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Mechanism and kinetics of curing of Diglycidyl Ether of Bisphenol a (DGEBA) resin by chitosan.

INTRODUCTION

Epoxy resins, characterized by presence of one or more oxirane rings in the structure, are one of the versatile thermosetting polymers which has a wide range of applications such as coatings, adhesives, and composites [1]. The presence of extremely strained cyclic three-membered ring in their structure, makes them highly reactive and form a three dimensional network when reacted with curing agents. Crosslinked epoxy resins are characterized by low shrinkage as there are no emission of volatile products during curing. In addition, they possess excellent adhesion to various surfaces, improved mechanical strength, good resistance to chemical and thermal attack, and high glass transition temperature and modulus [2], Some of the commonly available curing agents are aliphatic amines, cycloaliphatic amines, aromatic amines, amidoamines, polyamides, polyphenols, acid anhydrides, and catalytic curing agents [1],

One of the major environmental concerns associated with currently available curing agents is their toxicity. Some of the aliphatic amine based curing agents such as diethylene triamine (DETA), triethylenetetramine (TETA) are considered volatile and could cause severe skin and respiratory tract irritations. Aromatic amine based curing agents such as methylene dianiline (MDA) have been reported to cause toxic hepatitis and liver enlargement among the workers who handled it [3]. Even though anhydrides were considered to be replacing some of these amine based curing agents in many composite applications [1], they are still considered to be severe skin sensitizers [4]. Even though epoxy resins are cured, toxicity cannot be completely avoided as there are high possibility for incomplete reaction of the curing agent during the curing process [4]. On the other hand, solvents used in epoxy resin formulations such as methyl ethyl ketone (MEK), acetonitrile, toluene, and xylene are found to be skin irritants and could cause degreasing and dehydration of skin when it comes in contact [5], Hence, it becomes extremely important to avoid toxic curing agents and toxic solvents in processing of epoxy resins. Using a biocompatible material and a nontoxic solvent to cure epoxy resin could be the best alternative method to avoid toxicity.

Chitosan is a naturally occurring biopolymer obtained from crab or shells of shrimp. It is the second most abundant biopolymer on earth after cellulose. It is an N-deacetylated derivative of chitin, after the removal of most of acetyl groups using strong alkalis. Hence, it is a copolymer of glucosamine and N-acetylglucosamine [6, 7]. Presence of amine and hydroxyl groups in chitosan and their derivatives enables them to be modified for a wide variety of bio-medical applications such as wound dressings, artificial skin, and cosmetics [8]. Interaction of chitosan with various epoxy group containing polymers have been studied by various researchers in the past. Cestari et al. have studied the surface interactions of epoxy/chitosan-modified cement slurry/HCl for application of acidizing procedures in oil wells [9]. Shao et al. have studied chitosan nanocomposite crosslinked with epoxy groups bearing graphene oxide (GO), and have found improved tensile strength of chitosan membranes after incorporations of GO [10]. Chitosan functionalization by reaction with epoxy groups of allyl glycidyl ether have been carried out [12], Epoxy functionalized polyethylene has been used as compatibilizer in LDPE/chitosan blends [13].

Illy et al. conducted study on dry and waterborne DGEBA resin system using chitosan as a curing agent [11]. They found that interaction between DGEBA system and chitosan improved in their aqueous forms. However, detailed mechanism of the reaction between DGEBA and chitosan was not elucidated. In addition, relation between time, temperature, and crosslinking was not established. This necessitates a detailed study on DGEBA-chitosan curing system with prediction of cure kinetics which would enable successful formulation for processing and allow comparison with the conventional curing agents available.

In this work, FTIR technique was used to demonstrate the curing process and the cure mechanism of Diglycidyl Ether of Bisphenoi A (DGEBA) resins using chitosan as an environmentally friendly curing agent. Analysis was performed to establish a relationship between degree of crosslinking of DGEBA resin, curing time, curing temperature, and concentration of curing agent used. It was found that oxirane ring undergoes nucleophilic attack by the primary amine groups of chitosan and the degree of crosslinking increased with increase in curing time, curing temperature and concentration of chitosan. A four parameter kinetic model with two rate constants, developed by Kamal et al. [12, 13], was employed to study cure kinetics. It is given by,

d[alpha]/dt = ([k.sub.1] + [k.sub.2][[alpha].sup.m])[(1 - [alpha]).sup.n] (1)

