Volume 14, Number 2, May 1992, pp.13-17, graphs and illustrations
Since their introduction some years ago, flexible laminated vapor barrier films have revolutionized the packaging of materials as diverse as moisture sensitive military equipment and perishable foodstuffs. The ubiquitous single-serving box of juice owes its exceptionally long shelf life to an inner foil-laminate vapor barrier. This "bag in a box" prevents the ingress of oxygen that would degrade flavor, and it resists attack from the acids in the juice itself. Vapor barrier films come in a range of types for various specific applications and, on the whole, are easily available, inexpensive, usually heat sealable, and extremely efficient. How efficient? Consider that a single 6-mil sheet of most laminated vapor barriers will resist the transmission of water vapor (and other atmospheric gases) better than a sheet of acrylic glazing almost one-half inch thick. It was just a matter of time before conservators found ways of putting them to use.
For more than a decade, some museums have been lining the insides of shipping crates and exhibit cases with an aluminized polyethylene and nylon barrier film called Marvelseal 360 (Ludlow Corp.) with the objective of decreasing off-gassing from exposed wooden surfaces. At The Oakland Museum, most of our exhibit case decks and risers have been retrofit in this fashion, and the procedure of wrapping case elements in Marvelseal 360 now goes hand in hand with the final covering in washed fabric as part of our exhibition preparation protocols. In 1986, a traveling exhibit of over 600 California silver objects was prepared in this way and returned two years later with negligible tarnishing, in part due to these precautions.
When Therese O'Gorman at The Oakland Museum embarked on a research project to develop a passive low-cost microclimate exhibit case a few years back, it was an obvious extension to use Marvelseal 360 to line the desiccant space in the case bottom. Basically, a box-like bag of Marvelseal was fabricated to fit into and line the case bottom, with an aluminum U-channel and silicone gasket assembly attached at the top to receive the acrylic vitrine. A vapor barrier film has several advantages over a welded metal or acrylic box for this purpose: it is easier to fabricate, much less expensive, and flexible. This flexibility can compensate for pressure changes inside the case due to changes in temperature, a factor that otherwise can lead to air exchanges and decreased efficiency.
In addition to their growing applications in exhibit installations, vapor barrier films are ideally suited to the fabrication of passive humidity-controlled shipping or storage bags, and they are also now being employed to maintain low oxygen atmospheres in the treatment of insect infestations. Undoubtedly, other uses for these interesting materials are waiting to be discovered.
Manufacturers of laminate films combine materials to create a structure with specific properties that will, say, meet a military specification for vapor transmission, bursting or tear strength, and corrosion resistance; or can be printed with an eye-catching design for over-the-counter sales. For these reasons, laminated vapor barrier films are available with a variety of characteristics: opaque or transparent, antistatic, flame resistant, printable, or reinforced with fabric scrim or Tyvek (spun-bonded polyethylene) for additional strength, to name a few. See Comparison chart
In the basic laminate, one of the inner layers is usually a material that is highly resistant to water vapor transmission, often aluminum, but it could also be a transparent polymer film like Aclar, a polychlorotrifluoroethylene (PCTFE) made by Allied- Signal. Other polymers, such as Saran (polyvinylidene chloride) or Tedlar (polyvinyl fluoride, Du Pont) may also be used.
The water-vapor barrier layer is typically sandwiched between two layers of polyethylene. One of these layers ends up on the inside surface of the finished laminate film and serves as a film-to- film heat-sealable adhesive. The other layer of polyethylene acts as an internal "tie" layer that enables the water vapor barrier to be laminated to a final outside surface film. This outside surface material is chosen to provide additional properties to the completed structure, such as low gas permeability, puncture resistance, or printability. In the simplest laminated structure using aluminum foil as the water vapor barrier, an outer layer of oriented polypropylene or nylon protects the fragile metal layer from being easily stretched or broken and further serves to decrease permeability to oxygen, nitrogen, and carbon dioxide.
It is common to find an outer layer made of a polymer film that has been "oriented" by stretching and annealing. This "drawing" process aligns the random molecular bundles into a tighter, more crystalline arrangement and makes the film both tougher and less permeable to atmospheric gases. Films that have been stretched in two perpendicular directions are called "biaxially oriented" and are the most efficient barriers to atmospheric gases, as well as being exceptionally resistant to stretching, puncturing or tearing.
