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Synthesis and characterization of UV- and thermo-curable difunctional epoxyacrylates
Materials Chemistry and Physics 132 (2012) 540–549
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and characterization of UV- and thermo-curable difunctional epoxyacrylates Yu-Chieh Su a , Liao-Ping Cheng a , Kuo-Chung Cheng b , Trong-Ming Don a,∗ a b
Department of Chemical and Materials Engineering, Tamkang University, Danshui Dist., New Taipei City 25137, Taiwan Department of Chemical Engineering, National Taipei University of Technology, Taipei 106, Taiwan
a r t i c l e
i n f o
Article history: Received 28 March 2011 Received in revised form 2 November 2011 Accepted 24 November 2011 Keywords: Chemical synthesis Fourier transform infrared spectroscopy (FTIR) Adhesion Epoxyacrylate
a b s t r a c t This paper describes the synthesis and characterization of difunctional epoxyacrylate oligomers from the reaction of diglycidyl ether of bisphenol-A (DGEBA) and acrylic acid (AA) in the presence of triphenyl phosphine as catalyst. The reaction was investigated in situ by using Fourier transform-infrared spectroscopy in the temperature range of 60–120 ◦ C, and demonstrated to be an addition esterification between the epoxide group and the carboxyl group. The specific rate constants calculated from the second-order kinetics obeyed Arrhenius law very well, and from which the activation energy was found to be 63.9 kJ mol−1 . Moreover, various batches of respective epoxyacrylates, EA25 to EA100, were prepared by changing the equivalent ratio of carboxyl/epoxide from 0.25 to 1.0 in the feed under a temperature profile of 100 ◦ C for 2 h and another 2 h at 120 ◦ C. The epoxide conversions, acid values and epoxide equivalent weights of the produced EA25 to EA100 were all measured. The analysis by gel permeation chromatography revealed that the EA25 to EA75 consisted of unreacted DGEBA, monoacrylate-terminated epoxyacrylate and bisacrylate-terminated epoxyacrylate; while the EA100 was composed of only latter two. Increasing the initial carboxyl/epoxide equivalent ratio increased the epoxide conversion and the amount of bisacrylate-terminated epoxyacrylate. Yet, with an initial carboxyl/epoxide equivalent ratio of 0.5, the EA50 produced the maximum proportion (∼72%) of monoacrylate-terminated epoxyacrylate, which had one terminal double bond for UV-cure and one epoxide end group for thermo-cure. This dual-curable epoxyacrylate thus can be used as an adhesive sealant for LCD production. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Epoxy resins are one of the most important thermosetting polymers that exhibit many desirable properties, such as low cure shrinkage, low creep, excellent adhesive strength, good chemical resistance, high mechanical strength, thermal stability and excellent electrical insulation [1]. These properties have led to rapid growth of epoxy resins and their wide use in many applications, including surface coatings, structural adhesives, packaging of electronic products and matrix for (nano)composite materials [2–5]. Especially, the demand of epoxy resins has rapidly increased for the applications in the electronic products, such as the epoxy molding compounds used for the encapsulation of IC chips and the epoxy composites for printed circuit boards [6,7]. The thermosetting epoxy resin has also been used as an adhesive sealant to bind two glass substrates in the liquid-crystal display (LCD), i.e., a thin film transistor (TFT) substrate mounted
∗ Corresponding author. Tel.: +886 2 26293856; fax: +886 2 26209887. E-mail address: [email protected] (T.-M. Don). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.11.067
with the driving circuit and a color filter (CF) substrate for displaying colors. The liquid crystals (LC) and other components are thus confined between the two substrates. In the conventional LCD cell assembly process, the peripheral adhesive sealant is generally a thermo-curable epoxy. The outstanding adhesion is due to the existence of hydroxyl and ether groups in the structure of epoxy resin. However, this process has several problems such as the dislocation of the two glass substrates mainly caused by its high curing temperature and long curing time of epoxy adhesive sealant. Furthermore, it takes a long time to complete the filling of LC fluid through the injection hole [8]. In order to solve these problems, a new LC injection process, called onedrop-filling (ODF) technology, is currently developed and used in the LCD industry. In this process, LC is dispensed in droplets directly onto the glass substrate and then an adhesive sealant is applied to the periphery of the substrate. The two glass substrates are then bonded together by curing the adhesive sealant [9]. The main advantage is the great reduction of the LC-filling time, from 5 days with the traditional injection method to only 5 h with the ODF process when applied to a 30-in. panel. The consumption of LC can be also reduced by about 40%. Thus,
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utilizing the ODF method can simplify the overall process, leading to increases in both efficiency and yield [10]. In the ODF process, the adhesive sealant becomes very important. There are three types of adhesive sealant that can be used in the ODF process. The first-type sealant has double bonds in its structure and thus can be UV-cured with the help of photo-initiator. The UV-curable epoxy resins have several advantages over traditional thermal-curable ones such as rapid cure, solvent-free process, low temperature operation and low energy requirement [1,11]. However, the first-type sealant with only UV-curable property has suffered from low strength, significant curing-shrinkage and low moisture resistance [12]. The second-type adhesive sealant, which is a mixture of a UVcurable acrylate resin with double bond(s) and a thermo-curable epoxy resin with epoxide groups, has no such problems [13]. Yet, the dual-curable mixed resins need a great amount of UV energy because the existence of the second epoxy component can disturb the radical polymerization of the acrylate resin. The third-type adhesive sealant is also dual-curable, yet it contains both double bond and epoxide group on the same molecule, which by itself can be cured by applying UV and heat in sequence. In order to reduce the UV energy and increase the polymerization rate, a dual-curable oligomer having both UVcurable and thermo-curable functional groups in one resin is thus desirable such as monoacrylate-terminated epoxy oligomers [14–16]. In industries, epoxyacrylates are generally produced and used in the form of bisacrylate-terminated epoxyacrylates, also called vinyl ester resins [17,18]. They are produced by reacting stoichiometric equivalents of diglycidyl ether of bisphenol-A (DGEBA) and acrylic acid (AA) or methacrylic acid (MAA) [19]. During the reaction, the epoxide end groups of DGEBA react with the carboxyl group of AA or MAA and the resulting product, bisacrylate-terminated epoxyacrylate, thus has terminal reactive double bond at both ends. They can be categorized into the first-type sealant. The prepared bisacrylateterminated epoxyacrylates have been widely applied in the fields of coatings, structural adhesives and advanced composite matrices [20,21]. In recent years, the research of UV-curable epoxyacrylates has focused on nanocomposites [22], UV-curing flame retardant films [23] and waterborne epoxyacrylates [24]. Yet, by changing the equivalent ratio of AA or MAA to DGEBA, such as 0.5, it is possible to prepare the monoacrylate-terminated epoxyacrylate with only one double bond at one end, while the other end is still an epoxide group. The prepared monoacrylate-terminated epoxyacrylate thus can be used as a dual-curable adhesive sealant in the ODF process. However, very few studies were reported on the synthesis of the monoacrylate-terminated epoxyacrylate [14–16], and there is no report so far on the study of reaction kinetics from AA and DGEBA. This research was trying to synthesize a dual-curable oligomer containing both double bond and epoxide group on the same molecule, i.e., monoacrylate-terminated epoxyacrylate, from the reaction of AA and DGEBA. The reaction kinetics of AA and DGEBA catalyzed by triphenyl phosphine (TPP) in the temperature range of 60–120 ◦ C was carefully studied, and the activation energy of the reaction was reported. Subsequently, various batches of epoxyacrylates with different compositions were prepared by changing the equivalent ratio of carboxyl/epoxide in the feed. Structures of the produced epoxyacrylates were determined by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectrophotometers. Most importantly, we successfully determined the compositions of the prepared monoacrylate-terminated epoxyacrylates of various batches by gel permeation chromatography (GPC). Finally, the most suitable reaction condition for preparing a dual-curable difunctional epoxyacrylate was established in this study.
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2. Experimental 2.1. Material An epoxy resin, BE-188, based upon the diglycidyl ether of bisphenol-A (DGEBA) with an epoxy equivalent weight (EEW) of 188 g equiv.−1 , was supplied from Chang Chun Plastics Co., Taiwan. Triphenyl phosphine (TPP) also from Chang Chun Plastics Co. was used as the catalyst for the carboxyl–epoxide reaction. Acrylic acid (AA), crystal violet, tetraethyl ammonium bromide, methyl ethyl ketone (MEK) and perchloric acid (0.1 N solution in acetic acid) were purchased from Acros (Geel, Belgium). Hydroquinone (HQ) was obtained from Showa (Tokyo, Japan) and used as an inhibitor. 2.2. Isothermal kinetic studies and synthesis of epoxyacrylate resins Fourier transform infrared (FTIR) spectrophotometer (Nicolet model 550, USA) coupled with a heating cell was used to study the reaction kinetics of AA and DGEBA in situ. First, 1000 ppm TPP and 300 ppm HQ (based on the total weight of DGEBA and AA) were dissolved in the AA. The solution was then homogeneously mixed with DGEBA. The equivalent ratio of carboxyl group in AA to epoxide group in DGEBA (carboxyl/epoxide) in the feed was fixed at 0.5 for isothermal kinetic studies. The viscous liquid mixture was casted on a KBr plate and covered with another plate. The thickness of the liquid film was about 12 m by using a spacer. The sandwich-like salt plate was placed in a heating cell, which was then mounted in the spectrometer to carry out the reaction in the temperature range of 60–120 ◦ C. The heating cell was controlled by a Watlow series 965 model temperature controller to an accuracy of ±1 ◦ C. FTIR spectra were recorded at several specific times and the peak intensity of epoxide group was monitored to follow the extent of reaction in situ. The corresponding sample codes are listed in Table 1 as EA-T60 to EA-T120. Various batches of epoxyacrylates having different compositions, EA25 to EA100, were synthesized from AA and DGEBA by changing the equivalent ratio of carboxyl/epoxide in the feed from 0.25 to 1.0, respectively. The reaction was carried out in a 250 mL round-bottomed reactor fitted with a mechanical stirrer, a condenser and an air inlet. Typically, 1000 ppm TPP and 300 ppm HQ (based on the total weight of DGEBA and AA) were first dissolved in the AA. The solution and DGEBA were then introduced to the reactor under a controlled temperature profile, 100 ◦ C for 2 h and another 2 h at 120 ◦ C, and a stirring rate of 200 rpm. Sample codes and their basic experimental conditions are listed in Table 1. 2.3. Characterizations Structure analysis of the synthesized epoxyacrylates was carried out using a FTIR spectrophotometer and a 1 H-nuclear magnetic resonance (NMR) spectrometer. Samples were casted on KBr discs to obtain their transmission FTIR spectra. The recorded wavenumber range was from 4000 to 400 cm−1 with a resolution of 4 cm−1 . Proton 1 H NMR spectra were obtained with a 300 MHz NMR instrument (Bruker, Germany), using deuterated chloroform, CDC13 , as the solvent. Approximately 70 mg of sample was dissolved into 1 mL of CDC13 . The scanned range was 1–10 ppm. Epoxy equivalent weight (EEW, g equiv.−1 ) of the prepared epoxyacrylates was determined by the HClO4 titration method. Typically, 0.5 g of the epoxyacrylate resin was dissolved into 10 mL of methyl ethyl ketone. Tetraethyl ammonium bromide solution was prepared by dissolving 10 g of tetraethyl ammonium bromide in 50 mL of acetic acid. The tetraethyl ammonium bromide solution (5 mL) and 3 drops of crystal violet indicator solution were then added to the epoxyacrylate solution and followed by beginning
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Table 1 Sample codes and basic experimental conditions. Samplea
AA (mol)
DGEBA (mol)
[COOH]/[epoxide]
Reaction temperature (◦ C)
EA-T60 EA-T80 EA-T100 EA-T120 EA25b EA50 EA75 EA100
1.0 1.0 1.0 1.0 0.5 1.0 1.5 2.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0.50 0.50 0.50 0.50 0.25 0.50 0.75 1.00
60 80 100 120 100, 120 100, 120 100, 120 100, 120
a b
All reactions were carried out with the addition of 1000 ppm TPP and 300 ppm HQ for 4 h. EA25 to EA100 were carried out at 100 ◦ C for 2 h and followed by another 2 h at 120 ◦ C.
