Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
PCT/AU00/0l 131
- CA 02384878 2002-03-15 Received 12 November 2001
1
PROCESS FOR PREPARING FOOD CONTACT GRADE POLYETHYLENE
TEREPHTHALATE RESIN FROM WASTE PET CONTAINERS
Technical Field
This invention relates to methods for preparing polyethylene terephthalate
(PET) resin from plastic waste that includes PET containers. It also relates
to PET
obtained according to the process.
Background to the Invention
PET is a widely used polymer or resin with a broad range of appiications but
which has a particularly widespread use as a beverage container or bottle. The
use of
PET for beverage containers has increased rapidly over the last decade and has
to a
large extent replaced conventional glass beverage containers especially for
carbonated
soft drinks. Part of the widespread acceptance of PET has been attributed to
its ability
to be used for food contact as well as its light weight relative to glass of
comparable
strength and its ability to resist breakage.
Over recent years environmental pressures have increased and there is a
demand for the recycling of many materials, especially plastics. One common
source
of recyclable material is post consumer curbside waste. With PET containers
most of
the applications for recycled PET are for relatively low specification
products that use
a mixture of thermoplastic resins or polymers including PET. In these
applications
removal of contaminants is not important. It is desirable that PET containers
may be
recycled to produce PET resin that is suitable for the same applications as
virgin PET.
For example, it is~ especially desirable that the recycled resin may be used
for food
contact applications. However, for such applications there are strict limits
on the
presence of contaminants.
Various methods have been proposed for recycling PET resin. One such
method is disclosed in US Patent No. 5554657 which is assigned to the Shell
Oil
Company. In this patent a mixed polymer recycle stream that includes PET
polymers
is contacted with a solvent that selectively dissolves PET. This
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polymer solution is then separated from undissolved material and cooled to
allow the precipitation of the PET resin. While this process leads to
excellent
quality product it is expensive as it involves the use of organic solvents
which
need to be recycled themselves. Other recycling methods involve the separation
of particulate contaminants by filtration of a molten polymer. This filtration
method is facilitated by reducing the molecular weight and thus the intrinsic
viscosity of the polymer to allow the molten material to flow more readily.
However this reduction in molecular weight necessitates a final polymerisation
or condensation reaction stage to produce PET of the required viscosity and
molecular weight. This method also requires the frequent replacement and/or
cleaning of filters.
A further approach to recycling PET for food grade applications is
disclosed in Australian Patent Application No. 9478299. In this invention
beverage containers are produced having inner and outer skins. The inner skin
which is in contact with food is made from virgin PET and the outer skin is
made from recycled PET resin. This invention removes the necessity for the
recycled PET being suitable for food contact. However it is desirable that
recycled PET can be used for direct food applications without using multiple
skin production methods with their associated complexities and costs.
US Patent 5,876,644 discloses a process for preparing food contact grade
PET. The process involves the surface cleaning of comminuted pieces of post
consumer PET containers; followed by melting of the cleaned pieces; followed
by extrusion to form a melt and then blending of this melt with a melt virgin
polyester prepolymer. The combined melt is then solidified and polymerisation
is then effected while the pellets are in the solid state. The use of virgin
PET
prepolymer would have the effect of reducing the contaminant level as well as
allowing solid state polymerisation to take place to achieve the desired
intrinsic
viscosity increase. It is an important feature of this earlier invention that
molecular weight increase takes place in the solid state after extrusion. As
this
process requires a post extrusion solid state increase in intrinsic viscosity
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reduction in the intrinsic viscosity during extrusion is permitted or achieved
by
adding water prior to melting and extrusion (see column 7, lines 25-35).
PET originating from sorted collections of solid urban refuse can be
contaminated with a range of materials. The PET will include both food contact
grade PET such as containers as well as non food contact grade PET. The range
of non PET materials include other polymers especially polyolefins such as
I~PE. Other common contaminants are metals, particulate material such as
dirt, glues, paper, inks and remnants of materials stored in the containers.
It is
desirable that food contact grade PET may be obtained from such collections of
urban refuse.
