Non-corrosive phenolic foams are prepd. from an aqs. soln. of a phenol-formaldehyde resin of the resole type by (a) adding to the aqs. resin soln. (I) 1 - 5% wt. of an emulsion stabiliser, (II) a strong acid dissolved in an alcohol which is a solvent for the resin, as crosslinking agent, and (III) a basic agent in microcapsules with a retarded action for neutralising the residual acidity of the foam at the end of curing, the cover of plastic material of the microcapsules melting under the effect of the crosslinking heat to liberate the basic agent, which forms a water insoluble material salt with the residual acid, (b) crosslinking and expanding by heating the reaction mixt. at 30 - 60 degrees C for ca 1 hr., and (c) stabilising the foam by heating at 25 - 80 degrees C to eliminate volatile materials. Gives non-corrosive, neutral foams with a high content e.g. 95 - 98%, of closed cells.
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