Release Behavior of Benzimidazole-Intercalated α -Zirconium Phosphate as a Latent Thermal Initiator in the Reaction of Epoxy Resin

: The intercalation compound of benzimidazole with α -zirconium phosphate ( α -ZrP) was evaluated as a latent thermal initiator in the reaction of glycidyl phenyl ether (GPE) and hexahydro-4-methylphthalic anhydride (MHHPA). No reaction occurred at 60 ◦ C after 1 h. Upon increasing the temperature to 140 ◦ C, the conversion reached 97% after 1 h. The deintercalation ratio of Bim from the intercalation compound of benzimidazole with α -zirconium phosphate ( α -ZrP · Bim) was measured in the reaction of the GPE-MHHPA system. The deintercalation ratio increased upon increasing the temperature, reaching 97% at 120 ◦ C after 1 h. The storage stability at 25 ◦ C and 40 ◦ C in the reaction of GPE-MHHPA was tested and was found to be maintained for 14 days at 25 ◦ C. The intercalation compound of α -ZrP · Bim can effectively serve as a latent thermal initiator in the reaction of GPE-MHHPA. 20.3 Å (2 θ = 4.4 ◦ ) to 19.3 Å (2 θ = 4.6 ◦ ), showing that deintercalation of Bim from the interlayers of α -ZrP · Bim occurred. The ratio of C, H, and N of the product was C: 42.67, H: 4.61, and N: 3.87 and the composition was Zr(HPO 4 ) 2 · (C 9 H 12 O 3 ) 2.3 · (C 7 H 6 N 2 ) 1.1 as determined by elemental analysis. 31% of intercalated Bim was deintercalated from α -ZrP · Bim and the molar ratio of 2.3 of MHHPA to Zr was detected. The interlayer distance after the treatment of α -ZrP · Bim with MHHPA slightly decreased, indicating that MHHPA was immobilized on the surface of α -ZrP.

Imidazoles are widely used in industry as curing agents for epoxy resins found in electric devices, laminated plates, semiconductor sealing agents, etc. The equivalents of imidazoles in intercalation compounds of α-ZrP are 0.78 (Im), 0.96 (2MIm), and 0.65 (2E4MIm). However, after reaction with the GPE-MHHPA system, 35% (α-ZrP·Im), 48% (α-ZrP·2MIm), and 37% (α-ZrP·2E4MIm) of the intercalated imidazoles were deintercalated from the layers of zirconium phosphate [14]. Essentially, less than half of the imidazole was available for the reaction of GPE-MHHPA. 2 of 13 Benzimidazoles are known as good curing agents for epoxy-anhydride systems [16]. Using benzimidazole as an intercalation compound, Costantino et al. [17] reported a molar ratio of benzimidazole/zirconium phosphate of up to 1.9. Therefore, we anticipated that benzimidazole-intercalated α-ZrP would have better efficiency as a curing agent because it had a higher loading in α-ZrP. We prepared the intercalation compound of benzimidazole (Bim) and examined the capabilities of α-ZrP·Bim as a latent thermal initiator in the reaction of GPE with MHHPA. The release behavior of Bim from the interlayer of α-ZrP was studied in detail.

