Measurement of Polyethylene and
Polypropylene resin pellets

A manufacturer of polyethylene and polypropylene resin pellets presented their yellowness index measurement prob - lem to BYK-Gardner in the hope of finding a solution that would provide consistent, and cost effective results while being easy to use. What follows is how BYK-Gardner solved the problem using the color-view 45/0 spectro photo meter equipped with a sample rotation device and through the implementation of standardized operating methods.

Challenges of Pellet Measurement

Pellet characteristics:
Color measurement of plastic pellets can be a challenge. As
shown in figure 1, pellets come in a variety of shapes and
sizes. Because of the irregular nature of pellets, it is essen -
tial that multiple measurements are made and the average
used to represent the sample. With different shapes, sizes
and manufacturing methods for pellets, quantifying yellow -
ness can be difficult.a

Technician variables:
Similarly, measurement of pellets in the QC Laboratory can
be quite different than on the production floor due to
preparations by differing operators. With these factors in
mind, one must consider how pellet samples are prepared
for measurement.

Sample preparation and presentation:
Ideally, a new sample of pellets would be prepared for each
measurement. This is impractical due to time pressure on
the operator. Instead, often one sample is prepared and
measured. To make additional measurements, the same
sample is shaken or vibrated within the cup and then rein -
tro duced to the optical sensor. Using this technique, there
are several events occurring optically, which can contribute
to measurement variability. When a sample cup is filled with
pellets, pellets make contact with the bottom of the cup in
a random manner. When the cup is viewed from the glass
side, one sees a number of pellet surface contact points
together with a number of void spaces between the pellets.
Because this ratio of contact to voids is a random
occurrence, the number will change with each new filling.
The problem with the shake method is that when the cup is
shaken, pellets migrate to the bottom and compact or stack
up. Optically, the pellets simulate a near solid object rather
than a random collection of small elements. Variation in
shaking technique among instrument operators provides
another source of variation. Use of this method to accrue

data causes the average to skew and become unreliable
because of poor technique rather than actual yellowness.

Instrumental Design for Pellet
Measurement

Ideal instrument:
From an instrumental point of view, the ideal optical sensor for pellet measurement would have a very large area of measurement. Such an instrument would provide an optical average over a physically large area and would require fewer measurements to compute a representative average. Such an instrument would have its limitations such as a large footprint and not capable of measuring small samples.

Instrumental innovation through automation:
An alternative to a large area of view instrument is to use an instrument with a smaller viewing in conjunction with an automated sampling device. As mentioned before, pre - paring multiple samples for measurement required more time than the instrument operator had available. It also was
a source of measured variability. Mindful of these limita - tions, BYK-Gardner invented a device that would require fewer samples to be prepared while making multiple measure ments of each sample.

Automated sample rotator:
The sample rotator is mounted over the measurement port on the color-view spectrophotometer. An internal, offset motor rotates an optically clear sample cup at about 3-4 rpm that is inside the rotator and above the specimen plane. The sample cup is lined with uniformly white Teflon and once
filled with pellets, it is capped with a white Teflon cover. The cover excludes ambient light from the measure ment.

Standardizing the spectrophotometer for use with the
sample rotator.
It is required that the spectrophotometer is standardized
using its specific white and black standards. A standardi za -
tion technique is used so that the sample rotator is optically
incorporated. Using a quartz glass at the measurement
plane that is the same thickness as the bottom of the
sample cup, the instrument is standardized using its white
opal and black glass. Once this has occurred, pellet
measurement can begin.

Loading the automated sample cup:
Loading the large sample cell is very easy. Pellets are
poured into the cell to the top and leveled, leaving enough
room for the cap to be placed. Since the cup is n mm in
depth, it simulates an infinite thickness of pellets. Once
filled, the cup is placed onto the rotator for measurement.