The mechanical properties of epoxy resins

Crack propagation in a series of epoxy resins described in Part 1 has been studied as a function of testing rate and temperature. It has been found that crack propagation is continuous at low temperatures but that as the temperature is raised the mode of propagation becomes unstable (stick/slip). Features on the fracture surfaces at the crack arrest lines have been shown to be of the same dimensions as those expected for a Dugdale plastic zone. It has been suggested that the slip process takes place by slow growth of a crack through the plastic zone followed by rapid propagation through virgin material. It has been shown that the stick/slip behaviour is due to blunting of the crack which is controlled by the yield behaviour of the resin. A unique fracture criterion has been shown to be applicable to epoxy resins which is that a critical stress of the order of three times the yield stress must be achieved at a critical distance ahead of the crack. Electron microscope replicas of the fracture surfaces have been obtained and an underlying nodular structure can be resolved. However, no direct correlation between the nodule size and fracture properties has been found.     

Rheological and mechanical properties of poly(butylene terephthalate)-modified epoxy resins

Poly(butylene terephthalate) (PBT) was used as modifier for epoxy resin. With the incorporation of 7.5 wt% PBT, the resin became a gel as shown by rheological measurements in steady shear, thixotropic loop and dynamic shear mode. The gel was very stable on storage. An abrupt change of rheological properties occurred at the dissolution temperature of the PBT spherulites. The PBT modifier did not impair mechanical properties of the cured resins, yet a moderate improvement in toughness was achieved.     

混合型固化剂对环氧树脂室温和低温力学性能的影响

研究了一种刚性和柔性胺混合型固化剂（芳香胺DETD和聚醚胺D-400）固化环氧树脂浇铸体的力学性能、材料断裂表面的微观形貌和玻璃化转变温度等性能。结果表明:当D-400加入量占固化剂总量的40%时,其室温拉伸强度呈现最大值,为82.52 MPa,弹性模量为2.30 GPa,与未加D-400的体系相比分别提高了6.3%和14.4%,其低温冲击强度提高了14%。对冲击断面形貌进行扫描电子显微分析表明:D-400的加入致使断口形貌变得粗糙,抗开裂能力得到提高。热分析实验结果显示,体系的玻璃化转变温度随着D-400含量的增加而降低。此外,还探讨了环氧树脂体系低温增韧机制。      

含侧环氧基硅油复合改性环氧树脂的研究

采用侧基环氧化硅油(ES)及其改性物(PSA)来复合改性双酚A型环氧树脂(EP).通过测定复合改性固化物的冲击强度、拉伸强度、断裂伸长率和玻璃化转变温度(Tg),扫描电子显微镜对改性固化物的断裂面形态分析等,系统探讨了复合改性方法、有机硅组成及其含量等对复合改性材料性能的影响.结果表明,采用ES和PSA复合改性EP后,其韧性和耐热性均有不同程度的提高,且以环氧值高的ES和PSA的改性效果更好.其中环氧树脂经10份ES-16或10份PSA-16改性后,Tg由未改性的156.73℃提高到177.35℃,比改性前提高了近20.62℃,均达到了很好的增韧和提高耐热性的效果,符合电子封装等高性能材料的改性要求.     

The reinforcing effect of graphene nanosheets on the cryogenic mechanical properties of epoxy resins

The reinforcing effect of graphene in enhancing the cryogenic tensile and impact properties of epoxy composites is examined at a weight fraction of 0.05–0.50%. The micro-structure and cryogenic mechanical properties of the graphene/epoxy composites are investigated using scanning electron microscopy, transmission electron microscopy, small-angle X-ray scattering and mechanical testing techniques. The results show that the graphene dispersion in the epoxy matrix is good at low contents while its aggregation takes place and becomes severer as its content increases. And the cryogenic tensile and impact strength at liquid nitrogen temperature (77 K) of the composites are effectively improved by the graphene addition at proper contents. The cryogenic Young’s modulus increases almost linearly with increasing the graphene content. Moreover, the results for the mechanical properties at room temperature (298 K) of the graphene/epoxy composites are also presented for the purpose of comparison.     

