Dielectric cure determination of a thermosetting epoxy composite prepreg

Evaluation of degree of cure (DoC) of a glass reinforced epoxy composite prepreg used for manufacturing of printed circuit board (PCB) is an intensive issue because of its practical importance and cost reduction in industry. Typical techniques such as differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR) are destructive and require curing a material during a chosen time, quenching the sample to stop cure before performing analysis. Thus, it is necessary to remove the temperature influence on the determination of DoC. In this study, the feasibility of nondestructive dielectric sensing method as an in situ DoC measuring technique through cure monitoring of prepreg is presented, where a vacuum packing configuration has been established so as for the prepreg to evaluate accurately the DoC in a quenched state at an ambient temperature. The optimal curing condition to get the fully cured state of a prepreg material is determined by the dielectric cure monitoring based on the behavior of ion viscosity. The temperature effect compensated DoC of prepreg is correlated and compared with that evaluated by DSC and FTIR. The correlated DoC with ion viscosity has identified the curing behavior of prepreg by determining cure kinetic parameters. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44707.     

高密度聚乙烯导电复合材料介电性能的研究

用熔融共混和热压工艺制备了CB/HDPE,MWNT/HDPE聚合物基复合材料,研究了填料体积含量,测试电压,填料形貌尺寸对复合体系介电性能的影响.实验表明,当导电填料含量达到渗流阈值附近时复合材料的介电常数达到最大,测试电压达到一定值时,渗流阈值附近的复合材料介电损耗会迅速增加,相同填料体积含量的MWNT/HDPE复合体系比CB/HDPE体系具有更高的介电常数,利用渗流理论、Maxwell-Wagner界面极化效应和微电容模型解释了实验现象.     

Effect of cure temperature on the structure and water absorption of epoxy/amine thermosets

Dielectric and rheological measurements are reported on the effect of cure temperature on the water absorption of tris[(2,3-epoxypropoxy)phenyl] methane cured with a 1 : 1 stoichiometric ratio of 4,4′-diaminodiphenylsulphone. Analysis of the water absorption characteristics of these materials using a combination of dielectric and gravimetric measurements has indicated that water molecules can be found in two distinctly different types of environments. There are water molecules which are strongly interacting with polar groups and water molecules clustered together into sub-micro-scale cavities within the matrix structure. Changes in the final cure temperature have the effect of changing both the extent and distribution of the types of water molecules present in the matrix. Validation of the diffusion coefficients obtained from the dielectric analysis is based on a comparison with gravimetric data and the implications are discussed. Differences observed between these two different types of measurement are related to peculiarity in the dielectric method and its extreme sensitivity to interfacial phenomena.     

Characterization of model anaerobic adhesive cure using real-time fourier transform infrared spectroscopy and dielectric spectroscopy

A technique has been developed based on Fourier transform infrared spectroscopy (FTIR) in the attenuated total reflectance (ATR) mode, which is suitable for the investigation of the heterogeneous cure of surface-initiated redox polymerizations in thin and thick bondline situations. The results of the investigations into the surface-catalyzed and anaerobically promoted cure of some model adhesives using this method are reported. The systems under investigation were designed to exhibit different levels of cure heterogeneity or “cure through volume” (CTV) so that the influence of the bondline thickness and the formulation variables may be assessed. The analysis of the data provides evidence that supports our concept of the heterogeneous cure distribution existing in the form of a cure gradient. This concept of a cure gradient is developed further using dielectric spectroscopy. Here the cure gradient is characterized in terms of the parameters obtained using an empirical equation to fit the dielectric relaxation data. The results obtained using the infrared and dielectric spectroscopic methods are in satisfactory agreement and are shown to be most informative in characterizing and quantifying the CTV performance of the model anaerobic adhesives. © 1994 John Wiley & Sons, Inc.     

Dielectric Constants of SiO2 and RF Aerogels Measured by a Response Function Method

We measured the real and imaginary part of the dielectric constant of organic resorcinol-formaldehyde (RF)- and inorganic silica (SiO₂) aerogels. The samples were used as the capacitive part of a low pass filter. Values of were obtained from the response function of this filter in the frequency range from 1 Hz–50 kHz. The RF aerogels were made by polycondensation of resorcinol and formaldehyde, the SiO₂ aerogels via hydrolysis and polycondensation of tetramethoxysilane in water. The wet gels were supercritically dried in CO₂. Two density series of RF aerogels (density range from 380 up to 880 kg/m³) both differing in primary particle size as well as one density series of SiO₂ aerogels (3 samples, densities 120, 180 and 240 kg/m³) were investigated.The SiO₂ aerogels showed no dielectric losses, the relative dielectric constant did not depend on frequency. Evaluation of as a function of density/porosity showed that the data lie in between the theoretical limits, i.e. the serial and parallel connection of the two constituents, the SiO₂ backbone and air.The impedance spectra of RF aerogels are dominated by losses due to relaxation processes. The intensity of these losses and the frequency of maximal loss (_loss) are correlated to the amount of water adsorbed on the inner surface of the samples. Via the so called Maxwell-Wagner polarisation we were able to attribute these losses to adsorbed water layers, differing in size and extent.     