where (d[alpha]/dt) is the rate of curing, [alpha] is degree of curing, m and n are the reaction orders, [k.sub.1] represents the reaction rate at time, t = 0, and the influence of the reaction products on the conversion rate is given by the term [k.sub.2][[alpha].sup.m]. The results showed that the cure reaction is autocatalytic in nature. Kinetic parameters such as Arrhenius rate constants, reaction order and activation energy are determined for epoxy-chitosan cure reaction. In addition, thermal degradation behavior and stability of chitosan cured epoxy resin films were studied by Thermogravimetric analysis (TGA) and the results were compared against the analysis performed on neat epoxy and neat chitosan films. It was found that thermal stability of chitosan cured epoxy resin films increased with increase in crosslink density.

EXPERIMENTAL

Materials

EP1-REZ[TM] Resin 5522-WY-55, a waterborne Diglycidyl ether of Bisphenol A (DGEBA) resin, was supplied by Momentive Specialty Chemicals Inc. Weight per epoxide value for this resin is given by 607 g [eq.sup.-1]. A low molecular weight (50,000-190,000 Da based on viscosity) chitosan with Degree of Deacetylation (DDA) of [is greater than or equal to] 75% was purchased from Sigma Aldrich Inc. Chemical structures of these materials are shown in Fig. 1. Glacial acetic acid with purity >99.7% was supplied by Fisher Scientific Inc.

Sample Preparation

Five grams of chitosan was dissolved in 1% (v/v) glacial acetic acid solution by mechanically stirring for 1 h. A highly viscous solution was obtained. DGEBA resin and chitosan solution, measuring molar ratios (Epoxy:Chitosan) 1:1, 1:2, 1:3, and 1:4 were mixed together and stirred for 3 h. Molar ratios 1:1, 1:2, 1:3, and 1:4 will be termed as EPCH11, EPCH12, EPCH13, and EPCH14, respectively, in rest of the manuscript. The resulting mixture was solution casted onto 2.5 cm X 10 cm X 0.3 cm Al 2024-T3 coupons and they were cured in the oven. Temperature of the oven was increased from room temperature to 70[degrees]C and samples were maintained initially at 70[degrees]C for 1 h followed by 120[degrees]C for another 1 h, and the temperature was increased and maintained at one of the curing temperatures (160, 180, and 200[degrees]C) for 5 h. Step-wise curing procedure was adopted to facilitate the evaporation of acetic acid solution.

FTIR Characterization

To determine the chemical composition of samples, Nicolet 6700 FTIR instrument, equipped with a smart orbit ATR accessory with diamond crystal was used. ATR was performed over wave numbers ranging between 4000 and 400 [cm.sup.-1]. The FTIR spectrums were recorded at predefined time intervals (t = 0 to 5 h) at room temperature immediately after the samples were removed from the oven. It was assumed that crosslinking did not happen during the FTIR measurement given the temperature difference between cure temperature and room temperature. Different samples were used for each time interval to obtain the FTIR spectra. The initial time (t = 0) corresponds to the time when the chosen isothermal temperature is reached. Commercial software OriginPro 8.5 was used to determine the area of the FTIR peaks. Results for analysis were taken from average of three experiments.

The relative epoxy fractional conversion (a) was determined by following the intensity and area of the oxirane peak corresponding to 914 [cm.sup.-1]. In order to quantify the variation, an internal standard (band that remains unaffected during the cure reaction) corresponding to the stretching vibration of C=C of benzene ring (peak at 1506 [cm.sup.-1]) was chosen. Epoxy fractional conversion ([alpha]) was then calculated as per the Eq. 2 [21]. [A.sub.epoxy] and [A.sub.benzene] represent the areas of oxirane ring peak and benzene ring peak, respectively.

[alpha] = (([A.sub.epoxy(]/[A.sub.benzene]) with Chitosan/([A.sub.epoxy]/[A.sub.benzene]) without Chitosan) (2)

TGA

TGA was performed on neat epoxy, neat chitosan, and chitosan cured epoxy films to understand the thermal decomposition and stability of the films. TGA instrument STA 409 PC Luxx model manufactured by Netzsch Group was used. The sample purge gas and balance purge gas were maintained at 60 and 40 mL/min. The samples were placed on platinum pan, and the temperature was raised from room temperature to 700[degrees]C at a scan rate of 10[degrees]C/min to observe the loss in weight of the sample. Results were plotted as mass loss (%) curve and derivative mass loss curve as a function of temperature.

