Chemorheology of epoxy resin

A secondary hydroxyl group containing epoxy resin was reacted and cross-linked with polyurethane (PU). This PU-cross-linked epoxy resin was cured with a tertiary amine and the viscosity and dynamic mechanical properties were determined by a cone-and-plate rheometer. A dual Arrhenius viscosity model was modified to predict the viscosity profile with time before gelation during non-isothermal curing, and the calculated values coincided with the experimental data. The activation energy of this system (NCO/OH ratio=30 mol%) calculated by the modified model was 13kcal mol^–1 for the initial region, and 16.8 kcal mol^–1 for the final region. After gelation, the dynamic complex modulus was correlated to the reaction kinetics according to the rubber elasticity theory, and the activation energy calculated was 5.2 kcal mol^–1. These activation energies are all lower than those of the unmodified epoxy resin system. Consequently, the reaction rate of the PU-cross-linked epoxide system was less affected by temperature than that of the unmodified epoxide system. It was also found that the rate of increase of viscosity and dynamic moduli decreased with increasing PU content. The gelling point was estimated by rheological measurements.     

Physicochemical and mechanical interfacial properties of trifluorometryl groups containing epoxy resin cured with amine

A phenyl-trifluoromethyl (-Ph-CF₃) groups modified epoxy resin, diglycidylether of bisphenol A-fluorine (DGEBA-F), was synthesized and the physical properties, such as curing behaviors, thermal stabilities, and dielectric constant of the DGEBA-F/4,4′-diaminodiphenyl methane (DDM) system were investigated and compared with commercial DGEBA/DDM system. For the mechanical behaviors of the specimens, the fracture toughness and impact tests were performed, and their fractured surfaces were examined by using a scanning electron microscope (SEM). The dielectric constant values of the DGEBA-F/DDM system were lower than those of the DGEBA/DDM system and the mechanical properties of the casting DGEBA-F specimens were higher than those of the DGEBA specimens. This was probably due to the fact that the introduction of the -Ph-CF₃ groups into the side chain of the epoxy resin resulted in improving the electrical properties and toughness of the cured DGEBA-F epoxy resin.     

Poly(ether ether ketone) with pendent methyl groups as a toughening agent for amine cured DGEBA epoxy resin

A diglycidyl ether of bisphenol-A (DGEBA) epoxy resin was modified with poly(ether ether ketone) with pendent methyl groups (PEEKM). PEEKM was synthesised from methyl hydroquinone and 4,4′-difluorobenzophenone and characterised. Blends of epoxy resin and PEEKM were prepared by melt blending. The blends were transparent in the uncured state and gave single composition dependent T _g. The T _g-composition behaviour of the uncured blends has been studied using Gordon–Taylor, Kelley–Bueche and Fox equations. The scanning electron micrographs of extracted fracture surfaces revealed that reaction induced phase separation occurred in the blends. Cocontinuous morphology was obtained in blends containing 15 phr PEEKM. Two glass transition peaks corresponding to epoxy rich and thermoplastic rich phases were observed in the dynamic mechanical spectrum of the blends. The crosslink density of the blends calculated from dynamic mechanical analysis was less than that of unmodified epoxy resin. The tensile strength, flexural strength and modulus were comparable to that of the unmodified epoxy resin. It was found from fracture toughness measurements that PEEKM is an effective toughener for DDS cured epoxy resin. Fifteen phr PEEKM having cocontinuous morphology exhibited maximum increase in fracture toughness. The increase in fracture toughness was due to crack path deflection, crack pinning, crack bridging by dispersed PEEKM and local plastic deformation of the matrix. The exceptional increase in fracture toughness of 15 phr blend was attributed to the cocontinuous morphology of the blend. Finally it was observed that the thermal stability of epoxy resin was not affected by the addition of PEEKM.     

