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.     

Cure kinetics and catalyzed effect of hydroxyl group of LC diglycidyl ether of bisphenol‐S with aromatic diamines

The curing reactions of liquid crystalline 4,4′‐bis‐(2,3‐epoxypropyloxy)‐sulfonyl‐bis(1,4‐phenylene) (p‐BEPSBP) with 4,4′‐diaminodiphenylmethane (DDM) and 4,4′‐diaminodiphenylsulfone (DDS) were investigated by nonisothermal differential scanning calorimeter (DSC). The relationships of E_a with the conversion α in the curing process were determined. The catalyzed activation of hydroxyl group for curing reaction of epoxy resins with amine in DSC experiment was discussed. The results show that these curing reactions can be described by the autocatalytic ?esták‐Berggren model. The curing technical temperature and parameters were obtained, and the even reaction orders m, n, and ΔS for p‐BEPSBP/DDM and p‐BEPSBP/DDS are 0.35, 0.92, ?81.94 and 0.13, 1.32, ?24.45, respectively. The hydroxyl group has catalyzed activation for the epoxy–amine curing system in the DSC experiment. The average E_a of p‐BEPSBP/DDM is 67.19 kJ mol^?1 and is 105.55 kJ mol^?1 for the p‐BEPSBP/DDS system, but it is different for the two systems; when benzalcohol as hydroxyl group was added to the curing system, the average E_a of p‐BEPSBP/DDM decreases and increases for p‐BEPSBP/DDS. The crystalline phase had formed in the curing process and was fixed in the system. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Nonisothermal cure of bisphenol epoxy resin with a nonlinear aliphatic polyamine hardener

The amino terminated polypropylenimine dendrimer (DAB‐dendri‐(NH₂)₄) was employed as a new nonlinear aliphatic curing agent for diglycidyl ether of bisphenol A (DGEBA). Nonisothermal curing reaction kinetics of DGEBA/DAB was investigated with a differential scanning calorimeter (DSC). The apparent reaction activation energy E_a is about 56.7 kJ/mol determined using the Kissinger equation, and a two‐parameter (m, n) autocatalytic model ([icirc]Sesták–Berggren equation) was confirmed to be able to well simulate the reaction kinetics in the light of the Málek method. In addition, the relation between reaction activation energy E_a and curing degree α was obtained by applying model‐free isoconversional analysis with the Kissinger‐Akahira‐Sunose (KAS) method. As α increases, E_a reduced quickly from >80 kJ/mol to ≈60 kJ/mol up to a ≈ 15%, then decreased slowly to 55 kJ/mol till a ～ 75%, and finally dropped to 44 kJ/mol at full conversion. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

On the use of the generalized autocatalytic models: The thermal decomposition of 3,5‐dinitro‐4‐methylbenzoic acid

The thermal decomposition of 3,5‐dinitro‐4‐methylbenzoic acid is studied by means of differential calorimetric techniques (DSC). Its autocatalytic behaviour has been highlighted and the decomposition process has been described considering the generalized expression of the ?esták–Berggren model. A new procedure for the optimization of the initiation parameter along with the other Arrhenius constants and kinetic exponents starting from the knowledge of the classic ?esták–Berggren model is illustrated. Encouraging results point out the validity of the approach which has been verified considering both a series of numerical and real experiments. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1300–1308, 2015     

A novel imidazole derivative curing agent for epoxy resin: Synthesis,characterization, and cure kinetic

A novel imidazole derivative (named as EMI‐g‐BGE) was synthesized through the reaction of 2‐ethyl‐4‐methyl imidazole (EMI) and butyl glycidyl ether (BGE) and characterized by elemental analysis, FTIR spectroscopy, and ¹H NMR spectroscopy. The curing kinetic of diglycidyl ether of bisphenol A (DGEBA) epoxy resin with EMI‐g‐BGE as curing agent was studied by nonisothermal DSC technique at different heating rates. Dynamic DSC scans indicated that EMI‐g‐BGE was an effective curing agent of epoxy resin. The apparent activation energy E_a was 71.8 kJ mol^?1 calculated through Kissinger method, and the kinetic parameters were determined by Málek method for the kinetic analysis of the thermal treatment obtained by DSC measurement. A two‐parameter (m, n) autocatalytic model (?esták‐Berggren equation) was found to be the most adequate selected kinetic model. In addition, the predicted curves from the kinetic model fit well with the nonisothermal DSC thermogram. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Curing Behavior,Kinetics and Thermal Properties of o-Cresol Formaldehyde Epoxy Resin Modified by Liquid Crystalline Epoxy Resin

