Cure kinetics of carbon nanotube/tetrafunctional epoxy nanocomposites by isothermal differential scanning calorimetry

The cure kinetics of tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM) and 4,4′‐diaminodiphenylsulfone (DDS) as a cure agent in nanocomposites with multiwalled carbon nanotubes (MWNTs) have been studied with an isothermal differential scanning calorimetry (DSC) technique. The experimental data for both the neat TGDDM/DDS system and for epoxy/MWNTs nanocomposites showed an autocatalytic behavior. Kinetic analysis was performed with the phenomenological model of Kamal and a diffusion control function was introduced to describe the cure reaction in the later stage. Activation energies and kinetic parameters were determined by fitting experimental data. For MWNTs/epoxy nanocomposites, the initial reaction rates increased and the time to the maximum rate decreased with increasing MWNTs contents because of the acceleration effect of MWNTs. The values of the activation energies for the epoxy/MWNTs nanocomposites were lower than the values for the neat epoxy in the initial stage of the reaction. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3701–3712, 2004     

The epoxy–polycarbonate blends cured with aliphatic amine—I. Mechanism and kinetics

Reaction mechanism of the PC–epoxy blends cured by aliphatic amine has been investigated by varying PC contents in the blends. The transamidation reaction tends to convert nearly all the carbonates into N-aliphatic aromatic carbamates even at ambient temperature before normal curing. The remaining amine proceeds the normal curing with epoxy at a higher temperature (80°C). For the PC–epoxy/aliphatic amine blend containing 6 wt % PC, the yielded N-aliphatic aromatic carbamate further reacts with amine to produce the urea structure. The urea undergoes substitution reaction with the hydroxyl formed from the normal curing to give the N-aliphatic aliphatic carbamate. For the blend containing 12 wt % PC, the N-aliphatic aromatic carbamate converts into the N-aliphatic aliphatic carbamate via two different routes. For the blend containing lower molecular weight of the aliphatic amine, the N-aliphatic aromatic carbamate reacts with hydroxyl to form the N-aliphatic aliphatic carbamate directly. For the blend containing higher molecular weight of aliphatic amine, the N-aliphatic aromatic carbamate decomposes into the aliphatic isocyanate accelerated by the presence of the residual oxirane. The isocyanate formed then reacts with hydroxyl to yield the N-aliphatic aliphatic carbamate. The activation energy (E_a) and preexponential factor (A) of the PC–epoxy/POPDA blends decrease with the increase of the PC content. Kinetic study by thermal analysis by the method of autocatalyzed model is able to correctly predict oxirane conversion vs. time relationship for the neat epoxy/aliphatic amine and the PC–epoxy/aromatic amine systems because the dominant reaction is the normal curing reaction between amine and oxirane. The model fails to predict the PC–epoxy/aliphatic amine system because the system is complicated by several other reactions besides the normal curing reaction. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2169–2181, 1997     

Curing reaction of biphenyl epoxy resin with different phenolic functional hardeners

The investigation of cure kinetics and relationships between glass transition temperature and conversion of biphenyl epoxy resin (4,4′-diglycidyloxy-3,3′,5,5′-tetramethyl biphenyl) with different phenolic hardeners was performed by differential scanning calorimeter using an isothermal approach over the temperature range 120–150°C. All kinetic parameters of the curing reaction including the reaction order, activation energy, and rate constant were calculated and reported. The results indicate that the curing reaction of formulations using xylok and dicyclopentadiene type phenolic resins (DCPDP) as hardeners proceeds through a first-order kinetic mechanism, whereas the curing reaction of formulations using phenol novolac as a hardener goes through an autocatalytic kinetic mechanism. The differences of curing reaction with the change of hardener in biphenyl epoxy resin systems were explained with the relationships between T_g and reaction conversion using the DiBenedetto equation. A detailed cure mechanism in biphenyl-type epoxy resin with the different hardeners has been suggested. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 773–783, 1998     

Influence of substituents on the kinetics of epoxy/aromatic diamine resin systems

