Polypropylene (PP) is generally quite difficult to convert into film on the conventional upward blown film extrusion mainly due to poor melt strength, long cooling time and somewhat inferior optical properties. The thicker or heavy-duty PP film is produced from special PP grades of higher molecular weight with higher amount of other comonomers. However, blown PP film is not yet commercially developed in place of LDPE/LLDPE films.

Raw material producers have developed some specific PP grades that can be processed on conventional upward film extrusion. One of the grades called RB707F from Borealis is a terpolymer with random distribution of comonomers. This grade has 1.8MFR and can produce film with haze value less than 4-5. The clarity is much better than that can be produced from LDPE/LLDPE. PP film, because of its higher stiffness provides an opportunity to downguage compared to LDPE/LLDPE film. The lower thickness further provides better clarity. Similarly Dow has introduced Inspire 114 for upward blown film.

Earlier the melt strength of conventional PP was enhanced by addition of SEBS at about 20% level. However, the cost of this blend was high and hence the resultant film did not achieve the competitiveness of LDPE/LLDPE film. The present grades are reactor made PP elastomers and offer better cost economy. Of course PP offers higher heat resistance and therefore more suitable for retort packing or packing of material at higher temperatures. It also offers an opportunity for better grease resistance compared to LDPE/LLDPE films.

To achieve better processability, conventional film extrusion requires some modifications. Some of the major modifications are: higher L/D ratio (>=30). Die gap should be at least 1.7-1.8mm. Typically blowup ratio of 2.25-2.5 is required. Internal Bubble Cooler (IBC) and higher chilled air volumes (at temperatures of 8-10C) help in achieving better quality. These modifications also help in achieving similar outputs compared to LDPE/LLDPE. The typical LLDPE film equipments however, can produce PP film from these newer grades but at about 25-30% lower outputs. Interestingly all the major film equipment producers like Kiefel or Windmoller are now supplying film extrusion equipment for PP.

PP films can find increasing usage in IV bags, retortable pouches, stretch hoods or heavy-duty bags. PP bags can be used in hot fill packing of products. PP blown film can be used in place of cast PP film for lamination to PET/PE film for reverse printing. With these developments PP certainly will make inroad to the huge LDPE/LLDPE film market in the near future.

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