New epoxy resins. I. The stability of epoxy–trialkoxyboroxines triaryloxyboroxine system

A polymer with high aromaticity and/or cyclic ring structures chain backbone usually has high heat, thermal, and flame resistance. Two diglycidyl ethers of bisphenols were prepared from 4,4′ isopropylidenediphenol (DGEBA) and 9,9-bis(4-hydroxyphenyl) fluorene (DGEBF) for evaluation. Four boroxines—trimethoxyboroxine (TMB), triethoxyboroxine (TEB), triisopropoxyboroxine (TIPB) and triphenoxyboroxine (TPB)—were used as the curing agents. DGEBA and DGEBF cured with various boroxines indicate that the trend for their respective glass transition temperature (T_g's), degradation temperatures (T_d's), and gel fractions are TMB-cured epoxy ≈ TEB-cured epoxy < TIPB cured epoxy < TPB cured epoxy. The DGEBF system usually has a higher T_g, T_d, gel fraction, oxygen index (OI), and char yield than the related DGEBA system. DGEBF/DGEBA (80/20 mol ratio) shows a synergistic effect in regard to char formation. This effect exists not only in the copolymer system but also in blended homopolymers of the separately cured resins. A modified mechanism for the polymerization of phenyl glycidyl ether (PGE) with TMB has been proposed.     

The boron trifluoride monoethyl amine complex cured epoxy resins

The curing exotherm pattern is affected by the equivalent ratio of curing agent, boron trifluoride monoethylamine complex (BF₃ · MEA), to epoxy resin. The diglycidyl ether of 9,9-bis(4-hydroxyphenyl) fluorene (DGEBF) cures more slowly than the diglycidyl ether of bisphenol A (Epon 828). The glass transition temperatures (T_g's) of BF₃ · MEA cured Epon 828 are increased with inceasing concentration of curing agent (0.0450–0.1350 eq.) cured DGEBF. The activation energies for the thermal decomposition for BF₃ · MEA (0.0450–0.1350 eq.) cured DGEBF. The activation energies for the thermal decomposition for BF₃ · MEA (0.0450 eq./epoxy eq.) cured Epon 828 and DGEBF are almost equivalent 43 and 44 kcal/mol, respectively. DGEBF when added to DGEBA improves the T_g and char yield with the BF₃ · MEA curing system. The T_g of both resin systems can be increased by longer post cure, whereas the char yield does not appear to change significantly. No ester group formation is found for the BF₃ · MEA-cured DGEBF, although this has been previously reported for the DGEBA system. The BF₃ · MEA cure at 120°C is better than at 140°C because of vaporization and degradation of the curing agent at the higher temperature. The rapid gelation of the epoxy resin may be another reason for the lower degree of cure at high temperature.     

Synthesis and properties of trifluoromethyl groups containing epoxy resins cured with amine for low Dk materials

The study synthesized a trifluoromethyl (CF₃) groups with a modified epoxy resin, diglycidyl ether of bisphenol F (DGEBF), using environmental friendly methods. The epoxy resin was cured with 4,4′‐diaminodiphenyl‐methane (DDM). For comparison, this study also investigated curing of commercially available diglycidyl ether of bisphenol A (DGEBA) with the same curing agent by varying the ratios of DGEBF. The structure and physical properties of the epoxy resins were characterized to investigate the effect of injecting fluorinated groups into epoxy resin structures. Regarding the thermal behaviors of the specimens, the glass transition temperatures (T_g) of 50–160°C and the thermal decomposition temperatures of 200–350 °C at 5% weight loss (T_d5%) in nitrogen decreased as amount of DGEBF increased. The different ratios of cured epoxy resins showed reduced dielectric constants (D_k) (2.03–3.80 at 1 MHz) that were lower than those of pure DGEBA epoxy resins. Reduced dielectric constant is related to high electrronegativity and large free volume of fluorine atoms. In the presence of hydrophobic CF₃ groups, the epoxy resins exhibited low moisture absorption and higher contact angles. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The synthesis and characterization of a novel phosphorus–nitrogen containing flame retardant and its application in epoxy resins

