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Polyethylene terephthalate (sometimes written
poly(ethylene terephthalate)), commonly abbreviated PET, PETE, or
the obsolete PETP or PET-P, is a thermoplastic polymer resin of the
polyester family and is used in synthetic fibers; beverage, food and
other liquid containers; thermoforming applications; and engineering
resins often in combination with glass fiber.
Depending on its processing and thermal history, polyethylene
terephthalate may exist both as an amorphous (transparent) and as a
semi-crystalline polymer. The semicrystalline material might appear
transparent (particle size < 500 nm) or opaque and white (particle
size up to a few microns) depending on its crystal structure and
particle size. Its monomer (bis-β-hydroxyterephthalate) can be
synthesized by the esterification reaction between terephthalic acid
and ethylene glycol with water as a byproduct, or by
transesterification reaction between ethylene glycol and dimethyl
terephthalate with methanol as a byproduct. Polymerization is
through a polycondensation reaction of the monomers (done
immediately after esterification/transesterification) with ethylene
glycol as the byproduct (the ethylene glycol is directly recycled in
production).
The majority of the world's PET production is for synthetic fibers
(in excess of 60%) with bottle production accounting for around 30%
of global demand. In discussing textile applications, PET is
generally referred to as simply "polyester" while "PET" is used most
often to refer to packaging applications. The polyester industry
makes up about 18% of world polymer production and is third after
polyethylene (PE) and polypropylene (PP).
Uses
Sailcloth is typically made from PET fibers also known as polyester
or under the brand name Dacron; colorful lightweight spinnakers are
usually made of nylon.
PET can be semi-rigid to rigid, depending on its thickness, and it
is very lightweight. It makes a good gas and fair moisture barrier,
as well as a good barrier to alcohol (requires additional "barrier"
treatment) and solvents. It is strong and impact-resistant. It is
naturally colorless with a high transparency.
Plastic bottles made from PET are excellent barrier materials and
are widely used for soft drinks (see carbonation). For certain
specialty bottles, PET sandwiches an additional polyvinyl alcohol to
further reduce its oxygen permeability.
Biaxially oriented PET film (often known by one of its trade names,
"Mylar") can be aluminized by evaporating a thin film of metal onto
it to reduce its permeability, and to make it reflective and opaque
(MPET). These properties are useful in many applications, including
flexible food packaging and thermal insulation, such as "space
blankets". Because of its high mechanical strength, PET film is
often used in tape applications, such as the carrier for magnetic
tape or backing for pressure sensitive adhesive tapes.
Non-oriented PET sheet can be thermoformed to make packaging trays
and blisters. If crystallizable PET is used, the trays can be used
for frozen dinners, since they withstand both freezing and oven
baking temperatures.
When filled with glass particles or fibers, it becomes significantly
stiffer and more durable. This glass-filled plastic, in a
semi-crystalline formulation, is sold under the tradename Rynite,
Arnite, Hostadur, and Crastin.
While most thermoplastics can, in principle, be recycled, PET bottle
recycling is more practical than many other plastic applications.
The primary reason is that plastic carbonated soft drink bottles and
water bottles are almost exclusively PET. PET has a resin
identification code of 1. One of the uses for a recycled PET bottle
is for the manufacture of polar fleece material. Among its many
uses, companies such as English Retreads use the PET material to
line their products. It can also make fiber for polyester products.
Because of the recyclability of PET and the relative abundance of
post-consumer waste in the form of bottles, PET is rapidly gaining
market share as a carpet fiber. Mohawk Industries released
everSTRAND in 1999, a 100% post-consumer recycled content PET fiber.
Since that time, more than 17 billion bottles have been recycled
into carpet fiber.[4] Pharr Yarns, a supplier to numerous carpet
manufacturers including Looptex, Dobbs Mills, and Berkshire
Flooring,produces a BCF (bulk continuous filament) PET carpet fiber
containing a minimum of 25% post-consumer recycled content.
PET, as with many plastics, is also an excellent candidate for
thermal disposal (incineration), as it is composed of carbon,
hydrogen, and oxygen, with only trace amounts of catalyst elements
(but no sulfur). PET has the energy content of soft coal.