An electron microscopic study of the morphology of cured epoxy resin

An electron microscropic study of fracture surfaces and microtomed sections of a cured epoxy resin based on a difunctional bisphenol A type resin cured with different amounts ofm-phenylenediamine is presented. Heterogeneities in the range 5 to 100 nm are seen to be present and have relatively higher crosslink density compared to the surrounding matrix. It is observed that the fracture path is around the heterogeneity and not through it. The size of the heterogeneity is a function of curing agent concentration and also of cure cycle. The stoichiometric sample, which has the highest crosslink density and the highest glass transition temperature, has the smallest heterogeneities. On either side of stoichiometry, the heterogeneity size increases. Samples subjected to a more severe post-curing cycle have much larger heterogeneities. The possible physical basis for these differences is discussed.On leave from the Indian Institute of Technology, New Delhi 110016, India.     

Thermal expansion and swelling of cured epoxy resin used in graphite/epoxy composite materials

This paper presents results of experiments in which the thermal expansion and swelling behaviour of an epoxy resin system and two graphite/epoxy composite systems exposed to water were measured. It was found that the cured epoxy resin swells by an amount slightly less than the volume of the absorbed water and that the swelling efficiency of the water varies with the moisture content of the polymer. Additionally, the thermal expansion of cured epoxy resin that is saturated with water is observed to be more than twice that of dry resin. Results also indicate that cured resin that is saturated with 7.1% water at 95° C will rapidly increase in moisture content to 8.5% when placed in 1° C water. The mechanism for this phenomenon, termed reverse thermal effect, is described in terms of a slightly modified free-volume theory in conjunction with the theory of polar molecule interaction. Nearly identical behaviour was observed in two graphite/epoxy composite systems, thus establishing that this behaviour may be common to all cured epoxy resins.     

室温固化环氧树脂结构胶粘剂的研究

制备了室温快速固化综合性能较好的聚氨酯改性环氧树脂结构胶粘剂,探讨了增韧剂,固化剂等因素对胶 粘剂力学性能的影响,通过扫描电镜分析了胶粘剂断裂面的形貌特征,研究了胶粘剂形貌特征与力学性能的关系.研究结果表明:聚氨酯增韧环氧树脂既有物理交联产生"强迫互溶"和"协同效应",又与环氧树脂发生了化学反应,产生的化学键进一步加强了聚合物交联密度和聚合物的强度,当PU预聚体含量为环氧树脂40%时,聚氨酯与环氧树脂形成的互穿网络聚合物互穿程度最大,制备的胶粘剂力学性能最优,室温剪切强度达到20.16MPa.     

端氨基树枝状大分子PAMAM/环氧树脂体系固化物的性能

以端氨基树枝状大分子PAMAM作为环氧树脂固化剂, 通过拉伸试验、 冲击试验、 DSC、 TGA研究了配比和固化温度对PAMAM与环氧树脂E-44的固化物性能的影响。 结果表明, 最佳固化温度为140℃, 但随着固化温度升高, 配比的影响表现出不同的规律: 80℃固化时, 最佳配比为0.47, 此时拉伸强度和冲击强度最佳, 玻璃化转变温度最高, 交联密度最大; 而在80℃以上固化时, 最佳配比逐渐向低配比方向移动, 140℃固化时, 最佳配比为0.28, 此时拉伸强度和冲击强度最佳, 玻璃化转变温度最高, 交联密度最大。固化物的密度和体积收缩率都是配比为0.47时最大, 而热稳定性都是配比为0.28时最佳。利用滴定法测定了固化物的固化度, 结果表明, 随着固化温度的升高, 低配比体系的固化度迅速提高并接近化学计量点配比体系的固化度。      

Blends of thermoplastic polyimide with epoxy resin: Part II Mechanical studies

Bifunctional epoxy resins have been modified with thermoplastic polyimide in order to improve their toughness. The effects of polyimide content, curing conditions, curing agents, and types of polyimide on the fracture properties have been examined. Relationships between the microstructure of the cured resins and their mechanical properties have also been investigated. This revised version was published online in November 2006 with corrections to the Cover Date.     

