RECOMENDACIONES

The cone calorimeter tests were carried out following the procedures indicated in the ISO 5660 standard with a FTT cone calorimeter. Square specimens (100 mm x 100 mm), aluminium foil wrapped on the sides and at the bottom and placed in a holder, were irradiated with a heat flux of 50 kW/m2; under the holder, a load cell evaluated the mass loss during the experiment. The combustion gases that were produced passed through the cone and, after a filter system, reached the gas analysers [27]. The distance between the source and the specimens was kept constant and equal to 25 mm. Figure 5.3-1 shows the used equipment (Figure 5.3-1a) and the execution of a test (Figure 5.3-1b).

Figure 5.3-1 Cone calorimeter equipment used (A) and execution of a test (B).

Therefore, several combustion parameters were determined:

• Heat release rate (HRR), its mean value (HRRm), time to peak (TTP) and the total heat release

(THR). HRR is determined by measurement of the oxygen consumption derived from the oxygen concentration and the flow rate in the combustion product stream;

- 103 - • Peak of heat rate release (PHRR), which is considered as the parameter that best expresses the maximum intensity of a fire indicating the rate and extent of fire spread;

• Time to ignition (TTI) and fire performance index (FPI). This latter is defined as the ratio of

TTI to PHRR, and it is a parameter related to the time available to escape in a real fire situation [1, 2];

• Parameters related to the smoke evolution were also obtained, such as average of CO and CO2

emission, the loss of mass and the total smoke released (TSR), calculated by integrating the rate of smoke released (RSR) curve.

From the cone calorimeter tests, comparative plots of THR, HRR and mass loss percentage versus time are shown in the figures from Figure 5.3-2 to Figure 5.3-4, it is possible to observe that increasing the percentage of APP, the THR decreases; moreover, the HRR curves go down and move to the left. This means a decrease of HRRm, PHRR and TTP. A possible explication of this phenomenon is that

the flame retardant additive, while performing properly its action of retardant, triggers the advance ignition of the laminates.

In addition, it is interesting to analyze, by determining the percentage of mass loss, the different degradation of the samples. From Figure 5.3-4, the degradation rate results to be inversely proportional to the content of APP.

Then, the targeted action of APP on the fibres inhibits mass transfer in the combustion zone and thus improves the fire behaviour of the composites.

The results from the cone calorimeter, in terms of average and standard deviation, are reported in Table 5.3-1 and Table 5.3-2. From Table 5.3-1, concerning to the heat release parameters, it can be deduced that:

• time to ignition (TTI) is almost the same whilst time to peak (TTP) decreases with the percentage of APP;

• the heat released (HRRm and THR) and FPI are reduced with the increasing of additive, for

- 104 - Figure 5.3-2 Total heat release versus time for untreated and treated samples.

- 105 - Figure 5.3-4 Mass loss versus time for untreated and treated samples.

Sample TTI (s) HRRm (kW/m2₎ TTP (s) PHRR (kW/m2₎ FPI x 10-2 (m2_s/kW) THR (MJ/m2₎ APP0 21.2 [1.3] 121.6 [2.3] 100.1 [2.8] 720.5 [6.3] 2.9 68 [5.1] APP1 20.3 [2.2] 69.2 [1.9] 95.3 [1.6] 375.3 [5.8] 5.4 42 [3.3] APP2 18.1 [1.1] 59.5 [3.5] 88.6 [1.4] 293.8 [4.5] 6.2 33 [4.8] APP3 21.0 [0.9] 48.6 [1.8] 85.8 [2.2] 186.7 [3.4] 11.2 27 [2.7]

Table 5.3-1 Combustion parameters obtained from cone calorimeter tests (standard deviations between square brackets).

Sample Residual mass (wt.%) TSR (m2_/m2₎ CO emission (g/Kg) CO2 emission (g/Kg) CO2/CO ratio APP0 11.8 [1.2] 1612.2 [102.3] 65.5 [0.8] 1795.3 [15.3] 27.4 APP1 22.3 [1.9] 1233.3 [98.2] 131.3 [1.3] 1483.8 [13.3] 11.0 APP2 24.3 [2.1] 1104.5 [88.8] 153.2 [1.6] 1327.5 [10.5] 8.7 APP3 31.1 [2.4] 1006.7 [81.4] 164.1 [1.8] 1283.4 [11.2] 7.5

Table 5.3-2 Residual mass, TSR and average emission of CO and CO2 (standard deviations between square brackets).

A possible explanation of this behaviour is that, during the cone calorimeter test, the thermal degradation of APP leads to the formation of phosphoric acid, which combines with the hydroxyl function of hemp forming a phosphorus ester. This ester catalyzes the dehydration of the fibres and leads to the formation of a carbonaceous structure [3]. Hence, the vapour phase composition is changed and the released energy is lower.

- 106 - The parameters related to the residual and the smoke evolution are presented in Table 5.3-2; it can be deduced that:

• the presence of the additive generates a protective carbonaceous layer on the surface of the laminate, opposing the degradation process of the composite. This behaviour increases with the content of APP;

• the presence of APP also affects the emission of fumes; indeed, the TSR and the CO2/CO ratio

decrease with increasing the content of APP. The analysis of carbon oxides (CO and CO2) generated

during burning is useful in order to obtain some information on the decomposition mechanism. Lower values of the CO2/CO ratio suggest inefficiency of combustion inhibiting the conversion of CO to

CO2.

Finally, Figure 5.3-5 shows the residues of the analysed specimens, at the end of the test, in order to observe the structure of burnt laminate. It is immediately evident that, compared to the treated samples, the untreated one is completely burned (the residue is almost zero). This content underlines already the effectiveness of the flame retardant in question.

Figure 5.3-5 Residues at the end of the cone calorimeter test: APP0 (A), APP1 (B), APP2 (C) and APP3 (D).

It is possible to observe the phenomenon of intumescence, due to the presence of APP, that increases with the content of additive.

It is also possible to note that, despite what happens for the untreated sample, the treated ones show to resist into the flame without a complete consumption (for times longer than 600 s). In addition, starting from 8.88 wt.%, APP is a real barrier to the fibres, which maintain their integrity in shape. Figure 5.3-6 shows an unsound 3.15 wt.% sample (Figure 5.3-6 a), compared to a sound 8.88 wt.% sample (Figure 5.3-6 b).

- 107 - Figure 5.3-6 Top surface of the cone calorimeter specimens at the end of the test: APP1 (A) and APP2 (B).