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Fire Retardancy of Polymers
New Strategies and Mechanisms
By T Richard Hull, Baljinder K Kandola The Royal Society of Chemistry
Copyright © 2009 Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-149-7
CHAPTER 1
Synergy between Nanometric Alumina and Organoclay in Conventional Fire Retardant Systems for Ethylene– Vinyl Acetate
N. CINAUSERO, J.-M. LOPEZ-CUESTA, F. LAOUTID, A. PIECHACZYK AND E. LEROY
1.1 Introduction
Hydrated mineral fillers like aluminium hydroxide (ATH) or magnesium hydroxide (MDH) are used in the cable industry as flame retardants for poly-olefins such as ethylene–vinyl acetate (EVA) copolymers. The very high filler loadings usually required to obtain satisfactory fire properties, mean this results in a decrease in the mechanical performance of the materials. Nevertheless, enhancement of the efficiency of ATH or MDH may be achieved by partially substituting them with synergistic additives, in particular high-aspect ratio inorganic particles such as oMMTs or delaminated talcs. In addition to improvement in the mechanical properties, the presence of such lamellar particles leads to an intumescence phenomenon that occurs before ignition in cone calorimeter tests. A foam-like charred structure is formed as a consequence of heterogeneous bubble nucleation, increased viscosity and the promotion of charring of the host polymer. This structure leads to the formation of a porous protective residue (mainly inorganic), which limits both heat transfer and the diffusion of fuel and oxygen.
In a recent patent we showed that the addition of alumina nanoparticles improved the reactions to fire of flame retardant EVA compositions that contain metal hydroxide and oMMTs. In this chapter we present a detailed study of these complex systems and discuss the influence of the size of alumina particles.
1.2 Experimental
1.2.1 Materials
EVA [Elvax 260, containing 28 weight percent (wt%) of vinyl acetate] was purchased from DuPont. Magnesium hydroxide (MDH; Magnifm H10, d50 = 0.85 urn, specific surface area = 10m2g-1) and oMMT (Nanofil 5: distearyldimethyl-ammonium ion-exchanged bentonite) were supplied by Martinswerk (now Albemarle) and Süd Chemie (now Rockwood Holdings), respectively. Alumina particles of different physical properties were obtained from Degussa (ALU nano, Aeroxide Alu C, d50 = 13 nm, SBET = 86m2g-1) and Alcan (ALU micro, d50 = 0.47 µm, SBET = 6.5m2g-1), respectively.
1.2.2 Processing
Blending of molten EVA copolymer with the different minerals was performed using a Haake internal mixer at 160 °C and 60 revolutions per minute (rpm) for 10 minutes. Thick (4 mm) sheets were then compression moulded at 160 °C at a pressure of 100 bars (1 × 107 Pa) for five minutes. These sheets were cut to the size required for the experiment to be performed. For all the different compositions studied, the total filler content was 10% or 60% w/w. As an example (EVA 40/ MDH 50/ALU nano 5/oMMT 5) means a formulation that contains 40% w/w of EVA, 50% of MDH, 5% of ALU nano and 5% of oMMT.
1.2.3 Testing
Épiradiateur tests (AFNOR NF P 92-505) were carried out on 70 × 70 × 4 mm3 samples to determine the flammability and the self-extinguishability of the different compositions. The heat flux of épiradiateur measured using a flux meter is around 30kW/m2. This test allows the time to ignition (TTI) for the sample placed under the radiator (500 W) to be determined. After first ignition, the radiator is successively removed and replaced as soon as extinction occurs, the procedure being repeated for a period of 5 minutes. The mean inflammation period (MIP) and the number of ignitions (N) are then calculated from results obtained from four experiments for each formulation.
Cone calorimeter tests(ISO 5660) were performed on filled polymer samples (100 × 100 × 3 mm3) placed horizontally, using a FTT cone calorimeter. Irra-diances of 30, 50 and 70kW/m2 were used. Ignition is piloted by a spark generator in contrast to the épiradiateur test, during which ignition is spontaneous. TTI and peak of heat-release rate (PHRR) values are discussed later. The results given correspond to mean values obtained from two experiments for each formulation.