The curing kinetics of a bi-component system of o-cresol-formaldehyde epoxy resin (o-CFER) modified by liquid crystalline p-phenylene di[4-(2,3-epoxypropyl) benzoate] (p-PEPB), with 4,4-diamino-diphenyl ether (DDE) as a curing agent, was investigated by nonisothermal differential scanning calorimetry (DSC) method. The relationship between apparent activation energy, Ea, and the conversion α was obtained by the isoconversional method of Ozawa. A molecular reaction mechanism is proposed. The results show that the values of Ea in the initial stage are higher and tend to decrease slightly with the reaction progress. The primary amines have a higher Ea than secondary amines. The average curing Ea of o-CFER/p-PEPB/DDE system is 61.64 KJ/mol. These curing reactions can be described by a model proposed by ?esták and Berggren, which includes two parameters of m and n. Parameters such as reaction orders were evaluated using the ?esták-Berggren (S-B) equation and the following kinetic equation: dα/dt = Aexp(?Ea/RT)α ^m (1 ? a) ⁿ . The curing behavior of the system was studied by polarized optical microscopy (POM) and torsional braid analysis (TBA). The compatibility of the p-PEPB and o-CFER system is very good. Temperature of mechanical loss peak is higher by 63°C than the common o-CFER epoxy resin, when the weight ratio of p-PEPB with o-CFER is 4:100.     

Nonisothermal cure kinetics and diffusion effect of liquid-crystalline epoxy sulfonyl bis(1,4-phenylene)bis[4-(2,3-epoxypropyloxy)benzoate] resin with aromatic diamine

The curing kinetics of the liquid-crystalline epoxy resin sulfonyl bis(1,4-phenylene)bis[4-(2,3-epoxypropyloxy)benzoate] with 4,4′-diaminodiphenylsulfone was investigated by nonisothermal differential scanning calorimetry. The relationship between the apparent activation energy (E_a) and the conversion was determined, and the effects of the molecular structure and the order of liquid crystallinity on E_a are discussed in detail. Some parameters were evaluated with the autocatalytic kinetic model of the Šesták–Berggren (S–B) equation. The results show that there were some deviations of these simulation curves from the experimental curves at high heating rates and in the late stage of the curing reaction. The diffusion effect in the nonisothermal curing reaction is discussed, and a diffusion factor was proposed and introduced into the S–B equation. Then, a modified S–B equation was created, as follows: , where α is the conversion, t is the time, m and n are reaction orders, K is rate constant, C is the diffusion coefficient, and α_c is the critical conversion. The theoretical simulation curves agreed very well with the experimental data as determined with the modified S–B equation, which may be more useful for describing and predicting the nonisothermal curing reaction kinetics of epoxy resin. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Nonisothermal cocuring behavior and kinetics of epoxy resin/3‐glycidyloxypropyl‐POSS with MeTHPA

3‐Glycidyloxypropyl‐polyhedral oligomeric silsesquioxanes (G‐POSS) was prepared from 3‐glycidyloxypropyl‐trimethoxysilane (GTMS) by hydrolytic condensation. The cocuring behavior and kinetics of G‐POSS with bisphenol‐A epoxy resin (BPAER) using 3‐methyl‐tetrahydrophthalic anhydride (MeTHPA) as curing agent were investigated by nonisothermal differential scanning calorimetry (DSC) and torsional braid analysis (TBA). The face distribution of silicon in the cured products was characterized by energy dispersive X‐ray spectrometry (EDS). The results show that the compatibility of G‐POSS with BPAER is very well and can cocure. The curing mechanism was proposed. The relationship of E_a and conversion α can be obtained by the isoconversional method of Kissinger. These curing reactions can be described by the Šesták–Berggren (S–B) equation and can be depicted by the following equation: . TBA analysis indicated that T_g was decreased when the contents of G‐POSS is over to 30 wt%. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Influence of diaminobenzoyl‐functionalized multiwalled carbon nanotubes on the nonisothermal curing kinetics,dynamic mechanical properties,and thermal conductivity of epoxy–anhydride composites

