Experimental research on the feasibility of biodegradable polymeric insulating materials

Various environmental issues such as the global warming effect have become major public concerns. Eco-friendly biodegradable polymers have a potential ability to solve these problems. From this viewpoint, we have been studying various dielectric properties such as complex permittivity, electrical conductivity, dielectric breakdown strength, partial discharge (PD) resistance, and resistance to water imersion in comparison to the properties of low density polyethylene (LDPE) for the following various biodegradable polymers; starch ester (SE), poly-L-lactic acid (PLLA), polyethylene terephthalate succinate (PETS), polycaprolactone (PCL), polycaprolactone butylene succinate (PCLBS), polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), polybutylene adipate terephthalate (PBAT), polyhydroxybutyrate/valerate (PHB/V), and their blends. Among the nine polymers investigated, PLLA and PETS are in the glass state at room temperature, while the other seven polymers are in the rubber state. Partly because of this reason, the permittivity and conductivity of PLLA and PETS are much lower than those of SE, PCL, PCL-BS, PBS, PBSA, and PBAT. In contrast to such large differences in conductivity and permittivity, all the polymers investigated have an almost similar impulse breakdown strength at room temperature. As for dc or ac breakdown, in which the conductivity should theoretically play a more significant role than in the impulse breakdown, SE, PLLA, and PETS show a relatively higher strength than the others. The results of PD resistance show that a good PD resistance can be achieved by the sample with a high crystallinity and a low permittivity, and that the PD resistance is ranked as LDPE > SE > PLLA ≅ PETS > PBS > PCL-BS. The durability of biodegradable polymers against water is significantly reduced by the increase in water temperature. Regardless of the water temperature, SE shows the best durability of the five kinds of biodegradable polymers investigated, namely, SE, PLLA, PCL-BS, PBS, and PETS. All three blend polymers of SE with PCL, PBSA, and PBAT show resistivity and ac breakdown strength much higher than the values requested for 600-V class insulated wires. By taking the above-mentioned results into account, it is sumarized that SE can be an electrical insulating material if the surrounding atmosphere is relatively mild, while PCL, PCL-BS, PBS, PBSA, PBAT, and PHB/V can be plasticizers for SE.