Cure kinetics and physical characterization of epoxy/modified boehmite nanocomposites

Nanocomposites based on a cross-linked epoxy matrix with the addition of a reinforcing organically modified boehmite nano-phase were realized and characterized with the aim to produce systems possessing enhanced properties over commercial epoxy systems. Different amounts of a commercially available organically modified boehmite were added to a diglycidyl ether of bisphenol A (DGEBA) epoxy matrix. The rheological characteristic and kinetic behavior of the liquid nano-filled mixtures were analyzed and compared to those displayed by the un-filled resin. A mathematical model was applied to the experimental rheological data in order to assess the aspect ratio of the nano-filler. A proper equation was employed to model the cure kinetics of the nano-filled epoxy systems. The nanocomposites were heat-cured in the presence of an aromatic amine hardener. They were, then, characterized by scanning electron microscopy with EDS analysis, dynamic mechanical thermal analysis, differential scanning calorimetry, and Flexural and Hardness tests. Significant increase in the glass transition temperature, Shore D hardness and maximum flexural strength was found. The experimental results demonstrated the effectiveness of the o-boehmite nano-filler to improve the physical and mechanical properties of the epoxy resin. Further studies are in progress to verify the protective efficiency of the epoxy-boehmite nanocomposite when applied on different substrates as adhesive or coating for construction materials, such as porous stones, concrete, wood, and metal.     

Preparation of epoxy–clay nanocomposite and investigation on its anti-corrosive behavior in epoxy coating

An epoxy–clay nanocomposite was synthesized using a quaternary ammonium-modified montmorillonite clay and diglycidyl ether of bisphenol A (DGEBA) type epoxy resin, in order to produce anti-corrosive epoxy coating. Anti-corrosive properties of the nanocomposite were investigated using salt spray and electrochemical impedance spectroscopy (EIS) methods. The results showed an improvement in the barrier and anti-corrosive characteristics of epoxy-based nanocomposite coating and a decrease in water uptake in comparison with pure epoxy coating. Wide-angle X-ray diffraction (WAXD) patterns and transmission electron microscopy (TEM) analysis showed that the interlayer spacing of clays increased after addition of epoxy resin along with applying shear force and ultrasound sonicator. The best performance of this coating was achieved at 3 and 5 wt.% clay concentration.     

Cure kinetics of epoxy/anhydride nanocomposite systems with added reactive flame retardants

Montmorillonite nanocomposite systems obtained from epoxy cured using anhydride and the addition of a reacting flame retardant are studied in this paper. In particular, a thermokinetic analysis of the behavior of five different compounds was performed, using a differential scanning calorimeter. The isothermal tests showed double reaction peaks, due to the cure reactions of DGEBA/acid anhydride systems. The comparisons between dynamic thermograms (and between isothermal ones, too) for the different mixtures also showed that the addition of other active substances (such as a nanofiller or a flame retardant additive) does not change the mechanism of crosslinking from a qualitative point of view, but both the nanoreinforcement and the flame retardant seemed to exert an evident catalytic action on the cure reactions. A model describing the cure behavior of the aforementioned materials is proposed in this work. This model takes into account the fact that the reaction mechanism of each analyzed system is composed of a couple of parallel phenomena: the fast opening of anhydride ring (corresponding to a first exothermic peak and characterized by “n‐th order” kinetics) and resin networking (corresponding to a second exothermic peak and characterized by an “auto‐catalytic with zero initial velocity” behavior). The verification of the proposed model was performed by means of a comparison between experimental data (normalized curves derived from DSC thermograms) and theoretical data (derived from a numerical integration—using the second order Runge–Kutta method—of the model‐representative equation) and provided very good results. This allows one to apply such a model to any engineering process problem concerning the cure of DGEBA/acid anhydride/phyllosilicate nanocomposite systems. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1676–1689, 2004     

Mechanical and morphological properties of organic–inorganic,hybrid, clay‐filled,and cyanate ester/siloxane toughened epoxy nanocomposites