To obtain advanced materials with a high thermal dissipation, the addition of multiwalled carbon nanotubes containing diverse functionality groups, that is, as‐received multiwalled carbon nanotubes (AS‐MWCNTs) and diaminobenzoyl multiwalled carbon nanotubes (DA‐MWCNTs), to epoxy–anhydride composites was accomplished. According to nonisothermal differential scanning calorimetry analysis, the reactive functional groups present on the surfaces of the AS‐MWCNTs and DA‐MWCNTs accelerated the nucleophilic addition reaction of epoxy composites. Because of the difference in the reactivities of these functional groups toward epoxy groups, the distinction of fractional conversion and the reaction rate of the curing process were remarkably evident at the early stage. A suitable kinetic model was effectively elucidated with the Málek approach. The curing kinetics could best be described by a two‐parameter autocatalytic model as a truncated ?esták–Berggren model. The DA‐MWCNTs achieved effective load transfer and active heat conductive pathways; this resulted in good dynamic mechanical and thermal properties. As a result, the diglycidyl ether of bisphenol A/DA‐MWCNTs constituted an effective system with enhanced heat dissipation of materials for electronic applications. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43567.     

Toughening‐modified epoxy‐amine system: Cure kinetics,mechanical behavior,and shape memory performances

Shape memory epoxy resins are derived on reacting E51 with triethylenetetramine in presence of the toughening agent polypropylene glycol diglycidyl ether (PPGDGE). The curing behaviors are studied with differential scanning calorimetry. The toughening system shows a decrease in activation energy. ?esták–Berggren model is utilized to establish the kinetic equations. The fitting results prove that the equations can well describe the reactions. Tensile tests and dynamic mechanical analysis are used to analyze mechanical performances and thermodynamics. Shape memory properties are characterized by fold‐deploy tests. The elongation at break increases as the concentration of PPGDGE increases. The toughening materials have lower glass transition temperature (T_g). The fixable ratios of all systems are greater than 99.5%. The shape recovery time decreases with increasing the PPGDGE concentration. The optimal system can fully recover its original shape in about 2 min at T_g + 30°C, and exhibit the maximum fold‐deploy cycles as 13 cycles. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40853.     

Curing behavior of Azo‐containing twin liquid crystalline epoxy resins with anhydride

A series of novel azo‐containing twin liquid crystalline (LC) epoxy monomers were cured with anhydrides without extra catalyst, and the curing kinetics was investigated by non‐isothermal differential scanning calorimetry (DSC) technique. The results showed that the effect of phase behavior on activation energy (E_a) was enormous, which increased first and then decreased quickly with the curing reaction processing. The chemical kinetic control and diffusion‐control mechanisms dominate the curing together, which gives large values of E_a. Azo group also served as a catalyst to accelerate the curing reaction. The curing mechanism was confirmed by the UV–Vis spectra of azo‐doped curing system in which the absorbance values at 366 nm and 475 nm changed with the curing reaction processing. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Kinetic Analysis of Nonisothermal Reduction of Silica-Supported Nickel Catalyst Precursors in a Hydrogen Atmosphere

A series of silica-supported nickel catalyst precursors was synthesized with different SiO₂/Ni molar ratios. Reduction of Ni catalyst precursors with different SiO₂/Ni molar ratios under a hydrogen atmosphere was investigated at different heating rates. Kinetic parameters were determined using Kissinger–Akahira–Sunose isoconversional and invariant kinetic parameter methods. It was found that for all molar ratios, the apparent activation energy (E_a) is practically constant in the conversion range of 0.20 ≤ α ≤ 0.80. In the considered conversion range, following values of E_a were found: 134.5 kJ mol^?1 (SiO₂/Ni = 0.20), 139.6 kJ mol^?1 (SiO₂/Ni = 0.80), and 128.3 kJ mol^?1 (SiO₂/Ni = 1.15). It was established that the reduction of Ni catalyst precursors with different SiO₂/Ni molar ratios is a complex process and can be described by the ?esták–Berggren autocatalytic model. It was found that the reaction is more Langmuir–Hinshelwood type, as hydrogen dissociates rapidly on surface nuclei and the dissociated hydrogen reacts with the Ni–O active system. It was concluded that the reduction process proceeds through bulk nucleation, which is a dominant mechanism, where three-dimensional growth of crystals with polyhedron-like morphology exists. It was found that the Ni/Si ratio decreases after the reduction process. This has been explained by low Ni and higher Si surface concentrations. It has been disclosed that Ni dispersion decreases.     