Results and Discussion
The intercalation of benzimidazole (Bim) into the layers of α-zirconium phosphate (α-ZrP) was carried out by slightly modifying a previously reported method [14]. The α-ZrP was added to a solution of Bim in 1:1 water:methanol. The reaction mixture was stirred at 60 • C for 24 h. After the reaction, the intercalation compound was recovered by suction filtration. The ratio of C, H, and N of the product was C: 27.94%, H: 2.64%, and N: 9.28% and the composition was Zr(HPO 4 ) 2 (C 7 H 6 N 2 ) 1.60 ·0.50H 2 O as determined by elemental analysis. The interlayer distance of α-ZrP was calculated from the XRD patterns, which showed that pristine α-ZrP 7.6 Å (2θ = 11.7 • ) was expanded to 20.3 Å (2θ = 4.4 • ) as seen in Figure 1a. Thus, the intercalation of Bim into the layers of α-ZrP (α-ZrP·Bim) was confirmed.
To evaluate the catalytic activity of α-ZrP·Bim, the copolymerization of glycidyl phenyl ether (GPE) and hexahydro-4-methylphthalic anhydride (MHHPA) was carried out. The conversion of GPE was 97% at 140 • C for 1 h as determined by 1 H NMR analysis. The calculation of the conversion by 1 H-NMR analysis determined was shwon in ref. [14]. The intercalation compound α-ZrP·Bim showed good reactivity under heating conditions.
To evaluate the change of the layer structure after the reaction of α-ZrP·Bim, the product was washed with THF to remove the GPE-MHHPA products and the residue of α-ZrP (α-ZrP·Bim-RXN) was recovered. The interlayer distance of α-ZrP·Bim-RXN was increased to 22.9 Å (2θ = 3.8 • ) and the peak intensity was decreased, as shown in Figure 1b. This might cause the intercalation of the reaction products into the layers and the crystallinity of α-ZrP·Bim was decreased after the reaction.
The 31 P MAS NMR spectra of α-ZrP·Bim and α-ZrP·Bim-RXN are shown in Figure 2a,b. The peak of pristine α-ZrP is observed at δ-20.1. As shown in Figure 2a, the peak of α-ZrP·Bim is observed at δ-20.6. This chemical shift suggests that interactions between Bim and the HPO 4 group were not strong compared with that between alkylamines and the HPO 4 group [18,19]. In Figure 2b for α-ZrP·Bim-RXN, the signal at δ-21.2 and -23.6 were observed. This shift of the signal from δ-20.6 to δ-21.2 and -23.6 might due to the separation of Bim from the phosphate groups [11]. The 13 C NMR spectra of α-ZrP·Bim and α-ZrP·Bim-RXN are shown in Figure 3a,b. The 2-position of the imidazole ring at δ144.5 (N=CH-NH) in Figure 3a (assigned to 1) completely disappeared after the reaction (α-ZrP·Bim-RXN) and the corresponding peaks of the products of GPE-MHHPA appeared [ Figure 3b]. Therefore, Bim was completely deintercalated from the interlayers. This demonstrates that all of the intercalated Bim could be used to initiate the reaction of GPE-MHHPA. We have previously reported the intercalation compounds of α-ZrP·Im, α-ZrP·2MIm and α-ZrP·2E4MIm, showing that the deintercalation ratios after the reaction with GPE-MHHPA were 35%, 48%, and 37%, respectively [14]. Moreover, the copolymer of GPE-MHHPA was not formed after the reaction of α-ZrP·Im-RXN, α-ZrP·2MIm-RXN, and α-ZrP·2E4MIm-RXN. Substances derived from GPE were present in the interlayer of these three intercalation compounds of α-ZrP. In the case of α-ZrP·Bim, the copolymer was confirmed by the presence of ester groups at δ173.2 in Figure 3b (assigned to 1). Therefore, the products of GPE-MHHPA can be intercalated in the layers of α-ZrP·Bim. The intercalation compound of Bim (α-ZrP·Bim) was efficiently utilized in the reaction of GPE-MHHPA. FT-IR spectra of α-ZrP·Bim and α-ZrP·Bim-RXN are shown in Figure 4a aromatics (ν C-C at 1600, 1497 and 1458 cm −1 ), and ether groups (ν C-O-C at 1249 cm −1 ) in the products of GPE-MHHPA were clearly observed in Figure 3b.  The capabilities of Bim as a latent thermal initiator were examined in the reaction of GPE and MHHPA. The conversion of GPE and the deintercalation ratio of Bim from the layers of α-ZrP containing 3 mol% of Bim were measured at varying temperatures for 1 h, as shown in Figure 5. The deintercalation ratio of Bim from α-ZrP·Bim were calculated by decreasing the N content of α-ZrP·Bim by elemental analyses. The conversion did not occur at 60 °C after 1 h. Upon increasing the reaction temperature to 120 °C, the conversion improved to 97%. The deintercalation ratio increased with increasing reaction temperature. At 120 °C, the deintercalation ratio became quantitative, (i.e., all of the Bim in the interlayer of α-ZrP deintercalated). At 60 °C, the deintercalation ratio was 38% and the reaction did not proceed in 1 h. The capabilities of Bim as a latent thermal initiator were examined in the reaction of GPE and MHHPA. The conversion of GPE and the deintercalation ratio of Bim from the layers of α-ZrP containing 3 mol% of Bim were measured at varying temperatures for 1 h, as shown in Figure 5. The deintercalation ratio of Bim from α-ZrP·Bim were calculated by decreasing the N content of α-ZrP·Bim by elemental analyses. The conversion did not occur at 60 • C after 1 h. Upon increasing the reaction temperature to 120 • C, the conversion improved to 97%. The deintercalation ratio increased with increasing reaction temperature. At 120 • C, the deintercalation ratio became quantitative, (i.e., all of the Bim in the interlayer of α-ZrP deintercalated). At 60 • C, the deintercalation ratio was 38% and the reaction did not proceed in 1 h.
To study the reaction behavior of GPE-MHHPA, the effect of the layer distances of ZrP using GPE and MHHPA at 100 • C for 1 h was investigated. In the case of GPE, the XRD pattern is shown in Figure 7a  It is important to maintain stability under storage conditions. The stabilities were examined at 25 °C and 40 °C in the GPE-MHHPA system. The conversion of GPE was 51% for Bim and 22% for α-ZrP·Bim at 25 °C for 14 days, as shown in Figure 8. At 40 °C, the conversion was 50% for Bim and 21% for α-ZrP·Bim for 7 days, as shown in Figure 9. The storage stability for α-ZrP·Bim was maintained for 14 days (2 weeks) at 25 °C.
Accordingly, α-ZrP·Bim can serve as a latent thermal initiator in the reaction of epoxy-acid anhydride systems. In the reaction of GPE-MHHPA with α-ZrP·Bim, the conversion reached 97% at 140 °C for 1 h, and the storage stability was maintained for 2 weeks at 25 °C. All of the intercalated Bim could be deintercalated at 120 °C for 1 h. It is important to maintain stability under storage conditions. The stabilities were examined at 25 • C and 40 • C in the GPE-MHHPA system. The conversion of GPE was 51% for Bim and 22% for α-ZrP·Bim at 25 • C for 14 days, as shown in Figure 8. At 40 • C, the conversion was 50% for Bim and 21% for α-ZrP·Bim for 7 days, as shown in Figure 9. The storage stability for α-ZrP·Bim was maintained for 14 days (2 weeks) at 25 • C.
Accordingly, α-ZrP·Bim can serve as a latent thermal initiator in the reaction of epoxy-acid anhydride systems. In the reaction of GPE-MHHPA with α-ZrP·Bim, the conversion reached 97% at 140 • C for 1 h, and the storage stability was maintained for 2 weeks at 25 • C. All of the intercalated Bim could be deintercalated at 120 • C for 1 h.

