Raphaël Ménard

Claire Negrell-Guirao

Laurent Ferry

Rodolphe Sonnier

Ghislain David
email: ghislain.david@enscm.fr

Keywords: biobased flame retardants, epoxy networks, phosphorus-containing additives, POC-2014

Characterization of flame retardancy on epoxy

Introduction

Epoxy resins are widely used for their characteristics, such as excellent mechanical properties, adhesion or thermal resistance and have many applications in adhesive industry, coatings, electronic components and materials for energy. The flammability of these resins is one of their main drawbacks and implies the systematic introduction of flame retardant compounds (FR) in their formulation.

Phosphorus-containing FRs are known for their different mechanisms that occur during burning. They can act in gas and/or in condensed phases. Phosphorus atoms promote charring and the char formed acts as a barrier against heat and gas (O 2 and pyrolysis gases). Their action in condensed phase appears as a key parameter of the fireproofing material. An additional mode-of-action is flame poisoning. Organophosphorus compounds used as additives represent an important part of flame retardant market [START_REF] Walsh | ACS Symposium Series[END_REF] and are currently the most studied FRs mainly due to the restrictions of halogenated products imposed by REACH or RoHS regulations [START_REF] Reach | Commission of regulation (EC)[END_REF][START_REF] Rohs | Directive 2002/95/EC of the european parliament and of the council[END_REF]. It is likely to note that phosphorus FR release much less toxic compounds [START_REF] Lu | [END_REF] than halogenated ones.

The mode-of-actions of phosphorus-containing FRs depend upon several parameters and most often the action in the condensed phase is preferred. The aromaticity of the FR structure promotes the char formation during the thermal decomposition. The oxidation state of the phosphorus atom is also a key parameter. Indeed, some studies [5,6] showed that the higher the oxidation degree of phosphorus atom, the better the action in condensed phase. So, both phosphate and phosphonate are the most suitable phosphorus groups to promote charring during the thermal decomposition of the material.

Our previous study [7] achieved with phosphorus-containing oligomers allowed determining that an aliphatic FR used in an aromatic matrix led to a phase segregation which provides heterogeneous thermosets. Thus, the similarity between the FR and the matrix structure also appears as a key parameter to make available flame-retarded thermosets.

To the best of our knowledge, no biobased aromatic phosphorus-containing FR commercially available used in epoxy matrix [START_REF] Kyrioacos | Fire-retardants for polymers[END_REF]9] has been reported so far. Some studies found in the literature describe the synthesis of biobased phosphorus-containing FRs: Ma et al. [10] used the itaconic acid as bio-based resource and grafted the 9,10-dihydro-9-oxy-10-phosphaphenanthrene-10-oxide (DOPO) onto the C = C bond to afford both aromaticity and phosphorus group. Lligadas et al. [11] also grafted the DOPO onto fatty acid derivative.

However, some biobased resources naturally show an aromatic structure. Phenolic products coming from the depolymerization of lignins exhibit the required aromatic structure and are easily available. Indeed, lignins are the second most abundant raw material in the nature and the most abundant aromatic polymer. These phenolic products can be modified by functionalizing their reactive groups as aldehydes, carboxylic acids or hydroxy groups and these last ones can easily be functionalized via a Williamson etherification [12]. Using the same reaction, these reactive functions can also be grafted with phosphorus-containing compounds. Thus these products are good candidates to prepare some biobased FR compounds. Some studies found in the literature explored this way. Marosi et al. [13] carried out the selective phosphorylation and phosphinylation of hydroquinone and phloroglucinol with diethyl chlorophosphate and diphenylphosphonyl chloride by a nucleophile substitution reaction (SN2). Ren et al. [14] used the same reaction (SN2) to carry out the dimerization of catechol functionalizing the hydroxyl groups with the phenyl phosphonic dichloride to form a phosphorus-containing aromatic diphenol which is glycidylated to obtain a reactive phosphoruscontaining flame retardant.

The purpose of the present study is to synthesize and evaluate new biobased phosphate flame retardant additives for epoxy thermoset. To promote the charring effect, we have chosen to work with phosphate groups. Our strategy consists in functionalizing a biobased phenolic compound with different dialkyl chlorophosphate groups by a Williamson reaction. The chosen biobased resource for this study is the phloroglucinol thanks to its symmetrical structure, which allows the three hydroxyl functions to show the same reactivity. Different parameters were studied such as the influence of the R group present in P-O-R phosphonic ester or the thermoset phosphorus content.

The flame retardant properties of the synthesized FR and their impact on thermoset physical properties were assessed in an epoxy matrix commonly used, the diglycidylether of bisphenol A (DGEBA) cross-linked by the isophorone diamine (IPDA).

Experimental part

Reagents

Dimethyl chlorophosphate, diethyl chlorophosphate, diphenyl chlorophosphate, phloroglucinol, and potassium carbonate were purchased from Sigma-Aldrich. Triethyl amine was supplied from Acros organics. For epoxy resins components, diglycidylether of bisphenol A (DGEBA) resin and isophorone diamine (IPDA) as curing agent were purchased from Sigma-Aldrich.

Synthesis

Synthesis of phloroglucinol-tris(dimethyl phosphate): P3P(OMe)

Phloroglucinol (5.0 g, 0.040 mol, 1 eq) and potassium carbonate (21.9 g, 0.159 mol, 4 eq) (potassium carbonate was used to limit the water treatment during the purification step) were introduced with 60 mL of acetone in a 100 mL two-necked round-bottomed flask equipped with a stirrer, a rubber septum and a condenser connected to a high purity nitrogen source. The solution was heated at 60 °C under vigorous stirring. Dimethyl chlorophosphate (20.1 g, 0.139 mol, 3.5 eq) was introduced slowly. After 5 h reaction, potassium carbonate was removed by filtration, the filtrate solvent was evaporated and the residue taken up in a mixture of 150 mL of chloroform and 20 mL of water. The organic phase is dried with sodium sulfate and the solvent evaporated to obtain a yellow liquid. Yield: 60 %

P3P(OMe)

Synthesis of phloroglucinol-tris(diethyl phosphate): P3P(OEt)

Phloroglucinol (5.0 g, 0.040 mol, 1 eq) and triethyl amine (16.0 g, 0.159 mol, 4 eq) were introduced with 60 mL of acetone in a 100 mL two-necked round-bottomed flask equipped with a stirrer, a rubber septum and a condenser connected to a nitrogen source. The solution was heated at 60 °C under vigorous stirring. Diethyl chlorophosphate (24.0 g, 0.139 mol, 3.5 eq) was introduced slowly. After 5 h reaction, the system was filtered, the filtrate solvent was evaporated and the residue taken up in a mixture of 150 mL of chloroform and 50 mL of water. The organic phase was washed with water, dried with sodium sulfate and the solvent evaporated to obtain a brown oil. Yield: 66 %.

Synthesis of phloroglucinol-tris(diphenyl phosphate): P3P(OPh)

Phloroglucinol (5.0 g, 0.040 mol, 1 eq) and triethyl amine (16.0 g, 0.159 mol, 4 eq) were introduced with 80 mL of acetone in a 100 mL two-necked round-bottomed flask equipped with a stirrer, a rubber septum and a condenser connected to a high purity nitrogen source. The solution was heated at 60 °C under vigorous stirring. Diphenyl chlorophosphate (37.3 g, 0.139 mol, 3.5 eq) was introduced slowly. After 5 h reaction, the system is filtered, the filtrate solvent was evaporated and the residue taken up in a mixture of 200 mL of chloroform and 50 mL of water. The organic phase was washed with water, dried with sodium sulfate and the solvent evaporated to obtain a yellow solid. Yield: 74 %.

