Polyester or polyethylene envelopes, rather than other plastics such as polyvinylchloride (PVC), are used in conservation because they do not react with documents, or degrade or yellow over time. In contrast, PVC produces hydrochloric acid as it degrades and when kept in poor conditions it can melt or stick to photos and papers.
We use polyester and polyethylene enclosures as a preservation method here at The National Archives. One series that has received this type of treatment is the paper index cards found in WO 345.
WO 345 is a War Office series of about 50,000 Japanese index cards of Allied prisoners of war and internees from the Second World War. The cards record detailed information about prisoners of war so they are of great interest to family researchers. A common problem found in this series is the fragility and yellowing of the cards due to the poor quality paper. Some of these cards have been torn or fragmented over time.
In order to protect fragmented cards from further damage we are enclosing the most at risk in polyester. Polyester keeps the fragmented documents together, minimises the risk of further physical damage and keeps the paper index cards accessible for readers to request.
Protecting records in this way isn’t just applicable to the records here at The National Archives – it can also help you care for your collections at home. To best protect your collection, make sure when you purchase envelopes or folders you look for polyester or polyethylene on the packaging. If you have previously encapsulated any of your collection particularly before the 1990s this could be PVC, and if the plastic is yellowing this is a telling sign.