Comparison between CPP and BOPP

As we know CPP and BOPP exhibits different properties. Especially sealing properties are quite different. CPP exhibit better seal strength than BOPP. I request you to enlighten me on technical grounds why such difference is exhibited?

What will be the performance if we use same tool to seal CPP and BOPP?

Answer

The acronyms are as follows CPP = Cast PolyPropylene  and BOPP = Biaxial Oriented PolyPropylene.

Cast polypropylene is produced by extruding polypropylene and chilling it and so the polymer chains within the film are randomly distributed throughout the film in all three dimensions.

BOPP starts out in the same way, the polypropylene is extruded and chilled but then the polymer is re-heated and stretched in two directions hence the 'bi-axial orientation' in the description.  This stretching is done when the polymer is softened and so the polymer chains are able to rearrange themselves to some extent and so they become oriented into the two stretching directions.  This changes the mechanical performance of the polymer. The tensile performance is higher along the polymer chain length than it is across the polymer chains thus in either of the two stretched directions the tensile performance is improved but in the third axis the tensile performance is reduced.   In the ordering of the polymer chains many of them become ordered enough that they become crystalline. This too effects a change in the performance, as crystalline material is a better barrier than amorphous material.  Crystalline polymer is denser than amorphous polymer and the amorphous polymer will soften at a lower temperature than the crystalline.

Thus using the same heat sealing tool may well work OK but may need a higher temperature for the BOPP than for the CPP.

I hope this answer helps.