Effect of nanoclay and carboxyl-terminated (butadiene-co-acrylonitrile) (CTBN) rubber on the reaction induced phase separation and cure kinetics of an epoxy/cyclic anhydride system

The effects of nano clay, carboxyl-terminated (butadiene-co-acrylonitrile) (CTBN) liquid rubber and the combination of both on the cure kinetics of diglycidyl ether of bisphenol-A (DGEBA)-based epoxy resin/nadic methyl anhydride were studied. Cure kinetics studies were carried out by performing dynamic and isothermal differential scanning calorimetric (DSC) experiments. The dynamic DSC experiments were carried out at four different heating rates. Dynamic kinetic modeling was performed using Kissinger and Ozawa approaches. Since these methods are based exclusively on the maximum rate of cure, which occurs approximately at the beginning of the cure reaction, the activation energy calculated using these methods is valid only for the initial stage of the cure. The clay (3 phr) filled epoxy system has an activation energy 24 % lower than the unfilled system. The role of the surfactant chemistry on the initial stage of the cure reaction was also studied. A plausible reaction mechanism which involves the effect of the nanoclay surfactant as an accelerator of the cure reaction was proposed. The phase separated CTBN rubber hindered the cure reaction and has 3 % higher activation energy for epoxy/CTBN system than the unfilled system. In the ternary epoxy/3 phr clay/15 phr CTBN system, the accelerating effect of clay on cure was highlighted. The cure activation observed in the presence of clay overshadows the hindrance created by the phase separated CTBN. Isothermal DSC scans were carried out at five different temperatures. The experimental datas showed an autocatalytic behavior of the reaction, and the isothermal modeling was carried out by Kamal autocatalytic model. The results showed a very good agreement within the whole conversion range for the unfilled and all the filled systems. The evolution of the morphology and phase separation was also studied using optical and scanning electron microscope. Faster cure reaction resulted in smaller phase-separated CTBN particles in epoxy/clay/CTBN ternary system as compared with those observed in epoxy/CTBN binary blend.     

Fiber-optic epoxy composite cure sensor. I. Dependence of refractive index of an autocatalytic reaction epoxy system at 850 nm on temperature and extent of cure

We discuss the behavior of the refractive index of a typical epoxy-aromatic diamine system. Near 850 nm the index of refraction is found to be largely controlled by the density of the epoxy. Models are derived to describe its dependence on temperature and extent of cure. Within the range of temperatures studied, the refractive index decreases linearly with increasing temperature. In addition, as the epoxy is cured, the refractive index increases linearly with conversion to the gel point. From then on, shrinkage in the volume of the epoxy is restricted by local viscosity. Therefore the linear relationship between the refractive index and the extent of cure does not hold beyond the gel point.     

1,3,4-Oxadiazole epoxy resin-based liquid crystalline thermosets and their cure kinetics

Liquid crystalline thermosets were synthesized based on a difunctional liquid–crystal (LC) epoxy resin monomer, namely 2,5-bis(4-glycidyloxyphenyl)-1,3,4-oxadiazole with various tetrafunctional crosslinkers such as diaminodiphenyl methane, diaminodiphenyl sulfone, and diaminodiphenyl ether. The epoxy monomer was characterized by infrared and nuclear magnetic resonance spectroscopy. Cure kinetics of a stoichiometric mixture of epoxy monomer and diaminodiphenyl methane was investigated by differential scanning calorimetry (DSC) for specimens cured under various cure conditions. The nematic LC texture for the cured specimen was identified by polarized microscopy and confirmed by X-ray diffractometry. Phase diagram of cure time versus transition temperature was constructed based on the DSC data for epoxy/DDM system. The diagram displayed the changes of melting transition, isotropic transition, and glass transition temperatures as curing proceeds.     