A novel, halogen‐free, phosphorus–nitrogen containing flame retardant 2[4‐(2,4,6‐Tris{4‐[(5,5‐dimethyl‐2‐oxo‐2λ⁵‐[1,3,2]dioxaphosphinan‐2‐yl)hydroxymethyl]phenoxy}‐(1,3,5)‐triazine (TNTP) was successfully synthesized in a three‐step process, and characterized by FTIR, NMR spectroscopy, mass spectra, and elemental analysis. A series of modified DGEBA epoxy resin with different loadings of TNTP were prepared and cured by 4,4‐diaminodiphenylsulfone (DDS). Thermal gravimetric analysis and vertical burning test (UL‐94) were used to evaluate the flame retardancy of TNTP on DGEBA epoxy resin. The results showed that TNTP had a great impact on flame retardancy. All modified thermosets by using TNTP exhibited higher T_g than pure DGEBA/DDS. The loading of TNTP at only 5.0 wt % could result in satisfied flame retardancy (UL‐94, V‐0) together with high char residue (27.3%) at 700°C. The addition of TNTP could dramatically enhance the flame retardancy of DGEBA epoxy resins, which was further confirmed by the analysis of the char residues by scanning electron microscopy and FTIR. Furthermore, no obviously negative effect was found on the Izod impact strength and flexural property of DGEBA epoxy resins when TNTP loading limited in 5.0 wt %. DGEBA/DDS containing 2.5 wt % TNTP could enhance Izod impact strength from 10.47 to 10.94 kJ m^?2, and showed no appreciable effect on the flexural property (85.20 MPa) comparing with pure DGEBA/DDS (87.03 MPa). Results indicated that TNTP as a phosphorus–nitrogen synergistic intumescent flame retardant could be used for DGEBA epoxy resin. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41079.     

Thermo-mechanical behaviors and moisture absorption of silica nanoparticle reinforcement in epoxy resins

An investigation of the thermo-mechanical behavior of silica nanoparticle reinforcement in two epoxy systems consisting of diglycidyl ether of bisphenol F (DGEBF) and cycloaliphatic epoxy resins was conducted. Silica nanoparticles with an average particle size of 20 nm were used. The mechanical and thermal properties, including coefficient of thermal expansion (CTE), modulus (E), thermal stability, fracture toughness (K_IC), and moisture absorption, were measured and compared against theoretical models. It was revealed that the thermal properties of the epoxy resins improved with silica nanoparticles, indicative of a lower CTE due to the much lower CTE of the fillers, and furthermore, DGEBF achieved even lower CTE than the cycloaliphatic system at the same wt.% filler content. Equally as important, the moduli of the epoxy systems were increased by the addition of the fillers due to the large surface contact created by the silica nanoparticles and the much higher modulus of the filler than the bulk polymer. In general, the measured values of CTE and modulus were in good agreement with the theoretical model predictions. With the Kerner and Halpin-Tsai models, however, a slight deviation was observed at high wt.% of fillers. The addition of silica nanoparticles resulted in an undesirable reduction of glass transition temperature (T_g) of approximately 20 °C for the DGEBF system, however, the T_g was found to increase and improve for the cycloaliphatic system with silica nanoparticles by approximately 16 °C. Furthermore, the thermal stability improved with addition of silica nanoparticles where the decomposition temperature (T_d) increased by 10 °C for the DGEBF system and the char yield significantly improved at 600 °C. The moisture absorption was also reduced for both DGEBF and cycloaliphatic epoxies with filler content. Lastly, the highest fracture toughness was achieved with approximately 20 wt.% and 15 wt.% of silica nanoparticles in DGEBF and cycloaliphatic epoxy resins, respectively.     

