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Progress in Organic Coatings

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Cationic photopolymerization of bio-renewable epoxidized monomers

C. Noèa, S. Malburetb, A. Bouvet-Marchandb, A. Graillotb, C. Loubatb, M. Sangermanoa,⁎

a Politecnico di Torino, Dipartimento di scienza Applicata e tecnologia C.so Duca degli Abruzzi 24, 10129, Torino, Italyb SPECIFIC POLYMERS, 150 Avenue des Cocardières, 34160, Castries, France

A R T I C L E I N F O

Keywords:Biorenewable epoxy monomersCationic photopolymerizationBiorenewable coatings

A B S T R A C T

This paper reports the synthesis of epoxy functionalized bio-renewable monomers that can be applied in cationicphotopolymerization. The cationic UV-curing process was efficient giving rise to crosslinked coatings with awide range of thermo-mechanical properties. The final properties were analysed based on the monomer chemicalstructure.

1. Introduction

Recent years have witnessed an increasing demand for the re-placement of petroleum-derivate polymeric materials with bio-renew-able polymers [1–3], both because of an increasing environmentalconcern as well as because of the prediction of scarcity of petroleumresources. Epoxy polymers have been deeply investigated since they arelargely used in industrial fields such as aerospace and automotive in-dustries as matrix of structural carbon fibre composites, laminatedcircuit boards, electronic component, coatings and adhesives [4].

Among bio-renewable monomers, a large variety of vegetable oilshave been proposed since they contain several highly reactive sites [5].The epoxidation of castor oil and the cationic polymerization achievedby using a thermal-latent acid was reported by Park et al. [6]. The ki-netic of curing reaction of epoxidized vegetable oil systems thermallycured with biobased hardener was investigated. The effect of the au-tocatalytic process due to hydroxyl groups formation was demonstrated[7]. These polymer networks are potentially biodegradable in soil be-cause of hydrolytic cleavage of glycerol ester bonds [8,9].

Another class of bio-renewable epoxy monomer contains aromaticbackbone such as cinnamic acid [10], eugenol [11] and vanillin[12–14]. All these monomers have been investigated in thermal cross-linking reaction, designing a platform of biobased epoxy polymerscharacterized by high thermo-mechanical performance, due to thepresence of the aromatic backbone structure.

In the field of polymeric coatings, the UV-curing process of multi-functional monomers is an efficient method to generate three-dimen-sional crosslinked materials [15]. Among the advantages of this tech-nology it is possible to mention the high cure speed, the reduced energyconsumption and absence of VOC emission [16–18]. Therefore, UV-Curing technology can be considered the most rapid and environmental

method to transform, at room temperature, a solvent-free liquidmonomer into a crosslinked polymer. Photoinitiated polymerizationproceed by a chain-growth mechanism, involving the propagation of anactive centre, that can be either a radical or a cation [19].

Cationic photocurable systems shows important advantages withrespect to the use of radical process. First, an inert atmosphere duringcuring is not required since the carbocationic growing chains are in-sensitive to oxygen inhibition. Furthermore, cationic polymerizationmay continue after the light source has been removed, since the proticacid or Lewis acid species formed from the initiators are relatively long-lived and can continue the polymerization. This is in contrast to freeradicals, which are extinguished by a variety of termination reactionsand with no new radicals formed from the photoinitiator in the absenceof light.

While an intense scientific literature is available on thermal curingof bio-renewable epoxy resin [1–3], very few data are reported abouttheir reactivity towards cationic photopolymerization process. Crivelloet al. investigated epoxidized castor oil showing excellent reactivity inphotoinitiated cationic photopolymerization [20–22]. More recently,Lalevèe et. Al. reported the use of silyl radical chemistry for the free-radical-promoted cationic polymerization process of two epoxy mono-mers (epoxidized soybean oil and limonene dioxide) that are re-presentative of green monomers [23].

