Influence of the isothermal cure temperature on the nanostructure and thermal properties of an epoxy layered silicate nanocomposite

The cure kinetics of triglycidyl p‐amino phenol (TGAP) epoxy resin with a diamine (4,4‐diamino diphenyl sulphone [DDS]), reinforced with montmorillonite (MMT), has been studied by differential scanning calorimetry. The isothermal cure reaction consists of two parts: a rapid intra‐gallery reaction, attributed to homopolymerization of the TGAP catalyzed by the MMT and the extra‐gallery cross‐linking reaction of the TGAP with the DDS. Increasing cure temperature promotes the intra‐gallery reaction, which should promote an exfoliated nanostructure; this is confirmed by transmission electron microscopy. These results indicate that this system (TGAP/DDS/MMT) is an excellent candidate for achieving exfoliated polymer‐layered silicate nanocomposites and identifies a protocol for optimizing the degree of exfoliation. POLYM. ENG. SCI., 54:51–58, 2014. © 2013 Society of Plastics Engineers     

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     

Highly exfoliated nanostructure in trifunctional epoxy/clay nanocomposites using boron trifluoride as initiator

Epoxy/clay nanocomposites based upon a trifunctional epoxy resin, triglycidyl p‐amino phenol (TGAP), have been prepared by intercalating an initiator of cationic homopolymerization, a boron trifluoride monoethylamine (BF₃·MEA) complex, into the montmorillonite clay galleries before the addition of the TGAP and the curing agent, 4,4‐diamino diphenyl sulfone (DDS), and effecting the isothermal curing reaction. The BF₃·MEA enhances the intragallery cationic homopolymerization reaction, which occurs before the extragallery cross‐linking reaction of the TGAP with the DDS, and which hence contributes positively to the mechanism of exfoliation of the clay. The effects of isothermal cure temperature and of BF₃·MEA content have been studied, in respect of both the reaction kinetics, monitored by differential scanning calorimetry, and the nanostructure, as identified by small‐angle X‐ray scattering and transmission electron microscopy. It is shown that the use of BF₃·MEA in this way as an initiator of intragallery homopolymerization significantly improves the degree of exfoliation in the cured nanocomposites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40020.     

Comparison of the curing kinetics of the RTM6 epoxy resin system using differential scanning calorimetry and a microwave‐heated calorimeter

The cure of a commercial epoxy resin system, RTM6, was investigated using a conventional differential scanning calorimeter and a microwave‐heated calorimeter. Two curing methods, dynamic and isothermal, were carried out and the degree of cure and the reaction rates were compared. Several kinetics models ranging from a simple nth order model to more complicated models comprising nth order and autocatalytic kinetics models were used to describe the curing processes. The results showed that the resin cured isothermally showed similar cure times and final degree of cure using both conventional and microwave heating methods, suggesting similar curing mechanisms using both heating methods. The dynamic curing data were, however, different using two heating methods, possibly suggesting different curing mechanisms. Near‐infrared spectroscopy showed that in the dynamic curing of RTM6 using microwave heating, the epoxy‐amine reaction proceeded more rapidly than did the epoxy‐hydroxyl reaction. This was not the case during conventional curing of this resin. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3658–3668, 2006     

Comparison of the curing kinetics of a DGEBA/acid anhydride epoxy resin system using differential scanning calorimetry and a microwave‐heated calorimeter

The cure of an epoxy resin system, based upon a diglycidyl ether of bisphenol‐A (DGEBA) with HY917 (an acid anhydride hardener) and DY073 (an amine–phenol complex that acted as an accelerator), was investigated using a conventional differential scanning calorimeter and a microwave‐heated power‐compensated calorimeter. Dynamic cure of the epoxy resin using four different heating rates and isothermal cure using four different temperatures were carried out and the degree of cure and reaction rates were compared. The cure kinetics were analyzed using several kinetics models. The results showed different activation energies for conventional and microwave curing and suggested different reaction mechanisms were responsible for curing using the two heating methods. Resins cured using conventional heating showed higher glass transition temperatures than did those cured using microwave heating. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2054–2063, 2007     

