Recycling PET plastic
Quite a few sources on the Web state that the PET bottle for pressurized drinks was invented by Nathaniel Wyeth of Du Pont. This is the familiar thin-walled, flexible bottle found in every shop.
American patent 3733309 was published in 1973 and was basically for the machinery for making the “biaxially oriented” bottles. The material -- Polyethylene terephthalate -- is based on polyester, as used in clothing. Wyeth later recalled showing the misshapen results of an early experiment to the laboratory director, who wondered about spending so much money to get such a “terrible-looking bottle.” Wyeth was pleased that it was at least hollow. DuPont asked other companies if they wanted to manufacture it under license, but many initially refused, such as packaging giants Owens-Illinois and Continental Can.
PET, or PETE, consists of polyethylene terephthalate, a thermoplastic polymer, which is used 60% for synthetic fibres and 30% for bottles. What was important about it was that such bottles did not contaminate the contents. Other advantages were that the bottles were light and virtually unbreakable, unlike glass. The material is often also used for sturdy packaging such as oven ready meal trays.
They also, however, presented a problem with recycling. They take a very long time to degrade, and if not crushed empty cylinders take a lot of space in landfill. The first recycling attempts are thought to go back to 1977. Nowadays bottles, which are normally transparent or green (7-Up® for example), commonly have on the side or bottom a triangular symbol of arrows with a 1 in the middle with (in the USA) PETE underneath it. There are six categories, with number 7 being for those which are “other” and are not recycled. The American Society of Plastics Industry developed the symbols. http://www.wasteonline.org.uk/resources/InformationSheets/Packaging.htm#4 explains about the symbols as used in Britain.
This means that the correct bottles can be identified in automatic sorting, which is vital, as “plastics” vary in their composition, and the different types need to be sorted correctly. An increasing number of local authorities in Europe will accept categories 1, 2 or 3 for recycling. It is forecast that up to a million tons of PET will be collected in Europe by 2010.
The bottles are broken down into small “flakes” which are compressed into “bales” which are then used to make new bottles, or other products such as other kinds of packaging or textile fibres.
According to a recent patent application it has recently been discovered that there are advantages to making such flakes very small. They “exhibit unexpectedly superior processing properties for the production of new plastic articles”. This is my Patent of the Month, WO 2006/020603.
The company is Phoenix Technologies International, who are based in Ohio.
The problem with the tiny flakes, which are easy to decontaminate, is that they are difficult to transport and to handle. First the tiny flakes are created in the first place, using conventional equipment such as “grinders, ball mills, impact grinders, cryogenic grinders, pulverizers, attrition mills”, suggests the patent application.
The results is flakes of a size between about 0.013 mm to 1.3 mm. Then they are heated to the same, or slightly higher, temperature needed to create PET in the first place. This is called the glass transition temperature and is above about 70° C. The result is that rather than forming a hard, solid mass they loosely adhere to each other as what they call pellets. As there are gaps in the pellets they have the same surface to volume ratio as before the treatment (because of the spaces between the flakes), and have a superior ratio to traditional methods. This is apparently advantageous in using the material. Containers are recommended as a particularly suitable product to be made from the pellets.
The company now offers the pellets for sale to companies wanting a feedstock for their products.
Our Patent of the Month is provided courtesy of Steve Van Dulken from the British Library. Steve is a patent specialist at the British Library and has his own blog at http://britishlibrary.typepad.co.uk/patentsblog/
Steve has worked since 1987 as a patents librarian at the British Library. Author of Inventing the American Dream, Inventing the 19th century and Inventing the 20th century, both popular science, and of British patents of invention, 1617-1977: a guide for researchers and editor of Introduction to patents information.
