Synergistic flame-retardant effect between calcium hydroxide and zinc

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Aug 21, 2017 - calcium hydroxide and zinc borate in ethylene-vinyl acetate ... trihydroxide (ATH), magnesium hydroxide (MDH) are the most effective flame ...
Accepted Manuscript Synergistic flame-retardant effect between calcium hydroxide and zinc borate in ethylene-vinyl acetate copolymer (EVA) Mohamed Amine Oualha, Noureddine Amdouni, Fouad Laoutid PII:

S0141-3910(17)30277-X

DOI:

10.1016/j.polymdegradstab.2017.08.032

Reference:

PDST 8343

To appear in:

Polymer Degradation and Stability

Received Date: 19 June 2017 Revised Date:

21 August 2017

Accepted Date: 26 August 2017

Please cite this article as: Oualha MA, Amdouni N, Laoutid F, Synergistic flame-retardant effect between calcium hydroxide and zinc borate in ethylene-vinyl acetate copolymer (EVA), Polymer Degradation and Stability (2017), doi: 10.1016/j.polymdegradstab.2017.08.032. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Synergistic flame-retardant effect between calcium hydroxide and zinc borate in ethylene-vinyl acetate copolymer (EVA). a

a

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Mohamed Amine OUALHA , Noureddine AMDOUNI , Fouad LAOUTID ,

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a: Université de Tunis El Manar, Faculté des Sciences de Tunis, UR/11/ES/19 "Physico-chimie des matériaux à l’état condensé", 1068 Tunis (Tunisia) b: Laboratory of Polymeric & Composite Materials, Materia Nova Research Center, avenue Copernic, B-7000 Mons (Belgium)

* [email protected]

Abstract

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In this study, we developed a simple, fast and low-cost method for the preparation of lamellar calcium hydroxide particles (Ceg-Ca(OH)2) from chicken eggshell (Ceg) waste. The flameretardant effect of these particles has been evaluated into ethylene vinyl acetate copolymers (EVA) alone or in combination with zinc borate (ZnB). The incorporation of lamellar calcium hydroxide allows an important reduction of PHRR due to the formation of a cohesive mineral residue. Moreover, its combination with zinc borate enables for further reduction of peak of heat release rate (PHRR) and for an important increase of the cohesion of the mineral residue. XRD analysis of the structure of the residue formed during thermal decomposition has revealed the formation of calcium borate (B2Ca3O6). This latter is deemed responsible of the strong improvement of the residue cohesion.

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1. Introduction

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Ethylene-vinyl acetate (EVA) is a copolymer commonly used in cable industries that require high fire-resistance properties. For these applications, metallic hydroxides such as aluminum trihydroxide (ATH), magnesium hydroxide (MDH) are the most effective flame retardant additives. These mineral fillers act mainly through their endothermic dehydration which occurs between 180 °C and 200 °C for ATH and between 300 °C and 340 °C for the MDH [15]. However, a very high filler content is required in order to obtain satisfactory fire properties [6, 7]. Other alternatives to MDH and ATH, such as boehmite [8], hydrotalcite [8], hydromagnesite [9-11] and hydromagnesite/huntite hybrid compound [12,13], have been also reported by several studies. However, the flame-retardant action of these hydroxides is not effective above 400 °C because their endothermic degradation leads to the generation of noncohesive residues. Moreover, using such high mineral loadings generally results in low mechanical performance of the resulting composites.

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Therefore, in order to obtain a set of competitive properties and to reduce the global incorporation content, it becomes interesting to enhance the efficiency of the hydrated minerals by partially substituting them with synergistic additives such as nanoparticles [1419], delaminated talcs [20, 21], zinc borate [22,23], fumed silica [24], expandable graphite [25], zinc hydroxystannate [26] and ammonium polyphosphate [27]. All these combinations aim to promote the formation of cohesive and thermally stable structures, which can burn slowly and provide a better shielding effect.

