Cure kinetics of epoxy formulations of the type used in advanced composites

An analysis of the cure kinetics of three different formulations composed of tetraglycidyl 4,4′-diaminodiphenyl methane (TGDDM) epoxy resin and diaminodiphenyl sulfone (DDS) was performed. A series of isothermal tests was run, and the experimentally obtained results were checked against the proposed kinetic model. An autocatalyzed mechanism with the overall reaction order of 2 was found to adequately describe the cure kinetics. An increase in reaction rate was observed at higher temperature and higher DDS concentration. For a given formulation, the extent of reaction corresponding to the maximum reaction rate was independent of temperature. A secondary exotherm was detected, particularly in formulations with low DDS concentration, at approximately 40% conversion. At that point, the rate of primary amine–epoxide reaction decreases, and other reactions dominate the curing process. Such a mechanism is likely to cause a formation of an inhomogeneous thermoset morphology.     

Criteria for selecting cure cycles in autoclave processing of graphite/epoxy composites

Room temperature mechanical properties, such as flexural strength and impact resistance, of epoxies and graphite/epoxy composites go through a maximum as a function of epoxy conversion. For tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM)-4,4′-diamlnodiphenylsulfone (DDS) formulations, the recommended cure cycle prescribes a maximum temperature close to 177°C. The maximum extent of reaction that may be obtained at this temperature is determined from the vitrification curve. At this maximum conversion, balanced mechanical and physical properties are attained in the partially cured specimen. However, if the standard cycle is used to cure thick parts, the maximum temperature inside the sample increases beyond 177°C. This leads to a complete conversion in most of the part and a consequent impairment of resulting physical and mechanical properties. It is shown how numerical solutions of differential energy and mass balances may be used to propose alternative cure cycles such that the maximum conversion at every point remains bounded by the vitrification curve. An illustration for a particular thickness is provided.     

Cure kinetics of neat versus reinforced epoxies

An investigation was carried out into the cure kinetics of neat vs reinforced epoxy systems. The formulations were composed of tetraglycidyl 4,4′-diaminodiphenyl methane (TGDDM) epoxy resin and diaminodiphenyl sulfone (DDS). Glass was used as reinforcement. A series of isothermal differential scanning calorimetry (DSC) thermograms were run and analyzed by the proposed autocatalytic kinetic model. An increase in reaction rate was observed at higher temperature and higher DDS concentration in both neat and reinforced formulations. The presence of reinforcement had an effect on the cure kinetics. The observed effect, however, was not very pronounced. Slightly lower values of the reaction rate constant and longer times needed to reach the maximum reaction rate were recorded in reinforced systems. After reaching the peak value, the rate of reaction dropped off faster in reinforced formulations, resulting in lower average value of H_ult, the ultimate heat of reaction. It was suggested that the reinforcement imposes restrictions on the molecular mobility of reactive species.     

Thermokinetics and chemorheology of the cure reactions of the tetraglycidyl diamino diphenyl methane–diamino diphenyl sulfone epoxy systems

The cure behavior of commercial grade TGDDM–DDS mixtures of compositions ranging from 10 to 100 phr of hardener and the thermal polymerization of the epoxy component are analyzed by means of differential scanning calorimetry. The kinetic parameters and heats of reaction determined in isothermal and dynamic scans suggest that DDS primary amine addition and epoxide etherification dominate the cure reactions. The primary amine epoxide addition is characterized by overall heat of reaction (referred to the weight of the epoxy component) of 255 cal/g and by an activation energy of 16.6 kcal/mol. The corresponding values for the etherification reaction are, respectively, 170 cal/g and 41 kcal/mol. A method of derivation of the epoxide conversion from the heat evolved in DSC thermal scans of these systems is presented. The results are in good agreement with independent IR determinations. The steady shear and oscillatory viscosity measurements and the calorimetric analysis of the isothermal cure at 140°C, 160°C, and 180°C of a TGDDM–DDS mixture containing 35 phr of hardener indicate that gelation is principally governed by the primary amine addition. The gelation limits calculated in isothermal tests by combining the calorimetric analysis and the theory describing the nonlinear copolymerization of the tetrafunctional TGDDM with an essentially difunctional DDS were in good agreement with the values experimentally determined through rheological measurements.     

