Can I use carpet as a brassica collar?

I've just cut up loads of 6cm square pieces of carpet and fitted them to the base of all my brassica plants.

Now i've done it im concerned that perhaps carpet isn't the best material.  Does carpet leach chemicals/colouring into the  soil and cause damage to my little plants?

It will stop the Cabbage Root Fly from layig eggs, however. Just don't leave it down longer than you need to.

Can you gradually replace it with cardboard or something more friendly?