the titration with HClO4 solution until a sharp purple-to-green end point was reached [25]. EEW was then calculated as follows: EEW =
1000 × m (V1 − V2 ) × N
(1)
where m is the weight of the epoxyacrylate resin (g), V1 and V2 are the volumes of perchloric acid used (mL) in the titration of the epoxyacrylate solution and the blank solution, respectively, and N is the equivalent concentration of the perchloric acid solution (0.10 N). The acid value is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of the epoxyacrylate product. In other words, the acid value indicates the amount of unreacted carboxyl group after reaction. The acid value was determined according to ASTM D974 [26]. Molecular weight and composition of the produced epoxyacrylates were determined using gel permeation chromatography (GPC). The experiment was carried out by using an isocratic pump (Waters model 1515) with a RI detector (Waters 2414) and two serial styragel HR columns also from Waters. Polystyrene (PS) and chloroform were used as the respective standard and the eluent solvent. The flow rate was set at 0.8 mL min−1 , and the temperature was 40 ◦ C. To determine the composition of the reaction product, the chromatographic peak was deconvoluted into a series of peaks corresponding to the different structures by using PeakFitTM V4.12 software (Systat software Inc., San Jose, CA, USA). 3. Results and discussion 3.1. Structure analysis and isothermal reaction kinetics Isothermal reaction kinetics was studied by reacting AA and DGEBA with an initial carboxyl/epoxide equivalent ratio of 0.5 in the temperature range of 60–120 ◦ C (EA-T60 to EA-T120 in Table 1). The reaction between AA and DGEBA is basically a carboxyl–epoxide addition esterification [17–19]. In this study, TPP was used to catalyze the reaction. The probable reaction mechanism as proposed by Romanchick et al. [27] is shown in Scheme 1(a). The reaction starts by the nucleophilic attack of TPP, which opens the epoxide group, producing a phosphonium betaine. The betaine abstracts a proton from the acrylic acid. The carboxylate anion subsequently attacks the electrophilic carbon attached to the phosphorus, forming an ester linkage and regenerating the catalyst. Fig. 1 shows FTIR spectra of DGEBA, AA and a representative epoxyacrylate, EA-T100, obtained by reacting AA/DGEBA (carboxyl/epoxide = 0.5) at 100 ◦ C for 4 h. The absorption peaks at 913 cm−1 and 864 cm−1 in the spectrum of DGEBA are due to the epoxide group. The absorption peaks at 1700 cm−1 and 1631 cm−1 appearing in the spectrum of AA are attributed to the carboxyl group and C C stretching, respectively. In the spectrum of the produced EA-T100 epoxyacrylate, the absorption peaks of epoxide group (913 cm−1 and 864 cm−1 ) decrease significantly in
intensity compared with the DGEBA; yet, strong absorption peaks at 1721 cm−1 and 3467 cm−1 due to the respective ester group and hydroxyl group are clearly observed, arising from the reaction of the carboxyl group and the epoxide group. The additional peaks at 1631 cm−1 and 809 cm−1 are due to the terminal unsaturated C C double bond of the EA-T100 epoxyacrylate. This indicates that the carboxyl–epoxide esterification occurred and that the produced epoxyacrylate had the unsaturated C C double bond as well as the residual epoxide end group. This is because the equivalent ratio of carboxyl/epoxide in the feed was 0.5 for the EA-T100. Therefore, the theoretical maximum epoxide conversion could only reach 0.5, leaving half of epoxide groups un-reacted. Furthermore, because there are two epoxide end groups in one DGEBA molecule, the ideal structure of the final product would be a monoacrylate-terminated epoxyacrylate as shown in Scheme 1(a). The composition of the reaction products can be determined by chromatography which will be discussed later. The structure of the produced epoxyacrylate is further confirmed by the 1 H NMR spectra. Fig. 2 shows NMR spectra of pure DGEBA, AA and the EA-T100 epoxyacrylate. In addition to the epoxide absorption peaks at 2.74, 2.89 and 3.34 ppm (indicated as 1 and 2 in the spectrum), the spectrum of DGEBA exhibits the absorption peaks at 3.96 and 4.17 ppm which are caused by the methylene hydrogen ( CH2 O , 3). Furthermore, very small peaks are found around 4.10 ppm, which is assigned to the CH (OH) in the oligomeric units with n > 0 [28]. This confirms the existence of oligomeric fractions with n > 0 for the DGEBA used in this study, see Scheme 1(a). From the peak area in the NMR spectrum, it is possible to calculate the average n value and thus the EEW of the DGEBA [29]. The calculated average n value is 0.12 and the deduced EEW is 187 g equiv.−1 . The EEW value is in agreement with the results obtained from the titration method, 186 g equiv.−1 , and from the manufacturer, 188 g equiv.−1 . After reaction, the produced EA-T100 epoxyacrylate contained both epoxide end group as indicated at 2.74, 2.89 and 3.34 ppm (1 and 2 in the spectrum), and terminal CH CH2 double bond at 5.89, 6.18 and 6.42 ppm (8, 8 and 9). The additional proton absorption peaks at 4.05 and 4.36 ppm are assigned to the methylene group adjacent to the ester (3 ), CH2 O C( O), which confirms the formation of ester group. The structure analysis from the 1 H NMR spectra thus agrees with the results from the FTIR spectra, demonstrating that the reaction is a carboxyl–epoxide addition esterification. The EA-T60 and EA-T80 systems have all the same absorption peaks as the EA-T100, not only in the FTIR spectra but also in the NMR spectra. The only difference among these systems is the peak intensity due to their differences in the epoxide conversion. Furthermore, the NMR results suggest that most secondary hydroxyl group did not take part in the reaction with the carboxyl group or with the epoxide group when reacting at 100 ◦ C or lower for 4 h. Banthia and McGrath [30] also suggested that no side reaction occurred between the epoxide group with the secondary hydroxyl group at 100 ◦ C for the reaction between an epoxy resin and a phenol
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Scheme 1. (a) Addition esterification between the epoxide group of DGEBA and the carboxyl group of AA catalyzed by triphenyl phosphine (PPh3 ). The epoxyacrylate was produced by feeding the same molar amounts of AA and DGEBA, i.e., [COOH]/[epoxide] = 0.5. (b) Etherification between the epoxide group and the secondary hydroxyl group.
catalyzed by TPP. They believed this was due to the steric hindrance around the phosphorous atom that prevented the formation of any ion-pair complex between them and the pendant secondary hydroxyl group.
As in the catalyzed polyesterification of a dicarboxylic acid with a diol, the second-order kinetics has also been used to describe the carboxyl–epoxide addition esterification. Srivastava et al. [31] studied the kinetics of esterification of cycloaliphatic epoxies with
Fig. 1. FTIR spectra of (a) DGEBA, (b) acrylic acid (AA) and (c) an epoxyacrylate, EA-T100, prepared by feeding the AA/DGEBA with a carboxyl/epoxide equivalent ratio of 0.5 and reacting at 100 ◦ C for 4 h.
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Fig. 3. Epoxide conversion as a function of time for the reaction between DGEBA and AA at four reaction temperatures. The carboxyl/epoxide equivalent ratio in the feed was 0.5. The dotted line is the theoretical final conversion (=0.5) of the epoxide group.
group. Therefore, b equals a/2. Eq. (2) thus can be replaced by Eq. (3). d˛ = k1 dt
a 2
(1 − ˛)(1 − 2˛)
(3)
Integration of Eq. (3) yields
ln
1−˛ 1 − 2˛
= k1
a 2
t
(4)
Eq. (4) shows that the rate constant k1 can be calculated via the plot of ln[(1 − ˛)/(1 − 2˛)] versus time. The conversion of epoxide group (˛) at any time t can be calculated by the decrease in the epoxide absorption peak area at 913 cm−1 in the FTIR spectra as follows: ˛=1− 1
Fig. 2. H NMR spectra of DGEBA, acrylic acid and an epoxyacrylate, EA-T100, prepared by feeding the AA/DGEBA with a carboxyl/epoxide equivalent ratio of 0.5 and reacting at 100 ◦ C for 4 h.
