Mineral Reinforcement of Metallocene-Catalyzed LLDPE Film and Bags

F. A. Ruiz
Heritage Plastics, Inc.
1002 Hunt Street
Picayune, MS 39466 USA

Abstract
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This paper details the effects of adding 7.5% & 15wt.% calcium carbonate to three different film resins; two metallocene-catalyzed linear polyethylenes and a conventional Ziegler-Natta catalyzed PE film resin. An LLDPE-based pelleted 75% calcium carbonate concentrate was used to prepare dry blends with the base resin. These dry blends were extruded into 33µ (1.3 mil) films on a 63mm (2.5”), 24/1 L/D extruder fitted with a 225mm (9”) die and 1.4mm (0.055”) die gap.

Increasing mineral loading from 7.5% to 15% CaCO₃ did not have a significant effect on output rate or other extrusion conditions. Higher levels of mineral concentrate did increase dart impact strength, tear strength, and tensile yield for different film resin blends.
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Introduction

The blown film processing and product property enhancements possible with the use of calcium carbonate (CaCO₃) in particular and minerals in general as a reinforcing additives have been described in a number of papers and patents [1-8]. Mineral addition reduces the heat necessary to melt a given weight of material, and increases the thermal conductivity of the molten polymer. These papers also have discussed the mineral factors (particle morphology, particle size distribution, particle surface chemistry, and chemical purity) and polymer factors (molecular weight, molecular weight distribution, branching type and distribution, density/crystallinity, and polymer chemistry, e.g. polar/non-polar) which affect the processing and product properties with mineral addition. Proper mixing and dispersion of the mineral into the polymer matrix is a critical processing factor in the complete realization of the benefits of this technology. Most commercial extrusion equipment in good condition with modern screw designs has proven more than adequate to achieve the necessary level of homogenization.

Experimental

Polymers and Minerals Evaluated
The film resins were dry blended with commercially available concentrate containing 75wt.% of wet-ground ultrafine calcium carbonate. Heritage Plastics, Inc sells this material under the trademark “HM-10^®”. The mineral was treated with stearic acid by the supplier to form a hydrophobic coating on the surface. This allows the polyethylene to “wet” the mineral surface, allowing the dispersion of the calcium carbonate into the polymer matrix and the processing of the mineral/HDPE composites. Addition rates of 10% and 20% concentrate yielded 7.5wt.% and 15wt.% CaCO3 in the films.

Polymer Processing and Film Extrusion
Metallocene LLDPE/concentrate dry blends were extruded into film and converted into institutional trash bags using a commercial production line at the Heritage Bag Company plant in Villa Rica, GA. This line is comprised of a 70mm (2.75”), 24/1 L/D extruder fitted with a 225mm (9”) die and 1.4mm (0.055”) die gap. Film was converted in-line into bags on a Gloucester Model 418 bagmaking machine.

Results and Discussions

Changes in Polymer Processing Conditions with Calcium Carbonate Addition
The extrusion processing conditions are given in Table 1. No major changes were observed with mineral concentrate increase from 10 to 20% (7.5 to 15 wt.% CaCO₃).

Effects of Mineral Reinforcement on Film Properties
Dart Impact strength, as measured by ASTM D 1709, is commonly used as a measure of the ability of film to resist local failure in a loaded bag or package. Figure 3 shows the effect of increased mineral loading on dart impact. F-50 increases occurred with all resin, but most noticeably with the lower density metallocene film resin. The dart impact strength of film manufactured with this resin increased from 460 g to over 635 grams, the maximum dart weight available for testing. This is due to the toughening mechanism the mineral creates within the polymer matrix resin. This mechanism is described on a paper published in Polymer^9.

Figure 1: Dart Impact strength of films @ BUR & % Concentrate

Elmendorf Tear strength increased in both Machine Direction and Transverse Direction with mineral reinforcement, as shown in Figure 2 and 3.

Figure 2. MD Tear vs. BUR and % Concentrate

Figure 3: TD Tear vs. BUR and % concentrate

Tensile yield strength is a critical property of bag and can liner film, as it directly relates to the load-bearing capacity of a converted can liner or retail carry out sack. Figure 4 shows how mineral reinforcement effected the tensile yield strength of each resin. Mineral addition improved the film stiffness of both lower density resins.

Figure 4. Effect of Mineral Addition and Blow-up Ratio on MD Tensile Yield Strength

Summary

Mineral reinforcement of ZN and metallocene catalyzed film resins using fine-ground, surface treated calcium carbonate is shown to be a commercially viable method of modifying film properties. Differences in molecular weight, molecular weight distribution, and short-chain branching characteristics of commercially available film grade resins result in differing responses of film properties. In general calcium carbonate addition and changes in films extrusion conditions will yield improved properties.

References

1. Ruiz, F.A., “Effects of Polymeric and Particulate Variables on the Mineral Reinforcement of Polyethylene Film, Bags, & Liners”,

2. Ruiz, F.A., Mineral Reinforcement of LLDPE Film, Bags, and Liners, TAPPI Journal, Vol. 76, No. 1, January 1993, p. 174.

3. Ruiz, F.A. and Allen, C.F., “New Property Combinations Available with Mineral Reinforcement of Commodity Blown Films”, TAPPI Polymers, Laminations, and Coatings Conference, p. 365 (1987).

4. Ansari, D.M. and Higgs, R.P., “The Influence of Mineral Fillers on the Processing of LLDPE Films”, TAPPI Polymers, Laminations, & Coatings Conference, p. 173 (1997)

5. Johnson, S.L. and Ahsan, T., “Evaluation of Coated Ground Calcium Carbonate in Linear Low Density Polyethylene Film”, TAPPI Polymers, Laminations, & Coatings Conference, p. 471 (1997)

6. Arina, M., and Honkanen, A., “Mineral Fillers in Low-Density Polyethylene Films” Polymer Engineering and Science, Vol. 19, No. 1, pp. 30-39 (1979)

7. N.S. Murthy, A.M. Kotliar, J.P. Sibilia, and W. Sacks, Structure and Properties of Talc-Filled Polyethylene and Nylon 6 Films, Journal of Applied Polymer Science, Vol. 31, 2569-2582 (1986).

8. U.S. Patent 4,528,235 (Sacks et. al.)

9. Z. Bartczak, A.S. Argon, R.E. Cohen, M. Weinberg, “Toughness Mechanism in Semi-Crystalline Polymer Blends: II. High Density Polyethylene Toughened with Calcium Carbonate Filler Particles,”

Acknowledgements

The author wishes to acknowledge the assistance provided by the Heritage Bag Company for the use of their production line in Villa Rica, GA; to the employees of Heritage Laboratories for testing of the numerous film samples generated during this experiment; and to Ms. Myra Hayes for the assistance she provided in coordinating the testing of these film samples and compiling the data for analysis by the author.

Table 1 Processing Conditions 63mm (2.5”) extruder, 225-mm (9-inch) die with 1.4mm (0.055”) gap

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