Research Article pubs.acs.org/journal/ascecg
Hexahydro‑s‑triazine: A Trial for Acid-Degradable Epoxy Resins with High Performance Shusen You, Songqi Ma,* Jinyue Dai, Zhen Jia, Xiaoqing Liu,* and Jin Zhu Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, No. 1219 Zhongguan West Road, Zhenhai District, Ningbo 315201, People’s Republic of China S Supporting Information *
ABSTRACT: Hexahydro-s-triazine (HT) derivatives are acidsensitive compounds. 4,4′,4″-(1,3,5-Hexahydro-s-triazine-1,3,5triyl) tris(N-(2-aminoethyl) benzamide) (HT-A) bearing three amino groups was synthesized as a curing agent to prepare the acid-degradable epoxy resin. The chemical structures of HT-A and its precursor were confirmed by 1H NMR and 13C NMR. The synthesized HT-A, as well as the commercialized 4,4′diaminodiphenylmethane (DDM) (for use as the control) were used to cure diglycidyl ether of bisphenol A (DGEBA), respectively. The property examinations performed by dynamic mechanical analysis (DMA), differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), and tensile test demonstrated that the HT-A-cured epoxy presented high thermal and mechanical properties which were comparable with those of the DDM cured one. Meanwhile, degradation investigation indicated that the HT-A-cured epoxy could be controllably degraded with strong acid stimuli. Thus, HT-A has a great potential to produce recyclable thermosets with satisfactory properties. KEYWORDS: Degradable thermosets, Curing agent, High performance, pH-responsive
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INTRODUCTION In the past decades, thermosets have played an important role in polymeric materials, because of their excellent thermal and mechanical properties. Epoxy resin, as one of the most important thermosetting resins, is irreplaceable in many fields, such as coatings, adhesives, and composites.1,2 Because of their three-dimensional chemically cross-linked networks, epoxy resins cannot be reshaped or reprocessed by heat or with solvent.3,4 Worse more, epoxy-based materials are difficult to be recycled and degraded.5 Until now, incineration and landfill are the dominating methods to address the epoxy wastes, which will occupy much industrial land and cause serious environmental pollutions. Degradable plastics have been attracting increased interests, because of their environmental benefits and great potential applications in biomaterials.6 Developing degradable epoxy resin has been regarded as one effective solution to the recycle issue of epoxy-resin-based materials. So far, several types of degradable epoxy resins have been reported. For example, Shirai and his co-workers7 developed a series of thermal degradable epoxy resins based on secondary or tertiary ester bonds. Similarly, phosphorus,8 carbonates,9 and ethers10 were also tried in the preparation of thermally sensitive epoxy resins. These resins could be thermally degraded at temperatures under 200−300 °C, which was much lower than the initial degradation temperature (∼350 °C) of conventional epoxy resins.9 Besides the thermally degradable epoxies, those that could be degraded via chemical treatments were also reported in several literatures. For instance, disulfide as a fragile bond, which can be easily broken down by mercaptan © 2017 American Chemical Society
derivatives, was introduced into epoxy networks for utilization in extreme environments.11,12 Epoxy resin containing ketal or acetal linkage was also synthesized and it could be readily decomposed in diluted acid solutions.13 In Ma’s work,5,14 dicarboxylic and tricarboxylic acids were used to cure a biobased epoxy resin, and the resulted thermosets were sensitive to alkali solutions, because of the ester bonds formed during the curing process. Although more and more thermally or chemically degradable epoxy resins have been reported, preparing an easily degradable epoxy resin with good thermal and mechanical properties is still a great challenge. Hexahydro-s-triazine (HT) derivatives have rigid hexagonal structures and they will decompose to amines and formaldehyde when exposed to strong acids.15−18 Inspired by this fact, Garciá et al.4 synthesized an HT-containing thermoset (PHT) through the condensation of amines with formaldehyde. The resulted PHT possessed a storage modulus of ∼14 GPa, a glass-transition temperature (Tg) of >190 °C, and good degradability under strongly acidic conditions (pH
