Physical aging of polydimethylsiloxane-modified epoxy resin

The thermal expansion coefficient and physical aging behavior of cured epoxy resins modified with amino-terminated polydimethylsiloxane were investigated. The epoxy resin was tetramethyl biphenol diglycidyl ether. Two factors influenced the thermal expansion behavior and the free volume relaxation of the polysiloxane-modified epoxy systems. One was the miscibility between the epoxy resin and the polysiloxane and the flexibility of the chains in the network system. The intrinsic thermal expansion of the network chains and the volume relaxation increased as a result of building polysiloxane molecules into the network structure. The other factor was the size and concentration of the dispersed polysiloxane particles. The increased local free volume at the interface between the epoxy matrix and the polysiloxane particle resulted in a higher thermal expansion coefficient and led to a large driving force for the volume relaxation during annealing. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1291–1300, 1998     

Physical degradation by water clustering in epoxy resins

Water uptakes in sorption experiments on thin sheets of epoxy resins have been explained on the basis of theoretical models. The occurrence of an irreversible void formation process at higher humidities induces strong deviations from the Flory–Huggins analysis. This phenomenon can be successfully explained on the basis of cluster formation.     

Physical properties and shrinkage of epoxy resins cured with lactams

In order to reduce the shrinkage of epoxy resin during the curing process, lactam is incorporated into epoxy in a copolymerization reaction. In this study, various amounts of lactams and BF₃-MEA were added to epoxy, and the volume shrinkage of polymerization was investigated. It was found that shrinkage decreases with the increase of the lactam content and with the lactam ring size as well. Infrared spectroscopic analysis, scanning electronic microscopic analysis, and mechanical tests were used to investigate the structure and properties of the copolymers. The results show that the incorporation of caprolactam leads to an increase in tensile strength and elongation, but the Izod impact strength is not improved.     

Physical aging in graphite/epoxy composites

Matrix-dominated mechanical behavior of a graphite/epoxy composite has been found to be affected by sub-T_g annealing. Postcured (±45°)_4S specimens of Thornel 300 graphite/Narmco 5208 epoxy were quenched from above T_g and given a sub-T_g annealing at 140°C for times up to 10⁵ min. The ultimate tensile strength, strain-to-break, and toughness of the composite material were found to decrease as functions of sub-T_g annealing time. No weight loss was observed during the sub-T_g annealing. The time-dependent change in mechanical behavior is explained on the basis of free-volume changes that are related to the physical aging of the nonequilibrium glassy network-epoxy. The results imply possible changes in composite properties with service time.     

Physical aging for an epoxy network diglycidyl ether of bisphenol A/m-xylylenediamine

The physical aging of the epoxy network consisting of a diglycidyl ether of bisphenol A (BADGE n=0) and m-xylylenediamine (m-XDA) were studied by differential scanning calorimetry. The following aging temperatures have been used in this work: 60, 70, 80, 90, 100 and 110 °C. The glass transition temperature and the variation of the specific heat capacities have been calculated using the method based on the intersection of both enthalpy-temperature lines for glassy and liquid states. The endothermic aging peak, relaxation enthalpy and fictive temperature were also calculated for each aging temperature and aging time.     

Fracture and mechanical properties of epoxy resins and rubber-modified epoxy resins

Fracture and mechanical property data on a wide range of epoxy resin systems are presented. The extent to which toughening can be induced by heterophase rubber inclusions depends more on the curing agent used than on the resin component. The greatest improvements in toughness were obtained by rubber modification of epoxy resins cured with an anhydride. A preformed ABS polymer can be used to toughen many epoxy resin systems. With one major exception (where a large improvement was found) only small changes in tensile properties occur when small amounts of rubber are present.     

Physical aging of partially crosslinked RTM6 epoxy resin

The aging behavior of partially and completely crosslinked RTM6 epoxy resin samples in the glassy state is investigated by thermophysical and mechanical analyses. Curing degree, glass transition temperature, density, and micromechanical modulus are investigated as function of aging period, initial curing degree or depth below the sample surface. A clear increase of density and modulus with aging period as well as an enhanced surface stiffness is detected for all curing degrees. Also, the aging period necessary to achieve the steady state modulus is independent from the curing degree. In contrast, the degree of physical aging induced modulus changes shows a significant dependence on the curing degree. A discontinuity is detected in the so‐called transition region, which is related to the transition from rubber to glassy state during crosslinking. This emphasizes the importance of curing history for physical aging processes and the high potential of partial curing for the development of new processing routes, in particular for production of samples with low sensitivity to physical aging. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41121.     