The characterization of thermoset cure behavior by differential scanning calorimetry. Part I: Isothermal and Dynamic Study

A study of the isothermal cure kinetics of an unsaturated polyester resin by differential scanning calorimetry is described. An autocatalyzed second order kinetic model is adopted to elucidate the cure reaction and also assess the kinetic parameters. The rate constant, the maximum cure rate, the extent of cure, and the degree of conversion at the maximum cure rate, all increase with increasing cure temperature, while the half-life and the time required to reach the maximum cure rate both decrease. Discrepancies between the experimental results and the predicted values of some of the kinetic parameters, especially at high degrees of conversion, are attributed to the highly different-controlled cure reaction following the gel point. The activation energy (E) and the pre-exponential factor (1n A) of the polyester cure reaction were estimated, using differential graphical techniques, to be 131 ± 4 kJ/mol and 39 ± 1 respectively. An ASTM method (E698) produces erroneously low values of the Arrhenius parameters, suggesting that the assumptions of the method may be overly simplified.     

Kinetics modeling and time-temperature-transformation diagram of microwave and thermal cure of epoxy resins

Stoichimetric mixtures of a diglycidyl ether of bisphenol A (DGEBA)/ diaminodiphenyl sulfone (DDS) and a DGEBA/meta phenylene diamine (mPDA) were cured using both microwave and thermal energy. Fourier transform infrared (FTIR) was used for the measurement of the extent of cure and thermal mechanical analysis (TMA) was used for the determination of the glass transition temperature (T_g). The cure kinetics of the DGEBA/mPDA and DGEBA/DDS systems were described by an autocatalytic kinetic model up to vitrification in both the microwave and thermal cure. For the DGEBA/mPDA system, the reaction rate constants of the primary amine-epoxy reaction are equal to those of the secondary amine-epoxy reaction, and the etherification reaction is negligible for both microwave and thermal cure. For the DGEBA/DDS system, the reaction rate constants of the primary amine-epoxy reaction are greater than those of the secondary amine-epoxy reaction and the etherification reaction is only negligible at low cure temperatures for both microwave and thermal cure. Microwave radiation decreases the reaction rate constant ratio of the secondary amine-epoxy reaction to the primary amine-epxy reaction and the ratio of the etherification reaction to the primary amine-epoxy reaction. T_g data were fitted to the DiBenedetto model. A master curve and a time-temperature-transformation (TTT) diagram were constructed. The vitrification time is shorter in microwave cure than in thermal cure, especially at higher isothermal cure temperatures. For the DGEBA/mPDA system, the minimum vitrification time is two to five times shorter in the microwave cure than in the thermal cure. For the DGEBA/DDS system, the minimum vitrification time is 44 times shorter in the microwave cure than in the thermal cure.     

Fiberoptic intrinsic fluorescence for in-situ cure monitoring of amine cured epoxy and composites

