Analysis of the Media Resistance of Flexible Packaging Polyurethane Adhesives

Flexible packaging is often exposed to various media such as oils, sauces, acidic and alkaline flavoring liquids, and alcohols in fields like food and cosmetics. The media resistance of the adhesive directly determines whether the composite bag will experience delamination, wrinkling, swelling, or other adverse phenomena during long-term use.

 

The root cause of the media resistance lies in the cross-linked network structure of the adhesive layer. Polyurethane adhesives with an appropriate cross-linking density and polar-matching molecular chains can form a dense and stable adhesive film, preventing media from penetrating inward. If the cross-linking is insufficient, small molecule media can easily penetrate the adhesive layer, destroying the interface bonding force between the adhesive layer and the substrate; if the cross-linking is too high, the adhesive film becomes hard and loses its toughness, and is prone to cracking after being soaked in media.

The erosion mechanism of different media on the adhesive layer varies. For oils, it mainly relies on swelling, gradually softening the adhesive layer; acidic and alkaline liquids will catalyze the hydrolysis of polyurethane bonds, breaking the molecular chains; alcohol-based media have both penetration and plasticizing effects, easily causing a rapid decline in peel strength. In the formulation design, commonly used are oil-resistant polyol monomers, combined with an appropriate curing agent system, to optimize the density of the adhesive film.

 

During the selection stage, it is necessary to match the contents inside the packaging. For ordinary snack packaging, a general-purpose Polyurethane Adhesive can be selected; for products containing high oil, acidic fillings, or alcoholic contents, a high-media-resistant specification should be chosen, and an appropriate barrier substrate should be combined. Even if the adhesive itself is resistant to media, the composite structure defect and incomplete curing will significantly reduce the media resistance performance, requiring a coordinated control of the formulation and process.