Cure kinetics and rheology characterization of soy‐based epoxy resin system

A novel soy‐based epoxy resin system was synthesized by the process of transesterification and epoxidation of regular soy bean oil, which has the potential to be widely usable in various composite manufacturing processes. Cure kinetics and rheology are two chemical properties commonly required in process modeling. In this work, the cure kinetics and rheology of the soy‐based resin system were measured by means of differential scanning calorimetry (DSC) and viscometer. DSC was used to measure the heat flow of dynamic and isothermal curing processes. The cure kinetics models of the different formulations were thus developed. A Brookfield viscometer was used to measure the change in viscosity under isothermal conditions. A novel neural network‐based model was developed to improve modeling accuracy. The models developed for cure kinetics and rheology for soy‐based epoxy resin system can be readily applied to composite processing. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3168–3180, 2006     

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     

Thermomechanical and barrier properties of UV‐cured epoxy/O‐montmorillonite nanocomposites

Mixtures of an epoxy resin and organophilic montmorillonites were subjected to ultraviolet (UV)‐induced photopolymerization. Two types of commercially available nanoclays, namely Cloisite 30B and Cloisite Na⁺, were modified through interaction with organic compatibilizers (dodecylsuccinic anhydride, octadecylamine, octadecyl alcohol, and octadecanoic acid). The modified nanoclays, dispersed in the liquid epoxy resin at 5 wt%, were photopolymerized to get nanocomposite films. The kinetics of the photopolymerization was evaluated by means of real‐time Fourier transform infrared spectroscopy. The modified nanoclays and their nanocomposites were characterized through X‐ray diffractometry; transmission electron microscopy showed the presence of intercalated and partially exfoliated morphologies in the nanocomposites. Thermogravimetric and dynamic‐mechanical analyses showed an increase of the thermal properties and an increase of the glass transition temperatures of the nanocomposites compared with that of the neat UV‐cured resin. Finally, the oxygen barrier properties of nanocomposite films, coated on a polyethyleneterephtalate substrate, were evaluated; the decrease of permeability was correlated with the degree of exfoliation of the nanocomposites. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Intercalation of epoxy resin in organically modified montmorillonite

Various methods of preparation of epoxy resin/clay mixtures, before the addition of the crosslinking agent and curing to form epoxy‐based polymer layered silicate (PLS) nanocomposites, have been investigated to determine their effect on the nanostructure. Organically modified montmorillonite clay was used, and the mixtures were prepared by both simple mixing and solvent‐based methods. X‐ray diffraction shows that intercalation of the resin into the clay galleries occurs for all clay loadings up to 25 wt % and for both preparation methods, but the dispersion of the clay in the resin, observed by optical microscopy, is significantly better for the solvent preparation method. Differential scanning calorimetry (DSC) shows that the intercalated resin has the same molecular mobility as the extra‐gallery resin, but suggests that the intercalated resin does not penetrate completely into the galleries. Prolonged storage of the resin/clay mixtures at room temperature leads to changes in the DSC response, as well as in the response to thermogravimetry, which are interpreted as resulting from homopolymerization of the epoxy resin, catalyzed by the onium ion in the modified clay. This confirms and explains the earlier observation of Benson Tolle and Anderson (J Appl Polym Sci 2004, 91, 89) that “conditioning” of the resin/clay mixtures at ambient temperature has a significant effect when the crosslinking agent is subsequently added, and indicates that the preparation method has important consequences for the nanostructure development in the PLS nanocomposites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:3751–3763, 2006     

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     

Curing kinetics and viscosity change of a two‐part epoxy resin during mold filling in resin‐transfer molding process

The curing kinetics and the resulting viscosity change of a two‐part epoxy/amine resin during the mold‐filling process of resin‐transfer molding (RTM) of composites was investigated. The curing kinetics of the epoxy/amine resin was analyzed in both the dynamic and the isothermal modes with differential scanning calorimetry (DSC). The dynamic viscosity of the resin at the same temperature as in the mold‐filling process was measured. The curing kinetics of the resin was described by a modified Kamal kinetic model, accounting for the autocatalytic and the diffusion‐control effect. An empirical model correlated the resin viscosity with temperature and the degree of cure was obtained. Predictions of the rate of reaction and the resulting viscosity change by the modified Kamal model and by the empirical model agreed well with the experimental data, respectively, over the temperature range 50–80°C and up to the degree of cure α = 0.4, which are suitable for the mold‐filling stage in the RTM process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2139–2148, 2000     

Poly(ester‐amine) hyperbranched polymer as toughening and co‐curing agent for epoxy/clay nanocomposites