Eleven different epoxy/diamine systems, including tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM), triglycidyl p‐aminophenol (TGAP), and diglycidyl ether of bisphenol A (DGEBA) with 4,4′‐diaminodiphenylsulfone (DDS), diethyltoluenediamine (DETDA), dimethylthiotoluenediamine (DMTDA), and meta‐phenylenediamine (m‐PDA), were studied with near‐infrared spectroscopy at different temperatures. The reactivities of the epoxies were determined and found to be in the following order when reacted with the same amine: DGEBA > TGAP > TGDDM. When the primary amine was reacted with the same epoxy, the order was DETDA > DDS > DMTDA; for the secondary amine, the order was DETDA > DMTDA > DDS. The relative reaction rates of the secondary amine to the primary amine were compared and discussed in terms of the structural differences and the corresponding substitution effect. It was concluded that the increase in the secondary amine reactivity of DETDA and DMTDA was caused by the deconjugation of the benzene‐ring π electrons from the lone pair on the N atom. The overall order of the secondary amine relative reactivity was DMTDA > DETDA > DDS for the same epoxy and TGDDM > TGAP > DGEBA for the same amine. The m‐PDA systems had no significant positive or negative substitution effects. Molecular orbital calculations were performed, and the results showed the most significant deconjugation effect in the secondary amine of DETDA. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3143–3156, 2004     

气相生长碳纤维对环氧树脂固化反应的影响及其复合物固化动力学研究

用示差扫描量热方法研究了气相生长碳纤维作为填料对4,4′-二氨基二苯甲烷四缩水甘油环氧树脂(TGDDM)/4,4′-二氨基二苯基砜(DDS)等温固化反应的影响.与纯环氧树脂一样,气相生长碳纤维复合物的固化反应也属于自催化反应类型.气相生长碳纤维对环氧树脂的固化反应动力学影响很小.固化反应的过程可以用一种修正过的自催化动力学模型来描述,在整个固化反应过程中纯TGDDM/DDS环氧树脂及其气相生长碳纤维复合物模型拟合得到的结果和实验数据相当一致.     

THE MECHANISM OF CURE REACTION OF 4,4''''-DIAMINODIPHENYL METHANE WITH TETRAGLYCIDYL 4,4''''-DIAMINODIPHENYL METHANE EPOXY

The cure reaction of tetraglycidyl 4,4'-diaminodiphenyl methane (TGDDM) epoxy resin with 4,4'-diaminodiphenyl methane (DDM) has been studied by using DSC. Instead of one exothermic peak, two exothermic peaks, indicative of a complex reaction mechanism, are shown in the DSC curve of TGDDM-DDM mixtures in nonisothermal cure experiments when the content of DDM is lower than stoichiometric ratio. The result of the kinetic analysis of the cure reaction shows that the activation energy of the lower temperature exotherm peak is about 56 kJ/mol and that of the higher temperature exotherm peak is about 136 kJ/mol. The lower temperature cure reaction peak can be attributed to the primary amine-epoxide and secondary amine-epoxide reactions, and the higher temperature cure reaction peak can be attributed to the epoxide-hydroxy reaction under catalysis of tertiary amine in the TGDDM epoxy resin. Because the network density of the cured epoxy resin is determined by these two reactions, the content of DDM has little effect on the glass transition temperature of cured epoxy resin.     

Miscibility and cure kinetics studies on blends of bisphenol-A polycarbonate and tetraglycidyl-4,4′-diaminodiphenylmethane epoxy cured with an amine

Differential scanning calorimetry (DSC) has been applied to characterize the glass transition behavior of the blends formed by bisphenol-A polycarbonate (PC) with a tetrafunctional epoxy (tetraglycidyl-4,4′-diaminodiphenyl methane, TGDDM) cured with 4,4′-diaminodiphenylsulphone (DDS). A rare miscibility in the complete composition range has been demonstrated in these blends. Additionally, the blend morphology was examined using scanning electron microscopy (SEM) and a homogeneous single-phase PC/epoxy network has been observed in the blends of all compositions. Moreover, polycarbonate incorporation has been found to exert a distinct effect on the cure behavior of the epoxy blends. The cure reaction rates for the epoxy-PC blends were significantly higher due to the presence of PC. In addition, the cure mechanism of the epoxy blends was no longer autocatalytic. An n-th order reaction mechanism with n = 1.2 to 1.5 has been observed for the blends of DDS-cured epoxy with PC of various compositions studied using DSC. The proposed n-th order kinetic model has been found to describe well the cure behavior of the epoxy/PC blends up to the vitrification point. © 1995 John Wiley & Sons, Inc.     

