Liquid developer and image forming apparatus

1. A liquid developer which comprises an insulation liquid containing a fatty acid monoester, a polymer dispersant and toner particles comprised of a resin material, and the resin material containing a component of which molecular weight M is 10,000 or less, wherein the resin material is characterized in that:

a weight-average molecular weight Mw of the resin material is in the range of 20,000 to 200,000; and

an amount of the component contained in the resin material is in the range of 5 to 40 wt %,

wherein the polymer dispersant is represented by the following general formula (I):

where I is an integer in the range of 9 to 12, m is an integer in the range of 3 to 6, n is an integer in the range of 5 to 8, R represents —OH, R′ represents H— or CH 3 (CH 2 ) p CO—, and p is an integer in the range of 15 to 18.

2. The liquid developer as claimed in claim 1 , wherein when the weight-average molecular weight of the resin material is defined as Mw and a number-average molecular weight of the resin material is defined as Mn, Mw/Mn which is an index of a molecular weight distribution of the resin material is in the range of 1,4 to 40.

3. The liquid developer as claimed in claim 1 , wherein the resin material has ester bonds in its chemical structure.

4. The liquid developer as claimed in claim 1 , wherein the fatty acid monoester is comprised of a fatty acid component and an alcohol component which is reacted with the fatty acid component, wherein the fatty acid component contains a saturated fatty acid monoester of which carbon number is in the range of 8 to 20.

5. The liquid developer as claimed in claim 4 , wherein a carbon number of the alcohol component is in the range of 1 to 4.

6. The liquid developer as claimed in claim 1 , wherein an amount of the fatty acid monoester contained in the insulation liquid is in the range of 10 to 60 wt %.

7. The liquid developer as claimed in claim 1 , wherein an amount of the polymer dispersant contained in the liquid developer is in the range of 1.0 to 10.0 parts by weight with respect to the toner particles of 100 parts by weight.

8. A liquid developer which comprises an insulation liquid containing a fatty acid monoester, a polymer dispersant and toner particles comprised of a resin material, and the resin material containing a component of which molecular weight M is 10,000 or less, wherein

the polymer dispersant is represented by the following general formula (I):

where I is an integer in the range of 9 to 12, m is an integer in the range of 3 to 6, n is an integer in the range of 5 to 8, R represents —OH, R′ represents H— or CH 3 (CH 2 ) p CO—, and p is an integer in the range of 15 to 18, and

in the case where a part of the toner particles is taken out from the liquid developer, a weight-average molecular weight Mw of the resin material and an amount of the component are obtained by the following steps, the steps comprising:

subjecting the resin material contained in the part of the toner particles to a size exclusion chromatography to obtain a chromatogram and an integral distribution curve of the molecular weight of the resin material, the chromatogram having at least one peak, and the integral distribution curve plotted using a graph in which the ordinate axis represents a relative amount (%) of the resin material with respect to the total amount of the resin material (100%) and the abscissa axis represents the weight-average molecular weight Mw (log) of the resin material;

analyzing the at least one peak of the obtained chromatogram to obtain a first result which shows that the at least one peak is a peak corresponding to the resin material of which weight-average molecular weight Mw is in the range of 19,000 to 195,000; and

analyzing the obtained integral distribution curve to obtain a second result which shows that the amount of the component contained in the resin material is in the range of 5 to 40 wt %.