The marriage between hardness and flexibility of epoxy resins (improved toughness) is a desired feature, which broads their application in various industrial fields, especially for high impact resistance purposes. Accordingly, this work aims to improve toughness properties of epoxy resin (Epon‐828)/Ancamine (curing agent) system using amino‐terminated hyperbranched poly(ester‐amine) [Poly(PEODA‐NPA)] (HP) as toughening and/or co‐curing agent, in presence of organo‐modified Montmorillonite clay (OMMT) as a reinforcing filler. HP was synthesized via Michael addition reaction of poly(ethylene glycol) diacrylate (PEODA) to N‐methyl‐1,3‐propanediamine (NPA). Chemical structure and molecular weight of HP were elucidated using infrared (FTIR) spectroscopy and gel permeation chromatography (GPC) techniques, respectively. Epoxy/OMMT nanocomposites toughened with HP (at different concentrations) showed remarkable improvement in their toughness without any adverse effect on the other physico‐mechanical properties. The optimum concentration of HP and OMMT was found to be 20 wt % and 1–3 wt% of the epoxy resin, respectively. The extent of exfoliation and dispersion of OMMT platelets within the epoxy cured films was assessed by X‐ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. In addition, thermal gravimetric analyses (TGA‐DTA) of epoxy/OMMT nanocomposites toughened with HP showed a slight increase in their decomposition temperature, particularly at low OMMT loading. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Epoxy‐montmorillonite clay nanocomposites: Synthesis and characterization

Nanocomposites of epoxy resin with montmorillonite clay were synthesized by swelling of different proportions of the clay in a diglycidyl ether of bisphenol‐A followed by in situ polymerization with aromatic diamine as a curing agent. The montmorillonite was modified with octadecylamine and made organophilic. The organoclay was found to be intercalated easily by incorporation of the epoxy precursor and the clay galleries were simultaneously expanded. However, Na‐montmorillonite clay could not be intercalated during the mixing or through the curing process. Curing temperature was found to provide a balance between the reaction rate of the epoxy precursor and the diffusion rate of the curing agent into the clay galleries. The cure kinetics were studied by differential scanning calorimetry. The exfoliation behavior of the organoclay system was investigated by X‐ray diffraction. Thermogravimetric analysis was used to determine the thermal stability, which was correlated with the ionic exchange between the organic species and the silicate layers. The morphology of the nanocomposites was evaluated by scanning electron microscopy. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2201–2210, 2004     

Preparation,characterization, and nanostructural evolution of epoxy nanocomposites

Epoxy nanocomposites were prepared from the different organoclays with aerospace epoxy resin. A series of organoclays treated with alkylammonium chloride with different alkyl groups of different carbon chains were prepared, including SC4, SC6, SC8, SC10, SC12, SC16, SC18, and NC8, NC12, NC18. All of these organoclays, except for SC4, are very compatible with the aerospace Epon 862/curing agent W. The characterization from wide‐angle X‐ray diffraction (WAXD), small‐angle X‐ray scattering (SAXS), and transmission electron microscopy (TEM) confirms the exfoliated nanostructure. The six‐carbon chain length of the ammonium cation is enough to wet the surface of the clay gallery to make the organoclay compatible with epoxy resin. The clay with lower cation exchange capacity is more favorable for the polymer penetration inside the gallery and is dispersed better in the polymer matrix. The structural evolution of the aerospace epoxy nanocomposite was monitored by in situ SAXS. The 3% SC18/Epon 862/W, 3 and 6% SC8/Epon 862/W showed exfoliated nanostructure, while there is no exfoliation taking place for 3% S30B/Epon 862/W and 3%S25A/Epon 862/W up to 200°C. The acidity from the pendent group in SC18 and SC8 has a catalytic effect for the polymerization inside the gallery, while the organic pendent group of S30B and S25A does not. The faster reaction of the intragallery epoxy resin produced extra thermal heat inside the gallery to expand the gallery and is favorable for the migration of epoxy resin outside the gallery into the gallery where exfoliation took place. The exothermal heat of curing inside the gallery is an important factor for nanosheets exfoliation. Although exfoliation took place for both 3% SC18/Epon 862/W and 3% SC8/Epon 862/W, the detailed morphology development during the curing is different. For 3% SC8/Epon 862/W, the interplanar spacing between the layers is increased gradually, while 3% SC18/Epon 862/W experienced the disappearance of the ordered structure of the layered silicate in the beginning of the curing process and reappearance of the ordered structure of the silicate later. The glassy and rubbery moduli of the polymer‐silicate nanocomposites were found to be greater than the unmodified resin because of the high aspect ratio and high stiffness of the layered silicate filler. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2276–2287, 2003     

