this invention presents a process whereby thermoplastic polylactic acid polymers foams having the desirable properties for manufacture of thermoformed articles may be made . it has been found that introduction of a dual functional reactive agent into the melt will improve the relevant properties of the melt and thus , the resultant foam . an example of such an agent is pyromellitic di - anhydride , but it is envisioned that a wide number of dual functional reactive agents can be utilized . it has been found that such dual functional reactive agents do not shift crystalline melt point of the material by any appreciable amount . it has been found that , by carefully controlling such a reaction , melt strength can be increased sufficiently to produce stable foam at temperatures above the melt point of the polymer , to permit the production of foamed polylactic acid polymer and product formed therefrom on conventional process equipment .