Thiol-ene coupling: An efficient tool for the synthesis of new biobasedaliphatic amines for epoxy curing
Adrien Cornille 1, Vincent Froidevaux 1, Claire Negrell, Sylvain Caillol*, Bernard BoutevinInstitut Charles Gerhardt, Montpellier, UMR CNRS 5253, Equipe Ing�enierie et Architectures Macromol�eculaires, Ecole Nationale Sup�erieure de Chimie deMontpellier, 8 rue de l'�ecole normale, 34296 Montpellier Cedex 5, France
a r t i c l e i n f o
Article history:Received 25 May 2014Received in revised form28 June 2014Accepted 2 July 2014Available online 9 July 2014
Thiol-ene coupling interestingly allowed to synthesize reactive primary and multi-functional aminesfrom renewable resources with high yield and in mild conditions. These syntheses were performed intwo steps from triallyl pentaerythritol (PE-Al) by esterification of the hydroxyl function with a long ormedium alkyl chain and thiol-ene coupling with the cysteamine hydrochloride on the allyl functions. Thefirst step, the esterification allowed to fix a hydrophobic chain giving the water-insoluble characteristic tothe future amine to increase the yield of the extraction after amination by thiol-ene reaction. The secondstep, the thiol-ene coupling was realized under UV initiation with excellent yields. The synthetizedmultifunctional amines were used as hardeners with different aromatic biobased epoxy matrix: thephloroglucinol tris epoxy (PGTE) and the diepoxydized cardanol (NC-514). A traditional petroleum-basedepoxy matrix, bisphenol A diglycidyl ether (BAGDE), was also used for comparison and to precise thefunctionality of synthetized amines. Glass transition temperatures of each epoxy network are evaluatedfrom 10 �C for the NC-514 networks to 100 �C for the PGTE networks and the thermal stability was alsostudied by thermogravimetric analysis.
In 2001, Sharpless et al. [1] described a new concept for chem-istry reactions, which is focused on high selective (stereospecificityand stereoselectivity) and simple reactionswithout side products inmild conditions (solventless or aqueous solvent). Several efficientreactions, which are able to produce versatile synthetic moleculesand materials, have been grouped under the term “click reactions”[2,3]. Among these new reactions, Cu(I)-mediated Huisgen reactionbetween an alkyne and an azide [2,4e7] is one of the most studiedreaction according to easy reaction conditions, high yields andorthogonality ability. This reaction encouraged researchers to putattention on other “click-reaction” as DielseAlder cycloaddition[8e10], Micha€el reaction [11], and a series of thioleaddition re-actions such as thiol-ene [12e15], thiol-isocyanate [16e19], and,the most used reaction, thiol-ene [15,20e24], which could be an
).
ionic (thiol-Micha€el [25e28]) or radical reaction. This last reaction,known for many years, is simply a thiolation of a CeC double bondfollowed by a proton-exchange. It is especially used for the prepa-ration of polymers/materials as described byHoyle et al. [29,30] andBowman et al. [31,32]. However, it has been utilized for polymer/molecule functionalization. For example, Schlaad et al. have studiedthe post-polymerization grafting of 1,2-polybutadiene homopoly-mers [33,34] and AB diblock copolymers of 1,2-polybutadiene andpoly(ethylene oxide) [35] with a range of functional thiols. Theyalso used the thiol-ene tool for the modification of poly-organosiloxane microparticles by surface-initiated photochemistryin order to create some different core/shell particles [36]. The re-action of thiols with ene compounds has been employed for thesynthesis of amines, and it represents one of the most suitable re-actions for preparing these types of molecule. Indeed, recent yearshavewitnessed a growing demand on renewable resources-derivedamines, owing to increasing environmental concern, and restrictedavailability of petrochemical resources [37,38]. Indeed, existingamines often exhibit high toxicity and high volatility, which am-plifies their dangerousness. Some amineswill have to be replaced inthe years to come, such as 4,40-methylenedianiline (MDA) [39]. This
A. Cornille et al. / Polymer 55 (2014) 5561e55705562
