~ Experimental study of hydrothermal aging effects on intumescent coating

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Experimental study of hydrothermal aging effects on insulative properties

of 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 2012

Keywords:

Intumescent coating

Hydrothermal aging test

Fire test

Effective thermal conductivity

Fire resistance

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 andthat 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 it

allows the attractive steel structural form to be exposed. Intumescent

coatings are now widely used as passive fire protection to steel

structures and in countries such as the UK, the use of intumescent

coating dominates the passive fire protection market [3]. The coat-ings, which usually are composed of organic components contained

in a polymer matrix, are designed to decompose and expand when

subjected to high temperatures so as to provide an insulating, foamed

char to protect the underlying substrate.

When specifying intumescent coating fire protection for steel

structures, the following assumptions are made:

(1) the type and thickness of the intumescent are correctly

specified;

(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, but

the problem is not a technical one. Assumption (3) deals with

durability of intumescent coating. Since most of the chemical

components 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 coatings

deteriorates over time.

There are very few reported research studies in open literature

on durability of intumescent coatings. Sakumoto et al.[8] carried

out some accelerated aging tests according to the standards of

[7,2] to investigate the principal environmental factors that affect

the 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 experimental

study to provide some quantitative information on reduced fire

Contents lists available at SciVerse ScienceDirect

journal homepage: www.elsevier.com/locate/firesaf

Fire Safety Journal

0379-7112/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.

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

n Corresponding author at: University of Manchester, School of Mechanical,

Aerospace and Civil Engineering, PO Box 88, Manchester, M60 1QD, UK

E-mail addresses: [email protected],

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

Fire Safety Journal 55 (2013) 168–181

https://www.researchgate.net/publication/229464942_Fire-resistant_effect_of_nanoclay_on_intumescent_nanocomposite_coatings?el=1_x_8&enrichId=rgreq-fc1541b9-b94e-4718-99dc-8edd93c25fed&enrichSource=Y292ZXJQYWdlOzI2MzYwMDQ3MDtBUzoxNjQ4MDc2NTExMTA5MTJAMTQxNjMwNDYxNDI5Mw==

https://www.researchgate.net/publication/240378169_Effect_of_nanoparticles_on_the_improvement_in_fire-resistant_and_anti-ageing_properties_of_flame-retardant_coating?el=1_x_8&enrichId=rgreq-fc1541b9-b94e-4718-99dc-8edd93c25fed&enrichSource=Y292ZXJQYWdlOzI2MzYwMDQ3MDtBUzoxNjQ4MDc2NTExMTA5MTJAMTQxNjMwNDYxNDI5Mw==




protection performance of degraded intumescent coatings to

steel structures. Two series of tests have been conducted. In series

one (to be referred to as fire test), intumescent coating specimens

were subjected to different cycles of accelerated aging and then

tested in fire. The measured data include final expanded thick-

nesses and the substrate steel temperatures. From these tests, the

effects 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 intumescent

coatings were subjected to TGA, XPS and FTIR tests to measure

their mass loss, change of element contents and migration of

components in the intumescent coating system. The tests help to

explain the degradation processes.

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 Film

Thickness (DFT) intumescent coatings on all sides. Of the test

specimens, 16 were protected by type-U intumescent coating (to

be referred to as type-U specimens) and 40 were applied with

type-A intumescent coating (to be referred to type-A specimens).

The two different types of intumescent coating were supplied by

two different manufacturers operating in the Chinese market. The

principal components of type-U and type-A intumescent coating

are APP-MEL-DPER (aided with zinc borate) and APP-MEL-PER,

respectively; the acid resins of type-U and type-A intumescent

coatings are ethylene benzene–acrylic and single component

acrylic, 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 20

years in service). All specimens were coated with 1 mm DFT. For

the 40 type-A specimens, 20 were coated with DFT 1 mm coating

and the other 20 with DFT 2 mm coating. For each coating

thickness, four replicate tests were performed for each of the

following 5 cycles of accelerated aging: 0 (no aging), 4 cycles

(simulating 2 years in service), 11 cycles (simulating 5 years in

service), 21 cycles (simulating 10 years in service) and 42 cycles

(simulating 20 years in service). In all cases, the substrate steel

plate measured 200 mm by 270 mm by 16 mm thick. A primer

was applied to the steel surface first to act as an aid to adhesion of

the intumescent coating; this was then followed by different

layers 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 embedded

in each steel plate. Table 1 lists the main specimen parameters

and Fig. 1 shows the specimen dimensions, where d is the

intumescent DFT.