Multifunctional epoxy resins

The curing behavior of epoxy resins prepared by reacting epichlorohydrin with 4,4′-diaminodiphenyl methane (DADPM)/4,4′-diaminodiphenyl ether (DADPE) or 4,4′-diaminodiphenyl sulfone (DDS) was investigated using DDS and tris-(m-aminophenyl)phosphine oxide (TAP) as curing agents. A broad exothermic transition with two maxima were observed in the temperature range of 100–315°C when TAP was used as the curing agent. The effect of varying DDS concentration on curing behavior of epoxy resin was also investigated. Peak exotherm temperature (T_exo) decreased with increasing concentration of DDS, whereas heat of curing (ΔH) increased with an increase in amine concentration up to an optimum value and then decreased. Thermal stability of the resins, cured isothermally at 200°C for 3 h, was investigated using thermogravimetric analysis in a nitrogen atmosphere. Glass fiber-reinforced multifunctional epoxy resin laminates were fabricated and the mechanical properties were evaluated. © 1993 John Wiley & Sons, Inc.     

The effect of network structure on moisture absorption of epoxy resins

An investigation of the effect of modifying the network structure on moisture absorption of epoxies was performed. The network was modified by crosslinking the epoxides with various mono/diamine proportions, resulting in decreasing density and increasing free volume. It was discovered, however, that concomitant effects, i.e., the formation of a two-phase structure, and changes in resin polarity and in resin–water affinity predominated. Thus, compositions with higher chain molecular weights between crosslink points yet exhibited lower diffusion coefficients. Also, when drastic swelling conditions (such as a water-boil treatment) were employed, moisture absorption was facilitated by penetration via newly formed microcracks.     

Polyurethane-ductilized epoxy resins

Amine-cured epoxy resins were modified to improve their impact properties. Urethane prepolymers (PUs), in which terminal isocyanate groups were blocked with nonylphenol (NP) for easy handling, were used as modifiers. The synthesis of the elastomers were carried out at different NCO : OH ratios: 1 : 1, 2 : 1, and 3 : 1 (PU1, PU2, and PU3). Characterization of these materials by GPC and FTIR indicated that PU1 has a negligible amount of NCO-terminated chains and no unreacted toluenediisocyanate (TDI). PU2 and PU3 have free-blocked TDI in the mixture, even after distillation under a vacuum. The molecular weight and polydispersity of the prepolymer increases as PU3 < PU2 < PU1. Copolymerization was carried out by crosslinking with a mixture of cycloaliphatic amines, which react with the epoxy ring and with the NCO groups by deblocking and urea formation. Dynamic mechanical tests were used to measure the glass transition temperatures (T_g) of the copolymers. Two T_g were found if PU1 was the epoxy modifier, indicating that phase separation took place. This was correlated with a structure of PU1 of linear chains with a negligible amount of reactive groups. Flexural and compression properties showed negligible changes for PU2- and PU3-modified epoxy, but the critical strain energy release rate (G_1C) was improved if PU2 was the modifier. This behavior was explained by the linkage of elastomeric chains into the epoxy network. The PU1–epoxy copolymer showed a completely different behavior, with the bending modulus (E_b) reduced to almost one-half with respect to that of the epoxy matrix and with largely improved impact properties. This difference was attributed to the separation of an elastomeric phase, which favors the formation of shear bands in the epoxy matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1781–1789, 1998     

Radiopaque epoxy resins

Transparent, X-ray contrast (radiopaque) epoxy resins were obtained by dissolving up to 25 wt % triphenylbismuth in the commercial epoxy resin prepolymers EPON-815, DER-330, DER-383, and DEN-431 which were then hardened with diethylenetriamine. The radiopacities of the mixtures were found to be proportional to the molar concentration of the radiopaque additive. The systems follow the relationship, R = R_o + (R_a ? R_o) V _a M_a where R, R_o, and R_a are the radiopacities of the mixture, the pure epoxy resin, and triphenylbismuth, respectively (expressed in mm aluminum/mm resin); M_a and V _a denote the molar concentration and molar volume of the bismuth compound. R_a for triphenylbismuth was found to be 7.4 ± 0.2 mm Al/mm resin; the average value of R_o for the four epoxies equals 0.16 ± 0.1 mm Al/mm resin. The amount of amine required to harden the radiopaque resins was far less for the epoxy novolac resin DEN-431 than for the three bisphenol-A based epoxies. The concentration of triphenylbismuth required to impart a radiopacity equivalent to that of aluminum measures 14.6 wt % in EPON-815, 14.8 wt % in DER-330, 14.9 wt % in DER-383, and 15.9 wt % in DEN-431. The radiopaque resins remain transparent indefinitely, even when exposed to water. © 1995 John Wiley & Sons, Inc.     

