Polypropylene Pop Quiz

Suppliers respond to pP's readers' challenges and questions regarding polypropylene substrates.

By Susan Friedman

You asked, and polypropylene film material suppliers responded. After conducting a phone survey to collect pP readers' most-encountered challenges with this popular substrate, related questions were forwarded to suppliers for comment. Read on for their suggestions.

Available Grades

Q: Converters have expressed difficulty finding polypropylene below 60 gauge. Does 60 gauge meet most application needs, or will thinner gauges need to become more accessible in the coming year?

A: Though 50 gauge films are available from a number of suppliers, including Mobil, most BOPP manufacturing and converting processes have better efficiencies when the film thickness is greater than 60 gauge. The interest in thinner materials has been primarily driven by needs to find better economics, but the decrease in cost is not that pronounced for the thinnest gauges that are available. In addition, there are performance trade-offs below 60 gauge, such as increased web handling difficulties and reduced stiffness and water vapor properties.

Dru Kefalos, business manager, Americas Mobil Chemical, Films Division

Q: What types of thinner polypropylene film grades are on the horizon?

A: Super Film has a 50 gauge transparent in its SUPLAIN (label) film and SUPEX (surface print and laminating) film. The company is conducting trials with 45 and 48 gauge transparent co-extrusions. Our B gauge system with Bruckner lines permits thinner gauges with great accuracy and minimal line changeover.

Ross Wooldridge, sales director, Super Film of America

Design Developments

Q: What developments in polyprop formulations could offer improved surface characteristics?

A: Engineering co-extruded skin polymers by chemistry, formulations and by blends continue to be the most potentially successful routes to improving the surface performance of OPP films so as to improve wetting and increase adhesion. One of the hottest topics in polyolefins is extending the application of metallocene chemistry from the successes in polyethylene to the engineering of polypropylene film resins. The potential toughness of a metallocene PP may increase the overall perceived adhesion by increasing the strength of the base film itself. The biggest challenge with metallocene resins for OPP films is that they must be extruded, stretched and oriented.

Leighton Derr, technical service manager, AET Films

Q: One of the biggest challenges converters face with polypropylene is achieving surface adherence in pouches or other multi-layer constructions. What approaches, such as coatings, offer improvements?

A: Often the weak point in triplex laminations for adhesion is the metal-to-film interface. Mobil's efforts are focused on designing films that have treated or high-energy skins that provide better metal adhesion on one side and a good surface for extrusion or adhesive lamination bonds on the other. Coated surfaces are sometimes used in lieu of metal primarily to provide transparency for the package, and may provide benefits with adhesive laminations, but not necessarily better metal adhesion.

Dru Kefalos, Mobil Chemical, Films Division

Printing and Laminating

Q: What would you suggest to printers who find that corona treating of plain (uncoated) polypropylene is ineffective in improving surface performance?

A: Our experience is that flame treat provides a 44-plus dyne level for printing and laminating with water-based inks and adhesives, as well as PVDC adhesives. There is less dissipation in treatment and dyne level with flame treat than corona treatment, translating into a longer shelf life.

Overtreating can cause backside treat, thus affecting the sealing and machinability of the finished product. At Super Film, we have developed a flame treat metallized film and a flame treat transparent co-extrusion in response to customer needs for a better co-extrusion for use with water -based inks and adhesives as well as PVDC adhesives. For the label market, the material meets needs for UV inks and adhesives.

Ross Wooldridge, Super Film of America

Q: How can converters compensate for polypropylene's tendency to relax or 'snap back' under press tension controls, and elongate during lamination?

A: When printing on polypropylene, converters should set a minimum tension to keep the web in register. Once register is established, repeat length can be established. Converters can compare plate or cylinder circumference to the repeat length to calculate and compensate for snap-back.

On in-line laminators, curl is a huge issue directly related to polypropylene elongation during lamination. Converters can adjust the tension on the laminating web to make a flat web, so that register is not affected, and can adjust laminating tension to compensate for curl. In extrusion lamination, the softening point of polypropylene is 180°F, and the laminating extrudate's melt temperature ranges from 590°F to 620°F. Converters must favor the chill roll to eliminate softening or stretching of the more sensitive film.

