RESULTS
Breakdown Voltage
Untreated seed oils and their esters show breakdown voltage values up to 70 kV and thus fulfilling
both IEC requirements for mineral oils and synthetic esters (cf. Table II).
The power frequency breakdown voltage of an insulating liquid is primarily sensitive to its water
content. The samples we have tested had absolute water content between 150 and 200 ppm. Such
values are high if compared with typical values for unused mineral oils and similar to what is observed
for synthetic esters. Vegetable oils and esters, because of their chemical structure, have larger
saturation limits than mineral oils. It has been shown that the effect of water depends on the relative
moistness of the liquid [3]. So, an adequate comparison has to refer to relative humidities. At room
temperature, the water saturation limit of vegetable oils is about 750 ppm, and 40 ppm for mineral
oils ; an absolute moisture of 200 ppm in a vegetable oil reported to its saturation limit leads to a
relative humidity (RH) of 26.7 %. This RH value is equivalent to an absolute water content of 10 ppm
in mineral oil, which is commonly measured after a dehydration treatment. Accordingly, the measured
dielectric strength of vegetable oils and esters can be consistently compared with the value specified
for treated mineral oils. This result confirms the actual high dielectric strength of these vegetable
products.
Viscosity and Pour Point
The measurements (see Table II) show that pour point and viscosity values of unprocessed seed oils
exceed the specified values.
After esterification, the viscosity of the oils passes below the specified 35 mm2.s-1 value for synthetic
organic esters. The measured value tends towards 12 mm2.s-1 i.e. the specified value for mineral oils.
This is consistent with the fact that esterification corresponds to the separation of big triglycerids into
smaller molecules that flow more easily, improving the oil rheology.
The pour point is less affected by the modification of the molecular structure of the fluid, but it can be
noticed that the bigger the alcohol, the lower the pour point. The fatty acid esters, originated from
particular alcohol (referred as X-ol in the Table II below) reaction, present a pour point very close to
the referring mineral oil limit.
Table II : Measured Properties for three Seed Oils and some of their Esters
Breakdown
Pour point,