The cure reactions of epoxy-diamine and its composites are monitored in-situ using the intrinsic fluorescence of the aromatic diamine, diaminodiphenyl sulfone (DDS). With a fiberoptic fluorimeter, in-situ cure monitoring was performed via a single fiber, distal-end probe, in neat epoxy as well as in commercial grade prepregs containing graphite fibers and DDS curing agent. The prepregs were investigated during multiply lamination in an oven. The fluorescence excitation spectra were obtained by emitting at 420 nm with a scan range of 320 to 400 nm, and the DDS peak position was determined as a function of cure time and temperature. The DDS spectra show a progressive red shift up to 24 nm when the primary amine is reacted with epoxide to become the secondary and the tertiary amines. The spectral shift of the DDS is also correlated with the extent of epoxide reaction determined by the Fourier transform infrared (FTIR) spectroscopy. Both data exhibit a linear relation, consistent with the behavior of the DDS peak shift, which increases linearly with the amine reaction. The excitation spectra also show a temperature dependency such that the amount of red shift increases with the measurement temperature in a manner that can be described by an exponential function. The temperature effects also depend on the state of cure in the sample. The temperature correction can be made by the application of an empirically developed equation. Thus, a direct comparison can be made among the on-line data obtained under varying conditions of cure, by reducing the spectral data to any reference temperature. This intrinsic fluorescence technique is much simpler than the previously reported extrinsic fluorophore technique, which requires the addition of an extrinsic fluorophore and an internal dye, and can be applied to any commercial prepregs containing DDS, thus making it a very powerful and widely applicable monitoring tool for composite processing.     

Cure kinetics and mechanical properties of the blend system of epoxy/diaminodiphenyl sulfone and amine terminated polyetherimide-carboxyl terminated poly(butadiene-co-acrylonitrile) block copolymer

The cure kinetics of blends of epoxy resin (4,4’-tetraglycidyl diaminodiphenyl methane; TGDDM)/curing agent (diaminodiphenyl sulfone; DDS) with ATPEI (amine terminated poly-etherimide) -CTBN (carboxyl terminated poly (butadiene-co-acrylonitrile)) block copolymer (AB type) were studied using differential scanning calorimetry under isothermal conditions to determine the reaction kinetic parameters such as activation energy and reaction constants. Final cure conversion decreased with increasing amount of AB in the blends. A diffusion controlled reaction was observed as the cure conversion increased, and the curing reaction was successfully analyzed by incorporating the diffusion control term in the rate equation for the epoxy/DDS/AB blends. The fracture toughness was improved to about 350% compared to that of the unmodified resin at 30% of AB block copolymer. This is attributed to the formation of co-continuous morphology between the epoxy phase and AB block copolymer phase. By increasing the amount of AB, the modulus of the cured blends decreased, which was due to the presence of CTBN rubbery phases.     

Curing kinetics and thermal property characterization of a bisphenol‐S epoxy resin and DDS system

The kinetics of the cure reaction for a system of bisphenol‐S epoxy resin (BPSER), with 4,4′‐diaminodiphenyl sulfone (DDS) as a curing agent was investigated with a differential scanning calorimeter (DSC). Autocatalytic behaviour was observed in the first stages of the cure which can well be described by the model proposed by Kamal, using two rate constants, k₁ and k₂, and two reaction orders, m and n. The overall reaction order, m + n, is in the range 2∼2.5, and the activation energy for k₁ and k₂ was 86.26 and 65.13 kJ mol⁻¹, respectively. In the later stages, a crosslinked network was formed and diffusion control was incorporated to describe the cure. The glass transition temperature (T_g) of the BPSER/DDS samples partially cured isothermally was determined by means of torsional braid analysis (TBA) and the results showed that the reaction rate increased with increasing T_g, in terms of rate constant, but decreased with increasing conversion. It was also found that the  SO₂ group both in the epoxy resin and in the hardener increases the T_g values of the cured materials compared with that of BPAER. The thermal degradation kinetics of this system was investigated by thermogravimetric analysis (TGA). It illustrated that the thermal degradation of BPSER/DDS has nth order reaction kinetics. © 2000 Society of Chemical Industry     

Anomalous behavior of cured epoxy resins: Density at room temperature versus time and temperature of cure

The room temperature density (ρ_RT) of a difunctional aromatic epoxy resin cured with a tetrafunctional aromatic amine passes through a maximum value in the vicinity of gelation with increasing conversion. For a given cooling rate cure resutls in a unique value of ρ_RT for each conversion as long as the material does not vitrify on cure. The occurrence of vitrification during cure eliminates the one-to-one relationship because of the nonequilibrium nature of the glass transition region and of the glassy state. In the glass transition region there is competition between physical aging which increases the density and chemical aging which, after gelation, decreases ρ_RT. After gelation, prolonged isothermal cure and physical aging to well beyond vitrification result in limiting values of ρ_RT which decrease with increasing temperature of cure. The maximum in the ρ_RT vs. conversion relationship is discussed in terms of the effects of shrinkage due to cure, the corresponding nonlinear increase in the glass transition temperature with increasing conversion after gelation, and longer relaxation times in the glass transition region with increasing crosslink density. Other factors which affect room temperature density are discussed.     

