J B Jouenne

D Barbier

V Hounkpati

L Cauret

A Vivet

TWENTY-THIRD INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS (ICCM23) ANISOTHERM STUDY OF THE INFLUENCE OF FLAX FIBERS ON CURING KINETICS OF BIOBASED EPOXY RESIN

Keywords: Crosslinking, Composites, Natural Fibers

Composites materials made of biobased matrices reinforced with natural fibers are a solution considered to meet the environmental constraints of industry. Interactions between a partially biobased epoxy resin and flax fibers were investigated using Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis type trials (DMA). DSC analysis revealed that the catalyst effect of water increases the variation of entropy and lead to higher Eα values. DMA measurements highlight that composite with wet fibers has a higher Tg value after manufacturing and it is a consequence of the higher reactivity induced by water. By contrast, composite reinforced by dried flax fiber showed a lower curing reactivity and by consequence lower Eα values and Tg value after manufacturing. Both composites were stored under relative humidity conditions after manufacturing and evidenced an increase of Tg due to water absorption. A post-curing helped the composite with dried fiber to reach maximum Tg while it wasn't the case for composite with wet fibers due to chaotic network geometry caused by the entropic effect. Water contained by the flax fibers can act as a plasticizer after the post-curing as the Tg decrease with the absorbed water. This work gives a better understanding to the role of water naturally contained by flax fibers and point out the fact that monitoring the moisture of flax fibers is a key parameter to manufacture biobased composites.

INTRODUCTION

Interest for composite materials made of biobased matrices and reinforced with natural fibers is constantly growing. Besides their low environmental impact these materials exhibits good mechanical properties and low densities for mass reduction that are valuable for many industrial sectors [START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF]- [START_REF] Fogorasi | The potential of natural fibres for automotive sector -review[END_REF]. Among numerous natural fibers, flax fibers are the most widely used. This popularity is due to their versatility as they exhibit interesting properties like high tensile strength [START_REF] Bourmaud | Towards the design of highperformance plant fibre composites[END_REF], good vibration absorption [START_REF] Mathijsen | The renaissance of flax fibers[END_REF], for a well-mastered harvesting method [START_REF] Richely | A critical review of the ultrastructure, mechanics and modelling of flax fibres and their defects[END_REF]. With a better Life Cycle Assessment than many petroleum-based matrices reinforced by synthetic fibers [START_REF] Bensadoun | Environmental impact assessment of end of life options for flax-MAPP composites[END_REF]- [START_REF] Deng | Assessing the environmental impact of flax fibre reinforced polymer composite from a consequential life cycle assessment perspective[END_REF], bio-composites should already have overwhelmed the market of composite. But actually, the use of these eco-friendly materials remains limited due to technological locks such as the variability of fiber dimensions and the sensitivity to climatic changes. In addition to these physical issues, the presence of a natural fibers itself in a thermoset resin can modify its curing kinetics leading to lower performances [START_REF] Boutin | Influence of flax fibers on network formation of DGEBA/DETA matrix[END_REF].

The aim of this work is to study the influence of flax fibers on the crosslinking of a partially biobased epoxy resin during and after its curing. Crosslinking reaction of epoxy-amine systems consists in the epoxy ring opening from a primary amine by nucleophile attack. It is generally suggested that the curing mechanism is guided by two reaction paths [START_REF] Berges | Influence of moisture uptake on the static, cyclic and dynamic behaviour of unidirectional flax fibre-reinforced epoxy laminates[END_REF]: (1) reaction initiated by compound containing nitrogen -hydrogen bond known as non-catalytic reactions, (2) reaction catalysed by compounds containing oxygen -hydrogen bond known as catalytic reactions. On the one hand, some researchers showed that proton donor groups such as H2O and ROH tend to accelerate the reaction [START_REF] Choi | Effect of water addition on the cure kinetics of an epoxy-amine thermoset[END_REF], [START_REF] Johncock | Effect of absorbed water on undercured epoxy/amine thermosets[END_REF]. On the other hand, Canavy et al. recently showed that cellulosic compound of the fibers can interact with amine functions from the hardeners and lead to a lower network quality [START_REF] Canavy | Influence of flax fibers on epoxide-amine composites: Energetics of interphase formation[END_REF].

In this paper we investigate the role of the flax fibers and water that they contained on the curing kinetics of an epoxy-amine and their influence on network formation. For this purpose, samples of pure epoxy, epoxy with glass fibers, epoxy with wet or dried flax fibers were studied during curing of the resin and after composite manufacturing. First, Differential Scanning Calorimetry (DSC) was used to quantify the influence of wet / dried fibers on curing kinetics.Then, glass transition temperatures were studied using Dynamic Mechanical Analysis (DMA) to evaluate the influence on the network formation. From DSC results, an isoconversional method was used to obtain the activation energy and to have a better understanding of the phenomenon involved during crosslinking.

