How to extrusion-coat BOPP with PP

Q: How do you coat BOPP with LDPE/PP by extrusion-coating for low-end packaging applications? What precautions are required during processing?

A: I replied to the procedures and practices for coating in an earlier posting (Aug 12) for the general practice with LDPE, and did not address the use of PP for extrusion-coating. I spoke mostly about the process and what was necessary in my mind to make a good process without damaging the BOPP with the latent heat of the coating. To speak more directly about the use of LDPE and PP, we have to look a little more at the goals of the product.

LDPE will give a good low-temperature seal and will add moisture barrier based on its mass and the crystallinity of the LDPE chosen. It is widely used for these purposes and gives good bonding to the various substrates to which it is matched. In my experience, PP has not been used much for extrusion coating, and I would think in part that is due to its drawdown behavior (melt strength) and its general tendency to give lower bonding. Those comments are most appropriate for homopolymer and would be a little less true for various copolymers. It would add a higher melting-point seal, which would give the seal better resistance in retorting or other higher-temperature application, above 115 deg C where LDPE would be molten. There may be other reasons to use PP such as clarity and perhaps moisture barrier relative to LDPE, but I am less certain about that last point.

If I was to extrusion-coat with PP, I would likely try to use a two- or three-layer coating with a random copolymer surface for bonding to the BOPP, or other substrates such as PET, foil or paper. The copolymer may also add some melt strength to the structure, thus improving the coating process. Other alternatives would be to use an anhydride-graphed PP typical of tie layers in barrier coextrusion. These should help in forming a bond to metallized or printed surfaces while allowing lower coating temperatures to optimize melt strength and lower the heat load on the substrate.