I read all the comments. Some are biased against polyester, batting or fabrics. Polyester outlasts cotton and does not fade with use as cotton does. I would have no problems using the sheet for a backing.

I have used sheets as backing many times. Actually the 'thinner' sheets are easier to quilt because of the lower thread count. My kids have a couple that are many, many years old and have been very used, abused, laundered incorrectly and are still in good shape.

I put a poly-cotton back on my Confetti Quilt. It has been on my bed for at least 10 years. Soon after I made it, I found out that the back was full of pills. I just tried not to look at it. Then about a year or so ago I tried my little Dritz anti-pill machine. I was surprised at how easily those pills disappeared. I had to empty the collection basket/bin several times while doing this. It didn't take all that long to go over the entire quilt. Then I used the quilt for almost a year with the back side up. It was beautiful now too.

I always look for vintage 100% cotton sheets at Thrift shops...I usually only hang my quilts and the cotton weight from those Vintage sheets make them hang just right....