Modeling the chemorheological behavior of epoxy/liquid aromatic diamine for resin transfer molding applications

The analysis of the chemorheological behavior of an epoxy prepolymer based on a diglycidylether of bisphenol‐A (DGEBA) with a liquid aromatic diamine (DETDA 80) as a hardener was performed by combining the data obtained from Differential Scanning Calorimetry (DSC) with rheological measurements. The kinetics of the crosslinking reaction was analyzed at conventional injection temperatures varying from 100 to 150°C as experienced during a Resin Transfer Molding (RTM) process. A phenomenological kinetic model able to describe the cure behavior of the DGEBA/DETDA 80 system during processing is proposed. Rheological properties of this low reactive epoxy system were also measured to follow the cure evolution at the same temperatures as the mold‐filling process. An empirical model correlating the resin viscosity with temperature and the extent of reaction was obtained to carry out later a simulation of the RTM process and to prepare advanced composites. Predictions of the viscosity changes were found to be in good agreement with the experimental data at low extents of cure, i.e., in the period of time required for the mold‐filling stage in RTM process. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4228–4237, 2006     

Dielectric analysis of the cure of thermosetting epoxy/amine systems

A low molecular weight epoxy resin is cured isothermally with an aromatic amine hardener, and the dielectric properties are measured as a function of the frequency, reaction time, and cure temperature. At specific stages in the cure, small samples from the reacting mixture are quenched and subsequently analyzed for the glass transition temperature and epoxy group conversion by differential scanning calorimetry. In this manner, the change In dielectric properties can be directly correlated with the network structure. The ionic conductivity is modeled as a function of the cure temperature and the cure-dependent glass transition temperature using a Williams-Landel-Ferry (WLF) relation. Combining this WLF relation with the DiBenedetto equation, a comprehensive model relating conductivity with the extent of reaction and cure temperature has been developed.     

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     

环氧树脂和环氧/环硫树脂与胺的固化反应动力学

采用非等温DSC法对低黏度体系CY184/IPDA环氧树脂体系及对CY184/ES184/IPDA环氧/环硫树脂体系的固化反应动力学进行了研究。用高级等转化率Vyazovkin积分法求取活化能E_a,通过Málek法确定了固化反应机理函数和动力学参数,得到固化反应动力学方程。结果表明:CY184/IPDA环氧树脂体系的平均活化能为47.04 kJ·mol^-1;CY184/ES184/IPDA环氧/环硫树脂体系的活化能为48.97 kJ·mol^-1。两种体系的模型拟合曲线与实验得到的DSC曲线吻合得较好,均符合esták-Berggren(m,n)模型。     

A chemorheological model for the cure of unsaturated polyester resin

A chemorheological model is developed, using the free volume concept, for the prediction of viscosity during the cure of unsaturated polyester resin. We have incorporated into the development of the chemorheological model a mechanistic kinetic model of curing kinetics that predicts the degree of cure as a function of cure time. The mechanistic kinetic model uses an approach of free-radical polymerization that takes into account diffusion-controlled curing reactions, In order to test the usefulness of the chemorheological model developed, we have conducted cure experiments and measured viscosities of partially cured resin samples, using a general-purpose unsaturated polyester resin. Specifically, the following measurements were taken: (1) the quantity of ethylenic double bonds in the resin system before and after the cure reaction by infrared spectroscopy, (2) the glass transition temperature by differential scanning calorimetry (DSC) and (3) the viscosity as a function of shear rate, at several temperatures, using a cone-and-plate rheometer. It is concluded that the chemorheological model developed is very useful for predicting the variation of viscosity during the cure of unsaturated polyester resin.     

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.     

