Vitrification Times Research Articles

Influence of chemical composition on the rheological behavior of condensation reaction resins

AbstractIn this paper, the chemorheological and dynamic mechanical behavior of melamine‐formaldehyde (MF) resins of four different formaldehyde/melamine (F/M) molar ratios (1.25, 1.5 1.75 and 2.00) are investigated. MF resins polymerize via a polycondensation reaction involving formation of up to 10 wt% of H2O on cure. This typically results in rapid and extensive foaming of the resin when it is cured under atmospheric pressure. Experimental adaptation for the foaming behavior of MF resins is used to gather rheological information concerning the curing kinetics and the mechanical response of neat MF resins of different molar ratios. Likewise, the procedures developed allow curing of the resins under atmospheric pressure, hence allowing volatile evacuation as occurs during venting procedures (commonly used during compression molding of MF molding compounds) or as a result of absorption by hydrophilic fillers or substrates. The results show that increased moisture content in the B‐stage leads to faster reaction rates and greater foaming. Gelation and vitrification times are identified for each molar ratio, and are found to increase with decreasing molar ratio. The dynamic mechanical behavior of carefully molded neat MF samples of different molar ratios is studied using DMTA. Tg is found to be 200°C for the resin with the lowest formaldehyde content (F/M = 1.25), and around 230°C for the other resins. The storage shear modulus above Tg is studied, and the results show that the crosslink density increases with increasing molar ratio.

The effects of fillers on the chemorheology of highly filled epoxy resins: I. Effects on cure transitions and kinetics

AbstractThis work examines the effects of level of silica filler (at 0, 10, 30, 50 wt%) on the gelation and vitrification of a model silica‐filled diglycidyl ether of bisphenol F (DGEBF)/methylenedianiline (MDA) system. An increased filler level is shown to decrease the gelation and vitrification times at low temperatures (below 80 °C). FTIR cure kinetics show that the reaction rates are increased and the activation energies of gelation are reduced at these temperatures, indicating that network formation is made easier. Entropic and catalytic reasons for this phenomenon are discussed.© 2003 Society of Chemical Industry

The isothermal curing of a diepoxide‐cycloaliphatic diamine resin by temperature modulated differential scanning calorimetry

AbstractThe thermal properties and the isothermal cure of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with a diamine based on 4,4′‐diamino‐3,3′‐dimethyldicyclohexylmethane (3DCM) were analyzed by differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). The quasi‐isothermal TMDSC scans were performed at curing temperatures between 40 and 140°C for different periods of time, and the modulation conditions were amplitude of 0.5K and a period of 60 s. The heating rates used on the DSC scans were between 2.5 and 20 Kmin−1. The heat of curing measured nonisothermally increases when the heating rate decreases. An average value of 440 Jg−1 was estimated. The glass transition of the unreacted system measured by DSC was −40.4°C. The final glass transition temperature of the resin depends on the heating rate of cure and postcure conditions. Values between 143°C (measured by DSC) and 154°C (measured on the total heat flow by TMDSC) were obtained after isothermal curing and postcure at 10 and 1 Kmin−1, respectively. The vitrification was analyzed by the modulus of the complex heat capacity |C|, which decay gives the interval and the time of vitrification. These properties were used to build the time–temperature–transformation cure diagram. The intensity of the vitrification was measured by the change in |C|, which decreases quasi‐linearly with the curing temperature. The phase angle of the heat flow shows a peak in the vitrification region, which agrees with the vitrification time determined by |C|. The chemical kinetics may be fitted to a simplified autocatalytic model [f(α) = αm(1 − α)n, Sesták‐Berggren model] with an apparent activation energy of 58 kJmol−1. The analysis of the diffusion controlled step is studied using a mobility factor determined from the normalised variation of |C| during the vitrification. The simulated overall reaction rate, which is obtained from the product of the chemical kinetics and the mobility factor, agrees with the experimental reaction rate. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1263–1276, 2002

Cationic cure of epoxy resin by an optimum concentration of N-benzylpyrazinium hexafluoroantimonate

