Nonisothermal curing of a solid resole phenolic resin

A commercial solid resole phenolic resin was thoroughly characterized with Fourier transform infrared spectroscopy, NMR, and gel permeation chromatography, and its nonisothermal curing reaction was studied systematically with differential scanning calorimetry at a series of heating rates (βs) of 3, 4.5, 5.7, and 10°C/min. The results show that the solid resole had a higher molecular weight than conventional liquid resoles, and its reactive hydroxymethyl (CH₂ OH) and dibenzyl ether (CH₂ O CH₂) functionalities participated in the crosslinking reaction upon heating. The nonisothermal curing reaction of the solid resole exhibited a relatively constant reaction heat, whereas the onset, peak, and end curing temperatures increased gradually with increasing βs. In addition, the reaction kinetics of the solid resole was analyzed with an nth‐order reaction model, the global activation energy was determined with the Kissinger method, and the reaction order was derived from the Crane equation. The obtained rate equation was applied to simulate the reaction time, conversion, and reaction rate, with a good fit achieved between the experimental data and the model predications. In conclusion, this study provided us with new knowledge on solid resoles at a molecular level and was also a great help for the curing procedure design, property optimization, and practical application of this commercial solid resole. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of diluents on the curing behavior of vinyl ester resin

An acidic vinyl ester resin (～6 mg KOH per gram of solid) was prepared by reacting a bisphenol‐A‐based epoxy resin with acrylic acid in the presence of tributyl amine. The acrylated epoxy resin thus obtained was characterized by Fourier transform infrared spectroscopy. Five samples of vinyl ester resin containing styrene and methyl methacrylate (MMA) in the weight ratios 40:0, 30:10, 20:20, 10:30, and 0:40 were prepared at 30°C, and their curing behavior was studied by differential scanning calorimetry in the presence of benzoyl peroxide (2 phr). Curing behavior was dependent on the ratio of the two monomers used as reactive diluents. The kinetic parameters were determined by Ozawa's method. The energy of activation and frequency factor varied from 17 to 23 kcal mol^?1 and 8.47 × 10⁹ to 5.21 × 10¹² min^?1, respectively and were lowest for the samples containing 30:10 and 10:30 styrene/MMA weight ratios. The curing reactions followed first‐order kinetics and obeyed the Arrhenius rate expression. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1948–1951, 2003     

Use of isoconversional methods to analyze the cure kinetics of isocyanate‐ended quasi‐prepolymers with water

The curing of an isocyanate (NCO)‐ended quasi‐prepolymer by reaction with water in stoichiometric ratio was monitored by using differential scanning calorimetry both under isothermal and non‐isothermal conditions. A quasi‐prepolymer containing 16 wt % free NCO prepared by reacting trifunctional polypropylene glycol (PPG) with polymeric MDI was used in this study. The variation of the effective activation energy with the extent of the curing reaction was calculated by means of model‐free differential and integral isoconversional methods. Both isoconversional methods provided similar results showingthat the activation energy depends on the extent of thereaction of the quasi‐prepolymer with water. The dependence of the effective activation energy proved to be different under isothermal and non‐isothermal curing conditions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1049–1057, 2007     

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     

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     

Curing characteristics of urea–formaldehyde resin in the presence of various amounts of wood extracts and catalysts

The characteristics of urea–formaldehyde (UF) resin curing in the presence of wood extracts and a catalyst [ammonium chloride (NH₄Cl)] were investigated by differential scanning calorimetry (DSC). The effects of extracts from 16 wood species on resin curing behaviors were evaluated. A model developed in this study, T_p = 53.296 exp(?9.72C) + 93.104, could be used to predict the resin curing rate in terms of the DSC peak temperature (T_p) as influenced by the NH₄Cl content (C). The results indicated that the curing rate of UF resin increased as the catalyst content increased and reached a maximum when the catalyst content ranged from 0.5 to 1.0% (solid basis over liquid UF resin weight). Further increases in the catalyst content had no effect on the resin curing rate. The curing rates of UF resin in the presence of wood extracts increased with decreased pH values or increased base buffer capacities. It was also discovered that the activation energy could not fully explain the resin curing behavior when some species of wood extracts were present, and therefore, the pre‐exponential factor had to be taken into account. The concept of the equivalent catalyst content (ECC) of wood extracts to the NH₄Cl content was introduced in this study; ECCs ranged from 0.0022 to 0.0331% among the 16 wood species. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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     