methacrylic acid in the presence of TPP and showed that the reaction followed second-order kinetics. Hu et al. [32] also used second-order kinetics to describe the addition esterification of 1naphthylacetic acid with the epoxide groups attached to a random copolymer of ethylene, ethyl acrylate and glycidyl methacrylate in the melt state. Therefore, second-order kinetics was adopted in this study, in which the esterification can be considered as a reversible reaction, with the forward reaction being of first order with respect to the epoxide group and the carboxyl group, and the reverse reaction being of first order with respect to the newly formed vinyl ester group. Accordingly, the overall reaction rate thus can be described by d˛ = k1 (1 − ˛)(b − a˛) − k2 ˛ dt
(2)
where k1 and k2 are the forward and reverse reaction constants, ˛ denotes the conversion with regard to the epoxide consumption, a and b are the initial concentrations of the epoxide group and the carboxyl group, respectively. For the carboxyl–epoxide reaction, k1 was reported to be much larger than k2 [32]. This suggests that k2 can be ignored. For the reaction systems, EA-T60 to EA-T120, the initial epoxide group is twice the equivalent amount of the carboxyl
(A913 /A2872 )t (A913 /A2872 )0
(5)
where A913 and A2872 are the peak areas of the respective epoxide group and the methyl group. The methyl absorption peak is used as an internal standard to account for any variations in the sample thickness and viscosity since its amount does not change during reaction. Utilizing the FTIR heating cell for monitoring the in situ reaction and Eq. (5), the epoxide conversion as a function of time for the reaction between DGEBA and AA can be obtained. Since the epoxide group was twice the amount of the carboxyl group for the EA-T60 to EA-T120 systems, theoretically, the maximum epoxide conversion should reach 0.5 if all carboxyl groups reacted completely with epoxide groups as indicated by the dotted line in Fig. 3. Fig. 3 shows the reaction curves of DGEBA and AA at four reaction temperatures. The initial reaction rate and the epoxide conversion after 4 h of reaction were both increased with increasing reaction temperature. If the reaction temperature was too low such as at 60 ◦ C in the EA-T60, the reaction was very slow and the conversion could only reach about 37% after 4 h of reaction. When raising the reaction temperature to 120 ◦ C in the EA-T120, the reaction rate was so high that the epoxide conversion already reached 0.5 in a short period of time about 20 min, and still gradually increased with reaction time. The epoxide conversion exceeded 0.5 after 4 h of reaction, reaching a value of 0.58. This indicates that the reaction temperature is high enough to cause some other side reactions to consume more epoxide group. The most probable side reaction is the etherification between the epoxide and the secondary hydroxyl group, see Scheme 1(b). On the contrary, for the EA-T100 system, the epoxide conversion reached 0.45 after 1 h of reaction and increased very
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Fig. 4. Plots of ln[(1 − ˛)/(1 − 2˛)] versus reaction time of the reaction between AA and DGEBA catalyzed by TPP at four reaction temperatures. The carboxyl/epoxide equivalent ratio in the feed was 0.5. The rate constant k1 was calculated from the slope: k1 = 0.0014 kg equiv.−1 min−1 at 60 ◦ C; k1 = 0.0048 kg equiv.−1 min−1 at 80 ◦ C; k1 = 0.0161 kg equiv.−1 min−1 at 100 ◦ C; k1 = 0.0581 kg equiv.−1 min−1 at 120 ◦ C.
slowly since then. After 4 h of reaction, the epoxide conversion was 0.47, slightly less than the theoretical maximum value. This is because the reaction rate decreased greatly when most carboxyl groups were consumed. It has to be mentioned that at this temperature of 100 ◦ C, the secondary hydroxyl group did not take part in the reaction as proved previously in the NMR spectrum. The results in Fig. 3 also justifies that the reverse rate constant k2 could be neglected under the present reaction conditions; otherwise the conversion would not approach to the theoretical maximum value. Eqs. (3) and (4) then were applied, and plots of ln[(1 − ˛)/(1 − 2˛)] with time were obtained. The rate constant k1 was calculated from the initial slope. Fig. 4 shows that the plots are all linear at four reaction temperatures with very high correlation coefficients (R > 0.995). A slightly lower R value is observed for the EA-T120 system, probably due to the interference of the etherification side-reaction as mentioned previously. The results show that the rate constant (k1 ) and thus the reaction rate increased with temperature. The rate constant was increased by about 2.4 times when the reaction temperature was raised by 20 ◦ C. Since the rate constants at different temperatures are known, the activation energy (Ea ) thus can be calculated using the Arrhenius equation. k = Ae−(Ea /RT )
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Fig. 5. Plot of ln k1 versus reciprocal of temperature for the carboxyl–epoxide addition reaction between AA and DGEBA catalyzed by TPP.
By increasing the reaction time more than 4 h in the EAT100 system, the epoxide conversion probably could be further increased; yet, it would take a long time to reach the maximum value of 0.5. In this study, it was attempted to find the most suitable reaction condition through which the epoxide conversion could reach the maximum value in a time period of 4 h, because of the economic consideration in industrial production. It is interesting to find that by heating the reaction mixture at 100 ◦ C for 2 h first and followed by another 2 h at 120 ◦ C, the epoxide conversion of this EA50 (see Table 1) became very close to the theoretical value of 0.5 as indicated by the dotted line in Fig. 6. Most importantly, there was no etherification reaction observed. To validate the findings, FTIR spectra of different epoxyacrylates prepared with the initial carboxyl/epoxide equivalent ratio of 0.5 but under different temperature profiles were recorded and compared. As shown in Fig. 7, a weak absorption peak at 1120 cm−1 is noticed in the original DGEBA and assigned to the >C O(H) stretching due to the presence of pendent secondary hydroxyl group in the n = 1 and n = 2 homologues. After reaction, the relative increase in peak intensity is found to be proportional to the epoxide conversion for all systems except
(6)
where T is temperature (K), A is the frequency factor and R is the gas constant. Fig. 5 shows the plot of ln k1 versus the reciprocal of temperature, in which the Arrhenius law is obeyed very well. The slop of the plot obtained by regression analysis was used to calculate the activation energy for the carboxyl–epoxide esterification reaction, which was found to have a value of 71.5 (kJ mol−1 ). Since it was very possible that etherification might have occurred in the epoxide group when reacted at 120 ◦ C, the slope was re-calculated using data only in the range from 60 ◦ C to 100 ◦ C. The activation energy thus obtained was 63.9 kJ mol−1 . This value is slightly lower than those reported by Srivastava et al. [31]. They discussed the esterification of two cycloaliphatic epoxy resins CER1 and CER2 containing glycidyl and cyclohexane epoxy groups, respectively, with methacrylic acid using a stoichiometric ratio of 1/0.9 in the presence of TPP. They obtained the activation energies of 73.0 and 66.9 kJ mol−1 for CER1 and CER2, respectively. It can be seen that the difference in activation energy is caused by the structure factors of the reactants, especially the steric effect. The more bulky the reactant, the higher the activation energy is.
Fig. 6. Epoxide conversion as a function of time for the reaction between AA and DGEBA with a carboxyl/epoxide equivalent ratio of 0.5 under different temperature profiles. () EA-T120, 4 h at 120 ◦ C; (䊉) EA-T100, 4 h at 100 ◦ C; () EA50, 2 h at 100 ◦ C and 2 h at 120 ◦ C. The dotted line is the theoretical final conversion of the epoxide group (=0.5).
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Fig. 7. FTIR spectra of epoxyacrylates prepared with different reaction processes: EA-T100 (4 h at 100 ◦ C); EA50 (2 h at 100 ◦ C, and 2 h at 120 ◦ C); and EA-T120 (4 h at 120 ◦ C). All reaction systems had the same carboxyl/epoxide equivalent ratio at 0.5.
the EA-T120. This is because one additional secondary hydroxyl group is produced by the consumption of one epoxide group via the carboxyl–epoxide etherification as shown in Scheme 1(a). However, the increase is larger than expected in the EA-T120 system. This is explained by the occurrence of the etherification between the epoxide with the secondary hydroxyl group in the EA-T120, see Scheme 1(b). Though the etherification reaction consumes one secondary hydroxyl group, it generates another one. Moreover, the reaction also results in the structure of aliphatic ether C O C, whose asymmetric stretching is also at around 1120 cm−1 , and apparently stronger than the C OH stretching associated with the pendent secondary hydroxyl group. Consequently, the peak intensity is higher than expected in the EA-T120. Generally speaking, the etherification between the epoxide group and the secondary hydroxyl group is less reactive and a notable etherification is usually observed at temperatures around or greater than 200 ◦ C. Yet, because of the presence of the TPP catalyst, the reaction temperature for this etherification could be lower, such as 120 ◦ C in the EA-T120 system. Devi et al. [33] also found that the addition of TPP shifted the cure exotherm of phenol–epoxide reaction to lower temperatures. They studied the TPP-catalyzed curing of a diallyl bisphenol A-novolac epoxy resin system using differential scanning calorimetry (DSC). The maximum peak temperature for the phenol–epoxide reaction under a heating rate of 5 ◦ C min−1 in DSC shifted from 186 ◦ C to 129 ◦ C when 0.5 wt% TPP was added. Based on the above results, all following epoxyacrylates with different amounts of functional end groups were synthesized by this twostage heating process, 100 ◦ C for 2 h followed by another 2 h at 120 ◦ C.
compositions and intentionally to find out the system having the maximum proportion of the monoacrylate-terminated epoxyacrylate. Fig. 8 shows FTIR spectra of the produced EA25 to EA100 epoxyacrylates. The absorption peaks at 1720 cm−1 (C O stretching), 1633 cm−1 (C C stretching), 809 cm−1 (CH CH2 bending) and 3462 cm−1 (O H stretching) increase in intensity with an increase in the feeding amount of carboxyl group, but the absorption peaks at 913 cm−1 and 864 cm−1 from the epoxide group decrease. This is because increasing the equivalent ratio of carboxyl/epoxide would increase the extent of addition esterification reaction. The results thus indicate that by adjusting the initial equivalent ratio of carboxyl/epoxide, epoxyacrylates with different amounts of epoxide group and unsaturated double bond could be obtained as shown in Scheme 2. It is thus interesting to determine the properties and compositions of the reaction products from various reaction systems. It has to be mentioned that the relative change in the peak intensity at 1120 cm−1 was found to be proportional to the epoxide conversion in all EA25 to EA100 systems, indicating that there was no etherification occurred in these systems as explained in the previous section.
3.2. Synthesis of epoxyacrylates with different amounts of functional end groups The aim of this study was trying to synthesize monoacrylateterminated epoxyacrylate having one unsaturated group at one end for further UV-curing and one epoxide group at the other end for thermo-curing in a time period of 4 h. Therefore, various batches of epoxyacrylates were synthesized by changing the initial equivalent ratio of carboxyl/epoxide from 0.25 to 1.0 to obtain the corresponding EA25 to EA100 as shown in Table 1. The produced epoxyacrylates were then analyzed to determine their
Fig. 8. FTIR spectra of (a) pure DGEBA, and epoxyacrylates prepared with different equivalent ratios of carboxyl/epoxide: (b) EA25 (carboxyl/epoxide = 0.25), (c) EA50 (carboxyl/epoxide = 0.50), (d) EA75 (carboxyl/epoxide = 0.75), (e) EA100 (carboxyl/epoxide = 1.0). The reactions were all carried out at 100 ◦ C for 2 h and another 2 h at 120 ◦ C.
Y.-C. Su et al. / Materials Chemistry and Physics 132 (2012) 540–549
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Scheme 2. Chemical structures of the epoxyacrylates prepared by feeding AA and DGEBA with different equivalent ratios of carboxyl/epoxide. (A) DGEBA, (B) monoacrylateterminated epoxyacrylate, (C) bisacrylate-terminated epoxyacrylate.