Deoxybenzoin-based epoxy resins

The diepoxide of bishydroxydeoxybenzoin, termed BEDB, was prepared and used as a diepoxide in adhesive formulations with various aromatic diamine cross-linkers. These novel epoxy resins were characterized and compared to the properties of bisphenol A (BPA)- and 3,3′,5,5′-tetrabromobisphenol A (TBBA)-based epoxies in terms of their thermal and mechanical properties. Cured formulations were characterized to determine glass transition temperatures by differential scanning calorimetry (DSC). The char residue, heat release capacity, dynamic mechanical properties, fracture toughness, and adhesion strength of the cured resins were investigated by thermogravimetric analysis (TGA), microscale combustion calorimetry, dynamic mechanical analysis (DMA), plain-strain fracture toughness tests, and lap shear tests. The BEDB-based resins exhibited significantly higher fracture toughness and adhesion strength compared to the BPA-epoxy resins, as well as low heat release properties (i.e., lower flammability) despite the absence of halogen.     

Flammability of epoxy resins

Flammability determinations have been made for epoxy resin formulations. The oxygen index, defined as the volume fraction of oxygen in an oxygen/nitrogen atmosphere that is required just to sustain steady candlelike burning of a stick of the material, has been used as the measure of flammability. The formulations selected for study were those for which the chemical composition of the ingredients was known at least approximately and for which uniform cast slabs (1/8 in. thick) could be readily prepared. They covered a range of compositions of commercial interest. The results have been interpreted in terms of a proposed model for candlelike burning. Effects due to resin composition, cure conditions, fillers, and flame-retardant additives are discussed.     

Storage-stable epoxy B-stage resins

N-Substituted β-aminopropionhydrazides, RNHC₂h₄CONHNH₂, were prepared by the Michael addition of primary amines to acrylic esters. The adducts were converted to the corresponding hydrazides. The aminohydrazides react readily with terminal epoxides at moderate temperatures yeilding B-stage resins of excellent storage stability. When powdered B-stage resins are sprayed on hot metal (150-230°C.), a smooth cross-linked coating is formed in less than 3 min. The coatings have unusually high elasticity, they pass the Olson button test. For best performance the amine used should have the primary amino group attached to either a primary carbon atom or directly to an aromatic nucleus. Aminohydrazides with the following R. Groups have been prepared: methyl, n-butyl, sec-butyl, tert-butyl, phenyl, cyclohexyl, benzyl, octyl, dodecyl, and octadecyl.     

环氧树脂的增强增韧

1 前言 环氧树脂(EP)问世60年以来以其优异性能至今保持着高性能高分子材料的地位.但是与热塑性树脂相比,环氧树脂最大缺点是其脆性.所以对EP的增韧研究由来已久,改性方法也多种多样,如用液态弹性体增韧;用交联的橡胶粒子增韧;用有机弹性体一无机填料复合改性;用核/壳型橡胶粒子改性;用热塑性工程合金塑料改性等.     

The role of network architecture on the glass transition temperature of epoxy resins

A series of epoxy networks were synthesized in which the molecular weight between crosslinks (M_c) and crosslink functionality were controlled independent of the network chain backbone composition. The glass transition temperature (T_g) of these networks was found to increase as M_c decreased. However, the rate at which T_g increased depended on crosslink functionality. The dependency of M_c on T_g is well described by two models, one based on the concept of network free volume while the other model is based on the principle of corresponding states. Initially, neither model could quantitatively predict the effect of crosslink functionality in our networks. However, our tests indicated that both the glass transition and the rubbery moduli of our networks were dependent on M_c and crosslink functionality, while the glassy state moduli were independent of these structural variables. The effect of crosslink functionality on the rubbery modulus of a network has been addressed by the front factor in rubber elasticity theory. Incorporation of this factor into the glass transition temperature models allowed for a quantitative prediction of T_g as a function of M_c and crosslink functionality. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 387–395, 1997     

Heat resistance of epoxy resins

A torsion pendulum has been found to be a sensitive tool to detect the first steps of pyrolysis. It has been shown that some primary bonds break at elevated temperatures, but in the presence of oxygen, new crosslinks can form simultaneously. In resin heated in air, embrittlement due to these crosslinks starts on the surface of the resin and progresses inwards with time.