Biaxially Oriented Polypropylene (BOPP)
Innovia Films produces biaxially oriented polypropylene film using a unique 'bubble' process. Polypropylene film can also be produced using the stenter (or tenter) method. There are distinct differences in the processes and the finished product.
The Bubble Method
A circular die is used from the extruder to form a thick walled tube of polypropylene. This is then blown under air pressure, orienting the film in the transverse direction. At the same time, an equal orientation in the machine direction is achieved by the speed at which the tube is pulled downwards and collapsed. After orientation, the film is heat-set.
The bubble method produces film which is "balanced", that is to say it has the same mechanical properties in both the machine and transverse directions. Films produced in this way can have process advantages during conversion over stenter-made films.
The Stenter Method
Polymer is extruded as a sheet directly onto a chilled chrome roller. The film is then passed through a stretching unit by rollers moving faster than the rate at which the material is extruded. This orients the film in the machine direction.
The film is then fed into a stenter frame for transverse direction orientation. In the stenter, the film is gripped along each edge by clamps that are attached to moving chains. These move outwards to stretch the film in the transverse direction. After stretching, the film is heat-set to hold the orientation and then reeled up.
Features of Biaxially Oriented Polypropylene Film
The important properties of Biaxially Oriented Polypropylene (BOPP) film are:
• It has a high tensile strength that facilitates high-speed conversion
• It has a high gloss and clarity
• It has good puncture and flex-crack resistance over a wide range of temperatures
• It is a good barrier to water vapour
• It is resistant to oils and greases
• It is not affected by moisture and does not wrinkle or shrink with environmental changes
Advantages of Biaxial Oriented PP film versus Cast PP film
- Improved barrier properties
- Improved mechanical properties
Benefits of Bubble BOPP film versus Stenter film
- Balanced Orientation
- Excellent shrink tightening properties
- Improved film cutting (including die cutting)
- Access to both thin and thick film technology (down to 15 µm and up to 250 µm)
- High Machine Direction (MD) stiffness, high tensile strength (MD) and low elongation (MD) for conversion.