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Recently, calcium hydroxide (Ca(OH)2) has emerged as a new hydrated mineral enabling the extension of the flame-retardant effect of MDH or ATH to above 400 °C since its endothermic dehydration occurs at around 400 °C. Its use in combination with other metallic hydroxides enables the expansion of the temperature range of the flame retardant (FR) system action above 400 °C and the formation of a cohesive mineral residue [28, 29].

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Additionally, major advantages in using combination of zinc borates with other flame retardants have been reported by many authors. Their actions as afterglow suppressant, corrosion inhibitor, suppressant smoke, anti-tracking agent and synergistic agent have been stressed repeatedly [30-35]. In fact, the endothermic decomposition of zinc borate (503 kJ/kg) occurs between 290 °C and 450 °C releasing water, boric acid and boron oxide (B2O3) [3]. This latter (B2O3) softens at 350 °C and flows above 500 °C enabling the formation of a protective vitreous layer that improves residues cohesion. In the present work, we have tested the combination of calcium hydroxide and zinc borate in order to develop an efficient flame retardant system that can yield a cohesive residue as well as an endothermic effect that can cover a large temperature range. Therefore, we developed a novel, fast, simple and cost-effective method for the preparation of lamellar Ca(OH)2 particles from chicken eggshells (Ceg). Recycling this natural by-product 2

ACCEPTED MANUSCRIPT for the preparation of Ca(OH)2 particles presents an interesting way for upgrading this waste. These hydrated fillers have been used as flame retardant additives, alone and in combination with zinc borate, in EVA copolymer.

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Particular attention has been given to the structure of combustion residues in order to investigate potential chemical interactions between the two additives. To our best knowledge, no studies have been carried out on the use of eggshells waste for the synthesis of a halogenfree flame retardant. This work is the first to suggest recycling this material in this perspective. 2. Experimental section 2.1 Materials

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White chicken eggshells (Ceg) were collected from an egg breaker company.

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Ethylene-vinyl acetate (EVA) copolymer (Escorene TM Ultra FL 00014; MFI=0.25g/10min and vinyl acetate rate = 14 weight %) was kindly supplied by ExonMobil. Ethylene glycol (EG) was purchased from Fluka analytical, zinc borate Zn3BO6 (ZnB) from Sigma-Aldrich while both calcium carbonate (STD-CaCO3) and calcium hydroxide (STD-Ca(OH)2) were purchased from R & M chemical. 2.2 Preparation of Calcium hydroxide particles

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Chicken eggshells were first washed several times by hot water in order to remove the protein-based membranes. Once washed, the Ceg were dried in the oven at 100 °C for 12 hours and were then crushed to fine powders (d50=10 µm) with a ball mill (Retsch SM 200) and sieved at 210 µm with a mesh size 70.

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After that, the prepared fine Ceg powder was calcined at 800°C for 2 hours under atmospheric pressure in order to prepare calcium oxide (Ceg-CaO).

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The Ceg-CaO was then hydrated with an ethylene glycol solution (1 M) under vigorous stirring (1300 rpm). Ethylene glycol was used as surfactant because it is able to adsorb on the growing crystal surfaces of Ca(OH)2 particles and thus prevents the agglomeration of crystals [36-38]. The mixture was stirred for 1 hour, rested for 6 hours at room temperature and filtered on a PTFE membrane of 0.2 µm. The collected powder was dried in an oven overnight at 100 °C and was ground for 15 min grinding time using a ball mill (Retsch MM200) equipped with two grinding stations at a vibrational frequency of 25 Hz. The grinding tools (jar and balls) were of stainless steel. The final powder was labeled as Ceg-Ca(OH)2.

2.3 Melt processing

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EVA (wt %)

Ceg-Ca(OH)2 Mg(OH)2 (wt %) (wt %) EVA 100 0 20% Ceg-Ca(OH)2 80 20 40% Ceg-Ca(OH)2 60 40 60% Ceg-Ca(OH)2 40 60 55% Ceg-Ca(OH)2-5% ZnB 40 55 50% Ceg-Ca(OH)2-10% ZnB 40 50 45% Ceg-Ca(OH)2-15% ZnB 40 45 60% ZnB 40 Table 1: Formulations of flame retarded EVA composites.

