Cure kinetics and shrinkage model for epoxy-amine systems

Manufacture of most of epoxy resins implies that cure needs to be carried out under pressure. Due to the significance of knowing the influence of the pressure factor in cure kinetics, cure shrinkage of a stoichiometric epoxy-amine system was measured using a pressure-volume-temperature (PVT) analyzer. Recording the specific volume change in the range of temperature from 100 to 180 °C and a pressure of 200 bar we could model the cure kinetics. The Runge-Kutta method was applied to obtain the kinetic constants of the cure reaction. In addition, using the differential scanning calorimeter (DSC) for measurements of 1 bar and the PVT analyzer for pressures of 200, 400, and 600 bar, we also model the kinetic constants as a function of pressure. The results obtained show that the effect of the temperature on the kinetic constants is higher than the effect of pressure. Therefore, both PVT and DSC are complementary techniques to describe the full range of cure kinetic process of epoxy mixtures.     

An empirically derived model for further increasing microwave curing rates of epoxy-amine polymerizations

The reaction rates of common epoxy resins with diamine crosslinking agents in uniform microwave fields have been compared according to a variety of structural features. A statistically designed experimental matrix was used to determine that the curing rates were linearly dependent on only two significant variables, amine basicity, and degrees of rotational freedom (entropy) of the reactants. Surprisingly, the molecular polarizability, which is commonly understood to be responsible for the transfer of microwave electromagnetic energy to molecules with permanent dipoles, had no significant effect even as a dependent variable. A very high probability model was produced that accurately predicts the reactivities of epoxide and diamine reactants with respect to specific structural features. Further evidence is provided for a dominant linear pregelation polymerization and a uniform microwave reaction field.     

Fortifiers for epoxy-amine systems

The use of a range of additives is described which can increase the strength and modulus of cured epoxy-amine systems by up to 70 per cent. These additives, called ‘Epoxy Fortifiers’, are liquids or low melting point solids which are incorporated into the resincuring agent mixture, prior to curing, at loadings of 10–40 parts per hundred of resin.     

Real-time dielectric studies of network formation in thermally activated epoxy-amine and isocyanate-triol systems

Dielectric measurements are reported on the changes that occur in the nature of the dipole relaxation processes during cure of an epoxy-amine and a diisocyanate-triol system. The epoxy resin forms a vitrified solid in the final cured state, whereas the urethane retains its elastomeric properties. The initial behaviour for the epoxy resin is dominated by ionic conduction processes. Subtraction of the conductivity contribution reveals a dipolar relaxation process, which is analysed using the Havriliak-Negami (HN) equation. The characteristic exponent 1 - n of the HN equation changes in a similar manner to that found with other epoxy-amine systems. However, the dipolar process in the urethane occurs at very high frequency and a different form of the 1 - n dependence is observed. Sensitivity of the dielectric method for the detection of vitrification and gelation is critically assessed. If the vitrification is allowed to occur, then detection of a gel point may not necessarily be inferred from the dielectric data.     

Semianalytical solution for epoxy-amine curing in the presence of nonisothermal effects

The curing of epoxy-amine polymer has been modeled by the reaction of functional groups, and the mole balance of these is governed by a set of six nonlinear differential equations. In this work, we have first developed a complete analytical solution for isothermal curing. For nonisothermal curing, we have divided the domain of hydroxyl group concentration β into small increments Δβ and adopted our analytical results for this domain. In addition, we solved the energy balance equation analytically and obtained the temperature rise for Δβ. We have compared our results with those obtained from the Runge Kutta numerical solutions. We have shown that our semianalytical technique is about a thousand times more efficient and faster.     

Dielectric and viscoelastic studies of poly(ethyl methacrylate) containing cholesteric compounds

Poly(ethyl methacrylate) (PEMA) containing small amounts of cholesteryl additives was studied by dielectric and dynamic mechanical spectroscopy. Dielectric data from pure PEMA and PEMA + additive systems were fitted to the WLF equation. Using the WLF constants obtained from the data for pure PEMA, the To values for PEMA + additive systems were determined in order to get the best fit of the experimental points to the WLF curve. Values of tan δmax (dielectric and mechanical) for all the PEMA + additive systems shift to higher temperatures compared with that for pure PEMA. In the glassy region, the moduli of PEMA + additive systems are higher than that of pure PEMA. Viscometric studies of PEMA + additive systems indicate the presence of some interaction. FTIR spectra of the polymer + additive systems show no shift in carbonyl bond frequency. The shift of Tg to higher temperatures indicates that chain motion in this region is hindered greatly by these additives and the reason for this may be the fitting or aligning of the additive molecules in the polymer matrix.     

