Thermokinetic effect of the aging of epoxy matrix prepregs for high performance composites

The reactivity and processability of prepregs for high performance composites have been investigated as a function of time and storing conditions. The study has been focused on the stability of epoxy matrix carbon fiber prepregs, affected by exposure to controlled environmental conditions before their use in composite manufacturing. Effects of the aging on glass transition temperature, reactivity and processability have been investigated by calorimetry. Dynamic, isothermal and cure simulating tests have been performed to this aim. Results on toughened TGDDMDDS epoxy matrix prepregs are reported. A theoretical kinetic model proposed for the unaged system has been adapted for aged prepregs, by properly evaluating the variations of kinetic parameters with the aging time.     

Equivalent processing time analysis of glass transition development in epoxy/carbon fiber composite systems

The cure kinetics and glass transition development of a commercially available epoxy/carbon fiber prepreg system, DMS 2224 (Hexel F584), was investigated by isothermal and dynamic‐heating experiments. The curing kinetics of the model prepreg system exhibited a limited degree of cure as a function of isothermal curing temperatures seemingly due to the rate‐determining diffusion of growing polymer chains. Incorporating the obtained maximum degree of cure to the kinetic model development, the developed kinetic equation accurately described both isothermal and dynamic‐heating behavior of the model prepreg system. The glass transition temperature was also described by a modified DiBeneditto equation as a function of degree of cure. Finally, the equivalent processing time (EPT) was used to investigate the development of glass transition temperature for various curing conditions envisioning the internal stress buildup during curing and cooling stages of epoxy‐based composite processing. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 144–154, 2002; DOI 10.1002/app.10282     

Kinetics modeling of carbon‐fiber‐reinforced bismaleimide composites under microwave and thermal curing

This article focuses on the analysis of the curing kinetics of carbon‐fiber‐reinforced bismaleimide (BMI) composites during microwave (MW) curing. A nonisothermal differential scanning calorimetry (DSC) method was used to obtain an accurate kinetic model. The degree of curing, chemical characterization, and glass‐transition temperature of the resin and composites cured by thermal and MW heating were analyzed with DSC, Fourier transform infrared spectroscopy, and dynamic mechanical analysis. The experimental results indicate that MW accelerated the crosslinking reaction of the BMI resin and had different effects on the reaction processes, especially for the glass‐transition temperature and chemical bonds. However, the curing reaction rate of the BMI resin decreased when the carbon fibers were added to the BMI resin during thermal and MW curing. According to the experimental results, the curing kinetic model of the BMI composite was used to provide a theoretical foundation for MW curing analysis. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43770.     

Effect of NBR on epoxy/glass prepregs properties

Solid acrylonitrile‐butadiene rubber (NBR) was used in epoxy resin for toughening and also for increasing the tack of epoxy/glass prepregs. The NBR used in this study was a rubber with 33% acrylonitrile content. The changes in thermal and mechanical properties such as glass transition temperature (T_g), curing characteristics and lap‐shear strength have been studied. For this purpose, three types of prepregs with two levels of NBR content of 3 and 5%, were prepared. Prepregs were made by solvent type impregnation apparatus. In this method, resin impregnates satin textile glass fiber under the controlled and constant condition of line speed and oven temperature. Prepregs were B‐staged for about 3%. The cure characterization, T_g and flow behavior were evaluated using differential scanning calorimetry and rheological analysis. Results showed that increasing the rubber content caused the following effects: (a) delay in gel time of prepregs, (b) increase in activation energy of prepregs, and (c) decrease in total heat of curing reaction. It is interesting that NBR increased the tack of epoxy/glass prepreg but, had no effect on its resin flow behavior. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Resin transfer molding (RTM) process of a high performance epoxy resin. I: Kinetic studies of cure reaction

The cure kinetics of a high performance PR500 epoxy resin in the temperature range of 160–197°C for the resin transfer molding (RTM) process have been investigated. The thermal analysis of the curing kinetics of PR500 resin was carried out by differential scanning calorimetry (DSC), with the ultimate heat of reaction measured in the dynamic mode and the rate of cure reaction and the degree of cure being determined under isothermal conditions. A modified Kamal's kinetic model was adapted to describe the autocatalytic and diffusion‐controlled curing behavior of the resin. A reasonable agreement between the experimental data and the kinetic model has been obtained over the whole processing temperature range, including the mold filling and the final curing stages of the RTM process.     