含氟对苯型酚醛环氧树脂的制备与性能研究

将三氟甲基引入到对苯型酚醛环氧树脂体系的分子结构中,制备了新型舍氟对苯型酚醛环氧树脂(FPE)和含氟对苯型酚醛树脂固化剂(FPN).系统研究了含氟基团对环氧树脂固化物的耐热性能、力学性能和阻燃性能的影响规律,并对由其制备的环氧塑封料的综合性能进行了评价.结果表明,以含氟对苯型酚醛环氧树脂体系FPE/FPN制备的塑封料不但具有突出的绝缘性能,同时表现出优异的本征阻燃性.     

Investigating the mechanical properties of single walled carbon nanotube reinforced epoxy composite through finite element modelling

Varying experimental results on the mechanical properties of carbon nanotube reinforced polymer composites (CNTRPs) have been reported due to the complexities associated with the characterization of material properties in nano-scale. Insight into the issues associated with CNTRPs may be brought through computational techniques time- and cost-effectively. In this study, finite element models are generated in which single walled carbon nanotube models are embedded into the epoxy resin. For modelling interface regions, two approaches named as non-bonded interactions and perfect bonding model are utilized and compared against each other. Representative volume finite element (RVE) models are built for a range of CNTRPs and employed for the evaluation of effects of diameter and chirality on the Young’s modulus and Poisson’s ratio of CNTRPs, for which there is a paucity in the literature. The outcomes of this study are in good agreement with those reported available in the literature earlier. The proposed modelling approach presents a valuable tool for determining other material properties of CNTRPs.     

酚醛氰酸酯/环氧共固化树脂的结构和性能

研究了具有不同摩尔比的酚醛型氰酸酯（NCE）与双酚A型环氧（E-51）共固化树脂的化学结构与物理性能．结果表明,在共固化树脂的结构中,主要存在着三嗪、噁唑啉酮、异氰脲酸、醚键等化学结构．只有当氰酸酯的摩尔数大于环氧时,随着共固化体系中-OCN摩尔数的提高,化学结构中三嗪环含量增大,共固化树脂的力学性能、玻璃化转变温度（Tg）、烧蚀残留率、吸水率随之提高;当-OCN／epoxy的摩尔比大于1时,固化树脂的Tg比摩尔比小于1时提高30℃以上,力学性能的变化具有相同的规律．     

Improvements in mechanical and thermo-mechanical properties of e-glass/epoxy composites using amino functionalized MWCNTs

The prime objective of this work is to optimize the mechanical and thermo-mechanical properties of e-glass/epoxy composites by utilizing amino-functionalized multi-walled carbon nanotubes (MWCNTs–NH₂) through a combination of dispersion method. At first, 0.1–0.4 wt.% of MWCNT–NH₂ was integrated into SC-15 epoxy suspension using a combination of ultra-sonication and calendaring techniques. E-glass/epoxy nanocomposites were than fabricated at elevated temperature with the modified resin using hand layup and compression hot press. 3-Point flexural and dynamic mechanical analysis (DMA) results demonstrated a linearly increasing trend in properties from 0 to 0.3 wt.% loading. Micrographs of MWCNTs incorporated epoxy and e-glass/epoxy samples revealed uniform dispersion of MWCNTs in epoxy, good interfacial adhesion between CNTs and polymer, and improved interfacial bonding between fiber/matrix at 0.3 wt.% loading. An improved dispersion and hence an improved crosslink interaction between MWCNT–NH₂ and epoxy lead to the stronger shift of the mechanical and thermo-mechanical properties of the composites.     