New epoxy resins. III. Application of fourier transform IR to degradation and interaction studies of epoxy resins and their copolymers

In the Fourier transform infrared (FT–IR) study, diglycidyl ether of bisphenol A (DGEBA) did not show aldehyde or perester absorption when it was normally cured with trimethoxyboroxine (TMB) under a nitrogen atmosphere. Neither alumina nor gold surfaces would cause oxidation of the system. In air, alumina appeared to accelerate the oxidation when compared to gold. The cure of the epoxy in oxygen appeared to cause increased Claisen rearrangement when compared with the same cure in a nitrogen atmosphere. The DGEBA cured with TMB under air or nitrogen atmospheres showed differences in their degradation patterns. The TMB-cured DGEBA and diglycidyl ether of 9,9-bis(4-hydroxyphenyl) fluorene (DGEBF) copolymer had a higher degree of reaction and lower degradation than was shown by the synthetically generated spectra based on the FT–IR summation of the spectra of the respective homopolymers.     

Curing kinetics and properties of epoxy resin-fluorenyl diamine systems

Diglycidyl ether of bisphenol fluorene (DGEBF), 9,9-bis-(4-aminophenyl)-fluorene (BPF) and 9,9-bis-(3-methyl-4-aminophenyl)-fluorene (BMAPF) were synthesized to introduce more aromatic structures into the epoxy systems, and their chemical structures were characterized with FTIR, NMR and MS analyses. The curing kinetics of fluorenyl diamines with different epoxy resins including DGEBF, cycloaliphatic epoxy resin (TDE-85) and diglycidyl ether of bisphenol A (DGEBA) was investigated using non-isothermal differential scanning calorimetry (DSC), and determined by Kissinger, Ozawa and Crane methods. The thermal properties of obtained polymers were evaluated with dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). The results show that the values of activation energy (E_a) are strongly dependent on the structures of epoxy resin and curing agent. The curing reactivity of epoxy system is restrained by the introduction of rigid fluorene into chain backbone and flexible methyl into side groups. The cured DGEBF/fluorenyl diamine systems exhibit remarkably higher glass transition temperature, better thermal stability and lower moisture absorption compared to those of DGEBA/fluorenyl diamine systems, and display approximate heat resistance and much better moisture resistance relative to those of TDE-85/fluorenyl diamine systems.     

Synthesis and characterization of siloxane-modified epoxy resin

Epoxy resins of the EBS, a bis-p-phenol S modified diglycidyl ether of bis-p-phenol A and the ESBS, a siloxane modified EBS epoxy resin were prepared. Both structures of EBS and ESBS were elucidated with IR,¹H NMR, and¹³C NMR. The near perpendicular comformation of two phenyl rings of sulfone has been introduced into the epoxy resins of EBS und ESBS for Me increase of Me T_g. Some curing and thermal characteristics of these modified EBS and ESBS epoxy resins were studied. The curing patterns of ESBS and ESBS indicated the similarity with that of the DGEBA epoxy resins. T_g measurements resulted an increasing order of We ESBS (T_g of 141 °C), EBS (T_g of 135 °C) and then followed by Epons 1004 (T_g of 104 °C), 1001 (T_g of 101 °C) and 828 (T_g of 100 °C) in samples tested under the same conditions. Thus the substantial improvement of the thermal stability of the modified epoxy resins was indicated. Compatibility characteristics of the EBS and ESBS as indicated by the SEM/EDS is that an ESBS up to 30 % of the siloxane content was found to be compatible but not miscible with the Epon, a phenolic epoxy resin of DGEBA.     

Preparation and mechanical properties of modified epoxy resins with flexible diamines

