Residual Heat Of Reaction Research Articles

Effect of aging on the performance of solvent-based self-healing materials

Solvent healing of thermoset polymer matrices was suggested as an efficient and cost-effective alternative to other encapsulated self-healing systems. However, self-healing efficiency is not guaranteed over time. We report here on the autonomic self-healing of under-cured epoxy with ethyl phenylacetate-filled microcapsules under different aging conditions. We carried out quasi-static fracture tests and observed a decrease of healing efficiency from 77% for fresh samples down to 13% for samples aged at room temperature for 77 days. DSC results of pure epoxy matrix curing showed that the residual heat of reaction only decreased from 161 J g−1 to about 135 J g−1; this decrease was largely due to water absorption and could only partly explain the large decrease in healing efficiency. Furthermore, FTIR spectra did not indicate a change in conversion as observed by monitoring the associated oxirane peak in the pure epoxy matrix over time, confirming the above findings. While the microencapsulated solvent has good shelf life properties, evidence was found that solvent diffusion into the epoxy matrix prior to cracking was responsible for the reduced healing capability. DSC and FTIR results of aged self-healing epoxy confirmed the presence of the EPA solvent in the matrix even though no damage was introduced to the microcapsules. Therefore two phenomena responsible for the decreased solvent-healing capability in under-cured epoxy were identified: ambient moisture uptake and premature solvent diffusion through the microcapsule walls.

Study on the Cure Kinetics of Middle Temperature Curing 3234 Epoxy Resin System

The cure kinetics of middle temperature curing 3234 epoxy resin was investigated under both isothermal and dynamic curing conditions by Differential Scanning Calorimetry (DSC) technique. The kinetic equations for isothermal and dynamic curing were established respectively by analyzing experimental data obtained from DSC . By following the temperature procedure of the standard cure cycle of the resin system, the validity of the cure kinetic equations was verified by means of determining the residual heat of reaction of samples from different stages of the cure cycle. Results showed that the cure kinetics under both isothermal and dynamic conditions could be described by the autocatalytic kinetic model. Predictions by the kinetic equation for dynamic curing agreed well with the experimental results.

Measurement of Glass Transition Temperature, Residual Heat of Reaction and Mixing Ratio of Epoxy Resins Using near Infrared Spectroscopy: A Preliminary Study

As a measure of the degree of curing of epoxy resins, the glass transition temperature, Tg, and the residual heat of reaction, Δ Hr, are often used. In this study, near infrared spectroscopy and multivariate calibration [partial least squares regression (PLSR)] have been used to monitor the two variables, using differential scanning calorimetry (DSC) as the reference method. The epoxy under study was a commercial system consisting of the resin, trimethylolpropanetriglycidylether, and the hardener, 3-aminomethyl-3,5,5,-trimethylcyclohexylamine. Using samples cured under different conditions, calibrations resulted in root mean square errors of cross-validation ( RMSECV) of 18 Jg−1 for Δ Hr (range for Δ Hr: 6.1–231.3 Jg−1) and 7.2°C for Tg (range for Tg: 41.5–98.8°C). Also, a PLSR model for mixing ratio of hardener and resin was obtained, resulting in a RMSECV of 0.0040 (range for mixing ratio: 0.180–0.380).

Effect of the crosslinking degree on curing kinetics of an epoxy-anhydride styrene copolymer system

The cure kinetics of a high molecular weight acid copolymer used as a hardener for a commercial epoxy resin (DGEBA) was studied by DSC. The systems were uncured and partially cured epoxy poly(maleic anhydride-alt-styrene) (PAMS) at different periods of time. The state of cure was assessed as the residual heat of reaction and was varied by controlling both the time and temperature of cure. The conversion degree of crosslinking increased with time and temperature. Additionally, the activation energy and reaction order were calculated by the Freeman–Carrol relation and showed a dependence on the conversion degree of crosslinking. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2089–2094, 1999

Effects of Prior Cooling Rates on Tg and Sub-Tg Heat Flow Transitions of Cured Epoxy Resin

Effects of Prior Cooling Rates on <i>T</i>_g and Sub-<i>T</i>_g Heat Flow Transitions of Cured Epoxy Resin

