Study on the Dual Curing Mechanism,Structural Design,and Phase Change Properties of Double Bonds/epoxy Systems

Author:Lian Qing Song

Supervisor:cheng zuo


Degree Year:2018





Epoxy resin,which has been applied on various fields,is indispensable matrix materials in composites,adhesives,coatings etc.due to its excellent adhesive and mechanical properties,good chemical stability,and facile processing performance.Owing to the fact that the crosslinking network is formed by the ring-opening reactions of epoxy groups with curing agents such as amines,anhydrides,and mercaptons,the polar character makes epory resin mainly used for the adhesion of polar substrates like metal.On the contrary,due to the poor affinity between epoxy resin with non-polar materials such as rubber and long-chain alkyl compounds,it is difficult for epoxy resin to meet the needs of bonding or compositing polar and non-polar materials together in moderm techonology area.However,the chemical structure of commonly-used epoxy resin like diglycidyl ether of bisphenol A(DGEBA)is too single,so it is very hard to deal with the above issue through simply formulation design.Therefore,chemical modification for epoxy resin derived from molecular structure is the research hotspot.Diglycidyl ether of 4,4’-diallyl bisphenol A(DADGEBA)has the same skeleton structure compared with DGEBA,the difference between DADGEBA and DGEBA is that there are two allyl double bonds on the benzene skeleton of DADGEBA.Therefore,DADGEBA can react with rubber and long-chain alkal compound through double bonds to chemically bond epoxy resin with non-polar materials so as to achive the adhesion and compositing between epoxy resin and non-polar materials,while the excellent properties of epoxy resin can maintain at the same time.Besides,the import of allyl groups can not only give DADGEBA novel curing reactions,but also make it possible to design and prepare epoxy-based functional materials,which will greatly expand the curing systems and potential application prospects of epoxy resin.In this thesis,based on the special epoxy resin with double bonds side chains(DADGEBA),the dual curing mechanism of double bonds/epoxy groups/sulfur system,rubber vulcanization mechanism studied by double bonds/epoxy model compound,and the preparation of epoxy-based novel phase change materials(PCMs)by utilizing the designabilty of double bonds/epoxy groups system were detailed and deeply studied.Following are the main achievements.1.Dual curing mechanism of double bonds/epoxy groups/sulfur systsm.Dating back to 2004,our team achieved a breakthrough in double bonds/epoxy resin curing technology by discovering that DADGEBA and sulfur can undergo dual-cured reaction and we preliminarily proposed a dual curing mechanism of DADGEBA/S system.However,this mechanism has not yet been thoroughly proved due to the complicated reaction systems.In this thesis,DADGEBA was primarily synthesized by a one-step method.The structure of DADGEBA was characterized by FTIR and NMR.Secondly,2-Allylphenol(OAP)and DGEBA were selected as the model compound and built four research systems(OAP,OAP/S,OAP/S/DGEBA,and DADGEBA/S systems)to investigate the dual curing mechanism of epoxy(EP)/allyl compound(AC)/S system.When the temperature was above 170℃,DSC and FTIR data showed that S could cleave to form thiyl radicals,and FTIR,NIR,and 1H NMR measurements proved the disappearance of allyl groups and the generation of thiol groups by the thiyl radicals abstracting α-H atoms of allyl groups in the OAP/S reaction system.Real-time FTIR(RT-FTIR)results showed that the reaction of the generation of thiol groups is the dominant reaction in the two possible pathways of the OAP/S system;allyl groups and epoxy groups disappeared sequentially in the OAP/S/BADGE system.DSC curves also revealed the one-stage reaction for OAP/S system and two-stage reaction for OAP/S/BADGE system.These data were used to develop a detailed,experimentally validated pathway for the dual curing of EP/AC/S system,in which thiol groups are important intermediate,and the dual curing process included the crosslinking of double bonds that initiated by thiyl radicals and the ring-opening reaction of epoxy groups with thiol groups.Verification of the dual curing mechanism of EP/AC/S system can not only greatly enrich the dual curing systems of epoxy resin,but also gives theriotical guidance for exploiting sulfur and sulfur donors as novel curing agents of epoxy resin.More importantly,the dual curing EP/AC/S system can provide a new thought to solve the problem of bonding polar and non-polar materials,like the adhesion of rubber and metal.2.Research on the active center(radical or ion)property of rubber vulcanization mechanism.Rubber vulcanization mechanism remains controversial between radical theory and ionic theory due to the complex systems and difficulties in characterization.Based on the proved dual curing mechanism of EP/AC/S system,a series of epoxy model systems were used to study the sulfur cleavage behavior so as to verify the active center(radical or ion)in the vulcanization process,from which the study process of the complicated vulcanization mechanism was significantly simplified and a brand new sulfur-only vulcanization mechanism was revealed by FTIR and DSC analysis.Besides,the proposed brand new mechanism can also explain the reason why the vulcanization mechanism remains controversial between radical and ionic theory.The research shows