Dielectric modeling of the curing process

Dielectric data from an epoxy resin system were used as the foundation for dielectric modeling of the curing process. This resin system (DGEBA-polyamide) was chosen as an easily processible model system. Dielectric average relaxation times, defined as the reciprocal of the angular frequency at which the loss component of the dielectric constant reaches a maximum, were determined for a 40°C isothermal cure. The changes in the average relaxation time through the cure exhibited similar behavior to those for viscosity, which inspired the correlation of the two properties. The dielectric relaxation time was modeled using a six-parameter model analogous to that used for viscosity. The model parameters were in turn associated with both intrinsic properties of the system and reaction kinetics describing the cure. The possibility of extending the relaxation time model for use with single-frequency data by means of a time-frequency correlation was also investigated. Combined, these two modeling methodologies provide a powerful constitutive approach for describing dielectric properties of thermosetting polymers during cure.     

Dielectric studies of the cure of epoxy matrix systems

Dielectric spectroscopy was used to monitor the curing process of two epoxy resin systems. The basic system (system I) consisted of DGEBA (a difunctional epoxy) and a polyamide in a 50–phr mixture. In addition, a comparative analysis was performed on a high–performance resin system (system II) used primarily in unidirectional composite applications. This system contained TGDDM (a tetrafunctional epoxy) and DDS (a tetrafunctional amine) in a 25–phr mixture. The dielectric data were obtained using a simple yet functional sample cell electrode designed and constructed in the laboratory. For system I, isothermal dielectric data were used to determine apparent activation energies for the temperature range from 22 to 70°C. The data showed that the activation energy was a function of temperature and increased as the temperature of the cure increased. This indicated that the reaction mechanism was also a function of temperature. For system II, data were collected between 140 and 190°C and an overall activation energy for that temperature range was determined. The overall activation energies for both systems, calculated using dielectric spectroscopy, compared favorably to those obtained using differential scanning calorimetry. Also, using a wider frequency range (240 Hz to 2 MHz), Argand diagrams were constructed and modeled with the Cole–Cole empirical equation for systems with a distribution of relaxation times. This justified the calculation of average relaxation times, which could then be related to the bulk physical properties of the polymer, such as viscosity. Modified Argand diagrams, where ε″ is plotted against ε′ at one frequency as a function of time, were also constructed, which aided in the understanding of the curing processes for these thermosetting systems.     

Dielectric studies of three epoxy resin systems during microwave cure

Dielectric properties of three curing epoxy resin systems at an industrial microwave frequency, 2.45 GHz, were measured over a temperature range lower than the cure temperature. Extent of cure, which is determined by DSC, is used to describe the progress of the polymerization. It has been found that, normally, the real and the imaginary part of the complex dielectric constant increased with temperature and decreased during microwave cure. The changes of the dielectric properties during the reaction are related to the decreasing number of the dipolar groups in the reactants and the increasing viscosity. The Davidson-Cole model can be used to describe the experimental data. The Zong model is applicable to polymeric materials at high microwave frequencies and can be used to calculate the parameters of the Davidson-Cole model. The epoxy resins exhibit one γ relaxation, which can be described by the Arrhenius rate law. The evolutions of the parameters in the models are discussed.     

Studies on curing behavior and molecular motion of polyacrylate/epoxy-amine adduct complex system with additional crosslinker

The curing behavior of a three-component system—polyacrylate I, epoxy-amine adduct II, and polyglycol-2-ethylhexanol–blocked 2,4-toluene diisocyanate III (the crosslinker)—was studied by torsional braid analysis and in situ Fourier transform infrared (FTIR). Results show that the curing process consists of two close steps. The first step is a deblocking and curing period (the main period), in which the deblocked isocyanate group reacted with the hydroxyl in polyacrylate and epoxy-amine adduct to obtain a crosslinked network structure. The second step is a deep-curing period. The further deblocked isocyanate group reacted with NH group in urethane and the crosslinking density increased. The curing temperature of the first step could be lowered efficiently when the organotin catalyst was added into the system, and the curing time was shortened. Furthermore, the effect on the curing crosslinking extent of each system was studied when the types or content of the crosslinkers changed. The results show that, when the crosslinking density increased, the mutual molecular motion became more difficult and the glass temperatures (T_g) were heightened. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 247–254, 1998     

