High Clarity Linear Low Density Polyethylene for Packaging Applications
- Published: May 01, 2001, By Amy B. Hitchcock and Guy G. Luneau, Eastman Chemical Co.
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For most consumer items, “eye-appeal” is an important consideration in packaging. Consumers often make purchasing decisions solely on the appearance of a product. High gloss packaging increases the appeal of a package and increases the likelihood of purchase by a consumer. A package with tears or damage decreases the likelihood of its selection. Film extruders and converters are aware of the need for an abuse resistant and high clarity package especially for retail items.
The workhorse resin used by many extruders is an octene comonomer linear low density resin with a nominal density of 0.920 g/cm3 and a melt index of 1.0 dg/min. This resin offers a combination of reasonable clarity and toughness. It is also easy to process on most extrusion and converting equipment.
In the past 2-3 years, the packaging industry has experienced a surge in new designs. These range from zippered, stand-up pouches to high gloss form, fill, and seal packages for food items. Such packages require low haze, high gloss film for their sophisticated multicolor graphic printing. The haze and gloss of standard linear low density polyethylene (LLDPE) does not meet these requirements. Film extruders and converters who are comfortable processing standard LLDPE must make modifications to process narrow molecular weight distribution metallocene catalyzed polyethylene or use a totally different polymer in their films.
A new option is now available to polyethylene film extruders. This LLDPE has improved optical properties over a standard LLDPE and has good dart impact and tear strength. The polyethylene will extrude on conventional equipment with no modifications to temperature profiles, die gaps, or screw designs. This polyethylene has use as a skin, core layer, or both in a coextruded structure. It is also useful for a monolayer film. A 1.0 mil monolayer film made from this resin has <5.0 haze and >75 gloss properties.
Experimental
Blown films of 1 mil were produced on a monolayer line. Barefoot — no slip or antiblock additives — films and films with 2500 ppm of a silica antiblock were extruded for the study. Film testing used 23°C and 50% RH unless otherwise specified. Specific extrusion conditions were as follows:
- 2.5 in. (6.35 cm) commercial extruder with 24:1 L/D
- 2.4:1 blow-up ratio
- 4.7 lb/h/in. die circumference
- 6 in. commercial die with dual lip air ring
- 88 mil die opening.
Results and Discussion
The new polyethylene is a nominal 1.0 dg/min. melt index, 0.920 g/cm3 density hexene comonomer linear PE produced on a gas phase reactor. The resin used for comparison was a 1.0 dg/min. melt index, 0.920 g/cm3 density octene comonomer linear polyethylene. Films were extruded at nearly identical conditions for each resin evaluated.
Impact strength testing used ASTM D1709, Method A, with a dart drop test with a 26 in. dart height. The impact strengths for the 1 mil films produced from the octene LLDPE and the new clarity grade hexene LLDPE were very similar as Fig. 1 shows. Figure 2 shows that the optical property testing of the films indicated the new clarity hexene had significantly lower haze values. Tear testing on these same films conducted per ASTM D1922 using a commercial tear tester showed that the clarity grade hexene film had improved machine direction tear strength over the octene film.
Figure 3 shows the blocking test results using two temperatures — 100°F and 120°F. Because of the high surface gloss associated with films containing no slip or antiblock, 2500 ppm of a diatomaceous earth antiblock was added to all resins before evaluating film re-blocking performance. The addition of the antiblock increased haze by approximately 2% in all films.
Conclusions
The new clarity grade hexene LLDPE's unique properties allow its use in many packaging applications. Films produced from this PE have excellent clarity and good strength and sealing properties.
This new resin will find success in two primary packaging applications. The first type of application is a replacement for high pressure low density polyethylene in film structures where the clarity grade hexene resin will produce a clearer, stronger film. The second type of application is a replacement for conventional hexene or octene LLDPE in film structures. For these applications, optical properties can be significantly greater with no loss in film physical properties.