PET was patented in 1941 by the Calico Printers' Association of
Manchester. The PET bottle was patented in 1973 by Nathaniel Wyeth.
One of the most important characteristics of PET is referred to as
intrinsic viscosity (IV)
The intrinsic viscosity of the material, measured in deciliters per
gram (dℓ/g) is dependent upon the length of its polymer chains. The
longer the polymer chains, the more entanglements between chains and
therefore the higher the viscosity. The average chain length of a
particular batch of resin can be controlled during polycondensation.
The intrinsic viscosity range of PET
Fiber grade
0.40 – 0.70 dℓ/g Textile
0.72 – 0.98 dℓ/g Technical, tire cord
Film grade
0.60 – 0.70 dℓ/g BoPET (biaxially oriented PET film)
0.70 – 1.00 dℓ/g Sheet grade for thermoforming
Bottle grade
0.70 – 0.78 dℓ/g Water bottles (flat)
0.78 – 0.85 dℓ/g Carbonated soft drink grade
Monofilament
1.00 – 2.00 dℓ/g
[edit]
Drying
PET is hygroscopic, meaning that it naturally absorbs water from its
surroundings. However, when this 'damp' PET is then heated, the
water hydrolyzes the PET, decreasing its resilience. This means that
before the resin can be processed in a molding machine, as much
moisture as possible must be removed from the resin. This is
achieved through the use of a desiccant or dryers before the PET is
fed into the processing equipment.
Inside the dryer, hot dry air is pumped into the bottom of the
hopper containing the resin so that it flows up through the pellets,
removing moisture on its way. The hot wet air leaves the top of the
hopper and is first run through an after-cooler, because it is
easier to remove moisture from cold air than hot air. The resulting
cool wet air is then passed through a desiccant bed. Finally the
cool dry air leaving the desiccant bed is re-heated in a process
heater and sent back through the same processes in a closed loop.
Typically, residual moisture levels in the resin must be less than 5
parts per million (parts of water per million parts of resin, by
weight) before processing. Dryer residence time should not be
shorter than about four hours. This is because drying the material
in less than 4 hours would require a temperature above 160 °C, at
which level hydrolysis would begin inside the pellets before they
could be dried out.
PET can also be dried in compressed air resin dryers. Compressed air
dryers do not reuse drying air. Dry, heated compressed air is
circulated through the PET pellets as in the desiccant dryer, then
released to the atmosphere.
[edit]
Copolymers
In addition to pure (homopolymer) PET, PET modified by
copolymerization is also available.
In some cases, the modified properties of copolymer are more
desirable for a particular application. For example, cyclohexane
dimethanol (CHDM) can be added to the polymer backbone in place of
ethylene glycol. Since this building block is much larger (6
additional carbon atoms) than the ethylene glycol unit it replaces,
it does not fit in with the neighboring chains the way an ethylene
glycol unit would. This interferes with crystallization and lowers
the polymer's melting temperature. Such PET is generally known as
PETG (Eastman Chemical and SK Chemicals are the only two
manufacturers). PETG is a clear amorphous thermoplastic that can be
injection molded or sheet extruded. It can be colored during
processing.
Replacing terephthalic acid (right) with isophthalic acid (center)
creates a kink in the PET chain, interfering with crystallization
and lowering the polymer's melting point.
Another common modifier is isophthalic acid, replacing some of the
1,4-(para-) linked terephthalate units. The 1,2-(ortho-) or
1,3-(meta-) linkage produces an angle in the chain, which also
disturbs crystallinity.
Such copolymers are advantageous for certain molding applications,
such as thermoforming, which is used for example to make tray or
blister packaging from PETG film, or PETG sheet. On the other hand,
crystallization is important in other applications where mechanical
and dimensional stability are important, such as seat belts. For PET
bottles, the use of small amounts of CHDM or other comonomers can be
useful: if only small amounts of comonomers are used,
crystallization is slowed but not prevented entirely. As a result,
bottles are obtainable via stretch blow molding ("SBM"), which are
both clear and crystalline enough to be an adequate barrier to
aromas and even gases, such as carbon dioxide in carbonated
beverages.
