Curing behavior of epoxy resins in two‐stage curing process by non‐isothermal differential scanning calorimetry kinetics method

In this research, a new thermal curing system, with two‐stage curing characteristics, has been designed. And the reaction behaviors of two different curing processes have been systematically studied. The non‐isothermal differential scanning calorimetry (DSC) test is used to discuss the curing reaction of two stages curing, and the data obtained from the curves are used to calculate the kinetic parameters. Kissinger‐Akahira‐Sunose (KAS) method is applied to determine activation energy (E_a) and investigate it as the change of conversion (α). Málek method is used to unravel the curing reaction mechanism. The results indicate that the curing behaviors of two different curing stages can be implemented successfully, and curing behavior is accorded with ?esták‐Berggren mode. The non‐isothermal DSC and Fourier transform infrared spectroscopy test results reveal that two different curing stages can be implemented successfully. Furthermore, the double x fitting method is used to determine the pre‐exponential factor (A), reaction order (m, n), and establish the kinetic equation. The fitting results between experiment curves and simulative curves prove that the kinetic equation can commendably describe the two different curing reaction processes. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40711.     

Curing behavior of a novolac‐type phenolic resin analyzed by differential scanning calorimetry

Curing of a novolac‐type phenolic resin was studied by DSC. The kinetic analysis was performed by means of the dynamic Ozawa method at heating rates of 5, 10, 15, and 20°C/min. This analysis was used to determine the kinetic parameters of the curing process. The activation energy was found to be 144 kJ/mol. It was found that the Ozawa exponent values decreased with increasing reaction temperature from 3.5 to 1, suggesting a change in the reaction mechanism from microgel growth to diffusion‐controlled reaction. The reaction rate constant was found to range from 123.0 to 33.6 (°C/min)ⁿ. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1678–1682, 2003     

Curing kinetics of epoxy resin–imidazole–organic montmorillonite nanocomposites determined by differential scanning calorimetry

The curing kinetics of epoxy resin–imidazole–organic montmorillonite nanocomposites were investigated by differential scanning calorimetry (DSC) in the isothermal mode. X‐ray diffraction (XRD) analysis indicated the formation of a layered silicate–epoxy nanocomposite. The cure rates for the epoxy resin–imidazole–organic montmorillonite nanocomposite were lower than the values for the neat system at higher temperature (120 and 130°C), as indicated by the relation between the cure conversion and time. These results revealed that the autocatalytic model and the modified Avrami equation are both valid for describing the cure behaviors of epoxy resin–imidazole–organic montmorillonite systems. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2932–2941, 2003     

Cure kinetics of a glass/epoxy prepreg by dynamic differential scanning calorimetry

Dicyandiamide (DICY)‐cured epoxy resins are important materials for structural adhesives and matrix resins for fiber‐reinforced prepregs. Dynamic differential scanning calorimetry (DSC) with heating rates of 2.5, 5, 10, and 15°C/min was used to study the curing behavior of the epoxy prepreg Hexply 1454 system, which consisted of diglycidyl ether of bisphenol A, DICY, and Urone reinforced by glass fibers. The curing kinetic parameters were determined with three different methods and compared. These were the Kissinger, Ozawa, and Borchardt–Daniels kinetic approaches. The lowest activation energy (76.8 kJ/mol) was obtained with the Kissinger method, whereas the highest value (87.9 kJ/mol) was obtained with the Borchardt–Daniels approach. The average pre‐exponential factor varied from 0.0947 × 10⁹ to 2.60 × 10⁹ s⁻¹. The orders of the cure reaction changed little with the heating rate, so the effect of the heating rate on the reaction order was not significant. It was interesting that the overall reaction order obtained from all three methods was nearly constant (≅2.4). There was good agreement between all of the methods with the experimental data. However, the best agreement with the experimental data was seen with the Ozawa kinetic parameters, and the most deviation was seen with the Borchardt kinetic parameters. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Cure kinetics of epoxy/anhydride nanocomposite systems with added reactive flame retardants

