Polyethylene terephthalate

• PET
• PETE
• PETP (deprecated)
• Ethylene terephthalate
• Poly ( oxyethyleneoxy terephthaloyl )

Thermoplastic

Fixed

1.38 g · cm -3 ( 20 ° C)

250-260 ° C

70 ° C.

Partially crystalline

4500 N · mm 2 longitudinally and transversely

0.5 %

Practically insoluble in water

7:10 -5K- 1

Polyethylene terephthalate (abbreviated PET) is a manufactured by polycondensation thermoplastic material of the polyester family. PET has a variety of uses and is among other things for the production of plastic bottles (PET ) bottles, films and textile fibers. Global production is 40 million tons a year.

• 3.1 polyester film
• 3.2 Metallized polyethylene terephthalate ( MPET )

Properties

Physical Properties

PET is polar, thus are strong intermolecular forces present. The molecule is also a linear structure without cross-links. Both are prerequisites for semi-crystalline regions and fibers. Through these areas there is also a high tensile strength and dimensional stability results at a temperature above 80 ° C. However, the impact strength is low, the friction and wear behavior well. The glass transition temperature is about 80 ° C. In the semi-crystalline state (C - PET), PET is transferred at approximately 140 ° C. The unit cell is triclinic { a = 4.56 nm, b = 5.94 nm, c = 10.75 nm, α = 98.5 °, β = 118 °, γ = 112 ° }. The density of amorphous PET ( A- PET) is from 1.33 to 1.35 g · cm3 and of semi-crystalline C-PET 1.38 to 1.40 g · cm3. Compared to C-PET A-PET has a slightly lower stiffness and hardness, but a higher impact resistance. The density of the crystalline regions is dependent on the duration and the temperature of the solid phase polycondensation, which is performed by default in order to obtain higher molecular weight PET grades. The degree of crystallization hardly exceeds 70%. The melting point (depending on the degree of crystallization and the degree of polymerisation ) 235-260 ° C.

In case of strong heating produced by blowing a PET bottle pulls - like vacuum drawn yogurt cup made ​​of PS - partially back together.

Chemical Properties

Against the attack of strong inorganic acids, especially sulfuric acid or nitric acid and hydrochloric acid, PET is fickle. Beverage bottles may be filled with chemicals or other non- potable liquids, however, because of the likelihood of confusion.

Production

The monomers from which the PET is prepared, are terephthalic acid (1,4- benzenedicarboxylic acid ) and ethylene glycol (1,2- dihydroxyethane, ethane -1 ,2 -diol). The industrial production is carried out partly or by transesterification of dimethyl terephthalate with ethanediol. Since this is an equilibrium reaction, an excess of the glycol is used, which is distilled by the reaction again to affect the balance low. The melt-phase polycondensation does not lead to sufficiently high molecular weights ( in technically meaningful periods). Therefore, PET bottles or grades for industrial thread (eg, diols, Trevira ) subsequently by solid state polycondensation (SSP - solid state polycondensation ) can be further condensed. And ring-opening reactions of oligomers are possible, and no condensate is obtained quickly and high molecular weights of more than 100,000 g / mol can be obtained. These methods are still in development. In more recent method ethanediol is directly esterified with terephthalic acid. When processing PET is preferred in semi-crystalline form, however, it spontaneously crystallized very slowly so that a nucleating agent must be added for rapid crystallization.

PET often find traces of antimony ( III) oxide, which is used in the preparation of the catalyst.

Use

PET is used in many forms, and widely used. Among the best known uses include the manufacture of plastic bottles ( PET bottle manufacturing process injection blow molding, stretch blow molding) of all types and processing into textile fibers. For the production of film material, as it is used in cinemas, PET is used. Since the 1950s, PET is used for the production of very thin films, often under the name HOSTAPHAN ®, Mylar ®. PET has the recycling code 01

As a textile fiber ( polyester) PET is used for various useful properties. It is wrinkle-free, tear- resistant, weather resistant and absorbs very little water. The latter predestined PET fabric for sportswear that needs to dry quickly.

In the food industry, PET is preferably used. It can be processed amorphous and in this form is absolutely colorless and of high light transmittance. It is used for food packaging and bottles, such as the PET bottle.

However, in the production of PET bottles produced acetaldehyde which pass in small amounts into the content (including mineral water ) and ( sensory) it can change the taste. In the production of antimony often used (III ) oxide ( antimony trioxide ) may be solved in the liquid contents of a PET bottle. Investigations of bottled fruit juices in PET bottles were Antimontrioxidkonzentrationen from < 1 to 44.7 ug / L in undiluted juice concentrates. The applicable limit (so-called specific migration limit) for the transition of antimony trioxide plastic in food is 40 ug / L. This value was exceeded in finished bakery products partly because this particular can be solved easily at high temperature antimony trioxide. Antimony trioxide is used by the International Agency for Research on Cancer ( IARC) classified as possibly carcinogenic substance.

