A characteristics of steel; it will lose its structural strength at elevated
temperature (Sorathia et al., 1992). Thus, the
main function of intumescent coating is to protect the steel up to two hours,
where it can give ample time for evacuate and rescue process if there is any
fire accident happens (Jimenez et al., 2006a).
Moreover, the intumescent coating also act as thermal insulation for a building,
as it is can help to resist the fire spread to another building if the adjacent
building is on fire. Thus the bonding strength between the substrate and the
polymer is a fundamental aspect in intumescent coating. Poor adhesion between
the char and the substrate will lead to insecure substrate and finally the char
detached from the substrate (Ullah et al., 2011a).
As the result its increase the speed of heat transfers to steel substrate. A
strong bonding of intumescent coating is desired to form a good protective char
layer for steel substrate.
Intumescent coating is synonyms as passive fireproofing materials
which mean insulating systems designed to decrease heat transfer from a fire
to structure. Mostly, intumescent coating contained minimum of four ingredients;
a carbon source (pentaerythritol), a source of mineral acid catalyst (ammonium
polyphosphate), a source of nitrogen gas such as melamine or urea and resin
as binder (Bourbigot and Duquesne, 2007; Gu
et al., 2007). A series of chemical reactions occur with the release
of inert gases when an intumescent coating is exposed to fire, that result in
the low thermal conductivity of the substrate (Jimenez et
al., 2006a). A number of studies have established the use of filler
as reinforcing agent helps to increase the efficiency of the intumescent coatings
in terms of providing long term protection to the structural steel (Hao
and Chow, 2003).
The aim of this research is to study the synergistic effect of Aluminium Trihydrate
(ATH) and fumed silica on Expandable Graphite (EG) based intumescent coating.
Hence, the optimum performance of intumescent coating need to be find by conduct
several test and study the characteristic before and after test.
MATERIALS AND METHODS
Ammonium Poly Phosphate (APP) (Exolit AP422) is used as the acid source was
bought by Clariant (Malaysia) Sdn Bhd.
|| Composition of formulation (wt.%)
|EG: Expandable graphite, APP: Ammonium poly phosphate, MEL:
Melamine, BA: Boric acid, BPA: Bisphenol A epoxy resin BE-188, Hard: Hardener
H-2310 polyamide amine
Bisphenol A epoxy resin BE-188 (BPA) used as a binder with Hardener H-2310
polyamide amine were purchased from Mc-Growth chemical Sdn Bhd. Malaysia. EG
was bought from Clariant. Structural steel A36M was supplied by TSA industries
(Ipoh) Sdn. Bhd. Malaysia. Melamine (Mel) used as a blowing agent and Boric
Acid (BA) as an additive were bought from Sigma-Aldrich (M) Sdn Bhd. Malaysia.
Bhd. EG, Fumed Silica (FS) and ATH were purchased from Premier East West Malaysia
Coating preparation: All intumescent ingredients were mixed with their
respective weight percentage as stated in the Table 1. The
shear mixer was used for the mixing of coating at 40 rpm for 30 min. The structural
steel plate area 100 cm2 was used as a substrate. The coating was
applied using brush on the steel substrate and thickness of coating was maintained
at 1.5 mm and it was measured by digital vernier caliper. The coated substrate
was cured in the oven at 60°C for 1 h.
Bunsen burner test used for heat shielding at 950°C according to ASTM 119
and UTM machine was used for shear test. To study the char expansion carbolite
furnace was used for fire test at the temperature of 500°C.
Table 1 represents various formulations done in the research.
R is reference sample,
which is without inorganic filler. Symbol A
represent ATH and F
is fumed silica. A1, A2 and A3 represent 2, 4 and 6 weight (wt.) percentage
(%) of ATH, respectively. F1, F2 and F3 represent 2, 4 and 6 weight percentage
of Fumed Silica, respectively.
RESULTS AND DISCUSSION
Expansion of char: The expansion of the char and structure are very
important to common fire resistant properties of coating (Li
et al., 2007; Ullah and Ahmad, 2012). Figure
1, shown that ATH 4 wt.% gave the highest char expansion 7.5 times from
the original coating thickness. Fumed silica 4 wt.% gave 4.2 times char expansion
higher char expansion than others formulations of fumed silica, respectively.
||Char expansion of intumescent coating after furnace test at
Thus, these prove that adding inorganic filler give higher char expansion up
to a certain limit. Hence, those samples have been analysed by SEM to see the
structure of samples.
Scanning electron microscopy (SEM): Based on Fig. 2a
and b, both sample poses a span structure that contain tiny
hole that could help to trap inner gases that been released during burning process.
Moreover, there is no any defect in the char of A2 meanwhile an internal
crack occur on inner structure of F2. Thus, this defect reduced expansion
ability of fumed silica. From Fig. 2b, microstructure of F2
showed cracks and holes on the surface of char. It explains the dehydration
charring of APP, boric acid and frothing of melamine proceeds in the range of
rather appropriate temperature (Jimenez et al., 2006b).