demand for new biobased amines is increasing with the develop-ment of diamine-based polymers such as epoxy resins, poly-bismaleimide, polyureas and non-isocyanate polyurethanes(NIPU) synthesized by reaction between di-cyclocarbonates anddiamines. Only few biobased di-amines are already industriallyavailable but most of them are poorly soluble (except in water) orcontains some harmful by-product. Different ways does exist forsynthesizing amines. Thus, the Hoffman reaction [40,41] ofammonia on alkyl halide or alcohol compounds is one of the mostused ways. Moreover, it is possible to obtain an amine by Ritterreaction [42], which is a reaction between an alkene and a nitrile,followed by a basic hydrolysis [43]. The basic hydrolysis of isocya-nate gives also an amine [44]. Hydride reaction on amide [45],nitrile [46], nitro function [47] and azides or metal reduction onnitro [48] and nitrile are, also, widely used for preparing amines.Furthermore, another reaction of ketone/aldehyde with ammo-nium salt of formic acid, followed by an acidic hydrolysis, giveseasily the corresponding amine. All these reactions suffer for majordrawbacks, such as a high number of steps, and the use ofdangerous or expensive reactants. Therefore, one of the easiest andsafest ways to synthesize amines remains the use of aforemen-tioned thiol-ene reaction. Indeed, such hydrothiolation reactionscan be proceeded under mild conditions including radical pathway,as thermal [49] or radiation decomposition [50], catalytic processesmediated by nucleophiles, acids, bases [26,27] or less commonly,via supramolecular catalysis in water using b-cyclodextrin [51].Furthermore, thiol-ene coupling can be performed on a wide rangeof -ene compoundswith versatile thiols, including highly functionalspecies, such as Thioplast® and Thiokol® polysulfides. This reactioncould also be carried out in safe solvents such as ethanol or water.Furthermore, it enabled researchers to synthesize highly reactiveprimary amines [52]. Thus, someworks have been already reportedon the synthesis of biobased amines, from aromatic renewable re-sources such as vanillin derivatives [53], or from aliphatic renew-able resources [54] such as polyene [55], grapeseed oil [56] andcastor oil derivatives [57]. However, literature essentially reportsmolecular di-amines or poly-amine polymers. Therefore we tar-geted herein the synthesis of molecular biobased poly-amines inorder to obtain reactive molecular amine able to cross-link poly-mers. Thus, the thiol-ene reaction was a choice reaction to grafthighly reactive primary amines with cysteamine hydrochloride onpentaerythritol. Indeed, pentaerythritol is a biobased multi-functional reactant, non-toxic for human being, environmentalfriendly and biodegradable. Pentaerythritol was triallylated andthen cysteamine hydrochloride was added by thiol-ene coupling toobtain biobased molecular poly-amines. Since poly-amines arewell-known to be soluble in water [58], we modified pentaery-thritol with a biobased hydrophobic backbone to improve thewater-insolubility and at the same time, the yield. Then, the syn-thesis of biobased epoxy materials was carried out with two epoxycompounds and three different aliphatic amines. Thus, one of theobjectives of our paper is also to demonstrate the interest of thiol-ene coupling for the synthesis of reactive amines.
(�97%), hydrochloric acid 1 M (HCl 1 M) and all used solvents(>99.5%) were obtained from Sigma Aldrich. Rapeseed oil waspurchased from Novance. Triethylamine (>99.9%) was purchasedfrom Fisher, NC-514 diepoxidized cardanol was supplied by Car-dolite and phloroglucinol tris epoxy (PGTE) was obtained fromSpecific Polymers.
2.2. Characterization
Chemical structure of the molecules was determined by 1HNMR and 13C NMR spectroscopy in a Bruker Avance 400 MHzspectrometer equipped with a QNP z-gradient probe at roomtemperature. External reference is trimethylsilane (TMS) for 1HNMR and 13C NMR. Shifts are given in ppm. NMR samples wereprepared as follows: 10 mg of product for 1H and 100 mg for 13Cexperiment in 0.5 mL of CDCl3. For the PE-Al characterization, a600MHz Bruker Avance III apparatus equippedwith a BBFO probe,was used.
