Polyethylene terephthalate

From Academic Kids

Polyethylene terephthalate (aka. PET, PETE, PETP) is a plastic resin of the polyester family that is used to make beverage, food and other liquid containers, as well as for some other thermoforming applications. It is also 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. It can be synthesized by a transesterification reaction between ethylene glycol and dimethyl terephthalate.

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Chemical structure of polyethylene terephthalate
Contents

Uses

The main virtue of PET is that it is fully recyclable. Unlike other plastics, its polymer chains can be recovered for additional use. PETE has a resin code of 1.

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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 and solvents. It is strong and impact-resistant. Its is naturally colorless and transparent.

Mylar is often coated with aluminium to reduce its permeability, and to make it reflective and opaque. For soft drink bottles, PET is often blended with polyvinyl alcohol to reduce its carbon dioxide permeability (see carbonation).

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.

PET was patented in 1941 by the Calico Printer's Association of Manchester. The PET bottle was patented in 1973.

Intrinsic viscosity

One of the most important characteristics of PET is refered to as I.V.(Intrinsic Viscosity) The IV of the material is dependent upon the length of its polymer chains. The longer the chains, the stiffer the material, and therefore the higher the IV. The average chain length of a particular batch of resin can be controlled during polymerization.

an IV of about 0.6 would be approriate for fiber .65 - film .8 - bottles .85 - tire cord

Drying

PET is hygroscopic, meaning that it naturally absorbs water from its surroundings. However, before the resin can be processed in a molding machine, all moisture must be removed from the resin. This is achieved through the use of a dryer.

Inside the dryer, the air is run through an after cooler, because it is easier to remove moisture from cold air than hot air. The air is then dispersed into a dessicant bed. The air leaving the dessicant bed is cool and dry. The air then flows through a process heater. After that we pump our hot dry air into the bottom of the hopper containing the resin and it flows up through the resin removing moisture on its way by. The air leaves the top of the hopper and is run back through the same processes in a closed loop.

This process takes less time per batch when the drier is run at a higher temperature.

140 degrees Celsius air = 12 hours
145 degrees Celsius air = 6.5 hours
160 degrees Celsius air = 4 hours

Dryer residence time should not be shorter than 4 hours. This is because drying the material in less than 4 hours would require a temperature over 160 degrees Celsius. Exposure to such high temperatures will begin to degrade the outside of a pellet before its center is dry.

Copolymers

In addition to pure (homopolymer) PET, PET modified by copolymerization is also available. In many cases, a copolymer has fewer of the weaknesses of PET, but retains its strengths. For example, cyclohexane dimethanol (CHDM) can be added to the polymer backbone in place of dimethyl terephthalate. Since the cyclohexane ring is not aromatic, it does not participate in phenyl stacking with neighboring chains the way a terephthalate group would. This interferes with crystallization and lowers the polymer's melting temperature. This is helpful because high temperatures cause PET to degrade.

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. We are able to produce clear products 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.

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 opaque and white in most cases. Fiber drawing is among the few industrial processes that produces a nearly single-crystal product.

Degradation

When PET degrades, several things happen, including crystallization and the creation of acetaldehyde. Acetaldehyde is normally a colorless gas with a fruity smell. It is produced naturally in fruit. Acetaldehyde is produced in PET through the "abuse" of the material. High temperatures, pressures, extruder speeds and long barrel residence times all contribute to the production of acetaldehyde. When acetaldehyde is produced it becomes trapped in the walls of a container and through permeation it can change the smell/taste of the product stored inside. This is not such a problem for things such as shampoo(because it is not consumed) or fruit juice(since acetaldehyde already exists naturally in it) as it is in water or sodas.

With those two things in mind, think back to copolymers. By adding a catalyst, in this case CHDM, we can lower the melt temperature of PET. By lowering the melt temperature, we are able to plasticize the resin without causing degradation, thus avoiding crystallization and acetaldehyde content in our finished product.

Crystallization experiment

Find a plastic beverage bottle, and verify that it is PET through the recycling code on the bottom (PET or PETE and a code of 1). Hold a flame several inches below the bottle and slowly bring it closer. Once the temperature of the bottle is high enough, the polymer chains will begin to move and will fold up on themselves. Because the crystaline structure is more dense than the amorphous state, and due to surface tension, the plastic will seem to shrink. Also, the crystallites that appear will scatter light, giving the material an opaque white appearance.

See also

fr:Polythylne trphtalate nl:PET (plastic) ja:ポリエチレンテレフタラート pl:Politereftalan etylenu

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