Tough epoxy resins cured with aminimides

Aminimide compounds ( 1–4 ) thermally generating isocyanates and tertiary amines were found to be excellent curing agents for epoxy resin. Tensile behavior, glass transition temperature, and degree of curing for the combination of EPIKOTE 828 prepolymer with a series of curing agents ( 1–4 ) are reported. The resins exhibit a large elongation at breakage and a high fracture energy per unit volume. The epoxy resins (EP-AI) cured with 3 or 4 containing no hydroxyl group showed larger ultimate elongations (up to 15%) and higher fracture energies (ca. 8 J/cm³) than the resins (EP–AI_OH) cured with 1 or 2 . The curing reaction depends on the structure of aminimide (presence of hydroxyl group and generation of mono- or bisisocyanates). The origin of toughness and dependence of physical properties on the curing condition and the structure of aminimides were discussed. It was concluded that relatively slow curing at elevated temperature controlled by thermal decomposition of aminimides was a reason for the toughness.     

Kinetics and network structure of thermally cured vinyl ester resins

Bisphenol-A diglycidyl ether dimethacrylate was blended with styrene at varying concentrations and this model vinyl ester resin (VER) was compared with two commercial VERs. The VERs were characterized using gravimetry, FTIR spectroscopy, NMR spectroscopy, differential scanning calorimetry (DSC) and DMTA. NMR spectroscopy differentiated between a novolac epoxy-based multimethacrylate oligomer and the two bisphenol-A epoxy-based dimethacrylate oligomers. Reaction kinetics were studied using scanning and isothermal DSC and isothermal FTIR spectroscopy using benzoyl peroxide as the thermal initiator. The presence of oxygen was found to inhibit significantly the polymerization. Increased initiator concentration raised the rate of isothermal polymerization, but did not affect the final conversion while increased styrene concentration reduced the polymerization rate constant and increased the total conversion. This was interpreted in terms of the variations in the termination rate and the stability of the styryl radical on the cure rate and the effect of vitrification on the extent of cure. From measurements of the dynamic mechanical properties as a function of temperature, the breadth of the glass transition tan δ curve and the magnitude of the rubbery modulus was found to increase while the tan δ maximum decreased with increased crosslink density. The T_g, as measured by DSC, and the temperature of the tan δ maximum, as measured by DMTA, were not significantly affected by the styrene content in the resin per se, but were dependent on the combined effects of composition and crosslink density of the network.     

Polyoxometalate cured epoxy resins with photochromic properties

Novel photochromic thermosetting materials were facilely prepared by polyoxometalate, Keggin-type H₃PW₁₂O₄₀, cured epoxy networks with ethylene oxide blocks. The dual functions of polyoxometalate as both hardener and photochromophore were studied by differential scanning calorimetry, infrared spectroscopy, ultraviolet–visible spectroscopy (UV–vis), and generalized two-dimensional correlation analysis. Polyoxometalate initiates the cationic polymerization of epoxy resin through dissolving in either polyethylene oxide epoxy or organic solvents. When subjected to UV irradiation, the transparent thermosetting materials with ethylene oxide blocks change from colorless to blue, and could be bleached in air at various temperatures to recover its initial state. From the UV–vis measurements, all the resultant thermosetting materials demonstrated similar photochromic behavior after ultraviolet irradiation showing characteristic d–d transition band and intervalence charge transfer band. The 2D correlation analysis of the photochromic spectra clearly revealed the sequence of electron movements in the framework of PW₁₂ anion.     

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.     

Low temperature rearrangement of amine cured epoxy resins

Studies are reported on the rearrangements which occur in epoxy resins when heated above 497 K. The nature of the processes which occur in the resin and in model compounds is discussed with reference to changes in the ultraviolet, infra-red and ¹³C and ¹H NMR spectra. Chain scission is preceded by dehydration of the glycidyl segment and occurs at the CN and OC bonds, generating phenoxy radicals in the latter case. The observed coloration of the resin at high temperatures is associated with the formation of a polyenyl structure and the possible generation of quinoid and cyclic conjugated nitrogen-containing aromatic ring structures.     