Silica gel chromatography was performed on a Grace Reveleris.UV irradiation was performed in a Rayonet RPR-200 UV reactor
equipped of 16 UV-lamps of 35 W each of 254 nm wave length.Thermogravimetric analyses were performed using a Q50 from
TA instrument at a heating rate of 10 �C/min. Approximately, 10 mgof sample in an aluminum pan was heated from room temperatureto 500 �C under air atmosphere (60 mL min�1).
Differential scanning calorimetry (DSC) analyses were carriedout using a NETZSCH DSC200F3 calorimeter. Constant calibrationwas performed using indium, n-octadecane and n-octane stan-dards. Nitrogenwas used as the purge gas. 10e15 mg samples weresealed in aluminum pans. The thermal properties were analyzed at20 �C/min between �20 and 150 �C to observe the glass transitiontemperature at the second ramp. All the reported temperatures areon set values.
Dynamic mechanical analyses (DMA) were carried out on aMetravib DMA 25 with Dynatest 6.8. The DMA samples had arectangular geometry (length: 25 mm, width: 20 mm, thickness:0.5 mm). Uniaxial stretching of samples were performed whileheating at a rate of 3 �C/min from 20 to 200 �C, keeping frequency at1 Hz (viscoelastic region) and deformation at 5.10�4 m.
2.3. Synthesis of 3-(3-((2-aminoethyl)thio)propoxy)-2,2-bis((3-((2-aminoethyl)thio)propoxy) methyl)propan-1-ol: PE-NH2 (1)
2 g of PE-Al (7.80 mmol, 1 eq), 3.61 g of cysteamine hydro-chloride (46.81 mmol, 6 eq) were dissolved in the minimum ofethanol in the quartz reactor. The solution was stirred under UVlamp of 254 nm during 36 h. At the end of reaction, themixturewasdissolved in deionized water and K2CO3 was added to reach pH¼ 9.The solution was then extracted with dichloromethane, dried onanhydrous magnesium sulfate and concentrated under vacuum(Fig. 1). Yield: 13%.
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2.4. Synthesis of 3-(allyloxy)-2,2-bis((allyloxy)methyl)propyl 3,5,5-trimethylhexanoate: PE-C9-Al (2)
To a round-bottom flask, 10 g of PE-Al (39.0 mmol, 1 eq),4.34 g of triethylamine (42.9 mmol, 1.1 eq) were dissolved in90 mL of dry dichloromethane. The mixture was immersed in anice bath under nitrogen atmosphere. 3,5,5-trimethylhexanoylchloride (7.58 g, 42.9 mmol, 1.1 eq) was added dropwise to thesolution, with continuous stirring for 20 min. The reaction wasthen placed at room temperature for 5 h. At the end of reaction,the solution was filtered and the filtrate was washed three timeswith deionized water, dried over anhydrous magnesium sulfateand concentrated under vacuum. The crude product was purifiedby column chromatography with dichloromethane as eluent(Fig. 2). Yield: 98%.
2.5. Synthesis of 3-(allyloxy)-2,2-bis((allyloxy)methyl)propyl (E)-octadec-9-enoate: PE-C18-Al (3)
2.5.1. Saponification of rapeseed oil: (Z)-octadec-9-enoic acid: FA-C18
A round-bottom flask was charged with 100 g of rapeseed oil(113 mmol, 1 eq), 44.2 g of sodium hydroxide pellets (1.105 mol,10 eq) and 150 mL of ethanol. The solutionwas placed at 95 �C withcontinuous stirring during 2 h. Ethanol was then removed byevaporation under vacuum and the product was dissolved in themixture diethyl ether/HCl until pH ¼ 1. 100 mL of diethyl ether wasadded and the product was washed three times with brine. Theorganic phase was dried over anhydrous magnesium sulfate anddiethyl ether was removed on rotary evaporator (Fig. 3). Yield: 93%.