2.2. Hydrothermal aging test

Intumescent coating aging is an extremely complicated pro-

cess of physical and chemical interactions between the chemical

components 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 of

years. An alternative is to conduct accelerated aging test, in whicha real environmental condition over a long period of time is

represented 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 environmental

conditions that the product (intumescent coating) will be exposed

to, the length of time of the exposure and the performance

criterion based on which the effect of aging is assessed.

The accelerated aging test was performed according to the

European 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 Z1

for internal conditions which have above zero temperatures and

high humidity; and (d) type Z2 for internal conditions that have

above zero temperatures but humidity conditions that are not in

class Z1. The accelerated aging test reported in this paper adoptedexposure condition Z1, simulating the more severe exposure

condition of application around the coastal provinces in China.

Table 1

Main 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

(i ¼1–4)11 5 UI-1–11-i

(i ¼1–4)

21 10 UI-1–21-i

(i ¼1–4)

42 20 UI-1–42-i

(i ¼1–4)

A 1 0 0 AZ-1–00-i

(i ¼1–4)

4 2 AZ-1-04-i

(i ¼1–4)

11 5 AZ-1-11-i

(i ¼1–4)

21 10 AZ-1-21-i

(i ¼1–4)

42 20 AZ-1-42-i

(i ¼1–4)

2 0 0 AZ-2–00-i

(i ¼1–4)

4 2 AZ-2-04-i

(i ¼1–4)

11 5 AZ-2-11-i

(i ¼1–4)

21 10 AZ-2-21-i

(i ¼1–4)

42 20 AZ-2-42-i

(i ¼1–4)

Fig. 1. Specimen dimensions.

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




For exposure condition type Z1, each cycle of exposure is as

follows:

8 h at (4073) 1C and (9872)%RH;

16 h at (2373) 1C and (7572)%RH.

According to ETAG018, 21 cycles of accelerated aging is

equivalent 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.

2.3. Surface appearance

After the accelerated aging test but before the fire test, the

specimens were checked for their coating surface appearance.

Figs. 2 and 3 show typical appearance of type-U and type-A

specimens after having gone different cycles of accelerated aging

test. Type-U specimens did not appear to suffer any change in

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 noticeable

changes in appearance after every accelerated aging test. After

Fig. 2. Type-U coating appearance after different cycles of hydrothermal aging test. (a) UI-1-00, (b) UI-1-11, (c) UI-1-21 and (d) UI-1-42.

Fig. 3. Type-A coating appearance after different cycles of hydrothermal aging test. (a) AZ-1-00, (b) AZ-1-04, (c) AZ-1-11, (d) AZ-1-21 and (e) AZ-1-42.

Fig. 4. Furnace door with observation holes.

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




only 4 cycles, the surface of type-A specimens appeared uneven

(Fig. 3(b)). After 11 cycles of accelerated aging test, bumps

appeared on the surface (Fig. 3(c)). After 21 and 42 cycles, the

specimen surface was uneven, with very large bumps

(Fig. 3(d) and (e)). As will be shown later in this paper, there is

strong link between the surface appearance and fire protection

performance of an intumescent coating. Coating surface appear-

ance may be explored when determining a replacement strategy

in real applications.

2.4. Fire test

After the specimens were subjected to hydrothermal aging as

described in the previous section, they were placed in a furnace

(Fig. 4) and exposed to fire. The furnace temperature was

measured by four thermocouples and the average furnace tem-

perature was regulated according to the ISO 834 ([6]) standard

temperature–time relationship. The ISO 834 standard

temperature–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. The

steel temperature was measured by three thermocouples

embedded in the steel plate and recording was made every

minute continuously. Four observation holes were placed on the

furnace door to enable the fire tests to be observed and pictures of the surface of the specimens to be taken. Each test was continued

until 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 components

and the fire exposure condition, these reactions may happen in

sequence or together. Type-A intumescent coatings began to

expand earlier than type-U intumescent coatings. In the intumes-

cence (expansion) stage, bubbles that appeared on the surface of

type-A intumescent coatings were much larger than that those on

the surface of type-U intumescent coatings.

