Answers (2)
No, it cannot be stored in plastic!
"Helium cylinders should never be stored outside and rarely exposed to the elements.
While helium has many uses in the industrial world, most of us only use it for two things: joyously buoyant balloons and hilarious squeaky voices.
Because helium is a gas, it's always stored in pressurized tanks so that it can be readily used for filling balloons. However, like all high-pressure gas tanks, these cylinders need to be properly stored to prevent accident or injury from the tank rupturing.
Storage Room Conditions
Store helium tanks in a dry, cool room. A dry atmosphere prevents rusting and other corrosion, which in turn prevents the cylinder from weakening and failing. Heat can cause these canisters to rupture, so the room should be kept cool at all times. Helium cylinders should also be stored away from flammable materials, sources of ignition and other heat sources. Keep helium cylinders away from high-traffic areas or doorways, to minimize the risk of disturbing the cylinders while they aren't in use.
Helium does not expire or "go bad", nor does helium degrade. As long as the valve on the helium tank achieves an airtight seal, the helium in the tank will not escape
The helium atom is so small that it can pass through cracks that can not be detected by any other means.
You think "plastic" is a class of materials, but it really only means "flexible". A wide variety of materials are called "plastic", and characteristics vary a lot. If you look in a grocery store you can find plastic zipper bags called "sandwich", "storage", and "freezer", all seemingly made of the same material. But the sandwich bag is made of polyethylene and lets oxygen molecules go through, while the storage bag is made of vinyl and lets gas through but less than the sandwich bag. The freezer bag is made of thicker vinyl. Chips and helium balloons are made of mylar and coated with aluminum to plug tiny holes between the molecules, but helium balloons eventually go flat anyway.
Bottom line: buy helium from a gas store and follow their recommendations about storage.