Organic–inorganic hybrids involving cyanate ester and hydroxyl‐terminated polydimethylsiloxane (HTPDMS) modified diglycidyl ether of bisphenol A (DGEBA; epoxy resin) filled with organomodified clay [montmorillonite (MMT)] nanocomposites were prepared via in situ polymerization and compared with unfilled‐clay macrocomposites. The epoxy‐organomodified MMT clay nanocomposites were prepared by the homogeneous dispersion of various percentages (1–5%), and the resulting homogeneous epoxy/clay hybrids were modified with 10% HTPDMS and γ‐aminopropyltriethoxysilane as a coupling agent in the presence of a tin catalyst. The siliconized epoxy/clay prepolymer was further modified separately with 10% of three different types of cyanate esters, namely, 4,4′‐dicyanato‐2,2′‐diphenylpropane, 1,1′‐bis(3‐methyl‐4‐cyanatophenyl) cyclohexane, and 1,3‐dicyanato benzene, and cured with diaminodiphenylmethane as a curing agent. The reactions during the curing process between the epoxy, siloxane, and cyanate were confirmed by Fourier transform infrared analysis. The results of dynamic mechanical analysis showed that the glass‐transition temperatures of the clay‐filled hybrid epoxy systems were lower than that of neat epoxy. The data obtained from mechanical studies implied that there was a significant improvement in the strength and modulus by the nanoscale reinforcement of organomodified MMT clay with the matrix resin. The morphologies of the siloxane‐containing, hybrid epoxy/clay systems showed heterogeneous character due to the partial incompatibility of HTPDMS. The exfoliation of the organoclay was ascertained from X‐ray diffraction patterns. The increase in the percentage of organomodified MMT clay up to 5 wt % led to a significant improvement in the mechanical properties and an insignificant decrease in the glass‐transition temperature versus the unfilled‐clay systems. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Synthesis and characterization of diphenylsilanediol modified epoxy resin and curing agent

A series of diphenylsilanediol modified epoxy resins and novel curing agents were synthesized. The modified epoxy resins were cured with regular curing agent diethylenetriamine (DETA); the curing agents were applied to cure unmodified diglycidyl ether of bisphenol A epoxy resin (DGEBA). The heat resistance, mechanical property, and toughness of all the curing products were investigated. The results showed that the application of modified resin and newly synthesized curing agents leads to curing products with lower thermal decomposition rate and only slightly decreased glass transition temperature (T_g), as well as improved tensile modulus and tensile strength. In particular, products cured with newly synthesized curing agents showed higher corresponding temperature to the maximum thermal decomposition rate, comparing with products of DGEBA cured by DETA. Scanning electron microscopy micro images proved that a ductile fracture happened on the cross sections of curing products obtained from modified epoxy resins and newly synthesized curing agents, indicating an effective toughening effect of silicon–oxygen bond.     

Chemical modification of epoxy resins by dialkyl(or aryl) phosphates: Evaluation of fire behavior and thermal stability

The improvement of flame-retardation of thermosetted epoxy–amine resins was attempted by chemically incorporating phosphorus-containing reagents. By reacting 4,4′-diglycidylether of bisphenol A (DGEBA) with dialkyl (or aryl) phosphate, it was possible to chemically modify the epoxy resin and then cure it in the presence of 4,4′-diaminodiphenylsulfone (DDS) to obtain epoxy-amine resin with good flame-retardant and thermal stability behaviors. The quantitative aspect of the addition of dialkyl (or aryl) phosphate onto glycidyle oxiranes was evaluated by elemental analysis of the modified epoxy-amine resins. Flammability and thermal behaviors of modified DGEBA/DDS resins depend on the nature of phosphate groups (the best flame-retardation was observed on resins bearing phenyl phosphate groups) and their concentration in the material. In relation to DGEBA/DDS samples containing additives of the same structure [trialkyl(or aryl) phosphate], cured resins incorporating chemically bonded phosphate groups show a better flame-retardation. On the contrary to the nonomodified DGEBA/DDS [with or without trialkyl (or aryl) phosphate as additive], combustion of modified DGEBA/DDS resins is accompanied by formation of intumescent char. Chemical modification of DGEBA by dialkyl (or aryl) phosphates can be carried out in situ during the curing of epoxy resins without change in the fire behavior. © 1996 John Wiley & Sons, Inc.     

