Development of Sustainable Thermosets from Cardanol-based Epoxy

Research Article pubs.acs.org/journal/ascecg

Development of Sustainable Thermosets from Cardanol-based Epoxy Prepolymer and Ionic Liquids Thi Khanh Ly Nguyen,†,‡,§ Sébastien Livi,*,†,‡,§ Bluma G. Soares,∥ Guilherme M. O. Barra,⊥ Jean-François Gérard,†,‡,§ and Jannick Duchet-Rumeau†,‡,§ †

Université de Lyon, F-69003 Lyon, France INSA Lyon, F-69621 Villeurbanne, France § CNRS, UMR 5223, Ingénierie des Matériaux Polymères, F-69621 Villeurbanne, France ∥ Universidade Federal do Rio de Janeiro, PEMM-COPPE, Centro de Tecnologia, 21941-972 Rio de Janeiro, Brazil ⊥ Universidade Federal de Santa Catarina, Departamento de Engenharia Mecânica, Florianópolis, Santa Catarina, Brazil ‡

S Supporting Information *

ABSTRACT: In this work, the development of new biobased epoxy networks using phosphonium-based ionic liquids combined with dicyanamide and phosphinate counteranions as initiators of epoxy prepolymers was compared with a biobased epoxy−amine system. Ionic liquids (ILs) displayed high reactivity toward cardanol-based epoxy prepolymers, and the obtained biobased epoxy/IL networks highlighted a glass transition temperature of around 30 °C, an excellent thermal stability higher than 450 °C and higher hydrophobic behavior compared to biobased epoxy−amine networks. In addition, the use of a cardanol-based epoxy prepolymer as a modifier of epoxy/IL networks was also investigated. In this case, the final properties of epoxy/IL networks were improved by the presence of biobased epoxy compounds, especially their surface energy as well as their fracture toughness (+180%), suggesting a new way to develop partially biobased epoxy coatings. KEYWORDS: Ionic liquids, Epoxy networks, Cardolite, Mechanical properties, Initiator

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prepolymers14−19 or curing agents20−24) have been used by several authors with conventional comonomers of epoxy networks resulting in epoxy networks with excellent properties. Then, the combination of both cardanol-based epoxy prepolymers and a curing agent23 provides fully biobased epoxy networks suggesting a new way to develop environmentally friendly polymer materials. Recently, ionic liquids (ILs) well-known to have excellent thermal stability, good ionic conductivity, low saturated vapor pressure, and nonflammability have recently become a new target of academic and industrial research as additives within epoxy networks to develop new polymer electrolytes and anticorrosive coatings or for composite applications.25,26 In fact, the incorporation of a small amount of ILs can efficiently improve the properties of conventional epoxy networks such as thermo-mechanical properties,27−29 wear and scratch resistance,30 and ionic conductivity.31,32 Self-healing properties of ILs were also revealed on the surface of the epoxy/amine system after multiple scratch tests.33,34 In addition, the potential of ILs

INTRODUCTION Epoxy networks play a key role in the thermosetting polymer fields with a wide range of applications due to their excellent chemical and mechanical properties.1 Nowadays, the most popular epoxy prepolymer currently used in the epoxy applications is diglycidyl ether of bisphenol A (DGEBA) derived from petroleum products.1 However, the use of bisphenol A (BPA) in the polymers has recently raised concerns due to its toxicity.2 Moreover, the increase of oil prices and environmental problems also require the development of biobased epoxy systems derived from sustainable resources. Among all the available biobased epoxy systems, those derived from cashew nut shell liquid commercially denoted as cardanol-based epoxy compounds appear to be great candidates to substitute for DGEBA due to their commercial availability.2,3 In fact, the chemical structure of cardanol containing both aromatic rings and long aliphatic chains provides an exceptional combination of properties.4 Cardanol-based compounds can serve as modifiers of conventional epoxy networks in order to improve their processability5,6 and their flexibility7−10 (impact resistance and fracture toughness), as well as their chemical and water resistance.11−13 In addition, cardanol-based epoxy comonomers (epoxy © XXXX American Chemical Society

Received: July 8, 2017 Revised: July 28, 2017

A

DOI: 10.1021/acssuschemeng.7b02292 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Chemical structures of Cardolite cardanol derived epoxy prepolymer and phosphonium ILs. Preparation of Biobased Epoxy/Amine (CA/D230) and Epoxy/IL (CA/IL) Networks. To prepare biobased epoxy networks, Cardolite (CA) was mixed with ionic liquid (IL-DCA or IL-TMP) at 10 phr or with D230 at a stoichiometric ratio until a homogeneous mixture was reached. The concentration of active species calculated per epoxide equivalent was also calculated (see Table S1 in the Supporting Information). The mixtures were degassed at room temperature over 1 h before being poured into silicone molds. The curing protocol was chosen for all systems as 2 h at 120 °C, 2 h at 140 °C, and 1.5 h at 210 °C in order to complete the curing reaction. Preparation of CA-Modified Epoxy/Amine or Epoxy/IL Networks. In order to study the effect of CA incorporation in the properties of epoxy networks, CA-modified epoxy systems were prepared by adding 10 phr of Cardolite in the mixture of DGEBA and curing agent (D230 or ILs) at room temperature in order to obtain a homogeneous mixture. The mixture was then poured into silicone molds and cured in an oven. The curing protocol for CA-modified epoxy networks was 2 h at 80 °C, 3 h at 120 °C, and 3 h at 160 °C. Characterization Methods. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry thermograms (DSC) of CA networks and CA-modified epoxy networks were performed on DSC Q20 TA Instruments from 20 to 250 °C at a rate of 10 K min−1 under a nitrogen flow of 50 mL min−1. FTIR Absorption. FTIR absorption spectra were recorded on a Thermo Scientific Nicolet iS10 Spectrometer with a transmission accessory with IR absorptions from 400 to 4000 cm−1. Then, the conversion of the epoxide group was calculated from the relation between two absorption peaks at 914 and 1184 cm−1 for CA-modified epoxy networks43 or at 1244 cm−1 for CA networks16 based on the following equation:

as initiators to replace conventional curing agents of epoxy prepolymers has already been demonstrated.35−38 These new epoxy/IL systems requirea low content of initiators thanks to the high basicity of the counteranion of ILs toward epoxy prepolymers. In fact, our research group has demonstrated that a high basicity of the anion induced an increase in the polymerization kinetics. The obtained epoxy/IL networks have excellent thermal and mechanical properties combined with outperformed hydrophobic properties.35−38 Indeed, the strong dependence of the nanostructuration and properties of epoxy/ IL networks on the chemical nature of ILs38 makes them tunable for a wide range of applications. Nevertheless, due to the solvent effect of ILs, epoxy/IL networks are also excellent hosts of nanoparticles and particles in the preparation of composites or nanocomposites.39−42 Hence, the combination between ionic liquids and biobased materials in particular cardanol-based epoxy prepolymers can provide a new way to design and develop sustainable and environmentally friendly high-performance materials. Thus, this work will describe the preparation of cardanol-based epoxy/IL networks. In fact, a commercial cardanol-based epoxy prepolymer from Cardolite will be used to partially and totally substitute DGEBA in the epoxy/IL networks. Also, the effect of IL structure will be demonstrated through the incorporation of two phosphonium ionic liquids combined with dicyanamide and phosphinate counteranions as reactive components of epoxy systems. Moreover, the polymerization kinetics (DSC, FTIR) of epoxy/IL mixtures, thermal (ATG, DSC) and thermomechanical properties (DMA), as well as the morphology and surface properties of cardanol-based epoxy/IL networks will be investigated and compared to a reference cured with an aliphatic amine denoted Jeffamine D230. In addition, the influence of the presence of a cardanol-based epoxy prepolymer with a long alkyl chain on the fracture toughness of epoxy/IL networks will be highlighted.

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X% =

A0 − At × 100% At

(1)

where A0 and At are the ratios between the areas of two absorption peaks at 914 cm−1 and 1184 (or 1244) cm−1 (A914/A1184 or A914/A1244) of the system at the beginning (t = 0) and at a reaction time t, respectively. Thermogravimetric Analyses (TGA). Thermogravimetric analyses (TGA) of CA/IL networks and CA-modified epoxy/IL networks were carried out using a Q500 Thermogravimetric analyzer (TA Instruments) from 30 to 700 °C at a heating rate of 20 K min−1 under a nitrogen atmosphere. Dynamic Mechanical Analysis (DMA). Dynamic mechanical analysis (DMA) was carried out on Rheometrics Solid Analyzer RSAII at 0.05% tensile strain under a frequency of 1 Hz. The heating rate was 3 K min−1 for a temperature range from 30 to 250 °C. Surface Energy. The surface energy of biobased epoxy/IL networks was determined from the sessile drop method using a Dataphysics goniometer with water and diiodomethane used as probe liquids. Nondispersive (polar and H-bond) and dispersive components of surface energy were calculated using the Owens−Wendt theory.44 Fracture Toughness. The KIc of biobased epoxy/IL networks was determined using a compact tension specimen according to the ISO 13586 Standard. The initial portion of the V notch was obtained with a milling cutter, and the starter crack was introduced at the root of the notch by tapping with a fine razor blade. The ratio of crack length to

EXPERIMENTAL SECTION

Materials. A biobased epoxy prepolymer derived from Cardanol under the reference of Cardolite NC-514 (CA) was purchased from Cardolite, USA. Two kinds of ionic liquids, trihexyl(tetradecyl)phosphonium dicyanamidedenoted IL-DCA (Tdmax = 420 °C) and trihexyl(tetradecyl)phosphonium bis-2,4,4-(trimethyl pentyl) phosphinatedenoted IL-TMP (Tdmax = 380 °C)were kindly provided by Cytec. Chemical structures of Cardolite and ionic liquids are presented in Figure 1. Also, a conventional epoxy prepolymer diglycidyl ether of bisphenol A (DGEBA)-based epoxy prepolymer (DER 332) with an epoxide equivalent weight (EEW) of 175 g· mol−1was purchased from DOW Chemical Company. The curing agent used is an aliphatic diamine (Jeffamine D230) provided from Hunstman with an amine hydrogen equivalent weight (AHEW) of 60 g·mol−1. B


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Figure 2. DSC curves (10 K min−1, under nitrogen) of different epoxy-based reactive systems with DGEBA (■) and CA (□) containing different curing agents, (a) D230, (b) IL-DCA, and (c) IL-TMP. width (a/w) is to be maintained between 0.2 and 0.8. The KIc specimens were then tested in tension mode using a MTS tensile instrument with a 1 kN load cell at a speed of 10 mm min−1. Then, fracture toughness was calculated using the following equation:

KIc = f (a /w)

