I was interested now and again when the "Nylon" sandbags first came out. It seems starting sometime in 1971.
From the February 1971 issue of Army Digest.
Better Bags: Don't be too shook if your unit is having difficulties with rotting sandbags. An answer has been found. The latest is an acrylic or wash-and-wear model found to be unaffected by the sun's ultraviolet rays. When ordering this potential lifesaver, use Federal Stock Number 8105-935-7101.
(my note: Not sure what the wash and wear feature means? Probably means easy to wash after repeated use, to keep clean for continued servceablity etc. As noted these bags came out around this time, but I gather these only supplmented the old canvas or whatever they were made of in the 70s, I certainly seen and used both in 80-82.)