Crystallization of polymers occurs when polymer chains fold up on
themselves in a repeating, symmetrical pattern. Long polymer chains
tend to become entangled on themselves, which prevents full
crystallization in all but the most carefully controlled
circumstances. PET is no exception to this rule; 60% crystallization
is the upper limit for commercial products, with the exception of
polyester fibers.
PET in its natural state is a crystalline resin. Clear products can
be produced by rapidly cooling molten polymer to form an amorphous
solid. Like glass, amorphous PET forms when its molecules are not
given enough time to arrange themselves in an orderly fashion as the
melt is cooled. At room temperature the molecules are frozen in
place, but if enough heat energy is put back into them, they begin
to move again, allowing crystals to nucleate and grow. This
procedure is known as solid-state crystallization.
Like most materials, PET tends to produce many small crystallites
when crystallized from an amorphous solid, rather than forming one
large single crystal. Light tends to scatter as it crosses the
boundaries between crystallites and the amorphous regions between
them. This scattering means that crystalline PET is opaque and white
in most cases. Fiber drawing is among the few industrial processes
that produce a nearly single-crystal product.
PET is subject to various types of degradations during processing.
The main degradations that can occur are hydrolytic, thermal and,
probably most important, thermal oxidation. When PET degrades,
several things happen: discoloration, chain scissions resulting in
reduced molecular weight, formation of acetaldehyde and cross-links
("gel" or "fish-eye" formation). Discoloration is due to the
formation of various chromophoric systems following prolonged
thermal treatment at elevated temperatures. This becomes a problem
when the optical requirements of the polymer are very high, such as
in packaging applications. The thermal and thermooxidative
degradation results in poor processibility characteristics and
performance of the material.
One way to alleviate this is to use a copolymer. Comonomers such as
CHDM or isophthalic acid lower the melting temperature and reduce
the degree of crystallinity of PET (especially important when the
material is used for bottle manufacturing). Thus the resin can be
plastically formed at lower temperatures and/or with lower force.
This helps to prevent degradation, reducing the acetaldehyde content
of the finished product to an acceptable (that is, unnoticeable)
level. See copolymers, above. Other ways to improve the stability of
the polymer is by using stabilizers, mainly antioxidants such as
phosphites. Recently, molecular level stabilization of the material
using nanostructured chemicals has also been considered.
Acetaldehyde is normally a colorless, volatile substance with a
fruity smell. It forms naturally in fruit, but it can cause an
off-taste in bottled water. Acetaldehyde forms in PET through the
"abuse" of the material. High temperatures (PET decomposes above 300
°C or 570 °F), high pressures, extruder speeds (excessive shear flow
raises temperature) and long barrel residence times all contribute
to the production of acetaldehyde. When acetaldehyde is produced,
some of it remains dissolved in the walls of a container and then
diffuses into the product stored inside, altering the taste and
aroma. This is not such a problem for non-consumables (such as
shampoo), for fruit juices (which already contain acetaldehyde), or
for strong-tasting drinks like soft drinks. For bottled water,
however, low acetaldehyde content is quite important, because if
nothing masks the aroma, even extremely low concentrations (10–20
parts per billion in the water) of acetaldehyde can produce an
off-taste.
Antimony (Sb) is a catalyst that is often used as antimony trioxide
(Sb2O3) or antimony triacetate in the production of PET. After
manufacturing a detectable amount of antimony can be found on the
surface of the product. This residue can be removed with washing.
Antimony also remains in the material itself and can thus migrate
out into food and drinks. Exposing PET to boiling or microwaving can
increase the levels of antimony significantly, possibly above USEPA
maximum contamination levels.[9] The drinking water limit in the USA
for antimony is 6 parts per billion.[10] Although antimony trioxide
is of low toxicity when taken orally,[11] its presence is still of
concern. The Swiss Federal Office of Public Health investigated the
amount of antimony migration, comparing waters bottled in PET and
glass: the antimony concentrations of the water in PET bottles were
higher, but still well below the allowed maximum concentration. The
Swiss Federal Office of Public Health concluded that small amounts
of antimony migrate from the PET into bottled water, but that the
health risk of the resulting low concentrations is negligible (1% of
the "tolerable daily intake" determined by the WHO). A later (2006)
but more widely publicized study found similar amounts of antimony
in water in PET bottles.[12] The WHO has published a risk assessment
for antimony in drinking water.[11]
Possible toxicity of PET
Commentary published in Environmental Health Perspectives in April
2010 suggested that PET might yield endocrine disruptors under
conditions of common use and recommended[13] research on this topic.