Thermosets preparation

Different thermosets were prepared with the different FRs synthesized to evaluate the influence of phosphorus content and the nature of the phosphate groups on the flame retardancy. Table 1 summarizes data for thermosets preparation.

The FR was solubilized in DGEBA under stirring at 80 °C for 10 min. The mixture was cooled at 50 °C to avoid any crosslinking upon mixing with the hardener. A stoichiometric epoxy/amine ratio (2/1) was used. The system was stirred manually for 5 min. Lastly the mixture was crosslinked in a circular rubber mold The bold values represent the weight % of phosphorus contained in the epoxy resin.

according to the following temperature program: 4 h at 80 °C and 10 h at 150 °C. Thermosets were named according to their phosphorus content followed by the used FR: "x%P(used FR)". The reference thermoset was the stoichiometric system DGEBA/IPDA (2/1).

Characterizations

Chemical structures of the prepared compounds were determined by 31 P-NMR, 13 C-NMR (IG) and 1 H-NMR spectroscopy in a Bruker Avance 400 MHz spectrometer at room temperature in (CD 3 ) 2 CO solutions. External references were trimethylsilane (TMS) for 1 H-NMR and 13 C-NMR and phosphoric acid (H 3 PO 4 ) for 31 P-NMR. Shifts are given in ppm. Chemical structures of P3P(OEt) and P3P(OPh) were confirmed by elemental analyses. -Thermal analyses were performed using samples grounded with cryogenic grinder SPEX6770.

-Thermogravimetric analyses were performed using a Pyris 1 TGA Perkin Elmer at a heating rate of 10 °C/min under nitrogen atmosphere. -Differential scanning calorimetry (DSC) analyses were carried out using a NETZSCH DSC200F3 calorimeter. Constant calibration was performed using indium, n-Octadecane and n-Octane standards. Nitrogen was used as the purge gas. The thermal properties were analyzed at 10 °C/min between 20 and 200 °C to observe the glass transition temperature determined as the inflexion value in the heat capacity jump and the residual curing.

Samples were observed using a FEI Quanta 200 scanning electron microscope and phosphorus content was determined by energy dispersive X-ray spectroscopy (EDX) using Oxford INCA Energy 300 system and a detector of 133 eV (at Mn). Samples were characterized after metallization in high vacuum sputtering metallizer Bal-Tec CED 030 Blatzer. Flammability was assessed using a pyrolysis-combustion flow calorimeter from Fire Testing Technology (FTT UK). About 3 mg was placed in the pyrolyzer, undergoing an increase of temperature from 20 °C to 750 °C at 1 °C/s under a nitrogen flow. Decomposition gases were sent to a combustor heated at 900 °C under air flow (N 2 /O 2 = 80/20). At this temperature and with 20 % of oxygen, combustion was considered to be complete. Heat release rate was determined by oxygen depletion according to Huggett principle [15] (1 kg of consumed oxygen corresponds to 13.1 MJ of heat released).

Results and discussion

Synthesis of phloroglucinol tris-[di-alkyl/aryl(R) phosphate] compounds

Williamson etherification [16] is a well-known organic reaction regarding the O-alkylation of an alcohol with halide, leading to an ether bond. This reaction was performed between chlorophosphate and biobased phloroglucinol and showed a similar reactivity of the three hydroxyl functions. The use of a chlorophosphate and phloroglucinol allows the formation of phosphonic ester functions between the aromatic ring and the phosphorus atom. The synthesis of phloroglucinol tris-(di-R phosphate) was carried out by using a phloroglucinol/chlorophosphate ratio of 1.0/3.5 (Scheme 1). The formation of the phenolate ion is usually carried out by the introduction of a conventional base but can also be performed by the presence of metal as zinc [17]. Triethylamine was chosen for the syntheses using diethyl-chlorophosphate and diphenyl-chlorophosphate to obtain phloroglucinol-tris(diethyl phosphate) (P3P(OEt)) and phloroglucinol-tris(diphenyl phosphate (P3P(OPh)), respectively. K 2 CO 3 was used as the base for the P3P(OMe) synthesis. The conversion of hydroxyl functions was complete for any R groups and the average yield of these three syntheses was around 70 %.

NMR characterizations

Phosphate functionalities of P3P(OMe) and P3P(OEt) were determined by 1 H-NMR (SI.1.) and the degree of purity was assessed by 13 C-NMR (SI.2.). Results show a total conversion of hydroxyl groups and pure products after purification. However, for the compound P3P(OPh) the signals of all the aromatic protons overlapped; hence, the 1 H-NMR spectroscopy does not allow determining the phosphate functionality. Quantitative 13 C-NMR spectra can be obtained by inverse gated (IG) decoupling and long accumulating delays of 60 s between two pulses. Thus, quantitative 13 C (IG)-NMR was used and showed a total conversion of hydroxyl groups into phosphate groups (Fig. 1).

These three FR P3P(OMe), P3P(OEt) and P3P(OPh) possess different phosphorus contents, 20.7 wt%, 17.4 wt% and 11.3 wt%, respectively. The FR contents incorporated into the epoxy resin were adjusted to range the phosphorus content in the thermoset from 1 to 4 wt%P/thermoset. and corresponds to the P-O bond scission followed by the formation of phosphonic anhydride [18]; while, the second one around 450 °C corresponds to the reorganization of the formed anhydride. This difference can be explained by the mesomeric donor effect of the aromatic ring which stabilizes the P-O bond until higher temperature and allows the two mechanisms to occur at the same temperature. The char amounts at 700 °C are also different: the P3P(OMe), P3P(OEt), P3P(OPh), which contain 20.7 wt%P, 17.5 wt%P and 11.3 wt%P, respectively, undergo 47.5 wt%, 27.0 wt% and 16.2 wt% of char. Thus the higher the phosphorus content of the FR, the higher the intrinsic char amount at 700 °C. An apparent charring efficiency which correspond to the char content reported to the initial phosphorus content can also be determined: 2.29 for P3P(OMe), 1.54 for P3P(OEt) and 1.43 for P3P(OPh). Among them, P3P(OMe) presents the highest charring efficiency.

Thermal stability of the FRs

Formulations and characterizations of flame retarded resins

Incorporation in epoxy matrix and assessment of the compatibility FR/matrix

To evaluate their efficiency as FRs, phosphorus based additives were introduced in a conventional epoxy matrix, i.e., DGEBA, cured by IPDA (Fig. 3). The DGEBA exhibits an aromatic structure leading to rigid network with excellent mechanical properties and good chemical inertness. That is why DGEBA is widely used, especially in the production of coatings or high performance composites [19,20]. Isophorone diamine is a cycloaliphatic cross-linking agent and appears to provide a good balance between reactivity and network rigidity. The matrix was also chosen according to FR degradation temperature, which has to be higher than the curing temperature to avoid FR degradation during the thermoset cross-linking. Epoxy/amine ratio was fixed to 2/1 in order to optimize the cross-linking. The reaction between an epoxy function and a primary amine forms a secondary amine which is then able to react with another epoxy function [21]. Indeed, thermal stability of thermosets is directly dependent on the cross-linking density.

The curing conditions of this matrix were determined in our previous study [7]. Different thermosets were prepared with various FR and phosphorus contents. Thermosets were named according to their phosphorus content followed by the used FR: "x%P(used FR)". A possible incompatibility between the matrix and the FR could lead to a partial phase segregation. During early stages of curing, viscosity of the system decreases and may allow sedimentation or creaming of segregated phases. The homogeneity of the cured thermosets was studied by scanning electron microscopy with EDX.