Cure kinetics of epoxy resin used for advanced composites

The cure kinetics of an epoxy resin used for the preparation of advanced polymeric composite structures was studied by isothermal differential scanning calorimetry (DSC). A series of isothermal DSC runs provided information about the kinetics of cure over a wide temperature range. According to the heat evolution behavior during the curing process, several influencing factors of isothermal curing reactions were evaluated. The results showed that the isothermal kinetic reaction of this epoxy resin followed an autocatalytic kinetic mechanism. In the latter reaction stage, the curing reaction became controlled mainly by diffusion. Cure rate was then modeled using a modified Kamal autocatalytic model that accounts for the shift from a chemically controlled reaction to a diffusion‐controlled reaction. The model parameters were determined by a nonlinear multiple regression method. Copyright © 2004 Society of Chemical Industry     

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     

Material systems for rapid manufacture of composite structures

A manufacturing process is described that builds complex composite parts using a layered building process in which each layer of pre‐preg composite is laid and cured as the build progresses. In order to employ on‐line curing without molds, resin technologies that provide fast curing at room temperature—ultraviolet curable and epoxy/polyamide—were investigated. UV‐curable resins were tested for their ability to “shadow” cure by exposing carbon fiber composites to ultraviolet light to determine if the cure propagated from areas directly exposed to areas under fibers. Though ultraviolet curing showed advantages in cure time and low volatile production, very minimal “shadow” curing was achieved. A low temperature curing epoxy/polyamide mixture was tested for the effects of cure temperature, cure time, and mix ratio on the final degree of cure (%DOC) and glass transition temperature (T_g). Layers were made using different resin mixtures, partially cured, and used to build layered parts to determine curing characteristics during the lay‐up process. In the epoxy/polyamide mixtures, mix ratio had little effect on the reaction rate but did affect the T_g. A kinetic model was established for the resin epoxy/polyamide system for optimizing processing conditions during fabrication. However, the model failed to correctly predict the fabrication. The reaction of the material was different during the fabrication process than during the isothermal cure due to the presence of oxygen. During the build process, the degree of cure in each layer increased significantly over the prestaged degree of cure in less time than theoretically predicted. However, the final resin properties, such as T_g, were still below the specifications for high performance parts.     

Effect of surface area of nanosilica particles on the cure kinetics parameters of an epoxy resin system

Knowing of thermoset curing kinetics is essential for process development, quality control, and achieving desirable products. Hence, in this article, cure kinetics of an EPON 828 epoxy resin/dicyandiamide curing agent/diuron accelerator system is investigated. This resin system is usually used for the production of epoxy/glass fiber prepregs used in wind turbine blades. For this, differential scanning calorimetry analysis is used and the effect of temperature, weight percentage, and size of nanosilica is studied by conducting isothermal tests at several temperatures for samples with and without nanoparticles. An autocatalytic curing model is applied to describe the cure kinetic of system and then the variations in model parameters calculated by curve fitting using the MATLAB software. The results show that the increase in temperature, weight percentage of nanosilica from 0 to 6%, and surface area of nanosilica particles lead to the increase in curing rate, whereas the increase in the percentage and surface area of nanosilica particles significantly decreases total heat of reaction. At the end, the relation between each of model parameters and the total surface area of nanosilica particles, calculated by mathematical equations, is obtained. The allowable maximum surface area of nanosilica used in the mathematical equations is 12 m² g⁻¹. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47958.     

