Specific properties that separate engineering films from their commodity counterparts include greater tensile and impact strength; improved moisture and gas barrier characteristics; good heat resistance and weatherability; better bonding and lamination; and improved electrical ratings. One or more of these properties can be obtained by choosing from a number of different polymer films.
Several melt-processible engineering thermoplastic films such as oriented polyester, oriented nylon, and unoriented nylon, exhibit high strength especially at high temperatures. In addition, they provide toughness at low temperatures, stiffness and abrasion resistance, and good chemical resistance. Polyester film is made from the PET polymer, principally by Du Pont (Mylar) and ICI (Melinex). The monomer is polymerized, extruded, cast into a web, and biaxially oriented, forming a drawn polyester film.
Oriented polypropylene, a thermoplastic with low specific gravity, has excellent resistance, relatively high melting point, and good strength. Polycarbonate film is specified for its toughness, clarity, and high heat-deflection temperature.
Polyimides, both thermoplastics and thermosets, retain their principal properties over a wide temperature range. Polyimide films are available from ICI (Upilex) and Du Pont (Kapton). They have useful mechanical properties, even at cryogenic temperatures. At -453 °F, the film can be bent around a …-in. mandrel without breaking and, at 932 °F, its tensile strength is 4,500 psi. Room-temperature mechanical properties are comparable to those of polyester film.
To minimize transmission of moisture vapors, fluoropolymers are the best choice. This family of materials has a general paraffinic structure with some or all of the hydrogen atoms replaced by fluorine.
Polyetheretherketone (PEEK), a high-performance thermoplastic, offers outstanding thermal properties as well as resistance to many solvents and proprietary fluids. This film can be used for interbonding or cladding in PEEK structural components. Thermoplastic and thermoset acrylics are noted for exceptional clarity and weatherability, and also offer favorable stiffness, density, chemical resistance, and toughness.
Film produced from PEEK resins (ICI's Stabar) can be laminated to itself or to other substrates. Bond strength depends on surface preparation and adhesive type. The film is available in a transparent, thermoformable grade and in a higher-temperature, heat-stabilized version, which is more crystalline and less transparent (also thermoformable).
Plastic film can be manufactured from almost any resin, but not every resin produces an engineering film. Generally, a resin-based film's properties are related to the chemistry of the basic polymer; however, properties may be further affected by subsequent processing techniques. Manufacturers can choose from, and end users can specify, a range of process treatments that significantly enhance heat stability, mechanical properties, electrical characteristics, barrier properties, and bondability.
Coatings are typically applied by emulsion, solvent, and dry methods. Results vary according to the formula used: PVDC coatings improve barrier properties; polyurethane improves abrasion resistance; and aluminum coatings alter electrical characteristics.
Some processors have developed proprietary antistatic coatings that are cured by electron-beam radiation. Metal coatings produce conductive capabilities and also enhance barrier properties. Often, metallization is used to improve moisture-barrier properties for biaxially oriented nylon and polypropylene.
Surface treatment, which removes low-molecular-weight residue, improves adhesion and appearance. Several methods may be used. Corona discharge techniques position the film between an electrically grounded roller and a high-voltage electrode. A continuous-arc discharge (corona) is generated to clean and activate the film surface.
In gas-plasma surface treatments, film is placed in a reaction chamber. After evacuation, the chamber is charged with oxygen, argon, helium, or nitrogen while a radio-frequency field ionizes the gas. A resultant glow discharge creates free radicals on the surface, improving adhesion.
Film can be passed over a bank of flame jets to activate the surface and burn off contaminants. Other surface preparation techniques include polishing and embossing by roller.
Additionally, performance films combine base-resin properties with advantages unique to the film form. A film may be selected both for its dielectric properties and for its usefulness as a bonding material, thus serving two needs. Fabrication techniques such as coextrusion and lamination produce a film with the best properties of two or more individual films -- strength combined with barrier properties and optical clarity, for example.
The addition of various modifiers and pigments comprise another method for improving film performance. These are blended into the resin melt or introduced at the extruder, supplying the end film with specific characteristics. UV and heat stabilizers are among the most useful additives.
Applications: for engineering films span a broad spectrum of industrial uses. Film-based products appear in photographic, business graphic, reprographic, and electronic imaging systems. Magnetic-media manufacturers also rely on high-quality flexible films to produce their information-handling products. Packaging end users receive product protection, extended shelf life, and visual impact from a variety of films. In the electronics industry, films are used to construct components such as membrane switches.
Transparent PEEK film is used for a missile nose cone because of its microwave transmittance and resistance to rain erosion. Potential applications for the crystalline grade include helicopter-rotor blade cladding, fine-line microwave circuits, and aircraft interior panels.