Time–temperature–transformation (TTT) cure diagrams: Relationship between Tg and the temperature and time of cure for epoxy systems

A procedure is provided for estimating the time to full cure vs. isothermal cure temperature for vitrified epoxy systems. An equation relating the glass transition temperature of vitrified epoxy systems to the time and temperature of cure is developed.     

Time–temperature–transformation (TTT) cure diagrams: Relationship between Tg and the temperature and time of cure for a polyamic acid/polyimide system

Glass transition temperatures (T_g) were obtained vs. isothermal temperature (T_cure) and time of cure for a polyamic acid/polyimide system. A time–temperature–transformation (TTT) isothermal cure diagram was constructed to include the time to vitrification and iso-T_g curves. As for expoxies, the relationship between T_cure and the time to vitrification is S-shaped. Plots of T_g vs. T_cure show that solvent evaporation and chemical reaction are controlled by vitrification.     

Time-temperature-transformation (TTT) cure diagrams: Relationships between Tg,cure temperature,and time for DGEBA/DETA systems

Isothermal curing of a bisphenol A diglycidyl ether-based epoxy-resin-based, using an aliphatic polyamine, has been performed at temperatures between 20 and 60°C. Samples were cured isothermally at various intervals of time, and analyzed by differential scanning calorimetry (DSC). The glass transition temperature (T_g) and the conversion ratio cure determined by residual enthalpy analysis is used as an isothermal cure-controlled reaction. A time-temperature-transformation (TTT) isothermal cure diagram was carried out to include the time to vitrification and iso-T_g curves. © 1996 John Wiley & Sons, Inc.     

Competition between cure and thermal degradation in a high Tg epoxy system: Effect of time and temperature of isothermal cure on the glass transition temperature

The isothermal cure of a diglycidyl ether of bisphenol A with a tetrafunctional aromatic diamine has been studied in an attempt to achieve full cure (maximum glass transition temperature, T_g∞, ca. 170°C). Since high temperatures of cure are necessary for high T_g∞ systems (because of low reaction rates after vitrification), cure and thermal degradation reactions often compete. In this work T_g is used as a direct measure of conversion. An approach leading to a series of iso-T_g contours in a temperature vs. time transformation (TTT) diagram, which can be used to design time-temperature cure paths leading to particular values of T_g, is discussed.     

Time–temperature–transformation (TTT) cure diagram: Modeling the cure behavior of thermosets

The times to gelation and to vitrification for the isothermal cure of an amine-cured epoxy (Epon 828/PACM-20) have been measured on macroscopic and molecular levels by dynamic mechanical spectrometry (torsional braid analysis and Rheometrics dynamic spectrometer), infrared spectroscopy, and gel fraction experiments. The relationships between the extents of conversion at gelation and at vitrification and the isothermal cure temperature form the basis of a theoretical model of the time–temperature–transformation (TTT) cure diagram, in which the times to gelation and to vitrification during isothermal cure versus temperature are predicted. The model demonstrates that the “S” shape of the vitrification curve depends on the reaction kinetics, as well as on the physical parameters of the system, i.e., the glass transition temperatures of the uncured resin (T_g0), the fully cured resin (T_g∞), and the gel (_gelT_g). The bulk viscosity of a reactive system prior to gelation and/or vitrification is also described.     

Evolution of properties of an isocyanate/epoxy thermosetting system during cure: Continuous heating (CHT) and isothermal time—temperature—transformation (TTT) cure diagrams

The present article describes a methodology for examining the evolution of the properties vs. cure of a complex thermosetting isocyanate/epoxy reactive mixture which reacts through two consecutive but separable reaction regimes. The methodology is based on the use of the torsional braid analysis (TBA) technique and the continuous heating (CHT) and isothermal time—temperature—transformation (TTT) cure diagrams. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 15–25, 1997     

A methodology for characterizing reactive coatings: Time–temperature–transformation (TTT) analysis of the competition between cure,evaporation, and thermal degradation for an epoxy-phenolic system

A methodology, in terms of a macroscopic isothermal time–temperature–transformation (TTT) diagram, is presented for characterizing the changes that occur isothermally in a reactive thermosetting system in which evaporation, cure, and thermal degradation occur. Iso-weight loss contours show the progress of the loss of volatile material (mostly solvent). The gelation contour corresponds to the macroscopic viscosity rising to a definite level. The vitrification contour corresponds to the glass transition temperature (T_g) rising to the temperature of cure (T_cure). Iso-T_g contours show the progress of cure in terms of the easily measured T_g (rather than chemical conversion). The iso-T_g contours also show the influence of thermal degradation competing with cure. Degradation is responsible for difficulties in assigning the glass transition temperature of the fully cured material (i.e., T_g∞).     

