Electrical cure monitoring

Figure 6.29. (a) A photograph of an autoclave custom-modified to enable the accommodation of optical and electrical sensor systems for cure monitoring of advanced reinforced composites via contact and non-contact IR spectroscopy and measurements of residual strain and temperature. 

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. 

Radiometers are quantitative physical detectors of radiation, which convert the radiant energy to an electrical signal. They can be used for monitoring and controlling UV curing systems in order to produce the desired cure. 

Thermal Cure Reactions. Approximately 150 mg of VI or I were placed in 5 mm diameter tubes which subsequently were either sealed, kept open to the atmosphere or appropriately fitted so as to maintain a very slow stream of nitrogen over the reaction mixture. The tubes were placed in an electrically heated air bath and samples withdrawn periodically and monitored for disappearance of starting material and for formation of phthalo-cyanine rings as described in the Analytical section. The results are tabulated in Table I. 

Measurements of the electrical response of polymer materials has been used to elucidate molecular relaxations [1], monitor real-time changes in the chemical and physical state in polymers brought about by crystallization [2] and cure [3], measure moisture diffusion into polymers [4], and measure the moisture content and integrity of adhesive joints [5-8]. Our research applies this technique in order to obtain detailed measurements of interfacial diffusion of fluids into an adhesive bondline. 

Theoretical treatment of this polymerization is difficult because of the presence of both primary and secondary amine reactions as well as tertiary amine catalyzed epoxy homopolymerization. To obtain kinetic and viscosity correlations, empirical methods were utilized. Various techniques that fully or partially characterize such a system by experimental means are described in the literature ( - ). These methods Include measuring cure by differential scanning calorimetry, infra-red spectrometry, vlsco-metry, and by monitoring electrical properties. The presence of multiple reaction mechanisms with different activation energies and reaction orders (10) makes accurate characterizations difficult, but such complexities should be quantified. A dual Arrhenius expression was adopted here for that purpose. 

For metal or resin impregnated carbon.graphite materials, the extent of polymerization, cure cycles, and degradation can be assessed by measuring electrical resistance of the resin [lOlj. For example, the internal viscosity of a polymer can be determined by monitoring the electrical resistivity as a function of temperature. Electrical and electromagnetic properties of composites as well as NDT techniques are described next. 

Pucid, I. and Ranogajec, F. 1995. DC-electrical conductivity as a method for monitoring radiation curing of unsaturated polyester resins. 1. Measurement conditions and comparison with extraction analysis data. Radiat. Phys. Chem. 46 365-370. 

Conductivity or impedance measurements are also used to monitor the curing of an epoxy or a thermoset in the plastics industry. Here, the changes in the electrical properties of the materials give information about the chemical changes that take place in the curing process. 

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