Polyester film

Polyester film is a transparent, flexible film, ranging from 0.15 to 14 mils in thickness, used as a product component, in industrial processes, and for packaging. The most widely used type is produced from polyethylene terephthalate (i.e., Mylar). Polyester films based on other polymers or copolymers or manufactured by other methods are not identical, although they are similar in nature.

It is the strongest of all plastic films and strength is probably the outstanding property. However, it is useful as an engineering material because of its combination of desirable physical, chemical, electrical, and thermal properties. For example, strength combined with heat resistance and electrical properties makes it a good material for motor slot liners.

Polyester film is made by the condensation of terephthalic acid and ethylene glycol. The extremely thin film, 0.00063 to 0.0013 cm, used for capacitors and for insulation of motors and transformers, has a high dielectric strength, up to 236 x 106 V/m. It has a tensile strength of 137 MPa with elongation of 70%. It is highly resistant to chemicals, and has low water absorption. The material is thermoplastic, with a melting point at about 254°C. Polyester fibers are widely used in clothing fabrics. The textile fiber produced from dimethyl terephthalate is known as Dacron.

For magnetic sound-recording tape, polyester tape has the molecules oriented by stretching to give high strength. The 0.013-cm tape has a breaking strength of 3.4 kg/0.64 cm of width. Electronic tape may also have a magnetic-powder coating on the polyester. But where high temperatures may be encountered, as in spacecraft, the magnetic coating is applied to metal tapes.


Polyester film has excellent resistance to attack and penetration by solvents, greases, oils, and many of the commonly used electrical varnishes. At room temperature, permeability to such solvents as ethanol, ethyl acetate, carbon tetrachloride, hexane, benzene, acetone, and acetic acid is very low. It is degraded by some strong alkali compounds and embrittles under severe hydrolysis conditions.

Moisture absorption is less than 0.8% after immersion for a week at 25°C. Water-vapor permeability is similar to that of polyethylene film and permeability to gases is very low. The film is not subject to fungus attack and copper corrosion is negligible.

An outstanding feature of the film is the fact that good physical and mechanical properties are retained over a wide temperature range. Service temperature range is -60 to 150°C. The effect of temperature is relatively small between -20 and 80°C. No embrittlement occurs at temperatures as low as -60°C, and useful properties are retained up to 150 to 175°C. Tensile modulus drops off sharply at 80 to 90°C.

Melting point is 250 to 255°C; thermal coefficient of expansion is 15 x 10-6 in./in./°F; and shrinkage at 150°C is 2 to 3%.

Fabrication and Forms

Polyester film can be printed, laminated, metallized, coated, embossed, and dyed. It can be slit into extremely narrow tapes (0.038 mm and narrower), and light gauges can be wound into spiral tubing. Heavy gauges can be formed by stamping or vacuum (thermo-)forming. Matte finishes can be applied, and adhesives for bonding the film to itself and practically any other material are available.

Because of its desirable thermal characteristics, polyester film is not inherently heat seal-able. However, some coated forms of the film can be heat-sealed, and satisfactory seals can be obtained on the standard film by the use of benzyl alcohol, heat, and pressure.

Polyethylene terephthalate film is available in several different types:

A. General-purpose and electrical film for wide variety of uses

C. Special electrical applications requiring high insulation resistance

D. Highly transparent film, minimum surface defects

K. Coated with a polymer for heat seal-ability and outstanding gas and moisture impermeability HS. Shrinks uniformly about 30% when heated to approximately 100°C; after shrinking, it has substantially the same characteristics as the standard film

T. A film with high tensile strength (available in some thin gauges) with superior strength characteristics in the machine direction; designed for use in tapes requiring high-strength properties

W. For outdoor applications; resistant to degradation by ultraviolet light


The range of properties outlined above has made polyester film functional in many totally different industrial applications and suggests its use in numerous other ways.

The largest current user of the film is the electrical and electronic market, which uses it as slot liners in motors and as the dielectric for capacitors, replacing other materials that are less effective, bulkier, and more expensive. It is found in hundreds of wire and cable types, sometimes used primarily as an insulating material, sometimes for its mechanical and physical contributions to wire and cable construction. Reduced cost of materials and processing and improved cable performance result.

Magnetic recording tapes for both audio and instrumentation uses are based on polyester film. In audio applications they contribute toughness, durability, and long play; for instrumentation tapes, the film ensures maximum reliability. The film has proved to be a highly successful new material for the textile industry since it can be used to produce metallic yarns that are no tarnishing and unusually strong. They can be run unsupported, knit, dyed at the boil, and either laundered or dry-cleaned. Yarns are made by laminating the film to both sides of aluminum foil or by laminating metallized and transparent film. The structure is then slit into the required yarn widths.

As a surfacing material, polyester film is used on both flexible and rigid substrates for both protective and decorative purposes. Metallized, laminated to vinyl, and embossed, the film becomes interior trim for automobiles, for example.

With a special coating, the film becomes a drafting material that is tougher and longer-lasting than drafting cloth. It is used for map-making, templates, and other applications in which its dimensional stability becomes a significant factor.

Strength in thin sections makes the film advantageous for sheet protectors, card holders, sheet reinforcers, and similar stationery products. Pressure-sensitive adhesives make the properties of the film available for uses ranging from decorative trim to movie splicing.

The weatherable form (Type W) has a life of 4 to 7 years. It is principally used in greenhouses, where it cuts construction costs by as much as two thirds because a simple, inexpensive structure suffices and maintenance costs are at a minimum.

In the packaging field, polyester film serves in areas where other materials fail or have functional disadvantages. In window cartons it lasts longer and does not break as other materials do; its toughness permits transparent packaging of heavy items; and, coated with polyethylene, it has made possible the "heat-in-the-bag" method of frozen-food preparation.

An optically clear form of polyester film in thicknesses of 4 to 7 mils is used as a base for the coating of light-sensitive emulsions in the manufacture of photographic film. The outstanding qualities of toughness and dimensional stability make this film especially well suited as a base for graphic arts films, motion picture film, engineering reproduction films, and microfilm. Other advantages include excellent storage and aging characteristics.

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