Mike Carley, technical service engineer, AET Films,

Label Construction Issues

Q: How should converters select a cost-effective polypropylene label construction?

A: Converters must avoid over-engineering or under-engineering a polypropylene label construction so that material cost is balanced with performance. During the selection process, converters must consider gauge, opacity, flexibility, expected UV durability, and top coating needs for receptivity to the print process to be used. Thicker gauge, higher opacity and higher UV resistance all add cost.

Polypropylene is often selected for its good aesthetic quality, whiteness and surface smoothness. In a recent two-way decal application, an opaque polypropylene barrier material was selected to replace polyester because it was more cost-effective and provided a suitable base for metallization.

Carl Sittard, market development leader, graphic films, FLEXcon

Q: What types of adhesives will provide a pressure-sensitive polypropylene label with the ability to adhere to the greatest variety of surfaces?

A: A more aggressive adhesive, such as solvent/acrylic or rubber formulations, is more general purpose, but these often provide more performance than what is needed.

If cost is an issue, rubber adhesives might not be a viable choice. Rubber also tends to yellow, which isn't workable for the no-label look. Water-based adhesives might not provide the best bond, but they are clear and have a more desirable cost structure.

On rough surfaces, harder adhesives won't wet out to penetrate the surface as well as softer formulations such as water or rubber. Untreated surfaces with lower surface energy typically need more aggressive adhesives for initial tack.

PS polypropylene, which has a lower surface energy, typically transfers quickly to a higher-energy surface such as polyester. Rework, or open-time adhesives that don't immediately transfer from the printed substrate to the application surface provide a window to remove and reapply a label to a package or container.

Jeff Robinson, marketing representative, packaging, FLEXcon

Q: If a converter switches to a thinner polypropylene label construction, are dies used with the former label construction typically still usable?

A: On polypropylene label constructions with a minimum caliper of 1.5 mils, historically we have seen that the same die may be acceptable when a converter switches to a polypropylene construction that is less than 1 mil thinner than what is currently being used and the liner is unchanged. When the face stock change is greater than 1 mil thinner, retooling has typically been necessary. Downgauging the face sheet makes die-cutting much more critical.

Bruce Winter, technical marketing manager, Avery Dennison

Q: A line job currently printed on polypropylene is upgraded to four-color process. Will a new grade of polypropylene have to be specified?

A: If PP is top-coated it should be acceptable for four-color process with current inks. Converters have three options if polypropylene is non-top-coated: corona treatment on press; lay down primer on press; or buy top-coated face stock.

Solvent inks should not be a problem on non-topcoated polypropylene as they tend to bite into the surface better than water -based or UV systems. Depending on the ink system, a converter may be able to print on non-topcoated polypropylene without corona treating, laying down a primer or using a top coated product. We recommend a non-top- coated film to have a surface energy of more than 40 dynes for use with standard ink systems.

Bruce Winter, technical marketing manager, Avery Dennison

Technical Tips

Optimizing OPP

• Use OPP films within three to six months of receiving them to avoid additive migration and fading of surface treatment.

• To prevent shrinkage and reduced wetting tension, store both free film and laminated and/or printed shrink OPP films at temperatures less than 90°F, even for short periods of time.

• Use the lowest web tension settings that will maintain control over the web; a general recommendation for OPP is tensions of 0.25 to 0.50 lb./inch of film width.

• When drying the solvent from an ink or an adhesive coating, oven temperatures should not exceed 150°F to 170°F.

• To most effectively improve surface wetting and adhesion on OPP, bare-electrode corona treating systems should deliver an energy density of 2.4 watt-min/ft.2, while bare-roll systems need to be set at 4 watt-min./ft.2

• Slit OPP/BOPP films with a razor blade instead of shear slitting or score cutting; some opaque OPP films with very hard additives may require hardened-steel razor blades or ceramic blades.

Source: AET Films Product Manual