Dielectric studies of water absorption and desorption in epoxy resins: influence of cure process on behaviour

Little is known about the way in which the chemical structure of an epoxy resin influences its ability to absorb and desorb moisture. This issue is addressed in a study of dicyandiamide‐ and amine‐cured epoxy resins. The dicyandiamide‐cured material will have a significantly lower preponderance of pendant hydroxyl groups than the amine‐cured material and may exhibit different behaviour when exposed to moisture. The uptake and loss of moisture was monitored gravimetrically, using broad band dielectric, dynamic mechanical thermal analysis and thickness measurements performed as a function of time at various temperatures. A comparison of the uptake and loss profiles for the first and subsequent cycles indicated significant differences in behaviour attributed to the way in which water can plasticise the matrix. Stresses frozen into the matrix during the cure process are allowed to relax as a consequence of the water hydrating the matrix and create voids and also allow matrix densification. These processes occur during the first hydration cycle and are not reversible. Subsequent hydration and dehydration appear to be reversible after the first hydration cycle. Water in the polymer is distributed between free water which is to be found in microvoids and bound water which is attached to the polymer chain. The amine‐cured epoxy resin which contains pendant hydroxyl groups has a greater capacity for water absorption than the ether‐containing backbone of the dicyandiamide‐cured material. Copyright © 2008 Society of Chemical Industry     

Dielectric spectroscopy and dissolution studies of bioactive glasses

The first and rate-limiting step in the degradation of bioactive glasses is thought to be the ion exchange of hydrated protons in the external fluid with alkali metal cations in the glass. The activation energy (E_a) for alkali ion hopping can be followed by dielectric spectroscopy. The replacement of CaO by Na₂O resulted in a reduction in the E_a for ion hopping. In contrast, increasing the glass network connectivity or reducing the nonbridging oxygen content of the glass resulted in an increase in E_a. Substitution of K₂O for Na₂O had little influence on E_a. Mixing alkali metals increased the E_a as expected on the basis of the mixed alkali effect. There was no correlation between the E_a for ion hopping and the dissolution behavior of the glass. Furthermore, the activation energy for Si, Ca Na, and K ion release was found to be approximately a factor of three lower than that for ion hopping suggesting that another rate-controlling mechanism is important in the degradation of bioactive glasses. The presence of a second relaxation process suggested that bioactive glasses undergo amorphous phase separation into silica-rich and orthophosphate-rich phases and the two relaxation processes are due to ion hopping in the two phases.     

Dielectric dynamics of some nylon 6/CaCO3 composites using broadband dielectric spectroscopy

Broadband dielectric spectroscopy (10^?1 to 10⁶ Hz) in combination with TSDC were employed to study the relaxations and the conductivity phenomena in polyamide 6/CaCO₃ composites loaded with different contents and particle sizes of treated and untreated CaCO₃. In addition to the primary α‐relaxation associated with the glass transition, significant interfacial relaxation and ionic conduction processes have been revealed. The temperature dependence of the relaxation rates of α‐relaxation and Maxwell–Wagner–Siller‐process as well as that of the hopping rate of the conduction mechanism were found to follow the Vogel–Fulcher–Tammann‐model. This may support the idea that the three mechanisms are not completely independent. The modification of the filler does not add any effect on the resistivity values. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Dielectric relaxation spectroscopy of poly(ethylene terephthalate)

Contour maps of dielectric loss tangent within the ranges 0.1 Hz to 3 MHz and ?175 °C to +190 °C are presented for a commercial poly(ethylene terephthalate) (PET) in two initial states of crystallinity. Individual absorption regions resemble those for poly(butylene terephthalate) and are attributed to carbonyl‐driven α‐ and β‐relaxation processes and to Maxwell–Wagner–Sillars polarizations. Possible causes are considered for the asymmetry and structure apparent in the α‐peak of partially crystalline PET. © 2001 Society of Chemical Industry     

