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.     

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.     

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.     

Physical aging studies in epoxy resins. I. Kinetics of the enthalpy relaxation process in a fully cured epoxy resin

The physical aging of an epoxy resin based on diglycidyl ether of bisphenol-A cured by a hardener derived from phthalic anhydride has been studied by differential scanning calorimetry. The isothermal curing of the epoxy resin was carried out in one step at 130°C for 8 h, obtaining a fully cured resin whose glass transition was at 98.9°C. Samples were aged at temperatures between 50 and 100°C for periods of time from 15 min to a maximum of 1680 h. The extent of physical aging has been measured by the area of the endothermic peak which appears below and within the glass transition region. The enthalpy relaxation was found to increase gradually with aging time to a limiting value where structural equilibrium is reached. However, this structural equilibrium was reached experimentally only at an aging temperature of T_g-10°C. The kinetics of enthalpy relaxation was analysed in terms of the effective relaxation time τ_eff. The rate of relaxation of the system given by 1/τ_eff decreases as the system approaches equilibrium, as the enthalpy relaxation tends to its limiting value. Single phenomenological approaches were applied to enthalpy relaxation data. Assuming a separate dependence of temperature and structure on τ, three characteristic parameters of the enthalpic relaxation process were obtained (In A = ?333, E_H = 1020 kJ/mol, C = 2.1 g/J). Comparisons with experimental data show some discrepancies at aging temperatures of 50 and 60°C, where sub-T_g peaks appears. These discrepancies probably arise from the fact that the model assumes a single relaxation time. A better fit to aging data was obtained when a Williams-Watts function was applied. The values of the nonexponential parameter β were slightly dependent on temperature, and the characteristic time was found to decrease with temperature. © 1994 John Wiley & Sons, Inc.     

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.     

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.     

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.     

The effect of nanoparticles on structural morphology, thermal and flammability properties of two epoxy resins with different functionalities

The effect of layered silicate nanoclays, nano-silica and double-walled carbon nanotubes (DWNTs) on the thermal stability and fire reaction properties of two aerospace grade epoxy resins (a high temperature curing tetra-functional and a low temperature curing bi-functional resin) has been investigated using thermal analysis, cone calorimetry, LOI and UL-94 techniques. The morphology of the polymer-clay nanocomposites, determined by X-ray diffraction and transmission electron microscopy indicated intercalated structures. The addition of nanoclays (5-wt%) to both resins had a thermal destabilisation effect in the low temperature regime (<400 °C), but led to higher char yield at higher temperatures. The inclusion of nano-silica at 30-wt% significantly improved the thermal stability of the resins while DWNTs had an adverse effect due to their poor dispersion in the matrix. The nanoclays and carbon nanotubes significantly increased the fire resistance of the tetra-functional epoxy resin while a minimal effect was observed for the bi-functional resin.     

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.     

Network structure and glass transition of epoxy resins cured with active ester

The effect of network structure on the glass transition temperature (T _g) was examined by differential scanning calorimetry, thermomechanical analysis and dynamic thermomechanometry for epoxy resins cured with mixtures of curing agents consisting of an active ester, 1,3,5-triacetoxybenzene (TAB), and a polyfunctional phenol, 1,3,5-trihydroxybenzene (THB). Free hydroxyl groups are formed from THB after curing, whereas acetyl groups are left from TAB. TheT _g value of cured epoxy resins decreased with increasing TAB content in the curing agent, which is attributed to the looser network structure induced by the steric hindrance of acetyl groups from TAB in the curing reaction and also to the weaker intermolecular interaction and the internal plasticization of acetyl groups from TAB.     

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.     

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.     

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.     

Kinetics study of thermal decomposition of epoxy resins containing flame retardant components

Hyperbranched polyphosphate ester (HPPE) and phenolic melamine (PM) were blended in different ratios with a commercial epoxy resin to obtain a series of flame retardant resins. The thermal decomposition mechanism of their cured products in air was studied by thermogravimetric analysis and in situ Fourier-transform infrared spectroscopy. The degradation behaviours of epoxy resins containing various flame retardant components were found to be greatly changed. The incorporation of phosphorus and nitrogen compounds improved the thermal stability at elevated temperature. The kinetics of thermal decomposition was evaluated by Kissinger method, Flynn-Wall-Ozawa method and Horowitz-Metzger method. The results showed that the activation energy at lower degree of the degradation decreased by the incorporation of flame retardant components, while increased at higher degree of the degradation.     

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.     

Chemico-diffusion kinetics and TTT cure diagrams of DGEBA-DGEBF/amine resins cured with phenol catalysts

Curing of epoxy-amine resins with bisphenol A (BPA) as an external catalyst was studied from differential scanning calorimetry analyses in isothermal and dynamic modes. Both phenomenological and mechanistic models have been tested. The mechanistic model where epoxy cure is postulated to only occur through hydroxyl-catalyzed reactions, and assuming a different reactivity of both types of hydroxyl groups (from BPA and epoxy chains) provided a reasonable fitting of the whole set of experimental data. In particular, the latter model provides good predictive behavior for changes in the mixture composition (BPA content varying in the range from 3 to 10 wt.%, relative to the weight of hardener), contrary to the model based on the same reactivity of both types of hydroxyl groups.The isothermal time-temperature-transformation (TTT) diagram including the time to vitrification and iso-T_g curves of the complex epoxy system was also established.     

Dewetting of unreacted epoxy/amine mixtures on silica

We employ a direct method, time-of-flight secondary ion mass spectroscopy (ToF-SIMS), to determine experimentally the chemical compositions of the wetted and dewetted regions of an uncured epoxy thin film. Determining the composition of the dewetted region indicated the presence of a very thin sublayer of resin in what was thought to be a region devoid of resin. The capability of ToF-SIMS to probe small 65 x 65 microm(2) areas of the surface has permitted us to directly compare the SIMS spectra of the wetted and dewetted regions to the survey spectra of the reactants. This may indicate the strength of resin/silica interactions, which determine interface formation and properties.     

Modified amine curing agents of epoxy resins and materials on their basis

A review of the most commonly used amine curing agents of epoxy diane resins is presented. A method for radically improving the processing and performance properties of materials based on cured epoxy diane resins using the proposed technique for modifying curing agents is described. Examples of using formulated epoxy compositions in the building and cable-making industries are given.