2.4 Structural characterization

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Sample

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Blending the different calcium hydroxide particles within the molten EVA was carried out in a Brabender internal mixer at 180 °C (3 minutes at 30 rpm and 7 min mixing at 70 rpm). Plates (100*100*4 mm3) for mass loss cone calorimeter testing were compression-molded at 180 °C using an Agila PE20 hydraulic press. Precisely, the material was first pressed at low pressure for 200 s (three degassing cycles), followed by a high-pressure cycle at 150 bars for 180 s. The samples were then cooled down under pressure (80 bars). The investigated formulations are summarized in Table 1. For comparison purposes, a composition containing 60 wt% ZnB has been also prepared and tested. ZnB (wt %) 5 10 15 60

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The elementary analysis of the Ceg-based powders with different treatments as well as the standards has been carried out by energy dispersive X-ray spectroscopy (EDX) using a BRUKER spectrometer (BRUKER, S2 RANGER, Origin) equipped with an X-Ray tube of 50 W. The generated X-ray goes through a filter wheel. The selected mode was Powder. The analysis was conducted under helium using a XFLASH Silicon technology® detector. Infrared spectra (FTIR) were obtained by a Perkin Elmer spectrometer (spectrum Two). Scanning electron microscopy (SEM) analysis was performed using a JEOL JSM 6100 microscope at 10 kV in order to characterize particles shapes as well as their dispersion state into the polymer. EVA-based composite samples were prepared by cryogenic fracture and later coated with gold. 2.5 Thermal analysis and fire testing For thermogravimetric analysis, a labsys TG-DSC Setaram® differential calorimeter was used to study the thermal degradation of the flame retardants and the effect of their incorporation on the thermo-oxidative decomposition of EVA-based composites under air flow. The blend containing 60 wt% Ceg-Ca(OH)2 was analyzed under both air and nitrogen. About 20 mg of the sample was subjected to a temperature ramp of 150 to 900 °C at a heating rate of 10 °C/min with an air flow rate of 50 ml/min. The fire behavior was tested by a Fire Testing Technology (FTT) mass loss cone calorimeter to measure the heat release rate (HRR) on samples according to the ISO 13927 standard. A 100x100x4mm3 sheet was exposed to a radiant cone (50 kW/m2) using a forced ignition. 4

ACCEPTED MANUSCRIPT Results correspond to mean values obtained from 3 experiments for each formulation, for which a typical variation of 10% was observed. Time To Ignition (TTI), Heat Release Rate (HRR) curves as well as the amount of the final residue will be discussed. 3. Results and discussion 3.1 Particles characterization

Ca (wt%) 89.12 89.78 92.27 89.02 95.14 88.82

Na (wt%) 7.8 7.1 5.5 10.8 2.7 8.2

Mg (wt%) 1.1 2.1 1.4 1.4 2.0

S (wt%) 0.72 0.14 0.18 0.16 0.1

Cl (wt%) 0.31 0.06 0.10 0.11 0.1

K (wt%) 0.28 0.13 0.21 0.09

Al (wt%) 0.18 0.32 0.14 0.22 -

P (wt%) 0.16 0.10 0.09 -

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Samples/element(%)

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According to Energy Dispersive X-ray spectroscopy (EDX) analysis of the different powders (Table 2), chicken eggshells present a relatively pure source of calcium (90 %). Moreover, the chemical composition of both CaO and Ca(OH)2 obtained from chicken eggshells are very close to those of commercial products. Chicken eggshell waste can be thus considered as an interesting source for the synthesis of Ca (OH)2. Si (wt%) 0.07 0.09 0.18 -

Fe (wt%) 0.06 0.07

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Eggshell Ceg-CaO Ceg-Ca(OH)2 STD-CaCO3 STD-CaO STD-Ca(OH)2 Table 2: The chemical composition of the eggshell, Ceg-CaO, Ceg-Ca(OH)2, STD-CaCO3, STD-CaO and STDCa(OH)2.

