Thermo101 Recyrstal

A TECHNICAL ARTICLE 2004 VOLUME 23, #1

Recrystallization – What Does That Mean?1

In the last TF101, I mentioned recrys-
tallization. In this tutorial, I will
explain what it is and why it is important
in thermoforming.

Amorphous and Crystalline Plastics
– A Brief Review

In a very early lesson, we learned
that there are two general classes
of plastics used in thermoforming.
Plastics such as polystyrene, ABS,
polycarbonate and even rigid PVC are
considered amorphous. That is, they
are glassy-brittle at room temperature.
When they are heated to a general
temperature range called the glass
transition temperature, they become
rubbery. If we continue to heat them,
they become less and less rubbery
and more and more fluid-like. When
we cool these polymers to their glass
transition temperature, they immediately
become glassy-brittle again.
Amorphous polymers represent the
majority of plastics thermoformed
today.

But as we learned in that early
lesson, thermoformers are bent on
forming crystalline – or more correctly,
semicrystalline – polymers such as
polyethylene, polypropylene and PET.
For many years, polyethylene was the
only semi-crystalline plastic that was
widely thermoformed. PET is usually
formed in the amorphous state (as
APET). Special processes are needed to
produce crystalline PET structures.

Melt Forming PE

High-density PE has exceptional
hot melt strength above its melt temperature
of about 275°F (135°C). That’s
why the blow molder can extrude a
tube of polyethylene, then capture it
in a clamshell mold to make a bottle.
Thermoformers also rely on the hot
melt strength of it in sheet form. We
heat the sheet above its melt temperature
just prior to forming it, as a melt.
Polyethylenes are 50-80% crystalline
and the sleek shape of the molecule
allows very rapid crystallinity once
the formed part is cooled below its
melt temperature. As a result, HDPE is
the most successful semicrystalline
polymer thermoformed.

Solid State Forming of PP

Until recently, polypropylene recipes
did not have sufficient hot strength to
remain sheets in the thermoforming
ovens. As a result, PP was thermoformed
in the solid state. What this means is the
PP sheet was (and is) heated to just
below its melting temperature range,
which is about 330°F (165°C) for
homopolymer polypropylene
(homoPP). PP becomes rubbery in a
very narrow temperature range just
below the melting temperature.

There are two reasons for this
rubberiness. First, the crystallinity of
PP is about 50%, meaning that about
half the PP is not locked in crystallites,
but is instead in an amorphous state.
The glass transition temperature of
homoPP is about 15°F ( -10°C). So
when homoPP sheet is at a forming
temperature of about 320°F (160°C),
say, the amorphous portion of the
sheet is 305°F (170°C) above its Tg2.
Secondly, imperfect crystallites
tend to melt below the stated melt
temperature. This means that more
polymer is added to the amorphous
side of the equation, making the sheet
even more rubbery.

With sufficient pressure then, we can
squeeze, push and otherwise press PP
against the mold. Pressures of 50 to 100
psi have been used to do just this.

So, what is the problem with solid
state forming of PP? Really, nothing.
It just requires higher forming pressures
than what would be used for, say,
PS. Oh, and the product is not water
white but instead, about as translucent
as the original sheet. This is because
we don’t melt out the crystallites and
the crystallites are of sufficient size to
interfere with visible light (0.4 to 0.7
microns).

Melt Forming of PP

Copolymerization of polyethylene
in PP and now, short- and long-chain
branching of PP has greatly improved
PP hot strength. This lets us to thermoform
PP in the melt state, or the
state where all the crystallites are fully
melted. Copolymer PP or coPP usually
melts around 310-320°F (155-160°C).

But melt forming PP is not like
melt forming polyethylene. We run
into a very difficult problem. PP
recrystallizes at a much slower rate
than polyethylene. Even when coPP
is cooling at 9°F/minute (5°C/minute)
3, it recrystallizes around 210°F
(100°C), or about 100°F (60°C) below
its melt temperature. Small amounts
of recrystallization rate enhancers such
as sorbitols can increase the recrystallization
temperature by about 20°F
(10°C). While this may shorten the
hold time on the mold, we still need
to hold coPP on the mold longer than
we might think.

And more importantly, we need to
be concerned about recrystallization
that might continue long after we
remove the formed coPP part from
the mold surface. When the formed
part isn’t constrained, different areas
of the part can crystallize at different
rates and to different crystallinity levels.
Distortion, warping, cupping, and
general mayhem can occur long after
the part is formed. ¦

Keywords: Recrystallization, solid
state forming, melt forming, hot melt
strength

THERMOFORMING

101

1 Thermoforming 101 is designed to be a
tutorial on the basic building blocks of the
thermoforming industry. The first series of
lessons concluded in TFQ 21:3, 2002. This
is another in the second series of lessons
that have as their objective to fill in the gaps
from the first series of lessons.

2 Compare this with PS at its thermoforming
temperature of 350°F (175°C), or 140°F
(75°C) above its Tg of 210°F (100°C).

3 In practice, we would never cool PP this
slowly. The test equipment used to measure
melting and recrystallizing temperatures
operates at about this cooling rate.

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