Montmorillonite nanocomposite systems obtained from epoxy cured using anhydride and the addition of a reacting flame retardant are studied in this paper. In particular, a thermokinetic analysis of the behavior of five different compounds was performed, using a differential scanning calorimeter. The isothermal tests showed double reaction peaks, due to the cure reactions of DGEBA/acid anhydride systems. The comparisons between dynamic thermograms (and between isothermal ones, too) for the different mixtures also showed that the addition of other active substances (such as a nanofiller or a flame retardant additive) does not change the mechanism of crosslinking from a qualitative point of view, but both the nanoreinforcement and the flame retardant seemed to exert an evident catalytic action on the cure reactions. A model describing the cure behavior of the aforementioned materials is proposed in this work. This model takes into account the fact that the reaction mechanism of each analyzed system is composed of a couple of parallel phenomena: the fast opening of anhydride ring (corresponding to a first exothermic peak and characterized by “n‐th order” kinetics) and resin networking (corresponding to a second exothermic peak and characterized by an “auto‐catalytic with zero initial velocity” behavior). The verification of the proposed model was performed by means of a comparison between experimental data (normalized curves derived from DSC thermograms) and theoretical data (derived from a numerical integration—using the second order Runge–Kutta method—of the model‐representative equation) and provided very good results. This allows one to apply such a model to any engineering process problem concerning the cure of DGEBA/acid anhydride/phyllosilicate nanocomposite systems. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1676–1689, 2004     

Activation energy and curing behavior of resol‐ and novolac‐type phenolic resins by differential scanning calorimetry and thermogravimetric analysis

The thermal behavior, thermal degradation kinetics, and pyrolysis of resol and novolac phenolic resins with different curing conditions, as a function of the formaldehyde/phenol (F/P) molar ratio (1.3, 1.9, and 2.5 for the resol resins and 0.5, 0.7, and 0.9 for the novolac resins) were investigated. The activation energy of the thermal reaction was studied with differential scanning calorimetry at five different heating rates (2, 5, 10, 20, and 40°C/min) between 50 and 300°C. The activation energy of the thermal decomposition was investigated with thermogravimetric analysis at five different heating rates (2, 5, 10, 20, and 40°C/min) from 30 to 800°C. The low molar ratio resins exhibited a higher activation energy than the high molar ratio resins in the curing process. This meant that less heat was needed to cure the high molar ratio resins. Therefore, the higher the molar ratio was, the lower the activation energy was of the reaction. As the thermal decomposition of the resol resins proceeded, the activation energy sharply decreased at first and then remained almost constant. The activation energy of the thermal decomposition for novolac resins with F/P = 0.5 or F/P = 0.7 was almost identical in all regions, whereas that for novolac resins with F/P = 0.9 gradually decreased as the reaction proceeded. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2589–2596, 2003     

Effect of wood on the curing behavior of commercial phenolic resin systems

Differential scanning calorimetry (DSC) was used to study the effect of wood on the curing behavior of two types of commercial oriented‐strand‐board phenolic resins. DSC analysis showed that the curing behavior of the core resin differed significantly from that of the face resin in terms of the peak shape, peak temperature, and activation energy. The addition of wood to the resins moved the two separated peaks in the DSC curves of the core resin adjacent to each other. It also accelerated the addition reactions in the curing processes of both the core and face resins. The two peaks in the DSC curves were the result of the high pH values of the resins. These two peaks became either jointed together or overlapped when the pH value of the resin was reduced. Wood also reduced the activation energies for both the core and face resins by decreasing the pH values of the curing systems. Moreover, the effects of wood on the curing behavior of the resins among the five species studied were similar. The lowest activation energy for a phenolic resin probably appeared at pH 10–11 under alkaline conditions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 185–192, 2005     

