Kinetic method by using calorimetry to mechanism of epoxy-amine cure reaction

Some specific features of the thermochemistry of epoxy-amine curing at the later stages of the reaction are considered. Possible mechanism of cross-linking and the question about the driving force leading to the infinite network are discussed. The coupling of the reaction kinetics and rearrangement of the chains crosslinked into the rigid supramolecular structure is the essential feature of epoxy-amine vitrified system. It has been proposed that owing to the contribution from the side process, different curing temperatures can result in the structures with different T _g. It was also established that reaction of epoxy ring opening alone is not responsible for the residual curing. The latter is the result of the side processes. As compared with the reaction of epoxy ring opening the side processes are strongly dependent on the geometrical aspects. This revised version was published online in August 2006 with corrections to the Cover Date.     

Study of the influence of water absorption on different epoxy-diamine systems by DSC

The diffusive, calorimetric and thermal degradation behavior of different epoxy-amine systems was investigated during water sorption at different temperatures (23, 40 and 70°C). Experimental results showed that the water absorption at these temperatures fitted well to Fick’s law. Influence of water immersion during different periods of time on the glass transition temperatures was studied by differential scanning calorimetry. Thermal degradation of saturated samples was studied by thermogravimetric analysis. Dependence on the selected curing cycle was also checked.     

Physical ageing in ternary epoxy-amine curing systems of variable composition

Endothermal effects appearing close to the glass transition point in cured epoxy-amine systems of variable composition and connected with its physical ageing were investigated. The dependences of thermal effect parameters on ageing time and composition of curing system were determined.     

Reaction-induced phase separation in polyethersulfone-modified epoxy-amine systems studied by temperature modulated differential scanning calorimetry

In epoxy-amine systems with a thermoplastic additive, the initially homogeneous reaction mixture can change into a multi-phase morphology as a result of the increase in molecular weight or network formation of the curing matrix. Temperature modulated DSC (TMDSC) allows the real-time monitoring of this reaction-induced phase separation. A linear polymerizing epoxy-amine (DGEBA–aniline) and a network-forming epoxy-amine (DGEBA–methylene dianiline), both with an amorphous engineering thermoplastic additive (polyethersulfone, PES), are used to illustrate the effects of phase separation on the signals of the TMDSC experiment. The non-reversing heat flow gives information about the reaction kinetics. The heat capacity signal also contains information about the reaction mechanism in combination with effects induced by the changing morphology and rheology such as phase separation and vitrification. In quasi-isothermal (partial cure) TMDSC experiments, the compositional changes resulting from the proceeding phase separation are shown by distinct stepwise heat capacity decreases. The heat flow phase signal is a sensitive indication of relaxation phenomena accompanying the effects of phase separation and vitrification. Non-isothermal (post-cure) TMDSC experiments provide additional real-time information on further reaction and phase separation, and on the effect of temperature on phase separation, giving support to an LCST phase diagram. They also allow measurement of the thermal properties of the in situ formed multi-phase materials.     

Effect of the composition of polyacrylimide-forming copolymers on their processing

Thermomechanical studies of thermosetting copolymers of acrylonitrile with methacrylic acid and acrylamide with acrylic acid that form similar polyacrylimide foam plastics during curing have been performed. The dependence of the glass-transition temperatures of the copolymers on the degrees of intramolecular imidization has been calculated and confirmed experimentally. Procedures to control the curing rate have been proposed. The influence of plasticizing additives on the glass-transition temperatures, imidization, and foaming of foam-forming compositions has been studied. Plasticized powdered foam-forming compositions with delayed imidization have been developed, and temperature conditions for their processing have been determined.     

Energy Characteristics of the Surface of Modified Phosphorus-Containing Epoxy Polymers

The Fowkes and van Oss-Good approaches were used for assessing the energy characteristics of the surface of the epoxy-amine systems modified with triglycidyl phosphate. Variation of these characteristics with the modifier content and polymer curing conditions was studied.     

Formation of Strength Properties of Model Glue Joints Based on Epoxy-Amine Binders Operating by the Polymerization Mechanism

The kinetics of curing of an epoxy-amine compound by the polymerization mechanism were studied at various temperatures by isothermal calorimetry and rheokinetic procedures. The resulting kinetic data were correlated with the patterns of variation of the adhesion strength of glass-epoxy-amine compound-glass glue joints, of shrinkage stresses, and shrinkage defects, as well as with the nature of the glue joint failure.     