Summary of the Invention
This invention provides in one form a process for preparing food contact
grade PET from a waste stream containing PET and non PET materials
comprising the following steps:
sorting at least some of the non PET materials from the waste stream;
dividing the PET containers into flakes of preferable maximum size
approximately lOmm;
washing the flakes in a hot aqueous medium containing alkaline
materials and surfactants, preferably non-ionic, to remove particulate and
absorbed contaminants from the surfaces of the flakes;
de-watering and then drying the flakes to a moisture content of 0.1 % w/w
maximum, and more preferably 0.01 % w!w maximum;
optionally removing absorbed contaminants and moisture by heating and
vigorously mixing the flakes under vacuum, preferably 1- 10 millibar, more
preferably 2 - 7 millibar and at a temperature less than the melting point of
PET, preferably in the range 170-220°C for at least 30 minutes,
preferably at
least 60 minutes;
melting the flakes in a screw extruder under vacuum to remove absorbed
contaminant and;
extruding the molten material to form strands that are pelletised.
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Preferably the material in the extruder is maintained at 280 -
290°C with
a residence time of less than 60 seconds.
Preferably the heating to remove the absorbed contaminants is attained
by frictional forces from the vigorous mixing.
Detailed Description of the Invention
To meet FDA requirements for food contact grade PET the recycled PET
must have contamination sufficiently low than such that the level of
extractables
is less than 10 ppb. As well as contaminants from the presence of non PET
containers such as dirt, and other plastics, a range of materials may be
adsorbed
into or absorbed onto PET surfaces. These contaminants can include organo
metallic materials such as copper octoate. Absorbed materials may include
polar and non polar organic materials that have a range of volatilities.
It is an important feature of the present invention that the majority of
adsorbed and absorbed materials are removed while the PET material is in flake
form. We have found this facilitates the removal of contaminants that are
generally either on or near the surface of the PET flakes. We have found that
the removal of such contaminants after the material has become molten is much
less effective as the contaminants tend to become buried in the resinous mass.
It
is also an important feature of the present invention that water content is
reduced to low levels and is further reduced during the vacuum venting of the
PET melt in the screw extruder. We have found the low initial water level and
the vacuum venting in the melt extruder enables the equilibrium water content
to be reduced such that the molecular weight is increased. The presence of
excess levels of water at melt temperatures tends to cause hydrolysis of the
ester
linkage leading to reduced molecular weight and thus the intrinsic viscosity
of
the resin.
This invention will be further described by reference to preferred
processes.
An urban solid waste stream consisting of baled bottles are passed
through a debaler that singulates the bottles so that they can be fed at a
steady
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rate to the inlet of a pre-wash unit. The preferred pre-wash unit is one that
utilises elevated temperatures and alkaline surfactants such as one like a
Sorema
Bottle Pre-wash unit. However similar units that have either a continuous or
batch-wise mode of operation may also be used.
The feed rate is typically in the range 500 kgs to 2500 kgs per hour with
1500 kgs per hour being the optimum rate. The action of the pre-wash unit is
to
tumble the bottles using the rotary motion of the cylindrical tumbler unit
about
its longitudinal axis. Internal baffles in the tumbler ensure that all bottles
are
singulated by repeated impact of the bottles falling against the walls and
baffles
while they are exposed to hot water and steam. The internal temperature is
typically maintained in the range 90 to 100°C with temperatures greater
than
95°C being preferred. The residence time in the pre-wash unit is
generally in
the range between 3 minutes and 15 minutes with a typical time being 5
minutes. The water in the pre-wash may include cleaning agents such as caustic
soda and non-foaming detergents. Typical concentrations of the caustic soda
and detergent are 0.1 to 3% (ideally 0.5%) and 0.1 to 0.5% (ideally 0.2%)
respectively.
The preferred detergent or surfactant is non-ionic.
The wash bottles are then de-watered by tumbling them in a cylindrical
tumbler or similar device that allows the freed dirt and other contaminants
such
as labels and closures to pass through the perforations in the walls of the
tumbler. The water can be reused after it is filtered and treated to remove
foreign materials. The residence time in this de-watering unit is in the range
3
minutes and 15 minutes with a typical time being 5 minutes.
At this stage the bottles are clean externally except for a film of water
and are mostly free of plastic or paper labels through the action of the
mechanical handling of the bottles, the hot water and the cleaning agents.
The PET bottles are then sorted. The preferred process uses automatic
systems such as those made by Magnetic Separation Systems (MSS), ROFIN or
National Recovery Technologies Inc (NRT) although manual sorting can also be
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used. Particularly good results are achieved when a sequence of compositional
analysis and sorting modules are used to sort the PET bottles to give a level
of
purity of no more than 20 ppm of PVC. The level of sorting of non PET should
be to 99.998% purity. The MSS modules use modular sensors to detect the
presence of specific plastics and air jets to eject the bottles at a specific
station.