Cure behavior of an epoxy resin was investigated at different cure temperatures (110, 120, 130, 140, and 150°C) and cure times in the presence of 2 wt% of an N-benzylpyrazinium hexafluoroantimonate (BPH) cationic catalyst by means of differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The glass transition temperature (T g ) and chemical conversion (x) at the different temperatures were determined from DSC thermograms. TheT g andx vs. ln time data were superposed up toT g = 100°C andx = 0.70 by shifting horizontally at a reference temperature ofT g = 130°C. It is interesting that theT g andx of the superposed data increase rather slowly in the early stage of cure and rapidly thereafter. Therefore, the increase of theT g andx can be divided into two regions;R I = −18.4(=T g ) ∼ 5°C andR II= 5 ∼ 100°C inT g , andR I = 0 ∼ 0.24 andR II= 0.24 ∼ 0.70 in x. TheR I is closely related to the initiation reactions between BPH and epoxy and between hydroxy group and epoxy in this epoxy/catalyst system. From the kinetic analysis of theT g -shift, activation energy was 12.5 kcal/mol. The relationship betweenT g andx was also considered. The gelation and vitrification times for different cure temperatures were obtained from DMA curves.

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 Tg and fractional conversion, a independent of cure temperature. As a consequence, Tg 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.

Frequency dependent heat capacity in the cure of epoxy resins

Different temperature modulated differential scanning calorimetry ( TMDSC) approaches were explored for studying the frequency dependence of the vitrification phenomenon during the cure of a thermosetting epoxy resin. All approaches show the expected decrease in vitrification time with increasing frequency. Light ( heating) ( temperature) modulated DSC ( LMDSC), developed by Saruyama, offers the advantage that the frequency range for direct measurements is extended (from 0.01 to 1 Hz). However, at frequencies above 0.1 Hz, the results contain information about the changing thermal diffusivity (for samples above 100 μm thick). A complex saw-tooth modulation method, developed by Wunderlich, allows multiple frequencies (over almost one decade) to be measured in a single experiment, thus eliminating effects of sample reproducibility. For studying the frequency dependence during (chemical) transformations, such as cure, the desired invariance of the sample during at least one modulation period and thermal diffusivity effects (even for films of only 100 μm thick) limit a further extension of the frequency domain to lower and higher frequencies, respectively.

Influence of stage of maturation of bovine oocytes at time of vitrification on the incidence of diploid metaphase II at completion of maturation

The present study was to verify the incidence of bovine diploid oocytes when vitrified at various maturation stages. Bovine cumulus-oocyte complexes were recovered from ovaries at a slaughterhouse and then divided into five groups: control group (unvitrified oocytes), 0 h group (composed of oocytes vitrified before the onset of maturation) and 8, 12, and 22 h groups (vitrified respectively at 8, 12 and 22 h after the onset of maturation). The oocytes remained vitrified for 24 h, and then were thawed. In all groups, the oocytes completed 24 h of maturation. Subsequently, the cumulus cells were removed, and the denuded oocytes fixed on slides and stained with aceto-orcein. No differences ( P>0.05) in the incidence of diploid metaphase II oocytes were observed between the control, non-vitrified group (2.4%, 1/41) and oocytes vitrified at 12 h (6.9%, 3/43) or 22 h (2%, 1/50). However, significantly ( P<0.05) more diploid oocytes were detected after vitrification at 0 h (28.5%, 10/35) or 8 h (35.4%, 11/31) of maturation. These results suggest that the nuclear stage at which bovine oocytes are vitrified may affect the incidence of diploid oocytes, especially in oocytes vitrified before maturation or 8 h after the onset of maturation.