Proposed mechanisms include leaching of phthalates as well as
leaching of antimony. Other authors (FRANZ and WELLE) published
evidence based on mathematical modeling, indicating that it is quite
unlikely that PET yields endocrine disruptors in mineral water.[14]
Bottle processing equipment
There are two basic molding methods for PET bottles, one-step and
two-step. In two-step molding, two separate machines are used. The
first machine injection molds the preform, which resembles a test
tube, with the bottle-cap threads already molded into place. The
body of the tube is significantly thicker, as it will be inflated
into its final shape in the second step using stretch blow molding.
In the second process, the preforms are heated rapidly and then
inflated against a two-part mold to form them into the final shape
of the bottle. Preforms (uninflated bottles) are now also used as
containers for candy, and by some Red Cross chapters to distribute
to homeowners to store medical history for emergency responders.[15]
In one-step machines, the entire process from raw material to
finished container is conducted within one machine, making it
especially suitable for molding non-standard shapes (custom
molding), including jars, flat oval, flask shapes etc. Its greatest
merit is the reduction in space, product handling and energy, and
far higher visual quality than can be achieved by the two-step
system.
Polyester recycling industry
When recycling polyethylene terephthalate or PET or polyester, two
ways generally have to be differentiated:
The chemical recycling back to the initial raw
materials purified terephthalic acid (PTA) or dimethyl terephthalate
(DMT) and ethylene glycol (EG) where the polymer structure is
destroyed completely, or in process intermediates like bis-ß-hydroxyterephthalate
The mechanical recycling where the original polymer properties are
being maintained or reconstituted.
Chemical recycling of PET will become cost-efficient only applying
high capacity recycling lines of more than 50,000 tons/year. Such
lines could only be seen, if at all, within the production sites of
very large polyester producers. Several attempts of industrial
magnitude to establish such chemical recycling plants have been made
in the past but without resounding success. Even the promising
chemical recycling in Japan has not become an industrial break
through so far. The two reasons for this are: at first, the
difficulty of consistent and continuous waste bottles sourcing in
such a huge amount at one single site, and, at second, the steadily
increased prices and price volatility of collected bottles. The
prices of baled bottles increased for instance between the years
2000 and 2008 from about 50 Euro/ton to over 500 Euro/ton in 2008.
Mechanical recycling or direct circulation of PET in the polymeric
state is operated in most diverse variants today. These kinds of
processes are typical of small and medium-sized industry.
Cost-efficiency can already be achieved with plant capacities within
a range of 5 000 – 20 000 tons/year. In this case, nearly all kinds
of recycled-material feedback into the material circulation are
possible today. These diverse recycling processes are being
discussed hereafter in detail.
Besides chemical contaminants and degradation products generated
during first processing and usage, mechanical impurities are
representing the main part of quality depreciating impurities in the
recycling stream. Recycled materials are increasingly introduced
into manufacturing processes, which were originally designed for new
materials only. Therefore, efficient sorting, separation and
cleaning processes become most important for high quality recycled
polyester.
When talking about polyester recycling industry, we are
concentrating mainly on recycling of PET bottles which are meanwhile
used for all kinds of liquid packaging like water, carbonated soft
drinks, juices, beer, sauces, detergents, household chemicals and so
on. Bottles are easy to distinguish because of shape and consistency
and separate from waste plastic streams either by automatic or hand
sorting processes. The established polyester recycling industry
consists of three major sections:
PET bottle collection and waste separation—waste logistics
Production of clean bottle flakes—flake production
Conversion of PET flakes to final products—flake processing
Intermediate product from the first section is baled bottle waste
with a PET content greater than 90%. Most common trading form is the
bale but also bricked or even loose, pre-cut bottles are common in
the market. In the second section the collected bottles are
converted to clean PET bottle flakes. This step can be more or less
complex and complicated depending on required final flake quality.
During third step PET bottle flakes are processed to any kind of
products like film, bottles, fiber, filament, strapping or
intermediates like pellets for further processing and engineering
plastics.