Experimental characterization of a curing thermoset epoxy‐anhydride system—Isothermal and nonisothermal cure kinetics

This work describes in detail the kinetic model for the cure of an epoxy‐anhydride thermoset matrix resin system. The cure kinetics in both nonisothermal and isothermal modes has been characterized using differential scanning calorimetry. The Sestak–Berggren two‐parameter autocatalytic model was used to describe the nonisothermal cure behavior of the resin satisfactorily. The isothermal cure data was fitted with Kamal's four‐parameter autocatalytic model, coupled with a diffusion factor. These characterization data will form material property inputs for a multiscale modeling framework for the estimation of cure‐induced residual stresses in thick thermoset matrix composites. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

The structure‐property relationship of novolac cured epoxy resin/clay nanocomposites

Intercalated or exfoliated novolac cured epoxy resin nanocomposites were prepared with two different kinds of layered silicates – montmorillonite (PK‐802) and nontronite (PK‐805). The bifunctional modifiers (PI/BEN or MI/BEN) are used to modify the clays for improvement of the properties of polymer where benzalkonium chloride (BEN) acts as a compatibilizing agent and 2‐phenylimidazole (PI) or 2‐methylimidazole (MI) as the accelerators. Both the compatibilizer and accelerator are simultaneously intercalated into the gallery space of pure clays to form the modified clay. The novolac cured epoxy nanocomposites are prepared with these modified clays by crosslinking polymerization reaction. The properties of novolac cured epoxy/clay nanocomposites were characterized by wide‐angle X‐ray diffraction (WAXD), thermo‐gravimetric analysis (TGA), dynamic mechanical analysis (DMA), and transmission electron microscopy (TEM) methods. According to the measurement, these novolac cured epoxy‐clay nanocomposites have been shown the significant improvement in the thermal, mechanical, and barrier properties that may be applied to make printed circuit board. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Kaolin–epoxy‐based nanocomposites: A complementary study of the epoxy curing by FTIR and fluorescence

This work is focused on the study of the effect exerted by the presence of kaolin on the cure reaction of an epoxy‐based polymer, discussing the influence of different kaolin pretreatments. During the last few years, the interest on polymer matrix nanocomposite materials has sharply increased because generally they show improved properties when compared with those of the neat polymer (without filler). Among this sort of materials, polymer clay nanocomposites have been widely studied. However, there are not many works about kaolin‐based composites. Although several techniques have been used to monitor the cure process in epoxy‐based composites such as Fourier transform infrared spectroscopy (FTIR) or differential scanning calorimetry, only the use of the fluorescent response from a fluorophore seems to be adequate to monitor the reaction exactly at the interfaces at a molecular scale. In this work, FTIR and fluorimetry were used to monitor the cure reaction of the different composite systems at different curing temperatures. The analysis of FTIR experiments revealed that the presence of the reinforcement clay affects the extent of the cure reaction depending on the nature of its surface. On the other hand, the use of a fluorescent molecule chemically bonded to the reinforcement allows studying the curing exactly at the interface. Finally, with the collected data, a kinetic analysis was done and the results obtained were compared in terms of the technique used and the information source (interface or bulk). At the interface, the activation energy for the epoxy reaction is lower than that carried out in the bulk indicating that the reaction at the interface proceeds via a particular mechanism for which the reaction is favored. It seems that a higher amount of hydroxyl groups is capable of catalyzing the cure reaction. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Epoxy‐clay nanocomposites: Influence of the clay surface modification on structure

In this article, the effect of four different clay surface modifiers on the structure of epoxy‐clay nanocomposites was studied. Various organoclays were prepared via cation exchange reaction between inorganic cations naturally occuring in the clay gallery and different alkylammonium ions. Epoxy‐clay nanocomposites were prepared by in situ intercalative polymerization using a hardener of polyoxypropylenediamine type. It was found that various clay surface modifiers exhibit different catalytic effect on curing of epoxy inside the clay gallery as observed by measuring of the gel time with dynamic mechanical analysis. This was confirmed by monitoring the change in the d‐spacing by wide angle X‐ray scattering performed in situ during curing. Morphology of the cured systems was probed by transmission electron microscopy (TEM) and wide angle X‐ray scattering (WAXS). The degree of dispersion observed by TEM and WAXS corresponds with achieved mechanical properties of cured composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.