The term "acid-free" is not used in relation to plastics. The terms "inert" and "stable" are used to describe plastics that can be safely used in preservation.
A mil is one thousandth of an inch and refers to the thickness of the plastic. For example, something that is 4 mil = .004" thick.
The higher the mil, the stronger the plastic. Documents that require more support because they are oversized and/or fragile should be stored in higher mil plastic.
Archival polyester (PET) is also known by the brand names Melinex and Mylar. It is chemically inert with a high tensile strength and chemical and dimensional stability. It is extremely clear and will never yellow or become cloudy, making it ideal for encapsulation where a detailed view of the covered item is desired. Inherent static electricity helps to keep items from shifting in enclosures but also means that archival polyester should not come in contact with charcoal, pastels or other loose media.
For document storage, you will find envelopes, sleeves, L-sleeves, folders and page protectors made out of archival polyester. In sheets and rolls, it can be used to line wooden shelves as a protective barrier, to make book jacket covers and to cover rolled textiles. This versatile material is chemically inert and therefore will not break down over time, proving it to be an excellent choice for long-term protection of your artifacts.
A chemically inert material that is also heat-resistant and provides a highly protective barrier against moisture and vapors. It is slightly less clear than archival polyester but is clearer and more rigid than polyethylene.
A chemically inert material that is highly flexible and easy to work with. It has a filmy appearance but is an economical choice for items that need protection but not complete transparency.
Polyvinyl chloride (aka PVC and vinyl) is a thermoplastic that is widely used to make everything from plumbing pipes to children's toys. It has also been used to make photo storage pages. In order to make PVC more flexible, plasticizers have been added to the compound that should be considered dangerous and damaging to archival materials. These plasticizers will not only release chemical gases into the air, but will cause the breakdown of the material over a surprisingly short period of time. Photo pages will seem "oily" to the touch and will begin to soften and yellow. To protect your photos from damage, say no to PVC and look for polypropylene pages instead.