Thierry Vincent

Lo€ Ic Dumazert

Ludivine Dufourg

Claire Cucherat

Rodolphe Sonnier

Eric Guibal
email: eric.guibal@mines-ales.fr

New alginate foams: Box-Behnken design of their manufacturing; fire retardant and thermal insulating properties

Keywords: biopolymers and renewable polymers, foams, manufacturing, polysaccharides, thermal properties

A new method for preparing alginate foams with progressive release of copper in the presence of sodium lauryl sulfate (SLS, foaming agent) has been designed. Copper acts as the ionotropic gelling agent through the reaction of copper carbonate with gluconolactone. The process does not require freeze-drying contrarily to the conventional method used for preparing macroporous alginate foams. The new materials investigated in this study have remarkable thermal properties, including thermal conductivity lower than 0.041 W m 21 K 21 and low heat release (below 2 kJ g 21 ), which allows labeling these foams self-extinguishing materials. An experimental design methodology, based on a Box-Behnken plan with three parameters and three levels, is successfully used for evaluating the impact of the amounts of alginate, SLS, and copper carbonate on the productivity, apparent density, and shrinking at airdrying. It yielded an optimization of the process for the manufacturing of light, and stable/rigid insulating and thermally stable materials.

INTRODUCTION

The increasing demand for alternative materials produced from renewable resources is driving an intense research for substituting oil-based products with agriculture or marine subproducts. For example, in environmental applications biosorbents are being investigated for replacing synthetic resins for the removal and/or recovery of metal ions from dilute solutions. Biomass can be also used for binding metals and then used for supported catalysis. In the field of safety and fire protection, materials of biological origin have also received a great attention for the last decades. [1][2][3][4] Synthetic materials, especially chlorine and aromatic-containing ones as poly(vinyl chloride) and polystyrene, may produce toxic subproducts such as volatile organic compounds during combustion (even in well-ventilated conditions), 5,6 while the thermal degradation of agriculture waste or marine resources is generally more environmentally friendly. 7 Recently, alginate, a polysaccharide extracted from brown algal biomass has also retained a great attention due to its promising properties in terms of fire retardancy. [8][9][START_REF] Zhang | In Eco-Dyeing[END_REF][START_REF] Gady | [END_REF][START_REF] Palumbo | Proceedings of the 1 st International Conference on Biobased Building Materials[END_REF][13] Alginate is a heteropolymer constituted of guluronic acid and mannuronic units; this biopolymer rich in carboxylic acid is ionotropically gelled with divalent or trivalent metal cations to form stable hydrogels. The properties of the hydrogels strongly depend on the metal cations used for gelling. [14][15][16] Zhang et al. investigated the flame retardancy properties and pyrolysis products of calcium alginate [START_REF] Zhang | In Eco-Dyeing[END_REF] : they highlighted the relatively high value of the limiting oxygen index, close to 34, and the fact that alginate fibers do not show combustion in air. The heat release rate (HRR), effective heat of combustion, and total heat release (THR) are even lower than the relevant values of most of synthetic or natural polymers. The gelling of alginate with cobalt and manganese allows reducing HRR 8 : the presence of these metal ions is supposed to decrease the release of combustible gases. Similar trends were previously reported in the case of nickel-and copper-gelled alginate. 9 Zinc was also successfully used for preparing alginate-based materials with enhanced fire retardancy properties. [17][18][19] Kong et al. observed that the shape of calcium alginate fibers remained stable after limit oxygen index test (LOI) and that the rigid char contributes to form a barrier between the flame or heat and the material. 20 The incorporation of clay also contributes to improve thermal degradation properties. [START_REF] Gady | [END_REF] The HR values are decreased when the proportion of clay increases in the composite. 21 The freeze-drying procedure of alginate-based hydrogel leads to the formation of foam-like materials with good fire properties. While increasing clay proportion the apparent density of the composite obviously increases.

Foams, which are constituted by a continuous polymeric phase and a gaseous phase (as dispersed bubbles) have several advantages in comparison with unfoamed polymers: lightweight, high strength to weight ratio, thermal insulation property, and high thermal stability. 22 Apart synthetic industrial foams made of polypropylene (PP), polyethylene, or polystyrene base matrices, alternative materials have been used for preparing bio-based foams, for example poly(lactic acid). 23 Pantani et al. reported the combination of foaming agent with physical and temperature variations for elaborating biopolymer foams. 24 The elaboration of porous biopolymer scaffold has focused abundant research for the last decade using chitosan, 25 alginate, 26,27 or starch-based materials for biomedical application although the concept is now expanding to other applications, such as environmental use for metal recovery, [28][29][30] or manufacturing of fire retardant materials. 31 Usually sophisticated or expensive drying methods are required using, for example, freeze-drying 21 or drying under supercritical CO 2 conditions. 28,31 An alternative method has been developed for preparing alginate-based foams without freeze-drying process, for lower cost. The foaming of the alginate-based material is improved using a foaming agent, such as sodium lauryl sulfate (SLS) to produce lighter foam with improved properties of fire retardancy, lower thermal conductivity. The in situ gelling of the foam is operated in the presence of copper ions and gluconolactone; the slow dissociation of gluconolactone produces protons that react with copper carbonate, which, in turn, release copper ions in the core of the solutions and causes the progressive ionotropic gelation of the biopolymer. The copper-gelling allows producing more stable and rigid structure than conventional calcium gelling. [14][15][16] Despite, other metal cations can be used for alginate gelation, such as barium, zinc, which are very efficient for improving fire retardant properties of alginate (see above), the copper-alginate system is one of the most frequently used, and it is characterized by a high strength and rigidity. Calcium is even more frequently employed for manufacturing alginate-based gels; however, testing the selected procedure with calcium salt did not allow elaborating rigid gels; at air-drying (oven-drying) the porous structure irreversibly collapsed and the materials lose their morphology and shape limiting the possibility to use these materials for target applications. Another advantage at using copper salts is that copper bring bactericide properties that could contribute to prevent the early degradation of the foams.

The principal novelty brought by this study consists of the combined use of a foaming agent, the homogeneous ionotropic copper-based gelation mode, and the cost-effective air-drying method. The drying at 50 8C allows producing dried foams with stable structure that were characterized in terms of foam productivity, measured as the volume of dried foam produced with a given volume of preparation, apparent density and processing efficiency, qualified as the shrinking percentage at drying. A Box-Behnken (B-B) experimental design was designed for optimizing the synthesis procedure of alginate-based foams using copper carbonate, SLS: the concentration of alginate, SLS, and copper carbonate have been varied at three levels (-1, 0, and 11, see below) and the target characteristics are systematically determined in order to design the surface responses that fit experimental data. The processing equations (characterized by 10 specific coefficients, reflecting the individual impact and the cross-effects of experimental parameters) for the different responses are established, on the basis of a quadratic model equation. The iso-response curves are plotted to illustrate the effects of these parameters and suggest optimum conditions for highest production of light and stable alginate-foams. The discussion of these curves helps in identifying the relative importance of these experimental parameters on the different responses. The main criteria for evaluating thermal insulating foams are characterized by low thermal conductivity, low density, and low flammability. 32 Selected samples are also tested for thermal conductivity, thermal degradation, and mechanical stability. Scanning electron microscopy (SEM) is also used for characterizing the opening of foam cells.

EXPERIMENTAL

Materials

Alginate (Manugel DMB, sodium alginate) was supplied by FMC (Cork, Ireland). The commercial characterization of this material corresponds to a viscosity of 263 mPa s (for 1% w/ w concentration at 20 8C). Calcium chloride (CaCl 2 .2H 2 O) was obtained from Chem-Lab (Zedelgem, Belgium). Calcium carbonate, gluconolactone, copper chloride (CuCl 2 .2H 2 O) were supplied by Sigma-Aldrich Chemie GmbH (Steinheim, Germany). SLS was purchased from Carlo Erba (Milano, Italy). Sodium carbonate was supplied by VWR Prolabo (Fontenay-Sous-Bois, France). The extruded polystyrene foam (EPS) tested as a reference material is commercialized by Raja France (Roissy, France) as a packing material (reference PSE50C).

Preparation of Copper Carbonate

Copper carbonate was synthesized by reaction between CuCl 2 .2H 2 O and Na 2 CO 3 under pH control using the following procedure: 14 g of copper chloride were dissolved in 300 mL of demineralized water; the pH was adjusted to 10 using a 10% (w/w) solution of sodium carbonate. The solution was aged for 2 h before controlling the pH to 8 with 0.25 M HCl solution. The solution maintained under agitation and pH control for 3 h was finally adjusted to a final volume of 400 mL with demineralized water. Copper being in excess compared to carbonate the suspension contains copper carbonate and free copper: free copper will contribute to partial external ionotropic gelation of alginate while CuCO 3 will contribute to the delayed in-core ionotropic gelation (gluconolactone dissociation releases protons that will react with copper carbonate, see below). This suspension will be called CuCO 3 suspension in the next sections. It is noteworthy that commercial copper carbonate is not stable: it is rapidly dissociated in presence of moisture to form the so-called basic copper carbonate associating copper, carbonate, and hydroxide. In addition, the dried product appeared to be nonoperable when mixed with alginate; instantaneous precipitation, and agglomeration make impossible the processing of the mixture. The actual fraction of solid precipitate in the suspension has been quantified to 20 g L 21 by weight loss at 105 8C.

Manufacturing of Alginate Foams

The principle of the synthesis of alginate foams consists in the homogeneous ionotropic gelation of foamed alginate. [START_REF] Gady | [END_REF]21,33 Foaming was facilitated by the addition under strong agitation of SLS. The slow release of copper (for ionotropic gelation) was operated by the introduction of copper carbonate and gluconolactone in the alginate solution by analogy with the calcium carbonate/gluconolactone procedure of gelation: the progressive dissociation of gluconolactone releases protons that will interact with carbonate and, in turn, induces the release of copper and carbon dioxide, contributes to the internal ionotropic gelation of alginate in its expanded form (foamed suspension).

Copper carbonate was preferred to calcium carbonate since the copper gelation of alginate gives stronger and more rigid hydrogels. 15 Sodium alginate was first dissolved in demineralized water (4%, w/w) under agitation using a mixer/kneader (velocity: 150 rpm) for 12 h (until complete dissolution). SLS solution (1%, w/w) was added to a variable amount of the alginate solution (i.e., 140, 150, or 160 g): the volume of SLS (3.5, 6.5, and 9.5 mL; i.e., 35, 65, and 95 mg of SLS, respectively) and the amount of alginate solution (140, 150, and 160 g; corresponding to alginate dry weights of 5.6, 6, and 6.4 g, respectively) were varied for preparing different series of materials. Homogeneous copper carbonate suspension (10%, w/w; agitated) was introduced in the reactor varying the volume of suspension (i.e., 2.5, 5, and 7.5 mL, that corresponds to amounts of precipitates of 50, 10, and 150 mg of CuCO 3 , respectively). Demineralized water was added to a final volume of 200 mL. The suspension was maintained under agitation for 30 min. Then, 2 mL of CaCO 3 suspension (10%, w/w) were added to the mixture under agitation for 1 min and 3 g of gluconolactone were introduced under strong agitation (300-400 rpm) for 30 s and the wet foam was dropped into a rectangular mold (110 3 145 mm). This excess of carbonate (added as CaCO 3 ) serves to improve the fire retardant properties: the thermal degradation of carbonate produces carbon dioxide through endothermic reaction that may help in controlling fire behavior 34 ; or by heat absorption that increases to time to reach high temperatures around 500 8C. 35 The molds were finally dried for 72 h in an oven at 50 8C.