Curing Kinetics of Liquid Crystalline Epoxy Resins Based on Bisphenol-S Mesogen with DDE by Nonisothermal DSC Data

The curing kinetics for a system of Sulfonyl bis(4,1-phenylene)bis[4-(2,3-epoxypro pyloxy)benzoate] (p-SBPEPB) with 4,4′-diaminodiphenyl ether (DDE) were investigated by nonisothermal differential scanning calorimetry (DSC). The dependencies of the apparent activation energy Ea and the conversion α during overall curing reaction were revealed by Ozawa's method. The results shown the Ea decreased drastially from 107 to 75 KJ/mol with α in the initial stages (α = 0–20%), the average apparent activation energy Ea of p-SBPEPB/DDE is 82.81 KJ/mol and was relatively constant in the 0.5 to 0.9 conversion interval. Some parameters were evaluated using the two kinetic models of ?esták–Berggren (S-B) equation and JMA model. The liquid crystalline (LC) phase had formed and was fixed in the system during the curing process.     

Moisture curing kinetics of isocyanate ended urethane quasi‐prepolymers monitored by IR spectroscopy and DSC

The study of the kinetics of the curing of isocyanate quasi‐prepolymers with water was performed by infrared spectroscopy and differential scanning calorimetry. The influence of the free isocyanate content, polyol functionality, and of the addition of an amine catalyst (2,2′‐dimorpholinediethylether) in the reaction kinetics and morphology of the final poly(urethane urea) was analyzed. A second‐order autocatalyzed model was successfully applied to reproduce the curing process under isothermal curing conditions, until gelation occurred. A kinetic model‐free approach was used to find the dependence of the effective activation energy (E_a) with the extent of cure, when the reaction was performed under nonisothermal conditions. The dependence of E_a with the reaction progress was different depending on the initial composition of the quasi‐prepolymer, which reveals the complexity of the curing process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Cure Kinetics and Thermal Properties of Octa(aminophenyl)-POSS/Bromobisphenol A Epoxy Resin Nanocomposites

The tetrabromobisphenol A epoxy resin (TBBPAER) was synthesized and Octa(aminophenyl) polyhedral oligomeric silsesquioxane (OAP-POSS) was used as a curing reagent of TBBPAER. The cure kinetics, the glass transition temperature, and the flame resistance of OAP-POSS/TBBPAER nanocomposites were investigated. The results show that the curing reaction could be described by the autocatalytic ?esták-Berggren (S-B) model. The average activation energy E _a is 129.59 KJ/mol, the maximal mechanical loss temperature (T _p) is 166°C, when the molar ratio (N _s) of amino group to epoxy group is 0.5. The oxygen index for fire resistance is 46 ～ 49 for different amounts of OAP-POSS.     

DSC curing study of catalytically synthesized maleic‐acid‐based unsaturated polyesters

Unsaturated polyesters were synthesized based on ethylene glycol and maleic acid as unsaturated dicarboxylic acid, using a variety of saturated acids in the initial acid mixture, without or with different catalysts. The curing of the polyesters produced with styrene was studied using differential scanning calorimetry (DSC) under dynamic‐ and isothermal‐heating conditions. The FTIR spectra of the initial polyesters and cured polyesters were also determined. Curing is not complete at the end of DSC scan and the unreacted bonds were quantitatively determined from the FTIR spectra and by estimation based on literature data. The value of the mean degree of conversion (α) of all double bonds (styrene unit and maleate unit) was approximately α = 0.40. Using an appropriate kinetic model for the curing exotherm of polyesters, the activation energy (E_a), the reaction order (x) and the frequency factor (k_o) were determined. Because the kinetic parameters (ie E_a, k, x) affect the kinetics in various different ways, the curves of degree of conversion versus time at various isothermal conditions are more useful to compare and characterize the curing of polyesters. The kinetic parameters are mainly influenced by the proportion of maleic acid in the polyesterification reaction mixture and secondarily by the residual polyesterification catalyst. The degree of conversion of already crosslinked polyesters is greatly increased by post‐curing them at elevated temperature and for a prolonged time. © 2002 Society of Chemical Industry     

