Estimation of the cure rate of epichlorohydrin/bisphenol a type epoxy resins

Summary A new method is described for estimating the curing rates of epichlorohydrin/bisphenol A epoxy resins cured with complex amines or polyamides. The ground-up resins are suspended in p-dioxane and reacted for 30 min with an excess of hydrogen bromide generated in situ. The excess halide is then titrated potentiometrically with mercuric perchlorate, using a bromide-selective indicator electrode in conjunction with an expanded-scale pH meter. Results are in agreement with those obtained by following the disappearance of the oxirane absorption band at 10.95 m. Average error was 0.9%.

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Bestimmung des Härtungsgrades von Epoxyharzen vom Typ Epichlorhydrin/Bisphenol A

Zusammenfassung Ein neues Verfahren wird beschrieben für Epoxyharze, die mit komplexen Aminen oder Polyamiden gehärtet wurden. Die gemahlenen Harze werden in p-Dioxan suspendiert und 30 min der Reaktion mit überschüssigem, in situ erzeugtem Bromwasserstoff unterworfen. Der Überschuß wird dann potentiometrisch mit Quecksilberperchlorat titriert, wobei eine bromidselektive Indicatorelektrode in Verbindung mit einem pH-Meter mit Skalendehnung benutzt wird. Die Ergebnisse stimmen mit denjenigen überein, die durch Verfolgung des Verschwindens der Oxiran-Absorptionsbande bei 10,95 m erhalten wurden. Der durchschnittliche Fehler lag bei 0,9%.

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Work performed under the auspices of the U.S. Atomic Energy Commission.     

Mechanism of imidazole catalysis in the curing of epoxy resins

The mechanism of imidazole catalysis in the curing of epoxy resins was studied using the PGE/1-methylimidazole, 2-methylimidazole, and 1,2-dimethylimidazole model systems and another model system based on trichloromethylethylene oxide. It was demonstrated that imidazolium systems, generated in the curing reaction, show an inherent instability leading to cleavage of an N? C bond or the 2-C? H bond (2-unsubstituted imidazoles). Fourier-transform infrared spectroscopy was used to follow specific changes in the IR spectrum of the curing mixture during polymerization. The identification of carbonyl absorptions occurring during the polymerization led to the conclusion that ketone formation is a general occurrence in the cure of epoxides with nitrogen compounds. We have also shown that imidazoles are regenerated during the curing process by at least two routes. One pathway for the regeneration of the catalyst involves N-dealkylation of the imidazole via a substitution process. Another route, β-elimination, afforded carbonyl compounds, which account for the previously unexplained appearence of infrared bands in the 1650–1770 cm^?1 region during the curing process. These investigations demonstrated the true catalytic function of the imidazole. Possible mechanisms for the regeneration of the catalyst are also suggested.     

Luminescence spectroscopy as a tool for testing of cure kinetics of epoxy resins

In this paper, steady-state luminescence spectroscopy is used for the analysis of curing of epoxy resin. The advantage of this method is its rapidity, simplicity and sensitivity. Moreover, this method is contactless, and thus non-invasive. The aim is to analyze epoxy resin, mathematically describe its curing kinetics and determine its storage temperature. Using the photoluminescence method, a rapid procedure for obtaining the necessary technological data is achieved. This method is suitable for continuous measurement in production because there is no contact with the material, and the measurement itself can be performed very quickly. The elaborated mathematical model can serve as a basis for creating algorithms for automated data processing in case of fully robotic workplaces.     

Effect of tertiary amine accelerators with different substituents on curing kinetics and reactivity of epoxy/dicyandiamide system

Non-isothermal Differential Scanning Calorimetry and gel time measurements were employed to study the effects of four different tertiary amine accelerators with different molecular structure and substituents on the curing behavior and reactivity of epoxy/dicyandiamide (DICY) system. Results showed that the acceleration behavior depends on three factors: the bulkiness, the number of nitrogen atoms in molecular structure and the electron density of accelerator. Among these, the former two factors demonstrated stronger effect. The results of model fitting kinetic demonstrated that Šesták–Berggren model can well simulate the reaction rates especially for the samples accelerated by 2,4,6-tris(dimethylaminomethyl)-phenol and (Dimethylaminomethyl)phenol. The activation energy projected by Flynn-Wall-Ozawa and Kissinger–Akahira–Sunose methods exposed different amounts for each accelerator while both models revealed similar trends. Moreover, 1-methylimidazol-accelerated samples showed shortest storage life and gel time mainly due to the presence of two nitrogen atoms in a compact structure of accelerator.     

