Combustion measurements, such as heat release rate, critical flux, time-to-ignition, ignition temperature, thermal inertia, and kinematics—activation energy as well as preexponential factor—on epoxy polymer (Prime
Polymer-based materials are competing with metallic alloys in terms of cost and functionality such as durability, strength, and other physical and chemical properties. The shortened time of fabrication, toughness, strength, lightweight, and economic considerations are increasingly making polymer-based-material products comparable and even more usage-attractive and advantageous over their metallic alloy and ceramics counterparts [
In thermal applications, epoxy based materials are limited by their organic nature. When exposed to a heat source, the challenges of fire hazards result in undesirable changes that affect product functionality and durability. The single most important variable in fire hazard is the heat release rate (HRR) [
Quite a number of studies on heat effect on polymer composites are ongoing and findings are supporting refined and better methods in fabrication and performance of these polymer-based materials [
As part of research directed towards improved performance of resin matrix composites, the thermal behaviour of one particular epoxy polymer, Prime 20LV, with relatively low doses of EG inorganic filler of different weight percentages, was the focus of this study. The choice of Prime 20LV was simply based on its very low mixed viscosity and long working time that allows reinforcements like EG to be infused successfully. The polymer and its expandable graphite composites were investigated using the Cone Calorimeter and the thermogravimetric analysis (TGA) instruments. Thermal conductivity measurements were done using Linseis (Germany) THB100 Transient Hot Bridge thermal conductivity analyser.
The Prime 20LV (low viscosity) infusion epoxy resin and its slow Prime hardener with mixed density of 1.084 g·cm−3, glass transition temperature of 29°C, and viscosity of 23 cP at 20°C, were obtained from the Advanced Materials Technology (Pty) Ltd., in South Africa. The inorganic filler was expandable graphite (EG) of the trade name code ES250B5 with the following specifications: minimum carbon content of 90%, expansion volume of 0.25 m3 kg−1, and average particle size of 300
As per specifications from the supplier, the epoxy resin and its hardener were mixed and stirred vigorously in a mass ratio of 100 : 26, respectively, and poured into a Perspex mold of dimension of 100 × 100 × 5 mm3. The epoxy resin was then left, at room temperature, to cure for 24 hours.
Measured quantity of expandable graphite was carefully added to the mixture of the epoxy resin and its slow hardener and vigorously stirred continuously with a magnetic stirrer. When the viscosity of the composite began to increase, after 2 hours, the mixture was poured into the Perspex mold and left to cure for further 24 hours. The average thicknesses of the samples were 4.8 mm. In addition to the neat epoxy, the following sample compositions were fabricated for the various thermal measurements: Epoxy with 1 wt.% expandable graphite Epoxy with 3 wt.% expandable graphite Epoxy with 5 wt.% expandable graphite
The samples were subjected to the following set of external heat fluxes, which, according to [
The Cone Calorimeter is considered as the most significant bench scale instrument in fire testing due to the small sample sizes used for the test [
Thermal decomposition of the epoxy-EG composite was investigated further by Perkin Elmer TGA 4000 thermogravimetric analyser in an inert (N2) atmosphere for all samples, from neat to those with different EG compositions. TGA measurements of mass loss fraction versus temperature and time for different heating rates were done. Arrhenius plots for mass loss fraction (
Morphological studies of the epoxy resin EG composites were carried out using SEM JEOL 700 with accelerating voltage adjusted between 15 and 20 keV. The samples were carbon coated.
Heat is the driving force for ignition and development of fire. The following approximate equations describe how the measured parameters were obtained with the Cone Calorimeter and the TGA.
The time that a polymeric material can withstand heat flux radiated by a fire before it experiences sustained flaming combustion is called the time-to-ignition [
When exposed to a given net heat flux (
The net surface heat flux for the gasification period can be approximated as follows [
Equations (
The fraction of mass loss rate is assumed to be related to the decomposition rate of the polymer matrix as follows [
A similar analytical equation follows the Kissinger method and supposes a first-order kinetic was used by Régnier and Fontaine [
Equation (
Figure
Before (a) and after (b) Cone Calorimeter measurements of the epoxy-EG samples.
For a particular HRR profile (see Figure
HRR profile of neat epoxy subjected to different external heat fluxes.
As expected, the profile from the highest external heat flux (50 kWm−2) leads to the shortest ignition time (57 s), the highest HRR value (1359 kWm−2), and a more pronounced peak. The peak HRR values decrease with decreasing external heat fluxes [
External heat flux (50 kWm−2) leads to the shortest ignition time (37.5 s) and the highest HRR value (817.4 kWm−2). Similar to Figure
In Figure
External heat flux (50 kWm−2) again leads to the shortest ignition time (23.2 s) and the highest HRR value (360.8 kWm−2) in Figure
HRR profile epoxy-1 wt.% EG subjected to different external heat fluxes.
HRR profile epoxy-3 wt.% EG subjected to different external heat fluxes.
HRR profile epoxy-5 wt.% EG subjected to different external heat fluxes.
The summary of sample composition, time-to-ignition, and external heat flux are displayed in Figure
Summary of external heat flux and time-to-ignition.