The slow release of copper, mediated by the weak organic acid, is important for preventing the formation of aggregates, the heterogeneous gelation of the biopolymer. Weak acid promotes the slow release and lightly delayed gelation, which, in turn, allows processing the preparation for dipping in the molds. Adding a stronger acid would not allow getting the desired homogeneous structure for the foams and would make the processing of the materials complex with a partial gelation of the mixture before it is dropped in the mold. The protons (released during gluconolactone dissociation) convert carbonate anions in carbonic acid and carbon dioxide, which contributes to a kind of porogen effect in addition to the foaming effect of SLS during the strong agitation phase.

Preliminary tests allowed identifying three critical parameters for the manufacturing of alginate foams: the amounts of the biopolymer, the foaming agent (SLS), and ionotropic agent (copper carbonate). In order to optimize the manufacturing of the foam an experimental design methodology has been used: B-B design with three variables and three levels (see Table I). Fifteen formulations were prepared, including three replicates of the centered point; they are reported in Table II.

Several criteria were used for qualifying the production of the alginate foams: the volume of foam generated, both wet and dry, the density (kg m 23 ), and the shrinking percentage (%). The responses to variable parameters are also reported in Table II.

A first B-B experimental design was carried out with wider ranges of values; it served to focus on more precise selected values.

The model follows the equation [START_REF] Goupy | Plans D'exp eriences Pour Surfaces de R eponse[END_REF] :

y 5 b 0 1 b 1 x 1 1b 2 x 2 1b 3 x 3 1b 12 x 1 x 2 1b 13 x 1 x 3 1b 23 x 2 x 3 1b 11 x 2 1 1b 22 x 2 2 1b 33 x 2 33 ( 1
)
where y is the predicted response vector, x i are the independent variables, b i are the linear coefficients, b ij are the cross product coefficients, and b ii are the quadratic coefficients. The units of these parameters depend on the tested parameter (apparent density, volume, shrinking percentage), since in the equation the centered coded values are dimensionless. The values of the parameters have been estimated by matrix calculation [START_REF] Goupy | Plans D'exp eriences Pour Surfaces de R eponse[END_REF] :

b5 X t X ð Þ 21 X t Y ( 2
)
where b is the vector of coefficients (b i , b ij , b ii ), X is the socalled experimental matrix (15,10) (see Supporting Information section), X t is the transposed experimental matrix.

It is noteworthy that a specific set of parameters (b i , b ij , b ii ) was established for each vector of response; that is, dry volume, apparent density, and shrinking percentage.

The central point, repeated three times (runs 13, 14, and 15), shows a relatively good reproducibility for apparent density (i.e., 24.9 6 1.8 kg m 23 ; variation percentage: 7%), dry volume (i.e., 405 6 20 mL; variation percentage: 5%), and wet volume (i.e., 490 6 9 mL; variation percentage: 2%). On the other hand, for the shrinking percentage the variation is much Into brackets: amounts in mg for alginate biopolymer, SLS salt and CuCO 3 "equivalent" solid for a total volume of preparation of 200 mL.

greater (i.e., 16% 6 4%; variation percentage: 27%): surface deformation makes difficult the accurate determination of this parameter.

Characterization of Materials

The volumes of the foams were measured before and after drying for 72 h in an oven at 50 8C and the shrinking percentage was calculated by the volume loss after drying (in percentage). The apparent volumetric density was calculated by the measurements of weight and volume of dried foams. Obviously the shrinking during the drying step may cause, under certain conditions, a deformation of the surface of the dried foams; especially the surface that was in contact with the atmosphere; that is, not in contact with the mold. As a consequence the volumes may be determined with a certain "inaccuracy"; the maximum error is evaluated to 10%.

In addition, the macroporous structure (cell opening) of the foam has been characterized using SEM. Inner structure of foams was observed with a SEM (FEI Quanta 200 SEM). All images were obtained under high vacuum at a voltage of 10 kV and a working distance of 9-12 mm.

The analysis of the viscosity of alginate solutions was performed on a Visco Plus viscometer (Fungi Lab, Sant Feliu de Llobregat, Spain), changing both the mobile and the rotation speed with measured viscosity, in order to remain in the range of linearity of the equipment (scale 15%-90% of measurement range, as supplied for each mobile).

Fire Retardant Characterization

Fire behavior was assessed using UL94 vertical burning test. [START_REF] Xuan | [END_REF] Sample dimensions were 100 3 10 3 e mm 3 , where e is the thickness of the foam. The pyrolysis combustion flow calorimetry (PCFC) was also used to measure the flammability of samples at microscale (2-3 mg). This device was developed by Lyon and Walters. 38 The sample was first heated from 80 to 750 8C at 1 8C/s in a pyrolyzer under nitrogen flow (anaerobic pyrolysis-method A according to the standard ASTM D7309) and the degradation products were sent to a combustor where they were mixed with oxygen in excess at 900 8C. Under selected conditions, all gases were fully oxidized. HRR was then calculated by oxygen depletion according to Huggett's relation: 1 kg of consumed oxygen corresponds to 13.1 MJ of released energy. 39 Mechanical Characterization Compression properties were measured using a Zwick TH010 apparatus at 5 mm/min. 16 mm diameter samples were carefully cut from foams. The dimensions of the foams (including their thickness) depend on the conditions used for the preparation of the materials due to different shrinking percentages. The actual thickness of the sample was systematically measured and used for the determination of mechanical properties. Three to six samples were tested for each formulation.

Thermal Conductivity Analysis

Among the remarkable properties of EPS, which is extensively used as insulating material, its low thermal conductivity can be considered an appropriate reference for testing the alginatefoams produced in this study. Making profit of the equipment available in our research center some preliminary tests have been carried out for comparison with the reference material but also for evaluating the potential impact of foam density. Thermal conductivity was measured at room temperature using a thermal conductivimeter based on hot wire method (FP2C from Neotim).

RESULTS AND DISCUSSION

Morphology of Alginate Foams-SEM Analysis Due to their brittleness, foams were damaged to a certain extent during cutting and preparation for SEM observations. Figure 1 shows the inner structure of three foams with various densities. These cells can be roughly considered as having a closed-cell structure. 23 The air-drying process is favoring the formation of closedcells compared to freeze-drying process that forms more open cells. Actually, the freeze-drying was especially used for preparing macroporous alginate foams that were used as highly-percolating materials for the sorption of metal ions. 29 The freeze-drying conditions improve the release of the gas dissolved in the gel and the breaking of the bubbles leads to the formation of an interconnected porous network. This effect is even reinforced when using a blowing agent as SLS. With air-drying the interconnected porous network is not formed and the cells mainly remain closed. The open/closed cell structure may have a strong impact on (a) thermal insulating/fire retardancy properties by controlling the diffusion of gases and heat, but also on (b) mechanical properties. [START_REF] De Vries | department of Mechanical Engineering[END_REF] It is noteworthy that the denser foams exhibit smaller cells. For the foams of lower density (8.2 kg m 23 ) the order of magnitude of the cell size is higher than 4 mm while for intermediary density (i.e., 14.1 kg m 23 ) the cells have size between 1 3 1.5 mm and 1.5 3 3 mm. The smallest cells are obtained at the highest density; for density of 34.7 kg m 23 the cells are more regular with a size close to 1 3 1.5 mm. Mohebbi et al. classified, in the case of PP foams, the materials in several types depending on the cell size and the cell density 22 ; by analogy with PP foams and based on their size the cells can be qualified as conventional close cells (cell size being greater than 300 mm and the cell density being obviously lower than 10 6 cells/cm 3 ).

The thickness of the cell walls also increases when density increases. This is particularly obvious when comparing the densest foam to the least dense one. This may be related to the higher mechanical properties of denser foams (thickness from about 5 mm up to 18 mm); however, the actual mechanical behavior of the foams is controlled by both the changes in the thickness of the walls and the cell size (cross section). This would deserve a more complete study for optimizing these mechanical properties. Holes can be also found into cell walls (see top row of Figure 1). Higher magnification highlights the roughness of the scaffold (see for example, pictures for the foam with intermediate density).

Photographs of the size face of the foams (vertical cut) shows that the structure is globally homogeneous; however, for certain formulations, a thin layer of the foams appears less expanded at the center of the block (Figure S1, see Supporting Information section).

Characterization of Thermal Conductivity, Mechanical, and Fire Retardant Properties of Foams Fire Retardant Properties. The main benefit of alginate-based foams compared to expanded polystyrene foams is their low flammability. The flammability of alginate foams, fibers, or films has been already addressed. Depending on the divalent or trivalent cation used for gelation, V0 rating in UL94 test and LOI higher than 50 can be reached. 8,9,17,[START_REF] Liu | [END_REF]42 High char contents (>30 wt %) both in air and nitrogen were also measured using thermogravimetric analysis. Gelation with calcium and barium cations was found as very effective for flame retardancy. [43][44][45] Lyon et al. have studied correlations between PCFC and various fire tests from a large set of polymers. 46 For example, the probability to obtain V0 rating in UL94 test is high for THR and heat release capacity (HRC); that are lower than 17.8 kJ g 21 and 340 J g 21 K 21 , respectively. The HRC is the peak HRR dividied by the heating rate.

The measured properties in PCFC for alginate are much lower than the threshold values determined by the authors to pass UL94 test. HRR curve measured in PCFC is shown in Figure 2 for raw alginate powder (the analytical procedure is independent of the shape and size of the sample). There is no influence of the physical state (powder vs. film vs. foam) on the data measured by PCFC. Probably the exact composition (presence of SLS or other components) may have an influence. But, it must be noted that the HRR is very low and the very small differences from one sample to another one cannot be interpreted. Then a change from 10 to 20 W g 21 appears nonsignificant. Two main peaks are observed. Other small peaks can be considered as noise. Keep in mind that peak of HRR for common polymers for which PCFC was designed is generally comprised between 200 and 1200 W g 21 , 47 this means 10 to 50 times the value measured here. First peak is observed at 240 8C and the second one occurs at 440 8C. These two peaks correspond to two different in the thermal degradation profile as it would appear on TGA analysis. The first step consists of the thermal degradation of the material with simultaneous formation of a poorly stable residue, which will be degraded in a second step. However, it is noteworthy that an important fraction of carbonbased product remains in the residue that will be only thermally degraded under aerobic pyrolysis (with a heat release that may exceed 10 kJ g 21 ). Under anaerobic pyrolysis conditions, the THR is around 3 kJ g 21 and the weight loss does not exceed 70%. This means that the effective energy of combustion is close to 4 kJ g 21 . This value is very low; this means that released gases are highly oxidized even before the combustion. This is probably due to the presence of significant amounts of carbon dioxide and/or water (and possibly low amounts of carbon-containing gas poorly oxidized and that will be oxidized during combustion with supplementary heat release). Anyway, it is important to point out that the quantities of heat released during the anaerobic pyrolysis are very low; close to the detection limit, which makes the interpretation debatable. The THR (which represents the surface area under the curves) is below 2 kJ g 21 for the foams versus 15-44 kJ g 21 for common polymers, 47 and 8-9 kJ g 21 for cellulose. 48 For formulated foams, both peaks are also observed at similar temperature but their intensity is lower, especially for the second peak. The residue content is close to 70 wt %.