Synthesis, characterization, and cure properties of phosphorus-containing epoxy resins for flame retardance

An organophosphorus compound, 10-(2,5-dihydroxyl phenyl)-9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DHPDOPO), was synthesized through the reaction of 9,10-dihydro-9-oxa-10-phosphaphnanthrene-10-oxide (DOPO) and p-benzoquinone, and characterized by elemental analysis, Fourier transform infrared spectrum (FTIR), and ¹H-NMR and ³¹P-NMR spectroscopes. Consequently, the phosphorus-containing epoxy resins with phosphorus content of 1 and 2 wt.% were prepared via the reaction of diglycidyl ether of bisphenol-A with DHPDOPO and bisphenol-A, and confirmed with FTIR and gel permeation chromatography (GPC). Phenolic melamine, novolak, and dicyanodiamide (DICY) were used as curing agents to prepare the thermosetted resins with the control and the phosphorus-containing epoxy resins. Thermal properties and thermal degradation behaviors of these the thermosetted resins were investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Phenolic melamine-cured resins exhibited higher glass transition temperatures than the other cured resins due to the high rigidity of their molecular chain. TGA studies demonstrated that the decomposition temperatures of the novolak-cured resins were higher than those of the others. A synergistic effect from the combination of the phosphorus-containing epoxy resin and the nitrogen-containing curing agent can result in a great improvement of the flame retardance for their thermosetted resins.     

Synthesis of tri‐aryl ketone amine isomers and their cure with epoxy resins

Isomeric tri‐aryl ketone amines, 1,3‐bis(3‐aminobenzoyl)benzene (133 BABB), 1,3‐bis(4‐aminobenzoyl)benzene (134 BABB), and 1,4‐bis(4‐aminobenzoyl)benzene (144 BABB) are synthesized and cured with diglycidyl ether of bisphenol A and diglycidyl ether of bisphenol F in this work. Differential scanning calorimetry and near‐infrared spectroscopy reveal higher rate constants and enhanced secondary amine conversion with increasing para substitution attributed to resonance effects and the electron withdrawing nature of the carbonyl linkages. Glass transition temperatures increase from 133 BABB to 134 BABB, but decrease modestly for the 144 BABB hardener. With increasing para substitution, the flexural modulus and strength both decrease while the strain to failure increases but all BABB amines displaying higher mechanical properties than the corresponding 4,4‐diaminodiphenyl sulfone (44 DDS) networks. The thermal stability of the BABB networks is found to be modestly lower than 44 DDS, but char yields are significantly higher. Changes in thermal and mechanical properties are described in terms of molecular structure and equilibrium packing density.     

Liquid‐crystalline thermosets from mesogenic dimeric epoxy resins by tertiary amine catalysis

Liquid‐crystalline thermosets (LCTs) were prepared by the curing of difunctional liquid‐crystalline dimeric epoxy monomers with imine moieties in the mesogenic core and central spacers of different lengths. Tertiary amines were used as catalysts in different proportions. The locked mesophases of the LCTs were characterized by polarized optical microscopy and wide‐angle X‐ray scattering and identified as smectic‐C, regardless of their smectic‐A or smectic‐C initial state. The influence of a 7.1‐T magnetic field on the macroscopic orientation of these materials was studied by dynamic mechanical analysis, and the orientation parameter was determined by IR dichroism. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3916–3926, 2002     

Liquid crystalline thermosets from dimeric LC epoxy resins cured with primary and tertiary amines

Liquid crystalline thermosets (LCTs) were prepared by curing difunctional LC dimeric epoxy monomers with imine moieties in the mesogenic core and central spacers of different length. Primary diamines or tertiary amines were used as curing agents obtaining materials with different characteristics. The results obtained were related to the mesogen structure, since dipolar moments in the mesogenic cores affect the ability to form ordered networks.     

The study of esterifying reaction between epoxy resins and carboxyl acrylic polymers in the presence of tertiary amine

In this article, the fixed gel/nongel point and critical gel point experiments are designed for formulating the regulation of esterification reaction between epoxy resin and carboxyl acrylic polymer in the presence of tertiary amine in organic solvent. Ternary diagrams are employed to illustrate experimental results. It is disclosed that some factors, such as mole ratio of the ternary reactants, molecular weight of epoxy resin, even the hydrophilic solvent content of system, play important roles in the gelling effect of esterification reaction. In order to explain these experimental results, a suggested reaction process is put forward and, subsequently, a critical gel equation is derived from Carothers gelation theory. The equation provides an explicit connection between the critical gel composition and the relevant parameters of the reaction system such as average carboxyl functionality of acrylic resin, average molecular weight of epoxy resin, the base intensity of tertiary amine, and the solid content of system. The regressive critical gel curve based on the equation is well consistent with the experiment data. It expresses that the suggested reaction process is reasonable. Finally, based on the critical gel equation, the important factors that influence the esterification result are discussed qualitatively. All the achievements are beneficial to understanding the reaction process and avoiding useless gelation in preparing the water-reducible compositions.     

