The physical properties of bisphenol-a-based epoxy resins during and after curing. II. Creep behavior above and below the glass transition temperature

The creep behavior of a series of fully cured epoxy resins with different crosslink densities was determined from the glassy compliance level to the equilibrium compliance J_e at temperatures above T_g and at the glassy level below T_g during spontaneous densification at four aging temperatures, 4,4-diamino diphenyl sulfone DDS was used to crosslink the epoxy resins. The shear creep compliance curves J(t) obtained with materials at equilibrium densities near and above T_g were compared at their respective T_gs. T_gs from 101 to 205°C were observed for the epoxies which were based on the diglycidyl ether of bisphenol A. Creep rates were found to be the same at short times, and equilibrium compliances J_e were close to the predictions of the kinetic theory of rubberlike elasticity. Time scale shift factors determined during physical aging were reduced to T_g. At compliances below 2 × 10^?10 cm²/dyn, Andrade creep, where J(t) is a linear function of the cube root of creep time, was observed. The time to reach an equilibrium volume at T_g was found to be longer for the epoxy resin with lower crosslink densities. The increase of density during curing is illustrated for the epoxy resin with the highest crosslink density.     

Electrical properties of an epoxy resin during and after curing

By means of an epoxynovolac resin with BF₃ complex the suitability of measurements of electrical properties for the investigation of curing process and for the evaluation of cured resins is illustrated. The origin of electrical conductivity and polarization is elucidated.     

The curing of epoxy resins with aminophenoxycyclotriphosphazenes

Aminophenoxycyclotriphosphazenes have been used as curing agents for epoxy resins. The thermal curing was performed in stages at 120–125 and 175–180°C followed by postcuring at 225°C to give tough brown polymers. The thermal curing reaction was monitored using FTIR and differential scanning calorimetry. Thermogravimetric analysis of the cured resins has shown thermal stability up to 350–340°C. The char yield obtained in nitrogen at 800°C was about 55–42% and in air at 700°C was about 40–32%. Graphite cloth laminates were prepared. The mechanical properties evaluated were found superior to those of commonly used epoxy resin systems. These resins are useful for making fire- and heat-resistant composites, laminates, molded parts, and adhesives.     

Influence of curing conditions on the morphology and physical properties of epoxy resins modified with a liquid polyamine

A diglycidyl ether of bisphenol-A (DGEBA) epoxy resin has been stoichiometrically cured with cycloaliphatic amine 4,4′-diamino-3,3′-dimethylcyclohexylmethane (3DCM) and modified with an amine terminated oligomer polyoxypropylenetriamine (POPTA) at a concentration of 15 wt %. Mixtures, postcured at the same temperature, have been precured at different temperatures. Phase separation takes place before gelation at all precure temperatures used. The variation in the glass transition region of the mixtures has been analyzed by dynamic mechanical measurements. Mechanical properties and fracture toughness of the modified mixtures have been related to their microstructural spherical features. Results are compared to those for the unmodified mixtures cured with different precure temperatures. © 1997 John Wiley & Sons, Inc.     

Kinetics of curing reaction and properties of polymeric materials based on epoxy resins

Base epoxy oligomers of Bisphenol A type were cured by aliphatic and aromatic amines, as well as by anhydride and isocyanate hardeners. The glassy networks obtained were characterized by scanning calorimetry, linear dilatometry, IR spectroscopy and electron microscopy.     

Network build-up and structure in curing of epoxy resins

The available branching theories and their application to curing of epoxy resins are reviewed. Special attention is paid to theoretical treatment and experimental results of curing with polyamines, polyetherification, and to curing with poly(carboxylic acid)s and cyclic anhydrides.     

Anhydride curing of epoxy resins via chain reaction

In contrast to common curing reactions, the anhydride curing of epoxies follows a living anionic chain growth. The resulting consequences of this mechanism, i.e. (1) DP_n = a[M_o]/[I_o], (2) first-order kinetics and (3) Poisson chain-length distribution were tested with the phenyl glycidyl ether/phthalic acid anhydride system, using l-methyl imidazole. Overall agreement was found and the observed deviations could be explained with a modified Poisson process. Conformational properties of the resins were measured by static and dynamic light scattering and by viscometry. These were compared with the quantities of a corresponding branched system prepared with a mixture of phenyl glycidyl ether and bisphenol-A diglycidyl ether. Typical deviations to smaller dimensions were observed at high molar masses as a result of increasing branching.     