Effects of water on epoxy cure kinetics and glass transition temperature utilizing molecular dynamics simulations

Effects of water on epoxy cure kinetics are investigated. Experimental tests show that absorbed water in an uncured bisphenol‐F/diethyl‐toluene‐diamine epoxy system causes an increase in cure rate at low degrees of cure and a decrease in cure rate at high degrees of cure. Molecular simulations of the same epoxy system indicate that the initial increase in cure rate is due to an increase in molecular self‐diffusion of the epoxy molecules in the presence of water. Effects of water on the glass transition temperature (T_g) of the crosslinked thermoset are also studied. Both experiments and simulations show that water decreases T_g. Both types of results indicate that T_g effects are small below 1% water by weight, but that T_g depression occurs much quickly with increasing water content above 1%. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1150–1159     

环氧树脂/氧化石墨纳米复合物的等温固化行为研究

用示差扫描分析仪(DSC)研究了氧化石墨(GO)对N,N,N',N'-四缩水甘油基-4,4'-二氨基二苯基甲烷环氧树脂(TGDDM)/4,4'-二氨基二苯基砜(DDS)体系的等温固化反应的影响,用X射线光电子能谱仪(XPS)和傅里叶变换红外光谱仪(FTIR)研究了GO上存在的官能团及其对TGDDM/DDS体系固化行为的影响,用热失重分析仪(TGA)研究了天然石墨和GO的热力学稳定性.XPS、FTIR和TGA结果表明,GO上存在的大量羟基、羧基、环氧基等官能团能够影响环氧树脂的固化行为.DSC研究发现,环氧树脂/氧化石墨纳米复合物的固化反应属于自催化类型,随着GO含量的增加,达到最大反应速率的时间不断减小,初始反应速率不断增大,这说明GO对环氧树脂的固化反应有促进作用.Kamal模型计算得到的结果表明,随着GO含量的增加自催化反应初期阶段表观活化能E1先减小再增大,而自催化反应结束后表观活化能E2略微减小.经Kamal模型扩散控制函数修正后,整个固化过程中拟合得到的结果与实验数据相当吻合.以上结果说明,少量的GO对TGDDM/DDS体系的固化反应起着催化作用.     

Evolution of structure and properties of a liquid crystalline epoxy during curing

The evolution of structure, and thermal and dynamic mechanical properties of a liquid crystalline epoxy during curing has been studied with differential scanning calorimetry (DSC), polarized optical microscopy, x-ray scattering, and dynamic mechanical analysis. The liquid crystalline epoxy was the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene (DGEDHMS). Two curing agents were used in this study: a di-functional amine, the aniline adduct of DGEDHMS, and a tetra-functional sulfonamido amine, sulfanilamide. The effects of curing agent, cure time, and cure temperature have been investigated. Isothermal curing of the liquid crystalline epoxy with the di-functional amine and the tetra-functional sulfonamido amine causes an increase in the mesophase stability of the liquid crystalline epoxy resin. The curing also leads to various liquid crystalline textures, depending on the curing agent and cure temperature. These textures coarsen during the isothermal curing. Moreover, curing with both curing agents results in a layered structure with mesogenic units aligned perpendicular to the layer surfaces. The layer thickness decreases with cure temperature for the systems cured with the tetra-functional curing agent. The glass transition temperature of the cured networks rises with increasing cure temperature due to the increased crosslink density. The shear modulus of the cured networks shows a strong temperature dependence. However, it does not change appreciably with cure temperature. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2363–2378, 1997     

Do epoxy–amine networks become inhomogeneous at the nanometric scale?

