Chemorheology of thermosetting resins. I. The chemorheology and curing kinetics of unsaturated polyester resin

The rheological properties and curing kinetics of a general-purpose polyester resin have been determined during isothermal cure. Both steady and oscillatory shearing flow properties were determined using a cone-and-plate rheometer, and the curing kinetics were determined using a differential scanning calorimeter (DSC). It was found that, as cure progresses, the steady shear viscosity increases very rapidly with cure time at all shear rates investigated, and normal forces show negative values at low shear rates and positive values at high shear rates. The observed negative normal forces are believed to result from material shrinkage during cure, and positive normal forces from the deformation of large molecules, formed by crosslinking reactions during cure. Note that, in a cone-and-plate rheometer, the shrinkage force acts in the direction opposite to that of normal forces. It is, therefore, concluded that extreme caution is needed in the interpretation of normal force measurements with thermosetting resins, subjected to steady shearing flow. Dynamic measurements seem to offer some insight on the onset of gel formation. More specifically, we have found that, when the unsaturated polyester resin was cured at a fast rate, the time at which a maximum in the loss modulus G” occurs coincides reasonably well with the time t_η∞ at which the steady shear viscosity η approaches infinity. However, at a slow rate of cure, the time at which tan δ equals unity agrees fairly well with t_η∞. DSC measurement has permitted us to determine the degree of cure as a function of cure time and the kinetic parameters in an empirical expression for the curing kinetics advanced by Kamal and co-workers. By combining the rheological and DSC measurements, we have constructed plots describing how the viscosity increases with the degree of cure, at various values of isothermal curing temperature.     

Chemorheology of thermosetting resins. IV. The chemorheology and curing kinetics of vinyl ester resin

The rheological properties and curing kinetics of a vinyl ester resin have been determined during isothermal cure. Both steady and oscillatory shearing flow properties were determined using a cone-and-plate rheometer, and the curing kinetics were determined using a differential scanning calorimeter (DSC). Also determined were the rheological properties and curing kinetics of the resin when it had been thickened using magnesium oxide (MgO), in the presence of calcium carbonate (CaCO₃) as filler and polyvinyl acetate (PVAc) as low-profile additive. The steady shearing flow behavior observed with the vinyl ester resin was found to be very similar to that observed with a general-purpose polyester resin, reported in Paper I of this series [C. D. Han and K. W. Lem, J. Appl. Polym. Sci., 28 , 3155 (1983)]. However, a significant difference in the oscillatory shearing flow behavior was found between the two resins. We have concluded that dynamic measurement is much more sensitive to variations in resin chemistry than steady shearing flow measurement. DSC measurement has permitted us to determine the degree of cure as a function of cure time. By combining the rheological and DSC measurements, we have constructed plots describing how the viscosity increases with the degree of cure, at various isothermal curing temperatures.     

Chemorheology of thermosetting resins. III. Effect of low-profile additive on the chemorheology and curing kinetics of unsaturated polyester resin

An experimental study was conducted to investigate the effect of low-profile thermoplastic additives on the rheological behavior during cure and the curing kinetics of unsaturated polyester resin. For the study, a general-purpose polyester resin was used and two different types of thermoplastic additive, poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA), were used as low-profile additives. It has been found that, during cure, the resin/PMMA system exhibits shearthinning behavior even before the cure time reaches the critical value tη∞ whereas the resin/PVAc system does not. Also, both PVAc and PMMA help reduce the shrinkage of the resin during cure. However, our study shows that shrinkage control becomes effective only when the shear rate is greater than a certain critical value. The curing behavior determined with the aid of differential scanning calorimetry (DSC) shows that the rate of cure and the final degree of cure are decreased when the amount of low-profile additive is increased.     

The effect of resin chemistry on the curing behavior and chemorheology of unsaturated polyester resins

The effect of the structure of unsaturated polyester resin on its curing and rheological behavior during isothermal cure has been investigated, using three different grades of resin. In the investigation, the structure of the resins was determined, using nuclear magnetic resonance spectrometry (NMR), together with chemical analysis. Both a differential scanning calorimeter (DSC) and an infrared (IR) spectrometer were used to determine the curing kinetics, and a cone-and-plate rheometer was used to determine the variation of rheological properties during isothermal cure. On the basis of the experimental study, we have concluded: (1) at the same styrene-to-fumarate mole ratio, the resin having isophthalates cures slower than the one having none; (2) everything else being equal, the resin having a high styrene-to-fumarate mole ratio cures faster than the resin having a low one; (3) the higher the concentration of initiator, the faster a resin cures. It has been found that a resin that cures faster does not necessarily achieve a higher final degree of cure than one that cures slowly. We have found that a mechanistic kinetic model developed in our previous investigation is very useful for investigating the reactivity of unsaturated polyester resin, by determining the rate constant and activation energy of the propagation reaction. On the basis of rheological investigation, we have concluded that both t_η∞ determined from steady shearing flow measurement and t_{tan δ = 1} determined from oscillatory shearing flow measurement may be used as a measure of gel time.     

