About BOPP Films...

 

Biaxially Oriented Film -

Biaxially oriented polypropylene (BOPP) film is film stretched in both machine and transverse directions, producing molecular chain orientation in two directions. BOPP film is produced by a tubular process, in which a tubular bubble is inflated, or a tenter frame process, in which a thick extruded sheet is heated to its softening point (not to the melting point) and is mechanically stretched by 300–400%. Stretching in the tenter frame process is usually 4.5:1 in the machine direction and 8.0:1 in the transverse direction, although these ratios are fully adjustable. It is a widely used process, more common than the tubular process, and a glossy, transparent film is produced. Biaxial orientation results in increased toughness, increased stiffness, enhanced clarity, improved oil and grease resistance, and enhanced barrier properties to water vapor and oxygen. Impact resistance, low-temperature impact resistance, and flexcrack resistance are substantially modified. BOPP films are used in food packaging and are replacing cellophane in applications such as snack and tobacco packaging due to favorable properties and low cost (Fortilene Polypropylene Properties, Processing, and Design Manual, 1981; Goddard, 1993).

Oriented films can be used as heat-shrinkable films in shrink-wrap applications or can be heat set to provide dimensional stability. Heat sealing is difficult in BOPP films, but can be made easier by either coating the film after processing with a heat-sealable material (such as polyvinylidene chloride) or by coextrusion with one or more copolymers before processing to produce layers of film. Copolymers used in sealing layers must have high gloss and clarity and should have low sealing temperatures to prevent distortion of the oriented polymer during sealing. Random copolymers containing 3–7% ethylene are often used as sealing layers; the lower melting point (<132°C; <270°F) results in a 30% increase in line speeds, and they can be recycled with no significant loss of strength or clarity. Coating or coextrusion increases the barrier properties of BOPP film, decreasing its permeability to gases. Common barrier polymers are ethylene vinyl alcohol, polyvinylidene chloride, and polyamide; five or more layers may be coextruded or laminated, or the barrier polymer can be dispersed in the matrix polymer (Fortilene Polypropylene Properties, Processing, and Design Manual, 1981; Goddard, 1993; Polymers in Contact with Food, 1991).