Toughening of a trifunctional epoxy system: IV. Dynamic mechanical relaxational study of the thermoplastic-modified cure process

Dynamic mechanical spectroscopy has been used to investigate the cure of a thermoplastically modified trifunctional epoxy resin. The complex dissolution, curing behavior, and variations in the glass transition of the thermoplastic (PSF) phase were described, as was the T_g behavior of the epoxy phase. Prereaction of the PSF material with the epoxy resin was found to greatly increase the solubility of the PSF in the epoxy phase with little effect on the concentration of the epoxy monomer dissolving in the PSF phase. The curing behavior of the epoxy component in the thermoplastic phase was also investigated, in addition to changes in the mobility of the network at both gelation and vitrification. © 1997 John Wiley & Sons, Inc.     

Curing reaction of biphenyl epoxy resin with different phenolic functional hardeners

The investigation of cure kinetics and relationships between glass transition temperature and conversion of biphenyl epoxy resin (4,4′-diglycidyloxy-3,3′,5,5′-tetramethyl biphenyl) with different phenolic hardeners was performed by differential scanning calorimeter using an isothermal approach over the temperature range 120–150°C. All kinetic parameters of the curing reaction including the reaction order, activation energy, and rate constant were calculated and reported. The results indicate that the curing reaction of formulations using xylok and dicyclopentadiene type phenolic resins (DCPDP) as hardeners proceeds through a first-order kinetic mechanism, whereas the curing reaction of formulations using phenol novolac as a hardener goes through an autocatalytic kinetic mechanism. The differences of curing reaction with the change of hardener in biphenyl epoxy resin systems were explained with the relationships between T_g and reaction conversion using the DiBenedetto equation. A detailed cure mechanism in biphenyl-type epoxy resin with the different hardeners has been suggested. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 773–783, 1998     

Kinetic Study of Curing Bisphenol A Dicyanate Ester with Ionic Liquid Additive

A kinetic study of the trimerization reaction of bisphenol A dicyanate ester with an aromatic imidazolium‐based ionic liquid (IL) as additive is performed using dynamic and isothermal differential scanning calorimetry. The reaction follows second‐order autocatalytic kinetics, and a slight acceleration effect is observed in the presence of the aromatic IL relative to the neat resin. The activation energy also increases with the IL additive, whereas the glass transition temperature (T_g) is depressed, consistent with the Fox equation and a homogeneous one‐phase material. A model incorporating diffusion effects is able to describe the dynamic and isothermal curing data for both the neat resin system and that containing aromatic IL. A comparison with aliphatic‐based IL additive indicates that the reaction is more accelerated with aliphatic IL than with the aromatic IL in spite of the fact that the aliphatic additive phase separates during cure. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1315–1324     

Effects of curing on the glass transition temperature and moisture absorption of a neat epoxy resin

Apparent glass transition temperature (T_g) measurements were made on smaples of a neat epoxy resin that had been cured at four different temperature and for four different times at each temperature. The apparent T_g data increase with cure time toward an asymptote that was dependent on cure temperature. The asymptotic dependence of T_g on cure temperature may be explained by the effect of cure temperature on the reaction rates and available reaction sites. The asymptotic increase with cure time may be understood in terms of the resin's extent of cure. Moisture-conditioning studies were also made and the amount of moisture absorbed was correlated with the extent of cure. The absorbed moisture's interaction with the resin's molecular structure was deduced to by primarily at hydroxyl sites.     

Diglycidyl ether of bisphenol-A epoxy resin–polyether sulfone/polyether sulfone ether ketone blends: phase morphology, fracture toughness and thermo-mechanical properties