Etherification versus amine addition during epoxy resin/amine cure: An in situ study using near-infrared spectroscopy

The reactions between a multifunctional epoxy resin, tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) and a monofunctional amine, methylaniline (mAnil) are studied. Due to the existence of a tertiary amine catalytic center within the TGDDM molecule, the etherification reaction during cure of TGDDM is usually more significant than in other epoxide systems. The importance of this reaction relative to the amine addition reactions is investigated. In situ near-infrared spectroscopy is used to obtain kinetic data during the cure reactions. The reaction rate constants are calculated from linear regression analysis for both amine addition and etherification reactions based on the reaction mechanisms proposed. Arrhenius relationships are observed for all the reaction rate constants involved. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:895–901, 1998     

聚氨酯弹性体介电性能的研究

通过对比不同种类、不同相对分子质量多元醇以及不同异氰酸酯,研究了聚氨酯弹性体(PUE)结构与材料介电性能(体积电阻率、介电常数、介电损耗)的关系。结果表明,当多元醇种类变化时,聚氧化丙烯二醇(PPG)型PUE介电损耗最大;聚四氢呋喃二醇(PTMEG)型PUE、介电常数、介电损耗都最小;聚己内脂二醇(PCL)型PUE体积电阻率和介电常数最大。当多元醇相对分子质量增加时,PUE体积电阻率减小,介电常数和介电损耗增加;二苯基甲烷二异氰酸酯/1,4-二羟基丁烷(MDI/BDO)型PUE与甲苯二异氰酸酯/3,3’-二氯-4,4’-二氨基二苯甲烷(TDI/MOCA)型PUE相比,体积电阻率更低、介电常数和介电损耗更高。     

J-208厌氧型螺纹锁固密封胶的研制

介绍了J-208厌氧型螺纹锁固密封胶的组成和性能,该胶可用于多种金属表面包括纯铝、不锈钢、锌、镁等非活性表面试件的粘接,耐高低温、耐湿热、耐盐雾老化性能优异。     

Dielectric spectroscopy of conductive polyaniline salt films

Dielectric responses of polyaniline (PANI) salt films, made from two different methods, were compared to investigate the effect of a dopant and film-formation methods on the dielectric properties. One of the film-formation methods for emeraldine salt was the film-doping method, and the other, the solution-doping method. The conductivity relaxation and dielectric properties were measured by a microdielectrometer at 293 K in the frequency range of 1–100,000 Hz. Dielectric spectra were a function of the dopant in the case of the film-doping method, whereas it was a function of solvent–dopant combinations as well as the dopants in the solution-doping method. The dielectric responses of film-doped samples had similar patterns irrespective of the dopants, while those of solution-doped samples were varied, probably due to different conjugation conditions or/and the conduction mechanism. In the case of film-doped samples, dielectric loss and permittivity values were increased with decreasing dopant sizes. The dielectric relaxation times were also varied depending on the kind of dopants. The different solvent–dopant combinations led to variation of the dielectric responses in the solution-doped PANI due to an altered chain structure. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2760–2769, 2001     

Kinetic study of phenolic resin cure by IR spectroscopy

The cure reaction of a commercial phenolic resin (FRD‐5002, Borden Chimie S.A.) has been studied by IR spectroscopy (Fourier transform IR). A phenomenological approach was used to characterize the kinetic of reaction. Various kinetic models, including homogeneous reactions, diffusion‐controlled reactions, phase boundary movement, and nucleation and growth‐type kinetic, have been tested. Kinetic analyses using integral procedures on isothermal data indicate that the cure reaction data can be described above 140°C using the homogeneous first‐order reaction model. The activation energy has been found to be about 49.6 kJ mol⁻¹. At lower temperature, a diffusive mechanism was active, and the kinetic of reaction was well described by the Jander kinetic model. Because of the simple kinetic control active above 140°C, a mechanistic model for the resol cure at these temperatures has been also proposed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2703–2715, 1999     

后固化具有膨胀性的耐高温厌氧胶粘剂的研究

对N,N-间苯双马来酰亚胺、聚酰亚胺、甲基丙烯酸金属盐等对厌氧胶耐温性的影响进行了研究,研制出一种耐温230℃、后固化具有膨胀性的厌氧胶,该胶由乙氧化双酚A二甲基丙烯酸酯、N,N-间苯双马来酰亚胺及引发剂、促进剂、稳定剂等组成,该胶粘剂在第一固化温度梯度(常温)部分固化,在第二固化温度梯度(180℃)后固化时,N,N-间苯双马来酰亚胺与主体交联在一起,2小时后体积膨胀约0.2%,形成热固性材料,可耐温230℃。     