All foams exhibit self-extinguishing behavior according to UL94 test and are V0 ranked, irrespective to their thickness or density. In some cases, some parts of burnt materials fall but do not ignite the piece of cotton located below. Actually, the differences observed on different series were not considered sufficient for considering the fire retardant properties to be significantly affected; all the materials are characterized by V0 ranking (UL94) and low THR.

Actually, according literature, sodium alginate materials are not self-extinguishing (regardless of the form; i.e., foams or films) based on tests such as UL94 or LOI. The nature of the gelling agent (metal ion) affects this behavior, while copper and manganese cations do not bring self-extinguishable properties, 8 metal ions such as Fe(III), Cr(III), Al(III), [START_REF] Liu | [END_REF] Co(II), 8 Zn(II), 17 Ni(II), 9 or Ba(II) 45 allows converting alginate to selfextinguishable material after metal gelation. the block progressively degrades it appears that the material maintains its global structure compared to expanded polystyrene that melts and drips as soon as its temperature exceeds 100 8C.

Thermal Conductivity. Thermal conductivity is a key property of foams for insulating properties. Thermal conductivity was measured at room temperature for a set of foams exhibiting various densities. Values were measured in the range 0.028-0.039 W m 21 K 21 , corresponding to highly insulating materials. Similar value (0.041 W m 21 K 21 ) was measured for commercial EPS foam with apparent density of 10 kg m 23 . The analytical procedure used for the characterization of thermal conductivity does not allow detecting significant differences below 0.005 W m 21 K 21 . As a consequence, the differences observed between the different samples cannot be considered representative. However, it is important to remind that the objective of this test consisted of demonstrating that the thermal conductivity is of the same order of magnitude than the values obtained with a conventional commercial sample. These data are sufficient to confirm the interest of these foams in terms of insulating properties: thermal conductivities are comparable to the values of a critical reference material like expanded polystyrene foam.

Mechanical Properties. Mechanical properties are also important properties for foams. Among the different tests existing for characterizing the mechanical properties of polymer foams, [START_REF] De Vries | department of Mechanical Engineering[END_REF] the uniaxial compression test is a relatively simple test that can be readily used with appropriate reproducibility for the preliminary testing of mechanical strength and comparison of selected foams. Obviously more complete tests would be necessary for standard qualification of the materials. [START_REF] De Vries | department of Mechanical Engineering[END_REF][START_REF] Lu | National Laboratories, Mechanical Characterization of Rigid Polyurethane Foams[END_REF][START_REF] Lee | Polymeric Foams Science and Technology[END_REF] However, this simple test is sufficient for evaluating the relative impact of operating parameters through the correlation of the compression properties with the apparent density of the foams. Compression tests were carried out on a set of foams with apparent density ranging from 7 to 35 kg m 23 . Typical "stress-displacement" curves are shown in Figure 3. Curves for alginate foams are characterized by several peaks due to the brittle behavior of the foams. On the contrary, expanded polystyrene shows a smooth curve. Both maximum stress and modulus (measured typically when the force is between 0.04 and 0.4 N) increase with apparent density increase. This expected result gives an indication on the importance of optimizing the density of the foams taking into account mechanical resistance in addition to the apparent density of the materials; for the next step in the optimization of foam production see Optimization of Foam Manufacturing section.

Figures 4 and5 show the change in modulus and maximal stress with apparent density. Modulus is low for low-density foams (<200 kPa). A significant increase is observed for denser foams. For 15 and 35 kg m 23 , modulus is 430 and 840 kPa, respectively. In the case of EPS, the modulus is slightly higher (around 300 kPa) than corresponding alginate foams with similar density.

Linear increase of maximum stress with apparent density is more obvious for alginate foams. The maximum stress increases from 2 to 45 kPa when density increases from 7 to 35 kPa. Nevertheless, EPS exhibits a much higher maximum stress: at least 62 kPa versus 5 kPa for comparable foams (data not shown because the maximum stress for EPS was not reached at the end of the test). This critical issue is currently investigated with both the incorporation of cellulose-based fibers and the optimization of alginate concentration with the objective to increase mechanical resistance of composite material.

Optimization of Foam Manufacturing B-B Design. Table II summarizes the main results obtained in the morphological characterization of alginate foams in relation with the variation of experimental parameters (B-B design).

The coefficients of the model for the four responses are reported in Table III. In addition, the predicted and experimental values are compared in Figures S3-S6 (see Supporting Information section) for the density of the foam, the dry volume, the shrinking percentage, and the wet volume, respectively. The calculated equations fit relatively well experimental data, especially for the apparent density and the volume of production of wet foams: the determination coefficients are close to 0.986 and 0.991, respectively; much better than for the shrinking percentage (R 2 5 0.885).

The main objectives for processing optimized foam, in terms of practical characteristics, concern the apparent density of the foam and the shrinking percentage that should be as low as possible and the volume of dried foam produced that should be the highest possible. Obviously, the experimental parameters should take into account, in a second step, criteria such as the mechanical properties of elaborated materials, as well as fire retardant properties and insulating performance. Ongoing research will consider the incorporation of additives (fibers, fire retardant compounds) to improve relevant properties, based on the optimization of foam production.

In order to get a better visualization and understanding of the effect of individual parameters on these three criteria a series of iso-response curves has been plotted using the general equations (b ij ) for the three parameters at the three levels (-1, 0, and 11). In addition, Figures S7-S15 intermediary amounts of SLS, regardless of the amount of copper carbonate introduced in the composition of the production batch (Figure 6). When increasing the alginate concentration the amount of SLS should be increased to reach target objective.

These results mean that a key parameter for the design of "light" materials is strongly controlled by the critical parameter alginate/SLS, while the relative value of copper carbonate has a limited effect on this parameter. The levels of copper carbonate have a limited impact on the apparent density; however, best results are obtained with SLS in the range 6.5-9.5 mL (or 35-95 mg) for 200 mL preparation. The amount of alginate is not critical. Nevertheless, the optimal surfaces for the three levels of copper carbonate delimit a median axis that shows a relatively stable slope that corresponds to a mass ratio between alginate and SLS close to 40.

With the target of maintaining apparent density below 20 kg m 23 the best conditions correspond to the following proportions for 200 mL of preparation:

Alginate solution: 140 mL (4% w/w solution; i.e., 5.6 g) SLS solution: 6.5 mL (1%, w/w solution; i.e., 65 mg) CuCO 3 suspension: weak impact (centering on 5 mL of 10% w/w suspension; i.e., 100 mg).

Evaluation of Processing Parameters on Dry Volume of Obtained Foams. Figure 7 reports the iso-curves for the production of dry foams. This is also a critical parameter for optimizing the productivity of the process. Low levels of SLS are crippling: when SLS is set to 3.5 mL for 200 mL of preparation, whatever the levels of copper carbonate and Alginate the production of foams remains below 300 mL. At levels 0 and 11, (i.e., 6.5 and 9.5 mL of SLS) the production of foams increases with decreasing Alginate volume, while the amount of copper carbonate has a weaker impact.

When examining the effect of alginate amount at three levels it appears that the biopolymer should be below 150 mL (i.e., below 6 g) for SLS higher than 6.5 mL (i.e., 65 mg), while the amount of copper carbonate has a limited impact. This conclusion is corroborated by the last row of Figure 7 that examines the effect of copper carbonate: for the three levels, best results were obtained for amounts of alginate below 6 g and SLS higher than 65 mg. In the case of foam productivity the ratio alginate/SLS also sounds to be a crucial parameter. Best conditions correspond for 200 mL of preparation to:

Alginate below 150 mL (4% w/w solution; i.e., 6 g of alginate) SLS above 6.5 mL (1% w/w solution; i.e., 65 mg) Copper carbonate (5 mL 10% w/w suspension; i.e., 100 mg).

Evaluation of Processing Parameters on Shrinking

Percentage. The shrinking percentage is also an important criterion in terms of operability of the manufacturing. An important shrinking is frequently associated to a large deformation of the size and planarity of the foams that will make the material difficult to use for final application. In this section, the shrinking is roughly calculated as the reduction (in percentage) of the produced volume with reference to wet volume. Obviously, this parameter is not independent of the other parameters analyzed above but it seems interesting discussing the results under this specific criterion. Figure 8 shows the iso-response curves for shrinking percentage; the objective being to maintain this shrinking percentage below 20%.

The first row of Figure 8 illustrates the effect of SLS and copper carbonate at three levels of alginate. The optimum for the reduction of shrinking is clearly located at the center of the figures with intermediary values for SLS and copper carbonate. The optimum surface is considerably reduced in size when increasing alginate amount; this requires a more drastic control of the other compounds. The shrinking decreases when alginate amount decreases.

When considering the effect of SLS level on shrinking it appears that SLS should be around 6.5 mL (1% solution; i.e., 65 mg) with copper carbonate close to 5 mL suspension (10% w/w; i.e., 100 mg); under these conditions the amount of alginate has a limited impact on shrinking. However, when the SLS amount moves from this central value the shrinking strongly increases regardless the values of the other parameters. The third row of Figure 8 shows the iso-response curves for the three levels of copper carbonate amount; similar profiles are observed with a minimized shrinking obtained at the central value of SLS and with low levels of Alginate.

Optimal conditions are roughly found close to:

Alginate: below 150 mL (4% w/w solution; i.e., 6 g) SLS: 6.5 mL (1% w/w solution; i.e., 65 mg) CuCO 3 : 5 mL (10% w/w suspension; i.e., 100 mg).

Effect of Alginate/SLS Ratio. The analysis of iso-response curves has demonstrated that the critical parameters are alginate and SLS levels, while copper carbonate has a lowest impact on the global productivity, apparent density, and morphological stability (shrinking). The macroscopic observation of the different series also subjectively identified a number of materials having "appropriate properties," such as general aspect (correlated to shrinking), handling stability (correlated to mechanical stability), and apparent density. In Table II, the data in italics identify the experimental conditions that fit individual selected criteria; that is: Apparent density around (or below) 20 kg m 23 ; dry volume around (or higher) 400 mL; and shrinking percentage <20%. Data in bold correspond to conditions that satisfy all criteria. Actually, the best results correspond to the conditions for the preparation of 200 mL:

Alginate: 140 mL (4% w/w solution; i.e., 5.6 g) SLS: 6.5 mL (1% solution; i.e., 65 mg) Copper carbonate: 2.5 mL (10% suspension; i.e., 50 mg) Under these conditions, the three criteria are satisfied. Several other points also give relatively good manufacturing performance and stability properties as shown in Table II: including the central point: coded values (0, 0, 0) corresponding to alginate: 150 mL (i.e., 6 g), SLS: 6.5 mL (i.e., 65 mg), and CuCO 3 : 5 mL (i.e., 100 mg). The point (alginate: 140 mL; SLS: 6.5 mL, CuCO 3 : 7.5 mL) also gives good production parameters.

Copper carbonate appears to have a marginal effect on the synthesis and properties of produced foams (at least in the tested experimental range). Actually all the positive conditions correspond to a mass ratio between Alginate and SLS around 42-46.