RESULTS AND DISCUSSION

Fourier Transform Infrared (FTIR) Spectroscopy

FTIR spectra of neat epoxy and neat chitosan are shown in Fig. 2. Characteristic peaks of oxirane ring in DGEBA resin can be noticed at 914 and 863 [cm.sup.-1], and the characteristic peaks corresponding to Amide I and Amide II in chitosan can be noticed at 1652 and 1587 [cm.sup.-1]. Full band assignments for these materials are provided in Table 1.

In order to understand the structural changes occurring when the chitosan is dissolved in acetic acid and mixed with DGEBA resin, drying of EPCH14 was studied in situ by FTIR spectroscopy at room temperature. The same has been shown in Fig. 3.

Spectra were taken in real time and at room temperature when about 0.1 mL of EPCH14 solution was drying over ATR crystal which spans for about 5 min. It can be noticed that at time (t = 0), bands corresponding to aqueous acetic acid solution and water absorption band is predominantly visible and as the water evaporates from the mixture, bands of other functional groups start to discern. Disappearance of strong water absorption band at 1634 [cm.sup.-1] enables the visibility of peak at 1547 [cm.sup.-1] which corresponds to the symmetric stretching of [NH.sup.+.sub.3]. This presence of [NH.sup.+.sub.3] is expected as primary amine in chitosan will protonate to form chitosonium acetate ([NH.sup.+-.sub.3] OOCR) groups when dissolved in acetic acid solution. Similar behavior was reported by Fernandez-Saiz et al. [14]. In addition, peak around 3300 [cm.sup.-1] shows that vibration of O-H is overlapped with the N-H vibration. This reveals that primary amines get regenerated from their protonated form with the evaporation of aqueous acetic acid solution and water. Such a behavior has been observed by Andre et al. [15] and they concluded that protonated amine exist in equilibrium with the acid content present as per equation [15] (7).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

Their detailed studies on dissolution of chitosan in various carboxylic acids have shown that protonated amine are regenerated as the acid solution gets evaporated from the mixture and the regeneration is dependent on the nature of carboxylic acid used. In addition, they observed that this regeneration is more favorable with acetic acid when compared to using other carboxylic acids such as formic, butyric, or valeric acids. Formation of stable intermediary compound is also not favored as it may hinder the regeneration of amine groups of chitosan. This is significant especially in case of formic acid where chitosan forms formamide when treated with formic acid [15]. Thus, regenerated amine groups from chitosonium acetate will become readily available to crosslink with oxirane groups of DGEBA resin.

In observing the FTIR spectra of EPCH14 before (t = 0 h) and after (t = 5 h) curing at 200[degrees]C, (shown in Fig. 4), it can be noticed that the area and intensity of the oxirane peaks corresponding to 914 and 863 [cm.sup.-1] has decreased before and after curing. Appearance of new peak at 1738 [cm.sup.-1] corresponding to carbonyl ester stretch indicates possible esterification reaction of carboxylic anion with highly strained epoxy ring leading to ionized epoxide (shown in Fig. 5a). This ionized epoxide stabilizes itself by extracting a hydrogen from protonated amine groups ([NH.sup.+.sub.3]) of chitosan and thus leaving them with the regenerated primary amine groups (shown in Fig. 5b) [16]. These primary amine groups become readily available to undergo nucleophilic attack on oxirane groups of DGEBA, which results in secondary amine groups and increase in the hydroxyl groups (shown in Fig. 5c). The shift of hydroxyl peaks from a higher wavenumber 3419 [cm.sup.-1] to the lower wavenumber 3383 [cm.sup.-1], suggests possible increase in the strength of hydrogen bonding groups. This increase in strength is contributed by the newly formed hydroxyl groups (stronger hydrogen bonding) and the secondary amine groups (weaker hydrogen bonding) [17], Secondary amine undergoes further reaction with oxirane to form a tertiary amine (shown in Fig. 5d). Hydroxyl groups formed will autocatalyze the curing process [1]. Possible reaction scheme of the crosslinking process, and the representation of crosslinked network are shown in Figs. 5 and 6.

