Polyethylene terephthalate

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. It is one of the most important raw materials used in man-made fibers.

Depending on its processing and thermal history, it may exist both as an amorphous (transparent) and as a semi-crystalline (opaque and white) material. Its monomer can be synthesized by the esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct, or the 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 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.

Some of the trade names of PET products are Dacron, Diolen, Terylene, and Trevira fibers,^[1] Cleartuf, Eastman PET and Polyclear bottle resins, Hostaphan, Melinex, and Mylar films, and Arnite, Ertalyte, Impet, Rynite and Valox injection molding resins. The polyester Industry makes up about 18% of world polymer production and is third after polyethylene (PE) and polypropylene (PP).

Chemical structure of polyethylene terephthalate

PET consists of polymerized units of the monomer dimethyl terephthalate or ethylene terephthalate, with repeating C₁₀H₈O₄ units. It contains the chemical elements carbon, hydrogen, and oxygen. If fully burned, it produces only carbon dioxide (CO₂) and water (H₂O). PET is commonly recycled, and has the number "1" as its recycling symbol.

Uses

A PET soft drink bottle

PET can be semi-rigid to rigid, depending on its thickness, and 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 high transparency.

PET bottles 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.

When produced as a thin film (biaxially oriented PET film, often known by one of its tradenames, "Mylar"), PET can be aluminized by evaporating a thin film of metal onto it to reduce its permeability, and to make it reflective and opaque. 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.

Sails are usually made of Dacron, a brand of PET fiber; colorful lightweight spinnakers are usually made of nylon.

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, which makes them more easily identifiable in a recycle stream. 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. It can also make fiber for polyester products.

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 sulphur). 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.

Intrinsic viscosity

One of the most important characteristics of PET is referred to as I.V. (intrinsic viscosity).

The I.V. of the material, measured in deciliters per gram (dl/g) is dependent upon the length of its polymer chains. The longer the chains, the stiffer the material, and therefore the higher the I.V. The average chain length of a particular batch of resin can be controlled during polymerization.

An I.V. of about:

0.60 dl/g: Would be appropriate for fibre

0.65 dl/g: Film

0.76-0.84 dl/g: Bottles

0.85 dl/g: Tire cord

Drying

PET is hygroscopic, meaning that it naturally absorbs water from its surroundings. However, when this 'damp' PET is then heated a chemical reaction known as hydrolysis takes place between the water and the PET which reduces its molecular weight (IV) and its physical properties. 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 40 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.

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 neighbouring 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 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 packagings 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.

Crystals

Crystallization 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 produces a nearly single-crystal product.

Degradation

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. Acetaldehyde is normally a colorless gas 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, such as soft drinks. For bottled water, low acetaldehyde content is quite important, because if nothing masks the aroma, even extremely low concentrations (10-20 parts per billion parts of resin, by weight) of acetaldehyde can produce an off-taste. The thermal and thermooxidative degradation results in poor processability 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 reduces 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.

Antimony

Antimony trioxide (Sb₂O₃) is a catalyst that is often used in the production of PET. It remains in the material and can thus in principle migrate out into food and drinks. Although antimony trioxide is of low toxicity, 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 was higher, but still well below the allowed maximal concentrations.[1] (report available in German and French only) 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 by a group of geochemists at the University of Heidelberg headed by William Shotyk found similar amounts of antimony in water in PET bottles.[2]

The most recent WHO risk assessment for antimony in drinking water can be found here: [3]

Bottle processing equipment

There are two basic moulding methods for PET bottles, one-step and two-step. In two-step moulding, two separate machines are used. The first machine injection moulds the preform. The preform looks like a test tube. The bottle-cap threads are already moulded into place, and 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 mould to form them into the final shape of the bottle. Preforms (uninflated bottles) are now also used as containers for candy.

In one-step machines, the entire process from raw material to finished container is conducted within one machine, making it especially suitable for moulding 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.

See also

• Recycling of PET Bottles
• Biaxially oriented PET film
• Plastic recycling
• Polyester
• Solar water disinfection - a method of disinfecting water using only sunlight and plastic PET bottles

External links

• American Plastics Council info on plastic bottles, refilling, PET safety
• Information on PET, detailed information of PET uses in different bottles
• Chemical Engineering - Polyester Information Platform, literature collection about polyester