MATERIALS AND METHODS

Materials

Partially bio-based epoxy resin (56 %) GreenPoxy SR56 and hardener SD7561 were purchased from SICOMIN®. Flax fabrics used was a unidirectional untreated flax fabric with an areal weight of 210 g/m² and were kindly supplied by TERRE DE LIN®. Mat of glass with an areal weight of 300 g/m² were used as non-organic reference reinforcement.

Sample preparation

Flax fabrics were dried using an oven at 103°C for 24 hours to reach a theoretical water content of 0 % wt [START_REF] Abida | Effect of hygroscopy on non-impregnated quasi-unidirectional flax reinforcement behaviour[END_REF]. Then a part of the plies was impregnated with the epoxy resin to give samples GP/FF0% (Figure 1). The rest of the plies were stored in a controlled temperature and relative humidity chamber at 20 °C and 75 % RH until saturation to reach 10 %wt of absorbed water. When the water weight reached 10 %, plies were impregnated with epoxy resin to obtain samples GP/FF10\% (Figure 1). Glass fiber composite were directly manufactured from the glass fabrics to obtain samples GP/GF. All composite samples were manufactured using hand lay-up process. The volume fiber fraction was 30 % for each composite manufactured.

@20°C 240h@20°C 20h@60°C S1 X - - S2 - X - S3 X - X

Differential Scanning Calorimetry (DSC)

A fresh mixture of resin and hardener was made before each thermal analysis. Non-isothermal DSC runs were performed using a DSC Q10 from TA instruments to evaluate the behavior of pure epoxy, glass / epoxy composites and flax / epoxy ones with different water contents. Samples were sealed in aluminum pans and then heated from 25°C to 250°C at different heating rates β: 4 °C.min -1 , 6 °C.min -1 , 8 °C.min - 1 , 10 °C.min -1 and under a constant nitrogen atmosphere of 50 mL.min -1 .

An integral isoconversional method was used in this study to analyze the curing kinetics of pure epoxy and its composites. This method relies on the isoconversional principle that is: Reaction rate only depends on temperature for a given degree of conversion [START_REF] Vyazovkin | Isoconversional Kinetics of Polymers: The Decade Past[END_REF]. Therefore, the heat flow is directly proportional to the reaction rate, and so the degree of conversion can be determined following equation ( 2):

α = H t
H T where α is the degree of conversion, Ht is the enthalpy at fixed time and temperature and HT is the total enthalpy of the reaction. It is generally accepted that the reaction rate can be expressed by the Arrhenius equation:

𝑑α 𝑑𝑡 = 𝑘(𝑇)𝑓(α)
with the rate constant k associated to temperature defined as:

𝑘(𝑇) = 𝐴𝑒𝑥𝑝 ( -𝐸 α 𝑅𝑇 )
With A the pre-exponential factor that is the frequency of collision between molecules (entropy), R the gas constant, Eα the activation energy and T the temperature. If the sample is cured at heating β = dT/dt we obtain:

𝑑α 𝑑𝑇 = 𝐴 β 𝑒𝑥𝑝 ( -𝐸 α 𝑅𝑇 ) 𝑓(α)
Kinetic evaluation was done using the Kissinger -Akahira -Sunose (KAS) method [START_REF] Akahira | Trans. Joint convention of four electrical institutes[END_REF] as recommended by ICTAC committee [START_REF] Vyazovkin | ICTAC Kinetics Committee recommendations for analysis of multi-step kinetics[END_REF] The basis of the KAS method is the integral form of equation 4 that leads to an approximated form of the temperature integral:

𝑔(α) = 𝐴 β ∫ 𝑒𝑥𝑝 (- 𝐸 α 𝑅𝑇 ) 𝑑𝑇 𝑇 α 0 ≡ 𝐴𝐸 α β𝑅 𝑝 ( 𝐸 α 𝑅𝑇 )
This equation is integrated into a logarithmic form as:

𝑙𝑛(𝑔(α)) = 𝑙𝑛 ( 𝐴𝐸 α 𝑅 ) -ln β + 𝑙𝑛 [𝑝 ( 𝐸 α 𝑅𝑇 α )]
With the approximation:

𝑝 ( 𝐸 α 𝑅𝑇 α ) ≅ 𝑒𝑥𝑝 (- 𝐸 α 𝑅𝑇 α ) 𝐸 α 𝑅𝑇 α
and equation 5 can be rewritten:

𝑙𝑛 β 𝑇 α 2 = 𝑙𝑛 ( 𝐴 α 𝑅 𝐸 α 𝑔(α) ) - 𝐸 α 𝑅𝑇 α
Thus, the plot of ln (

β T α 2 ) versus 1 T α
following different heating rates β should give slopes equal to

-E α R
and intercepts equal to ln A α g(α) [START_REF] Perrin | Kinetic Analysis of Isothermal and Nonisothermal Epoxy-Amine Cures by Model-Free Isoconversional Methods[END_REF], [START_REF] Zheng | Mechanism and kinetics of thermal degradation of insulating materials developed from cellulose fiber and fire retardants[END_REF]. After the kinetic analysis a second run was performed to obtain the glass transition temperature TgDSC of each sample (heating rate 10 °C/min from 25°C to 250°C). Glass transition temperature Tg values was estimated using tangent method with the software Universal Analysis.

Dynamic Mechanical Analysis (DMA)

Dynamic Mechanical Analysis (DMA) type tests were performed on a Kinetech ® apparatus to study the mechanical relaxation associated to the glass transition of composite samples and to estimate the mechanical glass transition temperature TgDMA Principle of the test is to alternatively apply a positive and a negative torsion angle to the sample. At the beginning of a torsion step, the required torque to apply on the sample to obtain the torsion angle is measured and is called the instant torque Ci. The torsion is maintained for 2 seconds to measure the relaxed torque Cr. A relaxation curve is obtained by the difference of Ci and Cr and the analysis of this curve allows one to determine the region of glass transition. When the glass transition temperature of the material is reached, the relaxation curve shows a maximum that correspond to the TgDMA (Figure 2).

RESULTS

DSC curing monitoring

The curing of pure epoxy resin (GP), epoxy reinforced by glass fibers (GP/GF), epoxy reinforced by wet flax fibers (GP/FF10%) and epoxy reinforced by dried flax fibers (GP/FF0%) was investigated by non-isothermal DSC runs. Thermograms of each batch are shown in Figure 3. Three samples for each heating rate have been analyzed. Since repeatability was good, results below only show one representative curve for each heating rate. Values of onset temperature (Tonset), temperature of exothermic peak (Tpeak) and enthalpy are reported in Table 2. Tpeak and Tonset shift to higher temperatures as the heating rates increases for each sample. This is due to the fact that low heating rates allow the distribution of temperature to be more homogeneous, thus the curing starts earlier. Also at higher heating rates, high temperatures are quickly reached. This behavior is well known for epoxy curing [START_REF] Sun | Novel Spirocyclic Phosphazene-Based Epoxy Resin for Halogen-Free Fire Resistance: Synthesis, Curing Behaviors, and Flammability Characteristics[END_REF], [START_REF] Hardis | Cure kinetics characterization and monitoring of an epoxy resin using DSC, Raman spectroscopy, and DEA[END_REF]. It can also be observed that Tpeak and Tonset values of GP, GP/GF and GP/FF0% are very similar (Table 3). When fibers are wet, exothermic peaks shift for each rate to lower temperatures. A decrease in the enthalpy can be noticed for the composites with fibers in comparison with pure epoxy resin. This behavior indicates that fibers absorb a certain amount of heat during the reaction. Reaction heat flow obtained from kinetic analysis (Figure 3) was used in equation 1 to calculate the conversion α and the curing rate dα dt of each sample. The catalyst effect of water presence in flax fiber was highlighted by the plot of the curing rate versus the temperature at β = 10°C.min -1 (Figure 4) showing that the curing rate of composite with wet flax fibers increases earlier comparing to other samples. From DSC data, the activation energy Eα has been evaluated using the isoconversional KAS method (eq. 4). Results are plotted in Figure 5. A second run has been performed for each sample to determine the glass transition temperature of materials (Table 3).

Sample

TgDSC (°C) GP 92 ± 1 GP/GF 89 ± 1 GP/FF0% 92 ± 1 GP/FF10% 87 ± 1 Table 3: Average glass transition temperature for pure epoxy and its composites Despite a slower curing start from composite with dried fibers, we obtain the same TgDSC as pure epoxy sample indicating that there is no modification on the final geometry of the epoxy network. By contrast, TgDSC value of composite with wet fibers show a slight decrease that seem to be due to the fast and chaotic epoxy network formation [START_REF] Boutin | Influence of flax fibers on network formation of DGEBA/DETA matrix[END_REF], [START_REF] Sharp | Effects of water on epoxy cure kinetics and glass transition temperature utilizing molecular dynamics simulations[END_REF].