Figure 4 represents pictures and SEM pictures (× 2000) of 3 %P(P3P(OMe)), 3 %P(P3P(OEt)), 3 %P(P3P(OPh))-thermosets. SEM picture of 3 %P(P3P(OMe)) shows the presence of different phases (Fig. 4, 1-b) at the micrometric scale. This phase segregation visually results in an opaque material (Fig. 4, 1-a). The thermosets prepared with P3P(OEt) and P3P(OPh) are translucent (Fig. 4, 2-a, 3-a) and their SEM pictures show the presence of a single phase. These results evidence a better compatibility between the FR and the matrix for the P3P(OEt) and P3P(OPh) compounds compared to P3P(OMe).

Phosphorus content was determined at different depths from the upper to the lower surface of 3 %P(P3P(OMe)) and 3 %P(P3P(OEt)) (Fig. 5). The 3 %P(P3P(OEt))-thermoset shows a homogeneous distribution of the phosphorus content; such as 3 %P(P3P(OPh))-thermoset, whereas a significant evolution of phosphorus content is observed for the 3 %P(P3P(OMe))-thermoset. These results evidence that the biphasic structure observed in Fig. 4 allows the sedimentation of the phosphorus phases and leads to a gradient in phosphorus content of thermosets. The biphasic structure of the P3P(OMe)-containing thermoset have to be taken into account regarding the flame retardant properties of these thermosets.

DSC analyses of cross-linked thermosets

Physical properties of thermosets can be assessed measuring their glass transition temperature (T g ). The T g is the temperature needed to break the non-covalent inter-chains interactions. It corresponds to the relaxation of the network and the transition between the brittle and the rubbery states. Most often, high T g are targeted for the thermosets to reach both high thermal and mechanical properties.

DSC analyses were performed on cross-linked thermosets. Figure 6 shows the evolution of the T g values when increasing the phosphorus content for each thermoset. Two distinct behaviors are clearly observed. When both P3P(OEt) and P3P(OPh) are used as additives in the epoxy matrix, a decrease of the T g values with the phosphorus content is noted. Since these FR additives are homogeneously distributed into the matrix, they then act as plasticizers and create space between network chains, thus reducing the crosslinking density. For similar phosphorus content, T g values determined for the P3P(OPh)-and P3P(OEt)-containing thermosets are comparable even both FRs are incorporated in different amounts into the thermosets. On the one hand, the plastisizing effect of the P3P(OPh) should be more significant due to the larger FR size but on the other hand, this effect is counterbalanced by a higher rigidity induced by the FR aromaticity. The second behavior is observed when P3P(OMe) is used as additive. T g values determined for these thermosets do not follow the same trend than as that of all other thermosets. Indeed, for the same phosphorus content, i.e., 1 % w , T g values are 126 °C and 150 °C for P3P(OMe) and P3P(OEt), respectively.

The P3P(OMe) FR introduced in small amount in the material plasticized the network due to the formation a segregated microphase which avoids the intramolecular interactions between the chains. However, when P3P(OMe) was introduced in higher amounts, T g values were less affected and almost comparable to that of the reference one. Indeed, 3 %P(P3P(OMe)) and 4 %(P3P(OMe)) thermosets show T g values of 140 and 153 °C, respectively. The increase of the phosphorus content for P3P(OMe)-containing thermosets generates strong phase segregation and the FR does not act as plasticizer anymore.

Thermal properties and flammability of epoxy thermosets

Phosphorus-containing FRs are known for their different flame-retardancy mechanisms. They can act in gas and/or in condensed phases [22]. In the gas phase, they act by flame inhibition. Thermal decomposition of phosphorus group can generate radical species PO 2 .

, PO . and HPO . , which scavenge the H . and OH . radicals, slowing down the combustion and even reducing the combustion efficiency. In the condensed phase, chains dehydroxylation occur due to the action of phosphoric acid onto the hydroxyl groups, leading to conjugated molecules, and then to polyaromatic structures by Diels-Alder cyclization [START_REF] Lu | [END_REF]. The formed char can act as a barrier against heat and gas (O 2 and pyrolysis gases).

The flammability of the different thermosets was assessed according to different parameters such as high temperature char amounts, heat release rates or total heat released during decomposition. Thermosets are characterized by thermogravimetric and pyrolysis combustion flow calorimetry (PCFC) analyses to highlight the effects of FR on the thermal properties and flammability.

Thermogravimetric analyses of flame retarded thermosets

TGA analyses were performed to assess the influence of FR on thermal stability and charring of the flame retarded thermosets. T 10 represents the temperature at which the material loses 10 % of its initial mass. Figure 7 compares the thermoset without FR to the three thermosets, each containing 3 % w P. Thermosets 3 % w P(P3P(OMe)), 3 % w P(P3P(OEt)), 3 % w P(P3P(OPh)) exhibit lower thermal stabilities than that of the thermoset without FR. Indeed their T 10 are respectively 311, 307 and 304 °C compared to 369 °C for the thermoset without FR. These lower degradation temperatures are assigned to the early stage of decomposition for phosphorus-containing groups. Price et al. [23] described the phosphonic-ester decomposition ranging between 300 and 340 °C by alkyl chain loss. It may be assumed that phosphonic-ester decomposition generates radical species which initiate the matrix decomposition.

For a phosphorus content of 3 % w , the three thermosets 3 % w P(P3P(OMe)), 3 % w P(P3P(OEt)) and 3 % w P(P3P(OPh)) contains FR in different amounts, 14.9 % w of P3P(OMe), 17.2 % w of P3P(OEt) and 26.5 % w Fig. 7 TGA thermograms of 3 % w P-containing thermosets compared to the thermoset without FR.

of P3P(OPh), respectively. However these three thermosets displayed similar thermal stability, i.e., their T 10 are in the 304-311 °C range. This result was verified on other phosphorus content and thus shows that the thermal stability of thermoset is only modified by the phosphorus content, not by nature of the phosphate group. The influence of the phosphorus content on the thermal behavior of the thermoset is also studied. TGA thermograms of thermosets reference and P3P(OPh)-containing thermosets are presented in Fig. 8 and show that the higher the phosphorus content of a thermoset, the lower its thermal stability. Thus, thermal stability of the thermosets is impacted by the phosphorus content.

Figure 7 also exhibits that phosphorus-containing thermosets provide a higher amount of stable residue at high temperature than the thermoset without FR. The 3 % w P(P3P(OMe)) and 3 % w P(P3P(OEt))-thermosets provide equivalent char amount between 17 and 18 % w . The thermoset 3 % w P(P3P(OPh)) generates about 22.3 % w of char, i.e., three times higher than that of the thermoset without any FR.

Expected char contents have been calculated with an addition law by summing the respective char content of each component of the system. These calculated char content do not consider the action of the FR on the matrix. Results are given in Table 2. It is observed that experimental values are higher than the theoretical ones. It demonstrates the action of the three FR additives to promote charring of the matrix. However, even if P3P(OMe) displays the intrinsically higher charring efficiency, when the three FR are introduced into the matrix, P3P(OPh) allows a better charring effect on the matrix. TGA curves of the P3P(OPh)-containing thermosets (Fig. 8) also exhibit an increase in the char content when phosphorus rate increases. With 4 % w P brought by P3P(OPh), the epoxy thermoset generates more than three times the char amount obtained with the thermoset without any FR. It shows the ability of this aromatic biobased polyphosphate to improve the charring effect of an epoxy matrix during its thermal degradation. The same trend was observed on TGA thermograms of P3P(OEt)-containing thermosets. TGA thermograms of P3P(OMe)-containing thermosets are not presented in this study because of some reproducibility issue probably due to the heterogeneous structure as discussed above.