The effects of silica fillers on the gelation and vitrification of highly filled epoxy-amine thermosets

Highly filled thermosets are used in applications such as integrated circuit (IC) packaging. However, a detailed understanding of the effects of the fillers on the macroscopic cure properties is limited by the complex cure of such systems. This work systematically quantifies the effects of filler content on the kinetics, gelation and vitrification of a model silica-filled epoxy/amine system in order to begin to understand the role of the filler in IC packaging cure. At high cure temperatures (100°C and above) there appears to be no effect of fillers on cure kinetics and gelation and vitrification times. However, a decrease in the gelation and vitrification times and increase the reaction rate is seen with increasing filler content at low cure temperatures (60-90°C). An explanation for these results is given in terms of catalysation of the epoxy amine reaction by hydrogen donor species present on the silica surface and interfacial effects.

TTT cure diagram for an epoxy system diglycidyl ether of bisphenol A/1,2 diamine cyclohexane/calcium carbonate filler

Curing reactions of the epoxy system consisting of a diglycidyl ether of bisphenol A (BADGE n=0); 1,2 diaminecyclohexane (DCH); and calcium carbonate filler were studied to calculate a time–temperature-transformation (TTT) isothermal cure diagram for this system. Gel times were measured as a function of temperature using two experimental methods: solubility test and dynamic mechanical analysis (DMA). The results obtained are in fair agreement with that obtained for the two component system BADGE n=0/1,2 DCH. Vitrification times for the three component system was higher than those obtained for the system BADGE n=0/1,2 DCH and the conversions achieved were very close to maximum values. The activation energy for the overall polymerization reaction was calculated from the gel times obtained using the two above mentioned methods: solubility test (57.12 kJ/mol) and DMA (54.12 kJ/mol). These values are of the same order than those obtained for the two component epoxy system. It was established that the addition of calcium carbonate filler avoids the appearance of physical aging, thus making easier both the determination of glass transition temperatures, and the calculation of the TTT diagram.

Understanding vitrification during cure of epoxy resins using dynamic scanning calorimetry and rheological techniques

The gelation and vitrification transitions during cure of an epoxy-amine system are examined using rheological, modulated differential scanning calorimetry (DSC) and FTIR techniques. The results from dynamic mechanical analysis show that gelation is observed before vitrification at all temperatures where it can be rheologically defined. By comparing different rheological criteria for vitrification, it is seen that the vitrification transition is a gradual process that extends over a large part of cure at all temperatures where it occurs. Results from modulated DSC measurements show that the calorimetric vitrification times are longer than the vitrification times obtained from rheological measurements at cure temperatures above 100°C, but that at lower temperatures calorimetric vitrification occurs before, or at the same time as, rheological vitrification. Theoretical gelation times, estimated from FTIR conversion data, were found to be consistently shorter than the observed gelation times. Theoretical vitrification times agreed well with the observed times. The magnitude of the vitrification transition, expressed either as amount of change in heat capacity or as maximum value of loss tangent, was found to decrease approximately linearly with increasing cure temperature.

Study of the curing process of a vinyl ester resin by means of TSR and DMTA

The curing reaction of a vinyl ester resin (VER), using methyl ethyl ketone peroxide (MEKP) as initiator and cobalt hexanoate (HxCo) as promoter has been studied by thermal scanning rheometry (TSR) and dynamic-mechanical thermal analysis (DMTA) under isothermal conditions. The gel time, which is defined by several criteria, has been utilized to determine the apparent activation energy (Ea) of the process. Furthermore, an empirical model has been used to predict the change of complex viscosity ( η ∗) with time, and assuming a first order kinetics, a new value for Ea is obtained independent of HxCo concentration. Finally, the vitrification time has been obtained from DMTA experiments.

In vitro blastocyst formation of human oocytes obtained from unstimulated and stimulated cycles after vitrification at various maturational stages

