Intumescence is a strategy in flame retardancy which involves the formation
on heating of a swollen multicellular thermally stable char insulating the underlying
material from the flame action (Camino and Delobel, 2000).
It was known that steel structures are not stable at 500°C as it will lose
its structural properties within that temperature. If the load bearing elements
reach the temperature of 550°C, the structure will collapse and therefore,
this failure will lead to explosion as load bearing elements contain a highly
flammable material such hydrocarbons and natural gas.
In recent years, the release of toxic gases and smoke during the burning of
halogenated flame retardant materials (Han et al.,
2007). To find alternative flame retardants due to increasing environmental
and health concerns surrounding by using halogenated flame retardants. Expandable
Graphite (EG) shows attractive applications as intumescent fire retardant as
its large increase in volume with expansion factors more than 200 mL g-1
(Duquesne et al., 2002).
In previous studies expandable graphite was used as synergistic effect with
APP-PER-MEL and polyethylene coatings. Because of performance of anti oxidation
of APP-PER-MEL coating was not suitable (Li et al.,
2007). The synergistic effect is likely to come from reaction between pyrolyzing
In this study, EG used as a carbon source, APP as an acid source, melamine as a blowing agent and boric acid Kaolin Clay are as an inorganic additives.
The objective of this study is to investigate the effect of boric acid and boric acid kaolin on the heat shielding effect of coating, residue weight and structure of char of EG-APP-Mel-boric acid-epoxy-hardener intumescent coating formulations.
MATERIALS AND METHODS
Materials: Kaolin Clay supplied from Jinyang Shanxi Jinyang Calcined kaolin Ltd. China. Bisphenol A epoxy resin BE-188 (BPA) and ACR Hardener H-2310 polyamide amine purchased from Mc-Growth Chemical Sdn. Bhd. (MGC) Selangor, Malaysia. Ammonium poly phosphate supplied by clarinet Germany, Flake Graphite supplied by Insutex Industries Sdn. Bhd. Malaysia, Boric acid, structural steel coupons.
Preparation of intumescent formulation: Expandable Graphite was prepared
by the mixing of flake graphite with acetic acid, sulphuric acid and KMnO4
1: 2: 0. 5: 0. 07 with weight percentage, respectively in a conical flask. The
flask was stirred at 25°C for 1 h (Tushinsky et al.,
2002; Bhagat, 2001; Wang et
al., 2007). The mixture was washed with distilled water. Leave the expandable
graphite to dry in the oven at 60°C which is further used as a carbon source
in the intumescent coating composition.
|| Weight percentage composition of coating
|*Noted that (BPA+ACR hardener) refers to curing agent
|| Experimental flow chart
|| The FESM image of kaolin clay
Figure 1 showed the experimental flow chart. The intumescent
ingredients are mixed with their weight percentage composition homogeneously
by using high shear mixer. C1, C2, C3 and C4 are controlled formulations with
EG-APP-Mel-boric acid-epoxy-hardener. C4 formulation is further modified by
adding 3-5 wt% of Kaolin Clay (KC) described in the Table 1.
The Field Emission Scanning Electron Microscopy (FESEM) image of Kaolin clay
is illustrated in Fig. 2. The formulation was coated manually
on the steel substrate. The coated substrate was cured in the oven at 60°C
for 4 h. Bunsen was used for fire test.
Analysis and characterization
Heat shielding effect: Bunsen burner was used as a fire test source by using the standard UL 94. K-type thermocouple is used to measure the temperature with AMS-850 data logger. The flame temperature of bunsen burner is 780°C.
Scanning electron microscopy (SEM): Charring layer and their morphological structures were observed and analyzed by AMARY 1000 SEM.
Thermogravimetric analysis (TGA): The Thermogravimetric analyses of samples (approximately 10 mg) were carried out at 20°C min-1 under N2, over the whole range of temperature (50-900°C) by TGA Q50.
