Curing Kinetics of Biobased Epoxies for Tailored ... - ACS Publications

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Curing Kinetics of Biobased Epoxies for Tailored Applications Ammar Patel,† Anthony Maiorana,‡ Liang Yue,† Richard A. Gross,‡ and Ica Manas-Zloczower*,† †

Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New York 12180, United States

‡

S Supporting Information *

ABSTRACT: The curing kinetics of a family of biobased epoxies derived from n-alkyl diphenolate esters differing in ester side chain length were compared with diglycidyl ether of bisphenol A (DGEBA). Isothermal isoconversional analysis and Kamal−Sourour model fitting by differential scanning calorimetry (DSC) were utilized to obtain reaction constants. The biobased epoxides and DGEBA have reaction orders that are comparable while the autocatalytic rate constant of DGEBA was larger than those of the biobased epoxies. As the n-alkyl side chain length of diphenolate esters increased, the autocatalytic rate constant decreased. Furthermore, the non-autocatalytic rate constant for DGEBA is smaller than that of the biobased epoxies. The cause for the difference in rate constants is discussed, and applications are assigned to the epoxies based on curing kinetics.

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INTRODUCTION Polymer thermosets are used in various high performance applications. They are often selected due to their high glass transition temperature, chemical resistance, and high modulus.1 Epoxy resins mixed with a curing agent/hardener (diamines, disulfides, or anhydrides) represent the majority of thermoset polymers. These resins, once mixed, undergo a series of reactions that transform them from a fairly viscous liquid to a gel (formation of three-dimensional network) and, ultimately, to a cross-linked network.2,3 The combination of curing agent and epoxy type affects molecular parameters such as free volume,4,5 cross-link density, and, ultimately, the thermomechanical properties of the network.6−8 However, it is not only the final product properties that determine the end application. Processing parameters of the epoxy resin such as the rate at which the viscosity rises and time required to reach the gel point are all critically important. Quick curing epoxies are required for adhesives. In contrast, low-viscosity epoxy resins which require longer times to gel, i.e., when the viscosity remains constant for a long time, are more suitable for mold filling applications such as wind turbine blades and boat hulls. Thus, studying the curing kinetics of new epoxy resins is essential to determining their end application. In order to study the reaction kinetics, knowledge of the reactions taking place is necessary. A typical epoxy amine cure reaction occurs as follows: (i) the primary amine reacts with an epoxide forming a secondary amine and an alcohol, (ii) reaction of the secondary amine with another epoxide to form a tertiary amine and an alcohol, and (iii) the alcohol group reacts with an epoxide to give an ether (see Scheme 1). The primary and secondary amine reactions are well documented in the literature.9−12 The etherification reaction, © XXXX American Chemical Society

Scheme 1. Typical Epoxy−Amine Reactions

mentioned by a number of authors, is believed to take place at relatively higher temperatures and at later stages of cure.13−15 Thus, epoxy−amine curing involves multiple reactions each having different catalysts or initiators.16−18 Most epoxy reactions undergo autocatalysis because hydroxyl functional groups formed partly protonate the epoxy oxygen atom lowering the activation energy for ring-opening reactions.19 However, during initial stages of epoxy cure reactions, prior to the formation of a Received: June 13, 2016 Revised: July 13, 2016