TgDMA measurements

DMA measurement were performed to observe the evolution of the glass transition temperature over time in function of different type of treatments after manufacturing (Table 1). For samples stored following S1 conditions (Table 1), measured values are below Tgmax indicating a partial crosslinking (Figure 6a). GP/FF10% showed higher value than GP/GF confirming the faster network formation assumed in curing monitoring. GP/FF0% showed a lower value than GP/GF confirming latent crosslinking. After 10 days at 20 °C and 60 %RH (S2) TgDMA values of every sample increased (Figure 6b). We can observe that the average increase of TgDMA values for composites with natural fibers is more important (around 8 °C) than composite with glass fibers (around 4 °C). To complete TgDMA measurements under S2 conditions, comparing samples were stored under 75 %RH and 97 %RH to observe the evolution of water uptake for each composite. Results are shown in Table 4 Water uptake increases with higher relative humidity percentage and is more important for composites reinforced by natural fibers. For post-cured samples (S3) it can be observed that all samples have nearly reached the maximum Tg (92 °C). These results are in good accordance with glass transition temperatures values measured on the DSC second run (Table 3).

DISCUSSION

As shown by our results, water absorbed by flax fibers has a huge effect on curing kinetics of epoxy -amine system. Exothermic peak temperatures measured by DSC (Table 2) showed that water contained in flax fibers accelerates the crosslinking reaction of the epoxy -amine system. It is confirmed by the plot of the curing rate ( dα dt ) versus the conversion rate (α) (Figure 4) as the crosslinking reaction of the composite with wet flax fibers starts earlier comparing to other samples. This effect was first shown by Chen et al. [START_REF] Chen | Curing of epoxy resin contaminated with water[END_REF]. Moreover Choi et al. pointed out that only a small amount of water has a strong catalyst effect on the curing of the mixture (< 2 \% wt) [START_REF] Choi | Effect of water addition on the cure kinetics of an epoxy-amine thermoset[END_REF]. For higher amounts Sharp et al. [START_REF] Sharp | Effects of water on epoxy cure kinetics and glass transition temperature utilizing molecular dynamics simulations[END_REF] showed the creation of water molecules clusters preventing the crosslinking resulting in lower TgDMA values. As our fibers contained about 10 % of water, it could be expected that some water molecules clusters are formed in our samples. The addition of flax fibers in epoxy -amine system was studied by Boutin et al. [START_REF] Boutin | Influence of flax fibers on network formation of DGEBA/DETA matrix[END_REF]. They also observed the curing acceleration induced by water contained in flax fibers. From a kinetic point of view, it corresponds to an increase of the variation of entropy. Variation of entropy describes how much a system is irreversible and is proportional to the pre-exponential factor A of the Arrhenius equation presented in eq. 4 [START_REF] Wu | Effects of water on the curing and properties of epoxy adhesive used for bonding FRP composite sheet to concrete[END_REF]. Increasing variation of entropy leads to higher Eα values [START_REF] Sbirrazzuoli | Determination of pre-exponential factor and reaction mechanism in a modelfree way[END_REF]. In our study activation energy Eα calculated using KAS method showed "s" shape curves (Figure 5) similar to results found by Sbirrazuoli [START_REF] Sbirrazzuoli | A Study of Epoxy-Amine Cure Kinetics by Combining Isoconversional Analysis with Temperature Modulated DSC and Dynamic Rheometry[END_REF], Roudsari et al. [START_REF] Mashouf Roudsari | Study of the Curing Kinetics of Epoxy Resins with Biobased Hardener and Epoxidized Soybean Oil[END_REF] and Perrin et al. [START_REF] Perrin | Kinetic Analysis of Isothermal and Nonisothermal Epoxy-Amine Cures by Model-Free Isoconversional Methods[END_REF]. At low conversion rates (<0.2) the decrease of Eα is linked to a change in the curing mechanisms [START_REF] Cruz-Cruz | Influence of Epoxy Resin Curing Kinetics on the Mechanical Properties of Carbon Fiber Composites[END_REF], switching from non-catalyzed reaction to auto-catalyzed reaction due to increasing number of hydroxyl groups generated by amines reacting with epoxides. At higher conversion rates (>0.7) the second decrease of Eα is due to the diffusion-controlled reaction kinetics effect [START_REF] Yu | Tetrafunctional Epoxy Resin-Based Buoyancy Materials: Curing Kinetics and Properties[END_REF]. This dependency of activation energy on the increasing extent of curing is known as the kinetic compensation effect (KCE) [START_REF] Sbirrazzuoli | Determination of pre-exponential factor and reaction mechanism in a modelfree way[END_REF]. As the crosslinking starts earlier for composite with wet fibers (Figure 3), an irreversible state is quickly reached, increasing the entropy variation and so the pre-exponential factor A leading to higher values for Eα than for pure epoxy and epoxy with glass fibers (Figure 5). On the contrary, Eα values of composites with dried fibers are lower, highlighting the decreased reactivity observed on DSC with higher peak temperatures values for GP/FF0% (Table 2). It is strengthened by the recent study of Canavy et al. [START_REF] Canavy | Influence of flax fibers on epoxide-amine composites: Energetics of interphase formation[END_REF] showing that cellulose from flax fibers can interact with amine-based hardeners. The creation of hydrogen bonds between amine groups and hydroxyl groups from the fiber cellulose inhibits amino-Belfast, 1-6th August 2021 epoxy interactions. However, those interactions are not irreversible since TgDSC (Table 2) and TgDMA (Figure 6b) of GP/FF0% are equal to pure epoxy Tg values after a post-curing.

