Effect of the nature and associate structure of an epoxy oligomer on the rate of its curing with diamine

The supramolecular structure of a number of epoxy oligomers, monofunctional epoxy compounds, their mixtures, an oligoamine curing agent, and curing systems on their basis is studied by dynamic light scattering. In all of the starting oligomers and their mixtures, light scattering centers are detected that may be considered as the associates (aggregates) of oligomer molecules. The structure of associates that is determined by the flexibility of the oligomer chain, its branching degree, and the presence of proton-donor groups capable of hydrogen bonding is suggested. The relationship between the rate of curing of epoxy oligomers and their mixtures with oligoamine and the associate structure of oligomers and the nature of the substituent at the glycidyl group of the oligomer is ascertained.     

Curing rheokinetics of epoxy diane oligomer mixtures with polyglycidyl ethers of oligooxypropylenetriol

The rheokinetics of curing of epoxy diane oligomer mixtures with the polyglycidyl ether of oligooxypropylenetriol by an oligomer aliphatic amine is investigated. The process can be generally divided into two stages with respect to the rate of the increase in viscosity. It is assumed that the adducts of the diane oligomer with the curing agent preferably form at the beginning of the process and, then, the residual epoxy oligomers react in an equivalent amount in relation to each other.     

Synthesis and characterization of novel polythiourethane hardeners for epoxy resins

A novel thiol-terminated polythiourethanes were synthesized from low-molecular-weight di- and multifunctional mercaptans and diisocyanates and employed as curing agent of epoxy resin. The curing reaction of epoxy resin and thermal properties of cured products were investigated with differential scanning calorimetry. Evaluation of climatic ageing resistance was made by the change in mechanical properties. Mechanical studies indicated that the application of polythiourethane has toughening effect and significantly increases ageing resistance of the cured resins. The results of this study indicate that molecular structure and functionality of polythiourethane oligomers are of critical importance in governing the curing mechanism, structure of the network and final properties of epoxy compositions.     

Oligomeric epoxy binders with controllable molecular characteristics: Shrinkage upon curing

Experimental data and generalized dependences of the effect of molecular characteristics of epoxy diane oligomers (EDOs) and mixtures thereof of various compositions on the value of volume shrinkage are presented. It is established that EDOs and mixtures thereof with a low molecular weight (to 400) and the content of the first fraction over 90 vol % have the highest value of shrinkage by the moment of gel formation, which allows one to obtain polymeric composite materials and adhesive joints with a low level of residual stresses and high physicomechanical characteristics.     

Curing of epoxy oligomers by the eutectic mixture of aromatic amines

Kinetics of curing of structurally different epoxy oligomers (ED-20 and PDI-3AK resins) in a mixture with other low-molecular-weight epoxy oligomers and plasticizers by the eutectic mixture of aromatic amines UP-0638/1 is studied by the DSC method. The activation energy and the heats of curing reactions are determined. It is established that crosslinked epoxy polymers cured at moderate temperatures (40–80°C) are strong moisture-resistant compositions with different mechanical characteristics. Plasicized elastomers based on PDI-3AK resin with glass transition temperatures of ?78 and ?95°C are freeze-and heat-resistant materials.     

Estimating the molecular mass between epoxy-amine network junctions from glass-transition-temperature data

The average molecular mass between network junctions is estimated for several epoxy-amine systems of various compositions with comparable molecular masses of the epoxy oligomer and the curing agent. This estimation is based on the experimental data on glass-transition temperatures of curing epoxy-amine mixtures. The constant that relates these physicochemical characteristics is dependent on the nature of the epoxy-amine system and the rigidity of polymer chains located between network junctions. Correlations are found between the constant and the glass-transition temperature of the crosslinked epoxy-amine system and between the constant and the difference in the glass-transition temperatures of uncured and ultimately cured epoxy-amine systems.     

Refractometry of dian and aliphatic epoxy oligomers

The refractometric characteristics of dian and aliphatic epoxy oligomers and their mixtures in a wide range of concentrations and temperatures have been investigated. It has been shown that the temperature dependences of the refractive indexes of the oligomers and their mixtures are straight lines. Upon mixing the oligomers, their volume remains unchanged. The molar refractions of individual oligomers and their mixtures are determined from experimental values of refractive indexes and densities and on the basis of calculations of atomic and group contributions using the concentrations of terminal fragments and the ratio of components. The effect of terminal fragments of the oligomers on the refractive index, density, and molar refraction was taken into account using the reduced molecular mass values. Methods for estimating the molecular mass of linear epoxy oligomers on the basis of their refractive indexes are suggested.     