For DGEBA-based epoxy systems cured with ILs or amine, the exothermic peak occurs at the same temperature as that reported in a previous work, i.e., 140 °C for the reference DGEBA/D230 as well as for the blends with IL-TMP.37,38 ILDCA also exhibits high reactivity toward DGEBA with the exothermic peak at 194 °C for systems containing 10 phr of ILDCA. Then, DSC curves performed on the biobased epoxy blends (D230 or IL-TMP or IL-DCA) highlighted exothermic peaks, suggesting the existence of curing reactions between the curing agent or initiators and cardanol-based epoxy prepolymer (CA). Moreover, the substitution of DGEBA by CA has no influence on the reactivity of reference systems cured with D230 due to the similarity of the chemical structure of CA and DGEBA. The same results were observed and described by Chrysanthos et al. for epoxy blends containing cycloaliphaticdiamine.16 In opposition, CA/IL blends present a higher exothermic peak temperature than DGEBA-based ones. This reduction of reactivity can be explained by the chainwise mechanism of the reaction between the epoxy prepolymer and ILs proposed previously,37,38 differing from the stepwise mechanism in case of epoxy/amine reactions. In detail, ILs act as initiators to open the epoxide ring of the epoxy prepolymer to form an anionic reactive center (O−) which in turn reacts with the other epoxide groups in order to form the epoxy networks. Thus, the steric hindrance effect of the CA-based reactive center due to the presence of a long alkyl chain between two epoxide groups results in a decrease of the system reactivity compared to DGEBA. Moreover, similar to systems based on DGEBA, the reactivity of CA/ILs systems also depends on the structure of ILs. Concerning CA/IL-DCA systems, DSC curves display only one exothermic peak, which is shifted to higher temperature, indicating the reduction of the system reactivity due to the substitution of DGEBA by the cardanol-based epoxy prepolymer. In detail, when the same amount of IL (10 phr) is used, the exothermic peaks of epoxy systems cured by ILDCA are shifted from 194 °C for the DGEBA-based system to 210 °C for the CA-based system, respectively. On the other hand, the DSC curve of CA/IL-TMP displays three peaks, suggesting a more complicated curing mechanism. Indeed, Chrysanthos et al. have studied the CA prepolymer with size exclusion chromatography and have found a complex structure of this epoxy prepolymer with the existence of various oligomers.16 It should be noted that IL-TMP is more reactive than IL-DCA with a lower exothermic peak temperature for the blend with DGEBA;36 the ability of IL-TMP to react with all the components in the CA epoxy prepolymers leads to a complex enthalpy profile. In addition, the lowest value of the exothermic peak temperature of CA systems with 10 phr of IL-

FQ (2)

h w

where FQ (N) is the load at the initiation of the crack, a (m) is the crack length, w (m) is the width of the specimen, h (m) is the thickness of the specimen, and f(a/w) is the geometry constant: (2 + a)

f (a /w) =

(1 − a)3/2

(0.886 + 4.64a − 13.32a2 + 14.72a3

− 5.6a 4)

(3)

Transmission Electron Microscopy (TEM). Transmission electron microscopy (TEM) was performed at the Technical Center of Microstructures (University of Lyon) using a Phillips CM 120 microscope operating at 80 kV to characterize the dispersion of ILs and/or Cardolite in the biobased epoxy/IL networks. The 60-nm-thick ultrathin sections of samples were obtained using an ultramicrotome equipped with a diamond knife and were then set on copper grids. Scanning Electron Microscopy (SEM). Scanning electron microscopy (SEM) on a Phillips XL20 microscope with a tension of acceleration of 20 kV was used to characterize the fracture surface of KIc specimens. The samples were cleaned using ethanol45 and then coated with gold.

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RESULTS AND DISCUSSION Biobased Epoxy Networks. Reactivity of Cardanolbased Systems. The reactivity of cardanol-based epoxy/IL reactive systems was characterized using DSC from the reaction enthalpy peak. Thus, DSC curves of different IL cured epoxy (DGEBA or CA) systems were illustrated in Figure 2 with the reaction enthalpy and exothermic peak temperature presented in Table 1. Table 1. Exothermic Peak Temperature and Reaction Enthalpy (DSC, 10 K min−1) of Different Epoxy Reactive Systems samples DGEBA/D230 CA/D230 DGEBA/ILDCA-10 CA/IL-DCA-10 DGEBA/ILTMP-10 CA/IL-TMP-10

ΔH (J g−1)

ΔH (kJ eq−1)

141 140 194

447.7 211.4 370.6

105.0 97.2 71.3

210 140

108.1 251.9

47.6 48.5

1st peak (°C)

163

2nd peak (°C)

263

3rd peak (°C)

344 C


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ACS Sustainable Chemistry & Engineering TMP is 163 °C compared to 140 °C of DGEBA/IL-TMP blends, which also suggests a decrease of the system reactivity, similarly to IL-DCA-based systems. In order to compare the different systems, the reaction enthalpy was calculated in Joules per blend weight (J g−1) and kilojoules per epoxide equivalent (kJ eq−1). The enthalpy of reaction between CA and IL-TMP was not included due to the complexity of the DSC curve with several peaks at high temperatures. In the case of using D230, reactive systems based on DGEBA and CA have a similar enthalpy, which is in the typical range obtained for an epoxy/amine reaction at a stoichiometric ratio. This also confirms the equivalent reactivity of these two systems. On the other hand, the replacement of DGEBA by CA in IL-DCA-based blends leads to a reduction of exothermicity of the reaction from 71.3 to 47.6 kJ eq−1. In conclusion, similar to amine, ILs (IL-DCA or IL-TMP) are able to react with the cardanol-based epoxy prepolymer. The reactivity of epoxy/IL systems depends strongly on the chemical structure of monomers and initiators (epoxy prepolymer or ILs). FTIR Analysis on Cardanol-based Blends. In order to examine the epoxy conversion, FTIR analysis was only used to follow the reaction between the cardanol-based epoxy prepolymer (CA) and D230 or ILs since the systems based on DGEBA were widely investigated in the literature.36,38,46 Thus, FTIR spectra of each system were recorded at different reaction times during the curing process, and the epoxide group conversion of different epoxy systems was calculated according to eq 1 and is presented in Figure 3 as a function of curing time.

