Mineral Reinforcement of HMW-HDPE Film

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

Abstract
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LLDPE-based pelleted concentrates were used to add 0, 10, & 20wt.% fine-ground, surface-treated calcium carbonate (CaCO₃) mineral to three different high-molecular weight HDPE film resins. These dry blends were extruded into film on a 65mm grooved-feed extruder fitted with a 100mm die and 1.2mm die gap. Stalk height, blow-up ratio (BUR), and extruder screw RPM also were varied to determine how the addition of CaCO₃ affected film property response to variation in these process conditions.

Mineral addition yielded output rate increases of 8% at 10% CaCO₃ and 17% at 20% CaCO₃ while simultaneously decreasing melt pressure and motor load (current). Certain combinations of base resin, processing conditions, and mineral loading yielded increases in dart impact strength without a loss in tensile yield stiffness. Other advantages of CaCO₃ reinforcement of HMW-HDPE films included an increase in film coefficient of friction with CaCO₃ addition, which improves stacking of liner or merchandise bags.
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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]. In general, increases in processing efficiency are realized due to the thermal and rheological changes which occur with mineral incorporation. 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
Polymeric variables known to affect the physical properties obtained in HMW-HDPE film extrusion include molecular weight and molecular weight distribution, and density. To determine the effect of these variables, three commercial HMW-HDPE film grade copolymers in the range of 0.03 - 0.05MI and 0.948- 0.952ρ were utilized as the film base resins. Resins of this MI & density commonly are used for the manufacture of institutional liners and retail carryout sacks (“T-shirt” bags). These resins were dry blended with a concentrate containing 75wt.% of a wet-ground calcium carbonate with a 1.0µ mean particle size (MPS) and 8µ top-cut (maximum particle size). 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, greatly improving the dispersion of the calcium carbonate into the polymer matrix and the processability of the mineral/HDPE composites. Addition rates of 13% and 26% concentrate yielded 10wt.% and 20wt.% CaCO₃ in the films. Films without mineral were run as controls.

Polymer Processing and Film Extrusion
Neat resin and resin/concentrate dry blends were extruded into film on a 65mm grooved-feed extruder fitted with a 100mm die, 1.2mm die gap, and internal bubble cooling. Processing conditions were varied to determine how the addition of CaCO₃ affected film property response to variation in operating parameters. Stalk heights of 5X and 10X the die diameter, a blow-up ratio of 3:1, and screw speeds of 70 and 115 RPM were employed in selected combinations with the three film resins during the experiment.

Results and Discussions

Changes in Polymer Processing Conditions with Calcium Carbonate Addition
The effect of mineral addition to each of the three base resins on the extruder output rate at 70 RPM is shown in Figure 1, and at 115 RPM for Resin C in Figure 2. In general, mineral addition yields an increase in output rate an average of 8% at 10% CaCO₃ and 16% at 20% CaCO₃, with the exception of Resin B at 10% loading. This discrepancy can not be explained at this time.

Fig. 1. Extruder Output at 70 RPM Screw Speed

Fig. 2. Extruder Output at 115 RPM Screw Speed

These increases in output rate are achieved with either no increases, or actual decreases in motor current and melt pressure, as shown in Figures 3 through 6.

Fig. 3. Motor Current at 70 RPM Screw Speed

Fig. 4. Motor Current at 115 RPM Screw Speed

 
Fig. 5. Melt Pressure at 70 RPM Screw Speed

 
Fig. 6. Melt Pressure at 115 RPM Screw Speed

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 7 shows the effect of mineral addition on the dart impact of extruded films at low and high stalk height, film thickness, and output rate.

Figure 7. Effect of Mineral Addition, Resin Type, Stalk Height, Film Gauge, and Output Rate on Dart Impact Strength

As expected, even without mineral addition the three different base resins display varying dart impact strength under similar conditions, and different response to variations in processing conditions. This is due to differences in molecular weight, molecular weight distribution, and crystalline morphology. For instance, Resin A prefers a high stalk bubble configuration (slower cooling rate) while resin C prefers a low stalk configuration, all other parameters being equal. Due to time constraints, Resin B was run in the low-stalk configuration only.

Resin A showed an increase in impact strength in the high-stalk extrusion mode, but virtually no change in the low-stalk mode. Resin B had a peak impact strength at 10% mineral, which was reduced with further mineral addition. Resin C showed uniform decreases in impact strength regardless of extrusion processing conditions (stalk height, film gauge, or output rate).

Tensile yield strength is a critical property of HMW-HDPE films, as it directly relates to the load-bearing capacity of a converted can liner or retail carry out sack. In general, very little, if any loss in this property can be tolerated, as it would require an increase in film thickness to maintain load capacity.

Figure 8 shows the effect of mineral reinforcement on the effect of tensile yield strength under the same process variations as detailed for dart impact strength above. With the exception of Resin C at high rate and 18µ film thickness, tensile yield strength is observed to not change or actually increase with the addition of calcium carbonate.

Figure 8. Effect of Mineral Addition, Resin Type, Stalk Height, Film Gauge, and Output Rate on Tensile Yield Strength

Summary

Mineral reinforcement of HMW-HDPE films using fine-ground, surface treated calcium carbonate is shown to be a commercially viable method of increasing extrusion output rate without penalty in terms of processing difficulty. Depending on resin type and specific processing conditions, motor load and melt pressure may actually decrease while output rate is increased.

Differences in molecular weight, molecular weight distribution, and density between commercially-available HMW-HDPE film resins result in differing responses of film dart impact with calcium carbonate addition and changes in films extrusion conditions. These differences suggest that those interested in exploring the application of mineral reinforcement technology to their processes and products explore a wide range of base resins and processing conditions to determine the optimum combination for their given equipment and desired film properties.

References

1. Ruiz, F.A., “Effects of Polymeric and Particulate Variables on the Mineral Reinforcement of Polyethylene Film, Bags, & Liners”, 1994 ANTEC Proceedings

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. H.S. Katz, J. V. Milewski, Editors, Handbook of Fillers for Plastics, Van Nostrand Reinhold, NY, 1987.

10. R. Gaechter, H. Mueller, Editors, Plastic Additives, Hanser Publishers, NY, 1985.

11. OMYA Technical Bulletin No. US-PL-3

Acknowledgements

The author wishes to acknowledge the assistance provided by Hosokawa Alpine American Corp. for the use of their extrusion line in Natick, MA and the expertise and advice they provided; 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.

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