why do lampshades have a 'plastic' liner?

[mzgarden]

We have a number of pole lamps where the lampshades have an interior 'plastic' type of liner, with a fabric exterior.  The liners have become very yellowed.  I've tried washing, bleaching - for no improvement.  I don't want to buy new lampshades.

I'm wondering if the liners exist to prevent heat from incandescent bulbs setting the fabric on fire - and if we are using LED bulbs that produce almost no heat - is that liner even needed?  I could cut the 'plastic' liner out maybe and just go with the fabric shade?

Or - does anyone know a way to whiten the liner (washing/bleach made no difference)?

Thanks

[backwoodsman]

I would guess the plastic liner is there because it's the cheapest way to make the shade hold its shape.