Cure processes monitoring

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. 

Monitoring the polymerization of each substrate provided an informative picture of the effect of both substituents and of isomer distribution on the curing process. We first addressed the question of the relative rates of substrate isomerization and polymerization. We found that, for the parent monomer (PN and PX), the rate of isomerization greatly exceeds the rate of polymerization. Under conditions where PN and PX are fully equilibrated (195°C/15 hrs or 250 /l hr) there is still less than 20% polymer formation in the neat sample. We conclude that for PN or PX the composition of the mixture undergoing polymerization is essentially independent of the starting isomer. The observation that fully cured samples of either PN or PX show identical and C NMR spectra and indistinguishable SEC analyses, is consistent with this contention. 

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 situ frequency dependent electromagnetic-impedence measurements provide a sensitive, convenient, automated technique to monitor the changes in macroscopic cure processing properties and the advancement of the reaction in situ in the fabrication tool. This chapter discusses the instrumentation, theory, and several applications of the techniques, including isothermal cure, complex time—temperature cure, resin film infusion, thick laminates, and smart, automated control of the cure process. 

Second, by actually monitoring the state of the material, it is possible to control the fabrication process by data rather than a procedure, such as a set time temperature sequence. This means one can have a self-correcting, automated, intelligent cure process that can adapt to variations in material age, fabric permeability, tool heat transfer characteristics, and the like. 

It is also possible to introduce the idea of the rheological degree of conversion prh, l which allows the curing process to be monitored. This is... 

Polymer properties are very often dependent on the polymer preparation. So, a good monitoring of the polymerization process is the key step to obtaining good and reproducible materials. The extent of the polymerization can be controlled in different ways. IR is the most usual [27,30] but is not very accurate and requires the extraction of samples to analyze. Recently, an in situ monitoring of PMR-15 processing has been provided by means of frequency-dependent dielectric measurements [33,34]. This non-destructive technique allows the characterization of all the steps of the curing process and thus they can be optimized. 

Process Monitoring includes papers where, for the most part, commercial resin or prepreg systems are carried through nominal cure cycles. 

Baumgartner, W. E., Ricker, T. Computer assisted dielectric cure monitoring in material quality and cure process control, SAMPE J., 19, 6 (1983)... 

Analytical pyrolysis has been successfully used for monitoring the curing process of certain polyimide polymers. For example, the polyimide Matrimid 5292 is obtained from two components in a reaction as shown below ... 

The model satisfactorily described the cure behavior for the entire range as experimentally monitored by FTIR, DSC, and torsional braid analysis (TBA). This model satisfactorily explained the cure behavior of both catalyzed and uncatalyzed systems over a wide range of temperature and throughout the curing process. The authors proposing the kinetic model considered the reaction to be triggered by the adventitious water and phenol impurities (whose reactions with the cyanate ester is considered as an equilibrium reaction). Catalysis by the added metal ions, which stabilizes the imino carbonate intermediates by complex-ing, is also considered. The model has considered all possible reaction paths and intermediates as detailed in Sect. 4 and depicted in Scheme 14. Considering the various reactions, expressions could be obtained for the individual apparent empirical rate constants of the second order auto catalytic model in terms of the actual rate constants and equilibrium constants. 

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