Treatment of Polypropylene Films

By: Brian Reuter

Polymers generally, but notably polyolefins, have low surface energies and it can be difficult for inks and other coatings to adhere to them. In the case of polyolefins, such as polypropylene (PP), the problem is particularly severe since this polymer has a surface void of polar groups. Thus, only weak physical (dispersion-force) interactions can bond materials such as printing inks to the surfaces.

An additional problem arises in polyolefins caused by their wide molecular weight distribution. The surfaces of polymers such as PP are preferentially made up of low molecular weight chains, creating a cohesively weak boundary layer. Even if strong bonding occurs between polymer and a coating, such as printing ink, the weakness of the boundary layer will lead to easy destruction of the applied ink. Therefore the surface of PP must be treated prior to coating and the success of those surface treatments have to meet a dual challenge:

1) To introduce polar groups into the polymer surface, thereby bringing into play strong, non-dispersive, forces which strengthen the bond with applied agents, e.g., inks

2) To strengthen the cohesively weak boundary layer

There are three basic polypropylene resins: rigid homopolymers, impact grade block copolymers, and random copolymers. Two of these are used in film applications. The homopolymers for biaxial oriented PP (BOP) film and random copolymers for softer, non-oriented films. The homopolymer PP have a high melting point near 160?C. The random copolymers, based on ethylene and butane-1, have much lower melting ranges overlapping those of polyethylene and its copolymers.

To aid the processing of these films, surface lubricants are added, also called slip additives. These additives are typically amides and the common ones used are stearamide, oleamide, behenamide, and erucamide, with the latter being the industry preference. The levels at which slip additives are blended into a polyolefin resin is critical. Depending on final film gauges, the levels can range from 600 to 1800 ppm.

Slip additives are blended on a weight percentage basis. If two films of different gauges are extruded from the same resin, the thicker film may experience overcrowding of slip additives on the surface and multi-layer formation. If too much slip additive migrates to the surface, this can have an adverse effect on surface treatment. Some of the energy for the surface treatment will instead be spent on vaporizing or oxidizing the additives, thereby leaving a surface that is not yet suitable for printing or coating.

These additives must be treated as a precursor to printing, laminating, or metallizing. The treatment required is specific to each application. For printing, the problem is being compounded by the industry trend away from solvent-based inks towards water-based or UV curable inks.

Water based inks are still in the process of development. Some of the ingredients now in use may be replaced with new ones. The use of acrylics, as well as hybrid binders, will remain strong; binders that do not require amines will be more desirable. Water based inks are complex formulations of chemicals and additives. With future changes in environmental regulations, some of these may be restricted.

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