Adhesive Tie-Layer for Multi-Layer Films

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DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

Typically, the materials used include polyethylene (PE) – high-density polyethylene (HDPE), low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) blended with a variety of materials such as EVOH, polypropylene (PP) or PA providing the desired end-properties. Due to the difference in the chemistry between these materials, they do not fuse together to form a single structure leading to delamination of the multilayer film. This is where an adhesive tie-layer material comes into play that can bind these two layers together.

Tie-layer materials

The most common materials used as adhesive tie-layers are polyethylene-based materials that have been functionalized with either acid or anhydride groups by grafting these groups onto the polymer backbone. The presence of the grafted acid or anhydride groups allows this tie-layer to attach itself covalently to the surface of the barrier layer due to the polar groups present on it. This results in increased interfacial adhesive strength.

Typically, the tie-layer materials are dry blended with unmodified PE, which results in cost reduction while maintaining the required adhesion on the bonded film layers. BindEX is an adhesive tie-layer that is polyethylene functionalized with maleic anhydride groups by reactive extrusion process. Several methods exist for combining different materials to form a multi-layer structure. Of these, co-extrusion is a widely used method to obtain flexible multi-layer packaging material.

Co-extrusion

In co-extrusion, the plastic materials to be combined into multi-layer films are heated in order to melt them separately. This polymer melt is then allowed to pass through a die where they get combined and are extruded together to form a multilayer polymer web. Cast extrusion and blown extrusion are the two methods primarily employed for extruding multilayer films.

Cast extrusion

In the cast extrusion process, the polymer feed – LDPE or EVOH, and tie-layer are first passed through a feedblock where one or two feeds may be split up depending on the requirement of the multilayer structure. The feeds are then combined together in a feedblock. The feedblock geometry is adapted to suit the layer thickness requirement of the multilayer film. The feed is then passed through a die, which typically has a ‘coat-hanger‘ type geometry causing the feed to spread across the required film width.

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