The capacity of the alginate-based system to form foam is obviously correlated to its viscosity: a highly viscous biopolymer solution (corresponding to high alginate concentration) has more difficulties to foam in the presence of the foaming agent (SLS) than a "fluid" solution. Figure S16 (see Supporting Information section) shows the evolution of viscosity with alginate concentration: the viscosity should be below 4000 cP. Other parameters such as the surface tension of the alginate/SLS solution are probably playing an important role on the foaming of the suspension and then the morphological properties of the foams.

CONCLUSIONS

The foaming of alginate solution using SLS (foaming agent) in the presence of copper carbonate and gluconolactone (ionotropic gelation system) allows preparing expanded macroporous structures that retain their shape after air-drying. The production of foams, their apparent density, and their shrinking (during the drying step) are critical parameters in terms of productivity and processing. These materials have remarkable characteristics in terms of thermal conductivity (below the corresponding value of expanded polystyrene foam; i.e., 0.041 W m 21 K 21 ), and fire properties (self-extinguishing behavior according to UL94 test and classified as V0 ranked materials; THR below 2 kJ g 21 ). The mechanical properties, roughly defined by compression tests but also by the shrinking percentage and the collapse of the foam structure during the drying step, are logically correlated to the apparent density and the operating conditions should be selected taking into account a compromise between the mechanical stability and the apparent density. The SEM characterization shows that the "opening" of the cells of the foams decreases with apparent density.

According to the criteria selected for the optimization of the synthesis, the best conditions were determined. For the preparation of 200 mL of suspension, alginate should be set below 6 g, SLS should be around 65 mg while copper carbonate should be around 500 mg (in mass of active product; i.e., 150 mL of 4% w/w alginate solution, 6.5 mL of 1% SLS solution, and 6.5 mL of 10% w/w CuCO 3 suspension). These conditions allow maintaining the shrinking below 20%, the apparent density below 20 kg m 23 , and the production of dry foam exceeds 400 mL (for an initial volume of preparation of 200 mL). The critical mass ratio between alginate and SLS is around 42-46.

Current development consists of incorporating fibers and other additives in these alginate foams in order to increase their mechanical and fire retardant properties. An effort will be made for using green products (PLA, natural fibers, bioplasticizer) for improving the mechanical properties of the foams but also their surface properties (with more specifically the objective of decreasing the wettability of the material). A special attention will be also focused on testing the acoustic properties of these foams. The mechanical properties of the gel, the manufacturing conditions are strongly controlled by intrinsic parameters associated to the biopolymer such as the viscosity of the solution.

Playing with the type of alginate (G/M ratio) and the concentration is the key for appropriate processing; but a low concentration of the biopolymer would lead to a dramatically collapse of the foam structure during the drying step. The strength of the gel and the dried foam could be also controlled by the type of metal salt used for ionotropic gelation. These are parameters that could be tested with profit for improving the properties of the foams in addition to the incorporation of other adjuvants (as reported above).

Figure 1 .

 1 Figure 1. SEM observations of three foams. Effect of density on cell observation [top row, general morphology of foams at the lowest magnification (360); bottom row, view focused on the thickness of cell walls, and fine surface aspect of the scaffold at highest magnification (32000); pressure in the SEM chamber: 0.53 Torr; accelerating voltage: 10.0 kV].

Figure

  Figure S2 (see Supporting Information section) shows the change in the aspects of a foam block subjected to three successive flame exposition (individual irradiation time: 10 s). Though

Figure 2 .

 2 Figure 2. HRR curve for raw alginate in PCFC.

  (see Supporting Information section) show the three-dimensional curves at centered values of the third circular parameter. Evaluation of Processing Parameters on Apparent Density. Decreasing the apparent density below 20 kg m 23 (i.e., comparable to usual EPS materials; the tested EPS herein has a density of 10 kg m 23 ) requires using low amounts of alginate and

Figure 3 .

 3 Figure 3. Stress-displacement curve for several alginate foams and EPS (dimensions of the samples mechanically tested: fixed diameter of 16 mm; variable thickness: 12.1, 19.4, and 14.4 mm, for the samples of apparent density close to 7, 14.1, and 34.7 kg m 23 , respectively).

Figure 4 .

 4 Figure 4. Modulus versus apparent density for several alginate foams and EPS (the solid line shows the linear regression plot of modulus vs. apparent density for alginate-based foams).

Figure 5 .

 5 Figure 5. Max stress versus apparent density for several alginate foams (the solid line shows the linear regression plot of max stress vs. apparent density for alginate-based foams).

Figure 6 .

 6 Figure 6. Isoresponse curves for the effect of experimental parameters (coded variables) on the volume of apparent density (kg m 23 ) (-1/0/11 levels: alginate: 2800/3000/3200 mg; SLS 35/65/95 mg; CuCO 3 equivalent solid: 50/100/150 mg for the preparation of a volume of 200 mL wet foam). [Color figure can be viewed at wileyonlinelibrary.com]

Figure 7 .

 7 Figure 7. Isoresponse curves for the effect of experimental parameters (coded variables) on the volume of foam produced (dry volume, mL) (-1/0/11 levels: alginate: 2800/3000/3200 mg; SLS 35/65/95 mg; CuCO 3 equivalent solid: 50/100/150 mg for the preparation of a volume of 200 mL wet foam). [Color figure can be viewed at wileyonlinelibrary.com]

Figure 8 .

 8 Figure 8. Iso-response curves for the effect of experimental parameters (coded variables) on the shrinking percentage (%) (-1/0/11 levels: alginate: 2800/3000/3200 mg; SLS 35/65/95 mg; CuCO 3 equivalent solid: 50/100/150 mg for the preparation of a volume of 200 mL wet foam). [Color figure can be viewed at wileyonlinelibrary.com]

Table I .

 I Levels of Selected Parameters for Box-Behnken (B-B) Design

				Coded variable level
			Low	Center	High
	Variable	Symbol	21	0	11
	Alginate (amount, g)	x 1	140 (2800)	150 (3000)	160 (3200)
	SLS (volume, mL)	x 2	3.5 (35)	6.5 (65)	9.5 (95)
	CuCO 3 (volume, mL)	x 3	2.5 (50)	5 (100)	7.5 (150)

Table II .

 II Box-Behnken (B-B) Design with Actual Values of Variables and Experimental Corresponding Results (for a Total Volume of Preparation of 200 mL)

			Actual values of variables			Experimental results
					Apparent	Dry	Shrinking	Wet
		x 1 (alginate)	x 2 (SLS)	x 3 (CuCO 3 )	density	volume	percentage	volume
	Run #	(mL)	(mL)	(mL)	(kg m 23 )	(mL)	(%)	(mL)
	1	140	3.5	5	50.5	251	26.1	339
	2	160	3.5	5	145.7	168	38.8	275
	3	140	9.5	5	31.6	425	43.1	748
	4	160	9.5	5	22.0	408	37.7	654
	5	140	6.5	2.5	18.9	504	12.8	578
	6	140	6.5	7.5	20.0	441	17	531
	7	160	6.5	2.5	22.4	294	24.3	388
	8	160	6.5	7.5	28.4	289	25.4	388
	9	150	3.5	2.5	71.0	176	42.7	307
	10	150	9.5	2.5	31.6	404	40.7	681
	11	150	3.5	7.5	95.0	148	45.9	275
	12	150	9.5	7.5	23.8	431	30	616
	13	150	6.5	5	22.5	434	11.7	492
	14	150	6.5	5	25.4	390	20.2	500
	15	150	6.5	5	26.9	392	18	478

Table III .

 III Coefficients of the Model [eq. (1)] for the Four Responses (Apparent Density, Dry Volume, Shrinking Percentage, and Wet Volume)

	Coefficient	Apparent density	Dry volume	Shrinking percentage	Wet volume
	b 0	24.93	408.33	16.63	490.00
	b 1	12.19	257.75	3.40	261.43
	b 2	231.65	115.63	20.25	187.91
	b 3	2.91	28.63	20.28	217.96
	b 12	226.20	16.50	24.53	27.35
	b 13	1.23	14.50	20.78	11.80
	b 23	27.95	13.75	23.48	27.98
	b 11	2.30	21.54	20.08	7.71
	b 22	35.22	293.79	19.87	6.34
	b 33	24.81	224.79	3.32	226.71
	R 2	0.954	0.938	0.885	0.985


Raymond Hajj

Raphael Brunel

Rodolphe Sonnier

Claire Longuet

Francois Ganachaud
email: francois.ganachaud@insa-lyon.fr

François Ganachaud

Silicone-recycled pyrolyzed fillers for enhanced thermaland flame -resistant silicone elastomers

Keywords:

Introduction

Silicone rubbers exhibit a unique combination of maintained elasticity at exceptionally low temperatures and high thermal and thermo-oxidative stabilities at high temperatures [START_REF] Jones | Silicon-containing polymers[END_REF] . They are generally prepared by mixing silicone chains with reinforcing fillers and crosslinking the chains by different chemistries. The production of silicones has severely increased during the last decades [START_REF] Köhler | Industrial synthesis of reactive silicones: reaction mechanisms and processes[END_REF] and the end-of-life of these complex composite materials remains an open question nowadays. Usually, most silicones end up as landfill waste [START_REF] Rupasinghe | Full circle recycling of polysiloxanes via roomtemperature fluoride-catalyzed depolymerization to repolymerizable cyclics[END_REF] where they depolymerize within few days to few weeks [START_REF] Buch | Rearrangement of poly (dimethylsiloxane) fluids on soil[END_REF][START_REF] Graiver | A Review of the fate and effects of silicones in the environment[END_REF][START_REF] Varaprath | Speciation and quantitation of degradation products of silicones (silane/siloxane diols) by gas chromatography-mass spectrometry and stability of dimethylsilanediol[END_REF] . They breakdown naturally into harmless byproducts such as oligomers containing silanol groups, silicates, CO 2 and H 2 O [START_REF] Buch | Rearrangement of poly (dimethylsiloxane) fluids on soil[END_REF][START_REF] Graiver | A Review of the fate and effects of silicones in the environment[END_REF][START_REF] Varaprath | Speciation and quantitation of degradation products of silicones (silane/siloxane diols) by gas chromatography-mass spectrometry and stability of dimethylsilanediol[END_REF][START_REF] Furgal | Green routes to silicon-based materials and their environmental implications[END_REF] .

Recycling silicones is not new per se. Early on, a team proposed to grind thermally-aged silicone rubbers to reintroduce these into the neat rubber formulation [START_REF] Ghosh | Recycling of silicone rubber waste: effect of ground silicone rubber vulcanizate powder on the properties of silicone rubber[END_REF] . The addition of this powder at a loading of up to 60 phr caused acceptable losses in terms of tensile and tear properties in comparison to the genuine silicone rubber. Furthermore, the changes in hardness, hysteresis loss, tension set, and stress relaxation were found marginal.

