But often these seemingly simple barriers need four or more layers to provide the strength, permeability, and ease of use that manufacturers need. A new formulation of Ixef polyarylamide from Solvay Advanced Polymers, Dearborn, Mich. (solvayadvancedpolymers), lets engineers use two-layer barriers that meet standards in EPA's Partial Zero Emission Vehicles and regulations from the California Air Resource Board regulations. That's because Ixef resists fuel so well. Fewer layers also simplify construction and lower costs.