Thermal cure behavior of unsaturated polyester/phenol blends

Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to detect and simulate the cure behavior of unsaturated polyester (UP), phenol, and UP/phenol blends and to calculate and predict the cure rate, cure temperature, conversion, and changes in the glass‐transition temperature along with various cure orders in order to obtain the optimum parameters for processing. With dynamic scanning and isothermal DSC procedures and Borchardt–Daniels dynamic software, cure data for the UP resin were obtained, 90% of the conversion rate at 100°C being achieved after 15 min. However, for the phenol and UP/phenol blends, gradually increasing the temperature was found to be best for curing according to the DSC and DMA test results. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1041–1058, 2004     

Cure kinetics of neat and carbon-fiber-reinforced TGDDM/DDS epoxy systems

The cure kinetics of neat and carbon fiber-reinforced commercial epoxy systems, based on Tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and 4,4′-diaminodiphenylsulfone (DDS) were studied by means of differential scanning calorimetry (DSC). Analysis of DSC data indicated that the presence of the carbon fibers has a very small effect on the kinetics of cure. A kinetic model, arising from an autocatalyzed reaction mechanism, was applied to isothermal DSC data. The effect of diffusion control was incorporated into the reaction kinetics by modifying the overall rate constant, which is assumed to be a combination of the chemical rate constant and the diffusion rate constant. The chemical rate constant has the usual Arrhenius form, while the diffusion rate constant is described by a type of the Williams-Landel-Ferry (WLF) equation. The kinetic model, with parameters determined from isothermal DSC data, was successfully applied to dynamic DSC data over a broad temperature range that covers usual processing conditions. © 1996 John Wiley & Sons, Inc.     

CURING KINETICS AND THERMAL PROPERTY CHARACTERIZATION OF BISPHENOL-F EPOXY RESIN AND DDS SYSTEM

The curing kinetics of bisphenol-F epoxy resin (BPFER)/4,4′-diaminodiphenyl sulfone (DDS) system were studied by isothermal experiments using a differential scanning calorimeter (DSC). Autocatalytic behavior was shown in the first stages of the cure for the system, which could be well described by the model proposed by Kamal that includes two rate constants, k ₁ and k ₂, and two reaction orders, m and n. The curing reaction at the later stages was practically diffusion-controlled due to the onset of gelation and vitrification. To consider the diffusion effect more precisely, diffusion factor, f(α), was introduced into Kamal's equation. Thus, the curing kinetics could be predicted well over the whole range of conversion covering both pre- and postvitrification stages. The glass transition temperatures (T_gs) of the BPFER/DDS system isothermally cured partially were determined by means of torsional braid analysis (TBA), and the results showed that T_gs increased with conversion up to a constant value. The highest T_g was 406.2 K. The thermal degradation kinetics of cured BPFER were investigated by thermogravimetric analysis (TGA), revealing two decomposition steps.     

Effect of the extent of cure on the modulus,glass transition,water absorptio,and density of an amine-cured epoxy

The modulus, density, glass transition temperature (T_g), and water absorption characteristics of an amine-cured resin [diglycidyl ether of bisphenol A (Epon 828)/diaminodiphenyl sulfone (DDS)] were studied as a function of extent of cure. The glass transition is a function of the extent of cure and reaches a maximum temperature, T, when it is completely cured; specimens with different extents of cure were formed by isothermal cure below T, for different times. After slowly cooling, the density at each extent of cure was obtained at room temperature. Moisture absorption was monitored gravimetrically at 25°C for 2 months at several humidity levels. The room temperature density and modulus decreased with increasing extent of conversion whereas the glass transition temperature and equilibrium water absorption increased. The equilibrium water absorption increased linearly with relative humidity, and the absorptivity increased linearly with specific volume. An interpretation of these anomalous results is made in terms of the nonequilibrium nature of the glassy state. The glass transition temperature increases as the extent of cure increases resulting in a material that is further from equilibrium at room temperature and therefore has more free volume and a greater propensity to absorb water.     

