Cure, experimental techniques

The experimental technique used to obtain the cure curves and a detailed examination of the procedure used to generate the rate constants, an iterative procedure to determine the best values of and k based on the set of differential equations describing the cure process, for these systems will be discussed. 

Frequency dependent complex impedance measurements made over many decades of frequency provide a sensitive and convenient means for monitoring the cure process in thermosets and thermoplastics [1-4]. They are of particular importance for quality control monitoring of cure in complex resin systems because the measurement of dielectric relaxation is one of only a few instrumental techniques available for studying molecular properties in both the liquid and solid states. Furthermore, It is one of the few experimental techniques available for studying the poljfmerization process of going from a monomeric liquid of varying viscosity to a crosslinked. Insoluble, high temperature solid. 

In the past, impedance or dielectric studies have been examined as an experimental technique to monitor the flow properties, effects of composition, and the advancement of a reaction during cure [1]. Until a paper by Zukas et al [2], little emphasis had been placed on the frequency dependence except to note the shift in position and magnitude of impedance maxima and minima. Furthermore, most measurements on curing systems reported results in terms of... 

Combination of the results with solvent extraction/liquid chromatography data may elucidate the role of free, unreacted monomer in the post-curing process. The main conclusions have already appeared elsewhere (9). Here we report in much more detail on the experimental techniques as well as on new results on the delay of shrinkage with respect to chemical conversion. 

A major objective of this book is to evaluate the reported values of molecular electron affinities and their errors and to assign them to specific states. Prior to 1970 the magnetron and ECD methods were used to measure the majority of gas phase molecular electron affinities. An extensive compilation of unevaluated experimental, empirical, and theoretical electron affinities of atoms, molecules, and radicals was published before 1990 [9]. The electron affinities measured in the gas phase are now available on the Internet but have not been evaluated [26]. The molecular Ea in this list is defined and evaluated in Appendix IV. Values that are significantly lower than the selected values will be assigned to excited states. Semi-empirical calculations and the CURES-EC technique support these assignments. Unpublished electron affinities and updated electron affinities from charge transfer complex data and half-wave reduction potentials are given in Appendix IV. 

Experimental techniques for determining physical properties during cure... 

Therefore the most convenient way to proceed in order to establish the phase separation mechanism is to use different experimental techniques giving different size scales of the morphology generated, i.e. SAXS and LS [30], or, when possible, to observe the evolution of morphologies by SEM or TEM at the same cure times as LS observations [129]. 

In this chapter the interrelation between mechanical properties, molecular mobility and chemical reactivity is discussed. Examples of how the changes in charge recombination luminescence, heat capacity and rate constants of chemical reactions can be related to the evolution of viscoelastic properties and the transitions encountered during isothermal cure of thermosetting materials are given. The possible application of the experimental techniques involved to in-situ cure process monitoring is also reviewed. 

In the 1970s and 1980s, considerable amounts of effort were spent on investigating the chemorheology of thermosets. There are many experimental techniques that have been used to investigate the cure kinetics of thermosets differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, dielectric measurements, and rheokinetic measurements. There are monographs (Kock 1977 May 1983 Turi 1981) and a comprehensive review article (Halley and Mackay 1996) on the subject. 

Modification of the FT-IR analysis techniques to analyze coatings under isothermal cure conditions provides the data needed to determine the rate constants for each reaction. An effective method to generate the rate constants from the experimental data has been found and will be described. 

The only way to validate kinetic models is to measure experimentally the degree of cure as a function of time and temperature. It can be done on both macroscopic and microscopic levels by monitoring chemical, physical (refractive index [135], density [136], and viscosity [137]), electrical (electrical resistivity [138,139]), mechanical, and thermal property changes with time [140,141]. The most-used techniques to monitor cure are presented in the next two subsections. 

In Chapter 2 the DSC technique is discussed in terms of instruments, experimental methods, and ways of analysing the kinetic data. Chapter 3 provides a brief summary of epoxy resin curing reactions. Results of studies on the application of DSC to the cure of epoxy resins are reviewed and discussed in Chapter 4. These results are concerned with the use of carboxylic acid anhydrides, primary and secondary amines, dicyanodiamide, and imidazoles as curing agents. 

This paper describes the general methodology and some results of the study of thermoset systems by rheological techniques, chiefly in steady rotation. Analysis of the experimental cure curve is described in detail, and the use of chemorheologioal data to correlate and predict product performance as well as provide guidance for formulation, is discussed. 

IGC data of sufficient accuracy may be obtained only if certain experimental and acquisition/analysis techniques cure followed. 

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