From DMA experiment, samples stored under S1 condition (i.e. 24 hours after manufacturing) showed that samples with dried fibers have the lowest TgDMA values showing that the lesser reactivity (Table 2), the lower crosslinking density. This is in good agreement with Kuo et al. [START_REF] Kuo | Influence of cellulose nanofibers on the curing behavior of epoxy/amine systems[END_REF] that showed that glass transition temperature of thermoset polymers is dependent of the reactivity of the reaction and the crosslinking density. In the same way, composite with wet fibers has the highest TgDMA value (Figure 6a) as a consequence of higher reactivity induced by catalyst effect of water. Furthermore, TgDMA from S2 samples showed a global increase compare to S1 samples due to natural crosslinking (Figure 6b). It can be linked to the amount of water regained by samples (Table 4) allowing them to reach higher crosslinking density. This effect is more important for natural fiber reinforced composites as glass fiber are less sensitive to water ageing (Table 4). Lower crosslinking densities observed for GP/GF and GP/FF0% don't have consequences on the final shape of the network. A post-curing at 60 °C for 20 hours (S3) or a DSC second run can bring sufficient amount of energy to allow them to reach max glass transition temperature as shown by TgDMA value in Figure 6b and TgDSC values in Table 3. By comparison TgDMA and TgDSC values for composite with wet fibers did not reach the maximum value. It could be caused by water clusters as suggested by Sharp. et al. [START_REF] Sharp | Effects of water on epoxy cure kinetics and glass transition temperature utilizing molecular dynamics simulations[END_REF]. But we assume that this could also be linked to a formation of a more chaotic network. As water catalyzes the crosslinking reaction, a less perfect network is formed preventing GP/FF10% samples to reach maximum Tg.

Another effect of water contained by natural fibers is that water molecules that are still trapped in composite networks or regained after post-curing bring a plasticizing effect on sample. The presence of H2O molecules increases the mobility of macromolecular chains and leads to a decrease in the Tg value [START_REF] Le Guen-Geffroy | Physical ageing of epoxy in a wet environment: Coupling between plasticization and physical ageing[END_REF]. Many studies showed that moisture absorption of epoxies leads to degradation in mechanical properties and have effect on glass transition temperature [START_REF] Choi | Effect of water addition on the cure kinetics of an epoxy-amine thermoset[END_REF], [START_REF] Le Guen-Geffroy | Physical ageing of epoxy in a wet environment: Coupling between plasticization and physical ageing[END_REF]- [START_REF] Bockenheimer | New aspects of aging in epoxy networks. II. Hydrothermal aging[END_REF].

A weighing method was done on GP/FF0%, GP/FF10% and GP/GF samples and evidenced this plasticizing effect. Successive DMA type trials gave the evolution of TgDMA in function of water content (Figure 7). Initial Tg of post-cured samples was 92 °C. We observed that after the storage TgDMA drastically decreased to 55 °C for GP/FF10%, 67 °C for GP/FF0% sample and to 72 °C for GP/GF. However, the temperature applied to the sample during DMA procedure brought enough energy to the epoxy-amine system to allow water evaporation. As the water evaporates during DMA procedure, TgDMA increases. Figure 7 show that water quickly exits the glass fibers sample and the TgDMA reached its maximum after the third consecutive procedure. Composite with dried fibers needed five DMA procedures to recover the maximum TgDMA highlighting that water is absorbed deeper due to presence of flax fibers as they are a greater diffusion channel for water. Concerning the wet fibers composite, TgDMA did not reach maximum value confirming the assumption of an incomplete, chaotic resin network formation induced by water presence during curing.