Thermogravimetric analysis (TGA – Perkin Elmer PYRIS 1) was used to study the thermo-oxidative degradation of composites. Samples of typically 15mg were placed in alumina crucibles and subjected to a temperature ramp from 25 °C to 700 °C in air at a heating rate of 5 °C min-1.
1.3 Results and Discussion
To study the interactions between the various components of the formulations above, we performed three series of TGA.
Figure 1.1 shows the effect of the presence of oMMT, ALU micro and ALU nano on the thermo-oxidative degradation of EVA. Pure EVA shows two main mass losses, the first one corresponding to EVA deacylation and the second one to EVA main-chain degradation. In the presence of oMMT, the first mass loss takes place at lower temperatures, while the second one is shifted towards higher temperatures, in agreement with literature results. The main explanation of the acceleration of the acetic acid loss is the catalytic effect of hydroxyl groups on the clay. In contrast, the presence of ALU nano does not affect the temperature of either of the two mass losses, while the presence of ALU micro leads to a slight change in the temperature of the second mass loss.
Figure 1.2 displays the effect of nanofillers on the dehydration of MDH. Pure MDH dehydration takes place between 270 and 390 °C, with a maximum mass loss rate at 370 °C. In comparison, pure oMMT shows a mass loss between 200 and 390 °C, which corresponds to the degradation of its organic part, while ALU nano (and ALU micro) does not show any mass loss in this temperature range. When MDH is mixed with ALU nano (50/50 w/w powder mix), the dehydration of MDH starts at lower temperature, with a maximum mass loss rate at 355 °C. In comparison, when MDH is mixed with oMMT (50/50 w/w powder mix), the opposite occurs, with a maximum mass loss rate at 380 °C. In the meantime, the degradation of the organic part of oMMT does not appear to be influenced by the presence of MDH.
Figure 1.3 shows the TGA and differential thermogravimetric (DTG) curves for the flame retardant formulations. The first mass loss, which corresponds to both the deacylation of EVA and the dehydration of MDH, is slightly influenced by the presence of ALU nano (the maximum mass loss rate is shifted towards lower temperatures). This may confirm its influence on the decrease of the MDH dehydration temperature observed in Figure 1.2. When both ALU nano (or micro) and oMMT are present, a shoulder appears on the low temperature side of the mass loss rate peak. In this case, two phenomena can accelerate the mass loss: ALU nano seems to sharpen the reaction of deacylation of EVA catalyzed by oMMT, as observed in Figure 1.1; in addition, when incorporating both nanofillers, water release may be restricted, which contributes to the acceleration of mass loss at lower temperatures.
Furthermore, the presence of oMMT shifts the first maximum mass loss rate towards a higher temperature, in agreement with the results of Figure 1.2 (dehydration of MDH at higher temperatures). The second mass loss peak is clearly shifted towards higher temperatures in the presence of oMMT, a shift due to EVA charring.
From these TGA results we can make the assumption that the presence of ALU nano (or micro) in flame retardant formulations does not affect the flame retardant action of oMMT, but can modify the dehydration of MDH, which starts at a lower temperature. Besides, oMMT used with alumina particles accentuates the deacylation of EVA in the low temperature range (250–340 °C), which could have a favourable effect on further charring due to double bond formation at lower temperature.
The data obtained from the épiradiateur tests are presented in Figure 1.4. The reference flame retardant formulation (EVA 40/MDH 60w/w%) has the lowest TTI, which means the highest flammability, and the highest MIP, which means a poor auto-extinguishing ability. The introduction of oMMT clearly improves these two characteristics, while in the case of ALU (nano or micro) no significant improvement is observed. When both oMMT and ALU (nano or micro) are present, the TTI increases significantly, compared to formulations that contain only oMMT. The best increase in TTI is obtained for ALU nano. The size of alumina particles is therefore an important parameter.
Figures 1.5 and 1.6 show the cone calorimeter HRR curves obtained at various incident heat fluxes for (EVA 40/MDH 60), and (EVA 40/MDH 50/oMMT 10), formulations, respectively. The behaviour shown in Figure 1.5 is "classical": when the incident heat flux decreases, the TTI increases and the PHRR decreases. In contrast, the presence of oMMT (Figure 1.6) leads to an unusual behaviour at low irradiance (30kW/m2): the TTI becomes extremely long while the PHRR strongly increases. Similar behaviour was observed for the (EVA 40/MDH 55/oMMT 5) formulation, as shown in Table 1.1.