Effect of aromatic substitution on the cure reaction and network properties of anhydride cured triphenyl ether tetraglycidyl epoxy resins

A systemic study of the impact of aromatic substitution on the reaction rate and network properties of the isomers of a tetraglycidylaniline triphenyl ether epoxy resin cured with anhydride hardeners is presented here. The epoxy resins synthesized in this work were based upon N,N,N,N‐tetraglycidyl bis(aminophenoxy)benzene (TGAPB), where the glycidyl aniline and ether groups change from being all meta (133 TGAPB), to meta and para (134 TGAPB), and finally to an all para substituted epoxy resin (144 TGAPB). Increasing para substitution increased reaction rate, promoted the onset of vitrification and increased epoxide conversion. Thermal properties such as glass transition temperatures (T_g) and coefficients of thermal expansion (CTE) both increased consistently with increasing para substitution, although thermal stability as measured via thermogravimetric analysis decreased. Mechanical properties also varied systematically with flexural strength and ductility increasing with increased para substitution, while the modulus decreased. Indeed, the ductility almost doubled, as measured by the work of fracture and displacement at failure highlighting the importance of substitution on properties.     

Cure of epoxy resins with cyanoacetamides

Cyanoacetamides are a novel class of curing agents for epoxy resins. Since reaction products of epoxy compounds with cyanoacetamides have not yet been described, we investigated the reaction of phenyl-glycidylether (PGE) and N-isobutylcyanoacetamide (NICA) under the conditions of the epoxy cure (120–150°C). Twenty-two fractions of the reaction product have been separated by preparative TLC and characterized by FD and MS mass spectroscopy. The structures of 10 reaction product have been elucidated by MS, NMR, and IR techniques. They belong to the classes of cyclic urethanes, spiro-dilactones, cyclo-oxa-1-hepten-4-one-2, pyrimidones, aminocrotononitrile, and tertiary amine. This complex model reaction mixture does not enable us to propose a curing mechanism. However practical cure of Bisphenol A diglycidylether (BADGE) yields clear and tough solids with a glass transition temperature up to 200°C, good mechanical strength, and high adhesion to metal surface. Cyanoacetamides are latent hardeners requiring a curing initiator. Since N-4-chlorophenyl-N′-dimethylurea is a latent initiator, liquid, homogeneous, storage stable “one shot” systems can be formulated which harden quickly above 120°C. Heat aging properties of cured specimens are reported. A series of novel liquid, resinous, and crystalline cyanoacetamides and their potential as curing agent are described.     

Dilute solution properties of epoxy resins

Number-average molecular weights of fractions of epoxy resins were estimated by vapor-pressure osmometry and size exclusion chromatography coupled with multiple-angle light scattering. Potential reasons for differences between the two sets of data are examined. The molecular weight dependences of the intrinsic viscosity in tetrahydrofuran and chloroform are discussed in terms of theories which take into account the low-molecular weight character of poly(hydroxy ether) chains. The polymer-solvent interaction parameter is estimated. The impact of the presence of branched chains on the results of size exclusion chromatography is examined. It is shown that the universal calibration of size exclusion chromatographic columns by polystyrene is reliable at molecular weights above 2000 only.     

Carbonyldiimidazole‐accelerated efficient cure of epoxidized soybean oil with dicyandiamide

This study investigates the curing of epoxidized soybean oil (ESO) using dicyandiamide (DICY) and combinations of DICY with several accelerators as curing agents. The differential scanning calorimetry (DSC) results indicated that carbonyldiimidazole (CDI) is a highly efficient accelerator for the ESO‐DICY curing system. CDI accelerated ESO‐DICY curing system can gel within a short period of 13 min at 190 °C. The activation energies of the ESO‐DICY curing systems with and without CDI are 95 and 121 kJ mol^?1, respectively. Similar acceleration effect was observed in the ESO‐diglycidyl ether of biphenyl A (DGEBA) blending formulations. When the molar part of the glycidyl epoxy groups of DGEBA was equal to the internal epoxy groups of ESO in the mixture, gelation of the DICY curing system accelerated by CDI was achieved in 3 min at 160 °C. Furthermore, the DSC results with FTIR analysis suggest that the stoichiometric curing molar ratio was 3 ESO epoxy units per 1 DICY molecule. Two epoxy units reacted with DICY to give secondary alcohols, while the other one linked to the nitrile group. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 375–382     