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The mineral structures of CaCO3, CaO and Ca(OH)2 obtained from chicken eggshells have been determined by XRD analysis (Fig.1). XRD patterns demonstrated the presence of typical characteristic peaks of calcium carbonate, oxide or hydroxide. FTIR spectra (Fig. 2) also confirmed the presence of only typical absorption bands of calcium carbonate, oxide or hydroxide. Moreover, after the calcination of Ceg, the Ceg-CaO bands attributed to the CO32group decreased and became similar to those observed with a STD-CaO obtained from the calcination of STD-Ca(OH)2. No bands corresponding to C-C or C-H bonds have been detected. This result indicates that the preparation process allows a total elimination of the proteinic membrane.

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Ceg-CaO

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Ceg-Ca(OH)2

Figure 1: XRD patterns of CaCO3, CaO and Ca(OH)2 produced from chicken eggshell; (+ Ca(OH)2, ■ CaO; * CaCO3)

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After hydration of Ceg-CaO by EG, a band appeared at 3642 cm-1, attributed to the stretching of the -OH function of crystalline Ca(OH)2. The absorption band observed at 874-857 cm-1 is attributed to a carbonate-type (CO32- off-plan) bridging of Ca(OH)2 [39] and indicates a carbonation by atmospheric CO2 occurs during the storage. The Two other bands appearing at 1043 cm-1 and 1084 cm-1 were attributed to symmetrical and asymmetric C-H elongations of EG used as hydration agent.

Figure 2: The FTIR spectra of the eggshell and the standards, (a) :( Ceg), (STD-CaCO3); (b):( Ceg -CaO), (STDCaO); (c): ( Ceg -Ca(OH)2), (STD-Ca(OH)2).

Furthermore, the SEM observations performed on Ceg-Ca(OH)2 particles (Fig. 3) evidenced the presence of lamellar microparticles presenting an interesting aspect ratio owing to their small thickness (less than 300 nm). Therefore, Ceg-Ca(OH)2 particles are expected to present an interesting barrier effect during the combustion. It is worth mentioning the presence of some nanoparticles (less than 500 nm) in addition to the microparticles.

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Figure 3: SEM picture of the Ceg-Ca(OH)2 prepared from chicken eggshell.

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The thermal stability of Ceg-Ca(OH)2 and zinc borate was determined using TGA/DSC thermal analyzer under air flow at 10°C/min. TGA curves are shown in Fig. 4 and results are summarized in Table 3. Results indicate that calcium hydroxide decomposes in two steps. The first step occurs between 426 °C and 525 °C and corresponds to water release (16 wt%) with an endothermic enthalpy of 695 j/g. The second degradation step corresponds to the decomposition of calcite (9%), suggesting the carbonation of a fraction of calcium hydroxide during its storage. TGA curves revealed that the zinc borate endothermic thermal decomposition occurs at lower temperature (330 °C) and corresponds to the release of around 11.4 wt% of water. The results summarized in table 3 show that the endothermic effect provided by zinc borate (234 J/g) is lower than that induced by the thermal decomposition of calcium hydroxide (695 J/g). The combination of both additives has enabled the expansion of the temperature range of the endothermic effect from 330 °C to 525 °C.

Figure 4: TGA curves of Ceg-Ca(OH)2 and ZnB under air.

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ACCEPTED MANUSCRIPT Tonset (°C)

Enthalpy (j/g)

First Weight loss (%)

Second Weight loss (%)

Ceg -Ca(OH)2

426

695

16

9

ZnB

362

234

11.4

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Table 3: Thermal properties of Ceg-Ca(OH)2 and zinc borate under air.

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3.2 Dispersion of lamellar Ca(OH)2 and zinc borate in EVA matrix

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The incorporation of the two flame retardant additives into EVA has been performed using a Brabender internal mixer at 180 °C. SEM observations revealed that the used compounding process has allowed the breaking of the Ca(OH)2 and the zinc borate agglomerates. Both additives were maintained separately in the matrix without using any particles surface treatment (figure 5). Moreover, as expected from SEM observations performed on Ceg-Ca(OH)2 particles, SEM images confirmed the high aspect ratio of lamellar micrometric particles. The lamellar shape of the Ca(OH)2 particles makes them very attracting for this application. In fact, Lopez cuesta et al. [39] reported that using lamellar micronic talc particles, in combination with Mg(OH)2, enables similar flame retardant behavior to that of EVA/MDH/organomodified montmorillonite (oMMT)) composites.