环氧树脂/玻璃纤维模压预浸料固化反应动力学研究

采用非等温DSC方法研究了一种模压预浸料（环氧树脂/玻璃纤维）的固化动力学,应用Kissinger和Crane方程拟合求得固化动力学参数,并建立了该预浸料固化动力学唯象模型。通过无转子硫化仪测试预浸料在不同温度下的凝胶时间,通过线性拟合得到固化温度与凝胶时间的函数关系,并对预浸料的固化工艺进行优化。结果表明,通过Kissinger和Crane方法算得该预浸料的固化反应动力学表观活化能为89.9 kJ/mol,指前因子为1.17×10¹¹ min^-1,反应级数为0.93;预浸料在模具温度为150 ℃下预热40 s,环氧树脂具有一定的流动性,并在2 MPa压力下固化300 s,可制备综合性能良好的复合材料制品。     

Effect of cure kinetic simulation model on optimized thermal cure cycle for thin‐sectioned composite parts

In this article, the optimal design of the thermal cure cycle of a prepreg material has been presented. Three different kinetic methods have been considered to determine the kinetic parameters of the curing reaction of an epoxy/glass prepreg. The capability of these kinetic models in simultaneously predicting kinetic parameters in isothermal and non‐isothermal conditions differs with each other. For the simulation of the cure cycle, a zero‐dimensional model has been used and coupled with the genetic algorithm to optimize the cure cycle of thin composite parts. The effect of the kinetic models on the optimized cure cycle has been investigated. Of these models, only one model could predict the kinetic parameters outside the experimental temperature regions and resulted in a reliable and acceptable optimized cure cycle. The validity of the optimized cure cycle has been also verified experimentally. POLYM. COMPOS., 34:1172–1179, 2013. © 2013 Society of Plastics Engineers     

Thermal analysis of thermosetting phenolic compounds for injection molding

The differential scanning calorimetry technique has been applied to investigate the curing of injection molding phenolic compounds. The data obtained include degree of cure, rate of curing, and heats and temperatures of curing as function of various heating rates, rate constants, energy of activation, and glass transition temperature. The curing temperature and heating rate were found to affect both the curing reaction kinetics and the final structure of the crosslinked network. The glass transition temperature changes continously with the extent of curing, approaching the cure temperature.     

Cure modelling of polyester thermosets in a glass mould

The curing of unsaturated polyester was studied experimentally and using a model of the process. The kinetic parameters were calculated from the heat flow–time curves obtained by differential scanning calorimetry (Mettler Toledo DSC 823), operating in a non-isothermal mode. The temperature–time histories were studied in a cylindrical glass mould. A potential use of glass as a mould for polymer curing is found in the production of optical sensors. Here, glass was selected as a mould material because it is UV transparent, chemically inert and easy to clean. The thermal properties of glass moulds coupled with the intrinsic curing kinetics are of a significant interest in such investigations. Taking into account the heat transferred by the convection from the air to the mould surface and the conduction through the mould wall and resin, as well as the kinetics of the heat generated in the cure reaction, a numerical model has been constructed. The contributions to the rise in temperature from the heat conduction and chemical reaction are different in different parts of the composite, which can explain the temperature–time histories. The introduction of a carbonate based filler reduced the amount of heat released in the composite and, as a result, lowered the temperatures through the resin. A good agreement between experimental data and the predicted mathematical model of the curing process in the mould has been observed.     

In-process control of epoxy composite by microdielectrometric analysis

The curing of an epoxy prepolymer based on the diglycidyl ether of bisphenol A (DGEBA) with dicyandiamide (DDA) as the hardener and imidazole as the catalyst agent was analyzed using microdielectrometry, differential scanning calorimetry, viscosity measurements, and insolubles in THF for gel-point detection. Interpreting dielectric data with respect to chemorheology continues to be the subject of scientific discussion. The focus of this issue is to give an industrial point of view on the collected on-line dielectric measurements during an epoxy/fiber glass composite cure. Hence, isothermal polymerizations of DGEBA/DDA/imidazole resin were examined and dielectric properties such as ionic conductivity were related to the cure kinetics by conversion through an experimentally established equation. This mathematical model was used to predict reaction advancement of epoxy processing under nonisothermal cure conditions. This model is shown to be able to forecast both isothermal and nonisothermal cure data of unaged resin. According to these results, cure monitoring was carried out on prepregs. Whereas some deviations of the law were observed at the time of the last stage of the cure, good correlation was obtained for the reaction rate during the in-mold process curing time.     

