Felt is a style of fabric, not a type of fabric.
Just as satin does not mean silk, so felt does not mean wool.
A scapular is required to be made of wool.
| The small scapulars consist essentially of two quadrilateral segments of woolen cloth (about two and three-quarter inches long by two inches wide), connected with each other by two strings or bands in such a manner that, when the bands rest on the shoulders, the front segment rests before the breast, while the other hangs down an equal distance at the back. The two segments of cloth need not necessarily be equally large, various scapulars having the segment before the breast of the above dimensions while the segment at the back is much smaller. The material of these two essential parts of the scapular must be of woven wool; the strings or bands may be of any material, and of any one color. The color of the segments of woolen cloth depends on the color of the Carmelite habit, ... The Catholic Encyclopedia |
According to the Sacred Congregation of Indulgences wool is necessary and exclusively to be used in the making of scapulars. Cotton, felt, of wool woven into the fashion of lace will not suffice. A picture or decoration must never deprive the color of the Scapular of its predominance.