Transparent laminates, such as Marvelseal 1177 and Bell Fibre's Film O-Rap FR 7750, use Aclar PCTFE instead of aluminum. Bell Fibre's Film-O-Rap FR 3300 is a new and somewhat different structure with a polyester/polyethylene/Aclar composition. Because it does not use a polyethylene heat-sealing surface, it is flame retardant and its water vapor transmission rate is superior to the other films (<0.007). The Aclar inner surface is plasticizer- and additive-free, chemically stable, and inert, but more difficult to heat-seal than the films with polyethylene inner surfaces.
At The Oakland Museum, we have found that the two most useful types are the aluminized variety such as Ludlow's Marvelseal 360 or Bell Fibre's Film-O-Rap FR 2176, and the more expensive but transparent film such as Film-O-Rap FR 7750 (the analogous Marvelseal 1177 is not currently in production). The aluminized type is relatively inexpensive and is used wherever transparency is not important, such as in the sealing of wooden or microclimate case elements. Although we have used the transparent films for storage and shipping pouches as well as controlled- atmosphere treatment chambers, we principally use them to make bags for the anoxic treatment of infested artifacts in nitrogen atmospheres, so that both the object and the oxygen indicator can be observed during treatment. The two types can, of course, be used together to cut costs, such as by making bags with one side transparent and the other opaque, or even by sealing a small window into an otherwise opaque bag.
Performance levels for vapor barrier films usually must comply with military specifications for such things as breaking strength, puncture resistance, flame resistance, contact corrosivity, oil resistance, thickness, as well as water vapor or atmospheric gas permeability. A particular film must meet or exceed the required specification. For example, all the laminates discussed in this article are required to have a water vapor transmission rate of less than 0.02 grams of water per 100 square inches in 24 hours. This is usually tested at 100 degrees F with a relative humidity of 90% on one side of the film.
Flexible aluminized films achieve these levels because of the way small platelets of aluminum are deposited on top of each other to create a tortuous path through which water vapor penetration is impeded. In polymer barriers such performance is dependent on molecular structure and arrangement and is, of course, a function of film thickness. The following graph compares typical water vapor transmission rates for various polymers.
Transmission characteristics for atmospheric gases such as oxygen, nitrogen, or carbon dioxide are harder to find in the manufacturers' data sheets for most of these films, since the films are primarily considered water vapor barriers for storage of sensitive equipment. When such specifications are listed, the stats are usually in the form of milliliters of gas per 100 square inches per millimeter thickness in 24 hours, at normal temperature and pressure. Biaxially oriented polyester and nylon films, for example, might transmit less than 7 ml of oxygen per 1 mil thickness. The following graph compares typical atmospheric gas transmission rates for various polymers.
Such specifications are understandably hard to translate into practical terms. To get a sense for real world performance, we set up two extremely simple--if unscientific--experiments at The Oakland Museum lab three years ago. The results were impressive.
In the first test, two small bags were made using transparent Marvelseal 1177. The first bag was about 6 inches square and contained a few grams of dried indicating silica gel, filling less than an inch at the bottom of the bag, along with a cobalt humidity-indicator card. After sealing, the humidity card read around 5% RH and the silica gel was dark blue. Into the second bag, which was about 10 inches square, we poured a few ounces of water containing a little sodium benzoate as a biocide. The water filled a bit more than an inch at the bottom of the bag. Before sealing the larger bag, the smaller one was placed inside, where the relative humidity was sure to be near 100% (in fact, the silica gel sealed in the inner bag was actually immersed in the water of the outer bag). This was done in mid October, 1988.
For well over a year, no change could be seen in the color of the silica gel or in the reading on the cobalt RH strip. After more than two years, the color of some of the blue silica gel granules began to perceptibly lighten, and the reading on the card began to rise. Now, almost three and one-half years later, the reading on the humidity card indicates around 35-40% RH. It may be interesting to note that during this time, the bag has not just been hanging on a wall in the lab, but it has been moved around, handled, and generally played with.
In a more practical test we filled a small Gore-Tex zip-lock bag with silica gel conditioned to 15% RH and placed it into a transparent vapor barrier bag (Marvelseal 1177) along with a 6 inch dial recording hygrothermograph. Gore-Tex was chosen because it allows the passage of water vapor but not dust from the silica gel, but a muslin bag might have also been used. We wanted to see the general amount of fluctuation on a daily or weekly basis in a prototype storage or shipping bag containing a moderate amount of conditioned buffer. (In retrospect, we might have learned more if the silica gel was conditioned to 50% RH, but we wanted an environment in the bag that was widely different from ambient.) The bag was then placed on the roof of the lab and exposed to sun and rain, hot days and cold nights. We even placed the bag in the refrigerator for a few days, and near the ceiling when our building was hot during the summer. This went on for more than two years, until the pens began to write through the paper of the chart, and the outer polyester lamination of the bag began to separate. This delamination was most likely due to the extremes of exposure, and had no apparent effect on the water vapor transmission of the container. In the end, the extent of relative humidity fluctuation was never more than 7%. And some of that may have been due to slow upward drift as the bag progressively absorbed small amounts of water from its higher humidity environments.