Method for time–temperature–transformation diagrams using DSC data: Linseed aliphatic epoxy resin

A novel method to generate time–temperature–transformation (TTT) diagrams from Differential Scanning Calorimetry (DSC) data is presented. The methodology starts with dynamical DSC information to obtain the total transformation heat, followed by an isothermal‐dynamic temperature ramp that allows the inclusion of diffusion‐controlled reaction kinetic. The cure kinetics is modeled using an auto‐catalytic Kamal–Sourour model, complemented with a Kissinger model that allows the direct prediction of one energy of activation, DiBenedetto's equation for the glass transition temperature as a function of the cure degree and adjusted reaction constants to include diffusion mechanisms. The methodology uses a nonlinear least‐squares regression method following J.P. Hernández‐Ortiz and T.A. Osswald's methodology (J. Polym. Eng. 2004, 25, 23). A typical linseed epoxy resin (EP) presents two different kinetics control mechanisms, thereby providing a good model to validate the proposed experimental and theoretical method. TTT diagrams for EPs at two different accelerator concentrations are calculated from direct integration of the kinetic model. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40566.     

Theoretical evaluation and the effect of thermal degradation on the Time–Temperature–Transformation (TTT) diagram of bisphenol A epoxies

A brief discussion about the Time–Temperature–Transformation (TTT) diagram is presented. Using diamino diphenylsulfone‐cured diglycidyl bisphenol A as a representative example, its TTT diagram is completed by including the thermal degradation. The theoretical diagram as obtained from the kinetics of curing and thermal degradation is compared with experimental data. The agreement is good, slight deviations are observed only in the time to vitrify above 150°C and the maximum available glass temperature, which is due to side reactions and onset of thermal degradation during curing. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3894–3896, 2003     

Time–temperature–transformation cure diagrams of phenol–formaldehyde and lignin–phenol–formaldehyde novolac resins

In this study, the time–temperature– transformation (TTT) cure diagrams of the curing processes of several novolac resins were determined. Each diagram corresponded to a mixture of commercial phenol–formaldehyde novolac, lignin–phenol–formaldehyde novolac, and methylolated lignin–phenol–formaldehyde novolac resins with hexamethylenetetramine as a curing agent. Thermomechanical analysis and differential scanning calorimetry techniques were applied to study the resin gelation and the kinetics of the curing process to obtain the isoconversional curves. The temperature at which the material gelled and vitrified [the glass‐transition temperature at the gel point (_gelT_g)], the glass‐transition temperature of the uncured material (without crosslinking; T_g0), and the glass‐transition temperature with full crosslinking were also obtained. On the basis of the measured of conversion degree at gelation, the approximate glass‐transition temperature/conversion relationship, and the thermokinetic results of the curing process of the resins, TTT cure diagrams of the novolac samples were constructed. The TTT diagrams showed that the lignin–novolac and methylolated lignin–novolac resins presented lower T_g0 and _gelT_g values than the commercial resin. The TTT diagram is a suitable tool for understanding novolac resin behavior during the isothermal curing process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Master curve and time–temperature–transformation cure diagram of a polyfunctional epoxy acrylic resin

The curing reaction of a well‐defined glycidyl methacrylate‐co‐butyl acrylate statistical copolymer, prepared by atom transfer radical polymerization, and a commercial linear diamine (Jeffamine D‐230) was studied with the objectives of constructing and discussing a time–temperature–transformation isothermal curing for this system. Thermal and rheological analyses were used to obtain the gelation and vitrification times. Differential scanning calorimetry data showed a one‐to‐one relationship between the glass‐transition temperature (T_g) and fractional conversion independent of the cure temperature. As a result, T_g was used as a measurement of conversion. We obtained a kinetically controlled master curve for isothermal curing temperatures from 50 to 100°C by shifting T_g versus the natural logarithm time data to a reference temperature of 80°C. We calculated the apparent activation energy by applying two different methods, gel time measurements versus shift factors, suggesting a good agreement between them. Isoconversion contours were calculated by the numerical integration of the kinetic model. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Low temperature cure of epoxy thermosets attaining high Tg using a uniform microwave field