Thermodynamic Analysis of Phase Separation of a Thermoplastic in the Precursors of Different Epoxy-Amine Systems

The miscibility of a thermoplastic, polystyrene (PS), with the precursors of several epoxy-amine systems has been studied thermodynamically and experimentally. The epoxy-amine systems were different only in the origin of amino groups, which were provided by a monoamine (MA) and a diamine (DA) in different proportions. Cloud-point curves (CPC) at conversion zero were reported for five modified systems with different MA-DA proportion. All CPCs showed an UCST behaviour. CPC shifted to lower temperatures when the ratio of MA/DA in the system increased, meaning that the MA produces an increase in the miscibility of the system. A model based on the Flory-Huggins theory was used for the thermodynamic analysis, in which the dependence of interaction parameter on temperature and composition, χ(T,ϕ), and the polydispersity of components were considered. A general equation for χ(T,ϕ) also depending on the MA-DA proportion was reached and used to obtain the phase diagram of the different systems. A high level of agreement between theoretical and experimental CPCs was obtained.     

A free-volume-based approach to modeling thermoset cure behavior

The free-volume theory for the temperature dependence of transport properties of glass-forming polymers is extended to obtain their relationship to the extent of cure. This treatment centers on the unifying role of molecular mobility and yields a model which connects extent of reaction, viscosity, diffusivity, ionic conductivity and dipole relaxation time. The temporal dependence of these properties is expressed by coupling the extended free-volume model with a relationship for the rate of cure, which included diffusional limitations. Analyses based on this model are applied to the observed behavior of a model epoxy-amine resin system. The intrinsic kinetics of this model system are shown to be first order. It is shown that diffusional limitations strongly affected the progress of the reaction in the final stages of cure. The diffusion-modified rate expression predictions agree with extent of reaction versus time data over the range of experimental temperatures. The temporal dependence of viscous behavior of the curing resin is measured. The extended free-volume model accurately describes the evolution of resin viscosity during cure. The dielectric behavior is similarly characterized and is in close agreement with the predictions of the general free-volume expression. The results of this study indicate that the free-volume theory modified to account for molecular weight effects allows prediction of resin properties with a two-parameter model. The results show that a power-law relationship exists between viscosity and ionic conductivity. This result suggests that electrical properties may be used for on-line measurement of resin viscosity during cure.     

Kinetic method by using calorimetry to mechanism of epoxy-amine cure reaction

A combination of kinetic method and DSC measurements was used to examine the system of resorcinol diglycidylether-aniline. The purpose of this study is to obtain information about linear polycondensation in epoxy-amine system. The reaction of resorcinol diglycidylether (RDGE) with aniline falls into the family of epoxy-amine reaction mixtures, within of which the functional groups varies only. The molar heats and the rate constants for the three pathways were evaluated by nonlinear regression analysis of the data assuming that reaction mechanism proposed for simple molecular epoxy-amine system such as phenylglycidylether-aniline would be operative in the reaction between resorcinol diglycidylether and aniline. A feature of the present reaction system is that it proceed through the structural changes occurred with the heat effect. The loss of catalytic activity by the molecules of the reaction product was used as indicator for the structure forming in the reaction medium.     

An investigation of the interconnection between topological characteristics of physical and chemical networks of anhydride adamantane-containing epoxy polymers

The interconnection between topological characteristics of chemical and physical networks of adamantane-containing epoxy polymers of anhydride curing is studied. It is shown that the introduction of adamantane fragments into the network of an epoxy polymer by different methods affects the glass-transition point and stress-strain properties. The changes in polymer properties depending on the concentration of modifying agents are analyzed during comparison of the frequency parameters of the networks of chemical bonds and engagements within a cluster model of the structure of the polymer amorphous state.     