Intercalation of epoxy resin in organically modified montmorillonite

Various methods of preparation of epoxy resin/clay mixtures, before the addition of the crosslinking agent and curing to form epoxy‐based polymer layered silicate (PLS) nanocomposites, have been investigated to determine their effect on the nanostructure. Organically modified montmorillonite clay was used, and the mixtures were prepared by both simple mixing and solvent‐based methods. X‐ray diffraction shows that intercalation of the resin into the clay galleries occurs for all clay loadings up to 25 wt % and for both preparation methods, but the dispersion of the clay in the resin, observed by optical microscopy, is significantly better for the solvent preparation method. Differential scanning calorimetry (DSC) shows that the intercalated resin has the same molecular mobility as the extra‐gallery resin, but suggests that the intercalated resin does not penetrate completely into the galleries. Prolonged storage of the resin/clay mixtures at room temperature leads to changes in the DSC response, as well as in the response to thermogravimetry, which are interpreted as resulting from homopolymerization of the epoxy resin, catalyzed by the onium ion in the modified clay. This confirms and explains the earlier observation of Benson Tolle and Anderson (J Appl Polym Sci 2004, 91, 89) that “conditioning” of the resin/clay mixtures at ambient temperature has a significant effect when the crosslinking agent is subsequently added, and indicates that the preparation method has important consequences for the nanostructure development in the PLS nanocomposites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:3751–3763, 2006     

Curing studies of bisphenol a based bismaleimide and cloisite 15a nanoclay blends using differential scanning calorimetry and model‐free kinetics

Blends of 2,2‐bis[4‐(4‐maleimidophenoxy phenyl)]propane [bismaleimide (BMIX)] with different proportions (1, 2, 3, 4, 5, 7, and 9%) of the nanoclay Cloisite 15a were prepared with ultrasonication. Fourier transform infrared studies reveal the existence of interactions between the clay particles and the imide rings in BMIX. The difference in the melting characteristics and the decrease in the curing window caused by the incorporation of the clay particles in BMIX, as evidenced by detailed differential scanning calorimetry investigations, confirmed the existence of interactions between the nanoclay particles and BMIX molecules. The Flynn–Wall–Ozawa, Vyazovkin, and Friedman kinetics methods were used to calculate the activation energies (E_a's) for the curing of the BMIX materials. E_a for the polymerization varied, depending not only on the amount of clay loaded in the BMIX but also on the extent of the curing reaction. Because of the loss of interaction between the clay platelets and the imide rings of BMIX, a decrease in E_a at higher reaction extents was noted when there was lower clay loadings (1–4% Cloisite 15a) in BMIX. A reversal in the previous behavior was noted at higher clay loadings (7 and 9% Cloisite 15a) in BMIX and was attributed to the restriction of the molecular mobility due to the presence of increased concentrations of clay platelets and the decreased availability of reaction sites for polymerization. These two opposing factors played were equal at the optimum level of Cloisite 15a loading (5%) in BMIX, which was reflected in the constancy of E_a variation noted with increasing reaction extent. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Dynamics and thermal properties of epoxy resin cured by new diamino disiloxanes

Two disiloxane compounds, 3,3′‐(1,3‐dimethyl‐1,3‐diphenyl‐1,3‐disiloxanediyl)bis(benzenamine) ( C1 ) and 4,4′‐(1,3‐dimethyl‐1,3‐diphenyl‐1,3‐disiloxanediyl)bis(benzenamine) ( C2 ) were synthesized and used as new curing agents of DGEBA epoxy resin with an epoxy value of 0.51 ( E‐51 ). The curing kinetics of E‐51/C1 and E‐51/C2 systems was investigated by non‐isothermal differential scanning calorimetry (DSC) analyses. The activation energy (ΔE) and the characteristic cure temperatures of the two systems were determined. The two systems have the similar activation energy. The reactivity of E‐51/C1 is higher than that of E‐51/C2 . The reaction orders of E‐51/C1 and E‐51/C2 are 0.88 and 0.87, respectively, illustrating that curing reaction between the epoxy resin and curing agent ( C1 or C2 ) is complicated. The DSC result shows that E51 cured by C2 has higher Tg; whereas thermogravimetric analysis results indicate that E51 cured by C1 has higher thermal stability. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42385.     