Thermo mechanical behaviour of unsaturated polyester toughened epoxy–clay hybrid nanocomposites

The intercrosslinked networks of unsaturated polyester (UP) toughened epoxy–clay hybrid nanocomposites have been developed. Epoxy resin (DGEBA) was toughened with 5, 10 and 15% (by wt) of unsaturated polyester using benzoyl peroxide as radical initiator and 4,4′-diaminodiphenylmethane as a curing agent at appropriate conditions. The chemical reaction of unsaturated polyester with the epoxy resin was carried out thermally in presence of benzoyl peroxide-radical initiator and the resulting product was analyzed by FT-IR spectra. Epoxy and unsaturated polyester toughened epoxy systems were further modified with 1, 3 and 5% (by wt) of organophilic montmorillonite (MMT) clay. Clay filled hybrid UP-epoxy matrices, developed in the form of castings were characterized for their thermal and mechanical properties. Thermal behaviour of the matrices was characterized by differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA) and dynamic mechanical analysis (DMA). Mechanical properties were studied as per ASTM standards. Data resulted from mechanical and thermal studies indicated that the introduction of unsaturated polyester into epoxy resin improved the thermal stability and impact strength to an appreciable extent. The impact strength of 3% clay filled epoxy system was increased by 19.2% compared to that of unmodified epoxy resin system. However, the introduction of both UP and organophilic MMT clay into epoxy resin enhanced the values of mechanical properties and thermal stability according to their percentage content. The impact strength of 3% clay filled 10% UP toughened epoxy system was increased by 26.3% compared to that of unmodified epoxy system. The intercalated nanocomposites exhibited higher dynamic modulus (from 3,072 to 3,820 MPa) than unmodified epoxy resin. From the X-ray diffraction (XRD) analysis, it was observed that the presence of d ₀₀₁ reflections of the organophilic MMT clay in the cured product indicated the development of intercalated clay structure which in turn confirmed the formation of intercalated nanocomposites. The homogeneous morphologies of the UP toughened epoxy and UP toughened epoxy–clay hybrid systems were ascertained from scanning electron microscope (SEM).     

Curing kinetics of epoxy resin–imidazole–organic montmorillonite nanocomposites determined by differential scanning calorimetry

The curing kinetics of epoxy resin–imidazole–organic montmorillonite nanocomposites were investigated by differential scanning calorimetry (DSC) in the isothermal mode. X‐ray diffraction (XRD) analysis indicated the formation of a layered silicate–epoxy nanocomposite. The cure rates for the epoxy resin–imidazole–organic montmorillonite nanocomposite were lower than the values for the neat system at higher temperature (120 and 130°C), as indicated by the relation between the cure conversion and time. These results revealed that the autocatalytic model and the modified Avrami equation are both valid for describing the cure behaviors of epoxy resin–imidazole–organic montmorillonite systems. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2932–2941, 2003     

A new extra situ sol–gel route to silica/epoxy (DGEBA) nanocomposite. A DTA study of imidazole cure kinetic

Silica nanoparticles were obtained through the Stöber method, from mixtures of tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTS). The nanoparticles were dispersed in tetrahydrofuran (THF) and coupled to bisphenol A epoxy resin (DGEBA) through surface amino groups. After removing THF non-isothermal cure was performed at different heating rates (2–20°C/min), using imidazole (2–4 wt%) as curing agent. For the sake of comparison bare DGEBA epoxy polymers were also prepared with similar schedule A nanocomposite of well-dispersed silica nanoparticles (5 wt%) in a fully cured epoxy matrix was easily obtained. Lower cure kinetics were observed with silica addition. This was attributed to reduction of the imidazole volume concentration. Cure activation energy was not influenced by silica presence, whereas it changed with the imidazole content. Therefore, experimental results suggested that silica had only an indirect effect (the reduction of the imidazole molar concentration) on the epoxy matrix cure kinetics. Glass transformation temperatures, T _g, as high as 175°C were recorded. The nanocomposite glass transformation temperature depended on the heating rate of the cure process, the imidazole and silica content. T _g changes as high as 40°C were detected as a function of the heating rate. At higher imidazole content no differences in T _g values between bare polymer and the nanocomposite were observed. This suggests that a higher imidazole content assures a better interconnection between the compatibilizing epoxy shell around the nanoparticles and the epoxy matrix. The new proposed methodology is an easy route to engineer both nanocomposites structure and interfacial interactions, thus tailoring their properties.     