Effect of bentonite on the structure and mechanical properties of CE/CTBN system

Cyanate ester (CE) resins are among the most important engineering thermosetting polymers and have received attention because of their outstanding physical properties. However, their main drawback is brittleness. Toughening by rubbers is a notable way to improve the toughness but unfortunately it decreases the modulus and thermostability. The CE/carboxyl terminal butadiene-acrylonitrile (CTBN)/bentonite (BT, a kind of clay) composites were prepared by two-step melt blending of CE, CTBN and amino-modified bentonite. The nanostructure and mechanical properties of the CE/CTBN/BT composites were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and dynamic mechanical analysis (DMA). Both of the intercalated and exfoliated structures of BT existed in CE/CTBN/BT composites while rubber particles also could be observed in CE/CTBN/BT composites by TEM. When an appropriate amount (about 0.5 wt.%) of BT added into CE/CTBN (100/10 by weight) system, both of the modulus and impact strength increased. A 30% increase of the modulus was obtained without greatly sacrificing the impact strength of the composite when the BT content was 1 wt.%.     

Effect of loading rate on the creep behaviour of epoxy resin insulators by nanoindentation

In the present work, the effect of loading rate on indentation creep was studied. Indentation creep tests were conducted on epoxy resin to provide creep deformation under constant load, contact creep compliance and cut-off time using a Berkovich indenter. Several loading rates, ranging from 0.25 to 6 mN/s, were used to perform the tests. The results showed that there is a strong loading rate dependence on creep response of the epoxy resin under indentation. Contact creep compliance and cut-off time decreased with increasing loading rate. In contrast, an increase in reduced modulus, hardness, displacement variation and contact creep compliance variation during the holding time was noticed. The loading rate sensitivity on creep response under indentation can be attributed to viscoelastic response prior to holding segment and strain rate effect on yield stress of the epoxy resin. This study provided an insight to understand the loading rate dependence on creep behaviour of epoxy resin under indentation.     

超声振动对快速固化环氧树脂体系固化动力学特性的影响

通过非等温差示扫描量热法,结合黏度测试和傅里叶红外光谱分析,研究了不同超声波振动条件下环氧树脂体系的固化特性。基于Flynn-Wall-Ozawa/FWO、Kissinger-Akahira-Sunose/KAS和Boswell积分型动力学模型,计算了不同超声波振动下环氧树脂体系的活化能。结合Malek最大概然函数法,得到了超声振动下树脂体系的固化反应动力学方程,并与实测固化度对比进行了验证。研究表明,超声振动振幅越大,树脂体系黏度降低越明显,较小的超声波振幅振动下树脂体系活化能增大,而振幅增大后活化能有明显的降低。固化物的红外光谱分析表明,随着超声振幅的增大,羟基吸收峰减弱,表明超声效应加速了胺基加成反应或者羟基醚化反应。超声振动条件下的树脂固化反应模型符合自催化模型形式,但超声振动并不能改变树脂体系的固化反应机制。以上研究结果对设计和优化碳纤维增强树脂复合材料超声振动辅助树脂传递模塑成型(RTM)工艺具有一定的指导意义。     

环氧树脂灌封结构固化行为数值模拟和工艺优化

依据实验测试获得E51环氧树脂的固化动力学和力学性能参数,针对特定尺寸的E51树脂灌封结构,将数值模拟与实验方法相结合,优化树脂的常温固化工艺为“二阶段”中高温固化工艺。首先采用数值模拟方法,分别比较第一段保温平台和第二段保温平台不同温度幅值和固化时间对结构内部的温度、固化度和应变的影响,优选固化工艺参数;然后基于光纤布拉格光栅(FBG)监测技术,对优选和原始固化工艺曲线的结构内部的固化温度和应变进行实时在线监测,结果表明了数值模拟的可行性,显示了通过数值仿真优选固化工艺曲线的可靠性;最后实验测试比较优选工艺和原始工艺曲线下制造的E51树脂浇注体性能,结果显示优选工艺制造的树脂浇注体的拉伸强度、压缩屈服强度、弯曲强度和冲击强度相比原始工艺制作的试样分别提高了3.9%、1.5%、14.5%和16.2%。      