The properties of diglycidyl ether of bisphenol-A epoxy resin toughened with poly(ether sulfone ether ketone) (PESEK) and poly(ether sulfone) (PES) polymers were investigated. PESEK was synthesised by the nucleophilic substitution reaction of 4,4’-difluorobenzophenone with dihydroxydiphenylsulfone using sulfolane as solvent and potassium carbonate as catalyst at 230 °C. The T _g–composition behaviour of the homogeneous epoxy resin/PESEK blend was modelled using Fox, Gordon–Taylor and Kelley–Bueche equations. A single relaxation near the glass transition of epoxy resin was observed in all the blend systems. From dynamic mechanical analysis, the crosslink density of the blends was found to decrease with increase in the thermoplastic concentration. The storage modulus of the epoxy/PESEK blends was lower than that of neat resin, whilst it is higher for epoxy/PES blends up to glass transition temperature, thereafter it decreases. Scanning electron microscopic studies of the blends revealed a homogeneous morphology. The homogeneity of the blends was attributed to the similarity in chemical structure of the modifier and the cured epoxy network and due to the H-bonding interactions between the blend components. The fracture toughness of epoxy resin increased on blending with PESEK and PES. The increase in fracture toughness was due to the increase in ductility of the matrix. The thermal stability of the blends was comparable to that of neat epoxy resin.     

Effect of Non-Woven Carbon Nanofiber Mat Presence on Cure Kinetics of Epoxy Nanocomposites

Summary : An investigation was carried out into the cure kinetics of carbon nanofiber (CNF) mat-epoxy nanocomposites, composed of bisphenol-A based epoxy resin and diethylene triamine as a curing agent. It was observed that the rate of cure reaction for CNF mat-epoxy nanocomposites was higher than that for neat epoxy resin at low curing temperatures and the presence of the CNF mat produced the maximum influence at a certain curing temperature and time. At high curing temperature and long curing times, the effect of CNF mat on the cure rate was insignificant. The CNF mat-epoxy composite exhibited somewhat lower value of activation energy than that of the neat epoxy system at the beginning of the curing stage. The weight fraction of CNF mat also affected the cure reaction of epoxy nanocomposites at the same curing temperature. As the amount of CNF mat increased, the cure rate was higher at the same cure time. However, at high CNF mat loading, the cure reaction was retarded since the amount of epoxy and hardener decreased dramatically at high CNF contents together with the hindering effect of the CNF mat on the diffusion of epoxy resin and the curing agent, leading to lower crosslinking efficiency. Although the curing efficiency of epoxy nanocomposites dropped at high CNF mat content, the glass transition temperature (T_g) was still high due to the ultra-high strength of the CNF mat. The cure kinetics of CNF mat-epoxy nanocomposites was in good agreement with Kamal's model.     

TTT Cure Diagram

Curing reactions of the epoxy system consisting of a diglycidyl ether of bisphenol A (BADGE n=0) and m-xylylenediamine (m-XDA) were studied to calculate time-temperature-transformation (TTT) isothermal cure diagram for this system. Gel times were measured as a function of temperature using solubility test. Differential scanning calorimetry (DSC) was used to calculate the vitrification times. DSC data show a one-to-one relationship between T _g and fractional conversion, a independent of cure temperature. As a consequence, T _g can be used as a measure of conversion. The activation energy for the polymerization overall reaction was calculated from the gel times obtained using the solubility test (41.5 kJ mol^-1). This value is similar to the results obtained for other similar epoxy systems. Isoconversion contours were calculated by numerical integration of the best fitting kinetic model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Epoxy/poly(4-vinylphenol) blends crosslinked by imidazole initiation

Poly(4-vinylphenol) (PVPh) was used as thermoplastic modifier of epoxy resins. Binary epoxy/PVPh mixtures with high thermoplastic content rise high glass transition temperatures (T _g) after heating, due to the epoxy-phenol reaction. Blends with low PVPh percentages reach high T _g if 2-methylimidazole is added, which catalyses epoxy homopolymerisation and epoxy-phenol reaction. The cured blends do not present phase separation although the network structure depends on the epoxy/phenol ratio. At low PVPh percentage the main crosslinking reaction is epoxy-epoxy but, when the thermoplastic content increases, the epoxy-phenol reaction prevails, causing an important T _g increase and becoming less brittle.     