A

DOI: 10.1021/acs.macromol.6b01261 Macromolecules XXXX, XXX, XXX−XXX

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assume that primary and secondary amines have equal reactivity, and etherification reactions do not take place. The majority of all epoxy resins are based on bisphenol A (BPA). However, BPA may contribute to reduced fertility as well as the onset of diseases including cancer.31−33 This potential risk has sparked a demand for a suitable replacement.34 Also, BPA is derived from petroleum, and due to the nonrenewable nature of petroleum, increasing efforts are being made to replace petroleum and petroleum derived products with those obtained from building blocks derived from readily renewable sources (biobased).35,36 In recent years, numerous biobased epoxy resins have been synthesized. Examples include epoxy resins from esters of a polyol and 4-hydroxybenzoic acid,37 thermoset resins from pines and other plants,38 tannin derivatives,39 and isosorbide.40 Thus far, studies on the curing behavior of these epoxies have been on a peripheral level.41−44 Although the kinetics of epoxy curing reactions using the diglycidyl ether of bisphenol A (DGEBA) with a variety of different curing agents has been extensively studied,17,18,45−50 the recent surge of new-to-the-world biobased epoxy resins and curing agents requires a more thorough examination of their cure kinetics in order to determine if they indeed represent viable BPA replacements as well as to establish the application(s) for which they are best suited. Recently, we have successfully synthesized a promising family of biobased epoxy resins from n-alkyl esters of diphenolic acid (DGEDP epoxies). These epoxies, once cured, have similar or better mechanical and thermal properties to DGEBA.51 This paper describes a comprehensive study on the curing kinetics of this family of epoxy resins. Comparisons of biobased DGEDP epoxy cure kinetics were made with DGEBA and EPON 828, two bisphenol A based epoxy resins. The aim of these studies was to differentiate the reaction constants based on chemical structure. Methods used to assess epoxy cure include dynamic chemorheology, differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). Possible explanations for differences observed are discussed. The ultimate goal of this work was to provide a means to estimate what category of applications would best fit biobased DGEDP epoxy resins based on curing kinetics. For example, an epoxy resin with a high reaction constant would be suitable for adhesives, whereas those with smaller rate constants are more suitable for vacuum infusion molding type processes.

sufficient concentration of hydroxyl groups, the reaction is catalyzed by available hydrogen bond donor HX molecules such as water and impurities.16 In addition to concurrent multiple reactions, epoxy curing has further complications. The glass transition temperature (Tg) of the epoxy increases as the reaction progresses,20 and if the Tg of the material increases above the temperature of reaction, the reaction will become diffusion controlled and the rate will approach zero.21−23 Furthermore, gelation increases the viscosity by 3 orders of magnitude when the degree of conversion increases from 0.1 to 0.6.24 The diffusion coefficient will decrease with this viscosity rise25 which will affect the reaction kinetics.19 Thus, calculating the kinetic parameters unaffected by diffusion is essential if curing kinetics of epoxy resins is to be studied as an aid to processing during the initial stages. Since the curing kinetics of these epoxies have never been studied before, isoconversional analysis is initially used to determine the mechanism of curing. Analysis of the Ea by the isoconversional data analysis method is useful in identifying the rate-determining step of a reaction. There is no requirement for prior knowledge of the reaction order nor is it dependent on whether the chemical transformation occurs via a single or multiple steps. Thus, changes in the reaction mechanism associated with changes in the activation energy can be detected by isoconversional analysis.26,27 Apart from being used in epoxy curing reactions, isoconversional analysis has been successfully used in gaining insights into other phenomena such as degradation reactions,28 crystallization,28 and diffusion.19 Once, the reaction mechanism has been established, the curing parameters can be calculated based on previous studies done on epoxy curing kinetics. This has largely been investigated with two constitutive equations: nth order and autocatalytic. In nth-order kinetics, it is assumed that the rate of reaction is proportional to the concentration of unreacted materials described by

dα = k(1 − α)n dt

(1)

where k is the rate constant, α is the degree of conversion, and n is the reaction order. The nth-order reactions work under the assumption that the reaction rate is maximum at time = 0, and the reaction products do not influence the reaction. In contrast, autocatalytic reactions assume that at least one of the reaction products are involved in reaction catalysis. Consequently, autocatalytic reactions reach a maximum reaction rate after time >0 at around 20−40% degree of conversion. Autocatalytic reactions are often described by the Horie model29 and the Kamal−Sourour model30 shown in eqs 2 and 3, respectively: dα = (k1 + k 2α)(1 − α)(B − α) dt