Flammability analyses: pyrolysis combustion flow calorimetry (PCFC)

To assess the flame retardant properties of synthesized phosphate, thermosets were characterized by pyrolysis/combustion flow calorimetry. PCFC analyses correspond to anaerobic thermoset pyrolysis followed by the high temperature oxidation of decomposition products (complete combustion) [24]. pHRR is the maximal value of the heat release rate. Tα represents the temperature at pHRR. The total heat released (THR) was obtained by integration of PCFC curves. The effective heat of combustion (EHC) represents the released heat by lost mass. It is calculated as the ratio between THR/mass loss measured in TGA.

The reference values for pHRR and THR wee respectively 664 W/g and 34.2 kJ/g. For equal phosphorus contents, HRR curves of the thermoset containing P3P(OMe), P3P(OEt) and P3P(OPh) exhibit equivalent HRR curves (Fig. 9).

Figure 10 also shows the similar action of the three phosphates FR on the different parameters studied (pHRR, THR, Tα and EHC). Peak of heat release rate (Fig. 10a) is significantly reduced, from 664 W/g for the thermoset without FR to around 400 W/g for the thermosets containing 3 % w P brought by any of the phosphate FR, i.e., a decrease of 40 %.

The total heat released is also reduced (Fig. 10b) from 34 kJ/g for the thermoset without FR to around 27 kJ/g for the thermosets containing 3 % w P brought by any of the phosphate FR, i.e., a decrease of 21 %. The THR decreased linearly by increasing the phosphorus content, which increases the char content at high temperature. Indeed, the charring reduces the amount of decomposition gases which are able to participate to the combustion and thus reduces the total heat released by the thermoset. Nevertheless, the THR appears to be slightly higher with P3P(OPh) than with other FR, at least at high phosphorus content, despite the higher char amount of this thermoset.

Combining the char rate obtained by TGA to the THR measured by PCFC analyses allows determining the EHC for P3P(OEt) and P3P(OPh)-containing thermosets. Figure 10d shows a decrease of the EHC values starting from 2 to 3 % w P. If phosphorus content is lower than this value, EHC remains constant and highlights the action of the FR in the condensed phase. The Tα of the thermoset (Fig. 10c) is also reduced by the phosphorus amount. From 385 °C for the reference thermoset to around 330 °C for the thermosets containing 4 % w P. This loss of thermal stability is comparable to that observed from TGA analyses (Fig. 7).

However, for similar phosphorus contents, the different FR were not introduced into the same amount in the resin formulation. Thus, the similar behavior of the thermosets containing the same phosphorus content brought by the different FRs shows that the additive content and its nature do not affect the flammability of the thermoset. Moreover, the structural heterogeneity observed for P3P(OMe)-containing thermosets does not influence the action of the FR. This result could show that a micrometric repartition of the FR in the matrix is enough to ensure good FR properties. Nevertheless, it could also be assumed that a high temperature would better homogenize the RF into the thermoset. However, the incompatibility observed between the P3P(OMe) FR and the DGEBA-IPDA matrix has probably an influence on the migration of the FR in the crosslinked thermoset and favors a non permanent thermal behavior. In this study, the flammability of thermosets was studied just after manufacturing, not after ageing. Thus, it is observed that only the initial phosphorus content has an influence on the flammability of the thermosets.

Conclusion

Williamson etherification was successfully used on phloroglucinol to synthesize some phosphorus-containing biobased phosphates with different R groups in the phosphate groups P-O-R. The average yield of these synthesis is around 70 %. Flame retardant properties of these phosphates were evaluated in an epoxy matrix. The influence of R group of the phosphate P-O-R was evidenced by obtaining opaque thermosets which display phase heterogeneity using a methoxy phosphate. It showed the importance of the compatibility between FR and matrix structures to preserve visual aspect of the thermoset and to obtain homogeneous thermosets. A plasticizing effect was highlighted by the decrease of the glass transition temperatures of thermosets by introducing any of the FR. TGA and PCFC analyses showed that thermosets flammability mainly depends on the phosphorus content. A decrease in the thermal stability was observed for all phosphoruscontaining thermosets, and is attributed to the earlier decomposition of phosphorus groups. Nevertheless, this decrease in thermal stability is accompanied by a significant increase in char yield and reduction of flammability. Indeed, THR was reduced by 21 % and pHRR by 40 % with 3 wt%P compared to the that of thermoset without FR. The heterogeneity of the P3P(OMe)-containing thermosets does not influence the action of the FR on the matrix. However, this heterogeneity should promote the migration of the additive into the network and hence modify its FR efficiency. The chosen biobased ressource is a model molecule of the biophenol really targeted. So far, the pholoroglucinol is produced by bacterial fermentation of glucose and has a limited availability. However, some biobased phenolic compounds such as gallic acid are more available than the phloroglucinol and could be used from the same synthesis pathway for a possible development as new biobased FR.

Figure 2 Fig. 1 Scheme 1

 211 Figure2represents TGA thermograms performed for all FR. Concerning the thermal stability of the three FR, it shows that the main thermal degradation step starts beyond 150 °C for all FR. However, it is observed that the thermal behaviors for the three FR are rather different. R groups used on phosphate impacts the dissociation energy of the P-O bond. The P3P(OMe) and P3P(OEt) FRs, which bear an alkyl group bonded to their phosphate group show similar thermal stabilities; their T 10 (T 10 represents the temperature at which the thermoset loses 10 % of its initial mass) are 228 and 198 °C, respectively. However, P3P(OPh) FR, which has an aryl group bonded to the phosphate group, displays a better thermal stability. Indeed, its T 10 value is 364 °C. P3P(OPh) FR which exhibits only one type of P-O bonds decomposes in a single step beyond 400 °C, while the P3P(OEt) and P3P(OMe) FR which have two types of P-O bond, decompose in two steps: the first one is around 250 °C

Fig. 2 TGAFig. 3 (

 23 Fig. 2 TGA thermograms of the synthesized flame retardants P3P(OMe), P3P(OEt) and P3P(OPh) under nitrogen atmosphere at 10 °C/min.

Fig. 4

 4 Fig. 4 Pictures (a) and SEM pictures × 2000 (b) of thermosets 3 %P(P3P(OMe)) (1), 3 %P(P3P(OEt)) (2), 3 %P(P3P(OPh)) (3).

Fig. 5 Fig. 6 T

 56 Fig. 5 Evolution of the phosphorus content vs. the depth from the surface in the 3 %P(P3P(OMe)) and 3 %P(P3P(OEt)) thermosets.