Isothermal differential scanning calorimetry study of the cure kinetics of a novel aromatic maleimide with an acetylene terminal

An isothermal differential scanning calorimetry (DSC) study on the cure kinetics was performed on N-(3-acetylenephenyl)maleimide (3-APMI) monomer to determine a suitable cure model. The 3-APMI monomer reported in our prior article was a novel aromatic maleimide monomer with an acetylene terminal that would be an ideal candidate for heat-resistant composites. The isothermal DSC study was carried out in the temperature range 150–200°C. Although the cure temperatures were different, the shapes of the conversion curves were similar, and all of the cure reactions could be described by an nth-order kinetic model. In particular, the cure reaction at the initial stage was a first-order kinetic reaction. The cure kinetic parameters of the 3-APMI monomer, including the reaction model, activation energy, and frequency factor, were determined. This information was very useful for defining the process parameters, final properties, and quality control of the cured 3-APMI monomer. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Online near‐infrared measurements of an epoxy cure process compared to mathematical modeling based on differential scanning calorimetry measurements

An epoxy resin containing diglycidyl ether of bisphenol A, dicyandiamide, and an accelerator (diurone) was investigated under different cure cycles. The mathematical prediction of the degree of cure in a thermoset as a function of time and temperature was investigated and compared to measured data. Near‐infrared analysis was used to measure the conversion of epoxy and primary amine and the production of hydroxyl. Modulated differential scanning calorimetry was used to measure the changes in the heat capacity during cure. The measurements revealed differences in the primary amine conversion and hydroxyl production, and close relations to the measurements of heat capacity were found. The measurements of the degree of cure revealed that cure cycles initiated at 80°C produced a lower degree of cure than cure cycles initiated at 90°C, although all cure cycles were postcured at 110°C. These findings were to some degree supported by measurements of the primary amine conversion and hydroxyl production. The characteristics found were attributed to differences in the cure mechanisms. The mathematical model did not incorporate these differences, and this may have led to discrepancies between the predicted and actual values of the degree of cure. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Analysis of the cure reaction of carbon nanotubes/epoxy resin composites through thermal analysis and Raman spectroscopy

The effect of the incorporation of single‐walled carbon nanotubes (SWNTs) onto a diglycidyl ether of bisphenol A‐based (DGEBA) epoxy resin cure reaction was investigated by thermal analysis and Raman spectroscopy. The results of the investigation show that SWNTs act as a strong catalyst. A shift of the exothermic reaction peak to lower temperatures is, in fact, observed in the presence of SWNTs. Moreover, these effects are already noticeable at the lowest SWNT content investigated (5%) with slight further effects at higher concentrations, suggesting a saturation of the catalyzing action at the higher concentrations studied. The curves obtained under isothermal conditions confirm the results obtained in nonisothermal tests showing that the cure reaction takes less time with respect to the neat epoxy. The thermal degradation of cured DGEBA and DGEBA/SWNT composites was examined by thermogravimetry, showing a faster thermal degradation for DGEBA–SWNT composites. Raman spectroscopy was successfully applied to demonstrate that the observed changes in the cure reaction of the composites lead to a different residual strain on the SWNT bundles following a different intercalation of the epoxy matrix. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 452–458, 2003     

Curing of epoxy siloxane monomer with anhydride

Epoxy siloxane monomer, 1,3‐Bis[2‐(3‐{7‐oxabicyclo[4.1.0]heptyl})ethyl]‐tetramethyldisiloxane, was cured with methylhexahydrophthalic anhydride, and the catalysts, N,N‐dimethylbenzylamine (BDMA) and tetra‐n‐butylphosphonium o,o‐diethylphosphorodithioate (PX‐4ET), were compared. The curing reactivity of BDMA was higher than that of PX‐4ET, but the thermal stability of the polymer was lower. PX‐4ET caused less thermal discoloration, which increased in proportion to catalyst concentration. The optimum was 0.71–0.35 mol %. Maximum hardness and glass transition temperature as well as minimum coefficient of thermal expansion and thermal discoloration was achieved with equivalent amounts of epoxy and anhydride. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 946–951, 2005     

Differential scanning calorimetry characterization of urea–formaldehyde resin curing behavior as affected by less desirable wood material and catalyst content