Thermal characterization of carbonate rocks,Kozani area,North-western Macedonia,Greece

The curing of a thermoreactive alkyd-melamine-formaldehyde resin system was investigated by rheologycal, TG and TMA-analysis, in order to construct the time-temperature-transformation diagram. The points of the gelation curve were determined by measuring the increase in viscosity during isothermal curing at different temperatures. A power-function could be fitted to the gelation curve, which is suitable to estimate gelation at any curing conditions, as well as to establish storage conditions. The reaction in the resin matrix was followed by monitoring the loss of mass during isothermal curing at different temperatures. The final section of the resulted iso-curing temperature (iso-T _cure) diagrams could be fitted with logarithmic functions, which may be used for estimating the conditions needed to a given, desirable mass loss, i.e. conversion. The steepness of the curves increases with temperature suggesting the forthcoming of degradation during cure with increasing temperature. From these data the iso-mass loss curves of the TTT-diagram were constructed. For determining the iso-Tg curves of the TTT-diagram isothermal curing was carried out in a drying oven at different temperatures, followed by TMA measurements. The iso-Tcure diagrams served to determine T _{g ￥}, and to construct the iso-T _g curves of the TTT diagram. Vitrification curve is far beyond conditions of storage, curing and degradation, meaning that the resin matrix is in rubbery physical state before, during and after the cure. Curing conditions resulting degradation can also be estimated from the TTT-diagram. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mechanical and Thermal Properties and Morphology of Epoxy Resins Modified by a Silicon Compound

A silicon compound (GAPSO) was synthesized to modify the diglycidyl ether of bisphenol-A (DGEBA). The chemical structure of GAPSO was confirmed using FT-IR, ²⁹Si NMR and GPC. The mechanical and thermal properties and morphologies of the cured epoxy resins were investigated by impact testing, tensile testing, differential scanning calorimetry and environmental scanning electron microscopy. The impact strength and tensile strength were both increased by introducing GAPSO, meanwhile the glass transition temperature (T_g ) was not decreased and the morphologies of the fracture surfaces show that the compatibility of GAPSO with epoxy resin was very good and the toughening follows the pinning and crack tip bifurcation mechanism. The high functional groups in GAPSO can react during the curing process, and chemically participate in the crosslinking network. GAPSO is thus expected to improve the toughness of epoxy resin, meanwhile maintain the glass transition temperature.     

Curing reaction of o-cresol novolac epoxy resin according to hardener change

The investigations of cure kinetics and glass transition temperature (T_g) versus reaction conversion (α) of o-cresol novolac epoxy resin with the change of hardener were performed. All kinetic parameters of the curing reaction such as the reaction rate order, activation energy, and frequency factor were calculated. The curing mechanisms were classified into two types. One was an autocatalytic mechanism and the other was a nth order kinetic mechanism. The constants related to the chain mobility of polymer segments were obtained by using the DiBenedetto equation. We have tried to correlate the relationships between curing mechanism and molecular structures of hardeners from these results. © 1993 John Wiley & Sons, Inc.     

Cure kinetics of thermosetting bisphenol E cyanate ester

Resin injection repair is a common method to repair delamination damage in polymer matrix composites (PMCs). To repair high-temperature PMCs, the resin should have a very low viscosity, yet cure into a compatible adhesive with high temperature stability. Normally, thermosetting polymers with high glass transition temperatures (T _g) are made from monomers with high room temperature viscosities. Among the high temperature resins, bisphenol E cyanate ester (BECy, 1,1’-bis(4-cyanatophenyl)ethane), is unique because it has an extremely low viscosity of 0.09–0.12 Pa s at room temperature yet polymerizes as a cross-linked thermoset with a high T _g of 274°C. BECy monomer is cured via a trimerization reaction, without volatile products, to form the high T _g amorphous network. In this study, the cure kinetics of BECy is investigated by differential scanning calorimetry (DSC). Both dynamic and isothermal experiments were carried out to obtain the kinetic parameters. An autocatalytic model was successfully used to model isothermal curing. The activation energy from the autocatalytic model is 60.3 kJ mol⁻¹ and the total reaction order is about 2.4. The empirical DiBenedetto equation was used to evaluate the relationship between T _g and conversion. The activation energy of BECy from the dynamic experiments is 66.7 kJ mol⁻¹ based on Kissinger’s method, while isoconversional analysis shows the activation energy changes as the reaction progresses.     

