Description: 521 synthetic resins or natural rubbers -- part of the class 520 series / 521 synthetic resins or natural rubbers -- part of the class 520 series/() synthetic resins (class 520, subclass 1) (50) cellular products or processes of preparing a cellular product, e.g., foams, pores, channels, etc. (134) cellular product derived from two or more solid polymers or from at least one solid polymer and at least one polymer-forming system (136) at least one polymer is derived from an aldehyde or derivative or wherein the polymer-forming system contains the same type of reactant
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