What is OPP? Well OPP is the abbreviation of Oriented Polypropylene, a type of plastic polymer. OPP film is more transparent and has high mechanical and optical features compared to other plastic materials. It has been used for flower, textile, food and stationery packing. OPP sleeves/bags are very popular among stamp collectors in Malaysia.
Some stamp collectors have been questioning if OPP is acid free, or some of them claim that OPP is acid free. I was really scratching my head to read about this argument. I think "acid free" is not applicable to any plastic film/bag. It is understandable why stamp collectors are obsess of "acid free" because acid may damage stamps. So, stamp collectors are cautious about stamp albums, stamp cardboards and stamp holding cards etc., so that these paper-made stamp stationeries are free of acid.
Paper is produced from paper pulp that comes from wood. The pulping process, involving the bleaching with acid, is important to produce white clear paper (otherwise, the paper would be brown in colour). So, ordinary white paper contains acid residues from the pulping process. However, acid residues may be eliminated during the pulping process with additional steps, and paper produced by this type of pulp is called acid free paper. Therefore, acid free paper is more expensive compared to ordinary paper.
Nevertheless, there is no bleaching involves in making OPP plastic bag/film. How on earth "acid free" could be applied on OPP? It is simply an abuse of terminology "acid free".