Intumescent coatings for both types are highly ‘‘engineered’’ to

pass the standard fire resistance test when freshly applied.

Hydrothermal aging causes some chemical components in the

intumescent coatings to migrate to the surface, altering the

chemical reactions. In the intumescence stage, the blowing agent

in the intumescent coating decomposes to produce gas, a fraction

of which is trapped within the molten matrix to cause the coatingto expand. From the pictures taken of the specimens through the

observation holes on the furnace door, many bubbles appeared

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.





during the intumescence stage on both types of intumescent

coating after 0 or 4 cycles of hydrothermal aging tests, as shown

in Figs. 6 and 7(a). This means a large amount of gas was

produced due to decomposition of the blowing agent. After 11

and 21 cycles of hydrothermal aging tests, the number of bubbles

decreased drastically and the bubble distribution was much less

uniform, shown as Figs. 6 and 7(b). After 42 cycles, bubbles werealmost non-existent, see Figs. 6 and 7(c).

The observed phenomena for type-A specimens with both

1 mm and 2 mm DFTs were generally similar.

The most important parameters that directly reflect the fire

protection performance of intumescent coatings are the final

expanded thickness and internal structure of the char [11].

Fig. 8 shows the expanded heights of both types of coatings after

different cycles of aging test. These figures also give someindication of the consistence of the char.

Fig. 7. Bubble appearance on the surface of type-U specimens. (a) UI-1-00, (b) UI-1-21 and (c) UI-1-42.

Fig. 8. Cross sectional view of expanded intumescent char after different cycles of accelerated aging test. (a) AZ-2-00, (b) AZ-2-42, (c) AZ-1-04, (d) AZ-1-42, (e) UI-1-11

and (f) UI-1-42.





It can be seen from Fig. 9 that the expanded thickness

decreased greatly for both types of intumescent coating after 42

cycles of aging test. In addition, the integrity and consistency of

the char for the specimens after 42 cycles of aging test are poor

Fig. 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, the

measured final thickness in some cases is around the average

value. Fig. 10 plots the expansion ratio as a function of the

number of cycles of accelerated testing. It can be seen that the

expansion ratio decreases considerably after only a few cycles of

aging test. After 21 cycles (simulating 10 years in service), the

expansion ratios of all three groups of intumescent coatings were

about 60% of those without aging. The expansion ratio may be

used to give a measure of the effective thermal conductivity of

intumescent coating. This means that after 21 cycles, the effective

thermal conductivity of the intumescent coating is about 1.7 times

(1/0.66) the effective thermal conductivity without aging. After 42

cycles, 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.

It can be seen from Fig. 10 that the replicate tests give

generally consistent results even though some discrepancies

exist. The average values of temperatures of the steel substrates

will be used. Figs. 11 and 12 compare the average steel

temperature–time relationships to show the effect of aging on

steel substrate temperature.

It can be seen from Figs. 11 and 12 that compared to speci-mens without aging, there is sharp increase in steel substrate

temperature after a certain number of cycles of aging test. For

type-A coating (Fig. 11), 11 cycles (representing 5 years in service)

appear to mark the beginning of sharp increase in the steel

substrate temperature. For type-U coating (Fig. 12), the increase

in steel temperature appears to be more even over the entire

range of aging cycles.

Figs. 13 and 14 present the steel substrate temperatures after

different cycles of aging at the same time when the specimens

without aging reached 400 1C, 500 1C and 600 1C. Furthermore,

Tables 3 and 4 present fire resistance times that may be achieved

by the different specimens if the steel limiting temperature is

400 1C, 500 1C, 600 1C and 700 1C.

The increase in steel temperature or the reduction in fireresistance time, due to degradation in intumescent coating

performance is very high. A question may arise on when intu-

mescent coating should be replaced. On the assumption of

replacing degraded intumescent coatings after suffering a loss of

20% in its fire resistance rating, then type-A coating (Table 3)

would need replacing after 11 cycles (representing 5 years in

service) and type-U coating (Table 4) would need replacing after

21 cycles (representing 10 yes in service). If no loss of fire

resistance rating is allowed, the only alternative solution would

be to specify intumescent coating DFT based on the degraded

intumescent coating performance. For example, look at the results

in Fig. 14 for type-U intumescent coating. Suppose the steel

limiting 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 steel

limiting 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 hydrothermal

aging on fire performance of intumescent coatings. The effec-

tive thermal conductivity may be obtained from the following

0

12

24

36

48

0

number/cycles of hydrothermal aging

e x p a n s i o n r a t i o

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 2

Expansion ratios for specimens.

