Modern Plastics Worldwide - March 2010 - dae uptlax

EXTRUSION
whether they are coming in with
the resin or are created in the process.
The best way to find this out
is to run the virgin resin through a
clean, small extruder. It’s virtually
impossible for gels to form from
clean resin in just a few minutes.
Thus, if the product comes out
with gels from the beginning, and
the gel content stays more or less
the same for a while, they are in
the resin and you have a good case
for rebate or refund if it was sold
to you as clean. There is even a
machine on the market that blows
film and counts “discontinuities
per square meter of surface” to
quantify gel content.
You can also get the same
answer on production machinery,
but it is less precise. If the gels don’t
appear for a while, maybe a few hours or even a few shifts, and
the material and conditions stay the same, it’s likely that they are
forming in the system, most likely the adapter and die in places
where the melt is moving most slowly (large diameters, sharp
bends). Formulation matters: Some processing aids coat the
inside of the flow paths and discourage gel formation, as well as
reduce backpressure and thus allow lower melt temperature.
Resins vary in thermal stability, too, and less-stable materials
will degrade faster into gels and (eventually) black or brown
particles. Running a small product (low mass rate) in a large
machine will contribute to this, as the melt must move more
slowly and stay hot longer. If the gels start as soon as the suspected
resin is used and stay relatively constant, they are in the feed.
You can, of course, have both things going on at once, where
there are plenty of gels in the incoming resin, but after a while
you are adding to them by degrading materials in the head/die.
Q: Which of the screw segments or zones is very important?
AG: They are all important, of course, but the one I was referring
to as special was the rear (first) barrel zone. In this zone,
unlike the others, the barrel wall is not fully coated with melt,
but is still usually well above the melt temperature of the plastic,
so the particles can stick to the barrel as needed for good
“inpush.” Grooved-barrel extruders, typically used for HDPE
film, are an exception; the particles slip on the barrel but only
in the forward direction, but that zone is still independently
important.
I try to separate this rear zone from the others so that people
will consider it alone, or perhaps combined with #2 in a long
extruder with five or six zones, rather than move all the settings
up or down in unison. There is an optimum setting of
zone #1, not necessarily the hottest or the coldest, where the
sticking is best—any hotter and the particles melt on contact
and slide around self-lubricated, and inpush starts to fall off.
Figure 1. The screw-cooling fitting on the rear of the
extruder wobbled back and forth with each revolution,
showing the count, but the loose fit stressed the
connections.
This optimum will vary with the
resin, with the feed temperature,
and with the screw speed, so it
really is best found by “trial and
success,” and I am thus skeptical
of any proposed settings that
are alleged to apply to any one
material in all cases.
The rear barrel temperature
is also the “aspirin” of the troubleshooter—it’s
what you change
if you don’t really know what else
to do, especially when processing
semicrystalline polymers. I
have seen it resolve problems of
surging, excess pressure, or melt
temperature and air entrapment,
and wouldn’t be surprised to hear
other stories, including some in
which it made things worse.
Q: How and why does overheating HDPE make it stronger?
AG: When any polymer is heated, there is some chain breakage.
As might be imagined, the hotter the temperature and the
longer the time at that temperature, the more the degradation.
However, there is also a cross-linking and chain-growing reaction
that takes place where the loose, broken ends of a molecule
look for and find places on some other chain to attach.
This makes that molecule larger and stronger and compensates
for the weakening effect of chain breakage. In the case of some
HDPE grades, the viscosity actually increases (equivalent of
melt index decrease). But this can’t go on indefinitely, because
the breakage is normally inhibited by antioxidant in the formulation,
which is consumed as the melt remains at the high
temperature, and eventually is used up. After that, the breakage
occurs faster than the cross-linking and the viscosity starts to
fall again, representing a weakening effect.
I was involved in these experiments many years ago, when
we were trying to show that the use of “scrap” HDPE pipe
did not necessarily weaken the pressure pipe made from such
a mixture. We ran several HDPEs in a torque rheometer for
up to an hour, far longer than the usual residence time in an
extruder, and sure enough, the torque (measure of viscosity)
What’s a webinar?
Many of you know already that a webinar is a seminar held
via the Internet. Here’s how we do it. We limit them to 60
minutes. They are a free source of information. Our goal in
each webinar is to provide plastics processors with information
they can use to improve their business (not coincidentally, the
goal of this magazine, too).
During the webinar, you hear the presenter but cannot speak
to him (imagine 250 people around the world talking at once
on the same phone line). As an attendee you can submit your
questions via an instant messaging-type service. Questions are
answered during the event or via e-mail soon thereafter.
34 MARCH 2010 • MODERN PLASTICS WORLDWIDE plasticstoday.com/mpw