Curing behavior and network formation of cyanate ester resin/polyethylene glycol

The mechanism and kinetics of the curing reaction of cyanate ester (CE) resin modified with polyethylene glycol were investigated by means of differential scanning calorimetry (DSC) and Fourier‐transform infrared spectroscopy (FTIR). The relationship of heat flow versus conversion rate was used to evaluate the effects of polyethylene glycol (PEG) on the curing reaction of CE. DSC results showed that the addition of PEG decreased the curing temperature of CE effectively when its content was less than 20 wt %. The curing behavior of CE/PEG still complies with the self‐catalytic kinetic model proposed by Kamal. The effects of PEG content on the kinetics parameters and conversion rate of the curing reaction were discussed. FTIR results indicated that the –OH groups in PEG participated in the polymeric reaction of CE and formed –O–C (=NH) –O– structure through block copolymerization, which extended the chain length between triazine rings and reduced the density of triazine rings. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41841.     

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     

Polymers based on N,N‐diglycidylaniline. I. Investigations of the curing kinetics by dynamic differential scanning calorimetry measurements

N,N‐Diglycidylaniline was reacted with aniline (yielding polymer EP‐1) and the newly synthesized chromophore 4‐(phenylazo)aniline (yielding polymer EP‐2). The curing kinetics of these two epoxy resin systems was studied in dynamic experiments by means of differential scanning calorimetry. Kinetic parameters such as the activation energy and frequency factor were estimated with the Ozawa method [E(O) and A(O), respectively], the Kissinger method [E(K) and A(K), respectively], and the modified Avrami method [E(A) and A(A), respectively]. The activation energy and frequency factor of EP‐1 were much lower than those of EP‐2 estimated with the Ozawa, Kissinger, and Avrami methods. The activation energy and frequency factor for EP‐1 determined with the Ozawa method [E(O) = 55.8 kJ/mol, A(O) = 10 × 10³ 1/s] and the Avrami method [E(A) = 56.4 kJ/mol, A(A) = 9.2 × 10³ 1/s] were higher than those determined with the Kissinger method [E(K) = 51.0 kJ/mol, A(K) = 2 × 10³ 1/s]. In the case of EP‐2, the kinetic parameters calculated with the Ozawa model [E(O) = 140.4 kJ/mol, A(O) = 12.3 × 10¹³ 1/s] and the Kissinger model [E(K) = 139.9 kJ/mol, A(K) = 10.9 × 10¹³ 1/s] were higher than those calculated with the Avrami model [E(A) = 130.4 kJ/mol, A(A) = 7.9 × 10¹² 1/s]. The obtained polymers were characterized with Fourier transform infrared, ¹H‐NMR, differential scanning calorimetry, and ultraviolet–visible spectroscopy. The polymers exhibited low glass‐transition temperatures in the range of 57–79°C and good solubility in common organic solvents. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Time–temperature–transformation cure diagrams of phenol–formaldehyde and lignin–phenol–formaldehyde novolac resins

In this study, the time–temperature– transformation (TTT) cure diagrams of the curing processes of several novolac resins were determined. Each diagram corresponded to a mixture of commercial phenol–formaldehyde novolac, lignin–phenol–formaldehyde novolac, and methylolated lignin–phenol–formaldehyde novolac resins with hexamethylenetetramine as a curing agent. Thermomechanical analysis and differential scanning calorimetry techniques were applied to study the resin gelation and the kinetics of the curing process to obtain the isoconversional curves. The temperature at which the material gelled and vitrified [the glass‐transition temperature at the gel point (_gelT_g)], the glass‐transition temperature of the uncured material (without crosslinking; T_g0), and the glass‐transition temperature with full crosslinking were also obtained. On the basis of the measured of conversion degree at gelation, the approximate glass‐transition temperature/conversion relationship, and the thermokinetic results of the curing process of the resins, TTT cure diagrams of the novolac samples were constructed. The TTT diagrams showed that the lignin–novolac and methylolated lignin–novolac resins presented lower T_g0 and _gelT_g values than the commercial resin. The TTT diagram is a suitable tool for understanding novolac resin behavior during the isothermal curing process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Curing kinetics of medium reactive unsaturated polyester resin used for liquid composite molding process