Curing behavior and structure of an epoxy/clay nanocomposite system

The curing behavior of an epoxy/clay nanocomposite system composed of a bifunctional epoxy resin with an aromatic amine curing agent and an organically modified clay was investigated. Differential scanning calorimetry (DSC) was used to investigate the curing behavior of the epoxy/clay nanocomposite system. The curing rate of the nanocomposite system increased with increasing clay content. A kinetic equation, considering an autocatalytic reaction mechanism, could describe fairly well the curing behavior of the epoxy/clay nanocomposite system. The reaction kinetic parameters of the kinetic equation were determined by fitting DSC conversion data to the kinetic equation, using a nonlinear numerical method. Dynamic mechanical analysis was used to investigate the thermomechanical properties of the epoxy/clay nanocomposite system. The glass transition temperature of the epoxy/clay nanocomposite system increased slightly with increasing clay content. The structure of the nanocomposite system was characterized by X‐ray diffraction analysis and transmission electron microscope imaging. The formation of intercalated structures was observed dominantly in the epoxy/clay nanocomposites, together with some exfoliated structures. POLYM. ENG. SCI., 46:1318–1325, 2006. © 2006 Society of Plastics Engineers     

Cure kinetics and thermal stability of micro and nanostructured thermosetting blends of epoxy resin and epoxidized styrene‐block‐butadiene‐block‐styrene triblock copolymer systems

The compatibility of styrene‐block‐butadiene‐block‐styrene (SBS) triblockcopolymer in epoxy resin is increased by the epoxidation of butadiene segment, using hydrogen peroxide in the presence of an in situ prepared catalyst in water/dichloroethane biphasic system. Highly epoxidized SBS (epoxy content SBS >26 mol%) give rise to nanostructured blends with epoxy resin. The cure kinetics of micro and nanostructured blends of epoxy resin [diglycidyl ether of bisphenol A; (DGEBA)]/amine curing agent [4,4′‐diaminodiphenylmethane (DDM)] with epoxidized styrene‐block‐butadiene‐block‐styrene (eSBS 47 mol%) triblock copolymer has been studied for the first time using differential scanning calorimetry under isothermal conditions to determine the reaction kinetic parameters such as kinetic constants and activation energy. The cure reaction rate is decreased with increasing the concentration of eSBS in the blends and also with the lowering of cure temperature. The compatibility of eSBS in epoxy resin is investigated in detailed by Fourier transform infrared spectroscopy, optical and transmition electron microscopic analysis. The experimental data of the cure behavior for the systems, epoxy/DDM and epoxy/eSBS(47 mol%)/DDM show an autocatalytic behavior regardless of the presence of eSBS in agreement with Kamal's model. The thermal stability of cured resins is also evaluated using thermogravimetry in nitrogen atmosphere. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Microstructural effects and the toughening of thermoplastic modified epoxy resins

We have studied an epoxy resin formulation consisting of the diglycidyl ether of bisphenol-A (DGEBA), modified with phenolic hydroxyl-terminated polysulfone (PSF) and cured with an aromatic amine curing agent, diaminodiphenyl sulfone (DDS). A range of microstructures and fracture properties have been obtained by controlling the formulation cure conditions (cure temperature and cure cycle in an isothermal mode). The chemical conversion of the cured resins has been monitored by near-infrared spectroscopy (NIR). Although only a single material formulation was used, three distinct types of microstructure were identified by scanning electron microscope (SEM) observations on samples prepared at different cure temperatures. Surprisingly, the thermal and fracture properties of the cured samples did not vary noticeably, in spite of the significant microstructure variations. The consistency of these fracture toughness results with cure temperature changes was an unexpected result in the light of our earlier observations of a strong dependence of fracture toughness on cure temperature in neat resin systems. The difference in behavior between neat and modified resins reveals that the fracture toughness of the latter is dependent on a combination of the microstructure and the matrix resin properties. This hypothesis was also supported by an observation of high fracture thoughness in a sample cured in a two-step process, which we believe is due to the optimum microstructure and matrix resin properties, being achieved separately during precure and postcure, respectively. The increase in fracture toughness values caused by the modification (ΔG_IC) was calculated from the fracture toughness values of neat and modified resins, prepared under the same cure conditions, using a proposed theoretical equation. © 1993 John Wiley & Sons, Inc.     