group of CA/IL blends showed a higher reactivity of IL-TMP compared to IL-DCA. In fact, after the same curing time (1 h at 120 °C), CA epoxy systems based on D230, IL-TMP, and ILDCA exhibit conversions of epoxide groups of 76%, 14%, and 9%, respectively. In conclusion, FTIR results confirmed that the prepolymer derived from cardanol initiated by ILs led to a high conversion of the epoxide group. Surface Energy of Cardanol-based Networks. The surface energies of epoxy/IL networks (CA or DGEBA) were determined with the sessile drop method. The nondispersive and dispersive components of the epoxy networks calculated using the Owens−Wendt method from the contact angles with water and diiodomethane are presented in Table 2. Table 2. Contact Angles and Surface Energy of Epoxy Networks Measured by Sessile Drop Method samples

Θwater

Θdiiodomethane

γnondispersive (mJ m−2)

γdispersive (mJ m−2)

γtotal (mJ m−2)

DGEBA/ D230 CA/D230 DGEBA/ILDCA CA/IL-DCA DGEBA/ILTMP CA/ILTMP

72

50

9.2

27.5

36.7

81 102

53 63

5.1 0.5

27.9 27.2

33.0 27.6

103 91

66 44

0.7 1.0

24.3 37.3

25.0 38.3

95

61

0.9

26.7

27.6

In fact, the use of ILs as reactive additives for the epoxy prepolymer (DGEBA) was discovered to induce a more significant hydrophobic behavior thanks to the hydrophobic nature of ionic liquids compared to conventional epoxy/amine networks.37,38 As can be seen in Table 2, DGEBA networks cured with ILs (both IL-DCA and IL-TMP) display a low nondispersive component and surface energy compared to the DGEBA/D230 network. Then, the replacement of the conventional epoxy prepolymer by one derived from cardanol generates a significant reduction of the surface energy for all epoxy networks. CA-based networks present a lower surface energy than the DGEBAbased one, i.e., 33.0 mJ m−2 for CA/D230, 25.0 mJ m−2 for CA/IL-DCA, and 27.6 mJ m−2 for CA/IL-TMP. In fact, the chemical structure of Cardanol led to excellent water resistance compared to DGEBA thanks to a long aliphatic pending chain.4 In fact, many authors have used Cardanol-based epoxy prepolymers as modifiers for epoxy coatings.13,18 Moreover, these results proved that the use of ILs and cardanol-based epoxy prepolymers induced a more hydrophobic surface than systems cured with amine. In conclusion, the combination of two hydrophobic components, ionic liquids and cardanol-based epoxy prepolymers, results in outperformed hydrophobic systems, suggesting new perspectives for coatings applications. Thermal Behavior of Cardanol-based Networks. The thermal stability of CA/IL networks was investigated by thermogravimetric analysis (TGA; see Figure S1 in the Supporting Information). The degradation temperatures at 5% and 10% of weight loss and at the maximum of DTGA curves are summarized in Table 3. First, DGEBA-based networks present the same thermal stability as reported in the literature, in which DGEBA/IL

Figure 3. Conversion of epoxide groups versus reaction time of different CA-based systems containing D230 (■), IL-DCA (□), ILTMP (▲).

As expected, a decrease of the absorption peak at 914 cm−1 corresponding to the epoxide group is observed in the FTIR spectra of all CA-based mixtures (with D230, IL-DCA, and ILTMP), highlighting the opening of the epoxide ring during the curing process.25,36 At the end of the curing protocol, this absorption peak nearly disappears for all of the CA-based blends. Thus, it is evident that the applied curing schedule for CA-based systems is effective, allowing it to reach a final epoxy conversion of about 95%. Also, FTIR confirms the DSC results about the reactivity of the curing agent toward the CA prepolymer since the conversion of the epoxide group is more significant by using D230 with an exothermic peak at 140 °C than systems based on ILs. Then, the conversion of the epoxide D


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Table 4. Relaxation Temperature, Tα, and Storage Modulus in the Rubbery State, ER′, of Epoxy Networks

Table 3. Degradation temperatures used epoxy prepolymers and formed epoxy networks samples CA DGEBA DGEBA/D230 CA/D230 DGEBA/IL-DCA CA/IL-DCA DGEBA/ILTMP CA/IL-TMP

Td5% (°C)

Td10% (°C)

TdmaxDTGA (°C)

Td 2nd peak (°C)

247 256 350 325 384 349 401

296 272 360 344 426 376 430

454 335 376 450 458 465 458

365

322

360

466

366

367

samples

Tα (°C)

ER′ (MPa)