Photo-oxidation of epoxy resins cured by non-aromatic amines

The photo-oxidation of epoxy resins based on bisphenol A and cured by non-aromatic amines: diethylene triamine, aminoethyl piperazine and isophorone diamine results in carbonyl and amide formation, decrease of glass transition temperature, and the appearance of a new endotherm at 70–80°C in the DSC traces. The carbonyl and, essentially the amide yield depend strongly on the hardener structure and concentration. The mechanisms of formation of these groups are discussed.     

Determination of glass transition temperature,thermal expansion and,shrinkage of epoxy resins

On the basis of measurements of linear thermal expansion of hardened epoxy resins the influence of some modifiers on the thermal expansion of epoxy resin Epidian-5 has been examined. The glass transition temperatures of examined samples were determined.The paper presents also results of the examinations of changes in thermal and cure shrinkage for epoxy resins that occur under the influence of such modifiers as plasticizers and fillers.Five different compositions were examined. A simple and fast measuring method was applied, in which sample elongations vs temperature were determined with a cathetometer. Specific volume changes of liquid resins with temperature were measured with a quartz dilatometer and a cathetometer.     

Controlled shrinkage polymers: Characterization of epoxy resins cured with spirobislactones

The diglycidyl ether of bisphenol A (DGEBA) was cured with either an aliphatic or an aromatic spirobislactone using a tertiary amine catalyst. The products were characterized by FTIR, TGA, DSC, dilatometry, and single-fiber adhesion measurements, and their performance was compared to that of DGEBA cured with acid anhydrides. Both aliphatic and aromatic bislactones are effective curing agents for DGEBA. FTIR and dilatometry confirm that both lactone rings open early in the curing reaction and initially offset shrinkage caused by polymerization. After the bislactone has been consumed, oxirane reactions proceed in a normal fashion. The final shrinkage of cured DGEBA polymers, with or without addition of bislactones, is 3.0–3.5%. Bislactone-modified materials possess superior thermal properties, when compared to those of anhydride-cured materials.     

Curing behavior and kinetics of epoxy resins cured with liquid crystalline curing agent

This article describes the synthesis of a liquid crystalline curing agent 4,4′-bis-(4-amine-butyloxy)-biphenyl (BABB), and its application as a curing agent for the epoxy resin (DGEBA) in comparison with normal curing agent, 4,4′-diaminobiphenyl (DABP). BABB was investigated with polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scatting, and the results showed that BABB displayed smectic liquid crystalline phase. The curing behaviors of DGEBA cured with BABB and DABP were studied by using differential scanning calorimetry (DSC), polarized optical microscopy (POM), and dynamic mechanical analysis (DMA). The results indicated that BABB showed a higher chemical reactivity than DABP. The kinetics was studied under isothermal conditions using an isoconversional method, and the isothermal DSC data can be fitted reasonably by an autocatalytic curing model. The nematic droplet texture was observed for the resulting polymer network of DGEBA/BABB system, while the DGEBA/DABP system showed an isotropic state. The storage modulus of DGEBA/BABB system was enhanced in comparison with DGEBA/DABP system because of the formation of LC phase, whereas the glass transition temperatures decreased because of the introduction of flexible spacer group.     

Influence of the type of epoxy hardener on the structure and properties of interpenetrated vinyl ester/epoxy resins

The morphology–toughness relationship of vinyl ester/cycloaliphatic epoxy hybrid resins of interpenetrating network (IPN) structures was studied as a function of the epoxy hardening. The epoxy was crosslinked via polyaddition reactions (with aliphatic and cycloaliphatic diamines), cationic homopolymerization (via a boron trifluoride complex), and maleic anhydride. Maleic anhydride worked as a dual‐phase crosslinking agent by favoring the formation of a grafted IPN structure between the vinyl ester and epoxy. The type of epoxy hardener strongly affected the IPN morphology and toughness. The toughness was assessed by linear elastic fracture mechanics, which determined the fracture toughness and energy. The more compact the IPN structure was, the lower the fracture energy was of the interpenetrated vinyl ester/epoxy formulations. This resulted in the following toughness ranking: aliphatic diamine > cycloaliphatic diamine ≥ boron trifluoride complex > maleic anhydride. For IPN characterization, the width of the entangling bands and the surface roughness parameters were considered. Their values were deduced from atomic force microscopy scans taken on ion‐etched surfaces. More compact, less rough IPN‐structured resins possessed lower toughness parameters than less compact, rougher structured ones. The latter were less compatible according to dynamic mechanical thermal and thermogravimetric analyses. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5471–5481, 2004     

