Influence of Post-Curing on Solid State C NMR Spectra of Electron Beam

J. Ind. Eng. Chem., Vol. 12, No. 6, (2006) 972-975 SHORT COMMUNICATION

Influence of Post-curing on Solid State 13C NMR Spectra of Electron Beam-irradiated Epoxy Resins

Soo-Jin Park†, Gun-Young Heo, and Fan-Long Jin*

Department of Chemistry, Inha University, Nam-gu, Incheon 402-751, South Korea *Department of Chemical Engineering, Jilin Institute of Chemical Technology Jilin City 132022, People’s Republic of China

Received April 21, 2006; Accepted August 30, 2006

Abstract: In this work, difunctional epoxy resins were cured using electron beam (EB) irradiation. After EB curing, the specimens were cured thermally and the effect of post-curing on the network structure was investigated using nuclear magnetic resonance (NMR) spectroscopy. Decreases in the abundances of extant oxirane and secondary alcohol groups after post-curing were confirmed from analyses of solid-state 13C NMR spectra. These phenomena could be explained by the fact that the crosslinking density increased through the re-crosslinking that occurred during post-curing. In terms of the cure mechanism we believe that the main cure reactions during EB curing were the reactions mediated by the protonic acids and oxonium cations.

Keywords: electron-beam, post-curing, solid-state 13C NMR, protonic acids, oxonium cations

Introduction Meanwhile, the initiators are critical to EB curing of ep- 1) oxy resins; indeed, the range of applications of the final Electron beam (EB) curing, a non-thermal curing proc- EB-cured epoxy products depends largely on the choice ess, is a novel manufacturing process that offers many of initiator [7]. To date, several types of onium salts with - - - advantages over conventional thermal curing, such as re- complex metal halide anions, such as PF6 , AsF6 , SbF6 , - duced environmental and health concerns, shorter cure and BF4 , have been used as initiators for EB curing times, lower temperatures, dimensional stability, and un- [8-10]. These onium salts can also behave as excellent la- limited shelf life of resins [1]. The EB curing process can tent thermal initiators for cationic polymerizations of ep- be used to induce physical and chemical changes in oxy resins in the absence of any co-initiators [11,12]. materials. In the case of polymers, it can induce cross- Recently, most EB curing studies have focused on the linking and bond scission, depending on the type of poly- diagnosis of molecular dynamics and molecular struc- mer and energy supplied [1,2]. tures to control the properties of the cured specimens In recent years, with increasing use of EB-curable resins [13]. In this work, EB-irradiated epoxy resins based on and initiators, EB curing of thermosetting resins has been bisphenol-A were post-cured to examine the changes in studied intensively and applied widely in all industrial the chemical structure that were associated with the im- fields [3,4]. Most EB-curable resins consist of multifunc- provement of the physical properties. The effect of tional oligomers and monomers that polymerize to form post-curing on the molecular dynamics and molecular highly crosslinked polymer networks. The oligomers structures of the cured specimens was characterized us- used are generally acrylate functional resins, such as ing nuclear magnetic resonance (NMR) spectroscopy. acrylated urethanes, polyesters, silicones, and epoxides [5,6]. Experimental

† To whom all correspondence should be addressed. Materials (e-mail: [email protected]) The epoxy resin used for this study was the diglycidyl- Influence of Post-curing on Solid State 13C NMR Spectra of Electron Beam-irradiated Epoxy Resins 973

Table 1. Chemical Shifts of Pre-cured, EB-cured, and Post-cured DGEBA Chemical shift (ppm) Assignment of carbon type Before curing EB-cured Post-cured

C1 44.4 41.3 40.9

C2 50.6 49.7 -

C3 70.1 69.6 70

C4 157.7 156.7 156.5

C5 114.9 114.2 113.5

C6 128.5 127.5 127.2

C7 144.0 143.8 143.5

C8 42.3 41.3 40.9

C9 30.9 30.7 30.2

C10 71.5 76.3 75.7

C11 69.3 69.6 70

urement, the cured specimens were ground to obtain a fine powder. Solid-state 13C-CP/MAS NMR spectra were recorded using the combined techniques of cross-polar- ization, proton dipolar, and magic angle spinning at oper- ating frequencies of 300.149 and 75.477 MHz for 1H and 13C, respectively, on a Bruker DSX-300 solid state FT-NMR spectrometer.

