Glossary
Acid-free
Also called: acid free sleeves, acid free card storage
Acid-free means a material has a neutral pH and will not release acids that yellow, embrittle or stain a card over time.
The term comes from paper conservation, where acidic pulp breaks down and discolours anything stored against it. Applied to card supplies, it usually means the plastic will not off-gas acidic compounds into the card it is protecting.
It is worth knowing that the label is not independently policed. Preservation guidance from the Library of Congress and the Northeast Document Conservation Center notes that enclosures sold as "acid-free" or "archival" may still contain lignin, dyes, sizing agents, coatings or plasticisers that can damage what they hold.
A more reliable signal than the phrase is the named material. Polypropylene, polyethylene and polyester are chemically stable; PVC is not. If a product states its polymer, you can judge it. If it only says "acid-free", you are trusting the marketing copy.