DABCO- and DBU-intercalated α-zirconium phosphate as latent thermal catalysts in the copolymerization of glycidyl phenyl ether (GPE) and hexahydro-4-methylphthalic anhydride (MHHPA)
Introduction
There has been considerable interest in development of latent polymerization catalysts that are completely inert in the monomer or prepolymer under storage conditions up to 40 °C under indoor lighting [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. We have previously reported that primary alkylamines intercalated with α-zirconium phosphate [α-Zr(HPO4)2·H2O, α-ZrP], α-ZrP·Hed (1-aminohexadecane), α-ZrP·Oct (1-aminooctane), α-ZrP·Bu (1-aminobutane) and α-ZrP·Pr (1-aminopropane), can serve as latent thermal initiators in the reaction of glycidyl phenyl ether (GPE) [22]. Among them, the intercalation compound of α-ZrP·Hed showed the potential to initiate the reaction upon heating. While it did not react with GPE at 40 °C, increasing the temperature increased the conversion factor such that, at 140 °C for 1 h, conversion of 64% was obtained. Although we showed the potential of intercalation compounds to function as initiators, molar equivalents of the intercalated primary amines were generally needed to react with the epoxide.
In the case of intercalated tertiary amines, catalytic amounts are expected to serve as curing accelerators for epoxy resins. We prepared intercalation compounds of 1,4-diazabicyclo[2,2,2]octane (DABCO) and 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) with α-ZrP, and examined the catalytic activity of α-ZrP·DABCO and α-ZrP·DBU in the curing of GPE with hexahydro-4-methylphthalic anhydride (MHHPA).
In the course of writing this paper, we found a patent on intercalated tertiary amines serving as curing accelerators in epoxy resins. The patent described the products obtained by reacting zirconium phosphate, titanium phosphate, tin phosphate, or Magadiite (H2Si14O29 3H2O) with tertiary amines used as a hardening accelerators, e.g., tris(dimethylaminomethyl) phenol, DBU, imidazole, benzimidazole and 2-ethyl-4-methylimidazole [23]. The storage stabilities of the products were tested by spiral flow tests, namely, the time until spiral flow decreased to 10% of the initial value. The spiral flow test gave satisfactory results for the products after mixing the curing accelerator with novolac-type epoxy resin for 30 days. The abilities of the intercalated amines as latent hardening accelerators were evaluated by measuring the torque after 5 min from initiation at 175 °C using a curelastometer or by compactibility after storage for 2 weeks at 40 °C. Compared to the tertiary amines themselves, the intercalated amines showed faster hardening and better compactibility in addition to better storage stability. However, substantial differences were not found among these intercalated catalysts in the novolac-type epoxy resin curing at 175 °C. These tests are not enough to estimate the performance for curing commonly used epoxy resins at lower temperature. Therefore, for the estimation, it is necessary to examine the change at the molecular level. Our present study demonstrated that intercalated catalysts showed good performance and individual behavior regarding latent hardening accelerators for the reaction of GPE with MHHPA.
Section snippets
Materials
α-Zr(HPO4)2·H2O (CZP-100) was purchased from Daiichi Kigenso Kagaku Kogyo Co., Ltd.; DABCO and GPE from Aldrich Chemical Co., Inc.; and DBU and MHHPA from Tokyo Chemical Industries, Co., Ltd., Solvents were used as received without further purification.
Measurements
X-ray diffraction (XRD) patterns were obtained using a Rigaku RINT2200 with CuKα 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 using tetramethylsilane (TMS) as
Results and discussion
The basal distances and compositions as estimated from XRD and CHN elemental analyses, respectively, of the prepared intercalation compounds are summarized in Table 1 [26]. Intercalation compounds of DABCO and DBU with α-ZrP at a ratio of 0.56 and 0.83 per 1 mol of the phosphate compound were prepared. The basal distances of 16.1 Å (2θ = 5.5°), 14.4 Å (2θ = 6.2), and 10.9 Å (2θ = 8.1) for α-ZrP·DABCO and 20.0 Å (2θ = 4.4°) and 13.7 Å (2θ = 6.4) for α-ZrP·DBU expanded from that of pristine α-ZrP (7.6 Å, 2θ =
Conclusion
The catalytic performance of α-ZrP·DABCO and α-ZrP·DBU in the copolymerization of GPE and MHHPA was examined. α-ZrP·DABCO did not catalyze the reaction of GPE with MHHPA at 40 °C for 144 h, but did catalyze the reaction over 100 °C and gave a polymer with 90% conversion of GPE at 140 °C. α-ZrP·DBU catalyzed the polymerization at 100 °C or above and gave a polymer with 94% conversion of GPE at 140 °C. Although these catalysts showed individual behavior in the reaction of GPE and MHHPA, these
Acknowledgments
This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. We wish to thank A Rabbit Science Japan Co., Ltd., for CHN elemental analyses. We also thank Dr. M. Shizuma and Dr. S. Kawano of Osaka Municipal Technical Research Institute for GPC analyses.
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