In order to model cure kinetics (discussed below), variation in the area of oxirane ring peak corresponding to 914 [cm.sup.-1] was analyzed. Epoxy fractional conversion, calculated as per Eq. 2, was plotted as a function of time for all temperatures and molar ratios. It is shown in Fig. 7. Experimental values of a versus t was fitted to a four parameter logistic function that produces a sigmoidal curve of type [18]:

y = [[[A.sub.1] - [A.sub.2]]/[1 + [(x/k).sup.p]]] + [A.sub.2] (4)

Table 2 shows the parameters obtained as a result of fit. It can be noticed that at all temperatures and for all molar ratios, agreement between experimental data and the logistic function is very good as given by value of [R.sup.2].

Epoxy fractional conversion (shown in Fig. 7) calculated was found to increase as a function of curing time, curing temperature, and molar ratio of chitosan. Maximum epoxy fractional conversion value of about 70% was obtained for EPCH 14 at 200[degrees]C.

Factors like possibility to regenerate the protonated amine without any formation of stable intermediary compound during dissolution process, lower boiling point ([approximately equal to] 118[degrees]C) when compared with other carboxylic acids, and nontoxicity makes acetic acid a favorable choice to dissolve chitosan. Such dissolution also enhanced the processability to achieve proper mixing with waterborne DGEBA resin, which is very essential for successful curing of epoxy resins. This dependency by which mechanism of curing reaction is based on solvent removal from the system, acts as an inhibitor enhancing the pot life of the epoxy resincuring agent mixture. As the solvent removal occurs well above the room temperature, the process becomes more end-user controlled avoiding the necessity for refrigeration or any other precautionary measure for storing the epoxy resin-curing agent mixture [19].

Determination of Kinetic Parameters

Derivative of the plot (d[alpha]/dt) was numerically determined from the fitted data, to understand the variation in the rate of the reaction (d[alpha]/dt) as a function of time. The plots for the same are shown in Fig. 8. It was found that reaction rate attained its maximum during the initial stages of the reaction, and gradually decreased with increase in time. In addition, reaction rate was found to increase with increase in the temperature which helps us understand that reactivity of oxirane and primary amine groups are higher at higher temperatures.

It can be noticed that maximum reaction rate occurs at time t > 0 and also the reaction rate is finite at t = 0. This negates the simple nth order kinetics in the curing process and suggests the possibility of utilization of reaction products in the curing process. Hence, Eq. 1 was chosen to model the cure kinetics.

The nonlinear least squares method is the most commonly used method in estimation of kinetic parameters. To simplify the kinetic parameters determination, overall reaction order (m + n) has also been assumed to be two [20]. Kenny proposed a graphical method to determine the kinetic parameters without presuming the overall reaction order [21]. However, in this work, the kinetic parameters have been estimated without any constraints on them by using nonlinear least squares (Levenberg-Marquardt) method.

Temperature dependent rate constant is defined through an Arrhenius-type expression given by,

k(T) = A exp (-[E.sub.a]/RT) (5)

where A is the frequency factor, [E.sub.a] is the activation energy, R is the universal gas constant, and T is the absolute temperature.

The value of [k.sub.1] can be numerically obtained from Eq. 1. At t = 0, [alpha] = 0. Hence, rate can be given by [21, 22],

[[d[alpha]/dt].sub.t=0] = [k.sub.1] (6)

To obtain the remaining parameters [k.sub.2], m, and n, Eq. 1 was used to fit the conversion rate (d[alpha]/dt) versus conversion ([alpha]) plot by nonlinear least squares method. It is shown in Fig. 9. It can be observed that the fit obtained for the experimental data by the kinetic model is very good.

The values of [k.sub.1] and [k.sub.2] obtained were applied in Eq. 5 to establish the Arrhenius relationship (shown in Fig. 10) and the activation energies [E.sub.1] and [E.sub.2] were calculated from the slopes of their lines. Similar procedure was applied for other molar ratios and the activation energies were determined.

Figure 11 shows the plot of activation energies as a function of molar ratio of chitosan. Activation energies obtained were in the range of 25-50 kJ [mol.sup.-1]. For all the molar ratios, it was found that autocatalytic activation energy, [E.sub.2] is greater than noncatalytic activation energy [E.sub.1]. Values obtained are in close agreement with the values reported in the literature [23, 24],

Values obtained for reaction orders m and n for all four molar ratios are shown in Table 3. Overall reaction order (m + n) was found to be in the range of 2.5-3 and the average values for m and n were found to be 0.41 and 2.43, respectively. These are in very good agreement with the reported m and n values for various epoxy-amine curing systems in the literature [23, 25].