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Figure 5: SEM images of EVA containing 60 wt% Ceg-Ca(OH)2 (left) and 60 wt% zinc borate (right)

3.3 thermal behavior and fire properties 3.3.1 Effect of Ceg-Ca(OH)2 and zinc borate separately

The effect of the incorporation of different amounts of lamellar Ceg-Ca(OH)2 on the fire properties of EVA composites has been investigated by mass loss cone calorimeter at 50 kW/m². Figure 6 shows HRR curves vs time of EVA containing 20, 40 and 60 wt% CegCa(OH)2. The incorporation of 20 and 40 wt% Ceg-Ca(OH)2 induced only a slight reduction of the time to ignition from 40 s for pristine EVA to 32 s and 36 s respectively. Using 60 wt % of the same filler did not induce any changes on the composite ignitability. 8

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The most interesting effect of the incorporation of Ceg-Ca(OH)2 concerns the reduction of the PHRR. In fact, the incorporation of even relatively low amount of calcium hydroxide lamellar particles (20 wt%) results in a significant reduction of the PHRR of about 27% (from 1340 to 976 kW/m2). Moreover, the higher is the Ca(OH)2 content, the higher is the PHRR reduction. Using 60 wt% Ca(OH)2 results in a PHRR decrease of about 82 %.

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Figure 6: HRR curves of EVA containing different content of Ceg-Ca(OH)2.

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The reduction of the PHRR observed in the presence of calcium hydroxide has been already reported in the literature for both Polyethylene and EVA [28, 40] and was attributed to the formation of a mineral residue, mainly composed of calcium carbonate. The formation of this calcium carbonate has been also demonstrated to take place at relatively low temperatures (450 °C) in EVA [40] as a result of a competition between dehydration (CaO formation) and recarbonation (CaCO3 formation). CaO can also recarbonate, but the reaction is known to be quite slower. Above 750 °C, the decarbonation of the formed carbonate occurs leading to the formation of CaO. Hull et al [41] have also evidenced the formation of calcium carbonate at 450 °C during pyrolysis of calcium acetate. Calcium acetate was studied as a potential product resulting from the interaction between acetic acid and calcium hydroxide. Despite its limited specific area (only 8.2 m²/g), the lamellar calcium hydroxide produced from chicken eggshell can generate a relatively cohesive residue (Fig. 7). This finding contradict those reported in previous study [28] which states that the cohesion of the residue is enhanced when Ca(OH)2 with high specific surface area is used.

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The formation of a cohesive residue with Ceg-Ca(OH)2 particles prove that the lamellar shape of calcium hydroxide particles is an important parameter for enhancing the FR effect of Ca(OH)2.

Figure 7: Pictures of the combustion residue obtained after mass loss calorimeter test at 50 kW/m².

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Additionally, the presence of Ceg-Ca(OH)2 induces significant change of the composite thermal degradation as evidenced by TGA analysis. In fact, figure 8 shows that the first degradation step of EVA (350-420 °C), attributed to the evolution of acetic acid due to the decomposition of vinyl acetate groups, is significantly limited when Ca(OH)2 is used. This result suggests some interaction between acetic acid and Ca(OH)2 and supports the hypothesis proposed by hull et al [41] which suggest the formation of intermediary metal acetate complexes during the pyrolysis of EVA filled with hydrated minerals such as ATH, MDH and Ca(OH)2. This interaction is accountable for the conversion of acetic acid to corresponding metal acetates and acetone. Among the other metallic acetates, calcium acetate has the best thermal stability with the formation of calcium carbonate. TGA curves of EVA filled with various content of calcium hydroxide are similar below 550 °C but some differences could be pointed out at higher temperatures. In fact, a second decomposition step has occured above 550 °C attributed to the degradation of calcium carbonate formed during the composite thermal degradation. The formation of chalk in such conditions has been previously reported [29, 40].