A simplified chemorheological model of viscosity evolution for solvent containing resol resin in RTM process

The solvent content‐dependent chemorheology of the solvent containing resol resin for resin transfer molding (RTM) was investigated. The curing behavior of the resol resin was studied by in situ Fourier transform infrared spectroscopy together with rheology tests. The chemorheological behavior of resol resins with a series of solvent contents was measured under isothermal conditions. The four parameters of empirical dual‐Arrhenius equation regarding isothermal resin viscosity and reaction rate constant were found to be functions of the solvent content. A simplified chemorheological model involving only three parameters of curing temperature, time, and solvent content was first established to facilely describe the viscosity during precuring process. The simulated viscosity results during isothermal curing process agreed well with the experimental data which shows the simplified chemorheological model can be utilized to describe the viscosity evolution and offer guidance for optimizing the injection process and improving the design flexibility of RTM process. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45282.     

The relationship between dielectric and rheological behavior of epoxy resin during cure

Measurement of the frequency-dependent, vector voltage (V_c) provided an in-situ and non-destructive technique to measure continuously the rheological change of a resin due to polymerization, and can be used as the basis of real-time control. The vector voltage depends on the degree of polarization of the dipolar molecules and on the change of viscosity during cure; both result from the modified structure of the epoxy resin during cure, The initial stage of curing, represented by the former portion Of the Vc curve (divided at the minimum of the Vc curve), was caused mainly by the effects of temperature and viscosity. During the latter stage of the cure reaction, Vc alters because of the effect of the lightened matrix structure that inhibits alignment of dipoles. The duration of reaction. temperature of curing and degree of conversion all have the same effects on both vector voltage and viscosity, The minimum value of vector voltage is correlated to the minimum viscosity, and there is a nearly quantitative relationship between them, One can determine the viscosity of the epoxy resin during cure from reading of the vector voltage. Various reaction mechanisms may be explained based on the graphs of vector voltage of various types.     

Stress relaxation behavior of 3501-6 epoxy resin during cure

Epoxy resins and other thermosetting polymers change from liquids to solids during cure. A precise process model of these materials requires a constitutive model that is able to describe this transformation in its entirety. In this study the viscoelastic properties of a commercial epoxy resin were characterized using a dynamic mechanical analyzer (DMA). Specimens were tested at several different cure states to develop master curves of stress relaxation behavior during cure. Using this experimental data, the relaxation modulus was then modeled in a thermorheologically complex manner. A Prony (exponential) series was used to describe the relaxation modulus. An original model was developed for the stress relaxation times based on similar work by Scherer (16) on the relaxation of glass. Shift functions used to obtain reduced times are empirically derived based on curve fits to the data. The data show that the cure state has a profound effect on the stress relaxation of epoxy. More important, the relaxation behavior above gelation is shown to be quite sensitive to degree of cure.     

Torsional braid analysis of the aromatic amine cure of epoxy resins

The cure behavior of diglycidyl ether of bisphenol A (DGEBA) type of epoxy resins with three aromatic diamines, 4,4′-diaminodiphenyl methane (DDM), 4,4′-diaminodiphenyl sulfone (44DDS), and 3,3′-diaminodiphenyl sulfone (33DDS) was studied by torsional braid analysis. For each curing agent the stoichiometry of the resin mixtures was varied from a two to one excess of amino hydrogens per epoxy group to a two to one excess of epoxy groups per amino hydrogen. Isothermal cures of the resin mixtures were carried out from 70 to 210°C (range depending on epoxy—amine mixture), followed by a temperature scan to determine the glass transition temperature (T_g). The times to the isothermal liquid-to-rubber transition were shortest for the DDM mixtures and longest for the 44DDS mixtures. The liquid-to-rubber transition times were also shortest for the amine excess mixtures when stoichiometry was varied. A relatively rapid reaction to the liquid-to-rubber transition was observed for the epoxy excess mixtures, followed by an exceedingly slow reaction process at cure temperatures well above the T_g. This slow process was only observed for epoxy excess mixtures and eventually led to significant increases in T_g. Using time—temperature shifts of the glass transition temperature vs. logarithm of time, activation energies approximately 50% higher were derived for this process compared to those derived from the liquid-to-rubber transition. The rate of this reaction was virtually independent of curing agent and was attributed to etherification taking place in the epoxy excess mixtures. © 1994 John Wiley & Sons, Inc.     