TGA

TGA was performed on neat epoxy, neat chitosan, and chitosan cured epoxy resin films. The same is shown in Fig. 12. Occurrence of different slopes in the TGA plot and presence of more than one peak in derivative plot (DTG) suggests that degradation occurs in more than one step. For neat chitosan, degradation occurs in three steps, which are in agreement with results noted by earlier researchers [26, 27], First step which occurs up to 200[degrees]C corresponds to the removal of adsorbed water molecules and evaporation of solvent (acetic acid solution). Second step of degradation which occurs from 200 to 350[degrees]C corresponds to the elimination of side groups and beginning of depolymerization of chitosan. This step can result in carbonaceous residue which could undergo oxidative degradation in the third step from 350[degrees]C along with the degradation of polymer backbone [26, 27]. In case of neat epoxy film, single step degradation was noticed from 300 to 420[degrees]C. This corresponds to degradation of freely available DGEBA molecules [28], In the case of chitosan cured epoxy resin films, degradation occurred in three steps, and the characteristics of degradation reflected the crosslinks present in cured epoxy films. First step of degradation which begins from 220 to 350[degrees]C corresponds to the unreacted chitosan molecules present in the films. It can be noticed that the intensity of DTG curve in this temperature range, which can be correlated with the amount of unreacted chitosan molecules, increases with increase in the molar ratio of chitosan. The second step of degradation between 350 and 450[degrees]C corresponds to the degradation of unreacted DGEBA molecules. In this temperature range, the intensity of the DTG plot decreases suggesting decrease in the amount of free DGEBA molecules with increase in concentration of chitosan. This shows that utilization of oxirane molecules increases with increase in molar ratio of chitosan which agrees with the FTIR analysis results. Third step of degradation which starts from 450[degrees]C extending up to 550[degrees]C corresponds to the degradation of epoxy crosslinked network and this characteristic degradation was absent in neat epoxy film. In this temperature range, the intensity of the DTG plot, which can be correlated with the amount of crosslinked network in the chitosan cured epoxy film, was found to be the highest for EPCH14. Hence, EPCH14 is determined to be highly crosslinked than rest of the samples.

CONCLUSION

In this research, chitosan was used as a replacement for toxic curing agents to crosslink Diglycidyl ether of Bisphenol A resins. FTIR studies have shown that the primary amine group of chitosan becomes protonated when dissolved in acetic acid solution to form ([NH.sup.+-.sub.3]OOC) groups. The carboxylate anions formed reacts with oxirane to result in ionized epoxide which then stabilizes by extracting hydrogen from the protonated amine groups resulting in regeneration of primary amine groups. Reaction then proceeds by the nucleophilic attack of oxirane ring by the primary amine groups formed to result in crosslinked structure. Four different molar ratios at three different temperatures were used. It was found that degree of crosslinking increased with increase in curing temperature, curing time, and molar ratio of chitosan, as determined from the epoxy fractional conversion ([alpha]). Dissolving chitosan in acetic acid solution favored the processability to achieve proper mixing with waterborne DGEBA resins.

A four parameter phenomenological kinetic model with two kinetic rate constants ([k.sub.1] and [k.sub.2]) and two reaction orders (m and n) was employed to model the cure kinetics. It was found that reaction between DGEBA resin and chitosan is autocatalytic in nature and does not follow simple nth order cure kinetics. Also the rate of the reaction was found to increase with temperature and concentration of chitosan. Overall reaction order was calculated to be about 2.53-3.07 with the average values for m and n were found to be 0.41 and 2.43, respectively. Activation energy calculated from the Arrhenius plots of two rate constants was found to be in the range of 25-50 kJ [mol.sup.-1]. They are in good agreement with the values reported in the literature.

TGA performed on chitosan cured epoxy resin films showed that degradation occurred in three steps. Degradation in the temperature range of 220-350[degrees]C was found to be due to degradation of unreacted chitosan molecules, while degradation of unreacted DGEBA molecules was found to be in the range of 350-450[degrees]C and the degradation of chitosan crosslinked epoxy network was found to be in the range of 450-550[degrees]C.

ACKNOWLEDGMENTS

The authors thank and acknowledge the instrumentation assistance provided by Dr. Necati Kaval, Department of Chemistry, University of Cincinnati, OH.

REFERENCES

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Praveen Kumar Balasubramani, Jude O. Iroh

Department of Mechanical and Materials Engineering, University of Cincinnati,

Cincinnati, Ohio 45220, USA

Correspondence to: J. Iroh; email: irohj@ucmail.uc.edu

DOI 10.1002/pen.24463

Caption: FIG. 1. Schematic representation of (a) Diglycidyl ether of Bisphenol A and (b) chitosan.

