3.2. Flammability and Thermal Barrier Study
The flammability results of the control CRE sample evaluated by cone calorimetry at 35 and 50 kW/m2
heat fluxes are given in Table 2
, the important parameters relevant to evaluating the thermal barrier efficiency of surface coatings providing passive fire protection being time-to-ignition (TTI), peak heat release rate (PHRR), time-to-PHRR (TPHRR
) and total heat release (THR) values [7
]. The heat release rate (HRR) versus
time curves, from which these parameters are obtained, are shown in Figure 3
and Figure 4
. The control sample ignited at both heat fluxes, as expected. The TTI in CRE is reduced from 173 s at 35 kW/m2
to 106 s at 50 kW/m2
. Once ignited, the HRR starts increasing, reaching a peak value and then starts decreasing before reaching the minimum, representing the end of burning process. With the increase in heat flux, the PHRR value increased from 302 kW/m2
at 35 kW/m2
to 358 kW/m2
at 50 kW/m2
, while TPHRR
decreased from 218 to 174 s and THR increased from 32.1 to 36.3 MJ/m2
. The rationale for this behavior is related to the fact that the net heat flux on the exposed laminate surface considerably increases with the increasing incident heat flux. This leads to an accelerated increase in surface temperature, thereby considerably increasing the rate of resin decomposition. Thus, the critical volatile mass flux is achieved earlier, thereby providing ideal thermodynamic conditions that can sustain ignition [7
]. This behavior is also observed from mass loss results in Figure 3
b, where the mass loss occurs at an earlier time and at an enhanced rate.
The burning intensity of GRE composite laminates reported previously in reference [8
], prepared using the hand lay-up process (with resin content 50 wt %) ,was much higher than the CREs, igniting much earlier (104 and 48s at 35 and 50 kW/m2
, respectively) even in the absence of the spark ignition, having much higher PHRR (526 and 691 kW/m2
at 35 and 50 kW/m2
, respectively), but comparable THR (36.8 and 38.4 MJ/m2
at 35 and 50 kW/m2
, respectively), and slight differences in THR that are due to different resin contents. The higher flammability of the GREs is due to several reasons, these being: fabrication was done using the hand lay-up process, leading to a greater resin content, the use of a bifunctional epoxy resin, fewer glass fiber layers (8) compared to the 14 carbon fabric layers in the latter and the different area densities of the fabrics.
The cone results for ceramic particles coated laminates are given in Table 2
, and HRR versus
time curves at 50 kW/m2
are shown in Figure 4
. Since the small specimens (75 mm × 75 mm) were individually surface-coated, there are slight variations in the thicknesses of coatings (±20–45µm) as well as their uniformity, hence the variation in the cone results. The results of replicate specimens are given to demonstrate this effect. The samples with ceramic particles deposited during the semi-curing stage were also of small sizes (75 mm × 75 mm), but four specimens from each of the 150 mm × 150 mm laminates were obtained, hence the variation in coating thickness in those samples are less (±14 µm).
The effect of Flekashield on CRE composites is quite obvious. When the surface coating is on the surface of the laminate in sample CRE-P/FlekS, one out of three samples did not ignite at 35 kW/m2
, showing no PHRR and THR. For the two samples which ignited, the TTI was not affected, the PHRR was decreased and TPFRR
increased, but the THR increased. This behavior is usually observed from surface coatings providing passive fire protection, i.e.
