Plastic enclosures for conservation storage.
Because of their transparency plastics are commonly used for the protection of archival documents and photographs. However many of the first plastics used were later found to be harmful to the objects they enclosed. The most common plastic used was PVC (polyvinyl chloride). It has good transparency and can be welded with ease. The problem was that it contained a large amount of plasticiser to keep it flexible. In time this plasticiser comes to the surface and transfers a sticky acidic residue to the enclosed object. Plasticisers are not the only danger from poor quality plastics. Polypropylene is often recycled and can have fatty acid slip coatings to make it run smoother on production machinery.
Polypropylene, Polyethylene and Complex Plastics
Only virgin Polypropylene without this slip coating should be used for photographs or archival material. Virgin Polyethylene, without plasticisers or harmful additives that might off-gas, is suitable for archival storage but it lacks rigidity and clarity. When applied as a thin film to Polyester it gains strength and rigidity but the crystal like transparency is reduced. The recent inclusion of reactive copper and carbon particles in plastics now offers new possibilities for the long term protection of archival material. This is also the case with multi layer oxygen and moisture barrier films which combined with the use of vacuum or oxygen scavengers can greatly slow down the rate of deterioration of a variety of sensitive materials.
Polyester is known as the most stable and chemically inert plastic used for conservation storage. It offers optical clarity, strength, rigidity and a surface free of slip coatings.
Melinex® and Mylar® are the most commonly known brands of Polyester. The clarity, purity and resistance to degradation is prized by conservators, however it is much harder to weld and form into pockets and complex album pages than softer plastics such as Polyethylene and Polypropylene which have a lower melting temperature. Polyester can be welded with heat but the temperature melt band is very narrow, meaning, too much heat and it will crystallise, too little and it will not weld.