The mechanical properties of rubber-modified vinyl esters

“Vinyl esters” were prepared by reacting an epoxy resin (DGEBA) with acrylic and methacrylic acids, modified with Hycar VTBN, and cured at 90°C for 4 h with t-butyl perbenzoate. The resultant materials showed a two-phase structure in electron micrographs and dynamic mechanical spectra. The domain of the dispersed phase was estimated to be on the order of 10 nm in size. Infrared spectroscopy indicated extensive copolymerization of the internal double bonds or pendant vinyl groups in VTBN during the cure of the resin. The mechanical properties of the modified materials were measured, and the differences in mechanical behavior between the acrylic and methacrylic vinyl esters were discussed.     

Effect of the crosslinking degree on curing kinetics of an epoxy–anhydride styrene copolymer system

The cure kinetics of a high molecular weight acid copolymer used as a hardener for a commercial epoxy resin (DGEBA) was studied by DSC. The systems were uncured and partially cured epoxy poly(maleic anhydride-alt-styrene) (PAMS) at different periods of time. The state of cure was assessed as the residual heat of reaction and was varied by controlling both the time and temperature of cure. The conversion degree of crosslinking increased with time and temperature. Additionally, the activation energy and reaction order were calculated by the Freeman–Carrol relation and showed a dependence on the conversion degree of crosslinking. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2089–2094, 1999     

Toughening of epoxy resin systems using low‐viscosity additives

This study has evaluated three low‐viscosity epoxy additives as potential tougheners for two epoxy resin systems. The systems used were a lower‐reactive resin based upon the diglycidyl ether of bisphenol A (DGEBA) and the amine hardener diethyltoluene diamine, while the second epoxy resin was based upon tetraglycidyl methylene dianiline (TGDDM) and a cycloaliphatic diamine hardener. The additives evaluated as potential tougheners were an epoxy‐terminated aliphatic polyester hyperbranched polymer, a carboxy‐terminated butadiene rubber and an aminopropyl‐terminated siloxane. This work has shown that epoxy‐terminated hyperbranched polyesters can be used effectively to toughen the lower cross‐linked epoxy resins, i.e. the DGEBA‐based systems, with the main advantage being that they have minimal effect upon processing parameters such as viscosity and the gel time, while improving the fracture properties by about 54 % at a level of 15 wt% of additive and little effect upon the T_g. This result was attributed to the phase‐separation process producing a multi‐phase particulate morphology able to initiate particle cavitation with little residual epoxy resin dissolved in the continuous epoxy matrix remaining after cure. The rubber additive was found to impart similar levels of toughness improvement but was achieved with a 10–20 °C decrease in the T_g and a 30 % increase in initial viscosity. The siloxane additive was found not to improve toughness at all for the DGEBA‐based resin system due to the poor dispersion within the epoxy matrix. The TGDDM‐based resin systems were found not to be toughened by any of the additives due to the lack of plastic deformation of the highly cross‐linked epoxy network Copyright © 2003 Society of Chemical Industry     

Curing kinetics and mechanical properties of epoxy nanocomposites based on different organoclays

The effect of different organoclays and mixing methods on the cure kinetics and properties of epoxy nanocomposites based on Epon828 and Epicure3046 was studied. The two kinds of organoclay used in this study, both based on natural montmorillonite but differing in intercalant chemistry, were I.30E (Nanomer I.30E—treated with a long‐chain primary amine intercalant) and C.30B (Cloisite 30B—treated with a quaternary ammonium intercalant, less reactive with epoxy than the primary amine). The two mixing processes used to prepare the nanocomposites were (i) a room‐temperature process, in which the clay and epoxy are mixed at room temperature, and (ii) a high‐temperature process, in which the clay and epoxy are mixed at 120°C for 1 h by means of mechanical mixing. The nanocomposites were cured at room temperature and at high temperature. The quality of dispersion and intercalation/exfoliation were analyzed by scanning electron microscopy, transmission electron microscopy, and X‐ray diffraction. The heat evolution of the epoxy resin formulation and its nanocomposite systems was measured using differential scanning calorimetry at different heating rates of 2.5, 5, 10, 15, and 20°C min^?1. The cure kinetics of these systems was modeled by means of different approaches. Kissinger and isoconversional models were used to calculate the kinetics parameters while the Avrami model was utilized to compare the cure behavior of the epoxy systems. The cure kinetics and mechanical properties were found to be influenced by the presence of nanoclay, by the type of intercalant, and by the mixing method. POLYM. ENG. SCI., 47:649–661, 2007. © 2007 Society of Plastics Engineers.     