It is demonstrated for the first time that an epoxy thermoset resin can be cured at temperatures well below its T_g∞. This study compared the use of a uniform variable frequency microwave (VFM) field to standard oven curing at temperatures above and below T_g∞. Using T_g, tan δ, modulus, and FTIR measurements, it is shown that the reaction of BFDGE with MDA to attain a product with T_g∞ of 133 °C is achieved by VFM at temperatures from 100 to 140 °C; in contrast, the thermal cure normally requires 170 °C to attain the same T_g∞ and the same extent of cure. By following the pregel cure reaction with ¹³C‐NMR spectroscopy, it was determined that the lower cure temperatures of VFM cure predominately lead to chain extension and smaller amounts of crosslinking compared to the thermal cure. To explain these results, it is suggested that, after gelation, with VFM cure there is higher mobility from dipole rotations that continues the cure to completion without vitrification. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44222.     

Chitin derivatives. II. Time–temperature–transformation cure diagrams of the chitosan amidization process

The transformation of the salts of chitosan with acetic and propionic acid, chitosonium acetate and chitosonium propionate, into chitin or the respective homolog of amidized chitosan has been described on the basis of time–temperature–transformation (TTT) cure diagrams. The time to vitrification at various isothermal cure temperatures (T_c) was determined using dynamic mechanical thermal analysis. The time to full cure was derived using a T_g–T_c cure time relationship according to the method of Peng and Gillham, as well as by an extrapolation procedure. Consequently, TTT cure diagrams describing the temperature-driven regeneration process include full cure and vitrification curves. As in thermosets, this transformation displays an S-shaped vitrification curve, and the time to full cure increases with decreasing cure temperature. The time to full cure is very remote from the time to vitrification, and this is attributed to the tendency of vitrification to prevent full cure from being attained. The activation energies for vitrification of chitosonium acetate and chitosonium propionate derived from an Arrhenius equation are similar. This suggests that the same mechanism governs glass formation in the N-acetyl and N-propionyl-glucosamine derivatives. Additionally, the morphology of amidized chitosan and native chitin was examined using X-ray diffraction and FTIR analysis. X-ray diffraction results indicate that amidized chitosan is an amorphous material, whereas native chitin is crystalline. FTIR suggests the existence of hydrogen-bonded amide groups in native chitin but not in amidized chitosan. This difference in morphology between amidized chitosan and native chitin is accounted for in terms of the influence of glass formation in the former. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1879–1889, 1999     

Effect of time-temperature path of cure on the water absorption of high Tg epoxy resins

The effect of different time—temperature paths of cure on the water absorption of high T_g epoxy resins has been investigated. The resins were cured isothermally for different times, with the following results: as extent of cure increased, the glass transition temperature (T_g) increased, the room temperature (RT) modulus decreased, the RT density decreased, the RT diffusion coefficient appeared to decrease, and the RT water absorption increased. The decrease in RT density is related to an increase in free volume, which controls the amount of water absorbed. A qualitative model accounts for the increase in RT free volume with increasing cure. The model is based on a restricted decrease of free volume on cure due to the rigid molecular segments in the cured resin systems. The sorption isotherms can be characterized by the dual mode theory at low activities but at high activities the sorption is complicated by penetrant clustering. A thermodynamic approach, independent of the absorption model, can correlate sorption data at different temperatures. The diglycidyl resin was also cured for extended times at three temperatures, in an effort to achieve full cure at each temperature. For these, the higher the cure temperature, the lower the RT density, which could result from the lower initial density of materials cured at higher temperatures. The equilibrium water absorption increased with increasing cure temperature, consistent with the decrease in RT density. The systems studied were a diglycidyl ether of bisphenol A cured with an aromatic tetrafunctional diamine, trimethylene glycol di-p-aminobenzoate (T_g∞ = 156°C), and a triglycidyl ether of tris(hydroxyphenyl)methane cured with the same amine (T_g∞ = 268°C).     