Oligomer–oligomer modification upon the synthesis of epoxy polymers with a “shape-memory” effect

The results of studies of the oligomer–oligomer modification of epoxyamine compositions with amine adducts, oligomers, block oligomers, and oligocyclocarbonates differing in their structure and molecular weight upon hardening under the conditions of a general epoxide–amine polycondensation reaction, and competitive epoxide–amine and cyclocarbonate–amine reactions are given. The parameters of the structure and the elastic-deformation properties required for the preparation of high-quality thermally contracting epoxy polymers with a “shape-memory” and for their reliable application are established.     

Studying the curing of epoxyamine mixtures via dynamic light scattering and differential scanning calorimetry

The curing of epoxy oligomers with different structures and reactivity using an aliphatic amine curing agent was studied and quantitatively characterized by the methods of dynamic light scattering and differential scanning calorimetry. The presence of disperse phase in the initial reagents, as well as in the reacting systems is established. It is shown that the particles of the disperse phase are involved in the process of curing even in the early stages. By changing the width of the devitrification region, the change in the molecular homogeneity of epoxyamine systems during curing is estimated.     

Controlling the softening temperature of epoxy oligomers and their mixtures

The possibility was shown of decreasing the temperature of softening of ED-8 brand high-molecular-weight solid epoxy oligomer by means of creation of mixtures with DER-332, DER-330, ED-22, and ED-20 low-viscosity epoxy oligomers. It was shown that the softening temperature of epoxy oligomers and their mixtures can be controlled by directional variation of molecular weight, molecular-weight distribution, and content of the low-molecular-weight fraction.     

Epoxy-amine polymers obtained upon curing with adamantane diamines

The versions of modification of epoxy polymers upon curing of epoxy-diane oligomers with adamantane diamines and adamantane-containing imidazolines that are close in basicity to aliphatic amines are considered. The parameters of their manufacturability, life and curing times, and parameters that characterize the adhesion strength are defined. The effect of an increase in the functionality of epoxy compounds on the thermal stability of epoxy adamantane-containing polymers is established.     

Kinetics of curing of diane and aliphatic epoxy oligomer blends: IR-spectroscopy study

The curing of diane and aliphatic epoxy oligomers and their blends is studied by IR spectroscopy. Methods for the quantitative determination of epoxy groups in diane and aliphatic oligomers and their blends during curing are developed. It is shown that, during the joint curing of epoxy oligomers, the reactivity of a more active (diane) oligomer remains practically the same, whereas the reactivity of a less active (aliphatic) epoxy oligomer increases. The rate constants for the consumption of epoxy groups of oligomers and primary amine groups of the curing agent are determined.     

Fracture toughening of epoxy matrices with blends of resins of different molecular weights and other modifiers

The microstructure and fracture behavior of epoxy mixtures containing two monomers of different molecular weights were studied. The variation of the fracture toughness by the addition of other modifiers was also investigated. Several amounts of high‐molecular‐weight diglycidyl ether of bisphenol A (DGEBA) oligomer were added to a nearly pure DGEBA monomer. The mixtures were cured with an aromatic amine, showing phase separation after curing. The curing behavior of the epoxy mixtures was investigated with thermal measurements. A significant enhancement of the fracture toughness was accompanied by slight increases in both the rigidity and strength of the mixtures that corresponded to the content of the high‐molecular‐weight epoxy resin. Dynamic mechanical and atomic force microscopy measurements indicated that the generated two‐phase morphology was a function of the content of the epoxy resin added. The influence of the addition of an oligomer or a thermoplastic on the morphologies and mechanical properties of both epoxy‐containing mixtures was also investigated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3920–3933, 2004     

Residual stresses at hardening of disperse-filled epoxy systems

The influence of MS-VP-A9 brand hollow glass microspheres and curing temperature on the kinetics of growth of residual stresses in disperse-filled polymer composites based on DER-330 and ED-20 epoxy oligomers was considered. It was shown that the introduction of hollow glass microspheres and change of generalized parameters of structure leads to a decrease of the level of residual stresses by approximately half during curing, differing in average molecular weight, molecular-weight distribution, and content of associates.     

Study of the reaction mechanism of the copper chelate with DGEBA using DSC

The reaction mechanism of metal-containing and complex compound with epoxy oligomer of diglycidyl ether of bisphenol A (DGEBA) was studied using dynamic DSC technique. It is shown that cure reaction of the epoxy oligomers with copper acetate proceeds at two stages: through coordination of cation with the epoxy group, and through ionic polymerization at high temperatures. Mechanism of curing of DGEBA with copper chelate depends on equilibrium process of dissociation of the chelate which, in turn, depends not only on temperature of curing but also on concentration of the hardener. At the dissociation temperature of the hardener, polymerization proceeds according to ionic mechanism. Hardening of the epoxy oligomers due to interaction of epoxy groups with unconnected amine groups predominate at higher temperatures or at higher concentrations of the hardener. At low temperatures and small concentrations of the hardener, polymerization proceeds according to catalytic ionic mechanism. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study on the chemical modification of epoxy/anhydride thermosets using a hydroxyl terminated hyperbranched polymer