Figure 7: TgDMA values of GP/GF, GP/FF0% and GP/FF10% for different water content

CONCLUSION

The influence flax fibers on curing kinetics of a biobased epoxy resin have been studied here. Results showed that water contained by flax fibers has a huge influence on curing kinetics as it accelerates the crosslinking. This effect leads to faster irreversible state that increases the activation energy by compensation effect. On the contrary, a decrease in the reactivity of dried flax fiber composites is observed and can be induced by amine -cellulose interaction. The consequence is a lower glass transition temperature after manufacturing comparing composite with wet fibers. From this state, water absorb by both composites participated to the natural crosslinking as the Tg increased. After a post-curing both composites with dried fiber and reference with glass fibers reached maximum Tg value. The lower value observed for composite with wet fibers after post-curing has been attributed to formation of a more chaotic network geometry induced by entropic effect preventing a complete crosslinking. Amount of water still present after post curing or regained after post-curing tends to decrease the glass transition temperature by plasticization effect. This effect is greater with natural fibers as they are more sensitive to water absorption. Nonetheless, for glass fibers and dried flax fibers composites, a simple heating program can rise the glass transition temperature to its initial value by water evaporation. But, for composite with wet fibers water evaporation did not achieve to reach maximum TgDMA because of incomplete, chaotic network formation due to water presence.

Figure 1 :

 1 Figure 1: Sample preparation protocol DMA samples were stored in various conditions (table 1) to follow the evolution of glass transition temperature (Tg).

Figure 2 :

 2 Figure 2: Glass transition temperature estimation via Kinetech ®

Figure 3 :

 3 Figure 3: DSC thermograms of GP (a), GP/GF (b), GP/FF0% (c) and GP/FF10% (d)

Figure 4 :

 4 Figure 4:Comparison of the curing rate at β = 10°C.min -1 as a function of the temperature for each material

Figure 5 :

 5 Figure 5: Activation energy Eα dependence to conversion rate from anisotherm data Activation energy Eα varies differently with degree of conversion for pure epoxy and its composites.For every sample, the activation energy is higher in the first stage of the curing and decreases over time. Values from epoxy reinforced by dried flax fibers showed lower trend indicating that it has an influence on the curing reaction. By contrast, composite reinforced by wet flax fibers showed the highest Eα at every conversion rate. Values of Eα found here are consistent with the literature on biobased epoxy resins (45 -80 kJ.mol -1 )[START_REF] Lascano | Kinetic Analysis of the Curing of a Partially Biobased Epoxy Resin Using Dynamic Differential Scanning Calorimetry[END_REF]-[START_REF] Cruz-Cruz | Influence of Epoxy Resin Curing Kinetics on the Mechanical Properties of Carbon Fiber Composites[END_REF]. A second run has been performed for each sample to determine the glass transition temperature of materials (Table3).

Figure 6 :

 6 Figure 6: TgDMA comparison for S1 / S2 storage (a) TgDMA comparison for S1 / S3 storage (b)

Table 1 :

 1 Storage conditions of composite samples after preparation

	Storage condition S1, made by keeping the samples at a temperature of 20°C and a relative humidity of
	65 % for 24 hours, was chosen to allow the stabilization of the sample. Storage condition S2, 10 days
	(240 hours) at 20°C, allows samples to regain a sufficient amount of water to observe the effect of aging.
	Last storage condition S3, a post -curing at 60°C for 20 hours, gives fully cured composites.

Table 2 :

 2 DSC data from pure epoxy analysis and its composites

	Sample	Tonset	Tpeak	Enthalpy
		(°C)	(°C)	(J.g -1 )
	β = 4°C.min -1
	GP	54 ± 3	90 ± 3	404 ± 16
	GP/GF	53 ± 2	89 ± 3	234 ± 9
	GP/FF0%	53 ± 1	90 ± 1	303 ± 17
	GP/FF10%	46 ± 2	83 ± 1	224 ± 55
	β = 6°C.min -1
	GP	59 ± 1	98 ± 1	408 ± 11
	GP/GF	57 ± 1	95 ± 1	215 ± 17
	GP/FF0%	57 ± 1	96 ± 1	280 ± 15
	GP/FF10%	50 ± 1	89 ± 1	256 ± 11
	β = 8°C.min -1
	GP	64 ± 2	104 ± 1	412 ± 19
	GP/GF	60 ± 2	100 ± 2	224 ± 37
	GP/FF0%	62 ± 1	103 ± 1	245 ± 4
	GP/FF10%	54 ± 3	96 ± 1	282 ± 75
	β = 10°C.min -1
	GP	66 ± 1	107 ± 1	405 ± 25
	GP/GF	63 ± 1	105 ± 2	252 ± 54
	GP/FF0%	67 ± 1	109 ± 1	241 ± 28
	GP/FF10%	58 ± 1	98 ± 1	228 ± 24