It appears that these transparent vapor barriers perform admirably when used in the construction of storage or shipping containers. With a large amount of silica gel conditioned to a level near the mean ambient RH, the RH in the bag should be fairly stable for extended periods, certainly for a number of years. Further areas of investigation should address the question of interior pollutants, from the polyethylene liner or elsewhere, and the possibility of using laminated films to contain inert gas environments for extended periods.
Heat sealing a polyethylene-surfaced vapor barrier film is quite simple, but must be done perfectly to ensure an adequate seal. Problems with vapor barrier packages are almost always due to either a small puncture or a faulty seal.
As described above, one side of a laminate film is usually polyethylene and thus easily heat-sealable. Fortunately, the other side usually has a much higher melting point, so it won't stick to the iron. On the aluminum films, the dull polyethylene hot-melt side is easily distinguished from the shinier outside. Basically, sealing involves placing two edges dull-side together and applying heat. To make things even easier, especially for the transparent films where both sides look alike, all of the films have printing on the outside, and the transparent ones clearly state: "seal other side."
Sealing can be done with a tacking iron or other heating element that applies heat over an area at least 1/4 inch wide. At The Oakland Museum, we use a hand-held unit, much like a fat pair of canvas pliers, with a 1/2 inch x 6 inch heating element in each jaw. Typical recommendations call for a temperature of about 350 degrees F for one second at 40 psi. Somewhat lower temperatures for a longer dwell time will also give adequate seals, but too low or too high a temperature should be avoided. Thermal impulse heaters are also available, but are expensive.
Bell Fibre's Film-O-Rap FR 3300 has an Aclar (PCTFE) inner surface, which is harder to seal than polyethylene. The heat sealing conditions for Aclar must be at 485 degrees F for one second at 50 psi. For sealing FR 3300, thermal impulse heaters are recommended.
To find out if your seals are sound, you should first practice with the aluminum films. Seal two pieces together, let them cool, and tear them apart. An improper seal will come apart with minor damage to the laminates, while a good seal will delaminate the aluminum from one side onto the other. In other words, a good seal should bond the two films together at least as well as the internal laminations are bonded in the film itself.
It is important to make the seals fairly wide, at least 1/4 inch wide, but 1/2 inch would be better. This is not only to ensure against channels or wrinkles that may increase permeability; the polyethylene layers are the least efficient barriers in the laminate, more than 1000 times more permeable to some gases than the laminated film itself. If you imagine a laminate bag to be like a flat box, with the efficient vapor barrier laminate on the top and bottom, and polyethylene walls on the other four sides, you can see why the seal should be wide. The polyethylene "walls" need to be as thick as possible to approach the barrier efficiency of other layers.
In some situations when it would be very difficult to create an adequate seal everywhere, an imperfect solution may be acceptable. For example, when tightly wrapping a wooden exhibit case deck to decrease off-gassing, cut one piece of Marvelseal 360 the exact size of the deck and another piece several inches oversize. The large piece goes underneath the deck, heat-seal side up, and the smaller piece is centered on top of the deck, also heat-seal side up. In this way it is easy to wrap the lower piece around and seal it to the piece on the top. Because of the one-sided nature of the films, however, it is not possible to completely seal the corners. In this case, the corner seams can be finished with a wide pressure-sensitive tape, usually 3M #850 polyester, or a thick aluminum foil tape or even silicone RTV sealant. Such a solution would be inappropriate, however, for applications demanding an extremely efficient and long lasting barrier, since the corners would probably leak over time.
Certainly the most exciting use of vapor barrier films to date is in the treatment of insect-infested collection objects with low oxygen atmospheres. This technique uses nitrogen to replace most of the oxygen in a vapor barrier bag, and an oxygen absorber to further reduce the concentration to below 1%.