Fig. 8 TGA

 8 Fig. 8 TGA thermograms of P3P(OPh)-containing thermosets compared to the thermoset without FR.

Fig. 9 PCFC

 9 Fig. 9 PCFC curves of the 3 % w P-containing thermosets compared to the thermoset without FR.

Fig. 10 Evolution

 10 Fig. 10 Evolution of pHRR (a), THR (b), Tα (c) and EHC (d) vs. initial phosphorus content of all thermosets.

Table 1

 1 Compositions of the formulated epoxy thermosets.

	Thermosets	Weight composition	Used FR	Phosphorus content
		(% w ) (DGEBA/IPDA/FR)		(% w P/thermoset)
	Reference	(80/20/0)	None	0
	1 %P(P3P(OMe))	(76.0/19.0/5.0)	P3P(OMe)	1
	2 %P(P3P(OMe))	(72.1/18.0/9.9)	P3P(OMe)	2
	3 %P(P3P(OMe))	(68.1/17.00/14.9)	P3P(OMe)	3
	4 %P(P3P(OMe))	(64.2/16.1/19.7)	P3P(OMe)	4
	1 %P(P3P(OEt))	(75.4/18.9/5.7)	P3P(OEt)	1
	2 %P(P3P(OEt))	(70.8/17.7/11.5)	P3P(OEt)	2
	3 %P(P3P(OEt))	(66.2/16.6/17.2)	P3P(OEt)	3
	4 %P(P3P(OEt))	(61.6/15.4/23.0)	P3P(OEt)	4
	1 %P(P3P(OPh))	(73.0/18.2/8.8)	P3P(OPh)	1
	2 %P(P3P(OPh))	(65.9/16.4/17.7)	P3P(OPh)	2
	3 %P(P3P(OPh))	(58.8/14.7/26.5)	P3P(OPh)	3
	4 %P(P3P(OPh))	(51.7/12.9/35.4)	P3P(OPh)	4

Table 2

 2 Comparisons between calculated and experimental char content values for 3 %P(P3P(OMe)), 3 %P(P3P(OEt)) and 3 %P(P3P(OPh)) thermosets.

	Thermosets	Calculated char content a (% w )	Experimental char content b (% w )	Δ(exp-calc)
	3 %P(P3P(OMe))	14.0	17.3	3.3
	3 %P(P3P(OEt))	11.4	17.8	6.4
	3 %P(P3P(OPh))	10.2	22.3	12.1

a %Char calculated = (%FR/thermoset) x (%char FR) + (%matrix/thermoset) x (%char matrix). b Form TGA.


Patrice Lucas

Malvina Vaysse

Julien Aubry

David Mariot

Rodolphe Sonnier

Francois Ganachaud

François Ganachaud

Finest nanocomposite films from carbon nanotube-loaded poly(

Since their discovery in the early nineties, 1 carbon nanotubes (CNTs) have been the subject of numerous researches, owing to the combination of electric, mechanical, thermal and chemical properties. [START_REF] Dresselhaus | Carbon Nanotubes: Synthesis, Structure, Properties and applications[END_REF] The CNTs' large Van der Waals force density, wide interaction surface, long tubular geometry and low rigidity however favour their strong aggregation into bundles. [START_REF] Zakri | [END_REF] One major challenge remaining to be solved today thus lies in obtaining the best dispersions, particularly in polymer matrices, to take advantage of their outstanding intrinsic properties. Nanotube surface functionalization, 4 or co-extrusion with polymers 5 generally allows yielding satisfying composite mixture. However, such techniques generally weaken the strength and / or reduce the size of the nanotubes. 6 Recently, different techniques starting from aqueous dispersions of nanotubes have been proposed to prepare CNTs/polymer composites. CNTs aqueous dispersions were obtained by wrapping them with polyelectrolytes or by adding conventional surfactants; since there are no needs for chemical or mechanical inputs (apart from reasonable sonication to help at the dispersion), most CNTs retain their original shape and length. 7 The 'latex technology' consists in mixing together 1/an aqueous dispersion of surfactant-stabilized CNTs and 2/ a polymer latex stabilized with the same surfactant. Removing water by different means (e.g. lyophilisation) leads to a nanocomposite. 8 In some very specific instances, the latex was generated by in-situ polymerization of a monomer emulsion in presence of CNTs dispersions. 9 The 'coagulation technique' described by Winey et coll. 10 consists in co-precipitating in a non-solvent (typically water) a mixture of polymer (PMMA or PS) and carbon nanotubes preliminary dissolved in a common solvent, typically DMF. The difficulty here lies in the mixing of both solutes in the solvent while keeping a low viscosity solution, compulsory to safely mix both organic and aqueous phases during the coagulation step. We propose in this communication a different and simpler approach, called "polymer emulsification-coagulation" technique (PEC), a combination of the two techniques previously described. It consists in nanoprecipitating a polymer solution in an aqueous 50 dispersion of carbon nanotubes in tailored conditions so that the polymer traps the nanotubes into nanoparticles (Scheme 1). The number of CNTs entrapped in the nanoparticles (of typically 200 nm in size) ranges from 0 to 5 maximum, thus avoiding bundling of these (Figure S1). The final CNTs-loaded polymer 55 nanoparticles can be stored into powder and melt to generate nanocomposite films or materials. The outstanding dispersion of CNTs into the polymer matrix is similar to the aqueous dispersion the nanocomposite arose from (1 to 1 copy). The following communication describes the concept of PEC technique, the 60 resulting nanocomposite particles and films and some resulting thermal properties improvements. The nanoprecipitation process is a general method to generate surfactant-free nanosized polymer particles. The spontaneous emulsification via solvent shifting technique (so-called « Ouzo 65 effect » 11 ) requires to select a solvent and a non-solvent of the polymer which are fully miscible, and to track out concentrations where only nanoparticles are formed. 12 In the open literature, surfactant-free biodegradable polymer pseudolatexes are generated in a view of drug vectorization applications. 13 The 70 entrapment of water-dispersed solutes using this technique is however, to our knowledge, not widespread. We could only track one article where indomethacin-loaded magnetic particles were generated by nanoprecipitation in the presence of magnetite nanoparticles. 14 In the course of a fundamental study on the 75 nanoprecipitation of a model polymer, 15 we previously derived a phase diagram for the PMMA/acetone/water system (Figure 1). 1a) or CNTs (Fig. 1b) (x axis in log scale). The weight content of water is deduced from the other two values. The PMMA nanoprecipitation domain, depicted in blue, shows two characteristic limits: the binodal curve as a black line, best fit of black squares, and the Ouzo limit in blue dashed curve, best fit of the blue squares. CNTs dispersion stability domain, in pink, is delimited by the stability limit (red line deduced from red circles data). In both phase diagrams, the purple diamonds give the theoretical final mass fraction of solute (PMMA and CNTs) tested in this study (see actual values in Table 1 and text for details).

Two limits were experimentally derived on the polymer binary diagram in Fig. 1a (the content of water is deduced from solvent and polymer weight ratios): the binodal or trouble point curve, which separates the domain where the polymer is miscible in the mixture of solvent/nonsolvent (noted as 1φ in the phase diagram) from the domain where it demixes; and the Ouzo limit, which separates the domain where nanoparticles are exclusively formed (Ouzo zone in blue, Figure 1a), from those where solvent-filled lumps are also generated (noted 2φ in Figure 1; see ref. 14 for TEM photos). On Figure 1, we also reported on the second phase diagram the stability zone of aqueous dispersions of carbon nanotubes stabilized here by sodium cholate, one of the best dispersing agent of CNTs. Below the red line on Figure 1b, CNTs dispersions remain stable for at least an hour (pink zone in Figure 1b). 16 For diluted enough s dispersions in water (typically below 10 -5 in wt fraction), large contents of acetone (above 65% in wt.) are needed to provoke a rapid disrupting of the s dispersion. The purple diamonds represent theoretical final mass fractions of PMMA and CNTs in case of ideal composite formation (quantitative recovering of composite particles, 100% CNTs encapsulation). The actual results for these different formulations are given in Table 1 (vide infra). Figure 2 gives an overview of TEM and SEM photos of original nanocomposite particles and of films resulting from particles annealing (120°C, 30 min.). Not all particles entrapped a single CNT (white arrow, Figure 2a): this explains the polydispersity of particles shapes observed using SEM (Figure 2b). Some CNT protrude from particles as stressed by black arrows on a zoom photo (Figure 2c). Final dispersion of the CNTs in annealed films is remarkable, without any evidences of bundles in the sample (the entire surface was screened during analysis). Such dispersion cannot, to our knowledge, be achieved nowadays by using other techniques. It was also observed that increasing the final CNTs/polymer ratio enabled the encapsulation of a few CNT in a 40 same particle, still without aggregating them, or nanotubes protruding from two or more particles (see TEM photos in Figures S1 andS2, respectively). Different experimental conditions which enabled us to generate large contents of nanocomposites are given in the supporting 45 information section. We were able to generate nanocomposite particles at any solvent final weight ratio ranging from 0.2 to 0.6. It is worth noting that if the solvent is quickly removed right after nanoprecipitation, it not only concentrates the dispersion, but also favors a longer stability of the dispersion (in the range of hours).

