英文版 BOPP薄膜 简介

Biaxially Oriented Polypropylene Film (BOPP)

Many of the unique characteristics of polypropylene enhance its utilization in the application of film. The main application for polypropylene film are the packaging of food, textiles and tobacco products.

Biaxially Oriented Film (BOPP)

Biaxial orientation is a process in which a continuous cast film is heated to bring it to a stretchable temperature and thereafter it is stretched in machine and then in transverse direction.

Biaxial orientation of polypropylene (BOPP) film is done by two methods

•Tenter frame process

•Double bubble process

Tenter Frame Process

Tenter frame process is a very expensive way of processing BOPP film. However it has become popular due to higher production rate with very thin and wide width film (Fig. 1)

Extruders

Extruders with 150 to 200 mm dia screws having L/D of 26:1 to 30:1 are more popular.

Dies

600 to 1500 mm wide coat hanger type of dies are used

The Tenter Frame Process

Casting & Quenching

The molten polymer extruded from the die is placed against a chill roll or multiple chill rolls. The most popular being casting onto a chill roll partially submerged in a water bath. The chill roll and water bath temperatures are generally 20 to 25°C. Usually, these chill rolls are 1000 to 2000 mm in diameter and approximately 1000 to 1500 mm wide. The surface of the chill roll must be very smooth; pitting or irregularities on the surface of the roller will influence the film quality. An air knife is used to force proper contact of molten polymer against the casting roll. The cooled web proceeds towards the machine direction orientation section.

Machine Direction Orientation (MDO)

The base sheet is alternatively heated by passing it over the heated rollers (145 - 150°C) to achieve uniform heating of web throughout its thickness. These rollers are usually smaller in diameter (450 to 600mm) compared to the casting drums (chilled rolls).

When the film reaches the necessary stretching temperature, it is passed over a series of stretching rolls which are usually arranged in pairs. Each pair has a rubber nip roll to prevent slipping of the web while stretching. These first set of nip rollers are run at a speed similar to preheat rollers. The next set of rollers are operated at higher speeds than the first set of nip rollers. Usually the stretch ratio is kept between 4:1 to 6:1.

From the stretching rollers, the film is passed on to a series of annealing rollers, which are heated to a temperature, adequate to prevent shrinkage in the machine direction (MD) of the web.