plastic containers are treated to provide increased barrier properties to organics and gases such as oxygen by treating the interior surface of the container with so 3 gas . the so 3 gas is provided by inserting a solid pellet of poly - so 3 into the container and evaporating the so 3 such as with microwave energy . a solid neutralizing agent such as ammonium carbonate is then placed into the container and evaporated with microwave energy to form ammonia which neutralizes the sulfonic acid groups on the surface interior of the container .