Simultaneous monitoring of viscoelastic and dielectric properties of curing epoxy resins using a vibrating electrode

Both dielectric and mechanical dynamic analysis can characterize the curing of epoxy resins. As the reproducibility of this process is poor, data from separate measurements cannot be compared. To allow for a simultaneous measurement of frequency-dependent viscoelastic (0.5–8 kHz) and dielectric (50–800kHz) properties, a vibrating electrode is used. The curing measurements of four different epoxy-amine systems show a frequency-dependent drop in both the dielectric and compliance real parts. Higher frequencies are sensitive to the relaxation of smaller molecules, hence, the step due to vitrification occurs first at higher frequencies. Accompanied by a loss maximum the dielectric step shifts down over four decades to a lower frequency region and appears eventually in the mechanical spectrum. The simultaneous investigation reveals that the dielectric and the longitudinal compliance function have almost the same frequency dependent reaction kinetic behavior. The mechanical time behavior can be extrapolated in the frequency domain by a power law to obtain the dielectric time parameters and vice versa.     

Evaluation of furfurylamines as curing agents for epoxy resins

Epoxy resin adhesives are widely used because of their strength, versatility, and ability to bond a variety of substrates. Furfurylamines represent a potential, new class of epoxy curing agents. Furfuryl amine (FA), tetrahydrofurfuryl amine (THFA), and 5,5′-methylenebis-2-furanmethanamine (DFA) were studied as possible epoxy curing agents. The utility of FA and THFA are limited by their volatility at the temperatures needed to effect cure of diglycidyl-ether of bisphenol A (DGEBA) based epoxy resins. DFA is a very effective epoxy curing agent with the ability to cure DGEBA at rates similar to that of standard epoxy curing agents such as liethylenetriamine.     

Synthesis and properties of new polyfunctional curing agents for epoxy resins based on dimerized fatty acid

The synthesis and properties of a number of devised imino(amino)amide derivatives of dimerized fatty acid are described. It is shown that synthesized products exhibit the properties of plasticized curing agents for epoxy resins.     

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.     

Morphology, flexural, and thermal properties of sepiolite modified epoxy resins with different curing agents

A bisphenol A-based epoxy resin was modified with 5 wt% organically modified sepiolite (Pangel B40) and thermally cured using two different curing agents: an aliphatic diamine (Jeffamine D230, D230) and a cycloaliphatic diamine (3DCM). The morphology of the cured materials was established by scanning and transmission electron microscopy analysis. The thermal stability, thermo-mechanical properties, and flexural behaviour of the sepiolite-modified matrices were evaluated and compared with the corresponding neat matrix. The initial thermal decomposition temperature did not change with the addition of sepiolite. The flexural modulus of the epoxy matrix slightly increases by the incorporation of the organophilic sepiolite. The flexural strength of the sepiolite modified resin cured with D230 increased by a 10% while the sepiolite modified resin cured with 3DCM resulted in a lower flexural strength compared with the unmodified resin. The reduced flexural strength was attributed to the stress concentrations caused by the sepiolite modifier, which rendered the resins more brittle.     

胺固化环氧酚醛树脂对Hg~(2+)的吸附性能

螯合树脂能与金属离子形成稳定的配合物,在无机、冶金、分析药物、催化、海洋化学、放射化学、环境保护各领域都有非常重要的应用[1-3]。我们曾以酚醛树脂为大分子骨架与多胺反应制备了一系列螯合树脂并对其结构及对多种金属离子的吸附性能进行了探讨[4,5]。本文则以线型环氧酚醛树脂为大分子骨架、多乙烯多胺为固化剂,制备了五种不同氮含量的氨基环氧酚醛螯合树脂,并考察其对金属离子Ｈｇ2+的吸附性能。1　实验部分1.1　仪器与试剂ＮｉｃｏｌｅｔＭＡＧＮＡ ＩＲ550(ＳｅｒｉｅｓＩＩ)红外分光光度计,ＳＨＡ-Ｃ水浴恒温振荡器,ＰＥ2000…     