Table 4 :

 4 Average moisture absorption of composite materials after a storage in various humidity chambers

	:

rd International Conference on Composite Materials Belfast, 1-6th August 2021


Eden Jean-Marie

François Millet

François Millet Open

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Un autre usage identifié est celui de permettre structurer et de formaliser formaliser l'appartenance à un groupe, un projet ou un réseau, identifier et fédérer la participation et l'engagement dans des projets informels ou non, de reconnaître et réunir les communautés de pratique. Les badges associés à cet usage peuvent ainsi contenir les informations, la charte, les valeurs, le rôle, les objectifs, ... du réseau, du projet ou de la communauté.

Un des usages peut également l'invitation comme le maintien dans la participation. Cet usage est complété par celui de l'animation qui peut être mis au profit des autres usages dès lors qu'il s'agit d'animer des projets, un groupe apprenant ou l'engagement et la participation.

Enfin, un dernier usage et celui de l'évaluation ou à minima un enrichissement de la mesure d'impact d'un projet. Quelles sont les compétences les plus reconnues ou revendiquées ? Quel parcours d'activités ou d'apprentissage sont les plus efficaces, disponibles ou réalisés ? Quelle distribution ou maillage territorial peut avoir une formation ou un projet ? Autant de questions auxquelles l'utilisation des badges peut répondre en tout ou partie. Si créer un badge consistait uniquement à remplir une plateforme numérique, il y a longtemps que la démarche se serait généralisée. Mais ce n'est pas le cas. Il est nécessaire de passer par des étapes de réflexion sur la stratégie des open badges, par des adaptations à la culture de la structure qui veut les émettre, on doit réfléchir à l'écosystème dans lequel on souhaite agir. Cette phase de travail passe par des documents comme le "caneva", qui permettent l'accompagnement de celles et ceux qui créent et diffusent des badges, et dans une moindre mesure des personnes qui les endossent. Car si le badge n'est pas qu'un visuel, c'est avant tout un visuel, c'est avant tout ce qu'on perçoit. Cela peut rassurer de disposer de visuels existants, notamment pour les structures qui ne disposent pas de ressources pour créer ces visuels. En second lieu, le visuel est signifiant. La matrice est une grammaire commune, à l'intérieur de laquelle on va choisir à quel temps on veut parler, et à quelle personne, ça peut être quelque chose de très facilitant. Même s'il y a beaucoup de badges affichés dans cette matrice, on va tout de suite sur la reconnaissance. Qu'est-ce que je vais pouvoir reconnaître avec un badge, par rapport à la colonne, par rapport à la ligne. C'est précieux de pouvoir identifier directement l'usage que l'on va mettre en place, ce que l'on veut reconnaître. Pour cela, elle a été déclinée en plateforme numérique interactive.

C'est avant tout une clé de détermination, un guide pour accompagner la formalisation de l'usage que l'on souhaite avoir d'un badge et de trouver le badge type et idéal pour y répondre. la "matrice " est ainsi un des rares outils qui favorise l'autonomie des structures émettrices. Par exemple, dans les plateformes de création de badge, un certain nombre de champs sont obligatoires, avant même de pouvoir sauvegarder son brouillon. L'image du badge fait partie de ces champs ; et elle doit correspondre à un format particulier. Si vous n'avez pas un fichier image prêt à ce moment, vous ne pourrez pas enregistrer votre badge. Ce générateur le permet.

