The use of ammonium polyphosphate (APP) and expanded graphite (EG) as a flame retardant for polymer generally requires a relatively high amount of addition to achieve a flame-retardant effect, and is liable to cause a decrease in strength of the material. Therefore, in this study waste diatomite was used as an eco-friendly flame-retardant for high density polyethylene (HDPE) to reduce the amount of APP and EG and can reach the flame-retardant level. Moreover, the waste diatomite was heat treatment and surface modified by silane coupling agent to improve its compatibility with HDPE. The results show that the tensile strength of the HDPE drops sharply after the addition of the APP and EG. However, replace some of the APP and EG by the modified diatomite can slow down the decline and maintain the basic physical properties of the material. The impact strength of HDPE was also decreased by the addition of APP, EG and untreated diatomite. However, the impact strength of HDPE was increased after adding the surface modified diatomite. It can be seen from the experimental results that the addition of the waste diatomite modified by the silane coupling agent can reduce the usage of the flame retardant such as APP and EG, and increase the strength by increasing the compatibility between the plastic and the inorganic material. Moreover, this eco-friendly formulation can reach the UL-94 HB level, and it can be applied to interior decoration or as building materials in the future. Thus, it can not only recycle the wastes, but also reduce the threat caused by fire.
