Canada's Favorite Hydroponic Supply Store
Canadian Garden Supply says.....
All garden professionals use these bags, which are made from Mylar, to store and transport their crops.
Mylar is made from polyester resin that has high moisture and oxygen barrier properties.
After filling the bags, iron the bag closed. It can be cut open later at the first seam where it was ironed, and then ironed closed again.
A good tip to remember, is that after the bag is sealed, it will have a lot of air in it. Cut one little corner off and suck the air out with a straw or vacuum, and reseal it quickly. You can be sure that the bag doesn't have a leak if it doesn't let air back in.