Table 2 shows the epoxide conversion, acid value and EEW of the EA25 to EA100 epoxyacrylates. The epoxide conversion determined by FTIR increased from 0.246 to 0.843 as the initial equivalent ratio of carboxyl/epoxide was raised from 0.25 to 1.0. Correspondingly, this would cause an increase in the EEW value as confirmed by the measured EEW data using HClO4 titration. The epoxide conversions after 4 h of reaction are very close to the theoretical values except the EA100. The theoretical values of epoxide conversion of various reaction systems were calculated by assuming that all carboxyl groups reacted completely with the epoxide groups under the employed reaction conditions, i.e., the reverse reaction and the etherification were both negligible. The complete reaction of AA with DGEBA in the EA25 to EA75 systems is confirmed by the acid values shown in Table 2 which are all close to zero. The fact that the measured epoxide conversions did not exceed their respective theoretical values also validates that the epoxide group only reacted
with the carboxyl group without the occurrence of the etherification, in agreement with the FTIR and NMR results. However, it appears that AA had not reacted completely with the DGEBA in the EA100 system after 4 h of reaction. The epoxide conversion and its acid value are 0.84 and 38.3 (mg KOH g−1 ), respectively. This is mainly because the DGEBA was in excess for the EA25 to EA75 systems, assuring a more complete reaction of the AA with the DGEBA; whereas the initial carboxyl and epoxide groups were in stoichiometric amounts in the EA100 system. As the reaction proceeded, both carboxyl and epoxide concentrations became lower and lower. As a result, the reaction rate decreased greatly after 4 h of reaction, and the conversion thereby could not reach its theoretical maximum value in this time period. It is believed that the conversion can still be increased as the reaction time is extended. In the EA50 system, an epoxide conversion of 0.5 should be obtained when the carboxyl group reacted completely with the
Table 2 Epoxide conversion (˛), acid value and EEW of different samples prepared with different equivalent ratios of carboxyl/epoxide in the feed. Sample
AA DGEBA
[COOH] [epoxide]
˛theo.
˛a
Acid valueb (mg KOH g−1 )
EEWtit. c (g equiv.−1 )
EA25 EA50 EA75 EA100
0.5 1.0 1.5 2.0
0.25 0.50 0.75 1.00
0.25 0.50 0.75 1.00
0.246 0.518 0.737 0.843
0.2 0.0 0.5 38.3
224 410 684 1744
a b c
The epoxide conversion was determined from FTIR spectra. The acid value was measured by titration with KOH(aq) (0.10 N). The EEWtit. value was measured by the HClO4 titration of epoxide group. Pure DGEBA has an EEWtit. of 186 (g equiv.−1 ).
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Y.-C. Su et al. / Materials Chemistry and Physics 132 (2012) 540–549 Table 3 The compositions of the prepared epoxyacrylates determined by GPC. Sample
EA25 EA50 EA75 EA100b
[COOH] [epoxide]
0.25 0.50 0.75 1.00
˛FTIR
˛GPC
0.246 0.518 0.737 0.843
0.210 0.490 0.725 0.855
Composition (%)a A
B
C
61 15 9 –
36 72 37 29
3 13 54 71
a A is the un-reacted DGEBA; B is the monoacrylate-terminated epoxyacrylate; C is the bisacrylate-terminated epoxyacrylate. All reactions were carried out at 100 ◦ C for 2 h and then another 2 h at 120 ◦ C. b In the EA100, there was still some un-reacted AA left in the system after 4 h of reaction.
Fig. 9. Chromatograms of (a) pure DGEBA, and epoxyacrylates obtained from the reaction of AA and DGEBA with different equivalent ratios of carboxyl/epoxide in the feed: (b) EA25 (carboxyl/epoxide = 0.25), (c) EA50 (carboxyl/epoxide = 0.50), (d) EA75 (carboxyl/epoxide = 0.75), (e) EA100 (carboxyl/epoxide = 1.0). The reactions were all carried out at 100 ◦ C for 2 h and another 2 h at 120 ◦ C.
epoxide group, which was confirmed by the acid titration and FTIR measurements. Ideally, the as-prepared EA50 epoxyacrylate should have one double bond at one end and still have one epoxide group at the other end of the epoxyacrylate molecule, i.e., monoacrylate-terminated epoxyacrylate. However, it is very possible that two AA molecules have reacted with the two epoxide end groups on the same DGEBA molecule, thus producing a bisacrylate-terminated epoxyacrylate. Consequently, this would be accompanied by one un-reacted DGEBA molecule left in the system. Therefore, the reaction products would be composed of un-reacted DGEBA, monoacrylate- and bisacrylate-terminated epoxyacrylate, corresponding to the structure (A), (B), and (C) in Scheme 2. Fig. 9 shows the GPC chromatograms of DGEBA and the produced epoxyacrylates of EA25 to EA100. The composition of the reaction products could be traced by GPC, assuming that the amount being proportional to the corresponding peak area. This is a good approximation since they have almost the same responding factor in GPC detector due to their similar backbone structures. The DGEBA chromatogram contains two primary peaks at 24.48 and 23.39 min corresponding to the two homologues: n = 0 and n = 1. In addition, a very small peak at 22.65 min is also observed, which is assigned to the n = 2 homologue. According to the resolved peak area using a curve-fitting software, the proportion of the n = 2 homologue is only about 2%. By reacting with AA, the synthesized epoxyacrylate obviously has a higher molecular weight than the DGEBA. The peak at 24.48 min, attributed to the n = 0 oligomer of DGEBA, apparently decreases in intensity with an increase in the feeding amount of AA due to the carboxyl–epoxide esterification. It disappeared completely in the EA100 system. The peak at 24.13 min is assigned to the monoacrylate-terminated epoxyacrylate produced from n = 0 oligomer of DGEBA. This peak increases first and then decreases with increasing the equivalent ratio of carboxyl/epoxide from 0.25 to 1.0. The maximum occurs in the EA50 system when the initial equivalent ratio of carboxyl/epoxide is 0.5. In addition, the peak at 23.76 min is attributed to the bisacrylateterminated epoxyacrylate produced from the n = 0 oligomer of DGEBA. The peak intensity increases as the equivalent ratio of carboxyl/epoxide increases. It can be seen clearly that in the special case of the EA50 system, the n = 0 homologue of the DGEBA did not disappear completely and the peak of bisacrylate-terminated epoxyacrylate
seemed to appear in the chromatogram. This indicates that two AA molecules indeed reacted with both epoxide end groups on the same DGEBA molecule. Therefore, the reaction products would consist of un-reacted DGEBA (structure A), monoacrylate-terminated epoxyacrylate (structure B) and bisacrylate-terminated epoxyacrylate (structure C). In order to estimate the proportions of different structures, the broad chromatographic peak was deconvoluted into a series of peaks corresponding to the different structures by using a curvefitting software. The percentages of different structures (A, B and C) were then calculated based on their resolved peak areas divided by the total area as shown below: percentage of structure i =
A
i × 100 = Ai
Ai A0 + AM + AB
× 100 i = 0, M, B
(7)
where Ai is the peak area of individual structure, namely, A0 , AM and AB corresponding to the respective peak area of un-reacted DGEBA (structure A), monoacrylate-terminated epoxyacrylate (structure B) and bisacrylate-terminated epoxyacrylate (structure C). The proportions of different structures were calculated and listed in Table 3. The results clearly show that all reaction systems consist of three different structures of A, B and C, except for EA100 which contains only the latter two; and their proportions depend on the initial equivalent ratio of carboxyl/epoxide. Increasing the equivalent ratio of carboxyl/epoxide leads to a decrease of the structure (A) and an increase of the structure (C). However, for the monoacrylateterminated epoxyacrylate (B) which contains one epoxide end group and one terminal double bond on the same molecule, there is a maximum occurred in the EA50 system, approximately 72%. It has to be mentioned that the result is only an approximation by using the deconvolution method. To obtain a more accurate result, model compounds should be used and calibration curves be constructed. Nevertheless, when the conversions of EA25 to EA100 were calculated from the resolved peak areas, as can be seen from Table 3, they are in agreement with the results from FTIR spectra. Though there are still some DGEBA and bisacrylate-terminated epoxyacrylate in the EA50, undoubtedly, the EA50 can be subjected to UV-cure and subsequent thermo-cure to obtain a fully crosslinked structure by addition of photo-initiator and curing agent. Therefore, the EA50 epoxyacrylate has the potential to be used as an adhesive sealant for the LCD production. Structures and properties of the cured EA50 with and without the addition of silica will be reported in our next paper. 4. Conclusions In this study, various epoxyacrylates with different functional end groups were synthesized by addition esterification of the epoxide group in DGEBA with the carboxyl group in AA. It was found that the rate constant increased by about 2.4 times when the
Y.-C. Su et al. / Materials Chemistry and Physics 132 (2012) 540–549
reaction temperature was increased by 20 ◦ C in the temperature range of 60–100 ◦ C. The activation energy of the carboxyl–epoxide esterification catalyzed by triphenyl phosphine was obtained from the Arrhenius equation and found to be 63.9 kJ mol−1 . However, at a higher temperature of 120 ◦ C, the side reaction might occur which involved etherification of the secondary hydroxyl group with the epoxide group. The most suitable temperature profile was found to be 100 ◦ C for 2 h followed by another 2 h at 120 ◦ C, by which a complete carboxyl–epoxide reaction could be reached without the etherification reaction. Furthermore, the synthesized epoxyacrylates were proved to consist of un-reacted DGEBA, monoacrylate-terminated epoxyacrylate and bisacrylateterminated epoxyacrylate, for which the composition has been estimated from the GPC chromatograms. When the initial equivalent ratio of carboxyl/epoxide was set at 0.5, a maximum amount of monoacrylate-terminated epoxyacrylate (∼72%) was observed, which had one double bond at one end for UV-cure and one epoxide group at the other end for thermo-cure. Therefore, by applying UV irradiation and thermal treatment, the dual-curable epoxyacrylate can be cured to a fully crosslinked structure, which is useful as an adhesive sealant for LCD production. Acknowledgements The authors wish to thank National Science Council in Taiwan for the financial support and professor Ching-Chung Chen for his kind assistance in the synthesis of epoxyacrylates. References [1] C.A. May, Y. Tanaka, Epoxy Resins: Chemistry and Technology, Marcel Dekker, New York, 1988, p. 794. [2] B. Ramezanzadeh, M.M. Attar, Mater. Chem. Phys. 130 (2011) 1208. [3] H. Jin, G.M. Miller, N.R. Sottos, S.R. White, Polymer 52 (2011) 1628.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and characterization of UV- and thermo-curable difunctional epoxyacrylates Yu-Chieh Su a , Liao-Ping Cheng a , Kuo-Chung Cheng b , Trong-Ming Don a,∗ a b
Department of Chemical and Materials Engineering, Tamkang University, Danshui Dist., New Taipei City 25137, Taiwan Department of Chemical Engineering, National Taipei University of Technology, Taipei 106, Taiwan
a r t i c l e
i n f o
Article history: Received 28 March 2011 Received in revised form 2 November 2011 Accepted 24 November 2011 Keywords: Chemical synthesis Fourier transform infrared spectroscopy (FTIR) Adhesion Epoxyacrylate