Synthesis and properties of phosphorus-containing bio-based epoxy resin from itaconic acid

A phosphorus-containing bio-based epoxy resin (EADI) was synthesized from itaconic acid (IA) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO). As a matrix, its cured epoxy network with methyl hexahydrophthalic anhydride (MHHPA) as the curing agent showed comparable glass-transition temperature and mechanical properties to diglycidyl ether in a bisphenol A (DGEBA) system as well as good flame retardancy with UL94 V-0 grade during a vertical burning test. As a reactive flame retardant, its flame-resistant effect on DGEBA/MHHPA system as well as its influence on the curing behavior and the thermal and mechanical properties of the modified epoxy resin were investigated. Results showed that after the introduction of EADI, not only were the flame retardancy determined by vertical burning test, LOI measurement, and thermogravimetric analysis significantly improved, but also the curing reactivity, glass transition temperature (T _g), initial degradation temperature for 5% weight loss (T _d(5%)), and flexural modulus of the cured system improved as well. EADI has great potential to be used as a green flame retardant in epoxy resin systems.     

Epoxy/POSS organic-inorganic hybrids: ATR-FTIR and DSC studies

Organic-inorganic hybrid nanocomposites were prepared by reaction of an octaepoxy-silsesquioxane, OECh, with an epoxy-amine system. OECh was used to partially replace the thermosetting resin, diglycidyl ether of bisphenol A, DGEBA, in its reaction with an aromatic diamine, 4,4′-(1,3-phenylenediisopropylidene) bisaniline, BSA. The OECh was characterized by different techniques. The curing kinetics of ternary systems formed by DGEBA, OECh and BSA, was followed by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy, ATR-FTIR. All the mixtures were prepared with a stoichiometric ratio between epoxy and amine groups. The degree of reaction of glycidyl epoxy ring along the curing cycle selected was obtained from the infrared spectra. A peak-height method based on the ratio of the height of the characteristic to reference absorbance peak was used. The curing kinetic of different blends was obtained by differential scanning calorimetry, DSC. Three different methods, the differential of Kissinger, the integral of Flynn-Wall-Ozawa and the phenomenological model of Kamal, were used in order to obtain the kinetic parameters of the cure reaction. It is observed that the presence of POSS accelerates the rate of opening of glycidyl epoxy rings from DGEBA. The behaviour of the mixture during the curing process can be explained with an autocatalytical model, corrected with the contribution of the diffusion of the molecules during the course of the reaction.     

Dielectric properties of self‐catalytic interpenetrating polymer network based on modified bismaleimide and cyanate ester resins

Bismaleimide (BMI) resins with good thermal stability, fire resistance, low water absorption, and good retention of mechanical properties at elevated temperatures, especially in hot/wet environments, have attracted more attention in the electronic and aerospace industries. However, their relatively high dielectric constant limits their application in the aforementioned fields. In this work, a new promising approach is presented that consists of the formation of a self‐catalytic thermoset/thermoset interpenetrating polymer network. Interpenetrating polymer networks (IPNs) based on modified BMI resin (BMI/DBA) and cyanate ester (b10) were synthesized via prepolymerization followed by thermal curing. The self‐catalytic curing mechanism of BMI/DBA‐CE IPN resin systems was examined by differential scanning calorimetry. The dielectric properties of the cured BMI/DBA‐CE IPN resin systems were evaluated by a dielectric analyzer and shown in dielectric properties‐temperature‐log frequency three‐dimensional plots. The effect of temperature and frequency on the dielectric constant of the cured BMI/DBA‐CE IPN resin systems is discussed. The composition effect on the dielectric constant of the cured IPN resin systems was analyzed on the basis of Maxwell's equation and rule of mixture. The obtained BMI/DBA‐CE IPN resin systems have the combined advantages of low dielectric constant and loss, high‐temperature resistance, and good processability, which have many applications in the microelectronic and aerospace industries. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1123–1134, 2003     

Formation of the Interphase of a Cured Epoxy Resin Near a Metal Surface: Reactive Coarse-Grained Molecular Dynamics Simulations

Mesoscale molecular dynamics simulations are performed to analyze the curing process of an epoxy resin with polyfunctional amines on a generic surface. The coarse grained potentials were derived from all-atomistic molecular dynamics simulations using iterative Boltzmann inversion. The reactive scheme incorporates cross-linking between an epoxy resin and an amine, as well as amine adsorption on the surface. The structure of the cured network is examined and compared with equilibrium properties of the uncured system. Special attention has been paid on the implications of the surface that is believed to play a crucial role in the performance of epoxy systems.     