Specimen Initial thickness(mm) Final thickness(mm) Expansion ratio

Type-U specimens (U-group)

UI-1–00 0.95 28.00 29.47

UI-1–11 1.01 22.00 21.78

UI-1–21 1.04 19.00 18.27

UI-1–42 0.94 10.00 10.64

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

AZ-1–00 1.02 47.00 46.08

AZ-1–04 1.05 42.00 40.00

AZ-1–11 1.10 35.00 31.82

AZ-1–21 1.08 28.00 25.93

AZ-1–42 1.06 10.00 9.43

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

AZ-2–00 2.20 85.00 38.64

AZ-2–04 2.16 76.00 35.19

AZ-2–11 2.09 69.00 33.01AZ-2–21 2.18 52.00 23.85

AZ-2–42 2.22 35.00 15.76





equation ([4]):

l p,t t ð Þ ¼ d p V

A p c ara 1 þ f=3

1

yt ya,t

Dt

" #

½Dya,t þ e

f=10

1

Dyt ð1Þ

where

Dya,t is the increase in steel temperature during the time

interval Dt

l p,t is the effective thermal conductivity of intumescent

coating during the time interval Dt

d p is the initial DFT of intumescent coating

c a is the specific heat of steel

ra is the density of steel A p/V is the section factor of the protected steel section

yt is the furnace temperature at time t

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 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 14 28 42 56 70

time (minute)

AZ-2-00-1

AZ-2-00-2

AZ-2-00-30

200

400

600

800

0 12 24 36 48 60

time (minute)

AZ-2-42-1

AZ-2-42-2

AZ-2-42-3

0

200

400

600

800

0 20 40 60 80

time (minute)

t e m p e r a t u r e o f s t e e l

s u b s t r a t e ( ° C )











UI-1-00-1

UI-1-00-2

UI-1-00-30

200

400

600

800

0 12 24 36 48 60

time (minute)

UI-1-42-1

UI-1-42-2

UI-1-42-3

Fig. 10. Replicate temperature–time relationships of steel substrate.

0

200

400

600

800

0

time (minute)

AZ-1-00

AZ-1-04

AZ-1-11

AZ-1-21

AZ-1-42 0

150

300

450

600

750

0

time (minute)


s u b s t r a t e s ( ° C )



AZ-2-00

AZ-2-04

AZ-2-11

AZ-2-21

AZ-2-42

14 28 42 56 7020 40 60 80

Fig. 11. Effect of aging on steel substrate temperature–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 time

interval Dt

f ¼ c pr pd p A p

c araV

Dt r30 s

Since the specimen was exposed to fire on all sides, the section

factor was calculated as A p/V ¼142 m1.

In theory, the temperature dependent specific heat and density

of intumescent coating should be used when calculating the

effective thermal conductivity of intumescent coating using

Eq. (1). However, since the amount of heat stored inside the

intumescent coating is very small and may be considered to be

negligible compared to that in the steel substrate, FE0 so Eq. (1)

may be simplified to Eq. (2) below:

l p,t t ð Þ ¼ d p V A p c ara 1yt ya,t

Dt

" # Dya,t ð2Þ

For each specimen and for each time interval, the intumescent

coating temperature y p may be taken as the mean of the steel and

fire temperature so that

y p ¼ yt þya,t

2 ð3Þ

Figs. 15 and 16 present some of the results of coating thermal

conductivity–temperature curve.

It can be seen from Figs. 15 and 16 that the effective thermal

conductivity of both types intumescent coating starts to fall

sharply after the temperature of intumescent coating reached

about 100 1C, indicating chemical reactions starting at about

100 1C. The effective thermal conductivity of the coating becamestable after reaching temperatures over 400 1C, a clear indication

that the coating had reached full expansion. Afterwards, the

effective thermal conductivities increase with temperature, which

may be explained by increased radiation inside the bubbles at

increasing temperatures [11]. It is the stable, fully expanded stage

of intumescent coating that is providing the fire protection

function, and the discussions below will focus on this stage.

