We make a simple distinction between bags and sleeves. We define the bags as having a flap that wraps over the top, while the sleeves are open on the top.
In many instances, such as comic bags, you can select bags made from polyester or polypropylene. Which material is best for your needs? There are several factors to consider to make the best choice.
Both polyester or polypropylene are considered archival-safe by the U.S. Library of Congress (learn more). The different chemistry between polyester or polypropylene gives them different attributes as sleeves and bags. BoPET (biaxially-oriented polyethylene terephthalate) is a polyester film made from stretched PET (polyethylene terephthalate). BoPET is commonly called Mylar, the brand name of BoPET developed by the DuPont company. Polyester is inert and has no plasticizers or other chemicals that will damage collectible documents. Polyester is almost completely impermeable to liquids and gases. Polyester is transparent and does not yellow with age. Polyester film is the material of choice for archiving many of the United States’ most valuable historical documents according to the Library of Congress. Polypropylene is also archival safe, however it is gas permeable.
Polyester sleeves and bags are more rigid than polypropylene counterparts. This gives polyester bags a brilliant, shiny surface. Polyester bags are strong and unlikely to tear, while the flexibility of polypropylene bags may be a better solution for some needs.
If price is a consideration, polypropylene sleeves and bags are more cost effective than those made from polyester (Mylar). This makes polypropylene the popular choice for large collections.
For all of the bags and sleeves offered by BCW Supplies, we clearly define what material is used in its production. The description is found on the product’s respective webpage.