Besides, more recently, several academic labs have proposed to recycle silicones by chemically degrading their network using metallic catalysts [START_REF] Enthaler | Zinc-catalyzed depolymerization of end-of-life polysiloxanes[END_REF][START_REF] Enthaler | Iron-catalyzed depolymerization of polysiloxanes to produce dichlorodimethylsilane, diacetoxydimethylsilane, or dimethoxydimethylsilane[END_REF][START_REF] Weidauer | Iron-catalyzed depolymerizations of silicones with hexanoic anhydride provide a potential recycling method for end-of-life polymers[END_REF] , fluoride catalysts [ 3 , 12 , 13 ], Lewis acids [ 11 , 14 , 15 ], strong acids [START_REF] Booth | Cleavage of silicones by hydrogen fluoride[END_REF] , or strong bases [START_REF] Chang | Nucleophilic cleavage of crosslinked polysiloxanes to cyclic siloxane monomers: mild catalysis by a designed polar solvent system[END_REF] . The endproducts are mostly silicone monomers that can then be used to produce again silicone elastomers and resins. For example, Enthaler and coworkers [ 10 , 11 ] have investigated the use of iron salts in the depolymerization of polysiloxanes to produce various types of silanes (e.g. dichlorodimethylsilane, diacetoxydimethylsilane, or dimethoxydimethylsilane). Typically, depolymerizing polydimethylsiloxane (PDMS) in a solution containing 7.5 mol% of FeF 3 with benzoyl chloride (190 °C, 5 h) resulted in 86% yield of dichlorodimethylsilane. Chang and coworkers [START_REF] Chang | Nucleophilic cleavage of crosslinked polysiloxanes to cyclic siloxane monomers: mild catalysis by a designed polar solvent system[END_REF] examined the base-catalyzed aminolysis of crosslinked PDMS to produce cyclic monomers (hexamethylcyclotrisiloxane (D 3 ), octamethylcyclotetrasiloxane (D 4 ), and decamethylcyclopentasiloxane (D 5 )). The obliteration of silicone network occurred in a reactor containing potassium hydroxide as a basic catalyst, ethanol and diethylamine at room temperature to yield 80% of cyclic siloxane oligomers after 25 h.

Another conventional way of converting silicon-based polymers into useful materials consists in using them as precursors of ceramics, mainly for aerospace applications [START_REF] Ren | High-temperature properties and applications of Si-based polymer-derived ceramics: a review[END_REF] . Several studies have long focused on the improvement of organic-to-inorganic conver-sion of silicone rubbers by adding different catalytic systems into the network, so as to promote high residue yields and little volatile contents [START_REF] Delebecq | High residue contents indebted by platinum and silica synergistic action during the pyrolysis of silicone formulations[END_REF][START_REF] Liu | Efficient organic-to-inorganic conversion of polysiloxane by novel platinum-thiol catalytic system[END_REF][START_REF] Liu | Remarkable improvement of organicto-inorganic conversion of silicone rubber at elevated temperature through platinum-nitrogen catalytic system[END_REF][START_REF] Chen | Synergistic effect and mechanism of platinum catalyst and nitrogen-containing silane on the thermal stability of silicone rubber[END_REF] . For instance, Liu and coworkers [ 20 , 21 ] used platinum-nitrogen-and platinum-thiol-based catalytic systems as ceramifying agents in silicone rubber. Thermogravimetric analyses showed that the final residue increased from around 1 up to 47% under nitrogen atmosphere and from 30 to 60 wt% under air in presence of the different systems at 900 °C. Delebecq and coworkers [START_REF] Delebecq | High residue contents indebted by platinum and silica synergistic action during the pyrolysis of silicone formulations[END_REF] investigated the synergistic role of platinum and silica as a way to increase the final residue of pyrolyzed silicones. The suggested degradation mechanism of polydimethylsiloxane (PDMS) in presence of these two components involves three steps: i) crosslinking reaction between PDMS chains adsorbed on silica via radicals generated by Pt at about 400 °C, ii) volatilization of PDMS mobile chain parts between 400 and 600 °C, and iii) ceramization of the remaining compelled chain fragments from 650 °C leading to the formation of a SiOC ceramic. Liquid silicone rubber (LSR) reinforced with vinylated-functionalized silica were shown to produce residues as high as 70 wt%. An original work by Shen and coworkers [START_REF] Shen | Stable superhydrophobic surface based on silicone combustion product[END_REF] explored the superhydrophobicity of SiOC powder obtained by burning silicone which showed some abrasion resistance and electro-conductive properties that opens some practical uses in e.g. electromagnetism shielding materials.

To the best of our knowledge, silica and other ceramics resulting from the degradation of silicone rubbers have never been reintroduced as fillers in silicone materials. In this work, we propose to pyrolyze a conventional commercial silicone elastomer in two different atmospheres and use these residues as fillers with the aim of bringing enhanced thermal properties to thus-prepared obtained new silicone rubbers.

Experimental

Materials

The pristine room-temperature-vulcanizing silicone (RTV) used in this study is an industrial two-part liquid component kit, containing a polymer base (part A) and a crosslinking curing agent (part B). The RTV crosslinks by hydrosilylation, and contains vinylated silica that allows a high residue content upon pyrolysis [START_REF] Delebecq | High residue contents indebted by platinum and silica synergistic action during the pyrolysis of silicone formulations[END_REF] (vide infra). A PDMS gum of weight average molar mass of about 500 kg.mol -1 was purchased from an industrial partner (see full description of gum A in ref. [START_REF] Stricher | Influence of the microstructure of gums on the mechanical properties of silicone high consistency rubbers[END_REF] ). Luperox 101, 2,5-Bis(tertbutylperoxy) -2,5-dimethylhexane (DBPMH, technical grade, 90%) was purchased from Sigma-Aldrich. Commercial silica Aerosil A200 (hydrophilic fumed silica, specific surface 200 m 2 /g) and Sipernat 22S (precipitated silica, 180 m 2 /g) with silica contents of 99.8 and 97%, respectively, were kindly given by Evonik industries. All the products were used as received without any purification.

Preparation of fillers by RTV pyrolysis

In order to prepare high carbon content residues, the pristine crosslinked RTV silicone elastomer was cut into small parts of 2 × 2 × 1 cm 3 placed in two interlocking steel tubes in order to limit air exposure. These tubes were then placed in an oven mounted in up to 850 °C, maintained around 15 min, then cooled down to room temperature (see the procedure described in [START_REF] Hamdani-Devarennes | Calcium and aluminium-based fillers as flame-retardant additives in silicone matrices II. Analyses on composite residues from an industrial-based pyrolysis test[END_REF] ). The obtained black residue pieces (later named black ceramic powder or BCP) were then ground into powder. Furthermore, in order to prepare carbon-free residue, the same elastomer parts were pyrolyzed under air at 600 °C overnight. The obtained white residue was then ground into powder and pyrolyzed again overnight (still in air) to insure the total removal of the remaining carbon and finally obtain a white silica powder (abbreviated SP in the following).

Elastomers' preparation

The PDMS gum and a given filler were blended in a three-roll mill (EXAKT Advanced Technologies) for 20 min to obtain a homogenous paste and then, DBPMH was introduced at about 1% relatively to silicone fraction in the paste. The obtained mixture was vulcanized under thermal compression (8 MPa, 160 °C) for 30 min as in conventional high temperature vulcanization (HTV) procedure. Samples were then post-cured in an oven at 180 °C for 2 h. The addition of SP or BCP was adjusted to achieve a final filler content of 10, 30 and 50 wt%. For samples with mixed fillers, the PDMS was blended separately with 30% of either SP or BCP filler, and then the resulting silica-paste and ceramic-paste were mixed together in 25/75, 50/50 and 75/25 wt%. For comparative experiments, the PDMS gum was also blended with 30% of either Aerosil A200 or Sipernat 22S. Crosslinking procedure was the same for all samples.

Characterization methods

Fourier-transform infrared spectroscopy (FTIR) spectra were collected on a Bruker VERTEX 70 spectrometer with a diamond ATR probe at room temperature, and 32 scans were accumulated at a resolution of 4 cm -1 .

X-ray diffraction analyses (XRD) were performed at room temperature by using a BRUKER D8 Advance X-ray diffractometer. The X-ray beam radiation (Cu K α, 1.542 Å) was operated at 40 kV and 40 mA. Data were obtained in the range of 2 θ from 5 to 70 °. Solid-state nuclear magnetic resonance (NMR) analyses of aerobic and anaerobic residues were carried out on an AVANCE III 500WB spectrometer at 99.34 MHz ( 29 Si NMR) and 125.73 MHz ( 13 C NMR). Cross polarization magic angle spinning (CPMAS) and high-power proton decoupling (HPDEC) were performed on both nuclei. 29 Si CPMAS experiments were performed using a contact time of 5 ms, a recycling time of 5 s, a spinning rate of 10 kHz and 10 0 0 scans. In the 13 C CPMAS experiments, the contact time was 3 ms, the recycling time 5 s, the spinning rate 10 kHz and the number of scans 5100. 29 Si HPDEC experiments were also performed using a recycling time of 120 s, a spinning rate of 10 kHz and 1700 scans. In 13 C HPDEC experiments, the recycling time was 60 s, the spinning rate 10 kHz and the number of scans 400.

The composition of aerobic and anaerobic residues was investigated on an X-ray photoelectron spectrometer (XPS, PHI Quantera SXM) with a monochromatic Al-K α source. The detection angle was 45 °and the analyzed area diameter was 200 μm. The detection limit was estimated at about 0.1 to 0.5 atomic% and the accuracy of the quantitative analysis at about 2 to 5% depending on the content.

The morphology of the filled silicones was observed using a scanning electron microscope (SEM, FEI Quanta 200). Micrographs were obtained under high vacuum at a voltage of 12.5 kV, with a working distance of 10 mm.

Hardness Shore A was measured on elastomers using a ZwickRoell (attached) durometer. Three measurements were applied on each of the samples (thickness > 6 mm). Hardness values were recorded after 30 s of applying the device on the materials.

Thermogravimetric analyses (TGA) were performed under nitrogen (flow rate 90 mL.min -1 ) using a Q500 apparatus from TA instruments. Samples (5 -8 mg) were heated in platinum crucibles from 30 to 900 °C at a heating rate of 10 °C per minute.

Swelling experiments

Crosslink density and extractability of the prepared elastomers were evaluated through swelling experiments. About 0.3 g of an elastomer sample (W i ) was immersed for 5 days (until equilibrium weight was considered to be reached) at room temperature in 5 mL of methylcyclohexane (MCH). The swelled sample was gently wiped to remove the liquid solvent present on the sample surface and weighted (W s ) then dried under vacuum overnight at 70 °C and weighted again (W f ). The extractable fraction (E) and the molar mass between crosslinks in the material (M c T ) were calculated as described by Delebecq et al. [START_REF] Delebecq | Looking over liquid silicone rubbers: (1) network topology vs chemical formulations[END_REF] according to Flory -Rehner equation [START_REF] Flory | Statistical mechanics of cross-linked polymer networks I. Rubberlike elasticity[END_REF]. This molar mass considers both physical attachements, i.e. hydrogen bonds between silica surface and polymer backbone, and chemical nodes between PDMS chains. Swelling experiments were repeated under ammonia fumes to break the weak interactions between silica and PDMS chains [START_REF] Vondráček | Ammonia-induced tensile set and swelling in silica-filled silicone rubber[END_REF][START_REF] Vondráček | NH 3 -modified swelling of silica-filled silicone rubber[END_REF]. The molar mass between the chemical crosslinks (M c Ch ) was calculated as explained above. Each experiment was performed three times to ensure the reproducibility of the results.

Tens ile testing

Tensile testing on elastomers were performed on an Instron 4469 apparatus according to ASTM D412. The dog bone-shaped samples had an effective length of 20 mm, a width of 4 mm, and a thickness of about 2 mm. The tensile measurements were performed using self-tightening roller tensile grips, long travel extensometer and pulling speed of 500 mm.min -1 until sample failure. Five samples were tested on each of the prepared elastomers. The plotted tensile curves were selected as the best performance obtained by each elastomer.