Peculiarities of structure formation of epoxy-amine polymers upon curing of diglycidyl ester of diphenylolpropane with adamantane diamines

The peculiarities of structure formation of epoxy-amine polymers upon curing of diglycidyl ester of diphenylolpropane with adamantane diamines are discussed. It is shown that, at the initial stage of polycondensation, deceleration of the curing process takes place that is caused by glass transition of the polycondensation products. It is found that complete consumption of all functional groups is impossible without additional postcuring.     

Low Dielectric Thermoset from Redistributed Poly(phenylene oxide)

A curable low-molecular-weight poly(phenylene oxide) (PPO) was prepared by the redistribution of regular PPO with bisphenol-A (BPA) followed by etherification of the redistributed-PPO (BPA-PPO) with N,N-diallyl-2-chloroacetamide. The redistributed-PPO with allyl group (AL-PPO) was characterized by proton nuclear magnetic resonance, and Fourier transform infrared spectroscopy. The AL-PPO oligomers with reactive double bounds were cured with triallylisocyanurate (TAIC) and/or phosphorus-containing allyl-functionalized monomer (allyl-DOPO). The glass transition temperatures were measured by dynamic mechanical analysis (DMA). Electrical properties of cured resins were studied using dielectric analyzer (DEA). The flame retardancy was determined by a UL-94 vertical test. The effects of curing accelerator, amount of TAIC and allyl-DOPO incorporated into the network on the glass transition temperatures, dielectric properties, and flame retardancy of the resulting systems were investigated. The results indicated that AL-PPO cured with TAIC exhibited high glass-transition temperature (162–198°C), low dielectric constants (2.36–2.57 at 1 GHz) and dissipation factors (0.0039–0.0043 at 1 GHz). The AL-PPO/TAIC copolymerized with allyl-DOPO could achieve a flame retardancy rating of UL-94 V-0 at about 1.35% phosphorus content. The AL-PPO/TAIC resins have potential applications in the fabrication of printed circuit board.     

Network polymers based on 3,3-bis-(4′-hydroxyphenyl)phthalide and [2.2]paracyclophane-4,16-dicarboxylic acid

A linear ester is synthesized on the basis of phenolphthalein and [2.2]paracyclophane-4,16-dicarboxylic acid dichloride. The formation of network polymers via the heat treatment of this ester in a wide temperature range is studied. The physical properties of these polymers are investigated. The computer-aided simulation of several physical parameters (including the glass-transition temperature and the dielectric constant) is performed for both the linear polymer and feasible network structures. There is a good correlation between the experimental results and the calculated data.     

Reactive processing of syndiotactic polystyrene with an epoxy/amine solvent system

Syndiotactic polystyrene (sPS) is a new semi-crystalline thermoplastic which is believed to fill the price-performance gap between engineering and commodity plastics. In order to reduce the high processing temperature of sPS (>290°C), an epoxy-amine model system was used as a reactive solvent. Such a processing aid can be used to achieve a 50 to 500 fold lowering of the melt viscosity. When initially homogeneous solutions of sPS in a stoechiometric epoxy-amine mixture are thermally cured, Reaction Induced Phase Separation (RIPS) takes place, leading to phase separated thermoplastic-thermoset polymer blends. We focus our study on low (wt% sPS < 20%) and high concentration blends (wt% sPS > 60%) prepared by two processing techniques (mechanical stirring in a laboratory reactor or internal mixer/ reactive extrusion respectively). These blends have different potential interests. Low concentration blends (sPS domains in an epoxy-amine matrix) are prepared to create new, tunable blend morphologies by choosing the nature of the phase separation process, i.e. either crystallisation followed by polymerization or polymerization followed crystallisation. High concentration blends (sPS matrix containing dispersed epoxy-amine particles after RIPS) are prepared to facilitate the extrusion of sPS. In this case, the epoxy amine model system served as a reactive solvent. The time to the onset of RIPS is in the order of 7-9 min for low concentration blends, while it increases to 20-45 min for high concentration samples, as the reaction rates are substantially slowed down due to lower epoxy and amine concentrations. During the curing reaction the melting temperature of sPS in the reactive solvent mixture evolves back from a depressed value to the level of pure sPS. This indicates a change in the composition of the sPS phase, caused by (complete) phase separation upon reaction. We conclude that our epoxy amine system is suited for reactive processing of sPS, where final properties depend strongly on composition and processing conditions.