Vulcanized Rubber

Even large plastic particles can be charged electrostatically and would start sticking. The finer a plastic fragment was comminuted, the harder it might stay attached to available surfaces. In worst case, fine ground particles could not be brushed off anymore, they would just jump back to the surface after the brush passed this area.

To check the grindability, we precut the 2 mm thick sheet into fragments of ~ 5x5 mm. 2 grams of pre-cut sample was placed into the zirconium oxide mortar. The grinding set was fastened inside the cryo-box and we flushed the mortar with liquid nitrogen (LN2) via the cryo-box feeding funnel. The sample was ground with an amplitude of ~ 2,5- 3 mm and evaporated LN2 was replaced during the grinding process. In the beginning of the grinding process, higher amounts of LN2 are required until the sample, mortar and grinding ball are chilled. After ~ 3 minutes of grinding, the consumption of LN2 should sink significantly.

After the first minute of grinding, a lot of fragments were already < 1 mm fine. We estimated that after 5 minutes of grinding, only a small improvement in fineness would be found, so we stopped the grinding trial after 5 minutes.

The mortar and grinding ball were taken out and the cold sample was brushed off best possible. As estimated, finest sample dust could not be regained properly. Those fine particles remained inside the mortar or attached to the grinding ball.

Surely higher amounts of sample could be ground with shearing forces applied. With a gap setting of ~ 100 µm (for the Variable Speed Rotor Mill PULVERISETTE 14 premium line), we would expect less of fine ground particles.

Plenty of plastic samples can be ground to < 0,5 mm with our Variable Speed Rotor Mill PULVERISETTE 14 premium line. With cutting-mill-conversion, a lot of sample were ground successfully to < 0,2 mm in the past.

We used 10 grams of pre-cut sample (also < 5 mm manually cut) and placed the sample into a PTFE beaker. Liquid nitrogen was poured into the beaker to embrittle the sample. Afterwards, we started to process the sample with our Variable Speed Rotor Mill PULVERISETTE 14 premium line which was equipped with cutting mill equipment and a sieve shell set with 0,2 mm trapezoidal perforations and our Small Volume Cyclone (passive, without vacuuming). A cyclone is required to avoid air pressure when LN2 evaporates (even when only tine amounts were fed into the mill).

The embrittled sample was transferred into the feeding funnel with a laboratory spoon in smaller portions. It took 1 minutes until the complete sample was fed and ground. The sound of grinding was typical and vanished instantly, after the last fragment was fed. A few particles hurled out of the feeding funnel, because the protective screen above the feeding funnel was removed for a faster feeding of embrittled sample (googles were required).

With an active cyclone separator (our sample exhauster attached to the cyclone), more of ground sample should be transferred into the cyclones connected collecting glass.

The trapezoidal perforations of the used sieve shell set were not clogged significantly. Higher amounts of sample could be ground this way. Also after this trial, finest plastic particles could not be brushed off properly (because of electrostatic charge).

The cleaning was performed rapidly by vacuuming with a soft brush. Also compressed air could be useful to clean the grinding chamber. The complete cleaning process took about 3 minutes.