DGEBA/D230 CA/D230 DGEBA/IL-DCA CA/IL-DCA DGEBA/IL-TMP-10 CA/IL-TMP-10

100 36 126 31 150 20

16 4.6 31.2 2.6 104.0 1.1

410

(D230 or IL-DCA or IL-TMP) are consistent with the data of the literature.36,38 Regarding CA/IL networks, all systems display a low glass transition region included between 20 and 36 °C, which is on the same magnitude order as the relaxation temperatures of cardanol-based epoxy prepolymer cured by different amines, such as isophorone diamine,16,18,19,47 Jeffamine D40017 and T403,18,19 and biobased curing agents.23,48 In fact, the presence of a long aliphatic chain of cardanol-based epoxy prepolymers is known to lead at the epoxy/IL networks, having lower relaxation temperatures compared to those of DGEBA epoxybased networks.16 In addition, the storage moduli in the rubbery state of CA-based networks are considerably lower than those of DGEBA-based systems. The storage modulus in the rubbery state is representative for the cross-linking density of the epoxy network, suggesting a lower cross-linking density of cardanol-based networks.49 Indeed, the long aliphatic chain between two epoxide functional groups of cardanol-based epoxy prepolymers led to a decrease of the cross-linking density of related epoxy networks.16,17 Overall, the flexible structure of cardanol-based epoxy prepolymers resulted in epoxy networks with low glass transition temperatures and cross-linking density. Influence of Cardolite on the Properties of DGEBAbased Systems. Although ILs are able to form epoxy networks with biobased epoxy prepolymers, it is evident from the previous part that the properties of CA/ILs systems are not competitive with those based on DGEBA. The partial replacement of DGEBA by cardolite appears to be a new way to introduce biobased materials into conventional systems while keeping their properties. Thus, the following part is dedicated to investigating the influence of the addition of cardolite (10 phr) on the properties of DGEBA/ILs systems. Curing Behavior of CA-Modified Epoxy Blends. The effect of a cardanol-based epoxy prepolymer on the curing behavior of conventional epoxy blends was also investigated by nonisothermal DSC. DSC thermograms obtained under dynamic mode in the exothermal region of CA-modified epoxy blends are illustrated in Figure 4 with the reaction enthalpy and exothermic peak temperature presented in Table 5. DSC thermograms of epoxy blends before the curing process show one exothermal peak corresponding to the reaction between epoxide groups of both DGEBA and CA with the amine or ionic liquids (IL-DCA or IL-TMP). The exothermic peak temperatures of unmodified DGEBA/D230, DGEBA/ILDCA, and DGEBA/IL-TMP blends are in agreement with our previous works.36−38 Then, the addition of cardanol-based epoxy prepolymer (10 phr) has no influence on the exothermic temperature of the DGEBA/D230 system since similar values are obtained for DGEBA/D230 and CA/D230 blends (see Tables 1, 5). On the other hand, the introduction of CA in the epoxy/IL blends led to an increase of their exothermic peak temperatures. The same

networks exhibit better thermal behavior than those containing D230 thanks to the higher degradation temperature of ILs compared to D230.36,38 In fact, the degradation temperatures of the DGEBA prepolymer cured with IL-DCA and IL-TMP are both at 458 °C. Concerning the cardanol-based networks, the degradation temperatures determined at the maximum of the DTGA peaks are 450 °C for amine systems and over 460 °C for those cured with ILs, higher than epoxy networks based on DGEBA, especially for the case of D230. In fact, the difference of maximal degradation temperature can be explained by the better intrinsic stability of cardanol-based epoxy prepolymers characterized by a higher DTGA peak temperature of 454 °C compared to 335 °C of DGEBA (Table 3). On the other hand, the oligomers contained in CA prepolymers result in a lower degradation temperature at 5% and 10% of CA/IL networks compared to DGEBA-based networks. In addition, all CAbased networks display two peaks of degradation, which is consistent with the studies about cardanol-based epoxy networks cured with amines.17,23 Obviously, this multistep decomposition can be explained by the complex profile of cardanol-based epoxy prepolymers with the existence of several oligomers. Nevertheless, compared to cardanol-based epoxy/ amine networks (CA/D230) and reported systems in the literature, the use of ILs as initiators of cardanol-based epoxy networks also led to a higher thermal stability17,23 In fact, Darroman et al. have found a degradation temperature at 30% of weight loss at 360 °C for cardanol-based epoxy/amine networks. By using IL, we have obtained an increase of the degradation temperature of 40 °C, i.e., around 410−420 °C for the same weight loss. In conclusion, phosphonium ionic liquids can be considered as alternatives of curing agents for biobased epoxy prepolymers derived from cardanol. In fact, these new epoxy/IL networks have a similar thermal stability to that of conventional epoxy prepolymers based on DGEBA and better than the epoxy/ amine systems. Dynamical Mechanical Properties of Cardanol-based Networks. The influence of the chemical nature of the epoxy prepolymer (DGEBA versus CA) on the dynamical mechanical properties of epoxy networks was studied with DMA (see Figure S2 in the Supporting Information). The α-relaxation temperature, Tα, from the maximum of tan δ as well as the storage modulus in the rubbery state, ER′, of epoxy networks are listed in Table 4. In all cases, DMA spectra display only one relaxation peak, suggesting the homogeneity of all epoxy networks. The relaxation temperatures (Tα) of the DGEBA-based systems E


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Figure 4. DSC thermogram of CA-modified epoxy networks (■) containing D230 (left), Il-DCA (middle), and IL-TMP (right) compared to the neat ones (○).

