A thermally expandable composition, including at least one epoxy-functional polymer, at least one thermoplastic polymer, at least one chemical blowing agent, and at least one activator, wherein the epoxy-functional polymer is present in the composition before expansion with an amount of between 30 and 75 wt.-%, based on the total composition, and the epoxy-functional polymer includes at least 300 mmol epoxy groups per kg polymer, and wherein the chemical blowing agent is able to form at least one reaction product with at least two amino groups upon thermal decomposition, and the chemical blowing agent is in the composition before expansion with an amount of between 5 and 30 wt.-%, based on the total composition.