A Comparison of Primer Coating and Calcium Carbonate Addition Methods of Achieving LDPE Coating Adhesion to Clay-Coated Board

Introduction

Calcium carbonate addition to LDPE has been shown to increase coating adhesion to several substrates, particularly clay-coated board^1,2. Other papers^3,4,5 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. Commercial extrusion coating equipment in good condition with modern screw designs has proven satisfactory in achieving the necessary level of homogenization.

Discussion

Mineral and Polymer Selection
A 5.0 MI, 0.923ρ autoclave-process polyethylene homopolymer (Chevron 4517) was selected as the base resin. Materials of similar melt index & density are commonly used for coating clay-coated board used as cup stock, liquid packaging, and boxes for bakery goods and frozen foods.

A wet-ground calcium carbonate with a 1.0µ mean particle size (MPS) and 8µ top-cut (maximum particle size) was treated with a fatty acid by the mineral supplier to form a hydrophobic coating on the surface of the mineral. This allows the polyethylene to “wet” the mineral surface, greatly improving the dispersion of the mineral into the polymer matrix and processability of the mineral/LDPE composite.

Polymer Processing and Substrate Coating
Heritage Plastics H-TEC™ calcium carbonate concentrate, comprising 75wt.% of the surface-treated calcium carbonate described above compounded into an autoclave-process LDPE homopolymer, was used to prepare pellet/pellet dry blends with the base LDPE. This allowed processing of CaCO₃-containing coatings on a standard extrusion coating line. A dry blend of 40% concentrate and 60% LDPE base resin were prepared to yield a 30wt.% calcium carbonate loading. Samples of clay-coated board were coated at this mineral loading and with 100% LDPE. A 114mm (4.5”) 30/1 L/D extruder fitted with a flat die deckled to 710mm (28”) exit width was run at a constant line speed of 500 feet per minute to yield a coating weight of 14.7 lb./ream.

The board surface was flame-treated at a level of 14,000 BTU/in. prior to primer and LDPE coating.

A standard commercial polyethylenimine (PEI) primer coating was employed as the control method of promoting adhesion. One part of primer was diluted with one part isopropyl alcohol and three parts water, and coated onto the board using a direct gravure applicator method.

The processing conditions employed for each run are summarized in Table I. Mineral addition yielded an increase in extruder specific output. Extruder screw speed had to be reduced from 70 to 51 RPM to maintain a constant line speed and coating weight. As a result, extruder head pressure and motor current were also reduced, as shown in Figures 1 and 2. These results are consistent with what has been observed in previous tests and is a result of the increase in thermal conductivity of the melt with mineral addition. This effect is shown in figure 3.

Fig. 1. Extruder Screw Speed and Motor Current Response to CaCO₃ Addition to 5.0 MI/0.923ρ LDPE

Fig. 2. Extruder Head Pressure Response to CaCO₃ Addition to 5.0 MI/ 0.923ρ LDPE

Coating-to-Substrate adhesion comparison of primer-coated board and unprimed board coated with LDPE containing 30wt.% calcium carbonate
Coating-to-substrate adhesion was measured qualitatively by judging the difficulty in peeling the LDPE coating from the clay-coated surface of the board. Both methods of promoting coating adhesion yielded fiber-tear levels of adhesion.

A possible explanation for the increase in coating adhesion with calcium carbonate addition is the increased thermal conductivity of the polymer with mineral addition (see Fig. 3) allows faster heat transfer from the bulk of the web to the polymer/substrate interface. This allows the polymer to stay molten and improves the wetting of the polymer onto the substrate surface.

Modifications to LDPE Coating Properties with CaCO₃ Addition
The addition of calcium carbonate to the LDPE coating reduced the MVTR, as shown if Figure 4.

Fig. 4. Decrease in coating MVTR with CaCO₃ Addition.

This is due to the formation of a “tortuous path” for the passage of water vapor within the polyethylene, as it must diffuse around the calcium carbonate particles in the coating.

Calcium carbonate addition also reduces the coefficient of friction between the polymer-coated surfaces, as shown in Figure 5. This should prove advantageous in operations where the coated surfaces must slide over each other, as in sheet-fed printing or box-making processes.

Summary

Calcium carbonate mineral enhancement of extrusion coated LDPE is commercially viable method of achieving fiber-tear adhesion to clay-coated board, and for modifying coated board properties. Mineral addition yields this adhesion level to clay-coated board, when combined with substrate flame treatment, without the need to use primer coating or corona/ozone treatment. The reductions in coating MVTR and COF should be of value to flexible package converters and end-users.

Fig. 5. Change in Poly-to-Poly C.O.F. of LDPE Coating with CaCO₃ Addition

References

1. Ruiz, F.A., TAPPI 1997 Polymers, Laminations, and Coatings Conference Proceedings, TAPPI Press, p.555.

2. Ruiz, F. A., “Improving LDPE Coating-to-Substrate Adhesion With Calcium Carbonate Addition,” TAPPI Journal, 79 (5) 139 (1996).

3. Ruiz, F.A., TAPPI 1994 Polymers, Laminations, and Coatings Conference Proceedings, TAPPI Press, p.89.

4. Ruiz, F. A., Society of Plastics Engineers 1994 ANTEC Proceedings

5. Ruiz, F.A., “Mineral Reinforcement of LLDPE Film, Bags, and Liners,” TAPPI Journal, 76 (1) 174 (1993).

6. Ruiz, F. A. and Allen, C.F., TAPPI 1987 Polymers, Laminations, and Coatings Conference Proceedings, TAPPI Press, p.365.

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

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

Acknowledgements

The author wishes to acknowledge the assistance provided by the Chevron Chemical Company in preparing the polymer/concentrate blends, and in extrusion coating the samples of clay-coated board, and Ms. Myra Hayes and the staff of Heritage Laboratories for the assistance they provided in the testing of coating adhesion and properties and the preparation of this manuscript and its illustrations.

Table I: Processing Conditions

Sample ID	Control using Primer Coating	LDPE + 30wt.% Calcium Carbonate
Coating Weight, lb./ream	14.7	14.7
Line Speed, m/s (ft/min)	2.54 (500)	2.54 (500)
Extruder Screw Speed, RPM	70	51
Extruder Output, kg/hr (lb/hr)	189.6 (417)	189.1 (416)
Motor Current, Amperes	120	111
Melt Pressure, MPa (psi)	8.21 (1193)	7.33 (1064)
Melt Temperature, °C (°F)	319 (606)	313 (596)