AbstractIn this work, fire retardants (FRs) such as aluminum trihydroxide (ATH), zinc borate (ZB), melamine, graphite, and titanium oxide (TiO2) were loaded into the shell layer of a co-extruded polypropylene (PP)-based wood-plastic composite (WPC). The incorporated retardants reduced the peak of the heat release rate by 8 to 22%, depending on the type of FR. Other studied parameters, such as ignition time and mass loss rate, were improved after the FR loading. The total heat release decreased slightly (except for the graphite-WPC). The effective heat of combustion was independent of the presence of the FR or, in the case of graphite, slightly increased. Carbon monoxide production increased (ZB, graphite) or was not changed significantly (ATH, melamine, and TiO2). It was also observed that the tensile strength improved after the FR loading; however, the tensile modulus decreased, except for the graphite-WPC. The impact strength improved or was independent of the FR loading, as in the case of the sample with ATH. The wettability of the composites decreased with filler loading, except for ZB, which showed the highest water absorption value among the studied composites.