FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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(5v/v)fit must be considered as more realistic since it integrates errors
from all sources.
From a statistical point of view, the fit can be considered as very good and
this leads us to the conclusion that for PE having a density in the range of
0.92 to 0.97 g/cm , the room temperature (23°C), sound velocity is
proportional to density. We may now examine the possibility of using the
measurement of SDund velocity for the determination of density in PE with a
model such as p = Av + B. Working the regression analysis with p as the
dependent variable we obtain A = (0.0965 +_ 0.0012)10~ ? g sec/cm 4 and B =
(0.07218 +_ 0.0026) g/cm . The correlation factor is of course the same as
above (r = 0.998) and the standard error of estimate becomes 0.00098 g/cm
yielding the relative average value (ôp/p) = _+ 0.0010. As mentioned above
this integrates errors from all sources, and yet it compares quite well with
what can be expected when using a density gradient column for which
(6p/p)co^# = _+ 0.0007. Given this result and bearing in mind that the small
difference most probably originates in the additive content, we believe that
the ultrasonic approach will allow the determination of density in
polyethylene with the required accuracy for quality control purposes.
IV - CONCLUSION
We have demonstrated that in commercial grades of polyethylene, there exists a
linear relationship between the sound velocity at ultrasonic frequencies and
the density of the material. We have shown experimentally that the velocity
measurements can be performed with sufficient accuracy to allow the
computation of density. The method we describe is simple and since it is
based on the sole measurement of time delays, it can be made automatic through
digitalization. The advantages of the method are numerous when compared to
the classical density gradient column: a) no preliminary preparation, the
setup is completely portable, suffers little influence from the environment
and requires no special care to upkeep, b) the sample is not destructed and
can be kept for files, c) the measurement is fast, reliable and repeatable,
the accuracy is good (ôp = +^ 0.001 g/cm ) and the operation being simple
requires no special attention, care nor skill.
Further work is needed to check whether such a technique could be applicable
to other polymers and also to examine the influence of fillers (silica, carbon
black...). Further work is also needed in order to establish the physical
foundation for the observed behaviour.
ACKNOWLEDGMENT
The author wishes to acknowledge the gracious participation of Dr. P. Kelly
(DuPont Canada Inc, Kingston, Ontario) and Mr. I.R. Lambie (Union Carbide,
Sarnia, Ontario), who provided both the incentives and the samples for this
work.