(2)

dα = (k1 + k 2α m)(1 − α)n dt

(3)

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EXPERIMENTAL SECTION

Materials. DGEBA and isophorone diamine (IPD) were purchased from Sigma-Aldrich. The DGEDP epoxies were prepared by identical methods that are described elsewhere.51 EPON 828 resin was purchased from Momentive Specialty Chemicals Inc. All purchased chemicals were obtained in the highest available purity and were used as received. The epoxy resin and curing agent were mixed together using a glass rod for 3 min. For rheology and isothermal DSC, the instrument was equilibrated to curing reaction temperatures of 80, 90, 100, and 110 °C, and thereafter, samples were held isothermally for testing. Dynamic DSC curing was carried out from −25 to 230 °C at 4 ramping rates: 2, 3, 4, and 5 °C/min. The induction time between mixing of the epoxy and curing agent and starting the test was kept constant at 5 min for all epoxies. Instrumental Methods. Rheology. Rheological measurements were obtained on a TA Instruments ARES G2 rheometer using a 25 mm parallel plate geometry, at a frequency of 1 Hz, with a gap size of 1 mm and a strain of 0.2%. Each test was repeated 2−3 times to ensure reproducibility.

where k1 is the rate constant catalyzed by impurities (moisture, dust particles, etc.) present in the system, k2 is the rate constant catalyzed by hydroxyl groups that form during the course of reaction, B is the initial ratio of amine to epoxide content, m and n are reaction orders, and α is the degree of conversion. The Kamal−Sourour model, developed by Kamal and co-workers, attempts to describe an autocatalytic reaction when the Horie model fails. To do this, they introduced reaction exponent’s m and n to better fit the experimental data. Both these models B


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Figure 1. Evolution of the complex viscosity as a function of cure time for (a) DGEBA at different temperatures (y-axis linear scale). (b) DGEBA and DGEDP epoxies studied herein at 90 °C cure temperature (y-axis log scale). Differential Scanning Calorimetry (DSC). DSC was performed on test samples using a TA Instruments DSC Q100 under nitrogen flow using aluminum hermetic pans obtained from TA Instruments. The total area under DSC exothermic transitions was averaged for the different heating rates during dynamic curing experiments to determine the total heat of reaction (ΔHtotal). Reactions were assumed to be complete when there was no further change as monitored by DSC. The curves for heat flow vs temperature for all epoxies are presented in Supporting Information 2. Values of isothermal heat of reaction (ΔHiso) were determined for isothermal curing reactions. Values of (ΔHiso) were then used to determine the maximum degree of conversion (αm) at a particular temperature, which is given by αm =

ΔHiso ΔHtotal

phr =

1 ΔHtotal

∫0

(4)

t

⎛ dH ⎞ ⎜ ⎟ dt ⎝ dt ⎠

(7)

