Plastics are produced by reacting at least one compound containing sulfonic acid groups with at least one aldehyde in the presence of a hydroxy compound in an acidic concentrated aqueous solution having a water content of about 20 to 30% by weight. If a tenside is present the reactants will form a foam. The preferred sulfonic acid-containing compound is the sodium salt of diisobutylnaphthalene sulfonic acid, the preferred hydroxy compound is a phenol, vix. resorcinol, and the preferred aldehyde is formaldehyde which may be formed in situ from hexamethylenetetramine, paraformaldehyde, trioxane, or the like.
A phenolic foam having a density of from 10 kg/m.sup.3 to 100 kg/m.sup.3 and comprising a phenolic resin base part and a cellular part most of which is made up of a large number of fine cells, wherein the fine cells contain a hydrocarbon and have an average cell diameter of 5 .mu.m to 200 .mu.m, and the cell walls of at least most of the fine cells are formed of a smooth surface of the phenolic resin base. While the blowing agent is a hydrocarbon, the foam has a thermal conductivity comparable to that of a conventional foam made with a flon blowing agent, undergoes no change in thermal conductivity with time, exhibits excellent mechanical strength such as compressive strength, and has reduced brittleness.