Caption: FIG. 2. FTIR spectra of Diglycidyl ether of Bisphenoi A resin (top) and chitason (bottom). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. FTIR spectra of EPCH14 as a function of curing time. [Color figure can be viewed at wileyonlinelibrary. com]

Caption: FIG. 4. FTIR spectra of EPCH14 before and after curing at 200[degrees]C for EPCH14 at 200[degrees]C before (bottom spectrum) and after curing (top spectrum). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Reaction scheme for the curing of DGEBA-chitosan system. |Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Representation of crosslinked structure for DGEBA-chitosan system. [Color figure can be viewed al wileyonlinelibrary.com]

Caption: FIG. 7. Epoxy fractional conversion as a function of time for (a) EPCH11. (b) EPCH12, (c) EPCH13, and (d) EPCH14. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. Reaction rate as a function of time for (a) EPCH11 (b) EPCH12, (c) EPCH13, and (d) EPCH14. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. Conversion rate versus epoxy fractional conversion at various composition and temperature. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. Determination of activation energy for curing of DGEBA resin. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 11. Plot of activation energies for epoxy-chitosan curing as a function of composition. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 12. TGA of epoxy-chitosan crosslinking (a) weight loss, (b) derivative plots. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. FTIR band assignments for DGEBA and chitosan.

Material                             Band

DGEBA [22-24]            [approximately equal to] 3,4
                                       0
                                 2,962, 2,927,
                                     2,870
                                 1,606, 1,581,
                                     1,506
                                      914
                                      771
Chitosan 125-27]         [approximately equal to] 3,300

                                     2,871
                                     1,652
                                     1,587
                                     1,373
                                 1,150, 1,055,
                                     1,022
                                      891

Material                              Assignment

DGEBA [22-24]         Bending vibration of O--H

                      Bending vibration of C--H in
                        methylene
                      Stretching vibration of C=C in
                        aromatic ring
                      Vibration of oxirane ring
                      Rocking of C[H.sub.2] group
Chitosan 125-27]      Stretching of O--H overlapped
                        with N--H stretching
                      C--H stretching
                      C=O stretching of Amide I (--NH)
                      Bending of -N[H.aub.2] (Amide II)
                      Bending of C[H.sub.2]
                      Symmetric and asymmetric stretching
                        vibration of C--O--C bridge
                        (saccharide structure of chitosan)
                      Wagging of C--H

TABLE 2. Parameters obtained from fitting experimental epoxy
Fractional conversion data to four parameters logistic function.

Temperature ([degrees]C)      Parameters    EPCH 11    EPCH 12

160                            [A.sub.2]      0.91       0.59
                                   K         10.27       3.28
                                   P          1.05       0.91
                               [R.sub.2]      0.99       0.99
180                            [A.sub.2]      1.24       0.72
                                   K         13.87       3.55
                                   P          0.97       0.84
                               [R.sub.2]      0.99       0.99
200                            [A.sub.2]      0.90       1.04
                                   K          6.01       5.55
                                   P          1.05       0.75
                               [R.sub.2]      0.99       0.99

Temperature ([degrees]C)      EPCH 13    EPCH 14

160                             0.91       0.87
                                5.94       3.89
                                1.03       1.08
                                0.99       0.99
180                             1.07       1.05
                                4.23       3.79
                                1.06       1.01
                                0.99       0.99
200                             0.91       0.96
                                2.29       2.22
                                1.22       1.10
                                0.99       0.99

TABLE 3. Values of m and n calculated for epoxy-chitosan
curing system at various temperatures.

                                    Reaction order

            Temperature      M            N           m + n
            ([degrees]C)

EPCH11          160         0.33         2.30          2.63
                180         0.37         2.36          2.73
                200         0.41         2.47          2.88
EPCH12          160         0.39         2.44          2.83
                180         0.39         2.68          3.07
                200         0.45         2.60          3.05
EPCH13          160         0.39         2.68          3.07
                180         0.44         2.09          2.53
                200         0.46         2.40          2.86
EPCH14          160         0.40         2.61          3.01
                180         0.43         2.24          2.67
                200         0.45         2.23          2.68
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Author:Balasubramani, Praveen Kumar; Iroh, Jude O.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Aug 1, 2017
Words:5083
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