Differential scanning calorimetry was applied to investigate the curing behavior of urea–formaldehyde (UF) resin as affected by the catalyst content and several less desirable wood materials (e.g., wood barks, tops, and commercial thinnings). The results indicate that the reaction enthalpy of UF resin increased with increasing catalyst content. The activation energy and peak temperature of the curing UF resin generally decreased with increasing catalyst content at lower levels of catalyst content. However, with further increases in catalyst content, the changes in the activation energy and peak temperature were very limited to nonexistent. The hydrolysis reaction of the cured UF resin occurred during the latter stages of the curing process at both lower level (<0.2%) and higher level (>0.7%) catalyst contents. This indicates that there existed an optimal range of catalyst content for the UF resin. The curing enthalpy of the UF resin decreased with increasing wood raw materials present due to the effect of diffusion induced by the wood materials and the changes in the phase of the curing systems. This suggests that the curing reactions reached a lower final degree of conversion for the wood–resin mixtures than for the UF resin alone. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2027–2032, 2005     

Improvement of the thermal stability of polyhydroxybutyrates by grafting with maleic anhydride by different methods: Differential scanning calorimetry,thermogravimetric analysis,and gel permeation chromatography

The crystallization and thermal degradation behaviors of polyhydroxybutyrate (PHB) grafted with maleic anhydride (MA) by different techniques were analyzed with differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and gel permeation chromatography (GPC). The results of DSC, TGA, and GPC analyses indicated that the grafting method could affect the crystallization rate, crystallinity, and thermal stability of PHB because of changes in the molecular weight of PHB and the amount of MA grafted during the reaction. The reduction of the molecular weight of PHB that reacted during the processing followed this order of methods: melt grafting > solvent grafting > mechanical grafting. However, the grafting ratio of MA followed this order of methods: melt grafting > mechanical grafting > solvent grafting. All three grafting methods significantly improved the thermal stability, therefore increasing the crystallization rate and melting temperature of the as‐received PHB. A grafting ratio of MA as low as 0.07 wt % could result in a significant improvement in the heat resistance of PHB. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Highly exfoliated nanostructure in trifunctional epoxy/clay nanocomposites using boron trifluoride as initiator

Epoxy/clay nanocomposites based upon a trifunctional epoxy resin, triglycidyl p‐amino phenol (TGAP), have been prepared by intercalating an initiator of cationic homopolymerization, a boron trifluoride monoethylamine (BF₃·MEA) complex, into the montmorillonite clay galleries before the addition of the TGAP and the curing agent, 4,4‐diamino diphenyl sulfone (DDS), and effecting the isothermal curing reaction. The BF₃·MEA enhances the intragallery cationic homopolymerization reaction, which occurs before the extragallery cross‐linking reaction of the TGAP with the DDS, and which hence contributes positively to the mechanism of exfoliation of the clay. The effects of isothermal cure temperature and of BF₃·MEA content have been studied, in respect of both the reaction kinetics, monitored by differential scanning calorimetry, and the nanostructure, as identified by small‐angle X‐ray scattering and transmission electron microscopy. It is shown that the use of BF₃·MEA in this way as an initiator of intragallery homopolymerization significantly improves the degree of exfoliation in the cured nanocomposites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40020.     

Isothermal crystallization kinetics of high‐flow nylon 6 by differential scanning calorimetry

The isothermal crystallization kinetics have been investigated with differential scanning calorimetry for high‐flow nylon 6, which was prepared with the mother salt of polyamidoamine dendrimers and p‐phthalic acid, an end‐capping agent, and ε‐caprolactam by in situ polymerization. The Avrami equation has been adopted to study the crystallization kinetics. In comparison with pure nylon 6, the high‐flow nylon 6 has a lower crystallization rate, which varies with the generation and content of polyamidoamine units in the nylon 6 matrix. The traditional analysis indicates that the values of the Avrami parameters calculated from the half‐time of crystallization might be more in agreement with the actual crystallization mechanism than the parameters determined from the Avrami plots. The Avrami exponents of the high‐flow nylon 6 range from 2.1 to 2.4, and this means that the crystallization of the high‐flow nylon 6 is a two‐dimensional growth process. The activation energies of the high‐flow nylon 6, which were determined by the Arrhenius method, range from ?293 to ?382 kJ/mol. The activation energies decrease with the increase in the generation of polyamidoamine units but increase with the increase in the content of polyamidoamine units in the nylon 6 matrix. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009