To reduce permeation levels and meet PZEV, LEVII or Japanese regulations, multilayer fuel tanks have been developed. The manufacturing process is called multi-layer extrusion or coextrusion "COEX". This process is especially used for the production of tank shells with barrier layers limiting hydrocarbon vapor emission into the atmosphere.
Coextrusion thus allows us to meet the increasingly strict requirements set up by environmental standards in terms of hydrocarbon emissions.
Coextrusion consists in extruding a parison made up of several different materials layers with complementary properties. Each material is fed by a separate extruder and the layers are formed in the head.
Each layer of material has a specific purpose:
- Outer virgin HDPE allows components welding, ensures protection against UV and can have conductive properties when used for blow-molded filler pipes
 - Regrind is made of excess material generated by the manufacturing process
- Outer and inner adhesives ensure the cohesion of the structure and the regrind/EVOH compatibility
- EVOH barrier ensures permeation performance
- Inner virgin HDPE provides mechanical resistance and can have conductive properties when used for blow-molded filler pipes.
An EVOH (Ethylene-Vinyl alcohol) layer is introduced between HDPE layers, with adhesive to ensure the material’s integrity. EVOH is the material which creates a barrier against hydrocarbon emissions. EVOH does not adhere to Polyethylene: an intermediate layer is thus necessary to avoid the delamination of one layer with the other layers: this is the adhesive layer.
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