The effect of the chemical structure of low-profile additives on the curing behavior and chemorheology of unsaturated polyester resin

An experimental study was conducted to investigate the effect of the chemical structure of low-profile additives on the curing behavior and chemorheology of unsaturated polyester resin during isothermal cure. For the study a general-purpose unsaturated polyester resin was cured in the presence of t-butyl perbenzoate as Initiator. The curing behavior of the resin was investigated using differential scanning calorimetry (DSC). Three different thermoplastic low-profile additives were used, namely poly(vinyl acetate) (PVAc), poly(styrene-co-butadiene), which is also known as KRATON DX-1300, and dehydrochlorinated Isobutylene/isoprene copolymer, often referred to as conjugated diene butyl (CDB) rubber. Each of the these additives, about 30 weight percent, was first dissolved in styrene. The solution was then mixed with unsaturated polyester resin and CaCO₃. The CaCO₃ particles helped stabilize the emulsions consisting of resin and KRATCN, and of resin and CDB. For each resin formulation, a series of isothermal DSC runs were made at various levels of cure pressure. It was found that for all three low-profile resins investigated, the final degree of cure went through a maximum as cure pressure was increased from atmospheric to 6.21 MPa (900 psi). We have observed evidence that in the presence of an initiator generating free radicals, the unsaturated double bonds in the KRATON and CDB undergo grafting reactions with the styrene monomers and unsaturated polyester resin, increasing the glass transition temperature of KRATON and CDB, to an extent which varies with the cure conditions employed. Both steady and oscillatory shearing flow properties were determined using a cone-and-plate rheometer. The rheological measurements indicate that the resin/CaCO₃/KRATON and resin/CaCO₃/CDB systems give rise to gel times shorter than the resin/CaCO₃/PVAc system. It is concluded that both KRATON and CDB are more effective, both for enhancing the rate of cure of unsaturated polyester resin and imparting impact properties to the cured composites, than those thermoplastic low-profile additives that contain neither unsaturated double bonds nor a chemical structure that has rubber-like properties in the solid state.     

Thermokinetics of unsaturated polyester and vinyl ester resins

An experimental study was carried out to investigate the isothermal and non-isothermal curing kinetics of unsaturated polyester and vinyl ester resins, using differential scanning calorimetry (DSC). Emphasis was put on investigating the effect of low-profile additives on the curing kinetics of the thermo-setting resins. For the study, a general-purpose polyester resin and a vinyl ester resin were used, together with polyvinyl acetate (PVAc) as low-profile additive, benzoyl peroxide as initiator, and N,N-dimethyl aniline as promoter. It has been found that (1) the addition of the low-profile thermoplastic-additive decreases the rate of cure and, also, the final degree of cure of the resins, (2) the total heat of cure generated by isothermal cure is lower than that generated by non-isothermal cure, and (3) the resin/initiator mixture with promoter exhibits two major exotherm peaks during non-isothermal cure, but only a single exotherm peak during isothermal cure.     

Curing of unsaturated polyester resins: Effect of surface treatment of particulates

The effect of surface treatment of particulates on the curing kinetics of unsaturated polyester resin has been investigated using a differential scanning calorimeter. Two coupling agents, γ-methacryloxy propyltrimethoxy silane (γ-MPS) and phenyltriethoxy silane (PTS), were employed. The former reacted with the resin; the latter did not. A kinetic model of free radical addition polymerization was used. A correction factor was used to represent the effective free radical concentration in order to account for the formation of charge transfer complex between the glass beads and the free radicals in the resin. The results indicate that (1) the resin with treated particulates showed generally faster reaction rate and conversion than the resin with untreated particulates; (2) no obvious difference on the curing kinetics was observed between these two coupling agents; the primary role of coupling agent is to inhibit the influence of particulates on the curing kinetics of the resin by effective surface coverage; and (3) a minimum aqueous concentration of 0.05 wt% was suggested for surface treatment of particulates; the reaction rate of the resin was slightly improved when the concentration of the coupling agent was 0.01 wt%; this might possibly be due to insufficient coverage of the particulate surface.     