2.5.2. Synthesis of 3-(allyloxy)-2,2-bis((allyloxy)methyl)propyl (E)-octadec-9-enoate: PE-C18-Al
15 g of PE-Al (58.53 mmol, 1 eq), 1.01 g of APTS (5.85 mmol,0.1 eq) and 100 mL of toluene were introduced in a two-neckedround bottomed flask provided a Dean Stark and cooler. FA-C18(23.1 g, 81.94 mmol, 1.4 eq) beforehand dissolved in toluene(40 mL) was added dropwise to the solution. The mixture wasconducted for 48 h at 110 �C. At the end of reaction, the toluenewasremoved by rotary evaporator and the product was taken indichloromethane (100 mL). The solution was washed three times
Fig. 3. FA-C18.
with deionized water, dried on anhydrous magnesium sulfate andconcentrated under vacuum. The product was purified by columnchromatography with dichloromethane as eluent (Fig. 4). Yield:95%.
2.6. Synthesis of 3-(3-((2-aminoethyl)thio)propoxy)-2,2-bis((3-((2-aminoethyl)thio)propoxy) methyl)propyl 3,5,5-trimethylhexanoate:PE-C9-NH2 (4)
In a quartz reactor, 8 g of PE-C9-Al (20.2 mmol, 1 eq) and 13.48 gof cysteamine hydrochloride (118.6 mmol, 6 eq) were dissolved inthe minimum of ethanol. The solution was irradiated by UV lampof 254 nm of wave length during 36 h. At the end of thiol-enecoupling, the resulting mixture was dissolved in deionized waterand K2CO3 was added until pH ¼ 9. The solution was thenextracted with dichloromethane, dried on anhydrous magnesiumsulfate and ethyl acetate was removed under vacuum (Fig. 5).Yield: 73%.
2.7. Synthesis of 3-(3-((2-aminoethyl)thio)propoxy)-2,2-bis((3-((2-aminoethyl)thio)propoxy) methyl)propyl (E)-octadec-9-enoate: PE-C18-NH2 (5)
10 g of PE-C18-Al (13.3 mmol, 1 eq), 8.88 g of cysteamine hy-drochloride (78.2 mmol, 6 eq) were dissolved in the minimum ofethanol in the quartz reactor. The solution was irradiated during36 h by UV lamp of 254 nm. At the end of reaction, the mixture was
Fig. 6. PE-C18-NH2.
Fig. 7. 1H NMR spectra of A) PE-Al and B) protected PE-Al in CDCl3.
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dissolved in deionized water and K2CO3 was added to reach pH¼ 9.The solution was then extracted with dichloromethane, dried onanhydrous magnesium sulfate and concentrated under vacuum(Fig. 6). Yield: 76%.
The preparation of epoxy/aminematerials were carried out fromPE-C9-NH2 and PE-C18-NH2 and epoxidized compounds: BADGE,NC-514 and Phloroglucinol Tris Epoxy (PGTE).
2.8.1. Synthesis of BADGE/Amine materialsThe formulations of BADGE/Amine are calculated with equation
Equation 1 from 0.500 g of epoxy compound. The EEW of BADGE
of PE-Al in CDCl3.
Fig. 9. Reaction of acetalization with acetone on mPE-Al or dPE-Al.
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was fixed at 170 g eq�1 (Sigma Aldrich). The mixtures epoxy/aminewere carried out without solvent. Once the additions epoxy/aminewere performed, the mixtures were stirred during 3 min thenpoured into an aluminummold and heated at 80 �C during 12 h and120 �C during 8 h.
mAmine ¼ AHEWEEW
�mepoxy with AHEWtheoretical
¼ MAmineNumber of Hydrogen Actif Active
Equation 1: Amine amount determination for the formulationof the epoxy/amine materials.
2.8.2. Synthesis of NC-514/Amine materialsFor these formulations of materials, EEW of NC-514 [59] was
fixed at 356 g eq�1. The experimental protocol was the same as thepreparation of BADGE/Amines materials.
Fig. 10. Overlaying of spectra evolution for
2.8.3. Synthesis of PGTE/Amine materialsFor the preparations PGTE/Amines materials, EEW of PGTE was
fixed at 132 g eq�1 (Specific Polymers). The experimental protocolwas similar to the preparation of BADGE/Amine materials.