RESULTS AND DISCUSSION
Heat shielding effect: In this study, four samples with different compositions of intumescent ingredients were prepared. After the fire testing was done, it was found that C4 that contains 15% of expandable graphite, 20% of ammonium polyphosphate, 10% of melamine, 15% of boric acid and 40% of epoxy-hardener mixture gives the best intumescent effect after fire testing being performed onto the samples. By using similar composition with C4, another three coatings were prepared by adding kaolin clay 3-5wt%.
The temperature time curves and data for the fire proofing time of flame retardant coatings are illustrated in Fig. 3, 4 and Table 1. The uncoated mild steel plate can only sustain its properties for about 9 min after the fire. Figure 3, formulation 1, it has the back side substrate temperature 426°C after 60 min. Formulation 2 has the back side substrate temperature of 395°C while formulation C3 and C4 has the back side substrate temperature 378 and 377°C after 60 min, respectively. From the result obtained, formulation C4 has the lowest backside substrate temperature followed by formulation C3, C2 and C1. This is because of the weight percentage of every component for each formulation. As shown in the Table 1, by comparing formulation C1 and C3 when boric acid is constant, the back side substrate temperature of formulation C3 is lower than formulation C1; this is because of the weight percentage of each component used. Formulation C4 gives the best intumescent effect as it contains 15 wt% percentage of expendable graphite. Expendable graphite will gives a better intumescent effect that lead to better intumescent effect.
As C4 formulation was modified with 3 to 5 wt% of Kaolin clay, the fireproofing
time of the coating was increased dramatically. The result showed that the temperature
was lowered to 336, 304 and 302°C after 60 min of kaolin clay coating KC5,
KC6 and KC7, respectively.
||Thermal behaviour of coating EG-APP-Mel-B.A-epoxy and hardener
|| Thermal behaviour of formulation modified with kaolin clay
However, for samples containing Kaolin Clay, it shows that formulation KC7
gives the best intumescent effect as the temperature at the back of the steel
is 302°C after 60 min fire test. A ceramic-like B2O3
layer would slow mass and heat transfer to the pyrolysis zone while the water
vapour produced would serve to quench the flame, hence reducing its intensity
(Nyamboa et al., 2009).
This proved that, Kaolin clay as nano clay showed a better intumescent effect. This is the effect of adding the reinforcement material that will create a ceramic like protective barrier on the surface of the insulating material. Thus, it will give a better intumescent effect.
Scanning electron microscopy (SEM): The SEM micrograph of chars from
the formulation C3 C4 presented in the Fig. 5 and 6,
|| SEM image of formulation C3
|| SEM image of formulation C4
The multiporous structure the char layer depends on the resistance of the substrate
to fire. The expansion of the char and structure are very important to common
fire resistant properties of coating (Li et al.,
2007). From Fig. 5 and 6, microstructure
of C3 and C4 showed that the formation of bubbles. These bubbles expand the
char due the emission of N2 and ammonium gases (Jimeneza
et al., 2006). It explains the dehydration charring of APP, Boric
Acid and frothing of melamine proceeds in the range of rather appropriate temperature.
The intumescent charring layers with bubbles act as the effect of the flame
retardant, heat insulation and protecting inner matrix materials. can obstruct
heat transferring to the substrate and protect the substrate from heat. Transfer
speed of heat through. From Table 1, KC6, KC7 and KC8 modified
the C4 EG-APP-Mel-B.A formulation with 3, 4 and 5 wt% of Kaolin are more compact
than C3 and C4. The SEM images show the thick char structure of KC6, KC7 and
KC8, respectively in Fig. 7-9.
|| SEM image of formulation KC5
|| SEM image formulation KC6
|| SEM image formulation KC7
The cause is that kaolin Clay (Al2Si2O5(OH)4)
acts as a filler hindered the expansion of EG-APP-Mel-boric acid coatings. The
different aperture surface tensions in the route of gas cavities lead to the
irregularity of swelling, the surface tension rely on the viscosity and regularity
of the coating. Li et al. (2007) reported that
anti-oxidation of the coating was improved by adding EG and MoSi2
and SEM images showed that a good synergistic effect was obtained through a
ceramic-like layer produced by MoSi2 enclosed on the surface of open-cellular
The SEM results show that the char of the coating modified by Kaolin is covered by a ceramic like open cellular protective layer on the steel substrate showed in Fig. 7.
