Curing of epoxy systems at sub‐glass transition temperature

Three epoxy‐amine thermoset systems were cured at a low ambient temperature. Evolution of the reaction kinetics and molecular structure during cure at the sub‐glass transition temperature was followed by DSC and chemorheology experiments. The effect of vitrification and the reaction exotherm on curing and final mechanical properties of the epoxy thermosets was determined. Thermomechanical properties of the low‐temperature cured systems depend on the reaction kinetics and volume of the reaction mixture. Curing of the fast‐reacting system in a large volume (12‐mm thick layer) resulted in the material with T_g exceeding the cure temperature by 70–80°C because of an exothermal temperature rise. However, the reaction in a too large volume (50‐mm layer) led to thermal degradation of the network. In contrast, thin layers (1.5 mm) were severely undercured. Well‐cured epoxy thermosets could be prepared at sub‐T_g temperatures by optimizing reaction conditions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3669–3676, 2006     

The glass transition temperature of an epoxy resin and the effect of absorbed water

Summary The glass transition temperature, T_g, of a diamino-dipenyl methane (DDM, II) cured diglycidyl ether of bispenol-A epoxy resin (I, n=0) has been determined by proton NMR line width studies to be 410°K. In the presence of 2.5±0.1 w/o absorbed water T_g is reduced to 378°K. Such a large reduction of Tg by such a small amount of water requires specific interactions. It is postulated that segmental motion is increased as a result of the disruption of hydroxyl group hydrogen bonds by the absorbed water molecules.     

Measurement and prediction of the glass transition temperature during processing of copper-clad epoxy resin systems

The determination of the glass transition temperature of a cured resin system is necessary for the classification of the temperature region of usable service. For the printed circuit board industry, accurate and consistent properties of the copperclad laminates that are used as the substrate are essential. Variations in material properties of the finished laminates may become a hindrance in subsequent steps in the manufacturing of printed circuit boards and more importantly in the failure rate of the boards during service. In this research, a one-dimensional finite difference method has been implemented with constitutive models to simulate the progression of the degree of cure and glass transition temperature during the processing of copper-clad, multi-layered, fiber mat reinforced, epoxy laminates. The numerical results have been compared to experimental measurements and it has been found that the simulation predicts the experimental observations quite well.     

Morphology transition of bismaleimide‐modified novolac resin

The bismaleimide (BMI)‐modified novolac resin was synthesized by allylation of the novolac resin and its “Ene” reaction with BMI. The reactions were monitored by Fourier transform infrared analysis (FTIR). The morphology of the BMI‐modified novolac resin changed with degree of allylation (i.e., the amount of BMI used). On the basis of the studies done by dynamical mechanical analysis (DMA), differential scanning calorimetry (DSC), and field emission scanning electron microscopy (FE‐SEM), it was found that when the degree of allylation increased from 48 to 59%, the resin changed from a single‐phase structure to a two‐phase structure. From the research by thermogravimetric analysis (TGA) and DMA, it was found that the higher allyl‐content (when > 48%) caused a decrease of the thermal properties and mechanical properties of the resultant resin. The BMI‐modified allyl novolac resin with 48% degree of allylation has the best thermal properties and the highest dynamic modulus in the current research. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 76–83, 2006     

Effects of cure treatment on glass transition temperatures for a BADGE–DDM epoxy resin

The extent of cure for epoxy resin (BADGE) with a small excess of amine hardener (DDM, 4w/o) was determined by assay of the epoxy groups using near infrared spectroscopy. The effect of cure time at 100°C and postcure time at 180°C has been investigated. For the lower temperature cures the room temperature density, ρ_rt, increased with time, reaching a maximum at X_e = 0.37, and then decreased with extended cure times. For postcure, there was a monotonic decrease in ρ_rt that was unrelated to the extent of cure. There was an approximate linear relationship between glass transition temperature and In(t), which increased even when essentially all of the epoxy groups had reacted, that is X_e ∼ 0.99. © 1996 John Wiley & Sons, Inc.     