2.9. Protection of alcohol of PE-Al
5 g of PE-Al (19.5 mmol, 1 eq) and 0.03 g of APTS (0.02 mmol,0.01 eq) were dissolved in 30 mL of acetone in a round bottom flaskequipped with a condenser. The medium was stirred at roomtemperature for 24 h 0.03 g of potassium carbonate (0.02 mmol,0.01 eq) was subsequently added and left stirring at room tem-perature for 1 h. After evaporation of acetone, the product wasdissolved in ethyl acetate, washed three times with brine, driedover anhydrous magnesium sulfate and concentrated under vac-uum. Yield: 48%.
3. Results and discussion
The synthesis of various multi-functional amines from pen-taerythritol triallyl ether (PE-Al) by thiol-ene coupling are sum-marized in Scheme 1. The first compound (1) is obtained by a directthiol-ene coupling of PE-Al. Compound (2) and (3) are synthesizedby esterification of PE-Al respectively with 3,5,5-trimethylhexanoylchloride and FA-C18. Products (4) and (5) corresponds to the thiol-ene reaction of (2) and (3) respectively with cysteamine hydro-chloride by UV initiation.
4. Characterization of starting reactant: PE-Al
Before amine synthesis, a complete characterization of tri-allylpentaerythritol (PEeAl) has been carried out to determine
the synthesis of PEeC9-NH2 in CDCl3.
specific-polymers
Highlight
Fig. 11. Overlaying of spectra evolution for the synthesis of PEeC18-NH2 in CDCl3.
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the theoretical functionality of each synthesized amine. First, 1H 13CNMR analyseswere performed to calculate the proportion of mono-(mPE-Al), di-(dPE-Al), tri-(tPE-Al) and tetra-allypentaerythritol(tePE-Al) in the mixture. The spectra (Figs. 7A and 8), showed
Scheme 1. General scheme of aminati
that three different allylic compounds are present in the mixture.Indeed, we can observe three signals for b protons/carbons on the1H and 13C NMR spectra. Thus, we made assumption that there wassome tPE-Al in the mixture (the reactant is pentaerythritol triallyl
on of PE-Al by thiol-ene coupling.
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ether 70%) which contains also two other products among:mPE-Al,dPE-Al and tePE-Al. On the 1H NMR spectrum there is only twosignals for f protons, which means that there is two alcohol com-pounds, one of them being tPE-Al. Therefore, the third compoundcould be only tePE-Al, and the spectrum confirmed this results. Infact, on the 13C NMR spectrum there are only 17 peaks, with threepeaks for quaternary carbons a (~45,5 ppm) and two peaks for hcarbons. So, there is, for sure, tPE-Al and tePE-Al in the mixtureand, mPE-Al or dPE-Al.
To determine which one is present, a reaction of acetalization,with acetone in acidic conditions was performed. After reactionthere was a full disappearance of one peak of b protons and OHprotons (Fig. 7B). Thus, the third product is dPE-Al. Actually if it wasmPE-Al, it would remain two b and OH protons (Fig. 9). Therefore,dPE-Al, tPE-Al and tePE-Al are the three products in the mixturePEeAl. To know the exact ratio of the mixture we comparednormalized b integrations and h integrations respectively. Toconclude, there is 73% of tPE-Al, 20% of dPE-Al and 7% of tePE-Al.Therefore, only 2.87 amines maximum could be synthesized(Equation 2).
Theoretical amine Nb ¼ 0:785AllyltPE�Al þ 0;127AllyldPE�Al
þ 0:088AllyltePE�Al
¼ 0:733� 3þ 0:20� 2þ 0:067� 4
¼ 2:87 amines
Equation 2: Determination of theoretical functionality of syn-thesize amines.
