Frust-TIR: New Measurement Technique for Coating Kinetics
Introduction
n1 = 1.766 > n2 = 1.371 < n3
= 1.471
n here is a refractive index of a material, a is the
angle defining the direction of traveling light of wavelength
l and d is the coating (PPA) thickness
Flow instabilities in polymer processing operations such
as extrusion, injection
molding and others have a significant negative impact on product
appearance and performance. An important instability is sharkskin
melt fracture (left). The elimination of the instabilities
stimulated this research to explore novel materials and methods.
We developed a new technique for measurement of the coating
kinetics by polymer processing additives (PPA) during their
extrusion with commercial polymers, such as polyethylene (PE)
using a an optical phenomenon of frustrated total internal
reflection (Frust-TIR). Its principle is schematically shown
here (right).
Experimental Approach
Researchers have utilized traditional capillary rheometry
to understand flow instabilities for many years- however critical
questions remained unresolved. Thus our approach compliments
the traditional methods by incorporating optical measurement
technology. Our frustrated total-internal reflection apparatus
gives in-situ information about the coating of fluoropolymer
additives on internal die surfaces. Simultaneously, we conduct
high speed optical microscopy on the flowing polymer, which
gives us information about the velocity distribution of the
polymer and the sharkskin kinetics.
The direct, non-destructive and non-invasive nature
of the Frust-TIR technique allows acquisition of real- time
data under conditions of high temperature, pressure and flow.
By measurement of the intensity of the reflected light as
a function of angle, we can deduce the thickness of the coating
layer and also we can conclude that it has a striped nature.
Results
We capture the kinetics of the coating process (left). Here
we show simultaneously the elimination of the sharkskin defect,
the reduction in pressure necessary to extrude the polymer
at constant rate, and the growth of the coating layer (bottom
curve).
From this data and other measurements we proposed the coating
mechanism (right) and we developed a semi-quantitative model,
capturing the critical parameters describing the PPA coating
thickness in steady state. Further, we were able to identify
that the PPA/PE morphology, particularly, the size of the
PPA droplets dispersed in the PE matrix, is a critical variable
in optimization of the coating technology
When the reflectivity data are translated into the coating
thickness we obtain this 3-dimensional plot (lower left) in
which the coating (d) is plotted against the die circumference
(L) versus the extrusion time.
Our data indicate for the first time that the coating thickness
on the order of a PPA molecule appears to be sufficient to
completely remove the melt fracture
Publications
S. B. Kharchenko, P. M. McGuiggan, K. B. Migler
“Flow Induced Coating of Polymer Processing Additives:
Development of Frustrated Total Internal Reflection Imaging”
Journal of Rheology 2003, vol. 47, 1523
This work has won our team the Best Paper Award during the
Annual Technical Conference
of the Society of Plastics Engineers held in Nashville, TN,
2003
Contributors:
Kalman Migler, Sam Kharchenko, Patty McGuiggan
Collaborators:
S. Oriani (Dupont Dow Elastomers), M. Meillon & D. Bigio
(U. Maryland), C. Macosko (U. Minnesota)
Processing Characterization Group
Polymers Division
Materials Science and Engineering Laboratory