50

It was observed that the final particle size increases while increasing the final weight ratio of solvent and/or while increasing the final content of polymers. pH of the aqueous dispersion has also a huge influence on the system. High contents of hydroxide ions favor the generation of small colloidally stable 55 PMMA particles at polymer weight ratio as high as 0.1.

The selection of surfactant to stabilize the CNTs dispersion is even more crucial in this process, since the 1:1 copy strategy entails that the CNTs aqueous dispersions be the finest prior to PMMA entrapment. Sodium dodecylbenzene sulfonate 60 (NaDDBS), another common surfactant for CNTs, was likely used but did not lead to fine nanocomposites. Indeed, although the stability of the CNTs dispersion was found better than for those stabilized by cholate surfactant (see Figure S3), we were not able to destroy all CNTs bundles (which showed up on TEM 65 photos: Figure S4). The limited entrapment of NaDBBSstabilized CNTs into PMMA is obvious while centrifuging final composite particles, where a clear supernatant is the signature of 100% entrapment (Figure S5). The reason lies in the fact that the large bundles of CNTs cannot be trapped by the nanoparticles. imposed the initial CNTs/polymer ratio to be applied. Above 5wt%, the yield in dispersion decreases down to 50%, the rest being trapped into non-CNTs containing lumps. Resulting films from samples described in Table 1 were tested on a Pyrolysis

Combustion Flow Calorimeter (PCFC) 17 which measures the main heat release characteristics of small samples (1-4mg). It should be noted that PCFC is not sensitive to some physical effects like barrier effect or heat shielding. Since these two effects constitute the main modes of action of CNTs, the reduction in pHRR while incorporating CNTs into a polymer matrix is strongly lower than one would see in cone calorimeter. Still, it is obvious from Table 1 that the peak of heat release rate (pHRR) decreases with CNT contents. This value was found lower than the theoretical value predicted by a linear rule of mixtures (considering that HRR of pure CNT is 0). The temperature of the pHRR increases also (but very slightly) when CNTs are incorporated into the PMMA matrix to stabilize approximately at 2wt% CNTs (compared to polymer). Therefore we could assume that CNTs act not only as diluting filler, but also would trap radicals, indicating a very good dispersion of CNT (the developed interface between the CNT and the PMMA matrix favours radical trapping). Note that similar experiments using a commercial CNT masterbatch obtained by extrusion as starting material did not lead to such improvements (not shown). Such results were confirmed by TGA measurements, with a decomposition curve shifted towards higher temperatures while trapping cholatestabilized CNTs rather than mixing in the bulk the polymer and CNTs (Figure S6). Such shift was not obvious for CNTs stabilized by NaDBBS, showing the importance of the fine dispersion of CNTs on their thermal efficiency. PMMA (14700g/mol, polydispersity of 1.54) was purchased from Aldrich and used as received. Multiwalls CNTs are Baytubes purchased from Bayer, prepared by CVD with a high degree of purity (low concentration of residual catalyst and absence of free amorphous carbon). Other specifications are: inner mean diameter ~ 4 nm, outer mean diameter ~ 13 nm, typical length ~ 0.2-1 µm after sonification/water dispersion. They were used without further purification. Sodium cholate hydrate (98%) and Sodium dodecylbenzene sulfonate (NaDDBS, 98%) surfactants were provided by Aldrich. NaOH pellets (SDS, 98%), acetone (Analar Normapur), and HPLC grade water (Panreac) were likely used without modification. The diameter of the CNTs/PMMA nanocomposites was determined by retro-scattered light using a Nanotrac device. Particle size from 8 nm to 6500 nm can be recovered with this apparatus. According to the mass fraction of solvent of the sample, viscosity and refractive index of the dispersing fluid were deduced from tables and used for particle size analysis. The particle morphology was investigated using scanning electron microscopy (SEM). SEM samples were prepared by laying a drop of the dispersion on an aluminium foil and letting the drop airdried at room temperature. The resulted film consisting of PMMA nanoparticles on aluminium foil was then vacuum-coated 25 with gold and analyzed. Transmission electron microscopy (TEM) was also carried on samples in order to determine the distribution of CNTs in PMMA matrix. A drop of the dispersion was laid on a TEM grid and was left to dry gently at room temperature. Centrifugation of the dispersions was carried out 30 with a Sorvall GLC-2B General Laboratory Centrifuge. Microcalorimetry was carried out on a Pyrolysis Combustion Flow Calorimeter (PCFC) apparatus from Fire Testing Technology Ltd (FTT). The sample was heated under nitrogen stream up to 750°C at a high heating rate (typically 1K/s) from which released gases were evacuated in a combustor, burnt in presence of an excess of oxygen at 900°C. Heat release rate is calculated from oxygen consumption according to the Huggett's relation (1 kg of oxygen corresponds to 13.1 MJ of released energy). Each composition was tested in triplicate. Peak of Heat 40 Released Rate (pHRR) and temperature of the peak of heat release rate (TpHRR) were calculated.

Dispersion of CNTs in water

CNTs and the surfactant were put in water and the resulted solution sonicated under stirring (2 sec power on, 1 sec power off) during one hour with a power of 40%. This first sonication enables to divide big aggregates. The solution was then let to cool during 15 min and sonicated a second time during one hour with a power of 100% in order to divide CNTs bundles. The resulted 50 solution was black and the efficiency of the dispersion could be controlled with an optical microscope. A typical recipe consisted in mixing 0.5g of CNTS, 1g of surfactant and 100g of water. Different fractions of CNTs (issued from the 0.5% CNTs aqueous dispersion) were dropped in a NaOH aqueous solution, set at a 55 final pH 11. Acetone was added drop by drop thanks to a micropipette. After of each addition, dispersion is agitated and pipetted in a capillary. After a rapid visual observation, acetone was quickly added again and the same procedure carried out. The stability limit placed on the phase diagram was assigned when 60 black dots started to show up throughout the capillary. For larger contents of acetone, big lumps of CNTs aggregates were clearly seen without the need for capillary observation.

CNTs/PMMA nanocomposites formation

65

The nanoprecipitation consisted in mixing an aqueous phase and an organic phase containing a hydrophilic solvent (Scheme 1). The organic solution was first prepared by dissolving PMMA in acetone. The dispersion of CNTs diluted in the basic solution (water + NaOH) was added quickly to the organic solution under 70 moderate stirring. The solution became grey-black and the dispersion was used as his or after solvent removal by a rotavapor during 15 min at 30°C. A typical recipe consisted in first dissolving 1 g of PMMA in 100 g of acetone to obtain the organic solution. 8 g of the CNTs aqueous dispersion was then poured in 75 12 g of the organic solution.

Notes and references

Scheme 1 .