湿固化聚氨酯/环氧树脂固化体系的研究

以聚丙二醇(PEG)、甲基六氢苯酐(MHHPA)、异佛尔酮二异氰酸酯(IPDI)、二苯基甲烷二异氰酸酯(MDI)以及甲苯二异氰酸酯(TDI)为原料,分别合成出了三种末端含有异氰酸酯的反应性聚氨酯。评价了这三种反应性聚氨酯的物理机械性能,并将这三种聚氨酯分别与低黏度双酚A型环氧树脂按不同的比例混合,考察了三个不同的聚氨酯/环氧树脂固化体系的力学性能。研究表明,当反应性聚氨酯预聚体与环氧树脂的质量比为95∶5,固化条件为室温下7d或者室温下12h后,再在80℃放置4h时,这三种共混体系均能达到理想的黏结效果。     

APG工艺用环氧树脂体系的固化及流变模型

研究了自动压力凝胶工艺用环氧树脂混合体系在动态升温过程中的固化反应,采用Malek法确定了动力学模型并预测了相应的参数。该体系符合两参数自催化SB模型。用旋转流变仪测定了该体系在动态升温过程中黏度相对于温度的变化曲线,采用WLF和Macosko混合方程描述该体系的化学流变模型,用非线性拟合方法预测了模型中的参数。模型预测与实验数据比较吻合。     

Effects of variation in composition and temperature on the amine cure of an epoxy resin model system

Prior liquid chromatographic studies have shown that the reactions in epoxy resin model system phenyl glycidyl ether, p-chloroaniline, and Monuron include amine addition to epoxy, homopolymerization of the epoxy, and a chain-transfer reaction involving the hydroxy groups of the addition products. The present work examines the effect of variation in concentration of the accelerator Monuron, the amine-to-epoxy ratio, and the temperature on the competitive reaction mechanisms. The fraction of phenyl glycidyl ether reacting by homopolymerization increases with accelerator concentration and decreases with increasing amine-to-epoxy ratio and increasing temperature. The estimated contribution from chain transfer is much smaller and appears to parallel the homopolymerization reaction, as might be expected.     

Comparison of microwave and thermal cure of an epoxy/amine matrix

An investigation was carried out into the effect ofa microwave cure on an epoxy prepolymer with a cycloaliphatic diamine mixture, as compared to a standard thermal cure. The microwave waveguide and process (propagation mode TE₀₁) were adjusted to obtain large homogeneous samples. The extent of reaction, x, was measured during the microwave processing by size exclusion chromatography and differential scanning calorimetry. A good estimate of x was found using a modified DiBenedetto equation correlating x and the glass transition temperature T_g. The homogeneity of the samples was checked during the last steps of cure, showing the efficiency of the microwave processing and waveguide. The influence of the nature of the mold (metallic or dielectric) on the reaction kinetic was also investigated. Samples cured by both thermal and microwave processing were characterized by dynamic and static mechanical properties and then compared with those of fully crosslinked networks, i.e., postcured at a high temperature.     

Dielectric behavior of epoxy resin during cure: A vector voltage study

Vector voltage (Vc), measured across a plate electrode filled with resin, is a function of the resin matrix structure. The variation of Vc during the cure process at various frequencies was studied. A peak with minimum Vc value was observed. It was found that the initial stage of heating is mainly a temperature equilibrating step, where the viscosity decreased to a minimum owing to the rapid alignment of the dipole (and ion) species. Vc then ascended as the major cure reaction took place, as the mobilities of the electrical species were lowered in the rather tight matrix structure of the partially cured resin. The quantity of bound charge is an indicator of dipolar and ionic movements within the resin matrix, and was employed to describe the trajectory of the Vc curve. Bound charge conductance was also found to be the dominating factor in the Vc variation when tests were carried out in a low frequency AC field; whereas free charge conductance became more important for tests at high frequency. It was also observed that the cure phenomenon was not detectable from the Vc variation when monitored with an AC frequency of 38 kHz or higher.     