that the nature of rubber vulcanization mechanism is the formation of crossling network initiated by thiyl radicals and sulfur anions,in which thiyl radicals are derived from the homolytic cleavage of sulfur,while sulfur anions are transformed from the thiol groups which are generated from the reaction of abstracting α-H atoms of allyl groups by thiyl radicals.Therefore,the formation of thiyl radicals from the homolytic fission of sulfur plays the dominant role in the vulcanization mechanism.Besides,heterolytic fission of sulfur cannot occur as expected because this reaction needs 200℃.Nevertheless,sulfur anions,which are surely produced in the vulcanization process,are easy to be understood derived from the heterolytic cleavage of sulfur,this maybe the reason that vulcanization mechanism remains controversial for centuries.3.Preparation and properties of epoxy-based solid-solid phase change materials.Solid-solid PCMs owns the advantages of high reliability(no leakage risk),facile preparation,and low cost among all PCMs.However,at present,the types of solid-solid PCMs are very limited and the latent heat of solid-solid PCMs is relatively low.Therefore,exploiting novel solid-solid PCMs with high latent heat is a research hotspot.In this thesis,we used thiol-ene click chemistry to graft non-polar 1-octadecanethiol(ODT)onto the double bond of DADGEBA to form a side-chain crystalline epoxy resin named D18.The structure of D18 was characterized by FTIR and NMR,while the crystallinity of D18 was characterized by DSC and XRD.Next,we designed and prepared two types of epoxy-based solid-solid PCMs with high latent heat and filled the research blank of epoxy resin in this area.Besides,a brand new encapsulation mechanism for PCMs was proposed.Firstly,we designed and prepared the first type of epoxy-based solid-solid phase change materials(PCMs)based on DADGEBA,which not only achieved the functionalization of epoxy resin,but also filled the research blank of epoxy resin applied in this field.A series of novel epoxy-based PCMs(EPD18-x systems)were facilely prepared by blending DGEBA,D18,and poly(propylene oxide)diamine(Jeffamine D230)together followed by a one-pot thermal curing procedure.And thus the grafted ODT can be tightly locked in the reliable three-dimensional(3-D)crosslinking network of epoxy resin,which provides the PCMs with excellent shape-stable property.Therefore,EPD18-x systems are advanced solid-solid PCMs.The properties of the EPD18-x systems can be easily tuned by simply changing the mass ratio of DGEBA and D18.XRD and POM analysis proved that the crystallinity of the EPD18-x PCMs increased with the increasing content of D18.Phase separation behavior of the EPD18-x PCMs was characterized by visual images and SEM morphology.The result shows that microphase separation was observed in all the EPD18-x PCMs,and obvious phase separation was observed in the EPD18-25 system.However,the phase separation gradually disappeared with the increasing of the D18 content.Besides,with the increase content of D18,the latent heat of the EPD18-x systems increased from 14.8 to 70.5 J·g-1 and Tg decreased from 87.9 to 36℃,while mechanical properties including tensile strength,tensile modulus,and hardness decreased.Thermal recycling tests show that the EPD18-x PCMs can remain stable after 50 DSC thermal cycles.Due to the unique strong encapsulated epoxy curing networks,the EPD18-x PCMs have excellent thermal stability with the onset degradation temperature higher than 250 ℃.The tunable EPD18-x systems can be applied for room temperature used energy storage applications such as building insulation materials,thermoregulated fabrics,and so on.Secondly,based on the excellent compatibility between paraffin and D18,we constructed a paraffin/D 18/D23O phase change systems and facilely prepared a series of epoxy/paraffin composite PCMs(EPPa-x systems)by blending paraffin,D18,and D230 together followed by a one-pot curing process.The latent heat of EPD18-x PCMs were greatly enhanced by the importion of paraffin.Besides,a brand new encapsulation mechanism of PCMs,which can solve the contradiction between tradional form-stable PCMs(with high latent heat but high cost)and polymeric PCMs(with low cost but high latent heat),were proposed at the same time.Leakage test and SEM morphology show that EPPa-x systems can encapsulate 50%mass ratio of paraffin without leakage problem.Strong intermolecular forces between paraffin and D18(based on their good compatibility)and reliable 3-D crosslinking network of epoxy resin form a unique encapsulation mechanism,which provides the EPPa.x SSPCMs with excellent shape-stable properties and superior thermal stability(remain stable below 180 ℃)without sacrificing too much latent heat of paraffin(only 1.6%latent heat loss).Due to the combination of two melting process at 36 ℃ and 60 ℃ derived from D18 and paraffin,respectively,EPPa-X systems are unique multiple PCMs and the highest latent heat can be as high as 152.6 J·g-1.Besides,the supercooling extent of D18 decreased from 13.6 ℃ to 10.5 ℃ with the adding of paraffin due to the heterogeneous nucleation effect.The novel EPPa.x PCMs possess tremendous potential for a wide range of applications due to their facile preparation,low cost,high reliability,propriate phase change temperature,environmental friendly,and excellent phase change properties;while the unique encapsulation mechanism may open a new door for preparing other novel types of PCMs.