Besides this external (post-consumer) polyester bottle recycling,
numbers of internal (pre-consumer) recycling processes exist, where
the wasted polymer material does not exit the production site to the
free market, and instead is reused in the same production circuit.
In this way, fiber waste is directly reused to produce fiber,
preform waste is directly reused to produce preforms, and film waste
is directly reused to produce film.
[edit]
PET bottle recycling
see article: PET bottle recycling
[edit]
Purification and decontamination
The success of any recycling concept is hidden in the efficiency of
purification and decontamination at the right place during
processing and to the necessary or desired extent.
Generally, the following applies: the sooner foreign substances are
removed, in the process, and the more thoroughly this is done, the
more efficient the process is.
The high plasticization temperature of PET in the range of 280°C is
the reason why almost all common organic impurities such as PVC,
PLA, polyolefin, chemical wood-pulp and paper fibers, polyvinyl
acetate, melt adhesive, coloring agents, sugar and protein residues
are transformed into colored degradation products which, in their
turn, might release reactive degradation products additionally.
Then, the number of defects in the polymer chain increases
considerably. Naturally, the particle size distribution of
impurities is very wide, the big particles of 60–1000 µm—which are
visible by naked eye and easy to filter—representing the lesser evil
since their total surface is relatively small and the degradation
speed is therefore lower. The influence of the microscopic
particles, which—because they are many—increase the frequency of
defects in the polymer, is comparable bigger.
The motto "What the eye does not see the heart cannot grieve over"
is considered to be very important in many recycling processes.
Therefore besides efficient sorting the removal of visible impurity
particles by melt filtration processes is playing a particular part
in this case.
In general, one can say that the processes to make PET bottle flakes
from collected bottles are as versatile as the different waste
streams are different in their composition and quality. In view of
technology there isn't just one way to do it. There are meanwhile
many engineering companies which are offering flake production
plants and components, and it is difficult to decide for one or
other plant design. Nevertheless there are processes which are
sharing most of these principles. Depending on composition and
impurity level of input material, the general following process
steps are applied.[16]
Bale opening, briquette opening
Sorting and selection for different colors, foreign polymers
especially PVC, foreign matter, removal of film, paper, glass, sand,
soil, stones and metals
Pre-washing without cutting
Coarse cutting dry or combined to pre-washing
Removal of stones, glass and metal
Air sifting to remove film, paper and labels
Grinding, dry and / or wet
Removal of low-density polymers (cups) by density differences
Hot wash
Caustic wash
Caustic surface etching, maintaining intrinsic viscosity and
decontamination
Rinsing
Clean water rinsing
Drying
Air sifting of flakes
Automatic flake sorting
Water circuit and water treatment technology
Flake quality control
Impurities and material defects
The number of possible impurities and material defects which
accumulate in the polymeric material is increasing permanently—when
processing as well as when using polymers—taking into account a
growing service life time, growing final applications and repeated
recycling. As far as recycled PET bottles are concerned, the defects
mentioned can be sorted in the following groups:
a) Reactive polyester OH- or COOH- end groups are transformed into
dead or non-reactive end groups, e.g. formation of vinyl ester end
groups through dehydration or decarboxylation of terephthalate acid,
reaction of the OH- or COOH- end groups with mono-functional
degradation products like mono-carbonic acids or alcohols. Results
are decreased reactivity during re-polycondensation or re-SSP and
broadening the molecular weight distribution.
b) The end group proportion shifts toward the direction of the COOH
end groups built up through a thermal and oxidative degradation. The
results are decrease in reactivity, and increase in the acid
autocatalytic decomposition during thermal treatment in presence of
humidity.
c) Number of poly-functional macromolecules increases. Accumulation
of gels and long-chain branching defects.
d) Number, concentration and variety of non polymer-identical
organic and inorganic foreign substances are increasing. With every
new thermal stress, the organic foreign substances will react by
decomposition. This is causing the liberation of further
degradation-supporting substances and coloring substances.
e) Hydroxide and peroxide groups build up at the surface of the
products made of polyester in presence of air (oxygen) and humidity.
This process is accelerated by ultraviolet light. During an ulterior
treatment process, hydro peroxides are a source of oxygen radicals
which are source of oxidative degradation. Destruction of hydro
peroxides is to happen before the first thermal treatment or during
plasticization and can be supported by suitable additives like
antioxidants.