Diglycidyl ether of bisphenol A (DGEBA) is one of the most widely used epoxy resins for many industrial applications, including cryogenic engineering. In this paper, diethyl toluene diamine (DETD) cured DGEBA epoxy resin has been modified by two flexible diamines (D-230 and D-400). The cryogenic mechanical behaviors of the modified epoxy resins are studied in terms of the tensile properties and Charpy impact strength at cryogenic temperature (77 K) and compared to their corresponding properties at room temperature (RT). The results show that the addition of flexible diamines generally improves the elongation at break and impact strength at both RT and 77 K. The exception is the impact strength at 77 K filled with 21 wt% and 49 wt% D-400. Further, two interesting observations are made: (a) the cryogenic tensile strength increases with increasing the flexible diamine content; and (b) the RT tensile strength can only be improved by adding a proper content of flexible diamines. It is concluded that the addition of a selected amount namely 21–78 wt% of D-230 can simultaneously strengthen and toughen DGEBA epoxy resins at both RT and 77 K. However, only the addition of 21 wt% D-400 can simultaneously enhance the strength and ductility/impact strength of DGEBA epoxy resins at RT. The impact fracture surfaces are examined using scanning electron microscopy (SEM) to explain the impact strength results. Finally, differential scanning calorimetry (DSC) analysis shows that the glass transition temperature (T_g) decreases with increasing the flexible diamine content. The presence of a single T_g reveals that the flexible diamine-modified epoxy resins have a homogeneous phase structure.     

Thermal and Flame Retardation Properties of Melamine Phosphate-Modified Epoxy Resins

We have synthesized a series of epoxy resins containing melamine phosphate (MP) and investigated their thermal and flame retardation properties. MP functions as a hardener and a flame retardant or as an additive of the cured epoxy resin to enhance flame retardation properties of epoxy resins. The reactions of DGEBA cured in the presence of MP were monitored by NMR and FTIR. Our results show that in both reactive and additive modes, MP is effective in increasing limiting oxide index (LOI) and the char yields of epoxy resins at lower phosphorous content. We observed that flame retardation by MP in its reactive mode is better than in its additive mode; the same phenomenon was found also for the glass-transition temperature (T _g). Thermogravimetric analysis (TGA) demonstrated that epoxy resins containing MP decompose at relatively lower temperatures than those lacking MP; this decomposition results in a protective layer forming that prevents the epoxy resins from decomposing further by combustion.     

Preparation,curing kinetics,and thermal properties of bisphenol fluorene epoxy resin

Diglycidyl ether of 9,9‐bis(4‐hydroxyphenyl) fluorene (DGEBF) was synthesized to introduce more aromatic structures into an epoxy resin system. The structure of DGEBF was characterized with Fourier transform infrared and ¹H‐NMR. 4,4′‐Diaminodiphenylmethane (DDM) was used as the curing agent for DGEBF, and differential scanning calorimetry was applied to study the curing kinetics. The glass‐transition temperature of the cured DGEBF/DDM, determined by dynamic mechanical analysis, was 260°C, which was about 100°C higher than that of widely used diglycidyl ether of bisphenol A (DGEBA). Thermogravimetric analysis was used to study the thermal degradation behavior of the cured DGEBF/DDM system: its onset degradation temperature was 370°C, and at 700°C, its char yield was about 27%, whereas that of cured DGEBA/DDM was only 14%. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Thermomechanical behavior of epoxy resins modified with epoxidized vegetable oils

Biobased epoxy materials were prepared from diglycidyl ether of bisphenol A (DGEBA) and epoxidized vegetable oils (EVOs) (epoxidized soybean oil and epoxidized castor oil) with a thermally latent initiator. The effects of EVO content on the thermomechanical properties of the EVO‐modified DGEBA epoxy resins were investigated using several techniques. Differential scanning calorimetry indicated that the cure reaction of the DGEBA/EVO systems proceeded via two different reaction mechanisms. Single and composition‐dependent glass transition temperature (T_g) mechanisms were observed for the systems after curing. The experimental values of T_g could be explained by the Gordon–Taylor equation [Gordon M and Taylor JS, J Appl Chem 2 :493 (1952)]. The thermal stability of the systems decreased as the EVO content increased, due to the lower crosslinking density of the DGEBA/EVO systems. The coefficient of thermal expansion of the systems was found to increase linearly with increasing EVO content. This could be attributed to the fact that the degrees of freedom available for motions of the segments of the macromolecules in the network structure were enhanced by the addition of EVO. Copyright © 2008 Society of Chemical Industry     

Synthesis, characterization and properties of tetramethyl stilbene-based epoxy resins for electronic encapsulation

