ISSN:0032-3888    

DOI:10.1002/pen.760260502

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractDynamic dielectric analysis (DDA) has been used to study curing polymer systems and thermoplastics. Measurements have been made over a frequency range of six decades. This wide range of frequencies increases the amount of information which can be obtained. The data is analyzed in terms of the frequency dependence of the complex permittivity ε*, specific conductivity σ(ohm−1cm−1), and the relaxation time τ, parameters which are characteristic of the cure state of the material and independent of the size of the sample. Dynamic dielectric measurements have been used to monitor polymer processing in UDEL‐P1700, LARC‐160, polyphenyl quinoxaline (PPQ), and Epon 828 cured with Agent U. Dynamic dielectric measurements have been correlated with viscosity for the polysulfone thermoplastic UDEL‐P1700 and with viscosity and ultrasonic measurements on the DGEBA type epoxy Epon 828 cured with Agent U. The experimental results suggest that when ionic processes dominate the dielectric response, the intensive property σ is a good monitor of the resin's viscosity. The results show that the dielectric relaxation time τ can be used to monitor the state of the system and the extent and rate of the reaction. Solvent evolution can also be e

ISSN:0032-3888    

DOI:10.1002/pen.760260503

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractDielectric data from an epoxy resin system were used as the foundation for dielectric modeling of the curing process. This resin system (DGEBA‐polyamide) was chosen as an easily processible model system. Dielectric average relaxation times, defined as the reciprocal of the angular frequency at which the loss component of the dielectric constant reaches a maximum, were determined for a 40°C isothermal cure. The changes in the average relaxation time through the cure exhibited similar behavior to those for viscosity, which inspired the correlation of the two properties. The dielectric relaxation time was modeled using a six‐parameter model analogous to that used for viscosity. The model parameters were in turn associated with both intrinsic properties of the system and reaction kinetics describing the cure. The possibility of extending the relaxation time model for use with single‐frequency data by means of a time‐frequency correlation was also investigated. Combined, these two modeling methodologies provide a powerful constitutive approach for describing dielectric properties of thermosetting polymers dur

ISSN:0032-3888    

DOI:10.1002/pen.760260504

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractThe isothermal cure of a diglycidyl ether of Bisphenol‐A (DGEBA) epoxy resin with diaminodiphenylsulfone has been characterized by microdielectrometry and differential scanning calorimetry. The cure temperatures ranged from 410 to 460K. The behavior of the relaxed (or static) dielectric permittivity vs. cure time and temperature was determined from the microdielectrometry data. The DSC data was fit to an autocatalyzed reaction kinetics model, which was then used to predict reactive group concentrations as a function of cure time and temperature. The temperature dependence of the relaxed permittivity at constant chemical conversion was examined in the context of the Onsager theory for the relaxed permittivity of a system of independent dipoles. This analysis indicated that the dipoles in the resin are not independent, as assumed by the Onsager theory, and that the behavior is similar to that observed in polyethers. An empirical modification to the Onsager theory was used in conjunction with the kinetic model to estimate dipole moments for the epoxide, primary amine, and reacted (secondary and tertiary) amine groups. The relative and absolute values of the dipole moments were in good agreement with estimates based on the structures, leading to the conclusion that the observed decrease of the relaxed permittivity during cure of this epoxy/amine system is due to the changing concentrations of polar reactive group

ISSN:0032-3888    

DOI:10.1002/pen.760260505

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractWe report the use of microdielectrometry for monitoring the cure of the matrix resins in epoxy‐glass and epoxy‐graphite composites. The multi‐ply laminates are cured in a press using ramped temperatures. The microdielectrometer sensor is embedded in a cavity made by cutting a hole in each of the inner plies. For a brominated‐epoxy glass‐reinforced prepreg ramped to final cure temperatures as high as 200°C and at pressures up to 60 psi, a reproducible pattern of cure events is observed beginning with the flow of resin onto the sensor electrodes and ending with the characteristic stabilizing of the loss factor late in cure. The apparent dielectric properties (permittivity and loss factor) show good reproducibility for identically prepared samples, but their characteristics differ from those observed in neat resin. A ten‐fold increase in resin conductivity is noted when cures of glass‐reinforced prepreg are compared to cures of neat resin flaked off the prepreg. Special techniques are required in sample preparation when monitoring the cure of prepregs containing conductive fibers su

