Homa Abasian R&D manager from Moghadam Co. asks the following question: Would you please inform me, if we want to produce PP/tie layer/PP films, what is the tie layer between PP and PE layers?
This is a rich and interesting question and has a long history as well. I will speak of both cast and oriented films in the reply. In the 1960s, it was desired to make a bread-bag film with improved heat-sealing temperature range compared to LDPE film from a combination of cast polypropylene and LDPE. The film was made by coextrusion, which at the time was a novel process. This film was called TriKor and was produced by Kordite (which eventually became Mobil Chemical Films, where I worked). It is described on page 160 of “Plastic Films Technology” by W.R.R. Park published 1969 by Van Nostrand Reinhold.
The product was used for bread packaging and the product design was:
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0.4 – 0.45 mil Med density PE
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0.1 – 0.2 mil PP
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0.4 – 0.45 mil Med density PE
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The PP core gave the extended heat-sealing range relative to a cast-LDPE film and the LDPE outer layers the low seal-initiation temperature and low-temperature flexibility compared to a cast PP film. The weakness of this product was that the heat-seal strength was limited by the poor interfacial adhesion between the PP and LDPE layer.
Poor interfacial adhesion between LDPE and PP also limited the heat-seal strength of a coextruded and oriented PP film with a LDPE sealant. This was long before the existence of the copolymer and terpolymer PP resins used today, and therefore, heat-sealable OPP films were made by coating with acrylic and PVDC coatings.
In metallized and slip OPP films, it is possible to coextrude and orient a PP film with an HDPE layer directly to the PP-core layer, which has acceptable adhesion to the OPP core, and this film has found widespread use. This teaches us that some PE/PP coextruded compatibility is present, but it depends on the PE used and the method of preparation. In the case of OPP, the bond develops for the HDPE during orientation, but for LDPE the bond is insufficient for optimum heat sealing. In the OPP stretching ovens, both the HDPE and the LDPE will be molten while the PP is a solid. The HDPE is not used in heat-sealing applications, and the bond is sufficient. The differences in crystallization behavior and the final crystallinity of the HDPE and the LDPE relative to PP may be part of the difference in the adhesion as well as a potential epitaxial crystallization on the PP core. But I have no specific information about that, just a thought.
The opposite was true for oriented PE with a copolymer PP skin; in this case, the PP copolymer will not adhere to the HDPE core. In this case, both the HDPE and the Copolymer PP are solids during the orientation. In this case, the use of tie layers where the tie-layer density was less than that of the HDPE (blends of LDPE /HDPE and MDPE) as a tie layer allowed sufficient adhesion of the copolymer PP to the HDPE during orientation (US Patent 5885721).
So, for a tie layer between PP and PE, I would recommend trying blends of both layers as a bonding layer. For example, I would use a blend of a LDPE and a copolymer PP to help bind the PE and PP layers together. In the blend, there will be a continuous phase and a discontinuous phase depending on which polymer is in larger proportion. Then the blend is a little like the PE and a little like the PP and adhesion may be improved. This is what is often done for a peelable seal as well. It might be necessary to add two intermediate layers to obtain the highest interlayer adhesion between the PE and PP layers. Here you would have a PE rich layer against the PE layer and a PP rich layer against the PP layer, and the intermediate layer would have good adhesion as the blend components interact together.
Another example for your reference where a LLDPE layer was bonded to an OPP layer during coextrusion and orientation is given in US Patent 5,888,648, which is for an OPP film with cast film-sealing properties.