question on polypropylene bags

[eaglessbchamps-migration]

i noticed alot of my older polypropylene bags are kinda wrinkly, is that just from possibly being packed too tight or is it natural thing?

[FlComicFan]

I went through some modern books I had boxed about 5-6 years ago and two of the longboxes all the bags were like this plus they were starting to stick together somewhat. I'm going to assume they were due for a change out and already started rebagging the books. I usually change out cheap bags every few years anyways since they tend to yellow and you can see on the boards the chemicals being released wihtin the bags which will deteriorate your books paper quality at a faster rate. The few books that now warrant it will get mylars.