This strong increase of PHRR at low incident-heat flux caused by the presence of oMMT is likely to be a problem for cable applications of the material. Effectively, studies have shown a good correlation between the PHHR in the cone calorimeter at low incident-heat flux (typically below 50 kW/m2) and the passing of the FIPEC cable test, which is a vertical tray test using a 20 kW burner.
As Table 1.1 shows, the introduction of ALU nano allows the negative effect of oMMT on the PHRR at 30 kW/m2 to be decreased. In contrast, when ALU micro is used, the PHRR is increased, showing again that the size of alumina is an important parameter.
As regards ignition, when incorporating nanoclays in flame retardant EVA, an increase of TTI occurs that we have already observed in a previous study. If we now focus on the TTI values obtained in the cone calorimeter test at 30kW/m2, it is striking that they strongly differ from those obtained in épiradiateur tests, although the incident heat flux is nearly the same. Contrary to what is observed for épiradiateur tests (Figure 1.4), the introduction of ALU nano leads to a relative decrease of the TTI in cone calorimeter tests relative to that of the EVA/MDH/oMMT composition. These contradictory evaluations of the TTI are undoubtedly related to different experimental conditions: in the case of épiradiateur tests, the gases emitted from the sample are not aspirated, as in the cone calorimeter tests. To sum up, both the acetic acid and water are produced more efficiently owing to the effect of ALU on the deacylation of EVA catalyzed by oMMT observed in TGA, as well as the regulated water release from the dehydration of MDH. Therefore, we can assume that acetic acid and water may dilute the combustible gases at the surface of the sample during the pre-ignition period of épiradiateur tests. This is likely to delay ignition, which in addition is not promoted by a spark in this test. Then the effectiveness with which ALU nano increases the TTI of a cable in a real fire will depend on the fire scenario. Nevertheless, the "static" conditions of the épiradiateur are more likely than the forced flow of the cone calorimeter.
Let us now come back to the PHRR values at 30 kW/m2. Comparison of the shape of the curves in Figure 1.6 suggests that, for this low external heat flux, a more important flux of combustible gas evolves at the time of ignition, and results in a strong PHRR. Such a "critical phenomenon" observed for (EVA 40/ MDH 50/oMMT 10) could be explained by a stronger migration of clay platelets towards the surface at low irradiance. This forms a protective layer before ignition that becomes "unstable" after ignition because of the additional external heat flux provided by the flame. Such a "critical phenomenon" is not observed for (EVA 40/MDH 60; Figure 1.5) and (EVA 40/MDH 50/ALU nano 10; Table 1.1) formulations, and is significantly reduced when both ALU and oMMT are present (EVA 40/MDH 50/ALU nano 5/oMMT 5; Table 1.1). Eventually, this last formulation is the best compromise concerning TTI and PHRR. In contrast, when ALU micro is used (Table 1.1), the PHRR is increased compared to that of the (EVA 40/MDH 55/oMMT 5) formulation, which confirms the size dependence of alumina particles on flammability properties.
1.4 Conclusion
The effect of alumina particles on the thermo-oxidative degradation and the reaction to fire of conventional flame retardant formulations for EVA that contains MDH and oMMT has been studied. The introduction of alumina particles did not have any direct effect on the thermo-oxidative degradation of the EVA copolymer, but was shown to shift the dehydration of MDH towards lower temperatures. Besides, when mixed with oMMT, alumina particles may accentuate the deacylation of EVA catalyzed by oMMT. It was suggested that the restriction of water release and the acceleration of acetic acid loss could have an effect on the reaction to fire. Effectively, the épiradiateur test showed a strong increase in the TTI in the presence of both alumina particles and oMMT, the best improvement being obtained for nano alumina particles. In addition, the use of alumina nanoparticles allowed the PHRR of EVA/MDH/oMMT formulations to decrease at low external heat flux in cone calorimeter, which showed high values. This phenomenon is particularly relevant since the PHRR at low external heat flux is known to correlate with larger scale cable fire tests.
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Excerpted from Fire Retardancy of Polymers by T Richard Hull, Baljinder K Kandola. Copyright © 2009 Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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