Improvements in mechanical properties of a carbon fiber epoxy composite using nanotube science and technology

Carbon fiber reinforced epoxy composite laminates, with strategically incorporated fluorine functionalized carbon nanotubes (f-CNTs) at 0.2, 0.3 and 0.5 weight percent (wt.%), are studied for improvements in tensile strength and stiffness and durability under both tension–tension (R = +0.1) and tension–compression (R = −0.1) cyclic loadings, and then compared to the neat (0.0 wt.% CNTs) composite laminate material. To develop the nanocomposite laminates, a spraying technology was used to deposit nanotubes on both sides of each four-harness satin weave carbon fiber fabric piece for the 12 ply laminate lay up. For these experimental studies the carbon fiber reinforced epoxy laminates were fabricated using a heated vacuum assisted resin transfer molding (H-VARTM®) method followed by a 2 soak curing cycle. The f-CNTs toughened the epoxy resin-fiber interfaces to mitigate the evolution of fiber/fabric-matrix interfacial cracking and delamination under both static and cyclic loadings. As a consequence, significant improvements in the mechanical properties of tensile strength, stiffness and resistance to failure due to cyclic loadings resulted for this carbon fiber reinforced epoxy composite laminate.     

二氯二苯基硅烷改性双酚A型环氧树脂性能研究

以二氯二苯基硅烷对双酚A型环氧树脂进行改性,通过拉伸实验、冲击实验、DSC、SEM研究了二氯二苯基硅烷的含量对环氧树脂E-51固化物性能的影响.结果表明:用5.9份的二氯二苯基硅烷改性时,树脂固化物的拉伸强度达到了67.85MPa,断裂伸长率达7.59%,冲击强度达13.38kJ/m2,环氧树脂的玻璃化转变温度达146.56℃.分别比未改性提高了18.9MPa、3.6%、5.5kJ/m2、28℃.     

环氧型有机硅季铵盐的合成及性能

合成了含有环氧基团的有机硅季铵盐二乙基-2,3-环氧丙基-[3-(甲基二甲氧基)]硅丙基氯化铵(DEEPSAC),研究了该化合物的表面性能及抗菌性能,表面张力的测定表明该化合物具有很好的表面活性,测得其临界胶束浓度为2.6mmol/L;平板计数实验表明DEEPSAC浓度为0.10%时对大肠杆菌的抑菌率达100%.动力学研究表明,DEEPSAC的合成反应并不符合通常季铵化反应的二级反应规律,反应级数和速率受温度及体系极性的影响,温度较低时呈现阶段反应的特点.     

MWCNTs对环氧树脂及多尺度MWCNTs-碳纤维/环氧树脂复合材料力学性能的影响

通过对胺基化多壁碳纳米管（MWCNTs-NH₂）进行改性,得到改性MWCNTs悬浮液（MWCNTs-NH₂（M））。分别将羧基化MWCNTs （MWCNTs-COOH）和MWCNTs-NH₂（M）分散在环氧树脂（EP）中,采用热熔法制备了多尺度MWCNTs-碳纤维（CF）/EP复合材料。研究了MWCNTs对EP模量、韧性及EP与CF之间界面黏结强度的影响,并分析了MWCNTs与CF上浆剂的作用,评价了多尺度MWCNTs-CF/EP复合材料的力学性能。结果表明：官能团化的MWCNTs可对EP的模量和韧性起到更好的增强作用。MWCNTs接枝的-COOH或-NH₂可与CF上浆剂中的环氧基团发生化学反应,提高EP与CF之间的界面剪切强度。MWCNTs-NH₂（M）对多尺度MWCNTs-CF/EP复合材料力学性能的增强效果优于MWCNTs-COOH,当MWCNTs-NH₂（M）的含量为1wt%时,多尺度复合材料的0°压缩强度、90°压缩强度、弯曲强度、弯曲模量、冲击后压缩强度（CAI）分别提高了16.7%、16.3%、40.9%、30.3%、20.6%。     

Fractography of unfilled and particulate-filled epoxy resins

The objective of this work was to analyse and understand the types of fracture surface morphology found in unfilled and particulate-filled epoxy resins in the light of the thermomechanical history of the specimen (loading rate or duration of loading, temperature, strain at break). Short-term tensile tests and long-term creep tests were conducted at four different temperatures. The fracture surface features were analysed using the scanning electron and optical microscopes and, where suitable, an image analyser. In order to correlate these morphologies with certain regimes of crack velocity, fracture mechanics tests were also conducted, varying the crack speed between 10^–7 and 10² m sec^–1. In the case of the filled resin, the lifetime under static loading is governed by a phase of slow, sub-critical crack growth which is manifested by resin-particle debonding. Thereafter, the crack accelerates and finally may reach terminal velocities depending on the amount of stored elastic energy available at the moment of fracture.     