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Time-to-ignition (s) | |||
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Neat | 1 wt.% EG | 3 wt.% EG | 5 wt.% EG | |
25 |
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30 |
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35 |
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50 |
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Plots of sample composition, time-to-ignition, and external heat flux.
The decreasing time-to-ignition in increasing content of the EG findings, as in Table
Figure
HRR profile of different epoxy-EG compositions at external heat flux of 25 kWm−2.
For example, the epoxy-EG 5-wt.% has the least peak HRR value of around 200 kWm−2, peak width with a plateau over 375 s, before average background charring of 38 kWm−2. The residue charring increases somewhat with increasing EG content (see Figure
FIGRA measured values of epoxy-EG composites from external heat fluxes.
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FIGRA (kWm−2s−1) | |||
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Neat | 1 wt.% EG | 3 wt.% EG | 5 wt.% EG | |
25 | 4.0 | 3.9 | 2.3 | 1.9 |
30 | 7.8 | 4.7 | 3.3 | 3.1 |
35 | 11.6 | 9.4 | 7.3 | 4.7 |
50 | 23.9 | 22.4 | 13.2 | 13.2 |
HRR profile of different epoxy-EG compositions at external heat flux of 50 kWm−2.
Furthermore, the total heat release per unit area (THRR) for the composites subjected to assigned external flux, which is equivalent to the area under a HRR versus time profile, shows some variations, due possibly to the fact that the samples thicknesses were not the same. The general trend, however, shows that THRR decreases with increasing EG addition. See Table
Total heat release rate measured values of epoxy-EG composites from external heat fluxes.
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Total heat release rate (MJ·m−2) | |||
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Neat | 1 wt.% EG | 3 wt.% EG | 5 wt.% EG | |
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The THRR value of the neat is the highest (134.2 MJm−2) as compared to the 5 wt.% EG composite of 118.6 MJm−2.
From (
Figure
Ignition characteristics of epoxy-expandable graphite composite samples.
Sample | Critical flux, |
Ignition temperature, |
Thermal inertia, |
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Neat epoxy |
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452 | 7.0 | 0.076 |
Epoxy 1 wt.% EG |
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439 | 6.7 | 0.090 |
Epoxy 3 wt.% EG |
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434 | 6.4 | 0.111 |
Epoxy 5 wt.% EG |
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432 | 4.7 | 0.232 |
Critical heat flux of neat epoxy-0 wt.% EG composite samples.
Furthermore, from the slopes of plots of reciprocal of square root of time-to-ignition and external heat flux, according to (
Other similar plots, as in Figure
Thermal inertia of the neat epoxy samples.
All the values of the three parameters, namely, the critical heat flux, the ignition temperature, and the thermal inertia, seem to be decreasing with increasing content of EG in the composition. In the case of the critical heat flux, with the exception of the “odd” epoxy 1-wt.% EG sample (8.0 kW·m−2), the value of the neat epoxy was found to be 6.9 (±0.5) kW·m−2 decreasing to 6.7 (±0.2) kW·m−2 and further to 5.8 ± 0.4 kW·m−2 in the epoxy 3-wt.% EG and epoxy 5-wt.% EG, respectively. The overall decrease could be attributed to the decrease in the thermal inertia (and corresponding ignition temperature), from neat epoxy value of 7.0 kW2sm−4 K−2 to 4.7 kW2sm−4 K−2 for the epoxy 5-wt.% EG sample. Similar conclusions that critical heat flux decreases (from 435 to 370 kW·m−2) with increasing carbon fibre (from 56 to 59 vol%) in epoxy were reported by Dao et al. [
When each of the composite samples was subjected to a given irradiance, it vaporized at a certain rate. The steady-state mass loss per unit area (
The influence of expandable graphite in the epoxy-EG composite was further demonstrated from plots of external heat flux versus average specific mass loss rate (gs−1 m−2) from the Cone Calorimeter data, according to (
Heat of gasification of the composite samples. The slopes values for the samples neat, 1, 3, and 5-wt.% EG are 0.315, 0.294, 0.195, and 0.080, respectively.
Using the slopes in Figures
Heats of gasification (
Sample |
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Neat epoxy | 3.4 | 13.7 |
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Epoxy 1 wt.% EG | 3.8 | 13.2 |
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Epoxy 3 wt.% EG | 6.5 | 18.9 |
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Epoxy 5 wt.% EG | 10.5 | 23.4 |
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Heat of combustion of the composite samples. The slopes values for the samples neat, 1, 3, and 5-wt.% EG are 4.06, 3.49, 2.92, and 2.22, respectively.
The values of the heat of gasification obtained from Figure
The heat of gasification of neat epoxy (3.4 kJg−1) compares well with 2.4 kJg−1 obtained by Tewarson [
The increasing content of EG (as explained earlier) in the resin assists the initial ignition of the epoxy volatiles and, thereafter, forms thermal barrier that seems to prevent further gasification of resin molecules and, thereby, increases the heat of gasification (
The net flame heat flux (
The choice of mean peak heat release rate data from the Cone Calorimeter readings for each sample seems to give an energy release rate that is more consistent with steady burning as opposed to peak heat release. The intercept on the mean peak heat release axis gives the following:
These results show that EG fraction increase favours the transfer and the distribution of heat within the composite material. That is, there is decrease in inflammability and rather increase in the combustibility of the epoxy-EG composite sample.