Taking into consideration the above-mentioned chemical defects and
impurities, there is an ongoing modification of the following
polymer characteristics during each recycling cycle, which are
detectable by chemical and physical laboratory analysis.
In particular:
Increase of COOH end groups
Increase of color number b
Increase of haze (transparent products)
Increase of oligomer content
Reduction in filterability
Increase of by-products content such as acetaldehyde, formaldehyde
Increase of extractable foreign contaminants
Decrease in color L
Decrease of intrinsic viscosity or dynamic viscosity
Decrease of crystallization temperature and increase of
crystallization speed
Decrease of the mechanical properties like tensile strength,
elongation at break or elastic modulus
Broadening of molecular weight distribution
The recycling of PET-bottles is meanwhile an industrial standard
process which is offered by a wide variety of engineering
companies.[17]
Processing examples for recycled polyester
Recycling processes with polyester are almost as varied as the
manufacturing processes based on primary pellets or melt. Depending
on purity of the recycled materials polyester can be used today in
most of the polyester manufacturing processes as blend with virgin
polymer or increasingly as 100% recycled polymer. Some exceptions
like BOPET-film of low thickness, special applications like optical
film or yarns through FDY-spinning at > 6000 m/min or microfilaments
and micro-fibers are produced from virgin polyester only.
Simple re-pelletizing of bottle flakes
This process consists in transforming bottle waste into flakes, by
drying and crystallizing the flakes, by plasticizing and filtering,
as well as by pelletizing. Product is an amorphous re-granulate of
an intrinsic viscosity in the range of 0.55–0.7 dℓ/g, depending on
how complete pre-drying of PET flakes has been done.
Special feature are: acetaldehyde and oligomers are contained in the
pellets at lower level; the viscosity is reduced somehow, the
pellets are amorphous and have to be crystallized and dried before
further processing.
Processing to:
A-PET film for thermoforming,
Addition to PET virgin production,
BoPET packaging film,
Bottle resin by SSP,
Carpet yarn,
Engineering plastics,
Filaments,
Non-woven,
Packaging stripes,
Staple fibre,
Choosing the re-pelletizing way means having an additional
conversion process which is at the one side energy intensive, cost
consuming and causes thermal destruction. At the other side the
pelletizing step is providing the following advantages:
Intensive melt filtration
Intermediate quality control
Modification by additives.
Product selection and separation by quality
Processing flexibility increased
Quality uniformization
[edit]
Manufacture of PET-pellets for bottles (B-2-B) and A-PET
This process is, in principle, similar to the one described above;
however, the pellets produced are directly (continuously or
discontinuously) crystallized and then subjected to a solid-state
polycondensation (SSP) in a tumbling drier or a vertical tube
reactor. During this processing step, the corresponding intrinsic
viscosity of 0.80 – 0.085 dℓ/g is rebuild again and, at the same
time, the acetaldehyde content is reduced to < 1 ppm.
The fact that some machine manufacturers and line builders in Europe
and USA make efforts to offer independent recycling processes, e.g.
the so called bottle-to-bottle (B-2-B) process, such as URRC or
BÜHLER, aims at generally furnishing proof of the "existence" of the
required extraction residues and of the removal of model
contaminants according to FDA applying the so called challenge test,
which is necessary for the application of the treated polyester in
the food sector. Besides this process approval it is nevertheless
necessary that any user of such processes has to constantly check
the FDA-limits for the raw materials manufactured by himself for his
process.
Direct conversion of bottle flakes
In order to save costs, one is working on the direct use of the
PET-flakes, from the treatment of used bottles, with a view to
manufacturing an increasing number of polyester intermediates. For
the adjustment of the necessary viscosity, besides an efficient
drying of the flakes, it is possibly necessary to also reconstitute
the viscosity through polycondensation in the melt phase or
solid-state polycondensation of the flakes. The latest PET flake
conversion processes are applying twin screw extruders, multi-screw
extruders or multi-rotation systems and coincidental vacuum
degassing to remove moisture and avoid flake pre-drying. These
processes allow the conversion of undried PET flakes without
substantial viscosity decrease caused by hydrolysis.