It can be seen from Figs. 15 and 16 that there are some small

discrepancies between the results for the same three nominally

identical specimens. Nevertheless, the three replicate tests gave

generally consistent results. In the discussions to follow, the

0

200

400

600

800

0

time (minute)



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 50 60 70 80

time (minute)



AZ-1-00

AZ-1-04

AZ-1-11

AZ-1-21

AZ-1-42

350

400

450

500

550

600

650

700

750

10 20 30 40 50 60 70 80

time (minute)



AZ-2-00

AZ-2-04

AZ-2-11

AZ-2-21

AZ-2-42

530°C

592°C

506°C

400°C

567°C

643°C

500°C

711°C

600°C

675°C

443°C

621°C

530°C

400°C

556°C

641°C

500°C

600°C

658°C

450°C

420°C

630°C

424°C

532°C

630°C

Fig. 13. Temperatures reached when the specimens without aging reached 400 1C/500 1C/600 1C. (a) Type AZ-1 specimens and (b) Type AZ-2 specimens.





average effective thermal conductivities for intumescent coating

temperatures between 250 1C and 750 1C at 50 1C interval will be

used. In order to help clarify discussions, the raw results in

Figs. 15 and 16 are smoothed and the average results are shown

in Fig. 17.

The changes in effective thermal conductivity of all coatings

follow a consistent pattern as a function of the number of cycles

of 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 correspond

to the practically interesting steel temperature range of 500–

600 1C, may be used. The results are presented in Fig. 18.

The effect of hydrothermal aging is to cause the hydrophilic

components of intumescent coating to move to the surface of the

coating and then be dissolved by moisture. Hence, although type-

A coating of the same DFT (1 mm) performed better than type-U

coating if neither suffers any aging effect (as can be seen in Fig. 18

by the lower thermal conductivity of type-A coating at 0 cycle),

type-A coating suffers from hydrothermal aging much more

quickly than type-U coating due to it having a lower water

resistance as explained in Section 2.3. Whilst the thermal con-

ductivity of type-U coating increases very gradually as the

number of cycles increases, type-A coating suffers significantly

more loss in performance after only 4 cycles of hydrothermal

aging.

Comparing the performance of 1 mm DFT type-A coating with

that of 2 mm DFT type-A coating, the same degradation processoccurs, but the thicker DFT 2 mm coating delays the process

slightly so that the large change occurs between 11 and 21 cycles

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

cycles of hydrothermal aging for the thinner DFT.

4. Chemical analysis tests

A number of chemical analysis tests were carried out to further

examine the degradation process in more detail. The chemical

analysis 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 TGA

tests were conducted using a Pyris diamond TG/DTA instrument

under nitrogen at a heating rate of 20 1C mini within a tempera-

ture range of 25–800 1C. Fig. 19 presents the measured mass loss

results.

It can be seen from Fig. 19 that there is little difference in the

TGA test results after different cycles of hydrothermal aging. This

indicates that the hydrothermal aging tests did not cause the

chemical components to be any different. However, the optimum

matching of chemical components in the intumescent coating

changed due to migration of the hydrophilic components to the

surface. Hence, the expansion ratios of the fire test specimens

were different. This also suggests that when detecting changes in

intumescent coating performance over time, the TGA test would

not be suitable.

4.2. FTIR test results

FTIR (Fourier transform infrared spectroscopy) test was used

to investigate the migration of chemical components for both

types of specimens with different cycles of hydrothermal aging.

The FTIR test was conducted on samples extracted from the

surface layer of intumescent coating, using the EQUINOXSS/HYPERION2000 device. The experimental results are presented

in Fig. 20.

350

400

450

500

550

600

650

700

750

10 20 30 40 50 60

time (minute)

t e m p e r a t u

r e o f s t e e l


UI-1-00

UI-1-11

UI-1-21

UI-1-42

483°C438°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 4

Fire 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





Although the FTIR test results can only be used to gain a

qualitative understanding of the effects of aging, they can still

give some indication of the extent of aging in service.

