I'm using Prat Start presentation portfolios and 
so far so gd.

When you mention lignin, I presume you are 
referring to the paper inserts. Acid free 
doesn't necessarily mean lignin free. There may 
be lignin in the core of the paper but not the 
surface. It's hard to be sure but usually truly 
archival product (museum quality) will be 
advertised as so. We'll never really know. If 
you are uncomfortable, you can always replace 
the paper inserts with true archival 
paper/board. 

Polypropylene shd be PVC free, it's relatively 
inert. I rem that Prat sleeves are advertised as 
archival. 

There are other factors which are more damaging 
to your cel and act faster. e.g. humidity, UV 
light, trapped gases that causes warping and 
degeneration of cels. So store your cels in a 
cool, dry place, handle them with care and air 
your cels once in a while. ^_^