5. Esterification of PE-Al
Firstly, thiol-ene coupling of PEeAl under UV-lamp with anexcess of cysteamine hydrochloride was carried out to synthesizethe multi-functional amine PEeNH2. This amine was purified bywashing with basic water to remove the excess of cysteamine hy-drochloride. The overall yield of the reactionwas only 13%. This lowyield is due to the solubility of amines in the washing water afterextraction and basic treatment. To prevent the elimination of syn-thesized amine during the purification step and, therefore, to in-crease the yield of the extraction, a grafting step of a hydrophobicbackbone on the alcohol of PEeAl was evaluated. To increase hy-drophobicity of PEeAl, a model esterification reaction was per-formed with an acyl compound: 3,5,5-trimethylhexanoyle chloride(C9). Indeed, even if the acid chlorides are toxic and non-bio-based,this reaction is easy and leads to high yields. The obtained productis noted PEeC9-Al. From the 1H NMR spectrum, the esterificationreaction is quantitative. Indeed, the peaks between 3.5 and 4 ppmcorresponding to h (hn ¼ 2 and hn ¼ 3) protons (Fig. 10B) haveshifted. The reaction-yield is quantitative (98%). This small loss isdue to the purification of PEeC9-Al on chromatography column.
Such esterification reaction of PEeAl is easy to implement, butthe aim is to graft a bio-based hydrophobic chain. Therefore, asecond esterification reaction by azeotropic distillation was per-formedwith a fatty acid FAeC18 obtained from the saponification ofrapeseed oil. The obtained product is noted PEeC18-Al. The fattyacid contains about 1.5 double bonds in its aliphatic chain. Thesedouble bonds allow the fatty acid to be in a liquid state at roomtemperature therefore there are useful for materials formulation.The reaction was characterized by 1H NMR. The spectrum shows aquantitative esterification. Indeed, the peaks between 3.5 and4 ppm corresponding to h (hn ¼ 2 and hn ¼ 3) protons (Fig.11B) haveshifted. As previously, the yield of the esterification reaction is veryhigh (95%). This small loss is due to the purification of PEeC18-Al onchromatography.
6. Thiol-ene coupling of esterified PE-AL (PE-C9-Al and PE-C18-Al)
The thiol-ene coupling of esterified PEeC9-Al and PEeC18-Alrespectively was obtained with an excess of cysteamine hydro-chloride under UV-lamp, without photo-initiator. The productsformed are respectively noted PEeC9-NH2 and PEeC18-NH2. The 1HNMR spectra (Figs. 10C and 11C) show a disappearance of all peaksbetween 5 and 6 ppm, corresponding to allylic protons d and e,which means that the functionalization by thiol-ene reaction werequantitative. In case of PEeC18-NH2 a decrease of 45% of n, o pro-tons is observed, which means that thiol-ene coupling happenedon the double bond of fatty acid (C18). In order to purify the syn-thesized amines, the excess of cysteamine hydrochloride wasremove with basic water (pH ¼ 9). The yields of synthesis afterextraction were respectively 73% and 76% for PEeC9-NH2 andPEeC18-NH2.
The yields are higher than the thiol-ene reaction of PEeAlwithout grafting of hydrophobic backbone (13% after extraction).The grafting hydrophobic backbone on the PEeAl allows obtainingamines with higher yield. However, these yields are lower than100% since small quantities of amines were remove during the basicwater washing. Indeed, the starting product PEeAl contains around7% of tePE-Al that is converted into a tetra-amine, very soluble inwater. The maximal functionality of synthesized amines is lowerthan 2.87. This hypothesis could be verified by calculation of aminefunctionality with epoxy reactants with known functionality.
7. Epoxy materials
The synthesizedmultifunctional amines were used as hardenersfor bio-based epoxy resins. We particularly studied the synthesis ofbiobased epoxy resins since there is a high demand for bio-basedthermoset resins, especially epoxy resins, since these materialsare hard to recycle owing to their infusible and insoluble properties[60] and thus need a high content in renewable carbon to reducetheir environmental impact. Some interesting studies have alreadybeen reported on the synthesis of biobased epoxy resins fromnatural phenols such as flavonoids [61], lignin or, more recently,cardanol [62] which is extracted from cashew nut shell liquid(CNSL). Epoxidized cardanol is commercially available and wasalready fully characterized by our team with a functionality of 1.32[59] and was used for the creation of a new bio-based epoxy ma-terials. We chose that industrially available epoxy compound for itsability to lead to low Tg and flexible polymer for new adhesivematerials. To enlarge the range of material type, we tested bio-based phloroglucinol [63,64], which was epoxidized in one stepby reaction with epichlorhydrin [65], for its interesting aromaticstructure (triol-functions), and its ability to lead to high Tg and rigidcomposite materials. We have synthesized epoxy materials by re-action between our two multifunctional amines and the two epoxyreactants, respectively diepoxydized cardanol (NC514) and phlor-oglucinol tris epoxy (PGTE). In order to prepare the epoxy/amineformulations we have determined the amine functionality by in-direct method (DSC). Finally, the thermal properties of the mate-rials were compared to the one obtained from the most commonepoxy: bisphenol A diglycidyl ether (BADGE).