Effect of DOPO units and of polydimethylsiloxane segments on the properties of epoxy resins

New flame retardant epoxy resins containing phosphorus and/or silicon atoms were prepared by the introduction into the epoxy chemical structure of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and/or of polydimethylsiloxane segments. The structure and morphology of cured epoxy resins were evaluated by Fourier transform infrared spectroscopy and scanning electron microscopy analyses, respectively. The polymers exhibited good thermal stability, having initial decomposition temperature above 310 °C. Thermogravimetric analyses and limiting oxygen index values indicated that the incorporation of DOPO units/polydimethylsiloxane segments in the polymer structure significantly enhanced flame retardancy and thermal stability of char layer at high temperature. Broadband dielectric spectroscopy measurements over a wide range of frequency and temperature revealed two subglass transitions, γ and β. The experimental data were fitted to the Havriliak–Negami and Arrhenius models, and the obtained parameters were analyzed. An α relaxation process attributed to the glass transition was observed in the high temperature and high frequency ranges. The influence of DOPO units and polydimethylsiloxane segments on the properties of the polymers was examined.     

Analytical modelling of mechanical properties for rubber-toughened polymers

Using a new quasi-sphere model, an analytical method is presented to determine the material properties and stress concentration factors of rubber-toughened polymers. The calculation is simple and the closed form results fit quite well with experimental data and various results using the finite element method. This simple and efficient method can be easily used to calculate a huge amount of results to fit the requirement of engineering. Also, this type of modeling can provide a better understanding of deformation mechanisms for development of this important composite material.     

Acrylonitrile-capped poly(propyleneimine) dendrimer curing agent for epoxy resins: Model-free isoconversional curing kinetics,thermal decomposition and mechanical properties

Acrylonitrile-modified aliphatic amine adducts are often used as curing agents for room-temperature epoxy formulations (coatings, adhesives, sealants, castings, etc.), yet the curing reaction and properties of resultant epoxy systems still remain less fundamentally understood. Herein we systematically investigate our newly-developed acrylonitrile-modified multifunctional polyamine curing agent for bisphenol A epoxy resin (DGEBA): an acrylonitrile-capped poly(propyleneimine) dendrimer (PAN4). The impact of the molecular structure of PAN4 and a controlled poly(propyleneimine) dendrimer (1.0GPPI) on the curing reactivity, reaction mechanisms, thermal stability, viscoelastic response and mechanical properties of the epoxy systems are highlighted. Differential scanning calorimetry (DSC) confirms DGEBA/PAN4 shows markedly lower reactivity and reaction exotherm than DGEBA/1.0GPPI, and the model-free isoconversional kinetic analysis reveals that DGEBA/PAN4 has the generally lower reaction activation energy. To be quantitative, the progress of the isothermal cure is predicted from the dynamic cure by using the Vyazovkin equation. The isothermal kinetic prediction shows that DGEBA/PAN4 requires about 10 times longer time to achieve the same conversion than DGEBA/1.0GPPI, which agrees with the experimentally observed much longer gel time of DGEBA/PAN4. Subsequently, dynamic mechanical analysis shows that PAN4 results in the cured epoxy network with the lower β- and glass-relaxation temperatures, crosslink density, relaxation activation energy, enthalpy, entropy, but the higher damping near room temperature than 1.0GPPI. Finally, thermogravimetric analysis (TGA) demonstrates cured DGEBA/PAN4 is thermally stable up to 200 °C, and mechanical property tests substantiate that PAN4 endows the cured epoxy with much higher impact and adhesion strengths than 1.0GPPI. Our data can provide a deeper insight into acrylonitrile-modified aliphatic amine curing agents from the two good model compounds (PAN4 and 1.0GPPI).