 1 Scheme 1. Principle of entrapping of CNTs by the Ouzo-effect driven 80

Fig. 1

 1 Fig.1Binary phase diagrams showing the weight content of solvent (y axis) as a function of weight content of solute, PMMA (Fig.1a) or CNTs (Fig.1b) (x axis in log scale). The weight content of water is deduced from the other two values. The PMMA nanoprecipitation domain, depicted in blue, shows two characteristic limits: the binodal curve as a black line, best fit of black squares, and the Ouzo limit in blue dashed curve, best fit of the blue squares. CNTs dispersion stability domain, in pink, is delimited by the stability limit (red line deduced from red circles data). In both phase diagrams, the purple

Fig. 2 .

 2 Fig.2. Nanocomposite particles (top) and composite film after annealing (bottom) as observed by TEM (a, d) and SEM (b, c, e, f). Arrow in (a) shows individual CNT entrapped into polymer particles, whereas (b) and (c) shows the non-spherical shapes of the objects due to CNT protruding (showed by black arrows). After annealing, films were composed of long individual nanotubes, visible in the core of the material by TEM (d) or at the surface of the film by SEM (e). A fracture of the film breaks in some extent the CNTs (f). Scales are given vertically and horizontally in TEM and SEM photos.

Figure S1 :

 S1 Figure S1: Nanoparticles loaded with few carbon nanotubes in the same particle (typically less than 5).The ratio CNTs:PMMA was set at 1 wt% (top) and 5 wt% (bottom). The scale holds for 200 nm.

Figure S2 :

 S2 Figure S2: Examples of TEM photos showing long CNTs entrapped within fewer nanoparticles.CNTs:PMMA loading Top: 2wt% CNTs:PMMA loading, bottom: 5wt% (see Table1in the main text for details). Scale bar: 200 nm.

Figure S3 :

 S3 Figure S3: Same plot as Figure 1b in the main text, but with NaDDBS as the surfactant of the CNTs dispersion.

Figure S4 :

 S4 Figure S4: Bundles of nanotubes which were not destroyed while preparing the CNTs dispersion in presence of NaDDBS. These small aggregates were seen in the original dispersion or here after nanoprecipitation (see very small particles on the bottom left handside).

Figure S5 :

 S5 Figure S5: Efficiency of CNTs trapping by PMMA as observed by simple centrifugation experiments.Whereas the cholate sodium based sample was totally entrapped in the particles (on the left), the naDDBS-based sample was in some part still dispersed in the aqueous phase.

Figure S6 .

 S6 Figure S6. Normalized PMMA decomposition curves obtained by TGA representing PMMA weight percentage versus temperature for PMMA nanoparticles (black line), PMMA-CNT nanocomposites (color solid lines) and for mixtures of PMMA nanoparticles (prepared beforehand by simple PMMA nanoprecipitation) and dispersed CNTs (dotted lines). Sodium cholate based samples are in blue, those stabilized from NaDDBS in green.

70

  Table 1 summarizes different experiments where the CNTs loadings were varied. The final yield in composite nanoparticles (obtained after filtration, see experimental part for details)

Table 1

 1 Influence

		Initial	Final	Particle	Relative	pHRR
		CNT:PMMA	CNT:PMMA	yield	pHRR	temperature
		(%)	(%) b	(%) c	(%) d	(°C) d
		0	0	99	100	390
		1.0	-e	93	88	396
		2.3	1.7	78	83	400
		4.5	4.2	44	78	403
		7.5	-e	27	-e	-e
		10	8.9	12	74	401
		a Quantities: 12g of 1wt% PMMA acetone solution and xg of 0.5wt% of
	40	aqueous suspension (pH 11, 1wt% surfactant); b as given by TGA residue
		at 500°C (after subtraction of PMMA and surfactant residue); c measured
		after filtration on a paper filter; d measured by PCFC (see text for details);
		e not determined.
		In conclusion, we have developed a very simple technique to
	45	entrap CNTs into a thermoplastic polymer, in a view to generate
		the finest nanocomposite films. We are currently working on
		preparing films containing various CNTs contents for measuring
		electrical conductivities. 18, ‡ We also plan to generate
		nanocomposite particles in sufficient quantities to test such finest
		composites in a cone calorimeter. This work has been the subject
	55

of the content of CNT on the yield of dispersion and thermal resistance of the nanocomposite films. a 50 of a patent published recently. 19 FG thanks C. Longuet for her help on characterization, and the Nanoledge Inc. Company for financial support of a post doc grant (PL) and a master student grant (MV).


William Garat
email: william.garat@mines-ales.fr

Stéphane Corn

Nicolas Le Moigne

Johnny Beaugrand

Anne Bergeret

Stephane Corn

Improving the morphometric measurement of natural fibres cross-section to assess their mechanical properties

Dimensional characteristics of natural fibres are highly dependent on their botanical origin and extraction processes. Furthermore, their dimensions and physical properties could vary according to environmental conditions (humidity, temperature...). Advances in the development of natural fibres reinforced composites for engineering applications require a reliable prediction of their mechanical performances. From this point of view, the morphology of natural fibres should be assessed accurately so as to implement their intrinsic mechanical properties in mechanical predictive models. The dimensional measurement of natural fibres cross-section is usually based on assumptions of their sectional shape and rarely takes into account dimensional variations along their length. This study proposes a detailed dimensional analysis of various natural fibres species based on the comparison of two very different approaches: microscopic observations and an automated laser scanning technique.

INTRODUCTION

Currently, the industry is showing a growing interest in the development of biocomposites. Natural fibres of various botanic origins are used as reinforcement in polymer composites [START_REF] Mohanty | Natural fibers biopolymère and biocomposites[END_REF] as a solution to current environmental issues, such as the reduction of greenhouse gas emissions and pollution through the use of renewable and biodegradable resources and lightweight materials. However, natural fibres lack a scientific and technological history enough to claim a controlled and constant level of performances, and both agricultural and industrial sectors are investigating potential solutions. In order to be able to accurately model and predict the mechanical behavior of biocomposites from their manufacturing processes to their service use, it is essential to have a thorough knowledge of the morphological and mechanical properties of natural fibres and their evolution in specific environmental conditions. Indeed, natural fibres, whatever their origin and the scale considered can show wide variations in their dimensional features. [START_REF] Müssig | Industrial Applications of Natural fibres: Structure, Properties and Technical Applications[END_REF] investigated the evolution of the length, the width and the cross-sectional area (CSA) of several species of natural fibres. They observed a strong contrast of morphological dispersion between elementary fibres and fibre bundles. Several measurement methods have been used to assess the dimensions of natural fibres. However, they often rely on simplified assumptions of their shape that may be valid for synthetic fibres but not for the specific shape and dimensional irregularities of natural fibres [START_REF] Müssig | Industrial Applications of Natural fibres: Structure, Properties and Technical Applications[END_REF]. The determination of the mechanical properties of fibres are determined from the dimensional data collected with these methods [START_REF] Haag | Scatter in tensile properties of flax fibre bundles: influence of determination and calculation of the cross-section area[END_REF], which partly explains the large scattering of fibre mechanical properties reported in literature. The purpose of this study is to better assess the dimensions of natural fibres by considering their botanical origin and morphological features.