The first module uses an X-ray absorption to detect the chlorine atom in
PVC and the PVC bottles are ejected at this station. This module also removes
aluminium cans due to their strong X-ray absorption.
The second module uses infra-red absorption to detect HDPE (high
density polyethylene) bottles and these are then ejected.
The third module uses near infra-red absorption to detect PET and in this
module all the non PET bottles are ejected. This module will eject bottles
such
as PVC, HDPE, polypropylene, polystyrene and aluminium cans.
The fourth module uses X-ray absorption to detect the chlorine atom in
PVC and the PVC bottles are ejected at this station. This module also removes
aluminium cans due to their strong X-ray absorption.
A manual inspection is used to finally check that only PET bottles
proceed into the later stages of the process.
The sorted PET containers are then reduced in size using a wet grinder
such as a Sorema hot wash, separation and rinsing system or its equivalent.
The wet grinder uses multiple rotating knives to cut the PET bottles
against stationary knives in the presence of water that is at ambient
temperature
or at elevated temperatures (from 10 to 40°C, with 15 to 20°C
being most often
used), and which will contain caustic soda and low foaming surfactants and
antifoam additives. Typical concentrations of the caustic soda and detergent
is
0.1 to 3% (ideally 0.5%) and 0.1 to 0.5% (ideally 0.2%) respectively. Anti-
foam use is related to surfactant level and is usually in the range 0.01 to 1
%.
The PET bottles are cut against a screen with a hole size of lOmm to
30mm with 16 to 20 mm being the most common. This gives an intense
washing and simultaneous cutting effect on the PET bottles resulting in a
range
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of average flake sizes that varies from 3mm to l2mm with the most numerous
being under 8mm.
After grinding to this small size the mixture of PET flake and
polypropylene particles (from closures and neck rings) is fed into a hot wash
station such as Sorema Hot Wash Reactors (or similar) where the mix is
intensively washed for 10 to 20 minutes at temperatures from 75 to 95°C
with
90°C being the ideal. The flakes are fed into the reactors at a liquid
to plastic
flake slurry ratio of 90/10 to 65/35 with 75/25 Volume/Volume being most
common.
The reactors are designed to provide turbulent washing conditions where
particles impinge on each other through the use of opposed-rotor, dual-rotor
stirrers that are used in non-central positions to prevent laminar mixing from
occurring, thus ensuring the most intense washing effect on the PET flakes.
After washing the flakes are separated from the wash solution by the use
of a centifruge or screen, and the flakes are subjected to a sink-float
separation
in a tank of water where the polypropylene particles float due to their
density
being less than that of water (915 kg/m3) and the PET particles sink due to
their
density being greater than that of water (1400 kg/m3)
The separated PET flakes are then further rinsed at least twice in clean
water to remove the residual traces of surfactants and dilute contaminants.
The
pathway of the water and flakes is counter current to provide the maximum
rising effect.
The PET flakes are then de-watered to give a very low level of moisture,
i.e. down to 0.005% water.
This can be done by staged drying with fluidised bed driers to remove
apparent moisture followed by conventional recirculating air driers, desiccant
driers, agglomerators or other drying systems that may also use a dry gas to
dry
the PET at elevated temperatures (140 - 185°C).
The fluidised bed driers will remove the moisture from saturated levels
down to levels of less than 1% and typically 0.5%.
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The desiccant driers, agglomerators or other driers will reduce the
moisture level to at least 0.01 % i.e. 1000ppm of water with an ideal level of
0.005% of moisture, i.e. SOOppm. In conventional driers this may require the
use of long residence times as well as high temperatures, eg. temperatures of
the
order of 120 to 175°C for 8 to 5 hours with 170°C for 5 hours
proving optimum.
The use of pre drying of the PET flakes is important as it leads to an
increase
rather than decrease in the intrinsic viscosity (IV) of the polymer in the
following stages of the process. For example, the IV of the final PET pellet
with the pre drying step was 0.833 versus 0.749 without compared to the IV of
the flake itself of 0.767.
The dried flakes are then subjected to vacuum decontamination.