The cure kinetics of medium reactivity unsaturated polyester resin formulated for Liquid Composite Molding process simulation was studied by Differential Scanning Calorimetry (DSC) under isothermal conditions over a specific range of temperature. For isothermal curing reactions performed at 100, 110, and 120°C, several influencing factors were evaluated using the heat evolution behavior of curing process. We propose two‐ and three‐parameter kinetic models to describe the cure kinetics of thermoset resins. Comparisons of the model solutions with our experimental data showed that the three‐parameter model was the lowest parameter model capable of capturing both the degree of cure and the curing rate qualitatively and quantitatively. The model parameters were evaluated by a non‐linear multiple regression method and the temperature dependence of the kinetic rate constants thus obtained has been determined by fitting to the Arrhenius equation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Curing Behavior of Epoxy Resin Using Controllable Curing Agents Based on Nickel Complexes

Summary: The curing reaction kinetics and mechanism of the diglycidyl ether of bisphenol A (DGEBA) with three complexes of Ni(II) with diethylentriamine (Dien), Pyrazole (Pz) and Pyridine (Py) as ligands have been studied using differential scanning calorimetry (DSC). The curing reaction was characterized by high cure onset and peak maximum temperatures. The kinetics of the curing reaction were evaluated using the Ozawa method. The average values of activation energy for the three nickel complexes increased in the order: Dien‐based curing agent > Pz‐based curing agent > Py‐based curing agent. Three main curing mechanisms (catalytic, complex cation and free ligand polymerization path) have been proposed depending on the cure temperature. It was also shown that the cure kinetics of DGEBA with Dien‐ and Py‐based complexes could be described by the Sestak‐Berggren equation. The water absorption, chemical resistance and thermal stability of the thermosets were also studied. The results showed that the thermoset obtained with the Py‐based complex was more thermally stable than those obtained with the other two curing agents.

Activation energy versus conversion plots for the epoxy systems studied.     

Preparation and properties of cyanate ester modified by epoxy resin and phenolic resin

Stiff and brittle cyanate ester (CE) resin was modified by copolymerizing it with epoxy resin (ER) and phenolic resin (PR) to improve its toughness and flexibility. The cure process of the modified CE resin was characterized by gel time curves and differential scanning calorimetry curves. The Fourier transform infrared spectra of the modified CE resin showed its chemical structure during the curing process. The mechanical properties, thermal behavior, dielectric properties, and morphology of the modified CE resins were investigated. The results showed that an increase in epoxy and phenolic resins resulted in improved flexibility while maintaining thermal stability. When the mass ratio of CE/ER/PR was 70 : 15 : 15 (w/w), flexural strength and impact strength of the modified CE resin increased from 113.6 MPa and 5.2 kJ/m² to 134.5 MPa and 16.7 kJ/m², respectively. Little of the thermal stabilityand dielectric properties was sacrificed in the modification of the CE. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3150–3156, 2007     

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     

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     

Resin‐transfer molding of natural fiber–reinforced plastic. I. Kinetic study of an unsaturated polyester resin containing an inhibitor and various promoters

In this study an autocatalytic model was used to describe the cure of a polyester system containing various promoters and an inhibitor. The effect of the initiator concentration was investigated. Isothermal DSC measurements were used to determine the kinetic parameters for the curing reaction. The rate of curing increased with increasing initiator concentration. The parameters were found to be temperature dependent. The nonlinear regression analysis showed that by fixing one parameter at a constant value the temperature dependency of the other parameters was described by simple relationships. The model was then compared to the experimental data. The reaction rate could be predicted fairly well in a wide range of temperatures. These results will be used to model the cure of this resin in a resin transfer‐molding (RTM) process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2553–2561, 2003     

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