Chemorheology. Chemorheology is the study of the deformation properties of the reactive polymer system. It is used herein to determine cure properties. Figure 1a shows the evolution of complex viscosity of DGEBA with time during curing reactions conducted at different temperatures. The complex viscosity remains approximately constant for times that increase for lower cure temperatures. Thereafter, an exponential increase in the viscosity occurs. The temperaturedependent behavior of complex viscosity is expected since, as the cure temperature increases, so does the rate constant leading to faster reaction rates. Plots of complex viscosity as a function of isothermal cure time at 90 °C for the methyl, ethyl, butyl, and pentyl esters of DGEDP (DGEDP-methyl, DGEDP-ethyl, DGEDP-butyl, and DGEDPpentyl, respectively) and DGEBA are shown in Figure 1b. Values below 1 Pa·s were significantly noisy and therefore are not included in Figure 1b. By decreasing the ester chain length of DGEDP, the rise in viscosity for the biobased epoxies begins at shorter times (see Supporting Information 1.4). Correspondingly, increase in the complex viscosities of DGEDP-methyl and DGEDP-pentyl occurs at the shortest and longest times for this series of epoxy resins, respectively. Interestingly, the rise in viscosity for DGEBA occurs after DGEDP-pentyl. Similar patterns occur at all cure temperatures, and complex viscosity vs time curves at 80 and 100 °C are displayed in Supporting Information 1. However, once the viscosity of DGEBA starts to rise, the slope of the plots for DGEBA and DGEDP-ethyl are similar as can be seen in Supporting Information 1.3. Based on the curing behavior, DGEBA is an ideal candidate for infusion molding. Its viscosity remains approximately constant for the longest time among all the epoxies that allows for complete filling of the mold. But once the viscosity starts to ascend, the rate of viscosity increase occurs rapidly allowing shorter cycle times due to faster curing post initial processing. Among the DGEDP epoxies, Figure 1b reveals that the increase in complex viscosity for DGEBA and DGEDP-pentyl occur at similar times (748 and 644 s, respectively). It follows that DGEDP-pentyl is also a good candidate for infusion molding. In

The incremental area under DSC thermograms was used to obtain degree of conversions (α) at different times α=

AHEW × 100 EEW

(5)

where dH/dt is the heat flow in W/g. Both dynamic and isothermal experiments were repeated 2−3 times to ensure reproducibility. Fourier Transform Infrared Spectroscopy (FTIR). Sample analyses were performed with a Cary 680 FTIR equipped with a diamond ATR apparatus from 400 to 4000 cm−1. The nominal spectral resolution used was 4 cm−1, and 32 scans were averaged for each spectrum. All measurements were performed in the absorbance mode.

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RESULTS AND DISCUSSION The curing agent or hardener (IPD) and epoxy resins were mixed in stoichiometric amounts which were determined using the epoxide equivalent weight (EEW) and amine hydrogen equivalent weight (AHEW). Equations 6 and 7 were used to calculate the AHEW and the amount of IPD, respectively. The amount of hardener is given in terms of parts per hundred (phr) of epoxy. amine hydrogen equivalent weight molecular weight 170.30 = = = 42.575 number of active hydrogens 4 (6) C


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The isothermal integral (IsoInt) method, described by eq 8, was used for the current analysis:

contrast, the relatively rapid cure of DGEDP-methyl indicates it is a good candidate to develop high strength adhesives. Table 1. Comparison of Epoxy Equivalent Weight (EEW), Quantity of Amine Added for Curing Reactions, Zero Shear Viscosity, and Tg0 for All Epoxies

epoxy DGEBA DGEDPmethyl DGEDPethyl DGEDPbutyl DGEDPpentyl

EEWa (g/equiv)

amount of amine required (g per 100 g of epoxy)

zero shear viscosity (Pa·s)

Tg0c (°C)

174b 215

24.5 19.8

4 792

−20 −5

222

19.2

63

−14

243

17.5

18

−23

265

16.1

12

−28

ln tα , i = −ln

Epoxy equivalent weight (EEW) was determined by ASTM D1652. From certificate of analysis supplied by vendor. cTg0 is the glass transition temperature of the uncured epoxy resin measured by DSC as the midpoint of the drop in heat capacity during the heating cycle.

b

Differential Scanning Calorimetry. DSC analysis was performed to gain insight into reaction mechanisms and to obtain rate constants for the curing reactions. The total heat of reaction from dynamic scans and the maximum degree of conversion (αm) reached at different isothermal curing temperatures are shown in Table 2. Values of αm depend upon the Table 2. Total Heat of Reaction (ΔHtotal), Tg∞, and Maximum Degree of Conversion (αm) Obtained by Dynamic and Isothermal DSC maximum degree of conversion (αm)

epoxy DGEDPmethyl DGEDPethyl DGEDPbutyl DGEDPpentyl DGEBA EPON 828

Tg∞a,b (°C)