Rheology of unsaturated polyester resins. I. Effects of filler and low-profile additive on the rheological behavior of unsaturated polyester resin

Measurements were taken of the bulk rheological properties of concentrated suspensions of particulates in unsaturated polyester resins, using a cone-and-plate rheometer. The particulates used were clay, calcium carbonate, and milled glass fiber. With clay and milled glass fibers, shear-thinning behavior of suspensions was observed at low shear rates or low shear stresses as the concentration of particulates was increased, whereas concentrated suspensions of calcium carbonate exhibited Newtonian behavior over the range of shear stresses or shear rates investigated. The cone-and-plate rheometer was also used for measurements of the bulk rheological properties of various mixtures of polyester resin and low-profile additives. For low-profile additives, solutions, in styrene, of poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) were used. It was found that the bulk viscosities of all mixtures of polyester resin and PVAc solution lie between those of the individual components, whereas the bulk viscosities of some mixtures of polyester resin and PMMA solution go through a minimum and a maximum, depending on the composition of the mixture. While all mixtures of polyester resin and PVAc solution exhibited negligible normal stress, some mixtures of polyester resin and PMMA solution exhibited noticeable normal stresses. It should be mentioned that polyester resin follows Newtonian behavior. It turned out that all mixtures of polyester resin and PVAc solution exhibited clear, homogeneous solutions, whereas mixtures of polyester resin and PMMA solution exhibited optical heterogeneity, i.e., turbidity. When polyethylene powders were used as low-profile additives, suspensions of polyester resin and polyethylene powders exhibited negative values of normal stress as the concentrations of suspension reached a critical value. When both filler and low-profile additive were put together in polyester resin, the rheological behavior became quite complex, indicating that some interactions exist between the filler and the low-profile additive.     

Structural Carbon/Epoxy Prepregs Properties Comparison by Thermal and Rheological Analyses

Two different carbon/epoxy prepreg materials were characterized and compared using thermal (DSC, TGA, and DMA) and rheological analyses. A prepreg system (carbon fiber preimpregnated with epoxy resin F584) that is currently used in the commercial airplane industry was compared with a prepreg system that is a prospective candidate for the same applications (carbon fiber prepreg/epoxy resin 8552). The differences in the curing kinetics mechanisms of both prepreg systems were identified through the DSC, TGA, DMA, and rheological analyses. Based on these thermal analysis techniques, it was verified that the curing of both epoxy resin systems follow a cure kinetic of n order. Even though their reaction heats were found to be slightly different, the kinetics of these systems were nevertheless very similar. The activation energies for both prepreg systems were determined by DSC analysis, using Arrhenius's method, and were found to be quite similar. DMA measurements of the cured prepregs demonstrated that they exhibited similar degrees of cure and different glass transition temperatures. Furthermore, the use of the rheological analysis revealed small differences in the gel temperatures of the two prepreg systems that were examined.     

Effect of particulates and fiber reinforcements on the curing behavior of unsaturated polyester resin

The effect of particulates and reinforcement on the curing behavior of unsaturated polyester resin was investigated. Also investigated was the effect of surface treatment of particulates on the curing behavior of unsaturated polyester resin. We have found that (1) an increase in the surface area, by either increasing the loading of particulates for a fixed particle size or decreasing the size of particulates for a fixed loading of particulates, enhanced the rate of cure, and (2) the treatment of glass beads with γ-methacryloxy propyltrimethoxy silane enhanced the rate of cure of unsaturated polyester resin.     

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.     

Curing behavior of a novel polytriazole resin

The curing behavior of a novel low temperature curing polytriazole resin, prepared from p‐xylylene diazide and N,N,N′,N′‐tetrapropargyl‐p,p′‐diaminodiphenylmethane, was investigated by DSC and rheological analyses. The kinetics of the curing of the resin was studied by nonisothermal and isothermal DSC measurements and the kinetics parameters were obtained. The values of apparent activation energy E_a of the curing reaction obtained by nonisothermal and isothermal DSC are 80.7 and 75.3 kJ/mol, respectively. The curing of the resin was traced by the isothermal rheological analysis. The gelation times of the resin at 70, 75, 80, and 85°C are about 200, 150, 110, and 75 min, respectively. The viscosity equation for the resin was found as follows: © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Predicting the cure of thermosetting polymers: The isoconversion map

The modeling of the cure kinetics is widely used to predict the progress of the chemical reaction during processing of thermosetting resins. In this study, a new technique named the “Isoconversion Map” is proposed to predict thermoset curing from a series of differential scanning calorimetry (DSC) analyses. On the basis of the isoconversion methodology, it is possible to devise a model‐free technique to predict resin conversion for a given temperature profile. In this work, the cure kinetics of an epoxy resin has been measured by dynamic DSC tests to construct the proposed “isoconversion map.” The evolution of the resin cure for a given temperature profile has been determined by applying the proposed approach and then compared with the predictions of common cure kinetics models. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