7.1. Determination of the functionality of the synthesized multi-functional amine
In order to determine the best formulation ratios and thus thefunctionality of synthesized amines, we determined the optimumstoichiometry between these multi-functional amines and a die-poxy reactant with a well-known functionality such as BADGE, by
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measuring the maximum glass transition temperature obtained bythe material by DSC. Actually, this method is widely used in theliterature [59]. Themixtures epoxy/aminewere carried out withoutsolvent. Once the additions epoxy/amine were performed, themixtures were stirred during 3 min then poured into an aluminumpan to perform DSC analysis. For each amine, the variation of theratio epoxy/amine was studied between 0.8 and 1.5 from thetheoretical AHEWof the synthetized amines (Equation 1). The EEWof BADGE was fixed at 170 g eq�1 by SigmaeAldrich. For two net-works (BADGE/Amines), the variation of Tg, measured during thesecond DSC run, is plotted versus the epoxy/amine ratio in Fig. 12.
For the BADGE/PEeC9-NH2 network, the maximum Tg corre-sponds to the stoichiometry 1/1.2, thus the amine functionality inthe PEeC9-NH2 is closed to 2.5 available amines in the molecule. Adifference of 0.46 was observed between the theoretical value(2.87) and the DSC value, which confirms the loss of some multi-functional amines (tePE-C9-NH2 or/and tPE-C9-NH2) during thepurification step with water. For the BADGE/PEeC18-NH2 the curveexhibits a maximum Tg for the ratio equal to 1, which means that, ifwe consider the only mechanism of amine-epoxy addition, theamount of amine function was 2.9 equal to the theoretical valuecalculated by NMR. As for BADGE/PEeC9-NH2 network, the loss ofmulti-functional amine was demonstrated by the yield of the thiol-ene synthesis but the presence of a double bond on the C18 chainreactive during the thiol-ene reaction increases the amine ratio incomparison to PEeC9-NH2. According to these optimal stoichiom-etries, amine hydrogen equivalent weight (AHEW) was calculatedfor each amine following the formula: AHEWreal¼mAmine�EEW/mepoxy where m is the introduced mass of amine or epoxy in theoptimal formulation and EEW is the epoxy equivalent weight. Thereal AHEW of each synthetized amine was 126 and 127 g eq�1 forPEeC9-NH2 and PEeC18-NH2 respectively. With these DSC ana-lyses, the AHEW of the two synthesized amines were determined.These results will be useful to obtain materials with high cross-linking.
7.2. Elaboration of epoxy/amine materials
Six materials were synthesized: two with BADGE, two withdiepoxy cardanol NC-514 and, finally, two with phloroglucinol trisepoxydized (PGTE). For all the networks, the functionalities, pre-viously obtained by DSC determination, on the amines were used
Fig. 12. Evolution of glass transition temperature (Tg) versus epoxy/amine stoichi-ometry for BADGE/Amine networks.
for the calculation of optimum stoichiometry for the six formula-tions. The mixtures epoxy/amine were carried out without solvent,they were stirred during 3 min then poured into an aluminum panand cured at 80 �C during 12 h and 120 �C during 8 h. The picturesof the different formulated materials were shown on the Fig. 13. Allthematerials have a brown (in theweb version) coloration. The twonetworks made with NC-514 exhibit a high flexibility, giving firstinformation about a lower glass transition temperature.
