Construction Of Degradable Thermoset Materials Based On Hexahydrotriazine Rings And Their Properties

The degradation and recovery of thermosetting materials is an urgent scientific and technical problem to be solved.The introduction of acid-sensitive hexahydrotriazine（HT）ring structure into thermosetting resins is one of the effective ways to solve this problem.This paper introduces the HT structure into new thermosetting materials,and focuses on the study and discussion of the relationship between the structure and properties of the material,as well as the degradability.The specific research content and main results of the paper are as follows:（1）4,4’-diaminodibenzocrown ether（DACr）and 2,2-bis（P-aminophenoxy）ethyl ether（BAPE）were selected as diamine monomers,and they were used to prepare two kinds of polyhexahydrotriazine（PHT）films with different structural units through the processes of prepolymerized with paraformaldehyde and high temperature curing.The basic physical and chemical properties of the two PHT films（170℃-2h）were tested and analyzed,and the results showed that,the rigidity of the two structural units affected the completeness of the cross-linked structure of the HT ring,this consequently led to differencies in performace of PHT material.PHT containing BAPE structural unit could form a relatively complete HT cross-linked structure,and had the higher thermal stability（5 % thermogravimetric loss temperature is 296.8 ℃）,hydrophilicity and relatively low degree of acid hydrolysis,The PHT film based on the high rigidity of cyclic DACr structural unit had the higher mechanical strength（tensile strength 53.2 MPa）,hydrophobicity,resistance to organic reagents and the acid hydrolysis degree,,which could completely degrad in2 mol/L glacial acetic acid to obtain DACr monomer with high yield.Since the comprehensive influence by the structural integrity of the HT and the rigidity of the structural unit,the glass transition temperatures of the two PHT materials shown similar values（Tg are 117.8 ℃ and 117.9 ℃,respectively）.（2）The application of HT in degradable thermosetting materials was extended to the traditional thermosetting resin of polyurethane（PU）.The cross-linked curcuminbased polyurethane（Cur-PU）films containing HT structure were preparation by a step-by-step polyaddition process,and in the polymerization system,polyethylene oxide（PEG）,curcumin（Cur）,hydroxyethyl-hexahydro-s-triazine（THT）and isophorone diisocyanate were used as raw materials,nano-silica was used as a leveling agent.The influence of factors,including the ratios of NCO/OH groups in the reaction system and the ratios of hydroxyl groups from each hydroxyl raw materials,on the thermal properties,mechanical properties,degradability and other properties of PU was discussed,and the optimized synthesis system which could obtain the materials with good comprehensive performance was achieved.The optimized ratio of NCO/ OH and hydroxyl groups derived from each raw material（PEG-OH/Cur-OH/THT-OH）were 1.6/1 and 2/4/4,respectively.The Cur-PU film exhibited the characteristics of an elastic material,and its urethane bond and HT structure were degraded in a 2 mol/L phosphoric acid-ethanol solution,and the the raw materials of PEG and Cur could be recovered aftering degareation.In addition,Cur-PU had the property of UV photocrosslinking due to the existence of Cur structural units,and its modulus and thermal stability were effectively enhencced after formation of the photocrosslinking structure,and what’s unexpected was that,the degradation of the PU film was hardly affected by the crosslinked structure.（3）The application of HT in degradable thermosetting materials was further extended to the other traditional thermosetting resin of epoxy resin（EP）.Degradable epoxy resins were prepared by curing epoxy resins with self-made HT derivative triarylamine compounds（TAHT）.The self-made curing agent TAHT was synthesized by the click chemistry reaction between 1,3,5-triacryloyl-hexahydro-1,3,5-triazine and p-aminobenzenethiol,and the synthesis process was optimized by orthogonal experiment method.The optimized process was following: the ratio of alkenyl groups/thiol groups was 1/1,the content of triethylamine used as was 1 %,and the reaction temperature and time were 30 ℃ and 10 min,respectively.The selfmade curing agent of TAHT and bisphenol A epoxy resin E51 constitute a curing system according to the ratio of active hydrogen/ epoxy group = 1/1.The curing kinetics of this curing system was investigated by non-isothermal DSC method,and it was found that the activation energy of TAHT-epoxy resin curing was 50.68 k J/mol,and the curing reaction order was 0.79.Then,the optimized curing temperatures of epoxy resin were determined through fitting the non-isothermal DSC data,and the corresponding three-stage curing temperatures were 110 ℃,160 ℃ and 190 ℃.Through this optimized curing process,the resulted TAHT-epoxy cured product shown a similar thermal properties to that of traditional epoxy resin,and TAHTepoxy also had good degradability,it was completely hydrothermally digested in 3mol/L nitric acid aqueous solution.The thermosetting materials prepared in this paper have both good comprehensive properties and degradability,and are expected to be used as substitutes for some refractory thermosetting materials.Our work offer ideas for the design and preparation of degradable thermoset materials based on HT rings,and also provides experimental experience for the regulation of the structure and properties of thermoset materials.