Why polyethylene rather than polyproylene?
In an attempt to answer the many inquiries about this subject, I offer the following responses.
Polyethylene is inert, translucent and creates a lower static charge than polypropylene, and those are the three main characteristics which make it a superior archival or storage material. An inert substance, according to chemists, is not readily reactive with other elements, in that it cannot produce other chemical compounds. In other words, molds, mildews, and discolorations will not occur with an inert substance, so there's no way a polyethylene sleeve can damage your collectible, nor can damage readily occur to it. Polypropylene is also inert, but that's where the similarities end.
The translucent nature of polyethylene prohibits a the greater amount of light than the transparent polypropylene. Light can be particularly harmful to collectibles, especially magazines, comics, books, photographs, artwork or any kind of cover or jacket that has color and/or photos or illustrations. By inhibiting the overall amount of harmful light that actaully makes its way to your collectible, the risk of fading is minimized.
The lower static charge produced by polyethylene as opposed to the higher charge emitted by polypropylene means that polyethylene will attract much less dirt, dust and other foreign, organic elements. And it is those elements which produce damage to collectibles.
Polyethylene is more flexible than polypropylene. By contrast, polypropylene is stiff and hard, while polyethylene is soft and pliable. The relatively rough surface of polypropylne has the potential to produce scratches to the surfaces of collectibles while the ptoential for scratching by polyethylene sleeves is minimal.
Many of the polypropylene sleeves are less than 2 mil. Many of the sleeves being sold on the internet and by hobby and comic shops are very flimsy and range from .5 mil to 2 mil. The majority of commercially-available polypropylene sleeves are 1 to 1.5 mil. Our polyethylene sleeves are 2 mil and 3 mil, respectively.
In more chemistry-speak, propylene (CH3CH:CH2) is is a sub-stratum of ethylene. Ethylene is the purest form of the base chemical and thus the purer better of the two. Consider this example: Suppose you were looking for the perfect gene to clone a tough guy. Now, would you rather have the genes from the son of a tough guy or fom the original tough guy, in this case, the son's father? Obviously, you want the purest tough guy genes, so you would go with the father. Polyethylene is like the father; polypropylene is like the son. See definitions below.
Due to its rigitity, polypropylene sleeves are more prone to tear, especially at the seams. Try this experiment if you don't believe this. Take an ordinary piece of writing paper (rigid) and a common paper towel (pliable). Try to gently tear each one. The paper tears readily, while you have to give the towel a little more of a tug to tear it. It actually kind of resists tearing by having some flexibility. The same principle applies for propylene vs. ethylene. The ethylene base provides more "give" and thus stronger seams and less likelyhood of tearing.
A good deal of polypropylene is imported. Much of it is imported from China, while our polyethylene sleeves are manufactured in the United States of America. There is one dealer who advertises "Made in the USA" who sells sleeves that say "Product of China" right on the package. I don't want to devolve into politics or have this become a discussion of global economics, so I'll just make the statement and let you decide which is more appropriate for you.
Polyethylene costs more. Simple logic dictates that it should be a superior product and it is. It costs more because it is a product of higher purity (100% Virgin) in addition to the advantages already presented, and will outlast and outperform any derivative product. Polyethylene provides better protection for both the short and long term.