Objective: To evaluate the developmental competence and chromosomal normality of oocytes vitrified at various times after maturation culture. Design: In vitro model study. Setting: A university-affiliated hospital. Patient(s): Unstimulated women who underwent cesarean section or oophorectomy and infertile women who underwent a long protocol of GnRH stimulation. Intervention(s): Retrieved oocytes were vitrified at 0 or 48 hours after culture in unstimulated cycles and at 0, 8–15, or 24–28 hours after culture in stimulated cycles. Main Outcome Measure(s): Postthaw morphologic normality, maturation, fertilization, cleavage, blastocyst formation, and chromosome number. Result(s): In the 53 oocytes that were obtained from unstimulated cycles, no statistically significant differences were found in rates of morphologic normality (range, 56%–63%) or fertilization (range, 31%–37%) according to the time of vitrification. In the 50 oocytes that were obtained from stimulated cycles, more of those that were vitrified at 24–28 hours were morphologically normal than those that were vitrified at 0 or 8–15 hours. Regardless of these differences, high cleavage rates (83%–100%) were obtained that did not differ significantly among the treatment groups. In both cycles, 20%–43% of cleaved oocytes developed to the blastocyst stage by 6 days after IVF. All the karyotyped blastocysts, three from unstimulated cycles and four from stimulated cycles, had a normal number of chromosomes. Conclusion(s): Vitrified and thawed oocytes from unstimulated or stimulated cycles developed to the blastocyst stage, regardless of when vitrification occurred; the number of chromosomes in the blastocysts was normal.

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.

Viscoelastic properties and transitions during thermal and UV cure of a methacrylate resin

AbstractThe solidification behavior of a dimethacrylate system during UV and thermal cure at temperatures from the Tg of the monomer (T) to above the ultimate Tg of the polymer (T) is investigated using torsional dynamic mechanical analysis. Gelation and vitrification times are quantified using several criteria for the transitions, and the data are presented as a function of cure temperature in TTT diagrams. The influence of cure method and temperature on the mechanical properties, degree of conversion, and gel content of the cured samples is discussed. The results show that gelation occurs before or at the same time as vitrification at all temperatures, including T. With UV cure, gelation generally occurred within seconds, whereas the properties continued to develop for minutes or hours afterwards. The method of cure was found to influence the properties of the cured material. When prepared above T, the UV cured material had a higher Tg, a higher crosslink density, and a higher degree of conversion than the thermally cured material.

Isothermal curing of an epoxy resin by alternating differential scanning calorimetry

The quasi-isothermal curing of a diepoxide resin with a triamine of polyoxypropylene was studied by alternating differential scanning calorimetry (ADSC), which is a temperature modulated DSC technique. The complex heat capacity measurements allows to analyse the vitrification process at curing temperatures ( T c) below the maximum glass transition of the fully cured epoxy ( T g ∞ =85.8°C). Initially, the modulus of the complex heat capacity, |C ∗ p| , increases until a maximum (conversion between 0.42 and 0.56) and then decreases. This step is followed by an abrupt decay of |C ∗ p| , due to the vitrification of the system, which allows the determination of the vitrification time. This value agrees well with that determined by the partial curing method. The phase angle and out-of-phase heat capacity show an asymmetric wide peak during the vitrification process. The change in |C ∗ p| at vitrification decreases with the increase of T c becoming zero at temperature T g ∞ . This epoxy-triamine system shows a delay of the vitrification process respect to other model epoxy systems probably due to the presence of polyoxypropylene chains in the network. The decay of |C ∗ p| during vitrification may be normalised between unity and zero by defining a mobility factor. This mobility factor has been used to simulate the reaction rate during the stage where the reaction is controlled by diffusion. The observed reaction rate is simulated by the product of the kinetic reaction rate, determined by the autocatalytic model, and the mobility factor.

Cure chemo-rheology of mixtures based on epoxy resins and ester cyanates

Tetrafunctional epoxy and ester cyanate resins were cured together at several temperatures. Mixtures were investigated at different stoichiometric ratios. FTIR investigation of these systems through curing allowed us to analyze the reaction pathway of these reactive systems. Isothermal measurements performed by FTIR spectroscopy indicated that the curing reactions were dependent on the stoichiometric ratio of the mixtures. Differential scanning calorimetry (DSC) showed the dilution effect of the epoxy resin in the cyclotrimerization of the catalyzed cyanate ester resin and also the existence of several reactions during polymerization of the mixtures. Rheological characterization of the curing process indicated an evident delay of the vitrification time as epoxy content in the mixtures increased.

