Often book collectors are not aware that there are different grades of dustjackets available.  This is a very important consideration when deciding upon storage and preservation of their collections.  Keep in mind that bookstores will often use economy or standard jacket covers for their inventory.  Although these covers offer protection during shelving and handling, they are not intended for long term use as the materials in economy and standard jacket covers are not inert nor chemically stable over time.  Typically, economy jacket covers are a thin polypropylene or PVC [polyvinyl chloride] film merely to serve as basic protection from tearing.  Similarly, standard jacket covers will use a polypropylene or PVC film with the addition of a paper backing for some added protection.  As noted above, archival covers instead use higher grade polyester as well as inert paper backing and neutral adhesives.

Another very important distinction between archival and economy jacket protectors are the adhesives used for the sealed edges.  To ensure chemical stability, a polyvinyl acetate adhesive is used for archival covers.  In contrast, a hot melt adhesive is used for standard covers which will discolor and degrade over time.

About Mylar [Polyester], Polypropylene [PP], Polyethylene [PE] and Polyvinyl Chloride [PVC]

The term Mylar ® was coined by Dupont in 1952 with the design of their new extra strong polyester film based on the development of their earlier Dacron ® polyester fiber using modified nylon technology. 

Although jacket protectors are almost universally referred to as mylar covers, in truth this distinction should only correctly be reserved for archival covers which utilize this polyester film.  To review, polyester, or polyethylene-terephthalate, is a very strong film with high internal strength and good folding qualities.  Polyester will withstand heavy use and is hard to puncture or tear.   By comparison, at equivalent thicknesses, polypropylene is a softer, more flexible and conformable material.  Polypropylene is more economical than polyester, but does not offer the same strength or durability.

Polyvinyl chloride [PVC] is typically produced as a hard plastic.  In order to create a "vinyl" film, plasticizers must be added to turn this hard plastic into a flexible plastic.  These plasticizers -- or more correctly phthalate esters -- are derived from phthalic acid.  The most widely used phthalates are di-2-ethyl hexyl phthalate (DEHP), which is also the dominant plasticizer used in PVC, due to its low cost. 

The Preservation Directorate of the Library of Congress has established specifications for materials used in archival products.  We review these specifications closely when deciding upon archival products, collectors may wish to do likewise when comparing products for the storage of their collectibles.  These specifications include paper stock, card stock, board stock, mat board, polyester and other products used for storing collectibles.  Below is a link to this Library of Congress webpage.

Collectors should make a point to familiarize themselves with the many helpful preservation and conservation guidelines the Library of Congress has published on their website in the Caring for Your Collections section.