Synergy Effects between Polyethylene and high Density Papers in Gas Barrier Materials - a Morphological Study

Abstract

This work has its origin in efforts to optimise the barrier properties of packaging materials in which polyethylene extrusion coated high-density paper constitute the oxygen barrier. Examples of applications where this element is included, are containers for orange juice and some dairy products. The intention of present work has been to make contributions to increased understanding of the oxygen barrier mechanisms. A coating of polyethylene film onto high-density papers (greaseproof paper) happens to give especially good oxygen barrier, considerably better than expected from theoretical considerations, in spite of the fact that a polyethylene film and high-density papers separately have no oxygen barrier properties.

The prevailing hypothesis for the mechanism for the barrier property is that this property is achieved by blocking of the paper’s pores. The pore blocking theory implies that the dense fibre and fines reduce the effective area for permeation of polyethylene.

The basis for this work is briefly that the pore blocking theory alone cannot explain the oxygen barriers achieved. The hypothesis is that there are synergy effects between the paper and the polymer that can explain the barrier properties. A central element in this hypothesis is that a crystalline or transcrystalline layer will be formed in the polyethylene coating at the interface to the paper during extrusion coating and thus reduce the oxygen transmission rate through the laminate.

In the present work some grades of high-density papers have been extrusion coated with different grades of polyethylene. The coated samples were subjected to oxygen transmission rate measurements and morphological analyses such as 13C CPMAS NMR. Surface free energy measurements were done on the paper samples. The analyses og polyethylene extrusion coated high-density paper showed that the surface properties of the paper and variation in shear force affect the relative compositions of morphological fractions of the polyethylene, and that oxygen barrier property are affected by these morphological changes. When the monoclinic crystalline fraction is increased for a sample of a given material combination, the oxygen barrier is improved. Localised FTIR analyses showed monoclinic crystallites close to the paper surface, indicating that these structures have their origin in adhesion to the paper and shear forces. After delaminating the samples an uneven layer of crystalline structures was found by AFM analyses. SEM analyses of delaminated material showed a high number of open, flat bubbles in the coating at the paper interface, irrespective of barrier properties.

It has been found that for paper made of highly beaten sulphite pulps, not dried prior to paper making, a reduction in surface free energy (or reduced surface energy difference to polyethylene), leads to increased monoclinic crystalline fractions and an improved contact between the materials and thus better oxygen barrier. Papers made of dried sulphate pulps, or mixtures with wet and dried, gave generally poorer oxygen barrier properties and the relationship between surface free energy and oxygen barriers properties is found to be different. Coating of LDPE and HDPE different with respect to how they respond on storage and low temperature heat treatment. Storing improved the oxygen barrier of the laminates with LDPE, but there was no improvement of the HDPE coated samples. The reduced oxygen transmission rates of samples with LDPE were attributed to increased monoclinic crystalline fractions.

Measurements of mean lamella thickness by 13C T1 measurements showed that a paper surface with a low surface free energy of “wet” pulps made more optimal conditions for initiation and growth of crystallites. The water boned in the fibres of “wet” pulps was found to have higher mobility (by 1HT2 measurements) than in the more crystalline fibres of “dry” pulps where the water was harder bonded in the structures. Coating of the respective sides of paper samples shows that the side coated is not indifferent with respect to oxygen transmission rate achieved. These values correspond well with the surface free energy for the respective sides coated.

The hypothesis of the thesis was confirmed.