This process uses high levels of vacuum, preferably 1- 10 millibar, more
preferably 2 - 7 millibar, while the PET is subjected to elevated temperatures
(170 to 215°C) and mixing for controlled residence times of typically 1
hour,
although longer times may also be useful. This decontamination can be
performed in a shredder chamber modified to maintain a vacuum, or in a
fluidising mixer modified to hold a vacuum. Frictional forces between flakes
and parts of the equipment lead to heat build up and this is the preferred
method
of attaining the desired temperature. The impeller in the chamber is rotated
at
between 200 to 220 rpm converting mechanical energy into heat. Increasing the
speed generates high temperatures with typically a temperature gradient, the
higher temperatures being at the bottom. For example, when the impeller speed
was 220 rpm the temperature at the bottom was 199°C, in the middle was
189°C
and at the top 169°C.
Preferably the loading of the chamber with the PET flakes and the rotor
speed are selected so that the chamber was filled to a sufficiently high
level,
approximately 70%, that flake introduced through a vacuum lock could reside at
the top for a controlled residence time without the risk of immediately being
mixed into the bulk of the flake and being extruded with only a short
residence
time. The conditions in the chamber were balanced so that the PET flake was
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progressively exposed to higher temperatures while under high vacuum and
maintained at this condition for at least one hour after which it was
introduced
into the extruder.
The decontamination of the PET takes place in the mixer by the
combined action of the vacuum, elevated temperature and the residence time of
the PET under these conditions.
The decontaminated PET flakes are then fed to a single screw extruder
with an extrusion screw designed for the processing of PET and capable of
applying vacuum venting to the PET melt at 280 to 290°C. The process
could
be conducted in a similar twin screw extruder with vacuum venting or in a twin
or mufti-screw extruder with vacuum venting. The key requirement is the
capability to melt the PET (melt temperature in the range 265 - 300°C,
preferably 280°C) and to apply one or more stages of vacuum venting (at
preferably 1 millibar or less) without applying excessive shear to the PET in
the
melt stage through excessive mechanical working of the melt. Excessive shear
leads to a decrease in the IV. The application of the higher temperatures in
the
melt coupled with the vacuum venting allows removal of the least volatile
fluids
that may have been absorbed into the PET.
While this process has been described without the use of chemical chain
extenders to increase the IV of the PET, these chemical materials may also be
used. Chain extender materials are known and usually comprise one or more of
a polycarboxylic acid or anhydride, a polyol and an esterification catalyst.
For
example, we have found a mixture of pyromellitic dianhydride, anhydride,
pentaerythritol and antimony oxide in the weight ratios of 4:1:0.5 is a
particularly useful chain extender composition. In the process described above
the use of this chain extender composition has been able to increase the IV of
the PET to 0.930 when used at 0.3% w/w of PET. Higher levels, eg 1.0% w/w
increased the IV to 1.300.
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After melting and vacuum venting the melt is filtered by passing it
through fine metal mesh filters (usually the mesh size is 120 mesh or finer)
to
remove any particles.
The extruded melt may then be converted to pellets typically 3mm using
conventional techniques such as by using an underwater die face cutter or hot
die face and water ring that quenches the melt into pellets as they are cut.
Further steps may include crystallising the pellets and pre-drying the pellets
prior to moulding. The crystallising is carried out by heating the PET pellets
while they are kept moving via tumbling or agitation. The temperature is
maintained at from 120°C to 170°C for between 10 minutes and 1
hour.
Pre drying is carried out by heating the pellets at elevated temperatures in
hot gas that has a dew point of less than 40°C. The temperatures used
typically
vary from 140°C to 190°C for a duration that typically varies
from 4 hours to 7
hours.
The effectiveness of the recycling process of the present invention is
illustrated by introducing the following contaminants to PET containers:
toluene 10% v/v, chloroform 10% u/v, benzophenone 1 % v/v, methyl
stearate 1 % v/v and copper octoate 1 % v/v.
The concentrations of these contaminants after the various stages of the
process steps of the present invention are set out in Table 1. These results
show
that after the process of the present invention contamination levels in the
extruded pellets are acceptably low. The Table also shows the levels of
extractables from PET bottles made from pellets prepared as described above.
30
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Table 1
Contaminant Levels (Ppm) In Washed Flake,
Extruded Pellets And Food Stimulant (10°10 Ethanol)
Contaminant PET PET Flake Extruded Levels in Food
Flake After Wash PET PelletsSimulant (10%
Before Ethanol) After the
Wash Migration test with
Bottles made from
PET Pellets
Toluene 1768.0 360.5 7.0 <0.01
Chloroform 612.5 52.3 24.8 <0.004
Benzophenone 713.3 175 54 <0.005
methyl stearate81.2 16 1 <0.005
copper octoate230.3 8 5.0 <0.001
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