80 °C

90 °C

20.0

145

0.78

0.83

0.86

NA

19.6

125

0.83

0.85

0.87

0.92

20.3

112

0.86

0.91

0.95

0.97

20.5

97

0.89

0.93

0.97

0.98

22.4 20.1

165 163

0.70 0.71

0.75 0.74

0.78 0.77

0.81 0.81

(8)

where Aα is the pre-exponential factor, Ea is the activation energy, R is the universal gas constant, g(α) is an arbitrarily defined function, and Ti is the ith iteration of temperature. The complete derivation of eq 8 is given in Supporting Information 3.1. For each value of α, the slope of a plot of ln t at each temperature vs T−1 gives Eα/R. Since this analysis is performed isothermally, discrepancy in data at high conversion is expected due to effects of increasing glass transition temperature on diffusion relative to those at early reaction times and corresponding low conversions. Isoconversional analysis fitting parameters are listed in Supporting Information 3.2. Plots of Ea as a function of epoxy resin degree of cure are displayed in Figure 2a. For consistency, limits in the graph were defined for the material with the highest Tg∞ (DGEBA) and the lowest temperature (80 °C). Thus, the degree of conversion is limited to 0.7 as it is close to the theoretical maximum degree of conversion (αm) that can be achieved for DGEBA at an isothermal temperature of 80 °C. For DGEBA, the Ea first increases at low degrees of conversion until it reaches a maximum Ea at 0.20−0.25 degree of conversion. Increase in the degree of conversion above 0.25 results in lower Ea values. Similar results were reported in the literature for isothermally cured DGEBA with IPD.47 This phenomenon can be attributed to a change in mechanism from an HX molecule catalyzed reaction to an autocatalytic reaction. The initial increase in activation energy is due to the continued formation of hydroxyl groups when the primary amine reacts with an epoxide. Once a sufficient concentration of hydroxyl groups are formed, the reaction reaches the propagation stage, with autocatalytic behavior consistent with decreased activation energy. Previous reports of Ea’s for primary52 and secondary10 amine addition reactions are 55.1 and 52.3 kJ/mol, respectively. These values are close to activation energies determined herein. DGEDP epoxies show a similar trend to DGEBA at low levels of conversion. That is, the Ea increases at low degrees of conversion, indicating there is a change in mechanism. However, with further increases in the conversion, decreases in Ea values consistent with autocatalytic behavior is only observed for DGEDP-ethyl and -pentyl. DGEDP-methyl shows a slight increase in Ea values with an increase in degree of conversion. This increase is attributed to experimental error as the R2 value for the line fitting changes from 0.97 at 0.3−0.4 degree of conversion to 0.7 at degree of conversion of 0.7 (see Supporting Information 3.2). DGEDP-butyl exhibits small but regular increases in Ea values for conversions from about 0.3 to 0.7 at R2 values of ∼0.99. While this behavior differs from DGEDPethyl and -pentyl, it is to be noted here that the isoconversional analysis gives an estimation of the rate-determining step occurring in the reaction. There could be some yet to be investigated mechanism taking place that causes this increase. Previous reports describe how the sigmoidal shape of Ea versus degree of conversion plots can also be explained by autocatalytic behavior.53−55 Figure 2b displays plots of degree of conversion versus temperature from dynamic DSC scans recorded at a heating rate of 3 °C/min. DGEBA and DGEDP epoxies show an almost constant value of degree of conversion at low temperatures (30−50 °C) followed by a rapid increase that is consistent

a

total heat of reaction (kJ/epoxide)

Aα E + α g (α ) RTi

100 °C 110 °C

a

Measured as the midpoint of the drop in heat capacity in DSC thermograms during the heating cycle. bTg∞ is the glass transition temperature of the cured epoxy resin at a degree of conversion = 1 (e.g., fully cured).

