Tie Layer

Many of the resins used together in barrier packaging simply will not stick to each other on their own. Polyethylene and EVOH, for example, have very different molecular structures and surface chemistries, and if extruded directly against each other they would delaminate, meaning the layers would separate rather than stay bonded as a single film. A tie layer, usually a maleic anhydride-modified polyolefin or similar functionalized resin, is engineered to have chemical compatibility with both materials on either side of it, bonding to the polyolefin on one face and the barrier resin (commonly EVOH or polyamide) on the other.

Tie layers are typically among the thinnest layers in a coextruded structure, since their job is purely adhesion rather than contributing strength, barrier, or sealing performance. In a structure built around an EVOH barrier core, it's common to see a tie layer on each side of the EVOH, between it and the polyethylene layers, so the overall stack might read something like polyethylene, tie, EVOH, tie, polyethylene.

Without the correct tie layer resin matched to the specific materials on either side of it, a barrier structure can look fine coming off the line but fail later in distribution or use, when the layers separate under flex, heat, or handling stress. Selecting the right tie layer chemistry for a given resin combination is one of the more technical parts of designing a multi-layer barrier film.

A film design that adds an EVOH barrier layer between two polyethylene layers will always need a tie layer resin on each side of the EVOH, since polyethylene and EVOH won't bond to each other directly.

Tie layers are sometimes overlooked in casual film discussions because they don't contribute an obvious end-use property like strength or barrier, but skipping or under-specifying a tie layer is one of the most common causes of delamination failures in multi-layer film. The tie layer's chemistry has to be matched to the exact resins on both sides, not just any adhesive-type resin.