With the aim to fill the gap, in this paper it is reported the synthesisand cationic photopolymerization of different epoxidized bio-renew-able monomers. The reactivity of the starting monomers towards ca-tionic photopolymerization and the thermo-mechanical properties ofthe crosslinked films have been fully investigated; the results are dis-cussed in relationship with the monomer structures.

https://doi.org/10.1016/j.porgcoat.2019.03.054Received 24 February 2019; Received in revised form 26 March 2019; Accepted 27 March 2019

⁎ Corresponding author.E-mail address: marco.sangermano@polito.it (M. Sangermano).

Progress in Organic Coatings 133 (2019) 131–138

0300-9440/ © 2019 Elsevier B.V. All rights reserved.

T

2. Experimental

2.1. Materials

The bio-renewable epoxidized monomers: epoxidized castor oil(ECO, SP-3S-30-005), Phloroglucinol trisepoxy (PHTE, SP-9S-5-003)and Diglycidylether of vanillyl alcohol (DGEVA, SP-9S-5-005) weresynthesized as following reported and provided by SPECIFIC POLYM-ERS. Their chemical structures are reported in Table 1. The cationicphotoinitiator, triarylsulfonium hexafluoroantimonate, was purchasedfrom Aldrich (Milano, Italy). Vanillyl alcohol (98%), phloroglucinol(99%), amberlite® IR-120 hydrogen form, benzyltriethylammoniumchloride (TEBAC) (99%), anhydrous sodium sulfate Na2SO4 (99%),sodium hydroxide pellets (NaOH) and all solvents (> 95%) used werepurchased from Sigma Aldrich. Hydrogen peroxide aqueous solution(35%, w/w) and tetraethylammonium bromide (TEAB) (98%) werepurchased from Alfa Aesar. Acetic acid (99%) was purchased from abcrGmbH. All reagents, reactants and solvents were used as received.

2.2. Synthesis of the bio-renewable epoxidized monomers

2.2.1. Synthesis of epoxidized castor oil (ECO)Quantities were adjusted regarding the unsaturation content (eval-

uated by 1H NMR titration) of the castor oil (CO). Typically, theepoxidation is carried out under the following molar ratio conditionsC=C/Acetic Acid/H2O2 (35% w/w); 1/0.5/1.5eq with amberlite® IR-120 [24]. ECO was synthesized in one step as shown in Scheme 1.

A 1 L two-necked round bottom flask equipped with mechanicalstirrer and addition funnel was charged with castor oil (100 g,0.324mol of double bonds, 1eq) with 100mL of toluene. Acetic acid(9.728 g, 0.162mol, 0.5eq) was added to the reaction mixture. Then,after few minutes, amberlite® IR-120 hydrogen form (5.697 g) acting as

strong cation exchange resin was added and the mixture was stirred at510 rpm at 55 °C. Once the temperature was reached, hydrogen per-oxide aqueous solution (35%, w/w) (47.239 g, 0.49mol, 1.5eq) wasadded dropwise for 30min with continuous stirring. The reaction wasthus carried out at 55 °C for 24 h.

Once completion of reaction, the reaction mixture was filtrated toremove amberlite® IR-120 and 800ml of diethyl ether was added. Theorganic layer was washed three times with warm water (35 °C), driedon anhydrous Na2SO4 and then filtered. Solvents were removed underreduced pressure at 60 °C using a rotary evaporator. A clear viscousepoxidized castor oil (ECO) was obtained (102 g) with a yield of 98%and the product (Fig. 1) was characterized by 1H NMR.

The epoxy value was determined by NMR titration. The epoxy indexwas evaluated at 2.85 meq/g.

ECO: 1H NMR (300MHz, CDCl3, ppm) δ: 0.82 (t, 9H, H7); 1.18–1.78(m, 78H, H6); 2.25 (t, 6H, H5); 2.82–3.10 (m, 6H, H4); 3.81 (m, 3H,H3); 4.04–4.26 (2 dd, 4H, H2); 5.20 (q, 1H, H1.

2.2.2. Synthesis of phloroglucinol trisepoxy (PHTE)PHTE was synthesized according to a two-step protocol as shown in

Scheme 2 [25].

