The most reliable and convenient method of providing a completely isolated, or anoxic (oxygen-free) microclimate is to create a flexible container of barrier film.
Why not use a “baggie”?
Single layer films are generally quite permeable to most gases. Barrier films are usually multi-layer films that have been designed to be impervious to gas migration. Some common examples of barrier films will be found in any grocery store: most bags used for packaging snack foods are gas barrier bags; garbage, freezer, and sandwich bags are not. To understand the difference in practical terms, compare the longevity of a helium-filled balloon made of rubber to a balloon made of foil barrier film – the rubber balloon will shrink within hours, while the barrier film balloon will continue to hover about and haunt you with “happy birthday” or “get well soon” in mid-air for weeks.
Almost all gas barrier films are made of multiple layers. The outer/top layer will be a strong (usually transparent) plastic with a relatively high melting point, often made of polyester (Mylar, Melinex), nylon, or polypropylene. The inner/middle layer will be a thin layer of gas barrier material (see below). The inside/bottom layer is almost always low density polyethylene, which softens and sticks at relatively low temperatures to allow heat sealing. Additional layers are sometimes used to increase the barrier properties of the film, and adhesives may be used to cement the layers. Note that polyester, polypropylene, and polyethylene are all poor gas barriers on their own.
Barrier layer materials
Most of the thickness of a barrier film belongs to its inner (polyethylene) sealing layer. The outer support layer is often surprisingly thin, and the middle (barrier) layer can sometimes be of microscopic thickness.
Note that it is the composition of the barrier material, and not its thickness that determines its permeability. Metal foil (usually aluminum) is generally the most effective gas barrier material, closely followed closely by transparent ceramic barriers (Escal). Sputtered aluminum, EVOH, barrier nylon, and Saran are transparent plastic barrier layers often used in the food industry.
Sealing the film
Barrier films are designed to adhere when heated. Any tool that provides heat may be used to soften the sealing layer, however a wide element sealing press especially designed for barrier films, or a small “tacking iron” (for hand work), are recommended. As the inner layer of polyethylene is itself quite permeable, and oxygen molecules will slowly move through the seal, a wide seal is needed to create long-term oxygen-free storage. Impulse-type sealing machines designed for simple polyethylene bags use a narrow seal- this is not adequate for barrier film storage! Especially designed sealing clips are available as well, but a heat seal is more reliable.
Barrier films are used for storage of sensitive materials, but may also be used as a barrier layer to isolate showcase interiors from the showcase material (wood products).
Choosing the right film
When deciding which barrier film is correct for your application, these factors must be considered:
*What kind of microclimate do you wish to maintain? Most films control moisture well, but oxygen transmission varies.
*How long do you wish to maintain the microclimate? There’s no reason to use the most effective film if you need only maintain a controlled atmosphere for a few days, but long term storage demands better barrier properties.
*How will you maintain the microclimate? An active system is more forgiving than one based on passive absorbents.
*Do you wish to make visual inspections of the enclosed objects? Films vary in clarity and color, and foil based films (eg Marvelseal) are opaque.
*Will a difference in materials costs matter? While materials are usually the least expensive part of a project, cost savings could be substantial in a large project. We’ll be happy to help you determine the best film for your needs.
For more specific information on the two films available from us, see:
Escal Barrier Film- an excellent barrier film that is transparent.
Aluminum Foil Barrier Film- a particularly flexible foil based barrier film.