Investigation of isothermal and dynamic cure kinetics of epoxy resin/nadic methyl anhydride/dicyandiamide by differential scanning calorimetry (DSC)

Isothermal and dynamic differential scanning calorimetry (DSC) was exploited to study the curing behavior of diglycidyl ether bisphenol-A epoxy resin with various combining ratios of dicyandiamide (DICY) and nadic methyl anhydride (NMA). Curves of prepared samples indicated that the enthalpy of the reaction decreased with increasing the molar ratios (NMA/DICY) up to 40% after which an exothermic peak peculiar to the effect of anhydride appeared at a higher temperature. The curing behavior examination of the samples containing the aforementioned molar ratio of NMA/DICY (= 40%) was carried out using isothermal condition at different temperatures (130–145 °C) and dynamic condition DSC at various heating rates (2.5–20 °C min⁻¹). Under the isothermal condition, by constructing a master curve, the values of activation energy (E_a) and pre-exponential factor (A) were calculated 89.3 kJ mol⁻¹ and 1.2 × 10⁺⁹ s⁻¹, respectively. The activation energy of the curing reactions in a dynamic mode was obtained 85.32 kJ mol⁻¹ and 88.02 kJ mol⁻¹ using Kissinger and Ozawa methods, respectively. Likewise, pre-exponential factors were also calculated 3.35 × 10⁺⁸ and 7.4 × 10 ⁺⁸ s⁻¹, respectively. The overall order of reaction for both conditions was found to be a value around 3.

     

Frontal cationic curing of epoxy resins

We studied the frontal curing of trimethylolpropane triglycidyl ether (TMPTGE) using two BF₃‐amine initiators and two fillers, kaolin and fumed silica. In the case of kaolin, the range of concentrations allowing for frontal polymerization to propagate was dependent on its heat absorption effect whereas in the case of silica it was a consequence of the rheological features of this additive. However, for both systems the velocity and front temperature show the same trends; in all cases front velocities were on the order of 1 cm/min with front temperatures about 200 °C. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2000–2005, 2010     

Curing kinetics of liquid-crystalline epoxy resins

By endcapping mesogenic rigid rod molecules with reactive epoxy groups a novel class of liquid-crystalline thermoset has been obtained. In fact is has been shown that the nematic molecular arrangement is sustained over the crosslinking reaction of liquid-crystalline epoxy resins when the curing reaction is carried out in the thermal stability range of the liquid-crystalline phase. Calorimetric analysis was used in characterizing the isothermal cure. An unsophisticated model is proposed for evaluating the activation energies of the crosslinking reaction. For liquid-crystalline epoxy resins lower activation energies result with respect to the cure reactions for non liquid-crystalline epoxy resins.     

Dielectric properties of bisphenol-a epoxy resins

The α-relaxation process of seven commercial diglycidyl ether of bisphenol-A (DGEBA) epoxy resins of varying molecular weights, without hardener, was characterized by measuring the frequency and temperature dependence of the complex permittivity, (?ast;). The temperature dependences of the frequency of maximum dipole loss ?_max and the ionic conductivity σ are non-Arrhenius and can be described by the Williams-Landel-Ferry (WLF) equation. The frequency dependence of the molecular relaxation process is described by the Kohlrausch-Williams-Watts (KWW) relaxation function. The WLF C₁ parameter and the KWW β parameter, describing the temperature and frequency dependences, respectively, vary systematically with the molecular weight of the resin. These results are discussed in the context of recent theories of the glass transition.     

Predicting the phase behaviour of solvent-modified epoxy resins

 The Flory–Huggins equation has been used to model the equilibrium phase behaviour of a solvent-modified epoxy resin intended for the fabrication of porous components by chemically induced phase separation followed by evacuation of the solvent after curing. Points in composition–temperature space corresponding to a transition from homogeneous to heterogeneous postcuring microstructures have been identified for a variety of solvents using a thermal gradient oven. Assuming this transition to coincide with the cloud-point curve corresponding to the gel point of the resin, the data were used in conjunction with the predictions of the Flory–Huggins equation to estimate the interaction parameter and its temperature dependence for each solvent. Phase diagrams in composition–conversion space were then calculated for a range of curing temperatures. Received: 20 September 1999 Accepted: 21 December 1999