RESULTS AND CONCLUSIONS

Two methods of morphometric measurement were used and compared. The first method was proposed by [START_REF] Müssig | Industrial Applications of Natural fibres: Structure, Properties and Technical Applications[END_REF] and [START_REF] Thomason | Natural fiber cross sectional area, its variability and effects on the determination of fiber properties[END_REF]. It consists in embedding fibres in an epoxy resin pad which undergoes successive polishing so as to observe and measure their actual cross-sectional dimensions using an optical microscope. The second method uses an automated laser scanning device (FDAS), developed by Diastron Ltd (Andover, UK), to measure the transverse dimensions of fibres along their length on 360 degrees. Data collected by the FDAS device were correlated with those obtained by optical microscopy for five species of natural fibres (hemp, flax, sisal, palm and nettle). It was found that correction factors should be applied. These factors are associated to the morphometric characterics of natural fibres related to their botanical origin. As an example, Fig. 1 shows a correlation between the two methods.

Fig. 1 :

 1 Fig.1: Correlation between average apparent diameter from the FDAS and cross-sectional area from the optical microscopy.


Brahim Mazian
email: brahim.mazian@mines-ales.fr

Anne Bergeret

Jean-Louis Fanlo

Jean-Charles Benezet

Stéphane Cariou

Sandrine Bayle

Luc Malhautier

EFFECT OF DEW RETTING OF DIFFERENT HARVESTING PERIODS ON THE HEMP FIBERS QUALITY

This work assesses the impact of dew-retting duration (up to 9 weeks) on hemp fibers properties (color, chemical composition, mechanical performance, volatile organic compounds (VOCs) emissions). Different harvesting periods were compared (beginning and end of flowering and seed periods). Results show a change of color, an evolution in chemical composition with a decrease in the pectins and hemicellulose contents, an increase in cellulose contents, different behaviors in mechanical properties and evolution in VOCs nature and amounts. The retting duration differs as more than 9 weeks are required for fibers sampled during flowering while only 3 weeks are needed for fibers in seed period.

INTRODUCTION

Hemp fibers are increasingly used as reinforcements in polymeric materials (forming biocomposites) due to their challenging performances (low-density, renewability, good specific mechanical properties and biodegradability) compared to conventional fibers such as glass fibers. The hemp fibers are issued from the upstream processing termed retting. This treatment allows a subsequent fiber separation from the plant stems by a partial pectin degradation by microorganisms [START_REF] Ribeiro | Microbial diversity observed during hemp retting[END_REF]. This operation is currently very empirically carried out in the fields, so that a large variability in the fibers quality (mechanical properties, color, morphology, chemical composition…) is resulting. This variability hinders the biocomposites introduction in some competitive industries such as automotive. Furthermore, if controlled, dew retting could be favorable to good properties of hemp fibers. The mechanical performance of natural fiber depends on the chemical composition and fiber diameter [START_REF] Duval | Influence of the sampling area of the stem on the mechanical properties of hemp fibers[END_REF] which are linked to the harvesting period and the retting duration. For this raison, dew-retting of hemp fibers obtained from different harvesting periods (beginning, end of flowering and seed maturity) were performed to understand the variation of mechanical properties in relation to chemical composition, color, morphological features and VOCs emissions during growth and dew-retting. The aim of this present work is then to characterize hemp fibers of each weekly stems sampling during dew-retting. The fibers were characterized using multiple experimental techniques: biochemical (gravimetric) analysis, spectrocolorimetric measurements (pectins content), thermogravimetric, morphological analysis, mechanical testings and VOCs analysis.

RESULTS AND CONCLUSIONS

Variations in hemp fibers color versus retting time for different harvesting periods (beginning and end of flowering and seed maturity) are shown in Table 1. The color varies between light green for unretted fibers, yellow for low retted fibers and dark grey for highly retted fibers. Fibers need 9 weeks to be grey while 3 weeks only are required to obtain the same color in the seed period. This color change could be due to the development of microbial communities at the stems surface. It was given in evidence that the retting degree is linked to the weather conditions that were also recorded.

Figure 2a exhibits the evolution of chemical composition (based on solvent extraction) of hemp fibers cut at the beginning of flowering and retted. The cellulose content increases more than 15% during retting. Pectins, hemicellulose and ash content slightly decrease and then remain stable. Thermogravimetric analysis (Figure 1b) corroborates previous observations. Pectins and hemicellulose are degraded that leads to the increase of cellulose content. This result explains the increase of weight loss at 200-400°C with retting duration. This study shows that there are differences in color, chemical composition and VOCs emissions of the hemp fibers during retting for flowering period but also in mechanical properties.

Figure 1 :

 1 Figure 1 : Evolution of the biochemical content of fibers at the beginning of flowering A) Hemp fibers content variation using solvent extraction. B) Mass loss of hemp fibers content during dew-retting by thermogravimetry.

Table 1 : Color changes of hemp fibers. From unretted fibers on the left to retted fibers on the right

 1


Lata Soccalingame

Didier Perrin

Jean-Charles Bénézet

Anne Bergeret
email: anne.bergeret@mines-ales.fr

END OF LIFE OF PLA AND FLAX FIBER REINFORCED PLA BIOCOMPOSITE

This work aims to investigate the impact of artificial and natural weathering on the end of life of polylactic acid and flax fiber reinforced polylactic acid composite. The studied end-of-life scenarios are reprocessing, composting and incineration. Numerous characterization test results lead to understand the degradation behaviors involved through weathering and end-oflife treatments.

INTRODUCTION

With the ever-growing use of PLA and PLA biocomposites, their end-of-life issue is expected to get larger and increasingly difficult and expensive. Thus efficient waste management solutions are necessary. Although PLA is a compostable material, which would significantly reduce its waste issue, the knowledge about the material recycling and changes in the properties of PLA upon its multiple processing cycles is a relevant topic. Incineration is also a substantial end-of-life treatment.

Reprocessing usually involves mechanical and thermal degradation of both the matrix and the reinforcement. Several works highlighted PLA chain scission and natural fiber damage caused by multiple reprocessing cycles. Numerous papers reported good biodegradability of neat PLA and PLA biocomposites under composting conditions. Incineration is the most common PLA waste treatment nowadays as it enables to reduce efficiently the waste amount.

Moreover PLA and PLA/natural fiber biocomposite are well known to be sensitive to weathering. Thus, this work proposes to investigate the impact of an artificial humidity weathering or a natural climatic weathering on the end of life of PLA and PLA/flax composite by reprocessing, composting and energy recovery (incineration).

RESULTS AND CONCLUSIONS

The results from the flexural tests are shown in Fig. 1. Yield strength and deformation at yield were measured on PLA and PLA/flax compression molded samples after different combination of degrading conditions such as reprocessing and humidity weathering. The reprocessing cycles (C3) induce a strong decrease of neat PLA properties due to chain scission degradation. On the contrary, the PLA/flax composite exhibits no significant degradation. The humidity weathering (WC1) causes a significant drop of mechanical properties due to hydrolysis degradation by chain scission. Weathering is much more detrimental than reprocessing under these specific conditions. The reprocessing of weathered samples leads to a recovery of flexural properties of neat PLA thanks to macromolecular chain rearrangements. The PLA/flax composite presents a slight recovery but not sufficient to reach its initial values. This study shows that there is a beneficial effect of reprocessing thanks to a mechanical property recovery. Results will be presented for composting and incineration scenarios by taking into account degradation caused by weathering. This work leads to substantial results proving the ability of PLA and PLA biocomposite waste to be disposed efficiently.

Fig. 1

 1 Fig.1 Flexural properties of neat PLA (grey bars) and PLA/flax fiber composite (red bars) obtained by compression molding at the initial state (C1), after reprocessing (C3), after weathering (WC1) and after reprocessing of weathered samples (WC3)

ACKNOWLEDGMENTS

Authors are embedded to ADEME French organization for the financial support (convention n°1101C0066, acronym ENOLIBIO "ENd Of LIfe of BIOcomposites").