Fluorescence Based Temperature
Measurements Impact Industrial Polymer Processing
Temperature is the most important
parameter for controlling and modeling polymer processing. It plays
a fundamental role in determining flow characteristics, degradation
phenomenon, phase transitions, and morphology of the final product.
Accurate measurements of resin temperature are not obtained using
conventional methods such as thermocouples because such devices are
unduly influenced by the large thermal mass of the processing machine.
Resin and machine temperatures are never the same because the viscous
resin dissipates energy during flow causing its temperature to increase
significantly, sometimes approaching the degradation threshold. Our
method to measure the true resin temperature is based on fluorescence
spectroscopy. The technique has implications for materials testing,
for development of processing strategies and for process control.
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.
Errors in viscometric
data from failure to account for shear heating lead to inaccurate
modeling of the extrusion and injection molding processes. At shear
rates above 1000 s -1 , resin temperature was above degradation threshold.
“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.
For more information on
this topic:
“Fluorescence Based Temperature
Measurements and Applications to Real-Time Polymer Processing”, A.
J. Bur and S. C. Roth, Proc. Soc. Plastics Eng. Annual Technical Mtg.,
May 2000.
Technical Mtg., May 2000. “Fluorescence
Based Measurement of Temperature Profiles during Polymer Processing”,
K. Migler and A. J. Bur, Polym. Eng. Sci. 38, 213 (1998). Anthony
J. Bur and Kalman Migler
NIST Material
Science & Engineering Laboratory - Polymers Division