Measurements
X-ray diffraction (XRD) patterns were obtained using a Rigaku RINT2200 (Tokyo, Japan) with Cu Kα radiation over a scan range of 3-40 • at a rate of 2 • min −1 . NMR spectra in solution were recorded on a Varian Unity-300 spectrometer (Palo Alto, CA, USA) and a JEOL JNM-ECZS (400 MHz) spectrometer (Tokyo, Japan) using tetramethylsilane (TMS) as an internal standard. The 31 P MAS NMR and 13 C CPMAS NMR spectra were recorded on a JEOL ECA-600 NMR spectrometer (Tokyo, Japan). The contents of benzimidazole and water in the intercalation compounds of α-ZrP were measured using a PerkinElmer 2400II (Waltham, MA, USA). Gel permeation chromatography (GPC) was carried out on a Shodex GPC-101 (LF804*3 and KF-800RF*3, THF as eluent) (Showa Denko Co. Ltd., Tokyo, Japan) using polystyrene standards. The Fourier transform infrared spectroscopy (FT-IR) measurements were carried out with an ALPHA spectrometer (Billerica, MA, USA).

Typical Polymerization Procedure
A mixture of GPE (150 mg, 1.0 mmol), MHHPA (168 mg, 1.0 mmol), and benzimidazole intercalation compound with α-ZrP (α-ZrP·Bim) (9.0 mg, 0.019 mmol, content of benzimidazole: 0.030 mmol) was heated at 120 • C for 1 h. A small aliquot of the reaction mixture was dissolved in CDCl 3 , and its 1 H-NMR spectrum was acquired to determine the extent of the conversion of GPE and MHHPA. At 40 • C, a small aliquot of the sample was collected at determined times.

Conclusions
The intercalation compound of α-ZrP·Bim can effectively serve as a latent thermal initiator in the reaction of GPE-MHHPA under heating conditions. All of the Bim intercalated in the layers of α-ZrP was effectively deintercalated for 1 h. At 140 • C, the conversion reached 97% in 1 h. The storage stability was maintained up to 14 days at 25 • C. This investigation of the deintercalation behavior can be applied to other intercalation compounds of α-ZrP as latent thermal initiators.
Author Contributions: O.S. conceived, designed and wrote the article; S.S., K.K., S.K. and M.S. performed the experiments; A.O. and R.N. contributed to a helpful discussion.
Funding: This research received no external funding.