Research Highlights of Polymers Division

A new method for measuring temperature of a resin during processing has been developed. The technique addresses a long standing problem in polymer processing and materials testing to obtain accurate and true resin temperatures. Although measurements using thermocouples are the standard practice, it is well known that this measurement does not reflect the resin temperature because the device is unduly influenced by the large thermal mass of the processing machine. During processing, heat is dissipated in the resin by virtue of its own viscosity or by chemical reactions. Since temperature is the most important parameter for understanding and controlling the process, it must be measured accurately.

The new method is based on fluorescence spectroscopy and uses a temperature sensitive fluorescent dye that is doped into the resin. Concentration of dye in resin on the order of a mass fraction of 10 -6 dye is sufficient for using fluorescence spectra as a monitoring tool. When mixed with the resin at elevated temperatures, the dyes are soluble in the resin matrix so that the dye is completely surrounded by the resin at the molecular level. The concept regarding fluorescent dyes is that they are molecular probes, i.e., they respond to the molecular environment in which they exist and report conditions of the environment via their spectra. Thus, a fluorescence temperature is a true resin temperature.

Sensor hardware for interfacing with polymer processing machines consists of optical fibers that are inserted into existing instrumentation ports in the machine. For example, the standard half-inch sensor bolt for thermocouple or pressure transducers can be adapted to receive optical fibers. The fibers are used to transmit excitation light to the resin, to collect the resultant fluorescence light, and to transmit it to the detector. The NIST sensor head has been fitted with confocal optics so that temperature profiles of a resin flow stream can be obtained.

The fluorescence spectrum of the dopant die molecule is temperature sensitive. Its key spectral features are measured on-line during processing.

The optics is rugged and compact enough to fit into the standard extrusion instrumentation ports.

Temperature profiles during the reactive extrusion of a polyhydroxyl alkyl ether (PHAE) doped with BOS dye reveal new information about this process that can be used to avoid resin degradation and produce a higher quality product.

Two dyes we have used for process monitoring are perylene and benzoxazolyl stilbene (BOS). Intensity (arbitrary units) enough. Temperature sensitivity of the dyes arises from the fact that probabilities of fluorescence decay from an excited state to different ground state energy levels are temperature dependent. This is illustrated in the spectral plot of polycarbonate doped with perylene. The data taken at various temperatures between 180 o C and 295 o C display a continuous change in shape with increasing temperature.

An important application of the technology is temperature monitoring during reactive extrusion. The NIST confocal temperature sensor was positioned at the exit die of an extruder for this measurement. The standard uncertainty in the temperature data is ± 2 o C. The higher temperatures at 1.5 mm from the wall are due to a combination of shear heating and the exothermic chemical reaction.

In another application of the confocal temperature sensor, a capillary rheometer was instrumented as shown below. Here, excitation light was focused on the extrudate, polyethylene doped with perylene. The objective is to obtain viscosity as a function of shear rate at a constant temperature. The test provides basic materials rheological data that are used by U. S. polymer processors, particularly injection molders, to simulate process flows. This is an important step in setting up a new process and the simulation requires accurate input data. But, in order to use the results from this test, the data must be corrected for shear heating effects that occur at high shear rates. A temperature correction has not been possible heretofore because an accurate temperature measuring method has not been available. The impact of these new findings and the NIST measuring technique will be to change materials testing protocols so that accurate rheological data are made available for applications such as simulation software and process modeling.

Collaborations with Mobil Chemical, DOW Chemical and Datapoint Labs are yielding important information that is impacting on processing strategies and understanding.

“Fluorescence based temperature measuring methods developed by NIST address a long standing problem for obtaining accurate resin temperatures during rheological characterization.”… Hubert Lobo, Datapoint Labs

“Accurate measurements of resin temperature gradients during extrusion processing have not been possible in the extruder barrel until the development of the NIST fluorescence based method.”.......Walter Buzanowski, an Analytical Leader in the Analytical Sciences Lab of The Dow Chemical Company.