For type-A coatings (Fig. 20a), the troughs at 3321 cm1,

3176 cm1, 2957 cm1 and 1668 cm1 indicate N–H bonds

contained in MEL and APP, O–H bonds in PER, C–H bond in acrylic

acid resin and PER, and C¼O bonds in acrylic acid resin,

respectively. The trough at 1428 cm1 is the overlapping peak

of the absorption peak of CH2 group contained in acrylic acid resin

and PER and the absorption peak of triazing rings which are the

main structure of MEL. The troughs at 1253 cm1, 1078 cm1,

1013 cm1 and 889 cm1 indicate P¼O bonds in APP, C–O–H

bonds in PER, C–O–C bonds in acrylic acid resin and triazing rings

in MEL, respectively. Similar wave numbers of these bonds can be

observed in Fig. 20(b) for type-U coating.It is clear from Fig. 20 that the absorption peak of the above

mentioned 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 the

surface of the coating after different cycles of aging.

Compared to AZ-1–00, the absorption peak at 1428 cm1 of

AZ-1–11 was weakened whereas the width of the peak increased.

This indicates that the polymer binder degraded under the effect

of water and oxygen and some of the CH2 groups were oxidated

into C ¼O groups. This enhanced the absorption peak of C¼O

bonds at 1668 cm1 with increasing number of cycles of aging.

Degradation of the polymer binder (acrylic acid resin) was

present during the whole process of aging. The absorption peaks

at 1428 cm1and 889 cm1 were also enhanced, indicating

migration of MEL from within the coating to the surface of the

coating after different cycles of aging.

It is observed from the above analysis that the degradation of polymer binder (acrylic acid resin) and migration of flame

retardant system (APP-MEL-DPER) happened at the same time

0

0.05

0.1

0.15

0.2

0 200 400 600 800

temperatrure of coating (°C)

e f f e c t i v e t h e

r m a l c o n d u c t i v i t y

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


e f f e c t i v e t h e

r m a l c o n d u c t i v i t y

W / ( m •

° C )

UI-1-42-1

UI-1-42-2

UI-1-42-3

Fig. 15. Effective thermal conductivity (l p)–coating temperature (y p) 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


e f f e c t i v e t h e r m a l c o n d u c t i v

i t y

W / ( m • ° C )

AZ-1-00-1AZ-1-00-2

AZ-1-00-3

0

0.03

0.06

0.09

0.12

0.15

0 200 400 600 800


e f f e c t i v e t h e r m a l c o n d u c t i v

i t y

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


e f f e c t i v e t h e r m a l c o n d u c t i v i t y

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



W / ( m • ° C )

AZ-2-42-1

AZ-2-42-2

AZ-2-42-3

Fig. 16. Effective thermal conductivity (l p)–coating temperature (y p) 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.





0

0.03

0.06

0.09

0.12

0.15

200

temperature of coating θp(°C) e f f e c t i v e t h e r m

a l c o n d u c t i v i t y 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

e f f e c

t i v e t h e r m a l c o n d u c t i v i t y

λ p 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


λ p W / ( m • ° C )

UI-1-00

UI-1-11

UI-1-21

UI-1-42

300 400 500 600 700 800 900

200

temperature of coating θp(°C)

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



λ p W / ( m • ° C )

6 5 0 ° C ~ 7 5 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

w e i g h t l o s s ( % )

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

w e i g h t l o s s ( % )

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 fire

protective properties of intumescent coatings.

In practical application, when examining the effects of aging

on intumescent coatings, if on-site FTIR test shows little change in

the absorption peaks, then there is high confidence that the

effects of aging are minimal.

4.3. XPS test results

XPS (X-ray photoelectron spectroscopy) test gives information

on the amount of chemical elements being examined. For example,

Fig. 21 presents the amounts of Carbon and Nitrogen existent on

the surface layer of both types of intumescent coatings after

different cycles of aging. The XPS test was conducted using an

elemental analyzer VARIOEL 3.

C element is contained in MEL(C3H6N6) which acts as the

blowing agent and in DPER/PER and (C(CH2OH)4) acting as the

charring agent; N element is contained in MEL and

APP(NH4)nþ2PnO3nþ1) which act as the catalytic agent. Table 5

lists the percentage of Carbon and Nitrogen elements obtainedfrom the XPS tests for representative samples of both types of

intumescent coatings.

It can be seen from Table 5 that compared to specimens

without aging, the contents of C and N elements on the surface

layer of type-A and type-U specimens increased with increasing

number of cycles of aging. The change in N element is much more

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)

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

Fig. 20. FTIR test results. (a) FTIR test results for Type-A coating and (b) FTIR test results for Type-U coating.