Thermogravimetric analysis: The degradation of a pure APP has already
been described by Camino and Luda (1998). Ammonium Polyphosphate
(APP) will start to degrade to yield ammonia above 200°C (Cullis
and Hirschler, 1981). Boric acid degrade into two step, first step 100-140°C
it is decomposed into metaboric acid and boron oxide at 140-200°C in the
second step (Jimeneza et al., 2006). Melamine
will start to degrade at 290°C with the yield of ammonia and N2 (Jimeneza
et al., 2006). Graphite will start to degrade when temperature reached
at 250-650°C (Duquesnea et al., 2001). However,
the Kaolin clay will only degrade when the temperature is above than 1350°C
(Lee et al., 1989). Figure 10
TGA curve of C4 showed the decomposition of EG, APP, melamine, boric acid, epoxy
and hardeners and the weight loss of the formulation is about 39% in the later
It is also shown that the weight loss of the formulation C4 is 1% at the beginning
of the experiment (20-230°C), the moment is mostly due to fraction substance
and resin decomposing, other volatilization vaporizing. The medium-term of the
experiment (230-510°C) is the key weight loss region that the coating begins
to decompose largely, the weight loss gets to about 54%. The coating melts,
APP decomposes to release NH3, H2O and phosphoric acid
(the catalytic effect is considered as a benefit since to be efficient the intumescent
protective layer has to be formed in the early stages of a fire. For both resin/APP
mixtures, a thermally stable protective layer is formed later on in the higher
temperature range) thermally degrades, dehydrates to release poly-metaphosphoric
acid and pyrophosphoric acid with other organic materials which contains hydroxy
groups, dehydrates, to form the charring framework. At the same time, the vesicant
ammonia begins to release NH3 gas over 296°C (Gu
et al., 2007).
As the TGA maximum temperature is 900°C, the final weight percentage for samples with Kaolin clay are is slightly higher than samples without Kaolin clay because the Kaolin clay will only degrade when the temperature reach 1350°C. Thus, Kaolin clay will remain in the residual char.
The results of the TGA show the similar behaviour of thermal degradation for
each of the samples as the sample contains the same intumescent materials showed
in Fig. 10.
|| TGA curve formulation C4, KC5, KC6 and KC7
However, the rate of weight percentage loss slightly different as the weights
percentages of each formulation are differ from each other. The remaining weight
percentage of C4, KC5, KC6 and KC7 have 33.11, 38.7, 40.02 and 43.95%, respectively.
Based on these TGA result, it was observed that KC7 has the highest percentage of residual weight (43.95%) at the temperature 900°C. This shows that KC7 gives the best in tumescent effect compared to other formulations.
The fire retardant time of EG-APP-Mel-B.A coating modified by 5% wt kaolin clay was lowered the temperature of back side of substrate by various degrees. The KC7 showed the best result and the temperature of the backside of substrate was 302°C after 60 min. SEM result showed the structure of residue char was improved by adding kaolin clay. TGA showed that kaolin clay can enhance residue weight higher than that of EG-APP-Mel-boric acid-epoxy and hardener formulation C4. The largest improvement was achieved with 5 wt% of kaolin clay. Kaolin clay is a reinforcement material that will create a ceramic like protective barrier on the surface of the insulating materials. Thus, the efficiency of the heat transfer can be reduced and this will give a better effect of intumescent into the coatings.