Cone calorimeter

Fire behavior of silicones was assessed on a FTT cone calorimeter according to ISO 5660. 100 × 100 × 4 mm 3 samples were burnt horizontally in air using an incident heat flux of 50 kW/m 2 in presence of a spark igniter. Each test was performed twice to ensure the reproducibility of the results. The heat release rate (HRR) was calculated according to the Huggett's equation [START_REF] Huggett | Estimation of rate of heat release by means of oxygen consumption measurements[END_REF] which states that 1 kg of consumed oxygen corresponds to 13.1 MJ of heat release. The balance support of the apparatus enabled following the evolution of the mass loss during the experiment. The residue yield originating from the PDMS part of the elastomer was calculated as the following

Residue = W f -W f il l ers W PDMS × 100
where W f is the weight of the sample fully burned, W fillers and W PDMS are the weights of the fillers and PDMS in the elastomer before burning.

Thermal aging

Samples with 30% of different fillers were chosen to evaluate the aging properties of PDMS elastomers, together with pristine RTV. For this, five dog-bone shaped specimens were conditioned at 260 °C for 4 weeks consecutively. The samples were weighted before and after aging so as to measure the weight loss. Aging properties were assessed by tensile testing like described previously.

Results

Pyrolyzed powders

Preparation

The aerobic and anaerobic degradations of a commercial RTV silicone led to the formation of white and black powders, respectively. We have selected a specific grade that contains both vinylated silica and platinum, so that ceramization is greatly enhanced under inert atmosphere (see TGA spectrum in Figure S1). In aerobic degradation, we have pyrolyzed two times the materials, once using the original RTV material and one after grinding the residue, to remove all traces of carbon in the final powder (vide infra). The residues yield after aerobic and anaerobic degradation were 73 and 66%, respectively.

Characterization

FTIR spectra of both powders ( Fig. 1 a) were found close to the spectrum of a commercial silica (Aerosil A200). Two absorption bands were observed in the regions of 10 0 0 -130 0 cm -1 and around 800 cm -1 assigned to the Si -O asymmetric stretching vibration [ 31 , 32 ]. The band corresponding to the asymmetric stretching vibration of Si -C is detected only for black ceramic, as expected. XRD analysis showed that the two prepared powders are amorphous with some metal oxide impurities originating from the preparation process ( Fig. 1 b). The main peak at 2 θ around 20 °entails all Si -O bonds of the materials. The shoulder prolonging to 40 °in BCP indicates a large presence of SiOC units in the pyrolyzed ceramic.

XPS elemental analysis ( Fig. 2 ) and solid NMR characterization ( Figs. 3 andS2) were then confronted to provide finer information on the chemical structure of the pyrolyzed materials. 29 Si CPMAS and HPDEC routines were used here, the first non-quantitative but more sensitive technique to track the main peaks, and the second one to produce quantitative results (only the latter are commented below).

For the white powder, the surface of the ground sample presented an average composition of 28.3 ± 0.1 at% of silicon, 69.9 ± 0.3 at% of oxygen and some carbon traces around 1.8 ± 0.2 at% (at% = atomic percentage) ( Fig. 2 b). The Si2p peak shows a single component at 103.4 eV corresponding to silica [ 20 , 21 ]. The O1s peak decomposition shows the presence of two components where the majority of oxygen atoms (around 87%) at 532.7 eV corresponds to the Si-O bonds and the rest at 534.2 corresponds to adsorbed water. Despite the intensity of the noise, the weak C1s peak decomposition shows that carbon can be divided into three components: the first peak at 284.5 eV corresponding to Si -C or C -C bonds (around 70 at%), the second at 286.4 eV corresponding to C -O bonds (23 at%) and the third peak at 289.1 eV corresponding to O -C -O bonds (7 at%) [START_REF] Liu | Efficient organic-to-inorganic conversion of polysiloxane by novel platinum-thiol catalytic system[END_REF] . This is a clear indication that low content of residual carbon is linked mostly to silicon atoms without affecting the silica structure (vide infra). These observations are in accordance with 29 Si HPDEC 10 KHz solid state NMR ( Fig. 3 b) that shows Q units of silicon into three sub-forms: 6 mol.% of Q 2 at -90 ppm, 28 mol.% of Q 3 at -99 ppm and 66 mol.% of Q 4 at -108 ppm.

XPS elemental analysis performed on the surface of the black powder determined the presence of 27.8 ± 0.2 at% of silicon, 57.2 ± 0.3 at% of oxygen and 15.0 ± 0.2 at% of carbon ( Fig. 2 c). The Si2p peak shows two types of silicon units which are 67 at% of Q units at 103.4 eV and 33% of D units at 102.1 eV [ 20 , 21 ]. The O1s peak shows two components which are 85% of Si-O bonds at 532.6 eV and 15% corresponding to adsorbed water at 534.2 eV. The C1s peak decomposition shows that the carbon signal can be divided into five components, among which main ones are at 283.4 eV (48% of total carbon) that corresponds to an undefined structure of carbon, and the second (40% of total carbon) at 284.6 eV corresponding to Si -C or C -C bonds. The other three minor forms of carbon at 286.4 eV (8%), 287.6 eV (1%) and 288.9 eV (3%) are due to different types of C -O bonds [START_REF] Liu | Efficient organic-to-inorganic conversion of polysiloxane by novel platinum-thiol catalytic system[END_REF] (see Fig. 2 c for details). 29 Si HPDEC solid state NMR showed that silicon atoms are present under their four forms: 21% corresponding to M units at 0 ppm (SiOC 3 ), 18% of D units at -29 ppm (SiO 2 C 2 ), 26% of T units at -69 ppm (SiO 3 C) and 35% of Q units at -100 ppm (SiO 4 where Q 3 : 27.5% and Q 4 : 7.5%) ( Fig. 3 d). The 13 C CPMAS spectrum did not show peaks specific to a molecule or a group (Figure S2). Nevertheless, it was observed very broad bands in 2 areas of the spectrum, sp3 carbons between 10 and 50 ppm and sp2 carbons between 110 and 150 ppm, specific of aromatic compounds. The sp3 type of carbon could be mainly attributed to C -Si or C -C bonds observed in XPS while the sp2 type of carbon is attributed to aromatic compounds that could correspond to the unknown structure observed in XPS. Based on Kleebe and coworkers' works on the composition and the structure of RTV silicone residues obtained by pyrolysis under inert atmosphere [ 33 , 34 ], the appearance of aromatic compounds at 800 °C indicates the generation of a separate carbon phase from the ceramic one. Indeed, at this temperature, the pyrolysis induces a polymerization reaction of the methyl groups associated with removal of hydrogen atoms and a condensation reaction leading to the formation of aromatic compounds (as graphene for example).

In summary, XPS and 29 Si solid state NMR showed that the pyrolysis of an RTV elastomer under air proceeds via an oxidative crosslinking mechanism to essentially form large content of silica [START_REF] Liu | Remarkable improvement of organicto-inorganic conversion of silicone rubber at elevated temperature through platinum-nitrogen catalytic system[END_REF] . In the following parts of this study, this white powder is referred to as "silica powder", abbreviated as SP. The anaerobic degradation of same RTV led to carbon-carbon interchain crosslinking, due to the lack of oxygen, and produced a SiOC ceramic thanks to the synergic effect of silica and platinum. In addition, an independent aromatic structure of carbon a priori formed during the degradation process [START_REF] Delebecq | High residue contents indebted by platinum and silica synergistic action during the pyrolysis of silicone formulations[END_REF] . This "black ceramic powder" of rather complex structure is denominated BCP in the following.

New filled-elastomers

Preparation and microstructure

The two powders were introduced into a silicone gum via a three-rolls mill and crosslinked by peroxide activation to generate new HTV-type elastomer materials (see experimental part for details). All formulations tested in this study are summarized in Table 1 .

Macroscopically speaking, homogeneous materials were produced with respective color induced by the filler ( Figs. 4 a and 4 b). By SEM, both fillers were shown to have good compatibility with PDMS matrix ( Figs. 4 c and4 d). They exhibit random shapes with cutting edges, although XRD revealed that both fillers are amorphous, as anticipated from the moderate maximum temperature applied on pyrolysis (800 °C). As shown on SEM photos, the size distributions of fillers are qualitatively quite large, with particle sizes of silica powder estimated between 12 and 15 μm, and homogenous distribution within the matrix. On the other hand, we observed mainly two size categories for BCP elastomer, the first one is less than 5 μm while the second is around 20 μm. In fact, during the preparation of the pastes, the fillers are crushed between the two rolls so that the maximum size of the fillers is about 20 μm.

Crosslink density

Swelling experiments were carried out in normal atmosphere or under ammonia fumes, the latter which breaks interactions between the silanol groups on the filler and the silicone chains. All results are summarized in Table 2 . For SP samples, M c T (M c total, obtained in normal atmosphere) decreases from around 17,500 to 6400 g/mol when silica powder fraction increased from 10 to 50%, respectively. Chemical M c Ch (under ammonia fumes) shows the same trend with a systematic difference of 2500 g/mol compared to M c T for all samples. Interactions between silica and PDMS chains do not affect much the M c T , as expected from a filler with a relatively low specific surface (according to the average size of the filler particles). On the other hand, the results show that the crosslink density increases with silica content, which was an unexpected result. The extractable amounts were found constant, around 4% at all silica contents, indicating mostly full crosslink- Note that the black spots observed on the SEM micrographs are ascribed to an uncomplete metallization (see also Figure S3).

Table 2

Data collected from swelling tests in methylcyclohexane (MCH) under air atmosphere and under ammonia fumes (swelling content and extractives in %) as well as crosslinking density evaluation (total molar mass between two nodes M c T and molar mass between two chemical crosslinks M c Ch ). ing of the elastomer. Note that we checked that a formulation not containing peroxide precursor does not crosslink in the same conditions since we obtained after tentative vulcanization the same viscoelastic paste as originally prepared (not shown). Materials containing a PDMS matrix filled with black ceramic were difficult to crosslink. Molar masses between two chemical crosslinks in PDMS/BCP elastomers increase exponentially from around 25,0 0 0 to around 30 0,0 0 0 g/mol at increasing BCP content (from 10 to 50%) whereas the extractable amounts rises from 4% to around 49%. This is a clear indication that black ceramic impeded PDMS crosslinking. The black ceramic is composed in part of a carbon phase containing aromatic carbon rings that are known to kill radicals [START_REF] Bagdasar'ian | Polymerization inhibition by aromatic compounds[END_REF] , which may thus explain such difficult curing step. Typically, the sample of PDMS filled by 50% was not enough crosslinked to be further considered as a material. Note finally that at 10% of black ceramic, physical interactions seem to be negligible contrary to what was observed for SP-filled materials. Mixing both fillers in PDMS matrix led to a behavior similar to elastomers filled with silica. The molar mass between two chemical crosslinks is around 13,0 0 0 g/mol for PDMS filled with 30% silica only and rises linearly from around 14,0 0 0 to around 19,0 0 0 g/mol relatively to black ceramic fraction. M c T versus M c Ch is only about 20 0 0 g/mol difference, as for silica-filled samples. Finally, extractables amounts are less than 4% in these mixed formulations. Hence, it shows that even a small amount of silica in these elastomers is enough to allows a full crosslinking of the samples even in presence of black ceramic.

