Method for cleaning a polymerisation reactor

A process for cleaning a polymerization reactor for producing aqueous polymer dispersions by means of radically initiated emulsion polymerization of one or more ethylenically unsaturated monomers in the presence of one or more protective colloids and/or emulsifiers, comprising:

• initially charging an aqueous mixture to the polymerization reactor, the aqueous mixture comprising one or more ethylenically unsaturated monomers, one or more protective colloids and/or emulsifiers, but no oxidation initiator, wherein the proportion of ethylenically unsaturated monomers is 20 to 70% by weight, based on the total weight of the aqueous mixture,
• heating the aqueous mixture to a temperature of 50° C. to 100° C., and treating the polymerization reactor with the aqueous mixture for 5 minutes to 3 hours,
• adding one or more initiators and optionally further starting materials, removing and the aqueous polymer dispersion thus obtained from the polymerization reactor and post-treating the aqueous polymer dispersion to remove wall deposits detached during the process, wherein the process is carried out when the polymerization reactor exhibits 50 to 95% of the original cooling capacity without wall deposits.