Glossary
PVC-free
Also called: polyvinyl chloride, pvc sleeves
PVC-free means a sleeve or holder contains no polyvinyl chloride, a plastic that releases acids as it ages and can bond to the surface of a card.
The US National Archives is blunt about it: avoid PVC plastics, because they generate acids which fade images over time, and the plastic can stick to items inside. Their guidance specifically names printed items such as baseball cards, warning that the image can transfer onto the plastic.
This matters more than most collectors realise, because one of the most common card supplies is still made of PVC. BCW's standard 3 × 4 toploader is described by the manufacturer as rigid, high-clarity PVC. Toploaders are excellent for shipping and short-term handling. They were never designed as a decades-long storage solution.
Sleeves are generally the safer material. Polypropylene is the standard for penny sleeves and deck sleeves alike, and conservation guidance lists polypropylene, polyethylene and polyester as stable, non-damaging plastics.
The practical rule: use a toploader to move a card, and something polypropylene-based to store it.