Table 5. Reaction Enthalpy and Exothermic Peak Temperature (DSC, 10 K min−1) of CA-Modified and Unmodified Epoxy/D230 and Epoxy/IL Networks samples DGEBA/D230 CA modified DGEBA/D230 DGEBA/IL-DCA CA modified DGEBA/ILDCA DGEBA/IL-TMP CA modified DGEBA/ILTMP

reaction peak (°C)

ΔH (J g−1)

ΔH (kJ eq−1)

141 139 191 200

447.7 446.0 370.6 223.5

105.0 107.7 71.3 45.0

140 148

251.9 113.22

48.5 16.5

Figure 5. Epoxide group conversion as a function of curing time of CA-modified epoxy networks cured with D230 (■), IL-DCA (○), or IL-TMP (▲).

phenomenom in epoxy/anhydride systems reactivity was observed in the literature due to the presence ofa biobased epoxy prepolymer.5,50,51 Thus, the CA epoxy prepolymer has a lower reactivity toward ionic liquids compared to DGEBA due to the flexible chemical structure of the cardanol-based epoxy prepolymer (Reactivity of Cardanol-based Systems). In detail, the exothermic peak is shifted from 190 °C for unmodified DGEBA/IL-DCA blends to around 200 °C with the existence of CA. Similarly, an increase of exothermic peak temperature (9 °C) is observed when CA is added to the DGEBA/IL-TMP blends due to the reaction between IL-TMP with the CA prepolymer and its oligomers occurring at higher temperatures (Figure 2). However, CA/IL-TMP blends exhibited a complex exothermic peak. In fact, the exothermic peak evidenced in Figure 2 showed that the exothermic peak corresponding to the reaction between IL-TMP and CA is superimposed on that related to the DGEBA/IL-TMP reaction due to the low proportion of CA used in the blends. Similarly, the reaction enthalpy calculated by joules per epoxide equivalents decreases with the addition of CA into epoxy/IL blends, while the same values were obtained for systems containing D230. These results are explained by lower enthalpy of CA/IL blends compared to DGEBA-based ones (Reactivity of Cardanol-based Systems). Overall, the presence of a small amount of CA prepolymer induced a decrease of the reactivity of epoxy/IL blends resulting in an increase of the exothermic peak temperature. FTIR Analysis of CA-Modified Epoxy/IL Blends during the Curing Process. FTIR analysis was used to follow the curing process of CA-modified epoxy blends based on the absorption peak corresponding to the epoxide groups. Then, the evolution of epoxide group conversion of CA-modified epoxy blends as a function of curing agent (D230 or IL-DCA or IL-TMP) and curing time was shown in Figure 5.

As evidenced in Figure 5, after the curing process of 2 h at 80 °C, 3 h at 120 °C, and 3 h at 160 °C, a conversion of over 90% is obtained for all CA-modified epoxy systems. In fact, at the end of the curing process, the absorption peak at 914 cm−1 corresponding to epoxide group of both DGEBA and CA disappeared, indicating the incorporation of the cardanol-based epoxy prepolymer in the epoxy networks.10 Moreover, the evolution of this peak was affected by the addition of CA and the reactivity of amine or ILs. As expected, the existence of CA in epoxy/IL systems led to a decrease of their reactivity, indicated by the decrease of the epoxide group conversion. In detail, after 1 h of curing at 80 °C, the CA-modified DGEBA/ IL-TMP system has exhibited an epoxide group conversion of 30% compared to 50% of the unmodified system reported in the literature.38 These results can be explained by the exothermic peak corresponding to the reaction between CA and IL-TMP at high temperatures of over 160 °C (Table 5). In conclusion, the incorporation of CA in the epoxy blends led to a reduction of epoxide group conversion depending on the type of curing agent (amine or ILs). Morphologies of CA-Modified Epoxy Networks. The morphologies of CA-modified epoxy/IL networks were studied by transmission electronic microscopy with TEM. It should be noted that CA-modified DGEBA/D230 displayed no phase separation due to the high compatibility of CA in the DGEBAbased matrix;10 only TEM micrographs of CA-based epoxy/IL networks were presented in Figure 6. It was discovered from our previous works that the use of only 10 phr of IL (IL-DCA or IL-TMP) as initiators of DGEBA led to no phase separation due to the good miscibility between F


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Table 6. Thermomechanical Behavior of CA-Modified Epoxy Networks (D230 or IL-DCA or IL-TMP) samples

1st Tα (°C)

DGEBA/D230 CA modified DGEBA/D230 DGEBA/IL-DCA CA modified DGEBA/IL-DCA DGEBA/IL-TMP CA modified DGEBA/IL-TMP

100 90 126 107 104

2nd Tα (°C)

