They are cotton canvas wrapped around a sturdy (not flimsy) panel. Here is what W&N has to say about them (Bolding is mine):
"Originally a sketcher's product these superior quality canvas boards feature canvas laminated onto high quality boards of excellent stability. They are stretched by hand for correct tension and tailored corners and are triple coated with one coat of acid free sizing and two coats of highly pigmented primer formulated by Winsor & Newton. Artists' Canvas Board is correctly prepared for immediate use by the artist and are suitable for use with Oil, Water Mixable Oil, Alkyd and Acrylic colours. The medium grain surface is ideal for all techniques including heavier applications of colour. The balanced absorbency and tooth of the canvas board prevents sinking of oil colours and improves adhesion
Here is the link to above ground listing for them
You should Join OPAS and come out with the group on future paint outs!