Curing kinetics of phase separating epoxy thermosets studied by dielectric and calorimetric investigations: A simple model for the complex dielectric permittivity

Dielectric relaxation spectroscopy (3 kHz ≤ ƒ ≤ 3 MHz), differential scanning calorimetry, and temperature-modulated calorimetry have been performed during isothermal curing of an epoxy network (diglycidylether of bisphenol A crosslinked with diaminodiphenyl methane), and of two thermoplast modified epoxy resins (semi-interpenetrating polymer networks) consisting of the epoxy network component and different amounts (10 and 20 wt %) of a linear high Tg polymer (polysulfone or polyethersulfone). During reaction, the homogeneous-mixtures phase separate into an epoxy-rich and a linear polymer-rich phase with different mobilities of the electrical dipoles. The complex dielectric permittivity is composed of a contribution from the ionic dc-conductivity and a contribution from relaxations of the permanent electrical dipoles in the two phases. The decrease of the dc-conductivity in the initial stage of cure is related to the time for gelation or vitrification. The contribution of the dipole relaxations to the dielectric permittivity reflects an increase of the relaxation times with curing time for both phases. The time-dependent changes in the complex dielectric permittivity are described by a simple two-phase model based on two Havriliak–Negami functions combined with Vogel–Fulcher equations for the description of the curing-time dependence of the relaxation times. The increase of the relaxation times in the phases during isothermal curing is incorporated by time-dependent Vogel temperatures. The latter are related to the time evolution of the glass-transition temperatures in the two phases measured independently by calorimetry. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2461–2470, 1998

Effect of prereaction on curing of elastomer-modified epoxy resins

Three multifunctional epoxy resins novolac epoxy (EPN), triglycidyl paraaminophenol (TGPAP), and tetraglycidyl diaminodiphenylmethane (TGDDM) were modified with a carboxy-terminated acrylate-based liquid elastomer (PnBA). The effect of prereaction of the epoxy groups in the resin and carboxyl group in the elastomer on curing parameters, such as the cure reaction conversions; processing parameters, such as gel times and verification times; initial cure temperatures, and activation energy values were evaluated. Fourier transform infrared spectrometry (FTIR), dynamic mechanical spectrometry (DMS), and differential scanning calorimetry (DSC) were used for the study. The results indicated that while the cure reactions were slightly retarded in the presence of elastomer, they were slightly accelerated when certain prereaction catalysts were used. The gel times and vitrification times decreased due to the prereaction. The physical properties such as fracture toughness and modulus of neat resin castings and flexural strength properties of carbon cloth-reinforced laminates were evaluated. Results indicated that the toughness property increased and the modulus decreased with elastomer addition.

Kinetic and rheological relationships of dicyanate ester polycondensation for the resin transfer molding process

AbstractKinetic and rheological behavior of a liquid dicyanate ester catalyzed by a copper salt was studied with a view to controlling the RTM process. The kinetics are satisfactorily modeled with a simple second order model until vitrification occurs. Independent of the catalyst concentration, the relationship between glass transition temperature Tg and conversion x was well validated, indicating that vitrification occurs well before Tg(x) reaches the curing temperature. The rheological study allowed gelation (tanδ crossover) and vitrification times at different curing temperatures to be determined. The values of tanδ at gel point were found to be slightly higher than those usually found. In order to carry out a simulation of the RTM process, viscosity and conversion were also linked with an empirical model.

Determination of gel and vitrification times of thermoset curing process by means of TMA, DMTA and DSC techniques

In the present work, gelation and vitrification experimental data are obtained by TMA and DMTA techniques using the same thermoset based on an epoxy-amine system. The results show that the times obtained are not equivalent and depend on the technique used. An attempt has been made to compare both determinations using the degree of cure obtained by means of DSC technique. The principal conclusion that we want to emphasize is that it is the conversion degree and not the time of the phenomenological changes that take place during cure, that is the link to connect and interrelate the results obtained with different techniques. A method is also described for constructing the TTT diagram with only DSC and TMA or DMTA data.