surfaceobservations were made and additional chemical analysis (TGA,FTIR, XPS and SEM) tests were also carried out. The followingconclusions may be drawn:

1.

ent cy

Bumps with different degrees of unevenness appeared onthe surfaces of specimens applied with type-A intumescentcoatings after different cycles of hydrothermal aging tests.But no obvious change was observed for specimens appliedwith type-U intumescent coatings after 11 and 21 cycles ofaging tests. Slight wrinkles appeared on the surfaces ofspecimens applied with type-U intumescent coatings after42 cycles of aging. The surface appearance can be used togive a visual guide to the effectiveness of intumescentcoating performance in service.

2.
Both types of intumescent coating suffered considerablereduction in performance after 42 cycles of accelerated agingtest (corresponding to 20 years in service under the assumedexposure condition). For example, the expansion ratioreduced by over 70% and the steel plate temperature wasincreased by about 200 1C compared to the steel tempera-ture of 500 1C with fresh intumescent coating.
3.
The results from TGA test, FTIR test and XPS test show thatthe aging process did not cause the chemical components tobe any different, but the optimum matching of thesecomponents in the examined intumescent coatings changeddue to migration of the hydrophilic components to thesurface of the coating when exposed to the hydrothermalaging environment. This damaged the expanding ability ofthe intumescent coatings. From the chemical analysis testresults, the TGA test is not suitable for detecting changes foraging effect. The FTIR test can detect the qualitative changesof aging. The XPS test may be used to quantify the aging
cles of aging (a) 11 cycles; (b) 21 cycles; (c) 42 cycles.


effects, but much more extensive testing is required before aquantitative relationship between XPS test results and fireprotection performance results (e.g., changes in expansionratio/effective thermal conductivity) can be established.

4.
The SEM test is destructive but the results can be used toindicate that the effects of aging.
It should be pointed out that intumescent coatings aretop-coated in practice to protect them from environmen-tal damage. Their durability will be much better.

References

[1] American Society for Testing and Materials (ASTM) (2005) ASTM D 4587-91,Practice For Conducting Tests on Paint and Related Coatings and MaterialsUsing a Fluorescent UV-Condensation Light and Water-Exposure Apparatus,ASTM, Philadelphia.

[2] British Standards Institution (BSI) (1992) British Standard BS8202, Coatingfor Fire Protection of Building Elements, Part 2: Code of Practice For theAssessment and Use of Intumescent Coating System for Providing FireResistance, British Standards Institution, London.

[3] J. Dowling, Fire protection costs for structural steelwork, New steel Constr.(UK). Sep/Oct (2003) 8–9.

[4] European Committee for Standardization (CEN) (2002) ENV13381, Test

Methods for Determining the Contribution to the Fire Resistance of StructuralMembers, Part 4: Applied Protection to Steel Members, European Committee

for Standardization, Brussels.[5] European Organization for Technical Approvals (EOTA) (2006) ETAG018,

Guideline for European Technical Approval of Fire Protective Products, Part2: Reactive Coatings for Fire Protection of Steel Elements, European Organi-zation for Technical Approvals, Brussels.

[6] International Standards Organization (ISO) (1975) ISO 834: Fire ResistanceTests-Elements of Building Construction, Geneva.

[7] Institute Fur Bautechnik(1977). Procedures for Testing and Approval ofIntumescent Coating on Steel Members to Comply with F30 Fire Resistance

Category of DIN 4102, Berlin.[8] Y. Sakumoto, J. Nagata, A. Kodaira, Y. Saito, Durability evaluation of intumes-

cent coating for steel frames, J. Mater. Civ. Eng. 13 (1) (2001) 274–281.[9] Z.Y. Wang, E.H. Han, W. Ke, Effect of nanoparticles on the improvement in

fire-resistant and anti-ageing properties of flame-retardant coating, Surf.

Coat. Technol. 200 (2006) 5706–5716.[10] Z.Y. Wang, E.H. Han, W. Ke, Fire-resistant effect of nanoclay on intumescent

nanocomposite coatings, J. Appl. Polym. Sci. 103 (2006) 1681–1689.[11] J.F.,Yuan (2009). Intumescent Coating Performance on Steel Structures under

Realistic Fire Conditions, Ph.D. Thesis, University of Manchester.

wang 2013

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