Curing kinetics and reaction‐induced homogeneity in networks of poly(4‐vinyl phenol) and diglycidylether epoxide cured with amine

Mixtures of diglycidyl ether of bisphenol‐A (DGEBA) epoxy resin with poly(4‐vinyl phenol) (PVPh) of various compositions were examined with a differential scanning calorimeter (DSC), using the curing agent 4,4′‐diaminodiphenylsulfone (DDS). The phase morphology of the cured epoxy blends and their curing mechanisms depended on the reactive additive, PVPh. Cured epoxy/PVPh blends exhibited network homogeneity based on a single glass transition temperature (T_g) over the whole composition range. Additionally, the morphology of these cured PVPh/epoxy blends exhibited a homogeneous network when observed by optical microscopy. Furthermore, the DDS‐cure of the epoxy blends with PVPh exhibited an autocatalytic mechanism. This was similar to the neat epoxy system, but the reaction rate of the epoxy/polymer blends exceeded that of neat epoxy. These results are mainly attributable to the chemical reactions between the epoxy and PVPh, and the regular reactions between DDS and epoxy. Polym. Eng. Sci. 45:1–10, 2005. © 2004 Society of Plastics Engineers.     

Cure characterization of triglycidyl epoxy/aromatic amine systems

The curing of triglycidyl para-aminophenol (TGPAP) epoxy resin with three aromatic amine hardeners, diaminodiphenye sulphone (DDS), pyridinediamine (PDA), and toluenediamine (TDA), has been investigated. A series of iosthermal cures was conducted and analyzed by Fourier transform infrared spectrometry (FTIR) and differential scanning calorimetry (DSC). The chemical reactions occurring during cure were monitored at different temperatures by qualitative and quantitative estimation of different groups in the IR spectra, and the ratio of rate constants (k₂/k₁) were evaluated. Dynamic DSC analysis of TGPAP/TDA resulted in two exothermal peaks, indicating cure kinetics different from those of TGPAP/DDS and TGPAP/PDA systems, which gave a single exothermal peak. Various kinetic parameters such as total heat of reaction. ΔH′, activation energy E_a, Frequency factor z, and order of reaction n were evaluated for all the three systems. From the initial kick-off temperatures and activation energy values it was concluded that the rate of curing followed the order TDA > PDA > DDS. The reaction conversions during cure, evaluated from IR analysis, were exactly the same as those obtained from DSC Borchardt–Daniels kinetics. Using this model, the plots of time vs. temperature for different conversions were constructed for all the three systems; on the basis of these, the cure cycles can be fixed. © 1995 John Wiley & Sons, Inc.     

Characterization of thermoset cure behavior by differential scanning calorimetry. Part II: Effects of low-profile additives

An autocatalyzed second-order kinetic model was adopted to compare the isothermal and dynamic cure behavior of low-profile polyester resins in terms of kinetic parameters such as degree of cure, cure rate, half-life, onset cure temperature, reaction order, and Arrhenius parameters. The reaction orders of low-profile unsaturated polyesters appear to be almost independent of isothermal cure temperatures. The ultimate conversion, conversion at peak maximum, onset cure temperature, and Arrhenius parameters of polyesters are only slightly affected by the concentration and type of low-profile additives in the resins. Low-profile additives, in general, tend to retard the cure rate and suppress the exothermicity of polyester resins because of dilution effects. For low-profile additives such as poly(vinyl acetate) and polyurethane, which are “quite” compatible with polyesters before cure, the overall reaction rates of the resins are substantially enhanced over those of less compatible additives. However, the ultimate conversion of low-profile polyesters is found to be slightly greater than that of neat polyester, in most cases.     