尿素对UF树脂固化动力学影响的研究

为了研究尿素对弱酸性起始合成的UF树脂固化性能影响,采用DSC研究了弱酸性起始合成UF树脂中碱性加成阶段尿素添加次数和间隔时间对树脂固化反应动力学影响.计算了碱性加成阶段尿素添加次数和间隔时间不同的UF树脂的固化表观活化能和反应级数,并建立相应的固化反应动力学方程.研究结果表明:弱酸性起始合成UF树脂,碱性阶段尿素添加次数和添加间隔时间影响树脂固化,尿素添加次数增加和尿素添加次数相同而添加尿素时间间隔延长能降低UF树脂固化体系的活化能.弱酸性起始合成UF树脂的固化反应是介于0.92～0.93级之间的非基元反应.     

木质素基环氧树脂固化动力学模型及固化特征

采用工业碱木质素合成环氧树脂,在空气气氛下利用非等温热失重技术研究木质素基环氧树脂(LGEP)固化特征.采用自催化反应模型计算得到了LGEP体系的固化反应动力学参数,并得到LGEP体系的固化反应动力学模型.结果表明,自催化反应模型得到的模拟曲线与实验得到的DSC曲线的一致性较好.利用外推法得到了LGEP体系的固化凝胶温度Ti0 =454.88K,固化温度Tp0=507.55K,后处理温度T80=598.77K.通过比较得出实验结果与模型计算值较一致.     

Study of the cure kinetics of structural adhesives by ultrasonic interface waves

An ultrasonic method for in situ study of the curing kinetics of structural adhesives is described. The measurements are performed by ultrasonic interface waves, which produce shear oscillations in the plane of the bond line. The shear resistance of the interface changes in the curing process: softening of the prepreg and its transition to the liquid phase, and the beginning and the termination of the polymerization reaction. Measurements performed at temperatures from 60 to 120° C for the FM-73 adhesive showed that the activation energy of the curing reaction is 38.9 kJ mol⁻¹. On sabbatical leave at the Department of Welding Engineering, Ohio State University, 190 W, 19th Avenue, Columbus, Ohio 43210-1182, USA.     

The development of a low temperature cure modified epoxy resin system for aerospace composites

It has been recognised within the composites industry that there are economic advantages in moving away from a conventional autoclave cure at high temperature. A low temperature cure (LTC) resin system has been developed that can be cured at 85 °C initially then subsequently further cured at 175 °C, for a short period. For such resins there is a requirement to achieve the same toughness properties as conventional thermoplastic toughened high temperature cured (HTC) material. The development of an LTC resin system has required a special catalyst systems and appropriate choice of a poly(ethersulphone) co-polymer, in terms of molecular structure, molecular weight and chain ends. Toughness development in thermoset–thermoplastic blends requires an appropriate development of phase morphology during cure. Morphology control has been achieved in LTC materials. Application of linear elastic fracture mechanics measurements of toughness in association with transmission electron microscopy and scanning electron microscopy of the fracture surface has demonstrated links between the toughness and the phase morphology and the chemistry. Small angle neutron scattering has enabled the development of the phase morphology during the low temperature curing process to be monitored.The presence of phase morphology in the resin means that the influence of phase or interface boundaries on the environmental stress cracking (ESC) properties might be a critical issue. This issue has been addressed and investigated for three environments, namely air, demineralised water and dichloromethane at 23 °C. The LTC material is shown to perform in a similar manner to a comparable conventional HTC resin system.     