150 147 150 140

Then, the use of a cardanol-based epoxy prepolymer in the epoxy matrix induced a variation in the thermomechanical properties, including the relaxation temperatures (Table 6), which depend on the chemical nature of the initiator. The DMA curve of the CA-modified epoxy/amine network only showed one relaxation peak, indicating the miscibility of CA in the DGEBA-based matrix.10 The incorporation of CA in the epoxy/amine network reduced its relaxation temperature of about 10 °C. In fact, reductions of Tα have also been reported in the literature when bio-based epoxy prepolymers or oligomers with flexible backbones and free alkyl chains were introduced into conventional epoxy networks due to a reduction of cross-linking density.9,10,13,51−57 In addition, both relaxation peaks of DGEBA/IL-DCA networks are shifted to lower temperatures due to the presence of flexible CA epoxy prepolymers. Regarding the mechanism of the curing reaction between the epoxy prepolymer and IL-DCA36 and the close reactivity of IL-DCA toward both epoxy prepolymers (DGEBA or CA; Figure 2), the two curing reactions can take place at the same time, leading to the formation of a heterogeneous epoxy network containing both a CA and a DGEBA backbone. On the other hand, DMA confirms the phase separation in the case of IL-TMP-based systems in Figure 7 with the appearance of a second relaxation peak when CA was added to DGEBA/ILTMP blends. In detail, the addition of 10 phr of CA into DGEBA/IL-TMP involves two relaxation peaks at 104 and 140 °C, which may correspond to CA and DGEBA rich phases, respectively. The temperatures of these two relaxation peaks are between one of the neat phases of 150 °C and around 20−25 °C for DGEBA/IL-TMP and CA/IL-TMP networks, respectively (Table 4). The decrease of Tα corresponding to DGEBA/

Figure 6. TEM micrographs of CA-modified epoxy/IL networks cured with IL-DCA (a) and IL-TMP (b) containing 10 phr of the cardanolbased epoxy prepolymer.

epoxy and ILs.36 Then, the addition of 10 phr of CA into epoxy networks resulted in the formation of only one phase, which can be explained by the small amount and the good affinity of CA in the epoxy matrix. In fact, in the literature, many authors have observed either one or two phases of epoxy networks modified with biobased epoxy prepolymers depending on the compatibility between DGEBA and biobased epoxy prepolymers.9,10,59,52 For CA-modified DGEBA/IL-DCA and CAmodified DGEBA/IL-TMP systems, Figure 6 highlighted the presence of a small inclusions of 200−300 nm randomly dispersed in the epoxy matrix. These inclusions were identified as IL clusters based on our previous works.28,36 Dynamic Mechanical Properties of CA-Modified Epoxy Networks. Thermomechanical properties of CA-modified epoxy networks were evaluated by DMA with the evolution of the storage modulus (E′) and tan δ as a function of the temperature of CA-modified epoxy/IL networks as presented in Figure 7, and the relaxation temperatures from the maximum of tan δ are listed in Table 6. For unmodified epoxy networks, DGEBA/D230 and DGEBA/IL-TMP networks displayed a single relaxation peak at 100 and 150 °C, respectively,37,38 while two relaxation peaks are observed for those based on IL-DCA at 126 and 150 °C, respectively. These two relaxation peaks are attributed to the formation of an epoxy homopolymer and of a DGEBA/ILDCA network explained by the two competitive mechanisms proposed previously.38

Figure 7. DMA analysis (E′, tan δ) of unmodified (■, □) and CA-modified epoxy networks (●, ○) cured by (a) D230, (b) IL-DCA, and (c) ILTMP. G


Research Article


addition, the substitution of conventional amines by ILs reinforced the hydrophobic behavior of epoxy networks where energy surface values of 13 and 14 mJ m−2 have been obtained for CA-modified epoxy/IL networks initiated by ILTMP and IL-DCA, respectively. Overall, the results on the surface energy of CA-modified networks presented here suggest a great combination between ILs and cardanol-based epoxy prepolymers, leading to networks with emphasized hydrophobicity. Fracture Toughness of CA-Modified Epoxy Networks. The fracture toughness of all systems was studied using the CT specimen, and the values of the critical stress intensity factor denoted KIc of unmodified and CA-modified epoxy networks are presented in Table 8. SEM micrographs of the fracture

IL-TMP networks may be due to the existence of a small amount of CA which acts as a plasticizing agent. Indeed, Altuna et al. have discovered the plasticizing effect of epoxidized soy bean oil in the continuous phase of DGEBA.52 Vice versa, it should be noted that the CA amount is only 10 phr; there must be a great amount of chemically bonded DGEBA in this second phase resulting in a much higher relaxation temperature compared to CA/IL-TMP networks. An interpenetrating effect occurs between the two phases of CA-modified DGEBA/ILTMP systems, which is similar to the case of using thermoplastics as modifiers for epoxy networks.58 Thus, the incorporation of biobased epoxy prepolymer induces a phase separation in the epoxy/IL networks witnessed by two relaxation peaks. In addition, interpenetration has occurred between the two polymers, leading to the variation of the glass transition temperatures depending on the type of ILs and the amount of CA introduced. Surface Energy of CA-Modified Epoxy Networks. The surface energies of unmodified and CA-modified epoxy networks are characterized by the sessile drop method, and the nondispersive and dispersive components are shown in Table 7.

Table 8. Fracture Toughness (KIc) of Unmodified and CAModified Epoxy Networks

Table 7. Surface Energy of Neat Epoxy Networks and Epoxy Networks Modified by CA samples DGEBA/D230 CA modified DGEBA/D230 DGEBA/ILDCA CA modified DGEBA/ILDCA DGEBA/ILTMP CA modified DGEBA/ILTMP

Θwater (deg)

Θdiiodomethane (deg)

γnondispersive (mJ m−2)

γdispersive (mJ m−2)

γtotal (mJ m−2)

72 85

50 62

9.2 4.7

27.5 23.2

36.7 27.9

102

62.5

0.5

27.2

27.6

108

85.7

0.9

13.5

14.4

91

44.4

1.0

37.3

38.3

103

90.1

3.1

10.3

13.4

samples

KIc (MPa m1/2) ± 0.05

DGEBA/D230 CA-modified DGBEA/D230 DGEBA/IL-DCA CA-modified DGEBA/IL-DCA DGEBA/IL-TMP CA-modified DGEBA/IL-TMP