MCH

Mechanical properties

A summary of the different techniques and data gathered from this part is given in Table 3 .

First, materials' hardness was measured for all samples. Those containing SP showed fair hardness value improved by increasing SP content ( Table 3 ) to reach a value of 55 Shore A at 50% SP.

Tensile properties also clearly rised with SP contents ( Fig. 5 ). Stress at break increased from 0.59 ± 0.04 to 1.62 ± 0.11 and 2.82 ± 0.10 MPa for 10, 30 and 50% of SP, respectively. Strain at break was about 250% for all three samples. This shows that silica hardens the PDMS matrix thus leading to high mechanical properties [START_REF] Jones | Silicon-containing polymers[END_REF] .

For elastomers filled with black ceramic, stress at break increased from 0.69 ± 0.11 to 1.36 ± 0.05 MPa for materials filled with 10 and 30% of BCP, but in the meantime, strain at break increased from 450 ± 90 to 1050 ± 150%, respectively. At 50 wt% of ceramic filler, the material was so soft that tensile properties could not be assessed. Besides, hardness values were maintained around 17 Shore A ( Table 3 ). As discussed previously, black ceramic partly inhibits PDMS vulcanization that in fact limits the final mechanical properties of the elastomers (particularly tensile strength).

Tensile properties of materials prepared with mixed fillers were also studied ( Fig. 5 b, Table 3 ). Stress at break plateaued around 1.60 ± 0.10 MPa, whereas strain at break increased slightly while substituting a fraction of silica powder by black ceramic powder. These results are in accordance with the swelling observations. The presence of a small content of silica powder in the initial formulation was enough to reach a full vulcanization of the material and thus to bring sufficient toughness to the materials. As the black ceramic fraction increased, the crosslinking density declined, which can explain the slight rise of strain at break. Additionally, shore A hardness slightly declined because black ceramic softened the material ( Table 3 ).

Note that samples prepared with commercial silicas logically exhibited better tensile properties, thanks to their larger specific surface and thus higher reinforcing behavior [START_REF] Shit | A review on silicone rubber[END_REF] (Figure S4).

Thermogravimetric analyses

The different elastomers were analyzed by TGA under nitrogen atmosphere ( Fig. 6 and Table 4 ). Pristine (uncrosslinked) PDMS gum presents mainly one degradation step (according to the derivative curve -Fig. 6 b). The onset temperature of PDMS was observed at 400 °C and residual weight was negligible. Both incorporation of SP into silicone gum and crosslinking reaction increased significantly the thermal stability of the final material. Onset temperature rose to 410, 440 and 470 °C in accordance with incorporated silica rate into the elastomers, respectively 10, 30 and 50 wt% of SP. In a similar manner, thermal stability was much improved in presence of BCP. It rose to 460, 490 and 520 °C, respectively, while increasing the amount of black ceramic from 10 to 30 and 50 wt%. A significant jump of + 120 °C is thus observed between pristine PDMS and material at 50 wt% of BCP.

Looking at the derivatives of TG curves, one sees that the degradation process changed from silica-to black ceramic-based materials. A first degradation step occurs around 560 °C while a second one takes place around 660 °C. According to the intensities of the peaks in the derivative curves, the first degradation step of PDMS/BCP elastomers is less pronounced than in the case of PDMS/SP, whereas second peaks are higher to finally lead to almost similar residues. Therefore, black ceramic stabilized thermally the silicone elastomers and shifted the maximal degradation to higher temperatures. A slight increase in residual weight of about 4 to 8% was noticed compared to the expected one when considering the introduced filler content ( Fig. 6 a), especially with high amount of fillers, a priori thanks to the presence of residual platinum from RTV [START_REF] Delebecq | High residue contents indebted by platinum and silica synergistic action during the pyrolysis of silicone formulations[END_REF] , without any influence of the composition of the fillers ( Table 4 ). The effect of mixing black ceramic and silica powders on the thermal stability of PDMS was carefully studied ( Fig. 6 c). Elastomers with mixed fillers showed a two-step degradation process occurring at the same temperatures as mentioned previously for mono-filled samples. It was clearly observed that increasing the content of black ceramic in the mix would decrease the first degradation step intensity and increase the second one (see derivative curves in Fig. 6 d,Table 4 ). Moreover, increasing the amount of black ceramic in the mixes would increase the onset temperature of the material. In the following, the sample containing 30% of mixed fillers (25% silica and 75% black ceramic powders) was particularly selected for further tests as it shows both fair mechanical properties and high thermal stability.

The effect of other types of commercial filler on thermal properties of PDMS was also studied here for comparison (Figure S5 and Table 4 ). Onset temperatures of elastomers shifted only by + 30 to + 60 °C when filled by commercial silicas compared to PDMS, while in the case of recycled silica and black ceramic samples, it shifted by + 40 to + 90 °C for the same amount of fillers ( Table 4 ). Moreover, a single degradation step was observed around 560 °C for the elastomers filled with commercial silicas, alike PDMS case, which occurs at the same temperature as the first degradation of elastomers containing pyrolyzed fillers (Figure S5). All those results show a clear improvement of the thermal stability of elastomers filled with recycled powders in comparison to commercial silica.

Thermal aging resistance

Dog-bone shaped specimens of elastomers filled by 30% of several fillers were conditioned at 260 °C for 4 weeks consecutively. After that, samples were weighted and evaluated by tensile testing ( Table 5 , Figure S6). All samples clearly lost most of their properties in such harsh conditions, but differences could still be stressed out. Samples containing pyrolyzed fillers showed the best aging resistance among the tested fillers. Weight losses were between 7.5 and 9.5% after 1 month at 260 °C. Samples filled by 30% of SP showed the best mechanical properties after aging. The tensile properties of PDMS/SP went from strain at break of 310% and a corresponding stress of 1.6 MPa to 70% and 0.8 MPa respectively after aging ( Table 5 and Figure S6). Moreover, samples of PDMS/BCP and PDMS/mixed fillers had higher losses of properties to reach 40% and 0.7 MPa of strain and stress at break after aging.

For comparison, the Aerosil elastomer did not withstand thermal aging; it lost around 17 wt% during the aging process and became breakable during the preparation for tensile testing. The Sipernat elastomer had a better behavior with weight loss close to the other fillers, but sharp decrease of strain at break (compare Figure S4 and Figure S6). Pristine RTV lost around 15% in weight after 4 weeks of aging and the specimens were very brittle and breakable immediately when preparing tensile test. In summary, the pyrolyzed fillers brought enhanced resistance to long term thermal aging compared to other conventional materials.

Fire behavior

The influence of silica and black ceramic powders on the flame retardancy of corresponding elastomers was tested by cone calorimeter. In addition, the material prepared with mixed fillers (25% SP -75% BCP) was carefully chosen among this series of samples according to its best mechanical and thermal properties ( Fig. 7 and Table 6 ). For comparison, the materials prepared from the two commercial silica fillers have also been tested, as well as the pristine RTV (see Figure S7 and Table 6 ).

Elastomers filled with Aerosil A200 and Sipernat 22S ignited after 73 ± 0 s and 57 ± 3 s of exposure to heat flux respectively. The pHRR was reached 40 s after ignition and HRR maintained at around 200-220 kW/m 2 for around 150 s. After this plateau, HRR decreased, surely thanks to an insulating layer produced on the top of the sample (see white solid platelets in Figure S8) until the flame-out. THR was calculated around 19 kJ/g PDMS and residue yield around 40% (see experimental part for calculation). This residue yield calculation is specific to silicone materials since silicon-based volatiles oxidize during the test, resulting in an amorphous silicaash deposition on the surface of the samples [START_REF] Hshieh | Shielding effects of silica-ash layer on the combustion of silicones and their possible applications on the fire retardancy of organic polymers[END_REF] . The same general behavior was observed with pristine RTV, which ignited after 73 ± 2 s before reaching a plateau at HRR around 140 kW/m 2 for around 140 s, except that residue was much larger (around 70%) and THR smaller (12 kJ/g PDMS ).

Samples with pyrolyzed fillers (silica, black ceramic or a mix of both) exhibited a behavior significantly different ( Fig. 8 and Table 5 ). BCP-filled elastomers ignited relatively faster with a time-toignition around 40 -45 s for 30% of fillers. Few seconds after TTI, the pHRR reached 170-200 kW/m ² depending on the nature and the content of fillers, i.e. slightly lower than the values obtained for elastomers filled with commercial silica. But the pHRR decreased sharply and no HRR plateau was observed. This evidences that the insulating layer was forming faster and then protected more efficiently the underlying material than commercial silica. The THR was found around 16.5 kJ/g PDMS independently of the fillers' composition. Moreover, the residue yield for filler-recycled elastomers ranged between 46 and 51% relatively to silicone gum weight. This value is higher than for elastomers filled with commercial silicas.

Discussion

In this work, we have proposed to pyrolyze a commercial silicone RTV into two different powders to be tested as fillers in new high temperature vulcanization (HTV) formulations. Depending on the surrounding atmosphere, air or nitrogen respectively, either silica or black ceramic powders were prepared in quantitative yield. They were fully characterized by different techniques (ATR-FTIR, XPS and NMR) to confirm the overall microstructure of the residues. In the anaerobic process, a significant content of aromatic carbons was generated in addition to the SiOC ceramic, a result that was previously described [ 19 , 25 ]. Whereas, the aerobic process led in two steps to almost full silica material.

After grinding, both fillers were introduced in a high molar mass silicone gum to generate homogeneous materials (accord- ing to SEM photos) after peroxide crosslinking. The obtained elastomers were then tested by conventional techniques (swelling, tensile tests). Two opposite behaviors were obtained in terms of crosslinking reaction. Increasing the silica powder content resulted in more crosslinked materials, and thus stronger materials both in hardness in tensile strength. Contrarily, black ceramic was shown to inhibit partially the crosslinking reaction, limiting its use at high content. When mixing both of the fillers, crosslinking proceeded smoothly and mechanical properties were close to silica-filled materials.

We suspect that silica could participate to the crosslinking reaction in one way or another, generating strong materials even without reinforcement (large particle flakes mean low specific surface). Removing the peroxide initiator from the formulation did not lead to crosslinking, so silica is not able to promote crosslinking by itself. Two hypotheses can then be brought: 1. Crosslinking of the chains with the surface group of silica naturally reinforc-ing the materials; 2.'boosting' effect of silica powder towards the generation of radicals and thus enhanced crosslinking reaction. We have shown that inhibition induced by the black ceramic could be suppressed when adding even a small amount of silica. Also, the chemical crosslinking density of all samples containing SP is in direct correlation with the silica content ( Fig. 8 ). Finally, the extractive content decreased with silica powder content in both simple filler or mixed filler samples and these were found much lower than generally observed in peroxide-crosslinked materials. All this information pleads for an accelerating effect of radical generation when introducing silica powder. The chemistry behind that phenomenon still remains to be understood.

Mechanically speaking, some correlations can be established, independent of the content and nature of pyrolyzed fillers. We observed that the strain at break rises rather linearly when the molar mass between two chemical crosslinks increases, showing that the network density correlates well with the plasticity of the materials ( Fig. 9 a). Moreover, the tensile strength rises linearly with hardness shore A, both parameters depending directly on the strength of the elastomers ( Fig. 9 b). We also showed that the content of extractive materials was slightly depressed in mix-filled elastomers compared to SP and BCP filled ones, showing a synergy of properties between the two fillers in the mix material.