Microdielectric study of epoxy-amine systems: Gelation and relationships between conductivity and kinetics

A low-T_g epoxy-amine system, based on the diglycidyl ether of butane-1,4-diol (DGEBD) and 4,9-dioxadodecane-1,12-diamine (4D), and a high-T_g epoxy-amine system, based on diglycidyl ether of bisphenol A (DGEBA) and 4,4´-methylenebis(3-chloro-2,6-diethylaniline) (MCDEA), were studied during isothermal curing by means of microdielectrometry. The first one, which was investigated from 40°C to 60°C, exhibits only a gelation phenomenon. The latter was studied from 80°C to 150°C. At these temperatures, gelation and vitrification phenomena occur. To observe gelation phenomenon from the dielectric curves, three different approaches were developed (inflexion point as gelation criterion, percolation theory, and correlation between conductivity and viscosity) which fail to describe the complete evolution of the epoxy-amine reactive systems during cure. This is due to the fact that at the gel point the macroscopic viscosity diverges whereas the conductivity values involve the ion motions, thus the local viscosities. So, there is no manifestation of gelation in the dielectric curves. The values of logσ/logσ₀ vs. conversion x, where σ₀ is the conductivity of the initial monomer mixture, give a single curve which can be fitted by a model proposed recently. In addition, a linear relation exists between log σ(x)/log σ₀ and glass transition temperature, T_g(x). Thus, the combination of these relations with the modified Di Benedetto equation allows predicting kinetic and dielectric behaviors knowing the glass transition temperatures, the heat capacities, and the conductivities of the initial monomer mixture and of the fully cured network.     

Dielectric characterization of relaxation and curing behavior of oriental lacquer

Dielectric analysis was used to investigate the effects of temperature and humidity on the curing behavior of oriental lacquer and to characterize the dielectric properties of the lacquer film. It was found that the oriental lacquer could not cure to its hardened state at relative humidity less than 50% in ambient temperature and that the cure time could be shortened tremendously by increasing the curing temperature. In order to study the dielectric properties of oriental lacquer film, two films were prepared at different curing temperatures. The glass transition and secondary relaxation temperatures of ordinary oriental lacquer film, room temperature cured purified lacquer, were observed at 45 and −40°C, respectively. The high temperature cured purified lacquer film showed a secondary relaxation at around −50°C. The relationship between thermodynamic properties and chemical structures was explored based on the analysis of the dielectric relaxation behavior using Cole–Cole plots and the dielectric relaxation intensity Δϵ. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1804–1810, 2000     

Dielectric method for monitoring the curing of polyester-melamine coatings

A method has been developed for monitoring the curing of coatings by means of dielectric measurements. The features of the method have been demonstrated by monitoring the curing of polyester-melamine coatings.

A sensor has also been developed, consisting of two comb-electrodes for the dielectric measurements and a temperature-dependent resistance for temperature measurements applied on a glass substrate. This has allowed the use of a thin layer of clear or pigmented coating and therefore approaches coating application practice. During curing, both dielectric properties (the dielectric permittivity and loss) and temperature have been measured.

From the dielectric data, the electrical conductivity has been calculated. The conductivity has been shown to depend on the degree of curing as well as on the temperature and the amount of solvent in the coating. Most of the solvent was evaporated before curing started. The increase in the glass transition temperature of the coating during curing has been determined using the WLF equation and the data for uncured and fully cured coatings. The glass transition temperature has been used as a measure for the extent of the reaction.

Curing of two different polyester-melamine coating systems has been monitored. The first system was cured both quasi-isothermally at 130 °C and at four different heating rates. The second system, a coil-coating system, was developed for fast curing at high temperatures. This system was cured at four different heating rates, and also flash cured by placing for a few minutes in an oven at a temperature above 250 °C.

For both isothermal curing and curing at a constant heating rate, the extent of the reaction has been determined with time. Assuming simple kinetics, the reaction constant and the order of the reaction have been determined.