Synthesis and characterization of new epoxy and cyanate ester resins

New epoxide and cyanate ester resins with an aromatic ester backbone namely 1,3-[di-(4-glycidyloxy diphenyl-2,2^′-propane)]-isophthalate (DGDPI) and 1,4-[di-(4-cyanato diphenyl-2,2^′-propane)]-terephthalate (DCDPT) were synthesized and the intermediates were characterized by IR, 1H-/¹³C-NMR spectroscopic methods. The cured products from DGDPI and DHDPI exhibited higher T_g compared with standard epoxy system. The increase in the T_g may be due to the cyanate ester and rigid aromatic backbones present in the curing system.     

Using mechanical spectroscopies to study the glass transition dynamics in unsaturated polyester resins cured with different styrene contents

The control of chemical architecture has been one relevant parameter in the study of glass transition dynamics in macromolecular systems. In this study, two polyester resins differing in the styrene content that was added in the curing process were studied using two complementary mechanical spectroscopy techniques: dynamic mechanical analysis (DMA) and thermally stimulated recovery (TSR). Both techniques showed that the -relaxation is shifted to higher temperatures (longer times) with increasing styrene content. Master curves were obtained from the DMA data. The shift factors were used to obtain the temperature dependence of the apparent activation energy, E_a(T). The TSR results also permitted to obtain E_a(T) that also exhibited a maximum around T_g. This behaviour, apparently universally observed in thermally stimulated techniques, was explained by the shift from a Vogel-Fulcher-Tamman-Hesse to an Arrhenius regime. The data also allowed to calculate the fragility index of the two materials, which was found to be higher for the one with higher styrene content. Remarks are made on the dependency of the values of this parameter obtained from different techniques.     

Broad-line NMR studies of molecular motion in cured epoxy resins

The conformation of network chain segments in a rigid glassy DDM-cured bisphenol A epoxy resin has been determined by means of rotational isomeric-state model and confirmed by conformity of experimental NMR second moments with a theoretical estimate based on the model. The glass transition temperatures T_g have been determined from precipitate NMR line narrowing with increase in temperature due to the onset of rapid (>10⁴ Hz) main-chain molecular motion. The temperature dependence of the correlation frequency has been determined and the type and extent of molecular motion that occurs in such epoxy resins is discussed.     

Thermoplastic-modified epoxy resins cured with different functionalities amine mixtures. Kinetics and miscibility study

The kinetics of a poly(methyl methacrylate) (PMMA)-modified epoxy resin cured with different functionalities amine mixtures was analyzed using differential scanning calorimetry (DSC) in both isothermal and dynamic conditions. A delay in the reaction rate was observed which increased with PMMA content. An approach of kinetic features involved in curing was carried out. A linear dependence of preexponential factors of neat systems with modifier content was considered. The approach shows the contribution of other factors including the dilution effect of the functional groups to the observed delay. Fourier transform infrared spectroscopy (FTIR) indicated a noticeable change in the interactions present in neat systems due to the presence of PMMA. On the other hand, a significant influence of the ratio between each amine in the epoxy/amine mixtures on the final physical appearance was observed. At constant curing conditions, materials from completely opaque (phase separated) to transparent (miscible) were obtained with the increase in monoamine content.     

Liquid crystalline thermosets from dimeric LC epoxy resins cured with primary and tertiary amines

Liquid crystalline thermosets (LCTs) were prepared by curing difunctional LC dimeric epoxy monomers with imine moieties in the mesogenic core and central spacers of different length. Primary diamines or tertiary amines were used as curing agents obtaining materials with different characteristics. The results obtained were related to the mesogen structure, since dipolar moments in the mesogenic cores affect the ability to form ordered networks.     

