What is Mylar?

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The brightly colored balloons and plastic sheeting we use to protect paper and fabric are made from Mylar. “Mylar” is the trade name of a polymer whose technical name is “biaxially-oriented polyethylene terephthalate”, BoPET for its English name ( biaxially-oriented polyethylene terephthalate ). BoPET is obtained by processing polyethylene terephthalate, the PET widely used to manufacture containers of all kinds. Other trade names for BoPET are Melinex and Hostphan; It was developed in the 1950s by the American company DuPont, the English ICI and the German Hoechst, and used by Kodak as a support for photographic films.

How is the BoPET obtained?

BoPET is obtained from PET, (C 10 H 8 O 4 ) n , a polyester that is the result of the polymerization of terephthalic acid reacting with ethylene glycol. Terephthalic acid, C 6 H 4 (COOH) 2 , is an aromatic compound; six of its carbons form a benzene ring, while the remaining two carbons, located at opposite ends of the hexagon that forms the benzene ring, have a double bond with oxygen and the remaining one with an hydroxyl group. Ethylene glycol, C 2 H 6 O 2, has a hydroxyl (or hydroxyl) group on each of its two carbons; the hydroxyls of ethylene glycol and those on the side carbons of the benzene ring of terephthalic acid are replaced by an oxygen bridge, integrating the unit that repeatedly combines (see the figure below) to form the long polymer chain.

Polyethylene terephthalate monomer
Polyethylene terephthalate monomer

PET is a thermoplastic polymer; above a certain temperature it softens without degrading like thermosetting polymers, which allows its shape to be modified and the material to be processed by injection, by deposit on a mold or by extrusion. If it is cooled quickly after processing it crystallizes into small nuclei and a transparent material is obtained.

BoPET is obtained by processing PET powder. This powder is melted and laminated, and the resulting sheet is cold extruded so that the material loses its crystalline structure. Biaxial orientation is carried out on this sheet, laminating it again but this time at high temperature, using hot rollers and inside an oven. Hot rolling is repeated alternating the rolling direction perpendicularly. Then the material is heated to temperatures above 200 o C to crystallize it with the bidirectional orientation of the polymer macromolecules. With crystallization a transparent material is also obtained. Various additives are added to the material to improve its properties and surface finish.

The properties and uses of BoPET

BoPET is an electrical and thermal insulating material, has high resistance, is chemically stable and is impermeable to gases, constituting an insulator against odors, all qualities that have made it a material used in many different applications. Food containers in various shapes, sheets applied to paper and cloth as a protective layer and to give a glossy finish, thermal and electrical insulation, reflective material and part of decoration elements, are some of the applications of BoPET.

The use of polyethylene terephthalate to package edible products has generated concern in recent years due to the possibility that it could contaminate food with molecules or elements incorporated into the material in the manufacturing process, such as antimony, and that later migrate from the material. of the container.

Sources

María Anna Coniglio, Cristian Fioriglio, Pasqualina Laganá, Non-intentionally added substances in PET-bottled mineral water . Chapter 3; Polyethyleneterephthalate. Springer, 2020.

Mark T. DeMeuse Biaxial Stretching of Film: Principles and Applications . Elsevier, 2011.

Sergio Ribeiro Guevara (Ph.D.)
Sergio Ribeiro Guevara (Ph.D.)
(Doctor en Ingeniería) - COLABORADOR. Divulgador científico. Ingeniero físico nuclear.

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