Results and Discussion

Figure 1. Chemical structures of DGEBA and BQH. The chemical shifts in the solution-phase 13C NMR spectra of the uncured DGEBA epoxy resins obtained ether of bisphenol A (DGEBA, YD-128, Kukdo Chem., from the literature are summarized in Table 1 [16,17]. Korea). The epoxide equivalent weight of DGEBA was The C atoms of terminal methylene (C1, CH2O-) and ox- 185∼190 g/eq, and the density was 1.16 g/cm2 at 25 oC. irane ring (C2, -CHO-) units appear as peaks at 44.4 and N-Benzylquinoxalinium hexafluoroantimonate (BQH), 50.6 ppm, respectively. The C atoms of hydroxyl-sub- used as an initiator for EB and thermal curing, was stitued methylene groups (C3 and C10, -CH2O-) occur at synthesized as described previously [14,15]. The chemi- 70.1 and 71.5 ppm. The methyl group (C9, -CH3-) and C cal structures of DGEBA and BQH are shown in Figure 1. atom (C8, -C-) of the bisphenol occur at 30.9 and 42.3 ppm, respectively. The C atom of the secondary alcohol Sample Preparation (C11, -CHOH-) in the center of the DGEBA epoxy resin BQH (2 phr) was dissolved in acetone and mixed with appears as a signal at 69.3 ppm. The C atoms (C4, C5, C6, DGEBA. The mixture was stirred at 60 oC for 30 min and C7) of the aromatic ring appear as typical signals at and degassed in a vacuum oven for 30 min. The mixture 157.7, 114.9, 128.5, and 144.0 ppm. was poured into the mold and Mylar film covered the To obtain more detailed information regarding the sample. EB irradiation was performed using an accel- chemical changes resulting from the post-curing erator (Samsung ELV-4). The mixture was EB-cured at a performed in this work, the solid-state 13C NMR spectra dose per pass of 50 kGy for a total dose of 200 kGy at of the thermally cured, EB-cured, and post-cured room temperature. Post-curing of the EB-cured specimen o DGEBA epoxy resins were recorded (Figure 2). was performed at 150 C for 1 h in a convection oven. Irrespective of the curing method, the spectra exhibit two ranges of chemical shifts, from 0 to 100 ppm and from NMR Spectroscopic Analysis 100 to 180 ppm, which we assign to aliphatic and 13 For solid-state C-CP/MAS NMR spectroscopic meas- aromatic carbon atoms in the DGEBA, respectively. 974 Soo-Jin Park, Gun-Young Heo, and Fan-Long Jin

(a)

(b)

(b) Figure 3. Mechanisms for (a) EB curing and (b) thermal curing.

cation subsequently attacks the oxirane ring of DGEBA to produce the oxonium cation, as shown in Figure 3(b) [10,21]. (c) Consequently, the EB curing process differs from thermal curing in terms of the nature of the initiation species. Figure 2. Solid-state 13C NMR spectra of (a) thermally cured The initiation species of thermal curing are benzyl cati- DGEBA epoxy resins, (b) EB-cured DGEBA epoxy resins, and ons, but those of EB curing are protonic acids. However, (c) post-cured DGEBA epoxy resins (spinning side bands are both thermal and EB curing produce oxonium cations af- indicated by asterisks). ter the initiation step. Subsequently, hydroxyl groups and oxonium cations undergo nucleophilic addition reactions 13 Also, small shifts of the aborption bands in solid state C with the oxirane ring so that the epoxy resins are con- NMR spectra, compared with those recorded in solution, verted into hard three-dimensional cross-linked networks are quite common due to the overlap of chemical shifts throughout these cure reactions [21,22]. 13 [16-19]. The chemical shifts in the solid state C NMR As shown in Figure 2, the solid-state 13C NMR spec- spectra are summarized in Table 1. trum of the thermally cured DGEBA exhibits some dif- Generally, the mechanism for EB curing is different ferences compared with that of the EB-cured DGEBA. In from that for thermal curing. The EB curing reaction of the case of the EB-cured DGEBA, the C atom in the ox- DGEBA initiated by BQH is started by the homolytic irane ring (C2) and the C atom of the secondary alcohol and heterolytic cleavage of BQH, as shown in Figure (C11) in the center of the DGEBA epoxy resin are of low- 3(a). Both formations proceed with simultaneous release er intensities than those of the thermally cured DGEBA. + - of H , which reacts with SbF6 to generate protonic acids. This observation confirms that EB curing provides a These protonic acids generated in the initial step can shorter chain structure than thermal curing, due to in- subsequently attack epoxide groups and produce complete chain propagation by the reaction of the hy- oxonium cations [20]. droxyl groups, which increases the density of the curing However, in the thermal curing reaction of DGEBA structure. Therefore, in the case of EB curing, compared initiated by BQH, the high temperature decomposes with thermal curing, the cure reactions mediated by oxo- BQH into the benzyl cationic initiation species, which is nium cations may be superior in the propagation step to - coordinated by SbF6 , and quinoxaline. Then, the benzyl those mediated by the hydroxyl groups. Influence of Post-curing on Solid State 13C NMR Spectra of Electron Beam-irradiated Epoxy Resins 975

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