a b s t r a c t This paper describes the synthesis and characterization of difunctional epoxyacrylate oligomers from the reaction of diglycidyl ether of bisphenol-A (DGEBA) and acrylic acid (AA) in the presence of triphenyl phosphine as catalyst. The reaction was investigated in situ by using Fourier transform-infrared spectroscopy in the temperature range of 60–120 ◦ C, and demonstrated to be an addition esterification between the epoxide group and the carboxyl group. The specific rate constants calculated from the second-order kinetics obeyed Arrhenius law very well, and from which the activation energy was found to be 63.9 kJ mol−1 . Moreover, various batches of respective epoxyacrylates, EA25 to EA100, were prepared by changing the equivalent ratio of carboxyl/epoxide from 0.25 to 1.0 in the feed under a temperature profile of 100 ◦ C for 2 h and another 2 h at 120 ◦ C. The epoxide conversions, acid values and epoxide equivalent weights of the produced EA25 to EA100 were all measured. The analysis by gel permeation chromatography revealed that the EA25 to EA75 consisted of unreacted DGEBA, monoacrylate-terminated epoxyacrylate and bisacrylate-terminated epoxyacrylate; while the EA100 was composed of only latter two. Increasing the initial carboxyl/epoxide equivalent ratio increased the epoxide conversion and the amount of bisacrylate-terminated epoxyacrylate. Yet, with an initial carboxyl/epoxide equivalent ratio of 0.5, the EA50 produced the maximum proportion (∼72%) of monoacrylate-terminated epoxyacrylate, which had one terminal double bond for UV-cure and one epoxide end group for thermo-cure. This dual-curable epoxyacrylate thus can be used as an adhesive sealant for LCD production. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Epoxy resins are one of the most important thermosetting polymers that exhibit many desirable properties, such as low cure shrinkage, low creep, excellent adhesive strength, good chemical resistance, high mechanical strength, thermal stability and excellent electrical insulation [1]. These properties have led to rapid growth of epoxy resins and their wide use in many applications, including surface coatings, structural adhesives, packaging of electronic products and matrix for (nano)composite materials [2–5]. Especially, the demand of epoxy resins has rapidly increased for the applications in the electronic products, such as the epoxy molding compounds used for the encapsulation of IC chips and the epoxy composites for printed circuit boards [6,7]. The thermosetting epoxy resin has also been used as an adhesive sealant to bind two glass substrates in the liquid-crystal display (LCD), i.e., a thin film transistor (TFT) substrate mounted
∗ Corresponding author. Tel.: +886 2 26293856; fax: +886 2 26209887. E-mail address: [email protected] (T.-M. Don). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.11.067
with the driving circuit and a color filter (CF) substrate for displaying colors. The liquid crystals (LC) and other components are thus confined between the two substrates. In the conventional LCD cell assembly process, the peripheral adhesive sealant is generally a thermo-curable epoxy. The outstanding adhesion is due to the existence of hydroxyl and ether groups in the structure of epoxy resin. However, this process has several problems such as the dislocation of the two glass substrates mainly caused by its high curing temperature and long curing time of epoxy adhesive sealant. Furthermore, it takes a long time to complete the filling of LC fluid through the injection hole [8]. In order to solve these problems, a new LC injection process, called onedrop-filling (ODF) technology, is currently developed and used in the LCD industry. In this process, LC is dispensed in droplets directly onto the glass substrate and then an adhesive sealant is applied to the periphery of the substrate. The two glass substrates are then bonded together by curing the adhesive sealant [9]. The main advantage is the great reduction of the LC-filling time, from 5 days with the traditional injection method to only 5 h with the ODF process when applied to a 30-in. panel. The consumption of LC can be also reduced by about 40%. Thus,
Y.-C. Su et al. / Materials Chemistry and Physics 132 (2012) 540–549
utilizing the ODF method can simplify the overall process, leading to increases in both efficiency and yield [10]. In the ODF process, the adhesive sealant becomes very important. There are three types of adhesive sealant that can be used in the ODF process. The first-type sealant has double bonds in its structure and thus can be UV-cured with the help of photo-initiator. The UV-curable epoxy resins have several advantages over traditional thermal-curable ones such as rapid cure, solvent-free process, low temperature operation and low energy requirement [1,11]. However, the first-type sealant with only UV-curable property has suffered from low strength, significant curing-shrinkage and low moisture resistance [12]. The second-type adhesive sealant, which is a mixture of a UVcurable acrylate resin with double bond(s) and a thermo-curable epoxy resin with epoxide groups, has no such problems [13]. Yet, the dual-curable mixed resins need a great amount of UV energy because the existence of the second epoxy component can disturb the radical polymerization of the acrylate resin. The third-type adhesive sealant is also dual-curable, yet it contains both double bond and epoxide group on the same molecule, which by itself can be cured by applying UV and heat in sequence. In order to reduce the UV energy and increase the polymerization rate, a dual-curable oligomer having both UVcurable and thermo-curable functional groups in one resin is thus desirable such as monoacrylate-terminated epoxy oligomers [14–16]. In industries, epoxyacrylates are generally produced and used in the form of bisacrylate-terminated epoxyacrylates, also called vinyl ester resins [17,18]. They are produced by reacting stoichiometric equivalents of diglycidyl ether of bisphenol-A (DGEBA) and acrylic acid (AA) or methacrylic acid (MAA) [19]. During the reaction, the epoxide end groups of DGEBA react with the carboxyl group of AA or MAA and the resulting product, bisacrylate-terminated epoxyacrylate, thus has terminal reactive double bond at both ends. They can be categorized into the first-type sealant. The prepared bisacrylateterminated epoxyacrylates have been widely applied in the fields of coatings, structural adhesives and advanced composite matrices [20,21]. In recent years, the research of UV-curable epoxyacrylates has focused on nanocomposites [22], UV-curing flame retardant films [23] and waterborne epoxyacrylates [24]. Yet, by changing the equivalent ratio of AA or MAA to DGEBA, such as 0.5, it is possible to prepare the monoacrylate-terminated epoxyacrylate with only one double bond at one end, while the other end is still an epoxide group. The prepared monoacrylate-terminated epoxyacrylate thus can be used as a dual-curable adhesive sealant in the ODF process. However, very few studies were reported on the synthesis of the monoacrylate-terminated epoxyacrylate [14–16], and there is no report so far on the study of reaction kinetics from AA and DGEBA. This research was trying to synthesize a dual-curable oligomer containing both double bond and epoxide group on the same molecule, i.e., monoacrylate-terminated epoxyacrylate, from the reaction of AA and DGEBA. The reaction kinetics of AA and DGEBA catalyzed by triphenyl phosphine (TPP) in the temperature range of 60–120 ◦ C was carefully studied, and the activation energy of the reaction was reported. Subsequently, various batches of epoxyacrylates with different compositions were prepared by changing the equivalent ratio of carboxyl/epoxide in the feed. Structures of the produced epoxyacrylates were determined by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectrophotometers. Most importantly, we successfully determined the compositions of the prepared monoacrylate-terminated epoxyacrylates of various batches by gel permeation chromatography (GPC). Finally, the most suitable reaction condition for preparing a dual-curable difunctional epoxyacrylate was established in this study.
541
2. Experimental 2.1. Material An epoxy resin, BE-188, based upon the diglycidyl ether of bisphenol-A (DGEBA) with an epoxy equivalent weight (EEW) of 188 g equiv.−1 , was supplied from Chang Chun Plastics Co., Taiwan. Triphenyl phosphine (TPP) also from Chang Chun Plastics Co. was used as the catalyst for the carboxyl–epoxide reaction. Acrylic acid (AA), crystal violet, tetraethyl ammonium bromide, methyl ethyl ketone (MEK) and perchloric acid (0.1 N solution in acetic acid) were purchased from Acros (Geel, Belgium). Hydroquinone (HQ) was obtained from Showa (Tokyo, Japan) and used as an inhibitor. 2.2. Isothermal kinetic studies and synthesis of epoxyacrylate resins Fourier transform infrared (FTIR) spectrophotometer (Nicolet model 550, USA) coupled with a heating cell was used to study the reaction kinetics of AA and DGEBA in situ. First, 1000 ppm TPP and 300 ppm HQ (based on the total weight of DGEBA and AA) were dissolved in the AA. The solution was then homogeneously mixed with DGEBA. The equivalent ratio of carboxyl group in AA to epoxide group in DGEBA (carboxyl/epoxide) in the feed was fixed at 0.5 for isothermal kinetic studies. The viscous liquid mixture was casted on a KBr plate and covered with another plate. The thickness of the liquid film was about 12 m by using a spacer. The sandwich-like salt plate was placed in a heating cell, which was then mounted in the spectrometer to carry out the reaction in the temperature range of 60–120 ◦ C. The heating cell was controlled by a Watlow series 965 model temperature controller to an accuracy of ±1 ◦ C. FTIR spectra were recorded at several specific times and the peak intensity of epoxide group was monitored to follow the extent of reaction in situ. The corresponding sample codes are listed in Table 1 as EA-T60 to EA-T120. Various batches of epoxyacrylates having different compositions, EA25 to EA100, were synthesized from AA and DGEBA by changing the equivalent ratio of carboxyl/epoxide in the feed from 0.25 to 1.0, respectively. The reaction was carried out in a 250 mL round-bottomed reactor fitted with a mechanical stirrer, a condenser and an air inlet. Typically, 1000 ppm TPP and 300 ppm HQ (based on the total weight of DGEBA and AA) were first dissolved in the AA. The solution and DGEBA were then introduced to the reactor under a controlled temperature profile, 100 ◦ C for 2 h and another 2 h at 120 ◦ C, and a stirring rate of 200 rpm. Sample codes and their basic experimental conditions are listed in Table 1. 2.3. Characterizations Structure analysis of the synthesized epoxyacrylates was carried out using a FTIR spectrophotometer and a 1 H-nuclear magnetic resonance (NMR) spectrometer. Samples were casted on KBr discs to obtain their transmission FTIR spectra. The recorded wavenumber range was from 4000 to 400 cm−1 with a resolution of 4 cm−1 . Proton 1 H NMR spectra were obtained with a 300 MHz NMR instrument (Bruker, Germany), using deuterated chloroform, CDC13 , as the solvent. Approximately 70 mg of sample was dissolved into 1 mL of CDC13 . The scanned range was 1–10 ppm. Epoxy equivalent weight (EEW, g equiv.−1 ) of the prepared epoxyacrylates was determined by the HClO4 titration method. Typically, 0.5 g of the epoxyacrylate resin was dissolved into 10 mL of methyl ethyl ketone. Tetraethyl ammonium bromide solution was prepared by dissolving 10 g of tetraethyl ammonium bromide in 50 mL of acetic acid. The tetraethyl ammonium bromide solution (5 mL) and 3 drops of crystal violet indicator solution were then added to the epoxyacrylate solution and followed by beginning
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Table 1 Sample codes and basic experimental conditions. Samplea
AA (mol)
DGEBA (mol)
[COOH]/[epoxide]
Reaction temperature (◦ C)
EA-T60 EA-T80 EA-T100 EA-T120 EA25b EA50 EA75 EA100
1.0 1.0 1.0 1.0 0.5 1.0 1.5 2.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0.50 0.50 0.50 0.50 0.25 0.50 0.75 1.00
60 80 100 120 100, 120 100, 120 100, 120 100, 120
a b
All reactions were carried out with the addition of 1000 ppm TPP and 300 ppm HQ for 4 h. EA25 to EA100 were carried out at 100 ◦ C for 2 h and followed by another 2 h at 120 ◦ C.