Epoxy resins (DGEBA): The curing and physical aging process

AbstractDifferential scanning calorimetry (DSC) and infrared spectroscopy (IR) were used to monitor the degree of cure of partially cured epoxy resin (Epon 828/MDA) samples. The extent of cure, as determined by residual heat of reaction, concurred with that determined by monitoring the infrared radiation absorbance of the epoxide group near 916 cm−l. The fictive temperature Tf, g was found to increase with the degree of cure, increasing rapidly during cure until reaching a value near the cure temperature Tc of 130°C (approximately 80% cure) where the material vitrified. The greatly reduced reaction rate during the final 20% of cure was not only a consequence of vitrification but, as revealed by infrared spectroscopy, the result of the depletion in the number of reactive epoxide groups. The endothermic peak areas and peak temperatures evident during the DSC scans were used as a measure of the extent of “physical aging” which took place during the cure of this resin, and after, fully cured samples were aged 37°C below their ultimate glass temperature for various periods of time. The rate of physical aging slowed as the temperature increment (Tt,g − Tc) increased. Although an endothermic peak was evident after only 1 h of cure (Tf, g = 138.3°C), such a peak did not appear until fully cured samples were aged for 16 h or more. Enthalpy data revealed that for partially cured material, the fictive temperature Tf, a, reflecting physical aging, increased with curing time. In contrast, the Tf, a, for fully cured samples decreased with sub‐Tg aging time. The characteristic jump in the heat capacity ΔCp which occurred at the Tf, g decreased as curing progressed. This decrease appears to be dependent upon the rotational and vibrational degrees of freedom of the glass. Finally, a graphical method of determining the fictive temperature Tf, a, of partially and fully cured epoxy material from measured endothermic peak areas was developed.

Thermal analysis of photocurable materials

AbstractPhotocurable materials such as photoresists, UV‐curable inks and paints, etc., can be analyzed by a variety of techniques. Commonly, these analyses are performed in a discontinuous mode, i.e., samples are exposed to light for varying times and subsequently they are subjected to analysis. Rates of reaction, conversion, etc., are determined from the residual functionalities; in the case of differential scanning calorimetry, these parameters are derived from the residual heat of reaction. In order to determine “photo speed” or sensitivity, a number of exposures of varying duration are necessary to obtain rates of reaction. By combining an exposure unit with a differential scanning calorimeter, the photoreaction can now he observed in situ. Determination of speed or rate of reaction is reduced to a single experiment. As a consequence, effects of wavelength, intensity, temperature, and environment are quickly examined as is demonstrated here for a photoresist. During the second scan residual exotherms can be used to determine reaction extend or formation of new thermally active products. Conversion may be limited if Tg rises during polymerization to exceed exposure temperatures. In addition, the formation of inaccessable functionalities has to be accounted for during evaluation.

Optimalization of processing conditions for thermosetting polymers by determination of the degree of curing with a differential scanning calorimeter

AbstractThe curing reaction of thermosetting polymers is accompanied by exothermal heat which has been measured with a differential scanning calorimeter. The Perkin‐Elmer DSC‐1 B was used for recording the curing behavior of a diallyl phthalate molding compound and two unsaturated polyester molding compounds. The residual heat of reaction has also been determined by a dynamic DSC scan of partly cured products made at various mold temperatures and curing times. It was thus possible to calculate the degree of curing and to dictate the optimum processing conditions for the indicated molding compounds. The interpretation of results leads to the conclusions that the DSC can be used to set specifications and to perform quality control for the molding compounds and that the most significant processing parameter is mold temperature.

The effect of state of cure on bond performance for an epoxy resin system

A study of the influence of the state and mode of cure of an epoxy resin, diglycidyl ether of bisphenol A cured with 4,4′-diaminodiphenylmethane, has been made. The state of cure was assessed by differential scanning calorimetry as residual heat of reaction and a transition temperature T* corresponding to either the melting point or the glass transition temperature. The state of cure was varied by controlling both the time and temperature of cure. Bond strengths can be related to either Tg or residual heat of reaction, but the different cure schedules produce different relations; simultaneous measurement of both parameters is necessary to specify the state of cure. Some additional work has been carried out on the effect of delayed cure on bond performance and on the effect of varying the cure schedule for a phenol-formaldehyde adhesive. Both single- and double-overlap joints have been used and a constant single/double bond strength ratio has been found. This constant is different for the two adhesives, reflecting their different behavior in shear.