Mixtures of diglycidylether of bisphenol A (DGEBA) resin and commercially available hyperbranched polyester (HBP) Boltorn H30 were cured by anhydride to covalently bond the hydroxyl end groups in HBP with the epoxy resin. The curing mixtures were investigated by Differential Scanning Calorimetry (DSC) to study the curing evolution and to evaluate the kinetic parameters. DSC studies suggested that HBP could increase the curing rate of epoxy/anhydride systems at low conversions, but it produced a decelerative effect in the last stages of the curing. The influence of the HBP content and the proportion of anhydride on the curing conversions were discussed in detail. The addition of a tertiary amine was proved to decrease the curing temperatures. By Fourier Transform Infrared Spectroscopy (FTIR) the reaction of hydroxyl groups during the whole process was confirmed. By the determination of the conversion at the gelation, we could prove that it increased on increasing the proportion of HBP in the reactive mixture. By Thermomechanical Analysis (TMA) we could determine a reduction of the shrinkage after gelation.     

Study of the interfacial structure in epoxy bilayer films using neutron reflection

The interfacial structure in thin epoxy bilayer films was investigated with neutron reflection. For each experiment, a mixture of crosslinker and deuterated resin was spun onto a chemically similar, fully cured, protonated epoxy film. The reflectivity measurements were performed before and after curing the top epoxy film. We focused on the extent of penetration of the components of the top layer into the network of the bottom layer. The effect of the cure temperature of each layer was examined. In addition, the effect of the initial molecular weight of the oligomers in the top layer was probed by the partial curing of the mixture before spinning. As deposited, the components of the top layer penetrated the bottom layer to an extent that was largely independent of the aforementioned factors. The principal observation was that an additional penetration occurred with curing. This additional penetration was dependent upon both the molecular weight of the top layer and the cure temperature of the top layer relative to the glass‐transition temperature of the bottom layer. A decrease in the thickness of the top layer with curing was also observed, which likely indicates some evaporation of oligomers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1709–1718, 2002     

Curing of dicyanate ester/epoxy copolymers modified with polysiloxane and butadiene-acrylonitrile rubbers

The cyanate ester (CE) and epoxy (EP) resins were cured together at various mass compositions. The curing behavior of CE and CE/EP systems was studied by means of differential scanning calorimetry (DSC) in non-isothermal conditions. The DSC measurements indicated that the curing reactions were dependent on the stoichiometric ratio of the mixtures and showed the dilution effect of the EP resin in the cyclotrimerization of the catalyzed CE resin. The CE and CE/EP (70/30) systems were modified using reactive liquid butadiene-acrylonitrile copolymer (ETBN) and polysiloxane core?Cshell (PS) elastomer. The influence of ETBN and PS on the curing process and glass transition temperature (T _g) of CE/EP systems was determined. The impact resistance characteristics of the completely cured systems indicated the influence of the modifiers and the EP content in the mixtures on its impact resistance.     

Phase separation kinetics in mixtures of polymers and liquid crystals

Light scattering has been used to study phase separation kinetics in mixtures containing liquid crystals and epoxy resins. In the samples studied, phase separation was induced by the polymerization of the resins with an appropriate curing agent. Experiments were carried out at different compositions and at different temperatures. The results show that the kinetic mechanism of phase separation is composition dependent. For high liquid crystal content the data are in qualitative agreement with existing theories describing spinodal decomposition; at lower concentrations the mechanism is different. The physical properties of the resulting materials are independent of the decomposition mechanism. The data have also been analysed considering the scaling behaviour expected for late stages of phase separation in polyinduced meric mixtures. Samples obtained in a narrow concentration range, where the two kinetic mechanisms overlap, exhibit peculiar physical properties.     

Formation of fibril structures in polymerizable, rod-coil-oligomer-modified epoxy networks

This paper describes the in situ preparation of fibrils in epoxy networks in which the fibril-like structures are cured polymerizable rod-coil oligomers. The epoxy-terminated alpha,omega-modified PEO oligomers, which are ABA rod-coil-rod oligomers with a poly(ethylene oxide) coil unit and two aromatic azomethine liquid-crystalline rod units, were synthesized and then further blended with an epoxy precursor. Uniform nanoscale columnar structures were observed in the neat rod-coil oligomers as well as in the crosslinked liquid-crystalline state. During the curing of the blends, the supramolecular nanoscale columnar structures of the rod-coil oligomers are transformed into polymeric fibrils where the epoxy functional end groups have co-reacted with epoxy precursors to form a crosslinked network.