| Owing to the excellent mechanical property,bonding performance,chemical resistance,insulating property,barrier performance and so on,epoxy resins have been widely applied in electronic packaging materials,such as epoxy molding compound(Epoxy resins-Phenolic resins).However,as the curing agent,phenolic resins can only react with epoxy resins at a higher temperature,so the curing accelerator is essential to reduce the activation energy,and accelerate the curing rate.But the common curing accelerators,such as imidazole compounds,triphenylphosphine,tertiary amine and so on,they are all apparent curing agents and high activity,the curing of epoxy resins can be promoted even at room temperature,while elevating the curing reaction activity,they will also affect the storage stability of the system.Therefore,it is necessary to develop thermal latent curing accelerators,which can improve the storage stability at room temperature,and ensure the activity of one-component system at high temperature.As a commonly used curing accelerator,imidazole possesses the characteristics of low toxicity,cheap price and high activity.And besides,because of the numerous and beneficial to modified derivatives,imidazole is popular in application.However,due to its strong epoxy-opening activity,imidazole can initiate the polymerization of E/P system at room temperature,which greatly limits its application.Therefore,it is necessary to modify imidazole with thermal latency.Pyrrole-type nitrogen and pyridine-type nitrogen are the two positions possess high activity in imidazole,so it is necessary to chemically modify imidazole to improve its storage stability at room temperature and ensure its curing activity at elevated temperature.The main work includes the following two parts:1.Study on trimellitic anhydride modified imidazole as latent curing accelerator for epoxy resinsThe modified imidazole derivatives 2MI-T and EMI-T were prepared by the nucleophilic substitution reaction of 2-methylimidazole(2MI)and 2-ethyl-4-methylimidazole(EMI)with trimellitic anhydride acid chloride(TAAC),respectively.They were further used as latent curing accelerators of epoxy resin/phenolic resin(E/P)system.The successful synthesis of2MI-T and EMI-T was proved by 1H-NMR and FT-IR.The non-isothermal curing behavior of E/P system was studied by differential scanning calorimetry(DSC).The curing kinetics of E/P system was analyzed by Kissinger equation and Ozawa equation.The pot life of the one-component E/P system was investigated by FT-IR test at 60?for different time and DSC test at 30?for different time;in addition,the thermomechanical and thermal properties of the cured materials were tested by dynamic mechanical analysis(DMA)and thermogravimetric analysis(TGA).The results showed that compared to E/P system containing unmodified imidazole,the DSC curing exothermic peak of E/P system containing modified imidazole moved towards higher temperature about 40?,the curing reaction activation energy also elevated average 15 k J mol-1,and at the same time,the one-component system could be stably stored at 30?for more than four weeks,which further proved the excellent latency of the modified imidazole.This was due to the substitution of active hydrogen on pyrrole nitrogen by the introduction of the large trimellitic anhydride group;at the same time,the carbonyl group in the trimellitic anhydride group possessed strong electronegativity and formed a conjugated structure with the benzene ring,which could reduce the electron cloud density on the pyridine nitrogen and weaken its alkalinity,thus further reduced the activity of the imidazole compounds with epoxy groups.In addition,the addition of modified imidazole accelerators did not reduce the thermal mechanical properties and thermal stability of the cured products.2.Study on the imidazole derivative with intramolecular hydrogen bond as latent curing accelerator for epoxy resinsAn imidazole derivative 2-(2-hydroxyphenyl)-1H-benzimidazole(HBPI)with intramolecular hydrogen bond was prepared by the Schiff base reaction and oxidation reaction between o-phenylenediamine and o-hydroxybenzaldehyde,which was further used as latent curing accelerator for E/P system.HBPI was characterized by 1H-NMR,FT-IR,TGA and DSC,and the existence of intramolecular hydrogen bond was proved by variable temperature 1H-NMR.The non-isothermal curing behavior of the E/P system was studied by DSC,and the curing kinetics of the system was analyzed by Kissinger equation and Ozawa equation.The shelf life of the single-component E/P system was investigated by DSC test by storing at 30?for different time.In addition,the thermomechanical and thermal properties of the cured material were measured by DMA and TGA.The results showed that the latent curing accelerator possessed intramolecular hydrogen bond at low temperature,which blocked the activity of pyridine nitrogen and effectively inhibited the alkalinity,which made the one-component epoxy resin composition performed good storage stability at room temperature.At the temperature of about 160?,the intramolecular hydrogen bond of the imidazole derivative was broken,the base catalytic activity was released instantaneously,the curing exothermal peak was narrower,and the cross-linking curing of the E/P system was also accelerated.In addition,the thermosets cured by HPBI showed the similar glass transition temperature and thermal stability to those cured by common imidazole.The imidazole-type curing accelerator with intramolecular hydrogen bond was expected as a kind of high temperature promoter applied in epoxy molding compound,which could solve the problem of insufficient liquidity in the process of molding process owing to the addition of traditional curing accelerator,so as to improve the formability of epoxy encapsulation materials,and making it suitable for smaller,thinner,and more integrated advanced packaging standard. |
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