Two novel tetramethyl stilbene-based novolac (II and IV) were synthesized from 2,6-dimethyl phenol and chloroacetaldehyde dimethylacetal or chloroacetone, and then the resulted novolacs were epoxidized to tetramethyl stilbene-based epoxy resins (III and V). The proposed structures were confirmed by FTIR, elemental analysis, mass spectra, NMR spectra and epoxy equivalent weight titration. The synthesized tetramethyl stilbene-based epoxy resins were cured with 4,4-diaminodiphenyl methane (DDM) and 4,4-diaminodiphenyl sulfone (DDS). Thermal properties of cured epoxy resins were studied using dynamic mechanical analyzer, differential scanning calorimeter, thermal expansion analyzer and thermal gravimetric analyzer (TGA). These data were compared with that of the commercial tetramethyl biphenol (TMBP) epoxy system. According to the experimental data, the order of T_g for cured epoxy system is III>TMBP>V. The order of moisture absorption for cured epoxy system is V<III<TMBP. According to TGA, the 5% degradation temperatures in nitrogen atmosphere were in the range 370-377 and 397-412 °C for DDM and DDS curing systems, respectively. In air atmosphere, the 5% degradation temperatures were in the range 372-385 and 410-411 °C for DDM and DDS curing systems, respectively. The CTE is in inverse order with T_g, therefore, III/DDS<TMBP/DDS<V/DDS.     

Meta-bromobiphenol epoxy resins: Applications in electronic packaging and printed circuit board

Novel 2,2′,6,6′-tetrabromo-3,3′,5,5′-tetramethyl-4,4′-biphenol (TBTMBP), and its epoxy derivatives, were synthesized to incorporate the stable meta-brominated phenol moiety into epoxy resin systems. In electronic encapsulation and laminate applications, epoxy systems derived from TBTMBP have exhibited superior hydrolytic and thermal stability as compared with the conventional ortho-brominated epoxy resins. These properties have resulted in an extended device life for semiconductors and a high T_g with excellent blister resistance for the printed circuit board, while meeting flame retardancy requirements as well.     

Some factors influencing exfoliation and physical property enhancement in nanoclay epoxy resins based on diglycidyl ethers of bisphenol A and F

An investigation of the factors influencing the degree of exfoliation of an organically modified clay in a series of epoxy resins is reported. The use of sonication, choice of curing agent, effect of the moisture content of the clay, and the cure temperature were examined. The dispersion was characterized using a combination of rheological measurements, X‐ray diffraction, and dynamic mechanical thermal analysis. Rheological analysis of the clay dispersion in the epoxy monomer indicated that at high clay loads Herschel–Bulkley type behavior is followed. Higher cure temperatures and higher levels of clay moisture were found to influence the extent of exfoliation. Improvements in physical properties were observed through the addition of nanocomposites. The DGEBA/DDM and DEGEBA/DDS exhibited 2 and 4°C increase, respectively, in T_g per wt % of added clay. DGEBF showed virtually no enhancement. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Simultaneously increasing cryogenic strength, ductility and impact resistance of epoxy resins modified by n-butyl glycidyl ether

Epoxy resins are increasingly used in cryogenic engineering areas due to rapid developments of spacecraft and superconducting cable technologies as well as large cryogenic engineering projects (e.g., International Thermonuclear Experimental Reactor). Cryogenic mechanical properties are important parameters for epoxy resins to be employed in such areas. In this paper, a traditional reactive aliphatic diluent, namely n-butyl glycidyl ether (BGE, JX-013) with a low viscosity, was used to modify diethyl toluene diamine (DETD)-cured diglycidyl ether of bisphenol-F (DGEBF) epoxy system for enhancing cryogenic mechanical properties at liquid nitrogen temperature (77 K). The results showed that the cryogenic strength, ductility and impact resistance (impact strength) have been simultaneously enhanced by the addition of BGE with appropriate contents. Moreover, the comparison of the mechanical properties between 77 K and room temperature (RT) indicated that at the same composition, the tensile strength and Young's modulus at 77 K were higher than those at RT but the failure strain and impact resistance showed the opposite results. Finally, differential scanning calorimetry (DSC) exhibited that the glass transition temperatures (T_g) of the epoxy resins decreased with increasing the BGE content.     