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Figure 8: TGA curves of pristine EVA and EVA containing different amount of Ceg-Ca(OH)2 (under air).

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To further investigate the contribution of oxygen in the generation of chalk, a comparison of the thermo-oxidative and pyrolytic decomposition of EVA containing 60 wt% Ceg-Ca(OH)2 was conducted. TGA curves are presented in figure 9 and two major differences between these curves can be noticed. The first one concerns the fact that thermo-oxidative decomposition induces a premature degradation while the second one is related to the amount of the final residue that was higher in the presence of nitrogen. Moreover, it is worth mentioning that, under air flow, the first decomposition step (350 °C – 565 °C), attributed to EVA degradation and to the endothermic decomposition of calcium hydroxide leads only to a weight loss of about 39%. This value is lower than the theoretical one (Table 4) and suggests that the polymer does not undergo a total decomposition. The second decomposition step (565 °C – 835 °C) corresponds to a mass loss of 16% and could not be attributed only to the decomposition of the calcite part of Ceg-Ca(OH)2 particles (9%) since the thermal decomposition of calcite present in the original hydroxide accounts for only 5.4% of weight loss in the composite.

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Figure 9: TGA curves of EVA containing 60 wt% Ceg-Ca(OH)2 (under air and nitrogen flow).

Ceg-Ca(OH)2

60% Ceg-Ca(OH)2 Theoretical *

2nd weight loss (%)

Total weight loss (%)

16

9

25%

39

16

55%

5.4**

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60% Ceg-Ca(OH)2

1st weight loss (%)

49.6*

40% (EVA decomposition) + 16 % of 60 % (first decomposition of Ceg-Ca(OH)2) 9% of 60% (second decomposition of Ceg-Ca(OH)2)

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Table 4: experimental and theoretical TGA data under air

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The additional weight loss (10.6%) is thus attributed to a part of the polymer that has not been lost during the first decomposition step and contributes to the formation of calcium carbonate during the first decomposition step. The carbonation of calcium hydroxide during thermal decomposition of polymer has been also mentioned in some works [42, 43] reported by Rothon [44] to take place during oxygen index test. The additional weight loss may also be attributed to the presence of some char residue generated during EVA thermal decomposition. In order to investigate this point, we visually examined the residue formed during TGA analysis under air flow at 560 °C. Only a grey residue was obtained indicating that only a very small polymer remained in the form of organic compound at this temperature. Unexpectedly, under nitrogen flow, a higher residue content was obtained at 900 °C though it is supposed to contain only calcium oxide. 12

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However, a part of the polymer remained due to the formation of highly thermally-stable char residue. In fact, a black residue was recovered at the end of the TGA analysis under nitrogen flow. The second decomposition step under nitrogen flow (around 5%) corresponds to a weight loss of the calcite part of Ceg-Ca(OH)2. Under air flow, a part of the polymer is converted to calcium carbonate while this reaction does not occur under nitrogen flow. The comparison of TGA curves with the amount of residue formed during cone calorimeter test (Table 5) proves that the second decomposition step, corresponding to the formation of calcium oxide did not take place during the combustion. In fact, the amounts of the final residue obtained during fire test (31.2%, 43.2% and 62.6 %) were very close to those obtained after the first decomposition step during TGA analysis (26%, 42% and 61%) of EVA containing 20, 40 and 60 wt% of Ceg-Ca(OH)2 respectively. The thermal decomposition of EVA containing 60 wt% zinc borate is displayed in fig. 10. In comparison with the composite containing 60 wt% Ceg-Ca(OH)2, the first decomposition step occurred at lower temperature in the presence of zinc borate because its endothermic decomposition occurs at lower temperatures in respect to calcium hydroxide (Fig.4). At 550 °C, the amount of residue was lower with zinc borate (46% vs 39% with 60 wt% CegCa(OH)2) despite the fact that its endothermic thermal decomposition had a lower water release (11.4 % for ZnB and 16 % for Ceg-Ca(OH)2. The limited weight loss recorded after the first decomposition step of the composite containing calcium hydroxide stems from the consumption of a part of the polymer for chalk formation. However for ZnB- based composite, all the polymer degraded during the first decomposition step. Above 800 °C, the amount of the final residue decreased below that generated with ZnB due to the thermal decomposition of calcium carbonate. This result is important because it implies that the combination of calcium hydroxide and zinc borate can improve the thermal stability of the mineral residue above 800 °C. In this context, the combination of calcium hydroxide lamellar particles and zinc borate has been investigated.