2.2.2.1. Step 1- synthesis of glycidylated phloroglucinol(PHG). Phloroglucinol (50.0 g, 0.4mol, 1eq), epichlorohydrin(550.2 g, 5.95mol, 15eq) and Tetraethylammonium bromide (TEAB,62.5 g, 0.3mol, 0.75eq) were mixed in a 1 L three-necked reactorequipped with a thermometer, mechanical stirrer and a condenser. Thereaction mixture was stirred vigorously at 70 °C for 16 h. Then, themixture was cooled down at room temperature and poured indichloromethane (800mL). The organic layer was washed three timeswith deionized water, dried on anhydrous Na2SO4, filtered andconcentrated on rotary evaporator. Glycidylated phloroglucinol (PHG)was obtained as a red viscous liquid. This product was involved in thesecond step without any further treatment.

Determination of the chlorohydrin content was evaluated by NMRtitration. The chlorohydrin index was evaluated at 1.2meq/g

2.2.2.2. Step 2- synthesis of phloroglucinol tris epoxy (PHTE). PHG(116.6 g, 1.2eq of chlorohydrin) was solubilized in dichloromethane(1000mL) and TEAB (4.17 g) was added to the mixture. Then, a NaOHsolution (22 wt.%, 1.4eq. compared to residual chlorohydrin) wasadded dropwise for 30min with continuous stirring. The reaction wasthus carried out at room temperature for 3 h. Afterwards the mixturewas washed three times with 200mL of distilled water. The organiclayer was dried on anhydrous Na2SO4 for 12 h and filtered. The excesssolvents were removed under reduced pressure at 60 °C with a rotaryevaporator. Phloroglucinol tris epoxy (PHTE) was obtained as a pale-yellow viscous liquid (111 g) with a yield of 95% and the product(Fig. 2) was characterized by 1H NMR.

Determination of the epoxy value was carried out by NMR titration.The epoxy index was evaluated at 6.78 meq/g and the average-numberof repeating units was estimated here at n=0.15.

PHTE: 1H NMR (300MHz, Acetone-d6, ppm) δ: 2.70 (m, 3H, H4b);2.84 (m, 3H, H4a); 3.30 (m, 3H, H3); 3.83 (dd, 3H, H2b); 4.28 (dd, 3H,H2a); 6.20 (s, 3H, H1).

2.2.3. Synthesis of diglycidylether of vanillyl alcohol (DGEVA)DGEVA was synthesized according to a two-step one-pot process as

shown in Scheme 3 [26].A three-necked reactor equipped with a thermometer and a me-

chanical stirrer was charged with vanillyl alcohol (10 g, 0.065mol,1.0eq) and benzyltriethylammonium chloride (TEBAC, 1.5 g, 0.006,0.1eq). Epichlorohydrin (60 g, 0.65mol, 10.0eq) was added and themixture was stirred for 4 h at 30 °C until obtention of a limpid pinksolution. Then, this solution was cooled down to 15 °C A NaOH solution(33 wt.%, 15.0 eq.) was prepared and poured slowly into the cold

Table 1Chemical structure of the bio-renewable epoxidized monomers epoxidizedcastor oil (a) ECO, Phloroglucinol trisepoxy (b) PHTE and Diglycidylether ofvanillyl alcohol (c) DGEVA.

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mixture under vigorous stirring. The reaction was conducted overnightat 15 °C. Deionized water was added (60mL) to the mixture as well asethyl acetate (200mL). Then, the organic layer was washed two moretimes with deionized water and dried on anhydrous Na2SO4. Residualsolvents were removed on a rotary evaporator at 60 °C. Diglycidyl etherof vanillyl alcohol (DGEVA) was obtained as a white solid with a yieldof 94% and the product (Fig. 3) was characterized by 1H NMR.

Determination of the epoxy value was carried out by NMR titration.The epoxy index was evaluated at 7.28meq/g and the average-number

of repeating units was estimated here at n=0.03.DGEVA: 1H NMR (300MHz, CDCl3, ppm) δ: 2.64 (dd, 1H, H8b);

2.76 (m, 2H, H4b); 2.82 (m, 1H, H8a); 2.91 (m, 1H, H4a); 3.21 (m, 1H,H7); 3.43 (m, 2H, H6b, H3); 3.75 (dd, 1H, H6a); 3.90 (s, 3H, H9); 4.06(dd, 1H, H2b); 4.24 (dd, 1H, H2a); 4.52 (d, 2H, H5); 6.90 (m, 3H, H1).