Moisture absorption by cyanate ester modified epoxy resin matrices. Part V: effect of resin structure

A series of epoxy resins of varying functionality (in terms of the number of epoxide groups) were used to cure the cyanate ester resin, AroCy L10 (1,1-bis(4-cyanatophenyl) ethane). The effect of thermal spiking on moisture absorption and changes in glass transition temperature (T_g) were studied.With the tetra functional (MY720) epoxy a more polar crosslinked density matrix forms. However, after thermal spiking to 140 °C a second peak appeared in the DMTA spectrum which was attributed to partial hydrolysis. The other two systems were much more stable.All systems exhibited the enhancement of moisture absorption as a result of thermal spiking.The mechanism of this phenomenon has been discussed in detail and related to the distribution of unoccupied volume in the dry resin and the location of water molecules at hydrogen bonding sites. The redistribution of the water molecules throughout the sites during thermal cycling is considered responsible for the enhancement and the reduction in moisture contents at high thermal spike temperatures.     

酸酐固化侧链取代液晶环氧树脂的性能研究

采用DSC、DMA、TGA、SEM等研究了侧链取代液晶环氧树脂酸酐固化产物的动态力学和机械性能，并就侧链长度对液晶环氧树脂的性能影响进行了探讨。结果表明：侧链取代液晶环氧树脂表现出较高的模量和优良的耐热性能，其韧性也得到明显的改善。随着侧链长度的增加，固化产物的交联密度有所下降，随之下降的还有玻璃化转变温度、动态储能模量、弹性模量以及断裂伸长率。柔性链的增加也使其耐热性能下降，而其耐水性能则有所提高。     

An epoxy resin copolymer with zero shrinkage

Epoxy resin copolymer with zero shrinkage can be obtained by copolymerizing epoxy resin E51 with the expanding monomer of 3,9-di(5-norbornene-2,2)-1,5,7,11 -tetraoxaspiro[5,5]undecane (NSOC). The volume changes of an epoxy resin copolymerized with various amounts of NSOC during cure was measured with a dilatometer. The epoxy copolymer giving zero shrinkage was an epoxy resin with a E51:NSOC ratio equivalent to 5.881. This epoxy resin copolymer with zero shrinkage has been used as an adhesive to join the optical parts of a large optical telescope.     

Impact-modified epoxy resin with glassy second component

A resorcinol-based epoxy resin was modified by incorporating a glassy second component. The mixture showed a heterogeneous morphology with two clearly defined phases, one phase rich in oligomer, the other phase composed mainly of resorcinol epoxy resin. The fracture toughness measured asG _1c andK _1c values showed an increase from 174J m^–2 and 0.89 MN m^–1.5 S in pure epoxy resin to 431 J m^–2 and 1.36 MN m^–1.5 in 30% oligomer modified resins. The scanning electron micrographs showed that the oligomer-rich phase exhibited ductile failure behaviour and formed the dispersed phase at low concentrations while it was the continuous matrix when the concentration was 30%. Optical observations on the failure mode of thin films of the oligomer-modified epoxy resin showed the existence of both inter face failure and considerable distortion in both phase.     

氰酸酯／双马来酰亚胺／环氧树脂三元共聚物及性能研究

采用双马来酰亚胺和环氧树脂与氰酸酯共聚对氰酸酯树脂进行改性，用FTIR跟踪了其固化过程。性能测试表明，当BCE：BMI：EP为5：2：3时，其固化物的冲击强度达到了12．2kJ·cm^-2，韧性有较大的提高；改性树脂的耐热性随着EP含量的增加而下降，当EP含量为30％时，Tg为233．7℃；改性后的氰酸酯树脂在1MHz频率下的介电常数和介电损耗角正切都比纯CE的有所增加，但上涨的幅度较小，仍具有优异的介电性能；DMA分析表明，在瓦温度时，损耗因子和损耗模量达到最大值，其中，tanδmax为0．359。     