proximity of the curing temperature to the cured epoxy’s ultimate glass transition temperature (Tg∞). Analysis of Table 2 shows that the degree of cure increases with increasing isothermal cure temperature. Furthermore, the higher the Tg∞, the lower is the αm obtained at a particular temperature. Reaction Mechanism. The isoconversional technique is a method of data analysis that reveals the activation energy (Ea) as a function of conversion degree. The underlying principle of isoconversional analysis is that at a particular degree of conversion the reaction rate is solely dependent on temperature. D


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Figure 2. Plots for DGEBA and DGEDP-ester epoxy resins of (a) activation energy vs degree of conversion (isoconversional analysis) and (b) degree of conversion vs temperature at 3 °C/min (dynamic scan).

Figure 3. Plots for DGEBA at different isothermal curing temperatures of (a) reaction rate vs degree of conversion and (b) expansion of a region from the plot in (a).

dα = (k1 + k 2α m)(αm − α)n dt

with autocatalytic behavior. Similar trends were observed at other heating rates. Thus, we conclude that DGEDP epoxies undergo a transition from an initiator catalyzed mechanism to an autocatalyzed one. Kamal−Sourour Modeling. The curing parameters can be obtained using the Kamal−Sourour model which has been successfully applied for studies by others on DGEBA cure.16,19,50,56 Since the maximum reaction temperature is 110 °C, etherification reactions can be ruled out.13−15 Also, the presence of low level contents of oligomer in DGEDP epoxies51 led us to choose the Kamal−Sourour model over the Horie model due to ease of fitting. One of the widely accepted forms of this model is given by eq 3. The Kamal−Sourour model assumes that the maximum degree of conversion under isothermal conditions is 1. Recently, it was reported57 that for incomplete cures (1 − α)n should be replaced by (αm − α)n where αm is the maximum extent of cure and α varies between 0 and αm. Although this strategy does not explicitly describe vitrification, it accounts for the fact that the ultimate extent of cure is dependent on the cure temperature. Thus, the model used in the present system is

(9)

where k1 is the rate constant catalyzed by impurities, k2 is the rate constant catalyzed by hydroxyl groups, m and n are reaction orders, α is the degree of conversion, and αm is defined above. According to the Flory−Stockmayer theory for step growth polymerization,58 for a bifunctional epoxy (fa = 2) and a tetrafunctional (f b = 4) amine, the critical gel point occurs at αgel

⎛ ⎞0.5 1 ⎜ ⎟⎟ → αgel =⎜ ⎝ (fa − 1)(fb − 1) ⎠

⎛ ⎞0.5 1 =⎜ ⎟ → αgel = 0.577 ⎝ (2 − 1)(4 − 1) ⎠

(10)

It has been stated that epoxy curing kinetics past the gel point is complicated by diffusion effects, which causes the rate of reaction to slow relative to the rate that is uninhibited by diffusion.19 This is observed in Figure 3 where the experimental data begins to diverge from the fitting model at degree of conversions close to the theoretically determined αgel value (0.57). Hence, to E


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Figure 4. Bar graphs of rate constants (a) k1 and (b) k2 for DGEBA and DGEDP epoxy resins as a function of isothermal curing temperatures.

structure and the isothermal curing temperature are shown in Figure 4. The values of k1 and k2 can be used as indicators of processability. Since k1 is a measure of the non-autocatalytic rate at which curing reactions between amine and epoxy groups takes place at low conversion ( Ek2 and Ek1 < Ek2 have been reported. However, F