Cure kinetics modeling and cure shrinkage behavior of a thermosetting composite

In this work, the cure kinetics and through‐the‐thickness cure shrinkage upon curing of a carbon fiber‐epoxy composite (AS4/8552) were studied. The study is composed of two major parts. Firstly, dynamic and isothermal Differential Scanning Calorimeter (DSC) scans were performed to develop a new cure kinetics model. The most appropriate kinetic model that produces a nearly perfect fit of all data sets corresponds to a process with two single‐step parallel autocatalytic reactions with diffusion control. Multivariate kinetic analysis was used to evaluate the parameters. In the second part of the study, the coefficients of thermal expansion (CTEs), the glass transition temperatures (T_g), and the through‐the‐thickness cure shrinkage strain values of the partially cured unidirectional and cross‐ply composite samples were measured by using a dynamic mechanical analyzer (DMA). Cure strains were measured throughout the Manufacturer's Recommended Cure Cycle (MRCC) with the same method. Results indicate that glass transition temperatures of partially cured samples can be measured very closely by the two methods (DSC and DMA). The methods proposed were proved to be very reliable to predict the degree of cure and to measure the through‐the‐thickness strains during the cure cycle. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

A cure-rate model for the shuttle filament-wound case

An epoxy and carbon fiber composite has been used to produce a light-weight rocket case for the Space Shuttle. A kinetic model is developed which can predict the extent of epoxy conversion during the winding and curing of the case. The model accounts for both chemical and physical kinetics. In the model, chemical kinetics occur exclusively up to the time the transition temperature equals the reaction temperature. At this point the resin begins to solidify and the rate of this process limits the rate of epoxy conversion. A comparison of predicted and actual epoxy conversion is presented for isothermal and temperature-programmed cure schedules.     

DSC evaluation of epoxy and polyimide-impregnated laminates (prepregs)

Differential scanning calorimetry has been applied to the study of epoxy–glass and Kerimid 601 (polyimide–glass) prepregs used in making multilayer printed wiring boards. The data generated through thermal analysis are thermodynamic and kinetic. Thermodynamic properties of B-stage prepregs obtained are endothermal and exothermal peak temperatures, glass transition temperatures, and heats of residual cure. They were obtained from thermograms made at fixed scanning rates. Kinetic information obtained from isothermal cure scans gave rates of residual cure, cure times, and energies of activations. This information gives the process engineer useful information for controlling the processing of multilayer printed wiring boards.     

Glass transition temperatures in bismaleimide-based resin systems

The glass transition temperatures in bismaleimide-based resins were investigated using different stoichiometric ratios of 1, 1′-(methylenedi-4, 1-phenylene)bismaleimide (BMI) and 4, 4′-methylenedianiline (MDA). The resin cure involves a low temperature primary amine addition to the maleimide double bonds and a high temperature homopolymerization of the maleimide double bonds. The network topology and the glass transition temperature changes with resin composition and curing conditions were determined using differential scanning calorimetry (DSC). An empirical model was used to relate the glass transition temperature to the extents of the amine addition and the homopolymerization reactions in 1:1 and 2:1 BMI:MDA resins. The changes in thermal properties with resin post-cure were also examined.     

Consistent model predictions for isothermal cure kinetics investigation of high performance epoxy prepregs

An accurate kinetics model is essential for understanding the curing mechanism and predicting the end properties of polymer materials. Graphite/epoxy AS4/8552 prepreg is a recent high‐performance thermosetting composite modified with thermoplastic, which is being used in the manufacture of aircraft and military structures. The isothermal cures of this system along with another thermoplastic toughened high‐performance prepreg, the T800H/3900‐2 system, were investigated by real‐time Fourier transform infrared (FTIR) spectroscopy. The cure rate was quantitatively analyzed based on the concentration profiles of both the epoxy and primary amine groups. Three autocatalytic models were used to determine kinetics parameters for both composite systems. The model which utilizes an empirical term, the final relative conversion (at different isothermal curing temperatures), describes the experimental data of both systems more satisfactorily than the model which applies a diffusion factor. The modeling results suggest that the curing of epoxy within both prepregs can be assumed to be a second order process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008