Physical aging of partially crosslinked RTM6 epoxy resin

The aging behavior of partially and completely crosslinked RTM6 epoxy resin samples in the glassy state is investigated by thermophysical and mechanical analyses. Curing degree, glass transition temperature, density, and micromechanical modulus are investigated as function of aging period, initial curing degree or depth below the sample surface. A clear increase of density and modulus with aging period as well as an enhanced surface stiffness is detected for all curing degrees. Also, the aging period necessary to achieve the steady state modulus is independent from the curing degree. In contrast, the degree of physical aging induced modulus changes shows a significant dependence on the curing degree. A discontinuity is detected in the so‐called transition region, which is related to the transition from rubber to glassy state during crosslinking. This emphasizes the importance of curing history for physical aging processes and the high potential of partial curing for the development of new processing routes, in particular for production of samples with low sensitivity to physical aging. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41121.     

Effect of cyanate ester on the cure behavior and thermal stability of epoxy resin

Epoxy resin (diglycidyl ether of bisphenol A, DGEBA)/cyanate ester mixtures were cured with a curing agent, 4,4′-diaminodiphenylsulfone, and the effect of cyanate ester resin on the cure behavior and thermal stability in the epoxy resin was investigated with a Fourier transform infrared spectrometer, a rheometer, a dynamic mechanical analyzer, and a thermogravimetric analyzer. Cure reactions in the epoxy/cyanate ester mixture were faster than that of the neat epoxy system. The cure reaction was accelerated by increasing the cyanate ester resin component. Glass transition temperature and thermal stability in the cured resins were increased with increasing cyanate ester resin component. This may be caused by the increase of crosslinking density due to the polycyclotrimerization of the cyanate ester monomer to form triazine rings and the reaction of cyanate ester resin with the epoxy network. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 85–90, 1997     

Analysis of the cure kinetics of epoxy/imidazole resin systems

The reaction kinetics for the cure of epoxy resins with imidazoles were determined from Fourier transform infrared spectroscopy and differential scanning calorimetry studies. The diglycidyl ether of bisphenol A and phenyl glycidyl ether were cured with various concentrations of 2-ethyl-4-methyl-imidazole ranging from 4.0 to 100.0 mol %. The first step in the curing process is the formation of epoxide/imidazole adducts. These adducts initiate the etherification reaction which crosslinks the resin. The kinetics were determined and confirmed for both the adduct and the etherification reactions as a function of the imidazole concentration. A model was developed and used to predict the concentrations of the unreacted epoxide groups and the reaction products for a wide range of imidazole concentrations and cure temperatures.     

光引发阳离子聚合及其在环氧树脂固化研究中的进展

从光引发阳离子聚合的特点、常见阳离子型光敏引发剂及其引发聚合机理、光引发环氧树脂阳离子聚合体系及发展前景等几个方面进行了简要概述。     

Toughening of a trifunctional epoxy system. II. Thermal characterization of epoxy/amine cure

The cure of a trifunctional epoxy resin with an amine coreactant was studied using two thermal analysis techniques: differential scanning claorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). These techniques were used to monitor the development of both the thermal and mechanical properties with cure. Detailed kinetic analysis was performed using a variety of kinetic models: nth order, autocatalytic, and diffusion-controlled. The reaction was found to be autocatalytic in nature during the early stages of cure while becoming diffusion-controlled once vitrification had taken place. By combining the results obtained from DSC and DMTA, the degree of conversion, at which key events such as gelation and vitrification take place, were determined. A TTT diagram was constructed for this epoxy/amine system showing the final properties that can be achieved with the appropriate cure history. © 1996 John Wiley & Sons, Inc.