Le travail de design graphique réalisé avec l'agence Casus Belli a été très précieux et essentiel dans la diffusion de la "matrice". Aujourd'hui il existe plusieurs versions graphiques de la matrice, réalisées par différentes structures émettrices qui se la sont approprié et ont utilisé des éléments de la grammaire imaginée avec Casus Belli mais sous d'autres formes et déclinaisons. Très tôt, avec les membres de Badgeons la Normandie et l'association Reconnaître, la notion de cartographie a été évoquée comme étant un usage important. On parle de rendre visible les personnes et leur compétence, mais qu'est-ce qui permet de rendre visible le badge ? L'idée de pouvoir afficher ce badge sur une carte géographique s'est imposée également de façon très précoce dès 2017. L'objectif était de rendre visible sur un territoire les ressources et les compétences reconnues. Le principe de cette carte est de pouvoir intégrer et partager sur n'importe quelle plateforme et site Internet une carte facilitant l'identification de talents, de ressources, faciliter le recrutement. Aujourd'hui cet outil a été développé sur la plateforme Open Badge Passport (OBP), mais il n'est pas utilisé. Nous ne connaissons pas de site Internet qui se soit saisi de cet outil3 . L'outil "minimaps" également développé sur OBP est dans la continuité de la cartographie. Depuis longtemps, le Labo CIBC avait une vision de parcours rappelant la carte du métro. Autrefois, lorsqu'on rentrait dans une station de métro et que l'on souhaitait accéder à une autre, on pouvait appuyer sur un bouton de la destination et voir le parcours s'afficher. Le principe n'était pas de proposer uniquement des parcours, mais de proposer des destinations de parcours. En effet, il existe un concept de badges clés qui puissent ouvrir des portes, des serrures. mais le constat est qu'avec les open badges on ne fait que créer des clés, sans penser aux portes qu'elles peuvent ouvrir. L'outil minimap est à la fois un complément, un accompagnement et une alternative aux badges clés. Les outils "minimap" et "cartographie" viennent renforcer l'usage d'un autre outil qui est "la page" open badge qu'une personne bénéficiaire peut se créer sur OBP en sélectionnant, en agençant et en reliant certains de ses badges de façon thématique. Retrouvez l'intervention du 28 novembre 2022 en vidéo ici

  Semaine du badge 2022 -Caen Normandie Open badge : Développer l'accessibilité des outils, développer les usages Eden JEAN-MARIE et François MILLET Au-delà d'être un outil numérique qui nécessite une prise en main, l'Open badge est un objet souvent encore non-identifié et dont l'appropriation passe par des supports et des outils qui en clarifient les usages possibles. Ce n'est donc pas seulement son utilisation que doivent permettre les outils, mais également une lecture plus claire des champs ouverts par la création et l'obtention d'Open badges. C'est sur ce constat partagé que le Labo CIBC a intégré la résidence de projet du Dôme en 2018. Ce sont deux cultures distinctes mais complémentaires qui portent aujourd'hui un regard critique et réflexif sur la réelle utilité et les publics des solutions techniques qu'elles ont co-produites depuis plusieurs années.

  Cette orientation culturelle et scientifique pose plusieurs questions telles que : Comment rendre compte des apprentissages et des parcours d'éducation informelle liés à ces activités ? Comment assurer et maintenir les contributions individuelles dans ces démarches collectives ? Comment donner à voir les compétences des différents réseaux mobilisés sur ces projets ? Comment mesurer, reconnaître et représenter l'impact de telles activités sur un territoire ? Moins que l'ergonomie, la technique ou l'accessibilité des outils numériques, c'est donc sous l'angle des usages et de leur utilité que le sujet des open badges a été creusé, notamment avec le soutien d'un programme FEDER porté par l'Europe et la région Normandie. Une démarche également motivée par le fait qu'une partie des activités du Dôme se réclame d'une démarche de Living Lab 1 qui a pu être appliquée au sujet des usages des open badges.

L

  'écosystème d'usages : Traditionnellement, l'écosystème de confiance sur lequel s'appuie la reconnaissance avec les open badges s'appuie sur un triptyque dont il est supposé que pour être efficace, les usages des open badge doivent leur être utiles et partagés.

Figure 2 :

 2 Figure 2 : écosystème de confiance de la reconnaissance ouverte , d'après Serge Ravet

Figure 3 :

 3 Figure 3 : écosystème de reconnaissance élargis d'usages des open badges

Figure 4 :

 4 Figure 4 : exemples et évolution de canevas de conception d'open badge

Figure 3

 3 Figure 3.1 : répartition des outils canevas et communication dans l'écosystème de reconnaissance

Figure 5 :

 5 Figure 5 : la matrice open badge

Figure 3

 3 Figure 3.2 : répartition des outils matrice et générateur dans l'écosystème de reconnaissance

Figure 6

 6 Figure 6 : l'outil minimap sur open badge passport

Figure 3

 3 Figure 3.3 : répartition des outils cartographie, minimap et page dans l'écosystème de reconnaissance

Le Living Lab est une démarche d'innovation participative qui permet d'associer les usagers à la conception et la création des objets, usages et services dont ils seront bénéficiaires.

l'ensemble des outils suivants peut être retrouvé sur les sites https://badgeonslanormandie.fr et http://openbadges.ledome.info/

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