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In order to investigate the interest of combining zinc borate and calcium hydroxide, different blends containing various combinations of both additives have been prepared and tested. The total loading rate remained constant at 60 wt% for all the compositions. The effect of the incorporation of Ceg-Ca(OH)2/ZnB combinations on the fire properties of EVA has been investigated by mass loss calorimeter test (irradiance of 50 kW/m2). Results, presented in Fig. 11 and summarized in Table 6, revealed a synergistic effect between the two additives since their combination yielded very low PHRR (125 kw/m2 when 10 or 15 wt% Ceg-Ca(OH)2 is substituted by zinc borate). This value was lower than that obtained with 60 wt% calcium hydroxide (246 kW/m²) or zinc borate (200 kW/m²).

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Additionally, the combination of these two compounds resulted in the formation of homogenous and cohesive mineral residue at the end of the combustion (Fig. 7). It should be noted that the only limitation observed was a slight decrease of the time to ignition (TIT) which dropped from 40 s (pristine EVA) to 30 and 33 s when 15 wt% and 10 wt% of zinc borate were used.

Figure 11: Evolution of the heat release rate (HRR) during mass loss calorimeter test (50 kW/m2) for EVA compositions containing different combination of Ceg-Ca(OH) 2 and zinc borate (ZnB).

The analysis of the amount of the final residue (table 5) formed during cone calorimeter test revealed that the experimental weight (around 57%) was lower than the theoretical one (around 62%). This result suggests the presence of some interactions between the two 14

ACCEPTED MANUSCRIPT additives which limit the carbonation reaction occurring between calcium hydroxide and the polymer during thermal degradation. pHRR (kW/m²)

pHRR Reduction (%)

Residue content (%)

Theoretical residue content (%)

EVA

40

1340

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0

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20% Ceg-Ca(OH)2

32

976

27%

31.2

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40% Ceg-Ca(OH)2

36

400

71%

43.2

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60% Ceg-Ca(OH)2

42

246

82%

62.6

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60% ZnB

31

195

85%

56.4

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55% Ceg-Ca(OH)2-5%ZnB

35

147

50% Ceg-Ca(OH)2-EG-10% ZnB

33

126

45% Ceg-Ca(OH)2-15% ZnB

30

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Time to ignition (s)

89%

57.2

62.3*

91%

57.0

62*

56.9

61.7*

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Sample

125

91%

* = (100% of Ceg-Ca(OH)2) x 62.6 + % of ZnB x 56.4 62.6 = amount of residue formed with the composite containing 60% Ceg-Ca(OH)2. 56.4 = amount of residue formed with the composite containing 60% ZnB.

Table 5: Time to ignition and peak of heat release of EVA composites.

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Thermal stability of the composites containing zinc borate and calcium hydroxide combinations was studied by thermogravimetric analysis in order to investigate the nature of interactions between both additives (Figure 10 and table 6).

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Apart from the reduction of the composite thermal stability below 500 °C, due to the lower endothermic decomposition temperature of zinc borate and to the decrease of calcium hydroxide content, the combination of both additives did not produce any significant changes in the composites thermal behavior. However, it is worth mentioning that between 550 and 700 °C, the amount of residue was lower when 15 wt% calcium hydroxide was substituted by zinc borate. For this composition, the presence of zinc borate seemed to limit the formation of calcium carbonate. No significant changes have been observed for the other compositions. However, a related point to consider is that TGA analysis has been conducted on only few milligrams, so that some macroscopic effects could not be evidenced at this scale. Sample

60% Ceg-Ca(OH)2 55% Ceg-Ca(OH)2-5% ZnB 50% Ceg-Ca(OH)2-10% ZnB 45% Ceg-Ca(OH)2-15% ZnB 60% ZnB

T-10% (°C)

474 457 449 446 434

First Weight loss (%) 38.4 38.0 37.2 40.6 47.7

Second weight loss (%)

Residue (%)

13.4 17.6 16.2 12.0 ---

46.4 43.3 45.5 46.8 52.3

Table 6: Thermogravimetric data for EVA filled with different combinations of Ca(OH)2 and zinc borate (under air flow).