2.3. Photocuring of epoxidized bio-renewable monomers

The synthesized epoxy monomers were mixed with the cationic

Scheme 1. Synthetic route of ECO.

Fig. 1. 1H NMR spectra of the epoxidized castor oil (ECO) in CDCl3.

Scheme 2. Synthetic route of PHTE.

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photoinitiator at a content of 4 wt%.The formulations were coated on PP substrates and covered with a

PP foil, to ensure a homogenous thickness of about 100 μm as well as toprotect the specimens from the atmospheric moisture. The films werethereafter crosslinked by using Fusion lamp with a conveyor belt with aspeed of 6m/min and a light intensity of 224mW/cm2.

2.4. Characterization

1H NMR spectrum of the synthesized monomers were obtainedusing Bruker Advance 300 (300MHz) spectrometer equipped with aQNP probe at room temperature. Deuterated solvents used are given foreach molecule. The Epoxide Index (EI, number of moles of epoxidegroups per gram, eq. g−1) was determined according to 1H NMR ti-tration method. The method consists in solubilizing a known mass ofthe product and of an internal standard (3,5-Dinitrobenzoic acid (3equivalent H) in acetone-d6 whose signal is dissociated from theothers). The number of moles of epoxide functions per gram of productwas measured by comparing the integration of the standard (3 H) withthe integration of a signal accounting for all the oxirane rings (for in-stance H4a, H4b, H8a and H8b for DGEVA i.e 4 H).

The average number of repeating units (n) was also determined by1H NMR, comparing the epoxide integrations (3H per phloroglucinol(H3) and 4H per DGEVA (H4a, H4b, H8a, H8b)) to aromatic integrations(3(2n+1)H per molecule).

The photocurable formulation was coated on a silicon wafer(thickness of the mixture was about 25 μm), and FTIR measurements

were performed on a Nicolet iS 50 Spectrometer before and after irra-diation under the Fusion lamp. Spectra were recorded in transmissionmode. Data were collected at a scanning rate of 1 scan per 1.2 s with aspectral resolution of 4.0 cm− 1. The epoxy group conversion wasmeasured following the peak cantered at around 760 cm−1 [27,28].Data were recorded and processed using the software Omnic fromThermo Fisher Scientific.

DSC analysis were performed by using a Mettler Toledo DSC in-strument. Samples having masses of approximately 10mg were insert ina 100 μl aluminium pans with pierced lids in a nitrogen atmosphere.The applied heating rate was 5 °C min−1 in a nitrogen atmosphere (rate50ml min−1). Thermal behaviour of the samples was investigated usingtwo repeated heating-cooling cycles. It was used the following thermalprocedure: first ramp from -60 °C to 150 °C and then from 150 °C to-60 °C. Glass transition temperature (Tg) was determined from thesecond heating curve.

Dynamic thermal-mechanical analysis (DMTA) were performedwith a Triton Technology. Samples were cooled with liquid nitrogenand measurements were run with a heating rate of 3 °C min− 1 intensile mode.

Contact angle measurements were performed with a Kruss DSA10instrument, equipped with a video camera and an image analyzer.Analyses were made at room temperature by means of the sessile droptechnique. Three to five measurements were performed on each sampleand the values averaged. The measuring liquids were doubly distilledwater (γ=72.1mNm−1)

Fig. 2. 1H NMR spectra of the PHTE in acetone-d6.

Scheme 3. Synthetic route of DGEVA.

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3. Results and discussion

Three different bio-renewable epoxy monomers (ECO, PHTE andDGEVA) were synthesized, as reported in the experimental part.Considering the structure of each compound, theoretical epoxy valueswere evaluated at 3.24meq/g, 7.51meq/g and 10.2meq/g for ECO,DGEVA and PHTE respectively. However, the experimental epoxy va-lues obtained by 1H NMR titration were equal to 2.89meq/g, 7.28meq/gand 6.78meq/g respectively. The deviations between theoretical andexperimental epoxy values for each compound are discussed herebelow.