603环氧树脂体系固化动力学模型的建立与验证

采用非等温差示扫描量热法（DSC）研究了603热塑增韧环氧树脂体系的固化反应动力学。研究发现,在低升温速率测试条件下603环氧树脂体系固化反应的DSC曲线有两个重叠的放热峰,通过分离两个重叠的放热峰,研究了603环氧树脂体系固化动力学的特性。利用Kissinger方法和Kamal方程分别拟合得到603树脂体系固化反应的活化能和固化动力学参数,选择三种典型固化工艺制度下预测的树脂固化反应结果与实验数据对比,验证了所建立动力学模型的可靠性。基于不同升温速率的放热曲线,通过外推法得出该树脂占总反应比例70%的第一个反应固化温度为（177.3±2.2）℃,占总反应比例30%的第二个反应的起始温度和固化温度分别为（178.6±0.7）℃和（216.9±1.7）℃。研究结果对于多组分热固性树脂体系固化动力学的分析和复合材料成型工艺的优化具有重要的指导意义。     

Effect of carbon nanofibers on the cure kinetics of unsaturated polyester resin: Thermal and chemorheological modelling

We report the study of the effects of carbon nanofibers (CNFs) on the cure kinetics and on the chemorheology of unsaturated polyester resins (UP). Two main experimental techniques were utilized: differential scanning calorimetry and rheometry. Isothermal and dynamic tests were performed on the neat polyester resin and on two nanocomposite systems, with 0.5 and 1.0 wt.% of CNFs. Furthermore, a TGA study of the selected systems at different isothermal temperatures was performed to evaluate the styrene consumption and related loss rate during the curing process. Subsequently, a phenomenological model (autocatalytic approach) was applied to the experimental data, both in isothermal and dynamic conditions. In the case of dynamic curing, the evidence of multiple peaks in the heat flow curves was studied through the deconvolution of the overall reaction thermogram in single reaction steps. An empirical rheological model coupled with the reaction kinetics was successfully applied to simulate the viscosity changes of the UP matrix. Experimental and modelling results demonstrate the presence of a delay effect of the CNFs on the cure of the UP resin matrix.     

CTBN改性双酚A型氰酸酯树脂的性能

为改善氰酸酯树脂的冲击韧性,在双酚A型二氰酸酯(BADCy)树脂中混入了不同含量的端羧基丁腈橡胶(CTBN).用差示扫描量热法(DSC)及红外光谱法(FTIR)对CTNB/BADCy共混体系的反应性研究发现,CTBN能促进BADCy低温下的三嗪环反应,但是使BADCy的后处理温度提高.SEM分析表明,当CTBN的含量(质量分数)大于15%后,有CTBN颗粒从BADCy中析出,形成两相结构,且随着CTBN含量的增大,分散相的粒径增大,在断口处产生的银纹和剪切带使BADCy/CTBN共混体系的韧性提高.当CTBN的含量为25%时体系的冲击强度提高了2.3倍.     

Effect of strain rate on interlaminar shear properties of carbon/epoxy composites

In this paper, the effect of strain rate on interlaminar shear properties of laminates is studied. The material tested was a T300/5208 carbon/epoxy composite, and the range of strain rates explored was about 10⁻³ − 10³ s⁻¹. The specimens used were designed and optimized by finite element analysis, and the calculations are presented here. One of the specimens permitted the determination of the interlaminar shear modulus, G₁₃, and the other permitted the determination of the interlaminar shear strength, S₁₃. No influence of testing speed on interlaminar properties was observed at low, intermediate and high strain rates. Fracture surfaces were studied by scanning electron microscopy: a slight difference was observed between specimens tested at low and high strain rates.     

一种灌封结构用环氧树脂的固化行为表征和模拟

以中高温固化的E39D环氧树脂为研究对象,基于顺序耦合热传导-固化和应力位移模块的数值仿真方法,选择合适的实验方法测试环氧树脂的固化性能,引入相关假设,推导与热传导-固化和应力位移模块相关的树脂固化性能参数和模型;然后,建立典型E39D树脂灌封结构的数值模型,模拟结构内部观测点在固化过程中的温度和应力演变,并基于FBG...