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In order to investigate the origin of the synergistic effect observed when both calcium hydroxide and zinc borate were combined, the evolution of the mineral structure of residues, was studied at controlled thermal degradation at temperatures ranging from 25 °C to 900 °C. Approximately 10 g of each composite was heated in an electric muffle furnace (Nabertherm, Lilienthal) from ambient temperature to a target temperature corresponding to various transitions that occurred during TGA analysis (300, 500, 600, 770 and 900 °C). Samples were maintained in the furnace at the targeted temperature for a further 30 min then stored in a desiccator (under inert atmosphere) until their analysis by X-ray diffraction spectroscopy (Figures. 12, 13 and 14). As previously mentioned [28, 40], calcium carbonate is formed below 500 °C in the case of EVA/Ca(OH)2 composites, due to the reaction between calcium hydroxide and the polymer. Above 600 °C, calcium carbonate starts to decompose to form calcium oxide (Fig. 12). When combined with ZnB (Fig.14), no significant changes in the thermal behaviour of calcium were observed below 500 °C because the formation of calcium carbonate is ongoing at this temperature. However, above 500 °C, new peaks, attributed to the formation of calcium borate (Ca3(BO3)2) and zinc oxide (ZnO), emerged.

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The formation of calcium borate is evidenced by XRD to take place between 500 and 600 °C. However, since XRD evidences only crystalline structures, it is obvious that the interaction between calcium hydroxide/carbonate and zinc borate occurs below 500 °C. This interaction, which leads to the formation of zinc borate, is thus a competitive reaction to that leading to the formation of carbonate. This result explains why the theoretical residue was lower than the experimental one observed at the end of cone calorimeter test (Table 5).

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The interaction between zinc borate and calcium hydroxide limits the formation of calcium carbonate and promotes the generation of a more thermally stable calcium borate. The formation of this new mineral structure is at the expense of the exothermic carbonation reaction.

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600°C 770°C

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Figure 12: XRD diagrams of 60% Ceg-Ca(OH)2 at different temperatures. (+ Ca(OH)2; *Ca(CO)3;

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■ CaO).

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Figure 13: XRD diagrams of 60% ZnB at different temperatures. (+ ZnO;

■ B2O3; *Zn B2O4)

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Figure 14: XRD diagrams of 45% Ceg-Ca(OH)2 -15% ZnB at different temperatures. (+ Ca(OH)2; *Ca(CO)3;

● CaO; ○ B2Ca3BO6; ◘ ZnO)

Conclusion

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In this study, we developed a simple, fast and low-cost method for the preparation of lamellar calcium hydroxide particles from chicken eggshells.

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The potential flame-retardant effect of these lamellar calcium hydroxides has been evaluated in ethylene vinyl acetate (EVA) copolymer, alone and in combination with zinc borate. The incorporation of calcium hydroxide exerts some changes on the thermal stability of EVA depending on the test conditions. Under thermo-oxidative condition, a part of the polymer is converted into calcium carbonate. However, it has been found that this reaction does not occur under nitrogen flow. Consequently, the formation of this CaCO3-based residue enables an important reduction of PHRR. When combined with zinc borate, the barrier effect of this mineral residue become strongly enhanced due to the formation of more thermally stable calcium borate. The formation of this new mineral structure is at the expense of the exothermic carbonation reaction which is slightly limited. Reducing the amount of calcium carbonate for more cohesive and thermally stable calcium borate allows further reduction of pHRR and formation of more cohesive residue.

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