Foamed polylactam containing surfactant

A foamed polyamide is prepared by polymerizing a lactam in the presence of an anionic catalyst, a cocatalyst, a blowing agent and a surfactant, wherein the surfactant is, at least in part, of the general formula RN(XCOOM)2, wherein R is an aliphatic hydrocarbon group of 10 to 20 carbon atoms, X is an alkylene group and M is a salt-forming cation. The preferred lactam is e -caprolactam, which may be copolymerized with laurolactam, and the preferred anionic catalyst is the sodium salt of the lactam being polymerized. Specified cocatalysts are a toluene-2,4-diisocyanate-caprolactam adduct and a polycarbonate resin. The preferred blowing agents are allylic azides, e.g. dodecenyl and methyl benzyl azides, but volatile hydrocarbons, e.g. butane may also be used. Exemplified surfactants have M in the above formula as sodium, X as -C2H4- and R as lauryl, tetradecyl and stearyl, and these may be used alone or with, e.g. potassium stearate or a nonyl phenol-ethylene oxide adduct. The polymerization preferably takes place in a mould at 130 DEG C. to 200 DEG C. to produce a shaped article. A filler, e.g. CaCO3 may added to the mix.