Study On Synthesis, Structure And Properties Of Epoxy Resin Modified With Flexible Side Chain

Epoxy resin (EP) for its outstanding advantages on high thermal stability,excellent electric properties and technique as advanced resin matrix has been widelyused in high science and technique fields, such as aviation, sPaceflight and nationaldefence, etc. However, epoxy resin is generally rather bittle, because of its highlycrosslinked structure. Improving the toughness of this resin is very important forepoxy technology, a successful attempt is flexible crosslinking agent toughening orflexible crosslinking in main-chain of epoxy. However, the presence of flexiblechain may decrease the thermal stability, the strength and the modulus of thematerial. Whilst for adhesive applications such decreases in modulus andtemperature resistance are usually of no significance, in matrices forfiber-composites such effects can lead to unacceptable decreases in the properties ofthe firer-composite. In this thesis, the new approach to improve the toughness is tosynthesize EP with long polyether side chains, where the side chains don't undergophase-seParation on curing the blends. The synthetic technique, structure andproperties of EP with side chains, curing mechanism and structure and properties ofepoxy blends are studied. The toughening micromechanisms responsible for themarked increases in toughness arising from the presence of flexible side chain arethen identified.First, using toluene diisocyanate (TDI) as a coupling agent between the epoxyand hydroxyl-terminated polyethers forming a urethane linkage with the former andthe latter respectively, the EP with long polyethers side chains are synthesized.Polyethers used is diethylene glycol monoethyl ether (Mw=162). The progress of thereaction and the structural changes during modification process are studied using IRspectroscopy (FTIR), differential scanning calorimetry (DSC), viscosity data, andchemical analysis (epoxy value determination). The studies support the propositionthat TDI acts as a coupling agent between the epoxy and diethylene glycolmonoethyl ether forming a urethane linkage with the former and the latter. Usingmono methoxy polyethylene glycol (mPEG) of different molecular weight (Mw=450, 1200, 2000), EP with different length side chain are synthesized.The curing mechanism and kinetics of epoxy with polyether side chains reactedwith Me THPA are studied by non-isothermal differential scanning calorimetry(DSC). The curing reaction is proceeding in nth cure mechanism, the same as thepure epoxy. The order of curing reaction is 0.9～1, gained with kinetics calculation.Besides, the content of side chains don't affect in the cure mechanism.The activationenergy (Ea) and reaction frequency factor of the system are increased and reactionrate was decreased by the addition of flexible polyether side chains into the epoxyresin. Based on the kinetics results, the optimization of cure processing is discussed:100℃for 2h, 120℃for 2h and 160℃for 6h. DSC shows that the chain lengthdoesn't affect in the cure mechanism. The order of curing reaction is 0.9～1 also,gained with kinetics calculation. The urethane linkage will be decompounded underhigh temperature. The TGA data show that the thermal degradation temperature is220℃. The urethane linkage can't be decompounded on curing the blend for thetemperature of curing EP blends.The mechanical properties of epoxy modified with flexible sidechain arestudied. The mechanical properties of the cured blends show that the toughness ofepoxy modified with flexible side chain is increasing markedly. At that time, theway of flexible side chain toughening epoxy can retain the modulus of the curedepoxy resin. The pure epoxy polymer is very brittle with imPact toughness and thefracture toughness (K_IC) values of about 17.92 KJ·m^-2 and 2.34MN·m^-3/2, with istypical of a brittle thermosetting epoxy polymer. The addition of flexible side chaingives an immediate increase in toughness. There is a steady increase in toughnesswith increasing content of ITPE, and a maxium value of imPact toughness=24.03KJ·m^-2 and K_IC=4.32 MN·m^-3/2 is obtained at 10wt% ITPE, the measured toughnessstarts to decrease somewhat. On the other hand, the flexural strength and the flexuralmodulus of the cured epoxy resin retain invariable. The toughness of epoxymodified with flexible side chain is increasing more markedly under lowtemperature. The imPact toughness of epoxy with modified with 10wt% ITPE is18.72 KJ·m^-2 under -50℃. At that time, the imPact toughness of the pure epoxy isonly 7.55 KJ·m^-2。Dynamic mechanical thermal analysis (DMA) show that ITPE/ER blends clearly display two maxima on the tanδvs T curves. However, scanning electronmicroscopy (SEM) of the fracture surfaces shows that polyethers side chain doesn'tundergo phase-seParation upon curing the blends and there are not cavities inblends.The different Mw of epoxy and modified-epoxy leads to thermodynamicdifference during curing the blends. So, the network of the curing blends isasymmetric. It is the toughening mechanism for the flexible side chain modifiedepoxy. Besides, the polyethers chains were wrapped in the three-dimensionalnetwork of the epoxy. The imPact energy could be held back or delayed due to theexistence of the flexible chain. In this case, the dissiPation (or absorption) ofrelatively large Part of the imPact energy may be attributed to crack bifurcation andcrack Path alternation. The modeling analysis of the fracture energy (G_IC) shows thatthe toughness is linear increasing with the amount of side chain in the EP when theside chain is invariable.The maxium value of imPact toughness and flexural strengthare all obtained at 10～15wt%, making no difference with length side chain. Theresult show that the effect of side chain to epoxy resin lies on the amount of uniformchain node (-CH₂-CH₂-O-), but has nothing to do with the length of the side chain.