the titration with HClO4 solution until a sharp purple-to-green end point was reached [25]. EEW was then calculated as follows: EEW =
1000 × m (V1 − V2 ) × N
(1)
where m is the weight of the epoxyacrylate resin (g), V1 and V2 are the volumes of perchloric acid used (mL) in the titration of the epoxyacrylate solution and the blank solution, respectively, and N is the equivalent concentration of the perchloric acid solution (0.10 N). The acid value is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of the epoxyacrylate product. In other words, the acid value indicates the amount of unreacted carboxyl group after reaction. The acid value was determined according to ASTM D974 [26]. Molecular weight and composition of the produced epoxyacrylates were determined using gel permeation chromatography (GPC). The experiment was carried out by using an isocratic pump (Waters model 1515) with a RI detector (Waters 2414) and two serial styragel HR columns also from Waters. Polystyrene (PS) and chloroform were used as the respective standard and the eluent solvent. The flow rate was set at 0.8 mL min−1 , and the temperature was 40 ◦ C. To determine the composition of the reaction product, the chromatographic peak was deconvoluted into a series of peaks corresponding to the different structures by using PeakFitTM V4.12 software (Systat software Inc., San Jose, CA, USA). 3. Results and discussion 3.1. Structure analysis and isothermal reaction kinetics Isothermal reaction kinetics was studied by reacting AA and DGEBA with an initial carboxyl/epoxide equivalent ratio of 0.5 in the temperature range of 60–120 ◦ C (EA-T60 to EA-T120 in Table 1). The reaction between AA and DGEBA is basically a carboxyl–epoxide addition esterification [17–19]. In this study, TPP was used to catalyze the reaction. The probable reaction mechanism as proposed by Romanchick et al. [27] is shown in Scheme 1(a). The reaction starts by the nucleophilic attack of TPP, which opens the epoxide group, producing a phosphonium betaine. The betaine abstracts a proton from the acrylic acid. The carboxylate anion subsequently attacks the electrophilic carbon attached to the phosphorus, forming an ester linkage and regenerating the catalyst. Fig. 1 shows FTIR spectra of DGEBA, AA and a representative epoxyacrylate, EA-T100, obtained by reacting AA/DGEBA (carboxyl/epoxide = 0.5) at 100 ◦ C for 4 h. The absorption peaks at 913 cm−1 and 864 cm−1 in the spectrum of DGEBA are due to the epoxide group. The absorption peaks at 1700 cm−1 and 1631 cm−1 appearing in the spectrum of AA are attributed to the carboxyl group and C C stretching, respectively. In the spectrum of the produced EA-T100 epoxyacrylate, the absorption peaks of epoxide group (913 cm−1 and 864 cm−1 ) decrease significantly in
intensity compared with the DGEBA; yet, strong absorption peaks at 1721 cm−1 and 3467 cm−1 due to the respective ester group and hydroxyl group are clearly observed, arising from the reaction of the carboxyl group and the epoxide group. The additional peaks at 1631 cm−1 and 809 cm−1 are due to the terminal unsaturated C C double bond of the EA-T100 epoxyacrylate. This indicates that the carboxyl–epoxide esterification occurred and that the produced epoxyacrylate had the unsaturated C C double bond as well as the residual epoxide end group. This is because the equivalent ratio of carboxyl/epoxide in the feed was 0.5 for the EA-T100. Therefore, the theoretical maximum epoxide conversion could only reach 0.5, leaving half of epoxide groups un-reacted. Furthermore, because there are two epoxide end groups in one DGEBA molecule, the ideal structure of the final product would be a monoacrylate-terminated epoxyacrylate as shown in Scheme 1(a). The composition of the reaction products can be determined by chromatography which will be discussed later. The structure of the produced epoxyacrylate is further confirmed by the 1 H NMR spectra. Fig. 2 shows NMR spectra of pure DGEBA, AA and the EA-T100 epoxyacrylate. In addition to the epoxide absorption peaks at 2.74, 2.89 and 3.34 ppm (indicated as 1 and 2 in the spectrum), the spectrum of DGEBA exhibits the absorption peaks at 3.96 and 4.17 ppm which are caused by the methylene hydrogen ( CH2 O , 3). Furthermore, very small peaks are found around 4.10 ppm, which is assigned to the CH (OH) in the oligomeric units with n > 0 [28]. This confirms the existence of oligomeric fractions with n > 0 for the DGEBA used in this study, see Scheme 1(a). From the peak area in the NMR spectrum, it is possible to calculate the average n value and thus the EEW of the DGEBA [29]. The calculated average n value is 0.12 and the deduced EEW is 187 g equiv.−1 . The EEW value is in agreement with the results obtained from the titration method, 186 g equiv.−1 , and from the manufacturer, 188 g equiv.−1 . After reaction, the produced EA-T100 epoxyacrylate contained both epoxide end group as indicated at 2.74, 2.89 and 3.34 ppm (1 and 2 in the spectrum), and terminal CH CH2 double bond at 5.89, 6.18 and 6.42 ppm (8, 8 and 9). The additional proton absorption peaks at 4.05 and 4.36 ppm are assigned to the methylene group adjacent to the ester (3 ), CH2 O C( O), which confirms the formation of ester group. The structure analysis from the 1 H NMR spectra thus agrees with the results from the FTIR spectra, demonstrating that the reaction is a carboxyl–epoxide addition esterification. The EA-T60 and EA-T80 systems have all the same absorption peaks as the EA-T100, not only in the FTIR spectra but also in the NMR spectra. The only difference among these systems is the peak intensity due to their differences in the epoxide conversion. Furthermore, the NMR results suggest that most secondary hydroxyl group did not take part in the reaction with the carboxyl group or with the epoxide group when reacting at 100 ◦ C or lower for 4 h. Banthia and McGrath [30] also suggested that no side reaction occurred between the epoxide group with the secondary hydroxyl group at 100 ◦ C for the reaction between an epoxy resin and a phenol
Y.-C. Su et al. / Materials Chemistry and Physics 132 (2012) 540–549
543
Scheme 1. (a) Addition esterification between the epoxide group of DGEBA and the carboxyl group of AA catalyzed by triphenyl phosphine (PPh3 ). The epoxyacrylate was produced by feeding the same molar amounts of AA and DGEBA, i.e., [COOH]/[epoxide] = 0.5. (b) Etherification between the epoxide group and the secondary hydroxyl group.
catalyzed by TPP. They believed this was due to the steric hindrance around the phosphorous atom that prevented the formation of any ion-pair complex between them and the pendant secondary hydroxyl group.
As in the catalyzed polyesterification of a dicarboxylic acid with a diol, the second-order kinetics has also been used to describe the carboxyl–epoxide addition esterification. Srivastava et al. [31] studied the kinetics of esterification of cycloaliphatic epoxies with
Fig. 1. FTIR spectra of (a) DGEBA, (b) acrylic acid (AA) and (c) an epoxyacrylate, EA-T100, prepared by feeding the AA/DGEBA with a carboxyl/epoxide equivalent ratio of 0.5 and reacting at 100 ◦ C for 4 h.
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Fig. 3. Epoxide conversion as a function of time for the reaction between DGEBA and AA at four reaction temperatures. The carboxyl/epoxide equivalent ratio in the feed was 0.5. The dotted line is the theoretical final conversion (=0.5) of the epoxide group.
group. Therefore, b equals a/2. Eq. (2) thus can be replaced by Eq. (3). d˛ = k1 dt
a 2
(1 − ˛)(1 − 2˛)
(3)
Integration of Eq. (3) yields
ln
1−˛ 1 − 2˛
= k1
a 2
t
(4)
Eq. (4) shows that the rate constant k1 can be calculated via the plot of ln[(1 − ˛)/(1 − 2˛)] versus time. The conversion of epoxide group (˛) at any time t can be calculated by the decrease in the epoxide absorption peak area at 913 cm−1 in the FTIR spectra as follows: ˛=1− 1
Fig. 2. H NMR spectra of DGEBA, acrylic acid and an epoxyacrylate, EA-T100, prepared by feeding the AA/DGEBA with a carboxyl/epoxide equivalent ratio of 0.5 and reacting at 100 ◦ C for 4 h.
