Composition and method for developing electrostatic latent images

DETAILED DESCRIPTION OF THE INVENTION The electrostatic latent image developing composition of the invention includes (A) a toner component and (B) a carrier as defined above.
Carrier The carrier (B) used in the developing composition of the invention is a particulate carrier having a mean particle diameter of from 10 to 45 .
mu.
m, preferably 10 to 35 .
mu.
m, more preferably 15 to 30 .
mu.
m.
If the mean particle diameter of the carrier is in excess of 45 .
mu.
m, resolution would lower and the toner would readily scatter to cause considerable soiling of the developing unit.
If the mean particle diameter of the carrier is less than 10 .
mu.
m, more carrier would be dragged out.
The mean particle diameter used herein is a 50% particle diameter determined upon calculation of volume average particle diameter from measurements by the micro track method.
It is calculated from the data obtained by dispersing a particulate sample in water with the aid of a dispersant and carrying out measurement on a volume basis using a micro-track type STD 7991-0 (Leeds & Northrup Co.
).
The identity of the carrier is not critical to the invention.
The carrier may be formed of various soft magnetic materials such as iron, magnetite and various ferrites.
The ferrites used herein may be of various well known compositions include Mg-Cu-Zn ferrite, Ni-Zn ferrite, and Cu-Zn ferrite.
The carrier may have a coating of acrylic resin, silicone resin or fluoride resin, if desired.
The carrier may contain a binder such as a polyester resin and styrene acrylic resin like the toner which will be described later.
The carrier may have a coercive force Hc of up to 50 oersted (Oe) upon magnetization at 5000 Oe, preferably up to 20 Oe at 5000 Oe.
Carriers with a coercive force of more than 50 Oe would sometimes be unsatisfactory in carrying the toner.
The carrier may have a maximum magnetization .
sigma.
.
sub.
m of 25 to 220 emu/g, preferably 30 to 210 emu/g upon magnetization at 5000 Oe.
Particularly, ferrite carriers preferably have a maximum magnetization