Because of its good tissue compatibility PET is also used as a material for vascular grafts.

Polyester film

A big important application for polyethylene terephthalate ( PET) are films that are today produced in thicknesses of 1 to 800 microns. Your application goes through Dünnstfolien for plastic film capacitors, typewriter ribbons, stamping foils, packaging films for aroma-tight packaging, furniture foils, colored light shield film, motion picture film, photographic film, X - ray film, electrical insulating, Ankernutisolierfolie to the film for test strips in the pharmaceutical industry. A major application area is also support films for magnetic tapes.

PET film is produced from a raw material granulate which is dried first, or directly from the melt ( Uhde Inventa- Fischer method). The dried to avoid hydrolytic degradation during processing. By melting extrusion and filtration, a film is about a slot die prepared therefrom. The liquid melt is pressed by means of an electrostatic pinning film to a casting roll and the glass transition point of PET, which is about 65 to 80 ° C, cooled. The electrostatic pinning method is a application liquid for the melt film in which, for example, any of the bare wire in millimeter pitch is mounted in parallel with the film of melt. To the wire a high voltage of 5 to 10 kV is applied. By the dipole character of the PET molecules, the wire facing side of the film is positively charged. The opposite pole is grounded casting roll. With this method, the molten film is pressed against the casting roll, the air between the PET film and the chilled casting roll is displaced. This is important for rapid uniform cooling of the film. In this step, the so-called pre-film.

Your final mechanical properties of the film gets by the subsequent stretching process. The stretching is usually carried out in two steps, first in the longitudinal and then in the transverse direction. For the stretching, the film needs to be re-heated to above the glass temperature. To the longitudinal stretching, the film is passed over heated rollers, heated to stretching temperature, for example 85 ° C. and at a stretch gap with an additional IR radiator in the longitudinal direction by 2.5 - stretched to 3.5 times. The rolling after the extension gap rotate accordingly higher speed.

The second step is the stretching of wide. This is performed in a tentering frame, in which the film is held at the side edges of clip chains. Clip chains run parallel to the film web. At the individual chain links retaining clips are attached for grasping the film edges. First, the film is heated with hot air and then stretched in the width. The chains are guided on a guide rail such that, after the preheating of the distance between the dip chains to the 2,5 - to 4 - times is widened. The last step is the thermal setting of the film. Still clamped in the clip chain foil is heated to a temperature between 200 and 230 ° C. Voltages in the film are degraded. The heat setting the molecular threads are so close together that creates a physical connection between the molecular chains due to the small distance. This is the crystallite.

In addition to this standard process, there are various types, in order to produce special features such as increased strength in the longitudinal direction.

Should be used and remain dimensionally stable film later at high temperatures ( > 80 ° C), so it is advisable to pre-shrink these films in an oven above the later operating temperature, to make them tension free. In this process, the dimensions of the film according to their manufacturing process change. Therefore, some manufacturers also offer pre-shrunk films for high temperature applications.

Depending on the desired application of the raw material or pigments may be added. Thus, the winding properties of the final film to be improved. Also for dulling film for furniture such films are pigmented. Even colored pigments. Other soluble additives, there is also the UV stabilizer, and absorption or for coloring.

The raw material can also be modified by other polymer blocks. By replacing a portion of the terephthalic acid by isophthalic acid, so decreases the melting point of the raw material and the crystallization tendency and there is PETIP. The linear chain formation is disturbed. By co-extrusion of PET PETIP sealable films are produced.

The finished films are often coated or bonded with other films to Connected. Composites are, for example, aroma-tight film for coffee packaging.

Metallized polyethylene terephthalate ( MPET )

Metallized polyethylene terephthalate (abbreviation: MPET ), such as biaxially oriented PET ( boPET, Mylar ) is used for emergency blankets and has been used as flame retardants. His inadequate fire resistance has been identified as one of the causes of the heavy flight accident of Swissair flight 111 on September 2, 1998.

Recycling

The high value of PET and the profitability of recycling can also be seen that for high-tech sorting methods have been developed using the high-speed laser spectroscopy. To sort the fragments kleingeschredderter PET bottles which contain not only the main component of PET, other plastics ( film layer in the wall, cover), sorted for recycling in food quality. The method 2010 was nominated for the German Future Prize.

In 2009, 48.4 % of all PET bottles were collected for recycling in Europe. 40 % of recycled PET has been used for textile fiber, 27 % for films and thermoformed products, from 22 % in new bottles and containers were prepared and 7 % were used for the production of PP-straps.