High glass transition temperature bismaleimide‐triazine resins based on soluble amorphous bismaleimide monomer

A novel bismaleimide (DOPO‐BMI) with unsymmetrical chemical structure and DOPO pendant group has been prepared. The particular molecular structure makes DOPO‐BMI show an intrinsic amorphous state with a T_g about 135°C and excellent solubility in most organic solvents, which is beneficial to the processability of bismaleimide composite materials. A series of bismaleimide‐triazine (BT) resins have been prepared based on DOPO‐BMI and 2,2‐bis(4‐cyanatophenyl)propane at various weight ratios. The prepared BT resins show outstanding solubility in organic solvent and low viscosity about 10–671 mPa s at 180°C. The cured BT resins exhibit high glass transition temperature (T_g) over 316°C. As the weight ratio of DOPO‐BMI increases to 80% (BT80), the T_g can rise to 369°C (tan δ). The cured BT resins also show good thermal stability with the 5% weight loss temperature over 400°C under both nitrogen and air atmosphere. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42882.     

Influence of cure conditions on glass transition temperature and density of an epoxy resin

The glass transition temperature T_g and density of a TGDDM-DDS epoxy resin system were studied as a function of cure times at various cure temperatures. Both T_g and density asymptotically increased toward a maximum value with increasing cure time. The T_g and density measurements were related to the extent of cure, and the relationship in both cases was found to be independent of cure temperature.     

Cure and glass transition temperature of the bisphenol S epoxy resin with 4,4′‐diaminodiphenylmethane

The curing reaction of bisphenol S epoxy resin (BPSER) with 4,4′‐diaminodiphenylmethane (DDM) was studied by means of torsional braid analysis (TBA) in the temperature range of 393–433 K. The glass transition temperature (T_g) of the BPSER/DDM system is determined, and the results show that the reaction rate increases with increasing the T_g in terms of the rate constant, but decreases with increasing conversion. 1 The T_g of BPSER/DDM is about 40 K higher than BPAER/DDM. The gelation and vitrification time were assigned by the isothermal TBA under 373 K; in addition, an FTIR spectrum was carried out to describe the change of the molecular structure. The thermal degradation kinetics of this system was investigated by thermogravimetric analysis (TGA). It illustrated that the thermal degradation of the BPSER/DDM has n‐order reaction kinetics. 2 © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 794–799, 2000     

The curing mechanism and properties of bismaleimide–biscyanamide resin

Thermoset networks consisting of N, N-bismaleimidediphenylmethane (BMI) and bis [4-(4-cyanamide-3-methylphenyl)] methane (BCMM) were investigated with the aim of correlating macroscopic behavior and structure as network composition was varied. The glass transition temperatures of these cured resins obtained from the dynamic mechanical data and thermal expansion data had a distinct minimum at the 50/50 molar ratio of BMI/BCMM. Also, at this molar ratio, the decomposition temperature at a 5% weight loss in helium atmosphere for cured resins was the lowest. These results provided a rough idea of the thermoset network structure.     

Estimation of glass transition temperature of polysulfone-modified epoxy binders

The properties of polysulfone-modified epoxy binders, which are intrinsic for their high deformation characteristics, are given. The experimental results of the impact strength, bend strength, and glass transition temperature are given. The procedure of the measurement of the temperature range, at which there is a glass transition, is given. It has been shown that the amount of the introduced polysulfone to the binder has a greater influence on the temperature range where the change of the glass transition temperature occurs rather than its mean value.     

Influence of different concentrations of glycidylisobutyl‐POSS on the glass transition of cured epoxy resin

In this study, polyhedral oligomeric silsesquioxane glycidylisobutyl‐POSS was dispersed in epoxy resin by ultrasound, and the parameters of a phantom model, the Williams–Landel–Ferry (WLF) and Vogel–Fulcher–Tammann (VFT) equations were modeled using dynamic mechanical analysis (DMA) to evaluate their influence on the glass transition state. The relaxation and retardation time distributions were estimated using a nonlinear regularization method, and the estimated physical parameters were discussed based on the results obtained from transmission electron microscopy (TEM). The TEM analysis indicated higher POSS dispersion with a spherical shape. The POSS dispersion was associated with the formation of micelles due to their hybrid character. The micelles favored the interconnections of the nodular microstructure of the epoxy thermosetting, which led to an increase in their T_g values. These interconnections increased the structure's percolation, promoted a reduction in the thermal expansion coefficient and resulted in a more homogeneous glass transition, in terms of a cooperative distribution in the relaxation times at the time scale measured. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41453.     