Exploitation of introducing of catalytic centers into layer galleries of layered silicates and related epoxy nanocomposites. I. Epoxy nanocomposites derived from montmorillonite modified with catalytic surfactant‐bearing carboxyl groups

Montmorillonite (MMT) was modified with the acidified cocamidopropyl betaine (CAB) and the resulting organo‐montmorillonite (O‐MMT) was dispersed in an epoxy/methyl tetrahydrophthalic anhydride system to form epoxy nanocomposites. The intercalation and exfoliation behavior of the epoxy nanocomposites were examined by X‐ray diffraction and transmission electron microscopy. The curing behavior and thermal property were investigated by in situ Fourier transform infrared spectroscopy and DSC, respectively. The results showed that MMT could be highly intercalated by acidified CAB, and O‐MMT could be easily dispersed in epoxy resin to form intercalated/exfoliated epoxy nanocomposites. When the O‐MMT loading was lower than 8 phr (relative to 100 phr resin), exfoliated nanocomposites were achieved. The glass‐transition temperatures (T_g's) of the exfoliated nanocomposite were 20 °C higher than that of the neat resin. At higher O‐MMT loading, partial exfoliation was achieved, and those samples possessed moderately higher T_g's as compared with the neat resin. O‐MMT showed an obviously catalytic nature toward the curing of epoxy resin. The curing rate of the epoxy compound increased with O‐MMT loading. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1192–1198, 2004     

Cure kinetics of epoxy composite applied on stator bars insulation

Cure kinetics study of epoxy resin composite, employed as stator bars insulation system, was evaluated through differential scanning calorimetry using the dynamic methodology, different heating rates. These experiments provided some important information about the system as activation energy (E _a = 65.4 kJ mol⁻¹), glass transition (T _g) values on different curing stages including the final cured material information and, besides that, it enables the comparison of these data with new materials under development. The activation energy value allows the determination of different energy needs of the system under evaluation, specially temperature for the material cure, improving the possibility of comparison between different insulation systems in use in the high voltage insulation business. The composite conversion degree based on the cure enthalpy (ΔH _cure) at different time of cure was also subject of analysis and from that it was possible to comprehend the cure pattern which allows the cure state prediction of further samples of this type of material and the more accurate evaluation of similar samples acquired directly from stator bars.     

Isothermal cure characterization of dicyclopentadiene

Conversion (α) and the glass transition temperature (T _g) were investigated during the isothermal cure of endo-dicyclopentadiene (DCPD) with a Grubbs catalyst for different temperatures using differential scanning calorimetry. Conversion vs. In (time) data at an arbitrary reference temperature were superposed by horizontal shift and the shift factors were used to calculate an Arrhenius activation energy. Glass transition temperature vs. conversion data fell on a single curve independent of cure temperature, implying that reaction of the norbornene and cyclopentene ring of DCPD proceeds in a sequential fashion. Implications of the isothermal reaction kinetics for self-healing composites are discussed.     

Fracture behavior of core-shell rubber-modified crosslinkable epoxy thermoplastics

The fracture behavior of a core-shell rubber (CSR) modified cross-linkable epoxy thermoplastic (CET) system, which exhibits high rigidity, highT _g, and low crosslink density characteristics, is examined. The toughening mechanisms in this modified CET system are found to be cavitation of the CSR particles, followed by formation of extended shear banding around the advancing crack. With an addition of only 5 wt.% CSR, the modified CET possesses a greater than five-fold increase in fracture toughness (G _IC) as well as greatly improved fatigue crack propagation resistance properties, with respect to those of the neat resin equivalents. The fracture mechanisms observed under static loading and under fatigue cyclic loading are compared and discussed.     