It is concluded that the dielectric method is fast and convenient for monitoring curing, under conditions approaching coating application practice.     

Dielectric monitoring of the curing process in cyanoacrylate resin

The cure monitoring of cyanoacrylate resin using a dielectric technique shows that in addition to the usual fast anionic polymerization step, there may be a secondary process involved over longer time scales. Two peaks are distinctly observed in the isothermal curing curves, and from the position of these at various temperatures, the activation energies of 0.4 eV and 0.17 eV could be derived. In cured cyanoacrylate resin, three relaxations were observed, viz., α, β, and γ, occurring at 152°c, 87°c, and 47°c, respectively (for 1 KHz measuring frequency), with activation energies of 1.3 eV, 0.56 eV, and 0.4 eV, respectively. The various results have been explained on the basis of secondary bond formation through the nitrile group, for which some evidence could be obtained from the infrared absorption spectra.     

Dielectric and viscoelastic relaxations in poly(dicyclohexyl itaconate)

The relaxation spectrum of poly(dicyclohexyl itaconate) (PDCHI) was studied by dynamic mechanical, DC dielectric and thermally stimulated current measurements. Four relaxations, α, β, γ and δ, were obtained. The only method by which all four peaks were observed is that of dynamic dielectric measurements because of the broad range of frequencies employed. The β, γ and δ relaxations were characterized by the activation energy in a relaxation map. A tentative explanation of the molecular origin of each absorption is proposed. In the case of the α relaxation we have used two transformations, one from the permittivity to polarizability and the second from compliance to deformability in order to make evident the existence of this relaxation.     

Change of viscoelastic properties of epoxy resin in the curing process

This paper is concerned with the relation between the time and temperature dependences of the flexural properties and the curing conditions for the bisphenol A-type epoxy resin with acid anhydride hardener. Relaxation moduli of epoxy resin, prepared at several curing temperatures and times, were measured in the temperature range from T_g ?70°C to T_g. The master curves of relaxation modulus for the epoxy resin could be constructed, using their thermorheological simple properties. The time–temperature shift factors of the epoxy resin could be approximately expressed by the Arrhenius equation with the activation energy 59.4 kcal/mole. independent of its curing conditions. The curing time and temperature were equivalent, that is, the short curing time at high temperature corresponded to the long curing time at low temperature. The curing time–temperature shift factor could be approximately expressed by the Arrhenius equation with the activation energy 21.3 kcal/mole, which was higher than the activation energy 14.2 kcal/mole obtained in the measurements of gel times. The increase in the values shows that the temperature dependences of reaction rates increase with progressing gelation.     

Particle clustering in photocurable nanocomposites: Dependence of curing kinetics and viscoelastic properties

The aim of this article is to investigate the effect of nanoparticle clustering on the mobility of nanoparticles in nanocomposites, using spectroscopic methods (Brillouin and Raman). Special attention is paid to the effect of particle clustering on photocuring kinetics. The model system was poly(2‐hydroxyethyl acrylate) filled with fumed nanosilica in concentration range encompassing the percolation threshold. Results obtained from Brillouin spectroscopy show substantial changes in the sound velocity and the attenuation coefficient with increasing filler content. The damping of acoustic waves reaches the maximum at the percolation threshold (～15 wt %), which is related to changes in the mechanism of acoustic wave propagation. The formation of the cocontinuous silica phase strongly affects the curing kinetics of the monomer/silica system: the polymerization rate is the highest at a silica content corresponding to the percolation threshold. These results correlate well with the results of AFM surface roughness analysis. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39895.     

TBA studies of prepreg curing behavior

The torsional braid analysis (TBA) equipment has been used as an automated torsion pendulum to characterize prepreg materials in the form of single ply strips (2-1/2 × 1/8 in.). Compared to the use of coated glass braids, the main difference was a marked weakening of the gelation mechanical damping peak in isothermal runs. However, prepreg materials consisting of epoxy resins on glass, carbon or aramid fibers were successfully run isothermally to provide gelation and vitrification times as a function of temperature, or in constant heating rate scans to reveal the T < T_g and the T_g relaxations of the uncured resins, and at higher temperatures phenomena associated with gelation, vitrification and devitrification.