Linear viscoelastic properties of epoxy resin polymers in dilatation and shear in the glass transition region: 2. Measurement of the glass transition temperature

Measurements of the 20 second creep compliances in dilatation and shear, over a range of temperature spanning the glass transition region, have been made for six epoxy resin polymers. The linear thermal expansion behaviour has been determined in the same temperature range. Shear and dilatational compliances, and the linear thermal expansion properties may all be measured on the same specimen. Values of the glass transition temperature derived from 20 sec shear and bulk compliance data agree well with values obtained from thermal expansion data. When T_g was derived from creep data obtained at longer creep times (in part 1 of this paper), it was found that T_g in dilatation exceeds T_g in shear. It was shown that this is consistent with the difference in activation energies in dilatation and shear obtained in part 1 of this paper. An example is given of how systematic errors of the order of 1°C in temperature measurement may lead to maxima or minima when Poisson's ratio is calculated from measured values of shear and tensile compliances.     

Novel adamantane‐containing epoxy resin

A novel adamantane‐containing epoxy resin diglycidyl ether of bisphenol‐adamantane (DGEBAda) was successfully synthesized from 1,3‐bis(4‐hydroxyphenyl)adamantane by a one‐step method. The proposed structure of the epoxy resin was confirmed with Fourier transform infrared, ¹H‐NMR, gel permeation chromatography, and epoxy equivalent weight titration. The synthesized adamantane‐containing epoxy resin was cured with 4,4′‐diaminodiphenyl sulfone (DDS) and dicyandiamide (DICY). The thermal properties of the DDS‐cured epoxy were investigated with differential scanning calorimetry and thermogravimetric analysis (TGA). The dielectric properties of the DICY‐cured epoxy were determined from its dielectric spectrum. The obtained results were compared with those of commercially available diglycidyl ether of bisphenol A (DGEBA), a tetramethyl biphenol (TMBP)/epoxy system, and some other associated epoxy resins. According to the measured values, the glass‐transition temperature of the DGEBAda/DDS system (223°C) was higher than that of the DGEBA/DDS system and close to that of the TMBP/DDS system. TGA results showed that the DGEBAda/DDS system had a higher char yield (25.02%) and integral procedure decomposition temperature (850.7°C); however, the 5 wt % degradation temperature was lower than that of DDS‐cured DGEBA and TMBP. Moreover, DGEBAda/DDS had reduced moisture absorption and lower dielectric properties. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Photo-oxidation and photoprotection of the surface resin of a glass fiber–epoxy composite

The rapid photo-oxidation of the surface epoxy resin of a commercial seven-ply laminate (Scotchply 1009-26) is due principally to the epoxy novolac resin component. The photo-oxidation rate of this resin is eight times that of the other component, a bisphenol A epoxy resin. This rate depends on the conditions of cure, and photo-initiation occurs in part through aromatic carbonyl groups formed by oxidation of the methylene linkages of the novolac at the cure temperature (160–180°C). Inhibition of this thermal oxidation by vacuum cure or a chain-terminating antioxidant increases the photostability. Photoprotection of thin resin sections by the UV stabilizer 2-hydroxy-4-isooctoxybenzophenone and an epoxidized analog is assessed.     

Improvement of fracture toughness and glass transition temperature of DGEBA‐based epoxy systems using toughening and crosslinking modifiers

Several toughening and crosslinking modifiers were tested in two epoxy resin systems based on the diglycidyl ether of bisphenol A (DGEBA) with the objective to improve the critical stress intensity factor K_IC and the glass transition temperature (T_g) simultaneously. An amine hardener (isophorone diamine (IPD)) and a homopolymerization initiator (1‐ethyl‐3‐methylimidazolium acetate (EMIM Ac)) were used as curing agents. The highest effect on the K_IC value of the resin system DGEBA/IPD (K_IC = 0.72 MPa^1/2; T_g = 164°C) was achieved with the dendric polymer Boltorn P501 (10 wt%), but it decreased the T_g (K_IC = 1.39 MPa^1/2; T_g = 136°C). A high toughening effect with a low decrease of T_g was achieved with a combination of a self‐organized block copolymer (Nanostrength M22N) and silica nanoparticles (Nanopox F400) (K_IC =1.15 MPa^1/2; T_g =157°C). The K_IC value of the resin system DGEBA/EMIM Ac was improved from 0.44 to 0.66 MPa^1/2. An improvement of both, the thermal and mechanical properties was established for a combination of a poly(tetrahydrofuran) as toughening modifier (PolyTHF2000) with the post‐crosslinking modifier diethylphosphite (DEP) in the resin system DGEBA/IPD (K_IC = 0.86 MPa^1/2; T_g = 180°C). A system with chemical linkages between both modifiers was investigated for comparison but yielded inferior results. POLYM. ENG. SCI., 59:86–95, 2019. © 2018 Society of Plastics Engineers     

The influence of thermal properties on the glass transition temperature in styrene/divinylbenzene network–diluent systems

