Polypropylene Film Comparisons From Sequential and Simultaneous Tenter Bopp Processes, 1999 Polymers, Laminations & Coatings Conference Proceedings

R. Neil Campbell--The production of BOPP film on the Bruckner LISIM (Simultaneous stretcher = SLS) line in Siegsdorf, Germany yielded film with more balanced film properties than those obtained on sequential stretchers (SQS) at Lab 1, Lab 2, and previous Bruckner pilot line, Lab 3 [1]. When the stretch ratios of the sequential stretcher in this study were balanced between TDO (Transverse Direction Orientation) and MDO (Machine Direction Orientation), 7:1 MDO -7: 1 TDO, balanced physical properties of the BOPP (biaxially oriented polypropylene) film were not obtained.

A study of stretching variables on a SQS Lab 1 pilot line with different types of BOPP resins indicated stretch ratio and stretch temperature of the TDO unit had the major effect on film properties. There was some effect of stretch temperature in the MDO on final BOPP film properties when this temperature was too low. There is a definite correlation of film secant modulus and tensile strength with haze and clarity optical values. If TDO preheat temperature is too high then the film will have a high haze value and low clarity value.

The lab “batch” stretcher at Bruckner was used to compare BOPP resins of UCC UNIPOL
PP Process with BOPP resins from other competitive processes. The output from this stretcher was a measure of clip force when the sheet was stretched at predetermined air temperature. The low ethylene random (LER) resins (0.5% and 1% ethylene) of propylene and ethylene had lower clip force than the homopolymers with high atactic content; however, the homopolymers had improved tensile properties when compared to the 1% ethylene resin. Lab “batch” stretcher results can provide information which will help predict processability of BOPP resins on BOPP film continuous line stretchers.

Another advantage of the SLS line in addition to balanced physical properties is the use of low heat seal temperature BOPP resins that might otherwise stick to the MDO rolls. An experimental PP random copolymer with low melting temperature was supplied for a demonstration of this process feature. The resultant BOPP film had a seal initiation temperature of 85°C @ 200 grams/25mm seal strength and a hot tack strength @ 90°C of 279 grams/25mm.