Firstly, epoxidation of castor oil is not complete since residualdouble bonds are still observed in 1H NMR (Fig. 1). Residual doublebonds are evaluated at 0.12meq/g by the 1H NMR titration method.Moreover, additional side-reactions might occur during the epoxidationprocess leading to a lower epoxy content [29].

Regarding the DGEVA molecule, glycidylation process developedfor its production demonstrates a relatively low deviation, implying alow proportion of side-reactions.

However, in the case of PHTE, the glycidylation is accompanied byseveral side reactions which explains the large deviation between the-oretical and experimental epoxy values and that other peaks are ob-served in 1H NMR which do not correspond to the PHTE structure forn=0 (Fig. 2).

Compared to DGEVA, it appears that glycidylation of polyfunctionalphenolic compounds (trifunctional in the case of phloroglucinol) leadsto more secondary reactions than monophenolic compounds (such asvanillyl alcohol). According to H. Nouailhas and co-workers [30], oneby-product corresponds to the homopolymerization of the resin (PHTEwith n> 0). Indeed, oligomerization of di-epoxy phenolic monomers isa common phenomenon observed in industrial epoxy resins and con-duct to the repetition of the monomer unit (n). The molecular weightincreasing leads to formation of hydroxyl groups whose consequenceson the cationic photopolymerization reactivity and material properties

will be discussed later in this paper. Moreover, some other by-productsmay result from polyaddition of epichlorohydrin, formation of β-chlorohydrin, or α-glycol, explaining the large deviation in epoxycontents. As a matter of fact, all of these by-products have also beenobserved in the case of Bisphenol A glycidylation [31]. In order to get abetter understanding of the mechanisms at stake during the glycidyla-tion of both DGEVA and PHTE, further investigations are currentlycarried out and will be the subject of a forthcoming paper.

Regarding the DGEVA and PHTE monomers, the average number ofmonomer units determined by 1H NMR was estimated respectively ton=0.03 and n=0.15. Based on these values, the number of hydro-xylated functions could be evaluated at 0.109meq/g and 0.323meq/gfor DGEVA and PHTE respectively.

The reactivity of the different epoxy monomers was investigated byFT-IR analysis, following the decrease of the epoxy peak centred ataround 760 cm−1, after irradiation. The FT-IR before and after irra-diation is reported in Fig. 4A, 4B and 4C respectively for the monomerECO, PHTE and DGEVA. The final epoxy group conversions are col-lected in Table 1.

The different epoxy group conversion can be attributed to the oli-gomer structure as well as the presence of OH groups in the differentmonomers.

The ECO chemical structure gives rise to flexible polymeric network(see the Tg values below) which allows to reach a high epoxy groupconversion (about 85%) because of a high mobility of the polymericchains. The rigid aromatic structure present in the oligomer DGEVAleads to an early vitrification, with a decrease of the molecular mobi-lity, reducing the macro-carbocationic propagation rate; therefore, alower epoxy group conversion was reached (about 60%). Although thePHTE monomer contains as well aromatic structures (and furthermoreit is a trifunctional monomer, which should induce a higher crosslinkingdensity) it shows a similar final epoxy group conversion of ECO (about80%).

This can be explained taking into consideration that, besides to the

Fig. 3. 1H NMR spectra of the DGEVA in CDCl3.

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usual cationic epoxy ring-opening mechanism, known as the activatedchain-end mechanism (ACE), an additional mechanism can take place.This mechanism, described by Penczek and Kubisa [32,33], is called theactivated monomer mechanism (AM) and it occurs when the cationicpolymerization of epoxides is carried out in the presence of OH groups.

During the polymerization, the growing ionic chain-end undergoesnucleophilic attack by the OH group to give a protonated ether. De-protonation of this latter species by the epoxy monomer results in thetermination of the growing chain and the proton transfer to themonomer which can start a new chain. As a result, a higher epoxy group

Fig. 4. FT-IR spectra of the ECO (1 A), PHTE (1B) and DGEVA (1C) monomer, before (black spectra) and after (red spectra) UV-irradiation. Fusion lamp, 4 passes,velocity 6m/min, film thickness 25 μm, cationic photoinitiator content 4 wt%, light intensity 224mW/cm2 (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article).