Basically, there are two different approaches to extermination with atmospheric gases. One approach uses carbon dioxide, and the other a so-called inert gas such as nitrogen. Carbon dioxide, present in normal atmosphere at less than 1%, regulates breathing, and higher concentrations are lethal due to increased metabolism. (This is true for humans as well as insects, and so extreme caution should be exercised when using this gas). Concentrations above 50%-60%, while dangerous, are easy to achieve and are effective against insect pests in a matter of days. Except for the possibility of carbonic acid formation in humid atmospheres, there are no other reactions that could harm collection materials. On the other hand, such concentration levels are impossible to determine without some sort of gas detection equipment, which can be expensive. For this reason, carbon dioxide fumigation is best used in a monitored chamber or bubble where oxygen levels of less than 1% may be impractical to achieve.
The nitrogen approach, however, is ideally suited to the treatment of individual objects in vapor barrier bags. This method is virtually harmless to objects, essentially non-toxic (although care must be exercised when handling any suffocant gas), and easy enough to be used by the most remote and low- budget museum. Current research indicates that this technique is effective against common museum pests at all life stages, but optimum concentrations and exposure times are still being investigated.
What makes this technique so easy is the availability of Ageless, an oxygen absorber, and the Ageless Eye, an oxygen indicator, both manufactured by Mitsubishi. Ageless is a very inexpensive oxygen scavenger that is available in several grades, each one rated according to the amount of oxygen one packet will absorb. For example, a packet of Ageless 1000 will absorb 1000 ml, or one liter, of oxygen. Chemically it is made of ferrous oxide, potassium chloride, and a humectant in a small permeable packet a little larger than a business card. As purchased, many small packets are sealed together in a vapor barrier bag that also contains a small indicator tablet called an Ageless Eye. The Eye is an oxygen indicator that is blue in normal atmospheres, but pink when concentrations of oxygen are below 1%. This color change is reversible, so that a single Ageless Eye can be reused many times. The oxygen absorbing capability of each Ageless packet, however, is exhausted after exposure. The holding bag should be immediately resealed after any packets are removed to retain the efficiency of the unused portion. A pink Ageless Eye indicates that the unused packets are still active. It might also be noted that Ageless will absorb carbon dioxide, and therefore cannot be used in high carbon dioxide or nitrogen/carbon dioxide atmospheres.
In practice, treatment of an infested object by oxygen depravation is fairly simple. A custom bag is made from one of the transparent laminate vapor barrier films just large enough to hold the infested object. The object is placed into the bag, along with a few packets of Ageless, and an Ageless Eye. The bag is then sealed almost closed, leaving a small space at one corner to insert a tube from a cylinder of compressed nitrogen. The bag should be flushed with nitrogen gas several times to remove as much atmospheric oxygen as possible, and any large surplus squeezed out before the bag is sealed shut. As a rule of thumb, six packets of Ageless 1000 will remove all of the oxygen from a cubic foot of air. If half of the air in that volume were replaced by nitrogen, then only three packets would be needed, and so on. It is also important to remember that, as the Ageless absorbs the residual oxygen, the volume inside the bag will collapse correspondingly. If nitrogen flushing were not done, the bag would collapse by 21%, corresponding to the amount of oxygen present in our normal atmosphere. This decrease in volume should be considered to avoid any potential physical harm to the object being treated.
After a period of time, which may be several days depending on the volume of residual gas in the bag, the oxygen capacity of the object, and the number of Ageless packets inserted, the Ageless Eye will turn pink when the oxygen concentration drops below 1%. At this point, the bag should remain sealed for a period of time variously recommended as between one and two weeks--or more. The Eye should be checked periodically to ensure that it remains pink, since air could ingress through punctures or bad seals. After treatment, there seems to be no reason why the object could not just be put back into storage in its vapor barrier bag until needed If a conditioned silica gel buffer were included, such a container may provide an ideal storage or shipping environment, and research into the incorporation of activated charcoal or permanganate pollution scavengers may reveal additional benefits and applications.
Vapor barriers are sold only through authorized distributors who may have fairly large minimum order sizes. You should contact the manufacturer for regional sources. Conservation Materials Ltd. is planning on stocking smaller amounts of some of these films. Suppliers of vapor barriers can also refer you to vendors of hand-held and other types of heat-sealing apparatus.
Bell Fibre Products
P.O. Box 1158
Columbus, GA 31993
Laminating and Coating Division
1 Minden Road
Homer, LA 71040
Conservation Materials Ltd.
1165 Marietta Way
Sparks, NV 89431
Author AddressJohn Burke, Head Conservator