Epoxy–amine networks are known to be homogeneous. However, using new analysis tools that allow the observation scale to be reduced to a nanometric level, some authors have stated the opposite. In this work, the network morphology has been studied with atomic force microscopy in the tapping mode as a function of the hardener nature and the stoichiometry of the reactive blend. A very homogeneous epoxy network topography, similar to that of an amorphous thermoplastic, has been obtained. For comparison, a truly heterogeneous network topography, like that of unsaturated thermosets cured by free‐radical mechanisms, has been imaged. For the observation of surfaces on a scale smaller than a nanometer, caution must be taken:(1) the tips must be freshly cleaned so that distortion on the image is prevented and (2) the surfaces must be very flat so that the phase contrast is not influenced significantly by differences in the sample topography. This works gives guidelines on using atomic force microscopy in the tapping mode for epoxy–amine network characterization and discusses epoxy–amine network homogeneity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2422–2432, 2003     

Isothermal cure kinetics of epoxy/phenol‐novolac resin blend system initiated by cationic latent thermal catalyst

The investigation of the cure kinetics of a diglycidyl ether of bisphenol A (DGEBA)/phenol‐novolac blend system with different phenolic contents initiated by a cationic latent thermal catalyst [N‐benzylpyrazinium hexafluoroantimonate (BPH)] was performed by means of the analysis of isothermal experiments using a differential scanning calorimetry (DSC). Latent properties were investigated by measuring the conversion as a function of curing temperature using a dynamic DSC method. The results indicated that the BPH in this system for cure is a significant thermal latent initiator and has good latent thermal properties. The cure reaction of the blend system using BPH as a curing agent was strongly dependent on the cure temperature and proceeded through an autocatalytic kinetic mechanism that was accelerated by the hydroxyl group produced through the reaction between DGEBA and BPH. At a specific conversion region, once vitrification took place, the cure reaction of the epoxy/phenol‐novolac/BPH blend system was controlled by a diffusion‐control cure reaction rather than by an autocatalytic reaction. The kinetic constants k₁ and k₂ and the cure activation energies E₁ and E₂ obtained by the Arrhenius temperature dependence equation of the epoxy/phenol‐novolac/BPH blend system were mainly discussed as increasing the content of the phenol‐novolac resin to the epoxy neat resin. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2945–2956, 2000     

Cure Kinetics of a Tetrafunctional Rubber Modified Epoxy-amine System

In this work the curing kinetics behaviour of a rubber modified epoxy amine system is investigated through calorimetric analysis. This study is part of a wider investigation on new epoxy formulations to be used as matrices of composite materials. The aim is to enhance both the processing behaviour and the mechanical properties of the matrix in order to obtain higher performance composites for more demanding applications. The epoxy system is blended with a high molecular mass rubber containing functional groups reactive towards the epoxies. The formation of a rubber/epoxy network can be achieved by means of a 'pre-reaction' between the epoxy monomers and the rubber functional groups, carried out in the presence of a suitable catalyst and before the resin is cured with the amino hardener. In this work the influence of both the rubber and the catalyst on the resin cure kinetics is analysed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Epoxy resins and epoxy blends studied by near infra-red spectroscopy

The cure and the final network of epoxy resins have been investigated by numerous techniques, nevertheless a clear understanding of this network structure has not yet been achieved. FTIR analysis of polymeric materials provides highly precise measurements that are widely interpretable in terms of chemical structure. Yet the high absorption of fundamental bands requires careful sample preparation to reduce the thickness of the sample or special reflection techniques are needed. Furthermore, the occurrence of overlapping bands for epoxy resin (N-H and O-H vibrations in the 3000 cm⁻¹ region) renders the quantitative analysis in the region mid IR particularly difficult. However, the overtone and combination bands are 10–100 times weaker than the fundamental ones and are observed in near infrared (NIR) region. Longer pathlengths than Mid IR ones can be used allowing transmission analysis of thick samples (1-20 mm) without special preparation. NIR absorption bands have different intensities depending on the anharmonicity of vibrations. The strongest absorption bands are due to protons connected to carbon, nitrogen, oxygen. Hydrogen bonding due to inter- and intramolecular interactions can cause band broadening, peak position shifts and intensity variations. NIR spectroscopy is therefore a useful technique to investigate polymeric materials and was used to study the cure reactions of various epoxy resins cured with amine hardener. Using different NIR techniques (reflectance, transmission and microscopy) we will briefly present some results concerning hydrogen bonding between epoxy and amine hardener before curing, epoxy resins, glass/epoxy composites and epoxy/PES (polyethersulfone) blends.     