Epoxy resins. II. The preparation,characterization, and curing of epoxy resins and their copolymers

A polymer with high aromatic ring content in the chain backbone usually has high heat and flame resistance. Three diglycidyl ethers of epoxy resins were prepared from bisphenol A (DGEBA), phenolphthalein (DGEPP), and 9,9-bis(4-hydroxyphenyl)fluorene (DGEBF) in a study of the relation between the cured polymer structure and properties. The epoxy resin prepared from phenolphthalein was separated by liquid chromatography and three fractions were obtained. The fractions had a basic structure of 3,3-disubstituted phthalide and differed only in molecular weight. The DGEPP resin changed color from yellow to red after mixing with trimethoxyboroxine (TMB), the curing agent, and to orange after completing the curing cycle. To prepare a highly crosslinked material with good thermal stability, TMB with three active Lewis sites in a molecule was used as the curing agent. The reactivity of the three different resins toward TMB, measured by differential scanning calorimetry (DSC), was DGEBA > DGEBF > DGEPP. For the same curing conditions the order of crosslink density was DGEBA > DGEPP > DGEBF. To modify the flammability of DGEBA, the conventional epoxy resin, it was copolymerized with DGEPP and DGEBF, the higher-performance epoxy resins. The glass transition temperatures of poly(DGEBA-co-DGEPP) and poly(DGEBA-co-DGEBF) systems deviated from this relationship. The DGEBF copolymers showed an increased char residue (40 wt % at 700°C) at 20 mole % of DGEBF. This deviation may be due to the lower crosslinking density of this system.     

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     

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.     

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.     

In-situ monitoring of the curing of epoxy resins by Raman spectroscopy

Polymerization reactions are based on complex processes that are somewhat difficult to predict via mathematical models, especially without experimental data. A method to investigate the cure of epoxies via in-situ Raman spectroscopy has been developed.Differential Scanning Calorimetry (DSC) is the industry-standard method for determining the cure of a polymer, but it is a labor-intensive method that is also fairly slow. Raman spectroscopy was used to monitor the cure chemistry of DGEBA (Diglycidyl Ether of Bisphenol A) and to observe in-situ the evolution of the reticulation.     

Epoxy resins (DGEBA): The curing and physical aging process

Differential scanning calorimetry (DSC) and infrared spectroscopy (IR) were used to monitor the degree of cure of partially cured epoxy resin (Epon 828/MDA) samples. The extent of cure, as determined by residual heat of reaction, concurred with that determined by monitoring the infrared radiation absorbance of the epoxide group near 916 cm^?l. The fictive temperature T_{f, g} was found to increase with the degree of cure, increasing rapidly during cure until reaching a value near the cure temperature T_c of 130°C (approximately 80% cure) where the material vitrified. The greatly reduced reaction rate during the final 20% of cure was not only a consequence of vitrification but, as revealed by infrared spectroscopy, the result of the depletion in the number of reactive epoxide groups. The endothermic peak areas and peak temperatures evident during the DSC scans were used as a measure of the extent of “physical aging” which took place during the cure of this resin, and after, fully cured samples were aged 37°C below their ultimate glass temperature for various periods of time. The rate of physical aging slowed as the temperature increment (T_t,g ? T_c) increased. Although an endothermic peak was evident after only 1 h of cure (T_{f, g} = 138.3°C), such a peak did not appear until fully cured samples were aged for 16 h or more. Enthalpy data revealed that for partially cured material, the fictive temperature T_{f, a}, reflecting physical aging, increased with curing time. In contrast, the T_{f, a}, for fully cured samples decreased with sub-T_g aging time. The characteristic jump in the heat capacity ΔC_p which occurred at the T_{f, g} decreased as curing progressed. This decrease appears to be dependent upon the rotational and vibrational degrees of freedom of the glass. Finally, a graphical method of determining the fictive temperature T_{f, a}, of partially and fully cured epoxy material from measured endothermic peak areas was developed.