Apart from heat release rate being the single most important parameter in characterizing material hazard in a fire, smoke density as a by-product of thermal decomposition of the polymer matrix is the other hazard that affects visibility and produces toxic gas that needs to be analysed [
The variation of average smoke densities, measured by the decrease in transmitted light intensity of a He-Ne laser beam located within the Cone Calorimeter fume extraction duct and expressed in terms of average specific extinction area (SEA) [
Smoke density from the composite samples due to external heat fluxes.
The profiles in Figure
The total smoke production for the samples.
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Total smoke production (m2) | |||
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Neat | 1 wt.% EG | 3 wt.% EG | 5 wt.% EG | |
25 | 26.7 | 27.9 | 18.8 | 15.6 |
30 | 33.1 | 29.8 | 21.6 | 19.3 |
35 | 30.2 | 30.0 | 22.9 | 20.2 |
50 | 35.5 | 32.1 | 22.2 | 24.5 |
Further analyses of the thermal decomposition of the epoxy-EG composites were done with the TGA to evaluate their thermal stability [
A typical mass loss versus temperature profiles for neat epoxy and 1-wt.% EG-epoxy composite sample is seen in Figure
TGA thermograms for (a) neat epoxy and (b) 1-wt.% EG epoxy composites for different heating rates.
Both thermographs in Figures
As the samples begin to decompose and lose mass, they also vaporize [
Similar graphs, as in Figures
Activation energies (
Sample |
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Neat epoxy | 159.1 | 4.55 |
Epoxy 1 wt.% EG | 155.7 | 2.58 |
Epoxy 3 wt.% EG | 145.9 | 3.60 |
Epoxy 5 wt.% EG | 171.4 | 3.87 |
Arrhenius plots (a) neat epoxy and (b) 1-wt.% EG epoxy composite for mass fraction = 0.6 data obtained under nonisothermal conditions at heating rates 5, 10, 15, and 20 Kmin−1 from Figures
The activation energy, which is a measure of the reduction in molecular mobility within the resin structure [
Further analysis involving different EG compositions in the epoxy samples at the same heating rates was done. Typical example of one such thermogram, where the composites had the same heating rate of 5°C/min, is shown in Figure
TGA thermograms of different epoxy-EG composites at the same heating rate 5°C min−1.
The neat expandable graphite (EG) thermograph shows a multiorder kinetic with the first decomposition occurring from 210°C (dilution of the EG yielding water and sulphur dioxide). The effect of EG in the epoxy-EG composite is further displayed from temperatures above 400°C. There is increase in the residue mass fraction (char) for increasing EG content which is in agreement with the Cone Calorimeter results, as shown in Figures
The results of direct thermal conductivity measurement, at 24°C, using the Linseis THB100 Transient Hot Bridge thermal conductivity analyser show that the thermal conductivity of the epoxy-EG composite increases with increasing EG content. The results are shown in Figure
Thermal conductivity of with different EG composite contents.
Perhaps a well-homogenous dispersion of the EG within the epoxy resin matrix would have given a better linear graph.
The thermal degradation status of the samples is further demonstrated with Scanning Electron Microscope (SEM) micrographs.
SEM micrographs of the EG/epoxy composite before and after the Cone Calorimeter fire testing measurements are shown in Figures
SEM micrograph of neat epoxy resin after fabrication.
SEM micrograph of char from neat epoxy resin after the Cone Calorimeter thermal analysis.
SEM micrograph of EG after the Cone Calorimeter thermal analysis.
SEM micrograph of exfoliated graphite, residual after the Cone Calorimeter thermal analysis.
Figures
From the fire testing observations, the decomposition of the epoxy resin (solid) results in the release of volatile fuels which cause the resin to change phase from solid to liquid to gaseous under the constant heat flux leaving thin char remains. The SEM image of the char remain is shown in Figure
Figure
The residue of one of the composite samples after the Cone Calorimeter measurement is shown in Figure
Thermal measurements on decomposition processes of inorganic intumescent additives of EG in organic epoxy resin composite samples were done. Upon subjecting the composite samples to different heat fluxes (25, 30, 35, and 50-kWm−2), it was found that both the time-to-ignition and the peak heat release rate decrease from the lowest to the highest external heat flux, for a particular EG content composite.
Increasing the EG content in the epoxy composites, at a particular external heat flux, was found to lower the time-to-ignition, the critical heat flux, the ignition temperature, the thermal inertia, the smoke yield, and the peak heat release rate.
The activation energy of the decomposition of the epoxy-EG samples is found to decrease with increasing EG content. The measured activation energy of the neat epoxy was 159.1 kJmol−1 as compared to 145.9 kJmol−1 of 3-wt.% EG-epoxy composite.
The authors declare that they have no competing interests.
The authors wish to thank the Research and Innovation Committee, TUT, for the generous support rendered in the research into thermal measurements on polymer composites.