Looking at thermal properties, TGA showed that the black ceramic significantly shifted the onset of degradation of the materials of more than 100 °C, whereas the white silica powder increased the residue of only 50 °C. In the mix filled systems, a direct linear correlation is observed between main transitions and composition ( Fig. 10 ). Note that filler-recycled elastomers' TGA residues were only slightly increased, showing that platinum traces from RTV precursor hardly promoted ceramization of the new recycled materials. To compare these results with literature data, we have reported in Table 4 the temperature of degradation of 10% of the sample (T 10 ). Liu and coworkers [ 21 , 22 ] improved the T 10 values of silicones by 40 °C by adding Pt (Karstedt catalyst) and nitrogenous silane systems while Liu and coworkers [START_REF] Liu | Efficient organic-to-inorganic conversion of polysiloxane by novel platinum-thiol catalytic system[END_REF] improved it by 20 °C by adding 50 ppm of Pt (Karstedt catalyst) and 0.2 phr of (3mercaptopropyl)triethoxysilane. Here, new pyrolyzed fillers clearly improved the thermal stability of corresponding elastomers. During thermal aging, PDMS chains degrade to shorter oligomers in one hand, whereas on the other hand, radical coupling increases the crosslink density [START_REF] Liu | Remarkable improvement of organicto-inorganic conversion of silicone rubber at elevated temperature through platinum-nitrogen catalytic system[END_REF] . The result of these is an enhanced brittleness. For the two pyrolyzed fillers, the mechanical properties were greatly affected, particularly in terms of tensile strength, but still materials could be tested. Neither pristine RTV nor aerosil-filled sample sustained such long aging cycle, since their mechanical properties could not be tested.

Finally, flame retardancy studied by cone calorimetry. Even if the TTI is reached more rapidly for filler-recycled elastomers than conventional ones, the rapid generation of a thick barrier layer promoted slow burning, low heat release and a larger final residue yield. Such behavior is expected from the fact that introduced fillers are per se resistant to pyrolysis.

Conclusions

In this work, a new single-step solution of reusing RTV silicone waste was studied. Two types of residues were prepared and used as fillers in a silicone matrix. The silica powder brought some fair mechanical properties to the elastomers while the black ceramic incredibly improved TGA thermal prop-erties. Mixing both fillers in a single material brought sufficient mechanical properties while avoiding crosslinking inhibition observed for black ceramic formulations. The elastomer containing 30% of fillers (75% of black ceramic and 25% of silica) showed the best compromise in terms of mechanical and thermal properties. Moreover, these fillers improved the thermal resistance of elastomers compared to those containing commercial silicas. Finally, the filler-recycled elastomers prepared in this work showed better flame retardancy behavior than pristine RTV or elastomers filled with commercial silicas. It forms an insulating layer on the heat-exposed area earlier than commercial fillers which brings more protection to the matrix and results in less heat release. Overall, this work adds a new solution for one of the actual environmental issues by reducing silicone waste and reducing the use of raw materials for preparing fillers for silicone materials.

Fig. 1 .

 1 Fig. 1. (a) FTIR spectra of commercial silica Aerosil A200, silica and black ceramic powders and best gaussian fit(s) of main peak at about 800 cm -1 ; (b) XRD analysis of silica powder and black ceramic powder.

Fig. 2 .

 2 Fig. 2. (a) XPS wide scans for the silica powder and black ceramic powder; (b) Si2p, O1s and C1s XPS spectra for the silica powder; (c) Si2p, O1s and C 1 s XPS spectra for the black ceramic powder.

Fig. 3 .

 3 Fig. 3. Solid state NMR 29 Si 10 KHz spectra: (a) CPMAS of silica powder; (b) HPDEC of silica; inset: spectral decomposition. (c) CPMAS of black ceramic; (d) HPDEC of black ceramic; inset: spectral decomposition.

Fig. 4 .

 4 Fig. 4. (a, b) Photographs of the elastomers containing 30 wt% of silica powder (a) and black ceramic powder (b) after peroxide crosslinking; (c, d) Corresponding SEM micrographs of elastomers filled by silica powder (c) and black ceramic powder (d).Note that the black spots observed on the SEM micrographs are ascribed to an uncomplete metallization (see also FigureS3).

Fig. 5 .

 5 Fig. 5. Stress -strain curves of PDMS/SP or BCP elastomers (a) and PDMS/mixed fillers elastomers (b).

Fig. 6 .

 6 Fig. 6. Influence of fillers ratio on the thermal stability of the different elastomers prepared in this study: (a) TG curves of monofiller samples; (b) DTG curves of monofiller samples; (c) TG curves of mix-filled samples; (d) DTG curves of mix-filled samples.

Fig. 7 .

 7 Fig. 7. Influence of pyrolyzed fillers' type and content on HRR curves of PDMS elastomers (heat flux 50 kW/m 2 ).

Fig. 8 .

 8 Fig. 8. Influence of the silica powder content on the M C Ch for SP and mixed fillers elastomers.

Fig. 9 .

 9 Fig. 9. Influence of the network crosslinking on the strain at break versus M c T (a) and the hardness shore A versus stress at break (b) for all filler-recycled elastomers prepared here.

Fig. 10 .

 10 Fig. 10. Influence of silica powder and black ceramic powder content on onset temperature and temperature at 10% and 50% of degradation of PDMS.

Table 1

 1 All formulations tested in this study (as expressed in wt%).

		PDMS gum	Silica powder (SP)	Black ceramic Powder (BCP) Commercial Silica	DBPMH
	PDMS + 10% SP	89.20	9.91		0.89
	PDMS + 30% SP	69.51	29.79		0.70
	PDMS + 50% SP	49.75	49.75		0.50
	PDMS + 10% BCP	89.20		9.91	0.89
	PDMS + 30% BCP	69.51		29.79	0.70
	PDMS + 50% BCP	49.75		49.75	0.50
	PDMS + 30% mix (75% SP -25% BCP)	69.51	22.34	7.45	0.70
	PDMS + 30% mix (50% SP -50% BCP)	69.51	14.90	14.90	0.70
	PDMS + 30% mix (25% SP -75% BCP)	69.51	7.45	22.34	0.70
	PDMS + 30% Aerosil A200	69.51		29.79	0.70
	PDMS + 30% Sipernat 22S	69.51		29.79	0.70

Table 3

 3 Mechanical properties data: Hardness Shore A and tensile testing of the different elastomers.

		Hardness Shore A	Stress at break (MPa)	Elongation at break (%)
	PDMS + 10% SP	19 ± 1	0.59 ± 0.04	240 ± 20
	PDMS + 30% SP	31 ± 1	1.62 ± 0.11	310 ± 30
	PDMS + 50% SP	55 ± 0	2.82 ± 0.10	240 ± 10
	PDMS + 10% BCP	15 ± 1	0.69 ± 0.11	450 ± 90
	PDMS + 30% BCP	17 ± 1	1.36 ± 0.05	1050 ± 150
	PDMS + 30% mix (75% SP -25% BCP)	30 ± 1	1.68 ± 0.08	330 ± 20
	PDMS + 30% mix (50% SP -50% BCP)	27 ± 1	1.57 ± 0.04	350 ± 20
	PDMS + 30% mix (25% SP -75% BCP)	26 ± 1	1.51 ± 0.13	360 ± 30

Table 4

 4 Main data measured by TGA.

		T onset ( °C)	T 10 ( °C)	T max,1 ( °C)	T max,2 ( °C)	Peak dTG 1 (%/min)	Peak dTG 2 (%/min)	Residue at 800 °C (%)
	PDMS	400	420	510	N/A *	1.3	N/A *	0.3
	PDMS + 10% SP	410	450	580	N/A *	0.6	N/A *	11
	PDMS + 30% SP	440	480	560	660	0.3	0.2	38
	PDMS + 50% SP	470	510	560	660	0.2	0.2	54
	PDMS + 10% BCP	460	500	560	630	0.4	0.6	13
	PDMS + 30% BCP	490	530	550	660	0.2	0.6	36
	PDMS + 50% BCP	520	560	ND *	670	ND *	0.4	56
	PDMS + 30% mix (75% SP -25% BCP)	450	495	560	660	0.3	0.3	33
	PDMS + 30% mix (50% SP -50% BCP)	460	505	570	660	0.3	0.4	34
	PDMS + 30% mix (25% SP -75% BCP)	470	510	570	660	0.2	0.5	35
	PDMS + 30% Aerosil A200	460	490	570	N/A *	0.7	N/A *	32
	PDMS + 30% Sipernat 22S	430	480	560	N/A *	0.6	N/A

*

29 * ND = not determined; N/A = not available.

Table 5

 5 Tensile data before and after aging of samples containing 30% of fillers and pristine RTV.

			Before thermal aging		After thermal aging
			Stress at break (MPa)	Strain at break (%)	Stress at break (MPa)	Strain at break (%)	Weight loss (%)
	Filler type	SP	1.62 ± 0.11	310 ± 30	0.81 ± 0.04		70 ± 7	9.5 ± 0.4
		BCP	1.36 ± 0.05	1050 ± 150	0.70 ± 0.04		40 ± 2	7.6 ± 0.0
		Mix (25% SP -75% BCP)	1.51 ± 0.13	360 ± 30	0.73 ± 0.02		40 ± 1	7.9 ± 0.1
		Aerosil A200	8.67 ± 0.50	350 ± 20	NM *		NM *	17.1 ± 0.0
		Sipernat 22S	8.07 ± 0.54	330 ± 30	1.96 ± 0.07		20 ± 2	7.5 ± 0.3
	Pristine RTV	6.26 ± 0.34	600 ± 20	NM *	NM *		14.9 ± 0.1
	Table 6
	Main data from cone calorimeter.
			TTI (s)	pHRR (kW/m ²)	THR (kJ/g)	THR (kJ/gPDMS)	Residue (%)
	PDMS + 10% SP		59 ± 2	201 ± 11	17.1	19.0		45
	PDMS + 30% SP		46 ± 8	198 ± 13	11.4	16.3		46
	PDMS + 50% SP		44 ± 5	167 ± 19	7.7	15.3		59
	PDMS + 10% BCP	27 ± 4	198 ± 21	15.8	17.6		39
	PDMS + 30% BCP	39 ± 3	186 ± 11	11.6	16.5		50
	PDMS + 30% mix (25% SP -75% BCP)	45 ± 1	169 ± 18	11.5	16.4		51
	PDMS + 30% Aerosil A200	73 ± 0	219 ± 4	13.4	19.2		41
	PDMS + 30% Sipernat 22S	57 ± 3	202 ± 12	12.8	18.2		37
	Pristine RTV		73 ± 2	142 ± 4	9.0	12.7 a	71 a
	a on the basis of 29 wt% of silica in the RTV.

* NM = Non-measurable.

Acknowledgement

The authors acknowledge Loïc DUMAZERT, Gwenn LE SAOULT and Antoine ISHAK, from IMT -Mines Alès for their support on cone calorimeter and XRD analyses and the preparation of black ceramic powder. The authors also acknowledge Carol GROSSIORD from Science et Surface for her support on XPS analyses. Fernande DA CRUZ-BOISSON is acknowledged for her help interpreting NMR data, and Pierre ALCOUFFE for assistance on MEB analyses. RH was supported by a grant from ANR ( POLCADE ANR-17-CE07-0017 ).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.polymdegradstab. 2022.109947.