Kinetics and thermodynamics of isothermal curing reaction of epoxy‐4, 4′‐diaminoazobenzene reinforced with nanosilica and nanoclay particles

The kinetics of the cure reaction for a system of bisphenol‐A epoxy resin (DGEBA), with 4, 4′‐diaminoazobenzene (DAAB), reinforced with nanosilica (NS), and nanoclay (NC) by means of isothermal technique of differential scanning calorimetry were studied. The Kamal autocatalytic‐like kinetic model was used to estimate the reaction orders (m, n), rate constants (k₁, k₂), and also active energies (E_a) and pre‐exponential factors (A) of the curing reaction. However, the existence of NS and NC with hydroxyl groups in the structure improves the cure reaction and influence the rate of reaction and therefore kinetics parameters. The E_a of cure reaction of DGEBA/DAAB system showed a decrease when nanoparticles were present and therefore the rate of the reaction was increased. Using the rate constants from the kinetic analysis and transition state theory, thermodynamic parameters such as enthalpy (ΔH^#), entropy (ΔS^#), and Gibbs free energy (ΔG^#) changes were also calculated. The thermodynamic functions were shown to be very sensitive parameters for evaluation of the cure reaction. POLYM. COMPOS., 31:1442–1448, 2010. © 2009 Society of Plastics Engineers     

A kinetic study of the isothermal degradation process of Lexan® using the conventional and Weibull kinetic analysis

The degradation process of commercial grade Lexan® was investigated by thermogravimetric technique under isothermal experimental conditions at four different operating temperatures: 375 °C, 387.5 °C, 400 °C and 425 °C. The kinetic triplet (E _a , A, f(α)) was determined using conventional and Weibull kinetic analysis. The applied kinetic procedure shows that the investigated degradation process can be described by two-parameter autocatalytic ?esták–Berggren (SB) reaction model. It was established that the degradation process of Lexan® can be described by the following kinetic triplet: E _a? =?158.3 kJ mol^?1, A?=?8.80?×?10⁹ min^?1 and f(α)?=?α ^0.33 (1???α)^1.62. It was established that the operating temperature has an influence on the values of SB reaction orders (m and n) (0.27?m?n??1, represent the composite value from a complex degradation reaction and can not compare with the dissociation energy of the weak bonds in bisphenol-A polycarbonate. Also, it was concluded that the Weibull shape parameter (β) shows that the considered process occurs under the same reaction mechanism, independently on operating temperature (T), i.e. the change of rate-limiting step does not occur (β?ddf) of apparent activation energies for considered degradation process. On the other hand, it was shown that the experimentally evaluated density distribution function of apparent activation energies represents the intermediate case between the calculated density distribution functions at 375 °C and 425 °C.     

A study on the curing kinetics of epoxycyclohexyl polyhedral oligomeric silsesquioxanes and hydrogenated carboxylated nitrile rubber by dynamic differential scanning calorimetry

A new organic–inorganic hybrid material was prepared through reactive blending of hydrogenated carboxylated nitrile rubber (HXNBR) with epoxycyclohexyl polyhedral oligomeric silsesquioxanes (epoxycyclohexyl POSS). The structure of the composite was characterized by Fourier transform infrared spectroscopy (FTIR) and solid‐state ¹³C Nuclear Magnetic Resonance spectra (solid‐state ¹³C‐NMR). The differential scanning calorimetry (DSC) at different heating rates was conducted to investigate the curing kinetics. A single overall curing process by an nth‐order function (1 ? α)ⁿ was considered, and multiple‐heating‐rate models (Kissinger, Flynn–Wall–Ozawa, and Crane methods) and the single‐heating‐rate model were employed. The apparent activation energy (E_a) obtained showed dependence on the POSS content and the heating rate (β). The overall reaction order n was practically constant and close to 1. The isoconversion Flynn–Wall–Ozawa method was also performed and fit well in the study. With the single‐heating‐rate model, the average E_a for the compound with a certain POSS content, 66.90–104.13 kJ/mol was greater than that obtained with Kissinger and Flynn–Wall–Ozawa methods. Furthermore, the calculated reaction rate (dα/dt) versus temperature curves fit with the experimental data. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012