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Best Practices in PET Recycling Click here for printable PDF version Drying Methods and Requirements
PET
is an extremely hygroscopic thermoplastic,
in that it readily absorbs moisture from
the atmosphere. The presence of minute amounts of moisture
will hydrolyze PET in the melt phase, severely
reducing molecular weight. As a result, mechanical properties of PET decrease
and end-product quality is compromised.
Therefore, PET must be thoroughly dry just
prior to melt processing, and in some cases,
recycled PET may have to be crystallized
prior to drying.
Unlike
the other major packaging resins (e.g.,
polyolefins, polystyrene, and PVC), PET
is produced by a condensation reaction.
Various starting materials are used
and reacted in a series of steps to produce
PET. This reaction, which also produces water, is
reversible.
Therefore, when undried PET is melted,
the resin and water chemically react. Hydrolysis occurs and key mechanical properties of the PET are reduced.
This hydrolysis reaction also changes
PET melt viscosity and crystallization rate,
making it very difficult to process into
a quality end product.
PET
is a "semi-crystalline" thermoplastic,
meaning it has both crystalline and amorphous
regions within its molecular structure.
The crystalline portion develops
where the molecules can align themselves
in a very compact linear structure.
Otherwise, the molecules are set
in a random or amorphous pattern.
Virgin
PET resin is sold in crystallized form so
that it can be dried before being melt processed. Uncrystallized PET becomes sticky and clumps when its temperature
reaches 175dF. This is PET's glass transition temperature; the
point at which the amorphous portion begins
to soften. Recycled PET may have to be crystallized
prior to drying to avoid drying difficulties.
Crystallization permits trouble-free drying
in conventional equipment at 280dF-320dF. End-product
manufacture from clean, recycled PET flake
introduces an additional variable to be
considered when drying. A rule of thumb is that a crystallizer is not
required if the amorphous portion of the
feedstream is less than 40% of the total.
However, this generalization ignores
wide variation in the level of crystallinity
in flake. For example, flake from clear thermoformed
parts, trim scrap, or PET bottle preforms,
is highly amorphous. Flake from whole soda bottles will be a mixture
of crystalline and amorphous fractions,
while that from strapping will be highly
crystalline. Most
PET drying is done in dehumidifying hoppers
using hot air at a very low dew point. The
dehumidified air passes through a bed of
PET to extract moisture from the resin.
A desiccant material, such as silica,
absorbs moisture from the circulating air.
Dual desiccant bed systems are common,
so that one bed is on-stream while the stand-by
bed is
Due
to rapid growth in PET recycling, many new
plastic converters are manufacturing end
products from post-consumer PET. A number of these companies are just now learning
how critical drying is when producing high
quality PET end products.
Others may not be aware of the differences
in behavior between amorphous and crystallized
grades of PET.
It is important to note that inadequate
drying, (ppm) causes 60% of all quality
and molding problems in PET processing (1).
Best
Practices: Drying. PET must be dried to <100 parts per million
(ppm) moisture and maintained at this moisture
level to minimize hydrolysis during melt
processing. This is not optional with PET;
it is absolutely essential. A dry resin
will help control the Intrinsic Viscosity
(IV) loss, which should be less than 0.04
dL/gram. An IV reduction greater than this
can result in a product outside of the useful
range (0.70-0.80 dL/gram) for several recycled
PET applications.
Controlling IV loss is critical to
maintaining impact strength, stiffness,
chemical resistance, melt viscosity, and
other key properties of the starting material.
The
PET should be dried at 280-320dF, using
dehumidified air with a dew point of -20EF or less.
Higher drying temperatures can degrade the
resin and cause discoloration. Lower drying
temperatures will not dry the resin below
100 ppm moisture.
Dew
point is an absolute measure of air moisture
and is independent of air temperature.
Dew point should always be used to
control dryer performance, therefore, the
dryer should be equipped with a dew-point
monitor and alarm plus a temperature monitor
on the dryer inlet line.
Airflow to the dryer heats the resin
and absorbs its moisture. Sufficient air flow maintains the resin at
the proper temperature for its entire residence
time. Airflow must be maintained at one cubic foot of air/minute for every
pound/hour of PET being dried.
A volumetric flow indicator is recommended
to monitor airflow.
Pellets
should be dried for four hours, while regrind
should be dried for five to six hours.
Another
best practice for minimizing moisture-related
degradation of PET is to dry any blending
ingredients, colorants, additives, or internal
scrap that could potentially contribute
moisture to the base resin.
If any portion of the formulation
is hygroscopic, it must be dried according
to the supplier's recommendation.
Some non-hygroscopic components may
not have to be dried if their equilibrium
moisture content and percentage of the formulation
are small.
The
manufacture of end products from clean,
recycled PET flake may require that the
PET feedstream be crystallized prior to
drying. However, this generalization ignores
the fact that the level of crystallinity
in the flake is subject to wide variations. For example, flake from clear thermoformed
parts, trim scrap, or PET bottle preforms
is highly amorphous. Flake from whole soda bottles will be a mixture
of crystalline and amorphous fractions,
while that from strapping will be highly
crystalline.
The
sticking (or agglomeration) problem in dryers
will worsen as a higher percent of the mix
consists of amorphous material. A rule of
thumb is that a crystallizer is not required
if the amorphous portion of the feedstream
is less than 40% of the total. For some end-product manufacturers, it is not feasible or cost-effective
to maintain the amorphous portion at a content
level low enough to prevent this phenomenon.
The
clumps that form due to sticking of amorphous
or partly amorphous resin do not break up
as the temperature rises. In fact, they will stick to the container walls
and thermocouples and cause bridging as
the clumps grow. Drying of the non-sticking resin becomes inefficient
and some of the bridged material will heat
degrade and reduce product quality.
Crystallizers
are drying hoppers equipped with agitators
that break up the clumps.
The crystallizers generally are positioned
just above a series of dryers. Slow agitation is used to prevent agglomeration
and creation of fines.
The transition from amorphous to
crystalline PET takes 5-10 minutes at 270-300dF.
The crystallized material then is
conveyed into a hopper dryer.
Moisture
Control. Drying must be coordinated with production at
all times. Dryness of the PET must be maintained
until it enters processing equipment.
It is best to use dried material
right away so it does not absorb ambient
moisture. Depending on ambient conditions, dried PET
that is not kept in a sealed enclosure can
pick up enough moisture in five minutes
to defeat most of the benefits of drying.
To
maintain dryness, it is best to vacuum load
the pellets and/or regrind directly from
a centralized drying unit to feed hoppers
above the processing equipment.
In some instances, portable hopper
dryers are used and the dried resin is conveyed
directly to the feed hopper.
Moisture
Analysis. The Best
Practice for moisture analysis of dried
samples is to check two to three samples
at the end of each drying cycle to confirm
that moisture content is below 100 parts
per million.
Goodyear's test method R-123b is
used by several recycling companies. This procedure utilizes a Meeco electrolytic
moisture analyzer.
Reference: 1. Kozielski,
Gary. "Molding Reinforced PET: How
to Do It Right", Plastics Technology,
October 1988
Issue Date / Update: January 1998
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