The microscopy (SEM and OM) images of the polyolefins all show signs of the MPs being deformed and torn, with frayed edges and a distinct tortuosity in the overall shape of the particle (fig. , Supplementary Fig. ). This plastic deformation indicates that the material was not in a glassy state during breakage. This is more likely for these plastics as they represent the thermoset plastics with the lowest T g (∼-110 °C for PE and ∼-20 °C for PP). Friction between the milling balls and the plastics, as well as between plastics generates heat which may overcome the cryogenic cooling and heat the plastics above their T g and can cause them to be stretched or deformed before fracturing. Plastic stretching, flattening, and bending of the particles before breakage can result in particles with a pronounced AR and minimal solidity and, indeed, these features can be seen across the full size range of the LDPE particles (fig. ).