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Hexahydro-s-triazine: a trial for aciddegradable epoxy resins with high performance Shusen You, Songqi Ma, Jinyue Dai, Zhen Jia, Xiaoqing Liu, and Jin Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017
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Hexahydro-s-triazine: a trial for acid-degradable epoxy resins with high performance Shusen You, Songqi Ma*, Jinyue Dai, Zhen Jia, Xiaoqing Liu*, Jin Zhu Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, No. 1219 Zhongguan West Road, Zhenhai District, Ningbo 315201, PR China Corresponding authors：
[email protected] (Songqi Ma);
[email protected] (Xiaoqing Liu)
Abstract: Hexahydro-s-triazine (HT) derivatives are acid-sensitive compounds. 4,4',4''-(1,3,5-Hexahydro-s-triazine-1,3,5-triyl) 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
13
C NMR. The synthesized HT-A as well as the
commercialized 4,4’-diaminodiphenyl methane (DDM) for 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 due to 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 Due to 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 Up to now, incineration and landfill are the dominating methods to deal with the epoxy wastes, which will occupy lots of industrial lands and cause serious environmental pollutions. Degradable plastics have been attracted increasing 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 kinds of degradable epoxy resins have been reported. For example, Shirai and his coworkers7 developed a series of thermal degradable epoxy resins based on secondary or tertiary ester bonds. Similarly, phosphorus,8 carbonates9 and ethers10 were also tried to prepare the thermal sensitive epoxy resins. These resins could be thermally degraded under a temperature of 200-300 oC, which was much lower than the initial degradation temperature (around 350 oC) of conventional epoxy resins.9 Besides the thermally degradable epoxies, the ones 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 derivatives, was
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introduced into epoxy networks for extreme environments utilization.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 taken to cure a bio-based epoxy resin, and the resulted thermosets were sensitive to alkali solutions due to the ester bonds formed in 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 hexagon structures and they will decompose to amines and formaldehyde when exposed to strong acids.15-18 Inspired by this fact, García et al.4 synthesized a HT-containing thermoset (PHT) through the condensation of amines with formaldehyde. The resulted PHT possessed a storage modulus of about 14 GPa, a glass transition temperature of above 190 oC and good degradability
under
strong
acid
condition
(pH
