Method of making a homogeneous mixture of polyolefin solids and liquid additive

A method of making a homogeneous mixture of polyolefin solids and a liquid additive without melting the polyolefin solids during the making, the method comprising applying acoustic energy at a frequency of from 20 to 100 hertz (Hz) to a first heterogeneous mixture comprising at least one liquid additive and polyolefin solids for a period of time and at an acoustic intensity effective to substantially intermix the at least one liquid additive and the polyolefin solids together while maintaining temperature of the first heterogeneous mixture above the freezing point of the at least one liquid additive and below the melting temperature of the polyolefin solids, thereby making a first homogeneous mixture comprising the polyolefin solids and the at least one liquid additive without melting the polyolefin solids.