The depression of glass transition temperature in polystyrene and styrene/divinylbenzene networks, by the addition of ethylbenzene, ethyl acetate, and m-diethylbenzene, respectively, has been investigated by differential scanning calorimetry. The predictive quality of the Couchman–Karasz treatment for the composition dependence of T_g in compatible systems was found to provide excellent agreement with experimental results for the depression of T_g by ethylbenzene, provided certain modifications were made. Qualitatively, the theory predicts that for a given diluent, the depression of T_g is particularly sensitive to the incremental change in heat capacity of the pure polymer or network. The behavior of ethylbenzene provided good quantitative agreement with the theory, provided that the incremental change in heat capacity of the diluent at its T_g was approximated by ΔC_p = constant/T and that the incremental change in heat capacity of the network at T_g represents only those units capable of thermal activation. The anomalous behavior observed on plasticization by ethyl acetate and m-diethylbenzene could in part be explained by a reduction in solvent quality or syneresis of diluent in the highly crosslinked samples.     

The role of network architecture on the glass transition temperature of epoxy resins

A series of epoxy networks were synthesized in which the molecular weight between crosslinks (M_c) and crosslink functionality were controlled independent of the network chain backbone composition. The glass transition temperature (T_g) of these networks was found to increase as M_c decreased. However, the rate at which T_g increased depended on crosslink functionality. The dependency of M_c on T_g is well described by two models, one based on the concept of network free volume while the other model is based on the principle of corresponding states. Initially, neither model could quantitatively predict the effect of crosslink functionality in our networks. However, our tests indicated that both the glass transition and the rubbery moduli of our networks were dependent on M_c and crosslink functionality, while the glassy state moduli were independent of these structural variables. The effect of crosslink functionality on the rubbery modulus of a network has been addressed by the front factor in rubber elasticity theory. Incorporation of this factor into the glass transition temperature models allowed for a quantitative prediction of T_g as a function of M_c and crosslink functionality. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 387–395, 1997     

Substituent effect on the glass transition temperature of polyoxetanes

Summary The effect of the substituents on the glass transition temperature is analyzed for a series of polyoxetanes, with alkyl substituents in position 3. The results show a regular increase of that parameter with the Van der Waals volume of the linear substituents, although a departure from this trend seems to be observed for poly(3,3-dibutyloxetane), presumably because of the effect of internal plasticization. On the other hand, the increase of T_g for poly(3-terbutyloxetane) is specially high, as usual for branched substituents. The behaviour of the glass transition temperature along the series has been analyzed in terms of interand intramolecular interactions, concluding that the latter factor is the dominant contribution in determining the T_g in this group of polymers.     

The effect of pressure and temperature on time-dependent changes in graphite/epoxy composites below the glass transition

Effects of pressure and temperature on time-dependent changes in physical/mechanical properties of graphite/epoxy composites were investigated. Samples were cut from the eight-ply-thick laminates of commercially used composites, post-cured, and then quenched to environments of various temperature and pressure. Time-dependent changes in their properties were analyzed by thermal and thermomechanical (dynamic mechanical) measurements. An increase in the glass-transition temperature was found to occur as a function of time. The rate of this process was enhanced by an increase in temperature and/or a decrease in pressure. An explanation was offered in terms of types and mechanisms of molecular events that occur in the glassy state. Time dependent decrease in free volume (and enthalpy) takes place but is not the sole mechanism responsible for the observed increase in T_g. After a certain period of time (which depends on T and P of the environment), additional crosslinking appears to take place.     

Electrically conductive composites based on epoxy resin containing polyaniline–DBSA‐ and polyaniline–DBSA‐coated glass fibers

Four kinds of polyaniline (PANI)‐coated glass fibers (GF–PANI) combined with bulk PANI particles were synthesized. GF–PANI fillers containing different PANI contents were incorporated into an epoxy–anhydride system. The best conductivity behavior of the epoxy/GF–PANI composites was obtained with a GF–PANI filler containing 80% PANI. Such a composite shows the lowest percolation threshold at about 20% GF–PANI or 16% PANI (glass fiber‐free basis). The PANI‐coated glass fibers act as conductive bridges, interconnecting PANI particles in the epoxy matrix, thus contributing to the improvement of the conductivity of the composite and the lower percolation threshold, compared with that of a epoxy/PANI–powder composite. Particularly, the presence of glass fibers significantly improves the mechanical properties, for example, the modulus and strength of the conductive epoxy composites. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1329–1334, 2004