Influence of the isothermal cure temperature on the nanostructure and thermal properties of an epoxy layered silicate nanocomposite

The cure kinetics of triglycidyl p‐amino phenol (TGAP) epoxy resin with a diamine (4,4‐diamino diphenyl sulphone [DDS]), reinforced with montmorillonite (MMT), has been studied by differential scanning calorimetry. The isothermal cure reaction consists of two parts: a rapid intra‐gallery reaction, attributed to homopolymerization of the TGAP catalyzed by the MMT and the extra‐gallery cross‐linking reaction of the TGAP with the DDS. Increasing cure temperature promotes the intra‐gallery reaction, which should promote an exfoliated nanostructure; this is confirmed by transmission electron microscopy. These results indicate that this system (TGAP/DDS/MMT) is an excellent candidate for achieving exfoliated polymer‐layered silicate nanocomposites and identifies a protocol for optimizing the degree of exfoliation. POLYM. ENG. SCI., 54:51–58, 2014. © 2013 Society of Plastics Engineers     

Studies on the curing kinetics and thermal stability of diglycidyl ether of bisphenol‐A using mixture of novel,environment friendly sulphur containing amino acids and 4,4′‐diaminodiphenylsulfone

This article describes the curing behavior of diglycidyl ether of bisphenol‐A using Cysteine (A)/ Methionine (B)/Cystine (C)/ mixture of 4,4′‐diaminodiphenyl sulfone (DDS) and Cysteine/DDS and Methionine/DDS and Cystine in various molar ratios as curing agent. Differential scanning calorimetry was used to study the cure kinetics by recording the DSC scans at heating rates of 5, 10, 15, and 20°C/min. The peak exotherm temperature was found to be dependent on the heating rate, structure of the amino acids and on the DDS/amino acids molar ratio. A broad exotherm was observed in the temperature range of 150–245°C (EA), 155–240°C (EB), and 190–250°C (EC). Curing of DGEBA with mixture of amino acids and 4, 4′‐diaminodiphenyl sulfone (DDS) resulted in a decrease in characteristic curing temperatures. Activation energy of curing reaction is determined in accordance to Ozawa's method and was found to be dependent on the structure of the amino acids and on the ratio of 4,4′‐diaminodiphenyl sulfone (DDS) to amino acid. Thermal stability of the isothermally cured resins was evaluated using dynamic thermogravimetry in nitrogen atmosphere. No significant change has been observed in the char yield of all the samples, but it was highest in the system cured using either Cystine alone (EC‐1) or a mixture of DDS/Cystine (EC‐2, EC‐3, and EC‐4). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Variation in dielectric properties of an epoxy-novolac molding compound during dynamic cure

The variation in dielectric characteristics of an epoxy-novolac molding compound during dynamic cure was studied by means of dielectrometry (DE). Dielectric parameters such as permittivity (?′), loss factor (?″), and dissipation (tan δ) were observed to depend on various factors including temperature, frequency, the extent of cure (α, measured previously by using differential scanning calorimetry, DSC), as well as contributions from ionic conductivity (σ) and electrode polarization. The characteristic relaxation time (τ) and the relaxed permittivity (?_r), suggested in the literature as possible parameters for cure monitoring purposes, was found difficult to determine because of interfernce from ionic conduction and electrode polarization. In comparison, σ could be measured throughout the entire cure process and was observed to depend only on temperature and α. In combination with our previous DSC results, an empirical function relating α to temperature and α was constructed. Our analysis also indicated that the strong maximum in the ?′ and the ?″ curves during the course of dynamic cure was a direct result of the α-and the temperature-dependence of σ; the strong maximum of ?″ is directly related neither to gelation nor to vitrification.