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(moderate intensity, 1080−1360 cm−1) and ring-opened epoxy C−O stretching (strong intensity, 1050−1150 cm−1), overlapping stretching bands due to remaining N−H (3100−3500 cm−1), and formed secondary O−H groups (3200−3600 cm−1). The data were normalized with respect to out-of-plane C−H bending vibrations for p-substituted aromatic rings at 825 cm−1, which should not change in position or intensity as they do not take part in the reaction. Indeed, the ratio of the aromatic C−H bending peak to the CO vibrational band for the cured and uncured samples are 1.78 and 1.81, respectively. Hence, the ester moiety remains unchanged (i.e., does not undergo a reaction) during the cure reaction. Furthermore, the wavenumber at which the CO peak is located also remains unchanged for the cured and uncured samples. In effect, the ester chain does not chemically take part in the reaction. Similar results were obtained for the DGEDP-methyl, -ethyl, and -butyl epoxies which can be seen in Supporting Information 8. In order to determine if the presence of oligomers in DGEDP epoxies is the cause of differences in their behavior relative to DGEBA, EPON 828, which is a mixture of oligomeric and monomeric DGEBA, was also used as a control along with DGEBA. The k1 and k2 values for EPON 828 are included in Figure 4. Indeed, the presence of oligomeric species in DGEBA results in higher k1 values relative to the pure DGEBA resin. Furthermore, the presence of oligomeric species in DGEBA results in generally lower k2 values relative to pure DGEBA. The higher k1 of EPON 828 that corresponds to a more rapid initial reaction rate than pure DGEBA is explained by the presence of free hydroxyl groups in oligomeric molecules. The free hydroxyl groups have a catalytic effect on the curing mechanisms and thus accelerate the initial reaction leading to an increase in the value of k1. This can be also observed in Supporting Information 4 wherein the behavior of EPON 828 is similar to that of the DGEDP epoxies. The peak of the reaction rate appears at slightly lower degree of conversions for the oligomeric epoxies as compared to DGEBA. Thus, small amounts of hydroxyl moieties can have large effects on the reaction rates and corresponding epoxy resin viscosity. Furthermore, hydroxyl-rich small molecules or oligomers can form a hydrogen-bonded network that causes substantial increases in zero shear viscosity (Table 1) and high k1 values (Figure 4). The potential pathway toward hydroxyl-rich coproducts of DGEDP-Me is shown in Scheme 2. Another possible explanation for the extremely high k1 of DGEDP-methyl is the presence of 6 mol % of DGEDP glycidyl ester. The trifunctionality of the glycidyl ester may initiate branching at a lower degree of conversion leading to pseudoincreased rate constant values. To confirm these hypotheses, we are preparing DGEDP-methyl that is free of the oligomer fraction, DGEDP glycidyl ester, or both. The results of these studies will be reported as part of future work.

activation energy values for DGEBA systems have still been in the same range. For example, 52 and 42 kJ/mol61 and 61.5 and 51.3 kJ/mol62 have been reported. The results obtained herein are very similar to previous authors wherein Ek1 obtained was 47.5 kJ/mol and Ek2 obtained was 52.9 kJ/mol.48 The activation energy obtained from isoconversional analysis and that obtained from the Kamal−Sourour modeling also agree well with each other. The isoconversional analysis activation energy (seen in Figure 2a) starts at the same point as the Ea for k1 and rises to approximate values of the activation energy of k2. Although trends have been established for differences in rate constant between DGEBA and DGEDP, the explanation for why these trends occur remains unanswered. The two biggest differences between the DGEDP epoxies and DGEBA are the differences in structure and the presence of oligomers. For the former, this leads us to consider whether it is the presence of an ester chain for DGEDP that results in higher initial cure rate constants (especially for DGEDP-methyl and DGEDP-ethyl) relative to DGEBA. Furthermore, if the difference in the initial cure kinetics is indeed due to DGEDP-ester moieties, how is this observation explained on a molecular level? The DGEDP epoxies synthesized for this study were shown previously to contain a fraction, albeit small, of oligomers.51 In contrast, DGEBA was purchased as a high purity monomer. We also reported that preparations of DGEDP-methyl and DGEDP-ethyl contain 6 and 3 mol %, respectively, of a trifunctional DGEDP glycidyl ester.51 Furthermore, DGEDP oligomers contain low contents of hydroxyl groups that might act as accelerators during the initial curing reaction leading to higher initial curing rate constants. In order to determine if the difference in structure was the cause for the dissimilarity of initial rate constants, FTIR-ATR analysis was performed on DGEDP and DGEBA samples cured at 100 °C for 100 min. The only distinction between DGEDP and DGEBA uncured samples is the peak at 1724 cm−1, which is attributed to CO stretching of the ester group of DGEDP epoxies. FTIR-ATR spectra of cured and uncured DGEDPpentyl are shown in Figure 5. The most significant difference between the two spectra in Figure 5 are (i) the disappearance of a well-defined C−O−C peak in the uncured sample at 914 cm−1 corresponding to epoxy groups and (ii) the appearance in the cured sample of overlapping bands due to C−N stretching

