There are five different thermoplastic films used to fabricate mounts, sleeves and pages for displaying stamps and covers:
PET polyethylene terephthalate (marketed as "polyester" or "Mylar(r)")
PVC polyvinyl chloride
PE polyethylene
PP polypropylene
PS polystyrene
The commonly held notion is that PET is good, i.e., safe to come in contact with philatelic materials, and that PVC is bad. A recent article by William E. Souder, Director of the Center for Management of Science and Technology at the University of Alabama at Huntsville [1], concludes that these widely accepted ideas are unproved to date. This section is based on Souder's findings.
Much scientific information is available on these five thermoplastic films. However, much of this scientific information is not very practical for stamp collectors. Souder's research is a first attempt to fill this gap in our knowledge.
Archivists advocate the use of archival films; Dupont Mylar-D being a leading example. However, "archival," aside from the general meaning "chemically stable," is imprecise. After contacting seven leading international repositories of stamps and other historical documents, Souder concludes that "No standardized scale for measuring the degree of 'archivalness' exists." Though their needs and concerns differ from those of most stamp collectors, the experienced opinions of archivists must be considered as valuable advice for storing philatelic materials. But, for PET, our youngest and least experienced film, no scientific experiments can yet shed light on the results of storage over period of several decades, or longer.
PET films passed a test, cited by Souder, of ink transfer from stamps to plastics.
On the negative side, PET readily degrades in sunlight unless it contains ultraviolet inhibitors that may do damage to stamps. Indeed, a test in which stamps were heat and light tested in PET plastic products made for stamps showed yellowing and brittling of the plastic after four months of six hour/day exposure to summer sun.
Also, due to limitations in fabrication technologies, PET films are not widely available in the variety of forms desired by stamp collectors.
Collectors should be aware that a variety of imported products falsely labeled "Mylar" have recently entered the U.S. market. This has naturally occurred due to major shifts and consolidations in polyester (PET) global markets. These products have proved inferior and potentially damaging to stamps. Collectors who purchase Mylar/PET should insist on seeing evidence that these products are fabricated from registered DuPont Mylar-D films. These are the only films sanctioned by archivists.
PVC, popularly known as "vinyl," is the most controversial of the plastic films considered here. Even though it is one of our oldest and most successful plastics, in use for scores of household products, PVC has been the brunt of environmental and health criticisms and has been cited as destructive to collectible postage stamps.
In fact, there are two different types of PVC, plasticized (pPVC) and unplasticized (uPVC). pPVC is uPVC with plasticizers blended into it. This makes it more flexible and it is often called "soft" or "flexible" PVC. However, plasticizers are demonstrably bad for stamps. uPVC is often called "hard" or "rigid" PVC and it appears to be harmless to stamps. As Souder points out, the terms flexible/soft and rigid/hard are confusing because they refer to the polymer blend and not to products made from that polymer. Thus, highly flexible films can be made from "rigid" uPVC.
Although PVC, even uPVC, has not been subjected to every imaginable test, Souder is able to debunk the following eight misconceptions, commonly held about the film:
In summary, modern pPVC shows major improvements over poorly performing earlier versions. uPVC appears to be free of any problems for stamp collectors, as far as tests conducted so far indicate. Even so, Souder [1] is cautious to state that pPVC films should not be used with stamps. Though today's plasticizers are much less likely to migrate from their PVC blends onto stamps than earlier versions, the potential is ever-present. Even a careful collector may inadvertently subject a valuable collection to excess pressures and temperatures that could simulate some damaging migration of the plasticizer onto stamps.
Polyethylene films have the benefit of high flexibility. They soften at 150-230 degrees Farenheit and degrade at 175-250 degrees. This range of temperatures depends on the variety of coatings and additives present. These and other properties are summarized in the table reproduced here from Souder's article. As with all the plastics described here, more testing is essential.
Polypropylene films have high flexibility and they soften and degrade at high temperatures, similar to PET films. PP has failed tests of ink transfer and migration of plastics to stamps. PP films have sometimes performed poorly in long term storage tests, although contradictory test results exist.
Polystyrene films, like PP films, failed a test of ink transfer and migration of plastics to stamps, although polystyrol, a commercially engineered PP, subsequently passed a replication of the same test. They are flexible and have good stretch and ultraviolet resistance properties.
All of the films discussed here have performed without major problems in the conditions required by stamp collectors. However, more testing with stamp protection in mind is certainly required.
Collectors face many problems in identifying plastics. Even experienced polymer experts are often unable to distinguish one film from another without the aid of sophisticated laboratory tests. Collectors remain at the mercy of their philatelic product suppliers. They are well advised to become as informed as possible about philatelic plastics, and to use this information in asking penetrating and challenging questions. Avoid all products where answers consistent with this knowledge are not immediately forthcoming. In the natural evolution of knowledge and economic conditions, the current trend of replacing older plastics that damage stamps with safer ones will continue. Safe plastics for stamps are on the horizon as good pushes out bad. Until the time that is complete, collectors must use utmost care properly to protect and preserve their collections. Collectors can help this transition from bad to good plastics by a willingness to demand and pay for quality products. !
aDegradation refers to the chemical abasement of the material into a useless form, e.g., generally a charred or deformed mass. Softening refers to a physical plastic flow that alters the shape of the material. The temperature ranges here encompass the different versions of the films that are commercially available through various additives, coatings, and molecular configurations. bSlippery films do not hold stamps well. Some slip resistance could be provided by the incorporation of additives or coatings, but their chemical effects on the inks and papers of stamps is unknown. cThis is the potential for dust and dirt attraction through static charge. Some anti-static properties could be imparted with additives or coatings, but their chemical effects on stamp papers and inks are unknown. dLow permeability protects against water vapor/humidity ingress. Low permeability also restricts air passage, thus restricting the necessary opportunity for stamps to "breath." Due to their low permeability, PP and PE films often are used as barriers to keep out atmospheric gases and unwanted fumes. eFilms with little ultraviolet resistance yellow and degrade in sunlight and in the presence of other ultraviolet sources. fMany films can interact with the inks on stamps. Some films are believed to disturb the phosphor taggants on stamps. Other films have been observed to develop a whitish halo around a stamp. PET films may or may not exhibit these bad habits. Because our knowledge is so skimpy, these ratings are very tentative. |
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[1] William Souder, Waldo's Century, American Philatelist, 114 (December 2000), 1102-1106.