, they show their thermal barrier efficiency by the decrease in PHRR and increase in TPFRR
, whereas the burn time, THR and smoke production are increased due to slow and prolonged burning [7
]. If a coating, however, acts as a flame retardant system, the cone results should increase in TTI (preferably no ignition) and see a reduction in PHRR, THR, mass loss rate and smoke production. Similar behavior at 35 kW/m2
was also shown at 50 kW/m2
heat flux (Figure 4
When Flekashield is used in semi-curing stage (sample CRE-Semi/Flek), the TTI is increased from 173 s in the control to 269 s in the two specimens (55% increase) at 35 kW/m2
. PHRR is decreased from 302 to 230 kW/m2
(~24% w.r.t. control), TPHRR
is increased from 218 to 355 s (63% increase), however, the THR also decreases from 32.1 to 17.4 MJ/m2
(46% decrease). The effect is similar at 50 kW/m2
heat flux. The better thermal barrier performance of CRE-Semi/Flek than CRE-P/FlekS can be explained due to a higher concentration of Fleaeshield in the former (12.9 wt %) than the latter (1.7 wt %); see Table 1
. However, even in such a low concentration of Flekashield, in CRE-P/FlekS one specimen did not ignite, which is due to the fact that the Flekashield particles completely cover the surface. In CRE-Semi/Flek, while the particles are deposited when it is nearly cured, some particles may have penetrated the resin, hence the resin could be on the surface and thus, ignite.
Samples containing Recoxit also showed very good behavior. At 35 kW/m2
, the heat flux for CRE-PReS, one specimen did not ignite and in other two TTI increased to 250 s. However, for the sample which ignited, there was not much effect on PHRR, but the TPHRR
increased. At 50 kW/m2
, the effect was more pronounced, i.e.
, TTI and TPHRR
increased while PHRR and THR decreased. In the case of CRE-Semi/Re, all specimens ignited, but the TTI was increased to 262s, PHRR decreased to 289 kW/m2
increased to 306 s and THR decreased to 29.9 MJ/m2
. The improved thermal barrier efficiency of the coating in this sample compared to that in the CRE-PReS sample is due to a higher concentration of Recoxit particles in the former (see Table 1
The mass loss curves during the cone experiment at 35 and 50 kW/m2
also showed the thermal barrier effect of all the ceramic particle coatings. As clearly seen in Figure 5
, at 50 kW/m2
, all ceramic-coated samples significantly retarded the mass loss rate when compared to the control sample. The difference between surface-coated and semi-cured samples, and those between Flekashield and Recoxit particles, was dependent upon the quantity of particles on the surface. The trends observed are similar to those observed for other parameters discussed above.
On comparing Flekashield and Recoxit, Flekashield is more effective as a thermal barrier, even at low concentrations. This can be explained by the fact that at higher temperature (>350 °C), the glass frits melt on the surface, forming a thick, glassy coating [8
], which acts as an effective physical and thermal barrier. The layer on the surface remaining after the test can be clearly seen in Figure 6
a. The charred layer is thicker and fragmented in CRE-P/FlekS because all the particles embedded in phenolic resin layer were on the surface, the char is due to the phenolic resin. In the case of CRE-Semi/Flek, the melted glass layer on the surface can be clearly seen. Similar behavior is observed in the samples containing Recoxit where the surface-coated sample has some charring (mainly due to phenolic resin) and particles, whereas in CRE-Semi/Re ceramic particles on the surface are present, but the concentration is much less than that in CRE-Semi/Flek.
The effect of these ceramic particles as surface coatings on CRE and glass composites (GRE) cannot be directly compared because, in the case of the CRE samples spark ignition was used, whereas previously reported GRE samples were tested in the absence of a spark ignition. However, the fact that CRE-P/FlekS and CRE-P/ReS results are comparable and, in some cases better, than respective GRE samples, indicates the improved effectiveness of the particles on CRE composites, the mass of ceramic particles on both types (CRE-P/Flek or Re and GRE-P/Flek or Re) being similar considering the experimental error (Table 1
]). Out of the two specimens of CRE-P/FlekS, one did not ignite at 50 kW/m2
, and the other one which ignited had 34% and 15% reduction in PHRR and THR, respectively, w.r.t control CRE compared to GRE-P/FlekS, which had an 11%–22% reduction in PHRR and a 2%–21% increase in THR. CRE-P/ReS had 91 s increase in TTI, a 20% reduction in PHRR and a 14% reduction in THR w.r.t CRE compared to a 19–57 s increase in TTI, a 14%–18% decrease in PHRR and a 21%–28.6% increase in THR in GRE sample. These results show that at lower heat fluxes (≤35 kW/m2
), the effect of ceramic particles is similar providing they cover the surface of the substrate completely; however, at higher heat fluxes they are more effective on less flammable substrates.