Comparison of microwave and thermal cure of epoxy resins

Stoichiometric mixtures of DGEBA (diglycidyl ether of bisphenol A)/DDS (diaminodiphenyl sulfone) and DGEBA/mPDA (meta phenylene diamine) have been isothermally cured by electromagnetic radiation and conventional heating using thin film sample configurations. Fourier transform infrared spectroscopy (FTIR) was used to measure the extent of cure. Thermal mechanical analysis (TMA) was used to determine the glass transition temperatures directly from the cured thin film samples. Well-defined glass transitions were observed in the TMA thermograph for both thermal and microwave cured samples. Significant increases in the reaction rates have been observed in the microwave cured DGEBA/DDS samples. Only slight increases in the reaction rates have been observed in the microwave cured DGEBA/mPDA samples. Higher glass transition temperatures were obtained in microwave cured samples compared to those of thermally cured ones after gelation. The magnitude of increases of glass transition temperature is much larger for the DGEBA/DDS system than DGEBA/mPDA system. The microwave radiation effect was much more significant in DGEBA/DDS system than in DGEBA/mPDA system. DiBenedetto's model was used to fit the experimental T_g data of both thermal and microwave cured epoxy resins.     

New thermosets obtained by cationic copolymerization of DGEBA with γ‐caprolactone with improvement in the shrinkage. II. Time–temperature–transformation (TTT) cure diagram

Mixtures of diglycidylether of bisphenol A (DGEBA) with different proportions of γ‐caprolactone (γ‐CL) were cured with ytterbium triflate as initiator. The curing was studied with differential scanning calorimetry (DSC) and thermo mechanical analysis (TMA). The results are presented in the form of a time–temperature–transformation diagram. The kinetic analysis was performed by means of the isoconversional integral procedure and the kinetic model was also determined using the Coats–Redfern method. Gelation was determined by means of combined experiences of DSC and TMA. The relationship between the glass transition temperature (T_g) and the degree of conversion α was determined by DSC. Using the isoconversional lines and the T_g‐α relationship, the vitrificacion curve was obtained. The methodology developed makes it possible to obtain the TTT diagram using only no‐isothermal experiments with equivalent results to those using classical isothermal procedures. The addition of γ‐CL accelerates the curing and reduces the shrinkage after gelation and consequently the internal stresses in the material. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Modification of a two‐component system by introducing an epoxy‐reactive diluent: Construction of a time–temperature–transformation (TTT) diagram

Curing reactions of a three‐component system consisting of an epoxy resin diglycidyl ether of bisphenol A (DGEBA n = 0), 1,2‐diaminecyclohexane as curing agent, and vinylcyclohexene dioxide as a reactive diluent were studied to calculate a time–temperature–transformation isothermal cure diagram for this system. 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 α, 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 (58.5 ± 1.3 kJ/mol). This value was similar to the results obtained for other similar epoxy systems. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1190–1198, 2004     

Study of epoxy and epoxy–cyanate networks thermal degradation to predict materials lifetime in use conditions

Composite materials are used more and more for aeronautical applications and they have to be as thermally stable as possible. The thermostability of carbon‐fiber/epoxy–cyanate composites elaborated with an autoadhesive and autoextinguish prepreg were tested. Dynamical and isothermal aging tests were carried out to evaluate the composite thermal stability. Thermal degradation products were identified by chromatography/mass spectrometry analysis and the results obtained were compared with data known on the material network structure. The physicochemical network structure evolutions and the thermal aging data are correlated with the interlaminar shear strength (ILSS) mechanical results. For epoxy–cyanate composites, cracking appears after a longer time of aging than for epoxy composites. This new epoxy–cyanate material isothermal stability seems to be good and particularly good if it was postcured after processing. The comparison of chemical, mechanical, and crack formation results obtained by accelerated aging tests allowed us to determine models to predict long‐term behavior. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3142–3153, 2000     

Reaction during cure of a blocked isocyanate–epoxy resin adhesive

Model compound reactions of isocyanate sources with alcohols and an epoxy resin indicated that the major reaction product from the phenol-blocked methylenebisphenylene diisocyanate and epoxy resin-based adhesive dip for poly(ethylene terephthalate) cord was a polyurethane. A significant portion of the hydroxyl groups required for the reaction were formed by ring opening of the epoxide groups of the resin. The reaction rate for the unblocking of the isocyanate source was inhibited in the presence of polyester yarn finishes containing sulfated esters of fatty acids. Also, compounds containing carboxylic acid groups and sulfonic acid groups inhibited the unblocking step. Amines and their salts catalyzed the unblocking step. A mechanism for the polyurethane adhesive–polyester bond based upon physical interaction is postulated. The presence of certain nonsulfated ester finishes permitted good wetting of the polyester surface and penetration of the adhesive into the polyester. By contrast, sulfated ester finishes result in poor wetting and penetration by the adhesive on the polyester. By contrast, sulfated ester finishes result in poor wetting and penetration by the adhesive on the polyester. The latter finishes resulted in a weak boundary layer between the adhesive and the cord.