Process for producing an open celled foam

In a cigarette filter structure for the purification of air, the improvements comprising a filter material comprising a powder comprising particles of foam comprising urea formaldehyde-based copolymer obtained by a method comprising:

• mixing air and a foaming agent solution under pressure in a mixing part of an injection device, the mixing part being filled with a plurality of inert mixing bodies having dimensions in the range of 2 to 15 mm, so tat a non-cured foam is formed,
• supplying under pressure the non-cured foam from to mixing part and a prepolymer solution to a reaction part of the injection device so that a curable foam is formed,
• discharging the foam from the injection device and drying it to the air, so that a cured foam is formed.

In a process for manufacturing a cigarette filter for the purification of air, the improvements comprising providing a powder comprising particles of foam comprising cavities and interstitial spaces situated outside of the cavities, the cavities being defined by walls, the walls of the cavities being composed of a crosslinked urea formaldehyde-based copolymer and possible additives, the average largest dimension of the cavities being in the range of 30–350 μm, a considerable part of the cavities comprising two or more perforations in their walls, the perforations having an average diameter in the range of 5–300 μm, a considerable part of the perforations of adjacent cavities being positioned with respect to each other such that a continuous, non-liner path is formed, the foam having a porosity of at least 45 vol. % the porosity being defined as the volume percentage, based on the total volume of the foam, of that part of the foam accessible to matter passing through the foam, and the foam having a specific surface in to range of 0.1–15 m2/g.