Dielectric spectroscopy and relaxation phenomena of moistened and dry polysaccharides

The effect of hydration on the dielectric constant and loss factor tan δ of viscose hydrocellulose, hydroxypropyl cellulose, and dextran was studied in a wide range of temperatures and frequencies. The results obtained reveal that (1) whereas hydration shows little effect on the variation with temperature of both and tan δ of hydrocellulose, the presence of adsorbed water significantly modifies the dielectric behavior of the other two samples; (2) only one relaxation denoted as γ‐process is observed in the case of hydrocellulose sample irrespective of its moisture content; (3) two absorption peaks are recorded with moistened hydroxypropyl cellulose and dextran specimens, one below room temperature denoted as β‐process and a dispersion loss at high temperature; and (4) after drying of these samples, the dispersion loss peak disappears, the β‐peak diminishes, and the γ‐peak, which is hidden behind the sizable β‐peak, is observed and that, although with increasing frequency, the former peak disappears, the intensity of the latter one increases. The analysis of these results in view of the chemical and the fine structure of the test samples as well as the calculated activation energy indicates that the γ‐ and β‐relaxations recorded are of different natures. The satisfactory interpretation given clarifies to a great extent the contradictions reported in the literature concerning the relaxation phenomena in polysaccharides. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 571–582, 2005     

Cure kinetics,glass transition temperature development,and dielectric spectroscopy of a low temperature cure epoxy/amine system

This article reports a study of the chemical cure kinetics and the development of glass transition temperature of a low temperature (40°C) curing epoxy system (MY 750/HY 5922). Differential scanning calorimetry, temperature modulated differential scanning calorimetry, and dielectric spectroscopy were utilized to characterize the curing reaction and the development of the cross‐linking network. A phenomenological model based on a double autocatalytic chemical kinetics expression was developed to simulate the cure kinetics behavior of the system, while the dependence of the glass transition temperature on the degree of cure was found to be described adequately by the Di Benedetto equation. The resulting cure kinetics showed good agreement with the experimental data under both dynamic and isothermal heating conditions with an average error in reaction rate of less than 2 × 10^?3 min^?1. A comparison of the dielectric response of the resin with cure kinetics showed a close correspondence between the imaginary impedance maximum and the calorimetric progress of reaction. Thus, it is demonstrated that cure kinetics modeling and monitoring procedures developed for aerospace grade epoxies are fully applicable to the study of low temperature curing epoxy resins. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

聚苯胺包覆二氧化硅电流变液的介电弛豫谱研究

低温包覆聚苯胺的方法对球形二氧化硅粒子进行了表面改性,利用介电弛豫谱分析了聚苯胺包覆后的二氧化硅电流变液介电性质的变化。结果表明包覆聚苯胺后,电流变液的介电增量显著提高,弛豫频率向高频移动。两种变化均对体系的电流变效果起到积极的作用。另外还发现可以通过调整碱液的pH值改变电流变液的弛豫频率,从而提高改变电流变的电场响应能力。     

聚偏氟乙烯的宽频介电谱特征研究

采用熔融法制备了聚偏氟乙烯（PVDF）样品,通过X射线衍射仪、傅里叶变换红外光谱仪和宽频介电谱分析仪对PVDF样品的结构及介电性能进行了测试分析。结果表明,熔融法获得的PVDF主要含α相;在40～100℃范围内出现由玻璃化转变引起的α-PVDF的松弛现象, 在-100～-50 ℃范围内出现的δ-PVDF松弛,是由非晶区分子运动产生的极化现象。     

Orientation Polarization Spectroscopy—Toward an Atomistic Understanding of Dielectric Relaxation Processes

The theory of orientation polarization and dielectric relaxation was developed by P. Debye more than 100 years ago. It is based on approximating a molecule by a sphere having one or more dipole moments. By that the detailed intra- and intermolecular interactions are explicitly not taken into consideration. In this article, the principal limitations of the Debye approximation are discussed. Taking advantage of the molecular specificity of the infrared (IR) spectral range, measurements of the specific IR absorption of the stretching vibration υ(OH) (at 3370 cm⁻¹) and the asymmetric υ_as(CH₂) (at 2862.9 cm⁻¹) are performed in dependence on the frequency and the strength of external electric fields and at varying temperature. The observed effects are interpreted as caused by orientation polarization of the OH and the adjacent CH₂ moieties.