Heat shielding test was studied using Bunsen burner. Figure 3,
showed time and substrate temperature curves of R, A1, A2,
F1 and F2 respectively for 60 min fire test using Bunsen
burner. The highest temperature achieve for R is 140°C and yet A2
and F2 give lower value than R and others formulations. The substrate
temperature of A2 and F2 are 119 and 114°C, respectively.
However, during the test conducted, F2 had shown some failure, where
the char is partially detached from the steel substrate. The detachment part
is mainly at the fire zone. However, at non fire affected zone, the coating
are still well detached while A2 was not shown any sign of detachment
coating at any part.
Moreover, based on Fig. 3 the pattern of the graph is showed
when the temperature achieves the highest value, it will be reduce about 10%
and yet give an approximately constant value.
|| Inner structure of (a) Aluminium trihydrate (ATH) A2 and
(b) Fumed silica (FS) 4 wt.% Bunsen Burner test
|| Bunsen burner fire test
X-ray diffraction (XRD): During fire the char layer is gradually oxidized
at elevated temperature, hardly a few vague carbon and inorganic materials are
remained in the carbonaceous char (Wang and Yang, 2010).
The inorganic materials become the main protecting shield at later stage of
the fire. The residual char of sample A2 (ATH 4%) and F2
(fumed silica 4 wt.%) were analyzed using XRD technique.
Figure 4a and b showed the XRD peaks of
the residue char of sample A2 and F2, respectively burnt
at 500°C. Several XRD peaks of the residue char at 6.05, 3.68, 3.3, 3.17
and 2.25 were assigned according to JCPDS card. Those elements shown higher
peak in XRD result and give indicator that, the elements is dominant the composition
of intumescent coating during burning process.
The peak at 6.05 was allocated to boron oxide. The peak at 3.65237 assign to
boron phosphate and at 3.3 is assigned to graphite (carbon). The major peak
at 3.17 assigned to sassolite and 2.5 were assigned to boron phosphate oxide.
The dehydration of boric acid yield boron oxide while the reaction between APP
and boron oxide yield some boron phosphate in the charring inorganic elements.
The formation of sassolite (mineral acid of boric acid H3BO3)
that has been shown due to the dehydration to support the formation of B2O3,
glass-like material which increase fire retardancy of char (Jimenez
et al., 2006c; Ullah et al., 2011b).
B2O3 act thermal barrier and BPO4 act adhesion
for bonding strength of char with substrate steel.
Shear test: In shear test, all formulations were not giving a good result
as shown in Fig. 5.
The reference sample R showed 7.432 kN loading potential and A2,
F2 gave 2.623, 1.476 kN load capability which are better as compared
to other formulations. The visual inspection before the test, shown that all
the inorganic fillers formulation are not fully dried, compare to reference
sample R, where only need 3 weeks for draying. Hence, the coatings were not
strongly attached on the steel substrate.
Thermogravimetric analysis (TGA): TGA analysis was done to obtain the
residual weight for each of formulations. As the residual weight play important
role in intumescent coating (Amir et al., 2011).
When the residual weight is high, the anti oxidation and thermal stability of
char will become high. Hence, it is good for intumescent coating to have this
characteristic. Moreover, TGA was done, in order to know the degradation time
and temperature for each formulation.
A good intumescent coating must have a high residual weight. However, based
on Fig. 6, the residual weight of R, F2 and A2
are 23.2, 30.25 and 32.27 wt.%, respectively.
|| XRD result of (a) A2 (ATH 4%) and (b) F2
(FS 4 wt.%)
A2 has better residual weight while on fumed silica TGA result,
F2 have better residual weight from other formulation. Thus, a comparison
between A2, F2 and R (reference) were done in order to
find which formulation shown better in residual weight.
Hence, A2 have better residual weight from F2 and R based
on figure above. Thus, A2 is better in anti oxidation and thermal
stability characteristic. Based on the Fig. 5, shown that
intumescent without inorganic fillers gave the lowest value of residual weight
compared to A2 and F2. Furthermore, each of formulations
follow same pattern of graph, there are three phases can be seen. The phases
are, phase 1: between 30 to 300°C, phase 2: between 301 to 500°C and
phase 3: between 501 to 840°C.
|| Shear test result
|| Comparison TGA Result between R, F2 and A2
Those three phases are water elimination, degradation and residual process.
Hence, the degradation time and temperature of each formulation can be known
which is at 300°C.
As the result, intumescent coating with ATH give a better performance than
fumed silica and yet 4% of ATH (A2) gives the best performance than
others formulation in term of bonding strength. This is proven by the test that
been conducted, where on Furnace test, give highest char expansion, Fire Bunsen
burner test gave the lowest heat shielding effect without detachment of char
with substrate. On Shear test, although it does not withstand higher load than
reference value, yet it is the highest among others. Furthermore, on advanced
analysis using SEM and TGA, yet 4% of ATH give good result where it has less
internal crack inside the structure and have high residual weight for better
in anti oxidation and thermal stability characteristic.