10000

1000

2000

3000

4000

5000

6000

7000

8000

O1s

N1s

C1s

N ( E )

Binding Energy (eV)

AZ-1-00

0

1000

2000

3000

4000

5000

6000

7000

8000

O1s

N1s

C1s

N ( E )

AZ-1-21 AZ-1-42

0

1000

2000

3000

4000

5000

6000

7000

8000

O1s

N1s

C1s

N ( E )

0

1000

2000

3000

4000

5000

6000

7000

8000

O1s

N1s

C1s

N ( E )

UI-1-00

0

1000

2000

3000

4000

5000

6000

7000

O1s

N1s

C1s

N ( E )

UI-1-21

0

1000

2000

3000

4000

5000

6000

7000

8000

O1s

N1s

C1s

N ( E )

UI-1-42

800 600 400 200 01000

Binding Energy (eV)

800 600 400 200 0 1000

Binding Energy (eV)

800 600 400 200 0

1000

Binding Energy (eV)

800 600 400 200 0 1000

Binding Energy (eV)

800 600 400 200 0 1000

Binding Energy (eV)

800 600 400 200 0

Fig. 21. XPS test results.





sensitive to the change in C element. The current research is not

sufficiently comprehensive, but further extensive testing should

be done to ascertain whether it would be possible to link the

changes in C and N elements to the changes in fire protection

performance of different intumescent coatings.

4.4. SEM test results

SEM test gives some information on the change in internal

structure of chars after different cycles of aging. The SEM micro-

graphs of chars obtained from type-A coating after 11, 21 and 42

cycles 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 in

Fig. 22(a) and the number of pores decreases. For Fig. 22(c),

although the coating can still expand to form a char structure

after 42 cycles of aging, the aging process has damaged its

expanding effect and the ‘‘honeycomb’’ structure of intumescent

char does not exist.

5. Conclusions

This paper has presented the results of a series of fire tests on

intumescent coating protected steel plates after the intumescent

coatings have been exposed to different cycles of hydrothermal

aging according to exposure condition Z1 in European guide ETAG

018 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. The

results have been presented in terms of the expansion ratio, the

steel temperature and effective thermal conductivity. Surface

observations were made and additional chemical analysis (TGA,

FTIR, XPS and SEM) tests were also carried out. The followingconclusions may be drawn:

1. Bumps with different degrees of unevenness appeared on

the surfaces of specimens applied with type-A intumescent

coatings after different cycles of hydrothermal aging tests.

But no obvious change was observed for specimens applied

with type-U intumescent coatings after 11 and 21 cycles of

aging tests. Slight wrinkles appeared on the surfaces of

specimens applied with type-U intumescent coatings after

42 cycles of aging. The surface appearance can be used to

give a visual guide to the effectiveness of intumescent

coating performance in service.

2. Both types of intumescent coating suffered considerable

reduction in performance after 42 cycles of accelerated agingtest (corresponding to 20 years in service under the assumed

exposure condition). For example, the expansion ratio

reduced by over 70% and the steel plate temperature was

increased 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 that

the aging process did not cause the chemical components to

be any different, but the optimum matching of these

components in the examined intumescent coatings changed

due to migration of the hydrophilic components to the

surface of the coating when exposed to the hydrothermal

aging environment. This damaged the expanding ability of

the intumescent coatings. From the chemical analysis test

results, the TGA test is not suitable for detecting changes foraging effect. The FTIR test can detect the qualitative changes

of aging. The XPS test may be used to quantify the aging

Table 5

Contents 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 64.9N 7.9 8.7 10.5 9.9 10.1 13.4

Fig. 22. SEM micrographs of type-A intumescent chars after different cycles of aging (a) 11 cycles; (b) 21 cycles; (c) 42 cycles.





effects, but much more extensive testing is required before a

quantitative relationship between XPS test results and fire

protection performance results (e.g., changes in expansion

ratio/effective thermal conductivity) can be established.

4. The SEM test is destructive but the results can be used to

indicate 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 Committeefor 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 of Intumescent Coating on Steel Members to Comply with F30 Fire ResistanceCategory 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 infire-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 intumescentnanocomposite coatings, J. Appl. Polym. Sci. 103 (2006) 1681–1689.

[11] J.F.,Yuan (2009). Intumescent Coating Performance on Steel Structures underRealistic Fire Conditions, Ph.D. Thesis, University of Manchester.


Documents

~ Experimental study of hydrothermal aging effects on intumescent coating