Cure kinetics of epoxy-amine resins used in the restoration of works of art from glass or ceramic

The cure kinetics of two epoxy/amine resins, Araldite 2020 and AY103-HY956 widely used as adhesives in the restoration of works of art from glass or ceramic was investigated using FTIR spectroscopy. These resins are two-part adhesives, consisting of a resin - A, based on a diglycidyl ether of bisphenol A, and a hardener - B which is either a cycloaliphatic amine (isophorone diamine) for Araldite 2020, or a mixture of three aliphatic amines in HY956. The study was based on the collection of IR spectra, in the middle range (4000-600 cm⁻¹), of mixtures of resin and hardener at different proportions and isothermal temperatures (22-70 °C) as a function of curing time. A kinetic model was employed to simulate the experimental data using two kinetic rate constants. Diffusion control was incorporated to describe the cure behaviour at high degrees of conversion. From fitting to experimental data the kinetic and diffusional parameters were estimated, together with the activation energies of the kinetic and autocatalytic rate constants. It was found that higher degrees of curing are obtained at higher temperatures and increased amounts of hardener. Differences in the performance of the two adhesives are explained based on the type of the amines used as hardener.     

Acceleration of amine/epoxy reactions with N‐methyl secondary amines

Kinetic studies established that the monomethylation of a primary amine leads to significantly higher reaction rates with glycidyl ethers. The relative rates for approximately 25 amines were determined in an alcohol solvent under pseudo‐first‐order conditions (excess epoxy). The rates were referenced to aniline. For the aliphatic amines, reactivity consistently increased upon going from a primary amine to the corresponding N‐methyl secondary amine. This acceleration effect was not seen for aniline. The enhanced reactivity was also seen in curing systems, both with pure methylated amine curing agents and with complex mixtures obtained from the partial methylation of polyamines. Economically viable partially methylated amine curing agents were obtained by the reductive alkylation of commercial polyamines with formaldehyde and by the reaction of monomethylamine with 3‐(N‐methylamino)propionitrile in the presence of hydrogen and a hydrogenation catalyst. Although actual cure performance is based on a complex combination of several factors, the acceleration due to monomethylation could be a useful tool for enhancing amine/epoxy curing reactions. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 921–930, 2000     

Effect of the segmental mobility restriction on the thermoset chemical kinetics

The cure kinetics of an epoxy–amine commercial thermoset system have been investigated with the isothermal differential scanning calorimetry technique. In particular, a kinetic study has been performed in the glass–transition zone, in which diffusion phenomena compete with the chemical transformations and the overall reaction rate is partially slowed by the reduced segmental chain mobility. A generalized form of the Vogel equation has been formulated to account for the effect of the increasing glass–transition temperature on the chain mobility and, therefore, on the overall reaction rate. The kinetic model has been expressed with two factors representing the chemical reaction rate and the segmental mobility reduction. As the main result, the activation energy relative to the diffusion phenomena has been found to be very low, having a value of 42.5 K ≈ 0.356 kJ/mol, which is compatible only with the small‐angle rotation of the reactive unit. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3757–3770, 2002     

Near‐infrared spectroscopic studies on the cure behaviors of the CAE/DGEBA blend system initiated by a thermal latent catalyst

The latent properties and cure behaviors of an epoxy blend system based on cycloaliphatic epoxy (CAE) and diglycidyl ether of bisphenol A (DGEBA) epoxy containing N‐benzylpyrazinium hexafluoroantimonate (BPH) as a thermal latent initiator were investigated with near‐infrared (N‐IR) spectroscopy. The assignments of the latent properties and cure kinetics were performed by the measurements of the N‐IR reflectance for epoxide and hydroxyl functional groups at different temperatures and compositions. As a result, this system showed more than one type of reaction, and BPH was an excellent thermal latent catalyst without any coinitiator. The cure behaviors were identified by the changes in the absorption intensity of the hydroxyl groups at 7100 cm⁻¹ with different composition ratios. Moreover, characteristic N‐IR band assignments were used to evaluate the reactive kinetics and were shown to be an appropriate method for studying the cure behaviors of the CAE/DGEBA blend system containing a thermal latent catalyst. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 326–331, 2001