Structural investigation of e-beam cured epoxy resins through solid state NMR

In this paper the network structure of e-beam cured DGEBF based epoxy resins is investigated. Two epoxy systems, having different reactivity and cured in different process conditions, were analyzed through solid state NMR spectroscopy. The analysis shows that the more reactive system has higher cross-linking density and higher uniformity of network distribution. Similar information were obtained, in a previous work, on the same systems through dynamic mechanical thermal analysis. It is worth noting that unlike DMTA tests, which interfere with the molecular structure of the analyzed material, due to the heating during the analysis itself, more reliable information, without any artefact, are obtained by solid state NMR, carried out at constant room temperature.     

Thermal decomposition of fire retardant brominated epoxy resins cured with different nitrogen containing hardeners

Epoxy resins frequently have to meet a flame retardancy grade which can be accomplished by incorporating brominated reactive compounds, like tetrabromobisphenol A (TBBA) cured by a number of hardeners. A few brominated epoxy resins (BERs) have been prepared by curing a mixture of diglycidyl ethers of bisphenol A (DGEBA)/diglycidyl ethers of tertabromobisphenol A (DGETBBA) and different hardeners: dicyandiamide (DICY), 4,4′-diaminodiphenyl sulphone (DDS) and polyethylene polyamine (PEPA). The use of different hardeners strongly affects the thermal degradation behaviour of the BER.The main volatile products of pyrolysis, characterized by Pyrolysis-Gas Chromatography-Mass Spectroscopy (PY-GC-MS) at 423 °C were phenol, isopropyl- and isopropenylphenol, mono- and di-brominated phenols, bisphenol A, mono-, di-, tri- and tetra-brominated bisphenol A. No nitrogen containing volatile products or HBr were evolved whereas SO₂ is formed from BER cured with DDS (BER-DDS) and bromoethylene from BER cured with PEPA (BER-PEPA). Differences of 30-60 °C in thermal stability of epoxy network have been found, depending on the hardener. The experimental evidence suggests a cooperative action of bromine and nitrogen in chain scission of epoxy resins. In particular the ability of the hardener in fixing HBr, evolved from TBBA units, seems to depend on the basicity of the N atom of the hardener: the lower the basicity, the lower the scavenging effectiveness and consequently the higher the thermal stability.     

Syntheses and properties of cured epoxy resins containing the perfluorobutenyloxy group. I. Epoxy resin cured with perfluorobutenyloxyphthalic anhydride

Perfluorobutenyloxyphthalic anhydride (PFPA) has been synthesized as a new curing agent for epoxy resins, and the properties of epoxy resin cured with PFPA have been investigated. Good PFPA synthesis yields were realized by a dehydrating ring closure of perfluorobutenyloxyphthalic acid, which was obtained through the reaction of hexafluoropropene trimers with 4-hydroxyphthalic acid. Epoxy resin cured with PFPA was found to have several excellent properties. Its boilding water absorption was 0.45%, which is about a one-fourth that for conventionally cured epoxy resin. Its heat resistance was excellent, and its critical surface tension was almost the same as for PTFE.     

Preparation and properties of epoxy resins cured with silyl esters of phenol novolak and cresol novolak

The curing agents of epoxy resin, trimethylsilyl ethers of phenol novolak (TMSPN) and cresol novolak (TMSCN) were prepared by refluxing phenol novolak and cresol novolak respectively, with the mixture of hexamethyldisilazane and chlorotrimethylsilane in THF. The curing reaction of epoxy resin with these curing agents and the thermal properties of cured resins were examined. The Tg values of epoxy resins cured with TMSPN were a little higher than those cured with TMSCN. The maximum of Tg is 118°C for TMSPN-cured epoxy resin against 112°C for TMSPN-cured epoxy resin. The water absorption of hydrophobic epoxy resins cured with TMSPN was a little lower than those cured with TMSCN. The clear decrease of water absorption is attributed to the difficulty of the micro-void formation caused by the more tight primary structures of TMSPN. The water absorption at 25°C containing trimethylsilyl groups is about one-tenth of that of epoxy resins cured with conventional curing agents and even one-half of that of the epoxy resins cured with active esters. The low water absorption is attributed to the presence of trimethylsilyl groups, which are more hydrophobic than ester groups, and to the absence of hydroxyl groups of the cured resins. This revised version was published online in July 2006 with corrections to the Cover Date.