1.05 0.76 0.41 0.33 0.39 1.08

surface after the fracture toughness test of CA-modified epoxy networks are presented in Figure S3 (see Supporting Information) and are consistent with the obtained KIc values. According to the literature, epoxy cured by IL-DCA or ILTMP has a lower fracture toughness of around 0.4 MPa.m1/2 compared to 1.05 MPa.m1/2 for the DGEBA/D230 network due to the higher cross-linking density and higher glass transition temperature of these networks (Table 6).38 The incorporation of a cardanol-based epoxy prepolymer with a flexible chain is expected to improve the flexibility of these epoxy networks knowing that a decrease of the relaxation temperature of epoxy/IL networks (IL-DCA and IL-TMP) in Table 6 was observed, suggesting the formation of more flexible networks. However, despite the addition of the flexible cardanol-based epoxy prepolymer leading to a decrease of the relaxation temperature, a slight decrease of KIC was obtained for modified DGEBA/D230 and DGEBA/IL-DCA networks. Similarly, in the studies of Miyagawa et al., no change of fracture toughness of epoxy networks was obtained when epoxidized linseed oil was used as a substitution for DGEBA.59,56 The authors have explained the results by the lack of a rubbery separated phase. In opposition, a significant increase of the fracture toughness was obtained when 10 phr of CA was incorporated into DGEBA/IL-TMP networks. The KIc value of CA modified DGEBA/IL-TMP networks was 1.08 MPa m1/2, i.e., a relative increase of 180% compared to neat DGEBA/IL-TMP. Park et al.55 and Shabeer et al.51 have also observed an increase of the fracture toughness of epoxy networks due to the presence of biobased compounds and have explained their results by the decrease of cross-linking density. However, according to Maiorana et al., a significant improvement can only be achieved with the existence of a phase separation.9 In this case, a more homogeneously dispersed phase is observed when 10 phr CA are introduced into DGEBA/IL-TMP networks (Figure 8) than those cured with IL-DCA, while the flexibility of cured samples by both ILs is nearly identical (Figure 8a,b). Thus, the rubbery

In Table 7, the surface energies of unmodified epoxy systems cured or initiated with D230 or IL-DCA or IL-TMP are in agreement with our previous studies37,38 where epoxy/IL networks using phosphonium ionic liquids denoted IL-TMP and IL-DCA as initiators exhibited a low polar component compared to the epoxy/amine one. Then, the incorporation of CA into epoxy networks led to a reduction in their surface energy, indicating the formation of highly hydrophobic systems. The CA modified DGEBA/D230 network presents a lower surface energy due to the significant decrease of the nondispersive components compared to the neat one (4.7 mJ m−2 compared to 9.2 mJ m−2). In this case, the water resistance of the cardanol-based epoxy prepolymer is the major reason for the increase of the system’s hydrophobicity as previously observed for the CA/D230 network compared to the DGEBAbased one presented in Table 2. In the literature, many authors have demonstrated the use of cardanol-based epoxy prepolymers as surface modifiers for epoxy systems.11−13 Due to the chemical nature of cardanol-based compounds, relevant properties for coating applications such as the water resistance, water vapor diffusion, as well as the solvent resistance of modified epoxy networks were significantly improved. In H



Research Article

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ACKNOWLEDGMENTS

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REFERENCES

The authors gratefully acknowledge the financial support from Ministère de l’Enseignement Supérieur et de la Recherche (MESR).

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Figure 8. Flexibility test on CA-modified DGEBA/IL-DCA (a) and DGEBA/IL-TMP (b) networks containing 10 phr of CA.

dispersed phase is a decisive factor and has a great influence on the toughening effect of CA-modified epoxy networks.

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CONCLUSION In this work, phosphonium ILs based on phosphinate and dicyanamide counteranions were first investigated as initiators of biobased epoxy systems and compared to an amine reference system. Thus, CA/IL networks were processed with an excellent thermal stability (>460 °C), good thermo-mechanical properties combined with a high hydrophobic behavior thanks to the unique structure of the cardanol-based epoxy prepolymer with aromatic rings, a long backbone, and an alkyl pending chain. In a second part, Cardolite was used as modifiers of epoxy networks based on DGEBA/ILs. Thus, we have demonstrated that the incorporation of CA combined with the chemical nature of the phosphonium ionic liquids plays a key role in the properties of modified epoxy networks, especially the fracture toughness. In fact, the combination of hydrophobic IL, epoxy prepolymers, and a cardanol base led to a very low surface energy, suggesting promising candidates for coating applications. Finally, this work opens a new way to develop “sustainable materials” with outperformed properties and a new alternative to petroleum-based materials. Further studies are required to reveal the relevancy between ILs structure and the properties of biobased epoxy/IL networks.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02292. The concentration of the reactive additives for ILs and amine; TGA and DTG curves as well as DMA curves of epoxy based on DGEBA or CA initiated by IL-DCA and IL-TMP and cured with amine; SEM micrographs of fracture surface of unmodified and CA-modified epoxy networks containing 10 phr of CA (d,e,f) cured with D230, IL-DCA and IL-TMP (c,f) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sébastien Livi: 0000-0003-4959-982X Bluma G. Soares: 0000-0002-1273-7574 Notes

The authors declare no competing financial interest. I


Research Article

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Development of Sustainable Thermosets from Cardanol-based Epoxy

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