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.     

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.     

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.     

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.     

Residual stresses and the optimum cure cycle for an epoxy resin

Residual stresses in carbon fibre-reinforced plastic composites arise because of the large difference in thermal expansion of the fibres and matrix. It has been suggested that these stresses might be reduced in epoxy resin matrices by curing at a lower temperature. Experiments have been made on five curing cycles, to test this possibility and to study the variations of viscosity and volume changes of the resin under different cure conditions. DSC experiments have also been made to determine the degree of cure at various stages of the cycles. It was found that a cycle with the low peak temperature of 120°C required a prohibitively long length of time to produce a complete cure. Of the other cycles studied, it is predicted that it would be advantageous, in reducing residual stresses, to use a peak temperature of 160°C, rather than 175°C, provided that a dwell of 18 min is included at 160°C to ensure a complete cure.     

阳离子引发剂在环氧树脂固化体系中的应用研究

讨论了阳离子引发剂催化环氧树脂固化体系反应的原理,综述了新型阳离子引发剂,即镧系金属氟化物在DGEBA、DGEBA/γ-BL、DGEBA/MCB、DGEBA/MA、DGEBA/PC体系中的作用。浅析了固化促进剂的选用原则,并对镧系金属氟化物在环氧树脂/聚酰胺中的应用前景作了展望。     

The effect of cure on thermally stimulated discharge measurements in epoxy resin coatings

Thermally stimulated discharge (TSD) measurements are reported as a function of the degree of cure for a series of dicyandiamide cured powder epoxy resins. Data are presented on the effects of ageing and water uptake on the TSD traces. The observed effects are interpretated in terms of the generation of traps, creation of voids and changes in the bulk conductivity on the charge mobility. The TSD data are compared with dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) observations on these systems. These studies indicate that the method is very sensitive to changes in the state of the coating and has potential as a test method for the characterization of epoxy resin coatings.     

Effect of carboxyl-terminated poly(butadiene-co-acrylonitrile) (CTBN) concentration on thermal and mechanical properties of binary blends of diglycidyl ether of bisphenol-A (DGEBA) epoxy resin

Six blend samples were prepared by physical mixing of epoxy resin with varying concentrations of liquid carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer having 27% acrylonitrile content. The blend samples were cured with aromatic amine. A comparative study of Fourier-transform infrared (FTIR) spectra showed the modification as a result of chemical reactions between epoxide group, curing agent and CTBN. The tensile strength of cured blend samples decreased slightly from 11 to 46% where as the elongation-at-break showed an increasing trend with increasing rubber content, i.e., up to 25 phr, in the blend samples. Appreciable improvements in impact strength were also observed in the prepared blend systems. The glass transition temperature (T_g) of the epoxy resin matrix was slightly reduced on the addition of CTBN. The cured resin showed a two-phase morphology where the spherical rubber domains were dispersed in the epoxy matrix.     

The β-relaxation in epoxy resins; the temperature and time-dependence of cure

Internal friction and creep measurements have been used to reveal the mechanism of cure in epoxy resins cross-linked with diethylene triamine (DETA). The -relaxation is associated with the main glass transition of the undercured resin network. The glass transition temperature (T _g) is about 40° C above the maximum temperature of cure and the curing reaction slows down about 2 h after each increase of the temperature. At 25° C the cure is only about half complete and since in this resin, when fully cross-linked, T _g is at about 140° C, temperatures of 100° C or over are needed to complete cure.     

Advanced cure monitoring by optoelectronic multifunction sensing system

In this work, a dual functionality fiber optic sensing system for thermoset cure monitoring has been developed and successfully tested. Principle of operation relies on the use of a standard optical fiber in single ended configuration containing a single in-fiber Bragg grating. The change in refractive index directly related to material density at fiber end and the variation in the wavelength reflected by the fiber grating have been monitored simultaneously, throughout an In-Mould curing process of an epoxy resin. A compact and low cost in-fiber dual wavelength demodulation system has been developed and experimental results demonstrate the capability to on line identify the degree of cure, gel point, residual stresses and the glass transition temperature of the cured resin.