methacrylic acid in the presence of TPP and showed that the reaction followed second-order kinetics. Hu et al. [32] also used second-order kinetics to describe the addition esterification of 1naphthylacetic acid with the epoxide groups attached to a random copolymer of ethylene, ethyl acrylate and glycidyl methacrylate in the melt state. Therefore, second-order kinetics was adopted in this study, in which the esterification can be considered as a reversible reaction, with the forward reaction being of first order with respect to the epoxide group and the carboxyl group, and the reverse reaction being of first order with respect to the newly formed vinyl ester group. Accordingly, the overall reaction rate thus can be described by d˛ = k1 (1 − ˛)(b − a˛) − k2 ˛ dt
(2)
where k1 and k2 are the forward and reverse reaction constants, ˛ denotes the conversion with regard to the epoxide consumption, a and b are the initial concentrations of the epoxide group and the carboxyl group, respectively. For the carboxyl–epoxide reaction, k1 was reported to be much larger than k2 [32]. This suggests that k2 can be ignored. For the reaction systems, EA-T60 to EA-T120, the initial epoxide group is twice the equivalent amount of the carboxyl
(A913 /A2872 )t (A913 /A2872 )0
(5)
where A913 and A2872 are the peak areas of the respective epoxide group and the methyl group. The methyl absorption peak is used as an internal standard to account for any variations in the sample thickness and viscosity since its amount does not change during reaction. Utilizing the FTIR heating cell for monitoring the in situ reaction and Eq. (5), the epoxide conversion as a function of time for the reaction between DGEBA and AA can be obtained. Since the epoxide group was twice the amount of the carboxyl group for the EA-T60 to EA-T120 systems, theoretically, the maximum epoxide conversion should reach 0.5 if all carboxyl groups reacted completely with epoxide groups as indicated by the dotted line in Fig. 3. Fig. 3 shows the reaction curves of DGEBA and AA at four reaction temperatures. The initial reaction rate and the epoxide conversion after 4 h of reaction were both increased with increasing reaction temperature. If the reaction temperature was too low such as at 60 ◦ C in the EA-T60, the reaction was very slow and the conversion could only reach about 37% after 4 h of reaction. When raising the reaction temperature to 120 ◦ C in the EA-T120, the reaction rate was so high that the epoxide conversion already reached 0.5 in a short period of time about 20 min, and still gradually increased with reaction time. The epoxide conversion exceeded 0.5 after 4 h of reaction, reaching a value of 0.58. This indicates that the reaction temperature is high enough to cause some other side reactions to consume more epoxide group. The most probable side reaction is the etherification between the epoxide and the secondary hydroxyl group, see Scheme 1(b). On the contrary, for the EA-T100 system, the epoxide conversion reached 0.45 after 1 h of reaction and increased very
Y.-C. Su et al. / Materials Chemistry and Physics 132 (2012) 540–549
Fig. 4. Plots of ln[(1 − ˛)/(1 − 2˛)] versus reaction time of the reaction between AA and DGEBA catalyzed by TPP at four reaction temperatures. The carboxyl/epoxide equivalent ratio in the feed was 0.5. The rate constant k1 was calculated from the slope: k1 = 0.0014 kg equiv.−1 min−1 at 60 ◦ C; k1 = 0.0048 kg equiv.−1 min−1 at 80 ◦ C; k1 = 0.0161 kg equiv.−1 min−1 at 100 ◦ C; k1 = 0.0581 kg equiv.−1 min−1 at 120 ◦ C.
slowly since then. After 4 h of reaction, the epoxide conversion was 0.47, slightly less than the theoretical maximum value. This is because the reaction rate decreased greatly when most carboxyl groups were consumed. It has to be mentioned that at this temperature of 100 ◦ C, the secondary hydroxyl group did not take part in the reaction as proved previously in the NMR spectrum. The results in Fig. 3 also justifies that the reverse rate constant k2 could be neglected under the present reaction conditions; otherwise the conversion would not approach to the theoretical maximum value. Eqs. (3) and (4) then were applied, and plots of ln[(1 − ˛)/(1 − 2˛)] with time were obtained. The rate constant k1 was calculated from the initial slope. Fig. 4 shows that the plots are all linear at four reaction temperatures with very high correlation coefficients (R > 0.995). A slightly lower R value is observed for the EA-T120 system, probably due to the interference of the etherification side-reaction as mentioned previously. The results show that the rate constant (k1 ) and thus the reaction rate increased with temperature. The rate constant was increased by about 2.4 times when the reaction temperature was raised by 20 ◦ C. Since the rate constants at different temperatures are known, the activation energy (Ea ) thus can be calculated using the Arrhenius equation. k = Ae−(Ea /RT )
545
Fig. 5. Plot of ln k1 versus reciprocal of temperature for the carboxyl–epoxide addition reaction between AA and DGEBA catalyzed by TPP.
By increasing the reaction time more than 4 h in the EAT100 system, the epoxide conversion probably could be further increased; yet, it would take a long time to reach the maximum value of 0.5. In this study, it was attempted to find the most suitable reaction condition through which the epoxide conversion could reach the maximum value in a time period of 4 h, because of the economic consideration in industrial production. It is interesting to find that by heating the reaction mixture at 100 ◦ C for 2 h first and followed by another 2 h at 120 ◦ C, the epoxide conversion of this EA50 (see Table 1) became very close to the theoretical value of 0.5 as indicated by the dotted line in Fig. 6. Most importantly, there was no etherification reaction observed. To validate the findings, FTIR spectra of different epoxyacrylates prepared with the initial carboxyl/epoxide equivalent ratio of 0.5 but under different temperature profiles were recorded and compared. As shown in Fig. 7, a weak absorption peak at 1120 cm−1 is noticed in the original DGEBA and assigned to the >C O(H) stretching due to the presence of pendent secondary hydroxyl group in the n = 1 and n = 2 homologues. After reaction, the relative increase in peak intensity is found to be proportional to the epoxide conversion for all systems except
(6)
where T is temperature (K), A is the frequency factor and R is the gas constant. Fig. 5 shows the plot of ln k1 versus the reciprocal of temperature, in which the Arrhenius law is obeyed very well. The slop of the plot obtained by regression analysis was used to calculate the activation energy for the carboxyl–epoxide esterification reaction, which was found to have a value of 71.5 (kJ mol−1 ). Since it was very possible that etherification might have occurred in the epoxide group when reacted at 120 ◦ C, the slope was re-calculated using data only in the range from 60 ◦ C to 100 ◦ C. The activation energy thus obtained was 63.9 kJ mol−1 . This value is slightly lower than those reported by Srivastava et al. [31]. They discussed the esterification of two cycloaliphatic epoxy resins CER1 and CER2 containing glycidyl and cyclohexane epoxy groups, respectively, with methacrylic acid using a stoichiometric ratio of 1/0.9 in the presence of TPP. They obtained the activation energies of 73.0 and 66.9 kJ mol−1 for CER1 and CER2, respectively. It can be seen that the difference in activation energy is caused by the structure factors of the reactants, especially the steric effect. The more bulky the reactant, the higher the activation energy is.
Fig. 6. Epoxide conversion as a function of time for the reaction between AA and DGEBA with a carboxyl/epoxide equivalent ratio of 0.5 under different temperature profiles. () EA-T120, 4 h at 120 ◦ C; (䊉) EA-T100, 4 h at 100 ◦ C; () EA50, 2 h at 100 ◦ C and 2 h at 120 ◦ C. The dotted line is the theoretical final conversion of the epoxide group (=0.5).
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Fig. 7. FTIR spectra of epoxyacrylates prepared with different reaction processes: EA-T100 (4 h at 100 ◦ C); EA50 (2 h at 100 ◦ C, and 2 h at 120 ◦ C); and EA-T120 (4 h at 120 ◦ C). All reaction systems had the same carboxyl/epoxide equivalent ratio at 0.5.
the EA-T120. This is because one additional secondary hydroxyl group is produced by the consumption of one epoxide group via the carboxyl–epoxide etherification as shown in Scheme 1(a). However, the increase is larger than expected in the EA-T120 system. This is explained by the occurrence of the etherification between the epoxide with the secondary hydroxyl group in the EA-T120, see Scheme 1(b). Though the etherification reaction consumes one secondary hydroxyl group, it generates another one. Moreover, the reaction also results in the structure of aliphatic ether C O C, whose asymmetric stretching is also at around 1120 cm−1 , and apparently stronger than the C OH stretching associated with the pendent secondary hydroxyl group. Consequently, the peak intensity is higher than expected in the EA-T120. Generally speaking, the etherification between the epoxide group and the secondary hydroxyl group is less reactive and a notable etherification is usually observed at temperatures around or greater than 200 ◦ C. Yet, because of the presence of the TPP catalyst, the reaction temperature for this etherification could be lower, such as 120 ◦ C in the EA-T120 system. Devi et al. [33] also found that the addition of TPP shifted the cure exotherm of phenol–epoxide reaction to lower temperatures. They studied the TPP-catalyzed curing of a diallyl bisphenol A-novolac epoxy resin system using differential scanning calorimetry (DSC). The maximum peak temperature for the phenol–epoxide reaction under a heating rate of 5 ◦ C min−1 in DSC shifted from 186 ◦ C to 129 ◦ C when 0.5 wt% TPP was added. Based on the above results, all following epoxyacrylates with different amounts of functional end groups were synthesized by this twostage heating process, 100 ◦ C for 2 h followed by another 2 h at 120 ◦ C.
compositions and intentionally to find out the system having the maximum proportion of the monoacrylate-terminated epoxyacrylate. Fig. 8 shows FTIR spectra of the produced EA25 to EA100 epoxyacrylates. The absorption peaks at 1720 cm−1 (C O stretching), 1633 cm−1 (C C stretching), 809 cm−1 (CH CH2 bending) and 3462 cm−1 (O H stretching) increase in intensity with an increase in the feeding amount of carboxyl group, but the absorption peaks at 913 cm−1 and 864 cm−1 from the epoxide group decrease. This is because increasing the equivalent ratio of carboxyl/epoxide would increase the extent of addition esterification reaction. The results thus indicate that by adjusting the initial equivalent ratio of carboxyl/epoxide, epoxyacrylates with different amounts of epoxide group and unsaturated double bond could be obtained as shown in Scheme 2. It is thus interesting to determine the properties and compositions of the reaction products from various reaction systems. It has to be mentioned that the relative change in the peak intensity at 1120 cm−1 was found to be proportional to the epoxide conversion in all EA25 to EA100 systems, indicating that there was no etherification occurred in these systems as explained in the previous section.
3.2. Synthesis of epoxyacrylates with different amounts of functional end groups The aim of this study was trying to synthesize monoacrylateterminated epoxyacrylate having one unsaturated group at one end for further UV-curing and one epoxide group at the other end for thermo-curing in a time period of 4 h. Therefore, various batches of epoxyacrylates were synthesized by changing the initial equivalent ratio of carboxyl/epoxide from 0.25 to 1.0 to obtain the corresponding EA25 to EA100 as shown in Table 1. The produced epoxyacrylates were then analyzed to determine their
Fig. 8. FTIR spectra of (a) pure DGEBA, and epoxyacrylates prepared with different equivalent ratios of carboxyl/epoxide: (b) EA25 (carboxyl/epoxide = 0.25), (c) EA50 (carboxyl/epoxide = 0.50), (d) EA75 (carboxyl/epoxide = 0.75), (e) EA100 (carboxyl/epoxide = 1.0). The reactions were all carried out at 100 ◦ C for 2 h and another 2 h at 120 ◦ C.
Y.-C. Su et al. / Materials Chemistry and Physics 132 (2012) 540–549
547
Scheme 2. Chemical structures of the epoxyacrylates prepared by feeding AA and DGEBA with different equivalent ratios of carboxyl/epoxide. (A) DGEBA, (B) monoacrylateterminated epoxyacrylate, (C) bisacrylate-terminated epoxyacrylate.