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     

Effect of Phenol/Formaldehyde Stoichiometry on the Modification of Epoxy Resin Using Epoxidized Novolacs

ABSTRACT

Unmodified epoxy resins based on bisphenol A exhibit brittleness and low elongation after cure. This article reports the results of a study for improving the properties of epoxy resin by blending with suitable thermosets. Hybrid polymer networks of diglycidyl ether of bisphenol A (DGEBA) resin with epoxidized phenolic novolac resins (EPN) containing phenol and formaldehyde in different stoichiometric ratios were prepared by physical blending. The modified epoxy resins were found to exhibit improved mechanical and thermal properties compared to the neat resin. DGEBA resins containing 2.5 to 20 wt% of epoxidized novolac resins (EPN) prepared in various stoichiometric ratios (1:0.6, 1:0.7, 1:08, and 1:0.9) between phenol and formaldehyde were cured using a room temperature amine hardener. The cured samples were tested for mechanical properties such as tensile strength, modulus, elongation, and energy absorption at break. All the EPNs are seen to improve tensile strength, elongation, and energy absorption at break of the resin. The blend of DGEBA with 10 wt% of EPN-3 (1:0.8) exhibits maximum improvement in strength, elongation, and energy absorption. EPN loading above 10 wt% is found to lower these properties in a manner similar to the behavior of any filler material. The property profiles of epoxy–EPN blends imply a toughening action by epoxidized novolac resins and the extent of modification is found to depend on the molar ratio between phenol and formaldehyde in the novolac.     

Analysis of the cure of epoxy based layered silicate nanocomposites: Reaction kinetics and nanostructure development

The cure reaction kinetics of epoxy resin, with organically modified montmorillonite loadings of up to 20 wt % and with stoichiometric conditions, has been studied by differential scanning calorimetry with a view to understanding further the fabrication of epoxy‐based polymer layered silicate nanocomposites. The kinetic analysis of isothermal and nonisothermal cure shows that the autocatalytic model is the more appropriate to describe the kinetics of these reactions, and it is observed that a dominant effect of the montmorillonite is to catalyze the curing reaction. However, it was not possible to model the reactions over the whole range of degrees of conversion, in particular for nonisothermal cure. This attributed to the complexity of the reactions, and especially to the occurrence of etherification by cationic homopolymerization catalyzed by the onium ion of the organically modified montmorillonite. The homopolymerization reaction results in an excess of diamine in the system, and hence in practice the reaction is off stoichiometric, which leads to a reduction in both the heat of cure and the glass transition temperature as the montmorillonite content increases. Small angle X‐ray scattering of the cured nanocomposites shows that an exfoliated nanostructure is obtained in nonisothermal cure at slow heating rates, whereas for nonisothermal cure at faster heating rates, as well as for isothermal cure at 70°C and 100°C, a certain amount of exfoliation is accompanied by the growth of d‐spacings of 1.4 nm and 1.8 nm for dynamic and isothermal cure, respectively, smaller than the d‐spacings of the modified clay before intercalation of the resin. A similar nanostructure, consisting of extensive exfoliation accompanied by a strong scattering at distances less than the d‐spacing of the modified clay, is also found for resin/clay mixtures, before the addition of any crosslinking agent, which have been preconditioned by storage for long times at room temperature. The development of these nanostructures is attributed to the presence of clay agglomerations in the original resin/clay mixtures and highlights the importance of the quality of the dispersion of the clay in the resin in respect of achieving a homogeneous exfoliated nanostructure in the cured nanocomposite. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Curing of diepoxides with tertiary amines: Influence of temperature and initiator concentration on polymerization rate and glass transition temperature

The cure of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA), with benzyldimethylamine (BDMA), was investigated using differential scanning calorimetry. The kinetics showed a first-order behavior with respect to both epoxy and tertiary amine concentrations and a non-Arrhenius dependence on temperature. An activation energy could be defined only in the low-temperature range, i.e., from 80 to 120°C. Its value (E = 24.2 kJ/mol = 5.8 kcal/mol) indicates a very slight dependence on temperature. The glass transition temperature of epoxy networks decreased with an increase in both the tertiary amine concentration and the cure temperature. These effects are attributed to plastification by the free amine and to the decrease in the average length of polyether chains, which, in turn, increases the number of network defects.