8. Thermal characterizations of epoxy/amine materials
As a cross-linking agent, the amine molecule, depending on itsfunctionality and its chemical nature, has a strong influence on thethermosetting epoxy resin. This allows polymers to be producedwith properties associated with plastic materials ranging fromflexible, with low glass transition temperature, to strong and hardmaterials with more important Tg values [66,67]. Thermal prop-erties were firstly measured by DSC. Two dynamic temperatureramps were performed between �40 and 200 �C at 20 �C/minunder nitrogen flow. In the Table 1, the Tg obtained for BADGE/Amines networks, are 57 and 52 �C for PEeC9-NH2 and PEeC18-NH2 respectively. These values are lower than the Tg from BADGE/IPDA (isophorone diamine) (114 �C) [68] and slightly higher thanthe Tg of BADGE/Jeffamine D400 (47 �C) [69], among the most usedepoxy materials. The difference is due to the structure of amine,indeed Jeffamine D400 is a diamine constituted of linear aliphaticchains with propylene oxide units. Our synthetized amines arealiphatic but contain less methylene between two amine functionsand have a functionality higher than 2 that increases the cross-linking. For the materials with NC-514, the Tg were around11e14 �C, since the long aliphatic chain of the cardanol softens thenetwork. On the contrary, the PGTE/amines materials have a higherrigidity that comes from the epoxy functionality higher than 2. Wecan't obtain Tg values by DSC on PGTE/Amines networks, thereforewe used DMA analyses to measure Ta. We respectively measuredTa at 112 �C and 102 �C for PGTE/PEeC9-NH2 and PGTE/PEeC18-NH2, showing the hard and rigid characteristics of these systems.All these results allowed proposing three different types of mate-rials for various applications from coatings to composites. TGAwereperformed on optimum formulations in order to determine ther-mal stability of synthetized networks under air. The principal stepsof the mass loss as a function of temperature (until 500 �C) aregiven in the Table 1. The reference BADGE/Amines materials ther-mally degrade mainly through a simple step process (between 315
Fig. 13. Pictures of the different materials: A1-BADGE/PEeC9-NH2; A2-NC-514/PEeC9-NH2; A3-PGTE/PEeC9-NH2; B1-BADGE/PEeC18-NH2; B2-NC-514/PEeC18-NH2; B3-PGTE/PEeC18-NH2.
Table 1Thermal characterization of epoxy/amine materials.
A. Cornille et al. / Polymer 55 (2014) 5561e5570 5569
and 395 �C) as demonstrated Jaillet et al. [59] with the BADGE/Jeffamine400 network and with an initial degradation (Td5%) closeto 290e300 �C. The amine choice doesn't affect the thermaldegradation of materials. The TGA curve of the material with NC-514 shows two steps of degradation: the first one begins at290 �C and the second one beyond 400 �C. In the first step, thesample lost about 30% of its weight and in the second step it lostabout 55% of its weight. The char amount at 500 �C is close to 15%,slightly lower than for BADGE/Amines networks. The presence ofaliphatic chain decreases the amount of residue compared toBADGE system. The results for the PGTE networks demonstrate aTd5% similar to the NC-514 system and a unique decompositionprocess close to BADGE system. At the end, the char contents of thePGTE networks were around 28%, higher than the other networks,this result is important for applications needing a resistance to hightemperatures such as composites.
9. Conclusion
This study reports the interest of thiol-ene coupling for thesynthesis of various polyfunctional primary amines. We synthe-sized these amines by thiol-ene coupling of cysteamine hydro-chloride on triallyl pentaerythritol under UV. The direct coupling oftriallyl pentaerythritol gives low yield. This low yield is due to thesolubility of amines in the water. Therefore, in order to obtaininsoluble amine in water, and thus to increase the overall yield ofreaction (and specifically the extraction yield), a first step ofgrafting of hydrophobic backbone by esterification of hydroxylgroup of triallyl pentaerythritol was performed. These obtainedamines were then studied to determine their respective function-ality. Epoxy/amine materials were prepared from BADGE, with aknown functionality. These results showed that the synthetizedamines have functionality between 2.5 and 2.9. At last, epoxy/amine materials were prepared from two biobased epoxy: di-epoxydized cardanol (NC-514) and phloroglucinol tris epoxydized(PGTE). The thermal characterizations of the materials show thatthese networks may find applications from coatings to composites.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2014.07.004.
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