Table 2 shows the epoxide conversion, acid value and EEW of the EA25 to EA100 epoxyacrylates. The epoxide conversion determined by FTIR increased from 0.246 to 0.843 as the initial equivalent ratio of carboxyl/epoxide was raised from 0.25 to 1.0. Correspondingly, this would cause an increase in the EEW value as confirmed by the measured EEW data using HClO4 titration. The epoxide conversions after 4 h of reaction are very close to the theoretical values except the EA100. The theoretical values of epoxide conversion of various reaction systems were calculated by assuming that all carboxyl groups reacted completely with the epoxide groups under the employed reaction conditions, i.e., the reverse reaction and the etherification were both negligible. The complete reaction of AA with DGEBA in the EA25 to EA75 systems is confirmed by the acid values shown in Table 2 which are all close to zero. The fact that the measured epoxide conversions did not exceed their respective theoretical values also validates that the epoxide group only reacted
with the carboxyl group without the occurrence of the etherification, in agreement with the FTIR and NMR results. However, it appears that AA had not reacted completely with the DGEBA in the EA100 system after 4 h of reaction. The epoxide conversion and its acid value are 0.84 and 38.3 (mg KOH g−1 ), respectively. This is mainly because the DGEBA was in excess for the EA25 to EA75 systems, assuring a more complete reaction of the AA with the DGEBA; whereas the initial carboxyl and epoxide groups were in stoichiometric amounts in the EA100 system. As the reaction proceeded, both carboxyl and epoxide concentrations became lower and lower. As a result, the reaction rate decreased greatly after 4 h of reaction, and the conversion thereby could not reach its theoretical maximum value in this time period. It is believed that the conversion can still be increased as the reaction time is extended. In the EA50 system, an epoxide conversion of 0.5 should be obtained when the carboxyl group reacted completely with the
Table 2 Epoxide conversion (˛), acid value and EEW of different samples prepared with different equivalent ratios of carboxyl/epoxide in the feed. Sample
AA DGEBA
[COOH] [epoxide]
˛theo.
˛a
Acid valueb (mg KOH g−1 )
EEWtit. c (g equiv.−1 )
EA25 EA50 EA75 EA100
0.5 1.0 1.5 2.0
0.25 0.50 0.75 1.00
0.25 0.50 0.75 1.00
0.246 0.518 0.737 0.843
0.2 0.0 0.5 38.3
224 410 684 1744
a b c
The epoxide conversion was determined from FTIR spectra. The acid value was measured by titration with KOH(aq) (0.10 N). The EEWtit. value was measured by the HClO4 titration of epoxide group. Pure DGEBA has an EEWtit. of 186 (g equiv.−1 ).
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Y.-C. Su et al. / Materials Chemistry and Physics 132 (2012) 540–549 Table 3 The compositions of the prepared epoxyacrylates determined by GPC. Sample
EA25 EA50 EA75 EA100b
[COOH] [epoxide]
0.25 0.50 0.75 1.00
˛FTIR
˛GPC
0.246 0.518 0.737 0.843
0.210 0.490 0.725 0.855
Composition (%)a A
B
C
61 15 9 –
36 72 37 29
3 13 54 71
a A is the un-reacted DGEBA; B is the monoacrylate-terminated epoxyacrylate; C is the bisacrylate-terminated epoxyacrylate. All reactions were carried out at 100 ◦ C for 2 h and then another 2 h at 120 ◦ C. b In the EA100, there was still some un-reacted AA left in the system after 4 h of reaction.
Fig. 9. Chromatograms of (a) pure DGEBA, and epoxyacrylates obtained from the reaction of AA and DGEBA with different equivalent ratios of carboxyl/epoxide in the feed: (b) EA25 (carboxyl/epoxide = 0.25), (c) EA50 (carboxyl/epoxide = 0.50), (d) EA75 (carboxyl/epoxide = 0.75), (e) EA100 (carboxyl/epoxide = 1.0). The reactions were all carried out at 100 ◦ C for 2 h and another 2 h at 120 ◦ C.
epoxide group, which was confirmed by the acid titration and FTIR measurements. Ideally, the as-prepared EA50 epoxyacrylate should have one double bond at one end and still have one epoxide group at the other end of the epoxyacrylate molecule, i.e., monoacrylate-terminated epoxyacrylate. However, it is very possible that two AA molecules have reacted with the two epoxide end groups on the same DGEBA molecule, thus producing a bisacrylate-terminated epoxyacrylate. Consequently, this would be accompanied by one un-reacted DGEBA molecule left in the system. Therefore, the reaction products would be composed of un-reacted DGEBA, monoacrylate- and bisacrylate-terminated epoxyacrylate, corresponding to the structure (A), (B), and (C) in Scheme 2. Fig. 9 shows the GPC chromatograms of DGEBA and the produced epoxyacrylates of EA25 to EA100. The composition of the reaction products could be traced by GPC, assuming that the amount being proportional to the corresponding peak area. This is a good approximation since they have almost the same responding factor in GPC detector due to their similar backbone structures. The DGEBA chromatogram contains two primary peaks at 24.48 and 23.39 min corresponding to the two homologues: n = 0 and n = 1. In addition, a very small peak at 22.65 min is also observed, which is assigned to the n = 2 homologue. According to the resolved peak area using a curve-fitting software, the proportion of the n = 2 homologue is only about 2%. By reacting with AA, the synthesized epoxyacrylate obviously has a higher molecular weight than the DGEBA. The peak at 24.48 min, attributed to the n = 0 oligomer of DGEBA, apparently decreases in intensity with an increase in the feeding amount of AA due to the carboxyl–epoxide esterification. It disappeared completely in the EA100 system. The peak at 24.13 min is assigned to the monoacrylate-terminated epoxyacrylate produced from n = 0 oligomer of DGEBA. This peak increases first and then decreases with increasing the equivalent ratio of carboxyl/epoxide from 0.25 to 1.0. The maximum occurs in the EA50 system when the initial equivalent ratio of carboxyl/epoxide is 0.5. In addition, the peak at 23.76 min is attributed to the bisacrylateterminated epoxyacrylate produced from the n = 0 oligomer of DGEBA. The peak intensity increases as the equivalent ratio of carboxyl/epoxide increases. It can be seen clearly that in the special case of the EA50 system, the n = 0 homologue of the DGEBA did not disappear completely and the peak of bisacrylate-terminated epoxyacrylate
seemed to appear in the chromatogram. This indicates that two AA molecules indeed reacted with both epoxide end groups on the same DGEBA molecule. Therefore, the reaction products would consist of un-reacted DGEBA (structure A), monoacrylate-terminated epoxyacrylate (structure B) and bisacrylate-terminated epoxyacrylate (structure C). In order to estimate the proportions of different structures, the broad chromatographic peak was deconvoluted into a series of peaks corresponding to the different structures by using a curvefitting software. The percentages of different structures (A, B and C) were then calculated based on their resolved peak areas divided by the total area as shown below: percentage of structure i =
A
i × 100 = Ai
Ai A0 + AM + AB
× 100 i = 0, M, B
(7)
where Ai is the peak area of individual structure, namely, A0 , AM and AB corresponding to the respective peak area of un-reacted DGEBA (structure A), monoacrylate-terminated epoxyacrylate (structure B) and bisacrylate-terminated epoxyacrylate (structure C). The proportions of different structures were calculated and listed in Table 3. The results clearly show that all reaction systems consist of three different structures of A, B and C, except for EA100 which contains only the latter two; and their proportions depend on the initial equivalent ratio of carboxyl/epoxide. Increasing the equivalent ratio of carboxyl/epoxide leads to a decrease of the structure (A) and an increase of the structure (C). However, for the monoacrylateterminated epoxyacrylate (B) which contains one epoxide end group and one terminal double bond on the same molecule, there is a maximum occurred in the EA50 system, approximately 72%. It has to be mentioned that the result is only an approximation by using the deconvolution method. To obtain a more accurate result, model compounds should be used and calibration curves be constructed. Nevertheless, when the conversions of EA25 to EA100 were calculated from the resolved peak areas, as can be seen from Table 3, they are in agreement with the results from FTIR spectra. Though there are still some DGEBA and bisacrylate-terminated epoxyacrylate in the EA50, undoubtedly, the EA50 can be subjected to UV-cure and subsequent thermo-cure to obtain a fully crosslinked structure by addition of photo-initiator and curing agent. Therefore, the EA50 epoxyacrylate has the potential to be used as an adhesive sealant for the LCD production. Structures and properties of the cured EA50 with and without the addition of silica will be reported in our next paper. 4. Conclusions In this study, various epoxyacrylates with different functional end groups were synthesized by addition esterification of the epoxide group in DGEBA with the carboxyl group in AA. It was found that the rate constant increased by about 2.4 times when the
Y.-C. Su et al. / Materials Chemistry and Physics 132 (2012) 540–549
reaction temperature was increased by 20 ◦ C in the temperature range of 60–100 ◦ C. The activation energy of the carboxyl–epoxide esterification catalyzed by triphenyl phosphine was obtained from the Arrhenius equation and found to be 63.9 kJ mol−1 . However, at a higher temperature of 120 ◦ C, the side reaction might occur which involved etherification of the secondary hydroxyl group with the epoxide group. The most suitable temperature profile was found to be 100 ◦ C for 2 h followed by another 2 h at 120 ◦ C, by which a complete carboxyl–epoxide reaction could be reached without the etherification reaction. Furthermore, the synthesized epoxyacrylates were proved to consist of un-reacted DGEBA, monoacrylate-terminated epoxyacrylate and bisacrylateterminated epoxyacrylate, for which the composition has been estimated from the GPC chromatograms. When the initial equivalent ratio of carboxyl/epoxide was set at 0.5, a maximum amount of monoacrylate-terminated epoxyacrylate (∼72%) was observed, which had one double bond at one end for UV-cure and one epoxide group at the other end for thermo-cure. Therefore, by applying UV irradiation and thermal treatment, the dual-curable epoxyacrylate can be cured to a fully crosslinked structure, which is useful as an adhesive sealant for LCD production. Acknowledgements The authors wish to thank National Science Council in Taiwan for the financial support and professor Ching-Chung Chen for his kind assistance in the synthesis of epoxyacrylates. References [1] C.A. May, Y. Tanaka, Epoxy Resins: Chemistry and Technology, Marcel Dekker, New York, 1988, p. 794. [2] B. Ramezanzadeh, M.M. Attar, Mater. Chem. Phys. 130 (2011) 1208. [3] H. Jin, G.M. Miller, N.R. Sottos, S.R. White, Polymer 52 (2011) 1628.
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