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CONCLUSIONS Isoconversional analysis and Kamal−Sourour model fitting were performed to determine rate constants for a family of biobased alkyl DGEDP epoxies in comparison to DGEBA and EPON 828. The maximum degree of conversion achieved was dependent on the isothermal temperature as well as on the ultimate glass transition temperature of the fully cured epoxy. Reaction orders m and n follow the same trend through the epoxies with n increasing with temperature and m remaining relatively constant. The initial rate constant k1 is higher for all the DGEDP epoxies in comparison to DGEBA whereas the autocatalytic rate constant for the DGEDP epoxies was lower than DGEBA. While

Figure 5. FTIR spectrum for DGEDP-pentyl cured (bottom) and uncured (top). G


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Scheme 2. Synthesis of DGEDP-Methyl Ester, Carboxylate Anion Intermediate, and Potential Coproducts Responsible for the Large k1

comparing the rate constants among only the DGEDP epoxies, the DGEDP-ethyl, -butyl, and -pentyl showed similar values of rate constants for k1. DGEDP-methyl showed an unusually high k1 which is attributed to the presence of hydroxyl-rich oligomers or the presence of trifunctional DGEDP glycidyl ester. The rate constant k2 increased as the ester chain length of DGEDP epoxies decreased, which is consistent with their corresponding Tg∞ values. Based on viscosity rise and k1 values, DGEDP-pentyl and -butyl would be ideal substitutes for DGEBA in mold filling applications whereas DGEDP-methyl is well suited for use as a structural adhesive. The discrepancy between the rate constants of DGEDP epoxies and DGEBA was attributed to the presence of oligomers in the synthesized epoxies and the presence of free hydroxyl groups in the oligomers, causing an increase in k1. This was confirmed through a comparison with EPON 828. These results indicate that addition of hydroxyl-rich small molecules is a potential approach to increase initial rate constants leading to faster rise in epoxy cure viscosity, though further work will be needed to confirm this hypothesis. We further conclude that the presence of oligomers in DGEDP epoxies is sufficiently low such that the corresponding k1 values are below that of EPON 828. Hence, DGEDP epoxies are promising candidates for replacement of EPON 828.

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and -butyl epoxies; degree of conversion vs time at 80, 90, and 100 °C for DGEDP-methyl (PDF)

AUTHOR INFORMATION

Corresponding Author

*(I.M.-Z.) E-mail [email protected]; Ph (216) 368-3596. Funding

The authors are grateful for funding received from the National Science Foundation Partnerships for International Research and Education (PIRE) Program (Award #1243313). Notes

The authors declare no competing financial interest.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01261. Complex viscosity vs time for DGEDP and DGEBA epoxies; heat flow vs temperature for a dynamic DSC scan for all epoxies; isoconversional analysis derivation and results; plots of reaction rate vs degree of conversion for all epoxies; Kamal−Sourour fitting and results; activation energy calculation using Arrhenius equations; FTIR spectra for cured and uncured DGEDP-methyl, -ethyl, H


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Macromolecules

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