The effect of pressure on the curing behavior of unsaturated polyester resins

The effect of pressure on the curing behavior of unsaturated polyester resin was investigated, both experimentally and theoretically. The resin used was a general-purpose unsaturated polyester resin and the initiator used was t-butyl perbenzoate. A series of isothermal runs with differential scanning calorimetry (DSC) were made at various levels of cure pressure. It was found that the rate of cure was retarded under pressure, and that the ultimate degree of cure went through a maximum at a certain pressure as the cure pressure was increased from atmospheric pressure to 6.21 MPa (900 psi). It was interpreted that pressure has two competing effects on the curing behavior of unsaturated polyester resin; one is a free volume effect that hinders the curing reaction and the other is a thermodynamic effect that favors it. Therefore, when the pressure is higher than a certain level, the free volume effect becomes predominant over the thermodynamic effect, the ultimate degree of cure diminishing as the cure pressure is increased beyond that level. Theoretical interpretation of the experimental results is given, using a mechanistic kinetic model developed in our previous publication.     

Influence of oxidation treatment of carbon powders on the cure kinetics of amine cured tetrafunctional epoxy resins

A tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM) type tetrafunctional epoxy resin containing carbon powders was cured with the stoichiometric amount of a tetrafunctional curing agent, namely m-phenylenediamine (mPDA). Carbon powders were oxidised with air or nitric acid. The influence of carbon powders on curing of the resin was followed by dynamic mechanical analysis, Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Gelation and vitrification times were determined as a function of the variations of dynamic properties. The evolution of viscoelastic modulus during curing of the different mixtures showed that untreated carbon powder clearly accelerated the kinetics of curing whilst oxidation of carbon powders could remove their catalysing effect. These results were confirmed by monitoring the changes in conversion of epoxy and amine groups during cure using the FTIR technique. DSC experiments also showed the influence of carbon powder as a catalyst and the loss of the catalysing effect as a consequence of chemical treatment.     

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.     

Cure kinetics of aqueous phenol–formaldehyde resins used for oriented strandboard manufacturing: Analytical technique

The cure kinetics of commercial phenol–formaldehyde (PF), used as oriented strandboard face and core resins, were studied using isothermal and dynamic differential scanning calorimetry (DSC). The cure of the face resin completely followed an nth‐order reaction mechanism. The reaction order was nearly 1 with activation energy of 79.29 kJ mol^?1. The core resin showed a more complicated cure mechanism, including both nth‐order and autocatalytic reactions. The nth‐order part, with reaction order of 2.38, began at lower temperatures, but the reaction rate of the autocatalytic part increased much faster with increase in curing temperature. The total reaction order for the autocatalytic part was about 5. Cure kinetic models, for both face and core resins, were developed. It is shown that the models fitted experimental data well, and that the isothermal DSC was much more reliable than the dynamic DSC in studying the cure kinetics. Furthermore, the relationships among cure reaction conversion (curing degree), cure temperature, and cure time were predicted for both resin systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1642–1650, 2006     

Water vapor transport in an epoxy resin based on TGMDA and DICY

Water uptake has been measured in an epoxy resin based on tetraglycidylmethylenedianiline curved with dicyanidamide. The curing behavior of this system as elucidated by differential scanning calorimetry and Fourier transform infrared is complex. Based upon this information we selected curing temperatures and times in addition to the “standard” cure. The kinetics of the sorption of water by the materials which have undergone the standard cure indicate that the two modes of sorption are involved at high humidity and only a single mode at lower humidity (as seen by changes in the slope of the log M_t vs log t plots). The kinetics of the sorption in the resins which have undergone post cure at higher temperatures also indicate two or more modes of sorption at high humidities. However the slopes of the log M_t vs log t plots differ from those for the resin with standard cure. Subsequent sorption/desorption cycles on the standard cure resin showed marked increases in the initial sorption rate as well as changes in mode, suggesting that irreversible changes in the resin had occurred.     

The effect of curing of unsaturated polyester resin on the dynamic relaxation time

The effect of hot curing of unsaturated polyester resin on the dynamic relaxation time was studied using dielectric measurements along with two dynamic mechanical measurement methods. It was found that the dynamic response during cure was a material frequency dependent property and did not depend on the measurement method. All relaxation times, measured during cure, by all three measurement methods used, converged to a single equation: τ(t)_av=at^b where t= curing time, a, b=constants. The increase of the relaxation time during cure followed the same trend as a friction factor, which was found to increase with conversion. The crosslinking density was found to increase slowly with conversion, while the relaxation time increased exponentially. These two different modes of behavior during cure explain the high resolution of dynamic measurements as a cure monitoring tool, which can easily detect small curing changes. This behavior of the relaxation time was explained by the sharp rise of activation energy due to a parallel decrease of free volume at high conversion.     

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.