Cationic cure of epoxy resins via benzylsulfonium salts covalently bound to glass surfaces

The cure behavior of epoxy resins in the presence of glass fillers was investigated using differential scanning calorimetry (DSC). A novel benzylsulfonium salt capable of covalently bonding to glass surfaces through a trialkoxysilane moiety were synthesized. Coupling of the salt to silica gel (as a model glass surface), characterization of the bound material, and its ability to initiate the cationic cure of DGEBA resins were investigated. The bound material was characterized by solid-state ¹³C and ²⁹Si CP/MAS NMR, FTIR, and TGA. The sulfonium salt was coupled with silica as the Br anion form because of the insolubility of the SbF₆ salt. After anion exchange, silica-bound salt with SbF₆ counterion was shown to initiate cure of epoxy resins but only at temperatures much higher than with an analogous unbound salt (>200°C and <100°C, respectively). The inability to get complete anion exchange of Br anions for SbF₆ (necessary for cationic initiation activity) after coupling allowed formation of excess tetrahydrothiophene (THT) during heating through decomposition of the residual Br salt, causing temporary termination and a large delay in cure. The temporary termination mechanism involved reaction of THT and the active oxonium ion to give a primary alkylsulfonium salt. In addition, it was discovered that the silica gel itself had an inhibiting effect on the cure of epoxy resins cured with unbound initiator, giving low T_g materials. This was due to inherent surface surface interaction with the salt and not to chemical reaction with the surface or with a physically adsorbed contaminant (such as water). The degree of inhibition increased with increasing filler content. Low surface area glass beads also inhibited cure, although surface modification of the glass beads with bound benzylsulfonium salt (SbF₆ form) improved cure significantly, reducing onset delay and giving high T_g materials. The degree of delay was inversely dependent on the amount of silane coupled to the surface and varied with counterion.     

Toughening epoxy resins with polyepichlorohydrin

Polyepichlorohydrin (PECH) rubbers were found to toughen epoxy resins based on the diglycidyl ether of bisphenol A (DGEBA) and cured with piperidine. The degree of toughening depends on the molecular weight of the PECH and on the curing temperature. Best toughening was achieved with PECH of the highest nominal molecular weight of 3400 (Hydrin 10 × 2). Hydrin 10 × 1 (nominal molecular weight 1700) did not toughen the epoxy resin unless bisphenol A was also added, whereas Hydrin 10 × 2 toughened it in the absence of bisphenol A. Curing resins containing bisphenol A and Hydrin 10 × 1 at 160°C resulted in a slightly more brittle resin than when cured at 120°C. The effect of PECH rubbers on the T_g, modulus, and hot/wet properties is similar to that of carboxy-terminated butadiene-acrylonitrile rubbers (CTBN). Dynamic mechanical thermal analysis (DMTA) and scanning electron micrographs (SEM) of fractured surfaces show that the PECH separates as a discrete phase during curing. © 1993 John Wiley & Sons, Inc.     

Crystal structure,curing kinetics,and thermal properties of bisphenol fluorene epoxy resin

Diglycidyl ether of 9,9‐bis(4‐hydroxyphenyl) fluorene (DGEBF) monomer was successfully synthesized and characterized in detail. The crystal structure of DGEBF was measured by single‐crystal X‐ray diffraction analysis. Curing kinetics of DGEBF with 4,4‐diaminodiphenyl sulfone (DDS), thermal properties, and decomposition kinetics were investigated using nonisothermal differential scanning calorimetry (DSC) according to Kissinger, Ozawa and Crane methods. The glass transition temperature (T_g), thermal properties of cured polymer were estimated by DSC, dynamic mechanical analysis, and thermogravimetric analyses. Epoxy value of DGEBF monomer up to theoretical value leads to higher crosslink density of cured polymers. The cured DGEBF/DDS system exhibited obvious higher T_g and better thermal stability compared to those of DGEBF/diamine systems reported previously. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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