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Improving the effect and cost efficiency of barrier coatings against unwanted ingredients

Aim of the project was developing a concept to improve the effect and cost efficiency of barrier coatings against unwanted ingredients of paper for recycling (PfR) by optimally combining inorganic pigments with polymer coatings. The unwanted PfR ingredients concerned were petroleum-derived hydrocarbons (MOSH, MOAH), phthalates (DEHP, DBP, DiBP) and benzophenone.

Starting with 11 different barrier substances and 14 different pigments, coating trials and formulation tests were conducted in the laboratory. The samples produced were evaluated, and new test methods were developed and validated to better assess the barrier effects after long- and short-time exposure by means of Tenax® migration. Polyvinyl alcohol, ethylene vinyl acetate copolymers and cellulose derivatives (hydroxyethyl cellulose and hydroxypropyl methylcellulose) were identified as barrier substances with good or very good effects.

It could be demonstrated that the barrier effect of these substances against the above-mentioned unwanted ingredients can be increased by adding flaky pigments. At best, the migration of unwanted PfR ingredients was reduced by factors ranging from 2-6. As a broad tendency, however, the migration was somewhat greater than predicted by common theoretical tortuosity models. The differences could be caused by simplified model assumptions or small adsorption effects of the pigments.

The best results were achieved with kaolin pigments and very fine-grained mica combined with cellulose derivatives (hydroxyethyl cellulose and hydroxypropyl methylcellulose). The pigment orientation of especially kaolin was clearly visible in scanning electron microscopy images. To achieve the desired parallel orientation of the pigments, it was advisable to use substrates with high smoothness and relatively slow water absorption.

The laboratory results could be largely verified by pilot trials performed with a hydroxyethyl cellulose / kaolin system.

The hypothesis formulated in the research application, i.e. that it is possible to increase the barrier effect of pigment-containing coatings by adjusting the type, properties and amount of pigment as well as the polarity and free volume of polymer coating using the tortuosity effect, could be confirmed in all details. In particular, it could be shown that the approach leads to considerable cost savings of up to 50 %.

The second hypothesis put forward in the research application could be largely confirmed as well: the long-term effect of barrier properties was proved by means of selected examples. The coating formulations were applicable after identifying and optimising the coating conditions in each specific case.

The research project IGF 18656 BG of the research association PTS was funded within the programme of promoting “pre-competitive joint research (IGF)“ by the German Federal Ministry for Economic Affairs and Energy following a resolution of the German Bundestag and carried out under the umbrella of the German Federation of Industrial Co-operative Research Associations (AiF). We would like to express our warm gratitude for this support. We would also like to thank the paper and supply sector companies involved in the project for supporting the work.