Measuring Vapors To Monitor the State of Cure of a Resin

Excess curing time would no longer be needed as margin against uncertainty.

A proposed noninvasive method of monitoring the cure path and the state of cure of an epoxy or other resin involves measurement of the concentration(s) of one or more compound(s) in the vaporous effluent emitted during the curing process. The method is based on the following general ideas:

• The concentrations of the effluent compounds in the vicinity of the curing resin are approximately proportional to the instantaneous rate of curing.
• As curing proceeds at a given temperature, subsequent decreases in the concentrations are indicative of approaching completion of cure; that is, the lower are the concentrations, the more nearly complete is the cure.

The method could be utilized as the basis of a means of controlling the curing process to optimize the properties of the cured resin. It also could be utilized to minimize the cost of the curing process by ensuring a complete cure without the need to provide for excess curing time as margin against uncertainty in a prior estimate of required curing time.

A system to implement the method would include a sensor that produces electronic readouts of the concentrations of effluent compounds of interest. This sensor could be any of a variety of instruments, ranging from general-purpose full-size laboratory instruments capable of rapidly analyzing many compounds to microelectromechanical (MEMS) devices designed to detect effluent compounds specific to one epoxy. Either continuously or at regular intervals, the sensor would sample the effluent. Depending on the specific curing process, the sampling could occur at room temperature, at elevated temperature, under vacuum, or at atmospheric pressure: for example, in a case involving curing in a vacuum/heating apparatus, the sensor could be placed in the unheated tube from the vacuum bag to the vacuum pump.

The concentration(s) of the compound(s) of interest, and, thus, the rate of production of effluent would be monitored electronically and digitized to make a record of the curing process. Once the concentrations of the effluent compounds of interest decreased to predetermined levels, the cure would be considered complete and the operator would be so notified by the sensor circuitry. Alternatively, as depicted schematically in the figure, the sensor could be integrated into a control loop that would turn off the curing apparatus upon completion of the cure. Further, the control loop could be configured for active control to maintain the rate of curing at a predetermined level by monitoring the effluent-production rate and automatically adjusting the temperature and/or the pressure of the partial vacuum.

This work was done by K. Elliott Cramer, Daniel F. Perey, and William T. Yost of Langley Research Center. For further information, access the Technical Support Package (TSP) free on-line at www. techbriefs.com/tsp under the Manufacturing & Prototyping category. LAR-16695-1