Synthesis of hydroxyl‐terminated poly(ether ether ketone) with pendent tert‐butyl groups and its use as a toughener for epoxy resins

Hydroxyl‐terminated poly(ether ether ketone) with pendent tert‐butyl groups (PEEKTOH) was synthesized by the nucleophilic substitution reaction of 4,4′‐difluorobenzophenone with tert‐butyl hydroquinone with potassium carbonate as a catalyst and N‐methyl‐2‐pyrrolidone as a solvent. Diglycidyl ether of bisphenol A epoxy resin was toughened with PEEKTOHs having different molecular weights. The melt‐mixed binary blends were homogeneous and showed a single composition‐dependent glass‐transition temperature (T_g). Kelley–Bueche and Gordon–Taylor equations gave good correlation with the experimental T_g. Scanning electron microscopy studies of the cured blends revealed a two‐phase morphology. A sea‐island morphology in which the thermoplastic was dispersed in a continuous matrix of epoxy resin was observed. Phase separation occurred by a nucleation and growth mechanism. The dynamic mechanical spectrum of the blends gave two peaks corresponding to epoxy‐rich and thermoplastic‐rich phases. The T_g of the epoxy‐rich phase was lower than that of the unmodified epoxy resin, indicating the presence of dissolved PEEKTOH in the epoxy matrix. There was an increase in the tensile strength with the addition of PEEKTOH. The fracture toughness increased by 135% with the addition of high‐molecular‐weight PEEKTOH. The improvement in the fracture toughness was dependent on the molecular weight and concentration of the oligomers present in the blend. Fracture mechanisms such as crack path deflection, ductile tearing of the thermoplastic, and local plastic deformation of the matrix occurred in the blends. The thermal stability of the blends was not affected by blending with PEEKTOH. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 541–556, 2006     

Viscoelastic properties and morphology of dicyanate ester/epoxy co-polymers modified with polysiloxane and butadiene–acrylonitrile rubbers

Dynamic mechanical analysis was conducted on specimens prepared from cyanate ester (CE) and epoxy (EP) resins cured together at various mass compositions. Increase of amount of epoxy resin in composition was shown to have a disadvantageous effect on glass transition temperature (T _g). It was shown that post-curing procedure was needed to produce a polymer matrix with a single glass transition relaxation, but increase in post-cure temperature up to 250 °C resulted in slight reduction in T _g for epoxy/cyanate copolymers. TG results proved that the presence of epoxy resin reduces thermal stability of the cyanate/epoxy materials. The neat CE and EP/CE systems containing 30 wt% of epoxy resin were modified using epoxy-terminated butadiene–acrylonitrile rubber (ETBN) and polysiloxane core–shell elastomer (PS). The scanning electron microscopy (SEM) results showed the existence of second phase of ETBN and PS modifiers. Only in the case of EP/CE composition modified with ETBN, well-dispersed second phase domains were observed. Analysis of SEM images for other CE- and EP/CE-modified systems revealed the formation of spherical aggregates.     

Enhanced fracture toughness and mechanical properties of epoxy resin with rice husk-based nano-silica

This paper reports a novel approach to toughen epoxy resin with nano-silica fabricated from rice husk using a thermal treatment method with a particle size distribution in range of 40–80 nm. The nano-silica content was in the range, 0.03–0.10 phr, with respect to epoxy. The mechanical test showed that with the addition of 0.07 phr of rice husk based nano-silica, the fracture toughness of the neat epoxy resin increased 16.3% from 0.61 to 0.71 MPa m^1/2. The dynamic mechanical analysis test results showed that the glass transition temperature (T _g) of a 0.07 phr nano-silica dispersion in epoxy resin shifted to a higher temperature from 140 to 147°C compared to neat epoxy resin. SEM further showed that the nano-silica particles dispersed throughout the epoxy resin prevented and altered the path of crack growth along with a change in the fracture surface morphology of cured epoxy resin.     

Glass transition temperature for epoxy-diamine networks

The glass transition temperature of systems based on epoxy resin and a number of diamines has been determined by using a torsion pendulum. An equation relating composition and crosslink density with the glass transition temperature has been established which gives reasonable predictions of the glass transition temperatures for systems based on aliphatic or aromatic amines and methylated amines and for systems containing a monofunctional epoxy diluent. The equation may be used to predict T_g for systems with non-stoichiometric quantities of curing agent and blends of amines. Deviation of the predicted and observed values for T_g is interpreted in terms of differences between definitions of T_g used by other workers and, also the occurrence of competing side reactions during polymerization which lead to additional crosslinks.