ISSN:0032-3888    

DOI:10.1002/pen.760260506

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractVariation of molar mix ratio of the Epon 828/Diamino‐diphenyl sulfone (DDS) system was found to have significant influences on pre‐cure and post‐cure properties. Microdielectrometry showed that dielectric constant varies from about 9 to above 30 depending on DDS content before cure. Times to the 10 kHz loss peak during cures at 177°C were found to decrease with increasing DDS content. Above a ratio of 2/1 and below 1/2, due to the low molecular weight of the reaction products, loss peaks did not occur. Ionic conduction was shown to have a strong correlation with calorimetric kinetic cure data. The influence of both excess amine and excess DDS was to produce cured material with high ionic conduction levels indicative of loose networks. Deviations from ideal stoichiometry as small as 1.1/1 produced noticable differences in the finally cured n

ISSN:0032-3888    

DOI:10.1002/pen.760260507

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractWe have investigated the use of acoustic waveguide techniques to measure the cure and stress state of epoxy resins and composites. It is anticipated that the waveguides, which are incorporated within the structure, will also be valuable for nondestructive (NDE) evaluation of stress, strain, and damage caused to the structure during its life time. While dielectric techniques are commonly used to monitor the cure of composites, they suffer because metal electrodes intrude into the structure and poor signal‐to‐noise ratios may be encountered with graphite composites. Acoustic waveguides can be made compatible with the host structure and after the cure monitoring function is over, remain in the structure for NDE of flaws and voids, as well as to monitor other parameters. This paper presents results on experiments to monitor the cure and stress state of resins and composites with a 1.5 mm diameter polyester‐fiberglass acoustic waveguide in curing resin. A 1000 to 1 change in the peak value of a 71 kHz acoustic signal was found. Other parameters now being investigated are: frequency of transmission and phase change, transit time, waveguide size and shape, and acoustic wave attenuation characteri

ISSN:0032-3888    

DOI:10.1002/pen.760260508

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

摘要:

AbstractAs thermosetting resins and composites made from them begin to be used in critical applications, it becomes necessary to monitor and analyze the cure of the resin within the confines of the processing equipment. Such measurements have been carried out using a dielectric technique called dielectrometry or dielectric analysis. Conventional dielectrometry has certain limitations associated with the use of parallel plate geometry for electrodes. Forin situmeasurement of cure, intrusiveness of electrodes is a problem which may require placement of electrodes in non‐strategic areas. Since electrode spacing changes during cure, it is difficult to deduce permittivity and loss factor from the data. At lower frequencies the capacitive currents are small and signal‐to‐noise ratios are small, therefore, measurement at low frequencies (>100 Hz) typically require large electrode sizes. One approach to overcome the problems described above is the development of microdielectrometry. A solid state integrated circuit chip, 2 by 4 mm in size, is used as the sensor. The miniature sensor can measure the properties of a dielectric on its surface, therefore it need not intrude into the composite part. Transistors which are built into the integrated circuit are used to amplify the signal to make low frequency (>1 Hz) measurements feasible. The electrode geometry does not change, therefore loss factor and permittivity data can be deduced in real‐time. During cure of the resin or composite material, permittivity and loss factor are measured continuously at a series of preselected frequencies. Real‐time‐data is plotted on a strip chart and is also stored in a cassette for later analysis. Temperature of the curing material is measured either by a diode on the sensor or by externally placed thermocouples. Results on an aromatic amine cured epoxy resin and a dicyandiamide cured epoxy resin prepreg are presented in this report. The data obtained by microdielectrometry is compared and correlated to results obtained using differential scanning calorimetry, infrared analysis, and dynamic mechanical analysis. Results show that microdielectrometry can be used to follow the cure of epoxy resins and absolute measurements of permittivity and loss factor can be made to provide information on the mechanisms that produce the observed changes. Data at 1 Hz can be obtained but for certain lossy systems at typical cure temperatures, the data is often out of range of the instrument, thus restricting use to 10 Hz and higher frequencies. At very high loss factors (>100) a “blocking” phenomenon is observed due to charge pile up at electrode surfaces which results in erroneously high perm

ISSN:0032-3888    

DOI:10.1002/pen.760260509

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760260501

出版商:Society of Plastics Engineers    

年代:1986

数据来源: WILEY