Temperature dependence of fracture toughness of epoxy resins cured with diamines

The fracture toughness (critical stress intensity factor, K _Ic) of epoxy resins cured with four diamines has been measured as a function of temperature over the range from –35° C to above T _g. It was found that K _Ic for each epoxy-diamine system did not vary below room temperature, and in the higher temperature range K _Ic increased with increasing temperature to a maximum and then decreased. The temperature which maximized K _Ic, agreed with the temperature at which the flexural modulus of the epoxy resins abruptly dropped. This temperature was therefore considered as T _g. This temperature was found to be about 20° C lower than the heat deflection temperature under load (1.82 M Pa) of the resins.     

Mechanical fatigue of epoxy resin

In static bending fatigue tests, epoxy resins show practically no fatigue if the stress given to specimen is lower than a critical value, which is close to the bending strength of the specimen. In cyclic bending fatigue tests, on the other hand, the resins are easily fractured even though the stresses are far below the critical values. Some strain may be accumulated on the surface of specimen through cyclic deformations. However, the strain accumulated is reversible. If the specimen is allowed to rest, the strain disappears. If the strain reaches a critical value, an irreversible transition may be induced, probably in the arrangement of segments on the surface. A crack nucleus thus created may propagate and cause the final fracture of the specimen, following the fracture mechanics of elastic materials. The lifetime of epoxy resins under cyclic bending load is determined by the time required for creating a crack nucleus on surface.     

Thermally mendable epoxy resin strengthened with carbon nanofibres

This paper investigates the strengthening and toughening effects of carbon nanofibres (CNFs) on a self-healing thermoset/thermoplastic blend, i.e. an epoxy/poly (ε-caprolactone) (PCL) blend. The self-healing material system was prepared by polymer blending that produced a co-continuous phase-separated structure. The addition of CNFs altered the phase structures, leading to smaller domain sizes or even completely altering the phase separation mechanism, e.g. conversion from a co-continuous phase-separated structure to a particulate phase structure when the CNF content reached a certain level (0.3 wt% in this work). As the content of CNFs increased, the resulting nanocomposite became stronger and tougher, but the self-healing efficiency diminished; the optimal CNF content was found to be 0.2 wt%, which produced the highest strength, toughness and hardness, while achieving around 70% of healing efficiency.     

Epoxy resin cured with poly(4-vinyl pyridine)

Poly(4-vinylpyridine) (P4VP) was used as the macromolecular curing agent to prepare epoxy networks. The crosslinking structures were investigated by means of Fourier transform infrared spectroscopy (FTIR). It is identified that depending on the ratios of DGEBA to P4VP, different reactions dominated the formation of the crosslinking networks, which were involved the formation of pyridone (or cyclic amide) resulting from epoxide groups of DGEBA and pyridine rings of P4VP, the Diels-Alder reaction of in situ formed conjugated 3,5-diene in a 6-member ring and the homopolymerization of DGEBA initiated by pyridine moiety of P4VP. Differential scanning calorimetry (DSC) showed that all the DGEBA-P4VP co-crosslinked thermosets displayed single glass transition temperature (T_g), suggesting that the crosslinked networks are homogenous. In addition, it is noted that the T_g's of the DGEBA-rich network are greatly dependent on the molecular weight of P4VP used.     

碳质材料增强环氧树脂复合材料力学性能研究进展

综述了炭黑、碳纤维、碳纳米管等碳质材料增强环氧树脂复合材料的力学性能特征、论述了碳质材料的类型、用量,改性方法对复合材料抗张强度、抗拉强度、断裂伸长率、抗冲击强度以及韧性等力学性能的影响.讨论了表面化学修饰碳质材料、改进共混技术以及材料微观结构对复合材料力学性能的影响,碳质材料的分散性是影响复合材料力学性能和微观结构的...