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conversion is reached, and more flexible ether structures are found inthe crosslinked network. The PHTE monomer contains a higher hy-droxyl content (0.323meq/g for PHTE0, while 109meq/g for DGEVA)which will interact with the carbocationic growing chain via the AMmechanism delaying vitrification and reaching a higher final epoxygroup conversion. In all cases, it was possible to obtain a fully curedfree-standing film.

Thermal and dynamic-mechanical analyses were performed oncured films. While DSC analysis allows to characterize the thermalproperties of the crosslinked networks (data collected in Table 1),DMTA analyses permit the evaluation for the elastic and viscous com-ponents of the modulus of the material in a large temperature interval.Therefore, this technique gives a complete characterization of thethermal and viscoelastic properties of the material.

In Fig. 5 the tanδ curves are reported for the crosslinked epoxymonomers. The maximum of tanδ curves is assumed as the Tg of thepolymer network. The data are collected in Table 1. The Tg valuesobtained by DMTA analysis are higher than those obtained by DSC;similar results were reported previously and are attributed to a fre-quency effect [34].

From the Tg data collected in Table 1, it is evident that it is possibleto obtain a wide range of Tg values for the crosslinked film by simplyvarying the oligomer structure. The ECO crosslinked film showed a Tgcentred at around 20 °C, while PHTE showed a higher Tg of around75 °C. Finally, the DGEVA crosslinked film is characterized by a Tg ofabout 86 °C. The increase of the Tg value from ECO to DGEVA is due tothe presence of aromatic rigid structure in the polymeric backbone. Thehigher rigidity of this latter polymeric network is also the reason of thelower epoxy group conversion upon curing, as discussed above. ThePHTE crosslinked film shows a lower Tg value notwithstanding thepresence of aromatic structure. This can be explained because of thechain transfer reaction involving OH group, as previously discussed,which induces a polymer network flexibilization due to the formation ofthe flexible ether structures. The Tg data are, therefore, in agreementwith the epoxy group conversion measured by FT-IR analysis.

From the curves reported in Fig. 5 we can identify a sharp tanδ peakcantered at 20 °C for ECO, whereas the PHTE crosslinked networkevidenced a broader peak cantered at 75 °C A narrow tanδ peak reflectsthe quick formation of a homogeneous network. The broader peak ofcrosslinked PHTE is an indication of a highly crosslinked material [35].A different result was obtained for DGEVA-based network, for whichalong with an intense peak centred at 86 °C, a shoulder is evident athigher temperature (around 120 °C). This shoulder can be explained bythe formation of a highly crosslinked heterogeneous network as alreadydiscussed in literature by Coqueret [36].

Advancing contact angle measurements were performed with

distilled water. The values are reported in Table 2. The ECO networkshows the higher hydrophobicity (contact angle with water equal to94°) because of the long alkyl chains present in the monomer structurewhich imparts hydrophobicity to the polymeric network. The PHTE andDGEVA polymer network showed similar hydrophilicity with a contactangle with water between 73° and 75°.

4. Conclusions

In conclusion, this work presents an enhanced sustainable approachto the synthesis of epoxy functionalized bio-renewable monomers thatcan be applied in cationic photopolymerization. The cationic UV-curingprocess was efficient obtaining crosslinked coatings with a wide rangeof thermo-mechanical properties. The final properties were analysed onthe basis of the monomer chemical structure. ECO-based coatings couldreach a high epoxy group conversion due to the low Tg (around 20 °C)which allows a high mobility of the polymeric growing chain. TheDGEVA-based coating leads to an early vitrification possibly due to adecrease of the molecular mobility, caused by the presence of aromaticgroups on the polymer backbone. The PHTE crosslinked film shows alower Tg value notwithstanding the presence of aromatic structure. Thiscan be explained by the involvement of OH groups in the chain transferreaction, which enhance the epoxy group conversion taking advantageof the insertion of new polymeric flexible ether chains. The Tg dataagree with the epoxy group conversion measured by FT-IR analysis. Ourstudy successfully provides insight into the potentiality of the epoxybio-renewable monomers in the field of cationic photopolymerization.

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