Foamcore

Foamcore, foam board, or paper-faced foam board is a lightweight and easily cut material used for of photographic prints, as backing in picture framing, in for making scale models, and in painting. It consists of a board of polystyrene foam clad with an outer facing of paper on either side, typically white clay-coated paper or brown kraft paper.

The original white foamcore board was made in 1/8 inch (3 mm) and 3/16 inch (5 mm) thicknesses for the graphic arts industry by Monsanto Company under the trade name "Fome-Cor®" starting in 1957. Monsanto sold the business to International Paper in 1993, and the business is currently operated by 3A Composites. It is now widely used as backing in art and document mounting and picture framing, with archival-quality variants now available from several companies. It ranges in thickness from 1/16"–1/2". It is not recyclable or biodegradable in normal situations.

The surface of the regular board, like many other types of paper, is slightly acidic. However, for modern archival picture framing and art mounting purposes it can be produced in a neutral, acid-free version with a buffered surface paper, in a wide range of sizes and thicknesses.

Foam-cored materials are also now available with a of solid (non-foamed) polystyrene and other rigid plastic sheeting, some with a textured finish.

Foamcore does not adhere well to some glues, such as superglue, and certain types of paint. The foam tends to melt away and dissolve. Some glue works well in casual settings, however, the water in the glue can warp the fibers in the outer layers. Best results are typically obtained from higher-end spray adhesives. A hot glue gun can be used as a substitute, although the high viscosity of hot glues can affect finished projects in the form of board warping, bubbles, or other unsightly blemishes.

...
Wikipedia